# Simple Attenuators - Design And Testing



## JohnH

*Background*

Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve output stage develop, but without excessive volume.

The attenuator must present a load to the amp that is similar to a speaker and also maintain the tone as volume is reduced. It needs a consistent tonal and dynamic response from low attenuation, down to sub-bedroom level. This is where the simplest designs can be inferior, and the best commercial designs get expensive.

With feedback and testing by others I think we have a design that achieves this. For about $100-$120, anyone with workshop skills and the ability to follow a circuit schematic can build this. I want to thank everyone who has built one of these or contributed to this thread, with a special thankyou to @Gene Ballzz who was the first to see the potential and has been a great source of insight and practical help for everyone.

*An important point:*

Anyone who builds it does so at their own risk, and takes responsibility for working out their own wiring for their own private, non-commercial use, and completing it safely


*Summary: November 2022*

This thread focusses on the design of reactive attenuators for DIY building using simple construction and inexpensive parts. It started with just a few resistors and then developed into multistage resistive and reactive designs. The latest designs work much better than I’d first expected and have been built and tested successfully by many others. The design continues to evolve but the main principles have been constant for several years. This 1st post shows the current basic design M2, and a few possible additions to it.




It must be matched to the output tap of the amp, eg 8 Ohm or 16 Ohms. Component values for both are given, which differ by a factor of two:.

There are 4 attenuation stages, engaged or bypassed by switches.

Stage 1 is the key reactive stage and includes an inductor coil. This stage on its own, reduces power by a factor of 5, or -7db, reducing a 50W amp to 10W. The inductor coil is configured so that the impedance presented to the amp is similar to that of a real speaker (values based on various Celestions), particularly how impedance rises with frequency.

After Stage 1, three more stages are provided. These can be mixed and matched, but the design shown is based around additional -3.5db, -7db and -14db stages. By combining these switches in combination, and with Stage 1, reductions of up to -31.5db can be achieved in small, equal steps of -3.5db, at which point a 50W amp, at full power, is playing quietly at about 35mW.

On pages* 111* and* 112* are two construction layouts which may assist (designs are very close to above, not identical though). I recommend studying the diagrams above to understand the connections and then adapt the layouts to suit your needs. 





Simple Attenuators - Design And Testing


Wow this forum is amazing, im hoping to build a design for my amp, Im looking to have a headphone out as well as line out, becuase of the headphone out i want this section to have a cab sim, I was just wondering what was the point of m3 vs m2 does the m3 have a cab simulation as you keep...




www.marshallforum.com









Simple Attenuators - Design And Testing


Thanks @Markus Bender , that's very interesting indeed. Based on that, it looks to be just a simple resistive attenuator with some tone shaping. Once turned down, the amp sees almost none of that reactance. Here's a quote from their site: "We built into the RockCrusher a reactive load network...




www.marshallforum.com





Further features can be added, discussed in the thread, such as bass resonance circuit, foot-switchable stage, variable input impedances, bypass switch etc.

If you'd like to see a schematic with most of these further add-ons, go to Page 158, post 3146 from 19/11/2022.





Simple Attenuators - Design And Testing


Hi pietro, I reckon the case should not be grounded, and with the plastic Cliff jacks that we recommend, then the case is not in circuit at all. @JohnH , I have a "proposed" guess as to why we've encountered issues when the jacks get "grounded/connected together" through the metal enclosure...




www.marshallforum.com





*Component values and power ratings*

The table above shows the recomended power ratings for each resistor, based on up to 50W amps. The component ratings need to have a good margin above the actual power. I use a factor of at least 3 for case-mounted aluminium resistors, bolted (using thermal grease) to a heavy metal chassis or heatsink , and a factor of 5 or more for air-cooled resistors. These values fit with the spec in the schematic diagram above and also allow for overdrive of the amp.

Wire for hookup and also the winding of air-cored inductors should be 18 gage for 50W attenuators, and this is also OK for a 100W one, if built to the 16 Ohm values. For switches, use at least 5A rating (at 125V ac) for a 50W 8 Ohm build. The best jacks are plastic Cliff jacks, TRS (ie stereo type) which grip the plugs better than mono jacks.


*Cooling*

With amps > 30W at high power, the unit will heat up as it dissipates power. A good size die-cast aluminium case is best. Once components are positioned, then a number of additional large vent holes should be drilled, in the top and in the base, with feet to raise up the base. This will help to promote good convective flow of air out through the top, replaced by cool air at the base. The best colours for cooling are dark. For amps more than 50W, a fan should be added

Mount the coil without using a ferrous bolt, with a few mm timber or plastic spacer off the case surface.



Thanks for reading. If you are interested, and since this is a long thread, I suggest to read this post, look at the layouts and the most recent few pages, then make a post yourself and we'll be happy to discuss what would work best for you.

For nice collection of completed builds, see this thread:






Completed JohnH Attenuators?


Hey Fine Folks, The Simple Attenuator thread has become so lengthy that it is difficult to ascertain how many successfully completed builds there are of the fantastic @JohnH attenuator design. :thumbs: I'm starting this thread with the intention of showcasing all of these great builds. It is my...




www.marshallforum.com





And in the most recent post there (20/12/2022), @rowandg has presented sound samples stepping through attenuation settings, using an M2 built by @Gene Ballzz:






Completed JohnH Attenuators?


Now, these I would not be embarrassed to have sitting on top of my amp, onstage! Although, these two were built for a couple friends! My sincerest thanks to @DeluxeReverb for the inspiration to switch over to countersunk metric screws, for resistor mounting and to @Dblgun for the "gutter grate"...




www.marshallforum.com







*Attenuator M ( January 2019)*

The following is the original reactive design M which I built myself, with sound samples. M and M2 are closely related and perform the same, although M has two coils. It also has a bypass switch and a -3.5dB setting.

The build was in a case 170 x 120 x 55mm of thick aluminium:













AttenuatorM Outside 190217



__ JohnH
__ Feb 16, 2019


















AttenuatorM Inside 190217



__ JohnH
__ Feb 16, 2019


















Attenuator M 190110



__ JohnH
__ Jan 10, 2019
__ 1







*Simplified version M-Lite*

This was the same design, omitting the bypass switching and the 3rd output. Minimum attenuation is -7db. Its an easier build and it should still meet most needs.













Attenuator M-lite



__ JohnH
__ Aug 24, 2019







*Performance*

In the schematic above, there is a graph showing a calculated frequency response at each attenuation level from 0 to -31.5 db. These use a spreadsheet to calculate the signal at each stage of the circuit, as a series of voltage dividers, using complex number theory to assess magnitudes and phase angles. The speaker was represented, for analysis, by an equivalent load model, by Aiken:

http://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator

...adjusted to match the measured performance of a G12M 4x12 cab. The plots are based on small signals, with the amp output impedance assumed to be 20 Ohms, for an 8 Ohm tap, based on measurements of my VM2266C. These calcs were used to adjust the values in the design.


*Sound Samples*

The ideal is for volume to reduce, but with no change in tone or feel. This is best tested with a consistent loop, with attenuated sounds then normalised back to equal volume:

*Attenuator M: Max attenuation to non-attenuated:*


*Attenuator M: Normalised:*


It’s a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in 3.5db steps. The second file is based on the same recording, with each stage normalised for volume so you can hear any differences in the tone.

My VM2266c amp was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off a speaker.













Attenuator M Frequency Plots 190302



__ JohnH
__ Mar 1, 2019






The plots are taken from the sound sample posted above. The lower set of data are the basic plots, from full volume down to -31db (db scale is arbitrary, but relative db's are right).

The upper plots are intended to show the differences between responses. I took the -7db recording as the base case, so this is shown as a flat line. The others are the various other settings, with the -7db trace subtracted. The ideal for these traces is therefore also a flat line. And for all the traces below -7db down to -31db, this is what is happening, there is virtually no further tonal change at all as you attenuate down as far as you want. It measures as consistent.


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## tmingle

Here is the best attenuator comparison Ive seen so far.



Does your attenuator work better IYO than the MV? Did you try it with the 401?


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## JohnH

This was originally the first post, on 22/10/2017, moved here to allow my July 2019 summary in post 1




*A Fixed Attenuator*


This is about a simple fixed-level attenuator that I've been using with my Vintage Modern VM2266c. I built this mainly for low volume practice at home, in order to be able to get the volume up a bit but without disturbing others too much. Also, there's a lot written about attenuating a valve amp and how it does or does not change the tone, and I wanted to understand this better myself. The following is still in a testing form.



*Schematic*


I've tried a few recipes, but the current one that I've been using these past few months has three fixed resistances, combined to in theory take -7db off the input power, which is a x0.2 power reduction, so 40W becomes 8W. Each resistance is made up of several 5W power resistors. With 40W from the amp and an 8Ohm load, each resistor is dissipating between 2W and 4W.









The idea of the three resistances is to not only match the Ohms as seen by the amp, but also to control the impedance as seen by the speaker. This gives the speaker a bit of damping but not too much, as when directly connected to a tube amp. This is different to an SS amp which has super-low output impedance, which creates high damping and suppresses some of the speakers natural response.



*Construction*


It's currently built on tag-board, inside an aluminium box 120mmx90mm. Plastic jacks make sure the box is not connected at all.















*Sounds in use*


I like the sound of my VM2266c best at about 5 on the dial, at which point it is way too loud for home, but this box makes it feasible. At much lower settings, the attenuator squashes the residual hiss and hum and lets me get up to 2 or 3 for a reasonably quiet practice. At anywhere up to half-way volume, the resistors are not noticeably warming up at all.


Although, based on the nominal 5W ratings of the resistor, it could theoretically absorb the full amp output, as currently built it's not really set up for that. The resistors would need to be 10W types to give a margin, and it needs to get some better ventilation happening too. So I don't use it for rehearsal or gigs, and there really is no need with this amp since it doesn't need to be cranked to 10 to sound great.



*Tests*


Here are some tests from the VM2266c:



I miced up the amp with a Rode M1 and recorded via a neutral mixer into Audacity. I put a strum from a bridge Hb into a loop pedal, then recorded at Volume 5.5. I used LDR mode with Body at 6 and Detail at 8. Then with no change of settings and the loop still running, I closed down the amp, inserted the attenuator, switched on and re-recorded.



Normalizing both traces showed that Audacity was finding a -9.7db difference in the attenuated, rather than -7db as calculated.



This is the sound: There are three of the full strum, then three of the attenuated strum, to show what this approx -10db attenuation sounds like. Then there are three of each, full then attenuated but normalised to equal total volume. With those you can listen for tonal differences:


https://soundclick.com/share.cfm?id=13646224

This is the first part of the waveform, full and then attenuated and normalised:







They are very similar but you can see a few differences.

Then I exported the frequency spectrums into a spreadsheet to compare them, by subtracting one from another. This showed that, after adjusting for overall level, the attenuated sounds have a slight boost in the range up to about 1500hz and a slight dip above that. The differences are about 1 to 2 db. Its not much but you can just hear it. It's not enough to really change the character of the tone or turn a good sound bad, but it is a small noticeable difference, within range of EQ if needed.


This is the two spectrum plots. the green line is the difference in db, showing how the attenuated sounds has a db or so more mids and less treble:







--------------------------------------------------------------------------------------------------------------


That is another great vid from JS. I could hardly hear any difference in the normalsed tones right down to -30 db reduction. But his attenuator is more sophisticated than what Im testing.

Im using my fixed attenuator in conjunction with the MV on my VM2266c. For quiet playing, it sounds much better at volume 3 and using the attenuator, than at volume 1 without. At very low volume settings, my VM combo sounds a bit thinner than it might, and the residual hiss is more noticeable. The attenuator improves this significantly.

To be honest, I havn't yet tried it with the DSL401. I find this amp to sound great all the way to zero volume. But I do want to test it just for the sake of science. It has a very different power amp circuit, with a different ppimv design and no negative feedback. Im thinking tbat could mean a higher amp output impedance, less damping. I want to see what that does to the slight tonal shift that I found with the VM. I also want to test with an SS amp (Crate PowerBlock), just for info even though I'd not use it in practice.


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## SG~GUY

-ARACOM- best ive ever heard for a MARSHALL or Marshall platform
-MESA- -(Trans Atlantic)- whatever ALCHEMY,.. or Black Magic deal they have with satan that sacrifices power tubes,... Id gladly pay to have it in a stand alone unit right next to the big bag of power tubes thst need to be sacrificed,.. Its like a PERPETUAL MOTION MACHINE- the more attenuation that you use the better it sounds

---*(here's the sad part of the story, the very important missing ingredient with any such device,... they cant account for a pushed speaker,.. or a speakers character,.. or the sound a speaker makes with significant wattage)*---


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## JohnH

SG~GUY said:


> -
> 
> ---*(here's the sad part of the story, the very important missing ingredient with any such device,... they cant account for a pushed speaker,.. or a speakers character,.. or the sound a speaker makes with significant wattage)*---



Yep, there's no replacing the tone of a loud speaker except with a loud loudspeaker. But when it has to be quiet, id still rather have a bigger amp with all its features and full sized speakers than a small boxy-sounding one. My hope for an attenuator box like this it a one step change to bring a gig-sized amp down to the power of a 5W home amp, without losing too much and no need to buy the small amp.


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## SG~GUY

-im right there with you John, i think we both come from a time when a 50w amp was/is considered small. All else being equal I'll take the 100 watter every time. I dont understand the "lunch box" thing, they're full of features, they're cute & -- "sound" -- good, I guess,... ya know,... First it was our dogs,... Now,...its our amps.... We're next John,...... THERE GONNA CUT OUR BALLS OFF TOO.....


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## tmingle

SG~GUY said:


> -im right there with you John, i think we both come from a time when a 50w amp was/is considered small. All else being equal I'll take the 100 watter every time. I dont understand the "lunch box" thing, they're full of features, they're cute & -- "sound" -- good, I guess,... ya know,... First it was our dogs,... Now,...its our amps.... We're next John,...... THERE GONNA CUT OUR BALLS OFF TOO.....


I tried several of the lunchbox amps (Blackstar HT-5, VHT Special 6 Ultra, Peavey MH20) and found the DSL40C to be better at low volume than any of them.


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## SG~GUY

tmingle said:


> I tried several of the lunchbox amps (Blackstar HT-5, VHT Special 6 Ultra, Peavey MH20) and found the DSL40C to be better at low volume than any of them.




-but does it have balls?-


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## SG~GUY

JohnH said:


> *A Fixed Attenuator*
> 
> This is about a simple fixed-level attenuator that I've been using with my Vintage Modern VM2266c. I built this mainly for low volume practice at home, in order to be able to get the volume up a bit but without disturbing others too much. Also, there's a lot written about attenuating a valve amp and how it does or does not change the tone, and I wanted to understand this better myself. The following is still in a testing form.
> 
> 
> *Schematic*
> 
> I've tried a few recipes, but the current one that I've been using these past few months has three fixed resistances, combined to in theory take -7db off the input power, which is a x0.2 power reduction, so 40W becomes 8W. Each resistance is made up of several 5W power resistors. With 40W from the amp and an 8Ohm load, each resistor is dissipating between 2W and 4W.
> 
> 
> 
> 
> 
> 
> 
> The idea of the three resistances is to not only match the Ohms as seen by the amp, but also to control the impedance as seen by the speaker. This gives the speaker a bit of damping but not too much, as when directly connected to a tube amp. This is different to an SS amp which has super-low output impedance, which creates high damping and suppresses some of the speakers natural response.
> 
> 
> *Construction*
> 
> It's currently built on tag-board, inside an aluminium box 120mmx90mm. Plastic jacks make sure the box is not connected at all.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> *Sounds in use*
> 
> I like the sound of my VM2266c best at about 5 on the dial, at which point it is way too loud for home, but this box makes it feasible. At much lower settings, the attenuator squashes the residual hiss and hum and lets me get up to 2 or 3 for a reasonably quiet practice. At anywhere up to half-way volume, the resistors are not noticeably warming up at all.
> 
> Although, based on the nominal 5W ratings of the resistor, it could theoretically absorb the full amp output, as currently built it's not really set up for that. The resistors would need to be 10W types to give a margin, and it needs to get some better ventilation happening too. So I don't use it for rehearsal or gigs, and there really is no need with this amp since it doesn't need to be cranked to 10 to sound great.
> 
> 
> *Tests*
> 
> Here are some tests from the VM2266c:
> 
> 
> I miced up the amp with a Rode M1 and recorded via a neutral mixer into Audacity. I put a strum from a bridge Hb into a loop pedal, then recorded at Volume 5.5. I used LDR mode with Body at 6 and Detail at 8. Then with no change of settings and the loop still running, I closed down the amp, inserted the attenuator, switched on and re-recorded.
> 
> 
> Normalizing both traces showed that Audacity was finding a -9.7db difference in the attenuated, rather than -7db as calculated.
> 
> 
> This is the sound: There are three of the full strum, then three of the attenuated strum, to show what this approx -10db attenuation sounds like. Then there are three of each, full then attenuated but normalised to equal total volume. With those you can listen for tonal differences:
> 
> https://soundclick.com/share.cfm?id=13646224
> 
> 
> This is the first part of the waveform, full and then attenuated and nomalised:
> 
> 
> 
> 
> 
> 
> THey are very similar but you can see a few differences.
> 
> 
> Then I exported the frequency spectrums into a spreadsheet to compare them, by subtracting one from another. This showed that, after adjusting for overall level, the attenuated sounds have a slight boost in the range up to about 1500hz and a slight dip above that. The differences are about 1 to 2 db. Its not much but you can just hear it. It's not enough to really change the character of the tone or turn a good sound bad, but it is a small noticeable difference, within range of EQ if needed.
> 
> This is the two spectrum plots. the green line is the difference in db, showing how the attenuated sounds has a db or so more mids and less treble:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I hope that is interesting to some. I'm finding this useful, and it was a good learning experience to test it.




-the acoustic part sounds really good John, ive listened to it a few times, is it original?

-also some random things on attenuation-
-i have a EH 12AX7 that one day went south, amp went almost silent, i turned it up and it sounded really good but still quiet, ended up turning every nob on the amp up to 10.... Completely BAD ASS!!! but its a time bomb and i took it out and put it in a marked box, i pull it out every now and then for 15 minutes of the sound in my head

-always use a variac @ 80-90v-

-Mostly with a 2203KK in V-1. But works in every slot just not as well. Tried it on a JJ100 as well, same effect, same sound-( a heavy metal violin crying violently as her metal soul melts to liquid )-it scares the hell out of me thou, afraid. its gonna turn my amps to liquid lava!!!! And it seems i lost it in my move to the west coast, but i havent really looked for it either-(What could it be?)-

-theres a guy on TGP who has a socket adapter/s for this obscure power tube that reduces output to whisper levels, a friend has this set up and it sounds kinda similar to my -bad tube-


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## BowerR64

What about an Iso cab? is there a comparison of the 3? no attenuation, attenuation and then a miked Iso cab?


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## JohnH

SG~GUY said:


> -the acoustic part sounds really good John, ive listened to it a few times, is it original?
> 
> -also some random things on attenuation-
> -i have a EH 12AX7 that one day went south, amp went almost silent, i turned it up and it sounded really good but still quiet, ended up turning every nob on the amp up to 10.... Completely BAD ASS!!! but its a time bomb and i took it out and put it in a marked box, i pull it out every now and then for 15 minutes of the sound in my head
> 
> -always use a variac @ 80-90v-
> 
> -Mostly with a 2203KK in V-1. But works in every slot just not as well. Tried it on a JJ100 as well, same effect, same sound-( a heavy metal violin crying violently as her metal soul melts to liquid )-it scares the hell out of me thou, afraid. its gonna turn my amps to liquid lava!!!! And it seems i lost it in my move to the west coast, but i havent really looked for it either-(What could it be?)-
> 
> -theres a guy on TGP who has a socket adapter/s for this obscure power tube that reduces output to whisper levels, a friend has this set up and it sounds kinda similar to my -bad tube-



Thanks SG. I think what's happening is that the link to my small attenuator sample is putting up my whole play list, which is all various tests. If so, the one that comes up next is me testing a Digitech phaser - its electric but recorded without amp, and the riff is based on Crowded House 'Take the weather with you'

A variac is interesting - I wonder if it would affect any solid-state parts of the amp that were looking for certain supply voltages.


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## Micky

JohnH said:


> A variac is interesting - I wonder if it would affect any solid-state parts of the amp that were looking for certain supply voltages.


It will try...

A variac will lower the input mains voltage, which will in turn lower the output voltage of the PT.
If a PT has a tap for 12 or 5V to drive a solid state circuit (say, for switching or other control circuitry) it will lower that feed voltage as well. Sometimes there is a regulator circuit that will try to keep the voltage stable at a certain voltage, but many times there is not, and there will be a point where the SS stuff will become unstable.
It depends on how much you lower the voltage where that unstable point will be.


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## SG~GUY

-ive found when --ON-- indicator light goes dim or out that in 3 to 5 secounds the caps will drain & the amp fades away to silence-

-Marshall's -(especially JMP's)- could go the lowest in the high 70's to 80 volts area, usually keep it at 85-90 volts... 

-odd thing,... when lowering the voltage, the amps get very quiet, loses sensitivity to other electrical devices that create oscillation.... Exposes's witch doctor "power conditioners" show what they really are.. Uselesss placebo's.... Variacs are Useless against "DIRTY POWER" -(like 99% of power scrubbers unless you have big $$$ on something like is used in big raised floor server rooms)- i have "dirty power" from a 12v DC TRICKLE CHARGER...... had to put an off/on swith on it....


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## JohnH

I tried the attenuator box with my DSL401. I was quite interesting. This has an internal 16ohm V30, and I usually run it with another V30 in an extension cab, for an 8ohm load.

My attenuator, current version, is calculated for an 8ohm load. But using it with a 16ohm speaker makes only about an ohm of difference to what the amp sees, so its safe to try. So I ran it with one 16ohm speaker, via the attenuator, to the 8ohm amp tap.

It worked fine, but with a more significant tonal change than before (which was the 8ohm 2x12 pair on the VM). More loss of highs, and also a few db off the low bass. Not a bad sound, but not consistent.

Then I hooked up the second speaker, to restore an 8ohm load. Highs and bass were restored, with just the slightest treble roll-off, as with the VM before, equivalent to about 2 steps on the treble plot.

Then I dug out some specs and worked some numbers. All of this was predictable, and probably well known but it’s a learning process for me. I think one of the key parameters in a simple resistive attenuator, after power and impedance seen by the amp, is the impedance as seen by the speaker. This has two effects. At treble frequencies, it is part of an LR filter together with speaker inductance and dc resistance. Higher output Z of the attenuator reduces treble roll off in proportion to low frequencies. At bass frequencies, a higher attenuator output impedance reduces damping of the cone, allowing a bit more bass to develop. In the tests above, due to these effects, there was a greater tonal affect on the higher impedance 16ohm single speaker than the 8ohm pair.

The current box lets the speaker see an output impedance of about 5.5ohms. The next build will raise that slightly to about 7ohms for slightly more treble consistency. This one will be with case-mounted high power resistors (100w each in a ‘Pi' arrangement as before, 20, 8, 20) which I’ve ordered. It will be with a box drilled for ventilation and should be an interesting test and hopefully sound good.


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## mickeydg5

Are you finding the drop in wattage to the speaker worth the loss in frequencies/tone?


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## JohnH

mickeydg5 said:


> Are you finding the drop in wattage to the speaker worth the loss in frequencies/tone?



Yes indeed, particularly for the VM when playing quietly. It allows the volume to be raised smoothly to about 3, where the mv is sounding good and the tone is smooth. It also squashes out the residual hum and hiss, which is not at all high but its nice to knock it down. Tone change with the 8ohm pair is very small and is well compensated with 2 steos up on ths treble knob.

The dsl401 Ive only tried briefly for the testing above, but I think that this amp may deal better than the VM with very low volume.


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## JohnH

I’ve had my simple attenuator running with my VM2266C, as a test version for a few months. So now I’ve rebuilt it in a more permanent form with better robustness and cooling.

This one goes for a single reduction of -9db, which is a 1/8th factor on power, bringing a 40W amp down to a home-friendly 5W. There are three resistors, all nominally rated at 100W, two 10Ohms in parallel to make 5 Ohms, and a 4 Ohm, forming a voltage divider. With this and an 8 Ohm load, the amp sees 8 Ohms and the speaker sees about 3 Ohms. The resistors dissipate 9 or 13W each max.

For a -6db version, all resistors would be 8ohms.







It's all in a compact case of about 120mm x 90mm, so although it has plenty of electrical power rating, its useable capacity is controlled by cooling. There are several features to promote this:

1. Power resistors bolted with thermal grease to the underside of the top plate

2. Aluminium case for high conductivity and thermal capacity

3 Top plate drilled for internal ventilation, with holes in base for make-up air inlet

4. Base raised on feet to allow ventilation underneath

5. Outer surface black to increase heat loss by black-body radiation 











I've tested it and it sounds fine, nice and clear. I can play at 4 or 5 in a small room quite comfortably and fully crank if I want to. At these levels, the presence control is quite effective so there are ways to tweak the tone if needed.

But for me, half the interest in trying this is play through it and the other half is to test it and learn about how its working. 

I plan to run some tone samples to see how tone is changed. On cooling, so far I cant detect a noticeable temperature rise, but it would take sustained thrashing at high volume to heat it up. Another test I might try is to take it off the amp and run some dc through it to make it dissipate a known power. I can measure case temperature with an IR thermometer.


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## Micky

I'd like to see how hot you can get it.
300W of resistance is probably overkill...


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## JohnH

Micky said:


> I'd like to see how hot you can get it.
> 300W of resistance is probably overkill...



I agree, but you can get them for a dollar each from china!. Also, I suspect that that nominal rating will only apply if they are totally cooled to remain at ambient temp.


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## Micky

I use a couple of those mounted to a heatsink for a dummy load when testing the output of an amp with a scope. Even pushing a hundred watts continuous they barely get warm...


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## RickyLee

JohnH said:


> I’ve had my simple attenuator running with my VM2266C, as a test version for a few months. So now I’ve rebuilt it in a more permanent form with better robustness and cooling.
> 
> This one goes for a single reduction of -9db, which is a 1/8th factor on power, bringing a 40W amp down to a home-friendly 5W. There are three resistors, all nominally rated at 100W, two 10Ohms in parallel to make 5 Ohms, and a 4 Ohm, forming a voltage divider. With this and an 8 Ohm load, the amp sees 8 Ohms and the speaker sees about 3 Ohms. The resistors dissipate 9 or 13W each max.
> 
> For a -6db version, all resistors would be 8ohms.
> 
> 
> 
> 
> 
> 
> 
> It's all in a compact case of about 120mm x 90mm, so although it has plenty of electrical power rating, its useable capacity is controlled by cooling. There are several features to promote this:
> 
> 1. Power resistors bolted with thermal grease to the underside of the top plate
> 
> 2. Aluminium case for high conductivity and thermal capacity
> 
> 3 Top plate drilled for internal ventilation, with holes in base for make-up air inlet
> 
> 4. Base raised on feet to allow ventilation underneath
> 
> 5. Outer surface black to increase heat loss by black-body radiation
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I've tested it and it sounds fine, nice and clear. I can play at 4 or 5 in a small room quite comfortably and fully crank if I want to. At these levels, the presence control is quite effective so there are ways to tweak the tone if needed.
> 
> But for me, half the interest in trying this is play through it and the other half is to test it and learn about how its working.
> 
> I plan to run some tone samples to see how tone is changed. On cooling, so far I cant detect a noticeable temperature rise, but it would take sustained thrashing at high volume to heat it up. Another test I might try is to take it off the amp and run some dc through it to make it dissipate a known power. I can measure case temperature with an IR thermometer.



This one you have shown here is a bit closer to one I built many years ago. But this one I built is larger and has two rheostats/controls on it with its array of wire wound resistors.

I had settings/formulas worked out for decibel level adjusts as to where the two controls were ti be set. But all the info is on an old computer. Funny as I seen this thread as I have been thinking about digging that attenuator box out and using it in conjunction with these Eminence FDM speakers I now have . . . .


----------



## JohnH

RickyLee said:


> This one you have shown here is a bit closer to one I built many years ago. But this one I built is larger and has two rheostats/controls on it with its array of wire wound resistors.
> 
> I had settings/formulas worked out for decibel level adjusts as to where the two controls were ti be set. But all the info is on an old computer. Funny as I seen this thread as I have been thinking about digging that attenuator box out and using it in conjunction with these Eminence FDM speakers I now have . . . .



I looked up abouf those FDM's. A very clever idea based on dialing the distance between magnet and voice coil, to give up to -9db reduction. It would be interesting to know how the results compare to your switched attenuator, at least for the sake of science.

I was using my rebuilt box to do a bit of practising at whisper volumes before work this morning, while family was sleeping. It sounded fine for what it was doing, and it helped me start my day. Loud testing will be next weekend.


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## RickyLee

JohnH said:


> I looked up abouf those FDM's. A very clever idea based on dialing the distance between magnet and voice coil, to give up to -9db reduction. It would be interesting to know how the results compare to your switched attenuator, at least for the sake of science.
> 
> I was using my rebuilt box to do a bit of practising at whisper volumes before work this morning, while family was sleeping. It sounded fine for what it was doing, and it helped me start my day. Loud testing will be next weekend.



The last time I had my home made attenuator box out, I noticed the two controls had been changed. I am now curious to tinker with it, but I am wondering if it really matters for harms sake to the amp, what the ohms readings should be on the IN and OUT of this attenuator box.

If I have a close ohms reading at the IN (will use 8 ohms here, this is where the amplifiers out will be plugged in) with a speaker plugged into OUT ( 8 ohm speaker), then it should be safe to use?

I do remember there being well over 100W of wire wound resistors in a series/parallel/series type configuration inside this thing.


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## JohnH

I guess if the amp sees 8ohms then it's happy. Without knowing the design, can't tell how much total power it can take since Im guessing some resistors take more than others. Maybe watch for how hot they get.


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## tmingle

SG~GUY said:


> -but does it have balls?-


I know its been a while. I think the low volume thing has more to do with how your ears perceive the volume(Fletcher-Munson?) Ive mic'ed my amp & recorderdit at 4 different volumes. I then normalized the the volumes in Reaper to compare. It's closer than I thought it would be. Unfortunately, I can't find the clips right now to post.


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## JohnH

I've been trying some thermal tests on the new box, to see what happens. I haven't brought the IR thermometer home yet though for a proper measurement though.

I'm running a 9V dc power pack into the amp input, with no added load. So the attenuator has a resistance of 9 Ohms on its own. I'm getting 8.5V at the jack, so power = 8.5^2/9 = 8W.

It has definitely warmed up, but has taken about a half hour to get to a steady state. Its not hot though, not nearly as hot as the front plate on some amps can get. I guessing maybe a 20 degree C rise. but Ill test again properly.

That power, as a constant dc, would equate to an average dissipation from an amp over several minutes and a max dissipation of considerably more than. Physics says that doubling the power input should less than double the temperature rise, so I'm expecting that this box could soak up a flat out 40W amp, in practice, without adding much more than say 50C rise. TBC.


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## JohnH

I made a short sound clip, to find out how significant any tonal change is, at -9db. 

Its a low gain setting on the VM, bridge humbucker, played through a looper, with about 5 body and 7 detail. Volume is at 6. Recording is miced with a Rode M1. First without the attenuator, then with it and the recording level normalised later to match volume.

Actually I cant hear a difference in tone, but the room volume is very much less at this level of reduction:

https://vocaroo.com/i/s05Qix2IkSdw

Its not a great recorded tone for this amp, I usually like it thicker. The idea wasn't about absolute tone, but to try to reveal differences particularly in highs, due to the attenuation.


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## JohnH

Who likes numbers? Here's some more, based on better temperature measurements using an Infrared thermometer from the engineering school where I teach.

I ran dc power into the attenuator with measured voltage 8.55 V and measured resistance 9.2 Ohm. So power in was 7.9W

I ran for 30 minutes, at which time it had slowly reached a steady state with 35 C max on the top plate (about 33 C average) and average about 25C at the sides. Ambient in the room was 16.5 C.

That's as much dc as I can put through with what I have to hand. But based on these numbers, I did some maths with convection and radiation and I reckon that with an average of about 25W into it, it would get to about 55C. The hotter these things get, the better the mechanisms of passive cooling work. Given that this is an average value over at least 10 to 15 minutes, and it could easily absorb several times that for a shorter time, then that should be more than adequate for a 40 to 50W amp, given that any practical use has varying amplitude levels and gaps in the playing.

So I think this little unit is brick-solid electrically, thermally and mechanically, with no switches or moving parts, plus it sounds good. For a few $ I have a 5W Vintage Modern when I need it, running with its awesome 2x12 G12c speakers.

But Im sure the 1x10's in your 5w combos sound just fine!


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## iron broadsword

Great unit! I built a similar one from this pdf: http://www.hearditontheweb.com/images/pdf/attenuator.pdf

Two stages, -6db each. Used it for a while with my 900 and then got a weber mass so I could go even further. Now this guy is setup on my mesa which I use for recording. It's handy as hell.. no tone loss and 12db is enough to make a difference in pushing the output tubes past cold lifeless into hot and tasty. The box I used is pretty similar to yours and I also drilled it full of holes for heat dissipation. People talk a lot of shit about these types of attenuators but if all you do is 12db you'll never notice a loss in tone.


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## JohnH

iron broadsword said:


> Great unit! I built a similar one from this pdf: http://www.hearditontheweb.com/images/pdf/attenuator.pdf
> 
> Two stages, -6db each. Used it for a while with my 900 and then got a weber mass so I could go even further. Now this guy is setup on my mesa which I use for recording. It's handy as hell.. no tone loss and 12db is enough to make a difference in pushing the output tubes past cold lifeless into hot and tasty. The box I used is pretty similar to yours and I also drilled it full of holes for heat dissipation. People talk a lot of shit about these types of attenuators but if all you do is 12db you'll never notice a loss in tone.



That's a good arrangement, and it made me do some more tests yesterday with more attenuation, going from -9db to -12db. There's a very important thing to know about tone and resistive attenuators:

Assuming the load on the amp is correct, then the resistance seen by the speaker has a huge effect on tone, due to the speakers inductance. The two stage one above has this as about 4 Ohms.

We can use this in the design of the resistor network to control the tendency to lose high treble. If the speaker sees a higher output impedance, the effect is a bit more high end. I took my basic -9db L-pad and added another 4 ohms in series with the speaker. This took it down to -12db, and also lifted the high end, compensating for slight loses Id heard before. I did some modelling and it shows this too. The speaker is seeing about 6.5 Ohms. However, if you do -12db with a simple two resistor L-pad, then the speaker sees about 2 Ohms and it starts to sound dull. Within a range, you can design the resistor network to pick what output impedance you want for a given attenuation, and use this to balance the tone.


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## JohnH

I'm still working of this, exploring options and design paarmeters:

*Resistive attenuator designs*

As described above, I’ve been building and testing quite a few purely resistive attenuators, for use on my Marshall Vintage Modern 2266c. This is a 50W amp with a pair of 12” Greenback speakers for a nominal 8 Ohm speaker load. Its partly because I want one for use, and partly to learn by testing, and wishing to start simple.

The attenuators are very basic, just a single attenuation value with no switching. Typically, there are three resistors in a ‘star’ configuration. This allows me to pick values to target attenuation db, plus both input and output impedance. I maintain the nominal input impedance as seen by the amp at close to 8 ohms, including the speakers. I’ve most recently been targeting around -12db basic attenuation, bringing a 40W amp down to 2.5W for home use.

Here are three resistor layouts that all do -12db with around 8 Ohms seen by the amp.






Type A on the left, is a simple L-pad and it sounds noticeably dull. Type B in the middle sounds quite good, its clearer with much nearer unattenuated tones. Type C gets brighter again, see analysis below. The main difference between them is the output impedance as seen by the speaker. Based on nominal resistance values, speaker in Type A sees 2 Ohms, B sees 6.6 Ohms and C sees about 26 Ohms. Type C is an extreme example, I tested it and it is a bit bright and harsh, but A and B are quite feasible.

I’ve built Type B into a box and have been using it all volume settings. Comparison of recordings attenuated and not attenuated, using a looped clip then miced and normalised for equal volume show negligible audible tone difference (though a small response difference is measurable).

So what puzzled me is that I'm getting this good result by having quite a high output impedance from the attenuator, so far as I understand, 6.6 Ohms is much greater than most amps output Z.

*Amp with reactive load: response*

Aitkens page below shows how the electrical response of an amp varies with frequency when driving a reactive load.

http://www.aikenamps.com/index.php/parallel-attenuator-loads

This could be a real speaker, or a load box designed to mimic it. His page is focusing on how this changes if a further resistive load is in parallel, but Im just looking at the full response curves in green. For amps with or without feedback loops, the response varies in a similar fashion to that expected from the speakers reactive impedance. For an amp with a feedback loop (such as mine), there is a lift in response of around 3db from 500hz up to 5khz due to speaker inductance, and a boost in bass due to resonance, as shown here:






Clearly the real amp and reactive speaker load are interacting intimately to make this response.

With a resistive attenuators at -12db, amp and reactive speaker load are almost completely separated. The amp is seeing almost a totally resistive load and the speaker sees just a resistive output Z. About 15/16ths of the amp power is going into resistors in this case.

But Spice analysis of this situation shows how the reactive impedance of the speaker could interact with a pure resistive output Z to get very close to unattenuated frequency response, at an attenuated output level.

I used the reactive load box design from Aitken, scaled to an 8 Ohm version, to represent a real speaker, and tested the calculated frequency response, in terms of signal seen by the load, within the range of output impedances from 2 to 26 Ohms, as for designs A to C above:







Note: in the Spice program I use, I can’t vary more than 2 components at once, so the plots below show frequency response with varying amp output impedance Z but without controlling overall attenuation level – but it’s the relative shapes vs output Z that are important.






The plots show the signal seen by the speaker, with output Z from the attenuator varying from 2 to 26 Ohms.

The arrangement Type B that I built, with an output impedance of 6.6 Ohms, is very close to the red curve on that graph, but at -12db. It shows just under a 3db rise in treble from 500Hz to 5kHz, ie very close to the full amp/reactive load curve from Aitken. I reckon that helps to explain why it is sounding pretty good!

As the output Z increases further, about 3.5 more db of high and low relative boost is available.

This seems like a helpful idea, ie controlling Z to make simple attenuators sound more like an unattenuated amp at least in terms of basic frequency response. So what’s missing? I can’t see any undue risks, since resistor attenuators can be built very robustly and are fine for loading a valve amp. At moderate distortion levels, I think the match is reasonably valid, but at high power-amp distortion, there could be tonal differences. The tubes will still be saturating as they drive into the resistive load, and this will come through in the tone, but Id guess that some of the more subtle interactions of tube/OT/real speaker will be suppressed by the resistances between them.

*-12db Attenuator build
*
This is the current -12db build, based on the Type B design. The 5 ohm resistor is 2x10 in parallel, and those and the grounded resistor each dissipate around 12W based on a 40W amp output. They are oversized at 100W rating. The output series resistor is a 10W wire-wound, but it only dissipates 1.25W, so its all overbuilt.


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## JohnH

Not sure who's watching but I'm pretty excited. I just took this to a new level (lower), with a switch to go from -12db to -18db. And it sounds clear and full and I was just cranking the VM, maxed out in HDR mode, at about 0.8 of a Watt!

I've realised that the trick to getting good tone with high attenuation is to keep the speaker seeing a reasonably high output resistance, and a value of about 7 or 8 ohms is about the sweet spot for an 8 ohm speaker. Higher output resistance is brighter, lower is less bright. It's all to do with letting the natural reactance and resonance of the speaker develop, without too much damping (see discussion above).

Here is this weeks schematic:






I ganged two resistors for R1/R2, since this pair takes most of the power. Also, there is a 4Ohm and a 3.9Ohm shown at R3 and R5. There's nothing critical about that slight difference, its just that the big Chinese power resistors come in slightly non-standard values.

Assuming 50W total amp power, then the red resistors dissipate about 15W each,the green ones handle about 1.5W each and the blue dissipates less than 0.5W. I used 100w, 10W and 5W types, so it's well over-speced. The switch is a mini-toggle switch, DPDT with the two sides ganged in parallel to double current capacity. But it never sees the full amp power, so it should have no issues.

There's a couple of extra 'Easter Eggs' with this design:

1. Provided the amp is used with its 8 Ohm tap, you can safely plug any Ohms of speaker into it. There is enough resistor separation from amp to speaker that the amp always sees close to 8 Ohms, within about 1/3 of an Ohm.

2. My toggle switch is actually an on-off-on, so in the centre position, it is disconnected. This leaves the speaker seeing about 15 Ohms, a little brighter tone but also perfect for running a 16 Ohm cab at an attenuation of about -15db.

Here it is as built today:






This is a pretty handy and compact box as it is now, just right for home, bringing a 50W amp down to about 3W or 0.8W (-12db or -18db). With a bigger enclosure, a useful development could be to add a -6db option, and maybe a full bypass switch. A neat version with all of that would be a two stage attenuator, with one stage calculated for -6db and the other for -12db, each with an on/off switch. Then four settings would be possible, 0db, -6db, -12db and -18db.


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## gearhead

John, the pic you posted with the large power resistors looks like my dummy load for testing amplifier output. I have (2) 100 watt, 8 ohm resistors mounted to an aluminum heatsink with an 80mm fan on it. and have a cable with a 1/4" mono plug and alligator clips so I can connect one or both resistors for 4, 8 or 16 ohm load. I am only using it to look at sine wave clipping, voltage and form at power. Now it's a purely resistive load. Speakers are a reactive load, meaning that the resistance changes at certain frequencies (usually rated minimum ohms @ the resonance frequency of the speaker), due to the capacitance of the voice coil winding (it's an R-C circuit). An 8 ohm speaker is not 8 ohm ALL of the time. I've never tried to measure the impedance at different frequencies, that may be an interesting experiment one day.
Not sure I would want to run one of my expensive tube amps at close to full volume on my dummy load. But since you're connecting speakers with yours, then they give it some capacitance. Should be fine for what you are doing.


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## JohnH

A bit more:

I tested the approximate output resistance of the VM this week. As I have recently come to expect, its quite high and I was getting 16 to 20 Ohms with the 8 Ohm tap. It's is consistent with a valve output stage being close to a constant current source, until it clips. I did this test by loading the amp with the attenuator box and no speaker, which using the switch, gives about 9 or 8.5Ohms. A smallish signal was fed in, not wanting to push the amp in this situation. With the lower ohms load, the output voltage drops almost in proportion, and back figuring the implied voltage divider of amp output impedance and load, lead to these values.

This highish measured output Z is further evidence that if you want to make a simple passive/resistive attenuator sound half decent, you have to keep the output impedance up high, so the speaker sees something more like the original amp output impedance and is not damped by low value resistors. Doing this, gets the design to be capable of matching the full tone, at least for signals in the clean range.

This is this weeks schematic:






R7 was added, taking the attenuation values down another 3 db to -21db max, and brightening the tone with a 13 to 14 Ohm output resistance.

Inside:






I tested again, a looped signal, the VM fully cranked in HDR, volume at 8 and close miced. Jeez that's loud!, even with industrial ear protection. Then attenuated -21db, and its just enjoyably loud. But the frequency response and audible tones are very similar. The traces do vary a little (probably due to the speakers being driven or not) but there's no overall trend of losses of highs or lows and once recordings are matched for volume, they sound the same to me. I did another run with the presence cranked up, and that was a much more significant change in tone.


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## 1199RS

Hi john have been following this thread as it is very interesting reading. I have been using a hotplate for a while now and when pushing master volume i get two tubes red-plating so i made a resistive load box from 4 68ohm 100w resistors all wired in parallel. Then into a di box and into the front end of a jubilee set clean. This has eliminated the red plating but there is some ground loop buzz as i am not using an isolating transformer. Your little schematic looks like a far better and easier option. I have been playing with some values as i think i need to drop some more db as it is for a 100w plexi. I thought the 2nd voltage divider could change to 12 ohms to drop more voltage. My circuit calc app has output watts as 0.7 with it. Just curious as to how that will affect what the speaker sees and tonal implications.
Cant physically try it as i am currently stuck offshore due to fog
Thanks for the read


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## JohnH

1199RS said:


> Hi john have been following this thread as it is very interesting reading. I have been using a hotplate for a while now and when pushing master volume i get two tubes red-plating so i made a resistive load box from 4 68ohm 100w resistors all wired in parallel. Then into a di box and into the front end of a jubilee set clean. This has eliminated the red plating but there is some ground loop buzz as i am not using an isolating transformer. Your little schematic looks like a far better and easier option. I have been playing with some values as i think i need to drop some more db as it is for a 100w plexi. I thought the 2nd voltage divider could change to 12 ohms to drop more voltage. My circuit calc app has output watts as 0.7 with it. Just curious as to how that will affect what the speaker sees and tonal implications.
> Cant physically try it as i am currently stuck offshore due to fog
> Thanks for the read



Thanks for your interest. I don't know about the red-plating, generally that wouldn't be fixed by load boxes or attenuators, which encourage you to drive the amp harder.

But with this resistive attenuator scheme, Ive got spreadsheets happening to work them out to any level of db reduction. I'm on a roll, so I'm going to build another one, starting again. This will be 3 stage. A fixed -7db stage, then two switchable stages of around -7db and -13 db. That will give four settings in combination of -7db, -14db, -20db and -27db. based on a 50W amp, this will give power levels of 10W (rehearsal), 2.5W (loud home use), 0.5W (quiet home use) and 0.1W (late night). Im not building in a bypass, 0db switch. There's no point since if this is needed, you just don't use the attenuator. There will be 2x output jacks, so a pair of 16Ohm speakers can be used.

In all of them, the design needs to keep the ohms seen by the amp reasonably close to the spec. I'm allowing around 7.5 to 9 Ohms for the 8 ohm tap. Also, the output resistance needs to be kept up highish. Based on some tests and calcs, I'm targeting 18 to 20 Ohms output resistance in all settings for this new build, into an 8 Ohm speaker, to let the natural treble and bass resonance from the speaker develop.

Based on the design above that you saw, dropping the 18ohm to 12 will reduce power, at the expense of output resistance also dropping so it may sound a bit duller. Better to recalculate all value to suit. If you are interested to see what might be needed, give me some power or db values that you'd like to achieve with your amp and Ill run some numbers. Also what ohms cab would you use?

Note that my build is quite compact, and with the case-mounted power resistors and ventilation, Its fine for a 40-50W amp. but I think that for 100W, although the components may still be ok on paper, more serious cooling is needed which could mean a more spacious box with better ventilation or heatsinks.


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## 1199RS

Hi John thanks for speedy reply. I usually practice around 85db at night and during the day up to 100 is ok for the neighbors My amp is putting out around 120 or so db at full tilt but usually have master around 6 or 7. Using an 8ohm cab but have a standard 1960 as well.

I have some designs for an aluminium heat sink box to get rid of excess heat. Your box looks around 5" or so in length going by the resistors what temperature does it get up to with sustained playing?

The design you are working on now sounds just what I'm after, I thought I would parallel more resistors in the first stage to build in a factor of safety. 

Cheers, kev.


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## JohnH

The 100W resistors are 60mm body length, and the current box is 120x90 mm. The way I play, cleanish with breakup at about 6 volume, its hard to get it to heat up. But I did run a loud distorted thrash through it at max for a while and it heated up then, but not to a point where you couldnt touch it for an extended time, maybe 50C temp rise. But while the step from 50 to 100W is a small step in volume, Id expect its a big factor on heat, going from 'quite hot' to 'OUCH #@!!'.


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## 1199RS

Ha ha ye I imagine so as my load box gets to around 70 deg c after an half an hour or so of playing. I have not used thermal paste to seat the resistors yet as was waiting to see how it functioned first.
I have the first two stages cascaded on mine so only cleans I get are when I roll back guitar volume.
I think I will add cooling fins on the box this time much like the hot plate to help dissipate some of the heat.


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## JohnH

Im waiting on a new batch of power-resistors from China, which take a while. But given the interest, Ill post the new schematic soon.


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## 1199RS

Thats great john cheers


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## JohnH

This is what I have in mind:













Attenuator180601



__ JohnH
__ Jun 1, 2018






As described above, with three stages and two switches to give four attenuation settings. Ive shown the input impedance as seen by the amp, and the output impedance as seen by the speaker. Power dissipations are the max as calculated in any setting. For my 50W amp, I plan to use a 100W resistor for R1, and 5W for R6, 50W for the rest.


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## 1199RS

Great work John. Using the minimum variance of values too. Now im just waiting on RS delivering components so as to get it assembled.
Time to get started on the enclosure...


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## JohnH

You might have it done before me then. Chinese shipments are sloooow!

I'm putting in twin output jacks since my other amp uses a 16 Ohm speaker plus a 16 Ohm extension cab. But actually, I reckon it should also be quite safe to use this 8 ohm design for a single 16 Ohm cab. It would still go into the 8 Ohm amp output. At the minimum attenuation of -7db, the input resistance would rise to a nominal 9.5 Ohms, but that's still within 20% of 8 ohms. And in any case, if directly driving a speaker, the amp would see higher than 8 Ohms at low and high frequencies anyway.

The reason that this is OK is that with the first fixed stage, most of the load on the amp is determined and doesn't vary, reducing any mismatch due to the cab. At greater attenuation, the later stages keep the amp seeing closer to 8 Ohms.

If using a 16 Ohm cab, the natural treble and bass boost provided by the output resistance and speaker reactance will drop slightly, but maths suggest this will be only about a -1db difference so not too significant. eg, at 5khZ and at around 100hZ, we expect about a 4 to 5 db rise in signal reaching the speaker, as compared to 500hZ. I believe these boosts happen in most valve guitar amps, and help to compensate for the roll-off in speaker sensitivity at these ends of the spectrum.


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## JohnH

While I wait for parts, I thought I'd work out some wiring on this photoshop mash-up, based on a photo of some of the parts on top of the box:












Attenuator180602 Wiring



__ JohnH
__ Jun 2, 2018






it looks like I can use the same 120x90 box size I was using before. R1 and R2 which take most of the heat will be mounted under the top face and the other 3 larger resistors onto the side faces.


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## 1199RS

Very compact john. Ordered some 25 x 6 aluminium flat bar to weld vertically with spaces on top of the heatsink as vanes. Slightly concerned with heat generated so have doubled up the first 2 resistors (2x 30r & 2x 20r) 50w for the rest.


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## JohnH

That's cool if you have the gear to weld aluminium! I agree about those power ratings, for a 100W amp.


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## 1199RS

Cheaper to weld the Vanes on just now and if it works ok may get one machined from billet.
I can usually get my hands on most materials for nothing as my day job is rig welder. Most of my chassis are 2mm fully welded stainless steel. Better than aluminium or carbon steel. I cut them out of scrap electrical panel doors


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## JohnH

My parts came and so yesterday I built my new three-stage version:






Luckily it worked first time. Although it's not a particularly complicated circuit, getting everything fixed and connected properly in the Aluminium box was right at the limit of my ability with regard to vision and dexterity. A beam of clear winter sunlight coming in through the window was a big help!

So there's a single input jack to go to an 8 Ohm amp output and twin output jacks. There's an overall -7db attenuation, then two toggles to add a further -7 and-13db, for a maximum of -27db. At that point, a 50W amp is down to 0.1W. A quick test showed that this is is definately enough for the quietest noodling. I wasn't sure but now I know.

The twin outputs are mainly so I can plug in twin 16 Ohm speakers, from my DSL401 and its extension cab. But Im also going to try another 'wrinkle', which is just experimental so far:

Although the design is based around 8 Ohms, the circuit is also safe to run with a single 16 Ohm cab, using an 8 Ohm tap, due to the first fixed stage. But a single 16 Ohm speaker, being of higher impedance, may want to see a higher output Z for best tone (TBC). So, the twin sockets are wired so that if you plug into the right one, an extra resistor is added in series. But if you plug into the left socket, or into both sockets at once, it does not add this. This also compensates for what would be a slightly higher output using this design, for a single 16 Ohm cab. All this is done by using stereo type sockets for the jacks, which also grip the plugs better due to the additional spring contact.

I havn't tested that twin-socket idea yet, but stepping through the four available settings at 8 Ohms on the VM combo, they are all sounding fine so far. The steps of 6 and 7 db between settings are useful, but not huge. Im not missing having a continuous control or smaller steps. This amp seems to sound best to me at about mid-way on the volume knob, so for home practice I think the -20db setting may be the one I use most.

Apart from just having an attenuator for use, the biggest motivation for these designs is just to experiment and learn. When I've played it some more, Ill try to post some decent audio and measured tests.


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## 1199RS

Great work john looks great. Mine is a fair bit bigger to fit the 100w resistors in. Yours looked pretty smart in black so thought i would follow suit as had some textured paint laying around.
Waiting on a 50w 15r so cant test yet as must have forgotten to order 2.


----------



## 1199RS

Great idea about the switched out resistor. I was thinking about replacing the last 15r with a 50r 5w pot but that may be overkill for reducing volume as i never really play that quiet. I know it would affect the impedance as seen by speaker and amp, in any case it may not be required.


----------



## JohnH

That case looks awesome. Nice work on the welding. IRL Im a structural engineer so I appreciate the skill even though I never mastered it myself.


----------



## JohnH

Ive had it running with both my amps now. With the VM, I played for about a half hour at 7 volume and -27db, edge of breakup type of tone. Sounded good, definately with more happening in the tone than at lower levels, but this amp does great tone anywhere from 3 up depending how its set so Im spoilt for choice. The unit eventually got warm, but not at all hot.

With the DSL401, a 40W combo, I had it running with one and two 16ohm speakers out of the 8ohm amp tap. I was able to try the twin outputs with resistor 'trick'. With both speakers, providing an 8ohm net load, again all was great with a full clear tone. moving to just one x16ohm, I tried the two outputs, one of which adds another 6ohms in series. Subtle difference, adding the resistor creates a slight bass/treble boost - mid cut. Both good sounds though.

Maybe my ears are not critical enough, it all seems too 'good'. AlI I can say for sure so far is that the worst descriptions you read online about the tone of simple attenuators at very high attenuation are not happening in this case. But it really needs more quantitative and recorded tests.


----------



## 1199RS

Thats great john sounds like you have given it a pretty thorough shakedown already. Looking forward to trying my 100w through it.
Does the switchable resistor perform as you expected with a bit of mid cut?
Ye i have read a fair bit on attenuators and tone suck and it all seems a bit contradictory best thing to do is try it and see what you think. Glad your liking it so far.


----------



## JohnH

Yes that mid-cut effect fits the theory, but measurements are needed to be sure. The risk of plotting too much theory can be a tendency to hear what one expects! But yes Im pleased with it so far.


----------



## 1199RS

Ok got my resistor today so 5 mins work and its up and running, so first impressions... 

Hooked up VM head as its a fair bit quieter and straight away it sounded to me like i was connected straight into speaker cab without any attenuator in circuit. 
Both switches function as intended and must confess have never heard VM so clear with master dimed and gains on 7-8. Previous attenuator was an 8 ohm hotplate and it not totally transparent i know when its connected.

Hooked up my 100w and with my settings where i normally play its outputting 94db which im delighted with ha ha ha but the wife not so. Could do with another stage to drop to 80-85db for when the ball and chain is around . 

Very happy with the sound so far not much heat coming from it so far (10 mins)but will need a longer session to verify..

Great work john


----------



## JohnH

Great news! Thanks for the update.


----------



## JohnH

If you want to add another stage Id expect it would be fine, with no significant changes to tone or impedance. 

Currently, a 100W amp is in theory, being reduced down to 0.2W, ie a factor of 1/500 on power. Another -7db stage would bring it down by another factor of 5 to 0.04W. That would be by adding another resistor pair like R2 and R3. You could test that after the current output with a temporary wire-up. With the other stages all before it, there'd be almost no heat in it. If it worked ok, then it could be added permanently with a switch, before or after the current second stage.

Or maybe you have some other spare parts around? could maybe work something out to try.


----------



## 1199RS

Ye i was going to do just that and wire it in temporarily just before the jack.. or I could just turn the master down a bit ha ha ha. 
I think I will leave it as is just now and see how it operates first. I can always add another stage later if that's what I want.


----------



## J5684

This is cool, thanks for sharing.. I might have to give this a go on my 2466..


----------



## tmingle

I'm really interested in building 1 to use with the 16 ohm tap to 16 ohm speaker for my 40C. This amp sounds great as is but I can't crank it in a small room. 
Is there a final drawing posted? 
Thanks John.


----------



## JohnH

tmingle said:


> I'm really interested in building 1 to use with the 16 ohm tap to 16 ohm speaker for my 40C. This amp sounds great as is but I can't crank it in a small room.
> Is there a final drawing posted?
> Thanks John.



Hi tmingle, thanks for your interest

The schematic at post #42 (previous page) is the current/final version for using an 8 Ohm tap and 8 Ohm speaker. If you just want a 16 Ohm to 16 Ohm version, just double all values. May need to then adjust to the nearest standard values depending where you buy them. Power ratings would be the same. I'm happy to run any variation through my spreadsheet.

But the 8 ohm (amp tap) version I built is also sounding good with 16 Ohm and 2x16 Ohm speakers with the DSl401. It gives the option of using an extension cab with twin output jacks. Ill post how Ive wired that with an extra resistor over the weekend.


----------



## 1199RS

Have put a good few hours on the attenuator today and I am totally sold on it tried it back to back with my hot plate and it definitely sounded markedly clearer to my ears. Also with the hot plate I get v4 & v5 redplating slightly as I play with master above 6 this doesn't happen with the homebrew. 
This has been a real issue for my jubilee clone as I used a plexi power transformer which puts 500vdc on plates but with the homebrew was able to crank the master and lead master... totally different animal with some volume.
I think I will add another stage to drop some more volume on a switch for late night playing as I saw 96db on the max attenuation setting which is a bit loud for others in the house.
I would say the case got reasonably hot but not so I could not hold it. Maybe around 50 deg C not sure what that is in Fahrenheit. And that was after constant use for 2 hours approx. Actually my power transformer gets much hotter and I constantly check it as I had one go up in smoke not too long ago.


----------



## JohnH

That's great feedback, thanks, very encouraging!

Ive been blasting away on mine too, but with my 40-50W amp I cant get it more than warm. Even with my compact build, there's so much mass of case and aluminium-clad resistors that any heat gain takes quite a while to build up, and depends on an average power over at least 10-15 minutes rather than peak values. 

Hopefully this w/e I can run some tests to measure what I'm hearing.


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## Micky

tmingle said:


> I'm really interested in building 1 to use with the 16 ohm tap to 16 ohm speaker for my 40C. This amp sounds great as is but I can't crank it in a small room.
> Is there a final drawing posted?
> Thanks John.


It would be great to post a final 16-ohm design...


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## tmingle

Thanks John.
I was pretty sure that doubling the values would get me there. 
I have a bin full various power resistors I have been scrounging from old equipment & a lot of the values look familiar.


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## JohnH

tmingle said:


> Thanks John.
> I was pretty sure that doubling the values would get me there.
> I have a bin full various power resistors I have been scrounging from old equipment & a lot of the values look familiar.



Sure. It seems that power resistors often come in a different series of values than smaller ones. Im buying from a Chinese ebay retailer for the ones Ive used, and their range also includes exact x2 values too. But, in a standard range more usual elsewhere, 10, and 15 exist, but 20 and 30 do not, nearest being 18, 22, 27, 33. Lets see what you have in your spares box. Its all not super critical but picking the best combinstions from whats available helps to keep the attenuation steps and impedances as consistent as possible.


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## JohnH

Here are schematics for four variants of the three-stage attenuator:

*Design A* is the basic 8 Ohm version, with twin output jacks for either a single 8 Ohm or 2 x 16 Ohm speakers. The resistor values are as per the previous diagram. Power dissipations and attenuation levels are shown for an amp input of 50W.












AttenuatorA 180624



__ JohnH
__ Jun 23, 2018






*Design B* is as A, plus the extra resistor between the ground wires of the two outputs, aimed at balancing the output and tone if you use it with a 16 Ohm cab. This works by adding to the output impedance when Output 2 is used, which sounds a bit brighter with a 16 Ohm cab. If Output 1 or both outputs are used, this is bypassed. The effect is achieved using the ring connections on 'stereo' TRS jacks. Design B is the one I have built currently.












AttenuatorB 180624



__ JohnH
__ Jun 23, 2018






*Design C *is a simple 16 Ohm to 16 Ohm version, all values are as Design A x2 and so it should perform the same.












AttenuatorC 180624



__ JohnH
__ Jun 23, 2018
__ 2






*Design D* is also for 16 to 16 ohms but uses slightly different values, based on the more common series of standard resistors. The db increments are a bit smaller, typically 6db instead of 7db.












AttenuatorD 180624



__ JohnH
__ Jun 23, 2018







*General Notes*


Power dissipations are listed above based on 50W amp input. I’d suggest a factor of at least x3 on these for selecting components and use case-mounted resistors for those taking significant power.


The two switches used together can set up four attenuation settings. Eg, for Designs A and B running into 8 Ohms, a 50W amp can be brought down to 10W, 2W, 0.6W or 0.12W. If more reduction is wanted, then a further -7db switched stage can be added in the middle, based on values for R3 and R4. This would bring power down by another factor of 1/5th, to 0.03W (or 0.06W for a 100W amp).


There’s intentionally no ‘Off = 0db’ switch shown. My view is that if you don’t want to attenuate, then don’t use an attenuator! Having the first stage fixed, and taking most of the power, takes pressure off later stages so there is less current being switched and less power running in later resistors. Also, the amp always gets some load and there is no nasty transient moment when the switches either fully shunt or disconnect the amp. But if an ‘Off’ switch is wanted, I’d suggest a full bypass switch of the whole circuit, like a loop switch or a ‘True-bypass’ switch in a stomp box. But this switch would need to carry the whole current (eg, around 3.5A for a 100w into 8 Ohms), and should only be switched with the amp off or on standby. 


If anyone else tries these designs, it would be great to hear about them!


----------



## JohnH

I realized I cant do recordings until I get my main PC fixed. So instead I did some high and low attenuation listening tests on the DSL401 and VM. I set a clean preamp signal, max attenuation and master at about 8, tried some chords and arpeggios. Then I switched down to min attenuation and reduced the master to the same room volume. I got the same overall tonal balance but could clearly hear the cleaner tone / more headroom due to absence of the missing power amp harmonics. It happened with both amps. I guess that's the whole idea.


----------



## JohnH

Id like to ask for a bit of help from anyone building the attenuator shown above, either now or in the future. What amps have you tried it with and what is your impression of the tone?, eg too bright, thin, muddy, or just right? And if it makes a difference, at what settings?

I’ve worked it out to suit my amps and speakers (a VM2266C and a DSL401 with 1 or 2 x V30’s). I’m happy with it now but different rigs respond differently to various attenuator designs, plus there’s personal taste.

It would be possible to adjust this design further, and it’s also possible to introduce a control to change output resistance without affecting attenuation levels. This affects the extent to which bass and treble is lifted, changing the tonal balance. Any interest?


----------



## JohnH

1199RS said:


> Have put a good few hours on the attenuator today and I am totally sold on it tried it back to back with my hot plate and it definitely sounded markedly clearer to my ears. Also *with the hot plate I get v4 & v5 redplating slightly as I play with master above 6* this doesn't happen with the homebrew. .



Searching around to learn more, I found this link that may explain the red-plating with the Hotplate, and why this doesn't happen with a resistive attenuator. See posts by 'Club and country':

http://marshallroadhouse.com/viewtopic.php?f=2&t=2825&sid=6434bb5e971bb62827d124a8c58e6166


----------



## 1199RS

Thanks for that john. I had found that article a while back when the problem manifested. I went through a lot of changing of capacitors in my amp which didnt cure the red plating before searching for issues with the hot plate. Only thing that helped was running at 4 ohms into 8ohm hotplate and not getting above 5 or 6 on pre M.V.
Using your design my master is almost never off 10, and no issues.
Other than heat it does get hot now after an hour or so of playing its around 60 or 70 deg c almost too hot to hold your hand on.


----------



## tmingle

I found a 50 ohm rheostat in my pile of junk to use as the 2nd stage of John's 16 ohm version. Do I even need the 3rd stage? I also am going to add a line out to use with impulses & will probably just order the remainder of the items from Mouser.


----------



## JohnH

Hi tmingle. Nice idea about the reostat, but Its maybe not so easy. The key to the consistent tone of this design is the consistent impedances seen by amp and speaker, and using this could throw them off. But if you could let me consider further I might figure something. In which case:

is it just a two-lug unit or is it three lugs like a pot?
Power or current rating: is that a 1Amp rating? Lets say you are running at 10w after stage 1 into 16 ohm, then there is about 0.8Amp running, a bit too close for ir all to go through that piece?


----------



## tmingle

I located & purchased 100 watt resistors for John's 16 ohm version from a USA seller on Ebay for $38.
https://www.ebay.com/usr/mdflyelectronics
I will test them before I use them with my amp.


----------



## tmingle

JohnH said:


> Hi tmingle. Nice idea about the reostat, but Its maybe not so easy. The key to the consistent tone of this design is the consistent impedances seen by amp and speaker, and using this could throw them off. But if you could let me consider further I might figure something. In which case:
> 
> is it just a two-lug unit or is it three lugs like a pot?
> Power or current rating: is that a 1Amp rating? Lets say you are running at 10w after stage 1 into 16 ohm, then there is about 0.8Amp running, a bit too close for ir all to go through that piece?


It's 3 lugs. I was afraid of the same thing, so I ordered the resistors in my previous post. Since they were cheap, I went with all 100W resistors. My 40C starts to sound better above about 2-3 on the volumes. I have a small room & 2-3 is too loud. It really starts to cook above 4, which is way too loud. If I read the schematic correctly, the 40C MV is not the same as the 401.


----------



## JohnH

ok, thats great. so you can anyways build the all-switched version as an option. But Im interested to figure out how a pot can be incorporated, so Ill see what I can do.

Yes I think the 40C has a normal MV while the 401 had a PPIMV. There's also that thing about the C19 bright cap that folk around here seem to snip or reduce on a 40C.


----------



## tmingle

JohnH said:


> ok, thats great. so you can anyways build the all-switched version as an option. But Im interested to figure out how a pot can be incorporated, so Ill see what I can do.
> 
> Yes I think the 40C has a normal MV while the 401 had a PPIMV. There's also that thing about the C19 bright cap that folk around here seem to snip or reduce on a 40C.


I removed it and then put it back in while I installed my bias pots a few months back.. I don't use L2 much on the 40C.


----------



## JohnH

I had a go at designing the 50 Ohm pot into the 16 Ohm attenuator circuit, at Stage 3. Stages 1 and 2 are unchanged.

There are three other resistors around the pot, aimed at controlling impedances and also current flow through the pot. With Stage 2 off and Stage 3 at minimun, with 50W input, the greatest current through the pot is calculated at 0 73 Amp. Thats within a 1A rating, but it would be expected to heat up.

In general, I was going for ouput impedance in the range 2 to 2 5 x nominal ohms with these designs. This one peaks at about 46 Ohms, just under 3x nominal Should be Ok, but not tested yet.













Attenuator E



__ JohnH
__ Jul 9, 2018






With this arrangement, the pot is giving a 10 db range, providing from 3 to 13 db of attenuation when Stage 3 is engaged. Using this and the other stages together, you have -7db, and then any setting needed in the range from -10 to -27db.

Note that the switch for Stage 3 now needs two poles instead of 1.

I think this could be particularly useful for an NMV amp, or for those who just have to twiddle a knob. But for me, I think I prefer the switched-only versions, since they are simpler and more electrically robust.


----------



## JohnH

tmingle said:


> I also am going to add a line out to use with impulses



Its a good idea. I reckon just take your line-out directly from the speaker, via a 10k pot if you want its own control. That way you get all the electrical interactions of resonance and inductance from the speaker. Its almost like a full (expensive) inductive load-box, with the advantage of using the real speaker instead of an electrical model.


----------



## JohnH

My recording pc got fixed, and there was a short window of opportunity, so I've thrown together this clip of my attenuator:

https://vocaroo.com/i/s12fL9gJ6MGJ

What you hear is the Vintage Modern combo in LDR mode, body and detail at 6 and 8, LP bridge pickup. MV is at 7. The guitar is looped through my delay pedal, miced off one of the speakers with a Rode M1 and into the pc via a mixer set neutrally.

First, the build-up of attenuated levels, from -27db, -20db. -13db -7db and full volume. The attenuated ones I switched by hand in real time, then spliced on the full volume clip.

Then, exactly the same set of recordings with each section normalised for equal volume, so you can judge the consistency.

Im pleased! What do you think?


----------



## tmingle

JohnH said:


> My recording pc got fixed, and there was a short window of opportunity, so I've thrown together this clip of my attenuator:
> 
> https://vocaroo.com/i/s1nw8BeTxIo2
> 
> What you hear is the Vintage Modern combo in LDR mode, body and detail at 6 and 8, LP bridge pickup. MV is at 7. The guitar is looped through my delay pedal, miced off one of the speakers with a Rode M1 and into the pc via a mixer set neutrally.
> 
> First, the build-up of attenuated levels, from -27db, -20db. -13db -7db and full volume. The attenuated ones I switched by hand in real time, then spliced on the full volume clip.
> 
> Then, exactly the same set of recordings with each section normalised for equal volume, so you can judge the consistency.
> 
> Im pleased! What do you think?


Sounds good to my ears. I am waiting on parts right now & then will need to find time to build it. It probably wont happen for a month or so.


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## tmingle

I ordered jacks, switches & the stuff for the line out today. I am 80$ in at this point. Not bad considering most attenuators cost $300. 
https://www.ebay.com/itm/Power-Plug...attenuator&_from=R40&rt=nc&_trksid=m570.l1313
I forgot to mention that I purchased 1 of these a couple years ago. It sounded good, but it didn't have a line out and I was worried about the component quality for the price & sent it back. 


JohnH said:


> Its a good idea. I reckon just take your line-out directly from the speaker, via a 10k pot if you want its own control. That way you get all the electrical interactions of resonance and inductance from the speaker. Its almost like a full (expensive) inductive load-box, with the advantage of using the real speaker instead of an electrical model.


----------



## GrahamL

Keep up the good work guys.... interested in your findings. This pedal may have some good ideas to look at and at a great price of $39 (I don't have one myself). 
https://carlscustomguitars.com/True...hable-effects-loop-looper-pedal-True-Loop.htm


----------



## JohnH

tmingle said:


> I ordered jacks, switches & the stuff for the line out today. I am 80$ in at this point. Not bad considering most attenuators cost $300.
> https://www.ebay.com/itm/Power-Plug-6-12-Attenuator-direct-from-the-designers/151308056240?hash=item233aa9b6b0:g:eXwAAMXQOT5Q9PWE&_sacat=0&_nkw=power+plug+attenuator&_from=R40&rt=nc&_trksid=m570.l1313
> I forgot to mention that I purchased 1 of these a couple years ago. It sounded good, but it didn't have a line out and I was worried about the component quality for the price & sent it back.



I think for what I built, Im at about $50 Aus for the parts actially used, including the Al box and a bit for paint, wire and solder. But Ive got at least 2x that in spare parts bought while I was changing my mind!

Those 'Power Plugs' seem to be a pair of L-pads in series. They could be quite reliable if they use decent case-mounted resistors (as we are), and they are just as simple. What should make our version better is the thinking that went into chosing the arrangement and values of the resistors to control impedances both input and output. A string of the usual 6db Lpads ends up showing the speaker about 4ohms, for an 8 ohm load, enough to damp the highs and lows. Ours reverses the Lpads and offers 4-5x that output impedance, much nearer to many tube guitar amps.


----------



## JohnH

GrahamL said:


> Keep up the good work guys.... interested in your findings. This pedal may have some good ideas to look at and at a great price of $39 (I don't have one myself).
> https://carlscustomguitars.com/True...hable-effects-loop-looper-pedal-True-Loop.htm



Thanks for the encouragement.

Id be a bit worried about using a footswitch on a passive attenuator though. There might be nasty momentary conditions if switched while playing


----------



## tmingle

I added an adjustable line out & bypass switch to my build. Now I am waiting for some more parts. I may also add a speaker on/off switch to the attenuator portion. 
How do you guys draw schematics? I used a free tool at digikey to add the line out.


----------



## JohnH

I just draw schematics using the graphics tools that come with Microsoft Office; Word, Excel or PowerPoint depending what Im doing. On active circuits, I use a circuit analysis program, but the one I use (5Spice) doesnt represent switches in the graphics.


----------



## JohnH

This is to show some frequency plots from the attenuator, to investigate how consistent the tone is at different settings. The plots are based on this clip, previously posted above:

https://vocaroo.com/i/s12fL9gJ6MGJ

The lower frequency plots show the spectrum, as recorded from a miced Greenback, at each attenuation setting plus unattenuated. The plots are spikey because they they show the actual notes in the recorded riff, and the various harmonics generated from them, rather than from a smooth input spectrum. You can see the general stepping down of 6 or 7 db between settings, and that the traces are reasonably parallel.













FreqPlots180719



__ JohnH
__ Jul 18, 2018






The upper, flatter plots show the difference in db at each frequency, attenuated-full. These lines are quite parallel and reasonably flat, indicating that the tonal balance is well maintained, even down to max attenuation at about -26 db. There is a slight bump at around 6khz, in all the traces, suggesting a slight boost in attenuated treble there. But by this frequency, the speaker output has dropped away significantly.


----------



## tmingle

JohnH said:


> I think for what I built, Im at about $50 Aus for the parts actially used, including the Al box and a bit for paint, wire and solder. But Ive got at least 2x that in spare parts bought while I was changing my mind!
> 
> Those 'Power Plugs' seem to be a pair of L-pads in series. They could be quite reliable if they use decent case-mounted resistors (as we are), and they are just as simple. What should make our version better is the thinking that went into chosing the arrangement and values of the resistors to control impedances both input and output. A string of the usual 6db Lpads ends up showing the speaker about 4ohms, for an 8 ohm load, enough to damp the highs and lows. Ours reverses the Lpads and offers 4-5x that output impedance, much nearer to many tube guitar amps.


I bought this enclosure for mine. I just crossed the 100$ mark, but I will know what is inside. It will also allow space to mod to reactive at some point. 
https://www.ebay.com/itm/Black-Alum...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649


----------



## JohnH

tmingle said:


> I bought this enclosure for mine. I just crossed the 100$ mark, but I will know what is inside. It will also allow space to mod to reactive at some point.
> https://www.ebay.com/itm/Black-Aluminum-Project-Box-Enclosure-Case-Electronic-DIY-203x144x68mm-US-Stock/112522896551?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649



That looks like a very nice enclosure, with a nice bit of mass to it to spread out the Watts. Measure twice and drill once!


----------



## JohnH

*Reactive Attenuators*

My last attenuator has been working well this last few weeks, and Im not hearing any tonal difference between attenuated and non-attenuated sounds using the resistive design Type B (see sound clip and frequency plots above). I think getting the impedances right seen by the speaker and amp is most of the requirements for consistent tone, and this can be done by resistors.

But it can't be the whole story, particularly when the amp is cranking. Although the speaker is responding with the right response, the amp is seeing a purely resistive load. This should make a difference when the output stage tubes and OT start to saturate, bringing effective output impedance down and adding damping and compression at the peaks. As I said above, I cant actually hear this happening. But I think I can see it. Take a look at this output trace from the sound clips:













Attenuator180710



__ JohnH
__ Aug 11, 2018






This shows the build up of increasing volume as attenuation stages are switched out, followed by the same all normalised to the same volume. See how the last part of the second trace, full volume unattenuated, has smaller peaking transients than the normalised attenuated, as if they are getting compressed (Im sure that there as no clipping in the recording chain).

So to test this out, Im adding inductive components to the base 3-stage design, like this, for an 8 Ohm system:













Attenuator F



__ JohnH
__ Aug 11, 2018






The trick to adding the inductances is not to mess with the basic small signal tone, which based on the work before, is coming out very nicely with just the resistors.

Also, Im expecting that the most important reactive effect to capture back to the amp is the high frequency rise I impedance, since this will affect the most tonally noticeable ranges. Im leaving the bass resonance to be developed only by the speaker, as before. This means that only relatively small inductances are needed.

This design is not tested yet, but Im proposing two inductors as shown above. Both L1 and L2 add to inductance seen by the amp, replacing that of the speaker voice coil when directly connected. L1 also tends to trim high frequencies from the attenuatedspeaker output. But L2 while also adding to what the speaker sees, tends to increase highs at the attenuated output. So playing with modelling software, this balance of the two seems like the sweet spot:, adding reactance to what the amp sees but keeping output tone very much the same with two 0.4mH inductors:













Attenuator Fplot



__ JohnH
__ Aug 11, 2018






What you see there is the output from SPICE analysis, in which real speakers are represented by a reactive load box design (Aikens in this case), testing with or without the attenuator. The red trace is the voltage seen by an unattenuated speaker, driven by an amp of high output impedance (20Ohms, as measured on a couple of my amps). The lower traces show the lowest two stages of attenuation. Under the red trace, you can see the voltage at the amp output while driving the attenuator, which rises with frequency as with a real speaker, and which would be flat were it not for L1 and L2.

This the schematic of the virtual test rig:













Attenuator F Test Schematic



__ JohnH
__ Aug 11, 2018







The cool thing about this is that the two inductors can be added to the same components as drawn before. Ive ordered my inductors (18awg air coil, intended for speaker cross overs) and will test as soon as I can.


----------



## tmingle

JohnH said:


> That looks like a very nice enclosure, with a nice bit of mass to it to spread out the Watts. Measure twice and drill once!


I got mine wired up today. With all 3 stages engaged & the volume above 5-6, the 40C starts to cut out after a few minutes. When I disengage the 3td stage all is well.
This amp did the same thing with the attenuator I sent back a couple years ago as well. I have never cranked this amp this loud before, so I fear this particular amp is not up to the task. I have replaced the power tubes since the last attenuator. The line out works great & sounds pretty good with impulses. I will double check the wiring tomorrow.


----------



## JohnH

Sorry to hear the amp has such issues. At a given setting on the volume knob, the amp wouldnt know wherher the 3rd stage is engaged or not. Except presumably since it would then be louder without stage 3, the amp would be turned down?

At settings where the amp was ok, what did you think of the attenuated sounds through the speaker?


----------



## tschrama

I am late in this discussion, but is 20 Ohm a realiatic value for the output impedance of the amplifier? Is that a 100W EL34 power amp?


----------



## 1199RS

tschrama said:


> I am late in this discussion, but is 20 Ohm a realiatic value for the output impedance of the amplifier? Is that a 100W EL34 power amp?


Its a 50w vintage modern. So 2 x kt66
I have been using johns design on my 100w jmp and 2555. Works fine so far.


----------



## JohnH

tschrama said:


> I am late in this discussion, but is 20 Ohm a realiatic value for the output impedance of the amplifier? Is that a 100W EL34 power amp?



Ive measured 20-30 ohms with small signsls on my amps. One is an el84 powered dsl401 andtbe other is a VM with kt66's. You run a constant signal in and measure output voltage across a resistor load, then change the load slightly and remeasure, then do maths.


----------



## tschrama

OK, thanks... 20 Ohm is suprissingly high to me... EL34 is prolly about halve that value since they have twice the transductance.

Interesting thought that the output impedance drops as the PA clips due to dropping plate resistance... previously reasont that as the PA clips, feedback drops to zero and output impedance jumps.


----------



## tmingle

JohnH said:


> Sorry to hear the amp has such issues. At a given setting on the volume knob, the amp wouldnt know wherher the 3rd stage is engaged or not. Except presumably since it would then be louder without stage 3, the amp would be turned down?
> 
> At settings where the amp was ok, what did you think of the attenuated sounds through the speaker?


It sounds pretty good. I dialed the EQ knobs a little to get rid of a little flub & fizz. Since I bought it used,this was probably an issue from day 1 that I never exposed. I can only get 8' from the amp in my room so a 40W Marshall on 5-6 is insanity! Pic & possibly clips to come.


----------



## JohnH

It would be great to see it and hear a clip, particularly if its feasible to do attenuated and unattenuated.

If you are getting good tones then its good news. But in principle if needed, it should be easy to calm down the bass and high treble by a couple of db with a resistor across the speaker out. eg, with your 16ohm build, about a 39 ohm should have a small but significant effect. This resistor would take less than 1.5W at max.

I was going for quite high output resistance, to match my amps. But DSL's have a different circuit and perhaps a bit lower output Z in the attenuator may give it a better match, as noted by tschrama above.


----------



## tmingle

JohnH said:


> It would be great to see it and hear a clip, particularly if its feasible to do attenuated and unattenuated.
> 
> If you are getting good tones then its good news. But in principle if needed, it should be easy to calm down the bass and high treble by a couple of db with a resistor across the speaker out. eg, with your 16ohm build, about a 39 ohm should have a small but significant effect. This resistor would take less than 1.5W at max.
> 
> I was going for quite high output resistance, to match my amps. But DSL's have a different circuit and perhaps a bit lower output Z in the attenuator may give it a better match, as noted by tschrama above.


I did record a clip at all levels of attenuation. Unfortunately, the drop in volume influences my playing too much to make a good test. It does sound similar at all volumes though. I plan to record a clip at higher volume, capture a DI thru my Countryman DI & re-amp thru the DSL at different attenuation levels to get a better comparison. I have a feeling the difference will be inaudible & I get to learn something new.

I suppose the higher volume cut outs are power supply related(filter caps, bad connections on the filter caps, etc.). I'll have to wait till I can stand outside the room & crank the amp without the attenuator connected. 

Comparing my sub-amateur micing skills & SM57 with the line out ran thru an impulse, the impulse wins easily.


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## tmingle

JohnH said:


> Sorry to hear the amp has such issues. At a given setting on the volume knob, the amp wouldnt know wherher the 3rd stage is engaged or not. Except presumably since it would then be louder without stage 3, the amp would be turned down?
> 
> At settings where the amp was ok, what did you think of the attenuated sounds through the speaker?


I wound up changing the power tubes. Was it a coincidence? IDK but, I did check the attenuator in all settings with an Ohm meter before I tested it with the amp. The resistance is a stable 16 ohms in all positions. For all the louder I play, This amp seems to blow thru power tubes pretty quickly. In about 9 months, this amp has chewed thru a set of JJ EL34L & a cheap set of chinese tubes. I checked the B+ after the JJs went south, and found no issues. I set the bias at 32mv & observe for about 30 minutes before finalizing the bias adjustments. I suppose its due to cheap tubes. I have a spare set so I wont need to buy any right away, but the next tubes will be of a higher quality.

To be honest, the DSL's Master volume with the EQ in the loop works just as well as the attenuator. The next time the amp is on the bench, I will install the line out in the amp & use the resistors to make a dummy load. 

Maybe its time for something different? I never change channels, but I do use all 4 modes in the DSL.


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## JohnH

Thanks for the feedback and Im glad the attenuator worked. DSL's may be types that get their tone mainly in the preamp, so if it sounds fine a lower MV settings, there could be less need to attenuate. But when you need to have minimal volume, maybe it can sound better with a bit of atten to get it out of first gear.

So anyway, Ive now built another unit to test the inductive reactance idea above. This one has two inductors and just one -7 db stage, designed to plug into my multistage resistive attenuator if more is needed. It works and sounds good, but maybe not better than just the resistive, and im sensing its adding treble despite tbe modelling suggesting otherwise. careful testing needed to be sure!


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## JohnH

tmingle said:


> The next time the amp is on the bench, I will install the line out in the amp & use the resistors to make a dummy load.



A further though on this:

If currently your line-out for IR use is coming from the attenuated speaker output, then so long as the speaker is there, the speaker is shaping the signal to lift highs and bass, as it does when directly connected to the amp. If so, then the existence of the speaker is a key part of getting the tone right. The resistors alone as a load wouldn't do that and you would then need another way to correct the tone, hence the usual rabbit-hole of expensive reactive load boxes etc


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## tmingle

JohnH said:


> A further though on this:
> 
> If currently your line-out for IR use is coming from the attenuated speaker output, then so long as the speaker is there, the speaker is shaping the signal to lift highs and bass, as it does when directly connected to the amp. If so, then the existence of the speaker is a key part of getting the tone right. The resistors alone as a load wouldn't do that and you would then need another way to correct the tone, hence the usual rabbit-hole of expensive reactive load boxes etc


I put the line out before the resistors. I didnt really notice much difference with the IR in attenuated or full power. I haven't totally thrown in the towel yet. I merely disconnected it until I have a spare set of output tubes & room on my work bench to check the filament & B+ supplies while playing.


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## JohnH

This is my new reactive attenuator module, basically the first stage of the schematic below it:












Reactive Module180821



__ JohnH
__ Aug 21, 2018


















Attenuator F



__ JohnH
__ Aug 11, 2018






I tried it with various settings of connecting it to the 3-stage one, at various attenuated volumes and volume settings on the VM. I was trying to listen for changes in tone and dynamics with and without the reactive coils (ie shorting them out with alligator leads), Really any change was between zero and negligible. This was suggesting that any change in the amp response due to it seeing a reactive load was not significant. I was cranking it with the resistors getting hot. If I had a bias in my mind, it was to try to hear a difference, but honestly I really couldn't.


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## JohnH

I've run some more tests on the effects of the added reactive components (inductors) on the resistive attenuator. The inductors are designed so that the basic calculated frequency response from a clean amp at the speaker is not affected (because it was a very good match before), but the impedance seen by the amp now rises with frequency to match that of a speaker, instead of staying constant at 8 Ohms. So the only difference should be if the real amp reacts significantly differently seeing a reactive load.

I tested as before, with a looped strum, miced and recorded neutrally. This time, the amp is at about 6 volume and both tests are at -21 db. Resistive attenuator (inductors bypassed), then reactive:













Resistivereactivestrace180824



__ JohnH
__ Aug 23, 2018






You can see differences in the traces.

But here is the sound clip, resistive-reactive-resistive-reactive:

https://vocaroo.com/i/s14V0EcGUbuZ

Can you hear a difference?


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## tschrama

bit more bite in the inductive... I think...


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## JohnH

JohnH said:


> Can you hear a difference?



(posted before seeing tschrama's above, but I agree!)

Ive looked, listened and thought a bit more....

If you look at the first 20% of each trace, as the strum is developing, the reactive trace is higher. At this time, masses of high harmonics are being generated from the guitar and the preamp crunch. With a rising reactive impedance vs frequency, the attenuator is drawing more out of the amp at those frequencies. This is because max power transfer is between two matched impedances, and the amp has a higher effective output impedance than the nominal 8 ohms. This is adding more 'sizzle' early in the trace, as the power amp tries to react. Later in the trace, high harmonics have significantly died away, so the traces are then very similar.

Thats just my theory ....


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## The_Lucek

Wow John! Great job!
The inductive samples sure do "bloom". Kinda like the power sag that you get with valve rectifiers(or a R right before the reservoir capacitor, same effect).
Now, I'm not sure why that happens and frankly I'm worried that the "power" doesn't get reflected back to the OT.
On the other hand, the hotplate caused red-plating because it lowered it's resistance in the higher freq. So you may be safe there.
You could measure this proper with a signal generator and O-scope. If you don't have the equipment, time to find friends that do!
Will be watching this thread for sure!


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## JohnH

Ive been testing some more and trying to home in on how to control a slight abrasive high-end fizz that I was sometimes hearing with the reactive circuit. I was getting this most on the DSL401 with overdrive, which is driving a pair of V30's, so nothing out of character there. I found that a couple of resistors bypassing the inductors does the trick:













Attenuator G



__ JohnH
__ Aug 26, 2018






It keeps all the frequency and impedance curves where they should be up to about 5kHz, then smoothes it out above that.

Switching from reactive to resistive with this system is quite a small change. Its not like most attenuators where if you switch out the reactive parts, you get left with a dull-sounding L-pad. The reactive has a bit more happening in the tone when the signal is high, whereas the resistive-only feels like it has some extra headroom.


----------



## danman

I haven't posted here in awhile but a few weeks ago I ran across this thread and decided to build one. I gotta say that this circuit sound great with both my 50 watt Marshall and my Princeton Reverb. I have only had the chance to try a couple of attenuators in the past but was never very happy with them. This one works great and I really can't detect any loss in tone except for the normal effect of the lowered volume and speaker interaction. I went with the 8 ohm version but I am trying to understand the exact wiring for adding the extra 6 ohm resistor for use with my 16 ohm cab. I did use the stereo Cliff jacks in hopes of implementing this resistor on one of the output jacks.


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## JohnH

danman said:


> I haven't posted here in awhile but a few weeks ago I ran across this thread and decided to build one. I gotta say that this circuit sound great with both my 50 watt Marshall and my Princeton Reverb. I have only had the chance to try a couple of attenuators in the past but was never very happy with them. This one works great and I really can't detect any loss in tone except for the normal effect of the lowered volume and speaker interaction. I went with the 8 ohm version but I am trying to understand the exact wiring for adding the extra 6 ohm resistor for use with my 16 ohm cab. I did use the stereo Cliff jacks in hopes of implementing this resistor on one of the output jacks.



Thanks very much for posting that. Nice feedback and encouraging. Its good to hear about how it works with different amps, since I only have my two Marshalls. Which model of Marshall do you have?

On the use with 16Ohms, first, its safe to try without the extra resistor, provided that first fixed stage is in place. With that, a factor of x2 on the speaker impedance only results in a small increase in ohms as seen by the amp. The extra 6Ohms is just a small addition to the output resistance seen by the speaker, just to help it keep the tone. THis was the diagram for it:













AttenuatorB 180624



__ JohnH
__ Jun 23, 2018






The schematic tries to show the connections, So Out 1 is normally connected, Out2 has its tip connected to Out1, the two ring terminals are joined to each other (but not to anything else). Out 1 has the ground connection to ground, and Out2 has its ground connected via the extra resistor. So if you just plug into Out2, its tip is fully connected but its ground is through the resistor, adding some extra impedance in series. But if you plug in two cabs, then the ring connectors short out the extra resistor and both are fully connected.


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## Chrome

I play with a DSL 100 HR, the other guitar player in our band plays a DSL 40. his has plenty of punch when needs to be.


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## danman

Thank you John...I will try to wire it in this weekend and report back.


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## danman

I forgot to add that I was testing through a 1986 bass style circuit that I built several years back.


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## jeremybusby

I am going to have a go at the version with dual 8 and 16 ohm outputs. I have only 5R6 available instead of the 6R as shown. Would it make any significant difference to substitute th 5R6? Or would it be better to parallel a few to get the deisred 6R?
Thanks for all the hard work John!


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## JohnH

Hi Jeremy
5.6 is fine, and I think thats what I did too. Good luck with your build!


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## jeremybusby

JohnH said:


> Hi Jeremy
> 5.6 is fine, and I think thats what I did too. Good luck with your build!


Thanks John - I will give some feedback when done. Much appreciated.


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## JohnH

*16 ohm cab with 8 ohm attenuator*

Just on this extra resistor at the output, for using a 16ohm cab with the 8ohm design (and 8ohm amp tap):

I drew it as 6ohm, and 5.6 is fine. But also, it could go a bit higher, say to 8 or 8.2ohm. The higher it is, the slightly more treble is theoretically output, nearer to that of the full 16ohm version. Also, as this resistor goes up, slightly less output volume is produced.

If you plug a 16ohm cab straight into Out1 on the 8ohm attenuator (no added resistor), then it actually gets about 1db more power than an equivalent 8ohm cab. Add say an 8ohm in series with that 16ohm cab using Out2, and the cab gets about 1db less than the 8ohm cab.

I reckon all of this is safe to try, and fairly subtle changes involved, well within treaking range of adjusting with amp controls. Using the 16ohm cab, with added 8ohm resistor, with the 8 ohm attenuator is slightly raising the Ohms seen by the amp, to a max nominal value of 10.4ohms. Thats about 30% more than 8ohms but well within the normal range of a real speaker, depending on frequency.


*Reactive Design
*
So I have my reactive -7db box feeding either straight to the speaker, or into the resistive box for more db's cut. On the reactive, I reduced the bypass resistor on L1 from 68 down to 27 ohm.

I also added a switch to bypass the inductors for easier A/B testing. So Ive now fully maxed out what I can get into my two 90x120 Al boxes. Its neat enough, and still well ventilated though. But (note to self), if I build it again it will all go into a much larger case, at least just to make it easier to wire up. In that case, I might just build it with all identical -7db stages, and one extra. So then it adding attenuation, youd just flick more switches down and it wouldnt matter which ones.

At this point, based on the modelling, all the maths seems to hit a sweet spot. Predicted attenuated tone and impedance seen by the amp are matching that predicted for a directly connected speaker within a db at all frequencies and attenuation levels, other than I not attempting to feed the amp the bass resonance peak. Of course, there's plenty of assumptions in that statement, for example Im using Randall Aikens reactive load design as the calculation model for a real speaker.

But, I think Ive got it as good as I can do it for now, ready for more extended testing. Impressions from playing is that the switch between resistive and reactive is a very subtle change. Both sound fine and often its hard to pick the difference, or which is better.

For anyone wanting to try these designs, I think the resistive version is fine. If you want to try the reactive, leave some space to add the added parts and all the rest can stay the same. Id recommend the inductor bypass switch, just so you can hear the difference.

Ill update my diagrams soon.

Im testing with two amps. Interestingly, the DSL401 shows a greater change between resistive and reactive than does the VM, and I think thats related to it having no feedback loop in its power amp. So I think its a case of YMMV with different amps.


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## tmingle

tmingle said:


> I wound up changing the power tubes. Was it a coincidence? IDK but, I did check the attenuator in all settings with an Ohm meter before I tested it with the amp. The resistance is a stable 16 ohms in all positions. For all the louder I play, This amp seems to blow thru power tubes pretty quickly. In about 9 months, this amp has chewed thru a set of JJ EL34L & a cheap set of chinese tubes. I checked the B+ after the JJs went south, and found no issues. I set the bias at 32mv & observe for about 30 minutes before finalizing the bias adjustments. I suppose its due to cheap tubes. I have a spare set so I wont need to buy any right away, but the next tubes will be of a higher quality.
> 
> To be honest, the DSL's Master volume with the EQ in the loop works just as well as the attenuator. The next time the amp is on the bench, I will install the line out in the amp & use the resistors to make a dummy load.
> 
> Maybe its time for something different? I never change channels, but I do use all 4 modes in the DSL.


I finally managed to spend some more time with the 16 ohm JohnH designed & was quite pleased with the results. I must have had a bad hearing day the last time because, there is quite a difference between the 2 setups(attenuator vs MV/EQ). The attenuator sounds more natural to my ears for sure. I use Lead 1 with gain on 4, vol. 3 & the 1st 2 stages engaged, I managed to dial the amp to a really good crunch that is controllable with the vol & tone pots of the guitar. A boost pedal yields all the gain I could ever want.


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## tmingle

tmingle said:


> I finally managed to spend some more time with the 16 ohm JohnH designed & was quite pleased with the results. I must have had a bad hearing day the last time because, there is quite a difference between the 2 setups(attenuator vs MV/EQ). The attenuator sounds more natural to my ears for sure. I use Lead 1 with gain on 4, vol. 3 & the 1st 2 stages engaged, I managed to dial the amp to a really good crunch that is controllable with the vol & tone pots of the guitar. A boost pedal yields all the gain I could ever want.





JohnH said:


> Ive been testing some more and trying to home in on how to control a slight abrasive high-end fizz that I was sometimes hearing with the reactive circuit. I was getting this most on the DSL401 with overdrive, which is driving a pair of V30's, so nothing out of character there. I found that a couple of resistors bypassing the inductors does the trick:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator G
> 
> 
> 
> __ JohnH
> __ Aug 26, 2018
> 
> 
> 
> 
> 
> 
> It keeps all the frequency and impedance curves where they should be up to about 5kHz, then smoothes it out above that.
> 
> Switching from reactive to resistive with this system is quite a small change. Its not like most attenuators where if you switch out the reactive parts, you get left with a dull-sounding L-pad. The reactive has a bit more happening in the tone when the signal is high, whereas the resistive-only feels like it has some extra headroom.


Hi John,


JohnH said:


> Ive been testing some more and trying to home in on how to control a slight abrasive high-end fizz that I was sometimes hearing with the reactive circuit. I was getting this most on the DSL401 with overdrive, which is driving a pair of V30's, so nothing out of character there. I found that a couple of resistors bypassing the inductors does the trick:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator G
> 
> 
> 
> __ JohnH
> __ Aug 26, 2018
> 
> 
> 
> 
> 
> 
> It keeps all the frequency and impedance curves where they should be up to about 5kHz, then smoothes it out above that.
> 
> Switching from reactive to resistive with this system is quite a small change. Its not like most attenuators where if you switch out the reactive parts, you get left with a dull-sounding L-pad. The reactive has a bit more happening in the tone when the signal is high, whereas the resistive-only feels like it has some extra headroom.[/QUOTE





JohnH said:


> Ive been testing some more and trying to home in on how to control a slight abrasive high-end fizz that I was sometimes hearing with the reactive circuit. I was getting this most on the DSL401 with overdrive, which is driving a pair of V30's, so nothing out of character there. I found that a couple of resistors bypassing the inductors does the trick:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator G
> 
> 
> 
> __ JohnH
> __ Aug 26, 2018
> 
> 
> 
> 
> 
> 
> It keeps all the frequency and impedance curves where they should be up to about 5kHz, then smoothes it out above that.
> 
> Switching from reactive to resistive with this system is quite a small change. Its not like most attenuators where if you switch out the reactive parts, you get left with a dull-sounding L-pad. The reactive has a bit more happening in the tone when the signal is high, whereas the resistive-only feels like it has some extra headroom.


Hi John,
Im just wondering what the iinductor values & resistor values that bypass the inductors would be in the 16 ohm version I built.
Thanks


----------



## JohnH

Hi tmingle,

In principle, the inductors and bypass resistors for a 16ohm system would be x2 those for an 8ohm system. But relative to the post before, Ive played around with the values a bit more. Im currently settled on two 0.4mH inductors bypassed by two 27ohm resistors So nearest for a 16 version would be 0.8mH and 56ohms.

Im happy that these arrangements do a good job of showing the amp the right reactance at higher frequencies, and that its not worth bothering with the much bigger components needed to show a bass hump. 

But, Im genuinely getting to the point of concluding that, if you get the resistive parts right, adding the inductors either makes no difference or only a very slight and just a bit of a difference, not better!, and only in a very maxed out amp.

Id be very interested if you decide to try it though, its not much cost to add them.

Ive also been tuning up the way I work out resistor values. Ive worked out a stage for -3.5db, which together with the other three stages extends the range with finer overall adjustment, in about 3.5db steps from -7 down to -31db


----------



## JohnH

An update, for extended range and finer control of level:

*Attenuator Design H: 8Ohms -7db to -31db

Switching
*
This takes the previous reactive 3-stage design and adds another -3.5db stage to give finer control and further range. A total range of -31db takes a 50W amp down to about 40mW. There are now 1 fixed and 3 switched stages. Each sucessive stage does twice the db reduction of the previous stage. In combination, you can set any level of attdnuation in the available range, with 3.5db increments. Its like binary arithmetic: you start with all three switches off, then stage 1 on, then 1 off and 2 on, 1 and 2 on, then 1 and 2 off with 3 on..etc...













Attenuator H 181027



__ JohnH
__ Oct 27, 2018






All this allows 8 different settings, with 3 very simple and high capacity toggles, much more reliable than most rotary switches.

If all that binary logic is too fiddly, you can instead use a different combo, such as all three (or more) -7db stages and you just switch on however many you want in any order.
*
Reactive Module
*
The module has settled down with two 0.4mH inductors each bypassed with 27 Ohms. It makes a slight difference. If anyone builds it, do the bypass switch too as shown, so you can hear for yourself. I found it to make not much difference and the system works and sounds perfectly fine without the reactive parts. But it is feeding the amp a carefully judged reactive load, which at treble frequencies, is very close to that from a typical guitar speaker. So if there is any effect to find, it should be there. Different amps may react differently to my two Marshalls.

The inductors should be based on at least 18gage wire for current handling. The inductor bypass resistors R9 and R10 can be 10W.

If you don't want the reactive module, just omit the shaded area of the circuit and connect it all together in that zone.

*Component values*

All the main values are carried over from the previous design, so the added parts can be put into an existing build. R1 should be at least the amp power, and preferably 100W for a 50W amp. The others can be 50W.

For a native *16Ohm system*, all values to be x2. Values can be rounded to the nearest available, eg, 39 Ohms instead of 40, or 8 sub with 8.2, 80 with 82 etc. Omit the second jack and R13 if only 16Ohm is used.

With these values, performance should be quite consistent at the various settings. Setting aside reactive effects, the amp sees an input impedance at all settings within 10% of 8 ohms, and output impedance is always between 19 and 20 Ohms. Volume increments between any two adjacent levels are all within 0.15db of -3.5dB.

I have another overall set of values worked out too, to slightly stretch the effect of each stage out, for about another -5db in total combined range. This is for baby-bedroom lullaby-levels from your cranked Plexi. I can post on request.


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## Gene Ballzz

@JohnH 

I think it was on another forum that you suggested I check this long thread out. I've finally had the time to read it completely and digest it. Your design is so simple and generally inexpensive that I simply can't afford to *NOT* try one for myself! Many thanks for your diligent, in depth and careful R & D on this project. I currently have a Weber MiniMass and while it works fairly well, it does suck tone and dynamics in proportion to how far the volume gets squashed. It's also rather large and cumbersome. It funny though, most of my Marshalls don't seem to like the MiniMass, but the 5E3 Tweed Deluxe doesn't seem to mind it very much! I'll let you know how it works out for me. If I like it, I'll likely build one and install permanently in each of my amp enclosures, combos and heads!

Thanks Again,
Gene


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## JohnH

Thanks Gene, and good luck with it! Let me know any questions, I'm happy to help, work out further options etc, its all in the name of science!


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## tmingle

Gene Ballzz said:


> @JohnH
> 
> I think it was on another forum that you suggested I check this long thread out. I've finally had the time to read it completely and digest it. Your design is so simple and generally inexpensive that I simply can't afford to *NOT* try one for myself! Many thanks for your diligent, in depth and careful R & D on this project. I currently have a Weber MiniMass and while it works fairly well, it does suck tone and dynamics in proportion to how far the volume gets squashed. It's also rather large and cumbersome. It funny though, most of my Marshalls don't seem to like the MiniMass, but the 5E3 Tweed Deluxe doesn't seem to mind it very much! I'll let you know how it works out for me. If I like it, I'll likely build one and install permanently in each of my amp enclosures, combos and heads!
> 
> Thanks Again,
> Gene


After some initial concerns, I can attest to the fact that the 16 ohm resistive design works very well with my 40C. I also use a 10band eq in the loop for minor shaping. I rarely use boost pedals with this setup & have better control over the sound with the guitars volume/tone controls. I also installed a line out coming right off the speaker out of the amp & use impulses for recording.


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## Gene Ballzz

Hi @JohnH 

I want to build a standard,resistive, non-reactive unit, along the lines of what you've been doing. I do however want to incorporate a couple other features and small modifications and would sincerely appreciate some help in laying it all out. The largest amp will be no more than 50 watts, but I've always been advised/led to believe that attenuators are best designed for double the intended power handling, so please let's keep that in mind when sizing components? The most used amps will actually be in the 12 watt to 30 watt range. Here are the parameters/details/features I hope to cover:

_*A) *_Capable of supporting both 8Ω and 16Ω operation, via a single switch. For simpicity and convenience, I'm OK with always matching the cabinet impedance to the selected impedance setting/requirement of the amp. In other words, no complication of running 16Ω speaker from an 8Ω output, or vice/versa.
_*B) *_Four levels of operation:
_*1) *_Straight thru, basically a bypass.
_*2) *_Minimum attentuation, in the 5db to 7db range, whatever makes sense from standardly available components.
_*3) *_Medium attenuation, in the 10db to 14db range, again depending on components.
*4) *Maximum attenuation, in the 16db to 21db range, yet again, component dependent.​_*C) *_An additional *"post-attentuation"* line out. This could actually be another speaker level output or a line level, but I'm assuming the need for it to be isolated and/or buffered from the main speaker out to avoid any interaction or loading with the actual *"speaker"* output. Having a pot to control it's level would be nice/handy, but not necessary, if it complicates things too much. The purpose of this _*"line out" *_will be for driving the wet side of a wet/dry, dual amp setup. Using a *"Direct Box/DI" * that is capable of accepting speaker level signal and has phase reversal and ground lift fuctions between this "line out" and the input of my rack mount effects unit is likely part of the scenario.​While the line out would be nice, having it *"pre-attenuation"* or even leaving it out would be OK, if it's too complicated to get right! The intention of "post-attenuation" is tyo preserve the balance of wet & dry amp levels, although then we run into issues of having very low level input to the effects unit, so.......? There are plusses and minuses on both sides of that coin! The main and most important parts are *A & B.
*
I'm even OK with treating the *"line out"* as a separate project, though ideally it would be incorporated as it's own internal direct box, with the capabilities mentioned. It seems that the most important aspect is to make the *"line out"* as *"invisible"* as possible to the path of the speaker signal.

I Eagerly Look Forward To Your Thoughts,
Gene


----------



## Micky

Bugera PS-1


----------



## JohnH

Hi Gene, thanks, that's a great design brief!

Ill think of any specifics for it. But one option would be very close to Design H, ie, the one a couple of posts up. Leave out the -3.5db stage and the reactive stage if not wanted. 

Im also using 50W as a basis, and I size the parts with a factor of at least 2 for case mounted resistors and 3 for smaller air-cooled ones. I did a soak test the other day, feeding continuous hard distorted looped thrashing at about 40W, for about 20 minutes. An IR thermometer showed max 65C on the hottest power-resistor, which is fine. 

The post-attenuated line-out can be as simple as a pot of say 1k feeding an output. This would not affect the tone since its a high value compared to the speaker. But its still a low value compared to a standard line-in so all good. In its simplest form, its level would vary with the attenuation set, so if you move to less attenuation, youd need to turn down the line-out level. More switching could try to keep output more constant. In these scenarios, the speaker should stay connected, helping to provide load and the required tone shaping. Do you have specs for the DI box that you have in mind?

The toggles Ive been using are the simplest most robust arrangement I can think of. If you are sure that -21db is enough, then you could have a fixed -7db stage and two more switcheable ones (use either or both) to take you to -14 and -21db. The third toggle would be full bypass for 0db pass-through. Would that work for you? or would you need all the settings on a rotary?

Another thing to run by you. These 8ohm designs, with tweaks as described, work well with 16ohms. But at a given setting, the 16ohm setting is a couple of db lower than the 8 ohm. So if min attenuation with an 8ohm speaker is -7db, using a 16ohm speaker would give -9db. Is that too much attenuation as a minimum?

Finally for now, Ive been playing a lot on my version, and Im increasingly appreciating the slight differences of the reactive module more, which seems to add some extra edge when the amp is running hard, on the initial attack of notes. Its an independent part, so if you wish, you could consider leaving space in your build as an option later.

cheers
J


----------



## Gene Ballzz

Micky said:


> Bugera PS-1



I already own a Weber MiniMass and the Bugera gets similar reviews. It works well, but definitely affects tones and respones as attenuation gets increased. I am definitely impressed by the depth and level that @JohnH has tested and re-designed to ensure minmal deterioration of sound. My interest is also piqued by the hint that continuously variable may not be optimal, for several reasons. I must say that when using my MiniMass, there are definite _*"sweet spots" *_on the sweep and the spaces in between are at least a little less than stellar! I'm additionally hoping to build this in a fairly compact package and building multiples to mount into each amplifier cabinet and/or speaker cabinet I own. Once the design is nailed down, sourcing all the components in semi-bulk quantities and all from (hopefully) the same place will help keep the cost at a minimum.

And John, how much extra *"real estate"* is required for the reactive part of the circuit?

It is my considered opinion that every amp in the universe should have a built in and fully optimized (for that particular amp) attenuator installed in it! 

It's a little bit disappointing (for many reasons) that I'm at such a late stage in my life. I have a wonderful idea for a dual amplifier built into a single chassis and cabinet for use as either a stereo guitar amp or more importantly as a fully, self contained, wet/dry amp rig. The time and effort required for R&D would be massive and I don't think I have enough of either to accomplish it, before I go toes up!    I would, however, love to share and collaborate on some of the concepts, so that this truly innovative concept doesn't die with me! 

Thanks Folks,
Gene


----------



## JohnH

Hi Gene. The reactive parts are two inductors (about 40mm diiam. x 25mm high) and two 10W resistors. 

I should try to post some samples to show the difference (or not) that they make. Then it might be easier to decide whether to make provision. It depends on the amp and how hard its driven. What kind of amp tones do you like, eg cleanish, edge of breakup, or driven? What sort of volume settings best find your tone?

If the end game is a box for each amp or cab, do you need the 8/16 feature in each? Its simpler and a bit more consistent to make dedicated versions.

For size, my current builds are in diecast Al boxes about 120x90mm. One holds 3x resistive stages, similar to what you could have. Another has the reactive parts with another resistive stage. Its in those two parts since that was how it developed. Those boxes are neat but fiddly to work with. Im planning another build with everything together.


----------



## Gene Ballzz

Well @JohnH
My current *"Main Squeeze"* amp is a Tweed 5E3 Deluxe. My tones are generally achieved by being full on cranked (with a 5E3, that means volume on 9.5 out of 12, just before severe compression sets in) at the amp and guitar, but played with a very light touch for glorious, harmonically rich cleans and digging in a bit more for blazing overdrive. I also change it up a bit with a modded DSL 5, DSL20CR, DSL 40CR, Class5 and an older JTM30! All usually are played through either a late '60s, 4x12, 1960B, 16 ohm, with pre-Rola greenbacks or a 4x10 1965B at 8 ohm. When utilizing the wet/dry rig, the 4x10 gets split to two 16 ohm pairs or the 4x12 gets split to two 8 ohm pairs. In the wet/dry scenario, only the dry, core sound amp and associated speaker pair needs to be attenuated, as the wet side is driven by a small solid state amp and is only used for amplification as the tones/sounds were already developed in the dry amp.

As you can see, the flexibility for both 8 ohm and 16 ohm operation is kind of important. And yeah, one might think that a 13 watt Tweed Deluxe shouldn't need to be attenuated, but when fully cranked, it's one loud little sucker!

I guess one question is if the inductors need to be different values for optimal 16 ohm vs 8 ohm operation? Also, for optimal usage, do all the resistors need to be different, or just a single resistor change at the front and back end of the unit? It would not be a deal breaker to need two separate, complete attenuator circuits housed in the same box for the alternate impedances, if necessary! Although, just building a couple units of each optimized impedance, into conveniently swapped packages might make it easier.

My apologies if I'm making this too complicated.

Thanks For Your Thoughts,
Gene

Now that's odd! When I went to post this, I got a message telling me that the omega symbol for "ohms" is a banned symbol, but it was not a problem in my earlier posts? Very strange!


----------



## JohnH

Hi Gene, I crunched some numbers!

I reckon there are a couple of options, so let me know what you think of the following:

*Option 1: 8 Ohm attenuator, with option for 16 ohm output*

This would always run from 8 ohm amp taps when attenuating. It would have 3 similar stages, each based on a 15 ohm and a 10 ohm power resistor, with 2 of the stages switchable, plus overall bypass. So theres a bypass switch, and two other toggles. Youd get attenuation of 0db, -7db, -14db and -21db. It can have the reactive circuit too, added later if wanted. One input jack, one or two output jacks. it can have a pot feeding another output for DI, or take the DI from the second speaker out and control the level downstream.

For use with 16 ohm speakers, another 10 ohms is added at the output, and another resistor applied at the input to keep the amp seeing a suitable load. For fine tuning, these added resistors could get connected with a couple of caps so that the 16ohm tone matches the 8 ohm tone (very small difference, without these, the 16 ohm bass and treble is about 1 db down, which is arguably almost negligible). All other parts including inductors, stay the same. 

Engaging the 16ohm operation can be with a switch, or just by plugging into a dedicated jack with no extra switch.

Now heres the thing: All of the above should work very well and Ive tested almost all of it (not the caps idea yet). BUT, the output to the 16 ohm speaker will always be about 2.5db less than to an 8 ohm speaker. ie, the range changes from 7 - 21db, to 9.5 - 23.5 db. Or in terms of max attenuated power, a 50W amp goes down to 10W into 8 ohms, or about 6W into 16ohms. Also, bypass is still full bypass, but with a 16 ohm speaker, youd need to use a 16 ohm amp tap with bypass, and an 8ohm tap when attenuated.

For my builds, I havnt added a bypass. I figure that if I want full output, I just dont connect the attenuator. Also for me, Im generally using either an 8 ohm 2x12 combo, or a 1x12 combo plus 1x12 cab, also for an 8ohm net load. So the use at 16ohm is a nice-to-have added Easter-egg, wheras for you it is a key requirement. For your use, Im mainly concerned about that extra loss of 2 5 db with 16 ohm cabs. Please consider! (in my case, my 16 ohm 1x12s are V30's, so they are naturally 3db louder than my 2x12 Greenback pair, plus Im just an amateur and dont earn my living doing this)

*Option 2: Two seperate boxes*

To get exactly the same results at 8 and 16 ohms, you could just build two complete units, abd leave out all the tweaking for ohms changing. All the component values (including inductors if used) would double for a 16 ohm version, and youd run it from a 16 Ohm amp tap. The two builds would look identical except for component values There'd be no particular advantage in building them in the same box unless you wished to.

What do you reckon?


----------



## Gene Ballzz

Well @JohnH 

All of the amps mentioned have 16 ohm outs available and some have also 8 ohm and even 4 ohm taps. However, one has 16 ohm only and another is happiest at 16 ohms! None of my amps have 8 ohm only outputs. So it appears that my best option would be to always assume using a 16 ohm out from the amp, with the option of driving either an 8 ohm or 16 ohm cabinet.
Watcha Think?
Gene


----------



## JohnH

I think that will work, maybe quite well. Will get back to you after some more numbers....


----------



## JohnH

...yep!

Seems like this 16 to 8 design could work well, actually it seems easier to keep tone and volume more consistent than for the 8 to 16 version discussed before. So lets go with this?

There will be a switch to change cab impedances. The only watch-it will be if you are with an 8 Ohm cab, and want to use the full bypass switch, you'd need to use an 8 ohm amp tap unless you are ok with a mismatch. Otherwise, use a 16 tap and the amp will always see around 16 Ohms at mid frequencies, within about 15%. Attenuation is -7, -14 and -21db, very close within a db with both 8 and 16 cabs.

So assuming that's the go, the last thing to find out about is the DI box, specs and what kind of amp it needs to drive?, so that the output can be designed to suit it.


----------



## Gene Ballzz

@JohnH 
That "watch-it" should be no problem as the Deluxe has a 4/8/16 ohm switch and all the other amps have easily accessible 8 & 16 ohm output jacks. The reason the bypassis important for me, is that each attenuator will be mounted into it's own amp cabinet and adding to and removing from the circuit by plugging/unplugging could be cumbersome.

In the interests of simplification (for now), I think the line out/DI is best left as another, separate project for the future, if it may compromise the impedance picture? For the time being, I can simply use an actual, appropriate Direct Box, on either the in or out side. FWIW and for future thought/consideration, it would be most advantageous to have that output be a balanced XLR to easily facilitate phase reversal and it will almost always be driving a studio quality, effects processor (with selectable +4/-10, balanced/unbalanced inputs) and that effects processor will feed the second amp. If on the other hand, this DI is a no brainer?

I'm leaning towards incorporating the reactive part of the plan, right from the get/go. Do you have a good source/at reasonable price for the 0.4 mH inductors? I found these on the bay, but they don't list the physical size. Suitable or not?

https://www.ebay.com/itm/1PCS-0-8mm...=item4d5d8c834b:g:ckwAAOSwTf9ZUNUe:rk:29:pf:0

Also looking at this box in hopes it's big enough while still keeping the size under control:

https://www.ebay.com/itm/DIY-Alumin...-PCB-Instrument-Box-150x105x55mm/272979567800

Additionally, is there any reason (other than size) that the 6 ohm/12 ohm final stage resistor can't be another 50 watt unit?

Thanks John,
Gene


----------



## tmingle

Gene Ballzz said:


> @JohnH
> That "watch-it" should be no problem as the Deluxe has a 4/8/16 ohm switch and all the other amps have easily accessible 8 & 16 ohm output jacks. The reason the bypassis important for me, is that each attenuator will be mounted into it's own amp cabinet and adding to and removing from the circuit by plugging/unplugging could be cumbersome.
> 
> In the interests of simplification (for now), I think the line out/DI is best left as another, separate project for the future, if it may compromise the impedance picture? For the time being, I can simply use an actual, appropriate Direct Box, on either the in or out side. FWIW and for future thought/consideration, it would be most advantageous to have that output be a balanced XLR to easily facilitate phase reversal and it will almost always be driving a studio quality, effects processor (with selectable +4/-10, balanced/unbalanced inputs) and that effects processor will feed the second amp. If on the other hand, this DI is a no brainer?
> 
> I'm leaning towards incorporating the reactive part of the plan, right from the get/go. Do you have a good source/at reasonable price for the 0.4 mH inductors? I found these on the bay, but they don't list the physical size. Suitable or not?
> 
> https://www.ebay.com/itm/1PCS-0-8mm...=item4d5d8c834b:g:ckwAAOSwTf9ZUNUe:rk:29:pf:0
> 
> Also looking at this box in hopes it's big enough while still keeping the size under control:
> 
> https://www.ebay.com/itm/DIY-Alumin...-PCB-Instrument-Box-150x105x55mm/272979567800
> 
> Additionally, is there any reason (other than size) that the 6 ohm/12 ohm final stage resistor can't be another 50 watt unit?
> 
> Thanks John,
> Gene


I went with the larger 1 & am glad I did. I went with all 100W resistors & 15A switches for my build. I realize it's overkill but, I have seen to much electronic stuff fail due to under rated components. 
https://www.ebay.com/itm/Black-Alum...0:g:fhQAAOSwX8NakC-9:rk:1:pf:1&frcectupt=true


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## JohnH

Gene - the box you linked to is a bit larger than the ones Ive been using, but my builds are not the complete reactive circuit all in one box. Ive split it between two boxes 120x90 each, just due to the way it developed, and both are a bit too tight for convenience. Your link could be fine for just the resistive version, but larger would be needed for adding the inductors, and also the bypass switch etc. tmingles one looks like its a nice size.

The inductors you linked looks about right and similar to mine, but there's no definitive specs. Also, they come from China which may mean very long shipping time. I bought locally in Oz to get them quicker.Llets check out the US component suppliers like Mouser or Digikey.

For a 50W max amp, 50w is fine for resistors except for R1 which is better at 100W. (or use 2x 50W in series or parallel)

Also, please note that all component values are different to the 8 ohm version, mostly they are x2 (as tmingles 16 ohm), but the extra bits and pieces to bring it down to an 8 ohm switched option are different and need fine tuning, including the best inductor values and bypass resistors. Ive nearly got it, but will need a few more days to confirm.


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## Gene Ballzz

I was kinda figuring that at 55mm/2.16 inch thickness, there my be room to do it in two layers. One layer mounted to the top half and the other layer mounted to the bottom half? Maybe? I gues it would depend on how tall the inductors are. That one that @tmingle takes up a lot of real estate and is really tall. Remember that I want to mount this inside amp cabinets.

This one may be OK:
https://www.ebay.com/itm/Aluminum-D...lPaddedFlatRateEnvelope!89119!US!-1:rk:3:pf:0

though the one I linked earlier is also available in 13mm taller version.

And @JohnH where did you get your inductors, and do you have a part number? I couldn't find any at Mouser or DigiKey that looke even remotely like the ones you used?

Thanks Folks,
Gene


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## JohnH

This is what I actually bought (Aus ebay)

https://www.ebay.com.au/itm/0-4-mH-...m=152573764465&_trksid=p2056116.c100935.m2460

Its air cored, wound on a bobbin and 18awg wire. These are most commonly used for loudspeaker cross-overs, which is maybe a good way to search for them nearer to home. For your build, a higher value will be needed. One would be 0.8mH and the second may be the same or a bit more like, 1 2mH, TBC.


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## Gene Ballzz

Ya know @JohnH 
With all the different component/value swaps required to custom tailor this to my preferences, I feel like I'm asking an awful lot from you and may be putting you out a bit. On top of that, this isn't anything that you actually need, so I wouldn't expect you to actually build one and in turn, it won't be subject to your extensive and diligent testing. Unless you see a use for a full blown 16 ohm unit, might it be best for me to simply go ahead with your most recent 8 ohm version, with the 8 ohm/16 ohm speaker option? I sort of feel bad about asking you to re-invent a well functioning wheel! Unless, of course, you want to go ahead and build/test one for your advancement and increase of knowledge?

Thanks For All Your Thoughts & Efforts,
Gene


----------



## JohnH

Hi Gene, thanks for your thoughts.
Im happy to work up a design with you for the 16 to 8 ohm version. I find the design process to be fascinating and always benefits from trying new things and from input by others.

Now I wasn't planning to build this exact version myself, but the thing is, like most new designs, its 95% made of things that we know work and 100% on well known theory. All of my builds have been based on 8 ohms, but the basic x2 step to a 16 ohm version is simple. 

All the theory scales up to 16 ohms, and this has also been built by others in the basic resistive form. The options for the attenuation levels are just a matter of putting in the required stages, and any combination in the range will work.

I work most of this out by analysis, with spreadsheets to check through all the switched resistor combinations for attenuation, input and output resistance and power dissipation plus Spice models to assess frequency response. So Im able to build based on these calcs, and test the results. Then, with a verified basis for comparing the real build to the theory, I can work out a new design and if the analysis shows it to perform similar to a tested equivalent, then there is some confidence. With yours, the interesting new part is the switching down from 16 to 8, which is worth paying some attention to.

The main variable with your set up is all the different amps that you have which are different to my two. Im sure it will work but the way tones react to attenuation across different amps and volumes would be expected to vary to some extent.

Happy to carry on on this basis if you wish. If youd like to try something quicker and simpler as a first prototype for yourself, Id suggest maybe a basic resistive one, either the 8 or the 16 whichever is most useful. 
cheers,


----------



## JohnH

Heres maybe a good simple option to try:

Build a basic 16ohm version (with inductors if you want).
Its also safe to plug straight into it with an 8 ohm cab. In this case, at min attenuation, the amp will see about 14.5 ohms, within 10% of the nominal 16. 
The 8ohm speaker will see a higher output impedance, but at around 40ohms, its still within the range of some amps with an 8ohm tap (eg my DSL401). The consequence of this is a db or two more high treble and low bass, either negligible or easily adjustable at the amp. 

Its interesting, but wirh these designs, plugging a 16ohm speaker into the 8 ohm attenuator, while safe, definately dulls the tone a bit and hence the added output resistor to let the speaker see more ohms. But the opposite, plugging an 8 into the 16 attenuator, has a much smaller opposite effect. (Maths support this). The output level from this 8ohm speaker is about 1.5db down from that using a similar 16. Also not a big deal maybe.


----------



## Gene Ballzz

From a purely *"non-technical"* standpoint, utilizing my Weber MiniMass, I can confirm how differently different amps react to attenuation. Some amps will only tolerate/provide one sweet spot, while other amps allow two, three or even more. While I don't have an actual schematic for the Weber, I can make a layman's assumption as to why this is so. I'm betting they use the variable resistor/rheostat in place of your resistors 2, 4 & 6, and then use one fixed resistor in place of your resistors 1, 3 & 5. They then have a *"speaker motor"* (basically a magnet and voice coil) in place of one only one of your inductors. I think the *"sweet spots"* occur at the points where the sweep of the variable resistor reach a various optimal "balance" conditions with the fixed resistor. I'm also assuming that that "optimal" balance may be a little different for each amp?

Just Spit Ballin' Here, But It Kinda Makes Sense To Me!
Gene


----------



## Gene Ballzz

WOW! That was weird! Immediately after posting that last response and wanting to edit it, all four of my favorite websites went down, this site, Strat-Talk, TDPRI and MyLesPaul! Oh well, some kid getting off on messing with folks? Back to bidnes!

Let's see if in addition to a box, switches and jacks I've got my parts list correct, as per "Design C:" 16 ohm without inductors:
*1 - *30 ohm, 100 watt, R1
*2 - *20 ohm, 50 watt, R2 & R4
*2 - *30 ohm, 50 watt, R3 & R6
*1 - *12 ohm, 5 or more watt, R5​Now when it came to the reactive values, I caught that one inductor needs to likely be 0.8 mh, but the other one, by the way it was typed out, it wasn't clear if you meant 1.2 mH, 1 to 2 mH, or 12 mH. Also not sure which value inductor would go in which position and what the resistor values on "DesignG: 8 ohm" that are around the inductors need to be?

Thanks @JohnH For Your Patience With Me! 
Gene


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## JohnH

Ok I reckoned it was time for a new diagram, so here's 'Design K' for the win!













Attenuator K 181123b



__ JohnH
__ Nov 22, 2018






Its all as noted before, based on the 8 ohm designs, x2. But I did up-tweak one inductor to 1.2mH as discussed. Its L2, Ive pointed it out, the one in series wirh R1. Both inductors have a 56 ohm in parallel. This change is not super-critical though, just a slightly better treble consistency expected

We dont need the extra resistors at the output with this scheme. They were for running a 16 ohm cab from an 8 ohm unit.

All resistors except R1 can be 50W, aluminium clad, screwed to the case with thermal grease. R1 should be 100W if you want to max-out a 50W amp. In this case, its calculated power dissipated by R1 is about 30W, so plenty in hand.

*Switches
*
For the switching on and off of attenuation stages, Ive been using mini-toggles. You'd think these would be too small for such a power application, but I believe they are fine in this position. Its all about current handling. For a/c signals, they are rated at 3.5Amps per contact. Im ganging together two sides of a two-pole switch to double that. The calculated current is just over 1A max. I like the ones with flat levers.

But the bypass switch, which uses two poles (see diagram), carries up to 2.5A per contact at 50W, if at 8ohms. A larger switch is needed. If you liked mini-toggles, a 4 pole one doubled up in pairs could work, or something else with an a/c current rating of at least say 5A.

*Jacks*

These should be plastic body, so the box is not connected. Im using stereo jacks with the extra ring connection. In this design, its not used but I find the extra spring contact grips the plugs better than a mono socket.

*Inductors*

Need to be wound with 18gage (or a lower number= thicker) wire for low resistance.
*
Tweaking the design
*
Once its tested, if you want to tweak the reactive tone, the 56ohm resistors can be changed or omitted, for slight differences in high treble.

The resistor values should be quite common, but there is scope for changing if not available in a certain range. Special tolerances arent needed. 5% is fine.

We've based this on three equal 7db stages. For others who may be interested, it will also work with more stages, and different attenuation steps such as shown for other designs.

cheers


----------



## JohnH

Ive tried to follow as many attenuator schematics as I can find, and read as much as I can find about them, including the Weber ones. With regard to passive attenuators, Im not sure if Im missing something, discovered something or its just that the 'emperor has no clothes'. But so far as I can see, none of the known designs can possibly keep a consistent tone across different attenuation levels!. The key thing is to understand and maintain consistent impedance as seen by the amp and by the speaker. It starts with the resistive stages, and with the reactive module, its the reason for having two inductors instead of just one in these designs.


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## Gene Ballzz

Do those 56 ohm resistors (R7 & R8) want to be 50 watt also?
Thanks @JohnH 
Gene


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## JohnH

50W is fine, but less is ok for those, like 10W.

Do you reckon youll be able to find the parts ok?


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## Gene Ballzz

Yessir, I'm a big boy, just ask all the girls! Everything is already on it's way, except for the 56Rs!
Thank You,
Gene


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## Gene Ballzz

Well @JohnH
I guess I lied!  Everything is on the way, *EXCEPT* the inductors! The search provides me with more questions than answers. Of course I barely even noticed what an inductor was, or it's function, etc, etc, before this search. One odd thing is that even the 20 gauge units claim up to 200 watts capacity, but I realize that's when they're used in a different application, in a crossover network. I have found a couple in the 16gauge range, but they are generally huge and pretty darned pricey. Another question that comes to mind is how the heck does one "mount" these units to avoid having hefty bundles of copper wanting to bounce around? 

While I often source oddball components, this is rather new territory for me. I also realize that you are nearly at the opposite side of the planet from me!  Sourcing these items is really kicking my a$$! 

The Search Continues,
Gene


----------



## Gene Ballzz

JohnH said:


> Ive tried to follow as many attenuator schematics as I can find, and read as much as I can find about them, including the Weber ones. With regard to passive attenuators, Im not sure if Im missing something, discovered something or its just that the 'emperor has no clothes'. But so far as I can see, none of the known designs can possibly keep a consistent tone across different attenuation levels!. The key thing is to understand and maintain consistent impedance as seen by the amp and by the speaker. It starts with the resistive stages, and with the reactive module, its the reason for having two inductors instead of just one in these designs.



Well Sir,
The first _*"MOUSE TRAP" *_of passive attenuators was designed many years ago. I remember having an "ALTAIR" unit in the late '70s. I don't recall it's design , but it was very crude and although it would "SOAK" power, it absolutely destroyed the sound, even at minimum attenuation levels.

Now, as with most attempts to *"build a better mouse trap," *many folks only address the main issue of making sure that the mouse is dead, while ignoring disposal, sanitation, possible odors, convenience, size, etc, etc. Others still, overcomplicate the solution of the other issues, in lieu of doing the hard math, testing and practical intuition that you've brought to the table. Patience, diligence and stick to-it-ive-ness are virtues that can often pay big dividends of satisfaction, in the long run! My hat is certainly off to you for digging into this endeavor! 

Now, What To Do With That Nasty, Stinky Dead Mouse? 

Gene, In Las Vegas


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## Gene Ballzz

@JohnH
I did find this source, but they list the "mH" specs as a range, rather than exact and I'm not sure if they can even ship to the U.S. of A.

https://www.falconacoustics.co.uk/audio-inductors-ferrite-air-core-iron-dust.html

Then, of course there is Dayton:

http://www.daytonaudio.com/index.php/loudspeaker-components/crossovers-components/inductors.html?p=1
Cheers!
Gene


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## JohnH

Ok ill look. But first let me paste this link, to a US supplier. They seem to have the values, compact and good value. They are 19awg, but the resistance specs and values check out. Looks like you get a 0.8 and a 1 25mH

https://www.madisoundspeakerstore.com/air-core-19-awg-page-2


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## JohnH

I looked at your links. Madisound looks better to me.


----------



## JohnH

I fixed my coils in place with super-glue! (since it was a 'test' build). but with the bobbin style, a nut and bolt through the centre might do it.


----------



## Gene Ballzz

Yep @JohnH ,
I found those too, but was concerned about the wire size/gauge and wondering if 1.0 mH or 1.25 mH was "MO-BETTA!" I guess you answered that so, Madisound it is!
Thank You Sir!
Gene


----------



## tmingle

Gene Ballzz said:


> @JohnH
> I did find this source, but they list the "mH" specs as a range, rather than exact and I'm not sure if they can even ship to the U.S. of A.
> 
> https://www.falconacoustics.co.uk/audio-inductors-ferrite-air-core-iron-dust.html
> 
> Then, Of Course tere is Dayton:
> 
> http://www.daytonaudio.com/index.php/loudspeaker-components/crossovers-components/inductors.html?p=1
> Cheers!
> Gene


https://www.parts-express.com/cat/a...earchable","1")]&PortalID=1&showMoreIds=21806


----------



## Gene Ballzz

Thank You @tmingle 
I think I like the construction/format better on the ones from Madisound and their prices and shipping aren't too terrible.
Thanks Again,
Gene


----------



## Gene Ballzz

Hey @JohnH

Parts are starting to trickle in and I figured it's time to present a few option changes, in hopes that you'll share your opinion.

First, in the interests of guarding against operator error and figuring that "overkill" is always better than "underkill" I don't see any issue with using all 100 watt resistors, except for the two 56Rs in the reactive circuit. Any size difference is negligible as is price difference. You'll also see why this makes sense when you look at your drawing that I modified below. All of my modifications are in blue. Note that this gives all the same options as your original *"Design K"* except for maximum attenuation, without the reactive circuit, using three instead of four switches. I'm also making an assumption that the more dramatically we attenuate, the more important the reactive portion becomes? In this scenario, with all 100 watt resistors and switches rated for 6A @125VAC, for the DPDT needed to bypass the first stage/reactive circuit and 3A @125VAC, for the SPDT for the other stages, it won't matter which portion is taking the brunt of the attenuation, ergo guarding against operator error! I am still torn on switch choices.



Now, with all that said, before I decide to switch gears a little bit, I ordered a mix of 50 watt and 100 watt 30 ohm and 20 ohm resistors. Given that I will already have a small pile of 50 watt, if I were to build an additional unit with all 50 watt resistors, can I assume it safe for at least a 15 watt to 25 watt amp?

Thanks Again For All Your Help,
Gene


----------



## JohnH

Hi Gene

Your mods are fine, except that when bypassed, the full signal goes through all the switches, so they should prefferably all be rated for the higher current. 

An all 50W one will be fine wirh lower power. You might find its ok even with a more powerful amp than 25W, depending on how you use it. Electrically the resistors should be ok. Thermally, they respond and heat up slowly, averaged over time. I had to thrash my 50W VM with a continuous distorted loop for about 20min to get R1 to get hot. This was with my current test build where R1 is just 50W rated. The other one that got significantly hottish was one of the 56's, R7. This was just a 5W rated in the test build, which is why i suggested 10W for it. At 50w coming in, the estimated power dissipation of these two are 30W and 3W for R1 and R7 respectively. btw, the other 56 only gets about 1W in theory.

Another thing your scheme would limit is single-flick A/B testing to try to figure out how the inductors are affecting tone. Ill be very interested in your impressions running with your amps. Its never a very huge effect, based on my impressions.


----------



## Gene Ballzz

Yes @JohnH 
I see how I'll have to flip two switches at the same time to A/B, but that shouldn't be an issue, especially once I become familiar. I guess I settled on the switches. They are all rated 6A @125VAC/3A @250VAC. Should that be enough current capability or do I need more?
Thanks Again,
Gene


----------



## JohnH

Thats all good. The 6A /125Vac spec is fine. 50W into 16 ohm gives 28V, and current 1.8A, so there is a good margin.


----------



## Gene Ballzz

One other quick question. Is #18 wire heavy enough for wiring this up, or should I bump up to #16? My concerns are to be robust enough to handle the required current, yet small enough to fit the lugs of the components properly.
Thanks @JohnH
Gene


----------



## JohnH

I think 18 is fine, especially since these are only intended for up to 50W amps, and being based on 16 Ohm, current is less than for 8 Ohm.


----------



## Gene Ballzz

@JohnH 
Here's maybe a dumb question. Even though not marked as +positive/-negative, does the "polarity" of the inductor coils matter? They are wound on clear bobbins and it is easy to see which is the beginning/inside of the wind the end outside of the wind.
Thank You Sir,
Gene

Oh, and here's my planned layout, drawn in Sketchup:




In this box:

https://www.ebay.com/itm/Aluminum-D...rentrq:6ad573a41670a9cb2a4bbf6dfffb1456|iid:1


----------



## JohnH

Hi Gene

Nice diagram! Im only seeing one socket though? I reckon that the vent holes top + bottom should be as big as possible. Im using a 10mm bit, which might be over-kill but I like the look. Feet are key too, so air flows under and up and through

Inductors: It doesn't matter which way round they are connected.


----------



## Gene Ballzz

Yeah @JohnH
The jacks are a pain to draw, but the holes for the other two jacks are laid out, just can't see 'em at that angle view. The real down side of Sketchup is that there is no even partially sane facility for drawing wires. It is pretty handy for layout, measurements, etc, and has the bonus of being 3-D, so that you can rotate around with the mouse! If you like playing with such stuff, the free version is pretty powerful. The Pro version lets you save as DXF & DWG, AutoCad files, but it's kinda pricey. The way around that is to get the free trial of Pro (it has an hours used time limit) and only open the Pro long enough to import a drawing and save as the file you want and then close it. In eight+ years, I've used up less than an hour of the eight hour Pro freebie!

And thanks again for the advice on the polarity. Even though it doesn't matter, my an@l retentiveness will still make me orient them similarly, in respect to ground!  And in regards to cooling, with the gentle amount that I usually attenuate, my Weber doesn't even get slightly warm with the Tweed 5E3 Deluxe, and that's in a steel box with no heat sinking and no ventilation. It does, however, get a little bit *"toasty" * after a couple hours of fully crankin' my JTM30, squashed down to 75% to 80% attenuation! Those holes are all gonna be 3/16" - 4.7mm and when doing the math, that gives a fair amount of flow and spread fairly evenly around! I'll be strippin', drillin' and screwin' a bit today and just wating on the rest of the resistors that should be here Monday/Tuesday-ish!

Just Sayin'
Gene


----------



## Gene Ballzz

Well Sir @JohnH ,
Still waiting on a few parts, but I dug in to my layout and wiring scheme. WOW, did that kick my @ss, figuring all the switch interactions, minimizing wire lengths, etc. Hoping you could look at my drawing/diagram and make sure I haven't missed/messed anything? Note that the box ends and top lid are folded flat for the drawing.

I'm just one of those weird folks who likes to have all his ducks in a row, *BEFORE *picking up a tool!

Thank You So Much Sir,
Gene


----------



## JohnH

Sure, that deserves a close look, so Ill get back to you as soon as I can. Nice work!


----------



## JohnH

Right, I traced it through and I think it is all good, but with a couple of oportunities:

The two 20 ohm resistors #2 and #3, get directly connected to each other, at there lower ends. The diagram has this correct, via the switch lugs and two long red wires and a short blue one. Could delete one long red wire and connect the lower lugs directly to each other with a short wire.

The other opportunity is for a simplification. I believe one of the 56resistors can be omitted, and the tone should be hair-splittingly slightly more consistent. The one that may be left out is the one bypassing the 1.25mH. Maybe leave it out and try it with aligator clips to find out if it made a difference.

I found this with a new spreadsheet design tool that Ive made to analyze these designs. It uses the same classical circuit theory maths as in Spice circuit modelling, but hard-coded for this specific circuit design, with up to 4 stages and the reactive parts too, and using a speaker-sim model to represent a speaker. With it, all responses react instantly in real time and can be plotted, and then there is a macro to quickly cycle through and plot all the stage on/off options. Btw, all the values we picked for yours look like they will be just right!


----------



## Gene Ballzz

Yeah @JohnH ,
I drew it that way (Resistor 2 & 3) at first, but it just didn't seem to look right? While I realize that what's going on here electrically is about as simple as it can get, the physical implementation is a bit more complicated and daunting that one might expect!

Also, I forgot to number the 56R resistors. I've corrected and upated the previous diagram and I think it'll be easier to describe which reisitor to leave out? I'm assuming you refer to *R7*? I also updated the* R2* & *R3* resistor jump you suggested!

Additionally, is it OK to shorten the leads on the coils for neatness and then simply scrape off a portion of the enamel for soldering? I assume it may change the resistance, but so slightly so as not to matter?

Thank You Again,
Gene


----------



## JohnH

yep, R7 is the one that can be left off. No prob trimming leads if you wish.


----------



## JohnH

My heads still whirring around these designs, and im planning one more build to put it all together. Im thinking of a switching scheme based on an extended version of how Gene amended the base design, with fully bypassable stages. There will be one reactive -7db stage, which when engaged is at the front of the chain. Then a resistive stage of -3.5db, then either three more -7db stages, or a -7 and a -14. With this, you can go from fully bypassed 0db, in stages each of -3.5db increments, down to -31.5db. The maths is looking favourable in terms of consistent tone. All increments can be reactive except for the first 3.5db step, which is better just resistive anyway.

Also, working on Genes design revealed that there is an optimum balance between L1 and L2, where L2 should be about 30 to 50% more than L1. I actually unwound my L1 inductor this morning to bring it down a bit, and the tone got better. (its easy to do, especially given a meter with inductance readings) So my current build, at 8 Ohms, has L1 as 0.3mH and L2 as 0.4mH - and the estimated response matches some data I found on a Greenback cab. Genes design, for 16 ohms, is x2, plus a bit, and I think this puts it somewhere between typical for Celestion specs or some of the US brands that have a bit more inductance. We look forward to hearing how it goes!


----------



## Gene Ballzz

@JohnH
I wish I could give better reports/reviews, but I'm still waitin' on the *"slow boat from Chinesiawanoreanam"* for my main 20 & 30 ohm resistors!  Everything else is done as far as it can go.  I'm really glad that my input helped inspire even more depth for you. When it's done I'll give a full report.  

I think subsequent builds will use larger bodied switches, with larger lugs, as it is very difficult getting even #18 wire into the holes of the lugs of the minis! Yeah, I know I *COULD* just lay the wires alongside the lugs, but that's kind "janky" to me!

Thanks & I'll Keep Ya Posted!
Gene


----------



## Gene Ballzz

Well @JohnH ,
The rest of my parts arrived on Tuesday and I finished up my first unit and initial testing yesterday. I'm very impressed! Lots of little details, but I'll do my best to keep them organized. I've only had time to test/play it with my Tweed 5E3 Dekuxe and glorious JTM30. Remember also that I used all 100 watt resistors, except for the 10 watt/56 ohm in the reactive section.

The overall character of the amp is well retained throughout all attenuation levels. I was surprised by several aspects of the reactive circuit. It did not seem to have much affect on the high end of the frequency spectrum. It did however work quite well at retaining the "bounciness" and "springiness" of the bottom end that most folks would likely attribute to *"movement of air"* by the speakers when an amp is well cranked at high volumes. I was also pleasantly surprised that this affect on the bottom end was evident, even at minimum atteuation as well as maximum. I had assumed/estimated that it wouldn't be very noticeable at only *"one stage"* but I was incorrect! This leads me to the thought that subsequent units could be simplified by not bothering to incorporate a *"bypass"* for the reactive section. The reactive portion should be *"engaged"* all the time!

I can definitely see the wisdom and conveniece of adding a 3.5-ish db stage, as the 7db stages are a bit far apart from each other. What resistor values would be required for this?

Before plugging an amp into it, I plugged several different 8 ohm and 16 ohm cabinets into the output of the unit, with a meter attached to the input jack, measuring DC resistance. 8 ohm speakers that showed 6.8 ohms by themselves, showed between 14.4 ohms and 17.4 ohms at the various settings. 16 ohm loads that showed 12.9 ohms DCR by themselves, ended up with readings between 15.9 ohms and 17.5 ohms. This seems to be well within a suitable range for amplifier safety!

*Tweed 5E3 Deluxe Clone - *This amp responded very well at all attenuation levels, with very consistent tones throughout. At only 13 watts, an hour or more of pounding barely got the attenuator even luke warm! Still, and additional 3.5 db option to get *"in between"* the 7 db jumps would be pretty handy.

*JTM30 - *First, understand that this amp is truly _*"the little mouse that roars" *_and is only really happy when fully cranked with the channels and master wide open and is _*REALLY*_ *LOUD!* The amp sounded absolutely fantastic at all attenuation levels and may actually make it realistic to get back into the regular usage arsenal! Even though only "30-ish" watts, upon only 30-45 minutes of pounding, the attenuator got pretty close to "blazing" hot! I could still lay my hand on it, but not for very long! This leads me to think 30 watts or so is the maximum you'd want to use this for. 50 watts might be really pushing it a bit. Can I assume that the way to increase wattage capacity and reduce the heat would be to add wattage to the first stage resistors? Research shows 200 watt resistors to get really pricey, so I was thinking maybe two 15 ohm/100 watt in series for the 30 ohm and two 10 ohm/100 watt for the 20 ohm?

Also, I noticed that my layout diagram had the resistor numbers conflicting with your (and my modified) schematic!  I'll fix that!

Some crappy cell phone pics. I've not yet installed the feet, but had it well suspended during testing.

Thank You So Much John!
Gene







*TOP VIEW*




*BOTTOM VIEW*


----------



## JohnH

First, just want to say..Yay!!!!

Im really happy that this is working out. Thankyou for trusting the design and for building it so nicely, and for your ideas and detailed feedback.

Ill respond further on the questions.


----------



## JohnH

*-3.5 db stage*

For the 16 Ohm design, the values to provide a -3.5db stage are running at 60 Ohms to ground and 10 in series (instead of 30 and 20 respectively). But these need to form a stage downstream of the reactive module. The reactive module itself needs to be the first stage after the amp, and needs to be with the 30 and 20 values as you have, otherwise the maths that creates the nice tonal consistency drifts off. But, Im finding that if you just want an actual -3.5 attenuation, it should be fine to engage just this stage as a resistive module. I havnt tried a -3.5db stage myself yet though!


----------



## JohnH

*Heat*

Theres a certain amount of heat being absorbed by the whole unit, basicly its most of the amp power. So the whole box has to heat up, and overall, changing a component or two shouldnt make much difference globally. But splitting resistors into two should reduce local heat concentration. R1 does the most, about 2x the power or R2. The other one that may heat up some is the 56 ohm, smaller but lower power. I dont think theres anything to worry much about though. A safe test (except for your finger) would be if you can touch the components and count to three.

Its worth optimising overall cooling, maybe with bigger holes if possible next time? Theres a thing about holes whereby larger ones give less restriction to air flow than small ones, even with the same total area.

Also, thermal grease helps with heat transfee, and maybe if the resistors were under the top surface?

*Reactive Tone*

I totally relate to how you described it. Im finding it too, and its all about the ratio between inductors, which only became clear on working on your design. Im finding that sometimes, the reactive stage sounds no different, other times its just a bit more 'alive'


----------



## Gene Ballzz

Well Sir @JohnH ,

I don't see a need for any less than a 7 db cut for the initial stage, but to get in between 7 & 14, 14 & 21 and then even down to 24.5 (especially with the JTM30) for true bedroom levels would be handy. There are a couple/few aspects that I would consider important here:

*A) T*he resistors for the 3.5 db stage need to be robust/stout enough to withstand all the wattage left over, following only the 7 db cut of the reactive/first stage.
*B) *Having reached this point, it appears that your initial *"full unit"* bypass design may be the most appropriate. This would help facilitate:
*1) *Making sure that the first, higher wattage/reactive stage is always engaged and always the *"full brunt"* stage at the beginning of the circuit, whenever the attenuator is active.
*2) *Safe use of lower wattage resistors *"downstream"* of the first stage, remembering that my first prototype only utilized all 100 watt resistors to guard against *"operator error"* and to allow use of any/all stages, with or without reactive circuit. This seems to now be somewhat of a *"white elephant"* as it's my considered observation and opinion that the reactive part is such a great feature of the design that it should be *"in circuit"* all the time!​*C) *It also seems appropriate that the 3.5 db portion should be *"electrically"* located at the end of the circuit, just before the output jack. Kind of like having it as the *"cherry on top"* so to speak!​With all that said, this is by far thae best passive attunuator I've ever plugged into.  It absolutely blows away my Weber MiniMass and all other Weber units (and many others) I've tried and it's simple to use, without multiple, variable settings that need to get dialed in for good sound. I think one of the main keys to it's stellar operation is having switched steps that are optimized instead of being variable between *"sweet spots." *

The design works so well, that I could see being able to actually custom build these to order for fun and reasonable profit!  While I have no desire to start a *"big manufacturing concern"* I could envision doing a few per month, with a price in the neighborhood of US$200! I realize that's allowing quite a bit for labor, but there *ARE* a lot of man hours involved here! Should I start doing such a thing, I will happily throw you a few bones for all your input and initial design and testing! A PM for negotiation of percentage of profit would likely be a good idea! We ain't gonna get rich, but I could see it helping to enhance my *"broken"* old fart, retirement income!

Thank You Sir For All Your Help & Patience!
Gene


----------



## tmingle

I finally did a couple clips of the attenuator in action. Unfortunately, the mp3 files are too large to post here. I'll send to anyone that wants them as I don't have an online account for this stuff. I will say that the sound does change, but to my ears sounds better with mild attenuation.


----------



## JohnH

tmingle said:


> I finally did a couple clips of the attenuator in action. Unfortunately, the mp3 files are too large to post here. I'll send to anyone that wants them as I don't have an online account for this stuff. I will say that the sound does change, but to my ears sounds better with mild attenuation.



Sure do! PM sent
cheers
John


----------



## Gene Ballzz

@tmingle 
Where this design really shines is with the properly sized reactive circuit! It helps the whole system retain more of it's character, even at greater attenuation levels. Especially evident in the low end response and feel!
Just Attenuatin'
Gene


----------



## tmingle

Gene Ballzz said:


> @tmingle
> Where this design really shines is with the properly sized reactive circuit! It helps the whole system retain more of it's character, even at greater attenuation levels. Especially evident in the low end response and feel!
> Just Attenuatin'
> Gene


I plan on adding the reactive components soon. Some guitar practice is in order 1st though.


----------



## JohnH

ok well check back when you are ready to try them. Gene's build is helping us to better understand the best values to use.


----------



## Gene Ballzz

@JohnH 

I had said I would email you these pics and if you need them bigger or clearer, let me know and I will do so. Note the general simplicity, besides the *"speaker motor"* which is just a voice coil, spider and a magnet. The smaller rheostat is only for the line/headphone out. I've not yet put a meter on it, but I'm going to bet that the DCR in and out varies wildly as the main rheostat gets turned. I'm also going to guess that the lack of a delicate balance between the series and parallel resistances is why this unit only has "sweet spots" instead of good sound throughout the sweep! It would seem that having a truly consistent sounding, constantly variable unit would require a very delicate balancing and sourcing act of finding two rhoestats that could be ganged together. I think the stepped approach of John's design is far superior and if smaller increments are desired, it only requires the addition of extra stages to switch in and out. I surmise that the key to the performance of John's efforts is that the balance of series and parallel resistance is preserved, in addition to the reactive circuit.

This is really exciting stuff and I don't think anyone else has been approaching the subject in this manner!

Also, I must admit to not necessarily being the best and most objective tester. I was so enamored by how much better the one I built was compared to the Weber, that I may not have yet noticed any pitfalls or see any room for improvements. I have yet to try it in a band situation, so not sure if it has compromised any amount of *"cutting through the mix" *so to speak.

I hope I haven't said too much _*"in public" *_but I'm guessing that we should really start having our brainstorming R&D discussions via private email.

Just My Thoughts,
Gene


----------



## tmingle

I'm ready to order.the reactive parts.
2 - 56ohm resistors
1 - .8 uH inductor
1 - 1.2 uH inductor
Correct?


----------



## tmingle

1 thing I forgot to mention that really helps my low volume tone is the use of my HOF reverb in the loop set to a very low setting. It really helps with the perceived fizziness that my ears pick up & fills the sound out. I also use a boost pedal when playing. Ive tried a bunch of em & my favorite by a long shot is the Joyo Vintage OD. 
John, 
The clips I sent you are guitar into amp without any effects.


----------



## JohnH

Hi tmingle
For Gene's build, we used 0.8mH and 1.2 (or 1.25) mH (ie, not uH but mH which are 1000x more!). We use ones with 18 or 19 gage wire wound on plastic bobbins. The current builds are just using one 56 Ohm resistor, at least 10W. Relative to the last schematic on page 9, we use it in the R7 position and omit R8.

Note. my own build, which is 8Ohm based, uses 0.3mH and 0.4mH. Translated to a 16Ohm build, this would be 0.6 and 0.8 mH respectively. One of the most important things seems to be that there is about that ratio of difference between te two. Im liking my one too!. But Genes seems to be working well so I reckon go with his values. I don't know if a difference would be significant.


----------



## Gene Ballzz

Best source here in the states for nicely built and wide selection of coils is:

https://www.madisoundspeakerstore.com/air-core-19-awg

They only charge actual shipping charges, are fairly quick and seemingly a family owned, Ma & Pa business! They only sent half my order and when I called/emailed they sent the rest out express, on their dime! Any company can make a mistake, but how they handle/correct those mistakes is the true mark of that company! These folks were courteous and great!

Just Sayin'
Gene


----------



## tmingle

I went overkill again & purchased .8 mH & 1.2 mH in 14 gauge from Parts Express for $40. They have a bunch of 20W resistors on sale for $1.04 each, so I bought a few 20 ohm & few 8 ohm and will connect them in series. Hopefully they will arrive before my Xmas break.


----------



## tmingle

My parts are arriving tomorrow.


----------



## JohnH

Thanks, im very interested in your impression of any difference the inductors make!


----------



## tmingle

I installed the reactive components & a bypass switch for them today. I went with R7 56 ohm, L1 .8mH & L2 1.2mH. The difference is noticeable in the higher frequencies but, it is pretty subtle while playing. I recorded 1 track with the line out to compare with the waveform in Reaper seems to display more dynamics & sounds different than the previous take without the inductors. It could be my playing though.


----------



## JohnH

Thanks for trying that!
Your impression correlates with mine. A small but noticeable change.

Yours mine and Gene's are the only three reactive builds I know of. For others, the question will be whether they are worth adding. What's your view relative to your own use? After tweaking values to this point, Im now finding that yes I do prefer it with them, as does Gene based on his testing.

What's interesting to me is, its not night and day. With these designs most of action is in getting the resistors right, then the most important reactive interaction comes from the real speaker rather than depending on a reactive speaker model. But the added inductors add icing to the cake.


----------



## tmingle

JohnH said:


> Thanks for trying that!
> Your impression correlates with mine. A small but noticeable change.
> 
> Yours mine and Gene's are the only three reactive builds I know of. For others, the question will be whether they are worth adding. What's your view relative to your own use? After tweaking values to this point, Im now finding that yes I do prefer it with them, as does Gene based on his testing.
> 
> What's interesting to me is, its not night and day. With these designs most of action is in getting the resistors right, then the most important reactive interaction comes from the real speaker rather than depending on a reactive speaker model. But the added inductors add icing to the cake.


I need to hook up the looper again so the playing does not affect the playing.


----------



## tmingle

The difference is MUCH more obvious when I connected my 57 to my old Yamaha mixer & set proper levels. I did a comparison yesterday & screwed up the file during normalization. Damn multitasking anyhow! I don't see myself using the purely resistive setting anymore. My interface is a Scarlett 6i6 BTW. I monitor on Mackie MR5, MAudio BX5a & crappy stereo speakers. This recording stuff is fun & addictive.


----------



## JohnH

Thanks, so that seems to confirm our impressions of the extra reactive circuit. Worth adding, given that the coils required are modest in size and price.


----------



## JohnH

I'm gathering parts for my next build, here's a mockup (awaiting more resistors and coils to arrive):













20181231_103451_resized



__ JohnH
__ Dec 30, 2018



Attenuator mock-up





It will do each 3.5db step from 0 down to around -32db, all reactive except for the first step. It will run from an 8 ohm tap and work safely (and hopefully tonefully) with 4, 8 or 16 ohm speakers.

I got the box last week, larger than Ive used before, about 170x120x55, thick Aluminium.


----------



## tmingle

JohnH said:


> I'm gathering parts for my next build, here's a mockup (awaiting more resistors and coils to arrive):
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 20181231_103451_resized
> 
> 
> 
> __ JohnH
> __ Dec 30, 2018
> 
> 
> 
> Attenuator mock-up
> 
> 
> 
> 
> 
> It will do each 3.5db step from 0 down to around -32db, all reactive except for the first step. It will run from an 8 ohm tap and work safely (and hopefully tonefully) with 4, 8 or 16 ohm speakers.
> 
> I got the box last week, larger than Ive used before, about 200x140x55, thick Aluminium.


Yeah. I am going to upsize the box when I find something in the junk pile somewhere.


----------



## Matt Downing

A few days ago I ordered all the parts for an attenuator built around an L-pad, then I discovered this thread. I will definitely build JohnH’s latest design when he posts it, and probably the L-pad too when the parts arrive. Then I can post comparisons.

Many thanks to JohnH and everybody else here who has shared their knowledge and efforts so generously and patiently. 

I have spent the last two days reading this thread. Now I have to wait for updates in real-time! Looking forward to more.

Cheers,
Matt


----------



## JohnH

Matt Downing said:


> A few days ago I ordered all the parts for an attenuator built around an L-pad, then I discovered this thread. I will definitely build JohnH’s latest design when he posts it, and probably the L-pad too when the parts arrive. Then I can post comparisons.
> 
> Many thanks to JohnH and everybody else here who has shared their knowledge and efforts so generously and patiently.
> 
> I have spent the last two days reading this thread. Now I have to wait for updates in real-time! Looking forward to more.
> 
> Cheers,
> Matt



Hi Matt, thanks for posting. Im happy if you want to try one of these designs, and if you end up with an LPad one too, a comparison will be interesting for science. What's the LPad circuit design?

As you will have picked up from the thread, the design has settled down into a series of modular stages, each of which can be included or amended, and with various possibilities for switching, output and bypass. The one key central element is the first attenuator stage, at -7db. The consensus is forming that the reactive version of this is worth having, with its two inductor coils. This stage shows a reactive impedance to the amp, based on mimicking the mid and high frequency response of a real speaker, and feeds a mostly flat resistive output on to further stages, with an output resistance set to mimic that of a typical amp. For a minimum build, its fine to build just that one stage, and it can be used to divide the amp power by a factor of 5.

Further stages are purely resistive, and you can add as many as you like without changing tone. At the end of the chain, your real speaker responds reactively to the final output, based on its own impedance/frequency characteristics. This is where the design differs from more typical reactive attenuators, which usually depend on a component model of the whole speaker response, and that is what sets their tone. Instead, the tone you hear is created by your own speaker responding to a high impedance signal. This is why the pure resistive versions of the design work quite well, and why in the reactive version, its not important to build in the large and expensive components that model the low bass resonance - our real speaker can do that for itself. 

So you might like to think about what would be the most useful outcome for your own use, based on a series of choices:

The attenuator needs to be based around either an 8ohm or a 16ohm amp tap. The design is optimized for the same
ohms for the speaker, but it works fine with speaker ohms x2 or x1/2, with good tone and still reasonably close to the target impedance as seen by the amp.

How many steps of attenuation are needed, what are the least and greatest attenuations that you need, and does it need a full true bypass for 0db attenuation? 

What is the greatest amp power you want to run?

Do you want to run with different speaker ohm options?

What type of switches? The design falls most easily into a series of toggle switches. But rotaries may be more intuitive though its hard to find them economically with adequate capacity.

The choices affect overall size and component cost.


----------



## Gene Ballzz

I echo all of @JohnH comments and have to add that the one I built (in it's current three 7db stage form) is the most transparent, almost sonically invisible attenuation system I've ever tried, and I've tried a bunch! As far as costs, the switches and enclosure can be the most costly of the whole project. My next step is to incorporate a 3.5db stage taht can get added on top of the other stages. I'll keep everyone posted as to how it works. For simplicity of layout, constrution and operation, even though it will provide "finer" graduation of stages, it will still be an initial -7 db cut with all others being available at -3.5 db-ish increments. Maximum attenuation will be -24.5 db.

Reisistors found through fleabay, from "Chinesiawanoreanam" can be dirt cheap, but you really have to drill down and do a lot of scrolling to find the values you want/need at good prices. I've seen the exact same resistors go for only $.089, but also priced at over $5.00 from a different vendor!

For those who have access to the US market place (without outrageous shipping/import/export duties) the choke/inductor coils from https://www.madisoundspeakerstore.com seem to be the best value and in the best mounting/construction format, along with the widest variety of values. JohnH appears to have access to very similar products in his corner of the universe, but they don't seem accessible to us folks here in the USA.

If you build one cleanly, you will be amazed at how well it works! It really does retain the most overall tonal character and feel of any unit I've ever used! John's diligent attention to detail in his design and testing is truly brilliant, meticulous and second to none!

Just My -7, -14, Or -21 db!
Gene


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## mAx___

I've been following this thread with lots of interest. Although I already have a good attenuator, I'd love to try and build a 16 Ohm minimum version of this design. I'd only need one stage of attenuation: -3dB. How should I go about it? Can it be done within this design?


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## Gene Ballzz

mAx___ said:


> I've been following this thread with lots of interest. Although I already have a good attenuator, I'd love to try and build a 16 Ohm minimum version of this design. I'd only need one stage of attenuation: -3dB. How should I go about it? Can it be done within this design?



It seems from John's testing that the minimum, optimum attenuation level that interacts well with the reactive part of the circuit is around -7db. He may be able to shed some light upon your desires though.You be surprised though, -7db isn't quite as drastic as you may think.

And then @JohnH I have what may be a dumb question, just as a curiosity. Has anyone ever tested or considered how well an attenuator would operate as a parallel load for shedding volume, as opposed to our standard method of running it in series between the amp and the speaker? I'm just curious, as it would seem to allow the speaker and amp to operate with it's own "natural" symbiotic reactance, while simply "dumping" some of the electrical energy off as heat?

Just Askin'?
Gene


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## JohnH

mAx___ said:


> I've been following this thread with lots of interest. Although I already have a good attenuator, I'd love to try and build a 16 Ohm minimum version of this design. I'd only need one stage of attenuation: -3dB. How should I go about it? Can it be done within this design?



Hi mAx_

Gene is right in that for the design to date, -7db is a sweet spot, where both the amp and speaker can see the optimal impedance, and there is enough separation from amp to speaker that we can then add further simple stages without messing up the balance.

But if you just want a very small reduction, like -3db, then its possible, but as a separate unit rather than a as a front end for more extensive attenuation. Ive been working up a design (not tested yet) that has a -3.5db stage. For that one setting, its just two resistors, no coils active, and the numbers are looking like it will sound fine.

At -3db, even with no added reactive parts, about half the speaker reactance is still seen by the amp. Its not clear yet if there is any audible benefit of adding coils to optimize it further. It could be done though, I can work something out.

Gene - using the attenuator as a parallel load is fine, and safe to do. If you take your current build, set it to max attenuation, then it provides a close to 16Ohm load, even if no speaker is plugged into it. So that could go into parallel jacks on an amp set to 8 Ohm, with another 16 Ohm speaker. In theory, mids and treble should sound good and you may lose a couple of db of very low bass (because this design does not try to create a reactive facsimile of the low bass resonance).


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## mAx___

Thanks so much guys. I'll be reading your updates on that new -3.5dB design, John.


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## JohnH

*Single stage attenuators*

So this was interesting, at least to me. I decided to work out a consistent range of single-stage, fixed-value attenuators to get e feel for how the values should change as they move from -3 to -9 db. I targeted each db step, plus -3.5 db.

Provided we can accept a moderate amount (say 20%) of variation in the dc ohms seen by the amp, these can be as simple as just two resistors. Or, we can use the same resistors and add coils to also try to match the signal levels see at the amp at high frequencies, as with the other designs.

So here is a table and plots of component values, two resistors, two inductors and an extra bypass resistor (R3) on L1. As attenuation reduces, the required value of L1 goes down and L2 goes up.













Single Stage 190105



__ JohnH
__ Jan 4, 2019






The resistors are chosen so they can be made from the usual Chinese range of values, which are very good value online. But by plotting them on the graph, it should be possible to see the trends and find other values for an intended purpose, from a different range if needed.

Note: To make the graph plot clearly, the required inductors are shown x10. eg, for the -3db version, the required inductors are 0.3mH and 2.4mH.

The inductors are calculated values, not exactly standard values, so that the calcs can be consistent. But nearest available is fine, or, for more nerdiness, it is very feasible to unwind a coils a few turns to set a lower value. If picking from standard values, get the ratio of L1 to L2 close to as shown if possible.

The graphs at right take two of the recipes (-3db and -7db), and compare output response to that of a full speaker, plus signal seen at the amp, for both reactive and resistive versions. The plots and calcs are all based on small signal levels, and so they are not intended to show a tonal variation between resistive and reactive. But, the amp will know, and it will interact with the reactive version in the usual non-linear way, as we have found on the test builds so far, particularly at settings with higher power-amp drive.

But, particularly at low attenuations, I'd expect less variation between resistive and reactive than at higher attenuations, since some speaker reactivity comes back to the amp anyway.

The inductor values for these single fixed stages are optimized for this purpose, and are therefore lower than in the multistage designs. This is because there is still a significant amount of reactance coming through the attenuator back to the speaker at these lower settings. As part of a multistage attenuator, as discussed before, the inductors should increase x 1.3 approx (based on the -7db values).

Values are for 16 ohm amp and speaker. All x 1/2 if using at 8 Ohms. If trying just the resistive version, omit all component parts within the dashed box.


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## Mortfly

Thanks for your excellent work John, Gene et al.
Really awesome to be able to follow the progress so far.
I’ll be making one of these over the next few weeks too.
I’ve got a few amps with power scaling/ vvr. A combination of those too attenuating systems may be interesting tonally 
Cheers


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## JohnH

Here is a schematic for the next design that I’m planning to build, once parts arrive. It’s based on all the previous versions, with some extra features. 

It has 4 stages. Stage 1 is a -7db reactive stage. Stages 2 and 3 are further switched resistive stages of -7db and – 14db respectively. They always follow after Stage 1 to give overall reactive attenuation. Stage 4 is switched with -3.5 db attenuation. Its typically connected after stages 1 to 3, to bring total maximum reactive attenuation down to -7-7-14-3.5 = -31.5db.













Attenuator M 190110



__ JohnH
__ Jan 10, 2019
__ 1






There is a bypass switch sw1, to connect around stages 1 to 3, so stage 4 can also work on its own as a -3.5db resistive stage. By setting the switches, any 3.5db increment of reactive attenuation from -7 to -31.5 db can be set, plus resistive -3.5db and full bypass. Overall, this set-up should provide for quite a fine adjustment over a wide range down to less than 1/1000th of max amp power.

Two sets of component values are shown. Approximately, they vary by a factor of 2, but adjusted to suit available values. The higher values for use with a 16 Ohm amp tap, can be used with either 16 or 8 Ohm speakers.

With the 16 Ohm version, the amp will always see reasonably close to 16 Ohms, except with full bypass, where if an 8 Ohm speaker is used, the amp should also change to 8 to avoid a mismatch. Outputs 1 and 2 are parallel jacks, to use either a 16, an 8 or two 16 Ohm cabs

The values for an 8 Ohm amp tap also work fine with 4 Ohms. But for this version, there is also a 3rd output for using a 16 Ohm cab from an 8 Ohm tap. The added parts for this feature are optional, and are within the red box on the diagram. When using Output 3, two extra (yellow) resistors are engaged to maintain tone and impedances. One adds some extra series resistance to reduce speaker damping, and the other corrects input impedance and also high treble. If using Output 3 with 16 Ohms and either full bypass or -3 5db, use a 16 Ohm tap. At -7db and below or if using an 8 Ohm speaker, use an 8 Ohm tap.

The component values are mostly as before, taken from available ranges. Ive been a bit more ‘picky' with values to try to get the attenuation steps as even as possible. Hence Im showing 4.7 and 5.6 Ohms as part of stages 3 and 4 in the 8 Ohm version. These values are usually available as standard from component suppliers, but from better-value Chinese ebay stores the nearest may be 5 and 6. These are what I've ordered, but Ill test them and maybe add in parallel if I want to tweak them down.

Inductors are targeting a ratio of L2/L1 of around 1.5, as built recently by Gene B and tmingle. In my previous 8 Ohm build, I started with two equal 0.4mH inductors. I wasnt sure I was hearing much difference due to them, until it came 'alive' when I unwound L1 to reduce it to 0.3mH. This time, Ive ordered 0.4 and 0.6, and I may unwind them slightly to the values shown.

The graphs show the calculated signals at the speaker and at the amp, for an 8 Ohm version driving 8 Ohms. You can see how even and consistent the (predicted) frequency response and attenuation steps are.

The graph at right shows what the speaker sees, with the rise of impedance vs frequency, created by speaker inductance, maintained by the inductor coils as attenuation increases. When using speakers of different Ohm values to the amp, expect a db or two less overall output and slight differences in peak responses.

The calcs are done by a spreadsheet, based on classical theory, as used in Spice models. The real speaker is represented by a component model, by Aiken, adjusted to match a 4x12 Greenback cab. But most of the actual response is determined by the speaker itself, so when using different speakers, they should retain their own character.

Next, I have to build it!


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## Mortfly

Woo Hoo.....very timely schematic JohnH. Thanks


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## Mortfly

Got all my bits ..... time to wire it up......give me a while tho!!!!


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## JohnH

Mortfly said:


> Got all my bits ..... time to wire it up......give me a while tho!!!!



Great! Good luck with the build.

I'm waiting for one more delivery of Chinese resistors, to build the 8ohm version of 'M' above, so you might get there first.


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## born_hard

Hi guys, 

I´m interested in this attenuator but i dont have a possibility to build it myself. Anybody from Europe here who wants to build me this attenuator against payment for parts and working time of course.
PM me thanks Aron


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## Boysen

This is one of the best forum reads of the year (including 2018). What an amazing job JohnH! 

I plan to build your attenuator, but wish to incorporate DI functionaility for recording/amp testing purposes. Is it possible to siphon a line out signal right at the input between the jack and L1/R9, without messing with impedances, or are you better off building a separate DI box?

I've included a TT ToneHound Schem, similar to aikens reactive loadbox for reference. 

Kind regards
Christian


----------



## JohnH

Boysen said:


> This is one of the best forum reads of the year (including 2018). What an amazing job JohnH!
> 
> I plan to build your attenuator, but wish to incorporate DI functionaility for recording/amp testing purposes. Is it possible to siphon a line out signal right at the input between the jack and L1/R9, without messing with impedances, or are you better off building a separate DI box?
> 
> I've included a TT ToneHound Schem, similar to aikens reactive loadbox for reference.
> 
> Kind regards
> Christian



Hi Christian, thanks very much for your comments.

For a DI feature, taking a signal from the input as in that TT scheme is safe to do. Its a 50k load, which is negligible when applied across an 8 or 16 Ohm speaker system. I believe tmingle has already done something like that, and posted a sound clip earlier in the thread. It sounded good, but, not quite the same. There are a few things to consider if you want a DI out from which you can derive a natural speaker tone for recording or monitoring. This attenuator design is a bit different to others in this respect. Its really related to what you are able to do after this DI out.

To get a signal that mimics a miced speaker, it needs to have:

1. The natural boost in bass and treble, that a normal reactive speaker develops from being fed by a tube amp of high output impedance
2. Some form of cab-simulation, filtering or IR to shape the output from 1 to capture the acoustic response of a real speaker, particularly drop-off in treble above about 4.5kHz and also bass and mid shaping.

That TT/Aiken load box captures item 1 above, and has a reactive load that tries to provide both the bass resonance and the treble rise. So you sample that, and feed it into something else that does the cab-sim from point 2 above.

But my system, instead of modelling the entire speaker impedance with components, is designed to let your real speaker do that for itself. This lets you hear your own speaker doing its thing at attenuated levels, instead of being influenced by whichever speaker the load-box was based on. But it means that the only place in my design where that response in created for point 1 above, is right at the speaker itself. At the input to the attenuator, Im not trying to model the bass boost effect, it only captures the high treble rise, so that the amp can crunch it nicely. It seems to work! But a DI at that point may be lacking in low bass. If you can add a bass peak to it after the DI, then it should be good, as well as a cabsim.

But if I was taking a DI out from my design, I would do it at the speaker, since that's where the tone all comes together, then to a cabsim. The trick with that is that the DI signal would then be dependent on the attenuation setting, and you must always have some real speaker sound happening ie not dead silent recording. But that's hopefully not too limiting. You could play at low level in the room, and turn up the DI as needed to match the mixer input. I reckon a 10k pot with an extra 1k Ohms in series would do it.


*Some notes on design*

The arrangement I have gets around a consistent design problem that Ive noticed in the various reactive attenuator setups that Ive been able to figure out. Its this: If you want the amp to see an accurate reactive load, and also to get a correct frequency response at the speaker, its very difficult to achieve this with a range of attenuation settings in a passive system. There is too much interaction going on between load, speaker and attenuation. hence we find a lot of active systems that lightly sample a signal without changing it, which are all good but build up the complexity and cost since they need an amp somewhere. Or, they give a limited attenuation range to feed a speaker (there's a Two-notes design with just a single attenuated output to a speaker, plus DI), or, they try to offer a range but the tone varies away from the real tone at various settings.

So, what Im doing is:

Im only capturing the treble rise with reactive components. I don't worry about the bass peak and I let the speaker do that itself. This also takes out the need for the largest and most expensive reactive components. (note, Weber Mass designs do tis too, their coneless speaker motor component has no bass resonance)

There is a second inductor coil - and this is where my design is different. What this does is, the amp sees a reactive load, from L1 and L2, approximately matching that of a speaker at treble frequencies, but the balance and voltage divider effect of L1 and L2 then negate the reactive effect going forward through the attenuator. The multiple stages of following attenuation are just seeing an almost flat resistive response, which gets returned to the proper output shape, with bass peak too, when it gets to the speaker. Hence we keep consistent tone through multiple attenuation steps.


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## Mortfly

Hi Guys,
I finished building this attenuator and I've played it through various amps with both single coil and humbuckers.
Here's the build:



It works incredibly well at getting loud amps down to low volumes - so absolutely YAY!!! Those resistors stayed very cool - the inductors were warmer but nothing to worry about.
I've played it through a DC15 (AC15), Bassman (tweed), Tweed deluxe (5C3), and a London Power Standard amp. The DC15 and the LPS have power scaling/VVR. I also plugged into 10" and 12" speaker with all amps too.
_It rolled the highs off all the amps. The more attenuation the more the highs were lost. The Bassman sounded best regarding frequency balance. Then the Standard, DC15 and Deluxe in that order. The 3 1/2dB switch is a pretty small reduction. Probably better to stay with -7dB_
_I'd like to look into getting the highs being brought back in somehow._
Sorry folks the -3dB switch was faulty and the highs are actually all fine now  ....... well mostly.
It is a great design so far, bloody well done JohnH - you can really get that amp pumping without annoying the family.


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## JohnH

Great! Thanks for posting and that looks like a very nice build.

So that's the 8ohm version I assume, used with 8 Ohm cabs? or 16? It would be good to understand the highs issue some more. We haven't seen it before on earlier builds. If you have an opportunity, a couple of attenuated/non-attenuated sound clips might help figure it out. And Ill get mine built soon.


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## Michael Roe

Hey guys, just came over here after reading a post from Johnh on another thread. This looks really cool! I wish I knew/understand more about all the technical stuff you guys are talking about. My experience with tube amps and attenuators is...….I modded an Epiphone Valve JR and built an LPAD attenuator for it. These were mods by Bit-Mo and had clear directions. It was fun to do although I really had no idea what I was doing 
For my day job I own and operate a Fire Protection business and I service fire alarm systems. So my electronics skills are very primitive, lol.
I would definitely be willing to try and build one of these. It is very cool that you would put all this info out to the public.


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## Mortfly

JohnH said:


> Great! Thanks for posting and that looks like a very nice build.
> 
> So that's the 8ohm version I assume, used with 8 Ohm cabs? or 16? It would be good to understand the highs issue some more. We haven't seen it before on earlier builds. If you have an opportunity, a couple of attenuated/non-attenuated sound clips might help figure it out. And Ill get mine built soon.


Yep I should have mentioned it’s an 8 ohm build into 8 ohm cabs. I’ll check through my wiring once more - just in case there’s a fluff up somewhere!!
Edit*****
Yes I do have a error: the -3dB switch is broken .... I'l fix it today or tomorrow and edit the post above. Those old switches are cool tho'!!!!! it's a shame 
They were made when asbestos was safe for the whole family!


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## JohnH

I've been looking at your pictures. That really is a high-quality build!, very clean and methodical. I love the ventilated coil clamps.

I see you're in Adelaide. Where did you buy parts? - particularly the coils. I got mine from Queensland Speaker Repair, though to get the values on the diagram I have to buy a bit high then unwind them a slightly. (shouldn't make much difference though if they are in proportion to each other).

*Some random things to check:*

1. With an 8 Ohm cab plugged in, the dc resistance measured at the input should be in the range 7 to 9 ohms in all settings, with maybe 6.5ohms with full bypass measuring straight through to the speaker.

2. You could get some resistance measurements from the 8 ohm output, with no amp. We can work out what they should be. Also, other than the bypass and -3.5 stage, your build should be electrically almost identical to the current one that I have, which does 7db increments. So I can measure mine at -7, -14, -21 and -28 to compare.

3. Just confirm the coils are in the right order wrt their values. L1, which goes to the input jack is the smaller value. But it looks ok in the pic, with a few less winds apparent on L1 than L2.

*Work-arounds for more treble:* 

4. If you built the whole Design M shebang, it has the 16ohm output, which engages a couple of resistors via the switched jack that raise output resistance and bypass R1 a bit. Both should affect treble. Its ok to plug 8ohm cabs into the 16 ohm output, so long as you use at least -7 db or more attenuation (you can confirm by measuring input R), still going to 8 ohm amp tap. This will drop overall levels about 2db but treble only drop about 0.5db, so relatively a bit brighter.

5. We can consider cap-bypass at particular stages. Not complex but would need a bit of working out. It could be at Stage 1, to affect all settings of -7db and below, or Stage 3, which is the -14db stage, to run at settings -21db and below, or some of each. I haven't gone there yet but its how most other passive attenuators get extra treble.

*Other things to consider*:

6. As you found, the tone is dependent on the amp and speakers. The values have been worked out to try to show impedances to amp and speaker similar to a typical Marshall power-amp stage (which has a bit of NFB), and typical 12" Celestion speakers. As more attenuation is added, the real amp gets more separated from the real speaker, and what each one sees is then more dependent on the attenuator design. Some amps have no NFB, and I believe this applies to Vox circuits, and so maybe also to your DC15. This implies a very high output impedance for small signals, which with a reactive speaker, will result in higher treble due to speaker inductance. The attenuator is designed to provide a high output Z, but maybe not that high. This factor may also explain why the Bassman (as copied on early Marshalls) was the most consistent. I'm not sure how the power-scalling circuits will affect all this!

7. FM effects can mean than lower-volume sounds seem less bright. Im not trying to adjust for that though. Recordings at different levels, normalised for volume, can be used to investigate.

8. Some amps have a bright cap or similar, to boost treble at low volume, and at high volume where the power amp is saturating, the effective output Z changes. So comparisons to assess the attenuator must be made at consistent amp settings.

Just a few things to chew on!
cheers J


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## Mortfly

Cheers JohnH. I appreciate your kind words. And once again bloody awesome work you’ve done on this. It fills a gap!!
I got my resistors from mouser online. Selected for high tolerance and price. Inductors from Speaker Bug. The inductors are listed at 0.33 and 0.52. I didn’t bother to measure them but I could. 
Interesting for me: R1 and R2 are really small. Same size as a mosfet! Compare with the size of R7!!!!!
The -3dB switch doesn’t unlatch on one side so once that’s replaced I’ll check all the values listed above. 
Cheers and thanks. Gold star and kangaroo stamp for you mate.


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## tmingle

Here is my original build(Per JOHN H, Ive since added a couple inductors & a reactive/resistive switch). The line out is pretty hot, so I will be changing the values in the line out circuit at some point. S1 & S4 are actually a 2 pole switch.


----------



## tmingle




----------



## tmingle

I do wish I would have used a little larger box for this build. I used 100W resistors & installed 2 15 gauge inductors, so further additions will be difficult. I am currently looking for something from the junk pile at work to house this in. 
Based on the performance of this unit, my next amp will most likely be a 50W or 100W head rather than a "mini amp". Ive owned several & sold all of them. I can run the volume at 4-5 on my 40C with all the stages engaged & get really close to "bedroom volume". Anyone fighting the volume battle at home should consider building this unit. It does affect the sound to some degree, but I have found that more attenuation requires less gain, adjusting the BMT controls & a reverb pedal to help fill the sound.


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## Mortfly

tmingle said:


> Here is my original build(Per JOHN H, Ive since added a couple inductors & a reactive/resistive switch). The line out is pretty hot, so I will be changing the values in the line out circuit at some point. S1 & S4 are actually a 2 pole switch.
> View attachment 54473
> View attachment 54474
> View attachment 54475


Hi tmingle, you’re probably already all over the line out circuit but I’ll post this link anyway. Hope it helps someone 
http://www.dougcircuits.com/lineout.html


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## Mortfly

The -3dB switch has been replaced and all is well in the world of Design "M" Attenuators. 
This circuit is definitely worth building - thanks heaps for all your work JohnH and also for making the development of the circuit available to us! Much appreciated. Really. I mean it!.

tmingle's comments above are spot on: _"I can run the volume at 4-5 on my 40C with all the stages engaged & get really close to "bedroom volume". Anyone fighting the volume battle at home should consider building this unit. It does affect the sound to some degree, but I have found that more attenuation requires less gain, adjusting the BMT controls & a reverb pedal to help fill the sound."_

A deluxe version might have some sort of frequency bypass - let some highs thru (upper mids?) - possibly cut a bit of the lows.....maybe, I'd like to know how other players feel about it. ????? Wouldn't be as simple though.

A super duper deluxe version would include something to add that feeling in the body you get when exposed to gorgeously loud amps. How could that happen? Some sort of haptic body suit connected into it???? ....only joking.....but we live in hope!


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## JohnH

Thanks for the feedback. 

If you want to just try something, there's a tone-tweak that may or may not help but is safe to try. Just with an alligator lead, try bypassing L1, and the tone may get little brighter. Or, bypass L2 and it should reduce high treble. Bypass them both and you are back to a resistive design, but it is all safe if you are careful.

My last pack of resistors arrived, so maybe this w/e I might start to build the M version myself!


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## Mortfly

JohnH said:


> *Some random things to check:*
> 
> 1. With an 8 Ohm cab plugged in, the dc resistance measured at the input should be in the range 7 to 9 ohms in all settings, with maybe 6.5ohms with full bypass measuring straight through to the speaker.
> 
> Yeah so on testing these values just now I have found the switches (especially the new one!!!) add 2 1/2 to 3 ohms to the system = large for this circuit. Tested with a Fluke 177 DMM. [/QUOTE]
> 
> Edit.....
> the new switch is replaced with a new switch! - it all works properly now


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## JohnH

Hmmm...2.5 to 3 Ohms does sound like a lot if that's just due to switches. I just measured across the toggle switches that I'm about to use in my build. Its always hard to get accurate resistances for very low ohms, the meter resolution is 0.1 Ohms and the test leads read about 2.5 Ohms themselves, and to measure on the lowest Ohms scale, there's quite a lot of current draw from the meter battery so on my meter, the value drifts a little as it settles down. But the difference in ohms between meter leads touching each other, or through the closed contacts of a switch, appears to be of the order of 0.1 to 0.2 ohms max. 18gage stranded wire is about 20Ohms per km, so even a meter of that only adds 0.02Ohms.


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## JohnH

I started on my 'M' build, this could take a while!

As usual, I have been a bit over-optimistic about how much I can squeeze into a given box volume. When its done, I like it that its compact, but wiring it up does get tricky when it's squeezed. So here is a picture of how I plan to wire it, based on a photo of how I plan to place the parts. The switches and jacks are all pulled inside in this pic, since it was not drilled yet:













AttenuatorM Wiring190203



__ JohnH
__ Feb 3, 2019






The box is very thick and strong, and it has useful added bulges and ribs inside for screws and receiving pcb's, but in this case these features constrain where I can closely mount resistors with good metal/metal contact.

In the picture above, the parts are all just placed there, with the resistors stacked side-by-side across the middle, and they just fit. There will be a bit of spare real estate left over for drilled vent holes too. Luckily, at a given setting, only two of the large resistors will dissipate any significant heat, and they are never next to each other. I'm taking lessons from builds by Gene, tmingle and Mortfly (thanks!), and using the lid to mount the inductors and bypass resistor, and labelling all the parts.

Then, I started drilling, and so far I've just drilled for the main layer of resistors. But I find that the M3 bolts I got to mount them are too short for the thick heavy box that I got, so a pause while I get longer ones. No rush! I want to try to do this right, even though every build I try myself seems to get gets numerous little bugs in construction that need to be worked around.


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## Mortfly

JohnH said:


> I started on my 'M' build, this could take a while!
> 
> As usual, I have been a bit over-optimistic about how much I can squeeze into a given box volume. When its done, I like it that its compact, but wiring it up does get tricky when it's squeezed. So here is a picture of how I plan to wire it, based on a photo of how I plan to place the parts. The switches and jacks are all pulled inside in this pic, since it was not drilled yet:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> AttenuatorM Wiring190203
> 
> 
> 
> __ JohnH
> __ Feb 3, 2019
> 
> 
> 
> 
> 
> 
> The box is very thick and strong, and it has useful added bulges and ribs inside for screws and receiving pcb's, but in this case these features constrain where I can closely mount resistors with good metal/metal contact.
> 
> In the picture above, the parts are all just placed there, with the resistors stacked side-by-side across the middle, and they just fit. There will be a bit of spare real estate left over for drilled vent holes too. Luckily, at a given setting, only two of the large resistors will dissipate any significant heat, and they are never next to each other. I'm taking lessons from builds by Gene, tmingle and Mortfly (thanks!), and using the lid to mount the inductors and bypass resistor, and labelling all the parts.
> 
> Then, I started drilling, and so far I've just drilled for the main layer of resistors. But I find that the M3 bolts I got to mount them are too short for the thick heavy box that I got, so a pause while I get longer ones. No rush! I want to try to do this right, even though every build I try myself seems to get gets numerous little bugs in construction that need to be worked around.


Good planning is simply good!


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## Gene Ballzz

@JohnH 
Great to see your progress! This is a fantastic thread. My sincerest thanks for the help in getting my 4 stage units constructed. They are working flawlessly at 7db, 10.5db. 14db, 17.5db, 21db & 24.5db of attenuation for all of my amps over 10 watts. For my "baby" amps below 10 watts, I'm finding a need for something smaller and lighter duty, yet still want to incorporate the reactive inductor circuitry. My 4 stage units have a minimum attenuation of -7db, which is just fine for the bigger amps, but -7db is just too large of a cut for these "babies." What I now want to do is just a simple 2 stage unit with steps of 3.5-ish & 7-ish. Any thoughts or desire to look into it? I can easily do the hardware, wiring and building, but am clueless as to working out required values of components!
Thank You JohnH,
Gene


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## JohnH

Hi Gene
Seems like you have been busy! Im glad its working out.

For a -7 and -3.5 (and -10.5) version, it could be as simple as Design M (see page11), and omit stages 2 and 3. Power ratings could scale down.

M is the one im building, as has Mortfly. When its at -3.5, its just resistive, and im thinking that will be fine. I wonder if you could roughly rig up the two resistors for -3.5, just to check that for yourself with your amps?


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## Gene Ballzz

Ya know @JohnH
Just the one stage at 3.5db may be all that is required for my needs here! I'll give it a try without any inductors to see if it kills the sound! I bought a fairly good supply of resistors this last order, so I've got plenty for monkeying around! Funny thing about amps as the wattage goes down. The lower the wattage, it seems the more "fragile" the tone and response is and smaller changes at the output/speaker have even greater effect than on a bigger amp! It's likely as simple as the fact that 7db is a much larger percentage of the overall volume of a 5 watt amp than of a 50 watt amp!
Thanks Again JohnH!
Gene


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## JohnH

I just tried a very cautious aligator-clip lash-up of a -3.5db resistive stage, using the VM combo. Actually it sounded fine and properly lively, which is what I expected. Even the most tone-sucking schematic arrangements of resistors seem usually to be reported as being fine for a few db off. I was using 33 and 6 ohms, to run with 8ohm speakers and amp tap.


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## JohnH

Gene - btw, just going by the basic numbers (and perception is more important and may be different, plus other factors may be in play), comparing two amps of say 50W and 10W, the difference is -7db, and that is a 1/5th ratio of power. The next -7db step is to 2W, then 0.4W. We expect each step down of equal db value to sound about the same in terms of perceived volume change.

So if you have attenuators for amps up to 50W, then to get down to the same minimum power with a 10W amp, in theory one -7db stage can be omitted. Also, lets say you want to play a 50W amp at -21db, that will be 0.4W. A 10W amp at -10.5db should in theory, be louder than that at 0.9W.

But all of that is just in theory which is only useful if it helps to understand what actually happens!

Just thinking about what other factors might throw that off:

1. If we are dealing with combos or cabs with smaller speakers for smaller amps, then speaker efficiency could easily change actual volume by several db, leading to less attenuation needed for the smaller amps.
2. Output stage circuitry, might be very simple in the small amps, maybe single ended? Shouldn't change too much but maybe there is no NFB, hence very high effective output impedance for the amp and a naturally bright amp, to be maintained through the attenuator.
3. A different sweet-spot on the amp in terms of % of max power to find the desired tone.

All just maybe's...! The testing is the real proof for each setup.


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## tmingle

https://www.ebay.com/itm/AudioStorm...hash=item2cd2ba5a5c:m:mPGjNsDeGNinBxsby5DrMJg

Does this look familiar?


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## Mortfly

tmingle said:


> https://www.ebay.com/itm/AudioStorm...hash=item2cd2ba5a5c:m:mPGjNsDeGNinBxsby5DrMJg
> 
> Does this look familiar?


How new is the 125 unit I wonder? 
I really appreciate JohnH having all this info and testing available. Bloody good mate!!!! I'm using mine all the time.
Thanks


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## JohnH

tmingle said:


> https://www.ebay.com/itm/AudioStorm...hash=item2cd2ba5a5c:m:mPGjNsDeGNinBxsby5DrMJg
> 
> Does this look familiar?



Thanks, that's interesting! Obviously a similar train of thought, and he has some simpler resistive models too. That particular model seems to have been out a couple of years. Best wishes to him for having a go. This is from the website:
_
"The HotBox 125 is our flagship with four carefully chosen levels of reduction and a switchable inductor. This unit is at the cutting edge and we think it's the single best power attenuator in the world. Period."_

It would be interesting to know how he designs the resistive stages, and how the inductor coil is incorporated. Great to see such product confidence!

On this thread, there are only two main ideas:

1. Resistive stages need to take care not only of input impedance but also output impedance seen by the speaker
2. Twin inductors are used together, to show the amp a load that varies with frequency and show the ongoing stages a flat response until shaped downstream by the reactance of the real speaker.

My 'M' build is creeping along. This weekend I got the case drilled (about 50+ holes), painted (I got quite a nice satin black rattle-can), and all parts mounted. I also unwound the 0.4 and 0.6mH inductors slightly, to get them to the values noted. Easy with a meter that has a Henries scale. Wiring is next.
_
_


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## JohnH

Done it!

I got my 'M' version off the bench and onto the amp this afternoon. So far it seems to work all as expected. The new -3.5db setting and 3.5db increments seem to work fine, a smallish but useful step, and the tones seem clear and consistent, as before. It gets pretty loud and pretty quiet and everywhere in between.













AttenuatorM Outside 190217



__ JohnH
__ Feb 16, 2019


















AttenuatorM Inside 190217



__ JohnH
__ Feb 16, 2019






The way I have it, the left switch is bypass for the 7 and 14 db stages, and next to it is the -3.5db switch, which can work on its own or with other stages. So if both these close-together switches are up, the box is fully bypassed. Then, to the right are the -7 and -14db switches. All switches are orientated up = louder. All settings are reactive except bypass and -3.5db.

I learnt something about toggles today: I had thought that in all cases, when you flick down, the lugs on the back connect up, as they do in mini-toggles. Not so with these ones!


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## Mortfly

It’s a really good sounding unit. And loved by my family!!
The -3db increments are perfect I think. 
What did you reckon about the eq at high attenuation?


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## JohnH

Mortfly said:


> It’s a really good sounding unit. And loved by my family!!
> The -3db increments are perfect I think.
> What did you reckon about the eq at high attenuation?



I've only tried this one briefly, but it seems to fit in well with the previous versions. I'm not noticing an EQ issue, but that could be because I've designed it around the amps I'm using it with, or due to my cloth-like ears!

On My VM, using the attenuator, there's obviously a tendency to turn up the amp (that's the whole point right?). An when that happens, it makes the unattenuated tone of the amp change, and particularly the presence control gets much more effective.

To get a bead on it, I'll need to run some measured frequency response tests at consistent amp settings. Another test I want to do is to measure the attenuation at each step, to see whether they come out as consistent as calculated with my $1 Chinese resistors.

I know you were looking for an EQ option. It's possible. Would that be for use on all attenuation settings, or could it be limited to those below a certain range? eg, could use a switch to splice a cap across R6, to give an optional lift to settings where the -14db stage is engaged (which would be at -21db and below)

But I'm happy with it so far and it seems like it will be versatile with no obvious glitches or bugs revealed yet.


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## JohnH

I was playing with this last night, at -31db!

I think at a very quiet late-night time with no other noise in the house, our hearing and perception of sound is more sensitive. I could noodle away quite happily at this level, and appreciate the full range of tones. I had volume at about halfway, which unattenuated is very loud indeed and starting to compress. I figure the amp was putting out about 20W of which I was probably hearing less than 20mW!

But, with the small 3.5db increments, I can add another small dose of volume, and, I like that a bit better and its not much louder. So add another (and another) one and soon I've added 10x the power. Hearing readjusts and that late-night -31db setting then sounds too small.


----------



## JohnH

A few more checks today:

I wanted to check out the design and wiring by measuring the consistency of the attenuation steps, which are intended to be all 3.5db, from 0 down to -31.5db. The easiest way to do this was to feed in some dc from a 9V power supply, via a resistor to represent the amp output impedance, and adding a 8 ohm resistor at the far end to represent the speaker. Then I measured dc V and worked out db ratios as 20log(Vout/Vin). These all came out good, within 0.3db of as intended. I'm going to tweak R8, where I had to buy 6 Ohms, and 5.6 was the design value, so I'm splicing another 82 Ohm 5W resistor across it. R5 was another candidate, where I used 5 instead of 4.7, but this stage seems ok as it is.

But I found a 'watchit' about the inductor coils:

Before assembling, I'd careful adjusted the winds on L1 ad L2 to get them to what I wanted, using the inductance range on my meter. I wanted to recheck the values insitu, and I was surprised that they were both reading 40% high. What Id done was, fix them in position with M3 steel bolts. You wouldn't think it would make much difference, but it does. Being a ferrous element through the centre of the coil, it increases the inductance value. Pulling the bolts out brought them back to where I'd set them. This 40% extra is not enough to make a very large difference, but I want to test it as designed. So for now, they will be glued in place until I can find non-ferrous bolts.

Maybe its a way to make subtle tweaks to the tone. If L1 is higher than designed, high treble should be a bit less, but if L2 is higher then there should be a bit more treble.


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## JohnH

Here is the 'proof of the pudding'

*Sound Samples*:

*Attenuator M: Max attenuation to non-attenuated:*
https://vocaroo.com/i/s0QOpqTD3WAo

*Attenuator M: Normalised:*
https://vocaroo.com/i/s1GJvYK04i00

Its a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in nominally 3.5db steps. Then, the second file, which is based on the same recording, has each stage normalised for volume so you can hear any differences in the tone.

The VM2266c was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off the speaker with a Rode M1 into a neutrally set mixer.

What can you hear? All the attenuator settings are through the reactive circuitry, except the second to last which is -3.5db resistive, and the last, which is direct to speaker.


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## tmingle

JohnH said:


> Here is the 'proof of the pudding'
> 
> *Sound Samples*:
> 
> *Attenuator M: Max attenuation to non-attenuated:*
> https://vocaroo.com/i/s0QOpqTD3WAo
> 
> *Attenuator M: Normalised:*
> https://vocaroo.com/i/s1GJvYK04i00
> 
> Its a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in nominally 3.5db steps. Then, the second file, which is based on the same recording, has each stage normalised for volume so you can hear any differences in the tone.
> 
> The VM2266c was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off the spaeker with a Rode M1 into a neutrally set mixer.
> 
> What can you hear? All the attenuator settings are through the reactive circuitry, except the second to last which is -3.5db resistive, and the last, which is direct to speaker.


I certainly do not hear much difference. 
Again, Nice Work John
I use mine every day


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## BowerR64

JohnH said:


> Here is the 'proof of the pudding'
> 
> *Sound Samples*:
> 
> *Attenuator M: Max attenuation to non-attenuated:*
> https://vocaroo.com/i/s0QOpqTD3WAo
> 
> *Attenuator M: Normalised:*
> https://vocaroo.com/i/s1GJvYK04i00
> 
> Its a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in nominally 3.5db steps. Then, the second file, which is based on the same recording, has each stage normalised for volume so you can hear any differences in the tone.
> 
> The VM2266c was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off the spaeker with a Rode M1 into a neutrally set mixer.
> 
> What can you hear? All the attenuator settings are through the reactive circuitry, except the second to last which is -3.5db resistive, and the last, which is direct to speaker.



Dude that sounds really good


I was going to get an Ampeg VL501 GC had pretty cheap but after i watched a few videos to hear how it sounded by the time i went back it was gone.

It has a built in attenuator i thought ide get some pictures of it and post em in here.


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## JohnH

Sometimes it's interesting to see what things sound like....













Attenuator M Frequency Plots 190302



__ JohnH
__ Mar 1, 2019






These plots are taken from the sound sample posted above, for Attenuator M, which was my VM miced off one of the Greenbacks.

The lower set of data are the basic plots, from full volume down to -31db (db scale is arbitrary, but relative db's are right).

The upper plots are intended to show the differences between responses. I decided to take the -7db recording as the base case, so this is shown as a flat line. The others are the various other settings, with the -7db trace subtracted. The ideal for these traces is therefore also a flat line. And for all the traces below -7db down to -31db, this is what is happening, there is virtually no further tonal change at all as you attenuate down as far as you want. It measures as consistent.

The -3.5db and full-volume traces show some wiggles relative to -7db. The peaks are generally consistent and following a flat trajectory though. I think we are seeing extra resonances and distortions generated in the speaker itself at this high volume, and no attenuator can capture those. The -3.5 trace (resistive) shows a very slight treble fall-off, hard to hear in practice though.

Seems to work!


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## Gene Ballzz

@JohnH

Keep Up The Great Work!
Gene


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## GrahamL

So what is the cost and time put into making a unit these days? 
Oh, and when will they be up for sale?


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## JohnH

Hi Graham, good questions!

Its easy enough to list the parts, in the case of the 'M' version that I just built, there are:

4x 100W resistors
7x 25W resistors
2x inductor coils
4x insulated jacks
4x dpdt toggle switches
1x case
4x case feet
solder, wire, nuts/bolts, heatsink compound, tools etc

I get the resistors from China, free post and they are about $2 for 100W and $1 for 25W. They are fine but you can easily spend $10 on each plus shipping if you wish.
The coils from where I get them are around $9 each, plus shipping
Jacks, I use plastic bodied stereo switched jacks, for their good grip on the plug, $3 each.
Switches, about $4 each. Feet, say $2 each
The case should be aluminium, mine is around 170x120x50mm and I paid $35
Feet, say $2 each

I've not been particularly trying to save money or go for top quality, I just wanted to get the parts the best way I could get them, some online, some from shops.

Adds up to just over $100 (Aus), plus nuts/bolts thermal compound, solder and wire and paint, in the last one I built. Could be $120 total. Lots more spent in spare parts to try things with!

Time?? hours and hours and hours.... I'm not a particularly fast and definitely not a neat worker. I find the process relaxing though, in small quantities.

I don't have a plan to sell them. I have much better ways to earn a $ and this is a hobby. I do it to explore the design and I enjoy the collaborative feedback, which has helped a lot (thanks everyone!).

Anyone is free to try these designs at their own risk, and for their own personal use. The price then is to send a message so we can hear about how it went and which amps it was used for, and what you thought of it.

Clearly, if it had been my plan to try to make money on these, I would have been very reserved about posting all the info as I have. but then it wouldn't have been as fun nor turned out as it has.

I might consider selling a wiring diagram with parts list to help build them if there are such requests.

I do not offer general permission for commercial use of this design by others. But, to honest people who may wish to make them to sell, contact me by pm to arrange terms for using this design, which would be on a small $ per unit sold basis, no liability on my part but with ad-hoc advice given if needed.

cheers
John


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## TonyK

@JohnH

Hi John,

I was trying to design a good attenuator a couple of years ago and luckily got side-tracked. I just came across this amazing thread purely by accident and have read all the way through. I'll need to re-read to better understand the theory behind what is being discussed, but it seems that I won't have any choice now but to build this!

I'm just about to build a 5e3 Tweed Deluxe clone, but given shipping delays and so forth for the attenuator parts, the attenuator build might wait until after the 5e3. In any event, my 5e3 build will have a pre-PI Master Volume and a VVR as I'd always hoped to find a good balance of tone vs volume. However, I also have a Fender BassBreaker 15 head which has 4,8, and 16 ohm speaker outputs, going into a 16 ohm WGS ET65 speaker. And finally a Fender Hot Rod Deville 60W 4x10" combo, standard Fender 8ohm parallel/series configuration. Obviously, your attenuator would be very helpful for all of these amps in order to get those power tubes working nicely 

So just to help me contextualise your circuits before I closely re-read everything, I'm hoping that you can just clarify two things please. First, I guess I'd need to go with 8ohm in and 8/16ohm out right? And second, you mention throughout this thread "for amps up to 50W". Ok, the HRDv is only 10W more, but does this mean that I would ideally need to use 120W power resistors, or whatever is available larger? Or, as long as I never come close to diming the amp, would the 100W resistors be sufficient, perhaps with the major resistor replaced by two parallel 100W components?

You've done a fantastic job here, and I hope to be able to contribute a little back by way of my own experiences soon enough!

Cheers,
Tony


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## JohnH

Hi Tony, welcome to the MF and thanks for your interest in these attenuators.

On the power ratings, I think the values shown would be fine for a 60W amp, if built as I did, into a fairly heavy Aluminium case with the resistors bolted to it with thermal grease, and well drilled for ventilation above and in the base. There is plenty of electrical capacity with the values shown. For Attenuator M, R1 dissipates up to 30W for 50W input, so 36W for 60W in. There's still a factor of 2.8 against a 100W component rating. R7 (part of the -3.5db stage), also gets the same power if you use it on its own for just a -3.5db reduction. All the other values have greater margin. 

If you are choosing between the 8 Ohm or 16 Ohm builds, there's a couple of things to know:

The numbers work out a bit more nicely in terms of consistency of attenuation steps and tone when used with speaker ohms = amp-tap ohms. Eg, with 8 and 8, all the steps are very even (see tests above) at about 3.5db each. If you use 8 tap with 16 speaker, then all the attenuation levels are reduced by about a further 1.5 to 2db, eg 0, -5, -9, -12.6 down to -33db instead of -31db, similar steps but each level a tad lower. Not a big deal, but just so you know. The treble and mid tone should be consistent but the bass peak is maybe 1db down (ie not much). 

When you run with amp=speaker ohms, then all the settings give a good match for the amp. If they are different, then if you use a bypass switch, you need to match amp to cab, or not use the bypass. Also, if the 8 ohm attenuator is used with a 16 ohm speaker, at a -3.5db setting, it should use a 16 Ohm tap in that setting. 

But having said all that, then with your collection (two of which don't seem to have 16 ohm taps) then I agree, an 8Ohm build is the best one to go for and if you do so, I hope it works out well and Ill be interested to hear all about it. Happy to respond to questions too.


----------



## JohnH

JohnH said:


> Who likes numbers? Here's some more, based on better temperature measurements using an Infrared thermometer from the engineering school where I teach.
> 
> I ran dc power into the attenuator with measured voltage 8.55 V and measured resistance 9.2 Ohm. So power in was 7.9W
> 
> I ran for 30 minutes, at which time it had slowly reached a steady state with 35 C max on the top plate (about 33 C average) and average about 25C at the sides. Ambient in the room was 16.5 C.
> 
> That's as much dc as I can put through with what I have to hand. But based on these numbers, I did some maths with convection and radiation and I reckon that with an average of about 25W into it, it would get to about 55C. The hotter these things get, the better the mechanisms of passive cooling work. Given that this is an average value over at least 10 to 15 minutes, and it could easily absorb several times that for a shorter time, then that should be more than adequate for a 40 to 50W amp, given that any practical use has varying amplitude levels and gaps in the playing.



I thought id check out the thermal performance of Attenuator M, to compare it to my earlier ones that were in smaller boxes. So far, I haven't found much temperature rise with 'M' at all in practice.

The last time I ran a thermal test was at post 28, quoted above, which with 7.9W of DC going in, had a temp rise of 18.5C after 30 minutes, in an aluminium case 120x90x35mm.

The new one is 170x120x55, and much heavier. With about the same power going in, it rose 12C, but took 60 minutes to get there. So, back-figuring again with a bit of convective and radiative physics, I reckon average steady state of about 30 to 35W will get a temp rise of around 50C. I think in practice, given that temperature of this heavier build averages power over a long time, at least 30 minutes, this is fine to absorb the energy from a 50 to 60W amp, allowing for breaks and playing dynamics. 

For say a 100W amp, it would depend on playing style. To really thrash it continuously (not my style at all), some active cooling or extra finned heat sinking might be in order.


----------



## TonyK

Hi John @JohnH 

Thanks very much for the detailed response. This encourages me to now go back through the whole thread to try to piece together the evolution up to your Attenuator M circuit. I'm sure I might have questions as I go through, but more certainly when I actually start to source the build, so I really appreciate your openness to assistance. It's a rare positive quality! I was very interested by your thermal analyses - that really does make me feel better about simply using 100 ohm resistors especially as I'm not even sure there's any value in actually cranking the HRDv all the way to 12 on the dial. In the 20 years I've owned it, I've never actually gone above about volume 4! Although nowadays I play using a volume pot in the effects loop so that I can raise the level of preamp gain as much as I need (without the corresponding increase in power amp gain of course), but even in this configuration the amp remains pristinely clean until about 9 on the clean volume -- even my ES-335 doesn't push it much other than giving a little more bite at this level. For fun last night, I actually did dime the clean volume for the first time, and there is quite a nice break-up that creeps in around volume 11! You can imagine that I'm pretty excited by the prospect of hearing how it might sound with the power stage being pushed suitably as well. Strange, eh, that after 20 years, I've never actually experienced that sound!

Anyway, just to clarify the output transformers. The Hot Rod, as I said, is standard, using an 8ohm OT which switches internally to 4ohm in the event that an appropriate extension cab is added to the internal speakers. The BassBreaker 15 has a multi-tap OT but when I bought my ET65 speaker for it, the only one available was 16ohms (not a huge selection in the UK or Europe), and it's sitting in a provisional 2x12 cab right now in anticipation of the 5e3 build. For that build, I've bought a multi-tap OT so it, too, will have a tap choice of 4, 8, or 16ohms although I'd plan on using the ET65 16ohm speaker in the cab I'm building for the combo. So in fact, both of the smaller amps would have choice of OT taps, but most readily available, the 16ohm speaker. I think, though, that all of your thoughts above still apply, and that therefore, the 8ohm into 8ohm attenuator will still be the best choice. From what I recall from reading the thread, even if I choose to run the HotRod 8ohm tap into the 16ohm speaker (just for fun, instead of using the internal ten inch speakers), your design will still function safely.

Oh, and your point about the -3.5db setting is well taken, though as you can imagine, with these particular amps, such a reduction would be rarely audible given a drop perhaps from 115db (on the smaller amps) to 112db! So that said, I wouldn't really concern myself with that -- but I am much more interested in the higher attenuation levels for use at home or in a small practice space. Typically, even when I play at home, I register mid- to high-90s db levels. But my playing style is very dynamic, and like yours, thrash doesn't happen chez moi!

Many thanks again,
Cheers,
Tony


----------



## Gene Ballzz

@TonyK 
In building your Tweed 5E3 Deluxe Clone, I would highly recommend that you try it as a *"Bone Stock"* build, before implementing the mods you described! You can certainly lay it all out for the planned mods, but from my experience (several years of loving my 5E3), as soon as you change anything in the design (except for maybe a 12AX7 instead of a 12AY7 in V1) it becomes no longer a Tweed Deluxe and some of the magic gets lost! Try it stock first and I'll bet that after you implement the mods, you'll go back to stock!

And YEP, John's attenuator designs are "Da Bomb!"

Just Sayin'
Gene


----------



## JohnH

Tony - It does sound like the 8 Ohm version may suit you best. I did some more testing on mine today, with it feeding a 16 Ohm speaker, since I don't know of anyone else who has used this exact combination and most of my tests were 8 with 8.

I used the 16Ohm internal V30 in my DSL401, but with the 8 Ohm amp tap into the attenuator. I usually run this amp with an extension cab for an 8 Ohm load. The V30 sounded bright and clear as it should, using the tweaks that are built into the 8 Ohm M design to balance impedances and tone with a 16 Ohm speaker. And, there's an 'Easter Egg' too, since its also safe to run the 16 Ohm speaker into one of the 8 ohm attenuator outputs. The result is a slightly more 'middy' tone, and if its preferred then its fine to use as an alternative.


----------



## TonyK

@GeneBallzz Thanks for the advice, Gene. It seems to be a common theme for those who have already been down this path. I don't want to sidetrack this exceptional thread, but given that my goal was simply to be able to play the 5e3 at reasonable volume but with its natural breakup and overdrive, you are probably right. The master volume is simply a 1M ohm pot to replace the grid leak resistor so that when fully open, the circuit remains stock (per Rob Robinette's excellent web resource). And the VVR is a tiny add in PCB that again, when on maximum volume is a full bypass resulting in a stock circuit. However, the goal of John's attenuator is the same and indeed, doesn't touch the 5e3 circuit at all, and is therefore the preferred method! More importantly, it allows us to fully hear and experience the preamp and power amp saturation, as well as the OT transformer saturation, which is ultimately the holy grail. And if I understand the few observations posted here about the reactive circuit, it seems that the "feel" back through the guitar might emulate, to an extent, the speaker interaction as well. That means that I'd build the attenuator M first  Perhaps in a small tweed box?

@JohnH Thanks for testing that 8ohm into 16 ohm on my behalf, John. That was very kind of you. My own experience of running the HRDv's 8 ohm output into the 16 ohm speaker seems to parallel your findings. I understand that the impedance mismatch is minimal enough not to cause problems, and I can generally detect a very small drop in output volume (just a couple of db). But your observation of the slightly different tone due to a deliberate mismatch between the attenuator and the speaker is interesting! Only hearing would show if it is preferable of course, as you indicate. But yes, an "Easter Egg". I think I might have enough motivation here to build both attenuator and amp in time for actual Easter! Although like you, I tend to work very slowly and methodically. But a noble goal nonetheless. I can't wait to get back to you all with feedback, but please don't hold it against me if there is some delay


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## TonyK

Ah, I guess the first part of my previous post should have been addressed to @Gene Ballzz Still getting used to this forum as I've tended to be more active in the, well, other forums slightly more focused on the pre-Marshall brand


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## TonyK

So I've spent some time going over all of these pages again to see the evolution of Attenuator M. @JohnH you have done some incredible work here. Thank you!

I'm curious as to what level of attenuation you (as well as @Gene Ballzz @Mortfly @tmingle) find most useable for home use. I'm interested for the big 50W amps but also the few 5e3 Tweeds that I've seen talked about. I'm guessing that to get bedroom levels a full 28db of attenuation is needed but perhaps 14db might be okay for the smaller amps (though even they should theoretically still be pushing around 100db at that level if the amp is dimed). I'm working on the assumption that a 50-60W amp can deliver around 118db with a reasonably efficient speaker, and a 15W amp can still produce around 112db, so the former would give about 90db (still pretty darned loud, but very pleasurable to play!) and the latter around 84db

The reason for this question is that if I'm right, then in fact it might actually make sense to build a simple two-stage unit: the first stage comprising the reactive component modified to add in the existing 2nd stage (another 7db) for a total of 14db, and then the new 2nd stage could be the same as the existing 3rd stage for the second 14db of attenuation. I suspect that in a small gigging or rehearsal situation, one could live with just the 14db attenuation, and home, the choice of 14db or 28db depending on amp power. 

I'm all for simplicity, but how does this affect the resistor values? For example, in a 14db 1st stage, would one simply need to replace R2(10ohm) with a 20ohm to add in R4 and replace R1 with a 7.5ohm resistor (the equivalent of R1 + R3 in parallel)? 

And then can we simply take the existing 3rd stage (14db) as our new 2nd stage? It seems that R6 would have to be augmented by the additional 5.6ohm resistance of R8 to provide the proper resistance for outputs 1 and 2. Or might this totally change the dynamics of the resistance optimisation that JohnH has designed in to the circuit? In which case, leaving well enough alone with the multiple stages would be the way to go.

I'm sure I'm missing something as it's been a LONG time since I was clever at circuit analysis! 

Thanks for your thoughts 
Tony


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## TonyK

Oh, and if anyone has successfully sourced any of these components in Europe I'd appreciate some info! If not, which of the Chinese vendors have you had success with? Thanks!


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## JohnH

Hi @TonyK

The key part of the design is the first -7db reactive stage. You have to have that and although it could be recalculated for a different db, id suggest not to since it hits a sweet spot in the maths with input and output impedance and the balance of the two coils and two resistors.

After that, the following resistive stages get switched in and out so whether you actually build them or how many you have, or if they are hard-wired in, is a free choice. You could stack up a chain of -7db stages if with wished for example. The values of components don't need to change if you change the selection of stages in use.

Attenuator M goes down to -31db with all stages engaged. At home I'm using pretty much the full range with my two amps, which are about the same power but seem to respond differently with volume.

The DSL401 is a 40W 4xEL84 combo, and it has a particularly good PPIMV. When it was my only amp I never needed an attenuator with it. But since I have one, it is a bit better with about -7db or maybe up to -15db and within that range I can go as quiet as I like using the MV.

My VM2266c is 2xKT66 and nominally 50W (really just under 40W though) and its the amp I mainly used to design these boxes around. It also has a PPIMV, but at bedroom level it is not at its best. The best tones are from around 4 to 7 on the MV and I think that's how the amp was designed. At home I'm using the attenuators somewhere from -15db down to -31db for quiet late night noodling.

Attenuator M is the place where I've put all my design ideas so far, so it has everything in it that I've worked out. But I could certainly see that it would often be a good idea to strip it down to fewer elements. For example, for actual use, I can see that for many purposes there is not a need for a bypass switch, nor to achieve a setting of -3.5db. At home, I only use them just to see what they sound like. And to play out, Id probably use either -7db or no attenuator. 

Chinese resistors: This is the ebay link I use (Ebay.com.au but you can probably find it):

https://www.ebay.com.au/itm/25-50-1...hash=item239254cc9d:m:mDimLUo3wNHFIEXvjElODhA

They post quickly, and for free to Australia, though it takes about 3 weeks to arrive.


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## TonyK

Hi @JohnH, I was kind of afraid of that. Of course it makes sense too, and I thank you sincerely for taking the time to explain your logic here. Also on how you actually use the attenuator. In any event, I'm sold... I'll go ahead and build the first unit the way you have designed it, and I imagine that this will more than suffice for any needs I have in the foreseeable future. If I then choose to experiment with a simplified version, so be it, but for the time being, that's clearly not necessary.

Thanks also for the link. I have to work outside right now but will look into that later today, then get to ordering some components so that I can build the attenuator.

All the best and massive thanks!
Tony


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## Gene Ballzz

@TonyK and @JohnH
For my uses, I went with a 16 ohm version, as I don't have any amps that are 8 ohm only , but do have a couple that are 16 ohm only. The beauty of that is that the impedance seen by the amp only changes slightly when using either a 16 ohm or 8 ohm speaker load. I worked off of @JohnH *Design "K"* but modified slightly for my intentions.





Understand that I've not yet incorporated the main *"bypass"* switch, as I don't have a sturdy enough (for my tastes) one on hand. In this configuration, the first reactive, -7db stage is always active. This unit works fabulously for me on a wide variety of amps/wattages. The addition of the -3.5 db stage that can be switched in has been a big key to it's success for me. Here's a loose description of my hands on results with my most often used amps:
*A: Favorite, Bone Stock, 5E3 Clone. 13 watts and usually just a bit too loud for most situations!*
*1 -* Initial -7db + -3.5db = -10.5db gives a wonderful "cranking" band level volume without being deadly.
*2 - *First two -7db stages at -14db gives a nice "subdued" band-ish volume with maybe adding another -3.5db, for -17.5db, depending on circumstances.
*3 - *All three -7db stages, at -21db can give bedroom levels with this amp and adding the extra -3.5db, for -24.5db can get it "really" low!​*B: DSL20CR, Either With It's Internal 1x12, 1960 Greenback 4x12, or 1965 4x10.*
*1 - *Two -7db stages gets me right about to the same point as *"A/1"* above.
*2 - *Two -7db + -3.5db = -17.5db gets me to approximately* "A/2" *above, with the possible need for all three -7db stages, @ -21db.
*3 - *All three -7db stages + -3.5db = -24.5db gets me "just about" to bedroom levels in the amp's 20 watt mode, but very comfortably quiet in it's 10 watt mode.​*C: JTM30. Early '90s Combo, That's Officially Listed As 30 Watts, But I Can Guarantee That It Really Puts Out 40+ LOUD Watts! Reacts Similarly Through Internal Creamback, And Above Mentioned 4x12 And 4x10.*
*1 - *At least two -7db + -3.5db = -17.5db required to get to even partially sane band volumes, but sometimes, all three -7db stages needed for happiness @ about above mentioned *"A/1"*.
*2 - *I can "almost" get to "A/2" with all three -7db + -3.5db stages = -24.5db.
*3 - *I can't even touch bedroom levels until I activate all stages and start bringing down the Master Volume, which absolutely neuters this amp, although not totally terrible for personal practice.​With all the above comments, I must say that my overall tones and sounds, with all the listed amps (each with their own unique sound/character) have remained the closest to consistent across the different volume level settings as any other volume/SPL reduction method I've tried throughout my 50-ish years of *"Screechin' N Squawkin'" *with guitars through amps! It should be noted that all speakers used were 16 ohm, except for the 4x10, 1965 with G10L-35s at 8 ohm, but all amps were left at 16 ohm output setting. Obviously, we lose some air movement and wall shaking as the volume goes down, but with this general design developed by @JohnH the response of the amps really shines through as best as one could ever expect!

Thank You JohnH!
Gene


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## TonyK

@Gene Ballzz and @JohnH

Wow, what a glowing accolade! It's only going to make me get more impatient while components arrive Gene, is there any reason why you didn't build in a 14db third stage attenuation? Just curious. And when you say you haven't found a robust enough full bypass, are you talking about the capacity of the switch itself? I've found multiple 6A switches as I'm beginning to source my parts, but i was actually wondering whether it would be better to simply get all 15A switches.

Per A1 scenario above, your suggesting that usable band attenuation is 10.5 to 14db, so given that, for the sake of a switch (per John's advice about possibly making some stages non-switched if I desired), I can definitely see the value of keeping the stages as they are presented, each switchable.

John, thanks again for the Chinese link. I managed to find them on the UK ebay site but they don't sell via the French site (where I happen to be right now, anxiously awaiting the Brexit bell to toll). But that's not a problem as the UK offering also ships to France...

I have a quick question if you don't mind. In an earlier post you defined the minimal parts that it would take to build your Attenuator M, and you recommend 4x100W resistors and 7x25W resistors. Does this mean that for simplicity you are suggesting a 25W R9 (27ohm) instead of 10W? And (actually a second question) are you recommending the 100W resistors for R1 and R2, and R7 and R8 with 25W for all the others -- and as others have mentioned, is there any downside to using all 100W resistors? I would be building the "standard" 8ohm version so don't think I'd need the optional Output 3.

A final, more general question for which anyone is welcome to chime in... I really am thinking about building my attenuator in a tweed-covered enclosure, so that it will essentially look great with the 5e3 and also with the SurfyBear reverb unit that I hope to build (also in a tweed-covered box) afterwards (worth looking into if you like reverb and don't mind a superb alternative to a Fender tube-based unit or clone). So, if I were to build the attenuator onto a thick plate of aluminium (3mm perhaps) raised from the floor of a wooden enclosure with sufficient ventilation holes, do you think this structure would suffice given heat dissipation? I'd ideally like to have ventilation on the sides, bottom, and back rather than the top in order to maintain the best aesthetic, but I'm sure that if top ventilation is still recommended, I could find a way of putting some grill-covered slots in strategic locations.

Many thanks,
tony


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## TonyK

Sorry guys, one more question on the inductors. I'm having a hard time sourcing them in the UK or France, and the US ones looked great but came with extortionate shipping charges. What I am finding tend to be 0.8mm wire, which seems to be 20 gauge. 19 gauge has been explained as the ideal, but I wonder if this slightly thinner wire would make a difference. Also, one supplier can offer 0.33mH and 0.47mH: if I understand the tolerances that John has spoken about for L1 and L2, I would guess that this combination would be ok?

Thanks,
Tony


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## TrainableCat

I'd like to build one of these for a 100w amp. 

I assume the resistors need to be upsized to handle a 100w input? 

I read the first couple of pages and the last 7 and only saw references to 50w. 

Anyone built/used one on a 100w amp?


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## Gene Ballzz

And oh yeah, I forgot to mention! When used with the 13 watt 5E3 the unit stays cool to the touch at all levels, with the DSL20 it gets comfortably warm and with the JTM30, it gets fairly hot, though not alarmingly so. This is during long periods of cranking.

Also, my occasionally used even lower wattage amps can deal just fine with getting just a tiny bit *"shaved off the top"* with my Weber MiniMass, with little or no loss of tone/response.

And then there's my DSL40CR. I've not even bothered to try the attenuator with it, for several reasons:
_*1 - *_Given that most of the sound & character of this amp gets developed in the preamp section and there are multiple ways to tame the actual SPL/volume, it seems unnecessary.
_*2 - *_The overall sound of this amp, while it's a good, passable, middle of the road work horse, simply is not *"all that"* and will likely be going down the road soon, in favor of one of the three (as yet to be determined) Studio Series amps!​And to @TrainableCat (now there's a name that's an oxymoron ), I think all you would need to do is double the wattage ratings of all the resistors in any of the @JohnH designs, and you should be good to go.

@TonyK , I think JohnH had a reasonable source for those coils to NZ, but don't know about to the EU. And yeah, certainly no reason you couldn't use all 100 watt resistors, other than the footprint/size!

Just Sharon,
Gene


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## TrainableCat

> And to @TrainableCat (now there's a name that's an oxymoron ), I think all you would need to do is double the wattage ratings of all the resistors in any of the @JohnH designs, and you should be good to go.



Thanks, that's what I thought on the resistors but what about the inductor's size and rating?


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## JohnH

Hi guys
Thanks for all the comments and discussion. Ill jump in with more responses and info when I get some time tomorrow


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## JohnH

Starting on the questions

*Power ratings*

Fir my own M build I used just 100W and 25W resistiors. This was just because I could get everything I needed as case mounted types, and I found it easier to work with just two different sizes. I find the 50W ones more annoying to work with They aren't much smaller, but they have a lug on each side which makes them stack wider if next to each other.

But the Schematic on page 11 for Attenuator M shows 100, 50, 25 and 10W ratings. Those numbers are all still good and have a factor of safety of about 3 or so, for a 50W input.

If the switching is such that the -3.5db stage is never first in line (like in Genes build), then it could also drop down to 25W components.

*for 100W amps*

For a 100W amp, as Gene says, double all power ratings. For R1 and R2, you could use a pair of 100W (values chosen for series or parallel) for R1, and a single 100W for R2 (which only needed 50W before). All the 25W could become 50W. (Actually some could stay at 25, but I would need to dig out the maths to check)

Note: An attenuator for a cranked 100 W amp will need excellent cooling.


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## JohnH

*Cooling
*
I like the the thought of how a tweed attenuator might look, but I don't love the engineering of it! 

99% of an attenuators job is to turn electricity into heat. With a metal case It does that in three ways, being convection by air flow from the tnside, convection from the outside of the case, and black-body radiation from the outside. With only the first mechanism available, much more venting and space inside would be needed, if its to run with reasonably powerful amps.

The best flow is in through the base and out through the top, like a chimney effect.

How about a big finned heat sink on the rear panel and mount the large resistors to it?
*
*


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## JohnH

*Inductors in the UK*

These seem good, air-cored on bobbins with 1mm wire:

https://www.qtasystems.co.uk/loudspeaker-components/inductors.htm

Clunky site though! Seems to expect people to send them an email to order.


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## JohnH

*Switches
*
Most switches are rated with an Amps values at dc, 250V ac and 125V ac, which gets the largest current rating value. For the attenuators, only the bypass switches carry max current, which is 2.5A for 50W into 8 ohms, but only at around 20V, so the 125Vac ratings are conservative. 

Also, they are intended for actually switching that current under load. For an attenuator, we shouldn't switch to bypass with signal at high power, for the sake of the transformer. So if volume is muted on switching, wear on the switch is much less.

So I feel ok with 6A rating on switches, for 8Ohm and 50W, and also 16Ohm at 100W which uses the same current. 100W and 8 ohm could do with more current rating.


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## TonyK

@JohnH Hi John, as always you are incredibly helpful. 

Although you didn't actually say so, the fact that you've suggested a 1mm alternative implies that you're a little concerned with 0.8mm wire. I think you must be referring to the reinforced glass air-cored bobbins advertised on qtasystems, and they definitely look higher quality (but slightly lower tolerance) than what I had found yesterday in a fairly lengthy Google search for suppliers. I'm going to send them an email and see what they come back with as regards shipping. The earlier ones I had found are here: https://www.ebay.co.uk/itm/Air-Core-Inductors-5mH-8mm-wire-matched-pair/132908418604 Steve there offered me one each of the 0.5mH and 0.33mH for less than £10 including shipping! Bizarrely, I also found a pair of 1970s 0.5mH Celestial inductors for the same price! Not sure being vintage adds anything though  

I'll have to think a little more about the tweed enclosure I'm considering, although your idea of a heat sink might work. I'll give it some more though and if I come up with some ideas I'll share them of course. And I really appreciate your advice on the resistors, especially the point about case mounting versus tabs, as I wouldn't have considered that until they were in my hands ready for installation.

Time to get ordering!
Cheers,
Tony


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## TonyK

@Gene Ballzz Thanks again so much for your insights on the use you putting your attenuator(s) to. My guess is that I will end up using the 5e3 most of the time unless I find I don't bond with it (I know, heresy). But that's really good to know that there is so little heat diffusion at that level. Obviously I expect more if I were to use it with my HotRod, so I'll be very aware of that when I get there!


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## Gene Ballzz

@TonyK 
FWIW, when all is said and done, I will have at least three of these units, in addition to my Weber MiniMass. The JohnH designed units will be semi-permanently installed into the amps, instead of as separate boxes. I actually drilled and tapped the screw holes into my box and cut the screws to length, so there are no screw heads on the outside of the box, except for the two larger screws for mounting the coils. A lot of work, but for me, I thought the reward was worth the effort. Enjoy the build. I'm one of those guys who gets off on *"playing with"* all the guitars, hardware, bells & baubles surrounding guitars almost as much as actually making music on my many guitars! 

Just Sayin'
Gene


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## TonyK

Gene Ballzz said:


> @TonyK
> FWIW, when all is said and done, I will have at least three of these units, in addition to my Weber MiniMass. The JohnH designed units will be semi-permanently installed into the amps, instead of as separate boxes. I actually drilled and tapped the screw holes into my box and cut the screws to length, so there are no screw heads on the outside of the box, except for the two larger screws for mounting the coils. A lot of work, but for me, I thought the reward was worth the effort. Enjoy the build. I'm one of those guys who gets off on *"playing with"* all the guitars, hardware, bells & baubles surrounding guitars almost as much as actually making music on my many guitars!
> 
> Just Sayin'
> Gene


I'm with you totally, Gene. I have thought about doing the same, but for the moment like you I'll go with a stand-alone unit and then worry in the future depending on how my amp collection morphs -- ie, more builds, a few sales, or more accumulation! I am looking forward to doing this though, and with a bit of luck in 4-8 weeks I should have both Attenuator M and the Deluxe built to enjoy  @JohnH has been a real superstar in this thread and his kindness will definitely assure him wonderful musical karma


----------



## Nik Henville

JohnH said:


> Here is the 'proof of the pudding'
> 
> *Sound Samples*:
> 
> *Attenuator M: Max attenuation to non-attenuated:*
> https://vocaroo.com/i/s0QOpqTD3WAo
> 
> *Attenuator M: Normalised:*
> https://vocaroo.com/i/s1GJvYK04i00
> 
> Its a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in nominally 3.5db steps. Then, the second file, which is based on the same recording, has each stage normalised for volume so you can hear any differences in the tone.
> 
> The VM2266c was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off the speaker with a Rode M1 into a neutrally set mixer.
> 
> What can you hear? All the attenuator settings are through the reactive circuitry, except the second to last which is -3.5db resistive, and the last, which is direct to speaker.



Damned finest proof of the pudding I ever did here.
And consistent tone from a whisper to a roar. Excellent...


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## TrainableCat

@JohnH

So for R1 I'll use (2 100w 60ohm in parallel) or (2 100w 15 ohm in series) and same for R7 with appropriate values. 

And just to clarify: I don't need to upsize the size/value inductors L1/L2?

Thanks to everyone, this is a great project guys!



JohnH said:


> Starting on the questions
> 
> *for 100W amps*
> 
> For a 100W amp, as Gene says, double all power ratings. For R1 and R2, you could use a pair of 100W (values chosen for series or parallel) for R1, and a single 100W for R2 (which only needed 50W before). All the 25W could become 50W. (Actually some could stay at 25, but I would need to dig out the maths to check)
> 
> Note: An attenuator for a cranked 100 W amp will need excellent cooling.


----------



## JohnH

TrainableCat said:


> @JohnH
> 
> So for R1 I'll use (2 100w 60ohm in parallel) or (2 100w 15 ohm in series) and same for R7 with appropriate values.
> 
> And just to clarify: I don't need to upsize the size/value inductors L1/L2?
> 
> Thanks to everyone, this is a great project guys!



From those values, I'm assuming you're thinking of the 16Ohm version for 100W? Your resistor recipes seems fine. 

For the inductors the current through them would be the the same as for the 50W 8 ohm version. The Inductance values aren't affected by power, only by the nominal ohms. 18 gauge windings for inductors is ideal (or 19 gauge). 18 Gauge is 1mm diameter, and its what I've used. Its a nice thickness of wire, and it keeps the coil resistance low to reduce heating of the coil.


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## TrainableCat

@JohnH 

Yes, that was for the 16Ohm version. Thanks for the input, I'm off to order the parts!


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## TrainableCat

I'm building a version of Design M for a 100w amp input and a single 16ohm out. R1 and R7 are doubled up to avoid using a single 200w resistor. R1 is serial because I could not get a 60ohm from the supplier I used.

Is L1 and L2 directly connected where the line going from L1 to R2 crosses the line from R9 to L2?

Can you guys look over this drawing and make sure I've got it right?


----------



## JohnH

All fine. Yes L1, L2, R2 and R9 are all connected at the point where the lines cross.

For -3.5db stage, your diagram never puts this stage first in line (so it doesn't operate on its own). Theres never more than 20% of the power reaching it. So R7 and R8 could drop down to 50W and 25W respectively, if wanted.

Personally, I'd add another parallel output jack, its sometimes useful for two cabs. Also, you could use it as a line out to an IR or cab-sim.


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## TrainableCat

@JohnH Thanks for the review and suggestions. I changed R7/R8 and ordered the smaller resistors. 

I just don't think I would use the bypass, I'll just take out of circuit when I need full throttle. I also don't expect to have an amp that's not got a 16ohm jack and I can always build rev2 at that point.

Now I just need to wait patiently for China Post and get my enclosure worked out.


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## JohnH

@TrainableCat , I think you will have a nice simple and reliable design. I built everything into mine, because its a test-bed as well as an attenuator for using. But I also don't need the bypass nor -3.5db settings (though I do like having the -3.5 stage to run with the other stages) . Not having them means there is a lot of current that doesn't need to be switched. Also, with your diagram, each switch is just a single pole, and if you use dpdt switches, you can wire both poles in parallel for improved capacity and reliability. Plus, once its plugged in, the amp is always seeing a reasonable load even at the moment of switching, so you can switch stages in or out on the fly without worrying about hitting bypass by mistake.


----------



## TonyK

@TrainableCat Good work there! I've been sourcing parts and am just about to draw out my circuit but yours seems like it would be a good template except for the fact that mine would be an 8ohm to 8ohm basis.

I have a silly question about switches, which have long confused me in circuit diagrams. Your switches are simple ON-ON right? At one point in this thread I recall that @JohnH mentioned that he preferred ON-OFF-ON but I can't remember what his rationale was and I've been unable to re-find the specific post.

Also, John, on the same subject I'm having difficulty understanding the signal flow in your Attenuator M circuit through the switches. It looks to me that when the switches (in M) are engaged appropriately, the current is divided so that it passes through the series resistor (eg R4) and through the associated resistor (R3) to ground, thus engaging the attenuation stage. Whereas when the switch is toggled to the other pole, we are effectively bypassing (path of least resistance) R4 (in this stage) and disengaging the attenuation stage? Am I correct? I feel I have to understand what's going on to be able to successfully build the attenuator -- but of course, I'm also trying to learn or relearn electronics as I go forward with such projects!

This leads to deeper confusion regarding your ultimate bypass switch, Sw1A. As shown in your circuit (connected to the "right" pole), the signal passes straight into the inductor stage engaging the first stage of attenuation. But if the switch is toggled to the "left" pole, the solid signal line goes to Sw4B which is not important to me as I won't be using an Output, but the dotted line signal goes to Sw1B and this is where I'm struggling a bit to understand. As it's drawn, it seems to me that the signal effectively dead-ends as it seems to no longer pass through to the speaker output, so I'm sure I'm wrong here! On the other hand, if Sw1B is toggled to the right pole, then it seems as thought the signal would now pass through R8 and R7 providing only the 3.5db stage with no inductor stage. And if Sw4A is toggled to the right pole, then this stage is ignored providing the full circuit bypass. Could you please tell me if I'm right, nearly right, or just plain off-piste?

Thanks guys!!!

So for the record, I've completed most of the parts order and will complete it by ordering the appropriate switches as soon as I'm sure I understand the needs. In case it is of any use, either to European builders, or to others if the shipping is reasonable enough to provide a cost-effective solution, I was surprised that the eBay builder I referenced earlier, Stephen, came back to me with an offer to custom build two inductor coils to 1% tolerances using 1mm copper wire. His price was more than fair, and including shipping to Europe from the UK, I'm paying less than £15 for both. He is extremely friendly and easy to work with and I'd definitely recommend him if you're having difficulty finding the exact inductors you need. Here's his shop address: https://www.ebay.co.uk/usr/stevie2810?ul_noapp=true And to top things off, he's a bassist, so he can't be all that bad right? 

Cheers,
tony


----------



## Nik Henville

Just read right back through this thread...
@JohnH takes us on a journey and keeps it clear and simple all the way along.
Finally we get the "M" type attenuator... which looks/sounds to be better than many out there.
*Got to say thank you for all that work and knowledge and the willingness to put it out there.* Now I am just crossing my fingers that it becomes available commercially and I can buy a 100W version to run my 4145 through. Bloomin' marvelous work though @JohnH - makes a refreshing change from the usual inarticulate, uninformed technical gibberish found in so very many fora, and arcane black-magic spouted by so many commercial builders. Nice one...


----------



## JohnH

Just linking M again so its nsarer:













Attenuator M 190110



__ JohnH
__ Jan 10, 2019
__ 1






Hi @TonyK
The dashed lines on the diagram aren't signal paths. They show which switches are linked and operate together. Typically the bypass and -3.5db switches have two poles, which are physically on one switch, but shown seperately on the schematic so signal path is clearer. When sw1 is operated, the signal
bypasses the first 3 stages and feeds directly to the final -3.5db stage, or if that is off, straight to the speaker.

@Nik Henville , thanks for your comments.


----------



## TonyK

Thanks @JohnH I think that you have just explained in three lines what I was trying to express as my understanding!!! But particular thanks for the clarification of the dotted line. I should have known that! So just in case you see this shortly before nighttime Downunder, is there any particular benefit between a simple ON-ON switch and a ON-OFF-ON switch? 

Cheers,
Tony


----------



## JohnH

Tony - We don't want on-off-on switches in this design. These 3-position switches become totally disconnected in the centre off position. Useful in some cases, but not here, where we need to keep a load on the amp at all times.


----------



## TrainableCat

@TonyK Thanks! I ordered single pole On-On switches before @JohnH commented about using double pole. The switches I ordered are 15amp so unless one goes fails I should be OK. Next build I'll use doubles to add some redundancy.


----------



## JohnH

This is a table to help understand how each resistor dissipates power as heat and therefore:what power ratings are needed. All values are based on 50W amp power, so should be multiplied appropriately for other values. 













Attenuator M Power 190331



__ JohnH
__ Mar 31, 2019
__ 2






It shows, as best as I can calculate, the actual heat dissipation in each resistor, as a maximum value across all the different settings and speaker impedances.

Most of the power values are just based on resistive combinations and Ohm's law, since the power is dominated by mid frequencies where there are little reactive effects happening. The exception is R9, which only shunts high frequencies across inductor L1. The calc for this one is less certain since it depends on the balance of frequencies coming out of the amp. I put it as carrying 3W which I think is conservative. In an earlier build I used a 5W air-cooled ceramic resistor for it, and it definately heated up. So I think 10W case mounted is a good spec. In my current build I used 25W and its staying cool so far.

In general, to use the listed power values to pick component ratings, they need to be multiplied by a factor to allow for possibly overdriving the amp and for less than perfect cooling. For case-mounted resistors, I've been using a factor of x3 on these raw numbers. eg, R1 can dissipate 28W max from an amp running at 50W. 28Wx3 is 84W, hence the spec rating is 100W.

Note, all of the numbers are compatible with the various builds on this thread so far, so no worries with respect to anything to date.


----------



## TonyK

Hi @JohnH. Thanks for all of this extra work you're putting in. You're doing a fantastic service for all of us now and in the future who might read this thread. I did actually order ON-ON switches as I was concerned about the effect of that OFF position when switching, but thanks also to @TrainableCat for your confirmation... But for now, it's playing the Chinese waiting game  Oh, for this build I simply went ahead and ordered an aluminium case, 200x152x44 I think it was. I figured I can always sort something out later if I really feel the need for tweed!
Cheers,
Tony


----------



## TrainableCat

I received the resistors I ordered from China(ebay seller sun_gua) today in a blazing 18 days. 

I'm going to start assembly soon, is there any reason to use shielded wire for the interconnections?


----------



## JohnH

no need, just good 18gage insulated stranded is fine. Some short runs are also handy in bare solid wire instead, if they won't flop around.


----------



## TonyK

Ah, my components arrived over the past four days, too. Way faster than was indicated when I ordered! You've just answered a question that suddenly came to my head as well -- I have a lot of 22 gauge wire around as well as some far heavier duty (14 and 16 I think). So I'll need to get hold of some 18 gauge wire before going ahead. This will give me the time to draw out the layout and wiring diagram that I should already have done.

@JohnH I realise that the other thing I didn't think to order was heatsink compound. Do you think I should hold off construction until I've acquired some of this as well?

Cheers,
tony


----------



## JohnH

I think heatsink compound is important. Thicker wire is fine if you can work with it. A good source can be to strip out some old three-core mains flex. Another thing I ran out of several times was M3 bolts for fixing resistors

While you wait, the first job after planning a layout is to drill out the case for jacks, switches, resistors and plenty of vents, and then any painting.


----------



## TonyK

Thanks @JohnH Yes I'm ready for the drilling once I've done my layout of course, though the case is black so won't need painting at least! I have loads of household single-core cable which might work but is pretty thick! I probably have some three-core wire too so I'll check that out. Regarding the heatsink compound, my problem is simply that I'd rather find it locally but I can't even figure out what it would be called in French! And no one I know has any idea either!!! Also, we only have electrical wholesalers around, none that sell electronics components... Oh, and I'd better check my stock of M3 bolts...

Silly question for you or anyone... As I've mentioned before, I think, I'm about to build a 5e3 amp. I had bought 22.5 AWG wire (0.6mm, so a little thinner than the usual 22 gauge), pvc covered. But I've been reading in forums that people prefer cloth covered wiring as it is allegedly easier to solder (no plastic to melt for example), and given my need for 18 gauge wire for the attenuator, if this assessment is actually true, then I thought about getting a supply of cloth covered wire to do both jobs! Any thoughts are welcome!


----------



## TrainableCat

Thanks @JohnH, I plan to use thermal paste to mount the resistors to my way over-the-top heatsink. It was used and cheap on ebay. I'll build a custom case around it. I'm thinking about some type of vertical chimney design to get passive air flow out the top. Since I'm dissipating 100w of power I thought more would be better.


----------



## JohnH

@TonyK - maybe could order off ebay in France from a uk supplier, like this:

https://www.ebay.fr/itm/Arctic-Cooling-MX-4-Thermal-Paste-/111651079066

You have until (at least!) October 31 before it gets stopped at the English channel and impounded!

Also, this type of stuff must be available in most places since its necessary for assembling computers with their heatsinks and cooling. PC assembly is a local business these days and any shop that puts its own systems together should have some or know where to get it.


@TrainableCat - awesome heatsink! Self tapping screws might be a good way to fix to the back of that, so you don't need to feed bolt heads or nuts between those fins.


----------



## TonyK

Hi John, I did finally figure out what it's called in French after looking at a number of blogs here online, and only then did I see the link to the stuff they use in PC construction! So yes, it's easy to find here, but thanks so much for providing that link! And the Brexit humour - god knows we have to remain amused by those imbecilic shenanigans. Six more months of political insanity to end up a generation of further insanity whatever happens! 

So now, onto the layout...


----------



## TonyK

Hi @JohnH and others. First, here is my slightly revised schematic for an 8ohm attenuator but with stage 4 always after stage 1 and no third output. I'm using standard mono jacks. I'd like to use this for both my 8ohm speaker output for my HotRod Deville, so 8ohm into 8ohm, and I'm sure that this is the standard application. But I'd also like to use it with my BassBreaker 15, which has a multitap speaker output but currently runs into a 16 ohm speaker. First off, is this schematic correct? And second, would I need to set the BB15 output to 8ohm despite the 16ohm speaker or have I misunderstood the necessity for the 16ohm speaker-out sub-circuit? Finally, I think I can use both outputs 1 & 2 to drive either two 8ohm speakers or two 16ohm speakers????

Cheers,
tony


----------



## JohnH

Hi @TonyK 
Did you have a drawing? can't see it so far.


----------



## TonyK

oops...


----------



## JohnH

Hi Tony:

There are a few issues with your schematic:

L1 and L2 have their values reversed. L1 should be 0.33mH and L2 0.5mH
If you want the full bypass switch Sw1, then you need a second half of it with pole connecting to the speaker, ie it is a two-pole switch.(eg, see Design K on page 8, just for the bypass switch wiring) If you don't need it though, just omit Sw1 and if you want full volume, don't use the attenuator.

Other things to note:
Stage 3 is a -14db stage
In my 'M' design, the -3.5db stage has to go last so it can act on its own. You don't need that feature for yours, but the stage is still ok there at the end of the circuit. Id suggest when you lay out the switches, put them in some logical order that works for you, such as -3.5, -7, -14 etc

In use, I see the Hotrod Deville is a 60W amp. It will be fine through this set up, even though the values are nominally for a 50W amp, there is plenty of margin, no problem.

Your version does not have the correcting features for 16 Ohm output. But you can still use it safely, it was more of a tone-tweak. With a 16 Ohm speaker, you may hear a bit more mids relative to bass and treble.

So for your version, you should always use an 8 Ohm amp tap (unless fully bypassed in which case match to the speaker) , and then you can run it with any of the following speaker combos:
1x8, 2x16, 2x8, 1x4, 1x16 (but note as above)

Using it with 4 ohms out (ie 1x4 or 2x8) is equivalent to using the 16Ohm attenuator version with 8 Ohm out that Gene has done, and the maths work out well.

Final point, once you have built it, before you plug any amp into it, after triple checking it, run a set of resistance tests: plug an 8 ohm speaker to the output and measure resistance as seen by the amp, to check it is always within a range of between around 7 and 10 Ohms at all combinations of switch settings (could be a bit less with full bypass)

Good luck with the build!


----------



## TonyK

Hi @JohnH, Happy Easter to any who observe.

Thanks so much for looking over my schematic and finding the potential problems. I have suffered with MS for just over ten years and one of the more nuisance symptoms is what I call cognitive dyslexia. Sometimes I'll say a word the exact opposite of what I intend. In this case my brain told me that L1 and L2 were in the correct locations and properly labelled even after multiple readings. And then I read your analysis and of course my brain saw the issue right away  Anyway thanks very much for that.

Have you actually been able to discern a difference in the 16 ohm tone tweak circuit? I remember your earlier comments about running the 8 ohm attenuator into a 16 ohm speaker, so I'd be interested to learn what you think. And the rest of the speaker comments you're making make total sense to me!

I'll look back at Design K to see what you're saying regarding the full bypass. In truth I was debating whether or not it was actually necessary or not.

For the different stages, are you suggesting that I actually move them around on my schematic and then appropriately on the build to follow the more logical sense of attenuation, or are you saying build as is, but simply mount the switches in the more logical order?

Finally, I'l have to look into how to run the resistance test. It might have already been discussed earlier in the thread, but I do agree that it is an excellent approach to final testing.

Thanks so much,
Cheers,
tony


----------



## JohnH

Hi @TonyK

I just had a listen again on my DSL401, which has a 16 Ohm internal speaker (usually used together with a 16Ohm cab to make an 8 Ohm load). The tone difference using the 'fixed' 16Ohm output jack on the attenuator, as compared to one of the 8 ohm jacks is very subtle. I can clearly hear it, but its not like one tone is good and the other is a wet blanket. Both are fine, maybe leading to a tweak of the amp EQ if one was being picky.

The circuit is considerably more straightforward without the 16Ohm output fix. What you might do is just build it as you intended, without the extra 16Ohm output, but when planning out the layout, consider where you could add one more jack if needed later. 

On the order of stages, I was just thinking about the physical layout of switches. Electrically, provided the first stage with the inductors and R1/R2 is first, I don't think it makes much of a difference which order the other three stages are wired when there is no -3.5db overall setting needed.


----------



## JohnH

Further to the above, this is a very quick test to compare a 16 Ohm speaker plugged into the 16 Ohm output of Attenuator M, to the same plugged into one of the 8 Ohm outputs. Here is the setup:

Guitar: Strat Am Special, HSS with Bridge humbucker
Amp: DSL401 on OD1 channel 
Speaker: 16 Ohm Vintage 30
Mic: Rode M1, Xenix mixer into Audacity
Attenuator M, 8 Ohm version, plugged into 8 Ohm amp tap, set to -14db attenuation

First, speaker plugged into 16 Ohm output (which has the tone tweak circuitry)
Second, speaker plugged into 8 ohm output

The second clip was actually about 2.5db louder than the first, which is as expected. But to hear the tones more clearly, I normalized the volumes.

https://vocaroo.com/i/s1hvVfzISxKT

You can hear how when using the 8 ohm output (2nd clip), the treble is reduced a bit.


----------



## TonyK

Hi @JohnH Hope you're having a good day down under.

You've again gone beyond the call of duty and I thank you mightily! First, the sound of your two audio clips is wonderful given that you're using a full 14db of attenuation - almost every clip I've heard before from big brand attenuators has started to sound pretty poor at even -12db. And you're right, I can just about perceive the very slight treble reduction on the 8ohm speaker, but it is nevertheless a very good sound, so I'm going with the notion of leaving out the 16ohm tone tweak circuit for now.

So, I've attached (hopefully I'll remember this time) my schematic revised to reflect the proper L1/L2 values, and also to reflect the two pole switching necessary for the full bypass to function. I think I've "wired" correctly to account for this Sw1b. So if this is good, then I'm ready to build as I found some thermal paste in a local PC store.

So a question just for fun really. I imagine it would be relatively easy to connect an LED to each of the switches such that it is immediately clear which attenuation circuits are in or out of play. However, I've looked a little at such LEDs and I really can't work out how I'd choose the appropriate LEDs based on voltage and current requirements, but I assume that if that could be calculated, then the LED would be wired from the In-Use pole of each switch to ground in order to light it up. Does anyone have any ideas on this? And yes, I can accept that it might just be better to leave well alone


----------



## JohnH

Looks good now. That's a nice clear diagram and it will likely be helpful to others since it's nice and simple.

LED's, seems like too hard! powering from the signal, if would be hard to run them consistently, and they could possibly affect the sound. Otherwise, would need to supply power and use another pole on each switch.


----------



## TonyK

Thanks John. I'm glad that the simple diagram, which reflects my way of thinking these days, might of help, but especially thanks for checking it out for me. I can start building as soon as the missus allows us time off from the garden work!

I was afraid that the LEDs might need a power supply... oh well, nice idea in principle!

Take care, and I'll post back shortly with an update.
Best 
Tony


----------



## JohnH

That last test at -14db worked well for a quiet recording on the DSL401. I find this amp, as tweaked by me, gives its best tones at fairly moderate volume, needing less attenuation. But the VM, which on paper has about the same power, is more designed for pushing the power section, so I more commonly turn it higher, with more attenuation, maybe 21 to 28db. 

Without attenuation, the DSL actually does a better job at very low volume than the VM, while the VM needs to be turned up. Both have MV circuits but they are totally different amp designs.

But in any case, once the attenuator is at -7db or more, the speaker and amp are almost fully seperated. The amp sees a fairly consistent reactive load and the speaker sees a consistent output resistance. So the tone stays the same no matter how much added attenuation is included. Any number of stages could be engaged.

All of that is true from an engineering point of view. Human perception of tone at different volumes, due to Fletcher-Munson etc is a different matter, not addressed here!


----------



## Jon C

I second @Nik Henville’s comment too John …… Your 401 insight and advise has made all of the difference to my experience with Marshall…… You were the Guy who helped when I joined and you’re the reason I came back here……


----------



## Nik Henville

Jon C said:


> I second @Nik Henville’s comment too John …… Your 401 insight and advise has made all of the difference to my experience with Marshall…… You were the Guy who helped when I joined and you’re the reason I came back here……



I agree @Jon C - just wish they were commercially available, 'cause I _*want*_ one...


----------



## JohnH

hey @Nik Henville
Thanks but how about giving it a go yourself? The parts are very simple and robust, almost impossible to damage while soldering. The two main tasks are drilling and soldering.

The circuit itself is simple, literally 19th century technology. With a bit of planning, you can start with just one stage, five components plus the jacks, then build up. And before committing it to an amp, it can be tested with a meter.


----------



## BigDaddy23

Gday folks - this is my first post on this site. I've been looking intently through this thread as I'm really keen to build an attenuator like this for a 40w Hot Rod Deluxe but have a couple of questions:

1 - Is it possible to just build this with Stage 1 & 4? (Ie omit 2 and 3)
2 - Are the inductors absolutely necessary? If yes, what current rating is recommended? 
3 - Are Cliff type jacks necessary or can standard switchcraft be used?

I must add that this is a fantastic thread - thankyou to all for sharing your thoughts and experiences! Esp @JohnH

I'm looking fwd to getting some parts ordered asap. Where did you get the inductors and the case John? Jaycar ??


----------



## JohnH

Hi BigDaddy23

Welcome to the MF.

Yes you can just do those stages, which will total to -10.5 db, cutting power by a factor of just over 10. Id think that an HRD could still get fairly loud though, depending how you use it

The design works fine without the inductors (and without the resistor that bypasses L1). R Values are the same and the tone at clean volumes is not much affected. But the inductors add a bit of realistic dynamic sizzle as you push harder. Its not night and day, its just a bit more lively. Gene tried it both ways and preferred it with . Im using 18 gage inductor windings.

On jacks, any decent type should be ok Im using generic ones from Jaycar. I like the stereo ones better, even if just for mono since the extra spring contact grips the plug better.

I go to Jaycar for everything they sell, since there is one near where I work in Sydney. That includes solder, 18 gage wire, cases, M3 nuts&bolts, feet, jacks and switches. I get resistors from China and inductors from Queensland Speaker Repair. Actually, they didn't sell the exact values I specced, so I got 0.4 and 0.6mH, and unwound them a bit. But you could just use those higher values neat. It might be nearer to representing the US speakers in your Fender.

Good luck!


----------



## JohnH

Actually, im not seeing the bobbin-wound inductors that I got from QSR on their website. Might need more searching.


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## BigDaddy23

Thanks John - I’ll build it to stock specifications, I reckon.

Re the inductors , are the ones available from 'The Loud Speaker Kit' in NSW a suitable source locally? I cannot seem to insert a hyperlink in this post for some reason!


----------



## JohnH

I reckon they look fine. There's products LW18-33 and LW18-50 that look to be the buisness for L1 and L2. (Assuming 8 ohm build)

Have to figure out how to fix them down. Maybe with plastic zip ties? (not anything with steel bolts, which will throw the inductance values off)


----------



## BigDaddy23

Thanks! I have some plastic M4 screws that might work for that job.


----------



## JohnH

One more thing on jacks. They need to be plastic, like on a Marshall amp. Orherwise the case of the attenuator is live to the output.


----------



## Nik Henville

JohnH said:


> hey @Nik Henville
> Thanks but how about giving it a go yourself?...



Used to do this stuff for a living - my problem these days is finding the time.
67 years old and running fast to stand still - mainly financially...
...but I like to leave time to fire up the 4145 and let rip.
So, itt looks like I am going to have to give it a go.
Task #1 - scale up for 100Watts RMS (plus)...
Hey ho. Thought I'd left all this behind me.


----------



## tmingle

BigDaddy23 said:


> Gday folks - this is my first post on this site. I've been looking intently through this thread as I'm really keen to build an attenuator like this for a 40w Hot Rod Deluxe but have a couple of questions:
> 
> 1 - Is it possible to just build this with Stage 1 & 4? (Ie omit 2 and 3)
> 2 - Are the inductors absolutely necessary? If yes, what current rating is recommended?
> I made mine switchable. As far as current rating goes, I oversized everything(all 100W resistors, 15ga inductors & 15A switches.
> 3 - Are Cliff type jacks necessary or can standard switchcraft be used? I used switchcraft jacks.
> 
> I must add that this is a fantastic thread - thankyou to all for sharing your thoughts and experiences! Esp @JohnH
> 
> I'm looking fwd to getting some parts ordered asap. Where did you get the inductors and the case John? Jaycar ??


I'm in the USA & used these https://www.parts-express.com/cat/air-core-inductor-crossover-coils/297


----------



## TonyK

JohnH said:


> One more thing on jacks. They need to be plastic, like on a Marshall amp. Orherwise the case of the attenuator is live to the output.


Hi John, I didn't realise this so I'm glad you've brought it up. I knew that you personally preferred the plastic stereo hack sockets, but... I have a bunch of these around and was going to use them for my build. I thought that the beige insulating rings would isolate the socket from the chassis. Are you saying that this type of product won't work?


----------



## TonyK

@BigDaddy23 Hey BigDaddy23, welcome over here. I've seen a few of your posts in other forums as you know. This attenuator thread started by @JohnH is really superb. 

I've posted earlier (perhaps around page 14 or so) the details of the guy who build my inductors in the UK. He was very quick, did great work, and charges minimal postage to shop internationally. If this is of interest and you can't find the details just let me know! 

Like you, I also considered a scaled down version of the attenuator, but its simplicity and inexpensive parts persuaded me to do essentially the stock circuit. Good luck!


----------



## JohnH

I don't think those beige parts will separate it. The jack plug will enter the socket, and the outer plug barrel will be contacting the outer threaded part of the socket, which is what will be nutted off against the metal case.

In most cases, it should actually still work since OT secondary windings are floating, but it isn't right and I cant predict what sort of issues could arise if stray wires touched the case and an amp chassis, in an unusual amp.

You could use these jacks if they were mounted on an insulated panel - how about a 'Plexi' design?


----------



## BigDaddy23

TonyK said:


> @BigDaddy23 Hey BigDaddy23, welcome over here. I've seen a few of your posts in other forums as you know. This attenuator thread started by @JohnH is really superb.
> 
> I've posted earlier (perhaps around page 14 or so) the details of the guy who build my inductors in the UK. He was very quick, did great work, and charges minimal postage to shop internationally. If this is of interest and you can't find the details just let me know!
> 
> Like you, I also considered a scaled down version of the attenuator, but its simplicity and inexpensive parts persuaded me to do essentially the stock circuit. Good luck!



Cheers mate - Ive ordered the inductors from an Australian supplier and have the other resistors on the way. I'll build it to the stock schematic which looks like it will be as cramped as a 5e3 chassis 

Like John said, those jacks will ground to the chassis as opposed to the cliff jacks that are isolated. Out of interest, I used normal jacks on the L-Pad design attenuator (like the one u linked on the other forum)........I wonder if that is why I had a massive loss of volume? I should try Cliffs in that box too.


----------



## TonyK

@JohnH Thanks for that confirmation. You beat me to the deduction -- I just took one apart and can see the problem. So no biggie, I'll get the right ones (I might even have a few hanging around on obsolete equipment). The plexi idea was pretty good too though, and probably more practical than my Tweed idea 

@BigDaddy23 You're most welcome! And thanks for confirming what John said. And you might even be right on your L-Pad build issue too. Glad you're getting sorted.


----------



## BigDaddy23

Tried the cliff jacks - no dice with the L-Pad option. Literally passing no signal at all. Parts for the inductive attenuator aren't far away!


----------



## JohnH

Sorry to hear about the Lpad. Its not my fav way to build these things, but it should at least 'work'. Would it be a good idea to troubleshoot it, just for the sake of it?


----------



## BigDaddy23

It's a bizarre situation. It 'worked' with the normal mono jacks grounding the chassis - there was a massive drop in volume with it in series with amp/speaker despite being dialled to 'off.' As I turned the l-pad knob, volume predictably dropped until no sound was heard at all. I would have thought at the 'off' position, there would be a small loss of signal, but not this much! It makes me think that there is possibly a fault in the l-pad itself. It measures at 7.8 ohms across both tabs so that is correct.......it's a 100w model which is what is recommended for a 40w amp. 

Using an a volume box in the FX loop is a usable option (this is the current solution whilst I look at theses attenuators) but it essentially operates as a MV.....all preamp distortion. I'd like to get some more power tube OD in the tone. Hence building your great design!


----------



## TonyK

Well, I'm back here after a surprise visit from my daughter and her three young children during which time, absolutely nothing got accomplished  Well, except that her 6-yr-old son now wants electric guitar lessons!

So just to be sure, please, is this the correct style of jack sockets?

https://bit.ly/2ZMueLd


----------



## JohnH

hi @TonyK , yes that looks fine!


----------



## TonyK

Great. Ordered!


----------



## TonyK

I had to order the new jack sockets as the ones I had available were PCB mounted to an old solid state amp that still works, and I really didn't feel like sabotaging that amp!

I'm trying to lay out the enclosure with the components right now, while waiting for the new jack sockets to arrive. I'll try to post a drawing as soon as I can, but in the meantime, and please forgive what might be a dumb question, but given that the plastic sockets do not appear to ground to the chassis, does the general circuit have to ground somewhere else? Or is the ground bus simply a false "ground" that completes the circuit by connecting to all of the relevant components?

I might have other questions so I hope you can be patient... please 

Cheers,
Tony


----------



## JohnH

Hi Tony, actually the chassis doesn't get connected at all, and although the circuit has a common sort-of ground where the barrel connections of the jacks are joined, there's no real ground since both wires coming in come from output taps on the amp OT. 

Happy to answer questions.

J


----------



## TonyK

Thanks so much, @JohnH. That's what I thought after your discussion of the jack sockets themselves, but I prefer to get it right the first time 

As I've mentioned above (I think), I have a strange MS disorder that makes what used to be very simple things for me to do, now much more challenging to accomplish. So I'm still drawing up my wiring layout (probably being too perfectionist if not a bit anal), but this whole exercise (as well as my next-to-come build of the 5e3) is to help me relearn what's going on behind the schematics of electronics. So I should complete my diagram shortly and will publish it for review and, hopefully, benefit for some future builder. 

Cheers,
Tony


----------



## JohnH

*Amp Impedance Options
*
So far, the attenuators on this thread can deal with some different speaker ohms options, 4, 8 or 16 etc, but have to be dedicated to just one amp output value, either 8 or 16 Ohms, dependent on component values. Some commercial versions offer a range of options for amp taps though, and this could be useful if you have a selection of amps.

So I had a look at schematics online, and this often seems to be done crudely just by splicing in resistors (eg Weber Mass). This however, will blow out many other aspects of impedance, attenuation level and tonal balance.

But I'm thinking it will be possible to do this right, using one more switchable reactive stage. Its an advantage that I'm not trying to mimic a reactive bass peak (letting the speaker do that). To take the 8Ohm design and give it options for 4 and 16 ohm amp taps, it just needs another 8 Ohm resistor in series with 0.5mH coil, with coil bypassed by 27 Ohms. All that is switched either in parallel or series with tbe input, quite simple.

Sims suggest that this will make no difference to tone, and just result in a further -3 db drop. Quite manageable. I'm hoping to test it this w/e, rigged out of my previous and current builds. If it works out, it makes for a versatile rig, using any option of 4, 8 or 16 Ohm in and out, plus attenuation down to below -30 db.


----------



## TonyK

Sounds great @JohnH. If it works out I might have to change my schematic and layout diagrams yet again!!!

I'm using an aluminium enclosure with internal dimensions 200mm x 140mm x 40mm. The box is essentially a "U" shape, divided in half along the horizontal centre line. It's held together by two end panels. The height is tight of course, and it's possible that there will only be a few mm of space between the inductors (on the top of the box) and the resistors on the bottom. Basically I'm pretty comfortable with most of the wiring layout, but I certainly get a bit confused in the wiring for the switches, especially that full bypass switch, although internet research makes me fairly confident of what goes where, and there's always the trusty DMM to help. Also, with the stereo 6-pin jack sockets I'm assuming that the main signal is the furthest pin (either left or right?) from the jack entry, and the "ground" is the closest.

Cheers,
Tony


----------



## TonyK

Deleted as superseded by earlier edit


----------



## JohnH

Hi @TonyK

I'm very happy to help you sort out your wiring diagram, and I appreciate the work you have done on it and on the schematic. Its not far off but there are some issues with it so far though. I'll send comments by PM. Id prefer if you could send updates by PM too if that's OK by you?

Just in general for others in the future, I'm not planning on troubleshooting everyone's wiring, nor to post detailed wiring diagrams here. Every build is likely to be a bit different dependent on parts and design. Plus, the schematics are pretty simple, so being able to interpret them into a working layout is a bit of a test. Those who can follow a schematic and figure it out have a good chance of being able to check it and of making it work. But I don't want it to appear as simple as a 'paint-by-numbers' build, or else those who don't get the design will be getting it wrong and exploding nice tube amps. I hope that appears reasonable?


----------



## JohnH

JohnH said:


> *Amp Impedance Options
> *
> So far, the attenuators on this thread can deal with some different speaker ohms options, 4, 8 or 16 etc, but have to be dedicated to just one amp output value, either 8 or 16 Ohms, dependent on component values. Some commercial versions offer a range of options for amp taps though, and this could be useful if you have a selection of amps.
> 
> So I had a look at schematics online, and this often seems to be done crudely just by splicing in resistors (eg Weber Mass). This however, will blow out many other aspects of impedance, attenuation level and tonal balance.
> 
> But I'm thinking it will be possible to do this right, using one more switchable reactive stage. Its an advantage that I'm not trying to mimic a reactive bass peak (letting the speaker do that). To take the 8Ohm design and give it options for 4 and 16 ohm amp taps, it just needs another 8 Ohm resistor in series with 0.5mH coil, with coil bypassed by 27 Ohms. All that is switched either in parallel or series with tbe input, quite simple.
> 
> Sims suggest that this will make no difference to tone, and just result in a further -3 db drop. Quite manageable. I'm hoping to test it this w/e, rigged out of my previous and current builds. If it works out, it makes for a versatile rig, using any option of 4, 8 or 16 Ohm in and out, plus attenuation down to below -30 db.



I've now tested this out briefly, in a few different ways. (like myself), it seems to have legs, and very few hairs, ie, I think it could be a useful idea where these options are needed.

On the VM, setting the amp to 4 ohms, I ran my previous 8 Ohm attenuator as a load in parallel with the newer Attenuator M, and played that into the 8 Ohm speakers.

I also tried directly connecting the 8 ohm speakers to the amp, and just adding the parallel reactive load.

On the DSL401, I set the amp to 16 Ohms, and wired the 8 Ohm attenuator in series with the previous one, to make a 16 Ohm load. Then I ran that ether with a 16 ohm speaker or two, for an 8 ohm load.

At any of these arrangements, there is an extra -3db drop since half the power is going into the extra parts, easily adjusted for.

All the tones sounded fine, no obvious difference. In theory, with no added attenuation other than an extra parallel load, there is a small reduction in the bass peak, but from low mids up to high treble, tone is not changed. I couldn't hear it though.

*Conclusion:*
With a few more parts, effective options for input Ohms and amp tap settings can be added. With this in mind, the basic 8 Ohm design can be switched to 16 or 4, and the 16 Ohm circuit changed down to 8, all with a one-step loss of output, to be adjusted for. So potentially, the 8 Ohm design is the most versatile, but I still think its better to pick 8 or 16 based on the most likely or useful rig set-up. Both can then be set for 8 or 16 taps. Given those, Id expect not many will really need to also have a 4 ohm tap option.

This feature could be engaged either using a switch, or configured into a group of input jacks.


----------



## BigDaddy23

Just an update on my build - It works fabulously! I built it with Stages 1 and 2 only (thus far) and it has tamed the amp nicely. We used it at band practice Tuesday night and it has turned this amplifier into a much more versatile unit. I believe that having the extra 14dB in stage 3 will be perfect so I will implement that asap. Great design @JohnH !!!

With regard to stage 3, I was only able to get a 4.5R (instead of 4.7R) 25W resistor for R5. I don't think that will introduce any issues. Does anyone think that will be a problem?


----------



## JohnH

hi @BigDaddy23 

4.5 is a close sub for 4 7, no prob.

If you built Stage 1 and 2 off the diagram, then they are both -7db, and the next logical one to extend it could be Stage 3, -14 db. Then you can have -7, -14 and -21, then -28 to get bedroom levels.

But if you did the first -7db reactive stage, and then a -3.5, then the next one you want is a -7 resistive stage, to have all the steps down to -18.5 in 3.5 increments.

Except for just -3 5db as a single first step, you always want the Stage 1 -7db reactive stage to be on, to keep the tone and feel of the amp.


----------



## BigDaddy23

Thanks @JohnH - It's amazing what that reactive stage does! I've done -7dB reactive and the 2nd -7dB resistive stage. I reckon the 3rd -14db stage is all that will be required......although I will be trying it with some smaller 15w amps I use for recording too.


----------



## JohnH

Been thinking about different ways to use the attenuator for small reductions in volume. 

Design M has the ability to use the final -3.5db stage on its own, as a single stage resistive attenuator without engaging the first -7db reactive stage. No problem with that, and it works and sounds fine. But it adds a bit of complication to the wiring, and to the operation of the switches in that to get full bypass you have to switch both the bypass and -3.5db switches to off. It also means the -3.5db stage needs to be rated for the full amp power.

Some of you above are noting that -7db is fine as a minimum reduction for their purposes, and this simplifies the diagram.

So in such cases, if you then find a need for a reduction less than -7db , you can use it as follows:

*Use as a load-box for -3db
*
Most amps offer a choice of amp taps and at least one extra output socket. If you have an output tap at half the ohms of the cab, you can set the speaker direct to the amp using that, and separately plug the attenuator into the amp too. Set the attenuator to max attenuation, but without a speaker. It will now act as a simple reactive load box to absorb half the amp power 
*
*


----------



## matttornado

1st; Thank you so much for starting this thread & sharing the information!
My question is: If a line out is to be added, would it be before the attenuation or after?


----------



## JohnH

matttornado said:


> 1st; Thank you so much for starting this thread & sharing the information!
> My question is: If a line out is to be added, would it be before the attenuation or after?



I haven't tried it properly myself, but I think that the best place would be after the attenuator, ie from the speaker. Here it will pick up the response as influenced by the real speaker, including the bass resonance. It would be affected by how loud you set it though. Also, some cab-sim or IR loader would be needed to get the right tonal shape.

If there was a seperate output socket, it could have a 10k pot to control level.


----------



## TonyK

So I'm finally ready to solder! Thing is I have two choices of wire. The first is single core 1.5mm2 (different online sources indicate that this is between 15 and 16 AWG) and I can only imagine it will be very challenging to work with. The second is a multi-strand 0.75mm2: here my concern is that many online sites say that this is equivalent to 18 AWG which is what has been recommended in this thread, but some sites seem to place it more around 19 AWG. Does anyone have any concrete ideas on this? I think it's obvious that the 0.75mm2 wire will be far easier to work with  Thanks a lot guys!


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## JohnH

I think your 0.75mm2 multistrand would be fine. Good luck with the wire-up!
J


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## TonyK

Thanks John. I would have actually started but then my phone beeped with a news alert -- Theresa May's resignation... I think we need a political attenuator to cut down the volume of polarised voices at the top of most of our countries!!!


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## JohnH

I think an isolation box would be best, so I could go there for some peace and quiet.


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## assaf110

Finally have all of the parts for Design M 16 Ohm built.
Again, thank you JohnH, for the brilliant design and for taking us all for the ride.

My R9 is 50 Ohm, L1 / L2 are 0.68mH & 1mH, the rest is according to the original design.

Two questions before i start:

1. Have you considered making the -3.5 db stage foot switchable for a solo boost (ala Tone king Iron-man?)
I imagine its a simple case of adding a foot switch to the design to control the stage, but a led indicator would need a power source?
It's not relevant to my "ver. 1" build, but i think it will be effective especially for owners of NMV amps that get their gain from the power amp.

2. Lets discuss line out -
This is my plan - a voltage divider in parallel to output 2. I do have a 10k linear Pot (couldn't find 1/2w audio taper ).
What is the value of R21? According to http://www.dougcircuits.com/lineout.html the total sum of P22&R21 should be around 2k?
Is there a need for R23 (to make a minimal resistance level)? If so, what value?
I'm afraid that since the line out is taking the input from the attenuated speaker, at large attenuation levels there will be no audio at all at the voltage divider output. Too little resistance and the circuit may affect the speaker output..













Lineout



__ assaf110
__ May 28, 2019


----------



## JohnH

thanks @assaf110 for your post.

Yes i think you could have a footswitched stage. Maybe with your first version you could check out what setting would be most useful? It could be a seperate box after the end of the main unit, with just two resistors, and calculated for any db value chosen.

I think the attenuated signal should be loud enough for a line-out, though its not tested!

Take a 50W amp, played at max attenuation -31.5db. That's a factor of about 1/1400 of the power, or 0.026W. Its pretty quiet but still useable late at night. With a 16 Ohm version, the max output voltage is sqrt( 0.026x16) = 0.64V, which is still a good level for a line out.

I reckon the 10k should work, maybe with a about a 1k in the R21 position. Whether you need R23 could be based on testing with your rig.


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## assaf110

Great, thanks!


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## tmingle

I put my line out before the attenuation stages. It works really well with impulse responses.


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## Gene Ballzz

@assaf110 
If I were to make it foot switchable, I'd do it with a relay, as opposed to the long round trip from your amp to your pedal and back. There are multiple benefits and as you already determined, power supply will likely be required anyway!
Just Sayin'
Gene


----------



## JohnH

tmingle said:


> I put my line out before the attenuation stages. It works really well with impulse responses.



And from the clip you posted, it sounds good.

In theory, with this attenuator design, DI direct from the input would miss the bass resonant peak, which is only recreated at the output. But this may not be significant, or could be compensated by EQ combined with the IR.

And for info to others reading, DI out as discussed here, needs either a cab-sim or a speaker IR, or be reamped to a remote guitar speaker.



Gene Ballzz said:


> @assaf110
> If I were to make it foot switchable, I'd do it with a relay, as opposed to the long round trip from your amp to your pedal and back. There are multiple benefits and as you already determined, power supply will likely be required anyway!
> Just Sayin'
> Gene



Yes that would be best. And if the pedal is on the floor near other powered pedals, maybe power for the relay and LED could be picked up there, and sent through a footswitch up to the relay in the attenuator, so that there is still no need for power-supply to the attenuator itself.


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## Gene Ballzz

JohnH said:


> Yes that would be best. And if the pedal is on the floor near other powered pedals, maybe power for the relay and LED could be picked up there, and sent through a footswitch up to the relay in the attenuator, so that there is still no need for power-supply to the attenuator itself.



Leave it to you @JohnH , to be thinking truly *"Outside of the box"* on this one!  You sir, are one of the best thinkers I've ever encountered! 

Good On You!
Gene


----------



## Gene Ballzz

And oh yeah, the 16 ohm design choice that I made has produced a few of the most universally usable pieces of gear I've ever owned!

Thanks Again To @JohnH !
Gene


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## tmingle

I had a chance to crank the 40C today, so I recorded a semi clean & a dirty clip in Reaper. Both clips are Lead 1 with the gain 3, vol 4(pretty loud), Bass 8, Mids 8, Treble 2.5, Presence 7 & Resonance 7.5. The guitar is a 2011 Gibson Studio with Dimarzio SD bridge & PAF Pro neck. Cleanish sound is the neck pickup with guitar volume at 3-4. Dirty sound is the bridge on 10. The mic is an SM57 4" of the speaker 1/2 way between the center & edge of the speaker. The performance is pretty weak, as I haven't play much lately.


----------



## JohnH

tmingle said:


> I had a chance to crank the 40C today, so I recorded a semi clean & a dirty clip in Reaper. Both clips are Lead 1 with the gain 3, vol 4(pretty loud), Bass 8, Mids 8, Treble 2.5, Presence 7 & Resonance 7.5. The guitar is a 2011 Gibson Studio with Dimarzio SD bridge & PAF Pro neck. Cleanish sound is the neck pickup with guitar volume at 3-4. Dirty sound is the bridge on 10. The mic is an SM57 4" of the speaker 1/2 way between the center & edge of the speaker. The performance is pretty weak, as I haven't play much lately.




Thanks for those tests!

So I think you currently have the 16Ohm reactive version, and running with a 16 ohm speaker?
I'm guessing the steps in attenuation are 7db, starting from full volume and ending up at -21db?

Sounds good to me, both the dirty and clean. With good headphones, I can hear a slight reduction in high treble at lower levels for the dirty sample, do you hear that too? I'm hearing it's a bit more than I'm getting with mine, but its slight enough and doesn't seem to change the body of the tone. This kind of slight difference could be due to the different amp designs - I pitched it around measurements of my VM which I think is designed to run dirtier in the power stage than a DSL


----------



## tmingle

That is with the reactive engaged. I should do it again without the reactive. My favorite tone is with the amp on about 3 1/2 and just the 1st stage engaged. Its still too loud for my room though.


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## tmingle

I really need to practice & learn some new parts! LOL


----------



## Jordan Prysmiki

Hello guys! I built an 8 ohm reactive load attenuator and I seem to be having a problem in the third stage. The third stage is barely noticeable when I flip on the switch.

I have it built so that the on-off switch is the first stage which is -7, the second stage is also -7, and the third stage should be -14. 

Switching on stage 1 and Stage 2 are very noticeable and sound amazing so far, however there is no or very little noticeable difference when switching on the third stage. If anything switching on the third stage seems to just thin the tone a little bit while switching it off seems to fatten back up the tone again.

Can you advise of anything that I should check? I have already retraced my steps on the wiring and it matches the diagram perfectly.

This is a 20 Watt tube amplifier head. Is it at all possible that the amplifier needs to be loud enough in the first place in order to realize the decibel reduction in the third stage? Other attenuators I have used before are still noticeable at -32db even when the amplifier is at low volumes but I could be wrong.

What could be causing stage 3 (-14) to not be making a noticeable difference?


----------



## JohnH

hi @Jordan Prysmiki 

That -14db stage should be very clearly a big step down! What you describe, being not much reduction but a bit thinner, is consistent with the 4.7 Ohm resistor R5 (per Attenuator M), not getting connected down to ground when the -14 stage is engaged.


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## Jordan Prysmiki

Ok great I will check that!

Silly question.. Does the amplifiers actual decibels need to exceed 28 or 30 in order for me to notice/realize the third stage -14 in the 3rd stage? I suppose that would be logical, however I have no idea how many actual decibels are coming out of the amp as I was practicing at relatively low volumes to begin with.

I appreciate you and this thread so much!


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## JohnH

Thanks for your comments.

The attenuation works with any amp power. Its a ratio, every time you reduce by 10db the power is divided by 10. So at -30db you are getting just 1/1000 of the amp power. eg 0.1W from a 100W amp power or 0.02 W from 20W


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## Jordan Prysmiki

I think it's the resistor ..there is no continuity where as every other resistor have continuity on both ends. I also can't get an ohm reading.


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## TonyK

@tmingle Sounds really close, as JohnH said. I can hear a very slight high end roll off at maximum attenuation. Good job. Now there's no excuse to not play more eh? That was the real reason behind my entire interest in this project!


----------



## JohnH

TonyK said:


> @tmingle Sounds really close, as JohnH said. I can hear a very slight high end roll off at maximum attenuation. Good job. Now there's no excuse to not play more eh? That was the real reason behind my entire interest in this project!



I'm wondering wether that slight roll-off with the DSL is apparent when playing, and if so can it be adjusted for with the amp presence control? If needed, it should be possible to come up with a switch to give a slight treble lift on the attenuator. Would that be of any value? Im not needing it with my amps though.


----------



## TonyK

Just finished the attenuator build and, holy firkin crap, it seems to work like a dream  Many, many thanks to @JohnH for everything you've done and all the patience you've shown. I won't comment properly on the tones yet as there are two major things I need to take care of with my HotRod Deville 410 60W amp. 1) For years I've played with a 12AY7 in v1 to help control the volume and provide a more pleasant overall tone in both the clean and drive channels. So there isn't much breakup on the clean channel until vol 8-9, but by 12 it's a creamy overdrive (never heard this before as was totally unable to crank the amp). I might have to go back to stock with a 12AX7 for a more accurate reflection. 2) I have to spend enough time with it so that I can set the amp correctly for proper power tube saturation, and not the way I've always set it for low volume playability (I have no idea for example where the real "sweet spot" actually is). In this respect, I'll have to seriously reconsider also my pedal use, as finally, the tube screamer will perhaps be able to do what it's supposed to do.

I can report that for my purposes, -21db lowers the amp volume more than enough to actually dime it and remain within pleasant volume levels while sitting and playing directly in front of those 4 ten inch speakers! I can say that just by ear, I didn't notice any tone change whatsoever, even down to the full -31.5db attenuation. I can say it's firkin fantastic to have the amp dimed and be able to have a gorgeous Fender clean tone by rolling off the guitar volume. As soon as I'm able, hopefully within a few weeks, I'll try to post some audio samples. Running my 60W amp at full attenuation does cause the box to heat up, but it's still comfortable to put my hands on it -- sort of like a mug of hot coffee... I'm pretty impressed, especially as I'll mostly use less than full attenuation.

In terms of my experiences in building this attenuator. I have a auto-immune neurological disorder that causes many symptoms, but the most annoying for an electronics build like this is a trembling in the hands similar to Parkinson's (bizarrely doesn't show up so much when I play guitar). So working with these components and 18 gauge wire in a box 200mm x 140mm x 40mm is a real challenge! Also, I ended up with mini switches -- heed others' advice and go with full-size switches! Finally, because I'm basically new to translating a schematic into an actual build, I ended up with more wire in the box than was necessary as I didn't wire switches directly to each other, but rather went with the longer send/return wiring according to the schematic. JohnH has advised me that the direct inter-switch connections is the better way to go, but I was too far down the build path to revert to that -- if I build another I most certainly will!

So for the moment, here a few photos of the build... switches, left to right, are -- 1) full bypass/-7db; 2) -3.5db; 3) -7db; 4) -14db. As everyone is aware, these can be accumulated in any order, although the first -7db stage is on when the attenuator is engaged. I plan on designing an image for the top of the enclosure as well as labelling the switches and jacks on the sides. This will happen soon.

Only outstanding question so far: in the thread there has been talk of measuring the impedance at the output jacks into the speaker. I have no idea how to actually do this but would nonetheless like to do so. Is it as simple as plugging in the speaker (amp on or off?) and reading the resistance across the output jack socket tip and shield while engaging the different combinations of attenuation?

Cheers,
Tony


----------



## JohnH

Hi @TonyK , that's awesome! Thanks for the report, and for working through the build.

Sounds like its all working fine. But if youd like to do some measurements, just as a check, the best one is to measure resistance seen by the amp, so:

Unplug from amp, keep speaker plugged into attenuator, measure ohms across the input jack or across the tip and barrel of the plug that would normally go to the amp. Use a 200 Ohm setting.

Once you subtract about 0.5 Ohm for leads, the resistance should stay in the range around 7-10 ohms for the 8 ohm version, at all attenuation settings. With full bypass, it should read just tbe speaker, typically 6.5 to 7.5 Ohm.


----------



## tmingle

JohnH said:


> I'm wondering wether that slight roll-off with the DSL is apparent when playing, and if so can it be adjusted for with the amp presence control? If needed, it should be possible to come up with a switch to give a slight treble lift on the attenuator. Would that be of any value? Im not needing it with my amps though.


I had a Weber Mass for a while that had the switchable caps for treble compensation, but I really never used them. I had a 401 with a PPIMV at the time & it did not seem to benefit from the Weber. I always run the treble below 5 on the 40C, so compensation is easy. The 40C has a volume control for clean/crunch & a separate control for L1/L2, so I believe the attenuator is more effective with the 40C. When I have all 3 stages engaged, I am using the line out with IR's. I forgot to arm the track for the line out when I did the last test. The next time I have chance to crank the amp, I'll post a stereo track with the mic on 1 side & an IR on the other to see if anyone can hear the differences.


----------



## TonyK

JohnH said:


> Hi @TonyK , that's awesome! Thanks for the report, and for working through the build.
> 
> Sounds like its all working fine. But if youd like to do some measurements, just as a check, the best one is to measure resistance seen by the amp, so:
> 
> Unplug from amp, keep speaker plugged into attenuator, measure ohms across the input jack or across the tip and barrel of the plug that would normally go to the amp. Use a 200 Ohm setting.
> 
> Once you subtract about 0.5 Ohm for leads, the resistance should stay in the range around 7-10 ohms for the 8 ohm version, at all attenuation settings. With full bypass, it should read just tbe speaker, typically 6.5 to 7.5 Ohm.


Thanks John. I'll do that as soon as I can and hopefully the measurements will confirm that the unit is really functioning correctly. I have to admit that yesterday, when I did my initial checks, I started with very low volume on my BassBreaker 15 amp first, and then very slowly moved up the scale! 

The one thing that really surprised me (it shouldn't have but did), was that I heard some nasty feedback at maximum volume on the high gain stage of the BB15 at full volume - I guess as I was listening at low conversation level, I just wasn't expecting to hear this squeal!


----------



## TonyK

So as guided, I tested the impedance as seen by the amp. Using my 1x12 WGS 16 ohm ET65 I was showing 14 ohm with bypass which surprised me a little (nothing to do with the attenuator). But as soon as I engaged the attenuation switches, I read a range from 8.7 ohms to 10.6 ohms, which I think is pretty darned good given the amp wants to see 8 ohms in this case. Using my HotRod Deville 410, standard 8 ohm Fender configuration, I read 7.5 ohms at bypass, and then a range from 8.7 ohms to 9.9 ohms with attenuation engaged. Interestingly, given that at the moment I'm finding -21db and -24.5db to be the level of attenuation needed to come down to very low but pleasurable volume levels, both impedance readings (16 and 8 ohms) were 8.7 ohms. I'm very happy. I hope these sort of readings will help others considering what to build -- for the record, my build is the 8ohm in version.

i can't thank you enough John. So I'll drink a little toast to you this evening


----------



## JohnH

Great! that all sounds fine - rock on!.

By the time you are at -21db or more attenuation, the amp is seeing>99% attenuator and <1% the speaker


----------



## TonyK

Turn the heat up!


----------



## assaf110

JohnH said:


> Thanks for those tests!
> 
> So I think you currently have the 16Ohm reactive version, and running with a 16 ohm speaker?
> I'm guessing the steps in attenuation are 7db, starting from full volume and ending up at -21db?
> 
> Sounds good to me, both the dirty and clean. With good headphones, I can hear a slight reduction in high treble at lower levels for the dirty sample, do you hear that too? I'm hearing it's a bit more than I'm getting with mine, but its slight enough and doesn't seem to change the body of the tone. This kind of slight difference could be due to the different amp designs - I pitched it around measurements of my VM which I think is designed to run dirtier in the power stage than a DSL



I think I hear a difference in the attack of the notes, both dirty & clean.


----------



## matttornado

JohnH - Than you very much!
I want to build one of these. I have a 100 watt marshall I use with a 16 ohm cab. I used a marshall Powerbreak for years and loved it but I sold it.....

What design should I build? I like the one with the inductors.

do I simply double the R values of an 8 ohm design if using a 16 ohm cab? 

Please advise?

Thanks!


----------



## Jordan Prysmiki

I think we've successfully cleaned out the ebay US Stock 4.7 ohm 25watt wire wound resistors! 

My amp is 20W so do I even need a 25W resistor in my third stage or can I go down to 10W?

As started previously my stage 3 4.7 ohm resistor is bad from the get go so I currently only have -7 and -7 working. 

I ordered the 4.7 ohm 25 watt from China but don't want to wait the month..

I can get a local 4.7 ohm 10 Watt quickly if you say I can. Please let let me know!

And @ mattornado, go with a 3 or 4 stage reactive load!


----------



## matttornado

Jordan Prysmiki said:


> And @ mattornado, go with a 3 or 4 stage reactive load!



Yes Definately, Jordan! Good Call.
I'm not exactly sure what values and wattage ratings to use with a 100 watt amp and 16 ohm load. It might have been mentioned but I didn't see it. Sometimes it's hard to go over an entire thread that's multiple pages.


----------



## matttornado

So what if after building the attenuator you find that the attenuation stages are either to quiet or to loud for gigs? Fine tune it?


----------



## JohnH

Thanks for the questions - first, I'm just going to repost a couple of references to save hunting back for them:













Attenuator M 190110



__ JohnH
__ Jan 10, 2019
__ 1


















Attenuator M Power 190331



__ JohnH
__ Mar 31, 2019
__ 2


----------



## JohnH

matttornado said:


> JohnH - Than you very much!
> I want to build one of these. I have a 100 watt marshall I use with a 16 ohm cab. I used a marshall Powerbreak for years and loved it but I sold it.....
> 
> What design should I build? I like the one with the inductors.
> 
> do I simply double the R values of an 8 ohm design if using a 16 ohm cab?
> 
> Please advise?
> 
> Thanks!



The values for a 16 Ohm build are already given and quite a few guys have built it. Double the e power ratings relative to the 50W values to handle a cranked 100W amp. The design M gives you a first small step down of -3.5db, but most are saying that its fine not to have that and start at -7db, which will still be very loud at 20W out of 100W. If so, the 3.5db stage can be on the end of the others with lower power. But R1 should still be 200W, probably best done with two 100W in series or parallel. If you want to crank that 100w amp, build it big in a well vented aluminium box. 



Jordan Prysmiki said:


> I think we've successfully cleaned out the ebay US Stock 4.7 ohm 25watt wire wound resistors!
> 
> My amp is 20W so do I even need a 25W resistor in my third stage or can I go down to 10W?
> 
> As started previously my stage 3 4.7 ohm resistor is bad from the get go so I currently only have -7 and -7 working.
> 
> I ordered the 4.7 ohm 25 watt from China but don't want to wait the month..
> 
> I can get a local 4.7 ohm 10 Watt quickly if you say I can. Please let let me know!
> 
> And @ mattornado, go with a 3 or 4 stage reactive load!



And conversely, yes with a 20W amp, the 25W resistors can scale down to 10W. Will that be case mounted? if not, still do it but check how hot it gets, should be fine though.


----------



## JohnH

matttornado said:


> So what if after building the attenuator you find that the attenuation stages are either to quiet or to loud for gigs? Fine tune it?



The 3.5db steps are pretty small, so you'll probably find the volume that you want, and make any super-fine adjustments at the amp or guitar.


----------



## Jordan Prysmiki

@mattornado 

Go back to page 6 and start looking at the diagrams there for your parts list.

Yes I agree with you the thread is absolutely massive and finding just whst you need can be tough. My friend who told me about this thread admitted he spent an entire weekend reading it! I put every page into PDF when it was a few less Pages than it is now and it turned out to 196 pages on a PDF! I ended up just having a conversation with him to give me the long and short of it and here I am now with a sweet attenuator!

It would be helpful if one of us Consolidated all of the need to know stuff to one document but the digging is a lot if times where you learn. Also a lot of new discoveries and ideas have been brought up by John and others along the way.

I built mine with three stages. You *could* build a separate on off switch but I felt that was unnecessary so my first switch is the on-off switch as well as -7. When turned on Stage 2 is -7, stage 3 is -14. I also personally didn't feel the last stage being an additional -3 DB was necessary to justify another switch and partd which is why I went with a three-stage. 

Not sure what to tell you as far as the adjustability or for gigging. Gigging with an attenuated amp would kind of make this a moot point I would think, since that is the one place you can crank the amp. unless I suppose you just don't have a master volume amp or a way to tame your amp at a gig


----------



## JohnH

One of my plans is to go back to the very first post and add a summary, which I can update if any further developments occur. Also, to ask the mods if this thread can be stickied since it seems to have some ongoing life.

But just to build it, the schematic and table above are almost everything needed, once you know that you can reconfigure it if wanted in terms of bypass switching.

Other things are:
Wire and switches: for anything up to 50W 8 ohm and 100W 16 Ohm, I suggest 18 or 19 gage for wire and air-coil inductors, and at least 5 or 6 amp rating (@125V ac) for swirches.

In the meantime, I hope he enjoyed his weekend.


----------



## TonyK

matttornado said:


> So what if after building the attenuator you find that the attenuation stages are either to quiet or to loud for gigs? Fine tune it?


I can attest that with my 60W Fender amp, I am very happy that I went against my initial idea and installed the -3.5db stage which really does help fine tune the final volume desired, especially if the guitar volume and tone pots are used effectively. Remember that -3db is simply equivalent to halving the wattage, so a 100W amp behaves like a 50W amp -- only a tiny bit less loud. Of course if you find you find a situation where you actually need say 75W, then there's always the volume knob (but as you know volume rarely increases after a certain point when saturation and compression increase), or you'd need to mic the amp. I think you'd be very happy with the massive flexibility of this attenuator. I know I am! Good luck.


----------



## matttornado

Thanks for answers to my questions and replies! I really appreciate it.


----------



## Jordan Prysmiki

@TonyK - ya I can see the necessity of the -3db now.

@john - yes I agree a sticky is a great idea! And yes, he enjoyed his weekend. As far as the 25W to 10W 4.7ohm resistor on my 20 watt amp question. I have the build in a plastic project box mounted on a circuit board. The box is well vented. When playing with both -7's on, there was no heat at all. I know ideally the case should be metal and also aware of the suggested thermal paste but wanted to do this first and check since i have 2 20watt heard that maybe won't require a metal case.


----------



## Jordan Prysmiki

Heres mine!








The shrink wrap is not shrunk yet and the adhesive holding the inductors on is temporary.


----------



## _ripper_

Hey guys, love this thread, some great stuff going on here. 

I just wanted to ask, is there a simple way to tweak the design to make a reactive loadbox instead of an attenuator, for completely silent recording with IRs?

Thanks!


----------



## JohnH

Hi @_ripper_ 
You can use this attenuator safely as a silent load box with no speaker, just by setting it to max attenuation and take a line-out from the amp output (ie at the attenuator input) via a pot. It should work ok. But its not really intended for this use. In theory, you'd capture the treble reactance but could miss out on the bass resonance peak and may need to add it downstream with EQ.

The reason it works well as an attenuator is that the tone you hear depends mostly on the natural reactance of the real speaker that its driving rather than an electronic component model.. So if you dont have a speaker, you can lose this.

I haven't tried it, but the ideal way to use it with IRs would be to have a speaker connected, running at whatever attenuated volume suits, and take a line-out for the IR input from the speaker output.

But if you just want to build a reactive load box without attenuator, check out the one by Randall Aiken online.


----------



## _ripper_

JohnH said:


> Hi @_ripper_
> You can use this attenuator safely as a silent load box with no speaker, just by setting it to max attenuation and take a line-out from the amp output (ie at the attenuator input) via a pot. It should work ok. But its not really intended for this use. In theory, you'd capture the treble reactance but could miss out on the bass resonance peak and may need to add it downstream with EQ.
> 
> The reason it works well as an attenuator is that the tone you hear depends mostly on the natural reactance of the real speaker that its driving rather than an electronic component model.. So if you dont have a speaker, you can lose this.
> 
> I haven't tried it, but the ideal way to use it with IRs would be to have a speaker connected, running at whatever attenuated volume suits, and take a line-out for the IR input from the speaker output.
> 
> But if you just want to build a reactive load box without attenuator, check out the one by Randall Aiken online.



Cheers, thanks for taking the time to reply!


----------



## mAx___

Love this thread!
Question: I need close-to-no attenuation because my 4x12 lives in an iso cab. -3dB would as much as I need to protect the speakers but I'd also like to try -2dB. Can those levels be achieved by adapting values in John's design?


----------



## JohnH

mAx___ said:


> Love this thread!
> Question: I need close-to-no attenuation because my 4x12 lives in an iso cab. -3dB would as much as I need to protect the speakers but I'd also like to try -2dB. Can those levels be achieved by adapting values in John's design?



Sure, see page 11, Jan4th post. There are single srage designs down to -3db. Could do a -2 as well.


----------



## assaf110

JohnH,
I'm in the process of final assembly, wiring the attenuator. A silly question, why SW1 is not bypassing stages 1-4, only 1-3?
Is it to allow a minimum attenuation of -3.5db?
Sorry if it has been answered before and i missed it.
Also, just to clarify, is it a connection point between L1 & L2 (left of R2 on the diagram) or the cables (L1 to R2 and R9 to L2) bypass each other?


----------



## assaf110

.


----------



## JohnH

hi @assaf110 

Yes L1, L2, R2 and R9 are all connected to each other at that point.

As you describe, my bypass wiring was to give the option of running the -3.5 db stage on its own. Otherwise the first step is -7 db when the reactive Stage 1 is engaged but others are off. I figured that for some users who just want a small reduction for a gig, this could be useful. I wanted to try it in my build just to check it out. And it works fine, even though its just a resistive stage when on its own. You can hear it as the second-to-last stage in the sound clips a few pages back.

But for many, including my own actual use, a full bypass is fine.

I'm looking forward to hearing about how yours sounds with your SV!


----------



## matttornado

JohnH 

I'm a little confused. I usually set my amp to 8 ohms when using a single 16 ohm cab or 2 16 ohm cabs (amps stays on 8 regardless if I'm using one or two cabs).

So what do I build, the 8 ohm tap or 16 ohm tap? I still want to keep the amp at ohms but still use one or two 16 ohm cabs.

Please advise?

Thank you so much!


----------



## JohnH

Hi @matttornado 

Thanks for your post. Id say build the 8 Ohm version to use with your 8 Ohm tap. You can run 1 or 2 16 Ohm cabs out of that. 

The 8 Ohm version has, if you build it (parts in the dotted red box), an extra output socket for 16 Ohms and a couple of extra resistors to adjust the tone for use into a 16 Ohm cab. Its a subtle and optional tweak, and its also safe to use a 16 ohm cab out of the normal outputs too.

What kind of amp do you have? Obviously, there is a nominal mismatch when you use it at 8 Ohms straight into a 16 cab, but clearly it works for you. The attenuator will take care of that though, so you can run the 16 cab and 8 ohm tap with no mismatch seen by the amp. That might be a good thing, since presumably with an attenuator, you'll be running the amp at higher power than before.


----------



## matttornado

Thanks JohnH!

i play a 1974 100 watt superlead. it is a monster!
i have a 4x12 cab with 25 watt greenbacks & a 1x12 with a 150 watt Eminence Patriot speaker. 
sometimes i run a wet/dry rig along with my DSL too.


----------



## JohnH

Ok!
Well I'd expect if that was my rig, the Superlead just into the 4x12 would be the main rig, and I'd run that all at 16Ohm with the 16Ohm attenuator. With that, you can still plug a second 16 cab in.

The attenuator gets the most even attenuation steps and on paper, most consistent tone if attenuator, amp tap and speaker load all match. But both the 8 or 16 ohm versions, if matched to the amp tap, can run with 8 or 16 Ohm speaker loads.

An advantage of the 16Ohm version, when running a very powerful amp is that there is less current flowing than with the same power at 8 Ohms. So less stress on switches and jacks.

So what to build?

If you need to use an 8 Ohm tap, then an 8 Ohm attenuator.
Otherwise, build the one that matches the most important or commonly used rig, with a bias towards 16Ohm if no other preference.

Choices!


----------



## matttornado

I think I'm going to go with the 16 ohm tap. 

Now it's time to start getting the parts. My boss & I just made the two inductors.

Can I set the amp at 8 ohms using the 16 0hm tap?


----------



## Jordan Prysmiki

Hey guys just a quick follow up and a question for JohnH below.

I had a chance to replace my R5 resistor and now my -14 switch works. Wow, what a massive volume reduction with all 3 switches (-7, -7, -14) it's amazing!!

My goal was to be able to use the amps master volumes at higher levels and now I can absolutely achieve power amp saturation at bedroom volumes!

I might have noticed a bit of a high treble roll off but I could be mistaken as their were a few other distractions in the room. If what I noticed was accurate I put probably make that up with the treble and presence controls. I will know more after more playing.

With just a bit of playing I am now seeing the necessity for having that -3 switch in addition to what I have now, as you all said, for "fine tuning". Maybe I will build another.

@JohnH, do you know what value resistors I would need for -5 or -10 reductions @8ohms? Let's call them R9 and R10 to stay consistent with the diagrams. Could I just add a hypothetical R9 to ground and R10 anywhere after the R5/R6 circuit (or even sooner in the circuit)?


----------



## JohnH

matttornado said:


> I think I'm going to go with the 16 ohm tap.
> 
> Now it's time to start getting the parts. My boss & I just made the two inductors.
> 
> Can I set the amp at 8 ohms using the 16 0hm tap?



A bit of clarifying needed to answer the question: By 'tap', Im meaning the output of the output transformer (eg, which is tapped for 4, 8, 16 Ohm speakers), ie it is the amplifier output socket. You should match that to the version of the attenuator that you build.

So whats the plan?

Also interested in your inductors, what kind of wire and wire gage did you use? A photo would be great!

A couple of thoughts on the tone difference you are getting with 8 or 16 Ohm amp outputs: The way that the speaker impedance (which varies with frequency) shapes the tone depends on the real output impedance of the amp. The higher it is, the more the speaker will develop a bass resonance and a treble lift. Lower impedance out of the amp will, for a given speaker, reduce the highs and maybe the bass and favour the mids. I reckon this is what is happening when you plug a 16 Ohm speaker into your amp set at 8 Ohms. On my VM amp, the 8 Ohm output has an output impedance of about 18-20 Ohms for small signals, and with a 16 ohm output it would be 2x that. 

BTW, this effect is partly why some SS amps can sound lifeless, and why the usual L-pad speaker attenuators sound dull at low volume. Both of these get very low output impedance, squashing the ability of the speaker to develop its treble and bass tone. 

Our attenuator design takes account of these effects, and the current designs are matched to my amp measurements which I think are reasonable ballpark values for a range of amps. Whether you prefer the tone of a 16 or 8 ohm cab and which attenuator design it is into will likely depend mostly on the relationship between the attenuator and the speaker, rather than the amp and the attenuator. Luckily, once you are running the attenuator, it takes out any issues of mismatch to the amp and you can experiment.

So out of that, Id still recommend to match the nominal output Ohms of the amp (ie 8 or 16)to the attenuator version, just for the safety of your classic amp when you get to run it at high power


----------



## JohnH

Jordan Prysmiki said:


> Hey guys just a quick follow up and a question for JohnH below.
> 
> 
> 
> @JohnH, do you know what value resistors I would need for -5 or -10 reductions @8ohms? Let's call them R9 and R10 to stay consistent with the diagrams. Could I just add a hypothetical R9 to ground and R10 anywhere after the R5/R6 circuit (or even sooner in the circuit)?



If you wanted to have a switched stage to do -5 db, or -10db, the values would be, for an 8 ohm attenuator:

-5db
R9 (to ground) 25 Ohm
R10 (series) 8 Ohm

-10db
R9 10 Ohm
R10 15 Ohm

These could be wired in, like the other stages, anywhere after the first reactive stage


----------



## Jordan Prysmiki

JohnH said:


> The attenuation works with any amp power. Its a ratio, every time you reduce by 10db the power is divided by 10. So at -30db you are getting just 1/1000 of the amp power. eg 0.1W from a 100W amp power or 0.02 W from 20W



So I have a 20 watt amp. Are these calculations correct for the switches I went with?

-7db = 2.86 watts
-7db, -7db = .41 watts
-7db, -14db = .20
-7db, -7db, -14db = .03

I believe so as my last calculation for -28db is close to yours for -30db w/20Watt amp


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## JohnH

hi @Jordan Prysmiki 

Not quite the right maths. db's are a log scale. Although -10db is a factor of 1/10th, -7db is not a factor of 1/7th but is in fact a factor of 1/5th. ( actually 10^-0.7) 

-14db is a factor of 1/25th (ie like two -7db stages: 1/5 x 1/5 = 1/25.)

So starting with a 20W amp, you'll get....

-7db = 20W/5 = 4W
-7 -7db = 4W/5 = 0.8W
-7 -14db = 0.8W/5 = 0.16W
-7 -7 -14db = 0.16W/5 = 0.032W

...so you'll arrive in about the same place by the end! Each step will sound like a roughly equal reduction, since hearing perception of volume is also logarithmic.


----------



## Jordan Prysmiki

JohnH said:


> hi @Jordan Prysmiki
> 
> Not quite the right maths. db's are a log scale. Although -10db is a factor of 1/10th, -7db is not a factor of 1/7th but is in fact a factor of 1/5th. ( actually 10^-0.7)
> 
> -14db is a factor of 1/25th (ie like two -7db stages: 1/5 x 1/5 = 1/25.)
> 
> So starting with a 20W amp, you'll get....
> 
> -7db = 20W/5 = 4W
> -7 -7db = 4W/5 = 0.8W
> -7 -14db = 0.8W/5 = 0.16W
> -7 -7 -14db = 0.16W/5 = 0.032W
> 
> ...so you'll arrive in about the same place by the end! Each step will sound like a roughly equal reduction, since hearing perception of volume is also logarithmic.



Man you are wizard! I really appreciate that!

It's amazing how loud the 4 watts really is still. 

Keith Williams from the "Five Watt World" youtube channel says 5 watts is all you need for your home studio or practicing. I know that is subject to opinion but I completely agree. We naturally think wattage and loudness are a ratio and it's really not. As you said it's logarithmic and the attenuator really helped bring this home for me with knowing just how loud different wattages actually are.


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## Jordan Prysmiki

Alright alright. I'm as done as done can be. Here are the pics!

The case I built the attenuator In has mounting ears on the back panel. I chose this so I could mount the attenuator in my custom ©stagecraft.com BluesJack 2x12. I made custom connections from the cabinet's input jack, to the input of the attenuator and back to the speakers. I did this to eliminate extra cables on the rear of the amp and for cool factor.


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## matttornado

So simply put, with an 8 ohm tap amp output, i should build the 8 0hm attenuator and use either one or two 16 ohm cabs, right?

makes sense now. i will post some pics and info of the inductors.


----------



## JohnH

matttornado said:


> So simply put, with an 8 ohm tap amp output, i should build the 8 0hm attenuator and use either one or two 16 ohm cabs, right?
> 
> makes sense now. i will post some pics and info of the inductors.



Yes, that sounds like a plan!


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## matttornado

here are the inductors my boss & I made.


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## Jordan Prysmiki

Is the first pic the finished product or the stated project for the inductors?


----------



## matttornado

Jordan Prysmiki said:


> Is the first pic the finished product or the stated project for the inductors?



Yes they are the finished products. They were measured wit hthe meter set at 100Hz


----------



## JohnH

Interesting! they are wound onto cores, which means you get the inductance with far fewer turns. All the other builds are using air-cored, and I started with those since that is whets used in loudspeaker crossover networks, which is a similar application. I think the reason there is to avoid core saturation, and at that point im all out of tech! I don't know what the consequences would be except I cant imagine there's any risk, Can any one speculate on using a ferrite or iron cored coil in a loudspeaker application?


----------



## Nik Henville

JohnH said:


> ...Can any one speculate on using a ferrite or iron cored coil in a loudspeaker application?



If memory serves - cored coils start to behave non-linearly as the current increases. This in turn leads to distortion. Don't quote me, though - experts _*WILL*_ be along soon...


----------



## TonyK

@Jordan Prysmiki Very nice looking work there. I wish I could get my wiring dressed as tidily as that!

I've had a little time to try out the attenuator now with both my Hot Rod Deville 410 (60W) and my BassBreaker 15. So this is the first time I've ever heard my HRDv dimed! Disclaimer -- this ain't no Marshall!!! I've recorded a few samples so that people can hear the tone at different attenuation levels.

This is a very abbreviated "Cocaine". I recorded the riff clean into a Boss looper pedal, and then played it through a Hot Rod Deville 410 dimed. Starting attenuation was -14db (too loud to start with no attenuation) and then the following segments are respectively -17.5db, -21db. -24.5db, -28db, and -31.5db. The spl in my room started around 92db and finished up at between 70 and 75db -- easily bedroom level. 

The second sample is the same but with each music segment normalised to -1db. 

I'm posting this little piece to highlight just how good the attenuator really is, and also to show what a Hot Rod sounds like dimed! Obviously this is the clean channel, and I use a 12AY7 in V1 specifically to manage the volume and tone better at lower levels, but now I will probably put back in the 12AX7 to see what the more normal breakup is like.






Thanks a million to @JohnH


----------



## JohnH

Thanks @TonyK , very helpful and they sound great!

I appreciate the full volume issue. When I do maxed out sound samples I need to put on the same ear muffs I use when working with my chainsaw

This shows how you don't need to spend hundreds to get a small low powered amp with a tiny speaker to play at low volume, if you already have a full size amp and can wire up an attenuator circuit. Nor do you necessarily need an expensive load box plus digital IR processing and re-amping to simulate the tone of a real amp at low volume, when you cam just play a real amp at low volume. And as long as you can have a bit of volume (like at least vocal volume), you can mic it too.

That being said, the latest digital solutions to working with valve amps (like, OX, Boss etc) are wonderful devices with awesome capabilities and technology. Its just that if you just want to play the amp, there are alternatives . I kind of like how the simple robust technology of our attenuators fits in with the simple analogue tech of a nice old amp.


----------



## Jordan Prysmiki

JohnH well said bro!


----------



## Jordan Prysmiki

@TonyK , nice clips mang. 

I compared the -17db intro of your clip to the last -31.5db clip over and over on the -1 normalized version and heard what I thought were some tonal differences.

The difference was that the initial first clip had a longer tail to each note and stop as well as some more openness. I then realized that since you were recording at louder volumes in the first clip that the amp probably generated more room ambience for the mic to pick up actually adding to the perceived tone. 

It sounded good regardless, just pointing that out. 

Have you cranked your master volume yet to hear the power amp tonal differences at lower levels? If so what are your thoughts on that?


----------



## Edchen

I salute you John and all that contributed to the design of this fantastic attenuator!

I am very proud to confirm that I’ve built the 16ohms version using 100w rated resistors and the result is truly superb!!!

I cannot believe I almost forked out £1000 on The Ox when the cheap resistive attenuator I bought from Thomann ended up sounding extremely dull and lifeless.

I am finally able to fully appreciate the tonal capabilities of Victory V30 MK2 without blowing off the roof!

I have attached some pics of my build which will hopefully help others in the future.

Best regards
Eddie


----------



## JohnH

Hi @Edchen 
Thanks, and that's a great looking build. Awesome graphic and I like the drilling too! I'm glad it worked out.


----------



## Filipe Soares

can it be scaled up to 400w?


----------



## JohnH

Filipe Soares said:


> can it be scaled up to 400w?



All the theory scales up, but what kind of amp are you thinking of? is it a bass amp? Is it tube or ss? This design is based around tube driven guitar amps and guitar speakers.

And if there is 400W at play, much more attention to cooling would be needed (eg fan cooling) plus much greater current capacity in switches, inductors and wiring.


----------



## Nik Henville

Tasty looking build and box - nice work 


Edchen said:


> View attachment 59561


----------



## Jordan Prysmiki

@Edchen, nice build man! How you get the holes so perfect?


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## matttornado

That looks awesome!


----------



## Filipe Soares

JohnH said:


> All the theory scales up, but what kind of amp are you thinking of? is it a bass amp? Is it tube or ss? This design is based around tube driven guitar amps and guitar speakers.
> 
> And if there is 400W at play, much more attention to cooling would be needed (eg fan cooling) plus much greater current capacity in switches, inductors and wiring.



It's a Marshall Major, 200w. I said 400w because there's the thumbrule (Don't know how true it is) : attenuator capacity = 2x amp power. so there's why the humongous wattage


----------



## JohnH

Filipe Soares said:


> It's a Marshall Major, 200w. I said 400w because there's the thumbrule (Don't know how true it is) : attenuator capacity = 2x amp power. so there's why the humongous wattage



OK cool! (or hot) There'd never be an amp more in need of an attenuator than a Marshall Major.

Just a watchit, I'm thinking that's a old amp. So if you use an attenuator to allow it to be pushed, it may be working a lot harder than it ever did 

The basic component power ratings for 50W are chosen to have plenty of capacity for an actual 50W amp. So for the 200W Major, a x4 scale up is indicated. If a 16ohm version would suit, I'd suggest that instead of 8ohm, since less current is needed.

Where the 50W version uses a 25W resistor, use a 100W instead of the same value. Where 100W was used, you can use 4x 100W in series/parallel. Build it big and wide with lots of space and ventilation on a heavy Aluminium chassis, with all resistors chassis mounted using thermal grease

Inductors could be based on 14ga wire. Do you need full bypass switching? Maybe not needed? Even if -7db is the least attenuation, that's still a very loud 40W unit for a 200W input. 

If you do need full bypass, with a 16 ohm version, you are switching nominally about 3.5A 10Amp switch rating would be good. 

If you build it, I wish you luck and I reckon it will be awesome.


----------



## Filipe Soares

JohnH said:


> OK cool! (or hot) There'd never be an amp more in need of an attenuator than a Marshall Major.
> 
> Just a watchit, I'm thinking that's a old amp. So if you use an attenuator to allow it to be pushed, it may be working a lot harder than it ever did
> 
> The basic component power ratings for 50W are chosen to have plenty of capacity for an actual 50W amp. So for the 200W Major, a x4 scale up is indicated. If a 16ohm version would suit, I'd suggest that instead of 8ohm, since less current is needed.
> 
> Where the 50W version uses a 25W resistor, use a 100W instead of the same value. Where 100W was used, you can use 4x 100W in series/parallel. Build it big and wide with lots of space and ventilation on a heavy Aluminium chassis, with all resistors chassis mounted using thermal grease
> 
> Inductors could be based on 14ga wire. Do you need full bypass switching? Maybe not needed? Even if -7db is the least attenuation, that's still a very loud 40W unit for a 200W input.
> 
> If you do need full bypass, with a 16 ohm version, you are switching nominally about 3.5A 10Amp switch rating would be good.
> 
> If you build it, I wish you luck and I reckon it will be awesome.



first of all, thank you for your help. I do not want to push too hard on the metal grandpa. just enough for the tubes to work a little. just enought to work at half power without me being arrested, divorced or kicked out of the building. I have no plans to ¨use it as a plexi¨ , all on 10 like yjm. 

my major has a master volume installed, so the idea is to put the master volume at half and be able to play it without committing any felony LOL. 

I´ll start sourcing the material, maybe I´ll put a fan to force ventilation through the box.


----------



## tmingle

Sorry guys, I had to do it.


----------



## TonyK

Jordan Prysmiki said:


> @TonyK , nice clips mang.
> 
> I compared the -17db intro of your clip to the last -31.5db clip over and over on the -1 normalized version and heard what I thought were some tonal differences.
> 
> The difference was that the initial first clip had a longer tail to each note and stop as well as some more openness. I then realized that since you were recording at louder volumes in the first clip that the amp probably generated more room ambience for the mic to pick up actually adding to the perceived tone.
> 
> It sounded good regardless, just pointing that out.
> 
> Have you cranked your master volume yet to hear the power amp tonal differences at lower levels? If so what are your thoughts on that?


Jordan, you're right about the speaker output levels and the effect this can have on the perceived tone. That's exactly why I generally enjoy playing around a 90db level and not at bedroom levels. The whole experience is just... better But I am amazed at the overall tonal characteristics of John's attenuator design. I was planning on building a 5e3 (in a few weeks) but now I'm wondering if I'd be much happier with one of the larger tweeds given that I'd be able to push it into power tube saturation with ease!

Relative to your question on the master volume, I think you're talking about the (in)famous Drive and More Drive channels on the Hot Rod (how bad are they, really). I recorded a riff with the master on full, starting with the Drive at 3 and then gradually increasing it plus also repeating the riff with the More Drive each time. But when I'm playing it back, for some reason I can hear no difference at all - certainly a recording error. I'll redo the experiment as soon as possible, including, once the amp is totally dimed, then lowering the attenuation by -3.5db for each segment, so that the clip will also give another "hearing" to the actual attenuation. If it works, and with John's permission, (and with a link back to this thread) I'll post some of the clips on another forum that is more Fender orientated as it would be of potential use to those readers.


----------



## JohnH

Hi @TonyK , very happy if you'd like to post about it on other forums, and put a link to them here so we can follow.

On the last clips I posted, from -7db downwards there was no change in tone that I could see or hear, but from bypassed then to -3.5 and then -7db ( ie the first reactive setting), there was some relative ups and downs in response plots, but no general trend.

The consistency of the tone comes from the circuit arrangement and also the specific component values. It looks like we have hit on a sweet-spot that works well with a range of amps. But, to arrive there, there's some luck and judgement involved because I don't actually have a lot of data on which its all based. I only have two amps!

The key info relating to the amps is the small-signal output impedance, and its measured using a consistent, non-distorted input tone, feeding a resistive load, and recording or measuring output level as that load is changed. Its actually pretty simple to do with minimal kit. If anyone is interested to run such a test, I'd be grateful and can discuss further how to do it using whatever equipment is at hand.


----------



## assaf110

Ok, finished assembling a new brother to the family.
Got delayed cause of life getting in the middle..
Pics will come soon.
I made a couple of readings connected to a 16ohm cab (measured 14.3),
I’m not sure I got it right .
I measure at the amp in socket. It is a 16ohm version.
Completely bypassed ( sw1 sw4 bypass mode) - 15.5 ohm.
Sw4 on ( 3.5 stage) - 24.5 ohm
When sw1 is on (reactive stage connected) it goes to around 30ohm,
Sw1-3 on 37.3 ohm
All switches 36.8 ohm.
I will go over the connections, I think I have some error in my wirings - does the measurements make sense?


----------



## JohnH

ok thanks...it doesn't seem quite right! Ill think of some suggestions....


----------



## assaf110

View media item 10887View media item 10889View media item 10888View media item 10890View media item 10891


----------



## JohnH

The metalwork looks nice. How did you make the labels?

Those resistances measured from the input socket are too high, assuming that cab is plugged into the speaker out.

There's a quick check on full bypass: There should be infinite resistance from ground to hot input or to hot output. Also zero or very low ohms between hot input and hot output.

Im suspecting a problem with L2 and R1 being not connected. Not sure though, there may be other things. Good luck!


----------



## JohnH

BTW, I've edited the first post on the thread to make a summary of the main aspects of the current design. I couldn't add as much as I wanted since I've found there's a limit on post length of 10000 characters. I hope it may be useful though.


----------



## JohnH

Another easy test, use a 1.5V battery and touch it across the input, to make sure you get a credible thump in the speaker, same as direct when fully bypassed, and reducing as you engage the first few attenuation steps. Buit I'm still suspicious of L2 and R1!

With all stages fully off (no attenuation) and bypassed, no speaker connected, you should measure about 31Ohm from the top of L2, through R1, to ground. 

Now just switch Sw2 to attenuated, and it should put R3 in parallel for that same reading, and it should be just over 15 Ohm.

How about do those tests, unless you've found the problem already?


----------



## assaf110

thanks, will check, hopefully tomorrow...
The labels made with a simple p-touch label maker.
The retro one is 
*DYMO Office Mate II Embossing Label Maker*.


----------



## assaf110

Well, Bedroom Blaster is in the house!
I checked the wiring this morning, starting with stage1, I rewired the L2 R1 to be on the safe side, but the connection was solid to begin with. 
I then re measured the input resistance, it must have been the late hour and being so tired, but everything looks as it should now 
Made the battery check which turned up fine.
To be on the safe side, I first connected it to the Origin head, as I rather melt that transformer and not the SV 
Then made an unscientific test comparing to a L Pad passive attenuator & Jhs volume box in the loop.
First impressions -It’s awesome! 
First time I was able to hear the baby plexi in 20w mode like that.
It has more gain & harmonics compared to the volume box in the loop - as expected.
No real influence on tone ( was trying mainly in the 14db attenuation ) if anything it seemed slightly brighter when attenuated.
I don’t have the proper “technology” to make a recording (or the playing skill ). I will mess around with the attenuator some more and try to form a more detailed opinion later on.
As always, thank you JohnH for the amazing support and sharing. It is a very enjoyable & effective project!
View media item 10902


----------



## JohnH

Great news! And congrats for following it through. Yours is the first build I know of running from an SV20.


----------



## TonyK

Well, sorry for the delay in finally getting back to the earlier question of how the master volume and drive channels worked out on the Hot Rod Deville. My first recording attempt gave a bizarre result: the MV was at 12, dimed of course, and the drive was incremented in graduations from 2 up to 12, but the recording showed very little difference in sound/tone between each. Anyway, I scrapped that and recorded a very abbreviated version of the classic Alright Now onto my Boss looper, then played it back with the drive at 2, then 4, 6, 8, 10, and 12, each time recording once through with Drive engaged, and then again with More Drive engaged (so two versions for each level of Drive). The attenuator was set at -17.5db, which was as loud as I could manage (pushing around 100db in SPL). Obviously it would have been better if I could have started with no attenuation, but, reality prevailed! I figured that this is NOT a Marshall, but for classic rock tones, perhaps... Finally, for the clearer purposes of this thread, I left the amp totally dimed on More Drive and MV, and increased the attenuation in 3.5db increments. Guitar was my ES-335 (I've never owned a LP).

Enjoy - and sorry for the sloppiness!

Oops, warning -- even though the riff is very abbreviated, this is a long recording as there's so much going on!


----------



## TonyK

And just as an interesting comparison, I recorded a similar very simple riff through my BassBreaker 15 amp using -7db of attenuation on the medium overdrive structure. The MV is about 8 out of 10, and the gain is about 5.5. This amp is often said to be a Marshallesque attempt from Fender (not really true, but definitely not standard Fender fare either. So this is the very short sample again using my ES-335.



Would love to hear thoughts - but in my humble opinion, the gain tone is much more interesting than that of the HRDv


----------



## JohnH

Thanks, Im not seeing a link in the last post though.


----------



## TonyK

It's really bizarre. I started a thread on TDPRI and my first two audio samples worked perfectly well with links to SoundCloudm and the others I posted did the same as what's happening here. If I edit my post the link is there as expected but disappears as soon as I save the post. The actual links are live. I guess I'll look for an alternative to SoundCloud and repost but if anyone has some ideas please let me know.

Ohm the TDPRI link is: https://www.tdpri.com/threads/how-does-hot-rod-deville-410-really-sound.964930/#post-9219404

And just to try the music link again, it's here:


----------



## TonyK

go figure -- the SoundCloud link failed again 

For the moment, if you have access, please try this link to my DropBox account: https://www.dropbox.com/s/1qtbgpmv72jjnn0/alright now BB15.MP3?dl=0


----------



## JohnH

Thanks Tony, they are sounding good (just from my phone so far, since I'm travelling)


----------



## TonyK

I think I've figured out what's going wrong, so here's another attempt at just posting the easier to access SoundCloud version of the BassBreaker 15


----------



## tmingle

BUMPITY


----------



## kaludjerko

Maybe this was mentioned before, but I have a question:

How critical are exact values for L1 and L2 inductors?
For inductor L2=0.5mH, can I use L2' = 0.47mH or L2' = 0.56mH, as substitute, without adverse effects on the tone?
These two values I can obtain easily, but 0.5mH not.
What would be better option of these two values?

Thanks to JohnH for design and TonyK that posted this topic on TDPRI where i got this info.


----------



## Dogs of Doom

JohnH said:


> Thanks, Im not seeing a link in the last post though.





TonyK said:


> It's really bizarre. I started a thread on TDPRI and my first two audio samples worked perfectly well with links to SoundCloudm and the others I posted did the same as what's happening here. If I edit my post the link is there as expected but disappears as soon as I save the post. The actual links are live. I guess I'll look for an alternative to SoundCloud and repost but if anyone has some ideas please let me know.
> 
> Ohm the TDPRI link is: https://www.tdpri.com/threads/how-does-hot-rod-deville-410-really-sound.964930/#post-9219404
> 
> And just to try the music link again, it's here:


you put in a bad url...


----------



## JohnH

Hi @kaludjerko , and thanks for your interest.

The values are not super-critical, and I doubt you'd hear a difference across those values of L2.. Can you find the specified value for L1?

But, to make a choice: A higher L2, if you could hear a diffference, would theoretically be slightly brighter, and a bit more 'lively'. I'd go with the higher value of 0.56mH assuming L1 is as designed.

But its a good question. Ill run the numbers to let you know what difference it really would make.

Actually, if needed, you can reduce inductance by unwinding a few turns and re-taping the coil. Ive done that to make mine, but I have an inductance range on my meter to help do it.

And, you can also increase inductance by mounting a small steel bolt on the central axis of the coil.


----------



## JohnH

yes, going from 0.5mH to 0.56mH adds about 0.25db to the high treble. id use that but that's less difference than youll hear.


----------



## Gene Ballzz

JohnH said:


> Hi @kaludjerko
> 
> And, you can also increase inductance by mounting a small steel bolt on the central axis of the coil.




Hey @JohnH

Upon reading that, can I assume that my coil mounting method has increased the inductance? Should I be using plastic/nylon bolts here?






Thanks John,
Gene


----------



## Dogs of Doom

Dogs of Doom said:


> you put in a bad url...


I was going to bed last night when I fixed the link & typed this. For some reason, soundcloud would only let you post the link, as "in a set" (list). You can only embed a file, not a list, so, you had to remove the list reference. I see you figured that out, but thought I'd confirm it & make that clear to anyone else who might be interested...

see graphic below:




you only need to post the text & the forum will automatically format the player (same w/ youtube)...


----------



## Dogs of Doom

Gene Ballzz said:


> Hey @JohnH
> 
> Upon reading that, can I assume that my coil mounting method has increased the inductance? Should I be using plastic/nylon bolts here?
> 
> 
> 
> 
> 
> 
> Thanks John,
> Gene


how much heat does that generate?


----------



## Gene Ballzz

Dogs of Doom said:


> how much heat does that generate?



Get's only a little warm with a 20 watt DSL20 and stays generally cool with my 13 watt, 5E3, Tweed Deluxe. It gets pretty toasty (almost hot) with my JTM30, which although *"officially"* 30 watts, I'm betting it's every bit of 40 or more watts! I've actually rebuilt it, using lower wattage (physically smaller) resistors for all except the first, always on, -7db stage. 

Just My $.02,
Gene


----------



## JohnH

@Gene Ballzz
Yes, those inductors might be acting 20 to 40% higher! I don't think that's a problem if you are liking the results though, it doesn't change the basic mid-band ohms that the amp sees.

But you could try with the bolts out to see if you notice a difference. Nylon or stainless steel bolts wouldn't have that effect.

@Dogs of Doom
This design is worked out with a factor of at least 3 on calculated power going into each of the resistors compared to their spec, based on 50W amps. The first stage does most of the work, and the other stages take a lot less or may be off. With a heavy case and all resistors bolted with thermal grease, it takes a long time to heat up and so temperature rise caused by power dissipation is averaged over about 10-15 minutes on my build. With my 50W VM, I haven't been able to get it more than warm:












AttenuatorM Inside 190217



__ JohnH
__ Feb 16, 2019


----------



## kaludjerko

JohnH said:


> Hi @kaludjerko , and thanks for your interest.
> 
> The values are not super-critical, and I doubt you'd hear a difference across those values of L2.. Can you find the specified value for L1?
> 
> But, to make a choice: A higher L2, if you could hear a diffference, would theoretically be slightly brighter, and a bit more 'lively'. I'd go with the higher value of 0.56mH assuming L1 is as designed.
> 
> But its a good question. Ill run the numbers to let you know what difference it really would make.
> 
> Actually, if needed, you can reduce inductance by unwinding a few turns and re-taping the coil. Ive done that to make mine, but I have an inductance range on my meter to help do it.
> 
> And, you can also increase inductance by mounting a small steel bolt on the central axis of the coil.



Thank you for prompt answer and update with calculated data.
Will be ordering from banzaimusic (hopefully mentioning them doesn't break some rules here) and they have 0.33mH (for L1).
Already have resistors and am looking forward to this build.


----------



## Heph333

How would one go about sizing the inductors for much lower wattage? Let's say I want to make a version for a 5W single-ended amp. The resistors would be small enough that I could probably build on a PCB. Could I use a small axial leaded inductor? They're rated by amps, and without knowing the OT secondary max voltage, not sure how many amps the inductor needs to be rated for. Hell.... could probably just coil my own around a small sewing thread bobbin.
I'd also like to be able to use it as a load box DI. Has anyone tested this at full attenuation with the speaker disconnected? I imagine at -31db the speaker itself isn't contributing very much to the overall reactance of the output.


----------



## JohnH

Hi @Heph333 , thanks for your interest. An amp running at 5Wamp into 8 ohms sends out 0.8Amps. but for the inductors, the current rating is only part of the spec requirements. They also need to have low resistance, particularly L1 , or else the maths get messed up. We use 18gage wire for the base design. Id say you could go down to 20gage if its only for 5W (0.8mm dia)

You can safely use it as a load box with no speaker, when at max attenuation. The amp wont know about it. You miss out on the tonal effect of bass resonance however, which is not usually a big deal. In this design, this bass effect is generated at the speaker itself.


----------



## Heph333

JohnH said:


> Hi @Heph333 , thanks for your interest. An amp running at 5Wamp into 8 ohms sends out 0.8Amps. but for the inductors, the current rating is only part of the spec requirements. They also need to have low resistance, particularly L1 , or else the maths get messed up. We use 18gage wire for the base design. Id say you could go down to 20gage if its only for 5W (0.8mm dia)
> 
> You can safely use it as a load box with no speaker, when at max attenuation. The amp wont know about it. You miss out on the tonal effect of bass resonance however, which is not usually a big deal. In this design, this bass effect is generated at the speaker itself.



Thanks for the follow-up. That's exactly what I needed. I'll be using it with Impulse Responses, so bass resonance will be emulated.


----------



## matttornado

Back again! Finally getting my parts together for my build. Just ordered all of the resistors.

So with my 100 amp, will the first -7db reactive stage, if not bypassed, knock my amp down to ~20 -25 watts? What if that's too much?

My DSL 40CR on the 20 watt setting is usually loud enough but not always.
The other night I played in a fairly large sized bar with a a wide open dance floor & with my DSL set on the 40 watt setting, my master was at about 2 oclock.
I had a 1x12 extension cab hooked up too.

I'm comparing apples to oranges at this point I know, right?

I hear the new 20 watt plexi's are loud as shit so I don't know.


----------



## JohnH

matttornado said:


> Back again! Finally getting my parts together for my build. Just ordered all of the resistors.
> 
> So with my 100 amp, will the first -7db reactive stage, if not bypassed, knock my amp down to ~20 -25 watts? What if that's too much?
> 
> My DSL 40CR on the 20 watt setting is usually loud enough but not always.
> The other night I played in a fairly large sized bar with a a wide open dance floor & with my DSL set on the 40 watt setting, my master was at about 2 oclock.
> I had a 1x12 extension cab hooked up too.
> 
> I'm comparing apples to oranges at this point I know, right?
> 
> I hear the new 20 watt plexi's are loud as shit so I don't know.



Yes that's right, 100W reduced by -7db is 20W. That's still pretty loud on most such amps!

But in case you need to find a setting in between, if you wire up Attenuator M as drawn, then the -3.5db stage can act on its own, which will give you about 45W. ie you switch to bypass but still have the -3.5 stage engaged. This setting doesn't involve the coils, but it still sounds fine as a resistive stage for this small reduction. I put this feature into the design for just this reason. To do this, the -3.5 stage components are speced for full amp power.

If you are fully cranking that 100W through it, then the power ratings should be x2 compared to the spec, which was for 50W - what sort of resistors are you getting?


----------



## matttornado

JohnH said:


> what sort of resistors are you getting?



I'm using wire wound power resistors like what others are using. For R1, I'm using two 60 ohm 100 watt resistors in parallel. Same with R7 but with two 150 ohm / 100 watt resistors in parallel. R2 & R8 are 100 watt ratings also. All of he others are 50 watts except R9 is 20 watts.

Thanks JohnH!


----------



## JohnH

Sounds like it will be just right! Good luck with tbe build.


----------



## Musicmaniac

Glad I stumbled on this thread.


----------



## matttornado

JohnH said:


> Sounds like it will be just right! Good luck with tbe build.



Thanks but I'm still confused trying to understand the diagram. I don't want to build the "RED BOX" part & the switching has me confused.
maybe some earlier diagrams in some earlier posts will help me.


----------



## matttornado

So If I bypass the first 3 stages & want to use just the -3.5db stage, would I connect the BLUE wire between R6 & R8 before the switch? What if the 4th stage is not active? Will the signal just go straight out to the speaker(s)?
Sorry for the stupid questions..... It looks like if the last sage is off, not going to R7, it bypasses completely to the speakers for zero attenuation.
Am I on the right the track? LOL

So this diagram below shows that switch one bypasses everything if wanted. I want switch one to either go to all of the stages or just the last
like previously discussed.
S


----------



## Gene Ballzz

matttornado said:


> So If I bypass the first 3 stages & want to use just the -3.5db stage, would I connect the BLUE wire between R6 & R8 before the switch? What if the 4th stage is not active? Will the signal just go straight out to the speaker(s)?
> Sorry for the stupid questions..... It looks like if the last sage is off, not going to R7, it bypasses completely to the speakers for zero attenuation.
> Am I on the right the track? LOL
> 
> So this diagram below shows that switch one bypasses everything if wanted. I want switch one to either go to all of the stages or just the last
> like previously discussed.
> S



Mr Tornado,
The reason you are being confused is that this particular diagram depicts my own personalized version which does not allow for -3.5db attenuation all by itself. For that matter, the one that I'm currently using all the time does not even have that first, overall bypass switch. When it is plugged in, you are already automatically, by default, at -7db reactive attenuation. I opted for this configuration for several reasons:

_*A) *_The -3.5db cut is so small/subtle by itself that I deemed it to be able to be otherwise accomplished right at the amp through several other various methods. The main intention of this -3.5db stage was to *"fill in the gaps"* between the -7db stages, to make the individual jumps less drastic. It was my own (not @JohnH ) judgement call that if an attenuator was truly needed, at least -7db would almost always be the minimum requirement.
_*B) *_Even though this iteration is designed to be used out of the 16 ohm tap of the amp, I often use an 8 ohm cabinet and eliminating the bypass switch was intended as a safety/failsafe, to make sure I never accidentally run my 16 ohm amplifier output straight into an 8 ohm load! My advanced years have taught me to always allow for my brain occasionally passing gas! That's partly how I've been able to make it to this age! Subsequent builds will likely have a bypass switch, but will still incorporate the minimum -7db/reactive, default feature!
_*1) *_While considering *"brain farts"* we should ponder the fact that with all stages individually selectable, a person could inadvertently activate just one of the lower wattage rated -7db stages. *HEAT UP & GO BOOM! *A really good rotary switch could help here, but the search for the correct unit is a long, deep, dark rabbit hole that provides minimal, if not expensive results. Toggles do the job, but leave the operator with the need to be smarter than the box! Factoring possible operator error into the design/implementation is also not a bad idea, *IMHO!*​_*C) *_The above diagram also allows all resistors, other than the first *"ALWAYS ON"* stage to be both physically and electrically smaller. The other alternative could be to make every stage full wattage capable, but size could become an issue.
_*D) *_With or without the overall bypass switch, the above diagram sought to minimize the complication of wiring as well as the hands on operation of the switching. In other words, whenever the unit is active, the highest wattage portion is always active also!​Now, I belive that somewhere in this very lengthy thread (or maybe it was a personal email to me) @JohnH showed a way to increase the wattage of that -3.5db stage, to facilitate it's individual useage. The downsides of that scenario (for my tastes at least) were:
_*A) *_It required "non-use" of the of the reactive inductors in the -3.5db only mode.
_*B) *_If I recall correctly, it took away the option of being able to use an 8Ω load, as the -3.5db by itself reflected that 8Ω load unfavorably at the intended 16Ω input to this unit!​Sorry for the long rant! I just wanted to clear up and apologize for any confusion I may have caused by sticking my own  in here!

And really think about my considered opinion that if you are in a situation that attenuation is *"TRULY NECESSARY"* at least -7db will be the absolute minimum required and that the level of complication of making that -3.5db stage availble *"on it's own"* is simply not worth the effort, in the long run. This is especially true when thinking of the possibility of brains farting and maybe even someone else who doesn't understand, utilizing your rig!

Just My Multiple Piles Of $.02 & Likely Worth A Bunch Less!
Gene


----------



## matttornado

Thanks, Gene! yeah I think you're right that the first stage should automatically be on with -7db reduction right off the bat even though it'll take me down to 20+ watts. We all know that the difference in volume between a 100 watt & 50 watt amp isn't that much, right?

I was confused by the latest design "M" drawing with switch 1B, 4A, & 4B all clumped together - hard for me to follow the signal flow but now I don't even need to worry about it.

Should I be using ON - ON switches? I see people using on / off in some of the pictures.
Oh and can I use Cliff Marshall style "SWITCHED" jacks or do I need to use un-switched type? shouldn't matter, right?


----------



## JohnH

hi @matttornado , @Gene Ballzz makes good points.

Design M (see page 1 summary) is the most versatile because it does every 3.5db step and can run 4, 8, or 16 cabs in the 8 ohm version. But Gene's is the simplest and easiest to build and to use, if you don't need that first -3.5db stage on its own. Also, its absolutely essential to be able to follow the wiring and relate it to the schematic of whatever you build and, if the whole box starts with -7db always on, then there's nothing that can accidentally happen in use to the switches downstream that can hurt anything.

The toggle switches needed typically have 6 lugs and are in fact two switches side by side. The centre lugs connect to the upper or the lower lugs. To do the simpler version, you only need one side of the switches but you can link the two sides together with three wire links to double it up to get more current rating and reliability if you wish. Such switches may be called on-on or on-off, but if its six lugs and two positions it would be right.

Don't need switched jacks (I use one for the red-box section of my circuit though). Just basic mono jacks are fine for, but for the ones I bought I found the stereo version to grip the plug better.

ps... this thread has now had over 30,000 views, with more than 150 in the last 12 hours!


----------



## matttornado

Thanks again JohnH! 

Yes this thread is fantastic! Thanks for sharing and answering everyone's questions too! I should be starting my build within the next few weeks.
I still have a few parts to get and wait for all of my resistors to arrive. 

So you mentioned earlier that with the attenuation at its lowest setting, (the lowest volume), it can be used as a dummy load. Even if it normally would have sound coming out of it if a speaker were connected? 

The Marshall Powerbreak I had, when set to dummy load, would not put out any signal. That's why I'm asking.

I'm going to add a line out to my attenuator and might try slaving for the fun of it.


----------



## JohnH

yes well, when the attenuator is set to max attenuation, less than 0.1% of the power gets to the speaker. 99.9% of what the amp sees is the attenuator. So it then makes no significant difference to the amp if the speaker wasn't there at all.


----------



## JohnH

I thought it might be helpful to redraw Attenuator M, stripped back to minimal fetaures and less complexity, as discussed with @Gene Ballzz above:













Attenuator M-lite



__ JohnH
__ Aug 24, 2019






This is exactly the same as the full version, but without a third output and without bypass and with the -7db setting being the minimum attenuation. It's a very simple practical design, still with the same full range down to -31.5db. I use this max setting frequently late at night.

Also, maybe by looking at this diagram, it will be easier to see how the full featured version relates to it. Component references and all values are unchanged, except that the power rating for the -3.5db Stage 4 can be less in this 'lite' version. (also, Sw1 is deleted)

I kept two outputs shown. I think that is useful for running extension cabs, and its also another 'safety' reminder when setting up, as to which jack is in and which two are out.

One thing to note. I've drawn them with the order -7, -14, -3.5 in the three switched stages. There is a small electrical logic to this, and it is needed in the full version. But, that doesn't need to relate to the physical position of switches on the front panel - which is a free choice.


----------



## Gene Ballzz

Hey @JohnH

I've got what may be a dumb question, though I think there was brief discussion earlier in this lengthy thread. What happens if one of these attenuators gets used in parallel with the speaker load? Specific scenario, for discussion purposes: *16 ohm attenuator and a 16 ohm speaker plugged out of parallel speaker out jacks of the amp. This could be either two parallel jacks with a switch on the amp with switch set to 8 ohms or as on a DSL amp parallel out of the 8 ohm jacks or any other proper way to parallel the attenuator and the speaker, while having the amp set to the proper output tap for the type of use. 
*
I realize that there will likely still be some attenuation and possibly reactivity, but I'm guessing it won' be at all the same as running the attenuator in the more standard series manner! Would there need to be an added resistor at the output of the attenuator, to complete that circuit? Or.......?

Thanks For Any Thoughts? Just Curious Here?
Gene


----------



## JohnH

Hi @Gene Ballzz . I was just logging in to add something about exactly that scenario, and you went and raised it anyway! 

I think that could amount to a useful workaround if you do want to take just a few db off, and if the attenuator at hand doesn't have the -3.5db setting. Set the attenuator to max attenuation, so that it then can function as a reasonable load box with no speaker. So it will get half the power and the real parallel speaker the other half, being a -3 db attenuation. The amp will be perfectly happy with that. The tone should also be fine since this is a reactive box, It is designed to follow a real speaker impedance curve at least from low mids/high bass (say 150hz) upwards. The bass peak in the very low frequencies may be reduced by a few db.


----------



## Gene Ballzz

JohnH said:


> Hi @Gene Ballzz . I was just logging in to add something about exactly that scenario, and you went and raised it anyway!
> 
> I think that could amount to a useful workaround if you do want to take just a few db off, and if the attenuator at hand doesn't have the -3.5db setting. Set the attenuator to max attenuation, so that it then can function as a reasonable load box with no speaker. So it will get half the power and the real parallel speaker the other half, being a -3 db attenuation. The amp will be perfectly happy with that. The tone should also be fine since this is a reactive box, It is designed to follow a real speaker impedance curve at least from low mids/high bass (say 150hz) upwards. The bass peak in the very low frequencies may be reduced by a few db.



That, my good friend, is exactly where I was going with part of the question!  I was also considering the concept that the speaker is already, actually providing some reactance. My biggest concern was whether or not the output of the attenuator needed to be terminated in some way, or not?

I am also making an "assumption" that the nominal impedance of the attenuator should be generally trated as another speaker? So 16 ohm attenuator + 16 speaker needs to be driven from an 8 ohm tap and an 8 ohm attenuator + an 8 ohm speaker needs a 4 ohm amplifier output tap?

With that said, it is another example of why the 16 ohm version seems even more versatile, as many amps do not have a 4 ohm tap.

Thank You Again Sir!
Gene


----------



## JohnH

@Gene Ballzz , I dont think its necessary to plug anything extra into the attenuator when used as a load box like that, provided all stages are engaged. Without a speaker plugged in, it only makes a very small fraction if an ohm difference as seen by the amp.

Thats all true either for the ones I draw, with a -31.5db max, and also for yours where the -14 stage is subbed for another -7.

And I agree about the 16Ohm version, where that works with the rig. Its also good when uprated for 100W amps since it cuts current flowing through jacks and switches compared to the 8Ohm equivalent. In my case, I needed 8 Ohm build since my main amp doesn't have a 16Ohm tap unless I mod it. Also, an 8 Ohm version would work better with a pair of 8 Ohm cabs. So each user should pick their horses to suit their courses!


----------



## JohnH

*Talking about analysis....*

..so please bear with me here!

I designed this attenuator specifically around my Vintage Modern, and once Id homed in on how to find the best values, it works great, as it should, assuming my maths are correct.

But by now 8 months later, we have quite a growing collection of different builds made mostly by the good folks on this forum, and the design has been tested with multiple different amps at both 8 and 16 Ohms, including to our knowledge, several Fenders, and most of the key types of Marshall (including VM, DSL combos both old and new, Plexi's and now both new Studio amps. Different valves have been represented and different master volume arrangements NVM, MV and PPIMV, with and without negative feedback. All seem to work well. Why? Where are the limits? Its all good news but I've found this puzzling.

On the main circuit diagram, there are curves of frequency response, stepping through attenuation stages. You can see that they are consistent. They apply to small signals and are based on an amp output impedance measured from my VM, with an 'Aiken' model to represent speaker impedance. The trick is to match frequency response at the speaker with attenuation, with that of a fully connected full volume speaker.

But Im finding that having derived it for the measured output impedance of the VM of around 18 to 20 ohms (8 ohm tap), virtually the same consistency occurs if the amp is assumed to have either a lower or a higher output impedance, with the same component values. (This applies above about 150hz). This ability to adapt to different output impedances (and so different amps) and maintain consistency for that amp seems to be part of the key to this. 

But all of that is for small signals. When the amp is overdriven hard, its output changes enormously, tending to hit a hard limit and behave as a voltage source. This is hard to model fully. But an extreme analogy is to feed the circuit with transient square waves of low impedance. Here's my model, with some results, based on 2kz and -10.5db attenuation:













Squarewave Test 190825



__ JohnH
__ Aug 24, 2019






The model is of the type Ive been using to do the design throughout. It has two Aiken speaker models, signal sources and the attenuator, with some attenuator components at very high or low values if they are to be taken out of circuit. It's made in 5Spice software, but LTSpice would also do it.

I scaled the full volume amplitude so it overlays with the attenuated. Blue attenuated signal follows Red full volume very closely indeed. These are being fed out of a 1 Ohm amp impedance, like an amp so overdriven that its about to melt . But it matches just as well at different amp resistance too. The signal is getting through the attenuator and feeding the speaker, without changing its shape.

This type of model, is very sensitive for finding the optimum for R9 (R34 = 27 Ohm in the above, for an 8 Ohm build). The value adopted is ideal for a 10.5db attenuation. At higher attenuation, slightly lower at 22ohm is a bit better. But that is very close, you'd never hear a difference.


----------



## Gene Ballzz

Great Stuff @JohnH 

I realize that "standard" frequency for testing is often 1K and that your thoughts for testing may be to look at how smooth that 2K range is and that is certainly a valid consideration, but that frequency doesn't use involve much power/energy. It may be interesting look at the higher powered/harder working part of the spectrum, say down around the 200hz to 400hz region? That's where the tubes and iron get a good workout!  Easy to forget that the "fundamental" of notes on a guitar ends a little above 1.1khz, although most of the stuff guitarists obsess over can be well above that! 

Just Sayin'
Gene


----------



## Nik Henville

I
think
you just proved
that a good design is a good design is a good design...


----------



## matttornado

So at this point, I'm going to presume that if I had to pull two tubes in an emergency and run on just two instead of four, that I could set my amp to 8 ohms instead of 16 and still use the 16 ohm attenuator. Correct?

That was a good question too, Gene Ballzz! I ran my Powerbreak in parallel, set to dummy load, along with a 4x12. It knocked my volume down a little, nothing drastic though. I done that at few gigs when I still had my PPIMV installed.


----------



## JohnH

Hi Matt,
Yes I think that follows the usual advice about impedances when tubes are pulled.

If it was my amp, and in that situation due to some emergency, I think Id also take some pressure off the circuit by using a bit less attenuation so its not driven so hard. Nothing related to the attenuator, just being careful with the amp.


----------



## matttornado

JohnH said:


> If it was my amp, and in that situation due to some emergency, I think Id also take some pressure off the circuit by using a bit less attenuation so its not driven so hard. Nothing related to the attenuator, just being careful with the amp.



Yep! I agree! I can't wait to get all of my parts together so I can build this thing! I'm sure I'll have more questions when I'm ready to test.... It'll be a little while though.


----------



## Heph333

What I've found to work really well is a combination of tube adapter + attenuation. I'm a massive fan of Smicz adapters. They basically convert any power tube to 1w (single) or 3 watts (pair) using 6ak6 tubes. Cranked they sound like 6L6. Attenuating a pair of these will get power tube saturation, but with only a 5 watt peak load in your transformers.
Even the better known Yellow Jackets could accomplish the same if you like the sizzle of el84. Could drop a 50-100watt amp down to 18-20. Then attenuate that. Obviously you're driving the power tubes hard, so they will wear. But it's much easier on the trannies.


----------



## JohnH

*Analysis comparison: MiniMass and Attenuator M
*
Ive been designing the attenuators mainly using a circuit analysis, as shown in previous posts above. It uses an electrical model of a speaker at full volume and then through the attenuator, to compare the shape of waveforms and also frequency responses.

So I thought it would be interesting to do the same with another simple reactive design by way of a comparison.

Weber make some great products, and in various threads we are seeing good reports about them, particularly the MiniMass model. This is listed as 50W rating, suitable for up to 35W amps to allow for overdriving. I don't have one myself but by all accounts it's good value, well built and well liked. So its well positioned for all the 20W Studio amps.

*Weber MiniMass - Analysis*

The Mass series work reactively by using a cone-less speaker motor as part of the attenuation network, together with a high-power wire-wound pot.

Here is a link to a (non-official) schematic:

https://encrypted-tbn0.gstatic.com/...T8xqgRA2Xg15Y2AsJRV6wpzpBvAIzJzDRr9flIdFMu1k6

In the analysis, I'm representing the speaker motor with an Aiken speaker model, without the low resonance (which depends on the cone). Plots compare the frequency responses at full volume and attenuated -10db and -25db, with and without a +6db treble boost capacitor engaged (it also has a +3db treble boost):













MiniMassplot 190831



__ JohnH
__ Aug 31, 2019






The signal seen by a full-volume speaker is Red, the lowest (Blue) line is as seen by a speaker at -25db, and the Green line is the signal at the amp, with -25db attenuation. The lower of the two charts shows the same with treble-boost switch engaged. Also, similar plots at -10db are shown.

What can be seen is that, as the attenuation is increased, the usual bass peak and treble rise at the speaker, get flattened (although treble rise is partly maintained as seen by the amp). This is consistent with reports from users. Switching the treble boost does compensate at the speaker, but see how the signal seen by the amp (Green) drops right down, implying impedance to the amp is dropping at high frequency, due to a 'treble bleed' capacitor being added across the pot.

The plots are all based on the 16 Ohm setting. The unit also has 8 and 4 Ohm options which are engaged by putting fixed resistors in parallel with the input. This will add extra damping, making the net load more resistive than reactive.

*Attenuator M - Analysis
*
Here's the equivalent plot for the Attenuator M:













AttenuatorM Plot 190831



__ JohnH
__ Aug 30, 2019






Plots of speaker signals are all parallel at full volume, -10db and -25db, and signal at the amp is consistent above 200hz. This is suggesting a more consistent performance than the MiniMass. No treble compensation is needed.

So this is suggesting a good consistent result, which is what we find in practice, and likley more consistent than the Weber.

But of course, I may not be being fair to the Weber, since it all assumes that this analysis is actually a valid model, I don't have exact characteristic of the motor and in any case, Attenuator M has the benefit of having been designed using this model!


----------



## Nik Henville

More I read about 'em, more I want one.
Can't do the build malarkey - so how about:-

Anyone building one suitable for 100Watt RMS and 8.Ohm - build 2, yes... TWO, and charge ALL the parts to me, keeping one unit, sending one to me. Preferably Blighty based to avoid import duties and the like.


----------



## Gene Ballzz

@JohnH 
Great stuff! It's interesting that you chose -10db & -25db for your testing levels. Going by the positioning of the pot, I'm guessing that the range of -10db and less, along with -25db end up working out to a couple of the approximate *"sweet spots"* of operation that I've mentioned before. I'm going to guess that your numbers would look much *"uglier"* in the -15db and above -25db ranges! FWIW, with most amps there are only two or three of those *"sweet spots" *while a few amps seem to allow one or two more. And though I can't put my finger on exactly what the shortfalls are, in between those spots, the amps just seem lackluster and lifeless!

I'm also gonna bet that the impedance *"seen"* by the amp's output varies *"WILDLY"* with the sweep of the pot! Your designs seem to keep a fairly even (within a few ohms) impedance from one setting to another, with your 16 ohm, *"Design K"* seeming to be the most consistent!

While I don't have any sophisticated modeling/testing software, I do have a 50 watt MiniMass, various impedance speakers/dummy loads, a good Fluke meter and another OK EXTECH that I could take some readings with, at various settings? If this might be any help?

Let Me Know?
Gene


----------



## Gene Ballzz

Nik Henville said:


> More I read about 'em, more I want one.
> Can't do the build malarkey - so how about:-
> 
> Anyone building one suitable for 100Watt RMS and 8.Ohm - build 2, yes... TWO, and charge ALL the parts to me, keeping one unit, sending one to me. Preferably Blighty based to avoid import duties and the like.



Nik,

While that sounds pretty OK, it should be noted that these builds are fairly time intensive and I'd guesstimate the labor to have at least double the value of the price of all materials/components. And then, even with calling it all a "friends/family" arrangement, shipping may be fairly steep! While you don't tell us where you are, I'm kinda assuming somewhere in the British Isles? I would say that a wide ballpark would be a cost to you of between $150 & $200 US $$$ to get one to your door. Possibly even a tad more, given the extra size/weight of a 100 watt capacity build!

With all that said, used properly and as intended, these units are truly worth every penny of that!

Just My $.02,
Gene


----------



## JohnH

Hi @Gene Ballzz , thanks for the offer of some measurements in the interest of garage science!

The real variations in response and impedance come with frequency which are hard to meter, and I expect that what we are building tracks the impedance at mid to high frequencies much better, although the MiniMass does show the amp a rise in impedance with frequency, unlike fully resistive designs. But this rise appears not to transfer through to the speaker output, except at low attenuation, hence the treble switches provided. 

But with a meter, you can at least measure the dc conditions. You could try the following, which should give a check on my MiniMass model:

Set it to 16 ohms, and plug in a 16 ohm speaker of known measured ohms (usually about 13 Ohm). Then measure resistance at the input of the Minimass. Im expecting it will vary from about 10 Ohm at max and min, to about 16-17 Ohm max at around half turn? see what you get, and where on the pot turn is the max.

Then repeat without the speaker plugged in. id expect it to vary from 50 Ohm at min attenuation down to 10 ohm at max attenuation. 

Thanks!


----------



## Nik Henville

Gene Ballzz said:


> ...With all that said, used properly and as intended, these units are truly worth every penny of that!



Cold
not have
put it any better
than that, my friend.


----------



## TonyK

Nik, I'm basically blind without glasses, long-sighted, so that when my glasses move on my face everything goes in and out of focus all the time! Up close, this can be a nuisance to say the least. I have a neurological disorder that makes my hands shake a lot: annoying as this is when playing guitar, you can only imagine what it's like when holding a hot soldering iron in one hand and trying to hold solder at an electronics joint with the other! Then trying to do all of this in a tiny box... well, you get the picture. But if I was able to build this attenuator -- slowly for sure -- I'm sure that you could manage it. You might just want to consider it and have some fun as well


----------



## Nik Henville

TonyK said:


> ...You might just want to consider it and have some fun as well



Too
darned
busy running
really fast just to stand still financially.
There really just aren't enough hours in my days...
...and those few I do get spare tend to be spent playing guitar loud and proud


----------



## mike_lawyer

Hey guys, I am really impressed by all the work that has gone into this thread! I am planning on building one of the 8 ohm versions, and have a question about parts:

1. I am located in the US, is everyone purchasing wire wound resistors off eBay from China?

2. What size screws do you need to attach the resistors to the chassis?

3. What "thermal grease" is being used between the resistors and the chassis?

Thanks for all your help, I can't wait to start building.


----------



## Gene Ballzz

mike_lawyer said:


> Hey guys, I am really impressed by all the work that has gone into this thread! I am planning on building one of the 8 ohm versions, and have a question about parts:
> 
> 1. I am located in the US, is everyone purchasing wire wound resistors off eBay from China?
> 
> 2. What size screws do you need to attach the resistors to the chassis?
> 
> 3. What "thermal grease" is being used between the resistors and the chassis?
> 
> Thanks for all your help, I can't wait to start building.



Yes, FleBay is the cheapest/easiest source for resistors from Chinesiawanoreanam! You'll likely have to wait a couple weeks or so, unless you want to pay through the nose! Also, it pays off to check out many of the different vendors for variety, etc.

I use #6-32 screws and I drill/tap the fairly thick aluminum boxes I buy and then chop the screws to length. Makes for a fairly neat/orderly build. It's not unusual to have to drill out some of the resistors to fit these screws, but I think you'd not want to use any smaller!

Standard, silicone-based, heat sink compound does the job and is widely/easily available/cheap!

Though you didn't ask, the best selection/source in the U.S. for inductor coils is Madison Speaker Components https://www.madisoundspeakerstore.com

Have A Fun Build!
Gene


----------



## JohnH

Hi @mike_lawyer , great! and good luck!

(Edit: I see Ive crossed with Genes post above. Luckily its all consistent!)

Ive had good experiences with the chinese resistors, and have had no issues with them. At a dollar or two each for me in Aus, they halve the total build cost compared to local or western suppliers, and shipping is free. The orders go through and get sent quickly, but the journey can be slow, sometimes 3 weeks, but there may be a faster shipping option.

They mount neatly to a panel with M3 nuts and bolts. 10mm long is about the minimum but 15mm is less fiddle..

I dont know about the thermal grease products available in the US. So long as you use something intended for that purpose it should be fine. Mostly they are white, containing (I think) micaceous iron oxide. The tube I have now contains a silvery paste, filled out with aluminium dust. Just use the minimum amount for full contact.


----------



## JohnH

Id be interested to find out what toggle switches are being used by others. 

My current build uses full-sized toggles, bought locally. They look ok, work fine and have a good spec, but they were suspiciously cheap, less than $3 AUD retail. I found a couple that didn't feel right so I sidelined them. Then, due to a fumble, the box slipped off the amp and hit the floor. Two toggles got pushed in and then didnt feel right. I replaced them, but then opening up the damaged switches I found that the levers had punched through a rather thin internal bakelite panel.

Apart from using a better part, I think some kind of protection to projecting switch levers would be a good idea for units on the road, such as a projecting C shaped handle each side of the switch bank.


----------



## mike_lawyer

Thanks for all the replies, much appreciated! Does anyone have a favorite chassis for this project? I would like something aluminum and sturdy, but not too large.


----------



## JohnH

Aluminium is a good material, easy to drill and it has much better thermal conductivity than steel, plus it tends to be thicker. 

My tendency, every time I build, is to end up wishing I had a bigger case to work in. Mine is the full Attenuator M circuit, and its crammed into a 170x120x55 diecast Al case. It relies on the depth so that the coils can sit on the base, with the resistors mounted to the top and ends, jacks at the back, switches at the front. It would have been a more ideal case size for a 50W lite version.

But, once built, its all fine and barely heats up cranking a 50W VM.


----------



## Gene Ballzz

Here's the ones I use:

https://www.ebay.com/itm/Aluminum-DIY-Project-Box-Enclosure-Case-PCB-Amplifier-Electronic-200-105-55mm/142615103868?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

For a pic of how I laid it out, go to post #178. I have since pulled the one pictured apart, but general layout is still similar.

Hope This Helps,
Gene


----------



## tmingle

JohnH said:


> Id be interested to find out what toggle switches are being used by others.
> 
> My current build uses full-sized toggles, bought locally. They look ok, work fine and have a good spec, but they were suspiciously cheap, less than $3 AUD retail. I found a couple that didn't feel right so I sidelined them. Then, due to a fumble, the box slipped off the amp and hit the floor. Two toggles got pushed in and then didnt feel right. I replaced them, but then opening up the damaged switches I found that the levers had punched through a rather thin internal bakelite panel.
> 
> Apart from using a better part, I think some kind of protection to projecting switch levers would be a good idea for units on the road, such as a projecting C shaped handle each side of the switch bank.


https://www.mouser.com/ProductDetai...ha2pyFadujQBe3n7F%2B9Vvv1RYtAuHIjZfaEGn/ttN0=
I always purchase high quality switches. I also generally over spec them. I've replaced too many to remember at my job. I have a scrap microscope light that I am going to re-mount my build inside.


----------



## mike_lawyer

Gene Ballzz said:


> Here's the ones I use:
> 
> 
> For a pic of how I laid it out, go to post #178. I have since pulled the on pictured apart, bu general layout is still similar.
> 
> Hope This Helps,
> Gene



Thanks a bunch, I just ordered one of those boxes. Looks high quality and durable.


----------



## mike_lawyer

Gene Ballzz said:


> Here's the ones I use:
> 
> For a pic of how I laid it out, go to post #178. I have since pulled the on pictured apart, bu general layout is still similar.
> 
> Hope This Helps,
> Gene



Gene -

I looked at your post 178, it said you were having some serious heat issues with your build. Did you get that squared away?

Mike


----------



## Gene Ballzz

mike_lawyer said:


> Gene -
> 
> I looked at your post 178, it said you were having some serious heat issues with your build. Did you get that squared away?
> 
> Mike



Mike,
I'm not sure I'd call them *"serious"* heat issues, and even though it can get pretty hot, that is when I'm *"severely brutalizing"* the whole rig, much more than I would normally do in practicality! While it can heat up pretty well, it gets to a certain point and stays there, without getting any hotter. A couple thoughts for you:

*> *It would be a good idea to read at least a few of the posts following that #178, as there are some points fairly pertinent to the build you seem to be shooting for. One of those points is that in my future builds, I will be mounting the resistors on the inside of the top, instead of the bottom and that should improve the cooling substantially.
*> *I don't think you mentioned what amp you intend to use this with? It might help us help you, if we know this?
*> *As with all projects, you really want to embrace the *"Measure Twice, Cut Once"* concept here! Think really hard about where your switches and jacks want to be and especially whether you want it mounted into an amp or cabinet. For example, my next one will be intented for mounting inside my DSL20CR, at the bottom and I want the switches accessible through the slot at the bottom. This will require my switches to be on the "sides" as opposed to the ends. Of course then, the next one will be intended for installation in one of the Studio Series heads (not yet sure which one) and that will likely require some rethinking of the enclosure that I use and a redesigned rear panel for the amp!​And if you don't mind me asking, what general part of the planet are you located in?

Have Fun!
Gene


----------



## mike_lawyer

Gene Ballzz said:


> Mike,
> I'm not sure I'd call them *"serious"* heat issues, and even though it can get pretty hot, that is when I'm *"severely brutalizing"* the whole rig, much more than I would normally do in practicality! While it can heat up pretty well, it gets to a certain point and stays there, without getting any hotter. A couple thoughts for you:
> 
> *> *It would be a good idea to read at least a few of the posts following that #178, as there are some points fairly pertinent to the build you seem to be shooting for. One of those points is that in my future builds, I will be mounting the resistors on the inside of the top, instead of the bottom and that should improve the cooling substantially.
> *> *I don't think you mentioned what amp you intend to use this with? It might help us help you, if we know this?
> *> *As with all projects, you really want to embrace the *"Measure Twice, Cut Once"* concept here! Think really hard about where your switches and jacks want to be and especially whether you want it mounted into an amp or cabinet. For example, my next one will be intented for mounting inside my DSL20CR, at the bottom and I want the switches accessible through the slot at the bottom. This will require my switches to be on the "sides" as opposed to the ends. Of course then, the next one will be intended for installation in one of the Studio Series heads (not yet sure which one) and that will likely require some rethinking of the enclosure that I use and a redesigned rear panel for the amp!​And if you don't mind me asking, what general part of the planet are you located in?
> 
> Have Fun!
> Gene



Hi Gene, thanks for the info! I am located in Virginia here in the good old USA. I am going to be using this unit primarily with a 6V6 Plexi that I built about a year ago. I plan to use it beside the amp head, and not built into an amp. 

I like the idea of having the resistors on the top half of the chassis, that will allow for better circulation and cooling. I think I am going to place all of the switches on one end, with the input/output jacks on the other side.

I purchased the chassis and ordered the resistors from China. I ended up ordering all 100W resistors, which is probably overkill, but they were about the same price anyway. I still need to purchase the switches and the inductors. I am going to use that source in the US for the inductors.

Mike


----------



## JohnH

Hi Mike - all sounds good. what kind of power does your amp put out?

Its R1 and R2 that do most of the work in absorbing power, so to help spread heat, you can place them not next to each other, maybe with a less driven one in between. With all 100W resistors, they will add a lot of extra thermal mass which is good. it will take longer to heat up as so the average temperature that it all reaches will be over a longer time, so less max. Also, allow for lots of holes in top and bottom.


----------



## mike_lawyer

JohnH said:


> Hi Mike - all sounds good. what kind of power does your amp put out?
> 
> Its R1 and R2 that do most of the work in absorbing power, so to help spread heat, you can place them not next to each other, maybe with a less driven one in between. With all 100W resistors, they will add a lot of extra thermal mass which is good. it will take longer to heat up as so the average temperature that it all reaches will be over a longer time, so less max. Also, allow for lots of holes in top and bottom.



My 6V6 Plexi puts out about 22 watts, but it is the loudest 22 watts I have ever heard. That is great advice to spread out R1 and R2 to help dissipate heat. 

I would like to build a 1987 50 watt clone at some point, so this attenuator will come in really handy.


----------



## matttornado

Does anyone know or have an idea how many watts a full cranked metal panel 1959 Superlead actually puts out? It's over 100 watts, correct? I'm sure it varies from amp of the same model. Mine is a 1974 modified for EL-34s with 480 V on the plates.


----------



## matttornado

Check out this 100 watt/ 60 ohm resistor I got from China compared to the Arcol brand (16 ohm)! I'm a bit skeptical that these are actually 100 watt resistors. More like a 50 watt sized package, not 100 watt. 

If I have time, I'm going to pump 100 watts into these Chinese resistors (two 60 ohm in parallel = 30 ohms) here in the lab at work and see how hot they get. I wish I had a data sheet to go with these Chinese resistors....

A good Arcol brand resistor can cost an arm & a leg , especially for the power requirements we need for a 100 watt amp. One was Like 145 dollars from DigiKey.


----------



## JohnH

Ive not had any thermal issues cranking a 50W amp through this. The 50W spec , if running at 50W, puts 28W into R1, so there's a factor of about 3.6 against the 100W component spec. Yours is the same ratio. Itll need all the case contact for cooling though!


----------



## matttornado

I'm going to mount R1 resistors on a seperate aluminum plate first, prior to mounting to the case for extra heat sinking.


----------



## JohnH

I tried running the attenuator as a loadbox again yesterday, in parallel with a speaker, with amp set to half the speaker ohms. The attenuator was set to max attenuation It worked fine, nice and clear for a -3db reduction in speaker volume. I think thats a good trick!


----------



## Gene Ballzz

So, can I assume that while giving a -3db reduction, it ends up having all of the load and/or attenuation being reactive, as opposed to simply using the attenuator in between the amp and the speaker on a -3db, non reactive setting? If this is indeed the case, it is definitely a *VERY* _*"good trick!"
*_
Thanks Aagain @JohnH 
Gene


----------



## JohnH

Yes above about 150hz the amp thinks its driving a normal reactive speaker load of half the ohms and reacts to it normally, and out of the speaker comes the correct normal tone, at -3db reduction. 

What is lost however, and only in theory, is that the bass peak at the speaker is reduced by about 3db max, at about 100hz (depends on the speaker). But I don't think this really makes much noticeable difference - it only affects the fundamental tone of the very lowest guitar notes and not any of their harmonics. In any case, a tweak of a bass or resonance knob would fix it if wanted.

Interestingly, the -3.5db resistive reduction, if built, gives the right tonal shape including the bass peak, but the amp is not quite getting the full reactiveness to respond to dynamically.

But I couldn't tell any difference anyway. 


Try it and see what you think!


----------



## rockgod212

saw this thread, so I built a KF style air brake a while back. I use with my 50 watt 1987 NMV clone, love it.


----------



## JohnH

rockgod212 said:


> saw this thread, so I built a KF style air brake a while back. I use with my 50 watt 1987 NMV clone, love it.
> View attachment 61782



Thanks for posting that. It looks to be a nice build! When I was working out mine, I learned a lot by studying the Airbrake design.

Airbrakes are nice and simple. A good thing about them is that they don't get dull at higher attenuation, unlike most Lpad designs. Its because they reverse the Lpad, putting a resistor in series with the speaker instead of across it which would add too much damping. There's a sweet spot in its settings where the what the amp sees in terms of ohms, and what the speaker sees are similar to as if they were directly connected. At this point, the basic tone is in balance and close to that of a directly connected amp/speaker.

In my version, related to the Airbrake idea, this sweet spot is found at -7db in Stage 1. Instead of varrying the values to move away from this point, I added further stages to add attenuation while keeping this relationship between input and output ohms intact. This maintains consistent tone at each setting. Coils are then added, again in balance, to show the amp a correct reactive load without messing with what the speaker sees.


----------



## TonyK

Nik Henville said:


> Too
> darned
> busy running
> really fast just to stand still financially.
> There really just aren't enough hours in my days...
> ...and those few I do get spare tend to be spent playing guitar loud and proud


LOL, if I were busy enough playing guitar loud and proud, I probably wouldn't have put the time into building the attenuator either! But in my case, I am now able to play the guitar more frequently and with amp tones I can actually enjoy! Good luck!


----------



## JohnH

:This post is to assuage my OCD tendencies by exploring whether or not there are any further tweaks to the design values of components that could fine-tune its performance, and to explore its theoretical response in different scenarios to see how consistently it performs.

The core of the design is the first stage, particularly the two inductor coils L1 and L2, with L1 bypassed by a resistor R9, and how they interact with the resistor stages down-stream.

So this is mostly about the values of L1, L2 and R9.

*Short answer:* If you have one of these built to the current designs, its all good, no benefit in changing because any benefits from tweaks discussed below are typically only a fraction of a db.

But, we have to pick component values, so might as well pick the optimum, and sometimes the optimum is not available, or might depend on the use of the unit.

Reminder - here is the current design:












Attenuator M 190110



__ JohnH
__ Jan 10, 2019
__ 1






The values for 16 ohm and 8 ohm builds are in principal, a factor x2 different.
But above, the values of L1, l2 and R9 are 0.33mH, 0.5mH and 27 ohm for the 8 Ohm build, and 0.7mH, 1.1mH and 56 Ohm for the 16 Ohm build, so a bit more than x2. Also, @Gene Ballzz is having good results with 0.8mH, 1.2mH and 56 Ohm in his 16 ohm units.

So do these variations (20%) make a significant difference for good or bad?

*Benchmark reference: A 4x12 cab with G12M's*

To make any quantitative comparisons, we need a benchmark for reference, plus some criteria. I reckon the best target for response would be something like a classic 4x12 cab with Greenbacks in it.

Mike Lind on the TGP posted some measured load charts for various load boxes, plus actual cabs:

https://www.thegearpage.net/board/index.php?threads/attenuators-and-load-boxes.1947804/

So I traced his measured chart for a greenback cab (a 1960AX).

To bring it into a form I can use, I needed to represent this real impedance curve in the form of an equivalent electrical circuit. The best described is by Aiken, also referenced in a build thread for load boxes on TGP:

https://www.thegearpage.net/board/index.php?threads/aikens-reactive-dummy-load.1072793/

Direct link to Aikens page is at the start of that.

So I've been adjusting Aikens design to match Mike Linds measurements, like this:













G12m Ax CabZ2



__ JohnH
__ Sep 21, 2019






The schematic shows Aikens values for a 16 Ohm cab, with my amended ones in yellow above. This could form the basis of a reactive load-box to be built, as it was intended. But here, we don't need to actually build it. I'm just using the circuit in my analysis for virtual testing. Just as well, because to get that very sharp resonance peak, the required high-current capacitor is large and very expensive.

In my chart above, red is the calculated impedance vs frequency from the equivalent circuit, and green is a close trace from Mike L's chart. I reckon I got them very close indeed.

*Criteria and testing*

Back to the attenuator analysis. So using the response above as a target, the ideal is for the amp to see an impedance curve as close as possible to the equivalent virtual Greenback cab
, and the shape of the signal at the speaker also to match the benchmark, when normalised for volume.

Ive done a bunch of runs, based on 8 Ohm units, exploring L1, L2 and R9

Here is a plot of impedance, this time with the attenuator set at -14db plus the benchmark - scaled down by 2 for an 8 Ohm rig:













15db Zsp Vs Zin



__ JohnH
__ Sep 21, 2019






The values used were L1 =0.37mH, L2 = 0.55mH and R9 = 22 Ohms (equivalent 0.75, 1.1 and 44 in a 16 ohm build)

*Results*

In exploring many options, I've found a couple of consistent results:

1. R9 is very slightly better at 22 ohms than at 27 ohms. So we might as well use that (nearest would be 47 in a 16 Ohm build). So that can be the new default value although the difference it makes is less than 0.2db.

2. L1 and L2 keep consistently coming back to being in the ratio L2/L1 = 1.5.This seems to be more important than their actual values, within a close range.

So I tested the following L1/L2/R9
0.33/0.5/22
0.37/0.55/22
0.4/0.6/22
and also, for comparison with previous:
0.33/0.5/27

I was looking for consistency in the (somewhat arbitrary) frequency range 200hz to 7kHz, which is where these components have most influence, and being above the bass peak, which I am not controlling so closely. I was looking at signal level at the speaker and also at the amp output, based on a few different assumptions about the effective amp output impedance, the speakers and the amount of attenuation.

I tested at -7, -14, -21 and -28 db. I explored 16, 8 and 4 ohms cabs (with 8 ohm attenuator, the 8 and the 4 correspond to 16 and 8 respectively for a 16 ohm unit). With 8 ohm cab I took the amp effective output impedance at 20 ohms (per my VM) and also at x3 and x1/3 of that.
*
Conclusion*

Out of those design options across all those ranges, all the sets of values are very close to the benchmark, within 1.5db (and usually less) inside the frequency range. So they all work fine, and within about 0.3 db of each other.

The current L1 and L2 values of 0.33/0.5 were very slightly better than the others at -7 db, and also when a 16 Ohm cab is attached (using the dedicated output - diagram red box). Other than that, the higher value pairs 0.37/0.55 or 0.4/0.6 are fractionally better - but nothing much to chose between them.

I think it would not be possible to hear a significant difference between any of these options. The design seems to have hit a sweet spot, but it is a fairly flat one in this range.

In general, and also based on Genes experience, a great set of base values would be the 0.4 and 0.6mH for the 8 Ohm build, and 0.8 and 1.2mH for the 16 Ohm build. These are common values in most available product ranges. But if you pick a different value, try to maintain the 1.5 factor L2/L1.

R9, is slightly better at 22 Ohms (47 for 16 Ohm build)

But there's no need nor significant benefit in changing any built values.

*Some plots*

This plot shows full volume in red, signal at amp in green and signal at output in blue(normalised for volume), all at the -14db setting:













Attenuator Plots -15db 190922



__ JohnH
__ Sep 21, 2019






Top left is with the base assumptions, with very close matches throughout.

Lower left and top right show the same, with amp impedance x1/3 or x3. it is very interesting to see how the responses continue to track, illustrating how this design can respond to different amps and also dynamic changes in the amp.

Lower left is using half the cab impedance, ie a 4 ohm cab in an 8 ohm attenuator - a good results, a little bit brighter response as expected. This plot also shows what to expect from an 8 ohm cab into the 16 ohm attenuator.


----------



## JohnH

I've got a new idea for the front end of the attenuator circuit that reduces the number of coils needed, with the same performance achieved, and then leads to an enhancement with the abilty to add a resonant circuit.

To explain, it best to step through the journey so far. The following four circuits are based on the 8 Ohm design with four stages. They show the front end stage 1, then the other stages all consistent as per the current Attenuator M. Plots are shown of calculated frequency response.

Red is the signal seen by a speaker directly connected to an amp
The upper black and green lines are the signal at the amp feeding into the attenuator
The lower blue and black lines are the signal after the attenuator, using attenuation values of -7db and -14db.

Ideally, all these plots would be exactly the same shape, with the two attenuated plots being shifted down, and the upper plots of full-volume speaker and amp signal feeding the attenuator all identical. All should show the characteristic bass resonance peak, and the general rise in treble of a natural valve guitar amp driving a real guitar speaker

*Resistive attenuator


*

This is the simplest, and what is interesting about it is how it achieves a basically correct and very consistent tone at all settings, just with resistors. This is the necessary key to all these designs and is the main point of difference to most comparable commercial designs.

But, although nominally the right tone is coming out of the speaker, the amp itself is only seeing a flat resistive load. ie the upper curves below the red full-response curve are nearly flat, which means that the amp cant respond dynamically to the differences in impedance.

It still works though!

*
Attenuator M - reactive input stage
*
This is the current design since January 2019, now well tested by many builders. It captures the rise in treble impedance and lets the amp see this and respond dynamically to it. The bass resonance is developed at the speaker as in the resistive design, but is not seen at the amp. It continues to work well.




See how the green and black signals at the amp show a treble rise but not a bass resonance peak.

The basis of this is to split the treble inductance into two coils, one in series and one in parallel. This achieves a balance of impedances that allows further stages of resistive attenuation to be added, without affecting tone.

The coils are fairly compact and inexpensive, but to do this double arrangement while also capturing a bass resonance in each would be very expensive.


*Attenuator M2 - single treble coil
*
So here's the new idea. The front end of Attenuator M is amended so that the twin coil series/parallel arrangement is replaced with a single coil, and a resistor pair. This results in one less coil - simpler, cheaper and more compact. Plus, it appears that it will work just as well (not tested to date though):




(EDIT 14/10/19: R2B is better at 18 Ohms)

I don't see any down-sides to this variation. The numbers look very consistent and I believe it will sound virtually the same as the base attenuator M with twin coils.

*Attenuator M3 - with bass resonant circuit*

Having reduced the reactive circuits from two to one, it is now feasible to consider adding a bass-resonant circuit to it, like this:




(EDIT 14/10/19: R2B is better at 18 Ohms)

The circuit is based on getting close to the impedance of a a Marshall 1960 with 4x12 G12M's. Now the amp can see a bass resonant peak, and if it is driving hard, it should respond to it.

It should work fine. What I don't know yet is whether it is really worth doing this. The current circuits don't seem to lack for not having a resonant circuit. So this is just an exploratory proposal. Its not a cheap thing to add because to do the resonant circuit right, it needs capacitors and inductors with very low losses. Ideally, high quality bipolar film caps should be used, and two x 100uF may be needed to get the 200uF value specified. And the resonant coil should be iron core, preferably toroidal for very low dc resistance <0.5 ohm.

Interesting though I think! It would make the whole unit slightly better for use as a free-standing load box with no speaker, and might just add a few extra tads of authenticity to the bass-note overdrive tone.

*16 Ohm versions*

All the above is applicable at 16 Ohms, with inductors and resistors x2 and for M3, the capacitor value halved. This might make the cost a bit more feasible since one 100uF could be used instead of two to get 200uF.


----------



## Nik Henville

JohnH said:


> I've got a new idea for the front end of the attenuator circuit that reduces the number of coils needed, with the same performance achieved, and then leads to an enhancement with the abilty to add a resonant circuit....
> *[Magic aka Electrickery stuff*]
> ...The circuit is based on getting close to the impedance of a a Marshall 1960 with 4x12 G12M's. Now the amp can see a bass resonant peak, and if it is driving hard, it should respond to it.
> 
> It should work fine. What I don't know yet is whether it is really worth doing this...



Blimey - now _*THAT *_is impressive stuff...


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## Jordan Prysmiki

Hello John. I just purchased an amp with a reactive alternator built in. 15watt Frenzel HBX AC15. Friggin awesome amp by the way. The switch goes full power, half, and 1/3 power.

Wondering if you could help me figure out how to add inductors in to make it reactive?


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## JohnH

looks like a fun amp! Im not seeing the built-attenuator described on the website. Do you have a link?


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## Jordan Prysmiki

You know that's funny because I saw they offered three attenuator as a paid upgrade on eBay but don't mention it on their site.. 

They unfortunately would not send schematics either but I can maybe snap some pictures if that would help.


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## JohnH

OK, lets have a look, but Ill probably end up suggesting to make a separate attenuator, based on whichever one of the 4/8/16 output options suits you best, then ignore the built in one.


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## tristanc

Hi John, just a quick note to say thanks for all your work on this. I've just today finished your version M, before your recent updates (already had the parts). Works really well so far - great being able to blend between pre- and power-amp distortion easily.

I reduced the resistor wattages as my amps are 18W and 2W... And got the closest inductor I could for L2 (0.47). Other than that, it's stock in a Hammond enclosure.










Tristan


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## JohnH

A nice build! Thanks for sharing.


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## Jordan Prysmiki

tristanc said:


> Hi John, just a quick note to say thanks for all your work on this. I've just today finished your version M, before your recent updates (already had the parts). Works really well so far - great being able to blend between pre- and power-amp distortion easily.
> 
> I reduced the resistor wattages as my amps are 18W and 2W... And got the closest indutor I could for L2 (0.47). Other than that, it's stock in a Hammond enclosure.
> 
> View attachment 62590
> 
> 
> View attachment 62588
> 
> 
> View attachment 62589
> 
> 
> Tristan



Nice work bro!


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## JohnH

Ive been crunching more numbers on the M2 and M3 versions posted above. This confirmed what I posted, now checked at all attenuation levels and at various speaker ohms and amp output impedances. .

What I do is to put the whole circuit including a speaker sim into a dedicated spreadsheet that crunches all the resistances and reactances. Then I check it against a Spice model. Once that's working, i run macros to step through all the scenarios of stages on or off, with both 8 and 16 ohm speakers. I can then see plots of all the performance at once and quickly fine-tune the values. R2B got tweaked from 15 to 18, which I noted in the earlier post from last week.

Anyway, it's looking like M2 is definately a good alternative to version M. It performs at least as well and is a bit simpler and cheaper.

Version M3 is intriguing. It puts in the bass peak as seen by the amp, but this is already there in all versions at the speaker. But I might not be able to resist getting the big capacitors and iron-core inductor, just to try it, even though the cost of those parts alone more than doubles the cost of the whole build . 

Another way to assess this, requiring ear protection and an empty house, is to do more listening/recording tests with the current build, at full and attenuated volume to try to identify any difference in bass overdrive tone.


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## matttornado

Hi JohnH. 

I'm still gathering some parts.... question for you?
How come R2 is rated for 50 watts but R9 only 10 watts? They are in series, right next to each other so I'm just wondering if R9 should be at least 50 watts as well? 

Or does the L1 Inductor in parallel with R9 take up some of the power? 

Almost ready to start building!


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## JohnH

hi @matttornado , that's a good question. Actually neither L1 nor R9 dissipate much power. Most of it goes straight through them without stopping, just like a piece of wire can send a large amount of power through without getting very hot itself. And at say 500 hz, where most of the power is, L1 has an impedance of only about 1 Ohm, so it effectively shunts R9, bypassing it. Its only at high frequency that L1 has a higher impedance, channelling more of the current through R9

Good luck with the build!


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## matttornado

Thanks. That's what I kind of thought but I wanted to asked just to be sure.

Oh one more question for now if you don't mind?

I'm sorry if this question is a repeat.  I'm building the 16 ohm "amp tap" version. The diagram states I can use either a single 16 ohm cab or two 16 ohm cabs that equal 8 ohms or a single 8 ohm cab, correct? If the amp is set at 16 ohms and I'm running an 8 ohm load, isn't that a mis - match in the wrong direction? I understand an amp can be set on 8 ohms going into a 16 ohm cab but not the other way around.

Does the circuit compensate for that mis-match?


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## JohnH

matttornado said:


> Thanks. That's what I kind of thought but I wanted to asked just to be sure.
> 
> Oh one more question for now if you don't mind?
> 
> I'm sorry if this question is a repeat.  I'm building the 16 ohm "amp tap" version. The diagram states I can use either a single 16 ohm cab or two 16 ohm cabs that equal 8 ohms or a single 8 ohm cab, correct? If the amp is set at 16 ohms and I'm running an 8 ohm load, isn't that a mis - match in the wrong direction? I understand an amp can be set on 8 ohms going into a 16 ohm cab but not the other way around.
> 
> Does the circuit compensate for that mis-match?



No problem, thanks for asking. I appreciate that you need to be sure.

Its all OK. The short answer is that most of the power is being absorbed in the attenuator and only a small part gets to the speaker. So if that speaker has a different Ohms, only a small part of that difference reaches back to the amp.

The long answer is, every possible setting has been calculated, taking into account the reactive and resistive properties of the speaker and the attenuator. Here are the results for each setting, with impedance calculated at a mid frequency of 440hz. The upper table is with a 16 Ohm speaker and the lower is with 8. It stays close enough to 16 Ohms. The yellow column is for an 8 ohm cab into a 16 Ohm attenuator and 16 amp tap. It varies from about 15 to 17 ohms. This table assumes step 1 is -3.5db, per the full circuit. When with a 16 ohm cab, this first setting is 19 Ohms to the amp, still close enough to 16.




The other things you can see here are the steps of attenuation and the consistency of the response as you work the switches. With the 8 ohm cab, there is a db or 2 more high treble and bass, which you may or may not notice.


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## matttornado

Wow! Thank you so much, JohnH! I appreciate you taking the time to answer and explain it.
Should be starting the build next week sometime. Almost ready.


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## Gene Ballzz

JohnH said:


> Red is the signal seen by a speaker directly connected to an amp
> The upper black and green lines are the signal at the amp feeding into the attenuator
> The lower blue and black lines are the signal after the attenuator, using attenuation values of -7db and -14db.



@JohnH 

I am confused by the above quoted portion of your post #529? Of particular confusion is the phrase: _*"The upper black and green lines are the signal at the amp feeding into the attenuator" *_While I understand the general "jist" of it, I don't understand the difference of what the green line depicts, as opposed to the upper black line?

Please Illuminate?
Gene


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## JohnH

Hi @Gene Ballzz 
All those plots come from a Spice simulation, which is a modelling software. Its very powerful and user friendly with regard to what it can calculate, but the version I use has a few constraints on how it can plot, hence a bit of ambiguity creeping in! With these circuits, once I've figured out a general arrangement, I like to write my own spreadsheet from first principles, so then I get total control.

Anyway, those lines: The plot overall is representing what you would expect if you fed a sweep frequency through the amp, and put an oscilloscope or signal trace on the speaker, or on the output of the amp. The signal at the speaker is what the speaker reacts to and what we hear. The signal at the amp output shows what the amp is seeing and what it reacts to. Plots are provided at full volume and at two attenuated levels.

The red trace for full volume, is obviously what the amp outputs and this equals what the speaker sees, so one trace, and it represents the benchmark for comparison. 

The lower two traces are the speaker signal at two attenuated levels. You can see that the volume is lower, but the general shape is close to that of the red trace.

Now the actual answer!: The two upper black/green traces are the *signal coming out of the amp at the two attenuated levels* that are included in these particular runs. So it is high volume, but not necessarily the same shape as that of the red trace. The shape of these curves shows what the amp feels, and the ideal is that they are the same shape as the red.

So, with the all resistor version, although we get basically the right response at the speaker, avoiding the wet-blanket effect of most resistive designs, the amp is only seeing a mostly flat response as it interacts with the attenuator. The green line is with the lower attenuation, and you can see a bit of the rise and bass bump coming through but not much.

Then go to design M and the new M2 which is similar, you can see the upper green and black rising with high treble to match red. This pulls more out of the amp at the higher frequencies, making the amp react more, particularly if you play harder where the power stage then compresses more, which dynamically changes the response until the note decays. I think that is the main thing that our design does, and the benefit of a reactive system (thanks for being the first to really notice it earlier this year!). This effect is working well to match the upper curves from around 150-200 hz up.

The with the speculative M3 design, the same is added to the bass response. I think this is much less important to tone, and much more expensive to capture! I'm still thinking about how to confirm this.


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## Gene Ballzz

@JohnH
Thanks for that clarification, always nice to be speakin' the same language, so to speak! 

With that said, I think the graphs of the response show the most *"pleasing"* curve with the "M" version. Looking at it and thinking about it, it acts almost like a *"loudness"* switch on a stero. It allows for a slight accentuation of highs and lows at lower volumes. I see this as a beneficial byproduct of that particular design iteration!

I've been using mine, in anticipation of subsequent builds, I've been trying to decide about what (if anything) to change or do differently and decided no change was needed except for changing one of my two -7.0db stages to a -14.0db. I'm simply amazed at how natural and consistent all my amps sound through this attenuator at a widely varied range of volume levels!

I'm just now working on an updated, lower profile layout for at least one of my next builds. By the time I'm done, every one of my amps will have one of these (or similar, yet trimmed down version) installed in it. I may even install into a couple speaker cabinets!

Thanks Again John! 
Gene


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## JohnH

Hi @Gene Ballzz , 

I use my -14db stage most of the time, to get my 40/50W amps down to home volume.

I wonder, if you are looking to build a refined, slim-like build, you'd be interested in trying the M2 design? It's the same performance as M, but with just one coil, so cheaper and more compact. If so, we could work on fine-tuning the values for your use, using components from your preferred sources. Nothing wrong with the basic M though, if you prefer.


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## Gene Ballzz

Hi @JohnH ,

Yeah, with my DSL20 I toggle between -10.5db & -14db and with the Tweed Deluxe between -7db & -10db.

I guess I'd just need to know the values (wattage and ohms) of R1, R2A, R2B & L1, for a 16ohm build.

Thank You Sir!
Gene


----------



## JohnH

Gene Ballzz said:


> Hi @JohnH ,
> 
> Yeah, with my DSL20 I toggle between -10.5db & -14db and with the Tweed Deluxe between -7db & -10db.
> 
> I guess I'd just need to know the values (wattage and ohms) of R1, R2A, R2B & L1, for a 16ohm build.
> 
> Thank You Sir!
> Gene



Hi Gene,

I reckon, for a 16 Ohm build, Design M2 and up to a nominally 50W amp:

R1: 30 Ohm, 100W
R2A: 47 Ohm, 50W
R2B: 39 Ohm, 25W
L1 1.8mH (eg Madisound 19AWG, or 18AWG if from elsewhere)

They all allow for a factor of x2 on power plus and extra x1.5 to allow for overdriving

R1 is as before for Design M. 27 Ohm is also an alternative for it (loses about 0.5db) 

All other values as Design M, and can be 25W if you are skipping the '-3.5 db on its own' option.

(Note to others: if an 8 Ohm build, Its looking like
R1 = 15
R2A = 22
R2B = 18
L1 = 0.9mH)


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## matttornado

Finally got some parts in and some work done!

Working on the enclosure now. This will drop right in with four bolts through the bottom.
Inductors will be mounted on the side.

R1 is a 250 watt 30 ohm! Hopefully that'll handle a cranked 100 watt metal panel Super Lead?

I have never seen a resistor that big in any attenuator LOL

More pics to come. Hopefully I'll be doing some testing this weekend.


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## JohnH

Nice! That sure is one BIg Mutha of a resistor!

The trick will be getting the heat out of it, so lots of ventilation needed. Is there thermal grease between resistors and base?

It'll be very interesting to see how hot you can get it with that Plexi. Definately there's plenty of electrical power rating in those parts. And they can work at high temperatures too, more than you can touch. It'll take quite a long time if cranking to approach it's max temperature. On mine, the main resistors are direct to the thick aluminium case, and it takes about 30 minutes to get to max, which isn't very hot with my VM combo.

A guide to temperatures, if you can touch it for 3 sec, it's not too hot. With these simple conservatively -specced components, there's nothing that can suddenly blow.


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## matttornado

Yes I use thermal grease for each resistor. Here at my work, we use the 130 deg. F (54.4 Deg.C) as a max temp. for being able to touch something. I thinks it is an industry standard? 

Anyway, Yeah I can't wait to see how hot that big old 30 ohm gets. Hopefully I can finish wiring it up today or tonight and start taking some preliminary measurements before firing it up. Then, I'll have to finish drilling some holes in the enclosure.

I will be honest though- I will be very nervous trying it for the first time.


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## matttornado

When I add a line out, does the ground have to be isolated like the rest of the circuit (plastic jacks) or can I use chassis ground?

The volume pot for the line out is metal and will be chassis grounded. I was thinking of using a standard metal jack for the line out too.


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## JohnH

That's looking really good.

Down here in Australia, we call 54 degrees C a 'warm summer day'.

Im not sure what standards apply in industry, Im not in that field myself. But, Id say something that is inside a box would not need to apply such a consumer standard for the safety of touching it. There are plenty of much hotter things inside many appliances. I got an IR thermometer for some projects I run with engineering students, and its a great tool and not too expensive. Maybe you have access to one?

On the line out...hmmm...not sure. So far the chassis is not connected at all, but if its connected to your output ground, then it could be connected also to the amp tranny depending on how you wire the line out. A couple of other guys have used a line-out with this circuit, perhaps we could get advice as to how it went.

You'll have a pot, plus I assume a pad resistor in line with it? I think I would do a pad resistor at both ends of the pot, so there is no direct connection from pot, through the circuit, to the amp. I reckon get the attenuator going first, then experiment with caution.

As for the first test, its ok to be nervous. but I feel reassured once ive checked the wiring (again), if I can measure a reasonable resistance, with speaker plugged in the amp input. That way I know the amp cant get hurt.


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## tmingle

I used metal jacks & an aluminum case for mine. I have had no issues.


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## dudu

@JohnH, many thanks for putting this together! I have a couple of questions:

1. For a 100W amp using the 16 Ohm tap, I presume I need to double the power rating of each component yet again, is that correct?
2. Is the impedance curve (seen by the amp) in the graph only modelled in Spice or also measured, e.g. with the Dayton Audio DATS?


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## matttornado

tmingle said:


> I used metal jacks & an aluminum case for mine. I have had no issues.



Thanks tmingle! Is your attenuator circuit like the one mentioned in this thread w/ isolated ground with plastic jacks? 
Did you just use a metal jack for the line out?


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## matttornado

I finished wiring it up and took some preliminary resistance measurements with a 16 ohm resistor load prior to firing it up.
Here are my measurements:

I think I thought of all of the possible combinations?

Next I gave it a test run. IT SOUNDS AWESOME!!!! Even with the first stage dropping the power down by ~80%, it is still very loud, probably too loud for a gig. It must be more than 20 watts. I think my 100 watt amp is putting over 150 watts into the attenuator.
After playing for about an hour with maximum attenuation, ALL of the resistors got very hot meaning that I could touch them but only for a second.

I still need to add the line out and waiting to confirm how it should be grounded. Also, I'm thinking about adding a fan using the Marshall Powerbreak schematic. The fan is tapped right off of the input and rectified. Looks pretty simple. 

Not sure if I need to do that or not yet. I need to figure out what too hot is and if it is needed? 
I'd like to add some ventilation holes to the case as well but I don't feel like taking it apart now that it works so well! LOL


----------



## matttornado

Here is the Line out I will be using. I got this from the MetroAmp forum many years ago but never tried it.


----------



## JohnH

Hi @matttornado , that's great news and an awesome looking build! The resistance readings are totally looking right on the money given normal tolerances. I'm really glad that its working so well and I appreciate how carefully you have built it and I thank you for trusting the design.

On the temperatures, you are cranking a 100W Plexi and I reckon that attenuator is just cruising nicely. If you can touch the resistors for a second, then it would mean they are considerably less than 100C, eg, as an example of 100C, you cant touch a metal pan of boiling water without pain? 

When it gets a lid, vents are needed and air in through the base and out through the top is best. but I also think that self-powered fan idea could be a very good one to explore too, for a 100W build. Need to experiment to set it up right. I think it could be set so it only starts turning if the power gets up above say 30-50 W.

The line-out circuit looks right, and you might like to try it either at the input or the output. At the output, obviously its then on an attenuated signal but it'll pick up some more of the characteristics of your speaker, particularly the low bass resonance. But, we also know from @tmingle 's build that it works at the input too. At the input, you then have the option of not using a speaker at all, just set it all to max attenuation and use it as a load box. At -31.5db, the amp won't see any difference.


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## JohnH

dudu said:


> @JohnH, many thanks for putting this together! I have a couple of questions:
> 
> 1. For a 100W amp using the 16 Ohm tap, I presume I need to double the power rating of each component yet again, is that correct?
> 2. Is the impedance curve (seen by the amp) in the graph only modelled in Spice or also measured, e.g. with the Dayton Audio DATS?



Hi dudu, thanks for your interest. 

My numbers posted on this thread are for use with 50W amps and allow a factor of 1.5 for overdriving above 50W, then another factor of x2. The power ratings don't depend on the Ohms value, so double the ratings I list for use with 100W amps, either 8 or 16 Ohms. So for example, R1 gets uprated to 200W, which could be made from 2x 100W resistors, or as matttorando has done, one huge resistor.

I don't have any lab test gear other than a meter. My charts used to develop and present the design are calculated using Spice, or spreadsheets that I have written based on the same theory (and checked against Spice). There are also real frequency response tests which are miced recordings, then analysed using Audacity. 

But, to set up my analysis, I am using a real measured impedance curve as a reference, that came from Mike Lind on the TGF. Its based on impedance and frequency measurements from a real 4x12 greenback cab. I set up an equivalent circuit to match it very closely, then use that in my calcs to represent the speaker and to compare with.


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## yogi.1026

Hi JohnH and MF'ers, After following this thread closely I decided to bite the bullet and build this design. I stole Gene Ballz idea and went with a 50w/16 Ohm, three 7Db stages with bypass. I have a DSL 50 and 2266 Vintage Modern, with a 1960ax greenback cab, I was fortunate as this design was made with these amps in mind. I ordered all the parts from china, as I live in Western Australia, (3,500km from anywhere) and local stores only stock the same anyway. Everything was at my door within 3 weeks! After reading Genes post on temps I got a little worried, I did some reading of spec sheets and found that the 100w resistors unmounted are only rated at 40w, they dont rate at 100 until they are on a heatsink with 291sq/in.(12x12x1/4in) With this in mind I though I would seperate R1 and R2 onto seperate 3mm plates with HD heatsinks (from a old car amp)mounted underneath. I thought this would help even if techicnaly the only increased the rating by a couple of watts, I used a 200x145x55mm aluminium box and drilled top and bottom for some convection airflow. The 2266 amp LDR vol 8 for extended sessions only get warm to the touch at the resistor, the outside box is cool. Probably way overkill, but I'd rather be safe than sorry. I originally ordered 25amp toggle switches but in testing before use I was getting weird readings, I tracked this down to the DPDT bypass switch. One side wasn't contacting properly, this was at the time John reported that he broke a toggle by dropping it so I went with the rockers,(white were on sale 50% off) bit more involved than drilling for toggles, but hopefully bypass that problem if you are moving gear around all the time. Hows it sound? I was nervous at first(actually googled witch transformer was cheaper)so I went with the DSL. I was always happy with the sound of this amp, It was a case of going from vol1.5/2 to around 4,5/5. this filled the sound out ,less fizzy, more sustain, improvement but not outstanding, moving to the 2266 wow! this is more like it. Bear in mind I'm not going for super low bedroom levels, just to get some power tube goodness, If anyone is one the fence building one just do it. Parts are reasonably cheap, all parts minus the inductors were under US40$. I had to source my inductors locally( many thanks John), still worked out cheaper than China. The only caveat is the actual time to build, I made mine harder by fabricating the heatsinks salvaged from an existing amp, If I did this again I would use the smaller heatsinks mounted under plates with standoffs to secure to the box. Sorry for dribbling so much in a first post but I use this forum as a fantastic resource and would like at contribute something back FWIW. Many thanks to JohnH and contributors to the thread for all the different ideas. Some photos attached.


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## JohnH

Hi @yogi.1026 Thanks for posting, and thats a very nice build! Shame to put the cover on it.

Those rockers look to be a good option. Some more work to make the holes, but if looks like the final edges then get neatly hidden by the outer flanges of the switches?

I totally agree on the VM. You are the only other guy I know of who has run this with a VM.


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## yogi.1026

Hi JohnH, I forgot to add that the hiss reduction in the HDR is worth the admission price alone, even my 'special' 7025 ax7 tube didn't completely get rid of this. Very quiet with the attenuator. I also may have been a bit harsh on the DSL, having a chance to play with the settings brings out the best in this amp, definitely worthwhile. On LDR I found using the 14Db slightly better as this channel needs to be cranked on 21Db. I dont know if they all do this or it's just mine?(big vol difference in modes)


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## matttornado

Hello again.

With no load plugged into the speaker outputs, I measure 30 ohms across the input. Is that correct & Ok if used as a load box only?

Looking at the diagram I posted of the Line Out, it appears the the ground goes straight from the attenuator's input straight to the Line Out output (purple wire).
So if I use a plastic output jack like all of the others, the ground should not touch the enclosure. The pot is not connected to ground at all from what I understand by looking at it?


----------



## JohnH

Hi @matttornado 

30 Ohms would be the expected reading with no speaker, and all attenuation stages off. But to use it as a load box it needs to have all stages on, to max attenuation. Then you should find that the input reading is close to the nominal 16 Ohms even with no speaker.

The pot case is not connected in the circuit, so that can mount to the chassis with no issues


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## matttornado

I wired up the line out like the one I posted but it did not work correctly. I finally figured it out. Lug 3 must not be connected (grounded) to the pot & the ground from the attenuator's circuit gets connected to lug #3 instead. 

At first I connected it right from the amp's input but it was very powerful so I moved it to right after the reactive stage instead and that tamed it a lot.

Also, I believe an audio taper pot would work much better, making a more of a gradual increase in level, & would work fine connected to the amp input jack.


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## JohnH

Thanks for catching that, and I agree about the line out. Sorry I didn't spot it myself!


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## tmingle

matttornado said:


> Thanks tmingle! Is your attenuator circuit like the one mentioned in this thread w/ isolated ground with plastic jacks?
> Did you just use a metal jack for the line out?


Yes. Metal jacks for all.


----------



## tmingle

JohnH said:


> Hi @matttornado , that's great news and an awesome looking build! The resistance readings are totally looking right on the money given normal tolerances. I'm really glad that its working so well and I appreciate how carefully you have built it and I thank you for trusting the design.
> 
> On the temperatures, you are cranking a 100W Plexi and I reckon that attenuator is just cruising nicely. If you can touch the resistors for a second, then it would mean they are considerably less than 100C, eg, as an example of 100C, you cant touch a metal pan of boiling water without pain?
> 
> When it gets a lid, vents are needed and air in through the base and out through the top is best. but I also think that self-powered fan idea could be a very good one to explore too, for a 100W build. Need to experiment to set it up right. I think it could be set so it only starts turning if the power gets up above say 30-50 W.
> 
> The line-out circuit looks right, and you might like to try it either at the input or the output. At the output, obviously its then on an attenuated signal but it'll pick up some more of the characteristics of your speaker, particularly the low bass resonance. But, we also know from @tmingle 's build that it works at the input too. At the input, you then have the option of not using a speaker at all, just set it all to max attenuation and use it as a load box. At -31.5db, the amp won't see any difference.


I use the line out into impulse responses & have tried to match the miced sound. It can get pretty close with right impulse. I do get better results with some IR's that I purchased vs some of the free ones that are out there. I am trying to learn sends etc. in Reaper to allow for adding reverb etc. I do believe that with the right setup that you can actually create a virtual room & place the speaker wherever you want it in the room. I suck at it, but it beats watching TV.


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## 6StringStewie

JohnH said:


> Id like to ask for a bit of help from anyone building the attenuator shown above, either now or in the future.
> 
> ...Any interest?



Hi John. I am interested in building an attenuator for my JCM2000 TSL122 (100W 2x12 combo.) The amp has two Celestion V-Type speakers (8 Ohms, 70W each) in series. I would like to attenuate to 20W, 1W, and possibly a 1/2W output. I read that you have a spice model that you can run numbers in. Do you need any other measurements to run spice for my amp?

Would there be any modifications to values needed if I replaced the 70W speakers with 50W-60W speakers?

Looking forward to a fun project!


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## JohnH

Hi @6StringStewie 
I've got models all set up. But what I reckon would suit you would be a 16Ohm version of design M (tested many times), or the recent M2 (simpler, should perform the same but not tested yet!). If you don't need it, you can miss the full bypass switch. So from 100W, you'd get 20, 9, 4, 1.8, 0.8, 0.35, 0.16, 0.07 W using three switches. Might as well have a nice consistent range of equal db steps.

Or if you really just want fewer and specific steps, then two switches and I can work out values.

The speaker choice doesn't really affect the design, so long as it adds up to 16Ohms . But power ratings should be x2 compared to the 50W versions. I'd think though, that with a TSL, it's not needed to crank it fully to get it's best tone.


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## 6StringStewie

I finally got through the whole thread. I will have a notepad and pencil ready for the 2nd reading.

I am curious about Attenuator M3 with the bass resonant circuit. As I understand what you have posted, the bass resonant circuit is feeding the amp the bass response it is expecting to see of a speaker directly attached. Is that correct? What is the difference in behavior of an amp with and without the bass resonant feedback? Will this correct the bass dB loss at the speaker? Would it be cheaper to build a bass boost circuit instead?

I found this Jantzen toroidal inductor that looks like it might work.

Stewart


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## 6StringStewie

I have a question about the M model and the heat generated by R1. I could either get a 200W or greater 30 Ohm resistor, or use two 100W or greater 60 Ohm resistors. Let's say I get two 250W 60 Ohm resistors with heat sinks. By dividing the circuit between two resistors of a larger wattage, would this help tame the heat and keep the enclosure cooler?

Stewart


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## JohnH

6StringStewie said:


> I finally got through the whole thread. I will have a notepad and pencil ready for the 2nd reading.
> 
> I am curious about Attenuator M3 with the bass resonant circuit. As I understand what you have posted, the bass resonant circuit is feeding the amp the bass response it is expecting to see of a speaker directly attached. Is that correct? What is the difference in behavior of an amp with and without the bass resonant feedback? Will this correct the bass dB loss at the speaker? Would it be cheaper to build a bass boost circuit instead?
> 
> I found this Jantzen toroidal inductor that looks like it might work.
> 
> Stewart



Whether that bass resonance circuit is worth doing is still moot. Here are my thoughts so far about it:

The basic circuit (ie no bass res circuit) already corrects the output at the speaker to give a close match to a full-volume bass response, for all signal levels that are clean or not very distorted. This happens by the natural interaction of your real speaker with the resistors. So there's no benefit in adding EQ, since it's already basically right. You can see the bass peak in all output levels at the speaker.

But if you hit a bass note hard at high volume, it could saturate the output stage adding extra bass harmonics and compression, which then will decay dynamically. This is enhanced by the high bass resonance impedance, which will draw more from the amp at these frequencies. This may be audible, or it may not be. Design M3 should capture this accurately and feed it all through to the speaker. Design M or M2 will feed through a cleaner bass note, still of appropriate volume but with less bass flub/ harmonics. My belief is that M3 is more accurate, but M and M2, which will draw less from the amp at bass frequencies, will be like having a bigger more solid amp output with more headroom, for the bass frequencies. Ie, it could be a good thing.

The bass circuit is only active up to say 150Hz, and mainly only around a very few notes on the low frets of low strings. It's irrelevent if not playing those notes, and I suspect that, given the basic response is right without it, those bass harmonic variations are to some extent a quirk, and not fundamental to the character of the guitar amp tone which is all about low mids, mids and highs

As you've seen, the inductor and cap are not cheap. You may be in for $100 to $150 extra just to get them, whereas with careful shopping, everything else can be obtained for less than $100, soup to nuts!

I've not tried the bass resonance circuit myself (yet, but I might!). What I want to try first is to do some recorded tests on the low bass response first, to try to hear if there is any worthwhile difference between full volume and attenuated, using my Design M build. I'll need to figure out the bass resonance of my speakers, and then play some loud notes that try to provoke it.

But what we do know is that if you don't bother with the bass circuit, you can still build a great and very simple unit that you will like!


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## JohnH

6StringStewie said:


> I have a question about the M model and the heat generated by R1. I could either get a 200W or greater 30 Ohm resistor, or use two 100W or greater 60 Ohm resistors. Let's say I get two 250W 60 Ohm resistors with heat sinks. By dividing the circuit between two resistors of a larger wattage, would this help tame the heat and keep the enclosure cooler?
> 
> Stewart



I think using 2x100W resistors to give 200W capacity is a good idea. You can use two 15ohm in series or two 60Ohm in parallel, to get 30 Ohm.

In the wiring, you could place them not next to each other, using the space between for some of the less heated one's.

But I suspect that you won't need to crank your TSL to get it's best tone, since unlike the NMV Plexi's and also VM's, it's more based on preamp drive. The attenuator will help you get it out of first gear and cruising nicely


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## 6StringStewie

JohnH said:


> I think using 2x100W resistors to give 200W capacity is a good idea. You can use two 15ohm in series or two 60Ohm in parallel, to get 30 Ohm.



I am thinking I might want to try two resistors in series. In theory, doesn't the 15 Ohm resistor run cooler because there is less impedance per resistor?



> But I suspect that you won't need to crank your TSL to get it's best tone



Probably not, but I can't wait to try my amp on 11, lol.


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## JohnH

Either way that uses two equal resistors to make a combined 30 0hms will be fine, and share the same power equally. Power is volts x amps. Series connection halves the voltage on each resistor, parallel wiring halves the current, for the same net power.


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## 6StringStewie

JohnH said:


> *16 Ohm versions*
> 
> ... for M3, the capacitor value halved. This might make the cost a bit more feasible since one 100uF could be used...



Just curious to see if this would work and considerably reduce the cost of a build. JCM900 amps use 50uF+50uF dual can capacitors. Dual capacitors are two capacitors in one housing. Using both sides of the capacitor in parallel would yield 100uF (50uF + 50uF). These capacitors can be purchased for under $15. 

Thoughts?


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## JohnH

Not sure but Id expect probably not. The cap needs to be bipolar with high current rating and low losses, otherwise it could overheat, and also not achieve the resonant peak needed. The kind of cap needed would normally be used in the crossover network of high-powered speakers.


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## 6StringStewie

@JohnH

For a 100W amp, Build M R1 is spec'd at 200W. I found a couple of inductors with a value of 200W RMS. Those should work since the attenuator isn't expected to see all 200W, correct?

Are the L1 and L2 values the same for a 100W amp as a 50W amp? Have you run any Spice models with a 100W amp? (I thought I read earlier that L2 should optimally be 2x L1 for a 100W amp, but I haven't been able to find that post. Going to start a re-read of the thread this week.)


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## JohnH

Hi @6StringStewie
The required inductance values of coils and also the Ohms values of the resistors aren't affected by the amp power, they only relate with which speaker ohms tap you use on the amp. The inductors don't absorb much power if they are thick enough wire, nearly all of ths power is soaked up by resistors. What they need is thick enough wire to give low resistance. I'm using 18 Gage wire in my coils, which is definately fine with a 50W 8 ohm build. I think that will also be ok with a 16ohm build up to 100W. otherwise, go a greater thickness., say 16 Gage.

let's ask @matttornado what wire Gage are in his, for his 16Ohm build for a 100W Plexi, and if the coils are heating up much beyond having hot resistors nearby.


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## Marvelicious

Forgive me if this has been covered... I searched a little bit and there has been some discussion of other Lpad based designs, but didn't really find something that really spoke to it. It might be here, and I might be blind...

Anyhow, I've played around with Lpads and they tend to sound like a wet blanket on the speaker, but that is how a purely resistive load tend to sound. The key to this is the reactive stage 1. I'm wondering if you've tried feeding stage one into an Lpad for further reduction? The following stages are just resistive stages anyway, and theoretically shouldn't sound different than an Lpad...

Of course, as the saying goes: the difference between theory and practice is that in theory there is no difference.


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## JohnH

Hi @Marvelicious , Thanks for joining this thread.

What I realized is that the fundamental reason why L-pads sound like wet blankets is not because they are non-reactive, it's because they are not the best way to make a resistive attenuator. The key thing is that as you turn down, they show the speaker a very low impedance which damps down it's treble and bass resonant response. The resistive stages that I use keep the output resistance consistent and high, based in a real tube amp response. So a normal L-pad even after a reactive Stage 1 would still sound dull.

Doing the attenuation in switched stages means that each one can be tuned to get this correct. But I did work out a design for a final stage using a pot. Almost as consistent in the analysis, but never tried in practice.













Attenuator E



__ JohnH
__ Jul 9, 2018






That's from July 2018, before I had the reactive module worked out, but it should work ok as a final stage, after a reactive Stage 1. 

In my designs, the basic frequency response at the output is controlled by resistor choices. The reactive Stage then adds reactive response at the input, which corrects the dynamics of the amp.


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## Marvelicious

Interesting... I'll admit, I've never opened up an Lpad to see how they actually do their job. I hadn't considered the "back side" of the circuit.

In truth, the main reason for my interest in using an Lpad was to avoid the individual binary switches for packaging purposes. I originally considered a big multi-deck rotary switch, but the price for a switch that will handle the current is a bit prohibitive.


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## JohnH

That's where Gene and I got to too. Capable toggle or slide switches are easily obtained, but rotaries are either expensive, unavailable or under-specced. Three toggles will give you 8 settings and a simple clean wire-up. But a 3-pole 8-way rotary for several amps is not something that can be easily found and the build is a rats nest.

The design is set up for the amateur builder, and it's best if you love binary math too, whereby each stage is double the previous in terms of db's. Actually, it's easy to use once you get the feel for the volume influence of each switch, and the 3.5db steps are really quite small.


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## Marvelicious

I actually sketched it up with a 5P10T rotary with a bypass setting, 7 - 31.5db attentuation (I omitted a setting for non reactive 3.5db only) and a full mute setting with an additional 8 ohm dummy resistor. I managed to find a 5a rated switch for $70 - which isn't bad considering, and I'm tempted... there's something appealing about being able to simply turn the knob down. The switch is physically pretty big, so it wouldn't be the tightest rats nest I've wired.


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## JohnH

Sounds like a project! 

I think if it was mine, I'd do the bypass as a separate toggle, then the whole rotary is working at reduced current and you can better control transient issues as you switch to bypass. Also, break before make or make before break is relevant for the rotary. Probably want make before break.


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## Marvelicious

Yeah, the whole live switching issue complicates the hell out of it. Pretty much has to be make before break to keep it from briefly being an open circuit. The trouble is that when in between positions you wind up with some odd combo of the main circuit being paralleled with whatever odd ground leg (R3, R5, R7) it is switching from or to while the corresponding inline resistor is shorted. If that makes any sense... you obviously understand the issue even if I'm not phrasing it well.

Unfortunately, having to bypass to switch between setting pretty much kills the cool factor of the rotary switch as far as I'm concerned and I can buy a lot of toggle switches for $70. At the moment, I don't know whether those switches I found at that price are make before break or otherwise. The choice may make itself for me...


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## JohnH

I'd expect that if the bypass switch was separate, then you'd be ok to dial the rotary freely without operating the bypass. Even if there was an instant where all make before break contacts were connected, I don't think the amp sees less than about 7 Ohm for the 8 Ohm version.


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## Marvelicious

Oh, that would definitely work, it just makes it not worth the money for the expensive switch. The "cool factor" of just reaching up and turning the knob down is totally blown if you have to switch it into bypass mode first, then pick your setting, then kick it back on.

I'll probably build it as-designed and try it out first. If I like it enough, maybe I'll revisit the rotary, or maybe I'll decide I don't care...


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## JohnH

Good luck whichever you decide. What sort of amps would you use it for?

I built all the bells and whistles into my current build, just so I could test them out. But when I do another, it will probably be more stripped back, with no bypass switch and no -3.5db first setting. Omitting these takes significant pressure off the switching and simplifies the build. There are good work-arounds for both these. If a small first step is needed, the box can be wired in parallel as a load, to get -3db using a lower amp tap. And if I need fully loud for a party, I just don't use the attenuator.


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## Marvelicious

What amps... Well... There's the Ampeg V2, there's the Marshall 3203, there's that half built Carmen Ghia clone, and I have a JTM45 based idea sort of brewing in my head as well... Plus a pile of different combos that I could technically use it with.

The whole thing started with the intent to make a dummy load for testing purposes, which led me to doing a bit of reading on reactive loads, which led me to attenuators...


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## JohnH

The things I was wondering about was what power and what amp tap 8 or 16 ohms, would you build for? Would all your amp ootions be able to work with one choice? The designs so far are each dedicated to either 8 or 16 at the amp, and then at the output you can use either 8 or 16 on either version, or even 4 Ohms.

What we don't have so far is switchable input impedance. Commercial units that claim to offer this are often doing it just by putting a parallel resistor at the front, which works safely but kills off at least half of the effect of the reactive impedance for the amp. 

If you need an 8/16 input option, we could work out a reactive version that would work properly, although it would reduce by a further 3db when engaged.


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## Marvelicious

Nope, just going 8 ohm for the time being. Most of my stuff is switchable. My main use is really for testing purposes... being able to make sure there aren't any elusive issues at high volume without fracturing my ear drums. I have kind of a thing with small vintage combos, so I have plenty of things to play when I want low volume.


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## bodhi

First of all, thanks for such an amazing resource!

I recently purchased the parts for a build, with the idea to use a rotary switch to set the attenuation level. I bought a fairly cheap ceramic rotary switch that you can find on ebay, but based on the recent messages I'm starting to wonder what kind of current rating would be required for a switch to be safe to use in the attenuator? Unfortunately I'm more of a paint-by-numbers person regarding most of the electronics behind these matters and haven't yet worked through the math (or found the proper references) to figure out what kind of voltages and current are passed between an output transformer and a speaker.


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## JohnH

Hi @bodhi - thanks for that queston and Im glad you asked it before building!

The current through the switch will depend on the power of the amp, the ohms rating you are building to and whether or not the rotary is involved in creating a full bypass, or are you having a separate bypass switch (or no bypass)?

in terms of current and resistance, Power = Current^2xR, or Current = (Power/R)^0.5

Examples:

100W amp running at 8Ohms, with all of that going through the switch. Current = (100/8)^0.5
= 3.5A

50W amp running at 8 ohms, Current = 2.5A

50W amp running at 16Ohms, Current = 1.8A

10W amp running at 8 Ohms, Current = 1.1A

Now, if the switch in question is only after Stage 1, then it doesn't need to handle all of the power. With our design, all of the current valuess above would be reduced x 0.45.

Once you know all that, then to specify a switch, you then need at least an extra x2 factor on current handing.

So for those basic rotary switches, they are unlikely to be adequate if you want to crank your amp, unless it is very low Watts.

BTW: Switch specs often give you current rating for dc, and ac at 125V or 250V. If you have those specs, the 125V ac will be the highest value, and we can use that. The voltages we use are much lower.


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## bodhi

Thanks for the swift reply! Similarly to @Marvelicious I kind of thought to have it with the bypass included, just to be able to have a simple switch to turn it down, but might need to reconsider now with the proper calculations. After a while I ended up finding the same formula at the 18 watt forum.

I'm still at a bit of a loss regarding the expected voltage swing between the OT and the speaker though, since unless I'm mistaken this should allow for essentially a larger current rating than what the nominal x ampere @ 125 or 250 VAC that can be found. Typical OT impedance switches seem to be rated for 6A @ 125VAC, which is probably a safe value for the 100W Marshalls they might be used with, but I've seen some low wattage (the one I'm thinking of is 5w) with a common switchcraft slide switch, which seems to be rated at 3A @ 125VAC. Based on the calculations though, this seems to be easily within safe margins with the provided impedance settings.

It seems like the rating for similar rotary switches to what I've found are only 350mA or up to 0.5A, so it seems they're not really usable at all for this purpose. Not a lot of the common rotary switches seem to have a rating in the range required, or then there's not enough steps to get the full range available.

As for background, I'm probably going to start by building an external box to connect to the amps I have available, but I was considering integrating something like this into a couple of amp kit builds in the 18-25W range if it all seems to work out. I live in a fairly badly isolated apartment and need something like this to be able to play without the computer and headphones...


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## Gene Ballzz

@bodhi

First, welcome to the forum and one of the most enlightening and potentially liberating threads here. Constructed and used properly, these attenuator designs will allow you to use almost any amp, cranked well into its *"Sweet Spot"* at any chosen volume/sound pressure level, while retaining tone and tactile response!


Next, I feel obligated to ask: If _*"bodhi" *_is your first name, is your last name *"sattva?" 
*


Just Curious?
Gene


----------



## JohnH

Gene and I also tried to think through how to use a rotary, and where they might come from. but the numbers based on current vs switch positions vs size vs $ just didn't seem to stack up! And actually, once you get used to the 'binary' switching in either equal or x2 steps, it just works simply. You find an appropriate setting and leave it there and get on with playing the guitar. 

*Voltage swings coming out of amps:*

Power = V^2/R so Voltage = (Power x R)^0.5
for a 100W amp into 16 Ohms,
V= 40V
if the Power is RMS, then there is a further x1.41 factor to get to the peak voltage. I'm not sure whether switch voltage ratings are based on that or RMS

But I think that confirms how 125vac is a reasonable and safe voltage spec for deciding on switch current ratings. It doesn't tell us if at lower voltage, there may be a tad more current rating available. But it also is consistent with output switches being rated for 6A @ 125vac, though you'd be pushing up to the limit if cranking a 100W into 4 Ohms (=5A)

*What could you do with a 0.5A rotary?:*

ie, let's say you could accept a separate bypass switch, or none, what power amp could be handled?

I think I'd still want a minimum margin of say 1/1.5, so I'd let the current be 0.33A. By putting the rotary contacts only after Stage 1, then the current coming in from the amp could be 0.33/0.45 = 0.73A.
If I use a 16Ohm tap (to get more power with less current), I can deal with an amp power of
0.73^2 x 16 = *8.5W.*

So maybe there is scope for using simple rotaries with the low-watt amps.


----------



## Gene Ballzz

@JohnH

Ya know, I'm betting that *"PERFECT FOR OUR USE"* switches are out there and available at reasonable prices. I'm just not certain of all the search criteria/details to find them. When dealing with companies from *"Chinesiawanoreanam" *the language barrier, variations and discrepancies can make things especially confusing and/or complicated!

With that said,i've come to feel as John does about the *"binary" *toggle usage. While it may seem crude at first glance, they're laid out in a manner that keeps the user away from electronic trouble and/or possible amp damage! The only potential danger may be using any of these (16 ohm or 8 ohm amp tap version) with a different speaker impedance than the amp tap rating and then simply flipping a direct bypass switch. While the speaker impedance is not critical when attenuator is actually in use, an 8 ohm or 4 ohm speaker could potentially get connected to a 16 ohm amp tap, in bypass mode, etc!

Considering all that, if anyone finds an appropriate 8 to 10 position rotary switch, at reasonable pricing, please share the information?

Just My $.02,
Gene


----------



## bodhi

@Gene Ballzz Hi, thanks for the welcome! Regarding my last name, you could probably argue that you've guessed correctly, as the connection is made with the character the nickname is based on. There's a link to my actual name as well, but as it is with in-jokes, none of them get any better with explanation 

@JohnH Thanks for filling in the math, now I know what to look for properly! Looking at listings on Mouser and similar, there seems to be some relatively economical 2.5A rated rotary switches by C&K (< 10€ / $ / £ each) that could be useful in some schemes.

My end goal was to have something integrated into a combo amp, and in that case a rotary that can be easily found by reaching around to the back rather than individual switches would serve the purpose a lot better, I think. A separate bypass switch could still work, in that it's probably easy enough to find in practice with a bit of planning, so you wouldn't mistake it for a bypass or power switch.


----------



## Gene Ballzz

bodhi said:


> @Gene Ballzz Hi, thanks for the welcome! Regarding my last name, you could probably argue that you've guessed correctly, as the connection is made with the character the nickname is based on. There's a link to my actual name as well, but as it is with in-jokes, none of them get any better with explanation



Thanks for that and please understand that I had no intention of making fun of or mocking your or anyone else's spiritualism! Many (including myself) might enjoy a bit of enlightenment on our journey.

Thank You Again!
Gene


----------



## bodhi

Gene Ballzz said:


> Thanks for that and please understand that I had no intention of making fun of or mocking your or anyone else's spiritualism! Many (including myself) might enjoy a bit of enlightenment on our journey.



Absolutely no lines crossed at least on my behalf, it's perhaps a somewhat irresponsible username by me to take, but similarly no ill intents meant. The pop culture reference is close to 30 years old by now, so even that is really quite hit'n'miss...

Wrt. rotary switches, there also seems to be something like very high voltage rotary cam selectors used as spares for welding machines according to the description that might be possible to wire up properly with a full bypass option, these seem to be in the 10-20 $ range. Probably prohibitively large if one is going for compact box builds, but could again work in an amp chassis.


----------



## JohnH

I had another go at scouring Mouser for switches.
I reckon we need, given a separate bypass switch:

Two poles minimum (would calculate resistor values around using that)
At least 6 positions (8 would be better)
At least 2.5A at 125vac, provided Stage 1 Is before it.
Shorting contacts (=make before break)

Feeding all that into Mouser revealed this:

https://au.mouser.com/ProductDetail/Grayhill/44M45-02-1-06S?qs=sGAEpiMZZMvNbjZ2WlReYuoQqRoCNVYYEcH8GJq8Dfs=

More than $100 of my dollars!


----------



## Gene Ballzz

JohnH said:


> I had another go at scouring Mouser for switches.
> I reckon we need, given a separate bypass switch:
> 
> Two poles minimum (would calculate resistor values around using that)
> At least 6 positions (8 would be better)
> At least 2.5A at 125vac, provided Stage 1 Is before it.
> Shorting contacts (=make before break)
> 
> Feeding all that into Mouser revealed this:
> 
> https://au.mouser.com/ProductDetail/Grayhill/44M45-02-1-06S?qs=sGAEpiMZZMvNbjZ2WlReYuoQqRoCNVYYEcH8GJq8Dfs=
> 
> More than $100 of my dollars!



Yepper, $76US for me! Even though quite nice, it might be a bit overkill, particularly if doing a separate bypass switch. I'd be very interested in perusing some links links to the ones that @bodhi found?

Just Askin'
Gene


----------



## bodhi

This is the datasheet for the C&K switches, which seem to be the economical choice on the site. If I'm reading it properly, the A206 should be 2.5A @ 125VAC 2p6t with at least the possibility for make-before-break contacts https://www.mouser.fi/datasheet/2/60/arotary-1369275.pdf - but Mouser doesn't seem to stock that one...

The cam switches are things like these https://www.aliexpress.com/item/32829955717.html .

I had some first draft ideas about wiring schemes from a long time ago, but need to review my notes if I can find them as they were never verified. The thought was basically to use a rotary switch wired onto itself in different ways to ensure the proper combinations of components were connected in series...


----------



## JohnH

I did some thinking about, assuming suitable rotaries were available, what would the circuit look like?. My starting point is no compromise on the consistency of impedances and hence tones that the designs to date achieve.

To get the full range of steps, and capabilities assuming a fixed stage 1 (not involved with the rotary), then 8 switch positions are needed if we want to go from -7db to -31.5 db in equal -3.5db steps. 

The easist version to think about has 3 poles, so each pole does the job of one of the toggle switches from Stages 2 to 4. So that's a 3-pole, 8-position switch with shorting contacts, rated at 2.5A at 125Vac, for an 8 Ohm build and up to 50 W amp.

Then I wanted to see if a 2-pole switch could also work. I think it can, but not as easily as I'd hoped. To do it, I think many more resistors are needed, maybe 19, to replace 6 in stages 2 to 4. That's not so crazy, since they can be inexpensive (max power rating 25W), if it makes the switch more findable in reality.

If you can only get 6 positions, all of the above could still apply, maybe stretching out the increment steps to -4db each, and not quite so much max attenuation. You could go -8, -12, -16, -20, -24, -28 db.

Or, keep the -3.5 steps (or even reduce them to -3 db), and add another toggled switched stage, like a hi/low range switch.


----------



## Marvelicious

Hmmm... Yeah, more resistors should wind up being a lot cheaper than the switch in the end. I'm going to have to sit down with this again tonight... I'm old fashioned when it comes to circuits, so sitting down with a pad of graph paper and diagramming it out is how it makes sense to me.


----------



## JohnH

Marvelicious said:


> Hmmm... Yeah, more resistors should wind up being a lot cheaper than the switch in the end. I'm going to have to sit down with this again tonight... I'm old fashioned when it comes to circuits, so sitting down with a pad of graph paper and diagramming it out is how it makes sense to me.



I totally understand that! I leant to think 'schematic', using a pencil and paper, when I was an electronics nerd at high school in the '70's.

I have this idea now about how to use a 2-pole switch, I'll sketch it out so we can discuss it.


----------



## matttornado

My attenuator is working great, however, its gets too hot to handle after about an hour of full attenuation. Does anyone know how to add a fan powered by the attenuator? I measured about 60VAC peak at the attenuator's input from my amp with my 100 watt amp cranked. Not sure how accurate that measurement is but.... I'm thinking maybe a 12 volt DC fan.


----------



## JohnH

matttornado said:


> My attenuator is working great, however, its gets too hot to handle after about an hour of full attenuation. Does anyone know how to add a fan powered by the attenuator? I measured about 60VAC peak at the attenuator's input from my amp with my 100 watt amp cranked. Not sure how accurate that measurement is but.... I'm thinking maybe a 12 volt DC fan.



I think that's a great idea. The idea is to pull a little a.c. power from the input, using a resistor, high enough value not to mess with the load, then bridge rectifier to feed the dc fan. Ideally, it would start turning slowly while still at moderate power, and spin faster at full. Need plenty of air vents in and out.

I'll check out a schematic.

60V peak is 42V RMS, or 112W into 16 Ohms! All as it should be. (EDIT: actually, is that right? or is it swinging -30V to -30V, which is 21V RMS?)

I had a quick look at a few 12V fans. I saw current draw from 80 to 360mA at 12 V. I'd suggest to use one with low current. 80mA and 12V implies the fan would be around 150Ohm. Thinking the feed resistor from the amp input would be about 470 Ohms, but you'd need to test, it could be more or less, to get it running right. Then to a BR, with a 10uF smoothing cap, then the fan.

80mA through 470 Ohm gives 3W in the resistor. Need at least a 10W capacity by those numbers.

All of this needs to be tested in a mock up to get the values right. But at least, if it's all through that resistor of several hundred ohms, there's no risk to the amp to try it.


----------



## tmingle

JohnH said:


> I think that's a great idea. The idea is to pull a little a.c. power from the input, using a resistor, high enough value not to mess with the load, then bridge rectifier to feed the dc fan. Ideally, it would start turning slowly while still at moderate power, and spin faster at full. Need plenty of air vents in and out.
> 
> I'll check out a schematic.
> 
> 60V peak is 42V RMS, or 112W into 16 Ohms! All as it should be. (EDIT: actually, is that right? or is it swinging -30V to -30V, which is 21V RMS?)
> 
> I had a quick look at a few 12V fans. I saw current draw from 80 to 360mA at 12 V. I'd suggest to use one with low current. 80mA and 12V implies the fan would be around 150Ohm. Thinking the feed resistor from the amp input would be about 470 Ohms, but you'd need to test, it could be more or less, to get it running right. Then to a BR, with a 10uF smoothing cap, then the fan.
> 
> 80mA through 470 Ohm gives 3W in the resistor. Need at least a 10W capacity by those numbers.
> 
> All of this needs to be tested in a mock up to get the values right. But at least, if it's all through that resistor of several hundred ohms, there's no risk to the amp to try it.


Im pretty sure I saw a schematic for 1 of the commercial attennators that powered a cooling fan from the speaker out. I think it may have been the Marshall Power Brake.


----------



## Marvelicious

JohnH said:


> I totally understand that! I leant to think 'schematic', using a pencil and paper, when I was an electronics nerd at high school in the '70's.
> 
> I have this idea now about how to use a 2-pole switch, I'll sketch it out so we can discuss it.



Yeah, I'm jealous of people who can make electrical CAD programs work for them, but I wind up spending all my time figuring out how to make the program work and none of my time working on the circuit at hand. Still, now that I'm thinking it through, I'm pretty sure a 2-pole switch should work, I just have to draw it out to be sure.



matttornado said:


> My attenuator is working great, however, its gets too hot to handle after about an hour of full attenuation. Does anyone know how to add a fan powered by the attenuator? I measured about 60VAC peak at the attenuator's input from my amp with my 100 watt amp cranked. Not sure how accurate that measurement is but.... I'm thinking maybe a 12 volt DC fan.



I had an idea to use a Seebeck generator (basically the same thing as a Peltier module, but technically, it's the Seebeck effect if it's used to make power and it's the Peltier effect if it's used to cool something) to power a small fan. The sell versions of this designed for wood stoves, this would just be the baby version.


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## matttornado

For the fun of it, I alligator clipped a a AC to DC wall wart P.S. rated at 120VAC / 12DC to the input of my attenuator. I measured about 6 volts DC from the DC Power supply & hooked it up to a 5 volt DC fan. It would not power the fan. It wanted to. The fan would slightly move just a little. Once the fan started to spin but not at it's rated speed. So it can work. Looks like it needed more current maybe?


----------



## bodhi

Regarding steps and decibels, one of the original incentives for me to look for attenuators (and the idea of integrating into combos) came from Tone King amps and attenuators.

The one integrated into the Imperial (mk ii) amp has levels of 0db(bypass), -3db, -9db, -15db, -24db, -36db on a rotary switch. The separate attenuator in a box is called Ironman II, which has a Hi/Lo switch and a rotary

Hi range: -3db, -7db, -11db, -15db, -25db, -35db

Lo range: -6db, -10db, -14db, -18db, -28db, -38db

So both of those have 6 steps, and I would assume the specific levels were not chosen at random... Food for thought, haven't built the proper M2 attenuator to figure out which levels might be the most useful ones myself yet.

As for switching diagrams, realistically I don't have any time for drawing up plans that shouldn't really be spent elsewhere at least this week, but likely until Christmas taking the time of the year into consideration. Still haven't found my original wiring idea notes...


----------



## JohnH

Hi @matttornado , yes the idea should work! Different fans need quite different amounts of juice, so specific circuit values are TBC. But thanks for trying the concept.

@bodhi I think a hi/lo range switch is a good idea. I don't quite get the odd increments of the Ironman though. I may be to do with some peculiarity of its internal circuitry. I think it would be best if each step around a rotary is the same increment, and the 3.5db steps that we have are pretty small in practice.

6 position rotaries should open up the choices since they are available with two poles per wafer, with standard 30 degree rotation steps. 

You could have an overall bypass switch, a fixed -7db reactive stage, a high/ low toggle that did maybe 0db/-9db , then a 6-pos rotary doing say -3db steps 0db to -15db. That would give an overall range from -7db to -(7 + 9 +15) = -31db in consistent 3db steps. A -3db workaround is yo run the box in parallel with a speaker with amp set to half ohms.

Still havent found a real, suitable, available switch though, and it's still a complicated build compared to the previous designs!


----------



## Gene Ballzz

Hey guys,

This thread is so cool on so many levels!

Just My $.02,
Gene


----------



## Marvelicious

@JohnH
So, I'm doing a bit of sketching, but I'm having a little late night brain lock... You could just endlessly string instances of R7 and R8 for successive 3.5db drops, right? If this still makes sense when I look at it in the morning, it may actually be simpler in practice.


----------



## Gene Ballzz

So FWIW folks, I use one of these units almost constantly, with various amps and must say that it is as close to perfect as I think we can get, for a generally "passive" unit! With that said, a tiny bit of highs (the *"sparkle" *if you will) seems lost at attenuation levels of more than -21db, but I belive that to be more of a physical/air movement phenomenom than an electronic one. I'm betting that any attempt to correct for it *"electronically"* within the attenuator, will end up being a *"hit or miss"* situation, depending on the amp used as well as the attenuation level. This design is truly one of the:

Greatest Things Since Sliced Bread! 
Gene


----------



## JohnH

Marvelicious said:


> @JohnH
> So, I'm doing a bit of sketching, but I'm having a little late night brain lock... You could just endlessly string instances of R7 and R8 for successive 3.5db drops, right? If this still makes sense when I look at it in the morning, it may actually be simpler in practice.



That would work jn theory, you can stack any combination of the stages 2 to 4 end to end to make a combination. But I don't see it as the way to go for working with rotaries because each stage would need to have switch, which if translated to a rotary controlling all the stages, would need to be a separate pole, one per stage.

My current sketching around this is to have Stage 1 as designed before (design M or M2), then a single stage after that where each of the two resistors is a chain, selected by a switch, so two poles, and as many steps as can be found based on switch positions. If an 8 position 2 pole switch can be found, then that would do it. If it's a 6 position switch, then Id augment it with another high/low toggle switch to extend the range while keeping rotary increments small.

I have a heap of maths built into spreadsheets to deal with this stuff, but the trickiest step currently is to find a recipe that works with the limited range of resistance values that are available from normal suppliers. Definitely solvable, just a bit of a PITA!


----------



## JohnH

Gene Ballzz said:


> So FWIW folks, I use one of these units almost constantly, with various amps and must say that it is as close to perfect as I think we can get, for a generally "passive" unit! With that said, a tiny bit of highs (the *"sparkle" *if you will) seems lost at attenuation levels of more than -21db, but I belive that to be more of a physical/air movement phenomenom than an electronic one. I'm betting that any attempt to correct for it *"electronically"* within the attenuator, will end up being a *"hit or miss"* situation, depending on the amp used as well as the attenuation level. This design is truly one of the:
> 
> Greatest Things Since Sliced Bread!
> Gene



Thanks Gene, seriously many thanks for your perceptions, encouragement and trust. You were the first to believe in this design! 

Sparkle:
Many commercial units have some form of tone switch for treble. Usually this has to compensate for an inherent dullness that creeps in at more than a small amount of attenuation. I don't think this one has that fundamental issue. But there's lots of reasons why a treble switch might be helpful, just to give an option for a player who wants it. It might help to compensate for a particular amp, or adjust for FM effects.

Its not hard to do, particularly if the intent is to make that adjustment only at large attenuation more than -21db. In this range, the -14db stage will always be on, and below this it will always be off. So a bit of capacitor treble bleed around this stage would do it, and being downstream of at least Stage 1, the amp wont be messed up by it. Shall we work out a design? It could have a 3-position switch for 'off' and two options for treble lift.


----------



## Gene Ballzz

@JohnH
As mentioned, on my 16 ohm, K4 "Gene" unit it does not show up at -21db, but does at -24.5db and more. It is extremely rare for me to use level this quiet and I certainly never gig at such, so not a big issue in my case! Understand that everywhere I go, I tout the beauties and usefulness of this design and provide links to many non-ttmembers, so we might be getting some new members, simply for being able to ask questions.

I was going to suggest that it may be helpful/beneficial to start a new thread not intended for discussion and/or comments but instead to simply list the different versions, along with possibly the diagramatic [sic] evolution of each. Then if someone wants the full history and testing details they can read this thread that is becoming rather monstrous!

Just The Thoughts Of A Crazy Man,
Gene


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## Marvelicious

@JohnH




Excuse the chicken scratch, but this is based on the M design. This shows a 5 position rotary... The common of one deck wired to the output and the other to ground. The dots represent switch contacts... position one is bypass, two is the 7db reactive stage and each jump after adds 3.5db of drop. Add more stages as needed with more positions on the rotary.

Am I missing anything?

*Edit: As it turns out, I am missing something... I'll leave this schematic up for the moment, since I think I'm on the right track, but the grounded side of the circuit doesn't work as is... *


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## JohnH

hi @Marvelicious 

Thanks for the diagram, but I'm sorry that it has some hairs on it! 

Its OK up to Stage 1, but when additional stages are engaged, the ground connections to each of the previous stages which are still active, need to be maintained. That cant be done with just one pole to deliver the ground connection.

Also, the design works with the series resistors (eg R8) on the speaker side of the grounded resistors (eg R7). Its the key trick by which the right impedances are maintained. As drawn its shown the other way around, similar to most l-pads, which suffer tone problems in amp attenuators.


----------



## Marvelicious

Yep, I noticed it had some issues right after I posted it... still, I think I'm close to it...

Edit: I'm into a rut on it at the moment. Pretty sure it can be done with a 3rd pole, but I'm not making any headway with two right now. Time to walk away for a bit.


----------



## matttornado

I kind of got the fan to work but with some weird problems. 

1st: the fan circuit added a lot of extra attenuation when it worked. That's no good!
2nd: it would only work if grounded to the metal enclosure & I think the circuit's ground together?
3rd: when connected to just the circuit, not the enclosure, there was no sound. I guess it created a short?


These were quick tests so I need to go back and do some more experimenting. maybe the fan has to be connected to a different ground (enclosure) not sure what's up yet. I was kind of in a rush.


----------



## JohnH

Hi @matttornado , could you post your fan schematic again?
Thanks


----------



## matttornado

I got the idea from this Marshall Powerbreak schematic. I'm not sure if this is entirely correct or not.
I connected the fan between attenuator R4 & R6 and got lucky with the correct DC voltage between 5 & 6 volts DC. I was using a 5 volt DC fan. The fan seemed to work regardless of what attenuation setting I was using. I didn't put a diode in yet, just a cap.
I'll post a few pics of my set-up as soon as I can. 

Thanks, JohnH!


----------



## JohnH

did you have the bridge rectifier BR1 and the 56 Ohm resistor? Need to see it all drawn on our diagram.


----------



## JohnH

Here's how I reckon the fan would be wired: 




the value of R would be depend on what works. the powerbrake shows a 56 Ohm there


----------



## matttornado

Yep that's what I did but instead of adding the resistor, & connecting right at the input of the attenuator, I tapped power between R4 & R6 at the attenuator circuit. I wonder if that has anything to do with why it added more attenuation?

I can't wait to play around with it some more. Hopefully this weekend.


----------



## JohnH

Could be! It's better as I drew it, and then there's more juice avaikable and you can use a larger R to control it, so less attenuation added.


----------



## JohnH

*Rotary Switches*

I've been thinking about how to do this design using a rotary, with the best balance of fine control, range and minimum switch complexity. If I built one, and if a suitable switch can be found, I reckon it might be like this:













Attenuator M2 Rotary 201219



__ JohnH
__ Dec 21, 2019






It has a fixed Stage 1, based on design M2. Then a high/low range toggle in Stage 2, to change from say rehearsal or gig level, to home or studio level. I've made this a -10db stage. Finally, the rotary stage, to go from 0 to -15db in -3db steps. So the whole range is -7 db to -32db. The rotary for Stage 3 is 2 pole, 6 position with shorting contacts.

With an 8 ohm build and a 50W amp, the Stage 2 switch needs to have contact ratings of at least 2.5A at 125V ac (allowing a factor of a bit more than x2). The rotary can be rated at 2A. See how in position 1, where it is at zero, it's two poles are in parallel, sharing the current, which takes a bit of pressure off its loudest setting.

The Stage 3 has two chains of resistors to gradually increase attenuation. For each amp Ohms, I've given the best values I can figure out using either the standard range of resistors (like 10,12, 15, 18, 22 etc), or the slightly different range that I've being buying from an online seller in China:

https://www.ebay.com.au/itm/25-50-1...cc9d:m:mDimLUo3wNHFIEXvjElODhA&frcectupt=true

That's an Aus eBay link, but I'm sure you can find it.

All the above is untested so far, but follows tested principles for its design.


----------



## Skorpio

Hi, new to the forum. I will introduce myself in another thread but right now I have quick question because I had some thoughts about attenuation myself for the past few days and found this.

I run DSL15h as a pedal platform into 1x12 8 Ohm G12M-25 Greenback and since nowadays i play mostly at home, I wired simple resistors into the cab some time ago.

John, could you explain main DIFFERENCE that your design make, so that it's superior to classic wirings that i use ? Yes, I read thread from the first page but I _couldn't_ find simple answer

I have 8Ohm parallel to the +/- on the 8Ohm speaker (so i have 4Ohm) and 12 Ohm in series to back in up to 16Ohm, then 16Ohm parallel to have ~8,3 Ohm measured at the cable. I want it modify it, not some switches/bypass, simple hardwired resistors bacause i don't gig/don't jam with it anymore... and i can change it later if I do 

I read that you:
*1. *Wire resistors, so that resistor that is wired in *series* is_ closer to the speaker_, then *parallel* compensates for increased resistance and is _closer to the amp and grounded_
*2.* Ground connection - this means that the speaker and parallel resistors _additionaly _have to be connected to the ground/common ground to "see" proper impedance that thus work correctly without tone suck ?
I undersantd schematics and I calculate and additionaly check all resistances by myself if i need to, if you could explain where your design differs from l-pad/what i do.
Thanks!


----------



## JohnH

hi @Skorpio , thanks for your interest in these designs.

What you describe, with two parallel resistors (one across speaker, one across amp), plus a series one, is a decent design , called a 'pi' attenuator. Your numbers check out and its safe. It will take -12db off the volume.

Speakers have an impedance that varies with frequency. It rises at high frequency due to coil inductance, plus there is a resonance in the low bass. Tube amps react to this. Plus, tube amps have quite a high output impedance themselves, so the changing speaker impedance is reflected in the voltages across the speaker, and hence the frequency response that it creates. Solid state anps, and the dullest form of L-pad attenuators when at high attenuation, damp this natural resonpse down to a flat line. We get a muffled sound, and less dynamics in the tone. But yours is actually not bad. I fed it into one of my spreadsheets and produced this:





Red represents the signal as a frequency response in db, across a full speaker (a G12M), fully connected to a typical tube amp (mine), with no attenuation.

Blue represents the same speaker, with your attenuator, with the overall -12db taken out so the curves can be compared. You can see the same rise and peaks, but lower relatively. Id expect about 2 or 3 db less in the high treble and low bass. Actually, not a huge difference. Does it sound good? A response for a simple 2 part l-pad would be flatter.

The green line is what the amp is seeing - almost a pure flat resistive load. So when you dig in, it cant react in the same way as to a natural speaker.

My design adds a natural reactance for the amp to see, which in the M and M2 design comes into play above about 150hz. Also, the different resistor arrangement maintains the high output impedance to the speaker, so its response in maintained. here is an equivalent plot:




Now the response at the attenuated speaker (blue) is virtually identical to that of an unattenuated speaker (red), with the attenuation removed form the plot, and the amp is seeing a load that is much closer to that of a pure speaker, and reacting accordingly (green)

Does that help?


----------



## Skorpio

Wow, I didn't expect a frequency response for my 'design'  Subjectively, some highs may be taken out, but since I can increase master and my amp, and speaker (broken in) are bright (and honestly, presence control is for something, right?) it actually does sound better than I expected (not muffled and not fizzy)

And yes, this explains a lot, especially with graphs - i wonder if more flat green line with my (and any similar) design actually put more strain on the power amp, I heard some bad things about resistive load 

However, mainly I was refering to this quote:


> Also, the design works with the series resistors (eg R8) on the speaker side of the grounded resistors (eg R7). Its the key trick by which the right impedances are maintained. As drawn its shown the other way around, similar to most l-pads, which suffer tone problems in amp attenuators.



So, I suppose that wiring 8 Ohm in series to the '+' of a speaker for total 16, then parallel 16 Ohm to reduce to 8 and doing that again is gonna improve frequency response by itself and it's going to be basically yours 2-stage hardwired design ? Something along page 3 design with no switches. I was also using 2 adjustable resistors per l-pad calculator site double checking values, with different wiring however... (parallel on the speaker side) and values have to be way more precise so i'm more with this.


----------



## JohnH

Yes that proposal should work well. Its also -12db, but it would put your blue line right up to where the red line is.

But adding the inductor or inductors does make it better again.

Ive also read reports about damage from resistive attenuators, but they don't make sense and Ive read better experts debunk them.


----------



## Skorpio

Due to resistors I have at my disposal right now, I ended up wiring (modifying) something like this without adding inductor



I guess it's about -18db, tested it and i'm quite happy with it. I cannot crank the amp at 4 or 5 without neighbours going crazy so I can't really A/B how much response I lose with no attenuation. With my G12M it's doesn't sound too much though, many of todays reissue speakers (especially H or M type Greenbacks) are brighter anyway, so small teble loss is something I can live with for now - it's main reason right now I just sligthly modified original design with more power reduction. There's still plenty of bass but that could amp as well (I have deep switch which works like resonance at 100%)
Thanks for you input, I'll incorporate inductor and change wiring later on.


----------



## JohnH

Skorpio said:


> Due to resistors I have at my disposal right now, I ended up wiring (modifying) something like this without adding inductor
> View attachment 65480
> 
> 
> I guess it's about -18db, tested it and i'm quite happy with it. I cannot crank the amp at 4 or 5 without neighbours going crazy so I can't really A/B how much response I lose with no attenuation. With my G12M it's doesn't sound too much though, many of todays reissue speakers (especially H or M type Greenbacks) are brighter anyway, so small teble loss is something I can live with for now - it's main reason right now I just sligthly modified original design with more power reduction. There's still plenty of bass but that could amp as well (I have deep switch which works like resonance at 100%)
> Thanks for you input, I'll incorporate inductor and change wiring later on.



A good option with your parts, a bit better in balance with regard to ohms, would be based on your last diagram above, left to right, 16, 8, 16, 8 then omit that last resistor at right. That should give-12db and the amp sees 8 and the speaker sees 16 which is close to optimum.

EDIT: further on that, if you do in a similar arrangement, left to right, 16, 8, 16, 12, you get -13.5 db and the amp sees 8.2 ohm, with balanced tone.


----------



## Skorpio

I mean, I was happy enough with my old design to try even more attenuation, to see how it'll end up since it was easy to add resistors and get more power reduction. I like it actually but...



> EDIT: further on that, if you do in a similar arrangement, left to right, 16, 8, 16, 12, you get -13.5 db and the amp sees 8.2 ohm, with balanced tone.



Left to right, I suppose you're talking 16 series to the speaker and 8 parallel, gives 6 Ohm, then 16 series again to 22 and 12 parallel gives about ~7,8 Ohm seen by the amp, based on my quick math, correct ? Also, one 16Ohm and that 12 are adjustable (~15,8 Ohm max both), so we have some possibilities here, although not much I think. If we agree on this resistor arrangement as final, where would inductor go and what value ? Are they really necessary ?

EDIT; Whoa, got another idea - 8 Ohm in series to the speaker, then parallel 16 two times to give -12db and 8 Ohm (my original proposal that you said is good) and THEN another 8 Ohm in series (one that is adjustable set to) and connect it to the 16 Ohm amp tap. How about that ? This should give more power reduction.

But for now, Happy New Year!


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## JohnH

this is what I had in mind with your parts:



its about -13.5db

If you add the reactive front end, id suggest to use values form the M or M2 designs (two coils and three resistors, or one coil and three resistors respectively). These circuits are carefully balanced, so its important to use consistent values. The reactive stages at the front can add some extra dynamics as you dig in, pushing the amp. Most guys are finding it worthwhile.


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## Gene Ballzz

@JohnH 
Great work on the rotary layout!  For my own personal use, I've become so familiar with/used to the toggle scenario, that I'll stick with that. While I've only built one of these (design K4-Gene in post #267) I have multiple components to build at least two or three more. I've been living with this 16 ohm unit to get a good feel for changes, modifications and/or upgrades most pertinent to my universe. Though I've not yet started drawin', strippin', drillin' and screwin', I've determined my best plans and may need just a bit of help working out the *"parallel bypass feature"* I'm considering. So here's the poop:






Now, an addition I want to make is to have a switch to put this (ALL STAGES ACTIVATED) in parallel with the straight thru/bypassed signal. I realize that at initial face value, I need to switch my amp output to 8 ohms. I'm curious/hoping there is an effective way to put a resistor in series (when in what I'm calling_* "parallel bypass" *_mode. to be able tokeep the amp set at 16 ohms? My thoughts behind this are that putting this in parallel gives generally the only -3.5-ish-db option while still retaining at least some semblance of the reactive circuitry. Actuation could be done by either a separate switch, a three position bypass switch or even a separate output jack. While I can likely figure out the most appropriate switching scheme, I need help figuring out what components/values/layouts are required to keep the input at 16 ohms when this unit is paralleled with a 16 or 8 ohm speaker.

Thanks In Advance & Happy New Year!
Gene


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## JohnH

Hi @Gene Ballzz 
The maths is not so favourable!

The basic parallel idea with speaker andattenuator box In parallel, is a good way to get -3db, when using the lower 8 ohm amp tap. But to bring all that back up to 16 ohms , needs an extra 8 ohm series resistor, which takes off another 3db, leading to it being an overall -6db arrangement. That's not far off the-7db that we get anyway, but with sub-optimal impedance and reactive response.

Attenuators M can have the -3.5db stage on it's own, and it works fine. Maybe there's a nearer way to engage that arrangement?


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## Gene Ballzz

Aaahhh I see, said the blind man, as he picked up his hammer and saw!   But all is good, as long as I always change to the 8 ohm tap when using in parallel mode. Correct? That is not that big of a deal for me. On the plus side of using a parallel switch, at least I will avoid making a mistake while having to unplug and replug multiple speaker cables inside the back of an amp that has this attenuator mounted in it.

Thanks again for your computational assistance.
Gene


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## JohnH

Hi @Gene Ballzz

If you ae using an amp with both 8 and 16 taps, this might be a neat arrangement, for either a built-in unit or one that stays wired up:

Have two inputs, one into which you plug the 16 ohm amp tap and one for the 8 amp tap, both plugged in at the same time, but only one is connected based on switches.

The core of the attenuator, is one of the 16 ohm 'lite' versions, which go -7 to -31.5db, ie as you have done.

You have a full bypass switch, which is a dpdt toggle

And another dpdt toggle, which moves both the speaker and the attenuator input to the 8 ohm tap. This gives the parallel mode, for -3db. As it does this, it disconnects the 16 ohm tap. so there's no 'wrong' positions. In parallel -3db mode, you should switch to max attenuation, but its not a show-stopper if you don't, just a half step mismatch even if only Stage 1 is engaged.

So once all the leads are plugged, there's no rewiring to get the full range.

The one key thing to check is that the two amp outputs, 8 and 16, have their outer barrel connections in common. ie going to the same end of the OT secondary, which would be normal in any amp circuit I've seen

I'll draw it up if its of interest.


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## paul j melius

Hi, I joined this forum specifically because of this thread. 

I'm having a hard time finding similar inductors to what it seems like everyone in here has used. I'm looking on digikey, they always ship to me faster than mouser.

Can anyone direct me to exactly what I should be looking for? Also, I'm scaling up the design by 3, to hopefully be extra safe with running ~100w amps cranked. So 300w on R1, etc.


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## JohnH

Hi Paul, the inductors tend to come from places that sell cross-over components for loudspeakers. I believe we have sources found in the US, UK and Australia. Where are you?


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## paul j melius

JohnH said:


> Where are you?



PA, USA

Just wondering if there is a term or specific type that I should be looking for. I noticed air-core mentioned in one of your first posts, but I am unable to find any air-core inductors in the correct values on Digikey. I've not had much experience purchasing inductors haha.


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## JohnH

Gene found Madisound, and they seem to have a good range. If you are doing design M or M2, I'd suggest aircored and 16awg for 100W. Here's some;

https://www.madisoundspeakerstore.com/sidewinder-16-awg

Design M2 (see page 27) is neat since only one coil is needed. It hasn't been tested yet but all the analysis shows that it is just the same as M. It's the next one I'd build, if I do another.

It's great to have extra Watts capacity, but what is more important is a really good large heavy case, resistors bolted to it and we'll spaced (or heat sinks) with lots of space and vents above and below


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## paul j melius

Thanks for that link! That's perfect.

What got me here was ordering some junk power resistors from Amazon and throwing them in my Weber Z-matcher to use as a dummy load only to realize that they get way too hot, way too fast. Followed by days of researching readily available products and DIY options. So now my plan is to build a way over-spec and very heat-sinked attenuator/dummy load with line out based on this thread and the Aiken amps article. Will post results if I end up going the DIY route (likely).


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## tmingle

https://www.parts-express.com/cat/crossover-inductors/296


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## pilau

Here is a guide on creating a specific load circuit to your speaker using its Thiele/Small parameters (usually available from datasheets or google searches, otherwise you need something like a DATS to measure the speaker):

Simulate Speaker with Equivalent RLC Circuit

In essence, by adding two capacitors and 1 resistor in parallel with L2, the values of which can be calculated per speaker, it is possible to emulate the mass and mechanical behaviors of the speaker, meaning a much closer approximation of its response:






The funny abbreviations next to the components are some of the T/S parameters required. More information in the link above.


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## JohnH

hi @pilau Yes that is an interesting link showing the maths, and I saw it where you posted on TGP. It all leads to about the same equivalent form of circuit as Aiken.

Not many use TS parameters for guitar speakers, though you can find them for a few models, eg WGS. They don't know about the effect of the cab though, eg a closed back cab will raise the bass resonance frequency of the speaker

I worked out equivalent values for design based on Mike Lind's actual impedance plots for a real 4x12 cab. You can match that almost exactly, so I don't think the TS approach is really any better, if you have real measured cab data to start from.


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## pilau

Hi @JohnH, it's a pleasure to converse directly with the mind behind all of this 

Totally on the same page with you - over your impedance plots as well as the lack of widespread availability of T/S params for guitar speakers. Just wanted to help others who might be looking to more precisely model a particular speaker (a vintage model, some rarity, for example), thats exactly where something like a DATS which can measure everything from a single speaker to a full system (speaker(s) loaded in a cabinet, and even a crossover but that's rare in the guitar world).

I am still learning the theory however, and I wanted to ask you - all L-pad examples I've seen have the series resistor first in the circuit and followed by the parallel resistor. In your designs however it's the parallel first followed by the series. Is it because it's easier to introduce the bypass switches to the circuit like that? Or was is it easier to calculate the resistor values that way? Trying to work my head around it. Thanks!


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## JohnH

pilau said:


> Hi @JohnH, it's a pleasure to converse directly with the mind behind all of this
> 
> Totally on the same page with you - over your impedance plots as well as the lack of widespread availability of T/S params for guitar speakers. Just wanted to help others who might be looking to more precisely model a particular speaker (a vintage model, some rarity, for example), thats exactly where something like a DATS which can measure everything from a single speaker to a full system (speaker(s) loaded in a cabinet, and even a crossover but that's rare in the guitar world).
> 
> I am still learning the theory however, and I wanted to ask you - all L-pad examples I've seen have the series resistor first in the circuit and followed by the parallel resistor. In your designs however it's the parallel first followed by the series. Is it because it's easier to introduce the bypass switches to the circuit like that? Or was is it easier to calculate the resistor values that way? Trying to work my head around it. Thanks!



Thanks for your interest and input.

That arrangement of putting the series resistors on the speaker side, is the most important thing I realized about when I was experimenting. It is the key to keeping tone consistent at all attenuation levels, as low as you want. The response of the speaker depends greatly on the impedance and damping that it sees from the amp. This impedance is surprisingly high on most tube guitar amps. Normal L-pads show the speaker a much lower impedance at high attenuation, and as a result, the natural rise in treble at higher frequency is suppressed, resulting in a dull tone. And its pure simple maths to demonstrate it. My design is based on a credible output impedance as measured from my Marshall VM amp, and maintaining this closely at all attenuation settings, (while also showing the amp the impedance that it expects to see from a speaker). The best way to do this is with the series resistor after the parallel resistor.


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## Skorpio

@JohnH
I have been messing around and must say that all those circuits (my original 'Pi', the one modified by placing series/parallel/series/parallel resistors, and your last green schematic) have good sound, although I like your last the best - it seems to get the most balanced sound, I definitely will be thinking about building simple attenuator box with 2 power settings based on schematics here.

I was thinking about another thing though - is it hard to do line-out out of the amps' output ? Some modern tube amps have headphone out so I was wondering if it's that hard to do circuit with low-level signal at the other end, I would mainly use it with guitar interface and IR based cab sims (used it with solid-state amps with good results). Is it hard/worth to do this ?


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## JohnH

Hi @Skorpio 
It would be easy enough to add the line-out, use a resistor in series with a pot across the input wires from the amp, so you can adjust the line-out level. Maybe say 4.7k in series with a 10k pot? but it will be good to experiment with values to suit your use. Or, In theory, a more natural signal may be found at the speaker instead, but at lower level and provided a speaker is connected 

Can't say if it's worth it, but if you want a signal to feed into an IR or cab sim for mixing, recording etc, then something like that is what you'd need.


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## pilau

@JohnH thanks for getting back! Your reply is very interesting, and I've taken my time to read out and reinforce my knowledge before I felt comfortable to reply (now with even more questions ). As I said I'm still learning the theory. Excuse me for the complicated quotes, I'm just trying to make sense of your knowledge 



JohnH said:


> That arrangement of putting the series resistors on the speaker side [...] is the key to keeping tone consistent at all attenuation levels [...] The best way to [show the amp the impedance that it expects to see from a speaker] is with the series resistor after the parallel resistor


Is it just the resistance values adding up or is there more to it?



JohnH said:


> Normal L-pads show the speaker a much lower impedance at high attenuation, and as a result, the natural rise in treble at higher frequency is suppressed [...] And its pure simple maths to demonstrate it


Do you mind demonstrating that to this loyal student? AFAIU higher source impedance in relation to the load creates a low pass filter, which sounds like the opposite of what you're saying. Please don't take this as criticism, Your attenuator obviously works and is a great success and I am honestly only trying to understand.

@Skorpio concerning line out - might want to check this out: http://www.dougcircuits.com/lineout.html


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## Skorpio

@pilau, thanks for the link, a lot of good info here. By the way, you may check out my question/first post on the forum on the 32 page(a few posts back if you have different sorting or whatever) and curves that John provided for my design as well as his post on 2nd page where he shows comparison between 3 different resistor values/arrangement, including classic l-pad and correspoding impedances that speaker (not amp) sees.
The 'worst' design that I used was parallel-series-parallel so I can't say how much l-pad sucks tone - but I heard at -12db it's enough to dull everything.


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## Skorpio

I don't have pots needed but I got two ~8.2k resistors rated at something like 5 watts, so I slapped two in series across wires on the speaker side and wired output jack in between them, to the Behringer UCG interface - it has hi and lo input so I can change amount of signal going to it.
Here are quick sounds of a DSL15H boosted with 7-band EQ on red and clean channels, at volume 2, going to the Guitar Rig 5 with 3 IR cabs mixed
I'm sharing playlist, since I see individual files somehow won't show.



So yeah, this works well  Thanks a lot.


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## JohnH

pilau said:


> Do you mind demonstrating that to this loyal student? AFAIU higher source impedance in relation to the load creates a low pass filter, which sounds like the opposite of what you're saying. Please don't take this as criticism, Your attenuator obviously works and is a great success and I am honestly only trying to understand.



Heres a link with way more than we need to know about LR circuits:

https://en.wikipedia.org/wiki/RL_circuit

But this snip might give a bit of qualitative insight:




Think of Vin and R as the amp, with a certain output resistance R of the amp. L is an inductor, for which impedance rises with frequency. The speaker has inductance (as well as its own resistance etc, ignoring that here)

In the formula above, R is the resistance, and Ls is the impedance of L, rising with frequency. Ls it is a complex number with a phase angle as well as a value. But without getting into all that, just consider the formula qualitatively.

So, if that is all representing the amp and speaker, at low frequency, Ls is low, so VL (the voltage across L, equivalent to the voltage across the speaker in this over-simplified analogy), is low. As frequency rises, Ls rises and so does VL. If VL across the 'speaker' rises with frequency, we hear it as more treble.

If, instead of direct to amp, we make an attenuator for which its output impedance is also R, we get the same relationship as above, at lower volume, ie tone will match.

But with a normal L-pad at high attenuation, the equivalent R is very low, so even at low frequency, Ls/(R+Ls) is almost 1, and hardly rises above that, so relatively there is no rise in treble which we expect to hear.

All of that still applies with a more complicated circuit equivalent to a real speaker, and with true complex maths with phase angles taken into account, it's all built into the designs on this thread.

Short of getting right into that maths, the easiest way to explore it further is with a Spice circuit modelling program such as LT Spice.


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## Skorpio

By complete coincidence I stumbled across this Pete video, so I'd share it. 
One segment compares recording with impulse response (taken from line-out) with different loads on the OT (real speaker, hotplate, suhr reactive load etc.), difference here is very noticeable, especially first two samples. (comparison starts at 16:06)


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## JohnH

Hi @Skorpio , thanks for posting that.

There's a few further hidden insights available there too. The THD has that dullness to the tone, and it's the classic L-pad problem discussed above. The Airbreak sounds much better to me, even though only resistive. This one has it's series resistor inline with the speaker, like we do here, though it doesn't take much care of consistent input and output impedance.

In all cases, we hear a signal forced onto a speaker (or fed to our heaphones etc) at low impedance, so the best set ups have to try to capture all the real amp-speaker interactions electronically before doing this. This is different to our design, which allows the electrical and mechanical characteristics of the real attenuated speaker to do much of the work of creating it's own true tone, like it does unattenuated. This is why it can work so we'll, even as a very simple circuit.


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## Skorpio

I read that someone earlier in this thread also tried line-out but taken before resistive load and seemed to be happy. However, if this this is OT/tubes and speaker interaction back and forth since we're talking AC current I suppose that whole signal going from the amp is changed, not only that _after_ you placed your resistive load, but starting at the output jack, correct ? This is what seems to be happening with that box that Pete Thorn uses.

EDIT: By the way, I wonder how much was or could incorporated from Marshall's own Powerbrake on our design here - I heard both clips and seen schematics, it's far from being simple design.


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## JohnH

The PB is a very different design. But the fan system can be a basis for adding some cooling if you want to crank a big amp. The component values would need to be adjusted by testing though for each case.


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## mAx___

John, what would be the best alternative of all the great information found in this thread to attenuate a 200W amp to -7db? Just one level is all I'd need.


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## JohnH

mAx___ said:


> John, what would be the best alternative of all the great information found in this thread to attenuate a 200W amp to -7db? Just one level is all I'd need.



What kind of amp is tbat? That's an epic amount of power for a valve amp!

If it's like a Marshall major etc, then It would be to build just Stage 1 from Design M or M2, no switches, with power values x4 but same Ohms, with lots of heatsinking and probably a seperately powered fan. Inductors would be thicker wire, need to check, may be 14awg.


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## mAx___

Thanks John. It seems that a 400W resistor for R1 is not that easy to come by. I found one in either 10R or 25R, it's 182mm long, weights 1/2kg and costs $41 
Could this work? I'm trying to avoid getting three 10R for 30R.

http://www.farnell.com/datasheets/2...29.1994544898.1579131190-338014700.1579131190


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## tmingle

Skorpio said:


> I read that someone earlier in this thread also tried line-out but taken before resistive load and seemed to be happy. However, if this this is OT/tubes and speaker interaction back and forth since we're talking AC current I suppose that whole signal going from the amp is changed, not only that _after_ you placed your resistive load, but starting at the output jack, correct ? This is what seems to be happening with that box that Pete Thorn uses.
> 
> EDIT: By the way, I wonder how much was or could incorporated from Marshall's own Powerbrake on our design here - I heard both clips and seen schematics, it's far from being simple design.


Mine is before the resistors. I record the line out & add impulses. I also recorded a clip using a looper to compare different attenuation levels. I set the amp pretty loud, played a loop while changing the switches. I then split the clips & normalized them. The differences were very slight. I am convinced that the tone suck everybody speaks of is more hearing related then amp related. At least with this attenuator.


----------



## JohnH

mAx___ said:


> Thanks John. It seems that a 400W resistor for R1 is not that easy to come by. I found one in either 10R or 25R, it's 182mm long, weights 1/2kg and costs $41
> Could this work? I'm trying to avoid getting three 10R for 30R.
> 
> http://www.farnell.com/datasheets/2...29.1994544898.1579131190-338014700.1579131190



Looks ok! But there could be some advantages in using multiple in combination to make a required value. They may be more easily available, and you can spread them out to disperse the heat. 

I've not used more than a 50W amp, but everything I've tried myself has been with $2 100W (or less) from a chinese ebay seller.


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## Skorpio

@tmingle
What amount of attenuation are you using on what amp setting ? Out of curiosity, because we have similar amps (DSL40C vs 15H)
Right now I'm riding volume at 3 (-13db) at half power, sometimes full(i'm still amazed how loud this little amp can be), presence at 8, BMT at 5-8 depending on IRs.(on recording above I didn't really play with EQ so result ended up being scooped) and I tighten bottom end with EQ in front of the amp.


> The differences were very slight. I am convinced that the tone suck everybody speaks of is more hearing related then amp related. At least with this attenuator.


I agree, I never used typical l-pad, but that's because of how low values would be, so i'm more convinced with higher power non-adjustable resistor arrangement - still DSLs have enough treble in the first place.


----------



## tmingle

Skorpio said:


> @tmingle
> What amount of attenuation are you using on what amp setting ? Out of curiosity, because we have similar amps (DSL40C vs 15H)
> Right now I'm riding volume at 3 (-13db) at half power, sometimes full(i'm still amazed how loud this little amp can be), presence at 8, BMT at 5-8 depending on IRs.(on recording above I didn't really play with EQ so result ended up being scooped) and I tighten bottom end with EQ in front of the amp.
> 
> I agree, I never used typical l-pad, but that's because of how low values would be, so i'm more convinced with higher power non-adjustable resistor arrangement - still DSLs have enough treble in the first place.


I typically use lead 1 with the gain at about 2-3 boosted with a Joyo Vintage OD gain 0, level 10, tone about 65%. Volume varies. My DSL40C sounds its best with volume between 4-5 on the 40 watt setting. I typically run it around 4ish with 14db or 21 db of attenuation. I cant find the clip right now but, I double tracked to a backing track withe the takes panned hard left hard right. 1 track was an SM57 on the edge of the speaker & track 2 was the line out thru an IR. Withe the correct IR, the difference is very slight & the IR takes post effect better IMO. Check out Sonic Drive Studios on Youtube for some great IR tones in many styles.


----------



## chas.wahl

@JohnH : I'm thinking about building the M2 version of your design, but for somewhat lower-powered Fender-flavored amps, like 22 Watts or less; and I have a couple questions:

1. How might I scale the power requirements of the resistors and inductors from 50 W to, say, 25. Is the power required in linear proportion to wattage, or is it a "power of" (for instance, squared) relationship?

2. Is there anything about the difference between a Marshall architecture and that of a Fender that would suggest a different inductor value for use with the latter? How about single-ended vs push-pull configuration?

Thanks for your help,
chas.wahl


----------



## MurrayFarnhorn

JohnH said:


> I've got a new idea for the front end of the attenuator circuit that reduces the number of coils needed, with the same performance achieved, and then leads to an enhancement with the abilty to add a resonant circuit.
> 
> To explain, it best to step through the journey so far. The following four circuits are based on the 8 Ohm design with four stages. They show the front end stage 1, then the other stages all consistent as per the current Attenuator M. Plots are shown of calculated frequency response.
> 
> Red is the signal seen by a speaker directly connected to an amp
> The upper black and green lines are the signal at the amp feeding into the attenuator
> The lower blue and black lines are the signal after the attenuator, using attenuation values of -7db and -14db.
> 
> Ideally, all these plots would be exactly the same shape, with the two attenuated plots being shifted down, and the upper plots of full-volume speaker and amp signal feeding the attenuator all identical. All should show the characteristic bass resonance peak, and the general rise in treble of a natural valve guitar amp driving a real guitar speaker
> 
> *Resistive attenuator
> 
> View attachment 62397
> *
> 
> This is the simplest, and what is interesting about it is how it achieves a basically correct and very consistent tone at all settings, just with resistors. This is the necessary key to all these designs and is the main point of difference to most comparable commercial designs.
> 
> But, although nominally the right tone is coming out of the speaker, the amp itself is only seeing a flat resistive load. ie the upper curves below the red full-response curve are nearly flat, which means that the amp cant respond dynamically to the differences in impedance.
> 
> It still works though!
> 
> *
> Attenuator M - reactive input stage
> *
> This is the current design since January 2019, now well tested by many builders. It captures the rise in treble impedance and lets the amp see this and respond dynamically to it. The bass resonance is developed at the speaker as in the resistive design, but is not seen at the amp. It continues to work well.
> 
> View attachment 62399
> 
> 
> See how the green and black signals at the amp show a treble rise but not a bass resonance peak.
> 
> The basis of this is to split the treble inductance into two coils, one in series and one in parallel. This achieves a balance of impedances that allows further stages of resistive attenuation to be added, without affecting tone.
> 
> The coils are fairly compact and inexpensive, but to do this double arrangement while also capturing a bass resonance in each would be very expensive.
> 
> 
> *Attenuator M2 - single treble coil
> *
> So here's the new idea. The front end of Attenuator M is amended so that the twin coil series/parallel arrangement is replaced with a single coil, and a resistor pair. This results in one less coil - simpler, cheaper and more compact. Plus, it appears that it will work just as well (not tested to date though):
> 
> View attachment 62400
> 
> 
> (EDIT 14/10/19: R2B is better at 18 Ohms)
> 
> I don't see any down-sides to this variation. The numbers look very consistent and I believe it will sound virtually the same as the base attenuator M with twin coils.
> 
> *Attenuator M3 - with bass resonant circuit*
> 
> Having reduced the reactive circuits from two to one, it is now feasible to consider adding a bass-resonant circuit to it, like this:
> 
> View attachment 62401
> 
> 
> (EDIT 14/10/19: R2B is better at 18 Ohms)
> 
> The circuit is based on getting close to the impedance of a a Marshall 1960 with 4x12 G12M's. Now the amp can see a bass resonant peak, and if it is driving hard, it should respond to it.
> 
> It should work fine. What I don't know yet is whether it is really worth doing this. The current circuits don't seem to lack for not having a resonant circuit. So this is just an exploratory proposal. Its not a cheap thing to add because to do the resonant circuit right, it needs capacitors and inductors with very low losses. Ideally, high quality bipolar film caps should be used, and two x 100uF may be needed to get the 200uF value specified. And the resonant coil should be iron core, preferably toroidal for very low dc resistance <0.5 ohm.
> 
> Interesting though I think! It would make the whole unit slightly better for use as a free-standing load box with no speaker, and might just add a few extra tads of authenticity to the bass-note overdrive tone.
> 
> *16 Ohm versions*
> 
> All the above is applicable at 16 Ohms, with inductors and resistors x2 and for M3, the capacitor value halved. This might make the cost a bit more feasible since one 100uF could be used instead of two to get 200uF.



I found a 200uF cap at parts express - solen-200uf-400v-polypropylene-capacitor--027-630. Super expensive though, $73 with free shipping.


----------



## JohnH

chas.wahl said:


> @JohnH : I'm thinking about building the M2 version of your design, but for somewhat lower-powered Fender-flavored amps, like 22 Watts or less; and I have a couple questions:
> 
> 1. How might I scale the power requirements of the resistors and inductors from 50 W to, say, 25. Is the power required in linear proportion to wattage, or is it a "power of" (for instance, squared) relationship?
> 
> 2. Is there anything about the difference between a Marshall architecture and that of a Fender that would suggest a different inductor value for use with the latter? How about single-ended vs push-pull configuration?
> 
> Thanks for your help,
> chas.wahl



Hi Chas and welcome to MF. 

If you wish, you can halve the power ratings noted if you only need up to 25W. But they should still be of the type that bolts
to a chasis, (use thermal grease). Or, just keep with the full ratings, which Is what id suggest.

The inductor values are really aimed at representing the loudspeaker rather than the amp, and are derived from various Celestions such as Greenbacks. But most speakers have around the same for a 12" driver. But, this only affects what the amp sees, the response of the attenuated speaker that you hear is based on it's own characteristics. its not like a tone is forced onto the speaker based on design assumptions. 

The attenuator has been tried on quite a few different amps, including lower powered Fender circuits. @Gene Ballzz has tried a few and may comment!


----------



## JohnH

MurrayFarnhorn said:


> I found a 200uF cap at parts express - solen-200uf-400v-polypropylene-capacitor--027-630. Super expensive though, $73 with free shipping.



Yes indeed! that's the right kind of thing to add the resonant circuit to design M3. 

I'm tempted to get one, plus the coil to go with it, just to find out if if makes any difference. I'm still expecting that it doesn't though, since we get the resonant peak from the speaker anyway and it ould only make a difference to very low fundamental notes.


----------



## Mcentee2

Hi,

I have finally stumbled onto this thread after buying an sv20h!

I posted over on tdpri some time ago my very simple experiences with putting together a resistive attenuator, i knew it worked and sounded good to my ears, but couldn't follow the maths or physics of "why", and this thread is helping me now do that.

As John and others have said, the key is getting the impedances right that the speakers sees from the amp, ie amp - > parallel r -> then series r -> speaker.

However, i am stuck a little with two things:

1) i can work out the nominal impedance seen by the amp due to the resistors and speaker using ohm's law etc but how do i work out the reverse, that seen by the speaker ie the amp out ? 

2) i think the above requires me to know the actual amp out impedance? Without being able to measure that "easily", is there a common value for my simple Marshall power set up, 2x el34 ?

My small box is a simple 25% power reduction setup, for an 8r speaker, i have a parallel 16r resistor followed by a series 8r resistor.

It has a toggle to swap those to a 12r parallel and 16r series for a c10% power setting.

The amp sees 8r for both, but I would really like to know how to work out what the speaker is seeing.


One day i will get some inductors and add to this build!


----------



## chas.wahl

JohnH said:


> Hi Chas and welcome to MF.
> The inductor values are really aimed at representing the loudspeaker rather than the amp, and are derived from various Celestions such as Greenbacks.



Any notions about what happens when the speaker(s) are to be alnico rather than ceramic?

Also, I'm wondering, for the M2 version, whether the wattages sustained by the R1, R2A and R2B resistors remain the same as in your table included in the first (amended) post on p.1, or whether those should be revised in any way to match the different configuration. My table for an M2 build looks like this currently. I presume that the 50W amp wattage requirements for resistors R3 - R8 would remain the same, but don't really know about the first 3.


----------



## JohnH

Hi @chas.wahl

ill check and update the resistor values. In later post #549 I put:


JohnH said:


> if an 8 Ohm build, Its looking like
> R1 = 15
> R2A = 22
> R2B = 18
> L1 = 0.9mH




But let me review again.

Also note, the Watts numbers in your table look like they are based on the actual power dissipation calculated, but components that you buy need a factor of safety above that. I use at least x3. Also, the inductor is 0.9mH =0.009H, rather than 0.9uH

Hi @Mcentee2
I measured the output ohms of my amps to work out the design, but you don't need to unless you want to (Id be very interested if you did though). The reactive design is not critical to the
the exact value and works fine with different amps, adapting to them.

What you do is set the amp up to run very clean on a small sine-wave signal, and feeding into a pure resistive load. Set the volume so no distortion and maybe a couple of volts ac output to a meter, or better, record into a pc. Then change the load resistor (maybe x2 or x1/2, not dangerous at small levels) and see what the new output is. The output resistance is back calculated from knowing the two load resistances and the ratio of the two output signals


----------



## Mcentee2

JohnH said:


> Hi @chas.wahl
> 
> ill check and update the resistor values. In later post #549 I put:
> 
> 
> 
> But let me review again.
> 
> Also note, the Watts numbers in your table look like they are based on the actual power dissipation calculated, but components that you buy need a factor of safety above that. I use at least x3. Also, the inductor is 0.9mH =0.009H, rather than 0.9uH
> 
> Hi @Mcentee2
> I measured the output ohms of my amps to work out the design, but you don't need to unless you want to (Id be very interested if you did though). The reactive design is not critical to the
> the exact value and works fine with different amps, adapting to them.
> 
> What you do is set the amp up to run very clean on a small sine-wave signal, and feeding into a pure resistive load. Set the volume so no distortion and maybe a couple of volts ac output to a meter, or better, record into a pc. Then change the load resistor (maybe x2 or x1/2, not dangerous at small levels) and see what the new output is. The output resistance is back calculated from knowing the two load resistances and the ratio of the two output signals



Thanks, I might even have a lot of this around.

I can feed a small ac signal from my phone at about 500ma like a pickup.

I have a jettenuator that I can possibly use as the loads. It should be able to present a 4r, 8r or 16r to the amp, but isn’t a straightforward single resistor each time. Otherwise I could cannibalise my current attenuators mentioned in my previous post.


I need to see if I can measure across that load though, about 2vAC you reckon to start?

Also, do I need be wary of how I plug these into the amp? Ie should I plug them into the same 8r output on the amp, would that not hurt it ?


----------



## JohnH

Sounds fine. The exact voltage that you get at the output isn't critical, because the number crunching is based on a ratio of voltages rather than absolute values. Also, the two loads that you use can vary, so long as you can measure them. They don't need to be in an exact ratio. 

Athough the test loads may imply a load mismatch, if voltages are kept low it should be fine. eg, a 20W amp into 8 Ohms has a voltage of 9V., so keep it below 2V and it's not working hard.

All of that makes the maths a bit convoluted, so I usually set up a spreadsheet to help solve it. Basically, we are finding output resistance Rout, using voltage ratio V1/V2 and known load resistances R1 and R2. So, we need to solve for two voltage dividers. :

(R1/(R1 + Rout)) ÷ (R2/ (R2 + Rout)) = V1/V2

'Goalseek' in Excel is a quick lazy way.

But, As I noted above, doing this is optional. My two valve amps have very different output R values, by a factor of about 2. They both work well with the same ttenuator circuit. The key thing is that they are both much higher than that of an SS amp, and also both several x the nominal speaker ohms.


----------



## chas.wahl

@JohnH : Thanks for the later update on the R1 value for M2. I had missed that somehow, even though I read through from p.27 onward.

I have been sourcing resistors using your recommendation for 3x the wattage -- I just didn't put the safety factor into the table above.

Is this "mH" used for inductors then _milli_Henrys rather than _micro_Henrys? 0.9 mH would then be 0.0009 H, rather than 0.009 H, correct? I _have_ been looking for a 0.9 mH inductor, even though I wrote uH. Would it make much difference to use an 0.82 mH or a 1.0 mh inductor, both of which seem to be easier to find?

I'd still like some confirmation about the wattage requirement for M2's R1, R2A and R2B (assuming a 50 W amp output).

Thanks,


----------



## JohnH

Hi yes, appologies, it really is 0.9milli-Henries = 0.0009H.!

Yes it doesn't make much difference +/- 10%. So if it's easier to find, I'd go to 1mH. (could always unwind a few turns, but not necessary)

I'll check numbers more. But another point on power, the two resistors for the -3.5db stage are noted with larger power, because in the full design, they can be receiving the full amp power. But in the lite versions, they are always after Stage 1 so get much less.


----------



## Mcentee2

JohnH said:


> Sounds fine. The exact voltage that you get at the output isn't critical, because the number crunching is based on a ratio of voltages rather than absolute values. Also, the two loads that you use can vary, so long as you can measure them. They don't need to be in an exact ratio.
> 
> Athough the test loads may imply a load mismatch, if voltages are kept low it should be fine. eg, a 20W amp into 8 Ohms has a voltage of 9V., so keep it below 2V and it's not working hard.
> 
> All of that makes the maths a bit convoluted, so I usually set up a spreadsheet to help solve it. Basically, we are finding output resistance Rout, using voltage ratio V1/V2 and known load resistances R1 and R2. So, we need to solve for two voltage dividers. :
> 
> (R1/(R1 + Rout)) ÷ (R2/ (R2 + Rout)) = V1/V2
> 
> 'Goalseek' in Excel is a quick lazy way.
> 
> But, As I noted above, doing this is optional. My two valve amps have very different output R values, by a factor of about 2. They both work well with the same ttenuator circuit. The key thing is that they are both much higher than that of an SS amp, and also both several x the nominal speaker ohms.




Managed to get this done, but haven’t yet found an easy way to finish the equation without excel to hand (see edit)

r1 is 8.4r, V1 is 1.82vAC
r2 is 15.7r, V2 is 2.22vAC

edit: according to Wolfram, if I have entered it correctly this works out as Rout=5.31r

I can see why my home made Type C attenuator keeps that treble, looks like the load seen by the speaker is up at 12r at its 25% setting, and 20r at its 10% setting.


----------



## JohnH

Hi @chas.wahl

I checked through the analysis numbers last night. I'm happy that it's all good for design M2. Some of the values are such that they could click up or down half a step and since they are almost optimised, it makes very little difference. Eg, R1 can be 12 or 15 in the 8 Ohm build. The difference is about half an ohm as seen by the amp and about half a db. All negligible.

I also ran the Spice model to assess power dissipation. R1 and R2b do most of their work at low to mid frequencies, R2a does more at high frequencies, so it's harder to estimate since it's not obvious exactly what the frequency spread will be. But here's what I believe is a good safe recipe, resistance and estimated actual power dissipated at 50W.

R1: 15 Ohms, 27W
R2a: 22 Ohms, 17W
R2b: 18 Ohms, 7W

Spec = Power x about 3 leads to suitable ratings being 100W, 50W and 25W minimum.

For R7 and R8, if it's the lite version where they are always after Stage 1, their calculated power dissipations are 6W and 4W respectively.

For the ones I build, I've settled on just using 100W and 25W case mounted resistors. They arrange neatly into the case. I stopped using 50W ones since they are not much smaller than the 100's and I found them just a bit annoying geometrically when only one or two are needed.


----------



## JohnH

Mcentee2 said:


> Managed to get this done, but haven’t yet found an easy way to finish the equation without excel to hand (see edit)
> 
> r1 is 8.4r, V1 is 1.82vAC
> r2 is 15.7r, V2 is 2.22vAC
> 
> edit: according to Wolfram, if I have entered it correctly this works out as Rout=5.31r
> 
> I can see why my home made Type C attenuator keeps that treble, looks like the load seen by the speaker is up at 12r at its 25% setting, and 20r at its 10% setting.



I agree with the number crunching. 

Output resistance 5 Ohms (from 8 ohm tap) suggests this amp will be less affected by the kind of reactive effects under discussion. If you ran it with a simple resistive attenuator with a higher output resistance such as 20 Ohms, you'd expect about 2 or 3 db more high treble and bass resonance. Not a big difference, might be good based on opinion. And it supports your design values for a resistive version.

But interestingly, once you add the inductive coils (design M or M2) , it then compensates, and from low mids up to high treble, the response works out to be very much the same as the amp direct to speaker, ie it adapts to different amps, even though the design was based around a higher output resistance.


----------



## Mcentee2

JohnH said:


> I agree with the number crunching.
> 
> Output resistance 5 Ohms (from 8 ohm tap) suggests this amp will be less affected by the kind of reactive effects under discussion. If you ran it with a simple resistive attenuator with a higher output resistance such as 20 Ohms, you'd expect about 2 or 3 db more high treble and bass resonance. Not a big difference, might be good based on opinion. And it supports your design values for a resistive version.
> 
> But interestingly, once you add the inductive coils (design M or M2) , it then compensates, and from low mids up to high treble, the response works out to be very much the same as the amp direct to speaker, ie it adapts to different amps, even though the design was based around a higher output resistance.



Thanks for that insight, I was just thinking about your early posts on page 2 re the "Type C" set up and associated treble response, and thinking it couldn't be that far from a reactive response curve, at least to a first approximation.

This is probably asking a bit much, but is there any chance you could run some numbers or graphs to show this? No worries if its more convoluted than I am assuming!


----------



## chas.wahl

Thanks a lot for the M2 update @JohnH . Here's my revised table of values for a 25 W amp, with 8 Ohm speaker impedance:



I appreciate all the help!


----------



## Mcentee2

Mcentee2 said:


> Thanks for that insight, I was just thinking about your early posts on page 2 re the "Type C" set up and associated treble response, and thinking it couldn't be that far from a reactive response curve, at least to a first approximation.
> 
> This is probably asking a bit much, but is there any chance you could run some numbers or graphs to show this? No worries if its more convoluted than I am assuming!




Update, i have now found (ie my phone has now shown) the early graphs provided re source impedance re frequency response, so no need to provide more for my request above.

I am curious to find out if source impedance in a tube amp changes with frequency like a speaker's does, and also if it changes with power output/stress.

I don't know the former, but I think it would go down as the amp is pushed harder into clipping, not sure if that is gradual or a cliff edge, thus damping would increase (still probably negligible in the big scheme?).


Anyway, I think i will get a small inductor and put together an M2 version to see how my sv20h with its 5r output impedance behaves.


----------



## JohnH

Hi @Mcentee2 

I've been curious too on those questions, and have asked around on other threads. Here's a great thread, and see the latest from James Freeman;

https://www.thegearpage.net/board/i...tive-dummy-load.1072793/page-21#post-29861715

What I think happens , based on advice, is that the output impedance of the amp is essentially resistive, not varrying much with frequency, But it does vary with power, and it drops as the tubes are pushed into saturation. In doing so, the damping of the speaker by the amp is increased. So at high power amp drive there is a natural filter on bass and treble, which changes dynamically, as the note decays, becoming clearer. It's this dynamically changing effect that you don't get with a resistive attdnuator, but do get with a real speaker or a properly designed reactive attenuator.

Btw, those impedance measurements, are they the 8 ohm tap or the 16 on the SV20?


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2
> 
> I've been curious too on those questions, and have asked around on other threads. Here's a great thread, and see the latest from James Freeman;
> 
> https://www.thegearpage.net/board/i...tive-dummy-load.1072793/page-21#post-29861715
> 
> What I think happens , based on advice, is that the output impedance of the amp is essentially resistive, not varrying much with frequency, But it does vary with power, and it drops as the tubes are pushed into saturation. In doing so, the damping of the speaker by the amp is increased. So at high power amp drive there is a natural filter on bass and treble, which changes dynamically, as the note decays, becoming clearer. It's this dynamically changing effect that you don't get with a resistive attdnuator, but do get with a real speaker or a properly designed reactive attenuator.
> 
> Btw, those impedance measurements, are they the 8 ohm tap or the 16 on the SV20?



Good, all that makes sense, if src impedance is indeed fairly constant it would one the few simplicities in all this!

The readings were from the 8 ohm output.

I have now returned the Jettenuator, so will have to source other loads if i were to do more tests.

I'd like to try and *very simplistically" summarise what i understand is going on re the output impedance/speaker interaction (damping), vs the use resistive or reactive loads:

Firstly: the higher the amp src impedance the more the speaker is allowed to enhance its bass and treble responses, as it isn't damped by the src. From this you can tweak a resistive attenuator to enhance/reduce bass/treble heard eventually at the speaker.

This is what my Type C build does, it inherently has a higher src impedence (12-16r) than the amp itself (5r), and so enhances treble/bass at lower volumes, which sounds pleasing.

If your amp has a lower or higher src impedance, you might need to tweak your attenuator to get a good outcome.

I notice, John, you changed from a Type B to a Type C early on, was this related to the above and your later src impedance tests showing your amp to be up around 20r more like a Type C build?

As the amp starts to be pushed, or is heavily pushed, eventually its src impedance reduces and the resulting freq curves at the speaker change in line, ie drop in treble/bass response due to increased damping. Not sure how variable this is though.


2) The above doesn't take into account the amp's desire to match output power to the speaker load per frequency. Ie as freq goes up the speaker load increases and the amp takes up more power feeding it. Ie reactance.

So superimposed on top of the frequency curve of the speaker and src impedance we have an actual "how much" of each frequency the amp puts out and we end up hearing.

A purely resistive load cannot add this second bit, you just get the basic "constant" applied freq curve. The sound you get might "sound" ok, but is probably missing some element of unattenuated compression/bloom, high-mid to high overtones aren't as marked or overwhelming the amp output as much.

The reactive load element brings this back in, and allows the amp to push the attenuator at those frequencies with the "correct" power, bringing back more of the feel, compression/bloom.

Both of these combine to give you that amp/load response complexity.


Hope I have that at least semi accurate!


----------



## Mcentee2

@JohnH As well as my waffle above, I do have a technical querie about version M.

When did the resistor around L2 get dropped and why?


----------



## JohnH

Hi @Mcentee2 
That all sounds about right, to within the modest level of my own understanding anyway.

A few more related points:

One of the key drivers of the amp output impedance is negative feedback around the output stage. Without it, at small signals, my understanding is that a valve output stage is close to acting as a constant current source, implying very high output impedance. I have one amp like this, a dsl401 with 4x el84's and no NFB. It's unusual for a Marshall but is similar to a Vox AC30 in this respect. I measured 40 to 50 Ohms on this from it's 8 ohm tap. At this level, solving the numbers is very sensitive to values. This design gives a very clear and responsive output, which is probably what attracted me to the amp originally, though I didn't know why at the time.

Most Marshalls have some NFB, and this is affected by presence and resonance controls. So if you turn up presence, there is less NFB at higher frequencies and you get a treble lift, and also a raise in output impedance at high frequencies, which is relevant for attenuators and speaker response. (ie, it's not necessarily a flat response like I said before, unless NFB is neutral with frequency)

My VM has NFB with a presence control, and I measured 20 Ohms at the 8 ohm tap at mid frequencies.

Attenuator Design C was a resistive one for use with 16 ohm amp taps. Same as B but values scaled up x2. It's output R targets around 40 Ohm.

On Design M, it took a while to fine-tune it with calcs and tests, and this led to the result that a bypass resistor for L2 wasn't needed or didn't make it better, so I dropped it. Whereas the one at L1 turned out to be much more important.


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2
> That all sounds about right, to within the modest level of my own understanding anyway.
> 
> A few more related points:
> 
> One of the key drivers of the amp output impedance is negative feedback around the output stage. Without it, at small signals, my understanding is that a valve output stage is close to acting as a constant current source, implying very high output impedance. I have one amp like this, a dsl401 with 4x el84's and no NFB. It's unusual for a Marshall but is similar to a Vox AC30 in this respect. I measured 40 to 50 Ohms on this from it's 8 ohm tap. At this level, solving the numbers is very sensitive to values. This design gives a very clear and responsive output, which is probably what attracted me to the amp originally, though I didn't know why at the time.
> 
> Most Marshalls have some NFB, and this is affected by presence and resonance controls. So if you turn up presence, there is less NFB at higher frequencies and you get a treble lift, and also a raise in output impedance at high frequencies, which is relevant for attenuators and speaker response. (ie, it's not necessarily a flat response like I said before, unless NFB is neutral with frequency)
> 
> My VM has NFB with a presence control, and I measured 20 Ohms at the 8 ohm tap at mid frequencies.
> 
> Attenuator Design C was a resistive one for use with 16 ohm amp taps. Same as B but values scaled up x2. It's output R targets around 40 Ohm.
> 
> On Design M, it took a while to fine-tune it with calcs and tests, and this led to the result that a bypass resistor for L2 wasn't needed or didn't make it better, so I dropped it. Whereas the one at L1 turned out to be much more important.


Many thanks!

Can I ask re the value of r1 for an 8ohm M2 build, in the diagram on page 27 you have it as 12r, however the posts above still seem to confirm it as 15r ?

I have understood the change for r2b to 18r 

Thanks again.

Edit: I've ordered parts for an M2, using r1 of 15r.

This is a single reactive stage plus a further -7db stage.

I'll put a simple toggle around the 0.9mH inductor so I can easily do a reactive/cs resistive comparison  the parallel r3a/r2b 18r/22r pair come out as roughly the same as a single 10r single resistor in a resistive -7dB stage anyway


----------



## TonyK

@JohnH Hope you have been well, mate. I've been a little out of whack and out of touch with some weird health things going on.

Anyway, I'm about to pick up a hardwired replica of the 1957 Bandmaster 5e7 with the three 10" speakers in it. I suppose this might make my 5e3 project a little redundant but that's another story. So, the Bandmaster had a strange output impedance of around 2.6 ohms, but from memory, the OP transformer was the standard 4 ohm model used by some of the other Tweeds at that time, and this mismatch makes up a huge amount of the unique sound the amp produces.

You might remember that I build the 8 ohm in and out "M" variation. Do you think I could use this as is or would it take some tweaking?

Cheers,
TonyK


----------



## JohnH

TonyK said:


> @JohnH Hope you have been well, mate. I've been a little out of whack and out of touch with some weird health things going on.
> 
> Anyway, I'm about to pick up a hardwired replica of the 1957 Bandmaster 5e7 with the three 10" speakers in it. I suppose this might make my 5e3 project a little redundant but that's another story. So, the Bandmaster had a strange output impedance of around 2.6 ohms, but from memory, the OP transformer was the standard 4 ohm model used by some of the other Tweeds at that time, and this mismatch makes up a huge amount of the unique sound the amp produces.
> 
> You might remember that I build the 8 ohm in and out "M" variation. Do you think I could use this as is or would it take some tweaking?
> 
> Cheers,
> TonyK



Hi Tony, I hope you are feeling better.

Would you be running this from a 4 Ohm amp tap? does it have an 8 Ohm tap?


----------



## TonyK

JohnH said:


> Hi Tony, I hope you are feeling better.
> 
> Would you be running this from a 4 Ohm amp tap? does it have an 8 Ohm tap?


Hi John, actually in hospital right now, but no worries  Thanks for asking. I need to look closer at the schematic for the amp, but because the original amp uses the same OT as the Super Reverb, I'm 90% sure that it is a 4 ohm output tap. Vintage officiandos argue that one of the reasons that the amp has a distinct voicing compared to the SR or even Tweed Twins and Bassmans is specifically that slight mismatch with the the 2.6 ohms calculated for the three 10" speakers. If I find out more I'll let you know but tomorrow I'm being transferred to a different hospital in Bordeaux. Nothing too serious (I hope)!


----------



## JohnH

ok good luck!

With the amp, if it only has a 4ohm output tap, and you use an 8 ohm attenuator, then you have to make a call as to whether tbat mismatch is enough to be worried about. 

If if is of a concern, one option would be an extra 8ohm with inductor in series, connected across the input so the amp sees 4 ohm. This would take another 3db off the power. You could then run with the 2.7ohm speaker load after the attenuator, with a tendency to a fairly bright output.


----------



## TonyK

JohnH said:


> ok good luck!
> 
> With the amp, if it only has a 4ohm output tap, and you use an 8 ohm attenuator, then you have to make a call as to whether tbat mismatch is enough to be worried about.
> 
> If if is of a concern, one option would be an extra 8ohm with inductor in series, connected across the input so the amp sees 4 ohm. This would take another 3db off the power. You could then run with the 2.7ohm speaker load after the attenuator, with a tendency to a fairly bright output.


Brilliant. That's what I was thinking, just to add a little extra resistance even switchable if necessary. I'll give a lot more thought to this shortly! Thanks John!


----------



## Mcentee2

Well, still awaiting the arrival of my inductors, should be here today.

I've mocked up an old Biyang DS12 enclosure with all resistors and switches ready to drill, but one question I have is about the grounding of the jacks to the enclosure.

The jacks are the full bodied plastic type rather than switchcraft, but they still have a contact between the sleeve and the enclosure, continuity tested with my DMM.

Should they be completely isolated ?

I realise it's all AC at this point so ground/hot is moot I suppose but my older simple Type C attenuator build has the same set up and I haven't noticed any detriment or me being shocked etc by picking it up when it use......


----------



## JohnH

I think having the case live makes no difference to tone. It would just be of some risk in case something that is genuinely grounded happened to touch it, and still unlikely to be a problem, it's just a matter of what seems 'right'


----------



## Mcentee2

JohnH said:


> I think having the case live makes no difference to tone. It would just be of some risk in case something that is genuinely grounded happened to touch it, and still unlikely to be a problem, it's just a matter of what seems 'right'



Many thanks - I think operationally it works fine - as long as the safety aspect is fine too, I will be ok 

Edit: well, my inductors arrived in the post, I got 3 just in case I wanted to build an M as well as an M2 - 0.4 /0.6 / 0.9 mH.

Wow, the 0.9 is a lot bigger and heavier than I though it was going to be, I'll have to rethink my enclosure again to fit two stages in


----------



## RobinHugh

All
I love the DIY approach and am in the process of building a Bassman clone. I want a attenuator to be able to explore its tone at home. Do you have a recommendation for a reactive attenuator design which matches the 2 ohm output transformer / speaker arrangement ? 
Thanks.


----------



## JohnH

RobinHugh said:


> All
> I love the DIY approach and am in the process of building a Bassman clone. I want a attenuator to be able to explore its tone at home. Do you have a recommendation for a reactive attenuator design which matches the 2 ohm output transformer / speaker arrangement ?
> Thanks.



Thanks for your interest. In theory you can scale values according to the nominal ohms. So if you were doing design M or M2, at 2ohms, you could use values for the 8 ohm versions with resistances and inductances divided by 4. Current would be x2, so use thicker wire, resistor power is unchanged.

But we havnt tested that. I only know of 16 and 8 ohm builds to date.

Will the amp have any other taps? and what speakers do you want to use?


----------



## RobinHugh

Thanks for a quick response ! No other taps on the OT. It will run a 4x10 cab each being 8 ohms.


----------



## JohnH

Mcentee2 said:


> Many thanks - I think operationally it works fine - as long as the safety aspect is fine too, I will be ok
> 
> Edit: well, my inductors arrived in the post, I got 3 just in case I wanted to build an M as well as an M2 - 0.4 /0.6 / 0.9 mH.
> 
> Wow, the 0.9 is a lot bigger and heavier than I though it was going to be, I'll have to rethink my enclosure again to fit two stages in



Great! Out of interest, where did you buy them and what wire Gage are they?


----------



## Mcentee2

JohnH said:


> Great! Out of interest, where did you buy them and what wire Gage are they?



The UK guy on ebay mentioned earlier in the thread, Steve (stevie29810), wire gauge 1mm.

.9mH (Ebay item no. 133093852705)
.6mH (Ebay item no. 133093852705)
.4mH (Ebay item no. 133002963518)

The .9mH is on a 6cm diameter, 3cm tall bobbin, 167g

The .4 and .6 are on 4cm diameter, 2cm tall bobbins, 94g and 117g.

My new enclosure should be arriving today, Hammond 1590x 120x245x56, room for future extras I hope  first build is an M2. I just got the .4 and .6 as they were so cheap and combined postage, I haven't got the M stage 1 resistors yet.













IMG_20200131_064917368_HDR



__ Mcentee2
__ Jan 31, 2020


----------



## JohnH

RobinHugh said:


> Thanks for a quick response ! No other taps on the OT. It will run a 4x10 cab each being 8 ohms.



All fine! lets work it out for a 2 ohm version.. How much power would the amp put out?


----------



## RobinHugh

JohnH said:


> All fine! lets work it out for a 2 ohm version.. How much power would the amp put out?


The amp is about 45 watts of glorious tube power !
Based on what I’ve been reading about amp deign history, this amp design was what inspired early Marshall circuits.


----------



## Mcentee2

@JohnH

Enclosure is here and all is looking good, planning wise. I thought i would go back and refresh the maths etc and am puzzled by the -14db stage.

R5 4.7r
R6 15r

Given an 8r speaker, this seems to work out as only a 3.9r load that the first stage sees (4.7r parallel to (15r plus 8r))

This makes Stage 1 (7db) plus Stage 3 (14db) load seen by the amp as nearer 7r in total, which is probably close enough but seems a little low.

Is this right? Is there a "better" choice of values for the 14db stage to get closer to the desired 8r load, like the 7db and 3.5db stages do?

Another way to put it is, you would be able to run either of the 7db or 3.5db stages on their own with an 8r speaker into the amp 8r out, but not the 14db stage unless you plugged into the 4r amp out ?

Mind you, I am probably getting my maths wrong when combining the two stages....


----------



## JohnH

Mcentee2 said:


> @JohnH
> 
> Enclosure is here and all is looking good, planning wise. I thought i would go back and refresh the maths etc and am puzzled by the -14db stage.
> 
> R5 4.7r
> R6 15r
> 
> Given an 8r speaker, this seems to work out as only a 3.9r load that the first stage sees (4.7r parallel to (15r plus 8r))
> 
> This makes Stage 1 (7db) plus Stage 3 (14db) load seen by the amp as nearer 7r in total, which is probably close enough but seems a little low.
> 
> Is this right? Is there a "better" choice of values for the 14db stage to get closer to the desired 8r load, like the 7db and 3.5db stages do?
> 
> Another way to put it is, you would be able to run either of the 7db or 3.5db stages on their own with an 8r speaker into the amp 8r out, but not the 14db stage unless you plugged into the 4r amp out ?
> 
> Mind you, I am probably getting my maths wrong when combining the two stages....



Hi @Mcentee2 

Your resistor maths is completely right, and what you have noticed is intentional! The -14db stage does present about 3.9 ohms to the previous stage. But, it is never shown directly to the amp, it is always through the -7db stage. If you just work through the resistances, then you get about 7.2 Ohms at the amp. That in itself, is well close enough for an 8 ohm tap, its only about 10% off and even 30% would be ok.

But, there also some resistance in the coil or coils, and this adds about 1/2 an ohm.

So its all ok. but, if you want to fine tune this parameter, there are a couple of further constraints. One is that depending where you buy, only certain resistor values are available. Actually for my own build, I used 5 ohm instead of 4.7.

The other thing is, to create a general attenuator stage with a set input resistance, output resistance and attenuation level, you theoretically need 3 resistors in a 'Pi' or 'T' arrangement. For our use, at a specific attenuation of about -7db, one of the resistors is at zero, hence we have two resistors for stages 1 and 2 and they have a nice theoretical perfection. On this basis, the -14db stage should have three, the 4.7 would be 5.6, the 15 would remain and there would be a 3.3 before them, in a T. But that's an extra component, and its not really needed. and also, switching the stage to off then needs two poles on a switch, so more complication to go wrong.

You can see I think to much about it too...


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2
> 
> Your resistor maths is completely right, and what you have noticed is intentional! The -14db stage does present about 3.9 ohms to the previous stage. But, it is never shown directly to the amp, it is always through the -7db stage. If you just work through the resistances, then you get about 7.2 Ohms at the amp. That in itself, is well close enough for an 8 ohm tap, its only about 10% off and even 30% would be ok.
> 
> But, there also some resistance in the coil or coils, and this adds about 1/2 an ohm.
> 
> So its all ok. but, if you want to fine tune this parameter, there are a couple of further constraints. One is that depending where you buy, only certain resistor values are available. Actually for my own build, I used 5 ohm instead of 4.7.
> 
> The other thing is, to create a general attenuator stage with a set input resistance, output resistance and attenuation level, you theoretically need 3 resistors in a 'Pi' or 'T' arrangement. For our use, at a specific attenuation of about -7db, one of the resistors is at zero, hence we have two resistors for stages 1 and 2 and they have a nice theoretical perfection. On this basis, the -14db stage should have three, the 4.7 would be 5.6, the 15 would remain and there would be a 3.3 before them, in a T. But that's an extra component, and its not really needed. and also, switching the stage to off then needs two poles on a switch, so more complication to go wrong.
> 
> You can see I think to much about it too...


Many thanks, glad I had it the right way round!

Also, half the fun in this is thinking too much


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## Mcentee2

Box drilled and resistors/jacks/switches in.

Master switch, stage 1 always on if master on, reactive inductor on/off switch, stage 2 7db switch, spare switch for another stage later.

The box is deep enough for the inductor to sit above the resistors so there is plenty of room for stage expansion.

Just need to do a wiring layout and solder it up tomorrow:













IMG_20200201_180312769_HDR



__ Mcentee2
__ Feb 1, 2020


----------



## Mcentee2

All done, soldered up nicely.

The inductor is loose at the moment until I decide to add another stage or not, I am thinking of a small piece of velcro to tide me over....













IMG_20200203_145628974_HDR



__ Mcentee2
__ Feb 3, 2020


















IMG_20200203_145640217_HDR



__ Mcentee2
__ Feb 3, 2020






As for sounds, yes there is a definite difference/improvement with the inductor when you can do a direct A/B comparison. If I didn't have that toggle I think I would be hard pressed to really notice the difference if I had to connect and disconnect different boxes etc having said that....

There is a tangible high/mid content there with the inductor that feels more lively and moving with touch playing; things like harmonics, controlled feedback etc seem to come into being more smoothly or simply, also can control them back down.

I need to find where in the crunch/OD/distortion this is best heard, i suspect it is indeed in the area where it is crunchy then sustains into clean.

With a 20w SV20h run on max, and a single 8r G12H55, the box using both 7db stages gets luke warm at most after an hour or so, and that is only in the area where the resistors are mounted. At the moment the box is "upside down" as the inductor is loose, so the resistors are on the underside looking at it now with no open-air flow really.


----------



## JohnH

hi @Mcentee2 that's great news! it looks like a nice build and I'm very happy that it works well. This is the first completed M2 that I know of! Interesting too to hear your views on with and without the inductor.

On mounting the inductor, you probably picked up on the watch-it of not using a normal steel bolt. But you can use nylon or stainless bolts. Velcro could be ok too if its not going to be shaken around much. My ones for Design M sit down onto the base and I glued them there.

Even with just your two stages, but combined with the 20/5W switch that the SV and SC amps have, you have a good range of attenuation options down to about 0.2W.


----------



## Mcentee2

JohnH said:


> hi @Mcentee2 that's great news! it looks like a nice build and I'm very happy that it works well. This is the first completed M2 that I know of! Interesting too to hear your views on with and without the inductor.
> 
> On mounting the inductor, you probably picked up on the watch-it of not using a normal steel bolt. But you can use nylon or stainless bolts. Velcro could be ok too if its not going to be shaken around much. My ones for Design M sit down onto the base and I glued them there.
> 
> Even with just your two stages, but combined with the 20/5W switch that the SV and SC amps have, you have a good range of attenuation options down to about 0.1W.



Thankyou, its all your hard work really!

If i had some simple/easy way to record properly I would post some samples that could be analysed.

Thinking though, what would be a better source for a recording - would a line out from the attenuator show something more obvious than a miked speaker?

That might reduce the variables at least to offer consistence for comparison even if the sound itself isn't what you would hear from the speaker.

IIRC wasn't there an added Line Out design earlier in the thread ?


----------



## JohnH

Thanks for the thought.
On recording, I guess it depends what you have available. I use a vocal Mic into a small mixer, and then straight into a line-in on a pc, or even a mic input on a laptop, recording with Audacity.


----------



## Mcentee2

JohnH said:


> Thanks for the thought.
> On recording, I guess it depends what you have available. I use a vocal Mic into a small mixer, and then straight into a line-in on a pc, or even a mic input on a laptop, recording with Audacity.



I don't have any mike etc 

The SV does have a DI but it is already cab simmed inside, no idea how.

I use that already to feed a wet amp, but could try to send that to a recorder somehow, I guess it should still pick up any difference in tone between reactive and resistive, for comparison if not to show absolutes.

As a source, rather than playing, I would feed a suitable signal from say my phone, or looper if i had one, into the amp so it was consistent across tests.


----------



## JohnH

Ok thanks but not to worry then about samples etc. its difficult to interpret without a miced sound, to compare.
But I am grateful for your perceptive descriptions of how it works for you


----------



## Mcentee2

JohnH said:


> Ok thanks but not to worry then about samples etc. its difficult to interpret without a miced sound, to compare.
> But I am grateful for your perceptive descriptions of how it works for you



Np, we are susceptible to hearing what we want to hear though 

I'll think on it some more and see what I can come up with.

If and when i get some output files together, I have used Audacity before so it should show something. Would be happy to email them to you as well for more in depth analysis using your better tools!


----------



## matttornado

Nice build! I wish mine could be small like that. Mine is a beast being able to handle well over 100 watts!


----------



## JohnH

Hi @Mcentee2 , I'm thinking that your build, maybe with the addition of a -3.5db stage at the end, might be an ideal minimal attenuation option for the studio series. The extra stage will fill in the gaps between available levels, (useful particularly for the NMV Sv20), and take it all a tad lower when needed. -17.5db from the 5W setting works out as 0.09W, nice for use at home.

One thing I'm curious about, now tbat you have yours built: How is the tone of the SV at say 5W unattenuated, vs 20W and -7db (which is 4W)? or, how about 5W at -7 db (=1W) vs 20W and -14db? (=0.8W)


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2 , I'm thinking that your build, maybe with the addition of a -3.5db stage at the end, might be an ideal minimal attenuation option for the studio series. The extra stage will fill in the gaps between available levels, (useful particularly for the NMV Sv20), and take it all a tad lower when needed. -17.5db from the 5W setting works out as 0.09W, nice for use at home.
> 
> One thing I'm curious about, now tbat you have yours built: How is the tone of the SV at say 5W unattenuated, vs 20W and -7db (which is 4W)? or, how about 5W at -7 db (=1W) vs 20W and -14db? (=0.8W)



I'll have been thinking about an end -3.5db stage, and I can reuse the resistors from my old type c build 

I have some playing time tomorrow so can get my ear attuned to those comparisons.

I really must somehow get a recording scenario set up properly. I like the idea of repeated loops played through the attenuation settings, then volume normalised in audacity.


----------



## Mcentee2

Right...here goes.

Managed to get some "good" stuff this morning.

1) Taking a feed from the SV20h Line Out - no idea what cab sim Marshall uses, but it the same for all samples so comparisons should be valid.

2) guitar into High Treble Hi, patch from High Treble Lo to Normal Hi - Hi on full, Normal on about 3pm, presence/bass etc to taste but not changed across samples.

3) Guitar phrase looped into my DD500, simple stuff, repeatable and managable  crunchy chords, some picking and dynamics

4) Line out from SV20h goes into my dug-out-of-a-drawer Guita rig MObole interface, into a laptop running audacity.

5) Attenuator on the 8r speaker out, actually it is my new reactive/resistive atenuator into my old attenuator, into the speaker. this gives unattenuated, -7db, -14db, -21db, -24db levels

6) I took 4 sound run through recordings: one at 5w resistive only settings and one at 5w reactive settings, and similar pair for 20w

7) In audacity normalised each of the 4 recordings so each attenuation setting was at the same "level".

So with that dataset:

8) Selected a sample of the recorded phrase and ran the Audacity Frequency Analysis view, (Spectrum, Hanning, log, 1024 samples). Export to csv then import into excel

9) Repeat, rinse for (for this run through) unattenuated, 20w -7b Resistive and -7db Reactive, -24db Resistive and -24db Reactive

10) Massaged Excel data so show db deviation of each attenuated example against the unattentuated baseline (ie normalise against the unattenuated).

11) Simply plot them all out:

so Y axis (Vertical) is db difference, X axis is the frequency in linear (first pic) then log (2nd pic).













SV20h 20w



__ Mcentee2
__ Feb 5, 2020


















SV20h 20w 2



__ Mcentee2
__ Feb 5, 2020






Suffice to say - interesting!

Not 100% sure how to interpret the finer detail, but:

1) looks like the reactive load makes a significant difference in "retaining" the high end, but all traces show loss at that higher end as the attenuation increases

2) All the attenuated traces are lower than the unattenuated, even after audacity normalisation, that could be expected as tonal balance and therefore "loudness" shifts ?

3) At about 350hz all the traces most closely match the unattenuated, then all still drop off.

4) Poss the Marshall cab sim is doing something here as well, when I get chance I will put a Line out in the attenuator and try and get these without a cab sim on. I'll have to do another loop by then as the DD500 doesn't retain it when switched off 

5) Maybe I am misunderstanding Audacity Normalisation. I wonder if the Audacity normalising needs to be done per "test" and not per "file", at the moment I have normalised all the, for example, 20w reactive tests as they ran after each other, ie normalised the whole file - maybe I should have normalised each individual attenuated test, ie split them all out ?

EDIT: Actually, thinking this through, the output from the amp is via the Marshall Line Out which is *before* the attenuation stages, I was expecting this to remain constant, or at least approximately. I ran the tests again and did not normalise the recordings, one track has all attenuation settings for 20w resistive, one for 20w reactive, one 5w resistive and lastly 5w reactive. I did the graphs again and they pretty much match the normalised ones in shape but each is a little lower db. Each attenuated step has a lower output by a few db, not as much as the actual attenuated signal at the speaker itself.

This is the same comparison graph but with each sample "normalised" individually by just selecting its part on the track and "normalising" it:













SV20h 20w 3



__ Mcentee2
__ Feb 5, 2020






6) Maybe my single g12h55 is "off" from the calculated stuff from @JohnH from the models used (4x12 Greenbacks etc?)

At a very high level, I am at least showing that the reactive element makes a difference in the right direction

My values in the reactive attenuator are:
L1 0.91mH first stage -7db
R1 15r
R2a 22r
R2b 18r
R3 15r 2nd stage -7db
R4 10r

In my older model:

16r -6db
8r

Or

16r -10db
12r


The Resistive vs Reactive toggle just bypasses L1 so that R2 and R3 are in parallel, effectively the r2 10r of a normal -7db stage.

The whole audacity Project file for the above is zipped to a 160mb file, so not easily shareable!


----------



## JohnH

Thanks for running those tests. It takes a bit of thinking to draw conclusions, since the reference is a cab-sim signal etc.

in Audacity, normalizing happens on the selected part of the track, and the level gets boosted until it is 1db below max at it's highest peak. So if several seperate parts are on one track, and the idea is to compare them at equal volume, then they should be normalized one by one a separate selections.

The -24db tracks go a bit lower in the treble. This could be due to the second attenuator which has a lower output resistance than the base design. 

Are the attenuated signals recorded from across the speaker?

The key frequency range is up to about 5khz, after which guitar speakers and electric guitar signals from away

We don't know how the DI out is set up. How does a recording of it sound compared to the amp through a speaker? 

If there is significant power-stage distortion happening, then we expect the resistive tests to have less treble than reactive, as is apparent. If it is all clean, we would expect them to be more similar.


----------



## Mcentee2

JohnH said:


> Thanks for running those tests. It takes a bit of thinking to draw conclusions, since the reference is a cab-sim signal etc.
> 
> in Audacity, normalizing happens on the selected part of the track, and the level gets boosted until it is 1db below max at it's highest peak. So if several seperate parts are on one track, and the idea is to compare them at equal volume, then they should be normalized one by one a separate selections.
> 
> The -24db tracks go a bit lower in the treble. This could be due to the second attenuator which has a lower output resistance than the base design.
> 
> Are the attenuated signals recorded from across the speaker?
> 
> The key frequency range is up to about 5khz, after which guitar speakers and electric guitar signals from away
> 
> We don't know how the DI out is set up. How does a recording of it sound compared to the amp through a speaker?
> 
> If there is significant power-stage distortion happening, then we expect the resistive tests to have less treble than reactive, as is apparent. If it is all clean, we would expect them to be more similar.



Thanks for your insights!

Yes, the recordings from the line out always had the speaker live as well.

The sound of the cab sounds scooped to me vs the real speaker, but of course i am only using headphones for this vs the real speaker.

Yes, most of the loop was very crunchy/distorted, which would be power stage with the SV, with some clean picks at the end.

Edit: actually, a straight graph of all the samples against the real unattenuated one without it being zeroed would be useful just to see the "real" shape of the line out itself, only between 1-10kHz do you se the resistive dropping down a little.













SV20h 20w 4



__ Mcentee2
__ Feb 5, 2020


----------



## Mcentee2

Ok, again more thinking through the above.

My graph where all the individual samples are normalised is probably the closest comparison.

They are all from the Line Out, so really its the comparison rather than the absolute values that should be the take-aways - although of course I reserve the point that the Line out Cab Sim is "Unknown", and could potentially be "unknown" in a variable way 

Anyway, to recap the graph:













SV20h 20w 3



__ Mcentee2
__ Feb 5, 2020






Looking solely at the 100-5000hz, as @JohnH pointed out that is a good high end limit for analysis, it seems the attenuator puts in a mid-hump about 300hz which then steadily declines.

Above the 5k the drop off seems a "lot" but is probably not hearable.

The reactive plots seem to be more balanced than the resistive - the 300hz bump is not as pronounced in the reactive, nor is the gradual fall away from 500-5000.

The mid hump is about 0.8-1db above unattenutated at most.

Further, the -7db and -24db plots are actually pretty closely aligned up to 1200hz, above that they diverge but even at 5000hz they are only about 1db apart, despite overall -24db attenutation.

Overall, I see some very good things here.

From a stickler point of view though...


@JohnH - re the circuit, what do you think would be good tweaks for to look at with respect to the mid hump ?

I have my two spare inductors lying around, 0.4 and 0.6 along with my used 0.9mH so could combine the three somehow to get a few between 0.4 - 1.9 mH.

Recall I measured my amp output (input?) impedance as closer to 5r, I know the M2 presents a higher input impedance to the speaker than that, but I can't quite get my head around whether changing that up/down would reduce/increase the bump. Looking back at your graphs on page 2 (?) I higher input impedance seems to accentuate the bass/treble with a mid-scoop.

TBH - I think the only thing to do is get a microphone on the speaker I am using and redo all the tests, as that is the point of all this  ie what difference the attenuator makes on the speaker you are listening to. All the above speculation (from me) is really only due to me bypassing some of the chain and then trying to correlate data back to predictions, that will only get so far and I think we have probably got there.


----------



## JohnH

Hi @Mcentee2 I agree about the mic

I don't think that a set of tests purely from the line -out necessarily tells us anything much about the attenuator performance. we don't know how the line-out is wired internally on this amp.

A microphone is a cool thing to have! And yes, the main point of this whole thread is to match the actual audible sound of a real speaker at attenuated volume, to that at full volume.

Short of buying or borrowing a decenf mic, you could see what a phone recording can do with it's own mic being careful not to overwhelm it with volume. That may mean recording at low volume, not crunching in the power section. But, that's still a key check, consistency at low volume is very important.


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2 I agree about the mic
> 
> I don't think that a set of tests purely from the line -out necessarily tells us anything much about the attenuator performance. we don't know how the line-out is wired internally on this amp.
> 
> A microphone is a cool thing to have! And yes, the main point of this whole thread is to match the actual audible sound of a real speaker at attenuated volume, to that at full volume.
> 
> Short of buying or borrowing a decenf mic, you could see what a phone recording can do with it's own mic being careful not to overwhelm it with volume. That may mean recording at low volume, not crunching in the power section. But, that's still a key check, consistency at low volume is very important.



Took the plunge and will be picking up a Sennheiser e609 later, along with a Behringer Xenyx mixer. The mic should be fine, the mixer is low end but has some good "its not bad" reviews over the years.

Fingers crossed I can work through setting it all up, and getting some more results later today.


----------



## JohnH

Great! and thanks! 

Xenix mixers are good quality. I use a small one too, which has worked well for 10 years.


----------



## Mcentee2

JohnH said:


> Great! and thanks!
> 
> Xenix mixers are good quality. I use a small one too, which has worked well for 10 years.


Thanks, do you think a loop sample should be clean or driven to best show the differences if there are any ?


----------



## Mcentee2

Hurrah! Microphone and mixer worked first time, let's hope they are "transparent" as regards their output, I checked for clipping etc and it all seemed fine, no eq on the mixer or comp.

Anyway same workflow as yesterday, similar crunchy loop played.

Output as follows, first the whole spectrum graph from the resistive and reactive settings available vs the unattenuated sample baselined to 0db:












SV20h Normalised Vs Unattenuated



__ Mcentee2
__ Feb 6, 2020







It all hovers in the low deviation area (<2db) until about 8k, then it takes hike to the heavens  which I shall ignore for a while, although it is interesting to note that the db attenuation is more important here than reactive vs resistive - ie -7db are paired together, as are -14db etc, as @JohnH said yesterday this is probably my attenuator input impedance being higher than my real amp at those levels therefore driving that top end. Also interesting that within each pair the resistive is the lower one, at these frequencies and levels I think this is just "fluff" though.



Next is just a zoom in to the data from 0-6000hz, the more realistic area - apols for the left hand side, I have yet to master excel chart "cropping"!

This one is a log plot:













SV20h Normalised Vs Unattenuated 0-5000hz



__ Mcentee2
__ Feb 6, 2020






Same but linear:












SV20h Normalised Vs Unattenuated 0-5000hz Lin



__ Mcentee2
__ Feb 6, 2020
__ 2






At the low end, the reactive plots show better bass response than resistive.

Over c300hz they track better as db pairs again, ie the two -7db are more together, the -14db pair are lower down, and the -24db at the bottom, however in all three cases the reactive has a better tracking than the resistive.

They all start to come up again to another peak at about 900hz, then from 1000hz onwards they start to group into the reactive pair tracking the 0db nicely, and the resistive plots going up together - once again probably due to that pesky input impedance the speaker sees.

Final one is a pair of plots of -7db only resistive and reactive v the baseline unattenuated - these are based on a clean portion of the looped sample:













SV20h Normalised Vs Unattenuated Clean -7db



__ Mcentee2
__ Feb 6, 2020






It is a bit less cluttered and clearly shows the reactive tracking the baseline much better until about 5k

What I see from these plots for me is that the reactive circuit seems to better track the real unattenuated than the resistive - which is after all the aim of the game!

To emphasize the nitpicking though - deviations to unattenuated are all in the 1db range across the board really, until it gets to the high treble area >8000k, so whether it is really perceptible not 100% sure on that 

However, to be honest the resistive isn't very far out up to 6000hz, and of course at the quieter attenuation levels sometimes you want to accentuate the treble a little more anyway.

I still like to say the reactive "sounds" better, but my description of it having more punch due to the mid to higher end being pushed by the OT into the inductor reactance doesn't necessarily fit with the numbers here  in fact I would say I should be hearing that more with the resistive.


----------



## Mcentee2

Just wanted to add two more useful charts to show two things to bear in mind:

1) We are really nitpicking, below is the plot of all the real values for the unattenuated and attenuation samples taken today with the e609 mic.

They are *all* near enough spot on to the unattenuated - either resistive or reactive for me - any small bumps or deviance are hairline in this big picture, andI can;t take away any real conclusion as to if for me Reactive is "different" enough, never mind saying "better" or "worse".

Also all the very high treble anomalies above 8-20k are at so low a real db level anyway as to be negligible on both counts, frequency and level - I can't beleive the e609 picked them up from the speaker as I didn't think guitar speakers went that high even at that low level, however the deviations are certainly a "real" thing from somewhere and associated with lower volumes, ie the more attenuated the volume (20w vs 5w) etc the more those very high highs seem to be retained...













20w E609 Real Data For All Samples



__ Mcentee2
__ Feb 6, 2020






2) This one is the thought provoking one for any nitpickers worse than me, or those who might be taking these charts too literally, or implying they are showing "more" than they really are. ie "those attenuated plots show the attenuator is really messing with the amp tone".

The below shows plots of the 4 unattenuated samples, two at 20w and two at 5w.

Each of the pairs *should be exactly the same*, the same loop into the same amp settings (apart from 20w vs 5w) same microphone and placement, same room same mixer, same latptop etc etc etc etc)

However, something is introducing variability, I suspect it has to be with Audacity normalising and frequency analysis based on my human selections, you can see there is just as much difference between them both in main level and in variability.

I took one of the 20w samples as a zero base to show the deviations.

Note how the 5w samples show exactly the same sort of very high end behaviour....very interesting, as these are not "attenuated" by an external device, but by an internal tube plate voltage setup.

The size of the deviations is still smaller than the other plots between attenuated levels, but it does show up a tendency for sampling variance as well as real variance to a degree.

This really shows that we can only really take high level conclusions from this sort of testing, even though the detail is necesary to point us to them.













Unattenuated Comparison Vs Baseline



__ Mcentee2
__ Feb 6, 2020


----------



## JohnH

Hi @Mcentee2 

Thanks very much for posting those, they are very good information and they take our new science a step further! Im impressed at how quickly you get this all together.

Things Im seeing now:

Im looking at this one: http://www.marshallforum.com/media/sv20h-normalised-vs-unattenuated-0-5000hz-lin.11224/

The reactive plots do a better job of tracking the unattenuated than the resistive.

The process of normalising shows some dips down at mid frequencies and some boosting upwards at higher frequency across the various plots in general. But we could re-interpret that 
By setting all plots to a match at say 440hz, then the traces will be lifted up higher in the treble. There is some electrical logic in looking at it that way, since at low frequencies, we expect that the inductor has very little effect.

When driven hard, the amps output impedance drops right down, which reduces treble into a reactive load or a real speaker. Then as the note decays, or you pick lighter, the notes bloom and treble comes back. This is the dynamic effect of a reactive load. I think with this driven sample, we are largely looking at that part of the response

Im guessing that if the amp volume is set lower, then reactive and resisive plots will be closer to each other. If you wanted to try this, it could be done using an overdrive or distortion pedal to generate some harmonics to look at, but let the amp power stage run with it clean.

All plots have a series of close wiggles relative to the unattenuated. My guess is that this is the effect of the real unattenuated speaker starting to break up when driven loudly?


----------



## JohnH

In your last post with the 5W and 20W unattenuated plots, what made the change between the 1 pair and the 2 pair? was that an EQ change on the amp? or maybe a different loop sample?


----------



## Mcentee2

JohnH said:


> In your last post with the 5W and 20W unattenuated plots, what made the change between the 1 pair and the 2 pair? was that an EQ change on the amp? or maybe a different loop sample?



No changes at all apart from the High/Lo switch on the amp; same loop, same mic placement etc all into audacity and normalised by selecting each part of the trace.

Recording workflow was to record a loop of about 10 seconds of crunch chords/picking (AC/DC style) then same but lighter/cleaner. I ran this through the amp cycling on the single recording for no attenuation, -7, -14, -20, -24. then stopped the recording.

For each of the above it was 20w resistive, 20w reactiv, 5w resistive and 5w reactive.

So I have 4 tracks in audacity.

Each of those 4 tracks has at the beginning an unattenuated loop - so I have 2 of the 20w and 2 of the 5w  I thought it would be interesting to compare just those to see if any light could be shone on audacity variance itself, and yes there is.

Most of the excel stuff has been in 20w mode to hone in on anything, but on that last post I thought I would just compare across 20w and 5w as well to see if anything occurred, and it did re variance at unattenuated levels - ie nothing to do with the M2 build 

Whatever changes are in the traces - over and above any normalising etc - would mainly be down to amp tonal changes - the 5w is definitely different with its own thing going on up to 5000hz (must stress the small differences overall in single db number!)

The 20w/5w toggle changes the plate voltages on the el34 and the v3 phase inverter.

As a compensation the preamp v1/v2 sort of slightly go the other way (natural current/voltage thing I think).


----------



## Mcentee2

Quick follow up on the high anomalies on the plots yesterday:

1) The one from about 12k upwards is actually exactly the same on each unnormalised plot, at the same break point and same db level, so its noise somewhere in the USB etc. the normalised plots obviously make this bit relatively louder for each quieter loop.......doh!

2) the 8k-11k when filtered out and played by itself sounds very much like spring rattle or something, more rattly though than smooth tone. This is loudest on the loudest loops, and quietest on the quietest, but the quietest loops still have a more significant proportion of it (still tiny), ie whatever it is has a limit of rattling that it never goes above even when the cab is at its loudest.

I suspect the latter could be the Springs in the reverb tank inside my laney L5t that I am using as a cab, the L5 amp and reverb isn't being used of course.


----------



## JohnH

Hi @Mcentee2 , thanks for sending me your data. So I had a mess around with it!



I took the full volume, -7db and -14db data, subtracted the 20W full volume from all of them, and shifted them all to be nominally at zero at around 440hz. So full vol 20W is the zero base line.

5W unattenuated is green dashed, and the first thing I see is that Marshall have done a really great job of making it match the 20W at all frequencies.

The whole band of results is within about a 2db range - all settings are pretty good

The reactive settings are best and it looks like the design is coming out most optimised at around -7 db. Could maybe add a bit more to the inductor if higher settings are more used, at which less of the real speaker inductance is coming though to the amp. Or maybe the output resistance is a bit high, since its based on my VM and your measurements were lower. We can judge this if we get fully clean (in power stage) results plotted.

We can see a small peak in the bass, its as expected since we are not trying to represent the low resonant peak in design M and M2.

Good info and thanks for doing the testing


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2 , thanks for sending me your data. So I had a mess around with it!
> 
> View attachment 67213
> 
> I took the full volume, -7db and -14db data, subtracted the 20W full volume from all of them, and shifted them all to be nominally at zero at around 440hz. So full vol 20W is the zero base line.
> 
> 5W unattenuated is green dashed, and the first thing I see is that Marshall have done a really great job of making it match the 20W at all frequencies.
> 
> The whole band of results is within about a 2db range - all settings are pretty good
> 
> The reactive settings are best and it looks like the design is coming out most optimised at around -7 db. Could maybe add a bit more to the inductor if higher settings are more used, at which less of the real speaker inductance is coming though to the amp. Or maybe the output resistance is a bit high, since its based on my VM and your measurements were lower. We can judge this if we get fully clean (in power stage) results plotted.
> 
> We can see a small peak in the bass, its as expected since we are not trying to represent the low resonant peak in design M and M2.
> 
> Good info and thanks for doing the testing



Thankyou  I am glad the data was useful to you!

Lots of sampling, recording and mainly spreadsheet work today - I could go on (see earlier post lengths!) but after all the work there isn't much more to say except "more of the same".

One thing I did do today was to add my other 0.6mH inducter in series to my M2 build so now it is a large 1.5mH.

Chart below shows deviations from a 5w unattenuated baseline, plots are all at 5w at various levels of reactive attenuation, one each from the smaller inductor and one from the larger inductor each plot is coded with dots, or dashes, dot-dashes hopefully making it easier to read.

There is one lone 5w -7db resistive trace as well to show this is different.

The actual numbers on the Y axis are actually unimportant as I massaged these to get them all on the same sized picture 

The important bit is to compare the two traces small vs large at each attenuation step, to see if having a large vs a small inductor has actually made any/much difference. Ie does one track flatter than the other, are they parallel or do they deviate from each other as well as the baseline.













SV20 Large Vs Small Inductor



__ Mcentee2
__ Feb 7, 2020






Given the very small variances, les than 0.5db, you could possibly say that the large inductor is tracking a small smidgen better, but I am not conclusively convinced.

What I can say is that all the reactive plots are still tracking better than the -7db resistive in solid brown that varies a fair bit in comparison slowly rising from 500hz upwards.


----------



## JohnH

Very interesting. Which inductance do you prefer the sound of? or can you tell?


----------



## Mcentee2

JohnH said:


> Very interesting. Which inductance do you prefer the sound of? or can you tell?



To be honest, I can't really tell any difference at all between the two whilst playing 

Further, after all this playing and testing, if I didn't "know" any different I bet I probably couldn't tell the reactive from resistive in a quick blind test and be completely consistent, I would need a few good close listens to the top end to get any idea, they are a gnat's whisker apart at the speaker.

They are all very close together in numbers, and am wary of bias and placebo type things.

I took a good look at audacity's spectogram feature as it shows freq and level across time. I thought it might show if some reactive frequencies "bloom", but it wasn't stand out obvious.

Edit: re the inductors, i made sure when I connected them they were both in the same wind direction. I don't know if that is a "thing", but it made sense.


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## JohnH

Ok, I reckon to stay with the base design for now, since it's built up from analysis and has a solid basis. It shows the exact value of inductor is not too critical tbough.

Wind direction, I'd say that would come into play if the coils were close and on the same axis. I've got an inductance range on my meter, and next time I have two coils out I'll try a few measurements.


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## Esa Martikainen

I just bought 10R 200W resistors and 200W Stereo L-pad (two 8R/100W) to build resistive attenuator for 16 ohm cabinet. 

First half, two 10R and both L-pad 8R series with 16ohm cab comes 36 ohms. Other half, three 10R series comes 30 ohms. Parallel they come about 16,4 ohms.

Esa


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## Esa Martikainen

I have been thinking if there is any idea to connect an Output Transformer backwards as passive component to the attenuator circuit? Perhaps then its output coil Inductance could be used with RC circuit?

Naturally then there would come high voltage output out of the OPT primary but now perhaps another RC- or RCL-circuit (as OPT primary circuit has inductance as well) could be design that side as well to make it act as a loudspeaker?

Esa


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## Mcentee2

JohnH said:


> Ok, I reckon to stay with the base design for now, since it's built up from analysis and has a solid basis. It shows the exact value of inductor is not too critical tbough.
> 
> Wind direction, I'd say that would come into play if the coils were close and on the same axis. I've got an inductance range on my meter, and next time I have two coils out I'll try a few measurements.



Given the results I have now with the large inductor being flattest against the baseline, I think I am good to go. I don’t have the extra resistor to hand to change to an M build without cannibalising my older box.

One thing I have just noticed is the chart above is the dip down at 5.8k with larger inductor plots, hmmmmm wonder what that is if real or measuring/analysis anomaly.

Edit: that dip is on the unattenuated sample as well, so that's fine.

re Clean amp with OD pedal test, I will get to that this morning I hope. Please remind me again what you are expecting to see - closer matching between resistive and reactive or further separation ?

edit: found it..




JohnH said:


> Hi @Mcentee2
> 
> When driven hard, the amps output impedance drops right down, which reduces treble into a reactive load or a real speaker. Then as the note decays, or you pick lighter, the notes bloom and treble comes back. This is the dynamic effect of a reactive load. I think with this driven sample, we are largely looking at that part of the response
> 
> Im guessing that if the amp volume is set lower, then reactive and resisive plots will be closer to each other. If you wanted to try this, it could be done using an overdrive or distortion pedal to generate some harmonics to look at, but let the amp power stage run with it clean.


----------



## Mcentee2

Ok, final set of tests for me, as I don't think I can improve the data gathering or find anything that would "improve" my build or experience.

Charts for absolutes and deviations for -7db reactive/resistive vs baselines for Clean amp with an OCD in front vs the hot amp from yesterday's plots (both sets with the larger inductor in) :













SV20 Clean Vs Hot



__ Mcentee2
__ Feb 8, 2020






To me, there is little difference at all between Clean and Dirty with reference to parallel lines gradients diverging etc.


The SV20 had a measured output impedancec of c5r vs @JohnH VM at c20r, I need to sit down again and read back as to how that works and if my results hold that up or not.

All good to know though.


----------



## Mcentee2

Just tidying up a couple of things others might be interested in:

1) This a comparison between the SV20h DI with the Marshll Cab Sim, vs my Laney cab with its Celestion G12H55 - vs yesterday's Hot sample with large inductor - both unattenuated.

Can't moan about Marshall's take on it, looks pretty good.













SV20 Cab Sim Vs Real Speaker



__ Mcentee2
__ Feb 8, 2020







2) This one is for a Line Out I just put in the M2 attenuator box, installed at the output with fixed 22k and 2.2k resistors, again vs the same Hot large inductor through the G12H55

Clearly shows what the speaker does at low and high ends re damping and/or resonance.













M2 Line Out Vs Real Speaker



__ Mcentee2
__ Feb 8, 2020








I wanted to feed something to a delay pedal for a wet/dry set up, but not sure what my Wet amp would make of the M2 Line Out vs the Marshall DI in detail through its own speaker, vs a mixer ( or cab sim like a Mooer Radar) into a frfr setup either live or for recording.


... further project(s) for recording another day


----------



## Nik Henville

The
way
I am
reading
all that, is that they are damned close...


----------



## Mcentee2

Nik Henville said:


> The
> way
> I am
> reading
> all that, is that they are damned close...



Yes, that's what I take from it, TBH @JohnH has never shied away from saying the reactive is "icing on the cake", and is more suited to some amp designs than others (those with little to no negative feedback (NFB) in the power stage.

Amps with a presence control might vary with that too - the SV20h has a one like the old Plexi but that was one too many variables for me to look at!


----------



## Gene Ballzz

Mcentee2 said:


> Yes, that's what I take from it, TBH @JohnH has never shied away from saying the reactive is "icing on the cake", and is more suited to some amp designs than others (those with little to no negative feedback (NFB) in the power stage.
> 
> Amps with a presence control might vary with that too - the SV20h has a one like the old Plexi but that was one too many variables for me to look at!



_*FIRST: *_That *"icing on the cake"* encompasses many of those important, subtle nuances of tonal dynamics and response that are imperceptible to the audience (except for the inspiration perceived by the player) other than how much better a performer performs when everything is *"just right!"
*
_*And Next: *_I have used my attenuators with a wide variety of amps, some with and some without NFB/Presence! While the results and required compensations (slightly different amp settings than when not attenuated) are a little different with each amp, they tend to stay consistent throughout the low to high attenuation settings. Both my 5E3 Tweed Deluxe and DSL20 both love these attenuators. There will still always be the reality that the closer you get to *"baby sleeping in the next room"* volume levels, the less likely it will sound like a *"200 watt Marshall Major Stack!"

Just Sayin'
Gene*


----------



## Mcentee2

Gene Ballzz said:


> _*FIRST: *_That *"icing on the cake"* encompasses many of those important, subtle nuances of tonal dynamics and response that are imperceptible to the audience (except for the inspiration perceived by the player) other than how much better a performer performs when everything is *"just right!"
> *
> _*And Next: *_I have used my attenuators with a wide variety of amps, some with and some without NFB/Presence! While the results and required compensations (slightly different amp settings than when not attenuated) are a little different with each amp, they tend to stay consistent throughout the low to high attenuation settings. Both my 5E3 Tweed Deluxe and DSL20 both love these attenuators. There will still always be the reality that the closer you get to *"baby sleeping in the next room"* volume levels, the less likely it will sound like a *"200 watt Marshall Major Stack!"
> 
> Just Sayin'
> Gene*



I take your point!

That's why people quire like icing on their cake rather than not 

What I think all my analysis shows is that there are limits to what the methods and tools used to gather and present the data ate limited in terms if describing the icing.

Using audacity frequency plots only gets us so far and I think I have reached the end of that road and am looking for a better way of attacking the icing.

Audacity plot outputs are frequency vs level across the whole sample, there is no "time aspect" so you only get a general idea of frequency content that looks static and doesn't tell the whole story.

This clearly shows the difference between a resistive type and reactive type attenuator - hurrah - and it supports various theories of operation.

If there is a better tool for outputting that across the time domain it would provide a better way of looking at that icing.

We have standard track display of db vs time in audacity and that is what @JohnH provided way back, two freq/db images that showed differences from the same loop when normalised from different attenuation levels. Mine don't show any obvious similar difference, maybe I haven't looked hard enough 

Audacity does have a spectogram display which is freq db vs time, but I don't think it is granular or flexible enough to interpret, and also don't think it can be exported as data.


----------



## JohnH

Hi @Mcentee2 and thanks very much for all the testing. Lots of data! So I pulled out and plotted a sample which I think is quite telling about the reactive versions:




These are all at -14db, which is with two of the stages per attenuator M or M2, having an output resistance of around 18 to 20Ohms, showing about 14db attenuation relative to the 5W full volume signal. (is that right?)

The two reactive plots (low volume/clean and high drive) show good linearity, a basically horizontal plot through the most important frequencies. The attenuator is doing a good job of transferring the full response of this SV amp, to the response of a real speaker, at a power factor of about 1/25th.

The resistive plot is a bit different, and appears to rise with frequency relative to unattenuated. tmThis attenuator has a fairly high output impedance, and the amp has a much lower one, measured as 5 Ohms before. The resistive plot therefore shows a rise with frequency, it has no means of compensating for a different amp output impedance than that which it was designed for.

But the reactive design tracks the response at the amp, even as the amp has very different output R than expected, and also therefore, as the amp output R dynamically changes with signal level, which is what Gene is pointing to.

So the reactive designs are able to, well react. They track the response not only of different amps, but of given amps changing their characteristics at different drive levels. Ive seen this in the modelling calcs too, but I cant fully rationalise what is happening.


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2 and thanks very much for all the testing. Lots of data! So I pulled out and plotted a sample which I think is quite telling about the reactive versions:
> 
> View attachment 67289
> 
> 
> These are all at -14db, which is with two of the stages per attenuator M or M2, having an output resistance of around 18 to 20Ohms, showing about 14db attenuation relative to the 5W full volume signal. (is that right?)
> 
> The two reactive plots (low volume/clean and high drive) show good linearity, a basically horizontal plot through the most important frequencies. The attenuator is doing a good job of transferring the full response of this SV amp, to the response of a real speaker, at a power factor of about 1/25th.
> 
> But this attenuator has a high output impedance, and the amp has a much lower one, measured as 5 Ohms before. The resistive plot therefore shows a rise with frequency, it has no means of compensating for a different amp output impedance than that which it was designed for.
> 
> But the reactive design tracks the response at the amp, even as the amp has very different output R than expected, and also therefore, as the amp output R dynamically changes with signal level, which is what Gene is pointing to.
> 
> So the reactive designs are able to, well react. Ive seen this in the modelling calcs too, but I cant fully rationalise what is happening




Thankyou!

Yes the reactives all seem to track better 

What I was hoping to see was a visible difference between Clean Reactive vs Hot Reactive, that is what I mean by "little difference" 

I didn't see that on -7db, or on your -14db either. If anything it seems the Hot Reactive is closer to resistive, just. However I am sure I am misinterpreting that as representing something.

The relationship between the two reactives, clean vs dirty, is what I am trying get a feel for, where the gradients change or are parallel with each other and that is where my brain ground to a halt as it didn't seem to show anything obvious (for my data at least!)


----------



## Mcentee2

Edit: this turned into a longer post than intended, so this is my TLDR question:

@JohnH - does your simulator tell us anything about what input impedance the speaker sees for M or M2 circuits vs a straight resistive (across frequency?), I have checked the whole thread and can't see any comparisons vs a straight resistive circuit, apologies if I have missed it ?



.........

Having had a day's rest from actual testing I was thinking a lot yesterday about what all the above shows - TBH my head was in knots for a lot of it - and I came up with two things to note:

1) Given my clean/hot results were indistinguishable, I suspect that lends itself to showing that the SV20 output section isn't actually being "driven" very hard at all in terms of maxing out its current draw and "collapsing" into a "no more current left" state, therefore the output impedance isn't probably changing from that c5r I measured - either static or variable across frequency (recall, the thought was that this would probably change somewhat with the amp being stretched).

I don't know yet, but will search around google, but I think I recall seeing a few references in the past to a lot of talk about the Plexi power stage, whether the breakup comes form the phase inverter vs the power tubes , and where "best" to place a post-/pre master volume etc.

My SV20 has 6ac7 tubes in, not the origina el34 (they are in a drawer so still available when I have time to test), these are little more robust in terms of what/when they breakup, so that might be contributing to my results.

Finally, the SV20 is a cathode biased power section, not fixed, so there is some leeway in there for it to respond more dynamically when pushed and tend to balance itself out - by how much, I don't know, but its a known thing.


2) Regarding my results showing the reactive tracks better, but the resistive shows more treble responses. We have covered two main things going on at the same time and it is quite hard to distinguish both separately:

a) When seeing a reactive load the amp will push its high end output more (and low end resonance, but we are ignoring that in these designs!) whereas with a resistive load it tends to flatten out across the freq spectrum.

b) the speaker takes this set of freq output levels and depending on the amp output impedance (input to the speaker!) it develops a damping response in interaction with its own impedances/capacitances etc, and produces a sound. The higher the amp output impedance the less the speaker is damped and can "do what it wants rather than what the amp wants", the lower the amp output impedance the more "the speaker has to do what the amp tells it to do".

The balance of these two is what we are seeing in the results experienced.

This should be visible in the recorded mic samples, and I think I do get that in my results, my resistive attenuator input impedance is higher than the amp (13r vs 5r) so my speaker is more free to do what it wants, in this case ignore the amp's wish to control the high end a bit more - and here is the bit I was struggling with - even if with the resistive attenuator the amp is putting out *less high end as it is a flat response curve*!!!!

Now, with the reactive stage I can see clearly that the amp output has more high end (@JohnH 's simulator charts show this), however my recorded samples from the speaker show *less* overall high end kick vs resistive.

I'll run that again:

With a resistive load the am is putting out less high end than reactive, but my recorded samples show more high end than reactive.

With a reactive load the amp is putting out more high end than resistive, but my recorded samples show less high end than resistive.

In my case it seems that the speaker is the main influence on the resulting recordings - with the higher resistive load that it sees at its input it can do what it wants and outputs the amp's lower high end ouput at higher levels (@JohnH early graphs show this as well)

Now I am getting to my point and question for further work:

With a reactive stage - what does the speaker actually see in terms of input impedance across frequency ?

@Gene Ballzz 's experiecnes, and others of course, show the reactive build is "adaptable"...

@JohnH - does your simulator tell us anything about what the speaker sees for M or M2 circuits, I have checked the whole thread and can't see any comparisons vs a straight resistive circuit, apologies if I have missed it ?

Not only is the *amp* seeing a reactive load - so is the *speaker* (obvious,I know!), so the speaker is still "seeing" what it thinks is the (or "an") amp and is damped accordingly, even if the amp itself is putting out more high end.

In the M2 circuit if the inductor is bypassed then the attenuator basically has an input impedance of c13r.

In terms of "damping", the lower the input impedance the more the speaker is damped, so somehow the M/M2 reactive stage (in combination with the real amp) seems to be "presenting" itself to the speaker with "more damping" as frequency increases (ie lower input impedance closer to the real amp).

I can't work out from the M2 circuit how this works though 


Phew, another long post, apols.....


----------



## JohnH

in the current designs, the speaker sees about 18 to 20 ohms for the 8 ohm version


----------



## Mcentee2

JohnH said:


> in the current designs, the speaker sees about 18 to 20 ohms for the 8 ohm version



Is that for the reactive as well?

In your post back on page 11 (post 210), you provide the single stage work through, but standardise the input impedance at 440hz, hence my question, does it vary across frequency ?













Single Stage 190105



__ JohnH
__ Jan 4, 2019


----------



## Mcentee2

So, I wasn't 100% happy with my Clean vs Dirty comparison the other day, so I did it again, although a slightly cut down version of variants, I chose -14db only.

This time it was the exact same loop used for clean and dirty, no OCD this time on the clean, for the driven loop the amp was maxed out and boosted to hell, guitar into high treble high input patch to normal low.

For clean the amp was backed off a fair bit, not boosted, and guitar into Normal channel low, patched to high treble high.

In this test I also maxed out presence to give the output circuit something to work with.

I tried to get the eq sounding "right" - but I think I failed - On the clean test the low frequencies do look decidedly lacking  and it was quite ice picky! the driven sounded great  I think I under-did the bass on the amp and the channel mixing  however, it is a comparison of the same settings reactive vs resistive, so that stands ok.

Anyway:

Charts of actual values first followed by deviations against dirty resistive, this shows:

1) A very clear behaviour of clean plots being most trebley either resistive or reactive - that is probably down to my amp settings favourinig the high end for the clean run through.

2) Resistive plots are actually a fair bit higher than reactive in the driven plots, and is clearly audible up to 2db from 1000hz upwards (ish)

3) More crucially it shows that the reactive/resistive plots for clean are much closer together than they are for dirty.

(I didn't do an unattenuated loop this time).

This is exactly what @JohnH was describing 















Driven Vs Clean



__ Mcentee2
__ Feb 10, 2020






Secondly, this is a screen grab of audacity showing the tracks themselves - I hope they are clear enough to see.

Top is Dirty, bottom is Clean
Left is Resistive, right is Reactive

So I see - the reactive compression in the clean is there, you can see it most clearly on the two blocks before the last long fade and actually for quite a bit of the track. For the dirty it is not so obvious probably because of compression elsewhere inthe chain maybe rather the power section.













Driven Vs Clean Track



__ Mcentee2
__ Feb 10, 2020


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## JohnH

The impedance seen by the speaker in the reactive designs is pretty much always constant and resistive, not changing with frequency . It's one of the tricks in this design. The amp sees the reactive load, so it responds correctly, but the reactive phase lag is taken out by the balancing of the two coils in M, or the two resistors R2A and R2B in M2 or M3. Then this basicly resistive signal can be stepped down through the ladder of switched resistor stages, without changing it's tone. Then it arrives at the far end, and the speaker reacts to it with it's own resonance and inductance. At high attenuation, the amp and speaker are hardly connected to each other at all, eg at -30db, there's only 1/1000th of the power reaching the speaker. 

The mysterious bit to me is how, given almost no connection between the two, the changing impedance and response of the amp is correctly transferred, in miniature, to the speaker at the end of the chain! But the numbers and tests show that this is what occurs.


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## Mcentee2

JohnH said:


> The impedance seen by the speaker in the reactive designs is pretty much always constant and resistive, not changing with frequency . It's one of the tricks in this design. The amp sees the reactive load, so it responds correctly, but the reactive phase lag is taken out by the balancing of the two coils in M, or the two resistors R2A and R2B in M2 or M3. Then this basicly resistive signal can be stepped down through the ladder of switched resistor stages, without changing it's tone. Then it arrives at the far end, and the speaker reacts to it with it's own resonance and inductance. At high attenuation, the amp and speaker are hardly connected to each other at all, eg at -30db, there's only 1/1000th of the power reaching the speaker.
> 
> The mysterious bit to me is how, given almost no connection between the two, the changing impedance and response of the amp is correctly transferred, in miniature, to the speaker at the end of the chain! But the numbers and tests show that this is what occurs.



Yes, a mystery for me so far, still a lot to learn!

One thing I would appreciate, as i don't have a sim, is how my change to a 1.5mH inductor in the M2 might affect your curve predictions?

Thankyou


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## Mcentee2

@JohnH

I am trying to answer a question regarding the difference between a Line out at the input of the attenuator vs one at the end, I am not sure but I think I have the answer.

In your schematics you also present traces on the right hand side, if I am interpreting them correctly it shows that a Line out at the input would capture the red traces and the green and black ones.

Line out at the end would capture the blue and black lower traces.

So, firstly, does the Speaker trace capture just the signal on the speaker terminals? Or is it like what would be on a mic track / audacity frequency plot?

So to me, they are not that different apart from that low resonance not on the input.

I am not sure, but it seems the one at the input captures the amp signal (minus low peak), and would then need to be fed into something that provided the low resonant hump.

The one at the end captures the complete amp *and* the real speaker's response and just needs to be amplified?

Now this is also something I get confused by - what an IR brings to the table here ? Would it not be duplicating some of this ?

Am I close?


----------



## JohnH

The plots in all the calcs represent the elecrical signal that you would measure directly at the various points. 

The red is a full volume signal at the speaker and also at the amp (since there's no attenuator in that case), The lower blue and black are from a speaker after the attenuator, and green and upper black are at the amp into the attenuator. All of them do a good job from low mids up, but with the attenuated, only those from the right side (at the speaker) show a low resonance. But others have taken their line-outs from the left side and been happy with them, you may need to add a small bass boost.

But all of these are just electrical signals, they don't know about the mechanical/acoustic response of the speaker, nor the effects of the mic in the room. The most noticeable difference is that real guitar speakers cant reproduce the high frequencies that are in these signals above about 5khz. And if you took any of these traces and fed it to a full range speaker system, it would sound terrible. That is what a cab-sim or IR box needs to account for.


----------



## Mcentee2

JohnH said:


> The plots in all the calcs represent the elecrical signal that you would measure directly at the various points.
> 
> The red is a full volume signal at the speaker and also at the amp (since there's no attenuator in that case), The lower blue and black are from a speaker after the attenuator, and green and upper black are at the amp into the attenuator. All of them do a good job from low mids up, but with the attenuated, only those from the right side (at the speaker) show a low resonance. But others have taken their line-outs from the left side and been happy with them, you may need to add a small bass boost.
> 
> But all of these are just electrical signals, they don't know about the mechanical/acoustic response of the speaker, nor the effects of the mic in the room. The most noticeable difference is that real guitar speakers cant reproduce the high frequencies that are in these signals above about 5khz. And if you took any of these traces and fed it to a full range speaker system, it would sound terrible. That is what a cab-sim or IR box needs to account for.



Phew, good, I finally understand some of this 

Still not sure of what an IR truly represents though.

Presumably if you take that output side line out into an IR....is that the usual thing ?

And given that signal is coming from a what a G12h55 is inherently doing, having an IR of another speaker (cab type and mic setup is new of course) then it is sort of mixing them up in what you finally hear ?


----------



## JohnH

Some plots:

These are all based on -14db, but normalised to compare tone. Red is full-volume speaker, no attenuator, blue is attenuated , plotted at the speaker. Green dashed is the signal at the amp going into the attenuator.

This is M2 with 0.9mH and from a 20Ohm amp (eg my VM):



here it is with 1.5mH and 20Ohm amp:



Now from a 5 ohm amp, with 0.9mH:



And this is 5 ohm amp and 1.5mH:



You can see how with the assumed values, the 0.9mI shows a pretty good match both in terms of what the amp sees and at the speaker. When you add to make it 1.5, it doesn't really change the core small signal tone (blue is still close to red), but the amp is seeing more reactance which may chsnge its response.

Also, replotting with lower amp output impedance shows how the traces all drop down together, and stay reasonably close. This effect is relevant both to the use of different amps, and also how it responds when an Amps output imoedance changes dynamically. The latter, I can't model directly (would need to simulate the valve electronics), but it's inferred from these linear analyses.


----------



## JohnH

Have just editted the previous post to talk more about the plots.


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## Mcentee2

JohnH said:


> Some plots:
> 
> These are all based on -14db, but normalised to compare tone. Red is full-volume speaker, no attenuator, blue is attenuated , plotted at the speaker. Green dashed is the signal at the amp going into the attenuator.
> 
> This is M2 with 0.9mH and from a 20Ohm amp (eg my VM):
> View attachment 67370
> 
> 
> here it is with 1.5mH and 20Ohm amp:
> View attachment 67372
> 
> 
> Now from a 5 ohm amp, with 0.9mH:
> View attachment 67369
> 
> 
> And this is 5 ohm amp and 1.5mH:
> View attachment 67371
> 
> 
> You can see how with the assumed values, the 0.9mI shows a pretty good match both in terms of what the amp sees and at the speaker. When you add to make it 1.5, it doesn't really change the core small signal tone (blue is still close to red), but the amp is seeing more reactance which may chsnge its response.
> 
> Also, replotting with lower amp output impedance shows how the traces all drop down together, and stay reasonably close. This effect is relevant both to the use of different amps, and also how it responds when an Amps output imoedance changes dynamically. The latter, I can't model directly (would need to simulate the valve electronics), but it's inferred from these linear analyses.



Wow, it seems just the amp 5r vs 20r makes a large difference in the low end peak !!

From previous posts, the speaker will see it as 20r and feel less damped.

The inductor size has less effect overall, but I can see that the 0.9mH allows the amp to respond more accurately.


----------



## JohnH

If we go to the M3 design, then the low peak tracks too. But it's only relevant to the fundamental note of the very lowest strings at low frets, and not their upper hamonics . So far, I've been happy to avoid the extra $100 in parts.


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## Mcentee2

JohnH said:


> If we go to the M3 design, then the low peak tracks too. But it's only relevant to the fundamental note of the very lowest strings at low frets, and not their upper hamonics . So far, I've been happy to avoid the extra $100 in parts.



Well, TBH a bit of extra low end (even if really low) probably helps at lower real volumes.
..

Even just that little lift at 200hz....

Actually, even more relevant, looking back at my tests they do indeed show an increased bass response when attenuated vs not !!!


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## Gene Ballzz

@Mcentee2 & @JohnH ,

It should be noted that although my experiences have been politely and regularly referenced, my units are based on my own personal modifications of 16 ohm, DESIGN "K" version detailed in *post #267* of this thread. This may not track the same as the units you folks are testing. You guys are delving into areas that are far beyond my own understanding, even though quite interesting. Mine get used for mostly live performance and practice at home. My main unit gets used all the time and am constantly updating the various layouts/details/options of plans for future, subsequent units. The basic values for components will remain the same, differences are mostly in packaging, switching and number of steps, custom tailored to different wattage amps! 

It should also be understood that there are multiple reasons I chose to go with the 16 ohm version. One is that in my universe, I have one amp that wants a 16 ohm "only" load, none that want 8 ohm "only" and own only one 8 ohm cabinet. Also, somewhere along the way JohnH posted (either here or in a personal email) a list showing considerably more consistent impedances at both the amp and speaker, throughout the various attenuation levels, with DESIGN "K" as the basis! 

With all that said, I'd like to share a thought or two on the addition of a line out to the unit. While I can see the benefits and purpose of a "pre-attenuation" line out, in my frame of reference, a "post-attenuation" would be more useful, either instead of or in addition to a "pre-attenuation" send. In my case, it would get used as a dry send to an effects unit for a wet/dry rig and allowing the balance to remain constant throughout all attenuation levels would be quite handy! Maybe switchable from pre to post would be the coolest way to do it?

Bottom line is that this attenuator design has become the most liberating and useful addition to my guitar rig arsenal of my whole 50+ year career!

Just My $.02,
Gene


----------



## tmingle

Mcentee2 said:


> To be honest, I can't really tell any difference at all between the two whilst playing
> 
> Further, after all this playing and testing, if I didn't "know" any different I bet I probably couldn't tell the reactive from resistive in a quick blind test and be completely consistent, I would need a few good close listens to the top end to get any idea, they are a gnat's whisker apart at the speaker.
> 
> They are all very close together in numbers, and am wary of bias and placebo type things.
> 
> I took a good look at audacity's spectogram feature as it shows freq and level across time. I thought it might show if some reactive frequencies "bloom", but it wasn't stand out obvious.
> 
> Edit: re the inductors, i made sure when I connected them they were both in the same wind direction. I don't know if that is a "thing", but it made sense.



With my build, any difference between resistive & inductive is very subtle. It may be more obvious at higher volumes which, for my usage defeats the purpose of the attenuator.


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## Gene Ballzz

My two main amps are a 13 watt, 5E3 Tweed Deluxe and a DSL20. The attenuator is most often used to just "shave a little off the top" @-7db to -10.5db for the tweed and -10.5db to -14db for the DSL20. If I run 6V6s in the DSL, in place of theEL34s, it works out to almost identical SPL and usage as the 5E3.
Just Sayin'
Gene


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## Mcentee2

@Gene Ballzz Great to hear you have the line out at the output  I think that is the best place too for live use!


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## JohnH

Hi @Gene Ballzz
I think that for a line out signal, there should be plenty available even after the attenuator. Some experimenting would be advised. I'd imagine you could wire a pot of say 10k log in series with a resistor, minimum say 4.7k so there's no way the line out can short anything. That should give good control, and maybe a higher series resistor might be preferred.

A signal from there could feed either to another amp running a guitar cab, or into an IR or cab sim.


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## Mcentee2

@JohnH Hi, I would just like to say thankyou again for those plots above!

With my 1.5mH the results I am getting at the miced speaker do seem to track better than the 0.9mH on the higher rise.

Maybe my g12h55 has a different impedance curve to your 4x12 greenback model.

Trawling Google today, it seems Celestion don't provide impedance curves 

But luckily someone has taken the trouble to do a review/analysis of the g12h55 

https://audioxpress.com/article/Test-Bench-Celestion-G12H-GreenBack-12-Guitar-Speaker-Reissue

Particularly deriving an impedance
/Freq plot:







Either way, I am very happy so far 


Oh, and now I have read a bit more about this stuff I know I am out of my depth when things like voltage/current phase angle make an appearence!

And inductive/capacitive relational effects.


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## Nik Henville

This thread:-

The more I read, the more I learn... and the less (I realise) I know.

Rather than graphs, interesting though they are, can we not talk about the sound we hear from the various options?


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## Mcentee2

Nik Henville said:


> This thread:-
> 
> The more I read, the more I learn... and the less (I realise) I know.
> 
> Rather than graphs, interesting though they are, can we not talk about the sound we hear from the various options?


 

Sure, I'll give it a go - although it has probably been said in the thread 

For me:

Resistive makes the high end come through more than unattentuated - actually I would say "boosted", more uncompressed than even unattenuated - spikey ?

Reactive - smoother, high end is more like the unattenuated, has a compressed sound like the unattenuated. For my current build impedance it is possibly a hair more compressed than unattenuated.

Both have a very "minor" bass boost for me.

Can't say I have noticed the "bloom factor" as a "wow" thing, but it is most probably there, as the compressed audacity pics I took show - not been able to see this at a frequency level yet.

Both are great tones - you have to balance each against the volume loss and see what works better for you I suppose.

My preference has to be with reactive really - purely because of the compression aspect, it really is less fatiguing on the ears over a period in my small practice room.

I suppose sound clips are where its at really, but even then its all my gear and not your gear....as we have seen variances are there across all combinations of set up.


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## Gene Ballzz

Hey @JohnH

Today is the first time I've viewed your *"post #1"* that you updated/upgraded a few months back. So cool the way that you combined all/most of the important stuff (the meat and potatoes, if you will) into that first post!  I must add that your design is so timely as to be fantastic for use with the wildly popular Marshall Studio Series of amps and many others like it. It would be hard to find a better fit for 15-40-ish watt amps!

With that said and in consideration of how many folks seem interested in attenuators, I'm considering offering an assembly service for those in the continental United States that may lack the skills, time or desire to do it themselves. I'm looking for comments on what a fair price for such a service may be? I'm figuring on standardized builds of the "M" version with a full bypass mode/switch and a choic of either 8Ω or 16Ω. I have a couple of standard configuration/orientation choices in mind and any deviation from the standard, additional bells/whistles would likely incur a negotiable upcharge.

Part of the difficulty is that I've sourced multiple groups of parts for multiple variations and it's very difficult to figure the actual price of the parts, overall. Parts seem to come out to the $60 to $100 range, depending on a few variables and I'm kinda thinking that $75-ish for the time and labor would be reasonable. Do you think that materials estimate is in the ball park?

What Say You Sir? (PM me if you're more comfortable that way)
Gene


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## JohnH

Hi @Gene Ballzz 

That would be a helpful service to offer! Think of all the marriages that you might save, and all the babies who will grow up smarter, for having slept better while Dad jams quietly (but toneully) next door!

I also have a rough estimate of $100 in parts, AUD$ in my case. But it is hard to keep track of all the dollars and cents that go into a complete unit, including screws, a few dabs of thermal grease and a squirt of paint etc

For cost of hours, I think you'd need to control it so you enjoy the work. Otherwise I reckon you could make more $/hour stacking supermarket shelves, or even working as a musician.

But if you are well set up in the workshop, and maybe work in small batches, I reckon $100 for assembly would be generously good value for a buyer.

On my Post 1, I'm meaning to update it to include design M2 (and M3). But I've got up against a 10000 character limit per post, so every time I add I also have to condense.

But M2 works, as built by @Mcentee2 , and I think it functions exactly the same, with fewer parts. It's just that the maths to design it was harder to solve. If starts on p27.


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## Gene Ballzz

@JohnH 
I'm a fairly avid fabricator especially for making multiple units. Once I finalize a design/layout with prototypes, On a unit like this I'll save a buch of time and aggravation by making drilling jigs, to avoid having to measure/layout each piece. I love doin' repetitive stuff like this! It shares some similarities with hammerin' out a great musical groove on a great guitar with just the right amp, cranked to the perfect volume level through the use of a JohnH attenuator!  I estimate that once the jigs are finished I could knock these out in 4 to 6 hours each! Maybe even hold it to 4 hours, if doing just the right number of multiples?

Thank You Sir!
Gene


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## Mcentee2

Gene Ballzz said:


> Hey @JohnH
> 
> Today is the first time I've viewed your *"post #1"* that you updated/upgraded a few months back. So cool the way that you combined all/most of the important stuff (the meat and potatoes, if you will) into that first post!  I must add that your design is so timely as to be fantastic for use with the wildly popular Marshall Studio Series of amps and many others like it. It would be hard to find a better fit for 15-40-ish watt amps!
> 
> With that said and in consideration of how many folks seem interested in attenuators, I'm considering offering an assembly service for those in the continental United States that may lack the skills, time or desire to do it themselves. I'm looking for comments on what a fair price for such a service may be? I'm figuring on standardized builds of the "M" version with a full bypass mode/switch and a choic of either 8Ω or 16Ω. I have a couple of standard configuration/orientation choices in mind and any deviation from the standard, additional bells/whistles would likely incur a negotiable upcharge.
> 
> Part of the difficulty is that I've sourced multiple groups of parts for multiple variations and it's very difficult to figure the actual price of the parts, overall. Parts seem to come out to the $60 to $100 range, depending on a few variables and I'm kinda thinking that $75-ish for the time and labor would be reasonable. Do you think that materials estimate is in the ball park?
> 
> What Say You Sir? (PM me if you're more comfortable that way)
> Gene



If it helps here are my costs for my M2 build (just the bits I used!), I didn't shop around, just went on ebay and found what I wanted and bought it, I already had thermal paste and M3bolts/nuts/washers:

(Grrrr, Forum formatting for tables....grrrr)

Quantity Unit Pr Total 
Box Ham 1590x 1 £20.04 £20.04 
2 Cliff Jacks 2 £3.96 £7.92 
22r 25w Welwyn 1 £4.68 £4.68 
18r 25w Welwyn 1 £5.98 £5.98 
15r 50w TE/CGS 1 £5.65 £5.65 
10r 25w Welwyn 1 £5.57 £5.57 
15r 25w Welwyn 1 £5.40 £5.40 
.9mH 1 £5.50 £5.50 
2 switches 2 £4.53 £9.06 
£69.80 

Postage for two things £4.73 



@JohnH - you might have missed my earlier post (or I didn't ask!), but re the plot I found of the g12H55 impedance curve - is this different much to your 4x12 Aiken model, and could this account for my experience of the 1.5mH tracking better ?


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## JohnH

I don't think the G12H is really much different, it's impedance rises at about the same rate as the G12M so far as i can see. Plus what you hear is closely related to the speaker that you use. The only thing the inductor affects is the amp response when pushed. 
Dunno! the main thing is if you like what you hear..


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## Mcentee2

Thanks for you thoughts, so far it is working out very well  with either 0.6mH or 1.5mH when playing


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## matttornado

Does anyone no how to incorporate an effects loop to the attenuator?


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## JohnH

matttornado said:


> Does anyone no how to incorporate an effects loop to the attenuator?



the only way with these passive attenuators would be to set up a line out, then send that to effects and then onto another amp


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## matttornado

JohnH said:


> the only way with these passive attenuators would be to set up a line out, then send that to effects and then onto another amp



Ok, that what I do now and works well!


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## SnickSound

JohnH said:


> *Summary: September 2019*
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator M 190110
> 
> 
> 
> __ JohnH
> __ Jan 10, 2019
> __ 1
> 
> 
> 
> 
> 
> 
> *(EDIT 23/9/2019:* a minor tweak for a very slight improvement - see post #528 on p27:



Just curious, is is possible there is an error with R5 and R6?
For example, 15 and 4.7 (in the 8 ohm row) gives a combined impedance of 3.9 ohms with an 8 ohm speaker (1/4.7 + 1/(15+8) = 3.9), and by my calculations this is a 10.4dB attenuation (by power)

If you make R5 10 ohms and R6 32 ohms you get a perfect 8 ohms combined and 14dB attenuation. I notice that the 16 ohm row shows 10 and 30 which is pretty close, so perhaps that's where the error is. The 8 ohms value were pasted in the 16 ohm row.

Proper resistors for -14dB at 16ohms would be by my math would thus be R5 20ohms and R6 64ohm 

In any case, I breadboarded this circuit yesterday with some small changes (to have increments of -4dB on a rotary switch) and I'm sold! This circuit just works!


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## JohnH

hi @SnickSound 
Thanks for checking out this circuit and I'm glad your version works for you.

The values listed are all as intended, and checked several ways! You would be right about R5 and R6 except that it does not work on its own in isolation, It is always after other stages, at least the first stage and maybe the second stage. These bring the impedance seen by the amp back much closer to the intended nominal values. To judge the effect of attenuation, you also have to allow for the output resistance of the previous stages, and that will correct the attenuation to about -14db as intended. 

The reason all that is like that, instead of just using higher values, is to keep control of the output resistance of the stage, which is the third key parameter. It needs to stay consistent at the output of every stage, so the speaker reacts consistently no matter which final stage it sees. There is a theoretically purer form of that stage, which needs three resistors in a T configuration. With that, you can set attenuation, and the local resistance seen at the input and output. But its a another component to build in, and also in order to bypass it, it needs another pole on the switch. None of that is a big problem but then also it doesn't really give a real benefit either, so I kept it simpler.


----------



## SnickSound

JohnH said:


> hi @SnickSound
> Thanks for checking out this circuit and I'm glad your version works for you.
> 
> The values listed are all as intended, and checked several ways! You would be right about R5 and R6 except that it does not work on its own in isolation, It is always after other stages, at least the first stage and maybe the second stage. These bring the impedance seen by the amp back much closer to the intended nominal values. To judge the effect of attenuation, you also have to allow for the output resistance of the previous stages, and that will correct the attenuation to about -14db as intended.
> 
> The reason all that is like that, instead of just using higher values, is to keep control of the output resistance of the stage, which is the third key parameter. It needs to stay consistent at the output of every stage, so the speaker reacts consistently no matter which final stage it sees. There is a theoretically purer form of that stage, which needs three resistors in a T configuration. With that, you can set attenuation, and the local resistance seen at the input and output. But its a another component to build in, and also in order to bypass it, it needs another pole on the switch. None of that is a big problem but then also it doesn't really give a real benefit either, so I kept it simpler.



Ah, I see! Didn't think about the combined "output impedance" and how it reacts with the speaker. Makes a lot of sense.

I forewent the -14dB stage on mine, meaning it tops out at about 17.5dB of attenuation, just enough to bring a 50W amp down around 1W. Still fairly loud but this is strictly for stage use. 

If I decide to add that stage, I'll keep your advice in mind!


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## Gene Ballzz

SnickSound said:


> Ah, I see! Didn't think about the combined "output impedance" and how it reacts with the speaker. Makes a lot of sense.
> 
> I forewent the -14dB stage on mine, meaning it tops out at about 17.5dB of attenuation, just enough to bring a 50W amp down around 1W. Still fairly loud but this is strictly for stage use.
> 
> If I decide to add that stage, I'll keep your advice in mind!



Yes indeed sir, @JohnH is quite methodical in his perspective and attention to details! This unit is absolutely the most liberating piece of kit I've ever owned!

Thanks For Noticin'
Gene


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## matttornado

I absolutely love mine! I just finished getting around to painting mine and I added a fan too. I'll post some pics soon!


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## Xazrules

HI everyone, and thank you very much for your time JohnH you rock!!
i'd like to show you my take on your design with parts that i've got lying around, this:





i've used the thienevin equivalent circuit of the two last stages of attenuation to match yours, do you think i've got it right? or almost right? 
it is working right now and is a big improvement to a jet city jettenuator, but i don't know where i'm at actually, i'm very bad in impedances matching calculation.

Thank you!


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## Xazrules

In my mic preamp, eq and compressor project i use always the bridged-T attenuator, do you think it can work here too for our purpose?

sorry but i can't post link, don't have the rights


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## JohnH

Hi @Xazrules Sure! As a resistive version, yours should sound fine. I make it that with both the stages that you drew, the amp sees about 7 ohms and the speaker sees about 24, (depending on the amp) - not far off what I target and I expect it sounds nice and clear. You second stage is taking off about 17db, for a total of -24db combined with the first stage.

Will you extend your build with more stages or inductors?


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## JohnH

On T and bridged T attenuators:

I checked out various ways of doing the attenuation when I started this design. The two key ones are Pi and T, each with 3 resistors and you can make equivalents in either format.

But all the references to them that I've seen are based on making them symmetrical, with equal input and output impedance. Ie, you might have a stage that is 8 ohms in and 8 out, with whatever attenuation you want. This would certainly make a working safe attenuator for our amps, but having measured my amp I wanted an asymmetrical attenuator stage with greater output impedance than input impedance. eg, I want 8 as seen by the amp but 18 to 20 on the speaker side and that is what my designs do. 

The key is the first stage, which is targeting -7db. At this attenuation and with the desired input and output impedances, T and Pi designs converge and one of the resistors becomes zero, hence we have just the two 15 and 10 ohms. After Stage 1, we can relax the input ohm requirement to some extent since only a small part of any variation in stage 2 is seen by the amp.

Bridged T as i understand it, usually has 4 resistors, but you can condense it down to a standard T with 3, so I didn't find it helpful.


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## Xazrules

Thank you JohnH for taking time to analyze my circuit, at the moment i think to stay with it because the spl out from the amp is at a reasonable level whit the amp crancked and at the moment i'm in total tone heaven!

a friend of mine borrowed me a thd hotplate and as soon as i've some spare time i'd like to do a shootout, if you're instered i'd like to post the results here, but we already know who will be the winner.. 
tomorrow i can post the link to the attenuator calculator, no rights at the moment but the beauty of bridged-T is that with only one 2x6 rotary switch you have a clever layout on the front panel. if i find cheap power resistors i'm going to experiment


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## JohnH

Sure, it would be great to see a shootout.
Cheap power resistors can be had from Chinese ebay sellers, often about $2 for a 100W
But if you really want to compare what we are doing here, gotta have one of the reactive designs!


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## Xazrules

yes, so if i'd like the reactive circuit too, can i buy alredy made inductors or have i to wound them by myself?


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## JohnH

You can buy them, usually about $10 each. They are more usually used in crossovers within hifi speakers Which country are you in?


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## Xazrules

I live in Italy


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## TonyK

@Xazrules I'd strongly recommend Steve, the eBay seller in the UK. He builds to order and will do exactly what you ask (so 1mm wire for example) and his price including shipping to Europe is excellent (I'm in France). He's also very quick! Good luck with your build. https://www.ebay.co.uk/usr/stevie2810?_trksid=p2057872.m2749.l2754


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## TonyK

Hi John, Gene, Nik and everyone. As one of the "early adopters" of this attenuator, I'd also like to chime in. It's absolutely the best and most useful bit of kit I've ever owned. The fact that I built it myself just makes it even better  It's helped me find out where the sweet spot is on my Hot Rod Deville 410 60W amp, which until now was always so loud that it was impossible to find. It also allowed me to learn that a Deville cranked is not a sound I'd like to play with! But at around 7-9 on the Clean Volume, it's sweet. All of this is absolutely thanks to @JohnH of course. 

I've been largely absent for a while due to a couple of medical issues, the most recent being a disc hernia which has caused more pain than I thought was possible in my sciatic nerve. So now, for the first time in my life, I realise that I need not only to lose decibels, but I also need to lose kilograms (no, not me, from my gear). So I've considered replacing my Deville with the new Fender Tone Master Deluxe Reverb -- in case you haven't checked it out, it is possibly the way of the future, all digital, but all focused in a single amp modelling its tone at all levels of EQ and volume, and weighing just 10kg. Then I thought, darn, I won't need my attenuator any more. But there's a happy ending! Given the need to shed kilos, I will still need it. I'm back into building my Tweed Deluxe 5e3 project. I figure that I can build it at less than 10kg, significantly less if I were to use a NEO speaker. So although I can't actually play my amps (or guitars for that matter) right now, I'm happy again! It's definitely time to make my attenuator "permanent" and finally paint it. I'll post when it's done!

Thanks again to all of you who have helped!


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## matttornado

So I finally got around to finishing the assembly after a few months of use. I decided to add switchable 12VDC fan along with an LED that indicates that the fan is on. I used automotive paint for the enclosure and I powder coated the front and back face plates. 

Thanks again, @JohnH! This is by far my favorite piece of music gear that I built myself.


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## Nik Henville

Way I see it is this.
It's a far better attenuator than all those commercially available.
It looks like it could be made (semi)commercially for somewhat less than all those commercially available.
I am very, very interested in having one made, no frills, paint or fancy appointments, just a top notch working attenuator.

My Marshall "says" 100 watts - and that is RMS - in to 8 ohms ( both the combo it's in, and my 4x12 are 8 ohm ).

I live in England (Lincolnshire). Anyone wants to 'come at me' with an offer, we can talk, for sure...


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## JohnH

Thanks guys, and nice work!
This page is making me feel all warm and fuzzy....
....like a dimed Superlead driven by a '58 'Burst through a 4x12 of pre-rola Greenbacks, dialed down to perfect jam volume by a precisely calculated reactive attenuator.


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## TonyK

Nik Henville said:


> Way I see it is this.
> It's a far better attenuator than all those commercially available.
> It looks like it could be made (semi)commercially for somewhat less than all those commercially available.
> I am very, very interested in having one made, no frills, paint or fancy appointments, just a top notch working attenuator.
> 
> My Marshall "says" 100 watts - and that is RMS - in to 8 ohms ( both the combo it's in, and my 4x12 are 8 ohm ).
> 
> I live in England (Lincolnshire). Anyone wants to 'come at me' with an offer, we can talk, for sure...


Hi Nik, good luck in your search. I might consider doing it for you but I need a little rehabilitation from a very nasty slipped disc first! If you can find someone, remember that for this year, at least, you can pay anyone in the EU for no additional fees, but if you were to have the unit built in the USA (or Australia for that matter|), you'd need to add approximately 24% to cover the VAT and import duties plus the more minimal shipping. I know this as recently I've had the yearning to build a partscaster using Warmoth quality or better parts -- they're pretty much all made in the USA and as i looked into it, my project became ridiculously expensive very quickly because of these extra charges! Good luck anyway!


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## Nik Henville

TonyK said:


> ...add approximately 24% to cover the VAT and import duties plus the more minimal shipping...



Been there...
Bloomin' done that...
Got the bloody bills to prove it...
Maritime bunged me a Pistol Slapper - UK taxes etc were... eye watering.


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## TonyK

Sorry to hear that, mate. I was quite a ways down designing my perfect guitar before I realised just how impossible the project was! I was already questioning my sanity when the component prices alone started getting up into Custom Shop levels and then, wham! Well, hopefully someone lurking around the forum will be able to do something for you. You certainly won't regret it!


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## Mcentee2

Quick update, and mea culpa  i.e I'm still not great at htis self-build thing!

I've been getting a few issues with my line out, sometimes working ok, and other times having a huge loud buzzing. that last on is symptomatic of a ground loop or something.

On inspecting the attenuator and working through it all from scratch on paper and in my head, I realised I had used insulated jacks for the build..

....my new Line Out Jack therefore had not got the same "ground" point as the seaker out it was tied to.

The speaker out live has a 22k resistor to the Line Out live; the 2k resistor from Line out Live to ground was then using the "ground" of the Line Out cable to....whatever the Line out was plugged into, wither my DD500 or mixer etc.

So I just patched across the Line OUt ground to the rest of the attenuator ground and all is fine.

I still can't quite work out why it worked "sometimes".

Now its working fine, and seems to actually pass a healthier signal anyway- so much so that (and I think someone advised this, but I can't find the reference) my 22k/2k divider is actually abit hot, I wired a 100k pot onto the 20k so now I have an adjustable Line Out that drops it down to safe limits 

Finally - I swapped back to my 0.9mH inductor - I found that whilst the 1.5mH might "track better" to my setup when massaged in spreadsheet charts, actually in the room and playing the 0.9mH sounds "better", less compressed, almost as if the 1.5mH was strangling the sound a bit too much.


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## matttornado

Mcentee2 said:


> I've been getting a few issues with my line out, sometimes working ok, and other times having a huge loud buzzing. that last on is symptomatic of a ground loop or something.
> 
> On inspecting the attenuator and working through it all from scratch on paper and in my head, I realised I had used insulated jacks for the build..
> 
> ....my new Line Out Jack therefore had not got the same "ground" point as the seaker out it was tied to.



I had the same problem with my line out at first and then I realized I had the line out volume pot grounded to the enclosure. I scratched my head to for a while too. lol


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## Mcentee2

matttornado said:


> I had the same problem with my line out at first and then I realized I had the line out volume pot grounded to the enclosure. I scratched my head to for a while too. lol



My Line out pot *case* is connected to the enclosure, like the toggle switch mounting thread/nut would be, but not any of the three pot legs....

I can see the yawning gap approachnig of me researching "properly designed line outs", re transformers, isolated and lifting grounds etc, and depending on what I was plugging into re its ground......


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## JohnH

I'm glad you are getting it figured out. 

I haven't gone into line-outs for my ones yet. But my thoughts are:
I prefer the attenuator case not 'grounded' to the attenuator circuitry, because really that is . often not a ground at all, its just one of two wires with a/c signals coming off the amp output transformer. But for a simple unbalanced line-out, clearly the jack-barrel 'sorta-ground' wire from the attenuator has to go off to the ground connection on what ever is into the line-out 
On some amps, the speaker out may indeed have a ground though, and through the line-out this may end up connected to the ground of the recording rig, and so normal precautions about ground loops may be needed (running mains from the same supply etc) - I think it would normally not be a problem though. An isolation circuit of some kind, like a DI box may be a part of more complex scenarios.


----------



## SnickSound

Well adding a fan certainly makes a difference! With all aluminum body resistors screwed straight to a the chassis with thermal paste under, when playing my 50W 2204 cranked it would get warm enough that you can't leave your finger on it more than a few seconds, despite all the venting my case had. So I got the biggest fan I could fit in the remaining space: a 60x60x20mm 12V fan, running it via a pedal power supply so 9V which leaves it fairly silent but pushes a lot of air out of the case, and now it's just a little warm. 

Turns out though I like my 2204 around 4-5 on the Master though, just enough that it isn't thin sounding anymore but still has enough headroom for a tight low-end. 

I built mine without the -14dB step, so it tops out at -17.5dB. I've found that I can bring my 2204 down to 17.5dB no problem, but my 15W build (2204 preamp into cathode biased 6V6 power amp) doesn't sound really good past -14dB. My conclusion is that once you get under 1W of effective power, that's just not enough for the speaker to do a good job anymore. The 1W mark is still too loud to play at home while the wife is there, but it will be great on stage in smaller venues.


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## JohnH

That's good info. I think 'touch for a few seconds' is a good qualitative temp criterion, and probably indicates maybe 60 to 80C? We have a metal kettle and when it's boiling at 100C, obviously you can touch if at all.

It confirms my view that for a cranked amp, 50W without fan is a reasonable upper limit, unless the resistors in Stage 1 are divided and spread out in a large case. Fans make a big difference to convection. I'd like to e explore a fan powered from the signal too.

I think different amps have different sweet spots. My VM likes to be well cranked, but my DSL401 is best at about half vol.


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## matttornado

Here's the line out that I used if anyone is interested. Please note that on the drawing it shows that lug 3 is soldered to the pot's case. That must NOT be soldered or else you'll get weird ground loop noise. I hooked it up at the Input of the attenuator so its seeing 100% amplifier signal. Works well here or after the reactive stage. its says to use a Linear pot which I am using now but it is suggested that an audio pot be used instead. I will eventually try an audio taper type.


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## Nik Henville

JohnH said:


> I think different amps have different sweet spots.


.
.
.
Rarely were/are truer words spoken


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## Gene Ballzz

@JohnH & Other Interested Parties,
I'm getting ready to order parts/build a 16Ω M-lite, scaled up to 100 watts for a friend. When choosing *R1*, might it be best to go with two 60Ω, 100 watt in parallel or two 15Ω, 100 watt in series? The reason I ask is that a single, 200 watt resistor can be fairly cumbersome (size wise) and pricey! Something in the back of my addled brain is whispering that the parallel scenario may be better?
Thank Folks,
Gene


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## JohnH

hi @Gene Ballzz , I reckon spitting it in two is a good idea anyway, to spread the heat. For Design M, R1 split in two and R2 would share the main work and could be laid out across the case, with the others between. For M2, it would be R1 in two parts, and R2A and R2B.

For R1, series or parallel? I don't see a real difference. It might depend on what is available. 15Ohm is a value in all standard ranges while 60 may not be. Also which leads to better wire runs?

For a 100W if its really going to be cranked, I'd say big thick case, lots of large holes above and below, maybe move up to 16ga wire and coils and think about a fan.


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## SnickSound

I split R1 in two parallel resistors for mine. Parallel because it made the layout easier as I simply aligned all the resistors that have a connection to ground next to one another and run a buss of sort through their ends. 

I did spread the 3 main resistors (R1a, R1b and R2) over the complete width with the smaller ones in between, to prevent creating a hot spot of sorts. You have about 80% of the power dissipated through R1 and R2 so these are the one that will generate the most heat.


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## mattmoox

Hello John, and thank you for starting this awesome thread, and for keeping it alive for so long. I’ve read (most) of it, and am impressed and fascinated by the science of it all.

First, here are some pictures of a build of your design I created for a Fender Deluxe Reverb 65 Reissue.






I play in a band with three female singers and some other pared-down acoustic instruments. So, of course, I named my build “Something For The Ladies”. And the controls are labelled in pink. Not being too loud is something I am reminded of…frequently 

I hunted and gathered, exercised my frugal tendencies, built your attenuator, and life was good...at least during several recent rehearsals.

Last night however, I played a small venue in San Francisco, and the room was pretty warm. With the attenuator on -11.5 dB, during the last song of the hour-long set, the amp started cutting in and out, showing signs of death.

Today, I did some testing, and noticed that with the attenuator on any setting other than “bypass", the output transformer in the Deluxe Reverb is getting very hot. So hot, in fact, that I can’t touch it for more than a second or two. I’m deducing that the cutting in and out was perhaps due to the thermal cut off in the output transformer (?), but happy to hear other theories. 

With a thought that maybe something was off with my wiring, I ran some impedance tests in the hope that you could confirm whether they are as expected. Here are my results:




Questions:

Is the heat in the O/T normal?
Anyone else running this attenuator design with a Fender Deluxe Reverb?
Do my impedance readings look correct?
Any recommendations for how I can test whether I have done any permanent damage to the O/T? Running the amp with the attenuator in bypass sounds normal, and the O/T transformer doesn’t get nearly as hot as when the attenuator is on.

Thanks John, and everyone else that has contributed to this great project.

Cheers,
Matt


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## Gene Ballzz

@mattmoox
*First & Foremost >*  to the forum! Great looking work, fantastic chart and pics and nice clear description of your problem and tests!  You''ll be a welcome addition here, if you have the time to stick around a bit.
_*Next > *_I'd take those same readings in all modes again, with meter still on and wiggle every wire connection, cable connector and switch for intermittency! Stranger things have happened than to get a bad resistor or switch with intermittent connections.
_*Last, For now > *_You might consider that your power tubes might be going South and/or the bias is not set properly? Consider that when you are not using the attenuator, you are usually turned down to the *"You're too loud"* zone and not really working the power section or even whole amp very hard. Then when you use this attenuator, you get to crank the amp into its tonal *"happy zone"* thereby driving every thing much harder, and possibly showing up problems that weren't as easily evident when running it cool. Just a thought.

Keep us posted.

Just My $.02 & Probably Worth Even Less,
Gene


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## JohnH

hi @mattmoox and welcome to the discussion. That's a nice build indeed. What Gene says makes sense, if by using the attenuator, you are also turning up the amp louder. The readings all look about right, with the right tendencies up and down. Particularly the input R's with speaker connected are consistent to what id expect and also close to 8 within about 15%, which is very close and that is what the amp sees.

If you can touch the metal of the OT for a couple of seconds, then Id reckon it not as hot as 100C because you cant touch a boiling metal pot of water at all. But I don't know what to expect as normal

How hot does the attenuator get?


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## Gene Ballzz

@JohnH
You make a very good point, as I had not noticed no ventilation of any sort! Mine gets at least a bit warm to the touch even with fair to good ventilation at 20 watts. A good Deluxe Reverb puts out at least 22 watts , likely more. And if multiple stages are in use, it's shedding a lot electrical energy as heat. So no ventilation, coupled with being tucked in like that could cause some serious heat to build up! Add to that, the fact that we don't know how really robust these cheap resistors from Chinesiawanoreanam might be.
Great Catch!
Gene


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## Gene Ballzz

@JohnH & @mattmoox
Upon closer inspection of the build, I'm not seeing any R6, though I'm not seeing glaring anomalies of the tested numbers associtated with activation of that stage.

Hmmm, 
Gene


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## Gene Ballzz

Sorry guys, I found what I suspect to be R6 tucked away near what I suspect to be R9! 

OOPSIE!


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## BrokenBones

Can someone point me in the right direction on the switch types for Design M? I know they should be 5-6A but are SW1 and SW4 DPDT On/Off/On switches? SW2 and SW3 are just On/Off SPST? 

TIA


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## JohnH

The switches are all just basic two-position toggles. For the simple versions one pole only is used, but you can use dpdt.


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## mattmoox

Hi Gene, John,

Turns out that the issue with the amp cutting in and out was not the amp at all - it was the Xotic EP Booster on my pedal board and it had nothing to do with the attenuator. I'm not sure what's wrong with the EP yet, but it is intermittently failing.

That said, I'm still left wondering whether my O/T is getting too hot when the attenuator is on.

Thanks for the feedback and ideas. Certainly good to hear that my impedance readings look good. Other thoughts and comments at this stage:

This is Design M, with L1/L2 set to 0.4/0.6 mH respectively.
R6 and R9 are there for sure, hidden behind the wires in the pic 
I used plenty of conductive paste on the resistors.
These are not cheap Chinese resistors (I was too impatient ); these are all from Mouser.
In my early testing with the case open, the resistors were not generating any significant heat (I used John's resistor specs for a 50W amp - my amp is only 22W). Based on this, I haven't added any vent holes yet.
I need to wait for the house to be empty for a bit, and then will try cranking the amp to max, and setting the attenuator to -31dB - then I'll find out just how hot the O/T will get in a worst-case scenario.
Cheers,
Matt


----------



## Gene Ballzz

@mattmoox 
With all that said, I'm even more suspicious of power tubes and/or them not being biased properly. Fact is you are cranking the amp harder than usual, and that extra stress helps point out any weakness, especially in the power amp section. Just out of curiosity, model model and brand of power tubes are you running?
Just Curious,
Gene


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## TonyK

This thread just continues to getter bigger and better as an information resource, so I still visit every few days to keep abreast!

I have a sort of left-field question. Medical circumstances mean that climbing stairs and/or moving amps around is going to be an impossibility for a while, so I've been looking at alternative solutions so I can at least get back to playing some electric guitar. I've sort of come down to using my pedal board going through a Joyo American Sound and AcTone (plus reverb) and into a VERY small and light amp (SS) to listen either though headphones or from time to time through a lightweight 1x12 cab. The amp I think I've settled on is the Vox MV series. Price is good and as this isn't a "necessary amp" purchase that's important. Anyone who knows about these amps knows that they have multiple models, but the "Clean" represents essentially a Blackface sound and it comes with a built-in attenuator which many users claim to be a major reason for their acquisition of this amp. This is one option for me. The other option is their "Boutique" model which offers good but not BF cleans and very smooth Dumble-esque overdrives reminiscent of Robben Ford et al. This model doesn't have an attenuator and I'd have to rely on its Master Volume to keep the SPL down, however effective that might or might not be.

So this brings me to my question. Can anyone see any reason why I couldn't use my Attenuator M taking it's input from a solid state amp output (rated from 4-16 ohms)? If the answer is no, the "Boutique" might be a better option for me as it would give me a quite different tonal palette than I have with my actual tube amps.

Many thanks in advance!

Cheers, Tony
PS I hope you're all staying safe despite the Coranavirus pandemic!


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## JohnH

Hi Tony,
Good luck for getting back to your playing. 

I reckon the attenuator would be safe to use but, with most SS amps they work fine at low volume anyway, so would it be different to just turning down? Also, an Ss amp has a very low output impedance while the attenuator has a higher one like a tube amp, so it may change the tone. Do you have any Ss amps you could try it with first?


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## TonyK

Thanks, John, that's what I figured. In fact I've been doing more (too much) research and I think the "Boutique" model is perhaps not the better of the two models for my purposes. I need a little reverb when I play, and this tiny amp has no effects loop, so I'd only be abe to use reverb/delay type pedals at the lowest gain setting anyway, in which case I'm probably better off with the "Clean" model and getting my OD from the pedals before going through the reverb.

So I should probably stop researching and just place the order!

Thanks again!


----------



## Gene Ballzz

@TonyK 
Sad to hear of your "medical circumstances" and hope it improve soon for you! Although these have been off the radar recently due to many other new options, folks have raved about them for years, at only 4.4 pounds!
https://www.sweetwater.com/store/detail/THR5--yamaha-thr5-10-watt-2x3-inch-modeling-combo
Something To Think About,
Gene


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## TonyK

Hi Gene, thanks for your kind words (John's too). This was my other major option, but I still like the feel of a "real" amp through a decent speaker and these little THRs never seemed to give that to me when I tried them even though they sound decent enough with their tiny stereo speakers! They're back in vogue now as Yamaha recently released THR-II models with more power, all previous models incorporated into the same box, bluetooth and all sorts of other new bells and whistles!


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## JohnH

Before we get back to the important business of inventing attenuators to solve the worlds problems, Id just like to point out Katana 50 from Boss. My friend has one, not the very new model, but the previous which you can find at great prices. When we got together for a jam I was very impressed with it, and its a proper sized compact amp with great tone from a 12" speaker, with effects and its light too. We jammed with me on my 50w attenuated Marshal and he played through that.


----------



## LordoftheLivingRoom

I was about to build myself an "Elevenator" which is a L-pad based attenuator. But then I stumbled upon this epic thread! I've spent the last 6 hours binge reading the whole thing. Yikes! 

First off, Thank you JohnH! What an awesome thing you've done here. I'm blown away with the generosity you've shown here with your time and inventiveness. If I ever find myself in your neck of the woods I'd love to buy you a beer.

Secondly, now the bug has bitten me and I'd like to build a stock M2 version attenuator. My amps were built by me. I have a JTM45 clone running into a 2x12 with WGS ET65 and WGS Retro30, a single channel AB763 Deluxe Reverb clone (check out EL34world if you're into amp building) running into a WGS G12C, and a weird little single ended 5W franken-amp running into my spare ET65. Problem is, like most of you, I get the "turn it down" treatment from the family whenever I turn any of the amps above 1. Hence my search for an attenuator that won't require me to sell my first born. I often like running the DR and JTM45 together wet/dry, so I may need to build 2 attenuators.

Now to my question. Parts suppliers in Australia. Where do the Aussies here go for good quality aluminium enclosures and the air core inductors? 

Thanks.


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## JohnH

Hi @LordoftheLivingRoom , welcome and thanks, I'll take that beer! My home base is Sydney where I work, and I live south of it.

Your amps sound like great projects. This attenuator will be a piece of cake in comparison.

For the cases, I like the heavy die-cast Aluminium ones that Jaycar sell, see the pic in post 1. The only watch it is to work out the drilling to avoid internal ribs. They paint up well with a clean with wire wool and a squirt from a rattle can. On other parts from Jaycar, I find their jack sockets to be OK, and I use the stereo ones even if just for mono, for their better grip. I'm not too pleased with their full-size toggles but I like the mini toggles. A DPDT one of those with both side ganged together would be OK, particularly if no full bypass required

For inductors, I bought mine from Queensland Speaker Repair but they seem to have stopped selling the ones I used. Wagner electronics in Sydney sell what we need though, like these:

https://www.wagneronline.com.au/ind...ctors/inductors/electronic-components/111/fl/


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## jszair

Fantastic thread.

Question regarding R10 in design M. How does this resistor impacts treble?

Where is R10 connected to exactly? By plugging in a mono speaker to this stereo jack, are the L&R tap of the jack basically shorted?


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## JohnH

Hi @jszair and thanks for your interest.

When you plug a mono jack into output 3, it stunts the ring terminal of the jack to ground. This puts R10 in parallel with R1, resulting in a lower net value for R1and R10 combined. That then lowers the frequency at which L2 starts to add treble relative to lower frequencies, adding more relagive brightness to correct the tone for a 16 Ohm Speaker. But, it's optional, if sounds good without it too.


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## Addrs

Hi. Your project is amazing. 
While You are still discussing output 3... As I can see 16ohm versions has different inductors L1 and L2 than 8ohm version which is a clear - matter of calculations. 
So how usable is using output 3 for 16ohm on 8 ohm version for daily use? Should we use it just extra, or is it fully usable like standard 8 ohm? 
Because 8 ohm version has 2 advantages:
1) has cheaper inductors
2) has 2 options
Are the trebles going to be emphasised more/less or resonant peak? 80% of signal is "modelled" by the first -7dB stage so 8/16 ohm speaker choice has a smaller efect?


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## JohnH

I'd suggest to build the 8 or the 16 to match whichever is your most important set up, but it is important that it matches the amp tap.

It's fully useable any way. But if you can use all matched, like 8 or 16 for amp, attenuator and speaker, the attenuation steps are a bit more even. The 16 attenuators work very well with 8 Ohm speakers with no tweaking.

Also, for economy, the newer M2 schematic has only one inductor. It's a higher value but should still be less $ and space than the two in design M. M and M2 have identical performance according to the numbers.


----------



## What?

This is probably the coolest 'open' project I have ever seen for guitar with all the posts on the design and testing. Big thumbs up JohnH. I only wish that I had ran into it before I bought my Weber. Wondering now if I should sell the thing and build this. Also wondering if the design of this attenuator would be a good base from which to do impedance matching, i.e, using various speaker impedance loads with a given amp output impedance.


----------



## JohnH

Thanks @What? 
I've enjoyed this too, and sharing it has made if much better. In fact, I likely wouldn't have taken it so far without the great support and feedback from others.

Impedance matching, yes it can do that with different cabs, provided the attenuator model matches the amp tap. I'd suggest keeping it to a one step difference to keep tonal control, although this could be extended.


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## static86

I built the Simplified version over the weekend and it works great on both of my amps. I was originally going to just buy an attenuator but then I started digging more and came across this post. I can play my guitar more now since I'm not disturbing the entire house and the amps still sound good. Made an account just to say thanks JohnH (and anyone else who contributed) for this awesome design!


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## matttornado

FYI:
I used my design M as a load box only with a cranked 100 watt Super Lead.
I used my line out to slave the amp into my DSL combo. 
I added a 4x12 extension cab to the DsL as well.

- sounded phenomenal 
- attenuator barely even got warm with the fan on even with over 100 watts getting fed into it
- my amp ran surprisingly cool too, it would get much hotter with the power break i had.
- my amp seems to really like this design!

Thanks again JohnH!


----------



## Carl Schneider

Hello Everyone, I hope that you're all safe and healthy.
I was honestly going to just buy a reactive attenuator but I love building things so here I am.
Two questions I have if you don't mind? First, there is a lot to digest here and some recent updates that jump around etc. Can you possibly please make one page or PDF with the current/latest schematics and descriptions? I am little confused trying to follow this from front to back. The second thing is a thought I had that instead of using an external power supply to run a fan (as I have seen at least one person here do) couldn't you put a rectifier across one of the resistors at the front end and run the fan with the available voltage (or an AC fan filtered to allow only the specified frequency 50 or 60Hz?). I found this in the description of the THD Hot Plate "an internal cooling fan powered by ampliﬁer output".
Thank you for all of this hard work.


----------



## What?

Hey JohnH, at another forum you mentioned that this attenuator could be adapted for amps with lower output impedance. A member over there was inquiring about attenuating a Super Reverb. I would like to do the same. What would need to be changed to use your attenuator design with these amps? Also, would it be practical to have a single attenuator of this design that could handle amp output impedances of 2 ohm through 16 ohm? (2, 4, 8, 16). If so, it could maybe be the ultimate swiss-army-knife of attenuators, having your reactive design capable of handling those old Fender amps with lower output impedances. I can't safely use my Weber attenuator with my Super Reverb, and I'm not crazy about how the tone and feel changes with it any way.


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## What?

Carl Schneider said:


> The second thing is a thought I had that instead of using an external power supply to run a fan (as I have seen at least one person here do) couldn't you put a rectifier across one of the resistors at the front end and run the fan with the available voltage (or an AC fan filtered to allow only the specified frequency?).
> Thank you for all of this hard work.



I think I remember seeing JohnH mentioning somewhere about exploring the idea of running a fan from the power of an amp's output. It does seem like nifty idea worth exploring. I only wish that I understood more about electronics and how these things work so that I felt more comfortable tinkering and not possibly blowing up an amp.

Just throwing an idea out there from a naive perspective. Maybe the ultimate attenuator would be one that could do this sort of reactive attenuation and any impedance matching. On that last point for example, 2 ohm amp output to 16 ohm speaker load and the other way around. Maybe that sort of thing isn't practical though.


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## JohnH

I try to keep a summary going, but it takes some work! post 1 gives everything for design M with a reference to where to find design M2 and M3. M2 has the same performance as M with one inductor instead of two. M3 adds a resonant circuit, but the need for this is still moot since M and M2 work just fine.

*Fender circuits with low impedance:* So help me out, what are the combinations of output tap and max power that would need to be considered to cover the range? I think Ive seen Super Reverbs with 45W and 2 Ohms and deluxe reverbs with 88W and 4 Ohms, with Super Six at 2.8Ohms? And some at 100W or 135W?

*Self powered fans:* Definitely a great idea for powerful amps. It seems like these are not needed at 50W or less though, if you have a good heavy thick case with good vents. The circuit could be similar to Marshall Powerbrake, but the specifics will be very particular to the ohms, the fan and the power. Best to use a very efficient low power fan to suck as little as possible from the system so not to throw the response off. This really needs proving by testing, but I don't have any amps that would need it myself. 

cheers
J


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## What?

JohnH, old Fender amps range from 2 ohm to 8 ohm (2, 2.6, 4, 8). I would say that typically, Fender amps worth attenuating are going to be in the 50 watt range. The 135W amps for example are ultra-linear designs, designed to fight being overdriven. And even the earlier 85-100 watters silverface Twin Reverbs for example are clean design amps that aren't going to give up any overdrive goods worth having when cranked up. But it is popular to run blackface twin reverbs into overdrive (85 watts 4 ohm) and to mod silverface Twins to blackface specs (also 85 watts 4 ohm) but that it usually happening with 2 output tubes pulled and expecting an 8 ohm load. Any way, you can see specs for blackface amps at Rob Robinette's site (see the table labeled 'Table of Contents and AB763 Overview'): https://robrobinette.com/AB763_Model_Differences.htm But The Fender Amp Field Guide has more comprehensive specs for many Fender amps on separate pages per amp (relevant fields are Speakers/Load and Output): http://thevintagesound.com/ffg/


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## JohnH

A bit more thinking on impedance matching:

The designs shown to date are all based around matching the attenuator design to a specific amp ohms, typically 8 or 16, and 4 or even 2 could be derived by scale factors on components. But each build is specific to one amp tap. For speakers, you can use x2 x1 or x1/2 safely, with only a small tonal change. eg, and 8 ohm design can use 16, 8 or 4 ohms. Even 2 ohms is safe, but the tone drifts off another db or so in terms of lower mids vs treble and bass.

To adjust for input impedance, without too much complicating and without the bulk and expense of a matching transformer, I'm thinking as follows to add 16 and 4 inputs to an 8 ohm attenuator:

Create another circuit, probably with an 8 ohm resistor, about a 0.4mH coil in series and another bypass resistor across the coil. This would be tweaked to approximate the impedance curve of a an 8 ohm speaker from low mids to high treble (ie ignoring the bass resonance). Put this in parallel with the main circuit and the amp thinks its driving 4 ohms, put it in series and it thinks its seeing 16 ohms. Hence an 8 ohm build becomes usable with 16 or 4 ohm taps. The fairly small penalty is that when using 16 or 4, the new circuit is soaking up half the power so there is another -3db cut and the minimum cut would be -10db instead of -7 db. But a nice feature is that I think it is possible to do all this with just TRS jacks, no actual switches. There would be input sockets for 4, 8 and 16 and when you plug in with a normal jack plug, the correct impedance is presented.

So one box based around one of the 8 ohm designs could do 16, 8 and 4 ohm input and be used with 16, 8 or 4 ohm speakers. It would still be optimum to use it with 8 and 8 however for consistency, but should be very usable with other settings.

Scaling to other base builds, a Fender-based 8, 4, 2 package could be made with 4 ohms as the core design. Or based on the 16 ohm build, an 8 ohm input could be added.

The added input tap feature could be added to any previous version on the thread.


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## What?

JohnH, that sounds promising. It's very late here, so I'll reply tomorrow.


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## What?

JohnH, just to be clear, you are saying that a scaled 4 ohm design would be safe and have negligible impact on the sound when used with a 2 ohm amp and 2 ohm speaker load? But I take it that it wouldn't be a good idea to use this with a 2 ohm amp and a 4 ohm or higher speaker load.

Also, if it would be of any interest to you, I could open up with Weber attenuator (current model Mass 200) and provide pics and values. It might be interesting to see how your design graphs in comparison to a commercial attenuator. I have taken a look inside before, and there isn't much in there really. Just a a big rheostat, a couple of small components, switches and jacks, and a speaker motor (voice coil, spider, and slug. I didn't see anything moving in there, by the way, and I wonder if just an inductor wouldn't give the same result, where maybe the speaker motor thing might be a bit of marketing gimmick. When I bought the thing I chose the Mass 200 so as to cover all my bases in terms of power handling, not accounting for getting into old Fender amps with 2 ohm taps and speaker loads. If this thing covered impedance mismatches (like the z-matcher) and provided attenuation, it would have been worth the price. But it sounds like it is only really a 16 ohm reactive attenuator with added resistive attenuation for other loads (down to 4 ohms only).

You mentioned on another forum that the Weber design is 16 ohm, where lower impedance loads are covered by adding resistors. Is this what Weber means by 'impedance swamping'? Maybe this is where my issue has been with this attenuator. When I was using it, it was with an 8 ohm speaker that sounded pretty dang good when unattenuated (using the 8 ohm tap from my amp) but not so good when attenuated. I have yet to try it with my 16 ohm 412 cab, but now that I know, I will give it a try.


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## JohnH

hi @What? 

Id say if you had a 2 ohm Fender with 2 Ohm speaker set and just wanted a box for that, then you'd build just for two ohms, using 1/4 of the 8 Ohm values. But, since that'd be a fairly specialised piece of kit, the 8-4-2 system based around a 4 ohm version would be more useful. At 2 Ohms, the amp would be totally happy, and wouldn't know the difference. The minimum attenuation would be -10db instead of -7db, eg 50W becomes 5W. Also, totally safe to put a 4 Ohm speaker into such a box and it should sound fine since the core of the attenuator is a 4 Ohm design. Just don't have a full bypass to avoid sending a 4 ohm speaker direct to 2 ohm amp (although I don't know if that is really a problem)

When different ohms speakers are plugged in, very little of the difference is seen by the amp, so with a one step x2 difference there's only about an ohm change as seen by the amp. The tonal change is in theory, about a db of extra bass and treble if you use a lower speaker ohms, and about a db less with higher speaker ohms.


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## What?

JohnH, thanks a million. I think the -10db issue shouldn't really be an issue at all. Most people are probably looking to knock off more volume than at minimum any way, and I definitely am. Parts shopping tomorrow.


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## JohnH

What? said:


> JohnH, thanks a million. I think the -10db issue shouldn't really be an issue at all. Most people are probably looking to knock off more volume than at minimum any way, and I definitely am. Parts shopping tomorrow.



OK great - should we do a schematic with parts values for the 4 ohm option with front-end circuit to make it 8/4/2?


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## What?

Yea, a new schematic would be nice. I was intending to shop for component values as I close as I can get to half of the component values for the 8 ohm design. Is it going to make a noticeable difference going one way or the other (higher or lower than half the values for the 8 ohm design ) for the inductor values?


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## JohnH

What? said:


> Yea, a new schematic would be nice. I was intending to shop for component values as I close as I can get to half of the component values for the 8 ohm design. Is it going to make a noticeable difference going one way or the other (higher or lower than half the values for the 8 ohm design ) for the inductor values?



In a range of available values you often cant get exactly the intended ones. With resistors, some power resistors seem o be sold with the standard steps like 6.8, 8.2, 10 etc, while others such as those I used are in a different series. To chose based on scaling down ohms, try to keep those in each stage within the same proportions to each other, For inductors, id go a bit higher rather than lower. (can always unwind them a bit)

Are you going to try the extra front end circuit to give input ohm options yet? The inductor and bypass resistor values I suggested are likely to be worth some analysis to check the best values, which I haven't finished yet.


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## What?

Yes, I would like to try the input impedance options. I guess my horses were ahead of my cart, so I'll pull them over.


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## JohnH

I ran some analysis and the impedance options look like they'll work well and consistently. I think the best values will be, for an 8-4-2 input range as follows:

A 4Ohm resistor which should be sized with a rating of 1.5x the amp power (it takes half the power, then give it a x3 margin), in series with....

....An 0.25mH aircore inductor, 18 Gage or thicker, bypassed by a 10 Ohm resistor., probably 25W rating for a 50W amp. 

But I'll draw it.


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## What?

Sweet JohnH.


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## JohnH

*Attenuator M2v-with variable input options
*
Here's a schematic with three input and three output options, based on either 16/8/4 ohms or 8/4/2 ohm ranges.




I've picked values for the resistors based on the usual standard range, with power ratings for component based on up to 50W amps as before. I omitted a full bypass switch and also the -3.5db only option, to make sure there is a plenty of safe separation between input and output given multiple Ohms use.

I did include R10 and R11, to make the tonal correction when using the higher ohm speaker options. Its all fine to omit this or plug into the other output jacks instead. But the tonal adjustment does make a nice correction to lift treble and I was using it on mine this morning, so I think its good to have. I've also declared where using the high or low input taps, or the higher- ohms output jacks, reduce power slightly.

Note that if using this with a 2 ohm input setting, 50W delivers 5 Amps, so get really good jacks to handle that current


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## What?

Much appreciated JohnH! I'm looking forward to trying this with my Super Reverb. I will give it a better look over tomorrow when I'm off work.


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## What?

I'm lagging on ordering parts here, having some second thoughts on going ahead with the 8-4-2 attenuator version. It seems like I could get a much more versatile setup from the 16-8-4 version + z-matcher. Any thoughts on that? I have amps here with output impedances of 2 ohm, 4 ohm, 8 ohm, and 4/8/16 ohm selectable.


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## Freddy G

Hi JohnH!
GeneBallz over at the MyLesPaul forum turned me on to your design. I'm going to use your design M- 4stage reactive attenuator. Dumb question...Looks like I need the 0.33 and 0.5 mH inductors. Are there any other specs I need for these inductors? Would you perhaps be so kind as to point me to a digi-key part #?
Thanks! I'll report back after I have it built.
Freddy


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## Gene Ballzz

Hey @Freddy G

Digi-Key is not likely your best source. This place is one of the best sources (at least for US purchace, not sure about getting across the border to you?) in the most convenient format, on a bobbin.

https://www.madisoundspeakerstore.com/air-core-19-awg-page-2

As long as your intended amp is 50 watts or less, 19 guage air core is just fine!

Best Of Lick & Enjoy The Liberation!

Just Hopefully Helpin'
Gene


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## Gene Ballzz

And Oh yeah, @Freddy G ,

Welcome to the forum. Some cool guys here too!

Gene


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## JohnH

What? said:


> I'm lagging on ordering parts here, having some second thoughts on going ahead with the 8-4-2 attenuator version. It seems like I could get a much more versatile setup from the 16-8-4 version + z-matcher. Any thoughts on that? I have amps here with output impedances of 2 ohm, 4 ohm, 8 ohm, and 4/8/16 ohm selectable.



I think the z matcher uses a transformer to convert between impedances, which is the best way to do it. So if you had that, you wouldn't need the new front end of the attenuator circuit, just convert z to match the attenuator design, say 8 Ohms. The output then works with 16, 8 or 4. But you have to buy another box.

The only issue I could see is how youd deal with a 2 ohm cab if you have one, and if you also wanted to attenuate?. There could be another tweak on the output to make a 16/8/4/2 output version with 8 ohm input, with a bit more output loss at 2.


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## JohnH

hi @Freddy G , welcome to our attenuators! Gene has you covered with the advice above.


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## Freddy G

Wonderful! Thanks for the link Gene and for the warm welcome fellas!


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## Freddy G

Just to be sure....the schematic on the first post is the most up to date?


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## Gene Ballzz

@JohnH ,
Thanks for all your efforts here. It's absolutely amazing how this topic/thread seems to have taken on a life of its own! The advancements, improvements and testing over the course of time have been obviously inspirational to many, including myself.  Your diligent processes have freed me from the horror of the *"turn that damn thing down"* syndrome, along with the loss of tone (and enjoyment) that inevitably comes with it. 

Thanks Again JohnH,
Gene


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## Freddy G

@Gene Ballzz

Gene I have a 25 watt amp.....I'm assuming 50 watt resistors will do?

edit...silly me, never mind....I see the power ratings for the resistors in the chart.


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## Gene Ballzz

Freddy G said:


> @Gene Ballzz
> 
> Gene I have a 25 watt amp.....I'm assuming 50 watt resistors will do?



Yeah, I'd still go with 100w for R1 and 50w for R2 and even follow the rest as per the chart. R1 & R2 do most of the work and are always in circuit, excxept for the "-3.5db only" setting, though it's my personal opinion that that particular stage could be eliminated for most users, as -3.5db is barely noticeable and most often easily achieved through volume controls of the amp and/or use and dynamics by an experienced player. Also, don't be fooled into thinking that a 25 watt amp won't create a bit of heat. Make sure the resistors are properly heat sinked to a well ventilated enclosure. 

And Yessiree, the charts in the first post are on the money! John is righteously on top of keeping the designs well updated!

Have Fun & *DON"T* Have A Corona! 
Gene


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## Boysen

Hi Guys!

I lightly promised a friend last summer to build him a heavy rock amp. I had reasonly built an El84 cathode based plexi style amp, I could modify to his specs and wishes. The primary mods are as follows:

1) *FX loop* (LND150 TT kit)
2) *2 Button Ch.switch System* (Dual Relay TT kit):
a) Clean(single triode to PI) or Dirty(plexi style w/ cathode follower & TMB),
b) Cold clipper stage on/off in Dirty mode
3) *Fixed Bias & Choke*
4) *Reactive Attenuator (M2 John Hewitt style) w/ line out option*

The cleans are tweedish, with only 1 triode to the phase inverter, while the dirty channel is all out rock, into heavy with the boost engaged. I decided to try the M2 in the amp, and was thorougly surprised. It sounds amazing, with no coloration at any setting, giving you power amp grind at speaker volume. Without the Reactive Attenuator, this amp would never be able to play at its full potential, giving you a vast variety of sounds at any volume. I can't start to thank you (John especially), for making this possible.

Thank you!!


Kind regards
Christian


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## JohnH

hi @Boysen What a great project and thankyou so much for posting. Im really glad the attenuator part is doing its job!


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## JohnH

Freddy G said:


> Just to be sure....the schematic on the first post is the most up to date?



I try to keep the first post up to date as things develop. Design M has been nuilt it many versions now and is pretty much fixed. The diagram shown there is fine but note some very small tweaks referred to in the post 1 text. 

There's also a pointer there to Design M2. That one is newer, it has the same performance as M but with a different front end that only needs one inductor instead of two. The maths was trickier but it's a simpler more economical build.


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## Freddy G

JohnH said:


> There's also a pointer there to Design M2. That one is newer, it has the same performance as M but with a different front end that only needs one inductor instead of two. The maths was trickier but it's a simpler more economical build.



Thanks John. I cannot find Design M2 with one inductor. I see an M2v at the top of this page but that is with 2 inductors. Would you please link?


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## headcrash

hey FreddyG: http://www.marshallforum.com/thread...design-and-testing.98285/page-27#post-1877676


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## jejj21

Hey Guys,

So, I decided to take the extra COVID-19 tinkering time and build the M2 version of this attenuator. It works amazingly well and I am very happy with the results so I thought I would try and pay it forward some. 

Attached are my drilling templates and part layout in case anyone else wants to give it a shot. I laid everything out in AutoCAD so all parts are to scale and precise. I also created a wiring layout in DIYLC to help me keep it all straight. You can print the PDF out (Actual Size) and cut out the pages and tape them directly on the enclosure for center punching, which is how I did it. 

I did change the switch/resistor configuration to be in ascending order for my own sanity. I only included one output, since all of my amps can work on 8ohms. Other than that, all part values match with John's recommendation.

I used a Hammond 1590D enclosure and I bought it as well as all the resistors from Mouser. I could have probably found cheaper ones on eBay, but shipping to the USA was not in my favor. All wires are 18ga and the switches are all DPDT 6A ON/ON. The inductor is a 0.9mH 16ga Sidewinder Air Core from Madisound, held in place with a #10 stainless steel chassis bolt. I used #4 bolts for the resistors with the exception of #8 on the big 100W.

I'm attaching a couple pics of the final product so you can see how it looked once I completed it.

Thanks to John and Gene for the inspiration and knowledge.

Joey


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## JohnH

Hi @jejj21 , thanks for posting and for your pictures and diagrams. That looks to be a very nicely laid out clean build.

Seems like we now have a handful of M2's sprung to life and out in the wild!


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## jejj21

Thanks @JohnH it was a fun project and has definitely given me new hope for other things. My 4x10 Bassman build may now come to fruition since I'm confident I can tame the beast should I ever want. :O)


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## Freddy G

jejj21 said:


> Hey Guys,
> 
> So, I decided to take the extra COVID-19 tinkering time and build the M2 version of this attenuator. It works amazingly well and I am very happy with the results so I thought I would try and pay it forward some.
> 
> Attached are my drilling templates and part layout in case anyone else wants to give it a shot. I laid everything out in AutoCAD so all parts are to scale and precise. I also created a wiring layout in DIYLC to help me keep it all straight. You can print the PDF out (Actual Size) and cut out the pages and tape them directly on the enclosure for center punching, which is how I did it.
> 
> I did change the switch/resistor configuration to be in ascending order for my own sanity. I only included one output, since all of my amps can work on 8ohms. Other than that, all part values match with John's recommendation.
> 
> I used a Hammond 1590D enclosure and I bought it as well as all the resistors from Mouser. I could have probably found cheaper ones on eBay, but shipping to the USA was not in my favor. All wires are 18ga and the switches are all DPDT 6A ON/ON. The inductor is a 0.9mH 16ga Sidewinder Air Core from Madisound, held in place with a #10 stainless steel chassis bolt. I used #4 bolts for the resistors with the exception of #8 on the big 100W.
> 
> I'm attaching a couple pics of the final product so you can see how it looked once I completed it.
> 
> Thanks to John and Gene for the inspiration and knowledge.
> 
> Joey
> 
> View attachment 71701
> View attachment 71702
> View attachment 71703
> View attachment 71704
> View attachment 71705
> View attachment 71706


 Joey, that is beautiful work! and your layout drawings....could not be easier to follow. Excellent!


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## Gene Ballzz

jejj21 said:


> Hey Guys,
> 
> So, I decided to take the extra COVID-19 tinkering time and build the M2 version of this attenuator. It works amazingly well and I am very happy with the results so I thought I would try and pay it forward some.
> 
> Attached are my drilling templates and part layout in case anyone else wants to give it a shot. I laid everything out in AutoCAD so all parts are to scale and precise. I also created a wiring layout in DIYLC to help me keep it all straight. You can print the PDF out (Actual Size) and cut out the pages and tape them directly on the enclosure for center punching, which is how I did it.
> 
> I did change the switch/resistor configuration to be in ascending order for my own sanity. I only included one output, since all of my amps can work on 8ohms. Other than that, all part values match with John's recommendation.
> 
> I used a Hammond 1590D enclosure and I bought it as well as all the resistors from Mouser. I could have probably found cheaper ones on eBay, but shipping to the USA was not in my favor. All wires are 18ga and the switches are all DPDT 6A ON/ON. The inductor is a 0.9mH 16ga Sidewinder Air Core from Madisound, held in place with a #10 stainless steel chassis bolt. I used #4 bolts for the resistors with the exception of #8 on the big 100W.
> 
> I'm attaching a couple pics of the final product so you can see how it looked once I completed it.
> 
> Thanks to John and Gene for the inspiration and knowledge.
> 
> Joey
> 
> View attachment 71701
> View attachment 71702
> View attachment 71703
> View attachment 71704
> View attachment 71705
> View attachment 71706



Great work, effort and results! The round "speaker grill" pattern for vent holes in the bottom looks very cool! My only concern is that there are no holes drilled in the case, between the componets to provide flow through for the convection ventilation. Adding some feet may also be advisable.

Thanks For The Mention & Enjoy The Freedom!
Gene


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## jejj21

Thanks @Freddy G and @Gene Ballzz

I do have six small clear feet on the bottom to give it a little separation, but the gap is not huge. My amps are all 18W or less, so I haven't come close to warming this unit up yet. I will have to wait until this chaos is over to see how it does over a 1hr long jam session, but in my practice sessions so far, the case is not even the least bit warm using my Blues Jr. in the basement. I figure I can always drill more holes later if need be, but so far so good. I will continue to monitor and update the drill template if things start getting warm.

Joey


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## JohnH

Sounds fine! If you did need to add more cooling, then big holes in the base would do it. It lets air flow in below and out the top, without messing yr nice hole pattern.


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## jejj21

Hey @JohnH & @Gene Ballzz

After thinking more about this last night, I thought I would run some scenarios and see where in my design I could additional holes to give me a convection ventilation opportunity.

Attached is a V2 version of my drill pattern showing 9 additional 10mm holes. This pattern fits the layout and hopefully is more in line with what you are suggesting.

I think I can add the holes without removing all the components, but we shall see. Would've been a lot easier to add them when I drilled the others, so hopefully if we get this right it will help out the next guy.

Let me know what you think and I will try it out. 

Thanks,
Joey


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## JohnH

hi @jejj21 

Those holes look fine to me , so long as they don't clash with the case feet? 

I think heat and power and volume, although all related, follow their own trajectory's in terms of significance. Like, fully cranked 20W, 40W and 100W are all loud, and are just a fairly small step of 3 and 4 db of volume between each. But in terms of how hot things get and how much that might hurt, temperatures seems to take off as you get to about 30-40W continuous. Seeing your build, the case looks to be pretty much identical to the one I used. I have a pair of 40W Marshalls (a DSL401 and a VM2266c). They sound best a maybe 20W,but even setting up the nastiest continuous cranked distorted looped riff, and then leaving it to cook, I just cant get the attenuator past 'fairly warm'.

These thick cases take a lot of energy to get them warmed up, and the temperature that they get to is effectively based on an average of power in over maybe 15-20 minutes. So any pause in the blasted riffs, or short break or clean playing will reduce average power heating it all up.

I've also run dc through it and left it for an hour. 7W dc in raised the temperature by 12 C but by 15 minutes it had only raised half that. I do some thermodynamics for work, so back figuring convection, radiation, planetary alignment etc, at about 60C starting from 20C, there is about 27W coming out and going in steady-state average. 60C is a limit for touchablity for things like hot pipes etc, and nowhere near having any effect on our electrical parts. I reckon that power is enough for most practical uses of say a 40W amp, if built well ventilated using such a case. But, at 100C, which is way too hot to touch, there is about 60W steady state and so for powerful amps like 100W, if to be fully cranked, I think we start needing fans. 



I saw a difference in your switching compared to mine, with how the switches are wired. On my version, I did the stage bypass switches just with one pole, which disconnects the parallel resistor and shunts the series one, whereas yours uses the more normal two-pole bypass method. Not a big issue at all, given most switches have two poles. But I did it that way so there are less contacts being switched in the chain, and also it is possible to gang both sides of a switch into one for more reliability, halving the current through each contact.

Also, the reason I put the -3.5db stage last in the chain is for if it can be used independently such as in my M diagram on post 1. But physically, I also have them in logical 3.5, 7, 14 order. 

Last thing to point out, your R1 is 12 Ohm which matches my first diagram for M2, though now I show 15 Ohm for that. No issue at all the difference in the results is fractions of an ohm and db. I think the theoretically best value is in between, like 13.5 ohm but that's not an available value, But for a 16 ohm version, x2 at 27 Ohm is a good standard option.


----------



## jejj21

Hey @JohnH

The circle grid pattern I made is on the base plate of the enclosure, along with the clear feet. The new holes would go on the top of the box, where I have all the parts suspended.

All of the wiring I've done in the past used DPDT switches, so I just kept that same logic as it makes sense in my head.

I did pull the 3.5 only option out because I figured if I'm going to use the M2, I'm going to want at least a 7db cut and I can go from there. It also simplified the wiring and resistor sizing, so I went with it.

When I started the process of laying it out and figuring out parts weeks ago, 12ohm was the value I saw so I ordered it. I'll leave it for now and if there is compelling evidence to change it, the swap would be simple enough. I'm certainly going to build another one, so I will make that size change and maybe the switch change and A/B the two to see if I can tell a difference.

Thanks,
Joey


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## jejj21

What I get for trying to do this on my phone, lol.


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## JohnH

I see, that all makes sense. I also never use the -3.5db-only option, but I built it just to prove it up in case it was a key feature for someone else.


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## jejj21

Totally understand, being able to add the option into the mix is cool for sure. 

I will get out into the garage tomorrow and add some more holes and post a picture. I may try to do something cool around the inductor outline, just to give it some flare and extra venting near the 100W'r.


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## jejj21

Hey @JohnH

Thanks to a rainy Saturday morning, I got some time to piddle around on designing and drilling the extra vent holes. Decided to add some smaller holes vs. just the 9 bigger ones. I figured around the 100W/50W/Inductor side of the box would be my biggest heat concerns, so that's where I focused most of the holes. 

Check it out and let me know if you think that is sufficient. 

Thanks,
Joey


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## JohnH

I reckon that will run cooler if cranked now!


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## What?

JohnH, taking into consideration what you said previously about the Weber attenuators being 16 ohm with added resistive attenuation for lower impedances, I have been jamming my DSL 50 yesterday and today with a 16 ohm 412 cab, and it's rocking better than ever. I think I said on the forum somewhere yesterday that I wasn't digging my DSL, but after being able to crank the amp up to 7 again (without making my neighbors lose their minds) it makes such a big difference in the sound of this amp. I'm able to use the attenuator at down to around 2 out 10, and that gives me a manageable volume without any significant tone suck. Going lower than that starts getting flat and ugly. It's still on the loud side in a bedroom sized space with the attenuator set at 2 (more than loud enough to irritate people in the next room), but it is way below cop calling loud.

And I think I definitely will be building that last attenuator design for my Super Reverb. But on a related note to that point, with the DSL cranked up and boosted and shaped a bit with a graphic eq pedal, I'm able to get the same sort of cranked up tone (very much SRV tone ) that I get out of my Super Reverb out of my DSL on the green channel, which is unbelievably cool. I could never get that sort of sound with the amp down low. I always had the feeling that it was lurking in there, but not having a manageable volume level to be able to spend alot of time tweaking didn't allow me to find it. And with just a small amount of tweaking and switching to a humbucker loaded guitar, I'm able to get a nice fat and rich JCM 800 sort of tone from this same amp. Even the clean sounds so much better with the DSL cranked up, and being attenuated allows for a volume level for practically using it at home.

Any way, just saying thanks for dropping the knowledge here that allowed me to get this amp cranked up again and find some nice sounds. I had been jonesing for another Marshall variant lately, but I think the one I have had all along is doing what I want and then some with the attenuator. If I could only get some lows back into the red channel (lead channel design issue on these JCM 2000 amps) this would be the most versatile all around amp that does everything well for me.


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## Gene Ballzz

@What? 
Are you experienced enough to safely perform amplifier mods? If so, I'll share my approach to correcting the tone (especially the bottom end) disparity between the two channels of many/most DSL models.
Let Me Know?
Gene


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## jejj21

Here is the latest templates for my build in case someone wants to try it.


----------



## What?

Gene Ballzz said:


> @What?
> Are you experienced enough to safely perform amplifier mods? If so, I'll share my approach to correcting the tone (especially the bottom end) disparity between the two channels of many/most DSL models.
> Let Me Know?
> Gene



Yes, and it would be nice just to see if the red channel can get some oomph behind it. Looking at the signal flow diagram at http://www.lydian.ca/Marshall_DSL.html it looks like to me the problem is just before V1A. Maybe over here so as not to add noise to JohnH's thread about attenuators? http://www.marshallforum.com/thread...-green-channel-most-like.113262/#post-1966417


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## jchjch

@JohnH
Thanks for the great ideas. I haven't built it yet, but the idea occurred to me that two L-Pads connected in series, with the speaker side connected to each other, approximates a T-Pad. The impedance will be slightly below the L-Pad impedance at some point on the attenuation curve, but it will be minimal.

With that, I came up with the modified M2 design, using a stereo connected L-Pad for the 2nd stage of variable attenuation. Spice simulation seems OK, at both the Amp end and the Speaker end. The speaker never sees a load that is less than 8 ohms (going back toward the amp).

I'm attaching a drawing of the circuit, as well as a table showing how the L-Pad works. I'd appreciate some feedback.



In the table, I highlighted the lines with minimum impedance.

I also looked at what happens when the speaker is both open and shorted. At open, the amp sees 12 Ohms from R1, and it goes up with frequency. At short, the speaker sees 5.4 Ohms (Parallel combination of R1, R2a, R2b), and then it goes up with frequency.

Cheers!
Jeff


NOTES:
Rser and Rpar are calculated for an 8 Ohm L-Pad at the stated attenuation. 
Zattn is the DC impedance of the L-Pad network and speaker, where it connects to R2a and R2b. 
Zamp is the DC impedance of the entire attenuator network.


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## JohnH

hi @jchjch Thanks for your ideas. But to really know if it works and preserves tone you have to analyse it with all reactive components including an analysis model of the speaker, and take account and keep reasonably constant the impedance seen by the speaker. Have you checked it that way?


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## jchjch

JohnH said:


> hi @jchjch Thanks for your ideas. But to really know if it works and preserves tone you have to analyse it with all reactive components including an analysis model of the speaker, and take account and keep reasonably constant the impedance seen by the speaker. Have you checked it that way?


Absolutely - I use an electrical model with a resistor in series with an inductor which are in series with a parallel L-C. Actually tweaked the resonance frequency, inductance, and resistance.


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## _Steve

Hi - firstly thanks for all the effort that has gone into this. I've found it incredible reading through the thread.

Im going to try to build the M2v from the previous page. Just wondering with the resistors if there is a specific type I need to get? Although ive never bought power resistors before my understanding is that the big ones are usually wire-wound and I assume that will mean they also have more inductance and im wondering if that is going to be a problem? Come to think of it, different inductors will also have varying amounts of parasitic resistance too right!?

Which components are others buying and where from?


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## JohnH

Hi @_Steve welcome to our thread

Resistors need to be wire wound and aluminium clad, bolted with thermal grease to a substantial aluminium case or heatsink. You can see them in the various build pics. I work out ratings based on a 50W amp and the M2v post has the ratings on it for that, which include a safety margin of at least 3. I get mine from a Chinese eBay seller, very inexpensive and they seem fine. Or from a local supplier if you want them quicker but more $. 

Aircored inductors are trickier, they tend to come from suppliers who sell speaker components.. In the US Madisound has them, or here in Aus, theres Wagneronline. Use 18 Gage or 19 Gage minimum. They do have a bit of resistance but that is figured into the design.


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## _Steve

Great thanks so much! I've ordered everything, including case for a grand total of $71 USD.
Shipping is going to take a couple of weeks but I'll post an update and some pics when it's built.

Quick question: the blue wires on your schematic (M2v) - are they are connected to the terminal on the jacks that is closed when there is nothing plugged into it and open when something is? (I think its called a break switch or something?. Sorry for my lack of terminology im fairly new to this)


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## JohnH

TThe jacks shown in the digaram for M2v are intended to be stereo types like this:




They are plastic bodied so they don't connect the case, with 6 lugs. Usually these would be used for jobs such as a stereo headphone jack, which cuts out a speaker when you plug in. I use them for all jacks since I find that the ones I get grip the plug very well due to the extra contact.

But in this design and also the 'M', they are also used for a couple of extra things. When you plug a standard mono jack plug into one of these, the barrel of the plug connects the ground terminal to the ring terminal, and this is used to engage an extra connection in the 16 Ohm output and also configuring the input circuitry. The connections used would be on the far side of that picture. These jobs could alternatively be done using extra switches instead.


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## _Steve

Thanks for the detailed explanation!


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## telesto

Hi John,

I'm planning to build one of your attenuators (not sure which, M or M2). First let me say thanks for all the great research and info! I've been miserably using my home-made L-pad attenuator, and look forward to something that works better  And will probably eventually integrate it into an amp I'm designing and planning to build.

I have one question: I see you mentioned that the first -7dB stage with inductor is where the "magic" happens. I also see all the other resistive stages are based on a variant of -7dB (3.5, 7, 14)...you should change the name to the "Lucky 7" attenuator!  But my question is, it possible to swap the attenuation in Stage 1 for another value? eg. is it possible to put -3.5dB or -14dB instead of the -7dB? Or will the magic get lost?

For Stage 1 (in the original M attenuator, 8 ohm design), the 15/10 ohm resistors give 6 ohm resistance towards the OT, and if it was replaced by the -3.5dB stage using the 33/5.6 ohm pair from Stage 4, it would give 4.78 ohms towards the OT (and -14dB stage with 4.7/15 gives 3.4 ohms). So I guess these other variants would "ruin" the 6 ohms that the -7dB stage gives? (I see in M2 design the Stage 1 resistance also adds up to 6 ohms)

Since I have mostly smaller amps (5-25W range) would really like to shift the -3.5dB stage into the first stage, since I think -7dB is a bit too much initial attenuation for a small amp. I calculated that if I changed R1/R2 from 15/10 to 35/7 I would get -3.5dB drop and still show the OT 5.83 ohms, which I guess is close enough to the 6 ohms given by the 15/10 resistors in the -7dB stage.

ARCOL has 33/7.5 ohms for 6.11 ohms to the OT ...altho that would be a little more than 3.5dB (maybe 4dB or so), but I guess that doesn't matter? I guess the important part of Stage 1 is DC resistance value presented to the OT? (Together with the inductor, of course!)

I'd appreciate if you could give some feedback on that idea. Thanks! 



PS- One more question, if I go with the M2 design, I don't have a .9mH coil available, just 1mH or .82mH, does it matter which I take?

PPS- For the M2 design, to have about 3-4dB attenuation in Stage 1, I calculate R2A and R1A could be 15 ohms each, and R1 at 33 ohms. That would give the same 6.11 ohms to the OT, as the calculation for the original M-attenuator above.


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## JohnH

Hi @telesto , Thanks for your post

-7db is a sweet spot in the mathematics, based on having the right balance of input impedance and output impedance. Its possible to have more attenuation in Stage 1 (which would include another resistor) but not less unless values are allowed to drift away from target, which spoils the tonal consistency that this system has. It is this fairly rigorous approach to tone based on the maths that follows through to the end result.

Ive not found a front end for a multistage design with less attenuation than -7db that I wanted to put up. Could maybe stretch this to -6db.

Stage 2 is a resistive copy of stage 1, so it is also at this balance. Stages 3 and 4 have consistent output impedance seen by the speaker, but I allow their input impedances to vary since they are always used after stage 1 at least, and so little of this variation is seen by the amp.

That being said, Stage 4 which is the -3.5db stage can be used on its own as a resistive stage and the full M design on page 1 has this. A similar wiring could work in a version of M2 as well The amp sees about 10 Ohms when used this way which is close enough to 8. For this small initial step, the resistive only -3.5db stage works fine and in fact a good part of the real speaker reactance still reaches the amp. See the charts and sound samples on page 1. But the -3.5db stage is not a good enough basis for the front end for the whole system IMO. 

Actually, when you try it, -3.5db is a really small step, more like a fine tuning, and if you need attenuation, you probably want at least -7db. So for most users, that -3.5db first-step option is not needed.

But, if you do need a very small reduction, another good work around is to set the whole box to max attenuation with no speaker and use it as a load box in parallel with a speaker, setting amp to a lower ohm tap. 

Finally, just on your maths. When you have stage 1 or 2 with a 15 Ohm parallel and 10 Ohm series resistor, if you want to know what the amp sees, you also need to add the impedance of the speaker or following stages. eg, with stage 1 alone into an 8 ohm speaker, the basic resistive math is 15 in parallel with 10 plus 8, ie 8.2 so close to 8, rather than 6. 

The full analysis needs to include the inductive reactances and the speaker reactances, so a SPICE sim is the best simple way to explore ideas, or in my case I put it all into a huge spreadsheet.


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## telesto

JohnH said:


> Hi @telesto , Thanks for your post
> 
> -7db is a sweet spot in the mathematics, based on having the right balance of input impedance and output impedance. Its possible to have more attenuation in Stage 1 (which would include another resistor) but not less unless values are allowed to drift away from target, which spoils the tonal consistency that this system has. It is this fairly rigorous approach to tone based on the maths that follows through to the end result.
> 
> Ive not found a front end for a multistage design with less attenuation than -7db that I wanted to put up. Could maybe stretch this to -6db.
> 
> Stage 2 is a resistive copy of stage 1, so it is also at this balance. Stages 3 and 4 have consistent output impedance seen by the speaker, but I allow their input impedances to vary since they are always used after stage 1 at least, and so little of this variation is seen by the amp.
> .....
> Finally, just on your maths. When you have stage 1 or 2 with a 15 Ohm parallel and 10 Ohm series resistor, if you want to know what the amp sees, you also need to add the impedance of the speaker or following stages. eg, with stage 1 alone into an 8 ohm speaker, the basic resistive math is 15 in parallel with 10 plus 8, ie 8.2 so close to 8, rather than 6.
> 
> The full analysis needs to include the inductive reactances and the speaker reactances, so a SPICE sim is the best simple way to explore ideas, or in my case I put it all into a huge spreadsheet.


Hi John,
Thanks for the reply. I had such high hopes for my back-of-the-napkin idea, but then you had to throw cold water on it with your research and spreadsheets, LOL  That's OK, with your explanation, I think I understand now, that the -7dB is needed to "stabilize" things so that following stages don't throw things off too much. So your attenuator is indeed the "Lucky 7"  

Ok, I'll have a look at SPICE and play around with that maybe and see what other ideas come up.

Oh, and for the M2 design, I can't get a .9mH inductor, only .82 or 1mH, does it make a difference?
Thanks!


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## JohnH

For the inductor in M2, around the range in question it really makes very little difference to a point where you'd never hear it and maybe couldn't measure it either. But just based on my OCD analysis, 0.82mH would be my choice, the optimum being somewhere between that and 0.9mH. This is to target the closest performance to match typical Celestion speakers in terms of what the amp sees. I think some US speakers may be a bit more inductive. It doesn't have much influence on the basic clean tone, but begins to have an effect as the power amp starts to overdrive.


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## telesto

JohnH said:


> That being said, Stage 4 which is the -3.5db stage can be used on its own as a resistive stage and the full M design on page 1 has this. A similar wiring could work in a version of M2 as well The amp sees about 10 Ohms when used this way which is close enough to 8. For this small initial step, the resistive only -3.5db stage works fine and in fact a good part of the real speaker reactance still reaches the amp. See the charts and sound samples on page 1. But the -3.5db stage is not a good enough basis for the front end for the whole system IMO.


I was thinking about this...so theoretically, I could bypass the first 3 stages and go straight to Stage 4 to have only the -3.5dB resistive attenuation. Then to go lower, I could switch in Stage 1 (with inductor) and then have -10.5dB (and then Stage 2 and 3 in succession). 
...oh wait, this is what the dashed line and switch is doing in the original diagram...ah, ok, sorry, I have this habit of doing things first and then reading directions later 



JohnH said:


> For the inductor in M2, around the range in question it really makes very little difference to a point where you'd never hear it and maybe couldn't measure it either. But just based on my OCD analysis, 0.82mH would be my choice, the optimum being somewhere between that and 0.9mH.


Ok, thanks, I was leaning towards the .82mH myself


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## Gene Ballzz

@telesto
Operationally, the most logical and gradual progression for your intended/desired scenario and usage, from minimum atttenuation to maximim would be: *-3.5db resistive only > switch off the -3.5db which automatically activates the -7db reactive only (except when the whole unit is in full "bypass" mode) > then activate the -3.5db along with the -7db reactive (for -10.5db) > switch off the -3.5db and activate the second -7db (for -14db) > then switch in the -3.5db (for -17.5db) > then switch off both the second -7db & -3.5db and activate the -14db (for -21db) > then switch in the -3.5db (for -24.5db) > then switch off the -3.5db and activate the second -7db, along with the -14db (for -28db) >and then finally add in the -3.5db (for a total of -31.5db)! Remembering of course, that the -7db reactive stage is always active in all scenarios past the -3.5db only!*

Where this all gets kinda tricky is in the wiring of the switching system, in that any stage that would get used by itself *MUST* use the bigger 100 watt & 50 watt resistors, as it needs to handle the full wattage of the amp. This is why once the -3.5db is deactivated, the wiring/switching of *"Design M" *in the first post of this thread, *"forces"* the activation of that first -7db reactive stage at all times to bear the full brunt of the amp's full wattage. This is also why both that -7db reactive and -3.5db resistive only stages have the bigger/higher wattage rated resistors and why either one of theose two *MUST* be activated at all times, except when the unit is in full bypass.

Through multiple builds, testings and usage by multiple folks it seems to fairly well point to the idea that the -3.5db only option is inconsequential enough to not really warrant the extra complications and expense of it's implementation! Also, with the use of the -3.5db alone, the speaker impedance comes more impotantly into play.

And while @JohnH has redesigned in a number of upgrades, to mildly tweak the tonal responses and operational variabilities, it has become my considered opinion that the most practical, simple, convenient, economical, easiest to build and useful design is the *"Design M-Lite"* in that same first post, in either dedicated 2 ohm, 4 ohm, 8 ohm or 16 ohm amp tap configuration. One of the great benefits of this design is that when using it with it's designed for amp tap, it doesn't much care what the speaker impedance is (within intelligent reason), except that if the speaker doesn't match the amp tap, it must get the amp tap changed to match the speaker when fully bypassing the unit. Even though there has been a redesign to accommodate multiple amp tap impedances and speaker impedances and incorporating a pile of additional features, that simplest *"Design M-Lite"* is so simple, easy and inexpensive to build, putting together an additional unit or two for different impedance amp taps seems a no brainer to me!

I use a 16 ohm *"Design M-Lite"* installed in every amp I own and couldn't feel more happy, pleased and liberated by the results! And although I've not yet built an 8 ohm, or other impedance unit (as I don't have any amp that is 8 ohm only), all reports of the 8 ohm variants seem right in line with what I've experienced. FWIW, the redesigns to accommodate 2 ohm and 4 ohm amp taps are in fairly recent posts. And yes, the *"Design M-Lite"* preserves the tone, feel and response better than any of the vast multitude of passive attenuators that I've tried since the early '80s, as well as providing consistently safe operation of your 50 watt or less tube amp. The only down side is that power tubes will not likely last as long, given the ability to run your amp *"fully cranked into it's sweet spot"* at all times!

And again, multiple thanks to @JohnH for his generous donation of time and efforts towards this amazing project design, along with detailed testing and sharing of the results!

My Apologies For The Long Novel!
Gene


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## assaf110

Hi John!
I need a stripped down version of the attenuator -
50w max 
No need for switching stages - a constant 20db ish is fine.
One 8 ohm input, one 8 ohm out.
No compromise on tone.
Compact build.
How will you approach it? M2 design front end with a single 14db stage?


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## JohnH

Hi @assaf110 , yes that would work fine, to get -21db. If getting specifically -20 is needed, could make a small adjustment.

What's it for?


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## assaf110

Thanks for the Uber quick response.
I have the 16 ohm M design, use it on the Marshall SV. Works great.
I need to attenuate an incoming 35w amp MV (8 ohm 112 cab).
A grab & go small box would be great. I’ve noticed that I really don’t use the switches at all, once I settle on a certain attenuation level, I keep it more or less constant. The MV on this one should handle small changes, I will either plug the attenuator or unplug it if attenuation is not needed. -21 is great.


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## JohnH

*Attenuators M and M2 compared - Analysis
*
At this time there are quite a considerable number of *Design M* attenuators built in various forms, and now a few *M2* versions too. They are generally similar in many ways and both rely of putting inductance partly in series and partly in parallel with the input. But M uses two inductors while M2 uses only one. Although they have very similar performance they are not quite identical in how they work. So this is to compare them by analysis in a number of situations.

The benchmark is the tone of a full cab directly connected to an amp. An impedance model based on measured results of a G12M 4x12 is the basis.

In these charts, various attenuation levels are engaged but volumes are equalized to compare tone. The Red line is the signal at the fully connected real speaker, and also at the amp. Blue is the signal at a speaker after the attenuator, and green is the signal as seen by the amp. The ideal would be all three lines coincide.

The first two charts assume the effective output impedance of the amp is 20 Ohms, as measured on my VM from the 8 Ohm tap. -7db and -28db plots are shown.




Both designs show good tracking with red and blue lines very close. M2 tracks a bit better at very high frequencies, and the signal seen at the amp is a bit better too. Not much in it though.

But one of the interesting aspects of these designs is how they adapt to different amps, and to the same amps performing differently when overdriven. Amps with no NFB have very high output impedance at normal levels, while those with more NFB have less. Also, as an amp gets driven its output impedance drops dynamically. So here are three further tests, all at -14db, with the amp output impedance changing from 20 ohms, to 50, then 8 then 2 ohms:










See how in both designs, the blue output curves follow the red at frequencies above low mids, even at these very different amp values from those assumed in the design. Both are good but M2 seems to be a little better at the tracking.

Actually, there is some magic happening here. The output response curves are developed by the real speaker interacting with an output impedance, and the lower that impedance the flatter the response.. And in all these calcs, the attenuated speaker is always seeing the same impedance, and yet even through the attenuator the differences in response due to different amp impedance are feeding through fairly closely.

The Bass peak does not adapt in this way, but it does stay within about 3 db of the fjll-volume red curvs except for the extreme low output ohms.

That's it for now. M and M2 both seem fine and quite similar, M2 is my favorite at the moment (haven't built it myself yet though)


----------



## Mcentee2

JohnH said:


> *Attenuators M and M2 compared - Analysis
> *
> At this time there are quite a considerable number of *Design M* attenuators built in various forms, and now a few *M2* versions too. They are generally similar in many ways and both rely of putting inductance partly in series and partly in parallel with the input. But M uses two inductors while M2 uses only one. Although they have very similar performance they are not quite identical in how they work. So this is to compare them by analysis in a number of situations.
> 
> The benchmark is the tone of a full cab directly connected to an amp. An impedance model based on measured results of a G12M 4x12 is the basis.
> 
> In these charts, various attenuation levels are engaged but volumes are equalized to compare tone. The Red line is the signal at the fully connected real speaker, and also at the amp. Blue is the signal at a speaker after the attenuator, and green is the signal as seen by the amp. The ideal would be all three lines coincide.
> 
> The first two charts assume the effective output impedance of the amp is 20 Ohms, as measured on my VM from the 8 Ohm tap. -7db and -28db plots are shown.
> View attachment 72876
> 
> View attachment 72875
> 
> Both designs show good tracking with red and blue lines very close. M2 tracks a bit better at very high frequencies, and the signal seen at the amp is a bit better too. Not much in it though.
> 
> But one of the interesting aspects of these designs is how they adapt to different amps, and to the same amps performing differently when overdriven. Amps with no NFB have very high output impedance at normal levels, while those with more NFB have less. Also, as an amp gets driven its output impedance drops dynamically. So here are three further tests, all at -14db, with the amp output impedance changing from 20 ohms, to 50, then 8 then 2 ohms:
> 
> View attachment 72872
> 
> 
> View attachment 72874
> 
> 
> View attachment 72873
> 
> 
> See how in both designs, the blue output curves follow the red at frequencies above low mids, even at these very different amp values from those assumed in the design. Both are good but M2 seems to be a little better at the tracking.
> 
> Actually, there is some magic happening here. The output response curves are developed by the real speaker interacting with an output impedance, and the lower that impedance the flatter the response.. And in all these calcs, the attenuated speaker is always seeing the same impedance, and yet even through the attenuator the differences in response due to different amp impedance are feeding through fairly closely.
> 
> The Bass peak does not adapt in this way, but it does stay within about 3 db of the fjll-volume red curvs except for the extreme low output ohms.
> 
> That's it for now. M and M2 both seem fine and quite similar, M2 is my favorite at the moment (haven't built it myself yet though)



Well, my M2 works so well I haven't felt the need to contribute more to the thread  Lol!!!

Mind you, I am constantly looking at tweaks to make it more useful for me outside of the attenuation.

I added a line out, and am also thinking of adding a head-swap function, ie two inputs, one speaker out, with the attenuation (or none) on one side, and a simple 8r 100w load on the other.

I need to look at safe ways of switching before I attempt that, I think


----------



## telesto

JohnH said:


> Hi @telesto , Thanks for your post
> 
> -7db is a sweet spot in the mathematics, based on having the right balance of input impedance and output impedance. Its possible to have more attenuation in Stage 1 (which would include another resistor) but not less unless values are allowed to drift away from target


Hi John, coming back to this, I'm curious why would another resistor be needed for more attenuation in Stage 1? It's not possible to just adjust the values of the resistors without adding an additional one? For example, just replace the -7dB stage with the -14dB?


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## JohnH

hi @telesto 
Stage 1 needs to have particular input and output impedances. As a resistive (ie no inductor) network, this generally requires three resistors of defined values to achieve a given attenuation level. But at around -7db, with the resistance values I was targeting, one of those resistors gets to zero, leaving two. This is how Stage 1 works at low frequency.

If you consider the M2 Stage 1 at low frequencies, the inductor has no effect and R2A and R2B can be considered combined to one value of about 10 Ohms. That, and R1 at 15 Ohms do the-7db attenuation.


----------



## telesto

JohnH said:


> hi @telesto
> Stage 1 needs to have particular input and output impedances. As a resistive (ie no inductor) network, this generally requires three resistors of defined values to achieve a given attenuation level. But at around -7db, with the resistance values I was targeting, one of those resistors gets to zero, leaving two. This is how Stage 1 works at low frequency.
> 
> If you consider the M2 Stage 1 at low frequencies, the inductor has no effect and R2A and R2B can be considered combined to one value of about 10 Ohms. That, and R1 at 15 Ohms do the-7db attenuation.


Hi John, Thanks for the explanation, but I'm afraid I don't quite get it. In the original design R9 is shorted out by the inductor, is that what you mean by "going to zero"?
But then looking at the M2, do the resistor values need to be in any special formula, ie: -7dB and adding up the ohms to give the OT 8 ohms to see?
For example, in M2, if R2A and R2B are 60 and 68 ohms, and R1 is 10 ohms, the OT will see 8 ohms, and attenuation will be above -7dB (somewhere between -7dB and -14dB). Will that work?


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## JohnH

R9 was for a different job, at high frequencies,

In your example, the net result may be safe enough but will not sound right with many amps because it doesn't have the right output impedance. Gotta look at the whole system all at once, amp, resistors, inductors and speaker, which all interact. Changing a value or two in isolation can give a random tonal result. Personally that would be a something I wouldn't want to do, having done a lot of work and testing to keep tone consistent.


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## telesto

Is there a simple way to change R values in Stage 1 to make attenuation -14dB? Or that would require time consuming calculations, trial/error?
You are using AIM-Spice? I've now got it installed and learning basic things on it. There is no speaker component (that I can find), so I guess you use a resistor/inductor with fixed values in lieu of a speaker? And what input signal do you use for a 50W amp, 20VAC? Is your .cir file publicly available? If yes, I can play around with it and modify to see what happens.


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## Gene Ballzz

@JohnH has designed and rigorously tested a very nice wheel that rolls quite well, with some very useful options, bells & whistles. With the amount of effort and time that has been spent on it, in addition to John's gracious and amazing generosity and tolerance, I think that trying to re-invent it is an exercise of diminishing returns. It seems that the most useful effort might be well spent on standardizing the packaging and possibly a good source for a reasonably priced rotary switch that could accommodate the voltage and current being dealt with. John has already done a great job of developing the electonic end of this, any other innovations that may be desired really involve tweaking the GUI to make it as "idiot proof" as possible!

And @telesto , to achieve what you describe is really as simple as building it with two -7db stages or a first -7db stage and a -3.5db stage and just omitting the switching of the second one! that would make the first option a non-switchable -14db unit and the second one a non-switchable -10.5db unit. Maybe I'm not understanding you correctly? It seems that you are trying to save a few pennies by cutting down the number of very inexpensive resistors used?

Just My $.02 & Probably Worth Much Less!
Gene


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## telesto

Gene Ballzz said:


> @JohnH has designed and rigorously tested a very nice wheel that rolls quite well, with some very useful options, bells & whistles. With the amount of effort and time that has been spent on it, in addition to John's gracious and amazing generosity and tolerance, I think that trying to re-invent it is an exercise of diminishing returns.


Absolutely a hat-tip to JohnH for staring this interesting thread! I learned quite a bit, and lit my fuse to learn more and go farther with it. I've already found several other interesting RLC speaker modelling circuits (ok, here we lost the "C"  ). I'm not sure of John's intentions, if this is a kind of an open-source community project? Or is he just looking for Beta testers? He's put some interesting data out there, and I figured it's worth discussing the details, looking under the hood and kicking the tires. Of course I can just build a stock one, say "my amp rocks!" and call it a day  But as the title says "design and testing", well, he's put his circuit out there and I though some discussion about the design could be interesting  



Gene Ballzz said:


> And @telesto , to achieve what you describe is really as simple as building it with two -7db stages or a first -7db stage and a -3.5db stage and just omitting the switching of the second one! that would make the first option a non-switchable -14db unit and the second one a non-switchable -10.5db unit. Maybe I'm not understanding you correctly? It seems that you are trying to save a few pennies by cutting down the number of very inexpensive resistors used?


...not just pennies, but space too!  I'm cheap AND efficient


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## Gene Ballzz

Please understand that I was not chastising and please accept my sincerest apologies if it came off as such. Your intentions were not clear to me and I was simply suggesting that we don't need to go back to the original drawing board, to rework this well proven concept from the ground up.
Ideas & Frugality Are Always Great Things,
Gene


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## telesto

Gene Ballzz said:


> Please understand that I was not chastising and please accept my sincerest apologies if it came off as such. Your intentions were not clear to me and I was simply suggesting that we don't need to go back to the original drawing board, to rework this well proven concept from the ground up.
> Ideas & Frugality Are Always Great Things,
> Gene


Hey, don't sweat it, no need to apologize, it's all good


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## JohnH

hi @telesto , thanks for the link on speaker modelling. Heres how I'm doing it,based on a model by Randall Aiken:




His diagram is in black, but I use the values in the yellow boxes.

The blue curve are real physical impedance measurements (in Ohms) by Mike Lind on TGP, and red is the calculated trace from my spreadsheet to try to show a match, which is pretty good I think, This circuit is used in the modelling for the curves on the previous page. The values represent a 16 Ohm speaker, and we halve R and I and double C to make 8 ohm versions.

If you want to go to the next stage, you could build this circuit in your Spice program, feed it with an A/C source via a 20 Ohm resistor and see if you can get a db plot at the speaker the same shape as the red full volume curves in the attenuator tests.

BTW, here's a policy I have, after 35 years of engineering and teaching. Im happy to share a lot but there is an entry price! For example, in the case of the designs, the price is understanding the schematic enough to build it, so I don't put up paint-by-numbers build diagrams. As a result, everyone with the skills to read the diagram and tackle the build has been able to make a success by figuring out layout and wiring for themselves. Similarly with the analysis, I don't plan to put up analysis files but Im happy to discuss what is in them for those who wish to and can get their head around it.


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## telesto

JohnH said:


> hi @telesto , thanks for the link on speaker modelling. Heres how I'm doing it,based on a model by Randall Aiken:
> 
> View attachment 72949
> 
> 
> His diagram is in black, but I use the values in the yellow boxes.
> 
> The blue curve are real physical impedance measurements (in Ohms) by Mike Lind on TGP, and red is the calculated trace from my spreadsheet to try to show a match, which is pretty good I think, This circuit is used in the modelling for the curves on the previous page. The values represent a 16 Ohm speaker, and we halve R and I and double C to make 8 ohm versions.
> 
> If you want to go to the next stage, you could build this circuit in your Spice program, feed it with an A/C source via a 20 Ohm resistor and see if you can get a db plot at the speaker the same shape as the red full volume curves in the attenuator tests.
> 
> BTW, here's a policy I have, after 35 years of engineering and teaching. Im happy to share a lot but there is an entry price! For example, in the case of the designs, the price is understanding the schematic enough to build it, so I don't put up paint-by-numbers build diagrams. As a result, everyone with the skills to read the diagram and tackle the build has been able to make a success by figuring out layout and wiring for themselves. Similarly with the analysis, I don't plan to put up analysis files but Im happy to discuss what is in them for those who wish to and can get their head around it.


Hi John, sure, I understand what you're saying. I've worked in telecom for 25+ years, so I've got some technical background, but not exactly in this component-level stuff. But I learn fast...sometimes  And one thing I like about this tube-amp stuff, is that the technology doesn't change. Same speakers and amps from 50 years ago . I don't have to re-learn anything. With telecom, once I learned 2G, then 3G came out and I had to learn a whole new system again, once I learned that, then 4G came out, etc...  One bright side is I get to travel. Got to visit Oz in 2006 or 07 (?) when Telstra swapped CDMA to "NEXT G" (UMTS) all within 1 year. That was a clusterfk  

I saw the file Mike Lind posted of his Marshall cab many pages back. Yes, you have managed to match the lines pretty well. BTW have you seen LiveSpice? Looks interesting. I guess you can record whatever speaker you want there? Also have you seen the Spice plugins for Celestions on Duncan's page? They are 20+ years old (!!), Spice has changed since then, but the speakers probably haven't  I haven't figured out how to use the plugin exactly, but it looks like you can extract the info from the text file and put it in manually to a circuit. Just follow the connections points there ($N_)


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## telesto

Hi John, I've put the values into Spice and plotted the impedance. If I understand correctly, the 20ohm (R5) is the amp output 8ohm tap, R1 would be the DCR of the speaker (should 13.5 be for the 16ohm speaker tho?), and the RLC stuff is the unique impedance for the specific speaker. From what I saw Mike Lind gave you just an impedance plot, did you adjust the RLC values yourself to match Mike's plot? Am I on the right track so far?


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## JohnH

All looks good. Yes the values I used were adjusted by me to match Mikes measurements. Randall Aikens version have quite a different peak.

If you were doing this with an open-backed cab, the peak would be more like at 80 hz. One advantage of not trying to replicate the bass peak within the attenuator is that it is then left to the real speaker to make the peak, so it naturally comes out at the right frequency.

The Spice plot looks to be db output. The measurements were in Ohms impedance so in matching the model, I worked out the complex impedance by a spreadsheet.

Your spice plot seems to have the right rises and falls. 

What I then do is build all of that twice in a model and put the attenuator components on one of them between the R1 and R5. Then you can have two traces and see that one is at lower volume, but hopefully the same shape. Can also add a 3rd trace at R5 on the attenuated circuit, to represent amp output.

I find such Spice models to be great for exploring a new version. But once the basic idea is done and I'm ready to fine-tune components, I get more control by coding it all into a spreadsheet, set up to step through all settings.


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## telesto

Allright, thanks  Just to comapre notes, what flavor of Spice are you using? I'm trying out a few right now (kind of a headache). LT is what I used above, and seems to be the most common. Micro-Cap, which used to cost alot of money is now free (company folded), but I had trouble running the AC circuit in there, maybe a little too complex for my needs. I'm trying TINA now, looks pretty cool, but have to spend more time with it. I think you mentioned AIM-Spice? There was no GUI there, a little hard-core to put in all the data manually, but for a simple circuit, it could be OK. It may also be best if you want to import spread-sheet data, which is what you are doing. I don't see a way to do that in LT.

BTW, in LTSpice, you can toggle the Y-axis from dB to Linear:


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## JohnH

I use a simple one called 5Spice, its interface is more modern, like about 10 year ago instead of 20 years. Its not as adjustable as LT Spice though, also it seems to have been discontinued. 

Here's an example of a run to test M2 or M3:


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## telesto

Alright, nice set-up you got there!  I like the look of the 5Spice interface. I'll have to try it out. It seems it was discontinued in 2018 but is still available. Since ohm's law doesn't change, you can still keep using with no problem for quite a while  I also found Qucs, which looks interesting, but haven't tried it yet, I'm liking TINA so far. LT is easy and flexible, but has some quirks I don't like, plus the GUI is kind of ugly. I think LT is good for some quick-and-dirty work, but I don't think I'd want to do a long-term project in it (altho of course possible).

I was thinking about the general premise of your experiment. You modeled your speaker parameters in Spice to match the impedance curve of Mike's Greenback, which he gave you the actual measured plot, and you then also have the simulated plot, which both match very nicely. Then you stick the attenuator in, and compare the attenuated simulation with the real and simulated un-attenuated signal. And everything looks to match up very nicely. And that really is the main goal of an attenuator, to have the same plot as the un-attenuated signal. So you have done a great job in that sense, pop the champagine and pat yourself on the back  

However, in reality, the resonant frequency and impedance characteristics will vary depending on the actual speaker attached to the attenuator. Even Greenbacks will vary depending on the version, cabinet, etc. Have you tried changing the RLC parameters of the speaker in Spice and tested to see how your attenuator values work on different speakers? For example, plug in the original RLC values for the "speaker" from Aiken's dummy load and compare the original and attenuated plots? Do they match up perfectly as well?


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## JohnH

yes indeed I did. Actually, just the resistor values are enough to let the speaker develop its bass peak and also rise with frequency, and its the speaker that does this, so a different speaker will also work fine. What the reactive part does is set response at the amp and the way it responds dynamically to different signal levels. I did try it with the original Aiken values and also some significant tweaked versions, and it still tracks well. What is really surprising though is how if you change the 20 ohm representing the amp, to a very different value like 1/3 or 3x, the tracking of tone full to attenuated, still works - see the recent previous set of graphs a page or so back.


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## telesto

Alright, you did your due diligence  Sounds like a good, stable design. Ok, now I'll figure out which Spice to use, and re-create the full circuit in there and then wade deeper into the water. I'd like to play around a little with it and see what happens as some things change. If I find anything new, interesting or strange, I'll come back and share it with you.

You mentioned the inductor, and it's effect on different signal levels, this is something I'm also interested in. I suppose it helps more at weaker signals (like under 1W)? Maybe when the amps OT is too big to react to the weak response, the smaller coil gives it something to react to?

What's also interesting, is alot of amps I've seen that have power reduction switches are only resistive, which makes me wonder. For example, the VOX AC4TV is a 4W amp with resistive attenuation switch for 1W and 1/4W output. They do use a filter capacitor, but no inductor. Did they test with an inductor and find no big difference? Are they just being cheap? Or is the attenuator just an after-thought? But it's not just VOX, many other amp manufacturers also just have resistive attenuation.


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## JohnH

Those amp makers don't know what we know! But that Vox circuit is not too bad since it puts series resistance to the speaker, which was one of the first important things I understood early in this thread. I don't know about an AC4 but the classic Vox output stage has no NFB and a very high effective output impedance, so the attenuator needs to keep this high.

In our design, the reactive coil does two key things:

1. It shows the amp an impedance that rises with frequency. When an amp has itself a very high output Z, a load of higher impedance will suck more power out of the amp. If the amp is being driven hard, this pushes it to further toneful distortion at these higher frequencies, which dissipate as the notes decay. So this effect happens at high power, which is a place we can go now that we are attenuating. At low power, you can actually get a very close tone match just with the resistors provided they match the low-power amp output Z.

2. The way the inductor is balanced to be partly series and partly parallel at the input, while showing the amp the right value, allows the circuit to compensate for different amps of different output Z, or the same amp changing its response as it drives to the peaks. I can only show this on graphs (see the series I posted a few days ago) and hear it too, but I don't have a concise explanation, ie its magic. This happens from low-mids up, its not happening at the bass peak because we don't have a resonant bass circuit. But if we did (see design M3 on p27), then it happens over the full range. But my current believe is that the very expensive parts needed for the bass resonant circuit are not good bang/buck since this only happens at very low notes and only the fundamental tone of those notes. All the character of the tone is at higher frequencies and without the bass resonant circuit, we just end up with a more consistent solid bass tone, not a bad thing anyway.


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## _Steve

I dont understand 90% of what you two are saying but im loving very word of it!


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## telesto

JohnH said:


> I find such Spice models to be great for exploring a new version. But once the basic idea is done and I'm ready to fine-tune components, I get more control by coding it all into a spreadsheet, set up to step through all settings.


I just re-read this. So you export the impedance plots from Spice into Excel, and then re-run the calculations in there with some kind of formula/macro? (and I guess then re-import the final version into Spice to make visual plots?) Yikes, pretty hard-core  But on the other hand, yea kind of smart, probably faster/easier to experiment with different values like that. You put alot of time into this  I think most people, including myself, would have taken the horseshoes and hand-grenades approach of "close enough" and been happy with just similar looking Spice plots 



_Steve said:


> I dont understand 90% of what you two are saying but im loving very word of it!


#MeToo


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## JohnH

Actually, once I go into spreadsheet mode, I don't use the Spice models. It's all hand-coded as a big series/parallel circuit using complex number theory to represent reactive components and how they change with frequency. Then I make a series of rows each stepping through the frequency range, finally graphing the outputs in excel 

It's a good learning exercise, using maths from high school and year 1 engineering that I never properly grasped 40 years ago. But now I reckon I have it nailed!


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## telesto

Your teacher must be (finally) proud  I can see the advantages of using Excel regarding control and maybe speed/versatility of the impedance calculations, but isn't Spice just doing the same thing automatically? Or you are doing something different in Excel that is not happening in the Spice simulation?


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## JohnH

It's the same maths underneath it, but with the excel file I have a lot more control of plots and comparing things and it's a lot quicker to try values too. Also, a few macros can step through all possible switch settings and plot the whole lot. I'm sure the better Spice codes can do much of this too, but this works for me


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## telesto

Ok, yes, I can see how Excel has more advantages in speed/control/versatility, as you said. I don't know Spice well enough yet, and different versions vary, but yea, might be possible to do something close or similar there as well.


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## Dwayne Eash

SG~GUY said:


> -also some random things on attenuation-
> -i have a EH 12AX7 that one day went south, amp went almost silent, i turned it up and it sounded really good but still quiet, ended up turning every nob on the amp up to 10.... Completely BAD ASS!!! but its a time bomb and i took it out and put it in a marked box, i pull it out every now and then for 15 minutes of the sound in my head
> 
> *-always use a variac @ 80-90v-*


What are the sonic results of this, in terms of volume reduction and quality of tone? Since I am an inventive type, and have been wanting a variac for a long time, so that you say it's a good tool for the jamming guitarist, really interests me.

I have an '82 JCM 800 all tube, vertical inputs. Im trying not to fry my two 25 Watt Scumbacks, but at home jamming, I never get above 4 MV once in a blue moon if the neighbors are gone, so I only rarely go above 3 1/3 on the MV.

I have a newer pair of inexpensive 15" PA speakers for my living room music studio. They are ok sounding, and so my PA can get as loud as band practice, no problem. I can't wait to get my amp up to speed tho, so I don't have to play 4+ on the MV to sound good enough. That's just way too loud at home, so I need some attenuation.

So I am very interested how much quieter my JCM 800 might get, using a variac. I have no clear idea, what happens to the volume, other than going "somewhat" lower.


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## Dwayne Eash

After doing some calculations, going from 120V to 80V, would be roughly 40V of reduction. And that amounts to a 1/3 reduction of volts (which "effects" power or watts, but is not the same as watts) or 33.3333% voltage reduction. My amp is the 2204 circuit, 50 watt, '82 JCM 800 4010 but with two speakers. So there's two cabs, for my one amp.

I lucked out making the contact for the purchase, here on this forum about 10 years ago or so.

Hard to believe I've had it so long. I paid like $225-250, just to have it returned to stock, and to take some of the high end out. It was a two visit process, for that to happen.

He asked me which power tube to return it to, I opted for the EL34's, as that's the most notable tube of classic rock fame, at least by the mid 70's. So even if I would end up preferring some other power tube, I first wanted to hear the iconic glory of the original EL34 beast, itself. hehe

I realize a proper JMP Master (Super Lead) is what I should have, if I want the best in vintage and classic rock tone and feel. I played one in Chicago once, and OMG, it nearly melted me on the spot, hehe, I fell in love. It even had something golden with it, like the cloth was golden or the edge corner piping was gold, IDK.

And so, I have the strongest "gold" concept, in my head, because when I plugged into this thing with an SG Standard, it just sounded like gold, so much better and so vintage. The best amp I ever played. I'm for some dirt, so I'd live in the high input channel and just clean it up with the guitar vol.

But costing 2 and 3+ times more than a JCM 800, I decided on the purest old school JCM 800 was the best I could afford. And then put in the best NOS tubes for the first spot, and the best speakers. And I do not cloud up the signal path, keep things simple and pure for the best in organic guitar sonic bliss.

I am a wanna be tone hound, a poor man's version. Gotta love a proper tube amp guitar rig. 
Somehow, I figure you can relate.


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## telesto

Generally speaking, when you lower the plate voltages to the tubes, either by Variac or VVR, it causes the tubes to break up at lower volumes. EVH was famous for using a Variac on his Marshall on early VH albumns to get his "brown sound". You can also have your amp modded to get a VVR (Variable Voltage Reducer), which has a MOSFET connected to the tubes plate voltage so you can regulate it directly in the amp (the MOSFET soaks up the high voltage and gives a lower one to the tubes). So you can get cranked amp tone at low volumes. I haven't tried a VVR, but reviews are generally good. However, one thing to consider is that it's not pushing the output transformer as a cranked amp would, so the tone may be a little off. If you use an attenuator between the amp and speaker (eg. the one discussed in this thread), then you will be driving the OT hard as well, which would probably give a closer tone to that of the actual cranked amp. The other negatives of VVR is it's a little more complicated/costly, and I also hear the MOSFETs can become unreliable down the road. So in short, I would say skip the Variac and VVR, and just go with a good external attenuator if you want to crank your amp at lower volumes.


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## telesto

Hi John, This is my setup in TINA so far. I imported Mike's Greenback impedance plot (upper graph) and compared it to the measured impedance at the amp out (bottom plot). Two questions: why is the range bigger on the measured output? Mike's speaker is going from about 15 to 85 ohms, and I'm simulating between 20-40. It's the difference between the points being tested? Amp out vs speaker out? Or? And second question, why does the upper frequencies on Mike's speaker tail upwards, and in the Spice plot tail downwards?
Thanks


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## JohnH

TINA looks like its a powerful and useful free tool, I might try it.

But Im not sure what is happening in the lower plot, it doesn't look like impedance Ohms. Calculating impedance from first principles, I get the matching graph posted on the previous page. Actually the model looks great for plotting output signals and for adding the attenuator, but not for working out impedances, not sure how best to do that in SPICE. You may get the right shape if you up the amp impedance to a huge value and then plot voltage. Voltage will then be proportional to the circuit impedance. 

What is the SP1 speaker component? is it anything different to a 13.5 Ohm resistor? Also, there are upper and lower points that have to be matched if it is plotting impedance. At very low frequency, the impedance has to be 13.5 Ohms, since everything else is shunted by inductors that have no impedance at low frequency. Similarly, at very high frequency the model has to tend to a limit of 13.5+40+2.3 Ohms, being the sum of resistances except for R5 which is bypassed by the cap. That flattening of the upper tail is a correct interpretation of the Spice model and a difference to the real plot, but its at very high frequency above the acoustic response of guitar speakers. It doesn't matter to much to us, this speaker model is just a piece of test kit, not actually part of what we are designing. 

When Im tweaking a design to check performance of new arrangements and component values I look most closely a the range up to about 5kHz


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## telesto

The Y-axis on both my plots is Ohms. On the bottom plot I made a formula to divide the voltage of TP1 by current thru AM1 to get the impedance. The curves match up pretty good, but I was just wondering why the ohm values are different. Yea, maybe if I change the output power of the amp or something it could have an affect. I'll play around a little. (Actually, voltage of TP1 alone gives the curve shape, so I'm not sure if it's necessary to do the V/I ?)

SP1 is a "speaker" logo, but only contains ohm values, nothing else (it looks nicer than just a resistor  )

You said you calculate up to 5kHz, is that the effective range of a guitar speaker (I guess I can Google that, but wondering what your rational is) Looking at Mike's Greenback plot, the "ripples" seem to stop around 5kHz and smooth out after, so yea maybe after that something happens. Human ear can go up to 20kHz tho, so I guess it's still audible (?)

Here's a link to TINA, free download, just register: http://www.ti.com/tool/TINA-TI
I'm still playing with a few different Spice versions in parallel (Micro-Cap, 5Spice, LT, Qucs, TINA), but I find TINA to have a very powerful and intuitive UI. Yea, give it a shot


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## JohnH

I think the plot is confused by the 20 ohms amp resistance. You could try exactly what you have done with say a 1000ohm amp resistance, but it may still be off due to phase difference between current and voltage. 

Yes guitar speaker acoustic response drops off sharply above 5kHz


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## telesto

I think I am also confused by the 20 ohms amp resistance  Looking at Aiken's original plot, he doesn't have the amp resistance in the circuit, and is getting the impedance plot in Spice by doing V/I. His actual formula is V(V1:+)/I(R1). I am not using the ":+" in my formula, which may be why my impedance plot has the right curves, but is "shrunk" on the Y-axis (not sure, still looking into this).

I see in your circuit, you don't have an Ammeter attached (un-attenuated circuit), and so you take the voltage after the 20 ohms you defined for the amp, which also gives the right curves, because it still reflects the impedance/current changes of the speaker. (Then I guess you feed that into a spreadsheet and do the rest in there). These are two slightly different ways of doing things, but still getting a similar result. I don't know how much of a difference it makes in the end, would have to make simulations both ways and compare.


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## telesto

Hi John, I have a couple of observations/comments:

I've looked at some other attenuator/dummy-load circuits and none of them have the internal resistance of the amp figured into the calculation (aside from the need for nominal impedance matching). The circuits are just calculating the input signal coming from the amp. In your un-attenuated circuit, your 20 ohm internal amp load (R8) is directly in series with the 6.75 ohm speaker (R45). From the point of view of the circuit, it's no different than a 26.75 ohm speaker. 

Second strange thing I see, is that in your Stage 1 attenuator (and all other stages as well), you have the series resistor AFTER the shunt resistor. This puts it in series with the speaker instead of the input, which makes for a totally different DCR calculation than what I am used to seeing in speaker attenuators. I didn't go deeper into this yet, but was this intentional?

For example, take a typical L-Pad calculation from Weber's website:




In this case, R2 and the speaker are in parallel, so you would take the parallel calculation of them, and then add the R1 series circuit and get the total DCR. If you move R1 after R2 (as in your circuit), then R1 is directly in series with the speaker, so then you would add R1 plus the speaker resistance together, and then take that value in parallel with R2 to get the total resistance.


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## JohnH

it's all intentional. Getting the attenuator to feed the speaker a correct and consistenr output impedance is hardly written about at all, but it is the first and most important key idea in this design. 

Nearly all basic passive attenuators use the normal L-pad diagram, including Weber. As you turn it down, output impedance drops which flattens the bass peak and treble rise.


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## telesto

Ok, it works out, when I plug in 6.5 ohm speaker to your Stage 1, I get about 8.1 ohms. Whew, I was worried there for a second 
Actually, your post above mirrors what I just read on Aiken's site about L-Pads

There are two ways to make an L-type attenuator:

*(1) Match the impedance of the L-pad in the direction of the series arm.* This is the "traditional" L-pad configuration. In this case, the input impedance stays constant, and the output impedance gets lower as you increase the attenuation, down to a theoretical minimum of zero ohms. This configuration sounds like crap for a guitar amp, because the decreasing output impedance increases the damping factor and removes all "tubeyness" from the tone at lower volumes, because the amp no longer reacts to the variations in speaker impedance.

*(2) Match the impedance of the L-pad in the direction of the shunt arm. * In this case, the input impedance also stays constant, but the output impedance gets higher as you increase the attenuation. This is great for guitar amps, up to a point. The increasing output impedance lowers the damping factor, which enhances the interaction between the amp and the speaker, giving a natural bass and treble boost as you increase the attenuation (sort of a "built-in" Fletcher-Munson effect compensation!). The problem is that it gets to be too much, and you end up with too much bass and treble boost and "hangover effects" as it was called in the old days of audio, and you get a flubby, fizzy tone at high levels of attenuation. This can be alleviated by limiting the maximum output impedance with an additional shunt resistor at the output. 

This is an eye-opener for me, and need to get my head wrapped more around this. I've got a L-Pad wired into my amp in the first way Aiken describes (and the way Weber shows), and it indeed sounds like dog poop (but at least keeps my wife and neighbors from killing me  ) So from what I see, you have taken the second implementation that Aiken describes? My dislike for L-Pads are suddenly changing. By coincidence, I also saw Elektra has an L-Pad wired in together with some fixed value resistors as the MV on their 185 amp (have a closer look here). This looks really interesting to me, and something I want to explore as I design my own amp (eventually).

...altho I noticed the Elektra doesn't have any inductor (that I see), and Aiken mentions that his earlier amps had reactive attenuators, his next generation had only resistive, so makes me start to wonder what benefits the inductor really brings...


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## JohnH

RA's points in item (2) is one reason why each of our stages is fixed, and optimised to provide the intended consistent output impedance. The -7db stages also provide close to the right input impedance too. The other stages don't do this, but they are always after Stage 1 so their tendency to throw the input Z off is diluted by stage 1 and the amp sees an adequately consistent input Z.

The inductor is the second key idea, and the way it is provided partly in series partly in parallel is unlike any other design that I know of. If this is not correct, it can throw the tone off balance again. As designed either in design M, or M2, it shows the amp the right inductance so the amp reacts to it, then it balances itself out so the signal passes through the resistive parts with very little phase shift, ready to be recreated at lower volume when it gets to the real speaker. The inductor makes a difference when the amp gets driven hard and also helps to adapt to different amps. See my last series of plots.


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## telesto

Thanks for the explanation. While I'm digesting all of that, I'm also playing with Spice in parallel. I've put in a simple resistive only L-type attenuator, but no matter what R values I put, I keep getting the same waveform output, the only difference is it's shifted down on the Y-axis (ie: attenuated). I found the diagram below explaining impedance plots, and it also seems to imply that the DCR is not affecting the curve of the impedance plot, but just moving it up/down on the Y-axis, and it's the reactive inductance of the speaker shaping the signal. If this is correct, then I'm confused, as you are always tweaking to make input/output waveforms to match, so I was expecting to see some "bad" output plots when I threw in some crap values for the resistors. Am I missing some variable in Spice? (Changing power source settings also doesn't seem to have an effect.)


----------



## telesto

JohnH said:


> it's all intentional. Getting the attenuator to feed the speaker a correct and consistenr output impedance is hardly written about at all, but it is the first and most important key idea in this design.
> 
> Nearly all basic passive attenuators use the normal L-pad diagram, including Weber. As you turn it down, output impedance drops which flattens the bass peak and treble rise.


Ok, now I see. You're not just looking left-to-right (presepctive of the amp), but also looking right-to-left (perspective of the speaker. You started out with a U-shaped attenuator on Page 1, and progressed from there. Ok. I was also looking at U-attenuators, and found them used on some microphone attenuators, but true, don't hear it much talked about when it comes to guitar attenuators. Weber's L-pad design gives the correct 8 ohms to the amp, but the speaker is only seeing about 2 ohms, which can't be good. Ok, I will now read the technical summary (Page 1-46) with that in mind


----------



## Dwayne Eash

telesto said:


> ...
> 
> However, one thing to consider is that it's not pushing the output transformer as a cranked amp would, so the tone may be a little off. If you use an attenuator between the amp and speaker (eg. the one discussed in this thread), then you will be driving the OT hard as well, which would probably give a closer tone to that of the actual cranked amp. The other negatives of VVR is it's a little more complicated/costly, and I also hear the MOSFETs can become unreliable down the road. So in short, I would say skip the Variac and VVR, and just go with a good external attenuator if you want to crank your amp at lower volumes.


Seems to me, using a VVR and a power attenuator, both, is the better solution. Using the VVR lets the power tubes break up earlier, which in my case is the main thing sonically lacking. 

So the VVR get's me most of what I need. But also using an attenuator, pushes the OT in a more natural way than just VVR.

And since I'm also using VVR, I don't need to use quite as much power attenuation, and using less power attenuation, generally equates to better transparency to the amp, or, less disturbing the tone.


----------



## Dwayne Eash

telesto said:


> Generally speaking, when you lower the plate voltages to the tubes, either by Variac or VVR, it causes the tubes to break up at lower volumes. EVH was famous for using a Variac on his Marshall on early VH albumns to get his "brown sound".



If you can reduce volts before the power tubes, how about also increase the voltage before the OT, like we do in raising the bias to the power tubes, so they wont fail so quickly when we drop the volts so low, as was suggested to be done when using a variac to lower the volts?


----------



## telesto

Dwayne Eash said:


> Seems to me, using a VVR and a power attenuator, both, is the better solution. Using the VVR lets the power tubes break up earlier, which in my case is the main thing sonically lacking.
> 
> So the VVR get's me most of what I need. But also using an attenuator, pushes the OT in a more natural way than just VVR.
> 
> And since I'm also using VVR, I don't need to use quite as much power attenuation, and using less power attenuation, generally equates to better transparency to the amp, or, less disturbing the tone.


VVR gets earlier tube breakup, but doesn't push the OT. Attenuator will push your OT, but puts a metal wall between your OT and speaker so they don't interact naturally. Pick your poison. Or take both and get the worst of both worlds  Well, actually, I don't know, anythings possible, beauty is in the ear of the beholder. So play around and find what works for you. 



Dwayne Eash said:


> If you can reduce volts before the power tubes, how about also increase the voltage before the OT, like we do in raising the bias to the power tubes, so they wont fail so quickly when we drop the volts so low, as was suggested to be done when using a variac to lower the volts?


VVR reduces the power AT the power tubes, and the power tubes annode is connected to the OT primary. The VVR or Variac will raise/lower them together.


----------



## Dwayne Eash

telesto said:


> VVR gets earlier tube breakup, but doesn't push the OT. Attenuator will push your OT, but puts a metal wall between your OT and speaker so they don't interact naturally. Pick your poison*.* Or take both and get the worst of both worlds  Well, actually, I don't know, anythings possible, beauty is in the ear of the beholder. So play around and find what works for you.
> 
> ...


 But of course, I would use some sort of reactive circuit, so that the amp is fooled into thinking it's all good and speaker like. @@ I mean, we are chatting with the speaker reactive emulation man, hehe JohnH, after all.  DIY economy, and we're good. 

Can't be very hard to install a VVR, but where to get a good one?


----------



## telesto

Dwayne Eash said:


> But of course, I would use some sort of reactive circuit, so that the amp is fooled into thinking it's all good and speaker like. @@ I mean, we are chatting with the speaker reactive emulation man, hehe JohnH, after all.  DIY economy, and we're good.
> 
> Can't be very hard to install a VVR, but where to get a good one?


True, John has done alot of work to find a way to blaze a trail over or thru the metal wall of attenuation  
A VVR is basically just a pot and MOSFET, but you have to know exactly where to put it, and make sure it is done well, as you are tapping into the highest voltage in the amp, 300-500 VDC range. 
Someone on this forum installed one, you can read here: http://www.marshallforum.com/threads/18w-vvr-installation.71431/


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## JohnH

Dwayne, id suggest to take one step at a time and get it working. But are you thinking of doing VVR first rather than attenuator? 

I don't know anything about VVR's but if there is much more to discuss about them it might be better on another thread.


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## Dwayne Eash

JohnH said:


> Dwayne, id suggest to take one step at a time and get it working. But are you thinking of doing VVR first rather than attenuator?
> 
> I don't know anything about VVR's but if there is much more to discuss about them it might be better on another thread.


I was hoping you'd let me know and update on the diagram, so I can search out how much this should cost. I expect to spend a couple hundred on getting my amp going like it should. Here's hoping I can figure out which way is most effective, soon. Are you waiting on something from me?

I'd like to do both, but I also need to buy a set of tuners too as my set just failed, so no more playing until I fix the guitar.


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## Dwayne Eash

telesto said:


> True, John has done alot of work to find a way to blaze a trail over or thru the metal wall of attenuation
> A VVR is basically just a pot and MOSFET, but you have to know exactly where to put it, and make sure it is done well, as you are tapping into the highest voltage in the amp, 300-500 VDC range.
> Someone on this forum installed one, you can read here: http://www.marshallforum.com/threads/18w-vvr-installation.71431/


I can do intense bible theology, even textural criticism, and that's some heady deep stuff. I do alternative energy, and alternative health and medicine, I've built a dozen computers and a network from scratch, but I don't know electronic schematics.

But I can probably install what's needed, if someone points me to the right parts. How about a variac and just apply it to that spot, instead of a VVR? I prefer buying a variac for inventor experimenting.


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## telesto

Dwayne Eash said:


> I was hoping you'd let me know and update on the diagram, so I can search out how much this should cost. I expect to spend a couple hundred on getting my amp going like it should. Here's hoping I can figure out which way is most effective, soon. Are you waiting on something from me?


John's M2 diagram is here. Resistor power ratings are the same as the M1 I guess. Total cost should be well under $100 depending on where you shop and shipping costs, etc.



Dwayne Eash said:


> I can do intense bible theology, even textural criticism, and that's some heady deep stuff. I do alternative energy, and alternative health and medicine, I've built a dozen computers and a network from scratch, but I don't know electronic schematics.
> 
> But I can probably install what's needed, if someone points me to the right parts. How about a variac and just apply it to that spot, instead of a VVR? I prefer buying a variac for inventor experimenting.


I think your best best it to build John's attenuator, and get a Variac. VVR is "invasive surgery". Variac is just a step-transformer that you plug your whole amp into, so very simple. Just make sure you get one that can handle your amps power. Not sure how much one costs that can handle a guitar amps power, but I guess around $200 or so (?)

In any case, while it's interesting to compare VVR, Variac and attenuators, I think we shouldn't derail John's thread and keep the focus here on attenuators, and discuss the pros and cons of different volume reduction methods in another thread 



...coming back to the attenuator: I'm slowly re-reading this thread trying to understand the concept behind it. So far I'm up to Page 3, and understand that John is not just doing the traditional speaker to amp impedance matching, but also going the opposite way and matching the impedance of the amp to the speaker. Which I think is the crux of the design? Do I have it right so far John? I didn't get to the inductor part yet, but I suppose it will somehow enhance the dynamics of the impedance in a reactive way?

Oh, and I remember you mentioned somewhere about NFB increasing the impedance of the amp (?) Is this good or bad in terms of this attenuator? Or would it be best to adjust the Presence/NFB and find a "sweet spot" and leave it there? And does the NFB need to be coming from the same tap? Eg. NFB is usually taken from the 16ohm tap, but will it matter to the attenuator if the attenuator is connected to the 8 ohm output and NFB to the 16ohm tap?


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## Dwayne Eash

telesto said:


> John's M2 diagram is here. Resistor power ratings are the same as the M1 I guess. Total cost should be well under $100 depending on where you shop and shipping costs, etc.
> 
> 
> I think your best best it to build John's attenuator, and get a Variac. VVR is "invasive surgery". Variac is just a step-transformer that you plug your whole amp into, so very simple. Just make sure you get one that can handle your amps power. Not sure how much one costs that can handle a guitar amps power, but I guess around $200 or so (?)
> 
> In any case, while it's interesting to compare VVR, Variac and attenuators, I think we shouldn't derail John's thread and keep the focus here on attenuators, and discuss the pros and cons of different volume reduction methods in another thread
> 
> ...


I'm not doing the M2 design. If I do this, might as well have two footswitchable clean vol boosts, so I added two extra of the lowest attenuation stage for footswitchable control. I submitted to him MY modification and am waiting on him to ok the design so I can verify the part count and expense. Until he gives my design his pointers, I can't do anything on the design.

I can see your one of the smartest around when it comes to electronics, you should be proud of being such a professional. But I am am not familiar with electronic schematics, and even if I did, I don't know what each part does in the circuit. How would I know unless I work with it in a meaningful sense.

Just because I am asking for pointers so I can proceed with my project, does not mean I wish to derail the focus on this thread. That's a bit insulting. Just because I am not on your level, that should make you happy to welcome others. You continue in a discussion that only the few understand and can participate in.

I can pick up a variac of 500 or a 1000 watts, for about 90 bucks. I suppose I could make the attenuator for $150-175, but I don't know how much the footswitch end of things costs. So I was hoping he would help me find a suitable footswitch and relay to use, at least something to start with, as his designs do not seem to use footswitches, so I assume that unusual request on my part, has put a snag in his ability to verify parts for my design.

I think I based mine off of M3. So M2 is not exactly correct.

I'm glad your asking him questions, the design seems a bit simple for such a good result, but if it works, then I'm all for it. I feel certain Ted Weber's MASS attenuators is likely going to be hurt by this development. But more importantly, if we guitarist are better off, then I'm all for it.

I'm not here to disrupt, just to take care of my amp that lacks singing sustain is all. No worries, after I get answers, I'm outta hear. I bought my amp through this forum, and that was about 10-11 years ago. I have not been back until now as I needed to get my amp going again. I don't push myself on others. Maybe after I leave, I wont be back for another decade.


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## JohnH

telesto said:


> ...coming back to the attenuator: I'm slowly re-reading this thread trying to understand the concept behind it. So far I'm up to Page 3, and understand that John is not just doing the traditional speaker to amp impedance matching, but also going the opposite way and matching the impedance of the amp to the speaker. Which I think is the crux of the design? Do I have it right so far John? I didn't get to the inductor part yet, but I suppose it will somehow enhance the dynamics of the impedance in a reactive way?



yes, that's right.

@Dwayne Eash , I can help with the schematic, to include the relay footswitch control. It will be based on the M2 design (EDIT: I just saw your last post, you can also use M which is what you drew, but M2 is easier to build).

Please could you review each of the following:

Amp tap: 8 Ohms (this should match you main speaker or speaker pair. alternative is 16 Ohms)

Stage 1: -7db reactive always on

Stages 2, 3, 4: -3.5, -7, -14 db each with a switch

Stage 5 and 6: -3.5, -7 db each controlled by a foot-switched relay, with footswitch plugged into a jack. when the footswitch is not plugged in, or if dc power is not provided for the relays, these stages will be off ie bypassed, not attenuating

*Parts*

1 input jack
2 output jacks wired in parallel (use one for an 8 ohm cab or both with two 16 ohm cabs)
input socket to plug in a 9V wall wart

2 jacks sockets for footswitch cable at main and footswitch box

All jack sockets to be plastic bodied TRS (ie stereo with ring contact)

3 toggle switches rated at 5A minimum

Aluminium clad case mounted power resistors

Air-cored inductor 16ga or 18ga wire

hookup wire, 18 awg stranded

solder

M3 nuts and bolts to fix resistors to the case

Thermal grease

Feet for case

Spray paint if you wish

diecast Aluminium case for main box, a smaller one for the footswitch

stereo jack plugs and 3 core flex for footswitch carrying dc (could be a lightweight mains flex or a stereo audio cable)

*Questions:*
Do you want the option of running a single 16 Ohm speaker (through attenuator still using 8 ohm amp tap) ? Its perfectly safe to do it with no additional parts, but the treble tone is not optimised (you lose a bit). One more output socket and two more resistors fixes this.

I recommend not to have an overall bypass switch, and not to have attenuation less than the always on -7 db stage. its safer, less current flows and its simpler wiring. If you want full volume, just don't use the attenuator. please could you confirm if you agree.

What would be your sources for parts? are you in the US? do you have any retail electronics stores where you could pick out most of the common components? probably need to order at least the resistors and inductor, probably separately.


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## Dwayne Eash

To clarify, I downloaded the M3 design to make my mods. But I also downloaded another drawing. Maybe I used more than one drawing.


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## telesto

Dwayne, I think we got some wires crossed. I'm definitely not that smart, and you are definitely welcome to participate in the conversation  I wasn't aware you had asked John for a specific request and was waiting on that, I just assumed you were waiting for the M1 on the first page to be updated to the M2. But looks like John just got back to you on that 



JohnH said:


> yes, that's right.


Whew. So then I don't need to read the next 30 or so pages? 

Then just two questions:
1) Can you explain a bit on NFB affects on the attenuator?
2) How did you settle on the incuctor values? And don't say "complex mathimatical formula"  At least say what you based it on, eg. compensates or simulates impedance changes in the amp? Or did you just play with values until it "sounded right"? And on page 11 or so, the inductor only gave "subtle" improvement to the resistive design. Is that still true today? Or did some new revelation happen between page 11-49 regarding the inductor?


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## Dwayne Eash

I guess I don't understand the differences between the three different models. I saw the three output taps and prefer'd that option, that's why I thought I used M3. Opps, I guess it was M4 that I used. I downloaded more than one diagram.

This part is confusing to me, especially now, as I thought I did the M4 version.

I feel a little bit like I need an explanation of the differences between each version. I read what you wrote about them, but I still don't understanding the differences. 

I could use a wooden box, or better yet, a DIY metal plate, some something repurposed. I need to use anything I can re-purpose. 

Yes for three output jacks. 

Yes for gain stages as indicated. 

As to attenuation on off bypass. If all it takes is adding a switch, then no, I do not agree, it should be installed so I don't have to uninstall the attenuator in order to bypass it. I don't mind a slight hit on highs. I play an SG on a JCM 800. Both are high pitched instruments, so some loss on the high end seems nearly desirable. But you know the tonal loss better than I do.

Yes, I agree with 7 dB as minimum constantly attenuating. No problem.

I'm in the US, I am very unfamiliar with parts sourcing, but I sorta figure mouser might come into play, IDK.

Thanks for your time and energy invested. I believe this is a great project, so if I don't do this project presently, it would be done next time I have the money to do it. So this is not a waist of time. I want to use a minimum of attenuation (to keep the sound more transparent) combined with a variac, both.


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## Dwayne Eash

telesto said:


> Dwayne, I think we got some wires crossed. I'm definitely not that smart, and you are definitely welcome to participate in the conversation  I wasn't aware you had asked John for a specific request and was waiting on that, I just assumed you were waiting for the M1 on the first page to be updated to the M2. But looks like John just got back to you on that
> 
> 
> Whew. So then I don't need to read the next 30 or so pages?
> 
> Then just two questions:
> 1) Can you explain a bit on NFB affects on the attenuator?
> 2) How did you settle on the inductor values? And don't say "complex mathematical formula"  At least say what you based it on, eg. compensates or simulates impedance changes in the amp? Or did you just play with values until it "sounded right"? ...


Thanks for clarifying and offering a welcome. Much appreciated. I might stick around. But I am an inventor, and into alternative energy and alternative healing, and humanitarian causes. So I stay extremely busy. We the people deserve so much better than what we get.

You are asking the kind of questions I'd be asking if I could, but because I don't understand what things do to a circuit and you do, so I just stay out. You bring a functional reality to the discussion, however, I should not underestimate JohnH's intelligence. He also has a good grasp of electronic circuits and electronic theory.

We guitarists have been needing something like this forever. So I am all for it. I just find it odd that more guitarist's don't want foot-switchable clean boost(s). Am I the only one??? Nah, that's can't be true, as it could be the best in transparent clean boost, indistinguishable between it, and the amp. 

Because,, it is the amp.


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## JohnH

I started giving a letter to different designs once it looked like this would be a journey, starting with A. But when I got to M I stopped with letters, since I like M for Marshall.

M is the one most have built including me, it has two coils. It comes in a few versions depending on features included.

M2 was a newer idea last year and it performs the same but with one less coil. It's a nice idea and simpler. But what it does is the same as M.

M3 has additional components focussing on the low bass resonance, But it makes it quite a lot more expensive, and so far doesn't seem necessary. It's hasn't been built to my knowledge.

Any of then can have one, two or three output sockets

On boxes, attenuators heat up when an amp is driven hard, and a metal case (or a substantial heat sink) with plenty of vents is important for cooling.


*NFB
*
Negative feedback reduces the output impedance of the amp at normal volume, reducing the high treble and bass peak. without NFB, amp output Z would be much higher, and I've measured that effect on my other amp with no NFB The attenuator adapts to this, see charts on page 46. It means it helps to make each amp and speaker combo sound like itself, at lower volume. You can hear it and calculate it, but I can't explain it.

The inductance is picked so that the signal at the amp going into the attenuator is very close to that of an amp driving a speaker in terms of how it rises with frequency. The amp then can react to that changing impedance, but that's not modelled in the analysis.


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## telesto

There is a question about the amp impedance that is bothering me. You said somewhere (in the last 49 pages) that you measured your amps 8ohm output and found it to be 20 ohms. How is this possible? We put 8 ohm speakers on the 8 ohm taps because we need to do impedance matching for optimal signal transfer. If the amp is showing 20 ohms and we connect an 8 ohm speaker it would stress the amp and not sound very good. (Actually a 16 ohm speaker would be better.) So in this sense, your measured 20 ohms makes no sense to me 

You design the attenuator to still show the amp 8 ohms (why not 20?), but then to show the speaker an 18 ohm amp impedance. If there was no attenuator, the speaker would see the "natural" amp impedance, of what should (?) be 8 ohms. Some other attenuators show the speaker lower impedance, like Weber's L-Pad shows 2 ohms to the speaker. And my variable L-Pad shows probably even less, which makes for a very poor sound. Coming back to Aiken again:

In this case, the input impedance also stays constant, but *the output impedance (*of a reversed variable L-Pad) gets higher as you increase the attenuation*. This is great for guitar amps, up to a point. *The increasing output impedance lowers the damping factor, which enhances the interaction between the amp and the speaker, giving a natural bass and treble boost as you increase the attenuation* (sort of a "built-in" Fletcher-Munson effect compensation!). The problem is that it gets to be too much, and you end up with too much bass and treble boost and "hangover effects" as it was called in the old days of audio, and you get a flubby, fizzy tone at high levels of attenuation. This can be alleviated by limiting the maximum output impedance with an additional shunt resistor at the output. 

So, as you design your attenuator, I think the amp impedance doesn't matter, or at least you show it the 8 ohms it expects for successful impedance matching, so that is fine. But then you show the speaker a higher impedance, which is what Aiken is saying is "good" in respect to attenuators.

My next question then, did you experiment with showing the speaker other levels of impedance? eg. what if you show the speaker 30 or 40 ohms? Would it sound better or worse? ...ah, wait, yes you did, you tried with 26 and said it was to shrill. Based on your plots and listening tests, you were happy with about 6 ohms showing to the speaker. But now you have Stage 1 configured to show 18 ohms to the speaker, so what happened there? You changed your mind and decided 18 was better than 6?

Coming back to the 20 ohms as the amp resistance: You calculate with 20 ohms amp impedance, and then calculate Stage 1 to show the speaker 18 ohms. Using 8 ohms as amp resistance in the same calculation would mean showing the speaker 15 ohms. Not really a big difference I guess. Main thing I guess is that it's higher than the circa 2 ohms that some attenuators give, which end in anemic attenuated sound. So I guess the the "sweet spot" of impedance to show to the speaker would be somewhere in the 2-20 ohm range, or actually more in the say 6-18 ohm range. Your original U-pad attenuator on Page 1 was showing 8 ohms to the speaker, which made sense to me.

*edit: sorry for editing this post several times as I worked some things out in my head in real-time


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## JohnH

lots of questions!

Actual Amp output imoedance has nothing to do with what speaker ohms it is designed to drive. Impedance matching, although part of some devices design, is not a thing for guitar amps. An amp designed for an 8 ohm load could have any output impedance from almost zero (A typical solid state amp) to almost infinite ( a valve amp with no NFB, running clean). You've seen how this affects speaker response.

You can't measure output impedance directly, but it's not too hard to work it out. You feed a small steady signal into the amp and feed it into say an 8 ohm load and measure the output voltage. Then repeat unchanged but with a different load resistance, say 16 ohms. If the signal remains constant, amp output resistance is very low. It it increases in proportion to the load, then it's very high In between you can take the ratio and work it out.


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## telesto

Hmmm...ok. True, DCR of an OT is near zero. So what is the difference between 4-8-16 ohm secondary taps? Since it's a coil, I guess something to do with the winding ratio, and how it reacts to different impedance loads? 
Yes, sorry, lots of questions  Well, I'm also kind of thinking out loud, and finding some answers to my own questions, but yea, feel free to jump in, waters nice 
I found this somewhere, I guess you did the same or similar:

To measure the output impedance at the secondary of the transformer, as driven by the amplifier, do this:

Apply a 1 kHz signal to the input of the amplifier, so that the output is not too large, but easily measurable, perhaps 1/10 volt. Carefully measure the output voltage with no load connected. Next connect a variable load resistance to the output, and adjust its resistance until the output voltage is reduced by half. Measure the value of the load resistance. That value will be the output impedance.

Or you can use a fixed resistor. Measure the open circuit output voltage with sufficient drive to give you a small but easily measurable output voltage; call this voltage Vo. Now connect a resistor of suitable value; start with 10Ω. Measure the output voltage with the load resistor, RL, connected; call this voltage VL.

The output impedance is given by Zo = RL*(Vo/VL - 1).


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## telesto

Ok, so I was playing around with the fundamental concept of trying to show the speaker around 20 ohms, while still keeping the amp seeing 8. Keeping also the "simple" concept of just using 2 resistors in a reverse L-pad form. And it's a kind of tricky game to play! There are not that many possibilities. Using standard resistor values, I was only able to find 8 combinations that fit (chart below). I was focused more on keeping the amp seeing 8 ohms, and letting some "slack" with the values the speaker sees.

Do these look OK to you? I guess you also did some similar calculations over a wide range? You chose the -7 dB combo. I think you said to go with less attenuation would throw off the grand scheme when additional stages are added? So I guess the -9dB should be fine for Stage 1 as well? For your -3.5dB stage you went with 33 and 5.6 ohms, which shows the speaker a nice 18 ohms, but then shows the amp 9.6 ohms, which is maybe not the best option (?) I guess it's fine as additional stage, but in some early schematics, you bypassed Stage 1 and had the -3.5dB as a stand-alone stage. Or were you more focused on showing the speaker 18 ohms, and letting slack in what the amp sees?...Ok, yes, now I see you made a "single stage" chart like what I just did, but you did for 16 ohms, and were more focused on getting the speaker to see 18 ohms and giving slack in what the amp sees. What was the reason for that?


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## JohnH

You are reading my mind.

yes there has to be some slack in these numbers, given real resistor values and also not all may be available in high power types, and sometimes a different range is sold other than the standard series. I like my numbers to be as exact as possible, it helps me home in on how things are working, but then I use them to make the best available engineering compromise for a practical design.

People talk about mismatches between amp and speaker, and is it Ok or not, and its unclear enough to be a subject of discussion. But they are talking about x2 factors, 8 to 16 etc. In a tube amp, im pretty happy that a much smaller variation is definitely OK, so for an 8 Ohm load, if it happened to turn out between 7 and 10 then I don't see a problem for the amp, if it allows something to be achieved. So when I offer a -3.5db option, the amp sees about 10. Im not worried about that though Id rather it was nearer to 8. And when as is usually the case, that -3.5 stage is after the nicely balanced -7 stage, then it makes much less difference.


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## telesto

Ok, great 
Yes, I know there is some wiggle room with impedances. But why did you target 18 ohms for the speaker to see? You were focused on keeping that, and allowing the slack on the amp end. I would think the other way, since you were OK earlier with the speaker seeing 6 ohms, that if it sees now somewhere around 15 or 16 it's still good. 
In any case, yea, the 15 and 10 combo is a sweet spot of having both, 8 ohms to the amp, and 18 to the speaker, so I would agree that's the best choice.


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## JohnH

In the early posts, I was exploring (still am). I was happy that anything worked at all with just a few resistors, given all the BS and $ that get wrapped around this topic.

Once I'd learnt a few things, I wanted to push higher and I became more picky.

Other than the small compromise in the -3.5 only option, the design gives good control of both impedances at all settings


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## Dwayne Eash

JohnH said:


> I started giving a letter to different designs once it looked like this would be a journey, starting with A. But when I got to M I stopped with letters, since I like M for Marshall.
> 
> M is the one most have built including me, it has two coils. It comes in a few versions depending on features included.
> 
> M2 was a newer idea last year and it performs the same but with one less coil. It's a nice idea and simpler. But what it does is the same as M.
> 
> M3 has additional components focusing on the low bass resonance, But it makes it quite a lot more expensive, and so far doesn't seem necessary. It's hasn't been built to my knowledge.
> 
> Any of then can have one, two or three output sockets
> 
> On boxes, attenuators heat up when an amp is driven hard, and a metal case (or a substantial heat sink) with plenty of vents is important for cooling.
> 
> 
> *NFB
> *
> Negative feedback reduces the output impedance of the amp at normal volume, reducing the high treble and bass peak. without NFB, amp output Z would be much higher, and I've measured that effect on my other amp with no NFB The attenuator adapts to this, see charts on page 46. It means it helps to make each amp and speaker combo sound like itself, at lower volume. You can hear it and calculate it, but I can't explain it.
> 
> The inductance is picked so that the signal at the amp going into the attenuator is very close to that of an amp driving a speaker in terms of how it rises with frequency. The amp then can react to that changing impedance, but that's not modelled in the analysis.


That was very well put, awesome, and quite helpful. And I understood the vast majority. Thanks much.

As to the bass thing, uh oh, I'm kinda a bass nut. hehe I gotta have the warmth of my bass. I include a bass speaker in my rig, so my rig is probably warmer and has fuller mids than most. I feel lucky in my choice of speakers. So I don't know how transparent it already is.

So maybe I wouldn't mind one that includes lower frequencies, depending on the cost. How about a compromise, and do an in between version of M2 and M3? 

I want sonic transparency, or the least change to my tone.

Do you have sound clips of the amp, using the attenuator, and not using it, and volume equalized after the amp, like reduce the mic level. Might have to make a couple recordings to get the recording equalized, or maybe that's easily done in the software, IDK.

That sort of sonic example, with and without the power attenuator, and also volume equalized somewhere after the amp, would help me examine how transparent it treats the signal after attenuation.

I'm guessing you might attenuate using mid 20's dB. Wherever the sonic difference might be noticeable. And have that, compared to without an attenuator, without changing the amp settings, and just adjust the volume after the recording. Maybe you can manually do this by adjusting the mic volume, to result in the same dB's.

If need be, I would give up a clean boost pedal, to gain a more transparent attenuator. And I'd give up the bypass switch too if that helps the tone. If less switches means less tone loss, then I'd be willing to make more of the stages, continues stages. Let me know.

I'll have to go see if you already posted such an audio demo. Thanks again! Great post. Your design is a very cool project. Very glad to see this.


----------



## JohnH

Hi Dwayne,sound clips as you request are already done and posted in post 1. Its attenuator M which will be the same as M2. Not sure if you saw these before.

Here they are:

Attenuator M: Max attenuation to non-attenuated:

https://vocaroo.com/i/s1QgVDnl1XQi


Attenuator M: Normalised:

https://vocaroo.com/i/s1cnuucLz8yK

Its a looped riff placed in my delay pedal working as a looper, pushing the amp quite hard (harder than I usually do). The first recording is with me then stepping through each setting on the attenuator from max attenuation through to zero attenuation, recorded from the speaker through a dynamic mic with no other settings changed. Then the second one is the same recording, boosted in software so each setting is the same volume. The recording level at the mixer was set for the loudest setting, so it s pretty quiet recording to start with.

Obviously your amp and playing, set your way, will sound different, but just listen for the consistency. The riff is played twice at each attenuation setting.

There's actually nothing wrong with the bass in M or M2, which is why I have never built M3. You still get the resonant bass peak which is created at the speaker. If there is a difference, it would be in the added distortion of the bass tone when you hit the very lowest notes very hard. it would only be the fundamental note as affected by the power amp, not the harmonics coming from the guitar, nor from the preamp or any distortion pedals.

The only difference between M2 and M3, is another coil and large cap. But to do this right with the right parts is about another $100. But, if we base your design on M2, these parts could be easily added later if you leave space for them. 

There should be no tone loss with good switches. Can add it all as you wish. The reasons for suggesting not to have a bypass is that it is one switch that you need to be very careful with. Don't ever flick it while playing or else the amp can see transients. But that's not different in any such commercial attenuator of any type. I have literally never used my bypass switch on my 40 and 50W amps, nor has Gene who gigs regularly using smaller amps. Maybe if you have it, you could put it at the back while all the others are at the front? Other switches you can flick on and off at any time.


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## Dwayne Eash

JohnH said:


> Hi Dwayne,sound clips as you request are already done and posted in post 1. Its attenuator M which will be the same as M2. Not sure if you saw these before.
> 
> Here they are:
> 
> Attenuator M: Max attenuation to non-attenuated:
> 
> https://vocaroo.com/i/s1QgVDnl1XQi
> 
> 
> Attenuator M: Normalised:
> 
> https://vocaroo.com/i/s1cnuucLz8yK
> 
> Its a looped riff placed in my delay pedal working as a looper, pushing the amp quite hard (harder than I usually do). The first recording is with me then stepping through each setting on the attenuator from max attenuation through to zero attenuation, recorded from the speaker through a dynamic mic with no other settings changed. Then the second one is the same recording, boosted in software so each setting is the same volume. The recording level at the mixer was set for the loudest setting, so it s pretty quiet recording to start with.
> 
> Obviously your amp and playing, set your way, will sound different, but just listen for the consistency. The riff is played twice at each attenuation setting.
> 
> There's actually nothing wrong with the bass in M or M2, which is why I have never built M3. You still get the resonant bass peak which is created at the speaker. If there is a difference, it would be in the added distortion of the bass tone when you hit the very lowest notes very hard. it would only be the fundamental note as affected by the power amp, not the harmonics coming from the guitar, nor from the preamp or any distortion pedals.
> 
> The only difference between M2 and M3, is another coil and large cap. But to do this right with the right parts is about another $100. But, if we base your design on M2, these parts could be easily added later if you leave space for them.
> 
> There should be no tone loss with good switches. Can add it all as you wish. The reasons for suggesting not to have a bypass is that it is one switch that you need to be very careful with. Don't ever flick it while playing or else the amp can see transients. But that's not different in any such commercial attenuator of any type. I have literally never used my bypass switch on my 40 and 50W amps, nor has Gene who gigs regularly using smaller amps. Maybe if you have it, you could put it at the back while all the others are at the front? Other switches you can flick on and off at any time.



I've listened to them before, I just listened to them again, and although it is pretty transparent, it seriously lacks bass. Your not really playing lower notes, but the recording is rather decidedly not warm and fat.

If this is the only before and after attenuation I have to go by, I can not tell. Not enough bass and warmth was displayed. Could you point me to another clip, or someone else who made another clip? I trust my ears, not someone's word it handles bass well.

Bass is by far the hardest frequencies to handle well, and I don't like hearing you say, it handles the bass well, while at the same time you don't give an audio example where the low end is included.

Here's hoping we get to hear a sampling that includes the lower end, check out the warmth and if the bass remains tight, and accurate tone.


----------



## JohnH

Dwayne Eash said:


> I've listened to them before, I just listened to them again, and although it is pretty transparent, it seriously lacks bass. Your not really playing lower notes, but the recording is rather decidedly not warm and fat.
> 
> If this is the only before and after attenuation I have to go by, I can not tell. Not enough bass and warmth was displayed. Could you point me to another clip, or someone else who made another clip? I trust my ears, not someone's word it handles bass well.
> 
> Bass is by far the hardest frequencies to handle well, and I don't like hearing you say, it handles the bass well, while at the same time you don't give an audio example where the low end is included.
> 
> Here's hoping we get to hear a sampling that includes the lower end, check out the warmth and if the bass remains tight, and accurate tone.



OK so I can try another clip, maybe something in drop D, some 'Cinnamon Girl' maybe.

But lets get something totally crystal clear right now. I am providing free help to you, and having suggested a simple solution to your attenuator search, you are adding more and more requests and looking to be provided with a new more complex custom design to suit your needs with a heap of advice to go with it. That's all cool, I enjoy working things out. But I need some respect please. If I say it handles bass well, that is my opinion based on about a year with testing these designs, backed up by analysis. If you don't like to hear what Im saying I will stop.

Ok? If so, its all fine and we can continue...


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## JohnH

Have a listen to these, which try to show bass response. It's my attempt to play the start of Cinnamon Girl. First, attenuator bypassed, then switched to -21db, both from the same loop.

Full volume (attenuator bypassed), then -21db:
https://voca.ro/jwLCSg3wC9x

Here it is with the attenuated second part normalized in software back up to the same volume:
https://voca.ro/ncne6xsR8JS

Its miced off the VM2266c, Master volume at about 6.

I picked this since the dropped lower D gets frequencies down to 73 hz for low D, and has some notes at E#, G and A above that, so it should be in the range of the bass peak for comparison.

I hope that helps.

ps. another small nugget of info, this test was done using the -7 and -14db stages to make -21db. Audacity, which I record with lets me check that, and the actual difference in levels as miced, is -20.7db, so numbers are good.


----------



## Mcentee2

JohnH said:


> Have a listen to these, which try to show bass response. It's my attempt to play the start of Cinnamon Girl. First, attenuator bypassed, then switched to -21db, both from the same loop.
> 
> Full volume (attenuator bypassed), then -21db:
> https://voca.ro/jwLCSg3wC9x
> 
> Here it is with the attenuated second part normalized in software back up to the same volume:
> https://voca.ro/ncne6xsR8JS
> 
> Its miced off the VM2266c, Master volume at about 6.
> 
> I picked this since the dropped lower D gets frequencies down to 73 hz for low D, and has some notes at E#, G and A above that, so it should be in the range of the bass peak for comparison.
> 
> I hope that helps.
> 
> ps. another small nugget of info, this test was done using the -7 and -14db stages to make -21db. Audacity, which I record with lets me check that, and the actual difference in levels as miced, is -20.7db, so numbers are good.



Now *that* is why this build is so good......


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## Dwayne Eash

I thought I already gave this project serious props, saying something like, guitarists have been seriously needing a great project like this for eons,, and,, this design could possibly compete well with Ted Weber's MASS attenuators, because they seem to perform similarly, but your price is DIY low, and that is very cool!

I'm pretty sure I gave you another glowing remark about this excellent project, but if I haven't already said it, I think it's brilliant. And big thanks to Aiken amps for providing the solid state speaker emulation circuit for free. Both efforts deserve a round of applause from us all.

You seem to be giving it away to those who want it for the least price possible, and so I find that spirit of helping others, quite admirable.

So your open source attitude, is also admirable, more of the future, and should be supported. Whereas the next guy with the same info, could have setup a closed minded and self centered perspective, created a cool product, and then manufacture it, and sell it for a profit, and possibly get rich.

Because this project has low overhead, and relatively small parts, so the factory footprint would be pretty small, probably could easily be started in someone's home/garage/basement/pole-barn. Maybe do both open source DIY and a retail version?

But instead of profit before people, you put people before profit, so you were kind and smiled on us guitarists, and offered it to us, without entry fee, as a DIY project. Just show some respect, and don't hassle with change upon change.

That is very cool!!! That is a generous and helpful spirit. And much appreciated!

I've been wanting a reactive type power attenuator for 6-8 years. Just can't afford one yet.

I'm sorry if I had not made all this more clear to you earlier, but at least one of those compliments was probably given away from our conversation. Nothing but respect, for one of the finest DIY guitar projects, I've ever seen.

~~~

As for the audio clip, I hope your sitting, because that's another matter. hehe Things are becoming more clear to me now. I did a search on youtube, and listened to your amp model and, wow, I'm not sure how to say this,, gently,,

,, that is ,, I don't,, it's possibly,,

Gosh,, I'm trying to phrase this properly, but I'm having a hard time finding the right words.

Sorry, and remember, this is my personal taste test opinion. But I really really really dislike the tone and the sonic nature of the distortion, from that amp.

I sure hope we can get another audio clip. Please forgive my personal taste. The DSL is much more along the lines of what I'm used to, and appreciate.

Please, at your convenience, do another clip, using the DSL, that would help me, concerning an audio sample. I am sorry for being so unique, thus demanding.

~~~

As for evidence I wish to reduce the load, not increase it, I already did take back the request for foot-switchable clean boosts, if too many switches provide too much detriment to sound purity. 

And I also said I was also willing to give up the bypass switch, actually on more general purposes, if you feel strong enough it's somehow sonically better without it,, already showing I defer to your much more experience view, of your own technology and design.

I think it's brilliant, but,, so far,, not the audio sampling choice of amps. But maybe that's just me. Sorry, but maybe I just gotta say it. I never though a Marshall amp could sound that bad. That's some rough harsh distortion, and lacking tone. But that's just my personal opinion.

Best wishes always. I love great guitar sounds and great guitar projects. Thank you for showing us all such generosity, and what seems like a great project!


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## Dwayne Eash

Oops, update seems required. I just saw another video, and the amp has a lot of versatility I did not know about. My bad for judging too quickly. For my defense, this is the first one. I could not get through 3 minuets of it and I was offended too much. hehe No worries tho, seems this was just a bad example is all.

When this amp is dialed in, it can sound very sweet. So again, my bad for judging too quickly. I still would prefer a DSL clip of 70-80's oriented rock and roll gain area, but like you already did, to show off the low and warm half of the spectrum.

This video is what turned me off. But next up, what changed my mind!  But it's a different sound with those KT66's. When they sound good they sound out of this worldly good, but,, when they,, anyway,, nuff said. Please also watch the next video, to hear the same amp, sound much better, arguably great.


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## Dwayne Eash

And, here's a great video of the same amp sounding much better. Enjoy! 

Sorry about my previous length, so I put it in such a long post, as I do not wish to take up many posts on one page. I appreciate your time and space, and will keep things short and sweet. You've been a great help. Thanks again for the awesome project!


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## JohnH

Ok that's all good then.

My DSL is a DSL401, actually nothing like the tone of the more classic ones.
also my recording set up (and playing) is not great. its enough to show differences in the settings but doesn't necessarily do justice to a good amp. All the clips are for is to show to what extent what goes into the attenuator comes out at the other end.

To help the discussion, do you have a clip of the sounds that you like? either yourself or elsewhere online? (OK just saw what you posted above)


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## Dwayne Eash

Dig it man. After some reflection, I suppose a clean recording is the main thing. The distortion on the un-attenuated side, makes it harder to judge the attenuated tone, because there's not as much tone to listen to and compare.  Maybe use the same amp, but without the heavy distortion settings. I like the warm clean side of that amp. Thanks for the grace and great design.


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## Dwayne Eash

And I'm not kidding, if you build this out, and refine the offering of foot switchable clean boost(s) for the ultimate in clean boost transparency, then I do not doubt you could go into business selling these and become financially successful, because of it being cheaper to build it than a Weber Mass, yet does a very similar job as the MASS, at a discount in price.

People with an amp they like the sound of it, will love having an amp provided clean boost. That is simply genius for those who prefer the sound of their amp. I love how my amp sounds, so this option seems like a must have.

I believe one guy on this forum already installed a nice looking line-out, a feature many will like for their unit. You might also consider implementing a full dummy load option, as well. I know I would opt for that option if it was available.

Forgive me if you have no desire to mass produce these. But this DIY project, could easily be a worthwhile business venture. Too bad Ted did not have more of an open source mentality, or I could have built my own power attenuator years ago from an old speaker motor, and I would have thrown Ted Weber some of my money as well. But so far, nope, I held out for a better deal. Glad I did. 

We have too much self interest in profit over people these days. Thanks for demonstrating it's better to prioritize people over profit. But don't be afraid of making some money off these as well. If you can outsource the box, the rest is little more than soldering it together.

After parts are collected, probably doesn't take an hour to build the device in a manufacturing plant, start to finish setting. So that's only 10-15 bucks labor and handling per unit. Talk about nice overhead costs.

One of your finished units, with clean boost foot switching, and a line out, and dummy load, would easily fetch $200 as opposed to Weber's $275-$300 for his units of similar reactive power attenuation. If you could make them for around 150 per unit, including labor, you could make about 50 bucks a unit, and still afford to pay someone 12 bucks an hour to build them for you. You just sit back and handle orders and customer service if you want.

Something to think about. Peace!


----------



## What?

JohnH said:


> Have a listen to these, which try to show bass response. It's my attempt to play the start of Cinnamon Girl. First, attenuator bypassed, then switched to -21db, both from the same loop.
> 
> Full volume (attenuator bypassed), then -21db:
> https://voca.ro/jwLCSg3wC9x
> 
> Here it is with the attenuated second part normalized in software back up to the same volume:
> https://voca.ro/ncne6xsR8JS
> 
> Its miced off the VM2266c, Master volume at about 6.
> 
> I picked this since the dropped lower D gets frequencies down to 73 hz for low D, and has some notes at E#, G and A above that, so it should be in the range of the bass peak for comparison.
> 
> I hope that helps.
> 
> ps. another small nugget of info, this test was done using the -7 and -14db stages to make -21db. Audacity, which I record with lets me check that, and the actual difference in levels as miced, is -20.7db, so numbers are good.



Sounds pretty dang spot on here. I'm guessing that the noise in the normalized attenuated audio is due to the noise floor of the recording being raised along with the signal, i.e., that level of noise would not be there in person.


----------



## Dwayne Eash

What? said:


> Sounds pretty dang spot on here. I'm guessing that the noise in the normalized attenuated audio is due to the noise floor of the recording being raised along with the signal, i.e., that level of noise would not be there in person.


I have no problem with recording clarity. The slight distortion, or static from volume normalizing is perfectly acceptable to me. Just to clarify, and not that you are responding to my idea, but the distortion I was referring to, was not from the recording rig and mic, but the guitar amp settings with too much gain or boost buttons, so the signal is pushed into heavy distortion and overdrive.

Which would you rather have to listen to, to determine if the signal was sonically altered, a distorted signal, or a clear clean signal?

I say the choice is clear. Because of too much distortion in the amp, that makes judging the tonal transparency more difficult. I like to hear both distorted sounds, and clean sounds, to better gauge the effect on the sound quality. But at this point, I agree, the attenuator seems to do no noticeable harm to the sound quality, even using more warmer low end frequencies.

And that's encouraging news. I just can't clearly distinguish tonal variance, with a fairly highly distorted signal.

I look forward to a cleaner sample of warmth and low end, maybe including a bit of a bass roll, instead of more wide open sounding chords, because of overlapping, which can serve to cover up tones, instead of highlight them. But I like to hear some strumming too. It all helps.

I would like to start this project, ASAP. I have been wanting a good reactive power attenuator, like for most of a decade.


----------



## What?

Dwayne Eash said:


> Which would you rather have to listen to, to determine if the signal was sonically altered, a distorted signal, or a clear clean signal?



The sound of a given amp, at whatever volume it sounds best. That almost always means a cranked up amp and distorted tone. Some amps will be much more distorted than others when cranked up, so it's not a fixed type of tone (clean or distorted) across amps.


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## JohnH

Thanks for the comments guys.

The open source nature of this project is a key thing and actually we wouldn't have this design without it. The feedback is an enormous part of making it better. In fact, if @Gene Ballzz hadn't built one and noticed that it was doing a great job, I probably wouldn't have developed it beyond an early version. For example, for his first version, I tweaked the ratio of the inductors to better optimize the response based on the numbers. It brought it to life an he noticed it first while my then latest prototype had previous values that didn't bring out the proper benefit of the reactive circuit. 

On recordings and noise floor, yes boosting in software is bringing up everything in the signal, and the sound signal is embarrassingly tiny at -21db. For interest, here's how the traces for the last clips look like, for the original and normalized versions:




Clearly it would have been better to get the attenuated trace by boosting the mic level instead of downstream within the software recording. But it's interesting to see what a -21db signal reduction looks like. At just over a 1/100th factor on power, the signal voltage is reduced by factor of just under 1/10th, which is as you can see in the upper trace. And if you look at equivalent sections of the lower trace, there is much similarity between full volume and normalized signals, but they are not quite identical. The real speaker is obviously being pushed harder at full volume and is adding its own non-linearities.

There's anther noise-floor benefit, not picked up here. Any biggish amp has its own bit of residual hum and hiss even at zero volume. If the alternative to an attenuator is just to turn down volume, then you still get this coming through from the full power amp. But an attenuator squashes this back down in proportion. 

To test out a reactive attenuator, most is revealed in a signal where the power amp is saturating and that's what the clips so far have aimed at. This is where the varying impedance being fed back to the amp makes the amp react and compress. With clean signals its all a lot more linear and the comparisons can be less revealing of differences. But Ill put that on the 2do list. 


As to making $, sure I like it too. But to set up and run such a business properly, with warranties, taxes, paperwork, suppliers, shipping etc, for $50 profit per unit doesn't fit into my life right now. btw, IRL, I'm a structural engineer (electronics is a hobby since high-school). I design large buildings, and I teach it at Sydney university. It uses all my time and $50 wouldn't replace much of it.


----------



## Dwayne Eash

JohnH said:


> Thanks for the comments guys.
> 
> The open source nature of this project is a key thing and actually we wouldn't have this design without it. The feedback is an enormous part of making it better. In fact, if @Gene Ballzz hadn't built one and noticed that it was doing a great job, I probably wouldn't have developed it beyond an early version. For example, for his first version, I tweaked the ratio of the inductors to better optimize the response based on the numbers. It brought it to life an he noticed it first while my then latest prototype had previous values that didn't bring out the proper benefit of the reactive circuit.
> 
> On recordings and noise floor, yes boosting in software is bringing up everything in the signal, and the sound signal is embarrassingly tiny at -21db. For interest, here's how the traces for the last clips look like, for the original and normalized versions:
> 
> View attachment 73676
> 
> 
> Clearly it would have been better to get the attenuated trace by boosting the mic level instead of downstream within the software recording. But it's interesting to see what a -21db signal reduction looks like. At just over a 1/100th factor on power, the signal voltage is reduced by factor of just under 1/10th, which is as you can see in the upper trace. And if you look at equivalent sections of the lower trace, there is much similarity between full volume and normalized signals, but they are not quite identical. The real speaker is obviously being pushed harder at full volume and is adding its own non-linearities.
> 
> There's anther noise-floor benefit, not picked up here. Any biggish amp has its own bit of residual hum and hiss even at zero volume. If the alternative to an attenuator is just to turn down volume, then you still get this coming through from the full power amp. But an attenuator squashes this back down in proportion.
> 
> To test out a reactive attenuator, most is revealed in a signal where the power amp is saturating and that's what the clips so far have aimed at. This is where the varying impedance being fed back to the amp makes the amp react and compress. With clean signals its all a lot more linear and the comparisons can be less revealing of differences. But Ill put that on the 2do list.
> 
> 
> As to making $, sure I like it too. But to set up and run such a business properly, with warranties, taxes, paperwork, suppliers, shipping etc, for $50 profit per unit doesn't fit into my life right now. btw, IRL, I'm a structural engineer (electronics is a hobby since high-school). I design large buildings, and I teach it at Sydney university. It uses all my time and $50 wouldn't replace much of it.


 That was with you paying someone to do the work, you'd mostly just need to do watch over orders and customer service to keep the employee count down.

If you make more money as a building engineer, then that's perfectly understandable why you are not interested in smaller earnings. I'm glad you seem to be doing so well. I'm so poor, I can't pay attention. hehe But I have great ideas I feel the world needs. It's too much of an upside down world. Too much corruption.

I suggest you save up, and prepare to work for yourself. Because the future is not guaranteed. The economy could collapse because of this plandemic. After that, all plans for careers can be canceled if it's not essential enough. AI, and manufacturing automation, is gradually pushing out the worker, so there are less jobs, and more people without jobs.

Sooner or later, something has to give. I'm saying, be prepared for hard times. Lets hope it does not get that bad, but it could become another great depression.


----------



## JohnH

Sure, good points.

So what should your design be at this point?

A few extra things to know:

A line out is easy to add, which youd then need to feed through a cab-sim or IR processor to make a sound for recording or PA use.

Any if these designs can be a load box with no mods. You just set to max attenuation, and then the amp sees negligible difference whether or not a speaker is there (at -31db, less than 1/1000th of the power would reach the speaker anyway)

Can have the relay footswitching if you want it, which would cut the level further when engaged, then bring back up to level if off. An alternative to adding two more full switched stages with extea resistors, would be to have relays that could override the standard 7 and 3.5 switched stages when the footswitch is plugged in, saving the extra resistors.

Have the full bypass if you wish.


----------



## Dwayne Eash

JohnH said:


> Sure, good points.
> 
> So what should your design be at this point?
> 
> A few extra things to know:
> 
> A line out is easy to add, which youd then need to feed through a cab-sim or IR processor to make a sound for recording or PA use.
> 
> Any if these designs can be a load box with no mods. You just set to max attenuation, and then the amp sees negligible difference whether or not a speaker is there (at -31db, less than 1/1000th of the power would reach the speaker anyway)
> 
> Can have the relay footswitching if you want it, which would cut the level further when engaged, then bring back up to level if off. An alternative to adding two more full switched stages with extea resistors, would be to have relays that could override the standard 7 and 3.5 switched stages when the footswitch is plugged in, saving the extra resistors.
> 
> Have the full bypass if you wish.


Good points, thanks for clarifying. I don't need a line out, so I figure, no line out.

As for saving gain stages from the footswitch,, so far,, I think not,, but your right to bring it up as there's still too much attenuation, because of my foot-switches. I love the idea of two transparent clean boosts. And I like those transparent boosts, to act isolated from the attenuation volume functionality.

I think you once said that the attenuator options should probably be 3.5, 7 and 14, and I agree! But those settings are functionally "just" for volume level. Kinda like, set them, and forget them. Like controls on the amplifier!

The clean boost settings, are JUST for clean boost levels within a song, and so the two different functions are not really combinable in the same stage, since they each do different things. I don't want to sometimes render one of the clean boosts, unavailable. So the two functional sections, the set it and forget it part, and the foot switchable part, functionally speaking, should work independently from each other.

I would much rather pay for the extra relay switching system, and for two extra gain stages, in order to keep the foot switchable clean boosts functionally independent from the power attenuation options. 

Here's my first semi-final rough draft.

Speaker Emulation Stage (always on)
7 dB *
Attenuator Stages (set and forget per show/session)
14, 7, 3.5
foot switchable clean boost stages (frequent changes, even per song/musical number)
7, 3.5 either (A/B) and (on/off)

*Question, what's the lowest value that still works well for this spot? And what is the best value for this spot?


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## Dwayne Eash

I just realized, on the foot-switch relay stomp box, for the two foot-switches, I want one on/off relay switch, and one A/B selection relay switch. Here's how it works. Stomp on first stomp switch, to light up an LED showing activation of the stomp box.

This also actives one of the two stages. The other stomp box switch, designates which stage is being called upon, the 3.5 or the 7. That way, with one stomp, I can double the boost, or half the boost. Just one stomp. And with just one more stomp, I can shut the “boost” off, or turn it “on”.

If I had a separate on/off switch for each stage, then, just to go from one stage to the next, I would have to hit “two” foot switches, instead of “one”, and that's twice as time consuming, and clumsy.

~~~

I'm giving up a combined third stage in the clean boosts, and I'm very much ok with that. Question, can I add a (good quality?) line out to the unit later? Or is it best to add it when building it?

If it does not cost much time or money to add a quality line out, then cool, I'll add it, but I don't really have much plans for recording or re'amping.

I mostly just need a transparent power attenuator. So I'd prefer saving the money now, if I can add a line out later, just plan for it ahead of time, and leave enough space. Getting closer to design finalization!!!


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## JohnH

-7db is best as stage 1. Can probably squeeze it another db to -6 but needs checking out.

With the added foot switched stages, we don't really have a boost, just more cuts. Just need to be sure that its loud enough. I think it would be with a 50W amp going in.

So let's say the first stage can be -6db. That defines the loudest lead sound. A 50W amp is now like a 12W amp, still pretty loud, or 10W if Stage 1 remains at -7. Then, if your rythym volume is another 7 or 6db less, it is at 2 or 2,5W. Fine for home use, jamming, recording etc, but if you were giggiing, it might start to be limiting?. What do you think? There's always PAs though...


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## JohnH

As for line out, actually it's just a jack a pot and a resistor. Parts <$10. It's just easier to drill a case all before anything goes into it.

I don't use a line out, despite my tones, I Mic the cab since I don't need a silent solution and don't have the cab-sim or IR needed.


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## Dwayne Eash

Is it ok to change the stage sizes? If so, then..

First stage, speaker emulation
4 dB always on

Second and third stages, live performance
2 dB on/off foot-switch with LED
4 dB on/off foot-switch with LED, and another master on/off foot-switch with LED

Last stages, set and forget
2 dB of/off
4 dB on/off
8 dB on/off
12 dB on/off

For a total of 36 dB attenuation, including two clean boosts, and just 10 dB's of attenuation usually on. But I want to know what is the best dB value, for the first stage of speaker emulation, before I finalize my plans.


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## Dwayne Eash

JohnH said:


> -7db is best as stage 1. Can probably squeeze it another db to -6 but needs checking out.
> 
> With the added foot switched stages, we don't really have a boost, just more cuts. Just need to be sure that its loud enough. I think it would be with a 50W amp going in.
> 
> So let's say the first stage can be -6db. That defines the loudest lead sound. A 50W amp is now like a 12W amp, still pretty loud, or 10W if Stage 1 remains at -7. Then, if your rythym volume is another 7 or 6db less, it is at 2 or 2,5W. Fine for home use, jamming, recording etc, but if you were giggiing, it might start to be limiting?. What do you think? There's always PAs though...


I asked two things about the first stage, what is the least dB's, and what is the best dB's, for the first stage, not just what is best for that stage. So I have to wonder, how much sonic difference does it make, if it's more like 4 or 5 dB in the first, speaker emulation stage. Please let me know the least, and what the sonic sacrifice would be.

~~~

When I said "clean boost" for the last week or so, I never meant just a normal power boost, but switching off an attenuation stage. Talk about a case of miscommunication.

I've shared this idea with you, several times, at length, and since the beginning, about using two of the attenuation stages functionally in reverse as "clean boost" stages. I did this several times.

And I am passionate about it. And yet, you just acted like, you don't recall. Or that maybe suddenly I just don't know what I'm talking about, so you verify if I know the difference between a clean boost, and power attenuation.

Perhaps this just means, you don't really have time to remember what I have repeatedly said. No big deal, we are all human and make mistakes.

I'll try to be more clear in the future. I will illustrate for you, so no one will wonder. If you stomp the pedal, and the sound is reduced, that is attenuation, and if you stomp it again, it's clean boost. Clear as mud?


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## Dwayne Eash

You said 7 is best, maybe 6, and I think someone built one that worked that used 3.5 as the first stage, and you did not yet say what is the minimum acceptable, or maybe you partly answered that, not sure.

My design needs to get the combined first three attenuation stages, down to a lower number, even below 12 dB, and I've already reduced the clean boosts more than I wish to.

The speaker simulation dB's is the only thing left. So I wish the highest dB reduction for the speaker emulation stage, to be 5 if possible.

Speaker Emulation stage
5 dB always on

Live clean boost, 2nd and 3rd stages
2 dB foot switchable on/off
4 dB foot switchable on/off, and a master on/off switch

Last stages, set and forget
2 dB on/off
4 dB on/off
8 dB on/off
12 dB on/off

Features 37 dB of total attenuation, two foot-switchable clean boosts, and 11 dB "always on" attenuation.

Please expound the reasons for wanting higher dB in the first stage of speaker emulation. I want both best in transparent sound quality, and two usable clean boosts. I realize some compromise is likely called for.

What is the biggest down side, of not using 6-7 dB for the first stage?


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## JohnH

There's a lot of numbers to work out there, with 7 stages and all values different to what I've designed before. I understand what you want, it may not be possible but I'll get back to you when I can.


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## Dwayne Eash

JohnH said:


> There's a lot of numbers to work out there, with 7 stages and all values different to what I've designed before. I understand what you want, it may not be possible but I'll get back to you when I can.


On behalf of tone hound guitarist's everywhere, THANK YOU! This is kinda like a dream come true.

Sure, I could start out with just one clean boost and maybe that will end up being part of the compromise, but without a continuously variable volume control knob involved, adding more refined controllable options for each part of the unit, is very much desirable.

And I added one more stage to the "set and forget" attenuation batch, to allow more refined step of attenuation level options. I'd rather pay for the extra attenuation stages, to have more user selectivity, kinda like more closely emulating a variable resistor rheostat.

There is a very good reason why almost every single popular power attenuator has a continuously adjustable rotary knob on it, yet almost strangely, yours doesn't have that level of refined adjustment, which is a very real bummer. Hence the added attenuation choices, are nearly obligatory.

You deserve a cyber truckload of the best woman and ganja available! If I sincerely get two clean boosts out of this, and the entire unit is pretty darned tonally transparent through most of the settings, then this is destined to be one of the coolest guitarist DIY projects ever. I seriously need this in my life.

Godspeed, lightening and thunder, and off we go!


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## JohnH

Still looking at numbers.....

But meanwhile, I thought about the footswitches, and I think the functionality you need will be easier to do without relays. The final two resistor stages and and both foot switches can be in the stomp box. All we need to link that all back up to the main box is three wires, which can all be in one cable, easily made from a fairly light 3 core mains flex. No need for relays, relay sockets or power supplies except maybe for the leds. 

Standard footswitches used in stompboxes are rated (based on some I've bought) at 4Amps a.c., and you get three sets of contacts per switch, You could use one for the LED and two ganged together for the signal. 

The speaker plugs into the main box up near the amp, and if the stomp box is not plugged in, then it defaults to whatever the main switches are doing. Plug in the foitswitch and it gets inserted at the end of the circuit before the speaker, taking off more db's, or not, depending how it's set.

LEDs could run off a battery in the footswitch box. No big problem if the battery gets low, it's only doing Leds But, you could also add a 2. 1mm socket at the foitswitch to feed a bit of dc to it from a nearby pedal board daisy chain if you have one. Very little power needed for that.

I don't think there are any interference downsides with this, and likely less that from switching relay coils


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## Dwayne Eash

JohnH said:


> Still looking at numbers.....
> 
> But meanwhile, I thought about the footswitches, and I think the functionality you need will be easier to do without relays. The final two resistor stages and and both foot switches can be in the stomp box. All we need to link that all back up to the main box is three wires, which can all be in one cable, easily made from a fairly light 3 core mains flex. No need for relays, relay sockets or power supplies except maybe for the leds.
> 
> Standard footswitches used in stompboxes are rated (based on some I've bought) at 4Amps a.c., and you get three sets of contacts per switch, You could use one for the LED and two ganged together for the signal.
> 
> The speaker plugs into the main box up near the amp, and if the stomp box is not plugged in, then it defaults to whatever the main switches are doing. Plug in the foitswitch and it gets inserted at the end of the circuit before the speaker, taking off more db's, or not, depending how it's set.
> 
> LEDs could run off a battery in the footswitch box. No big problem if the battery gets low, it's only doing Leds But, you could also add a 2. 1mm socket at the foitswitch to feed a bit of dc to it from a nearby pedal board daisy chain if you have one. Very little power needed for that.
> 
> I don't think there are any interference downsides with this, and likely less that from switching relay coils


No, I do not agree. I wont run the signal chain any further than it needs to. Your version is sorta asking for noise issues I wish to avoid. This is probably more true with higher powered speaker cables, but short speaker cable runs are a first rule priority, because no one likes a noisy guitar rig, no one. And long speaker runs is a great way to get unwelcome noise.

It all needs to be housed in the unit by the amp with some sort of remote control foot switching. And I want three foot switches, one for each stage, and one master for them both. Live, I would mostly use the master footswitch, and just use the other two as selecting the level amount, 2, or 4, or 6 dB boost.


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## Dwayne Eash

I don't mind paying extra for clean electronic switching. I apologize for my demanding tone hound ways, but purity of signal and tone, is a must. I can't approve of long speaker runs on purpose, that's out of the question. Jack up the price for the proper electronic switching, absolutely.


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## _Steve

Dwayne Eash said:


> I don't mind paying extra for clean electronic switching. I apologize for my demanding tone hound ways, but purity of signal and tone, is a must. I can't approve of long speaker runs on purpose, that's out of the question. Jack up the price for the proper electronic switching, absolutely.



Hey Dwayne, why dont you just build what you have in mind and report back your results?


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## Dwayne Eash

JohnH said:


> Still looking at numbers.....
> 
> But meanwhile, I thought about the footswitches, and I think the functionality you need will be easier to do without relays. The final two resistor stages and and both foot switches can be in the stomp box. All we need to link that all back up to the main box is three wires, which can all be in one cable, easily made from a fairly light 3 core mains flex. No need for relays, relay sockets or power supplies except maybe for the leds.
> 
> Standard footswitches used in stompboxes are rated (based on some I've bought) at 4Amps a.c., and you get three sets of contacts per switch, You could use one for the LED and two ganged together for the signal.
> 
> The speaker plugs into the main box up near the amp, and if the stomp box is not plugged in, then it defaults to whatever the main switches are doing. Plug in the foitswitch and it gets inserted at the end of the circuit before the speaker, taking off more db's, or not, depending how it's set.
> 
> LEDs could run off a battery in the footswitch box. No big problem if the battery gets low, it's only doing Leds But, you could also add a 2. 1mm socket at the foitswitch to feed a bit of dc to it from a nearby pedal board daisy chain if you have one. Very little power needed for that.
> 
> I don't think there are any interference downsides with this, and likely less that from switching relay coils


Just to clarify, you are doing great. I am so happy to have found this project! I held out from buying a Weber MASS for MANY years. And so you are helping to make this looooooooong wait, worth while!!! So I am very happy with your help and this project.

I want it to be what I need and most of what I want, especially for such a low DIY price. This is sooo cool, I love it. I will post videos and sound clips, I will also show off the other attenuator I just purchased! It's a magnetic speaker attenuator, from Eminence Speakers, the British ReignMaker. It handles 75 watts, and can attenuate the sound to about in half, or nearly 9 dB's.

They simply move the spring magnet further away, so the voice coil becomes less and less efficient at creating sound at the same input signal power. So it's simply letting the speaker, push softer and quieter as much as you wish until half volume.

But I need this project, to bring back to life, my pair of greenback clones, which I love how they sound! So here's "hoping" we finalize fairly soon. The big problem is that I also need to purchase, or build a 2x12 cab. I have the tools to build it the unfinished box. But not to build it well, like with dove tale joinery.

I'm hoping to order all the parts by early next month, IDK. In any case, I am really looking forward to this project. And will let me use my greenback clones without worrying about blowing them out. They've served me well for years and years!


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## Dwayne Eash

_Steve said:


> Hey Dwayne, why dont you just build what you have in mind and report back your results?


Yes, I wish I could or I would start on my own. But I need the parts list first. My design is a bit different than normal. And JohnH is willing to verify the correct parts for me. I have no experience replacing diodes and resistors and all that. But I can put it together as DIY kit project, no problem.

I plan on providing detailed results, audio clips, and possibly a video also, IDK. I am a big fan of reactive loads being more natural to the amp ever since I heard about Ted Weber's reactive load. But I never had an extra 300 for it, bill collector  always on my back. So I was always hoping to find a deal instead. This is the deal. !!!

Mine has 2 clean boost stages, included as footswitch operated, so it's not just a power attenuator, it's also a double stomp, triple setting, clean boost pedal, with 2dB, 4dB and 6dB clean boost options. So I essentially get two devices, in one DIY unit! Very nice.

World class transparency, since it's IS the amp making the sound the entire time. Perfectly transparent boosts (other than is slightly muffled or blanketed by the use of a power attenuator).

So far tho, this is one of the most transparent attenuators I've heard of, and it has no tone compensation, presumably because it's not needed.


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## JohnH

Hi Dwayne

The problem that we have is that this switching system that you describe has become very complicated with all the relays, footswitches LEDs and maybe other electronics parts to control and power the relays. The only way to develop it is through prototyping it and testing. Its out of my range and I cant come up with a design and just tell what all the parts are, id have to build it and test it and change it. Id probably get there in the end but its just not something that I can take on. it would then depend on using specifically the same parts , and when you built it, youd have to figure out and troubleshoot any problems. That why I was trying to suggest something simpler that you could build and get going, and so it looks that that would not include footswitches.

I have gotten somewhere with resistor values for a -5db first stage, and then smaller steps switched down from that. Here's the results of calcs Ive done:




Red is represents the full volume speaker, its a G12M assumed but in use, it will follow whatever the actual speaker is, as will the lower traces. then you can see the first stage coming in at 5db lower, then fairly even 2 db steps below that. This gets down to -19db, but only three switched stages are in the calc, -2, -4 and -8. There'd be another one too which could be -12 or -16, to drop the whole group down lower.

So if you want to build something based on that I can provide the parts list, then you could experiment with working out and adding footswitch systems later on your own work bench.


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## Dwayne Eash

JohnH said:


> Hi Dwayne
> 
> The problem that we have is that this switching system that you describe has become very complicated with all the relays, footswitches LEDs and maybe other electronics parts to control and power the relays. The only way to develop it is through prototyping it and testing. Its out of my range and I cant come up with a design and just tell what all the parts are, id have to build it and test it and change it. Id probably get there in the end but its just not something that I can take on. it would then depend on using specifically the same parts , and when you built it, youd have to figure out and troubleshoot any problems. That why I was trying to suggest something simpler that you could build and get going, and so it looks that that would not include footswitches.
> 
> I have gotten somewhere with resistor values for a -5db first stage, and then smaller steps switched down from that. Here's the results of calcs Ive done:
> 
> View attachment 73971
> 
> 
> Red is represents the full volume speaker, its a G12M assumed but in use, it will follow whatever the actual speaker is, as will the lower traces. then you can see the first stage coming in at 5db lower, then fairly even 2 db steps below that. This gets down to -19db, but only three switched stages are in the calc, -2, -4 and -8. There'd be another one too which could be -12 or -16, to drop the whole group down lower.
> 
> So if you want to build something based on that I can provide the parts list, then you could experiment with working out and adding footswitch systems later on your own work bench.


Right on. I don't mind doing the project one step at a time. I can develop the foot-switches later. I will not give up until two of the finest stages become footswitchable. 

Until then, I'm happy with a very transparent attenuator. 

The graph doesn't really tell me much, right, just the nature of the steps of attenuation involved, and I'm already aware of the gaps involved, so I'm not sure why the graph was presented. I assume that's dB's over something, IDK. 

I'm glad the 5 dB request, for the speaker emulation circuit, seems to be working out. Do you think it will perform as good as a 7dB version?


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## Dwayne Eash

JohnH said:


> Hi Dwayne
> 
> ...
> 
> Red is represents the full volume speaker, its a G12M assumed but in use, it will follow whatever the actual speaker is, as will the lower traces. then you can see the first stage coming in at 5db lower, then fairly even 2 db steps below that. This gets down to -19db, but only three switched stages are in the calc, -2, -4 and -8. There'd be another one too which could be -12 or -16, to drop the whole group down lower.
> 
> So if you want to build something based on that I can provide the parts list, then you could experiment with working out and adding footswitch systems later on your own work bench.


Hey John, I want to do that. However, for some reason your graph shows 8, not 7 attenuation stages, plus red which is not attenuated.

So to clarify, I want the first stage to work great, kinda close to best design specs, but I seek a lower value in dB's than what you have been talking about, but without ruining the speaker emulation effect. I can't accept it if it's half as good. I hope it's only a small change, like 1/8 or so.

What I would loose, in terms of sonic transparency, or tone sacrifice, if it was just 3.5 db for the first stage?

If it's only a minor issue, like perhaps 10-20% less accurate, but still works well and sounds nearly as good, then I would want the first dedicated speaker emulation stage, to be,, 4 dB attenuation!

But if it needs closer to 6-7 dB's to work decently and works best at 9, then probably 5 is what I'm willing to try at first, and then if it doesn't perform well enough in the emulation circuit, I can always replace it with a larger value, until it works well enough.

I think I'd like to complicate my design, to include 3 and 5 speaker emulation stages. Because, if we can make a good enough emulation stage at just 3dB's, that would seriously make me happy.

So until I get answers on the 3 dB emulation stage suggestion, I figure it's the following attenuation stages.

dB . . state . . . . type of attenuation stage
5 . . . always on - speaker emulation (*PLEASE make this 4 if possible!)

2 . . . default on - (eventually) user selectable during a song/performance
4

2 . . . default off - set and forget
4
8
12

For 36* or 37 dB total attenuation, 10 or 11 dB default attenuation.

This design allows for yet-future foot switching added, for two clean boost stages, which offers three combined boost levels, 2 (on/off), 4 (off,on) and 6 (on/on). I seriously still wish the first stage was lower than 5 dB.


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## JohnH

On that graph, each trace is calculated based on a set of component values. What we look for is that the attenuation steps are even, and also tbat each trace is close to being the same shape, which they are pretty close showing tbat the tone is likely yo be close to being constant. I also look at these traces overlaid, to see any differences. 

Then I check these out with different assumptions about the real amp output imoedance, which happens with different amps or if a given amp gets driven hard.

This is where going to -5db starts to result in a bit of divergence in tone, not much, less than a db, whereas the ideal front end of -7db seems to keep better control in a wider range of scenarios.

i think the -5db will be fine but I don't want to put up a design with less than -5db as a first stage.

In any case as I understand it, you have a 50W amp and don't play out? At -5db you still have 16W which is more than loud enough for most gigs anyway.


----------



## SnickSound

In all honesty, from my experience, any scenario where an attenuator is wanted or needed rarely benefits from just 2dB of attenuation.

It's a surprisingly small volume difference.


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## Gene Ballzz

SnickSound said:


> In all honesty, from my experience, any scenario where an attenuator is wanted or needed rarely benefits from just 2dB of attenuation.
> 
> It's a surprisingly small volume difference.



Some honest facts right there. Even a -7db reduction is rarely enough of a difference to stop a club owner from complaining!

Just Sayin'
Gene


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## Trouble

I agree the 2 db and multiple steps just complicates a perfect design. 

Some of this stuff over the last 4 pages hasn't been very practical anyways 

Also you don't introduce noise through speaker cables, even if you run them with power wires.


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## Trouble

John I have been a huge fan and have followed this thread since nearly the beginning, I have always been amazed and learned so many things in the written words and pictures. I have to say.......You are a godsend and a total genius. 

Instead of talking about building an attenuator I just did it. You have dedicated so much time to this, I feel It would be disrespectful not to build one. So here it is, small simple and perfect. 

it was a hard decision which amp goes first, But as soon as I plugged it in I knew from the first note It was everything everyone said it was in the first 48 pages of this thread. I have tried a couple amps now, I will likely have to build another for 100 watters. I like the idea of having different Ohms outputs that you have talked about already. 


Thank you so very much................. 
Travis


A few Pictures.


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## JohnH

hey @Travis398 that's great, made my day!

A nice clean open SIMPLE build, with plenty of air.

Love the name, so does it have any other hidden features to help achieve that? (like, cooks dinner, remembers your anniversary etc?)


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## Trouble

JohnH said:


> hey @Travis398
> 
> A nice clean open SIMPLE build, with plenty of air.
> 
> Love the name, so does it have any other hidden features to help achieve that? (like, cooks dinner, remembers your anniversary etc?)



Hey you said SIMPLE build, all them other things require too much work......

although if you could design one for me......... ha ha


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## Trouble

I didn't realize the small resistors were going to be so small, I think I would get higher wattage next time. The terminals were pretty small for the wire.


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## Dwayne Eash

JohnH said:


> On that graph, each trace is calculated based on a set of component values. What we look for is that the attenuation steps are even, and also tbat each trace is close to being the same shape, which they are pretty close showing tbat the tone is likely yo be close to being constant. I also look at these traces overlaid, to see any differences.
> 
> Then I check these out with different assumptions about the real amp output imoedance, which happens with different amps or if a given amp gets driven hard.
> 
> This is where going to -5db starts to result in a bit of divergence in tone, not much, less than a db, whereas the ideal front end of -7db seems to keep better control in a wider range of scenarios.
> 
> i think the -5db will be fine but I don't want to put up a design with less than -5db as a first stage.
> 
> In any case as I understand it, you have a 50W amp and don't play out? At -5db you still have 16W which is more than loud enough for most gigs anyway.


Right, it's a 50 watt amp but later I might play out. And wrong about my default attenuation, because your right about the always on, but your wrong about usually on, which includes the 2nd and the 3 stages.

So usually on is 11 db's, baseline attenuation, not 5 dB as we both wish it was, but since I appreciate clean boosts, I differ accordingly.

You see, if I feel that less attenuation is better, I can give up one of my two clean boosts, and that brings the total dB's down to either 7 or 9 dB's. At least I would still have one clean boost left and the sound would be less squished.

This is why we have been wanting for the first stage to go even lower. It purely rubs me wrong to end up with 37 instead of 36 dB's which is resolvable down to 3. I sorta love it when 3 is involved in my designs. We were so close if only we could have gone to 4 dB reduction in the fist stage of speaker emulation.

For the last time, what would I loose when going lower??? I need to know what, or else I will experiment using 4 instead of just 5 in the circuit! Because it might be good enough! PLEASE TELL ME, I'VE ASKED LIKE THREE TIMES ALREADY.


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## Dwayne Eash

How about this, maybe I should include "two" speaker emulation circuits instead of just one. One would be 4 dB's and the other is around 6 or 7 dBs, probably 6, or whatever is optimum functionality, and yet also favoring a minimal value of dB's. 

Or would that cost too much?

That way, I can choose between a "proper" speaker emulation circuit, and a mostly / partly correct emulation circuit, sacrificing some sonic transparency, for a much needed smaller total dB startup or default attenuation. 

I honor your view and expert advice, and I also feel the clean boosts should be more central to such a great design. Or maybe just design it with one clean boost stage. To each their own. 

I happen to appreciate transparent clean boosts, as I love how my amp sounds better than any overdrive pedal or distortion. So for me, the best clean boost, comes naturally from the amp, so I am passionate about wanting transparent clean boosts.


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## JohnH

You have asked and I have explained. I am not offering any more options. The first stage loses the consistency of tone as we force it to be smaller db's. If you want the values for -5db I will PM them to you. I am not working out any more options, nor any designs that I don't believe in. I am not going to work out a -4db option.


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## Dwayne Eash

JohnH said:


> You have asked and I have explained. I am not offering any more options. The first stage loses the consistency of tone as we force it to be smaller db's. If you want the values for -5db I will PM them to you. I am not working out any more options, nor any designs that I don't believe in. I am not going to work out a -4db option.


So was I mistaken in the understanding that someone built your design using a 3.5 speaker emulation stage??? I listed that as an example for why going lower in size for the speaker emulation stage, has been tried and apparently they didn't complain much about it not working right, so I assumed it worked at least partly ok.

If I was mistaken about that, then I wish that would have been explained a long time ago, as I have been asking about this for several times and I think the point deserves an answer instead of holding unanswered presumptions against me.

And I already got the general idea that tone is sacrificed by going smaller in size. I never asked you to believe in my design, I just asked to see if it can be done. Some people don't appreciate what others do, and I accept that you apparently don't appreciate some things like I do.

Maybe I should not have followed this project as closely as I did, and asked questions about it so I could understand, because of the way some of my questions were overlooked and not answered. I still do not know if I was correct in assuming someone did make your design using 3.5 dB's of attenuation for the speaker emulation circuit. And that presumption was crucial to my quest for lower dB's in the first stage, but you didn't touch that, so I have been left wondering.

That is why I have repeatedly asked what is lost if we go lower in the first stage dB's of attenuation, besides some tone losses. If it's going from 3% tone loss, to 9% then I might be ok with that trade off, but if it's like going from 3% tone loss, more like 30%, then no way, that's way too much tone loss.

So how much tone is lost, is an important question when considering engineering trade off's for a smaller dB attenuation first stage.

If it's too much of a bad thing, I'm good with learning whatever the answer actually is, but not answering is not teaching is not learning, is not my fault because I asked, so please don't blame me for not knowing and for being interested in replicating your project my way.

I guess you don't care about that either... Sorry if I offend you for caring about your project in a way that you don't BELIEVE IN. I had no idea I had to sell belief in my version of your project. Are you saying everyone must think about your project like you do or it's not something to believe in? That's an odd way of addressing someone who lavishes praise on your work. You don't believe in what I'd like to do with your project. That is a curious notion.


----------



## Dwayne Eash

R U just jealous because my idea of foot switchable clean boosts functionally doubles the value of your project, but I thought of it and you didn't? Just kidding, but you seemed to have taken this discussion to a different place.

Because I must have asked about the trade off for going lower in dB's several times, including the 3.5 example, that I did not ask to go even that low!

So if someone else was fine with building your design using 3.5 dB's at the first stage, then it's eminently reasonable to ask the question, how much tone is lost when going down to a higher level than 3.5, like say 4 or 5.

Because you never answered about my presumptions that 3.5 was already done, maybe it was, maybe it was not, thus I have zero knowledge to go off of, other than my cloudy presumption I asked you about several times and usually not no response on the 3.5 dB version. If you had just responded to that repeated comment slash question, then I would not have pursued what should not be pursued.

I am just struck by the fact that my asking to replicate your project, is met by disapproval for not already knowing what is or is not acceptable over it. So many pages, so many graphics, M version, M2, M3 M4 but which is the correct updated version... IDK. Some are probably just differences in the number of output jacks, so that's a natural amendment.

You know your own project the best, that's why we ask the questions that we ask. I could ask the same question 4 or 5 times, but as some point, asking should no longer be viewed as the problem to not be believed in. But as other humans who are genuinely interested in your awesome project.

If you had answered my question a week or two ago, probably none of this would have happened. Maybe the 3.5 version of this project is top secret,, IDK. 

But I'm surely wrong for asking..


----------



## JohnH

Dwayne. So I'll chat with you further in PM, but not here anymore. This discussion is filling up this thread too much.

Nobody that I know of has built a reactive front end less than the -7db of the base designs. I know you wish for less, and I have worked on your request from last weekend:



> The speaker simulation dB's is the only thing left. So* I wish the highest dB reduction for the speaker emulation stage, to be 5 if possible*.
> 
> Speaker Emulation stage
> 5 dB always on
> 
> Live clean boost, 2nd and 3rd stages
> 2 dB foot switchable on/off
> 4 dB foot switchable on/off, and a master on/off switch
> 
> Last stages, set and forget
> 2 dB on/off
> 4 dB on/off
> 8 dB on/off
> 12 db on/off



And working that out was several hours work. And I can see the tonal control reducing at -5 db so -4 will be less good again so I'm not going there. I'm out of time, I'm not designing any more. If you still want this one with -5db front end, send me a PM.


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## LordoftheLivingRoom

On a positive and uplifting note, I've finally received all the parts to being building a 16 ohm M2.

The enclosure I bought is steel with back plastic front and black panels (aliexpress). I've had to get creative with the inductor mounting due to the steel enclosure, and I think I've come up with a good solution. Cable ties and silicon adhesive will fix the inductor in place. Speaking of which, if you are in Australia and you want air core inductors, try Speakerbug. Luckily for me he's only a few minutes drive form my place, but he's still cheaper than ebay or aliexpress, even when you include shipping. Nice guy, good service.

I'm staying true to the standard M2. -7dB standard, no bypass. Wiring the attenuation switches in a BCD style layout (reminds me of octal programming... 

I'm very thankful for your work Mr. JohnH.


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## Dwayne Eash

You can PM me if you want to know what it feels like to be not believed in and blame-shifted against.


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## SnickSound

Dwayne Eash said:


> You can PM me if you want to know what it feels like to be not believed in and blame-shifted against.



WTF does that even mean? 

John has done nothing but try to help you, on his own time, but you won't accept his answers because you are looking for a does-it-all device with minute changes in volume that anyone with experience with an attenuator knows is wasted. He told you why less than 7dB first stage is not recommended but you just read past that and keep pushing his buttons.

This is just a fun thread for a fun project that people can do at home at minimal cost. John is not your personal head of engineering. 

And thank you John for all your help in this thread. Feel free to ignore this person in the future we will understand.


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## Dwayne Eash

SnickSound said:


> WTF does that even mean?
> 
> John has done nothing but try to help you, on his own time, but you won't accept his answers because you are looking for a does-it-all device with minute changes in volume that anyone with experience with an attenuator knows is wasted. He told you why less than 7dB first stage is not recommended but you just read past that and keep pushing his buttons.
> 
> This is just a fun thread for a fun project that people can do at home at minimal cost. John is not your personal head of engineering.
> 
> And thank you John for all your help in this thread. Feel free to ignore this person in the future we will understand.


I accepted his answers until he blamed me for him not answering my question about the 3.5 version of his design, a question that was posed in different ways several times and he never answered that part of the question but then blamed me for seeking the answer to it. 

I NEVER showed the man one iota of disrespect, then I find out, he's not been believing in me and my project, thus exposing why he just ignores some of what I say to him, only to have him blame me for it. If you go back and read what was said, you would not have this question, or false view.


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## JohnH

I'll just say this. What I meant was, I don't want to offer any design of mine that I don't believe would work well. There's no problem having a vision of what you want, that's how we get new ideas, But when it is found that the engineering can't deliver it properly, then I don't want to put it out there. It wasn't about beleiving in your idea.


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## Dwayne Eash

So he's the reason I wrote so much asking for the same information repeatedly. If he doesn't want extra posts on his topic, them maybe he should not blame shifting and start answering clear and repeated questions of central importance. 

I also did not want to fill up his thread with my endless questions. That was not purely my fault but him avoiding my project which he does not believe in, HIS PROJECT Im replicating with HIS help, HE doesn't believe in that. 

That is not my doing or fault, so stop blameshifting against me, your own issues. I asked about the 3.5 build probably 3 different former times, and he never said, I know of no such design or build, yet that was the presumption going on in my head that it might work partly well enough, but maybe not ideal. IDK until someone clues me in, and so I asked about that, but I never got any answers on it, just the same old generalization that less dB's in the first stage is a bummer on the tone. 

Right, that's was not the question, the question was, how much of a bummer is it, like if you don't go quite as far as 3.5? He never ever touched that question and I asked it possible four times as it was central to my project. So to blame me for too much text and asking for what has already been answered, is simply not true. 

Did we have a communication problem, obviously, but to turn it into such a personal affront, seems petty and unproductive. And in my case, simply not true. I am on other forums and other threads here, bragging about this project. 

So you don't believe in it. WTF is that??? Is what should be asked. Im trying to do your project, and the guys is like, nope, I don't believe in it. 

I suggest people stop getting triggered so easily. But who am I, one not to be believed in!!! LOL


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## Dwayne Eash

JohnH said:


> I'll just say this. What I meant was, I don't want to offer any design of mine that I don't believe would work well. There's no problem having a vision of what you want, that's how we get new ideas, But when it is found that the engineering can't deliver it properly, then I don't want to put it out there. It wasn't about beleiving in your idea.


Ok, thanks for clarifying.


----------



## JohnH

LordoftheLivingRoom said:


> On a positive and uplifting note, I've finally received all the parts to being building a 16 ohm M2.
> 
> The enclosure I bought is steel with back plastic front and black panels (aliexpress). I've had to get creative with the inductor mounting due to the steel enclosure, and I think I've come up with a good solution. Cable ties and silicon adhesive will fix the inductor in place. Speaking of which, if you are in Australia and you want air core inductors, try Speakerbug. Luckily for me he's only a few minutes drive form my place, but he's still cheaper than ebay or aliexpress, even when you include shipping. Nice guy, good service.
> 
> I'm staying true to the standard M2. -7dB standard, no bypass. Wiring the attenuation switches in a BCD style layout (reminds me of octal programming...
> 
> I'm very thankful for your work Mr. JohnH.




Looks good and thanks for the tip about the coil. The place where I bought them for my two builds in in Queensland, but don't list them anymore. But for another Australian link, I've been noting Wagner online in Sydney for next time.

And your coil saddle looks like a good idea, especially with a steel case since it looks like it keeps the axis of the coil away from the ferrous metal of the case.


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## Dwayne Eash

I was going to call it, the proper speaker emulation circuit, and the partial one. That way no one would be confused as to the partial one was the one you meant as the proper one. It's designed as a not really right version, but it's smaller in size.

That's why it would not sound quiet as good. However, in practice, because we are talking about the modest end of attenuation, and this change makes it even more modest, and so the amp itself actually sounds better when you attenuate less. So in fact, and in practice, it's partly not as good sounding, and it's partly better sounding, both at the same time, such that it's likely that few might be able to notice this sonic difference. 

Frankly, you mostly notice this sort of issue, the more you attenuate, but this issue, does not require heavy power attenuation, and that is not the purpose for this design, so it seems you are concentrating, on what I am not wanting to do. Which I guess is your free will option, if that is your preference.

At the minimum, the idea was worth investigating. However, I do not wish to impose on others, or ask too much. That was never my intent. My personal interest demonstrated, and the quest to learn more, was sincere. I never changed loving this project and wishing to make my own..

.. Until I was welcomed to be rejected, for something I did not do, but felt forced to be a part of, our soggy communications drug out for sooo long, because my questions were sometimes repeatedly avoided, because of a lack of belief. I did not realize I had to connect with you spiritually, or on a one on one belief system of share values or something. hehe

That can not be held against me, for not answering my questions, clearly and repeatedly offered. But, if you must hold that against me, be your guest. I don't push myself on others. Sorry for sincerely asking. It was because I truly wanted to know. But if you are offended by my inquiry, then sadly, I accept it.


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## Dwayne Eash

And it can not be said I asked too much as I always qualified wishes that I am most passionate about, with, but, if I must make due with only one boost stage, then so be it, in order to make the default on attenuation stages, low enough to ensure better sound quality options.

We don't want to put out a design that is attenuated so much that although it does what it says, it's quieter, but it starts out sounding kinda damp in tone, and it just gets worse. No, we prefer, it starts out a little quieter, but the tone is still quite natural and good.

So, even though so far you have been allowing me two clean boost stages, that luxury, increases the "constantly on" dB's of attenuation, which functionally trades tone purity, for attenuation, and if you do that too much, the attenuator could get a reputation for, yes it works, but it's kinda drab on the tone, and we don't want that.

I feel bad if you spent so much extra time, and yet if all were told, perhaps some of that was spent on things not really resolved upon between us, or because of "disbelief" in something I was never told required belief in.

I agree that you do not want to put out something that would not work well, but so far, I don't think I'm asking that. I pretty much always said, if my wishes or plans do not work out, then ok, I defer to your expert understanding. I had to demonstrate this to you, and you accepted it. 

~~~

You don't have to answer me if you don't want to. And you can listen to others who welcome you to reject me, if you want to. I am not here to push myself on anyone. I didn't realize there is a personal aspect, involving unspoken personal beliefs, concerning getting straight answers to repeated questions, about replicating this project, we all love, or are passionately interested in.

I find it "amusing" that personal belief, ever was considered,, you judging my person, and discovered that I am not worthy of your cooperation with replicating your project.

I realize and openly acknowledged that I was likely pushing the envelope, and I even though you wrongly assumed that I was not respectful enough of your expert advice, so I showed you how how I already demonstrated that was not true. And you said something to the effect of, very well, and we moved on.

But this is not about me wishing to blame you, that is why I just kept repeating the same question instead of making a personal judgment. It is not for me to judge why you avoid some of what I say. That is on you. But you will not blame me for the extra text involved because you did not answer what I had thus to repeat and repeat.


----------



## Trouble

Dwayne It might be time to move on. Start your own thread about attenuators and you can discuss any unhelpful, Unrealistic, Complicated attenuator you want. 

This is maybe the best thread on the forum and you have added nothing. Perhaps go back and read this thread and understand what is going on here. Read the Title "Simple"

Despite your idiotic ideas and left handed compliments John has done nothing but help you, time and time again.
Every time he gives you something you don't like it or change your mind to something else.
Or you need proof by wasting his time making recordings for you to diss his amp, again not helpful, and not up to him to prove his design to you. If you don't like or appreciate what he has done here, more specifically what he has done for you over the last 4+ pages than move on.

Maybe if you hired an Engineer to do your designs he would be willing to design complex time consuming crap.


----------



## Gene Ballzz

@Dwayne Eash

Those of us who've been fighting the cranked tube amp being too loud battle for years (I've been a steadily gigging singer/gutarist for +50 years) have been through most of the same trials, tribulations, *"new"* ideas and schemes that you're cooking up. We've found what works and what certainly doesn't and are simply sharing our combined experiences with you. This has been an ongoing war since guitars first got plugged into amplifiers and got *"THAT SOUND!"*

One of the best additions to your rig involves how you use your guitar and in the long run boils down to developing very specific and controlled playing techniques and skills. One of those skills is hard earned playing/picking dynamics. I can plug a guitar with no volume and/or tone controls into a massively powerful amplifier cranked into its sweet spot and be *"almost"* whisper quiet and clean, with amazing clarity, tone and blissful harmonic overtones and tonal complexity by simply softly and judiciously *"controlling"* what my hands do on said guitar! I can then dig up a little dirt by *"digging in"* a tiny bit harder and then I can kill small pets and vaporize Rosemary's baby (she should know better than to bring a baby to a rock show, but that's another story) by really hammering on my guitar! Unfortunately, that blazing volume that gets produced is way more than most sane people (including venue operators) are willing to endure!

A bit more about dynamics. If we make a scale where *"0"* is silence and *"10"* is as blazingly hard as you can slam, many (MOST) folks keep their dynamic *"average"* (rhythms, etc.) somewhere between *"6"* and *"8"* and are then disappointed that slamming to *"10"* doesn't put their solo or other part *"out front"* in the mix. There's really not a very big, audible difference in *"volume/sound pressure level"* from *"7"* to *"10"!*

Now let's look at a more effective and beneficial way of controlling our volume. If a player develops the technique of keepin their dynamic average somewhere in the *"3"* to *"4"* range, dropping to *"1"* or *"0"* is still pretty dramatic and allows for nice dynamically apparent *"stabs"* into that *"6"* to *"7"* range, while still allowing blazing stings and/or solos at the *"9"* to *"10"* range. This is how many of the really great players have attacked this for years!

Now, I can certainly understand the attraction of stepping on some switch to make up for not wanting to put in the long hard hours of developing great techniques. The main purpose/intent of these great attenuator designs that @JohnH has worked so diligently and hard to develop and generously share with the world at large (for free) is to allow us crazy guitarsts and tone hounds to use larger than practical amps at sane and acceptable volume levels, without losing our precious tone, nuances and dynamic response to playing techniques! It also helps to tame our volume levels when our exuberance in the heat of performance gets the better of us! It does not seem that the intent was ever to turn these attenuators into some new stomp/foot/pedal/thingie/toy to try to make up for the hard, diligent practice required to become a truly stellar artist! Might I politely suggest that if stomping on a pedal to control your sound is what you're really looking for, maybe you should look into high end modeling amps and/or boutique stomp boxes!

For me, these designs have been the most liberating pieces of gear I've ever owned! I will never be without one and plan to build enough of them to end up having one permanently installed and custon tailored for each in every amp I own!

Just My $.02 & Likely Worth Much less!
Gene


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## Dwayne Eash

Gene Ballzz said:


> @Dwayne Eash
> 
> Those of us who've been fighting the cranked tube amp being too loud battle for years (I've been a steadily gigging singer/gutarist for +50 years) have been through most of the same trials, tribulations, *"new"* ideas and schemes that you're cooking up. We've found what works and what certainly doesn't and are simply sharing our combined experiences with you. This has been an ongoing war since guitars first got plugged into amplifiers and got *"THAT SOUND!"*
> 
> One of the best additions to your rig involves how you use your guitar and in the long run boils down to developing very specific and controlled playing techniques and skills. One of those skills is hard earned playing/picking dynamics. I can plug a guitar with no volume and/or tone controls into a massively powerful amplifier cranked into its sweet spot and be *"almost"* whisper quiet and clean, with amazing clarity, tone and blissful harmonic overtones and tonal complexity by simply softly and judiciously *"controlling"* what my hands do on said guitar! I can then dig up a little dirt by *"digging in"* a tiny bit harder and then I can kill small pets and vaporize Rosemary's baby (she should know better than to bring a baby to a rock show, but that's another story) by really hammering on my guitar! Unfortunately, that blazing volume that gets produced is way more than most sane people (including venue operators) are willing to endure!
> 
> A bit more about dynamics. If we make a scale where *"0"* is silence and *"10"* is as blazingly hard as you can slam, many (MOST) folks keep their dynamic *"average"* (rhythms, etc.) somewhere between *"6"* and *"8"* and are then disappointed that slamming to *"10"* doesn't put their solo or other part *"out front"* in the mix. There's really not a very big, audible difference in *"volume/sound pressure level"* from *"7"* to *"10"!*
> 
> Now let's look at a more effective and beneficial way of controlling our volume. If a player develops the technique of keepin their dynamic average somewhere in the *"3"* to *"4"* range, dropping to *"1"* or *"0"* is still pretty dramatic and allows for nice dynamically apparent *"stabs"* into that *"6"* to *"7"* range, while still allowing blazing stings and/or solos at the *"9"* to *"10"* range. This is how many of the really great players have attacked this for years!
> 
> Now, I can certainly understand the attraction of stepping on some switch to make up for not wanting to put in the long hard hours of developing great techniques. The main purpose/intent of these great attenuator designs that @JohnH has worked so diligently and hard to develop and generously share with the world at large (for free) is to allow us crazy guitarsts and tone hounds to use larger than practical amps at sane and acceptable volume levels, without lossing our precious tone, nuances and dynamic response to playing techniques! It also helps to tame our volume levels when our exuberance in the heat of performance gets the better of us! It does not seem that the intent was ever to turn these attenuators into some new stomp/foot/pedal/thingie/toy to try to make up for the hard, diligent practice required to become a truly stellar artist! Might I politely suggest that if stomping on a pedal to control your sound is what you're really looking for, maybe you should look into high end modeling amps and/or boutique stomp boxes!
> 
> For me, these designs have been the most liberating pieces of gear I've ever owned! I will never be without one and plan to build enough of them to end up having one permanently installed and custom tailored for each in every amp I own!
> 
> Just My $.02 & Likely Worth Much less!
> Gene


LOL, best post ever. You can paint it any fancy color you want, but you better play it well or it will not sound so good. hehe Might sound simple and trite, but it's nice to know others agree, the magic comes from within, and from your fingers, more than from a fancy stomp box or new digital effect.

I am for just me and the amp for the best in organic sonic ear candy. That's why a clean boost provided by the power attenuator, just releasing one attenuation stage, is transparent brilliance. You double the value of the project, with world class boost transparency, because "it's just your amp" the entire time.

It's just using, what is already naturally available, in a more convenient way is all. I LOVE foot-switch operation because I'm busy making magic with the hands. Sorta frees me up to do more, and stomp boxes for clean boosts is a match made in heaven.

Nice to meet another tone hound guitar purist.


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## Gene Ballzz

Oh & FWIW, at 65 years old, I play in a hard rock & rockin' blues band and still practice at 3-4 hours for every hour I perform! Playing guitar *WELL* is a life long commitment to a lot of hard work and not an act of instant gratification! Also remember that an audience will never notice the nuances of tone that we all obsess over, but finding that great sound is simply helpful to make us more comfortable on stage to in turn play our best and be inspired to do so. *THIS* is what an audiance really notices!
Just sayin'
Gene


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## Dwayne Eash

Gene Ballzz said:


> Oh & FWIW, at 65 years old, I play in a hard rock & rockin' blues band and still practice at 3-4 hours for every hour I perform! Playing guitar *WELL* is a life long commitment to a lot of hard work and not an act of instant gratification! Also remember that an audience will never notice the nuances of tone that we all obsess over, but finding that great sound is simply helpful to make us more comfortable on stage to in turn play our best and be inspired to do so. *THIS* is what an audiance really notices!
> Just sayin'
> Gene


Dig it man. No worries, I jumped a link to a new thread where I await John's answer to my question over my version of his design. We just got our signals crossed and pow, it's not linked up yet, so,,, it's all good. I can't wait to fire that thing up and let my greenbacks work again.

Until then, it's just my attenuation speaker from eminence. Not bad, but not a sweet pair of guitar and bass greenbacks either. So I totally agree, I need a power attenuator, to pull out the best in those old Marshall amps.

Holy cr@p, I turned it up to MV 6, and it was like, beam me up Scotty, feels like I have the ultimate power of the universe, at my fingertips, but more importantly, the lush singing overtones, and that's a nice,, way to treat your inner guitarist. 

You get in a zone, and the music, sorta transforms the moment, it's like your in a timeless space, where purity and life meet.

I feel more welcome at my place. So here's a link to see more about my project goings ons. What becomes of the first stage, determines what stays or does not stay in the second and third stages. So I respectfully wait for John's answer, over there to (once again) demonstrate respect for this topic.

http://www.marshallforum.com/threads/johnh-reactive-power-attenuator-replication.114015/


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## Martin56

This is a great design. I ran across it after sourcing parts for an airbrake, and made sort of a hybrid with this idea, the airbrake, and what I had. Very handy.

I wondered if one could add some flexibility to the key first stage, if one added a resistor. Here are the optimal values for an 8 ohm load, treating the amp as 20 ohms. While this confirms John H's good choice of 15 ohms for R1 and 10 ohms for R2, with standard value resistors, it also gives some other options.

Martin


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## JohnH

hi @Martin56 , thanks for posting. Understanding how Airbrakes work and how they don't get dull at low volume was a stepping stone for my designs.

Your sketch shows variations on a 'T' pad attenuator, which is a standard module in electronics (guessing you know that!). The variations do approximately -8, -12 and -16 db, and you keep it consistent so the amp always sees 8 ohm and the speaker always sees 20 Ohm. I have a spreadsheet that generates these too. I found that by accepting a variation of an ohm or so, the lower limit in db's reduction for R0= 0 can be brought down from -8 db to -7db or a bit less. 

Switching between your three versions needs switches on three legs, so that's why I started going for adding new switched stages after the first, so only one switch pole is needed per stage. Also, its hard to get the reactive coil or coils incorporated and properly balanced unless the first stage remains fixed.


----------



## Martin56

Sure thanks. I was only thinking of the first stage actually. The T as the first stage lets one keep the desired load at both ends for different values of dB reduction (but, yes, always more dB reduction in the first stage than 7 or 8dB), for the price of one more resistor. But your idea works great as is. Very nice. Martin


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## Dwayne Eash

Where are people getting the parts here in the USA? Seems like china has the supply for cheap, but that's a long shipping time..


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## Boysenman

Hi JohnH, thank you a lot for the schematic. I already tried your design in one of my amps with great success. 

I wish to create an attenuator, where the edcor 10W transformer in my 5f2a clone reacts the same way, as it does to the Weber 10A100 speaker installed. 

I've used LTspice, to recreate the impedance plot of your M3 design, along with series-parallel RLC circuit, simulating the weber speaker at the end of the attenuator. I've gathered the impedance plots of similar 10" alnico speakers, with a resistance of 5.6ohm, visually replicated the plot. It's no trouble to replicate the plot, but it's impossible also get a total impedance of 8 ohm. However i put the components, summing the two phasors will never yield 8 ohm and a good looking impedance plot. 

Also, your m3 design (and Aiken) has a treble slope reaching it's peak amplitude much earlier than my circuit, and similar impedance plots on the internet. What's the reasoning behind designen the circuit like that - It is better sonically?

Hope you understand my questions.

Kind regards
Christian Boysen


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## JohnH

hi @Boysenman Thanks for your message. I'd be happy to investigate adapting the design for 10" speakers.

May I ask what did you build before? ie which design?

Also, what network of LRC do you find best captures the 10" speaker? is that the one in the plot above?

The basic designs adapt quite well to different speakers and different amps, even without changing values. But if I remember right, typical 10" speakers have a lower coil inductance than similar 12" ones, so the L1 inductor might be smaller. Can you offer an impedance plot of a 10"?

I havnt built M3 myself and I don't know of anyone who has. But M and M2 have been built. The treble rise matches measurements from a 12" Greenback cab and the impedance hits around 8 ohms at about 400- to 500 hz, as with a normal speaker. It varies by part of an Ohm dependent on settings.

I see you are modelling the turns ratio of the OT. In order to capture the amp properly, at least at low volumes, it's output impedance also needs to be included, which I use as 20ohm for an 8 ohm tap, based on my own amp. Do you have any parameter included for that?

Inteteresting, I'm happy to explore further with you


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## Boysenman

Hi JohnH.

I posted similar pictures earlier in this thread, on a different account(that I forgot the password for). Heres the back panel of the previous build. A fixed bias 2xEl84 Marshall plexi/jcm800 style 3 channel amp with fx loop and your M2 attenuator.

The 10" speaker impedance plot, is the plot you see in green. It is the circuit you see down to the right in the schematic. Your impedance plot is shown in blue.
The RL series circuit simulates voice coil parameters, where the RLC parallel circuit simulates suspension resistance, suspension inductance, and mass/air lod capacitance, respectively. As for now I've only graphically tried to replicate the 10" speaker plots found on the internet. Next step would be to solve the equavalent 2nd order differential.

Please school me on the 20ohm impedance? The turns-ratio is 8K:8, and afaik I know, it should be reflected back to the primary by your M3 circuit? Primary resistance is 147ohm, and secondary is 0.5, as per datasheet specs by the way.

It should be possible to make a spreadsheet that computes all necessary component values, given basic speaker and amp parameters, for any type of amp and speaker combination.

I look forward to working with you.

Kind regards
Christian Boysen


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## JohnH

The real output impedance varies per amp, dependent on its design and how much NFB it has, and also how hard it is being driven. My calcs aim at getting the small signal tone right, and then showing the amp a reasonably close impedance so that when it gets driven, it responds as normal. its too hard to model the real over-driven performance (unless you build the output circuit into Spice, its possible, haven't gone there though). So we focus on the small signals. If a valve amp has no NFB, its small signal output tends to be like a constant current source. Double the load ohms, current stays constant and signal voltage is doubled. This is equivalent to virtually infinite output impedance. Compare to an SS amp, which has extremely low output impedance almost zero. If you double the ohms of the load, current halves, voltage stays the same. A real tube amp with some negative feedback is in between. The more NFB, the lower the output impedance (which has little to do with whether its designed for 8 or 16 ohm speakers.

This effective output impedance is vital to the tone and its ignored in most (dull sounding) attenuators. If the speaker sees a much lower output impedance than the real amp, its bass peak and treble rise cant develop, they are squashed flat.

You can work it out by feeding a small pure signal through the amp and measuring output voltage using two different resistive loads. Then you do maths. I have a DSL401 with no NFB and I work out very high output impedance with this, 50ohms or more. My VM gave me 20 Ohms, which has turned out to be a good value to work with. But I suspect other amps with EL34 outputs and normal NFB are less. But luckily the circuit adapts to them well.

im not sure about the plots of yours. The speaker model has a 1mH coil as its main coil inductance, and this seems very high compared to typical parameters for a nominally 8 ohm 10" speaker. Im also not sure about how LTSpice is plotting impedance. From first principles, at say 10kHz, this coil inductance dominates and its impedance should be around 0.001 x 10000 x 2Pi = 63 ohm, which is what I have for it using my spreadsheet with your model of the speaker.

I have a spreadsheet that analyse the whole circuit, and switches stages on and off cyclically with a macro. The maths is too hard (for me) to 'solve' to directly get component values, but it lets you quickly test and home in on a good design.


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## MAD64

Hi JohnH,
first of all - great work and thank you very much for sharing and supporting this!
I've built the M version and found, that 7 + 3,5dB ist best for my needs (I don't want to bring down my amps to bedroom level, half volume is great for my needs) and is also very transparent with my NFB and non-NFB amps. If I remember correctly, somewhere in this thread is a table with different values for the the first (L+R) stage to get different dampings but it ends with 9dB. Can you give me the values for the 2x L and 3x R (for 8 ohms tap and speaker) from your magic Excel sheet when I want that 10.5dB from one stage? And for 16 ohms tap and speaker (all values x2, if I understand it correctly), is it useful to add R11 and R12 from M2(v)?


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## JohnH

Hi @MAD64 Thanks for posting.

I think for a -10 5db fixed version, might as well just hard-wire a -3.5 stage after the-7db Stage 1 from version M or M2.

Those added R10 and R11 parts are great if you want to run a 16 Ohm cab from an 8 Ohm attenuator. It just fixes input and output impedance so the amp keeps seeing not too much more than 8 ohms and the 16 ohm speaker sees the right amount to keep its treble and bass correct. Without them, a 16 ohm cab gets noticeably less high treble It all safe, and fine if you want a softer tone, But I like the clarity. It works with either M or M2. I use this feature to run a 16 ohm V30 cab from my VM combo which doesn't have a 16ohm tap.

For a general 16ohm attenuator, yes all resistances and inductances double. But in practice, you have to adjust slightly to use available values.


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## MAD64

OK, will do as suggested, I just wanted to save on parts ;-)


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## JohnH

MAD64 said:


> OK, will do as suggested, I just wanted to save on parts ;-)



When I figured out the single-stage versions
(Ie no switching and designed to have a speaker straight after), I found that the configuration worked ok in the range -3 to -9db, by adjusting values. After -9 db, the output impedance as seen by the speaker was drifting outside of the target range, which would tend to start changing the tone. So it would need more parts anyway.


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## Boysenman

Hi Again JohnH, sorry for the late response. The way LTspice works, is simply plotting the step response of the circuit, with a sine wave sweep from 50 to 20khz (in my case). The Green picture you have seen, is simply the voltage across the speaker sim circuit at the end, divided by the current through the input resistor R6 to speaker sim

Again, I have not yet taken the time to actually sit down and meassure the actual values for the components, all I have done, is figuring out what component values looks most like the picture I've found on the web. This turned out to be with a coil inductance of 1mH, albeit that is very big. Smaller coil values, even when compensating the rest of the circuit, lowers the bass resonance, and gives a feeling of the bass-resonance being equally wider, due to the lower R11 values. 

Kind regards
Christian Boysen


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## JohnH

The plot is labelling itself in some kind of db scale, which suggests a logarithmic scale. is there a way to plot on a linear scale to represent Ohms?


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## Boysenman

Hi JohnH. With a little investigating, I figured out how to make it linear and plot ohms.

Blue curve is with 0.25 voice coil inductance, and no leak resistance
Green curve is with 0.7 voice coil inductance, and 33 ohm leak resistance

I have also included an original plot from a 10" alnico P10R style speaker.
Green plot seems to capture the visual representation a little better, but I don't know how much I should read into that.

kind regards
Christian Boysen


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## Baby Thomas

Hello, I've been reading this thread for months and I learned a lot. Thanks to all contributing with JohnH ahead of everyone. 

I built my version maybe two months ago and it's working great! My idea was to have simpler reactive only device with only 8ohm capabilities and 3 stages, but with the ability for one of the stages to be relay-foot-switchable for solo boost. For now this is not implemented yet (as well as the active cooling part) until I decide if I will use the device permanently. I never play with amps at home so I will only use the attenuator for rehearsals/live where I need to take off a few dBs and/or bump the MV 10-20% up without pealing the paint. I altered the stages so I have -3.5; -3.5; -7 dB and the first (bypass) stage is -3.5 which is fine for my needs.

I used it with a few amps (SLOClone and H&K Puretone for example) and working great. It's definitely better than Bad Cat The Leash (which I had recently but it's gone now) and much better THD Power Plate (which I had years ago). I'm not familiar with other reactive attenuators firsthand so I can't compare it with these devices. My device is based on Design M and honestly I can't hear ANY difference EQ-wise (which is not the case with the other aforementioned attenuators), however there's some... stiffness or lack of dynamics after the device is engaged. I'm not sure it's the attenuator though, maybe it's just the amp reacting different at higher MV volume levels. The change (if there's any) is minimal but I can't decide if I prefer the lower MV setting without attenuation. My "goal in life" is the purest signal path possible with maximum clarity and dynamics.


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## JohnH

hi @Baby Thomas thanks for posting and im glad that's working for you. 
And that is one bad-ass mutha of a heat sink! Does it heat up?


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## Baby Thomas

Sorry, I forgot to explain about the heat sink. This was the only size I was able to find that is big enough to fit all the resistors and is somewhat cheap at the same time. It's taller than I would want/need, but... this is it. It doesn't get warm *at all*, even after 2 hours of playing. The amp was with two KT88 power tubes, however the HT voltage is "only" 470vDC or so and the MV is at 30-40%. I didn't measure how much watts it can produce, but I don't think it will be more than ~50W of clean power. The attenuator was used like you see it on the last picture - without case/chassis which helps (I think). I went for the bigger wattage resistors so I can run safely 100W head (if needed) and both the resistor and the heat sink are polished and there's a thermal compound applied between them. If someone doesn't know - the power ratings of these resistors are valid only if they're mounted on a proper heat sink, without one their rating drops to maybe... 30%-40% of the max (mine are cheap Chinese resistors, I don't know if there are valid datasheets). I definitely went crazy with the cooling, but I prefer to have safe margin.


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## Boysenman

Hi Again JohnH. I'm sorry for coming up daft, and taking up your time like this. Would it be possible for you to send me the spreadsheet, or in any way point me in the direction of how to calculate the right parameters for the loadbox, for the small 10 inch speaker? When looking at a similar speaker, the eminence 1028K, they use a 6oz plug, meassuring 0.5mH of voice coil inductance. I understand why you thought 1mH was high. The Weber 10a100 uses a 7oz plug, so by a rough estimate, I'd guess 0.6mH would be adequate. The amp is a 5f2a as said, so the output impedance should be fairly high per your statement. 

My understanding is that your loadbox design adapts quite well to various systems, and thus I'd assume only slight chances would have to made to approximate a small magnet and speaker cone. 

Kind regards
Boysen


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## JohnH

hi @Boysenman 

I'll have a go at matching to a 10" speaker. (I don't send my spreadsheet out), it will be interesting. I expect that the resistors won't change, and also that even with no change, the tone will still be fine. You'd get the tone of the true 10" speaker out of an amp that thinks it's driving a 12. Probably not far off really.


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## sympatico

Hi JohnH and everybody else contributing to this thread.
I´ve built an M2 and I can say it delivers a very (for lack of a better word) "tactile" response, far better than my other attenuator.
A big THANK YOU JohnH for sharing!!!

Now, my build has a strange ringing/oscillation going on that I need to get rid of. It´s that raspy grainy fizzy high frequency artefact riding on top of the guitar sound that we all know and dislike. At first I was suspecting my amp (homegrown ab763 Deluxe Reverb), started messing with tube bias and PI caps and so on. Then I discovered that my other attenuator doesn´t reproduce this ringing. Next I found myself unwinding the M2´s inductor, trying to blame it for misbehaving. All to no avail.

Hopfefully I´ll manage to upload a short mobile phone video of my (cheap) oscilloscope in action of visualising the clipping. First part is without the M2. Amp into dummy load, fed with a sinewave from my (cheap) tube audio generator. Next to some children playing on the street you will hear a bit of the sinewave that is emitted by the whole installation even without a speaker connected. After the cut in the video it´s with the M2 in circuit. I imagine everybody will agree to congruence of visual and auditory phenomena...



My guess is - maybe it´s the very cost-effective (dirt-cheap) switches????
Anyone had a simliar problem?
Any advice and suggestions will be greatly appreciated!


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## Gene Ballzz

@sympatico 
First,  to the forum!

Do you have another amp you can test this with?

Just Askin'
Gene


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## Trouble

Post a picture of what you have.

I don't think I have that problem with mine.

My signal generator died so when the replacement arrives I can test it on the scope.

Hopefully someone else will beat me to it.


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## sympatico

Gene Ballzz said:


> @sympatico
> First,  to the forum!
> 
> Do you have another amp you can test this with?
> 
> Just Askin'
> Gene



Hi Gene, thanks for the welcome!
Yes, do have and will test with other amps. But my M2 is currently not in a working state - had to cut a wire in order to be able to wind up the inductor again. For now I´ll wait until the new switches arrive, there´s other work to do also...
By the way, I did some tests with just letting the amp rip vs attenuated. Absolutely no pronounced fizz without attenuator. The offending frequency was there but not as pronounced and not appearing in the same manner, meaning if for example you play a chord and let it ring out, the fizz doesn´t ride on top of the notes and doesn´t (hard to put into words: ) have this sputtery flakey trails going on that slowly disappear in a sort of wavy fashion... if that makes any sense...


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## sympatico

Travis398 said:


> Post a picture of what you have.
> 
> I don't think I have that problem with mine.
> 
> My signal generator died so when the replacement arrives I can test it on the scope.
> 
> Hopefully someone else will beat me to it.


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## Trouble

sympatico said:


> My guess is - maybe it´s the very cost-effective (dirt-cheap) switches????


I am going to guess them switches also, I tried some similar ones when I started mine. one of them fell apart after soldering. I decided on heavy duty ones after that.


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## sympatico

Travis398 said:


> one of them fell apart after soldering



ja, had that happening, too. One contact gave up with 3 seconds of 220°C on it. If you look closely, these switches do not have the contacts epoxied (?) in. Since there´s no room for big switches I now ordered Miyama MS series, these are usually very nice! I will post results when it´s done - the order may take 7 days or so to arrive here...


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## sympatico

The new switches do have the better contacts:


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## JohnH

hi @sympatico and welcome to our thread.

Looking at those traces, it looks like the difference is in cross-over distortion? (or something like it except it only happens at high drive levels?), ie its the small up/down as it crosses from + to - rather than at the peaks? Its there with the dummy load too , and you also note that the frequency is there just with a speaker, but the reactive M2 is enhancing it. If the amp is doing that then maybe the bias of the amp is an area to look at.

Id doubt the issue is with switches, although good switches are important.

From the traces, when the signal gets big enough to clip at the peaks, I'm not seeing anything unexpected.

If you have crossover distortion, then for that instant as it crosses over, the output is uncontrolled and the reactive circuit has free reign to respond. A resistive load wouldn't do this.

But, clearly what you have found from playing indicates a difference in M2 vs a real speaker. There's a couple of things you can try:

First, make sure there is no steel bolt through the coil, that makes a huge difference to the inductance!

Second, with M2 you can simply shunt the coil, ie bypass it with a solid wire and this will leave you with a fully resistive load, but of the right ohms and it should still sound fine. That may bring this added tone back down to where it normally is. (Id still think you want to get rid of this tone at the amp though). If that test worked, you can instead, bypass the coil with another power resistor in parallel with the coil which will just tame it a little at high frequencies. What value for this resistor? If you have an 8 Ohm build using a 0.9mH coil, then I'm thinking about 22, 27 or 33 Ohms to trim its effect above 4 or 5 khz.

Good luck and let us know!


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## JohnH

JohnH said:


> hi @Boysenman
> 
> I'll have a go at matching to a 10" speaker. (I don't send my spreadsheet out), it will be interesting. I expect that the resistors won't change, and also that even with no change, the tone will still be fine. You'd get the tone of the true 10" speaker out of an amp that thinks it's driving a 12. Probably not far off really.


:

Hi @Boysenman Here's some stuff relating to using a 10" speaker:

*10" Speaker model*

This is the model I came up with to match the plot that you posted:




The original plot is at left. I pulled a few key points from that and plotted them, as the pale grey line on the right chart. I base my calcs on 16" ohm speakers, so everything is x2 compared to 8 Ohms.

The yellow values are the model, based on Aiken, and the plot of that is in red, showing a reasonable match. Blue is the measured values for the G12M cab from Mike Lind.

The peak in the 10" plots represented free-air or open-back performance. It would be higher frequency in a 10" closed back system.

*M2 adjusted*

To match the impedance of the M2 design to this 10" speaker, I dropped the L1 inductor from 0.9mH to 0.6mH. This is the impedance as seen by the amp, in Ohms. Red is the 10" speaker model, Blue is M2 set at -14db:





*Output curves*

Here's the output, stepping through all the attenuation settings, with red being the full speaker:





So, to match to a 10" speaker, the basic tone output actually doesn't change from the 12" for small signals which is as I expected. But to match the impedance a bit closer for the amp, L1 can reduce to 0.6mH.


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## sympatico

Hi JohnH, thanks for the welcome and for your comprehensive input.

- crossover distortion: I already did tests with biasing (I even changed out the coupling caps to the output tubes, thinking maybe one is leaky...) for the same reasons you mentioned. The graps where recorded with a fairly high negative bias voltage resulting in low bias current of around 15ma per tube. Typically these amps run with 20 to 25 or so. High crossover distortion was to be expected. I came to set such low current in a very unscientific way by playing a chord that would provoke lots of the "unpleasant stuff" and dialing in different bias voltages. Low bias current gave less "unpleasant stuff" with different volume settings.

You are right with saying the difference of the two graphs is mainly in the crossover, but there also sems to be more ringing in the compressed peaks. (screenshots attached)
(Whatever that means, I don´t really understand a lot about analysing these graphs really.)

- inductance: I did use a nylon screw with a stainless nut (non magnetic). I did unwind the inductor to see wheter it´s too much. Then I did shunt the coil with an alligator clip. No effect concerning the flaky stuff.

- tone is in the amp: Ja, I mentioned the offending _frequency_ is in the amp even without attenuator, but it´s not behaving the same way. Well... if I think about it again I realise I cannot know that because it is just so much quiter that it _maybe _even does behave the same way and just gets masked.

Still, my assumption is there must be somthing else giong on. 

Things I will reconsider and investigate:
- do messing-around-with-bias-voltage test again and check for crossover distortion specifics.
- do purely resistive attenuator testing
- definitely do test with another amp (!!)

I will report back!

Edit: 
- I wonder if perhaps a loosely wound inductor could be a factor??


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## telesto

So...with all the riots and Coronavirus going on, I decided to hide inside from the world and do some electro-resistor-therapy  Impressed by my build quality, no doubt?  I played with some different configurations, and went with a 7dB+13dB+13dB. That gets my 10W and 22W amps down nicely to bedroom-levels. I played for about 5-10 minutes and my a-hole neighbor didn't bang on the wall, so it works good  ....or he's not home...one or the other 

Anyway, audio quality is MUCHO better than with the L-Pad that I had wired between my amp and speaker  I had the volume up in the 7 to 9 area to get the power tubes to breakup, and the big 50W resistor did get pretty warm after 5-10 minutes. Ok, it's not "heat-sinked" to a metal box, I guess that would help instead of my "al fresco" design (please nobody ask who Al Fresco is, I won't respond 

Actually, I did buy a metal box to make a final build in. Since I don't need all the stages, just a "one stage" attenuator of ~33dB, I'd like to see if I can reduce the number of resistors needed. My guess right now, is that it probably won't be possible with fewer resistors to keep a ~8ohm load on the amp, and have the speaker still seeing impedance in the ~18ohm area. But I'll play around with some numbers and see what I can come up with. If John or anyone has done some math already and already figured something out for that, please let me know 

Big thanks again John, great project!


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## _Steve

Love it!


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## telesto

sympatico said:


> Now, my build has a strange ringing/oscillation going on that I need to get rid of. It´s that raspy grainy fizzy high frequency artefact riding on top of the guitar sound that we all know and dislike. At first I was suspecting my amp (homegrown ab763 Deluxe Reverb), started messing with tube bias and PI caps and so on. Then I discovered that my other attenuator doesn´t reproduce this ringing. Next I found myself unwinding the M2´s inductor, trying to blame it for misbehaving. All to no avail.
> 
> Any advice and suggestions will be greatly appreciated!


I read/heard somewhere inductive attenuators and amps with NFB/Presence may have oscillating issues. Does your amp have NFB? Or else maybe try to take the inductor out of the attenuator and see how it sounds just with resistive components?


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## JohnH

Hi @telesto
if you just want a fixed attenuation with no switching, then with the 8 ohm and 18 ohm values as a target, you can make any attenuation you want as a single stage so long as its more than 7db. Youd use 3 resistors in a T or Pi arrangement, and they can be direct equivalents to each other. So if you are just wanting a resistive version, its just three resistors. If you want the inductive stage 1, Then you can pick that from design M or M2 and then have three resistors for the rest. The two-resistor stages used in designs so far are a simplification, but work fine when hidden behind Stage 1 and within the range 3.5 to 14.

But are you going for -23 or -33 db total?


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## telesto

Hi John, from testing today, -33dB was the good value for me, so I'm shooting for that, with no inductor (oops, I put 23dB in the earlier post, that was I mistake, I corrected it now). Yea, I was also starting to look at a T-attenuator. An arrangement of 7ohm/1ohm/7ohm looks interesting, I didn't work out the dB yet, but the calculation with Watts I made would bring a 10W amp to about 0.01W...oh wait, I guess that would be 30dB now that I look at it  (oops, no it's 24dB according to the T-calculator) What's also interesting is that with the 1ohm shunt resistor, I can put any other resistor in series with the speaker to reduce more power, and it has no affect on the impedance the amp sees.

...oh, I forgot to do anything with the inductor. I do have some now (0.8mH, 0.39mH and 0.56mH). I wanted to work out the volume level first with the resistors. Maybe if there's more riots next week, I'll stay home and play with the inductors


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## JohnH

Cool, so I can do that in a spreadsheet. All it is is a few simple series/parallel calcs, then using 'goal seek' to home in on a solution. Since we want three things being two impedance results and an attenuation, you goal-seek for one result at a time, varying one resistor for each parsmeter, and keep cycling it and it all converges. Put all that as a macro and link it to a button and it's a fun high-speed auto attenuator designer!

Anyway, for -33 db, 8 ohm in and 18ohm out, and assuming the amp is effectively also 18 ohm:

As a 3 part T attenuator:
Amp 18 ohm equiv
First series resistor 7.4
Shunt resistor 0.58
Second series resistor 17.4
Speaker 8 ohm

To tweak that to nearest standard values, it's 8, 0.56, 18 .

Let's say you have stage 1 at -7db to get the inductor Then you want another -26 db, you need: 6.8, 1.3 and 16.8. Nearest standard values 6.8, 1.5, 18.

Both of those theoretical T designs have exact equivalent 'Pi' designs too.( ie, shunt, series, shunt)
-33db 8.3, 247, 19.4
-26db 8.6, 110, 21.3

Again, nearest standard values gets you close.

So have fun rummaging through the spares box!


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## telesto

Thanks John, I had actually just worked out roughly the T-attenuator values myself, and you are already on the Pi-Attenuator, LOL, you are like some calculator/Excel sheet Terminator I see 

Actually, back to the T-Attenuator, I was thinking to put a 7ohm first series, 1 ohm shunt, 17ohm second series. Does that look OK to you? I have to re-check later, wrapping up for the day now.


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## JohnH

ok, "I'll be back!...."

...when I've checked those values out.


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## JohnH

I plugged those in, 7, 1 and 17

It looks like that hits the 8 and 18 impedance targets and offers about -28 db attenuation.

But who ever heard of a 17ohm resistor? Or is it a pair of 33's in parallel?


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## telesto

Thanks Terminator. I was just looking at "ideal" values, yea, didn't consult actual resistor values. Ok, -28dB is a few short of 33, but maybe still OK, I'll have to try it and see. 17ohm doesn't seem to be available for power resistors, but in the circuit, it will have way under 1 watt on it, so they will be small and cheap 2W resistors needed (ie: not big power resistors), so I can play around with resistors in series or parallel to come up with the needed value, shouldn't be a problem. The first series resistor will take the brunt of the power (~90%) I can get 6.8ohm, and the shunt will take about 10% I can get 1ohm, and the speaker and second series have almost nothing in terms of power. Actually, the second series resistor can vary widely and have almost no affect on the 8ohms that the amp should see. Only the "reverse" impedance (what the speaker sees) will be affected by that value. Actually, the speaker impedance wouldn't matter much either, eg. I can connect a 4,8 or 16ohm speaker, and the amp should still see about 8ohms, if I am correct.

Question: how far above/below the target of 18 ohms is still "acceptable"? And which is "worse"?.. to go above or below 18ohms? I saw with the L-pad that going too far below (eg. 1 ohm or less) is a very bad "tone-sucker". I would guess if the speaker is seeing something +/-10dB of the 18dB target it should still be OK in terms of acceptable tone?

PS- The idea of building a Stage 1 with inductor and adding a T-attenuator after is an interesting idea, I'll have to go into that one a little deeper maybe...


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## JohnH

With the output impedance, going higher tends to relatively reduce mids and add highs and bass resonance, lowering it does the opposite, adding mids.cutting high treble etc

I found that a factor of two is very significant, but smaller variations, I dont know, it would be down to how it sounds with your amp and how you like it.

If you have some spare resistors around, you could try putting 20 ohms extra in series and then alternatively in parallel, with the speaker just using your current wire-up, just to explore.. Or for a smaller change, a larger parallel one or a smaller one in series


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## telesto

JohnH said:


> I found that a factor of two is very significant, but smaller variations, I dont know


I don't understand what you mean, can you elaborate on the "factor of two"?


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## JohnH

Well I noticed a big difference with putting a 16 ohm speaker into the 8 ohm attenuator, so the speaker is seeing half the impedance it expected. Not a terrible sound, but a significant change. It would be the same if an 8ohm speaker was seeing say 9 or 10 instead of 18 or 20 ohms


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## telesto

JohnH said:


> Well I noticed a big difference with putting a 16 ohm speaker into the 8 ohm attenuator, so the speaker is seeing half the impedance it expected. Not a terrible sound, but a significant change. It would be the same if an 8ohm speaker was seeing say 9 or 10 instead of 18 or 20 ohms


Ah Ok, I got you now. You're saying to treat the "reverse" impedance (speaker looking at amp) the same as the "forward" impedance (amp looking at speaker)


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## telesto

Hey, I tried the T-Attenuator, 7ohm series, 1 ohm shunt, 17ohm series, and liked the sound of it  I was thinking to stick an inductor in there and see how it sounds. What if on the first series resistor (7ohm) I take two 14ohm resistors in parallel and stick a coil in between, like you did on the M2? Do you see any downside to that, or any better idea?


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## JohnH

telesto said:


> Hey, I tried the T-Attenuator, 7ohm series, 1 ohm shunt, 17ohm series, and liked the sound of it  I was thinking to stick an inductor in there and see how it sounds. What if on the first series resistor (7ohm) I take two 14ohm resistors in parallel and stick a coil in between, like you did on the M2? Do you see any downside to that, or any better idea?



You can try but the result may be random and sound bad/not good/not quite right/something missing. Reactive Stage 1 needs to be very carefully balanced between inductor and resistors so the inductance is partly in series and partly in parallel with the later stages.


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## telesto

Ok, how did you reach the conclusion about the inductor needing to be part series, part parallel? Did you base it on some theory/calculation? Spice sim? Or did you just simply experimented until you found something that worked good?


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## JohnH

Spice sims are probably the quickest way to test these things, and see clearly what is happening. Having the Inductance in series or parallel can both show the amp a reasonable reactive imoedance, but all in series will reduce treble fed to the speaker, while all in parallel will tend to boost treble. Hence a combination works best, either two inductors as I'n M, or one inductor feeding via two resistors as in M2.


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## telesto

Ok, interesting, thanks for sharing that


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## Boysenman

@JohnH

Sorry for not getting back sooner. You're really incredibly helpful regarding the 10" speaker design. I really appreciate it.

So, if I get you right, if I wanted the design the M3 attenuator, with regards to the small 10" speaker, I would only have to reduce L1 (in the M3 schematic) from 0.9mH to 0.6mH, while all the resistors stay the same? Would I need to make any changes to L2 or C1 as well in this case?

Also, I want to understand why Aiken has chosen the bass resonance with an impedance of around 80-100 ohm, while your schematics are placed lower if I'm not mistaken? An impedance of 50-60ohm is similar to that of the impedance plot of the alnico speaker, you simulated. I would assume that a small voice coil inductance, compared to a similar leak resistance, would have an equally lower bass resonance impedance? Of course, I could for all intend and purposes make a switch that selected 3 different notches of bass resonance amplitudes, and likewise, 3 different levels of treble attenuation by varying the leak resistance. Either way, it seems fairly easy to tweak afterwards. The coils and the capacitor are much more expensive to come by.

kind regards
Boysen


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## JohnH

Hi @Boysenman

A bit about M2, M3 and the M3 resonance circuit:

On the pure load box designs, such as Aikens, designed to work with no speaker, then to get that resonance you have to have the big cap and the big coil. It's the same with the active or reamped attenuators such as most of the expensive ones including OX and Boss TAE etc.

But for our design, the need for it is far less compelling, in fact, no one has built it yet to my knowledge and no one has noticed any problem. In M and M2, we get that resonance directly and naturally by the interaction of the attenuator with the real speaker. This means that if you change to a different speaker, 10 or 12", or closed-open cab, the result adjusts with no changes to parts. The only difference is that at these very low frequencies, the amp doesn't see it. The result is probably a more solid and consistent bass, theoretically with less bass distortion if you pump those very low notes.

But does if make an audible difference at all? so far I'm thinking not, and I haven't ponied up the $100 extra in parts to build it.

But if you go back to my last set of clips on page 50, 23rd May, I was really trying to reveal some difference full volume to attenuated in these low frequencies. Ignore my playing, tone and recording quality, but just try to listen for any difference in the low bass. The recording was through a 2x12 open back combo, probably with a resonance at about 80hz, and I was playing in drop D with several notes around there. I can't hear any difference in the normalized clips. And I was pushing the amp to try to provoke some result

So I suggest not to go for the full M3 until youve tried it just based on M2. Also, to make the M3 resonance circuit ideal, it has to be matched to the true resonance of your own speaker and cab. If it doesn't, it's all fine, still works etc, but it's not doing what's intended and might as well omit it since it sounds fine without

Also, getting a very sharp bass resonance peak at the amp needs big cap and big inductor, so more cost, particularly with the 8 ohm design That might be where designs you've seen are less than the theory


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## telesto

Regarding speaker size or type, I don't think that will have much difference in regards to an attenuator, if I am correct. John used the 12" Greenback as a reference, but could have been any other speaker and results should be the same, or very similar. But yea, good to verify by testing some other types just to be sure. I think you verified that in the previous page?

One other comment regarding speakers, be aware that 10" speakers usually have less efficiency than 12" speakers. Most 10" have a speaker efficiency of about 95dB and 12" are around 100dB. That's the reference amplification at 1 Watt. So if you switch from a 12" speaker to a 10" speaker, you will get some dB's of attenuation "built into the speaker". Just something to consider


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## telesto

Ok, I went an upped my game today and screwed the resistors to a board instead of leaving them loose. Wasn't really a good idea having them loose, as they would shift around and were in danger of shorting out (kids, don't try that at home!)

I stuck the inductor in as well, but was kind of disappointed, as I didn't really notice any difference with or without. I experimented with it on the T-attenuator, and same thing, no difference. Or maybe even better without it. I tried with two different amps, same thing. Is it a subtle difference? Have other people experimented with/without the inductor? I have a fixed 28dB attenuation, maybe the inductor is more noticeable at higher volumes? At the moment, I see no need for it, and will probably leave it out of my final build.

*edit*
ok, I went back to page 6-7 and see John also had the impression that the impact of the inductor was negligible. ..so then why leave it in?

*edit 2*
Putting the issue of tone aside for moment, would the inductor have any impact positive or negative on the amp itself? Meaning an amp is meant to interact with an inductive speaker load. If it sees just a fixed 8ohms without impedance swings, is that good, bad or doesn't matter, from the amp point of view?





(Picture has inductor removed from the circuit. To put it in, I would take the wire on the left side of the top resistor connecting to the resistor below it, and connect it to the free inductor connection. The wire that looks like it's going into the screw holding the inductor is actually connected to the other inductor lead)


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## JohnH

I think an amp is safe going into a purely resistive load. The inductor is a subtle difference, sometimes noticeable sometimes not, depends on the amp and how you play. It is most likely to be noticeable if the preamp is not distorting too much and the power amp is driving hard, and you are using a wide dynamic range in your playing, sometimes playing lightly, sometimes digging in, letting notes ring out etc. Check out page 9 where Gene first got his going. My own first impressions were before I had the values quite right.

Also, discovered much later, but the reactive circuit seems to be able to adapt to different amps much better. Its there to be seen in the analysis but hard to explain.


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## telesto

Ok, I did some further A/B testing with/without the inductor, and yea, I can hear a little difference, but very little, almost negligible in my tests. Of course depending on amp, playing style, etc your mileage may vary. But in my opinion, it's not really needed.


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## JohnH

I still have a prototype resistive version, this one, from 2 years ago:




This has three stages of -7, -7 and-14 db so -28db max. I did some A/B comparison against my last build, Design M with the coils and -31db max:













AttenuatorM Outside 190217



__ JohnH
__ Feb 16, 2019






The M is bigger and heavier, with several more features as well as reactive input. But the core resistive design is the same.

Running them both again with the VM, both set at -28db, the resistive design isn't bad at all, the tone is consistent and clear. But the reactive one is just a bit more lively, more present and responsive. The resisistive one has the tone, but its more constant, more like a good sim of an amp rather than the real deal.

Another thing I noticed is how the presence control on the amp is significantly more effective into the reactive input which makes sense since it is changing the amps output impedance at high frequencies as it varies NFB.

Its all quite small differences, it's not like one is a wet blanket and the other isnt. Each may have a place


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## telesto

Hi John, one observation/comment regarding the inductor: I'm using amps with relatively low wattage, 10W and 20W amps respectively (Laney Cub 10 and Jet City 20H). I vaguely recall reading somewhere (Aiken?) that inductors have less effect on lower watt amps (?) Maybe this is the reason I'm not noticing much difference with/without the coil? You are doing your testing mostly with a 40W amp, is that correct? 

Another remark, I'm using a 0.82mH 1mm thick Visaton coil in the M2 configuration. I also played around and tried a "split coil" formation (0.4mH with 22R series, and 0.6mH shunt) together with a T-attenuator, but the coils sounded saturated or distorted or something. I didn't really do any sim or math, I just sort of mimicked the original design Stage 1, but with different resistor values. Didn't work out well, but I was just playing around and experimenting.


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## JohnH

Yes my main amp is a 50W Vintage Modern, though its apparently really in the high 30's of Watts in comparison to others.

I found the schematics for your amps online:
https://www.thegearpage.net/board/index.php?media/laney-cub-10-schematic.8667/full
https://usermanual.wiki/Document/JCA20HSchematic.1420896408

Looks like the Cub10 has 6V6 output valves and EL84s for the Jet City 20H? So different from the EL34s of most Marshalls, or the big-ass KT66s in my VM. Also, both amps have negative feedback loops? The Jet City looks like it offers you a presence control with it, with the Cub10 doesn't? 

Both of these features, small tubes and negative feedback will affect the real output impedance of the amp. If it becomes relatively low due to the design, then this could explain why the inductance has little effect. It may corelate to Aikens comment too. What is good is that even with these two very different output stages, the attenuator is hopefully giving a reasonably good result with or without the coil?

If you are interested, for the sake of science, you could measure your amps output impedances. What you do is feed in a steady clean signal, make sure its at very low level, no distortion, and measure its output level into a resistive load, with a meter, scope or record into a pc. Then you repeat changing only the resistive load value. The result needed is the ratio of the two signal levels (so absolute measurements aren't needed) and the value of the two resistor loads. Then its maths.

BTW, my other amp also has EL84s, but no NFB so it has a very high output impedance 

If you were trying the M front stage with the two coils, it needs to also use the intended resistor values or else it may be out of wack tonally


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## telesto

Ok, I'm game, I have a multimeter. I can probably download some signal generator app to my phone to generate for example a steady 1khz tone to feed into the amp input, put the amp volume on , what, like 2 or some low volume? And connect just a 8 ohm dummy load (without the speaker) Then measure the AC voltage coming out of the amp. I can then remove the dummy load and put, say, a fixed 10 ohm resistor in it's place, and repeat the measurement. So then I have AC voltage and Resistance as known values. Well, I can calculate current (I=V/R) from that as well. But then where does the amps resistance come into the picture? What ratio?

Oh, and yea, the resistive attenuator sounds great  Well, I can only compare to the variable L-Pad that I had connected before, I don't have any other attenuators.
I do have a 100W Sovtek MIG in my basement, I could test on that and see if the reactive attenuator has better performance, but I don't really want to drag that beast out of the cellar  Besides it had some previous issues, I bought it in a non-functional state for cheap, and managed to get it working, but I'm still a little skeptical of it. I also have a Hiwatt clone that uses 6V6 or KT66, but I only have 6V6 tubes, KT66 tubes are kinda expensive, and I don't need that kind of power, since I mainly just play at home. I also have a couple of 5W heads. Maybe if I have time I can make some further tests. But ok, I'll probably start with the Laney tomorrow and see how that goes...


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## JohnH

OK game on! and I will be very grateful indeed to see what data you can create. A pure 1 khz is a good value to use, probably set it up so you are getting around 1 to 2 V at your output loads (2V into 8 Ohms is 0.5W, so not stressing anything).

Obviously, being super careful to make sure everything is hooked up before powering up, and power down or go to standby between changing loads but without adjusting any knobs.

Best to measure those resistors too and use the measured values, subtracting the resistance of the meter leads. 

*Basic idea:
*
The amp is thought of as making a fixed constant output voltage V0 (which we don't know and don't need), in series with an output resistance R0 (which we want to know).

The output load is Rl (ie, the 8 or 10 ohms)

Voltage that you measure is Vout = V0 x Rl/(Rl + R0)

so lets say you have two values of Vout, and your loads really are 8 and 10 ohms

Ratio of the measured voltages Vout(10) / Vout(8) = (10/(10+R0)) / (8/(8+R0))

The only unknown there is R0, and we use the exact values for the load resistors.

Solve it by high-school maths, or being lazy, I use excel and do a goal seek function to find R0


If you tried this with a SS amp, you would get Vout not changing much at all, so ratio 1 and the solution would be R0 = almost 0. If you had the output ratio approaching the load ratio, ie 10/8, it implies R0 is very high. In between, the maths should deliver the answer. You get a bit more resolution with a greater difference in the loads, but 8 and 10 should work fine.


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## Baby Thomas

IMHO the difference between reactive and resistive can be smaller/bigger depending on the quality of the gear used and the loudness you play at. An inexpensive 20W amp/cab played at home can mask the differences compared to 50-100W high end amp played on big stage outside. In the first scenario maybe you could hear any difference, maybe not. In the second scenario the difference will be more noticeable. Same thing for guitar, cables, speakers, cabinet (don't underestimate the cab!)...

In telesto's case - JCA 20H is not great amp (I have 50H and even after a bunch of mods is still mediocre) and playing it at home is not the best way to find out what the reactive design gives you in addition to the resistive one. In the same time if this is the way you play - why bother if one is 2% arguably better than the other and you can't hear it.  It will be interesting to test the attenuators with your other amps as well.

Best regards!


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## telesto

Hi John, ok, I made some measurements...

With a 1khz tone input and a fixed resistor of 4.7, 8.2 and 15 ohms (all measured exact values, well done ARCOL  ) amp was set to a fixed output volume (around 3) and not touched between measurements. I let the amp warm up for a few seconds, then ran the 1khz tone for about 5 seconds or so until the VAC seemed to be stable.

Laney Cub 10

4.7 ohms = 0.05 VAC
8.2 ohms = 0.07 VAC
15 ohms = 0.125 VAC


My Jet City 20H

4.7 ohms = 0.75 VAC
8.2 ohms = 0.92 VAC
15 ohms = 1.08 VAC

Using the 8.2 and 15 ohm values, I got about 20ohms for the Laney, and 22 for the Jet City. (My algebra skills are rusty, so I got some help from here  )
Feed it into your Excel and let me know if I got it right. If yes, then I can measure my other amps as well.


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## JohnH

Hi @telesto , thanks for the data.

Since you did three loads per test, we can run the numbers between three pairs of results for each one to see if they are consistent.

Based on that, the Jet City does come out fairly consistent at 3.6, 3.8 or 4 ohms based on 4.7-8.2, 4.7-15. and 8.2-15 load pairs. But quite a lot less than your numbers! But if around 3.6 to 4 ohms is right, it could definitely explain why the inductor doesn't make much effect with this amp. 

But the Cub10 numbers are all over the place. Just solving them as they stand, I get 10, 32 and 265 Ohms for the three pairs!. Probably the most likely is the 32 since it based on the widest spread of loads 4.7 and 15. But it is very clear that the Cub 10 has a much higher output impedance than the Jet City though. 

When you go from 4.7 to 15, you more than treble the load ohms 15/4.7. The Cub10 is almost tracking this with an output change ratio of 0.125/0.05 = x2.5. But Jet City is only changing 1.08/0.75 = x1.44 

When the resulting output impedance is tending to be a high value, it becomes very sensitive numerically to small variations in the numbers. It might be best to wind up the Cub a little more to get more around 1V out, and record all the voltage digits. Also, I suspect that basic multimeters reading very small ac voltages may get fairly inaccurate 

That's if you are interested to pursue this further down the rabbit hole. I'm well aware that people have a limit with regard to how far they want to explore the things that I get very interested in.

Meantime, if you wanted to listen for changes due to the inductor, it looks like the Cub10 is probably more likely to reveal them than the Jet City. 

And I need to solve the maths better instead of using goal-seek. It boils down to a quadratic equation, which is not rocket science to solve.


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## telesto

Hi John, I've been teaching myself the last few years about amps, built a kit amp, and now designing my own, so all this stuff is very interesting to me, so count me in on any experiments or tests 

I re-tested the Laney now with Volume a little higher (on 5):

15ohms = 0.32 VAC
8.2ohms = 0.21 VAC
4.7ohms = 0.15 VAC

Using the 15 and 8.2 ohm results, I plugged it into a formula calc to solve for the voltage ratio, and shows the R0 variable (x in the equation) to be either -2 or -21. How are you figuring the Jet City to be 4ohms? And I thought you were always saying that the amp should be about 20 ohms, so now I'm really confused, LOL.


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## Caspercody

I have a Marshall DSL40C (40 watts all tube amp), with a 16 ohm speaker connected. Can you tell me what is the best layout you have for this amp? 

Thanks
Rob


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## JohnH

Hi @telesto

Your online calculator gets a C- grade and needs to see teacher after school.

Actually, it is solving what it sees, but if you take the width of the division lines as the order of the divisions, it is working from bottom to top. What we need is the middle division line to be the widest, ie, the one that is done last. so 15/(15+x) is divided by 8.2/(8.2+x) = 1.52

Solved this way, it works with x = 25.8 ohms

Another clue is the negative ohms results, which are not a thing.

The new results are getting better, still quite wide apart, but I'm getting 9.5,16.1 and 25.8 from the three pairs. I think it still needs a bit more output (like 1V or so) to get the meter to read it right, but its homing in on a credibly higher value.

But amps do all differ. The value I used was based on one of my amps, the other is much higher since it has no NFB. But the inductive part of the attenuator seems to help it adapt to different amps in terms of relative treble response. Back in January, we got values with @Mcentee2 from an SV20, which gave a 5 ohms result (starts post 680), he went on to run frequency tests showing how the M2 version helped to track the response.


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## JohnH

Caspercody said:


> I have a Marshall DSL40C (40 watts all tube amp), with a 16 ohm speaker connected. Can you tell me what is the best layout you have for this amp?
> 
> Thanks
> Rob



hello, welcome to this thread. Are you interested in an attenuator design for your DSL40C? if so, the choices are similar to as for one of mine, a DSL401, which has a 16 ohm speaker but also has an 8 Ohm tap and I use it with another cab under it at 8 Ohms. Would you be intending it to be with different amps or different cabs? or just the internal speaker? also, what kind of uses? gigging or at home?


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## telesto

JohnH said:


> Hi @telesto
> 
> Your online calculator gets a C- grade and needs to see teacher after school.
> 
> Actually, it is solving what it sees, but if you take the width of the division lines as the order of the divisions, it is working from bottom to top. What we need is the middle division line to be the widest, ie, the one that is done last. so 15/(15+x) is divided by 8.2/(8.2+x) = 1.52


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## Caspercody

Thanks for the reply. I am just using the speaker that came in the cabinet, a 16 ohm. I am just using it at home.


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## JohnH

Caspercody said:


> Thanks for the reply. I am just using the speaker that came in the cabinet, a 16 ohm. I am just using it at home.



I reckon basing it on 16Ohms will be the go then. And if you ever add an extension cab it will run that too. Id suggest the one I call M2, in a simple version. There's diagrams on the thread but I'm planning to update them in the next few days. No design changes planned but just to pull a few options together. If you're interested, might want to wait for that.


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## Caspercody

I will wait. Curious if you happen to have a bill of material in the mean time?


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## LordoftheLivingRoom

Hi Everyone! I recently completed two 16ohm M2 attenuators, one for each of my home built amps (Deluxe Reverb clone, and JTM45 clone). I often play with them both fed from a splitter. And If I can build one attenuator for the price of one, why not build two for double the cost?  I'm impressed. Tube amps definitely sound better when they are turned up above 1 on the MV... Anyway, here are some pics. My enclosures are steel, hence the vertical mounting of the inductors.


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## JohnH

Hi @LordoftheLivingRoom those look awesome!
What is your technique with the faceplate writing? (and I appreciate the name check)
Also, do you find any difference in how they are working on your amps? Two classic amp circuits there!

I'm nearly there with new cleaned-up diagram. No changes though, its basically what you built.


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## JohnH

Here's a new drawing of Attenuator M2, stripped back to the basics, in 8 and 16 Ohm versions.













Attenuator M2 200702



__ JohnH
__ Jul 1, 2020






I think it is basically what @LordoftheLivingRoom has just built at 16 Ohm. There's no changes really to previous.

Ive not put in the full bypass, nor -3.5db-only resistive settings. These could be added (see M diagram on post 1), but after about 18 months, I don't think they are needed for most cases and leaving them off simplifies both for construction and use , reduces parts and electrical contacts, makes it much less likely to hook something up wrongly and takes the pressure off switching current for more reliability. 

(work arounds if needed: To get a small -3db reduction, use the attenuator as a load box set to max attenuation, in parallel with the speaker, using 1/2 the amp tap ohms. And if you want full bypass, just unplug it and don't use it at all!)

I kept the 3rd output for using a 16 ohm cab with an 8 ohm attenuator. This is a useful tonal correction, but its quite safe to use the other outputs instead with a 16 ohm cab, you get a bit less treble, more mids. If you don't need this, or are building a 16 Ohm version, ignore everything in Red. And an 8 Ohm cab into a 16 attenuator should work fine as it is.

As before, power ratings for resistors allow for a cranked 50W amp, assuming case mounted resistors with thermal grease. For a 100W amp, values are the same, power ratings to be x2 and it might warrant a fan TBC.

The inductor is air cored, 18awg or thicker, and don't mount it with a steel bolt

I think I'm going to build another one like this!


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## Gene Ballzz

@JohnH 
Did we determine that the 19awg units that are easily and cheaply available from MADISOUND https://www.madisoundspeakerstore.c...wg/madisound-1.8-mh-19-awg-air-core-inductor/ would be robust enough in 16Ω unit for up to 50 watt amps? I thought we decided it was not completely necessary to double the price by bumping to 16awg, as well as the 16awg is a bit harder to mount through the switch and resistor lugs?

Just Re-Confirming?
Gene


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## JohnH

Gene Ballzz said:


> @JohnH
> Did we determine that the 19awg units that are easily and cheaply available from MADISOUND https://www.madisoundspeakerstore.c...wg/madisound-1.8-mh-19-awg-air-core-inductor/ would be robust enough in 16Ω unit for up to 50 watt amps? I thought we decided it was not completely necessary to double the price by bumping to 16awg, as well as the 16awg is a bit harder to mount through the switch and resistor lugs?
> 
> Just Re-Confirming?
> Gene



Sure I think those ones are fine, and anyway they have worked well for you so far. Looking at different ranges from suppliers, 19awg seems quite rare, and 18 is more common. But a 1.8mH 18 gage that I can order has a resistance of 0.8Ohms, and the Maddisound at 19 gage is 0.85 Ohm, ie virtually the same. Also, with the M2 design, there is less total wire in it than the two coils in M and I think there is less current flowing through the coil.


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## Gene Ballzz

Thanks for that @JohnH ,
I'm kinda planning my next couple builds and generally intending to do 16Ω versions of your simplified M2, so I need a different choke. All I have on hand is a 1.25 & a .8. Although I may just do one more M-Lite with the parts I have on hand! Please remind me (without me needing to re-read the whole thread) if there is any downside to the two choke M-Lite design over the single choke M or vice-versa? I think we already determined that the 1.25 & .8 (that I already have) substituted for the 1.1 & .7 made little discernible difference in your testing?

In all honesty, I have all the parts for three more of these (except only one pair of chokes), but have been playing around with a few different cofigurations & layouts, as all of mine are destined for semi-permanent installations and somewhat custom tailored to their applications. For example, I could never see a need for any stage #3 for my 5 watt amps! 

Thanks Again John,
Gene


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## JohnH

hi Gene, this post....

http://www.marshallforum.com/thread...design-and-testing.98285/page-46#post-1973252

...was where I tried to tease out any likely differences between M and M2 types. The charts show that they are not quite identical but any differences are likely to be within a fraction of a db over a range of assumptions about amps etc. So really no difference. But we haven't had an M and and M2 in the same place at the same time yet, maybe until you or I get there.

But I like the M2 aesthetically. Understanding how to relate the output response to the triangle formed by the coil and the two resistors R2A and R2B was a nice bit of math (Star-Delta transform), and its front end although looking simple, has a nice WTF? aspect about it.


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## LordoftheLivingRoom

JohnH said:


> Hi @LordoftheLivingRoom those look awesome!
> What is your technique with the faceplate writing? (and I appreciate the name check)
> Also, do you find any difference in how they are working on your amps? Two classic amp circuits there!
> 
> I'm nearly there with new cleaned-up diagram. No changes though, its basically what you built.



Faceplates... my old nemesis. With these I just did up a quick thing in Gimp. They are simply printed out on paper stuck to the plastic front panel of the enclosure. And credit where credit is due.

I never really got much opportunity to crank either amp before. I can get both amps turned up to half or a bit more now. Before I was barely at 1. Totally different feel.


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## Gene Ballzz

@LordoftheLivingRoom
Fantastic looking builds!  I'd love to see a pic of the jack panel end. Did you wire two output jacks in parallel, or ????

It also looks like there is no venting on the top? Have you noticed any heat issues during extended use, particularly with the JTM45?

Thanks & Nice Work! 
Gene


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## MAD64

JohnH said:


> When I figured out the single-stage versions
> (Ie no switching and designed to have a speaker straight after), I found that the configuration worked ok in the range -3 to -9db, by adjusting values. After -9 db, the output impedance as seen by the speaker was drifting outside of the target range, which would tend to start changing the tone. So it would need more parts anyway.


In the meantime I've built the 7+3.5 dB 16Ohms version for a friend's vintage AC30 and he is happy. And it turned out, that 10.5dB was a bit too much so in the end I've added a switch to bypass the 3.5dB stage ;-)


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## MAD64

@JohnH I found some measurements of tube amplifier output impedances in a German book that might be of interest (all for 8 ohms and 1kHz): 
AC30 - 70 ohms
Marshall 18W - 90 ohms
Tweed Deluxe - 80 ohms
Super Reverb (w/o NFB) - 180 ohms
Super Reverb (w NFB) - 17 ohms


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## MAD64

@JohnH What are the limitations of your model - will it work to just divide the components values for 8 ohms by 4 to build a 7+3.5 dB version for my Tweed Bassman's 2 ohm tap? I haven't read each and every post in this thread, so I apologize if this has already been discussed...


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## Filipe Soares

what's the latest version of the attenuator? I think I'll have to build one for 100w...


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## JohnH

*Impedance values*

hi @MAD64 , thanks very much for those impedance values. Its very valuable to see them. Real output impedance is such a significant parameter that it is amazing that there is almost no info about it online. Your post above has just at least doubled the total English-language data pool!

Does the book have any such values for a classic EL34 Marshall circuit? We are finding that in some cases, the values for those may be much lower.

As a side comment, despite electric guitars being invented by Americans, I'm always very impressed by the very interesting engineering analysis of guitars and their systems that has been done in German.

But anyway, it confirms that the base assumptions used in these designs are right on the money. Also highlighting the value of the reactive circuit to allow the output signal to adapt to such a wide range.

*2 Ohm Design*

Yes in principle, values should just scale down, divide inductor and resistors by 4 based on values for 8 Ohms. But then it may be necessary to adjust values to suit the available range and for that, the aim should be to keep the ratios of resistors within each stage about the same. eg, take Stage 1 which is the most important one. R1, R2A and R2B would go from 15, 22 and 18 to 3.75, 5.5 and 4.5 which would round up to standard values 3.9, 5.6 and 4.7. But power resistors can come in different series values so you might end up with 4, 6, 5 for example.

Power ratings would be the same, scaled according to the amp power.

But current through the attenuator coming from a 2 Ohm tap for a given power, is x2 that for an 8 Ohm version. If its a 50W amp at 2 Ohms then the full current is nominally 5A so that puts pressure on jacks and switches, and Id suggest 16 awg wire for the coil. If you were running it all with a full bypass switch, you'd be needing at least a 10A rated switch. That's a big advantage of not having full bypass (see page 56 diagram, Stage 1 is fixed), since only 20% of the power is being switched. So we are only switching 10W and for 8 Ohm the switch current is nominally 1.1A, so 2.2A if for a 2 Ohm build (so use at least a 5A switch).

Also, you could have a look at page 44 where there is a version of M2 with variable input Ohms. This was done by scaling to 4 Ohms then adding another 4 Ohm dummy load at the front. But if its to be dedicated to a 2 Ohm amp, id just go straight to 2 Ohm though.

But, although there have been quite a number of successful builds at 8 and 16, I don't know of any at 4 or 2 although they have been discussed a few times. I cant think of any other factors that would prevent this though.


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## JohnH

Hi @Filipe Soares , see page 56 for latest, also linked from page 1. 

If it works for your amps, id suggest building at 16 Ohm for a 100W amp, and all power rating values get doubled, and use 16awg for the coil.

R1 would then be rated at 200W, but id suggest to use 2 at 100W in series or parallel, they are much more compact and also more commonly available, and you can spread them apart to mount in the case.

At 100W, cooling is critical and my starting point would be to mount resistors onto a big thick aluminium case (much better than steel for heat), with lots of wide holes in the top and the base. And/or, depending on your use and how its built, it may benefit from a fan.


----------



## Filipe Soares

JohnH said:


> Hi @Filipe Soares , see page 56 for latest, also linked from page 1.
> 
> If it works for your amps, id suggest building at 16 Ohm for a 100W amp, and all power rating values get doubled, and use 16awg for the coil.
> 
> R1 would then be rated at 200W, but id suggest to use 2 at 100W in series or a parallel, they are much more compact and also commonly available, and you can spread them apart to mount in the case.
> 
> At 100W, cooling is critical and my starting point would be to mount resistors onto a big thick aluminium case (much better than steel for heat), with lots of wide holes in the top and the base. And/or, depending on your use and how its built, it may benefit from a fan.


Thanks john, your attenuator is in my personal projects pipeline for a while. I think I'm going to do it next week.


----------



## MAD64

Thanks for your comprehensive answer. Gauge and switches/jacks is not an issue, e.g. my bypass switch is specified for 125V/15A. I'll give it a try with my tweed bassman and will report the results  The impedance values I've quoted are from this book (see last page of this chapter, diagram 10.5.20). I'm not aware of measured values for an EL34 powered amp (neither with nor w/o nfb) but maybe I find some if I check my books more thoroughly...
https://gitec-forum.de/wp-content/uploads/2017/08/pde-10-5-7-ausgangs-widerstand-impedanz.pdf


----------



## LordoftheLivingRoom

Gene Ballzz said:


> @LordoftheLivingRoom
> Fantastic looking builds!  I'd love to see a pic of the jack panel end. Did you wire two output jacks in parallel, or ????
> 
> It also looks like there is no venting on the top? Have you noticed any heat issues during extended use, particularly with the JTM45?
> 
> Thanks & Nice Work!
> Gene



Thanks Mr. Ballzz. The rear panel is just 3 jacks, one input and two output. Pretty boring hence no pics. The two output jacks are in parallel. I will one day run my Deluxe reverb clone 1x12 combo cab in parallel with the 2x12 I usually use with my JTM45 clone... It will be legendary. Parallel output jacks give me a ton of speaker combo options! 

The enclosures do have vents in the top. Compressing the images must have hidden them. The top vents are manufactured slot vents, the bottom vents are my dodgy drilled holes. 

I did get the JTM45 attenuator to begin getting quite warm after about 15 minutes with the amp cranked (everything on 11!!!), but I like the tone better with things backed off and the amp sitting just into overdrive for simple blues noodling. Then again, there are some early AC/DC tones in there to discover... It's amazing that I can attenuate down to living room levels with a JTM45 turned up to just over half (with some efficient WGS speakers too) and have some extra attenuation on tap.

Extended use... I don't get any of that these days. And I only know half of about 3 or 4 songs.... Too much building amps and stomboxes, not enough guitar practice. That'll begin to change soon.


----------



## Gene Ballzz

LordoftheLivingRoom said:


> Thanks Mr. Ballzz. The rear panel is just 3 jacks, one input and two output. Pretty boring hence no pics. The two output jacks are in parallel. I will one day run my Deluxe reverb clone 1x12 combo cab in parallel with the 2x12 I usually use with my JTM45 clone... It will be legendary. Parallel output jacks give me a ton of speaker combo options!
> 
> The enclosures do have vents in the top. Compressing the images must have hidden them. The top vents are manufactured slot vents, the bottom vents are my dodgy drilled holes.
> 
> I did get the JTM45 attenuator to begin getting quite warm after about 15 minutes with the amp cranked (everything on 11!!!), but I like the tone better with things backed off and the amp sitting just into overdrive for simple blues noodling. Then again, there are some early AC/DC tones in there to discover... It's amazing that I can attenuate down to living room levels with a JTM45 turned up to just over half (with some efficient WGS speakers too) and have some extra attenuation on tap.
> 
> Extended use... I don't get any of that these days. And I only know half of about 3 or 4 songs.... Too much building amps and stomboxes, not enough guitar practice. That'll begin to change soon.



Yeah, well I'm headed out the door to a 3-4 hour jam that'll let even my Tweed Deluxe warm mine up a bit.  Not hot, mind you, just a tad warm after a time! Now if I were using my JTM30, that'd get it mostly toastly with it's at least 40-ish watts!
Enjoy!
Gene


----------



## JohnH

MAD64 said:


> Thanks for your comprehensive answer. Gauge and switches/jacks is not an issue, e.g. my bypass switch is specified for 125V/15A. I'll give it a try with my tweed bassman and will report the results  The impedance values I've quoted are from this book (see last page of this chapter, diagram 10.5.20). I'm not aware of measured values for an EL34 powered amp (neither with nor w/o nfb) but maybe I find some if I check my books more thoroughly...
> https://gitec-forum.de/wp-content/uploads/2017/08/pde-10-5-7-ausgangs-widerstand-impedanz.pdf



That paper is interesting. I read some of it by pasting into Google-translate, which did quite a good readable job.

It does have some plots for JTM-45:




The lower plots show output impedance dependent on presence setting, very low at low frequencies and maybe about 5ohms at 3khz if presence set at 5. Upper plots are if NFB is disconnected 

Its much lower than I found on my VM, even though JTM-45 is its grandfather with similar tubes. But looking at the circuits, I think the VM may be using much less NFB at most settings. The JTM and VM feedback about the same fraction from the output tap, but JTM uses a 16ohm tap while VM uses the 8 ohm tap, so less signal. Also, at the settings I use, around 6-7, the PPIMV is probably reducing level by about another 50%


----------



## em07189

Well i arrived very late to this post so i have some questions, what components should i use to make a attenuator for 100 W amp?

Is it possible to make a attenuation level been activated by a jack footswitch?, im asking this because im thinking also as a boost for solos, for amps that dont have 2 master volumes.


----------



## JohnH

Hi @em07189 

The latest diagram is on p56 and also see the first post and response to Filipe on this page above. The first thing is to decide which amp and cab Ohms to use, ideally 16, then use values for that, with x2 power rating.

For a boost switch, you'd build the full design as it is then add one more switched stage to give the variation you want, maybe another-7db stage? - with a footswitch. I don't think anyone has built this yet but there's a few ways it could work, It could be all in the main box and relay controlled, or a separate single stage in a floor box daisy chained after the main circuit just before the speaker, or via an insert jack.


----------



## What?

I still intend to build one of these for my Super Reverb, but I'm trying to make sure I have my 'Marshall' squared away first.

Seeing this thread brought something to mind though. As transparent as this attenuator design seemingly is, would this sort of resistor and inductor network be suitable for speaker level matching? For example, if you have a speaker with a 95db efficiency and another with a 100db efficiency, the 100db might drown out the 95db speaker. Could the 100db speaker be transparently tamed to better match the 95db speaker level?


----------



## MAD64

@JohnH The book is really useful, in some areas maybe a bit overloaded with math. Some parts of the German version are officially available for free on the web and there is also a nearly complete english version freely available. Re your thoughts on output impedance: As Manfred Z writes it's not only depending on the tubes and OT but also screen voltage (more than on anode voltage) and the operating point. And it's depending on input signal size (especially for class AB, so for nearly all guitar amps), but I assume for our usage here, the measured values for small signal input are good enough...


----------



## em07189

JohnH said:


> Hi @em07189
> 
> The latest diagram is on p56 and also see the first post and response to Filipe on this page above. The first thing is to decide which amp and cab Ohms to use, ideally 16, then use values for that, with x2 power rating.
> 
> For a boost switch, you'd build the full design as it is then add one more switched stage to give the variation you want, maybe another-7db stage? - with a footswitch. I don't think anyone has built this yet but there's a few ways it could work, It could be all in the main box and relay controlled, or a separate single stage in a floor box daisy chained after the main circuit just before the speaker, or via an insert jack.



Hi John! A volume boost of -7db or plus 7b from the selected level of attenuation, this option will give more flexibility to the power attenuator.


----------



## JohnH

What? said:


> would this sort of resistor and inductor network be suitable for speaker level matching? For example, if you have a speaker with a 95db efficiency and another with a 100db efficiency, the 100db might drown out the 95db speaker. Could the 100db speaker be transparently tamed to better match the 95db speaker level?



I reckon it could. I worked out a few single stage attenuators using the two-coil arrangement.













Single Stage 190105



__ JohnH
__ Jan 4, 2019






Could likely simplify them down to one coil now.

Or for just a few db, Id expect that just a couple of resistors would do it in most cases,without a coil


----------



## JohnH

MAD64 said:


> @JohnH The book is really useful, in some areas maybe a bit overloaded with math. Some parts of the German version are officially available for free on the web and there is also a nearly complete english version freely available. Re your thoughts on output impedance: As Manfred Z writes it's not only depending on the tubes and OT but also screen voltage (more than on anode voltage) and the operating point. And it's depending on input signal size (especially for class AB, so for nearly all guitar amps), but I assume for our usage here, the measured values for small signal input are good enough...



Yes I think the small signal values are what we need. Any attenuator design needs to get the tone right at lower cleanish volume too and as the notes decay, so that's the best place to do the main checks. But then I think it is good to explore the designs at different amp impedances too to capture these variations.


----------



## What?

JohnH said:


> I reckon it could. I worked out a few single stage attenuators using the two-coil arrangement.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Single Stage 190105
> 
> 
> 
> __ JohnH
> __ Jan 4, 2019
> 
> 
> 
> 
> 
> 
> Could likely simplify them down to one coil now.
> 
> Or for just a few db, Id expect that just a couple of resistors would do it in most cases,without a coil



Very interesting. That could open up a world of speaker combinations that weren't previously practical.


----------



## telesto

MAD64 said:


> @JohnH I found some measurements of tube amplifier output impedances in a German book that might be of interest (all for 8 ohms and 1kHz):
> AC30 - 70 ohms
> Marshall 18W - 90 ohms
> Tweed Deluxe - 80 ohms
> Super Reverb (w/o NFB) - 180 ohms
> Super Reverb (w NFB) - 17 ohms


Aside from the Super Reverb with NFB, none of these amps internal resistance is even close to 18ohms. If someone wants to build an attenuator specifically/primarily for one of these amps, wouldn't it be better to change the 18ohm speaker-to-amp impedance target to something higher?


----------



## JohnH

My 18 to 20 Ohm target was just based on my amp, but I now think it is by chance, a good value that works for a much wider range of amps. With regard to the amps with no NFB (and my DSL401 is another), there's the following:

1. to try to make an attenuation stage with a very high output impedance but not too much higher than 8 Ohm input imoedance, the attenuation had to be quite high. The minimum attenuation could not be as low as -7db, limiting the versatility for moderately loud use. 

2. Once you have about 20 Ohms, being speaker 8 ohms x2.5, there's not that much more change to tonal balance as you go much higher.

3. The reactive circuit is self compensating for this effect, over the important ranges from low mids up to high treble. So the variations that are remaining are only for low base resonance

4. When the amp is driven hard, its output changes, dropping output impedance.

5. By not going too high with output impedance, the circuit is also within range of dealing well with much lower impedances, such as is evident on most other Marshalls


----------



## Freddy G

Got all my parts in finally! I have everything bolted in and am ready to wire up. I wonder if one of you guys would be so kind as to look over my layout for any wiring errors. I have always had a hell of a time reading switch schematics. I always end up getting something backwards or plain wrong!
Anyway, I know the layout is a mess, so forgive me for that!






View media item 11798


----------



## Gene Ballzz

Freddy G said:


> Got all my parts in finally! I have everything bolted in and am ready to wire up. I wonder if one of you guys would be so kind as to look over my layout for any wiring errors. I have always had a hell of a time reading switch schematics. I always end up getting something backwards or plain wrong!
> Anyway, I know the layout is a mess, so forgive me for that!
> 
> 
> 
> 
> 
> https://app.box.com/s/e4q1e1mdxruk4iffful8fc3x25xl5ldl



Hey @Freddy G LOL!

Switch scematics throw my mind into a tailspin also!  One of the things that can add to the confusion is some of the common connections of the ins and the outs of some of the resistors and whether it is more convenient to jumper them at the resistor lugs or the lugs of the switches. It sometimes seems/feels like playing a "multi-dimensional" chess game!

Can we assume you are building an 8 ohm version of _*"Design M" *_and are simply opting to have only one 8 ohm output jack? If that's the case, I'll pore over your layout diagram and see if it makes sense. I'll bet however that @JohnH will show up to make quick work of pointing out any snafus!

Answer My Question & We'll Try To Answer Yours?
Gene


----------



## Freddy G

Gene Ballzz said:


> Hey @Freddy G LOL!
> 
> Switch scematics throw my mind into a tailspin also!  One of the things that can add to the confusion is some of the common connections of the ins and the outs of some of the resistors and whether it is more convenient to jumper them at the resistor lugs or the lugs of the switches. It sometimes seems/feels like playing a "multi-dimensional" chess game!
> 
> Can we assume you are building an 8 ohm version of _*"Design M" *_and are simply opting to have only one 8 ohm output jack? If that's the case, I'll pore over your layout diagram and see if it makes sense. I'll bet however that @JohnH will show up to make quick work of pointing out any snafus!
> 
> Answer My Question & We'll Try To Answer Yours?
> Gene




Thanks Gene!
Yes I'm using the "Design M- 4 stage Reactive Attenuator" schematic Jan 10, 2019 posted by John in the first post of this thread.

I have only one 8ohm and one 16 ohm output jack. So since my speakers are 16 ohm I'm using the "output 3 jack" (what I labelled 16ohm in my drawing) and the circuit has the 8 ohm tap values.


----------



## Gene Ballzz

@Freddy G 
I realize you are likely itchin' to get strippin', drillin', mountin' & screwin', along with warmin' up yer solderin' iron. It may take me as long as til tomorrow morning to go through it all (a few times) to feel confident enough to give a clear "go ahead!" The drillin', mountin' & screwin' part looks good though!
Have Fun!
Gene


----------



## Freddy G

Gene Ballzz said:


> @Freddy G
> I realize you are likely itchin' to get strippin', drillin', mountin' & screwin', along with warmin' up yer solderin' iron. It may take me as long as til tomorrow morning to go through it all (a few times) to feel confident enough to give a clear "go ahead!" The drillin', mountin' & screwin' part looks good though!
> Have Fun!
> Gene



Itchin'?

You bet!


View media item 11797


----------



## Gene Ballzz

@Freddy G 
Given that I've only built straight 16 ohm units without that elaborate switching at the output jacks I'm a bit stumped. The #3 output appears to be a TRS switching Cliff type jack and I'm not at all certain if the actual intention is to send that R10 to ground (through the ring terminal) as the drawing seems to indicate? We may need to wait for @JohnH to show up for help?

So far, the rest is looking pretty good.

I'll Keep You Posted,
Gene


----------



## Freddy G

Gene Ballzz said:


> @Freddy G
> Given that I've only built straight 16 ohm units without that elaborate switching at the output jacks I'm a bit stumped. The #3 output appears to be a TRS switching Cliff type jack and I'm not at all certain if the actual intention is to send that R10 to ground (through the ring terminal) as the drawing seems to indicate? We may need to wait for @JohnH to show up for help?
> 
> So far, the rest is looking pretty good.
> 
> I'll Keep You Posted,
> Gene



Yes I missed that when I ordered the jacks, I just got a bunch of tip sleeve jacks. Luckily I have some TRS jacks kicking around. It looks to me like R10 goes to the ring of jack 3 so as to keep it isolated from the ground bus of the other output jacks. Yet since we use a TS plug anyway for a speaker, that ring connection will actually be a sleeve connection. I hope that's correct anyway!


----------



## JohnH

hi @Freddy G , nice work so far and a good time to check in. Id hold off going further until we have assessed the wiring a bit more.

A few comments and questions:

So I assume this is the full 'soup to nuts' version of Design M, to run off an 8 Ohm amp tap and use either 8 or 16 Ohm speakers?

What sort of amps do you want to use? what power?

Is there thermal grease under the resistors? (I don't see the usual ooze that I always create despite trying to be neat) If not, and if you want to go up to 50 Watts, then now is the time to get some in.

What is the a/c current rating on your switches? In this version S1 and S4 get to handle the current associated with the full power of the amp which if its 50W into 8 ohms is 2.5A. (Switches usually give you a current rating at 125V ac)

The case looks great, nice and big so should be an easy wire up. In use, will the components be under the top of the case, or at the base? Thinking about holes for ventilation.

R10. Yes its the only one that uses the ring terminal of a TRS jack at Out3. When the normal mono plug is inserted it gets grounded which puts R10 in parallel with R1, keeping input impedance close enough to 8 when using a 16 cab, and also helping to correct the tone. The other tweak for 16 Ohm is R11, helping to raise output impedance to that expected for the 16 Ohm speaker.

A watchit for using the 16Ohm output (also on the diagram), when you use full bypass or -3.5db on its own, and a 16 Ohm cab, you need to use a 16 Ohm amp tap to avoid mismatch.


----------



## JohnH

I checked it through, and the connections seem to be right so far as I can tell! I'm sure it will be less spaghetti he way you'll build it. And you can test with a multimeter to check resistances before committing it to an amp. With an 8 ohm speaker plugged in, the input should measure somewhere between 8 to 10 in all switch positions except full bypass, which will be your speaker maybe 6.5 o 7 ohms.


----------



## Freddy G

JohnH said:


> hi @Freddy G , nice work so far and a good time to check in. Id hold off going further until we have assessed the wiring a bit more.
> 
> A few comments and questions:
> 
> So I assume this is the full 'soup to nuts' version of Design M, to run off an 8 Ohm amp tap and use either 8 or 16 Ohm speakers?
> 
> What sort of amps do you want to use? what power?
> 
> Is there thermal grease under the resistors? (I don't see the usual ooze that I always create despite trying to be neat) If not, and if you want to go up to 50 Watts, then now is the time to get some in.
> 
> What is the a/c current rating on your switches? In this version S1 and S4 get to handle the current associated with the full power of the amp which if its 50W into 8 ohms is 2.5A. (Switches usually give you a current rating at 125V ac)
> 
> The case looks great, nice and big so should be an easy wire up. In use, will the components be under the top of the case, or at the base? Thinking about holes for ventilation.
> 
> R10. Yes its the only one that uses the ring terminal of a TRS jack at Out3. When the normal mono plug is inserted it gets grounded which puts R10 in parallel with R1, keeping input impedance close enough to 8 when using a 16 cab, and also helping to correct the tone. The other tweak for 16 Ohm is R11, helping to raise output impedance to that expected for the 16 Ohm speaker.
> 
> A watchit for using the 16Ohm output (also on the diagram), when you use full bypass or -3.5db on its own, and a 16 Ohm cab, you need to use a 16 Ohm amp tap to avoid mismatch.



Hey John, thanks for the assistance and this wonderful project!

Yes, "soup to nuts" version. I have an amp that only has an 8 ohm output but I only own 16 ohm speakers. Home brew push-pull EL34s running about 22 watts.

No thermal grease.

Switches are 5A @120VAC

The case will be mounted upside down in the head cabinet of the amp. So the lid will be at the bottom, I will have to remove the lid to mount the case to the inside of the head shell.


----------



## Freddy G

@JohnH @Gene Ballzz







It's alive! Running some tests on it now...Stage 4 is cutting -3.5db, but stage 1 alone is cutting -10.5db. Supposed to be -7db no? could I have miswired something? It's funny. stage 1 sounds great, but I'd really rather have about 7db attenuation....-10.5 is just too much! Anything I should try? maybe different resistor values?
Oh and BTW it's barely getting warm!


View media item 11804


----------



## Gene Ballzz

@Freddy G
That's one of the reasons I've avoided the extra bells & whistles of *"full bypass"* combined with the bypass of the reactive stage to allow for a *"-3.5db, resistive only"* stage. The added complication of switching and wiring for those two "features" increases the possibility/probability of error exponetially, at least from my perspective. You're a really smart guy and I'm certain that between your tracing/re-tracing, combined with a little insight from @JohnH you'll figure it out.

On the other hand, if you were to back up a bit and simply rebuild it as an "M-Lite" (also in the first post of this thread) with the simple addition of the impedance correction portion for 16 ohm speaker use, all would be happy and good! In your situation of only an 8 ohm amp output with a 16 ohm only speaker load, you can't ever really use the *"-3db only"* or *"full bypass"* modes anyway? Add to that the fact the a -3.5db reduction is easily accomplished by as experienced a player as yourself by playing dynamics and careful volume setting of the amp. -7db is pretty much a perfect place to start the attenuation process!

I'm a bit more fortunate, as in my universe, the only single output tap amp I have is 16 ohm only and the 16 ohm version of this attenuator design doesn't really care if the speaker is 16 ohm or 8 ohm!

Oh and by the way, I think the reason its only "it's barely getting warm" is that some small wiring snafu is combining the attenuation levels of the heavier wattage resistors of the first reactive, -7db stage and the -3.5db stage into being all part of the first stage, thus spreading out the dissipation a bit! That is likely also why you are seeing a -10.5db drop as your first stage. That's the place to inspect and troubleshoot for oopsies!

Of course, John is the real expert here, and I'm just a meatball hack who knows how to strip, drill, mount, screw and solder! 

Just My ,
Gene


----------



## Freddy G

Gene Ballzz said:


> @Freddy G
> That's one of the reasons I've avoided the extra bells & whistles of *"full bypass"* combined with the bypass of the reactive stage to allow for a *"-3.5db, resistive only"* stage. The added complication of switching and wiring for those two "features" increases the possibility/probability of error exponetially, at least from my perspective. You're a really smart guy and I'm certain that between your tracing/re-tracing, combined with a little insight from @JohnH you'll figure it out.
> 
> On the other hand, if you were to back up a bit and simply rebuild it as an "M-Lite" (also in the first post of this thread) with the simple addition of the impedance correction portion for 16 ohm speaker use, all would be happy and good! In your situation of only an 8 ohm amp output with a 16 ohm only speaker load, you can't ever really use the *"-3db only"* or *"full bypass"* modes anyway? Add to that the fact the a -3.5db reduction is easily accomplished by as experienced a player as yourself by playing dynamics and careful volume setting of the amp. -7db is pretty much a perfect place to start the attenuation process!
> 
> I'm a bit more fortunate, as in my universe, the only single output tap amp I have is 16 ohm only and the 16 ohm version of this attenuator design doesn't really care if the speaker is 16 ohm or 8 ohm!
> 
> Oh and by the way, I think the reason its only "it's barely getting warm" is that some small wiring snafu is combining the attenuation levels of the heavier wattage resistors of the first reactive, -7db stage and the -3.5db stage into being all part of the first stage, thus spreading out the dissipation a bit! That is likely also why you are seeing a -10.5db drop as your first stage. That's the place to inspect and troubleshoot for oopsies!
> 
> Of course, John is the real expert here, and I'm just a meatball hack who knows how to strip, drill, mount, screw and solder!
> 
> Just My ,
> Gene



@Gene Ballzz I can't see anything wrong with the wiring. Unless there is something wrong with the layout I drew up. But I have an idea....I bought enough to build two of these (because I run two amps in stereo) so I'm just going to put together another one, but first stage only with output 3 load for a 16 ohm speaker and see if there's a difference!

You may be a meatball, but I'm the cheese! lol

Oh, and another thing...the amp squeals when I touch the toggle bats or the attenuator chassis. If I ground the chassis to the sleeve of the input jack it stops. Normal?


----------



## Gene Ballzz

Freddy G said:


> @Gene Ballzz
> 
> Oh, and another thing...the amp squeals when I touch the toggle bats or the attenuator chassis. If I ground the chassis to the sleeve of the input jack it stops. Normal?



Funny you should mention that. I have experienced similar "artifacts" but only under somewhat extreme attenuation (-10db/-14db or more) with the amp fully cranked. I wrote it off to the massive amount of power being dissipated by a resistor/inductor network in close proximity to my guitar and possibly the amp itself. That *"oscillation"* seems to go away when I get my guitar a couple feet farther away from the amp and attenuator! No one else has reported this. I'm suspecting it my actually be some sort of mechanical or other physical vibration/oacillation within the unit itself. You know, kinda like how humbucker pickup coils/magnets/covers can sometimes vibrate, causing a squeal if the bobbins are not securely tightened and/or cover properly damped! Could also be the cheap, cheesy, tiny switches (I used 'em too) screamin' for mercy, no matter what their ratings may claim to be! And then of course, there's those VERY affordable resistors made in Chinesiawanoreanam! There's also the thought of the inevitable magnetic fields created by the inductor coils in such close proximity to all else? I wish I were a rocket scientist, but I'm sadly not! Although the name of my current band is *Rock It Science! *

Just Thinkin'
Gene


----------



## telesto

Freddy G said:


> Stage 1 alone is cutting -10.5db. Supposed to be -7db no?


I'm just curious, how are you measuring that so exactly?



Freddy G said:


> Oh, and another thing...the amp squeals when I touch the toggle bats or the attenuator chassis. If I ground the chassis to the sleeve of the input jack it stops. Normal?


I was looking at your inductors and wondering if the coils are isolated? ie: is the metal coil touching the chassis directly? Or is the red color some insulator coating and the wire is under that? I would guess it's probably not a good idea to have the raw coil touching the chassis, if that's what's happening there.

I was playing around with some other configurations (my own different configs, not John's design), and switching the coil in and out, and noticed sometimes I would have some squealing with the coil in, but when I switched it out I never had it. If I were you, I would look at the coil first. If it's really causing problems, try to un-solder it and bypass it with just a straight wire and see if it stops. At least you can rule it in or out as the source.


----------



## Gene Ballzz

telesto said:


> I'm just curious, how are you measuring that so exactly? AC Voltage with a multimeter, or..?
> 
> 
> I was looking at your inductors and wondering if the coils are isolated? ie: is the metal coil touching the chassis directly? Or is the red color some insulator coating and the wire is under that?



Freddy G is a highly experienced and advanced recording/studio/live sound engineer and likely has access to measurement accuracy that mere plebes can only dream of. My apologies to Freddy if I'm sharing too much?

And it kinda looks as though he may have put some sort of plastic isolator/insulator between the coils and the case. Yes, the coil wires are at least lacquer coated/insulated, but I'm not sure I would want to depend on that, for the long term!

Just Sayin'
Gene


----------



## JohnH

hi @Freddy G 
Build looks great, all of that happened while I got a few hours sleep!

On the db ratios, there's a few things we can discuss. Yours is the first build I know of where the use of 16 Ohm speakers in an 8 ohm version is the main purpose of it. Usually its either been omitted or just adopted as a nice-to-have extra. I do have it on mine though, identical to yours and it works. But I haven't written as much about this feature. One of the things that happens is that when using the 16 Ohm speaker, there's about another 2db loss, so nominally -9 db for the first stage. So it could be just that though you are measuring a bit more. But, how are you measuring that difference? if its to compare the first stage output to the full bypass, would that involve the 16 Ohm speaker being sent through directly to the 8 ohm amp tap? If that's the case then it could explain the difference, but consider the mismatch 8 to 16 Ohms.

Although you are looking for a bit more volume, how is the tone that you are getting?

Things to try:

1. You can plug the 16 cab into the 8 output and run it in the -7db setting, its all safe and you should actually get a bit louder, like -6db. You may or may not hear a small reduction in high treble and maybe bass. If its OK, or you can compensate at the amp EQ then that's one good way. If you have them, the best controls on the amp for such EQ tweaking would be resonance and presence, or else something in the effects loop.

2. Using the 16 output again, You can bypass the 10 Ohm resistor R11. This will get stage 1 back to -7 db and bring back a bit of treble compared to option 1. Or if R11 goes to 5 Ohm Stage 1 is at -8 db with a 16 cab.

3. We can run a full resistance check on your build and compare it to mine and to the theory. Plug in the 16 Ohm cab to the 16 Ohm output as it is, then without the amp, measure resistance across the input in all settings from full bypass down to max attenuation. Allow for the resistance of meter leads by touching them together and subtracting that reading. Ill do the same on mine. They wont be identical but we should see if there is anything really different.


----------



## JohnH

hi @telesto 

As amateur shade-tree electronics tinkerers we may not have access to accurate db measurements in absolute terms. But we can make comparative measurements if a few ways. One trick I use for relative db's is to make some recording into the computer running Audacity. Then, I select a portion and tell it to 'amplify', and it will report how many db's it is going to offer me before clipping. If I do that on two traces I can subtract these values to get the db difference, to within 0.1 db. I've also used this method for the amp impedance measurements instead of a meter, working back from db ratios to voltage ratios.


----------



## Freddy G

Gene Ballzz said:


> Could also be the cheap, cheesy, tiny switches (I used 'em too) screamin' for mercy, no matter what their ratings may claim to be! And then of course, there's those VERY affordable resistors made in Chinesiawanoreanam! There's also the thought of the inevitable magnetic fields created by the inductor coils in such close proximity to all else? I wish I were a rocket scientist, but I'm sadly not! Although the name of my current band is *Rock It Science! *
> 
> Just Thinkin'
> Gene



Could be the components, but I doubt it...I think in a circuit like this they either work or they don't. I did however think it might be a good idea to separate the two coils so proximity to each other would not cause problems. Rock It Science....I love it!



telesto said:


> I'm just curious, how are you measuring that so exactly?
> 
> 
> I was looking at your inductors and wondering if the coils are isolated? ie: is the metal coil touching the chassis directly? Or is the red color some insulator coating and the wire is under that? I would guess it's probably not a good idea to have the raw coil touching the chassis, if that's what's happening there.
> 
> I was playing around with some other configurations (my own different configs, not John's design), and switching the coil in and out, and noticed sometimes I would have some squealing with the coil in, but when I switched it out I never had it. If I were you, I would look at the coil first. If it's really causing problems, try to un-solder it and bypass it with just a straight wire and see if it stops. At least you can rule it in or out as the source.



How do I measure? I just stuck a mic in front of the speaker, recorded a riff into a looper and played it back while I recorded it at all the different attenuations. I know that my mic pres are very linear because I built them, they have tons of headroom. Also I sent a sine wave signal through the amp and recorded it as well, just to cross check.

Yes the coils are isolated although it's hard to tell from the pics....good eye! But regarding the squealing, as I said, all I have to do is ground the chassis and it cures it. So I guess the simplest and most effective fix is right in front of me!



Gene Ballzz said:


> My apologies to Freddy if I'm sharing too much?
> 
> Just Sayin'
> Gene



Nah....all good!



JohnH said:


> hi @Freddy G
> Build looks great, all of that happened while I got a few hours sleep!
> 
> On the db ratios, there's a few things we can discuss. Yours is the first build I know of where the use of 16 Ohm speakers in an 8 ohm version is the main purpose of it. Usually its either been omitted or just adopted as a nice-to-have extra. I do have it on mine though, identical to yours and it works. But I haven't written as much about this feature. One of the things that happens is that when using the 16 Ohm speaker, there's about another 2db loss, so nominally -9 db for the first stage. So it could be just that though you are measuring a bit more. But, how are you measuring that difference? if its to compare the first stage output to the full bypass, would that involve the 16 Ohm speaker being sent through directly to the 8 ohm amp tap? If that's the case then it could explain the difference, but consider the mismatch 8 to 16 Ohms.
> 
> Although you are looking for a bit more volume, how is the tone that you are getting?
> 
> Things to try:
> 
> 1. You can plug the 16 cab into the 8 output and run it in the -7db setting, its all safe and you should actually get a bit louder, like -6db. You may or may not hear a small reduction in high treble and maybe bass. If its OK, or you can compensate at the amp EQ then that's one good way. If you have them, the best controls on the amp for such EQ tweaking would be resonance and presence, or else something in the effects loop.
> 
> 2. Using the 16 output again, You can bypass the 10 Ohm resistor R11. This will get stage 1 back to -7 db and bring back a bit of treble compared to option 1. Or if R11 goes to 5 Ohm Stage 1 is at -8 db with a 16 cab.



OK...haha full disclosure! I used the 8 ohm output to do my testing. Here in my shop I have a 2 X 12 cab loaded with 16ohm speakers to make an 8ohm cab. But I won't be using that cab with my amps or these attenuators. I have two single 12, 16ohm cabs at my rehearsal studio that I use for my rig. 

The tone is great with the stage 1 reactive load. Just too much of a cut.



> 3. We can run a full resistance check on your build and compare it to mine and to the theory. Plug in the 16 Ohm cab to the 16 Ohm output as it is, then without the amp, measure resistance across the input in all settings from full bypass down to max attenuation. Allow for the resistance of meter leads by touching them together and subtracting that reading. Ill do the same on mine. They wont be identical but we should see if there is anything really different.



That sounds like a great idea. I will do that. But I'm knocking off for the day, back tomorrow. Thanks guys!


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## JohnH

Here's my resistance values:




More than might be needed, but in the table above, there's DC resistance values at the input for a 16 Ohm cab into Out 3, and 8 ohm cab into out 1/2 and also no cab. Also I've put in the theoretical db attenuations at mid frequencies based on the component values.

The DCR columns are calculated values assuming perfect resistors and no losses in wire, contacts or meter leads (they do include coil resistance). The measured values are as measured by me today and are all higher by up to an Ohm or more, a variable amount. This is due to meter accuracy, component tolerances and all the other factors. I think its hard to get very good reliable values of small resistances on a basic meter. I don't think these variations are a problem, but I'm showing them just to illustrate that on another build, there may be other variations measured and it could still be all fine.

So if you try these, id expect some different variation but with the same trends. I think some of the most useful checks might be the ones with no cab, because the values do change distinctly and if there is something to be found, it may reveal it.

Another chart to look at is on page 1, where I recorded loops and then plotted out the db vs frequency graphs and also the differences in dbs to see the reduction steps. As you go from full volume down to the attenuated, the overall trend stays consistent but there is considerable wiggle happening between full and -7 db traces, which could be speaker breakup and room effects happening at loud volumes. So if you are interpreting db changes in terms of peak levels, then this could add to the attenuation values you are noting.













Attenuator M Frequency Plots 190302



__ JohnH
__ Mar 1, 2019






Lets explore what you have, and either find some build glitch in it, or else use it to figure out what should change to suit you better once you have tried it out with the full rig.


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## telesto

Freddy G said:


> How do I measure? I just stuck a mic in front of the speaker, recorded a riff into a looper and played it back while I recorded it at all the different attenuations.


Microphones...man, you rock-star guys get all the fancy equipment 

LOL, just kidding . Ok, sounds pretty much the same like what John does, record a riff on a loop and mic it into a PC and see the levels there, makes sense. I tried measuring the difference with a SPL meter about 3 feet in front of the speaker, but I didn't really see it register much difference between some of the different attenuation settings


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## Freddy G

JohnH said:


> Here's my resistance values:
> 
> View attachment 76310
> 
> 
> More than might be needed, but in the table above, there's DC resistance values at the input for a 16 Ohm cab into Out 3, and 8 ohm cab into out 1/2 and also no cab. Also I've put in the theoretical db attenuations at mid frequencies based on the component values.
> 
> The DCR columns are calculated values assuming perfect resistors and no losses in wire, contacts or meter leads (they do include coil resistance). The measured values are as measured by me today and are all higher by up to an Ohm or more, a variable amount. This is due to meter accuracy, component tolerances and all the other factors. I think its hard to get very good reliable values of small resistances on a basic meter. I don't think these variations are a problem, but I'm showing them just to illustrate that on another build, there may be other variations measured and it could still be all fine.
> 
> So if you try these, id expect some different variation but with the same trends. I think some of the most useful checks might be the ones with no cab, because the values do change distinctly and if there is something to be found, it may reveal it.
> 
> Another chart to look at is on page 1, where I recorded loops and then plotted out the db vs frequency graphs and also the differences in dbs to see the reduction steps. As you go from full volume down to the attenuated, the overall trend stays consistent but there is considerable wiggle happening between full and -7 db traces, which could be speaker breakup and room effects happening at loud volumes. So if you are interpreting db changes in terms of peak levels, then this could add to the attenuation values you are noting.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator M Frequency Plots 190302
> 
> 
> 
> __ JohnH
> __ Mar 1, 2019
> 
> 
> 
> 
> 
> 
> Lets explore what you have, and either find some build glitch in it, or else use it to figure out what should change to suit you better once you have tried it out with the full rig.



Thanks for doing this @JohnH ! 

just to be clear, is this your order of increasing attenuation?

Full
1- stage 4
2- stage 1
3- stage 1 & 4
4- stage 1 & 2
5- stage 1,2 & 4 
6- stage 1 &3
7 -stage 1, 3 & 4
8- stage 1, 2 & 3
9- stage 1, 2, 3, 4


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## JohnH

That seems right. There were a few electrical reasons for putting them in that order, the biggest was so that stage 4, which is the -3.5db goes last so that it can work on its own when wanted


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## Freddy G

@JohnH 

well look what we have here!

View media item 11807


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## JohnH

hmm..I can't seem to read that due to some permissions issue?


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## Freddy G

JohnH said:


> hmm..I can't seem to read that due to some permissions issue?


Sorry @JohnH , just figuring out how to use albums....try this:














Attenuator Check 08_07_20



__ Freddy G
__ Jul 8, 2020


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## Freddy G

Spotted my wiring error! R6 was not connected to S3. Man.....I remember when I was drawing that layout and I continued the yellow line from R6 to S3 to S1 i thought to myself "self, maybe you better make the line a different colour from S3 to S1 or you might miss that that is a connection" 
And wouldn't you know it, that's exactly what I did when I wired it. D'OH!


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## JohnH

Good catch! That would leave R6, which is 15 Ohms, in series with the speaker and never bypassed, which would explain another -3.5db.

Might be worth stepping through those tests again at least for settings 1-4 and Full. It looks like there may still be something funky happening at the 16 Ohm output 3, where at full bypass it should read the speaker DCR of about 13-14 ohms and at setting 1 it should be around 15-16 ohms


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## Freddy G

JohnH said:


> Good catch! That would leave R6, which is 15 Ohms, in series with the speaker and never bypassed, which would explain another -3.5db.
> 
> Might be worth stepping through those tests again at least for settings 1-4 and Full. It looks like there may still be something funky happening at the 16 Ohm output 3, where at full bypass it should read the speaker DCR of about 13-14 ohms and at setting 1 it should be around 15-16 ohms



I think it's all good. here are the new numbers:




Yeah, this thing sounds and feels just great! I'm so happy.

One last request @JohnH ....I would greatly appreciate it if perhaps you might plug in what the resistor values need to be if I wanted to change that first stage (reactive -7db) to a -5db cut!


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## JohnH

That's all great. I'll try what might be possible with the resistor values tomorrow.


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## Gene Ballzz

@Freddy G & @JohnH 
Let me see if I've got my understanding correct?
_*A) *_The minimum amount of attenuation that allows effective use of reactive, inductance coils is about -7db?
_*1) *_Any smaller level of attenuation (by itself) is not only not benefitted by inductance, but may actually introduce a degradation of tone, response and/or feel.
_*2) *_Smaller amounts of attenuation are best achieved through resistance only?​_*B) *_It seems that the most important factor in John's design is a carefully maintained *"ratio"* of series and parallel resistance? This is one of the reasons why a continuously variable control becomes much more complicated and cumbersome to implement.
_*1) *_The choice of attenuation levels is somewhat dictated/limited by the standardly and conveniently available values of resistors in the required power/wattage ratings, although this could be adjusted through intelligent use of series and parallel arrangements to achieve specific resistance values?​Please correct me John, if I'm out in the weeds with my cursory understanding.
Thanks,
Gene


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## JohnH

Hi @Gene Ballzz , kinda like that!

It is difficult to keep everything in balance across a wide range. That's after realising what parameters need to be controlled. inductance and resistance seen by the amp and the speaker, and keeping them adequately in control as you step down. This is where the stepped stages work much better in a device that is simple to build, since they can be tuned for their specific job without requiring any exotic parts.

I find that the standard resistor ranges work OK provided ratios are maintained. Some of the online Chinese selections need a tweak at certain values, maybe two used together or one of higher value in parallel to bring it down.

But keeping the ideal balance is quite a trick. The current designs M and M2 start with the -7db stage which means that the sound is at 20% max power. The amp sees the attenuator and somewhere between 0 and 20% of the speaker. So the reactance at the amp is not varrying too much and can be optimised for the general range -7db down. 

As we go towards a smaller first stage reduction, the speaker starts to appear more to the amp. This either needs the tone to be allowed to vary a bit wider, or it needs a different balance of components just for those settings alone. In the last couple of pages, I posted some single stage versions, tuned to just one attenuation, but these would not be ideal for a general full-range build.

The -3.5db stage in M working alone, as built by Freddy and myself, is a workaround, and it works quite well even though its only resistive. It would be a bit better with a bit more inductance in it, but it's not worth the complication of doing that just for this one setting, that is rarely used.

For @Freddy G we are talking about whether a couple more db s could be added to the maxmum attenuated volume, and I'm assuming this is to focus on use of 16Ohm cabs. I've got a few thoughts for next post....


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## Gene Ballzz

Thanks @JohnH for clarifying that. It confirms, at least in my mind, that I'm only partially full of poo! 
Thanks Again,
Gene


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## JohnH

No worries Gene!

Hi @Freddy G

It seems like your build is running on all cylinders now, and the resistance checks seem a lot more consistent than my own ones, being very close to the theory after allowing for meter leads. As such, its all in balance and for a box covering the full range that it does, I’ve not been able to further improve the design in the last year and a half, which includes trying to get another db or two as max attenuated volume. So to squeeze the design in that way, tends to make it pop slightly out of whack elsewhere. That could be OK, and its obviously most important that it works as well as it can for the range and use that you need.

I'm happy to play wit resistor numbers but, I think it would also be best to explore options that keep its core performance as intact as possible.

The extent that it reacts differently to different amps depends on the amp design. The most sensitive are those without negative feedback, such as one of mine, AC30’s, some small Fenders and Marshalls etc. What can you say about the output stages in your amps? Are they similar to others and do they have NFB? Any resonance and presence controls?

Reason for asking is that you might find some easy solutions, once you fire it up with the 16 Ohm cab. The as-designed Output 3 will cut another 2 db from a 16 Ohm cab, so it becomes -9db, relative to a 16 cab straight into a native 16 Ohm amp tap. But if you put that 16 cab into Output 1, then you get -6db. I hear that as an OK but not so edgy sound on my amps, and OUT3 corrects for that tone difference. But on a Marshall Plexi for example, it may make less difference ,and presence and resonance might also fix it. Worth a try. Using Out1 with a 16 cab is safe with stage 1 on (ie usually the -7db).

Also, when you run your whole rig in stereo, there’s another +3db right there from the second amp?

Here’s another way to wire it up, just thought of it:

I've talked before about using the attenuator as a load box, set to max attenuation, no cab in attenuator, to get a -3db setting when in parallel with the speaker. This was to use an 8 Ohm cab, and use a 4 ohm amp tap. Here’s the new idea. same setup, but using your 16 Ohm speaker direct to the 8 Ohm amp tap, and use the attenuator in parallel (ie if you have a second amp out socket or can rig one). For this, set the attenuator to setting 2, - 7db, and with no cab into it, it provides a credible reactive 16-ish Ohm load and you get -3db reduction. Thel load on the amp is now 16 from the speaker and 16 from the attenuator/load box so set, so 8 overall.

EDIT. I just tried this myself, it works. I used the 8Ohm tap on my VM, and plugged my 16 Ohm V30 cab into it and also the attenuator, set at -7db with no cab. Maybe that will get close to what you need, but anyway its another useful trick.


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## Freddy G

JohnH said:


> No worries Gene!
> 
> Hi @Freddy G
> 
> It seems like your build is running on all cylinders now, and the resistance checks seem a lot more consistent than my own ones, being very close to the theory after allowing for meter leads. As such, its all in balance and for a box covering the full range that it does, I’ve not been able to further improve the design in the last year and a half, which includes trying to get another db or two as max attenuated volume. So to squeeze the design in that way, tends to make it pop slightly out of whack elsewhere. That could be OK, and its obviously most important that it works as well as it can for the range and use that you need.
> 
> I'm happy to play wit resistor numbers but, I think it would also be best to explore options that keep its core performance as intact as possible.
> 
> The extent that it reacts differently to different amps depends on the amp design. The most sensitive are those without negative feedback, such as one of mine, AC30’s, some small Fenders and Marshalls etc. What can you say about the output stages in your amps? Are they similar to others and do they have NFB? Any resonance and presence controls?
> 
> Reason for asking is that you might find some easy solutions, once you fire it up with the 16 Ohm cab. The as-designed Output 3 will cut another 2 db from a 16 Ohm cab, so it becomes -9db, relative to a 16 cab straight into a native 16 Ohm amp tap. But if you put that 16 cab into Output 1, then you get -6db. I hear that as an OK but not so edgy sound on my amps, and OUT3 corrects for that tone difference. But on a Marshall Plexi for example, it may make less difference ,and presence and resonance might also fix it. Worth a try. Using Out1 with a 16 cab is safe with stage 1 on (ie usually the -7db).
> 
> Also, when you run your whole rig in stereo, there’s another +3db right there from the second amp?
> 
> Here’s another way to wire it up, just thought of it:
> 
> I've talked before about using the attenuator as a load box, set to max attenuation, no cab in attenuator, to get a -3db setting when in parallel with the speaker. This was to use an 8 Ohm cab, and use a 4 ohm amp tap. Here’s the new idea. same setup, but using your 16 Ohm speaker direct to the 8 Ohm amp tap, and use the attenuator in parallel (ie if you have a second amp out socket or can rig one). For this, set the attenuator to setting 2, - 7db, and with no cab into it, it provides a credible reactive 16-ish Ohm load and you get -3db reduction. Thel load on the amp is now 16 from the speaker and 16 from the attenuator/load box so set, so 8 overall.
> 
> EDIT. I just tried this myself, it works. I used the 8Ohm tap on my VM, and plugged my 16 Ohm V30 cab into it and also the attenuator, set at -7db with no cab. Maybe that will get close to what you need, but anyway its another useful trick.



Thanks for all that. Very interesting ideas!
So, I was just spitballing when I asked about less than 7db cut. You're right, I will be running two amps in stereo. 7db might be just right, I won't know until I play the rig with the band. I should do that before I start asking to re-invent the wheel!

I have a 47k NFB resistor. Resonance circuit is fixed, 220K resistor in parallel with .0047 cap. Presence control 5k pot with a .1uf cap. I have been thinking about tweaking this though....maybe try a 33k or 27k NFB resistor. I'm finding that the time based effects in the FX loop are not quite as clean as I would like and hopefully more NFB (ie smaller value resistor) will clean it up but also it will knock off a db or two I think. Could be a win-win!


----------



## telesto

Hi All,
I was doing some research on power resistors and their effective ratings. In the past, I read alot of people say doubling the value is the way to go (eg. 100W resistor for a 50W amp). In some pro-grade attenuator schematics, I see 4x power ratings (eg. 400W resistors to take 100W amp) along with motorized cooling fans.

I saw something interesting from a Vishay resistors catalog that I thought I would share here. They say in order for a resistor to meet it's power rating spec, it needs a specific heat-sink, which for 50W resistors and up is a required 290 sq. inches of surface area (!). Without the heatsink, the resistor is rated at only 40% of it's specified rating, up to 25°C, after which the rating starts to decrease to about 30% at 100°C (see chart below). For temp comparison, 25°C is about room-temperature, 50°C is a nice summer day in Saudi Arabia, 100°C is a good sauna and 200°C is literally an oven.

Even if you mount the resistors to a metal chassis, it probably won't come near the surface area required, so I would stay with the 40% rated value, and considering the box may get a little warm inside, I think it's safe to lower it down a little more (and add ventilation to help offset that). So with this in mind, I think John's recommendation of 1/3 power is indeed accurate.

...keep in mind this is Vishay resistor recommendations, if using cheap Chinese resistors, your experiences may vary


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## JohnH

Thanks @telesto that's interesting indeed. 

I'll be happy to stick to my recipe. But one thing that most are finding is that under about 50W, with the case mounted resistors, its hard to actually get it to heat up much, and it takes a long time. I think that unless you play continuous thrash metal with no break, the average power is considerably less and any heating takes about the length of a set.


----------



## telesto

Yea, I also read some people saying that your attenuator stays pretty cool and was a little surprised by that. R1 is taking about 50% of the power. On my test circuit, I think it was getting kind of warm with a 50W resistor and 10w amp after a few minutes, if I remember correct (I was playing with many different configurations), but then mine is mounted on a wooden board. I'll have to go back and test it again.

The guy who built the Aiken dummy load said his 200W resistor was hot enough to "fry an egg on" with his 50W amp, but then that's taking 100% of the power, but still the resistor is rated 4x higher than his amp...hmm...


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## JohnH

I think the experience of heat vs power is quite different to sound volume vs power. 

Doubling power adds a tad more volume experience, but if it doubles temperature rise from 40C degrees up to 80 C, then temp rises from say 20 to 60 degrees which won't hurt, but then doubles up to a rise of 20 to 100 C, then it will burn. Not enough to fry an egg but at least enough to boil one!

My expectation remains that a heavy, thick aluminium case with thermal grease and ventilation holes above and below, will take good control of temp rises from amps up to 50W. Much above that, it depends on the use but a fan may be indicated 

Aluminium is far better than steel for conducting heat away. it is about 5x as conductive, and a given case could be 2 to 3 times thicker, so you can easily get a x10 improvement. its why your stainless steel pots have an aluminium base. Copper would be even better.


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## telesto

Switching gears away from resistors; have you thought to make a line-out on the attenuator, like on the Aiken dummy-load?


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## telesto

...shifting gears back to resistors again (sorry for jamming the gears) . When an attenuator is rated eg. "100W", is that the power rating of the resistors? Or the rating of the amp? It's always been confusing to me. I always understood it to be the amp power it can handle, however, I think even the manufacturers are not in agreement on this. Looking at a Tube-Town "100W" attenuator schematic, they use 4x 150W power resistors (600W total!) implying it can take a 100W amp (or even 150W probably). But then looking at Weber Mini-Mass, they say this (which means the Weber "200W" attenuator has power resistors equaling 200W):

IMPORTANT NOTE ABOUT HIGH GAIN AMPS: High gain amps such as Marshall or clones (with heavy distortion) are hard on attenuators. Unfortunately, an amp's volume dial is not a good indicator of how much power is being produced. Many amps can reach full power at 3 or 4 on the volume dial, so it is a good suggestion to get an attenuator with 4x the power handling. For example, if you have a 50w high gain amp, you should opt for the MASS 200. If you have a 100w amp, you should still go wtih the MASS 200 but you should not crank the amp.

https://tedweber.com/minimass


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## JohnH

hi @telesto 

*Line-outs*

Timely that you mentioned that, ive been experimenting a bit. There has been some line outs built on the thread, but i haven't done one myself. 

For my use, I'd want to have a cab-sim too, and that's where I've been investigating. 

My initial use would be for with my guitar lessons, which I've been doing online using Zoom. I just need to send my teacher a credible reasonably clean signal without killing him! I've been doing this with the DI out from my DSL401, which is actually not bad, but I'd like to use the VM if I could. And I could just use the mic instead, but i also need this set up for speaking so its not at the cab.

Im starting, as with the early posts in the whole thread, with low expectations and expecting that results will be ok at best, but hopefully usefull. Then we'll see whether 'ok' can be upgraded to 'reasonably good' or better.

Line outs with or without cab sim can be drawn either from the input (ie amp output), or from the speaker. They tend to have a resistor in series with a pot of 1k to 10k to set up a line level. Up for testing is what the ground should be doing, and whether the line out should be grounded to the mixer, attenuator case, amp ground, ground lifted or some combination.

I've been testing by taking a signal from one of the attenuator speaker outs. With a fair amount of attenuation it is already at line level-ish and it can go into a mixer. Its a super crispy-clear tone on a clean signal, and pretty nasty with distortion. But at the speaker out, its picking up several aspects of the speaker itself including the bass resonance, treble rise and various interesting quirks across the range due to the speaker.

I've been recording that with a loop, and also recording an identical miced signal. Then I compare the frequency responses and plot the difference using Audacity and Excel to see what a possible cab-sim circuit should be doing. This is currently telling me to apply a bass reduction below about 200hz, a steady reduction in treble above about 300hz , a shallow dip around 1000hz and a sharp drop after about 5000hz. Applying that as an EQ curve in Audacity sounds good, not quite the same as the mic, but credible. 

Then Im developing a real circuit to create that EQ shape, tested in a Spice sim. I have something for that which has reached 'not bad' status using a passive LRC network. Still experimenting. If i end up wanting more EQ wrangling I'll go to an active circuit, but I like the idea of needing no power.

But these days, it seems most will use a line-out to drive some form of impulse-response box, and maybe with no direct speaker. The line-out then can just be the resistor and pot. But the question remains where to take it from. The current M and M2 designs could provide this from the input or the output, but if from the input, the bass resonance is not developed there, so if required would need to be added down-stream. Design M3 would have this since it adds the bass circuit at the front.


----------



## JohnH

*Power and Heat*

I think the way manufactuers express power handling on these devices is subject to some variation and optimistic values, with a temptation to overstate. Is it the the real power of the amp? driven how?, or the nominal output? or the listed rating on the internal components? Weber lists his products as 50, 100, 200 etc, but is honest in advising about what amo rating they can really handle in different circunstances, which is often less.

Our designs here if built right, are brick-solid with very simple robust parts. But when you look at some of the commercial designs, and the way they are built and the parts used, youd have to wonder about them when they get cooked by a powerful amp.


----------



## telesto

JohnH said:


> My initial use would be for with my guitar lessons, which I've been doing online using Zoom. I just need to send my teacher a credible reasonably clean signal without killing him!








JohnH said:


> But these days, it seems most will use a line-out to drive some form of impulse-response box, and maybe with no direct speaker. The line-out then can just be the resistor and pot. But the question remains where to take it from. The current M and M2 designs could provide this from the input or the output, but if from the input, the bass resonance is not developed there, so if required would need to be added down-stream. Design M3 would have this since it adds the bass circuit at the front.


I vauguely remember the M3 design, but can't find it, you have just the M1 and M2 linked at the beginning of the thread, can you add it to the first page, or link to it somehow?  Why does it have a bass circuit if you take the bass from the speaker?


----------



## JohnH

M3 is on page 27, also linked on page 1. But here it is:




(R2B would be 18 and R1 15)

its the same as M2, with the extra bass resonant circuit. In theory, it let's the amp react to the bass resonance and also helps to adapt to different amps. But the two added parts are not cheap and probably double a build cost. The expensive commercial units have something equivalent but I'm not expecting they make much difference in practice to our design.

I don't think anyone has built it yet. But im tempted to, just to test it.


----------



## Freddy G

Band practice last night with my stereo rig. I kept flipping between -7db and -3.5db cut. -7db sounded great, but left me no headroom....step on a boost pedal and nothing got louder! So it looks like -3.5db cut is all I need. 
I was running my 16 ohm speakers from output 3. Am I correct in understanding that just running them from that output cuts a couple of db? So with a 3.5db cut I'm actually hearing more like -5.5db?


----------



## JohnH

ok. Good that it works. One thing to consider:

If the 16 ohm cabs were running with just the -3.5db stage only, then the amp sees about 16 ohm. Once you are at -7db or more, then the 16 cab appears closer to 8 to the amp.


----------



## Freddy G

JohnH said:


> ok. Good that it works. One thing to consider:
> 
> If the 16 ohm cabs were running with just the -3.5db stage only, then the amp sees about 16 ohm. Once you are at -7db or more, then the 16 cab appears closer to 8 to the amp.



Dog-gone it! That might explain why the -7db sounded good. Well, maybe I will have to look at some new output transformers. The ones I have now (Mercury Magnetics) only have the 8ohm tap, and I originally bought them to run 6v6 tubes so the trannies are 8k primaries. But I didn't like the sound of 6v6....so I'm running EL34s and I suspect that the 8k primary is not ideal for EL34s anyway. Can anyone confirm or deny I might be going down a stupid rabbit hole? lol....


----------



## Esa Martikainen

8k OPT impedance is fine for two EL34 push pull achieving 50W output when anode voltage is close to 500VDC and G2 is around 450V.

This link has an excellent calculator to test tubes, impedances, voltages etc.

Esa

https://www.vtadiy.com/loadline-calculators/loadline-calculator/


----------



## Gene Ballzz

@Freddy G
For your current situation, it would seem best to have built the straight 16Ω version of the M-lite and run it with all stages activated, in parallel with your 16Ω speaker(s). It would be perfect for your 8 ohm amp tap and give you about -3.5db, *WITH* the benefit of the reactive inductor coil circuit. An additional benefit is that you could still progressively switch off stages to lessen the amount of attenuation, if -3.5 was a bit too much of a cut. I'm not sure what the numbers would be and I don't think @JohnH has ever posted them? My guess is that each step turned off would be small *"baby steps"* for fine tuning your volume from -3.5db to almost no attenuation at the -7db only setting. Another small benefit of the 16 ohm version is that at least in series use, it doesn't seem affected very much by using 16 or 8 ohm speakers!

Now on the other hand, if you *DID* go ahead and upgrade your output transformers to something with with multiple (at least 8 ohm & 16 ohm) taps, more options become available. You could use the 8 ohm amp tap for parallel operation from -3.5db down to nearly no attenuation or use the 16 ohm amp tap for the more standard *"series"* operation to give -3.5db steps from -7db, all the way up to -31.5db of attenuation. In my considered opinion, this would be the best way to achieve a -3.5db or smaller cut and still retain the benefits of the reactive nature of this design. Smell what I'm steppin' in here? 

Of course, if you went with an output transformer with 4/8/16 ohm taps, you could use the attenuator(s) you've already built, in parallel on the 4 ohm tap. My concern here would be the stack up of variances caused by having a unit designed to present an 8 ohm load to the amp while using a 16 ohm speaker load. This could likely be minimized by eliminating (or *NEVER* using) the -3.5db only option.

Unfortunately, all these scenarios leave a gap/jump between the -7db only and -3.5db choices and it seems that somewhere right in that zone is what you wil end up ideally shooting for!

*On a different note:* I find it kinda funny and very indicative of how different people's tastes can be, with your preference of EL34s over 6V6s. One of my main amps is a Marshall DSL20, that is designed for EL34s and cathode bias, to allow the use of *"most"* standard octal power tubes. And given that it has enough heater current available for the EL34s, anything with lower heater current draw is a no brainer, from a safety standpoint. I found several benefits (for me anyway) by using 6V6s in place of the EL34s. They brought to the table a little bit of the typical *"clarity"* (not clean-ness) that many Fender amps exhibit, while slightly reducing the *"grindy-ness" *and lack of definition that EL34s tend to bring! The 6V6s also dropped the output power by a couple watts! The amp now sits in a very similar volume range as my 5E3, Tweed Deluxe! This demonstrates the validity of the phrase: *"Different strokes for different folks!"  *

Just My $.02 & Probably Worth Even Less!
Gene


----------



## Freddy G

Gene Ballzz said:


> @Freddy G
> For your current situation, it would seem best to have built the straight 16Ω version of the M-lite and run it with all stages activated, in parallel with your 16Ω speaker(s). It would be perfect for your 8 ohm amp tap and give you about -3.5db, *WITH* the benefit of the reactive inductor coil circuit. An additional benefit is that you could still progressively switch off stages to lessen the amount of attenuation, if -3.5 was a bit too much of a cut. I'm not sure what the numbers would be and I don't think @JohnH has ever posted them? My guess is that each step turned off would be small *"baby steps"* for fine tuning your volume from -3.5db to almost no attenuation at the -7db only setting. Another small benefit of the 16 ohm version is that at least in series use, it doesn't seem affected very much by using 16 or 8 ohm speakers!
> 
> Now on the other hand, if you *DID* go ahead and upgrade your output transformers to something with with multiple (at least 8 ohm & 16 ohm) taps, more options become available. You could use the 8 ohm amp tap for parallel operation from -3.5db down to nearly no attenuation or use the 16 ohm amp tap for the more standard *"series"* operation to give -3.5db steps from -7db, all the way up to -31.5db of attenuation. In my considered opinion, this would be the best way to achieve a -3.5db or smaller cut and still retain the benefits of the reactive nature of this design. Smell what I'm steppin' in here?
> 
> Of course, if you went with an output transformer with 4/8/16 ohm taps, you could use the attenuator(s) you've already built, in parallel on the 4 ohm tap. My concern here would be the stack up of variances caused by having a unit designed to present an 8 ohm load to the amp while using a 16 ohm speaker load. This could likely be minimized by eliminating (or *NEVER* using) the -3.5db only option.
> 
> Unfortunately, all these scenarios leave a gap/jump between the -7db only and -3.5db choices and it seems that somewhere right in that zone is what you wil end up ideally shooting for!
> 
> *On a different note:* I find it kinda funny and very indicative of how different people's tastes can be, with your preference of EL34s over 6V6s. One of my main amps is a Marshall DSL20, that is designed for EL34s and cathode bias, to allow the use of *"most"* standard octal power tubes. And given that it has enough heater current available for the EL34s, anything with lower heater current draw is a no brainer, from a safety standpoint. I found several benefits (for me anyway) by using 6V6s in place of the EL34s. They brought to the table a little bit of the typical *"clarity"* (not clean-ness) that many Fender amps exhibit, while slightly reducing the *"grindy-ness" *and lack of definition that EL34s tend to bring! The 6V6s also dropped the output power by a couple watts! The amp now sits in a very similar volume range as my 5E3, Tweed Deluxe! This demonstrates the validity of the phrase: *"Different strokes for different folks!"  *
> 
> Just My $.02 & Probably Worth Even Less!
> Gene



A lot to consider here.
Different strokes indeed! The "grindy-ness" of the EL34 is what I love, but then I've always been a Marshall kinda guy, never a Fender guy. And I will submit that they have _better _definition...at least what I regard as definition.


----------



## Gene Ballzz

@Freddy G
While I've also generally been a *"Marshall kinda guy"* I've discovered as of late, that I prefer riding that fine line between Marshall and Fender Tweed tones! Even though the Tweeds give some of *"that Fender"* sound, they tend to be a bit more *"mid forward"* than the black face Fenders. It's pretty cool being able to slide between the two worlds on either amp! Since installing a passive effects loop in my 5E3 Deluxe, it and the DSL20 are pretty much interchangeable with each having just a slightly different character of its own! I don't bother using the on board reverb of the DSL20 (even though it works well), but instead do all my reverb and other time based effects in my Lexicon MX300 and use a ZOOM G3 out front for chorus and subtle overdrives, along with a well tweaked VOX wah! Tap tempo delay is a must for this *"Echo Junkie!" *Both amps have Pre-Rola 25 watt/75hz/16Ω greenbacks to help get what I'm usually looking for!

Its funny how just 13 to 18 watts can be just a bit too loud!  The Tweed usually gets attenuated to -3.5db/parallel or -7db/series and the DSL20 gets either -7db or -10.5db/series attenuation. At home, with the wife sleeping, the attenuator is typically -21db or -24.5db. At -28db, I could watch TV and at -31.5db, I can almost hear the strings as loudly as the amp!

With This Attenuator, Life Is Good! 
Gene


----------



## TXOldRedRocker

Gene Ballzz said:


> @Freddy G
> While I've also generally been a *"Marshall kinda guy"* I've discovered as of late, that I prefer riding that fine line between Marshall and Fender Tweed tones! Even though the Tweeds give some of *"that Fender"* sound, they tend to be a bit more *"mid forward"* than the black face Fenders. It's pretty cool being able to slide between the two worlds on either amp! Since installing a passive effects loop in my 5E3 Deluxe, it and the DSL20 are pretty much interchangeable with each having just a slightly different character of its own! I don't bother using the on board reverb of the DSL20 (even though it works well), but instead do all my reverb and other time based effects in my Lexicon MX300 and use a ZOOM G3 out front for chorus and subtle overdrives, along with a well tweaked VOX wah! Tap tempo delay is a must for this *"Echo Junkie!"*
> 
> With This Attenuator, Life Is Good!
> Gene



I'll give you an "Amen" to this. I obviously completely agree. Enjoy both worlds of Marshall and Fender Tweed, switching between them. (I use them in stereo sometimes too.) Though I haven't modded my Fender like you have.


----------



## JohnH

Hi @Freddy G There's a couple of things to try ahead of rebuilding the amps!

We are talking if fairly small changes in level and small attenuations. Also your application has arrived at the combination of two features which were both add-on extras, kind of lucky work-arounds, being the use of the 3.5 resistive stage on its own, and the use of 16 Ohm speakers with the 8 Ohm build. Together they give about a 16 Ohm load to the amp, and also when you use the 16 Ohm speaker in the 8 ohm build, there is about another 1.5 to 2db of attenuation, so compared to what a 16 Ohm speaker coming out of a 16 Ohm amp might have seen, you might have been experiencing about -5db with how you had it wired.

(Side note: All of that is also within range of the kind of variations you get from different speakers. eg, my V30 1x12 is supposedly about 2 or 3 db louder than the G12M's in my combo)

I want to show you more numbers and plots from 'under the hood' that I don't normally post, to avoid excess WTF? effects from readers:

This one shows calculated results from the 8 ohm build that you have, running either an 8 Ohm speaker into the standard 8 ohm outputs (lower blue chart), or the 16 Ohm speaker into Output 3 (upper yellow chart):



In this table you can see results for a full volume speaker, and attenuated settings 1 to 9, with 1 being the nominally -3.5db setting and 2 being stage 1 nominally -7 db.

*8 ohm Speaker into Output 1*

Left to right on the lower chart, we have the calculated attenuation, the increments (all pretty close to 3.5 db), impedance seen by the amp at 440hz (labeled 'R440' all not too much above 8 Ohms), then rises in the bass peak and at 5khz in db, for tonal comparison. The graphs show attenuation vs frequency, with red being full volume.

*16 Ohm speaker into Output 3*

Now move to the top, with an 16 ohm speaker in Output 3.

The tones are reasonably consistent with full volume but all the levels are dropped a bit, -5db at setting 1 and -9db at setting 2. Highlighted green, the impedance seen by the amp in Setting 1 is 16.18 Ohm, but at settings 2 to 9 its still close enough to 8.

*16 Ohm Speaker into Output 1*

Here's what happens if you put your 16 Ohm speaker into Output 1, ie without the tone tweaks that Output 3 has




Have a look at the graphs and the bass and 5khz rise values. You can see that they are a bit more suppressed compared to full volume which is why I added Output 3. In my amps you can hear this but it does depend on the amp and also how you set it. You might find it is not a problem with your amp, or that you can adjust for it with presence.

See how the attenuation levels (2nd column) are restored. Setting 2 has gone from -9 db to -6.1 db. If the tone is acceptable, then Setting 2 using output 1 might do the job for you.

Also see Setting 1, louder again but its impedance is at 13.12 Ohms, kind of in between values, your call if its OK to use.

*Parallel attenuator as load box *

This was the new thought from a page or two ago, and I don't have a plot for it. Set the attenuator to Setting 2 ie the stage 1 reactive is on and all others are off. (this would nominally be -7db for normal use). This is now so far as I can tell, a decent 16 Ohm reactive load box. Run it in parallel with your 16 Ohm speaker for a -3db reduction, which should have good tone too.

When used in this parallel way as a load box, I think its best either as above in setting 2, or as Id originally thought, put all attenuation to max to make an 8 Ohm load for running with a speaker into a 4 Ohm tap. I don't think there's any thing useful in between as increments.


----------



## Gene Ballzz

TXOldRedRocker said:


> I'll give you an "Amen" to this. I obviously completely agree. Enjoy both worlds of Marshall and Fender Tweed, switching between them. (I use them in stereo sometimes too.) Though I haven't modded my Fender like you have.
> View attachment 76695



Yeah, in my situation I'm able to get full on Marshall and mostly *"Tweed-ish"* sounds from the DSL20 (fully cranked of course) and full on Tweed sounds, along with some mostly *"Marshall-ish"* sounds from the Tweed (again fairly cranked to its *"sweet spot"*)! That's the sliding between that I'm speaking of. Which amp I choose is dependent on the mood of the day and/or which of my multiple bands I'm playing with.

Again, Life Is Good!
Gene


----------



## Freddy G

JohnH said:


> Hi @Freddy G There's a couple of things to try ahead of rebuilding the amps!
> 
> We are talking if fairly small changes in level and small attenuations. Also your application has arrived at the combination of two features which were both add-on extras, kind of lucky work-arounds, being the use of the 3.5 resistive stage on its own, and the use of 16 Ohm speakers with the 8 Ohm build. Together they give about a 16 Ohm load to the amp, and also when you use the 16 Ohm speaker in the 8 ohm build, there is about another 1.5 to 2db of attenuation, so compared to what a 16 Ohm speaker coming out of a 16 Ohm amp might have seen, you might have been experiencing about -5db with how you had it wired.
> 
> (Side note: All of that is also within range of the kind of variations you get from different speakers. eg, my V30 1x12 is supposedly about 2 or 3 db louder than the G12M's in my combo)
> 
> I want to show you more numbers and plots from 'under the hood' that I don't normally post, to avoid excess WTF? effects from readers:
> 
> This one shows calculated results from the 8 ohm build that you have, running either an 8 Ohm speaker into the standard 8 ohm outputs (lower blue chart), or the 16 Ohm speaker into Output 3 (upper yellow chart):
> 
> View attachment 76694
> 
> In this table you can see results for a full volume speaker, and attenuated settings 1 to 9, with 1 being the nominally -3.5db setting and 2 being stage 1 nominally -7 db.
> 
> *8 ohm Speaker into Output 1*
> 
> Left to right on the lower chart, we have the calculated attenuation, the increments (all pretty close to 3.5 db), impedance seen by the amp at 440hz (labeled 'R440' all not too much above 8 Ohms), then rises in the bass peak and at 5khz in db, for tonal comparison. The graphs show attenuation vs frequency, with red being full volume.
> 
> *16 Ohm speaker into Output 3*
> 
> Now move to the top, with an 16 ohm speaker in Output 3.
> 
> The tones are reasonably consistent with full volume but all the levels are dropped a bit, -5db at setting 1 and -9db at setting 2. Highlighted green, the impedance seen by the amp in Setting 1 is 16.18 Ohm, but at settings 2 to 9 its still close enough to 8.
> 
> *16 Ohm Speaker into Output 1*
> 
> Here's what happens if you put your 16 Ohm speaker into Output 1, ie without the tone tweaks that Output 3 has
> 
> View attachment 76693
> 
> 
> Have a look at the graphs and the bass and 5khz rise values. You can see that they are a bit more suppressed compared to full volume which is why I added Output 3. In my amps you can hear this but it does depend on the amp and also how you set it. You might find it is not a problem with your amp, or that you can adjust for it with presence.
> 
> See how the attenuation levels (2nd column) are restored. Setting 2 has gone from -9 db to -6.1 db. If the tone is acceptable, then Setting 2 using output 1 might do the job for you.
> 
> Also see Setting 1, louder again but its impedance is at 13.12 Ohms, kind of in between values, your call if its OK to use.
> 
> *Parallel attenuator as load box *
> 
> This was the new thought from a page or two ago, and I don't have a plot for it. Set the attenuator to Setting 2 ie the stage 1 reactive is on and all others are off. (this would nominally be -7db for normal use). This is now so far as I can tell, a decent 16 Ohm reactive load box. Run it in parallel with your 16 Ohm speaker for a -3db reduction, which should have good tone too.
> 
> When used in this parallel way as a load box, I think its best either as above in setting 2, or as Id originally thought, put all attenuation to max to make an 8 Ohm load for running with a speaker into a 4 Ohm tap. I don't think there's any thing useful in between as increments.



Thanks John. Everything is getting clearer. I will try the parallel set-up and see how that goes!


----------



## Gene Ballzz

@Freddy G 
Just want to confirm that you caught the memo concerning the fact that this attenuator is *"in effect"* another 8 ohm load and when you use it in parallel with your 16 ohm speaker you get *"nominally"* 5.33 ohms that the amp is seeing. Of course I'm sure you are aware that a multimeter will read a bit lower than those numbers, as DC ohms! An 8 ohm speaker typically measures ast between 6 & 7 ohms DC.
Just Remindin'
Gene


----------



## JohnH

Gene Ballzz said:


> @Freddy G
> Just want to confirm that you caught the memo concerning the fact that this attenuator is *"in effect"* another 8 ohm load and when you use it in parallel with your 16 ohm speaker you get *"nominally"* 5.33 ohms that the amp is seeing. Of course I'm sure you are aware that a multimeter will read a bit lower than those numbers, as DC ohms! An 8 ohm speaker typically measures ast between 6 & 7 ohms DC.
> Just Remindin'
> Gene



Hi Gene, It should be OK, using the new 'wrinkle'. When the attenuator is set to max attenuation, then with no speaker in it, it does indeed make an 8 Ohm load. But if instead its set to just stage 1 attenuating (usual-7db) , other stages off, then it happens to make a pretty accurate 16 Ohm load. Basically, its R1 (15 Ohm) plus the two coils in Design M in series and if there's no speaker plugged into the box, it doesn't see anything else. This would also work in design M2 too.


----------



## Gene Ballzz

WOW! @JohnH 
I must have missed that *"wrinkle"* in the details you provided earlier, or possibly discounted its relevance! Still a bigger fan of the 16 ohm M-lite and the multiple options it can provide, as long as the intended amp is not 8 ohm only! I'm especially fond of it not being as dependent on speaker impedance for optimum performance as the 8 ohm version.
Just Sayin'
Gene


----------



## littlewyan

Just wanted to say a quick thanks for starting this thread. I've been working my way through the posts and have learnt a LOT! Very informative and incredibly valuable information in here! I plan on building myself one of your M2 attenuators when I get time. In the meantime I've got my custom Airbrake which I've now modded to sound better at high levels of attenuation. The bedroom control on these often sounds a bit fizzy because the impedance the speaker sees is too high (188Ohms at the highest setting!!), so I've modified it to stick a 40R resistor (two resistors in series in my case) across the speaker when the bedroom control is in use and also removed the 0.1uF bypass cap on the rheostat. Huge improvement.

Thanks again! I'll post when I start building my attenuator.


----------



## _Steve

Hi again!

So after a very long boat from China and some sitting around on my workbench for a few weeks I finally built an M2.

Just wondering however if there is a safe way to test it before plugging it into an expensive amp?


----------



## JohnH

sure yes. Plug the speaker in but not the amp, then measure resistance between hot and ground at the input. It should be between 7 and 10 in all settings (or x2 if its a 16 ohm version)


----------



## _Steve

JohnH said:


> sure yes. Plug the speaker in but not the amp, then measure resistance between hot and ground at the input. It should be between 7 and 10 in all settings (or x2 if its a 16 ohm version)



Got it. Thanks!!!


----------



## telesto

_Steve said:


> Hi again!
> 
> So after a very long boat from China and some sitting around on my workbench for a few weeks I finally built an M2.
> 
> Just wondering however if there is a safe way to test it before plugging it into an expensive amp?


Sure: plug it into a cheap amp first


----------



## nicnac

Hey guys! 

Thanks to John for this great forum and awesome design and hard work and to the guys for all the testing and thoughts! I have enjoyed the journey. 

I took the plunge and built the multi ohm (4/8/16) in/out M2 version (decided to build 5 and sell 4 off at cost to get mine free basically) and the tests have provided very big smiles so thanks again. This is my perfect studio solution as i run multiple cabs and heads in search of sounds.

I went for a bigger alu case 190 x 190 and split my initial resistors to 2 x 50w as I noted a post that someone mentioned they are roughly the same size as the 100w versions and my thinking is a little more heat sinking can't hurt in the long run and the bigger box has plenty space. I didn't cut any corners on parts so name brand res and coils from coilsnstuff (Steve) eBay. I got most of my bits from switch electronics uk and farnell uk. Over rated switches and branded wire too. Came to around 150 quid per unit (saved a bit on the bulk buy).

It is a tad tedious drilling etc but the step drill bit is your friend for switches and jack inputs. I also bought a 60w soldering iron as the resistors will suck up the heat as you solder so it makes life a little easier.

I quite like the line out after the R2 point in the circuit as it sees the inductors and benefits from some resistance. Will also reiterate with all the others that it will require a cab sim (Don't expect it to sound like the amp in your room).

I have run my 5e3 tweed deluxe, champ clone, ac30 and silver face 100w bassman all have sounded brilliant and no noticeable heat issues.

I 'll post pics soon. 

Thanks again, especially JohnH.


----------



## JohnH

hi @nicnac 
That's great and thanks for posting. I'll look forward to the pix in due course. Id also be very interested in your impression of the multi-ohm features, particularly at the input using the 4 or 16 inputs given the core of the unit is for 8 ohms. Id expect a small volume step down, but hopefully the tone stays reasonably consistent?

*Line outs and cab-sims*

The raw signal from a line out on any such unit is very bright indeed, but these days people seem to send it off into an IR setup to provide the cab sim. But I'm now working up an analogue cab sim, just to see how good I can get it. I'm hoping I can get good results just as a passive circuit. 

So far, Ive been recording miced samples off of the speaker, and simultaneously a direct output. Then I normalise them and make frequency plots of each, and I subtract them from each other in excel. This gives me a guide to the ideal transfer function for a cab sim that will take the raw signal and twist it to approximately match the miced signal.

Then, I'm concocting an LCR network to try to follow that ideal shape, analysing that in SPICE and also spreadsheets to find out its response, and feeding that response back into my Audacity recordings to see if it will bring the raw signal down to one close enough to the miced signal for use in PA, silent headphone playing or recording.

Im finding you can get an acceptable sound, you can definitely tell from listening that it is not quite the same, but it still sounds reasonably close to a miced cab, at least for some uses.

This plot has the wiggly line based on the theoretical change in frequency response derived from the samples (based on various played samples and frequency sweeps), and the theoretical response of my LRC network: 




By adjusting component values, I can shift the peaks and dips up and down and sideways.

Clearly, the trace based on real tests has lots of ups and downs as different resonances occur, and you can see these also in the plots provided by speaker makers. But its interesting to discover how important or unimportant these variations are in getting a good tone.

All work in progress, not physically built yet.


----------



## telesto

Hi John,

I've been playing around with various configurations of attenuators, and am now going into the area of the load-box, so since your M3 with resonance circuit effectively uses similar Caps+Coils, I can give one a try for you and let you know how it sounds (I've spec'd mine with 0.85mH treble and 8mH bass, and a cap in the 180 to 220uF range).

I'm searching for parts now, and found a reasonably priced coil 8mH with 1mm (18 gauge) wire with 0.5ohm resistance for 9EUR I think for the coil about 1mm/18AWG was agreed to be ok for a 50W amp (for 100W maybe a little thicker wire?)


..and for the cap, I'm a little confused about the specs. I read several times what Aiken wrote about the ripple current, but not all caps have the ripple current in their specs. Some give ripple voltage (VAC rating), and I guess from there the ripple current can be derived by ohms law (divide VAC by calculated Z of cap). For example, this 220uF cap has a 63VDC/23VAC rating, and Z = 1 /(2*(pi)*f*C), a low E-string is 88Hz, so plugging it all in:

Z = 1 /2*(3.14)*88*.00022
Z = 8.22

23VAC/8.22 = 2.8A

...so am I correctly calculating the ripple current to be 2.8A? Aiken calculated ripple current of 3.5A for a 100W (28VAC) amp, so if I divide 28VAC by (Z) 8.22 I get 3.4A so I guess it's correct.

I was looking at this 220uF rated at 23VAC cap that costs only 4EUR. A 50W amp puts out about 20VAC, so the 23VAC rating may be a little close, but maybe still OK (altho Aiken reccomended x1.5 the min value for extra headroom). Or what capacitor specs did you decide on for a 50W amp? I guess any with a rating over 20VAC or ripple current of 2.8A?


I was looking at this one, which actually specs the amp output rating at different ohm loads, which is interesting/nice (less math for me  ). These are sold at 2 pieces for 20EUR by me, so not too bad and should be well within spec for a 100W amp (@8ohm out). So with 10EUR for a 10mH coil and 10EUR or less for the cap, it's only 20EUR to put the bass circuit in, which isn't too bad really.

On a side note, the schematic/build I see at TGP is a little strange, the cap and bass coil looks to me way too big and over spec (not that that it's bad, just raises cost unnecessarily) and then they use a 200W power resistor, that is actually rated at only 80W, and then is below spec for a 100W amp. Strange. I should maybe ask about it on that thread, but I'm too busy/lazy right now to make an account there


----------



## JohnH

Hi @telesto 

I also get unsure about those cspacitor specs. The one you found looks ok though. If i built an M3 I was going to try big polypropylene caps with very low losses, but they are a lot more $. So I think your selections look good.

In comparison to Aikens design and the one derived from it on TGP, our box only sends half the current down through R1, while in the Aiken design, everything has to go through the main resistor and the cap.


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## telesto

JohnH said:


> In comparison to Aikens design and the one derived from it on TGP, our box only sends half the current down through R1, while in the Aiken design, everything has to go through the main resistor and the cap.



Yea, that was actually something I was thinking about and forgot to mention. Actually, my attenuator design reduces the signal even more, which means the signal hitting the RLC circuit will be kind of low. While this means I can actually have lower ripple tolerances for RLC components, I'm wondering about the effect Aiken wrote about that low signals hitting relatively high resistance in the LC will "kill the Q". The resistance of the cap+coil I was looking at was about 1 ohm or less, so I don't think it's too bad. The Spice SIM looked OK. But I guess I'll never know for sure till "the rubber hits the road"...or "the signal hits the coil"  Anyway, I'll let you know in a week or two once I get the parts and the rubber and coils start to fly


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## Grouville

I first came across this thread a month or more ago, and it seems just what I need to tame my Egnater Rebel 30 for home use. It is the combo with a 16ohm speaker. Love the tones from this amp, but the neighbours are tired of listening to me practise the same Pink Floyd number over and over! 

Am planning to build the M2 design posted in July? I ordered the resistors from China and 1 metre of 18AWG copper wire to hand build the inductor coil. I sat back and awaited the deliveries...
And now I cannot find the post which detailed the diameter and turns I would need to build the 1.8mH coil! 
I have searched and searched - perhaps I had originally found instructions on another forum?

Either way - could someone please enlighten me? I have used some online calculators, but they all seem to indicate I will need a coil 25 inches in length with 5000 turns! (I exaggerate, but not not by much!).

I was sure that 1m of 18AWG would be enough - was I wrong?


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## JohnH

Hi @Grouville , thanks for your interest. Im sorry but we haven't posted any turns/wire length numbers for the inductors on this thread. Generally we just order a coil of the required inductance and wire gage from a supplier, and there are a few of them depending where you are.

But I have seen calculators for multilayer coils. I'll have a look but id expect a lot longer than 1m is needed.


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## JohnH

this one looks useful:
https://www.teslascientific.com/products/coil-inductance-calculator/

I threw in a guess of 300 turns in a coil with 15 layers, diameter 20 internal to 50 external, and 20mm long. I got a bit over at 1.9mH. That's about 33m of wire.


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## Grouville

Hmm. I obviously completely misunderstood the other resource I read, or it was wildly off the mark.
Could anybody recommend a UK supplier for a ready built 0.18mH inductor coil?

Have not used such a thing before... 

Found a number of coils on eBay, some are wire, some are copper tape?.
Is the AWG significant? Some quote resistance?

Any recommended products?


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## JohnH

Im in Australia, but we have had a few builds in the UK. Maybe someone will chip in.

But for the 16Ohm M2 attenuator, you need a 1.8mH air core inductor with 18 gage wire, which means the wire is 1mm diameter. (in the US, there's Madisound that make a range with 19 gage wire, they seem good too for up to 50w amps and Gene has been using them) 

The resistance is not a spec that we need to specify directly, but if you get that wire gage then the resistance is reasonably low.

Here's a couple of UK links that look OK to me. 

https://www.falconacoustics.co.uk/a...-air-core-audio-151-180mh-audio-inductor.html

2nd from the bottom on this page:
https://www.qtasystems.co.uk/loudspeaker-components/inductors.htm

There were quite a few others too. They tend to be sold by people who sell loudspeaker cross-over parts, often wound in-house. Ive looked on ebay but could never find ones that had all the specs clearly.


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## Grouville

Thanks John. Have gone with the second of those two (min qty 2 for the first). Looking forward to building this - will let you know how it turns out.


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## Esa Martikainen

When here in Europe I have searched audio components often I have found them from from Holland and for my attenuator 15mH coil came from "audio.nl" and its link is here. Nowadays shipping is fast but quite costly as well.

https://audio.nl/Aprodview.asp?type=product&Product=1342

Esa


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## Esa Martikainen

And the 1.8mH coil is this "air core". It is huge having 100mm diameter (mounting case is 130) and 55mm thick. Wire is 2mm and resistance 0.25 ohms and it did cost 33 euros.

https://audio.nl/Aprodview.asp?type=product&Product=3604

That previous post 15mH Torobar is rated for 600W/8ohms and is 0.29ohms but it has a ferrite core and it is toroid wound diam 95mm and 50mm thick. It did cost 46 euros.

For capacitors I found a damaged and trashed circuit board which was used on a powerful "switching power supply" and there are eight 68uF capacitors which I beliece should handle current very well. I think that current handling comes better when more smaller capacitors are used than one or two big. But does it cause "stray noise" i don't know.

Esa


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## genuinemrjay

Hi John. First off, I want to say your project is amazing! I have read through all sixty some pages of it in the last two days, and can't say enough about your knowledge, capabilities, and creativeness in this arena. I am going to build your 16 Ohm M2 attenuator for my AC30 (I know, it's a Marshall forum so please don't boo me everyone). With a 30 watt/ 16 ohm amp would the following values below suffice (the number after the - is Ohms)? Thank you for your time and effort John!


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## JohnH

genuinemrjay said:


> Hi John. First off, I want to say your project is amazing! I have read through all sixty some pages of it in the last two days, and can't say enough about your knowledge, capabilities, and creativeness in this arena. I am going to build your 16 Ohm M2 attenuator for my AC30 (I know, it's a Marshall forum so please don't boo me everyone). With a 30 watt/ 16 ohm amp would the following values below suffice (the number after the - is Ohms)? Thank you for your time and effort John!
> View attachment 77408



Thanks for your message. Nice drawing and the values look fine. I see you scaled down the power ratings a bit, for your AC30. Should be OK if you get them bolted onto a thick aluminium chassis with thermal grease and plenty of vents. I've not see the 75, 30 and 15W resistors though where I've shopped. For the inductor, I'd stick with 18 (or 19) gage wire.

My other amp is a DSL401, and like an AC30, it also has 4xEL84 with no negative feedback. The tone of this type of output circuit is very sensitive to what its driving, so I'm glad you picked this attenuator design since it works well with it! Good luck with your project and we look forward to hearing how it works out.


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## JohnH

Hi @Esa Martikainen , thanks for your links. That looks like a good source for parts in the EU. Have you built your project yet?


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## genuinemrjay

JohnH said:


> Thanks for your message. Nice drawing and the values look fine. I see you scaled down the power ratings a bit, for your AC30. Should be OK if you get them bolted onto a thick aluminium chassis with thermal grease and plenty of vents. I've not see the 75, 30 and 15W resistors though where I've shopped. For the inductor, I'd stick with 18 (or 19) gage wire.
> 
> My other amp is a DSL401, and like an AC30, it also has 4xEL84 with no negative feedback. The tone of this type of output circuit is very sensitive to what its driving, so I'm glad you picked this attenuator design since it works well with it! Good luck with your project and we look forward to hearing how it works out.



Thanks for the reply John! I was able to find all the resistors except the 30W for R2A and R2B online at Mouser, so those will have to be changed to 50W. I also used draw.io for the diagram which was pretty handy. Prior to discovering your thread, I planned on buying a Two Notes torpedo. Downside was that it only has one fixed dB cut, and several other features I really don't need. I just need to tame the wildness that is 4xEL84, and your attenuator design looks like a perfect fit. I'll be sure to document the build and post in detail here in the near future. Thanks again!


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## Esa Martikainen

JohnH said:


> Hi @Esa Martikainen , thanks for your links. That looks like a good source for parts in the EU. Have you built your project yet?



Not yet! I just received those coils but soon I put it together.

I also thank You for all the research you have done and inspiration I have read here!

Is it easy for you to explain why is it better to use half the capacitor and double the inductance and resistance values when changing from 8 ohm to 16 ohm attenuator?

I understand how resonant frequenzy depends of capacitance and inductance but not how they and the loudspeaker impedance or attenuator go together.

If I use 15mH which I already have and 200uF (which I build using at least 4 caps) resonant circuit it is going to have about 92 Hz resonance but the peak seems to come quite narrow, narrower than majority of Jensen loudspeakers have. Jensen webpage contains best graphs and data of all manufacturers!

My 1.8mH series inductance has 3,2ohms around 280Hz where 15mH/200uF also has 3,2ohms which is the "low point" of whole circuit if I have understood it right?

My plan now is to measure the resonance peak of the loudspeaker I am going to use and tune the circuit resonant frequuency about the same switching more or less capacitors to the resonant circuit.

Esa


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## JohnH

Hi @Esa Martikainen 

I'm assuming you're building a 16Ohm version of the M3 design? with the resonant circuit?

That's great, you should know that I think you will be the first to try this one though there are quite a few M2's now as well as many M's

With your parts, the 1.8mH wound with 2mm wire is great, obviously thicker than we've been using so far, no problem there. With the resonant circuit, 15mH and 200uF, I agree that leads to 92 hz as you noted, which is close to many open back cabs, and less than most closed back ones. If you have this circuit, then it affects what the amp responds to, but at the speaker, the output will be more determined by the actual resonance of the real speaker. Ideally these will match. But this is why the other designs M and M2 still work very well even with no resonant circuit, the speaker provides it itself.

Ive done sound clips with my M design (no bass circuit) , comparing tone from the full unattenuated speaker with that from reduced volume, trying to drive all the bass notes very hard, and really I couldn't hear a tone difference at all.

When you go from an 8 Ohm build to 16, all resistances and impedances have to double, so resistors and inductors x2. But impedance of a capacitor is dependent on 1/C actually (1/(2Pi.f.C), so it halves, and with L x2 and C x 1/2, resonant frequency stays the same.

With those caps, note that the capacitor gets current through it both directions, so needs to be bipolar. Most large caps are polarized electrolytic so you should check out what you have. If buying, I would get the big polypropylene ones or bipolar electrolytic but we need to make sure it can handle high ripple current. Also, almost the full amp power passes through the cap. I have read however that if you put two polarized electrolytics in series, pointing in opposite directions, its equivalent to a bipolar cap of 1/2 the value


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## Boysenman

Hi @JohnH. I finally got around to building the M2 attenuator (with line out) for the 10" speaker. It works like a charm! 

I look forward to building more variations of these designs.

Thank you!


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## littlewyan

I'm getting parts together for your M2 build and am wondering about the rating of R2A and R2B. Is 50W necessary? Does R2A get warm at all in your build John? And how do R2A and R2B work together? I worked out that in parallel they give you 10Ohms, but I'm perplexed as to how they work with the inductor...


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## JohnH

Hi @Boysenman , very neat build it looks great.

@littlewyan R2A and R2B may be a bit over-specced at 50W but its better than under. Together they take up to about 20W if 50W is coming in, and I go x3 as a factor. How much power they each take depends on the frequencies running through, and how you play, how much distortion etc. More goes into R2A at high frequency. These two are different from the other resistors where the power can be assessed easily by proportion. 

So im sure 50W is plenty for each but I wasn't sure if 25W would be enough. My own build is the earlier M type, which has the same performance but needs two coils. M2 is more economical but was harder maths!

In both M and M2, the inductance is partly in series and partly in parallel with the circuit downstream. Thats the key to both versions.If it was only in series the speaker would get a dull sound, while if only in parallel it would be too bright.


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## littlewyan

If they're already over spec then actually it may not be an issue. Reason I ask is, I'm going to use a Hammond 1444-972 chassis which isn't very thick and won't be suitable as a heatsink. So if I use these chassis mount resistors then I need to really go by the 'Watts with no heatsink' rating to be safe. Luckily for the other resistors I can use these https://www.mouser.co.uk/datasheet/2/418/5/NG_DS_9-1773453-2_C-727458.pdf
I may use 75W chassis mount resistors instead for R2A and R2B just to be safe.


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## peefnik

Hey all! I've been investigating various load boxes/attenuators for a while now. I'm currently using mesa boogie mark Vs ... 35W, and the big boy. I also have a Helix HX stomp ... which is pretty sahweet. I get great sounds from that thing. Regarding the Mesas ... I say they are the Fender amps I always wanted. I've been looking at ox, waza, and recently the captor x.

I like the captor x (in theory) but feel like two attenuated levels would probably fall short in most/different situations.

Anyway, since I'm into the engineering aspect of this stuff I've done a deep dive into the spice modelling. I figured I'd start with the M3 8ohm version. All real tube amps are 8 ohms 

I'm using LTSpice but this is really applicable to most spice programs.

I created an M3 subckt (w/symbol) and also an M3 schematic with an identical (to the subckt symbol) 'block' symbol which just pulls in a schematic as a subckt under the hood. The good thing about the latter is that you don't have to edit a subckt manually to make changes on the fly. You can just modify the schematic.

I have run sims using just a voltage source and an actual deluxe reverb output section (transformer and all) into the great JH attenuator circuit. Its way too much fun, or I'm way too much of a nerd.

The thing with the resonance impedance facing the amp is you have to model it depending on the speaker you are using, or you get some funky multiple peaks facing the amp. I also have a complete fender deluxe simulation which I'm playing with ... thats where I pulled the output section from.

Anyway, here are some fine images of the results of these shenanigans  I apologize if these images don't come thru ... I'm unfamiliar with how this interface (image/attachments) really works.


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## peefnik

Heres a grab with the full deluxe reverb showing impedance @ the amp and output dB at the speaker.


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## JohnH

Hi @peefnik 
Thanks very much for this study! Getting the attenuator modelled at the end of a full output stage model, is not something I've done myself.

I'll need to look on a proper screen, I'm currently just on a phone. I'm likely to ask more about it, but here's a few initial Q's:

Does it include a speaker model? and if so what's in it?

With the M3 circuit, do you see a difference in how closely the attenuated signal tracks the shape of a full-volume one with the ougput stage model, as compared to just a voltage source/impedance?

Can this provide any insights to the benefits or not, of including the resonant M3 circuit as compared to the M2 which has only one coil?

Anyway, thanks again.


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## peefnik

@JohnH
I'm using a generic 8ohm speaker model ... as you have posted here (from aiken?).
The signal tracks pretty well ... but you have to tweak the resonance values to match the speaker or you can get multiple/offset peaks at the different resonance frequencies.

I tweaked the resonance inductor in the attenuator to 18mH to bring the peak down to the frequency when bypassing the attenuator (e.g. just the speaker load).

Here is the speaker schematic and a plot with the attenuator bypassed, then with only first stage. The top green line is both the amp and speaker when bypassed. The top blue and lower green are at the amp and speaker respectively when the attenuator is not bypassed.

Now that I can see the amp model working, I'll probably just use the output stage w/transformer model which eliminates all of the preamp filtering, etc.

_NOTE: This is with bass and treble both at 5. Fenders, at least the classic ones, are mid scooped as the tone stack has a fixed mid resistance. They only approach being flat when bass and treble are at 0. It is also on the vib channel which is considerably brighter than the normal channel._


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## JohnH

next morning and Im looking at it more clearly on a 24" screen.....

very impressed....

I just saw your latest post that answers a lot more. it looks like a good match.

But with the output-stage-only model, it would be worth building into it the NFB circuit from the full model. NFB totally changes the effective output impedance of the amp. Its where my modelling with a fixed 20Ohm output Z, based on one of my amps, will differ in comparison to various real amps. With just the M2 circuit, most of the frequency response seems to track with different values for this 20 Ohms. But the bass response tends to stay constant. With M3, my models suggest that it will all track better including the bass resonance. But if you are able to make an output stage with the NFB in it, then this can be analysed properly, and tested with different NFB amounts.

With the speakers, my values have been targeting measured response of a closed back cab, which raises the resonant frequency. I agree that if the bass res circuit goes in, and is intended to be correct, it needs to match the cab. Whether it matter in listening tests is up for consideration.

If you like this stuff, Ill float past you the following issues as subjects for investigation:

1. These attenuators have been well liked by folk using very different amps to those that I designed it with. This issue of response to different output stages with different NFB may be key and worth exploring
2. Also, is it really worth it to put in the resonance circuit in real builds? Builds so far have not had it
3. Given a model of a real output stage, what happens if you drive into non linearity? That's actually the point of having an attenuator, to drive the amp hard but quietly! This would probably need a transient analysis using a single frequency and the plotting of waveforms. Could the models be wrangled to do that?


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## peefnik

I'll throw the PI (long tail pair) into the tube output only sim. Then I can have the NFB as well.

Once I get that in I'll see what happens when I drive it harder. Not sure what the models do in that case to be honest.

Even for clean signals at the low end the resonance being present would certainly present a more realistic condition to the amp. Is the effect enough for people to hear or feel in some way as the player ... I don't know.


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## JohnH

I think from the bits I know about Spice calcs, with the plots so far, all signals are based on them being small signals staying linear, even if numerically they are large. So to explore large signals going into the tubes, its a different type of run but the same model. I use a simpler Spice version called 5Spice (cant handle this tube model though, I need to get into LT) and its called a transient analysis there, where you feed in a waveform such as a sine at a chosen frequency, and you get an output waveform like on an oscilloscope.


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## JohnH

peefnik said:


> Even for clean signals at the low end the resonance being present would certainly present a more realistic condition to the amp. Is the effect enough for people to hear or feel in some way as the player ... I don't know.



That's the thing! does it make a difference? here was my attempt to explore it a couple of months ago:



JohnH said:


> Have a listen to these, which try to show bass response. It's my attempt to play the start of Cinnamon Girl. First, attenuator bypassed, then switched to -21db, both from the same loop.
> 
> Full volume (attenuator bypassed), then -21db:
> https://voca.ro/jwLCSg3wC9x
> 
> Here it is with the attenuated second part normalized in software back up to the same volume:
> https://voca.ro/ncne6xsR8JS
> 
> Its miced off the VM2266c, Master volume at about 6.
> 
> I picked this since the dropped lower D gets frequencies down to 73 hz for low D, and has some notes at E#, G and A above that, so it should be in the range of the bass peak for comparison.
> 
> I hope that helps.
> 
> ps. another small nugget of info, this test was done using the -7 and -14db stages to make -21db. Audacity, which I record with lets me check that, and the actual difference in levels as miced, is -20.7db, so numbers are good.



With those clips, need to ignore the choice of music, the playing, the recording, the hiss when the attenuated signal is normalized and the actual tone! Just listen for the differences in the low bass notes as I was trying to provoke them to make things happen on the A and drop D lowest strings, right around where the resonance would be. This was with an M attenuator without the specific resonant circuit.


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## peefnik

There doesn't seem to be any noticeable difference when listening to the clips. Since the resonant hump is recreated at the spkr this may be enough ... from a listeners perspective? 

Having the resonance at the amp would, in theory, recreate the reflected impendance seen by the output stage and make the closed loop behavior of the system closer to normal. Again, would the player notice this difference. And mayhap it is not enough of a difference in the end?

In your clip the attack is pretty soft ... I wonder if a more percussive attack on the bottom end would feel different in the two scenarios? Does it chug, for instance. I can't really answer these questions as I don't have a unit built.

I added the PI to the DR output section only sim. Also created an M2 block. I have the input level set to get 5.3Vpp at grid of U4 (top, inverting tube in PI). This results in about 22 W into the load.


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## peefnik

Mayhap, since this is a simple attenuator, all of this resonance circuit stuff is pushing it out of that classification. The M2 sounds good as is and adding a resonance peak may not be worth the effort or $. Not to mention, if the peak were ever to be too high it could present a reflected imedance to the output transformer, tube stage which could potentially damage the amp. If it was designed correctly that wouldn't be an issue ... it just adds a level of uncomfortability if something were to go wrong. I guess that is true in any case if parts failed and resulted in a high impedance load.


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## JohnH

I agree with all of that, and its why I haven't pushed to build an M3 myself. The resonant parts can be relatively expensive, particularly the cap if its as high a spec as it probably should be. If the cap and L2 are there, then from low mids upwards, almost the whole power of the amp has to pass through the cap.

But, if the resonant circuit is used, I've tried to figure out what its implications would be and what benefits it might offer. Here's what I reckon:

1. If the unit is being used as a load box, with a line-out taken at the amp side, then the line out signal misses out on the bass peak if its not there. But a few have done this and not found it a problem. 

2. While the attenuator has been designed based on an effective amp output Z of 20 Ohms (for the 8 ohm version), real amps vary from much less, even 5 ohms, up to much more, almost infinite if there's no NFB. This affects the voltage vs frequency seen by the speaker, which has a peak and rise matching the speaker impedance if amp Z is very high, down to a flat response with a zero amp Z like an ss amp. Somehow, the first reactive stage compensates for this and you can see it in analysis. You should see it too with your NFB model. Zero NFB youll get high amp Z and a rising signal at the speaker, high NFB, low amp Z and the response is flattened. It keeps in match with the full volume version, except that with no resonant circuit, the bass peak does not adapt and if the amp is showing less that an output Z of 18-20 ohms, the bass peak stays higher

3. Last thing I can think of: When the load has a higher impedance, with a tube amp of moderate of high Z, it will actually draw more power out of the amp, and so at those frequencies, and when the signal is high with the amp working hard, there will be more distortion, dynamics and sizzle. This adds the realism and feel which we found added when we moved from the early resistive designs on this thread. Its relevant if the amp is running not clean in the power stage, at least for high signals. Designs M and M2 get that right from about 150-200 hz up. But if there is anything like that happening in the bass, only M3 will find it. It would be heard as some compression and maybe harmonics of the bass notes. But I cant hear any so far. I think the implication of not have this effect for the bass end is that the bass stays a bit more solid, less distorted flub like its got more behind it, not a bad thing really!


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## peefnik

You can see the amp output Z, spkr output, vs NFB in this image. The lower green line in outputz is with standard 820 ohm NFB resistor. I stepped it 820, 2.2k, 1T (e.g. no NFB). The changes in amp Z are small but you can definitely see the effect how it reacts less to the spkr impedance changes and smooths it out.


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## Oliver Gardiner

I may be missing something but I'm struggling to understand why R5 is only 4R7 as this means the load presented to the preceding stage is only 4R. By the same token, R7 seems high at 33R and I would have expected something like 20R which would get you back down to 8R in parallel with the 13R6 you get with the 8R load and the 5R6 of R8.

As a hybrid approach, would there be anything intrinsically bad about using a Monacor AT-62H L-Pad after the 1st reactive stage of M2?

Thanks


----------



## JohnH

Hi @Oliver Gardiner , thanks for your message.

Those resistor values are fine. Although its right to note that the third stage does present a low resistance to the previous stage, that value is never passed on to the amp because it is always separated from the amp by Stage 1, and also Stage 2 if its on. So the effect on ohms to the amp is quite small. Similarly, the high value of R7 is never changing the ohms seen by the amp excessively.

What is more important is output impedance of these stages, which needs to be kept quite high like a tube amp output stage.

The key thing with the resistive aspects of these designs is to keep reasonably consistent ohms at the amp, and also as seen by the speaker, and then to set it a up so even attenuation steps can be made. It works for every combination of switch settings.

The idea with an Lpad may have a problem shared by many attenuators, in that the output impedances is not consistent, and gets very low at high attenuation. This damps the speaker, making it sound dull.


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## Oliver Gardiner

Many thanks - that makes sense. I've not really looked at attenuators before - I can see that the first stage will clearly dominate the impedance seen by the amp but had not considered the effect of output impedance. Have been playing with a variety of amp designs and have always used a master volume set up so far - am now thinking that an M2 would be a better approach. I've got a volume/clean boost last in the FX loop anyway so can tweak levels there.

Thanks again!


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## Baby Thomas

Hi guys!
Last night I was watching the last Tone-Talk show (ep. 77) and John Suhr was talking about his Suhr Reactive Load and some other similar products. He shared his thoughts about these devices, impedance, frequency response, resonant peak and so on. Maybe it will be interesting for some of you.
Have a nice day!


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## peefnik

So, after more analysis, I've come to the conclusion that trying to recreate the resonance hump as an inline attenuator is not practical/possible. This is due to the way the cab/spkr load interacts with the reactive/resonance load. If they are not matched, exactly, you get multiple resonant humps at the amp output that are surely not desirable. You would have to make an different attenuator based on the cab/spkr you were using. This is with a speaker model that is most likely not a 'cab' model but the end result is, I think, the same. The response of the actual cab/spkr is really an unknown for a generic attenuator.

You could have a switchable option for attenuator/load that would switch in the resonance components ... but that adds a bit more complexity.

The only reactive loads creating the low frequency resonant hump, AFAIK, are loads only, not attenuators (Suhr, TAE, etc). I'm not sure exactly what the TAE is doing but given the fact it has 4 settings for resonance implies the they are creating/shifting the low frequency response. I don't know if this is a hump or just a general low frequency bump. The TAE is definitely a load only device as it reamps the amplifier signal via a SS amp ... it does not pass it directly thru.

The multiple low frequency hump effect can be seen in the following images. First is with the stock M3 values, then adjusted to more closely match the speaker load.


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## JohnH

Mike Lind makes passive fully reactive attenuators, with the bass resonance. He posts often on TGP:

https://www.thegearpage.net/board/i...rplant-a2xr-reactive-attenuator-load.2100557/

He's also been very generous with comments and also his cab measurements, which have been a great help here.

He has a switch to tune the resonance, and I think the key thing is whether its to be based on open-back or closed-back. With most 12" speakers, this seems to make a shift from around 80hz to around 110hz. 

My hunch is its not too significant in practice, and I still haven't found a compelling reason to it in my ones, since M and M2 seem to work fine just letting the speaker develop the bass peak.


----------



## Gene Ballzz

JohnH said:


> Mike Lind makes passive fully reactive attenuators, with the bass resonance. He posts often on TGP:
> 
> https://www.thegearpage.net/board/i...rplant-a2xr-reactive-attenuator-load.2100557/
> 
> He's also been very generous with comments and also his cab measurements, which have been a great help here.
> 
> He has a switch to tune the resonance, and I think the key thing is whether its to be based on open-back or closed-back. With most 12" speakers, this seems to make a shift from around 80hz to around 110hz.
> 
> My hunch is its not too significant in practice, and I still haven't found a compelling reason to it in my ones, since M and M2 seem to work fine just letting the speaker develop the bass peak.



So true sir! In real life practice, these JohnH units will usually be used for light (-7db to -14db) volume cuts and once it gets much lower than that the response of the attenuator becomes somewhat of a moot issue, when considering all the other aspects of such low sound pressure level! And sure, the resonance can be felt to be a tiny tad different at each setting, but is easily compensated for through a knob and/or technique tweak.

These attenuators are the greatest thing since sliced bread!

Thanks Again John!
Gene


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## BrokenBones

I've never worked with inductors. Are there any 'must follow' procedures as far as mounting or housing? I did buy some stainless washers and bolts. Can the bare inductor wire be touching the chassis?


***UPDATE***

I was able to find some pointers on mounting air coil inductors over on the audio karma forum. They give a lot of good tips on proper placement and materials.

https://audiokarma.org/forums/index.php?threads/mounting-air-core-inductors.639281/


----------



## JohnH

BrokenBones said:


> I've never worked with inductors. Are there any 'must follow' procedures as far as mounting or housing? I did buy some stainless washers and bolts. Can the bare inductor wire be touching the chassis?



Any secure mounting is fine, and I note you picked upon not using normal steel bolts, which can greatly increase the inductance. The insulation on the wires is quite thin and so the wires should not touch the chassis. Depending what the situation is, maybe some spacers, or thicker insulation sleeve, or mount the coil the other way up? If you scan through the thread you'll see a few versions.


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## Esa Martikainen

I put this together fast but it works so good that it deserves a neater housing. It is basically a reactive load which I first saw on Aiken site. In the middle is an aluminium 10 ohm 200W series resistor. On left is 1,8mH and 100ohm high frequenzy slope circuit. Top right there are 100ohm, 15mH and four blue capacitors total of 220uF resonant circuit.

Attenuation is done using a line transformer which is on bottom right and it is parallel with this reactive unit and loudspeaker is connected to it. It has three inputs and three outputs but in practise there are five different attenuations possible. I bought it after I tested how normal output transformer attenuates signal and it sound good but it did attenuate too much because of higher winding ratios.

Picture link did turn it 180 decrees so slope curcuit is top right, resonant circuit is on bottom left and output transformer on top left corner.

Esa


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## Esa Martikainen

That line transformer belongs to "100V standard" and can be used both ways. It has 12,5W, 25W and 50W pins and 4, 8 and 16 ohm pins and when I did put 100VAC to 25W pin there was 7.0VAC on 4 ohm, 10,0V on 8 ohm and 14.0V on 16 ohm pins

When 100V to 25W pin out came 10,0V 14,2V and 20,0V
And 100V to 50W pin out came 14,3V, 20,0V and 27,8V

I have not calculated attenuations yet because I am not used to them well enough 

Edit: If I understand it right, and assuming it is 16 ohm load where 50W is inputted there comes 3,7W from smallest attenuation output to 16 ohm load and 0,24W out of biggest attenuated output without losses the transformer cause. 

Esa


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## JohnH

Hi Esa, thanks for showing us those, it looks nice and simple to wire up. Sound clips would be interesting to hear,


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## Esa Martikainen

Thanks! I try to arrange some but it takes a while... Compared go resistive voltage divider load with a L-pad I used before this one seems to cause way less high frequencies drop. I did not build Your circuit yet when my resistors shipment did not include all I ordered and need.

Transformer winding ratios allow five different attenuations between 11dB...22dB.

This transformer/loudspeaker impedance must have some effect when its least winding ratio (11dB attenuation) is only 3.6 but without more study I can not say more. Using 16 ohm loudspeaker smallest input impedance is 207 ohms. Parallel with reactive unit the total mpedance is 14.8 ohms if reactive unit is assumed 16 ohms which seems reasonable? I don't know this line transformer inductance but it is rated for 50W and its -3dB frequency range is 25-20 000Hz.

If someone of You do have an output transformer and want a "bedroon quiet amp" you should test it as attenuator with a reactive load. And if you buy one I think you need only 10W or less transformer which are quite cheap.

This kind of "line transformers" also could be found cheap from various sellers but shipping might be expensive because they might be heavy? They were very common in buildings which did have a centralized annunciation loudspeaker systems like for example schools but they are still made new as well.

Because this mine attenuates at least 11dB next I will test mains transformers hoping that they do not restrict bandwidth too much...

Esa


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## What?

Excuse my excursion here. I'm wondering what JohnH thinks of Eminence's FDM approach to attenuation:



It appears that knob on the back of the speaker is threaded and pulls the slug part way out out of the voice coil as it is turned so that the speaker becomes less efficient but still gets the full signal from the amp.

There is an image posted with thiele small parameters halfway down the page here: https://www.tdpri.com/threads/looking-at-the-eminence-maverick-fdm.717638/


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## JohnH

Hj @What? thanks for posting that, which is very interesting.

I like the idea of the FDM speakers, nice simple direct method acting on the speaker, that anyone can understand. 

I can see it should have its place, but I can't see it as being a general solution to attenuation, because:
1. The tone changes significantly as you dial it down
2. There's only 9db attenuation available. That's enough for dialing down for a gig or rehearsal but not enough for home studio use 
3. You have to reach to the back of the speaker to adjust it, so except for cases where you can set it once and then leave it, its only adjustable on open-backed speakers with enough access, which would mostly rule out Marshall cabs and many combos too.
4 You can only use it with the cab or amp its built into
5. It doesn't have the potential for adding line-out functions, which are becoming popular on the wish lists of users 
6. We can get more consistent results over a wider range, working with any amp or speaker, using a coil and a few switches and resistors.


----------



## aceofbones

Hey John,
This is a fantastic thread and an amazing project! Thanks for all your work and thanks to everyone else who has contributed! I‘ve spent the last week reading it in its entirety. 
I will be building the M2 and had a question. Has anyone built an fx loop into their attenuator? I saw it asked once way back in the thread and the answer was use a line out for re-amping. Does it make sense to have a powered loop in the same unit?

I ask because I am building a Trainwreck Express clone. Apparently they don’t play nice with a built in loop, and since I’ll be building an attenuator, I figured I should look into it. I know some units such as the Fryette Power Station have this ability, so I wondered what the feasibility of such an option was. 
I like how the M2 works as it is, so I don’t need to alter it, I was just curious. 
Thanks very much,
Nick


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## JohnH

hi @aceofbones and thanks for your interest.

Attenuators that have fx loops are generally the ones that work by reamping the signal, so that there can be a nice controlled line-level, low power signal to send out and receive back, before the amp stage. The signals in a passive attenuator like ours are not good for that, too much power flowing for units in such a loop. But as noted, you can have a line out and then feed that into an external amp system.


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## aceofbones

Ah, ok. Thanks for the clarification. It’s much appreciated. 
I’ll find the last schematic with the line out on it and make sure I get the needed parts when I order everything else. Thanks again!


----------



## Oliver Gardiner

Hi John,

Thanks again for all your work here. While I was waiting for the power resistors to come from China, I knocked up an L-Pad using the Monacor AT-62H part - the design I followed included a switchable treble bleed cap, presumably as an attempt to compensate for lack of tone at high attenuation. It certainly does a job but I also discovered (which should have been obvious given the necessarily wire-wound elements) that its linear tapers give you no more than a quarter-turn of usable control. I eventually got all the bits for the M2 the other day and so have now given it a try. I've never had a particularly golden ear but I was genuinely surprised at how much difference there was between the two. Definitely worth the effort!


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## JohnH

hi @Oliver Gardiner , very nice build!. Thanks for posting. Which line-out circuit did you use?


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## Oliver Gardiner

Not sure what variants there are - it's currently just a 10k pot across the speaker. I've always used a mic before and will probably be a while before I could try it with a PA. It's probably there for completeness and there was plenty of space no the front panel... What are the recommended options?

Thanks,

Oliver


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## JohnH

A 10k pot is a good starting point. usually there will be a resistor in series with it on the hot side (ie from the speaker to the hot outer lug of the pot), maybe 2.2k (not critical). its job is mainly to be sure that nothing plugged into the line-out can short out the full signal.

If its at the end of the attenuator, at the speaker, it will pick up some speaker characteristics which is good, but its level will depend on how you set the attenuator. Some guys are putting them at the attenuator input = amp output instead. I think both are worth trying and you should experiment. But in any case, the line out wont be a good signal to go direct to a PA since it will have far to much treble because it doesn't have the acoustic roll off of a speaker. You'll need some form or cab-sim device, or better, an IR box.

One project I have is a passive cab-sim but I haven't tested it yet.


----------



## Filipe Soares

@JohnH what do you think about this build:

200 to 400w (to be decided) 16ohm reactive load to mute the amp, line leve transformer, class D ebay board to the speaker. making it 16ohms it would make it usable with 4 and 8 without harming too much the amp and the volume control would be totally seamless through the class d amp. no new idea here, that's what the waza amp do, but it's possible to make it super cheap and even with an added fx loop between the line level transformer and the class D amp board.


----------



## JohnH

hi @Filipe Soares 
The idea seems fine. I don't think you'd need a transformer, just a pot would do to set the input level to the Class D to match your amp output after the load. That would allow it to be set for different power tube amps. Then you'd have your main pot for setting the reduced output level.

I think the Class D amp has to be designed for the lowest speaker ohms you want to use.

The real design questions are around how much effort you put into the reactive load. It does two things, being a general rise in impedance with frequency to match coil inductance, and also the bass resonant peak. For the first, you need a $10 air-cored inductor - cheap and its the most important part. If you want to capture that bass peak too though, you also need a much bigger iron-core coil and a big-a$$ capacitor, and those alone can double the build cost. Google 'Aikens reactive load': 

https://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator

https://www.thegearpage.net/board/index.php?threads/aikens-reactive-dummy-load.1072793/

The bass peak is around 100 hz, so it only comes into play if you are down on the low strings and low frets. If its not built in, you can always compensate a bit with a some more bass or resonance on the amp.

On this thread here, the designs that guys have built (M an M2) have just the treble inductor, the bass peak is generated by the real speaker interacting with the attenuator, but with an SS amp for the output, this would not occur. We also have design M3 with the bass circuit. Your reactive load could be a lot like Stage 1 of any of the designs on this thread. Happy to keep talking.


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## Oliver Gardiner

JohnH said:


> A 10k pot is a good starting point. usually there will be a resistor in series with it on the hot side (ie from the speaker to the hot outer lug of the pot), maybe 2.2k (not critical). its job is mainly to be sure that nothing plugged into the line-out can short out the full signal.
> 
> If its at the end of the attenuator, at the speaker, it will pick up some speaker characteristics which is good, but its level will depend on how you set the attenuator. Some guys are putting them at the attenuator input = amp output instead. I think both are worth trying and you should experiment. But in any case, the line out wont be a good signal to go direct to a PA since it will have far to much treble because it doesn't have the acoustic roll off of a speaker. You'll need some form or cab-sim device, or better, an IR box.
> 
> One project I have is a passive cab-sim but I haven't tested it yet.



Thanks again for the advice and good shout on the safety resistor on the pot. I may use the line out in the unlikely event that I wanted to record my dodgy playing but I'll keep an eye out for the passive cab sim and perhaps build that in at some point.


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## aceofbones

I’m very interested in @Filipe Soares‘s idea. That’s basically what I’m looking to do. After your(JohnH) answer to my question about an fx loop, I decided to just build the M2 with a line out and make a small power amp to feed a second cabinet. However, if there’s a feasible way to do it in one box that would be fantastic. I have already ordered all the parts for the M2.
Do you see this as a possible other direction someone could take your work or would it end up compromising the functionality of an already proven design?


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## JohnH

hi @aceofbones
There's definitely a possible design in what you say. My view is that to get it from idea to finished design is probably not a one-step process, but rather one that needs some testing,/prototyping to get it right and find and iron out any bugs.

Id suggest to get the M2 working with a line-out, and make a test-bench build of the amp section separately, so you can check for levels at different stages, power supply, tone and what values of pots or any trimmers needed and how the fx loop will work. Then if it seems like its worth pursuing you can chose whether to build it into the M2 or to build it as a separate unit.

For this arrangement you need line-out via a pot from the attenuator input ie the real amps output

My question about it is, as for Filipe, how it will sound in the low bass without the bass resonance circuit since the speaker won't be interacting with the attenuator to create the bass peak, as it does in the M and M2 passive designs.

It might be handy to have both passive and active options, but if you end up just using the active amp output, then you might not need all the switched passive stages.

If I ever get any time, I can test some of this using my M build as a load box, and getting a line-out signal to feed into the poweramp section of my old PowerBlock amp, which when you plug in after its preamp is just a linear clean Class D.


----------



## Oliver Gardiner

JohnH said:


> A 10k pot is a good starting point. usually there will be a resistor in series with it on the hot side (ie from the speaker to the hot outer lug of the pot), maybe 2.2k (not critical). its job is mainly to be sure that nothing plugged into the line-out can short out the full signal.
> 
> If its at the end of the attenuator, at the speaker, it will pick up some speaker characteristics which is good, but its level will depend on how you set the attenuator. Some guys are putting them at the attenuator input = amp output instead. I think both are worth trying and you should experiment. But in any case, the line out wont be a good signal to go direct to a PA since it will have far to much treble because it doesn't have the acoustic roll off of a speaker. You'll need some form or cab-sim device, or better, an IR box.
> 
> One project I have is a passive cab-sim but I haven't tested it yet.



Presumably, the output of the first 7dB stage would also be an option in terms of the level being stable - will have a play. Having done a bit of poking around, I think I'll start with the Simple Cab Sim R2 by Lart but I'll keep it external to the attenuator


----------



## JohnH

Oliver Gardiner said:


> Presumably, the output of the first 7dB stage would also be an option in terms of the level being stable - will have a play. Having done a bit of poking around, I think I'll start with the Simple Cab Sim R2 by Lart but I'll keep it external to the attenuator



Can try it, but it may not be best and may sound a bit dull. 

One of the tricks of the M and M2 designs is that the tone that travels through the attenuation stages after Stage 1 is mostly neutral, without the reactive phase effects which are balanced out by the part series part parallel arrangement of the coil or coils.. This enables it to step down using resistors without changing. Then when it reaches the speaker, the speaker recreates the correct tone by its own reactance, as it would if fed directly from an amp. 

So the best tonal balance for taking a line out is at the speaker, or at the input where it is affected by the coil.


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## aceofbones

Thanks very much. I will complete the M2 then play around with some ideas. 
Cheers!


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## Mcentee2

Hi all, back with a question I should already know.

I have been thoroughly enjoying my M2 build for a few months now, but have recently got an extra 8r cabinet that plugs into my SV20 head along with its current 8r cab.

I can plug them both directly I to the sv20 in the "2x8r" sockets, ....hmmmm I would need two attenuators here!

or I can plug one into the extension jack of the other and take that back to the "1x4r" on the sv20.

Hurrah!

..but, I built an 8r M2.

With the 4r parallel speaker cabs plugged into the back of the M2 I am reading this on the amp input jack:

-7db 7.6r
-14db 8.6r

So, is this ok to plug into a 1x8r output on the SV20 for both?

With a 4r actual speaker load, are the M2 stages still -7db and -14db, or has this changed ?

IIRC from previous posts, quite a few pages now, from the speakers' point of view, now a total 4r, they are happy with the input impedance of the amp/attenuator anyway.

I know if I use the M2 bypassed I will need to repatch it to the 8r output until I can build a 4r version.

Which brings me to :

is there a quick and easy way to get my M2 to a 4r version ?

I'm thinking it isn't as simple as toggling a high power 8r in parallel with the whole thing?


----------



## JohnH

Hi @Mcentee2

Good questions, and there's good news:

Provided that at least the first -7db stage is on, it should work fine with a four ohm load on the attenuator, and using the 8 Ohm amp tap. You checked the resistance seen by the amp and it still stays close to the 8 ohms that the amp expects. The attenuation levels stay close to the same but with about another -1to -2 db attenuation through the mids compared to bass and high treble. You may not be able to hear this difference, but if you wanted to compensate, a small reduction in amp resonance and presence should fix it.

For using different amp taps, weber uses just a parallel resistor, but if you do that you partly loose the benefits of the reactive input. I worked out an add-on comprising another coil and two resistors which you can put in series with the input to convert to 16 ohms or in parallel to convert to 4 ohms. But in either of these cases, all attenuations drop another -3 db.

The diagram below has values to take a base 8 Ohm M2 and use it with 16, 8 or 4 ohm taps, or to start with a 4 Ohm M2 and use 8, 4 or 2 ohm amp taps (useful for certain old Fenders)













Attenuator M2v 200420



__ JohnH
__ Sep 20, 2020


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## Gene Ballzz

WOW @JohnH !
That is a really great revision! With a "post attenuation" line out, that would be a pretty much one size fits all (well two really, 16-8-4 and 8-4-2) with all of the bells, whistles and options!
Great Job!
Gene


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## JohnH

Hopefully it's a useful idea for those with multiple different amps. I dont think it should affect the tone significantly. Just have to take account that the added front end parts take half the power when they are running so hence the added -3db drop.


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2
> 
> Good questions, and there's good news:
> 
> Provided that at least the first -7db stage is on, it should work fine with a four ohm load on the attenuator, and using the 8 Ohm amp tap. You checked the resistance seen by the amp and it still stays close to the 8 ohms that the amp expects. The attenuation levels stay close to the same but with about another -1to -2 db attenuation through the mids compared to bass and high treble. You may not be able to hear this difference, but if you wanted to compensate, a small reduction in amp resonance and presence should fix it.
> 
> For using different amp taps, weber uses just a parallel resistor, but if you do that you partly loose the benefits of the reactive input. I worked out an add-on comprising another coil and two resistors which you can put in series with the input to convert to 16 ohms or in parallel to convert to 4 ohms. But in either of these cases, all attenuations drop another -3 db.
> 
> The diagram below has values to take a base 8 Ohm M2 and use it with 16, 8 or 4 ohm taps, or to start with a 4 Ohm M2 and use 8, 4 or 2 ohm amp taps (useful for certain old Fenders)
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator M2v 200420
> 
> 
> 
> __ JohnH
> __ Sep 20, 2020



Many thanks!

Glad I planned ahead, as I have a spare 0.6mH inductor left from my M2 build and maybe space on the enclosure 

I also have 2x16r, 1x12r and 1x8r all 25w, which I put together to make:

2x16r in parallel = 8r 50w for R12
1x8r and 1x12r series = 20r 50w for R13

These aren't exact but are probably close enough.

The extra -3db isn't something I was looking for, but might be liveable with.


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## crashcarlson

@JohnH 

All the work you've done here is just awesome! I did have one question on your schematics - I'm familiar with soldering, but I had a question on your symbols you're using. Specifically, I want to make "Design C" on post #68 way back on page 4 (for a 16 ohm amp output and a 16 ohm cab). I'm just not sure which is tip (signal) and sleeve (ground) for the jacks. Would I be right in assuming that the "top left" side (input from amp) has the tip (signal) of the jack connecting to R2, and the sleeve (ground) connects to R1? And then the bottom part of the circuit (R1 to R3 to R5) ends up terminating at the sleeve on the output jack? Is there any other grounding (like to the enclosure itself), or is that it?

Thanks!


----------



## Gene Ballzz

@crashcarlson 
 to the forum!

All *+/Positive/Hot* AND *-/Negative/Ground* connections must be connected by wire and isolated from the chassis/box/enclosure.

Just Sayin'
Gene


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## JohnH

hi @crashcarlson 
Yes as Gene says, and you are right about the jack connections.

There's no chassis grounding wanted here. Should use plastic jacks. The two wires that come out of the amp are usually from just the transformer windings and are not grounded either. so we dont need or want either of them being live to the attenuator box.

That design C is a couple of years ago, but its still a good version if you just want resistive design as simple as possible. Get resistors ated at least 3x the power dissipations shown, assuming a 50W amp.


----------



## crashcarlson

@Gene Ballzz @JohnH - Thanks for the tips! I did order 3x or 4x above the dissipation shown for the resistors (I do have a 50W amp). Good to know about separating hot/gnd from chassis, I don't have any plastic TS jacks so I'll have to order those as well. I am going with the simpler one first to see if I like how it works with my amp (never used an attenuator before)...I'll probably end up doing the 'final version' you posted with the inductors somewhere down the road as well.

Thanks again!


----------



## crashcarlson

@JohnH

I decided instead to build the M2 version with the single inductor, that you have laid out in this post (only mod would be a single output jack instead of two, since I have a 16 ohm amp driving a 16 ohm cab)
https://www.marshallforum.com/threa...design-and-testing.98285/page-56#post-2000746

I’m not the best at reading schematics, but after reading through the posts, and looking at all the build pictures, I think I understand it. I hope you don't mind answering some quick questions just so I know I've got this right. The one part that I am not 100% sure on, is how the switches connect the resistors for the different stages. I took this from your M2 diagram and made my annotations:





(I have SPDT On-On switches, so 3 poles)

This is how I’m understanding the schematic - could you let me know if this is right?

Wire comes off of pole 3 on the switch by R3/R4, connects to right lug on R4
Wire comes off right lug of R4, connects to pole 2 (common) on switch by R5/R6
Wire comes off left lug of R6, connects to pole 2 (common) on switch by R5/R6 (same as above)
Wire comes off of “top” lug of R5, connects to pole 1 of switch by R5/R6
Wire comes off of pole 3 of switch by R5/R6, connects to right lug of R6
Lastly - in this diagram, when R5 is connected via poles 1 and 2 on the switch - is this attenuation stage on or off?

Thanks again!


----------



## Oliver Gardiner

Yes, that is correct. When the switch is connected between pins 2 and 3, the wire connection simply bypasses the series resistor (e.g. R4) and the shunt resistor (e.g. R3) is disconnected - hence the stage becomes just a piece of wire and there is no attenuation. In the alternate position, both resistors are in circuit and the attenuation is created by the series resistor (e.g. R4) in series with the "speaker" (i.e. the load presented by the next stage which, simplistically, can be taken to be around 8 ohms). The shunt resistor is in parallel with the two series loads (e.g. R4 + 8) and serves to bring the impedance seen by the *preceding* stage back to around 8 ohms. Because the stages are cascaded, yes, pin 3 of each switch is also connected to pin 2 of the next switch as well as the output of the series resistor - it shouldn't really matter how you wire it as long as everything is robustly connected with wire rated for enough current (like your speaker wire).


----------



## Grey

JohnH, I haven't read through the entire thread but I think I did see simulations. Have you posted those for download? I'm a spice junkie and wouldn't mind playing around with them.

I like the versatility of the M2v. Have you thought about removing 2 of the jacks from each end and replacing them with option switches for the various input and output impedances?


----------



## crashcarlson

Oliver Gardiner said:


> Yes, that is correct. When the switch is connected between pins 2 and 3, the wire connection simply bypasses the series resistor (e.g. R4) and the shunt resistor (e.g. R3) is disconnected - hence the stage becomes just a piece of wire and there is no attenuation. In the alternate position, both resistors are in circuit and the attenuation is created by the series resistor (e.g. R4) in series with the "speaker" (i.e. the load presented by the next stage which, simplistically, can be taken to be around 8 ohms). The shunt resistor is in parallel with the two series loads (e.g. R4 + 8) and serves to bring the impedance seen by the *preceding* stage back to around 8 ohms. Because the stages are cascaded, yes, pin 3 of each switch is also connected to pin 2 of the next switch as well as the output of the series resistor - it shouldn't really matter how you wire it as long as everything is robustly connected with wire rated for enough current (like your speaker wire).



This is perfect, thank you for the explanation! Got all the parts ordered, can't wait to build it and try it out!


----------



## JohnH

hi @crashcarlson ,yes I think you have it all right. Im glad your building the M2!, it's a bit better than C, andbits worth it' for very little added complexity or cost


----------



## JohnH

Hi @Grey
I haven't posted the simulation files, but there are quite a few charts of simulation results Most of them are from a dedicated spreadsheet which has all the theory but i dont post that But these circuits are pretty simple so easy to set up in any Spice program that you use.


----------



## Grey

JohnH said:


> Hi @Grey
> I haven't posted the simulation files, but there are quite a few charts of simulation results Most of them are from a dedicated spreadsheet which has all the theory but i dont post that But these circuits are pretty simple so easy to set up in any Spice program that you use.


I was just curious because it's easier to go from a starting point than scratch. I'll probably do an M2 16ohm version myself. The weird part is I'd be doing it for a 15 watt amp but will have to over build it so I can also use it on a friends Bugera head with a 4x12. I'll have to check his impedances and wattage Sunday as I'm not sure what they are. This is my first tube amp so I'm not familiar with it all yet, I'm more into design power supplies.


----------



## RoadShow

Where can I get the 900uH inductor? I found some at Mouser like
https://www.mouser.com/ProductDetai...=sGAEpiMZZMv126LJFLh8y2AtSXNc0tXos1ZVI4wA1hY= 
but the wire gauge is 20 and not 18 as prescribed.


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## Oliver Gardiner

RoadShow said:


> Where can I get the 900uH inductor? I found some at Mouser like
> https://www.mouser.com/ProductDetail/Hammond-Manufacturing/1540M22?qs=sGAEpiMZZMv126LJFLh8y2AtSXNc0tXos1ZVI4wA1hY=
> but the wire gauge is 20 and not 18 as prescribed.


https://www.ebay.co.uk/itm/0-1mH-3-...var=611258617282&_trksid=p2057872.m2749.l2648

1mm is close enough to 18awg


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## JohnH

On the inductor, apart from the gage, that mouser link looks to have a core of some kind, but what we need is an air-core ie no core at all! 

The ebay one linked by Oliver above looks fine to me. Ive not bought inductors from ebay myself because the ones I see are never clear about the wire gage, but that one does state it and 1mm diameter is 18 gage (tables quote 18 gage = 1.02mm, same thing)

In general, these coils are sold at places where components for speakers are sold, and they are used in speaker cross-over networks.

Ive bought mine from a couple of places down here in Oz. But in the US, we have had success buying them from Madisound. They do a range at 19 gage, which is OK. id be happy with that up to about a 50W amp and Gene has built a few using them. Good value too.

https://www.madisoundspeakerstore.com/air-core-19-awg-page-2

They seem to list a 0.85mH and a 1mH. To be honest, either of those would be fine for the 0.9mH inductor in the M2 design. Ive sometimes bought a bit bigger then unwound it a little to reduce the value (easy if you have a meter that measures inductance), but thats being picky.

For a use with a more powerful amp , I think they do a 16 gage range too.


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## RoadShow

Thx guys,

I appreciate the guidance!!


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## JohnH

hi @Grey

Here's a picture of the kind of model I use, I this case its in 5Spice. Im sure you would use a more powerful sim program but this works for me.



What I do is:

A frequency sweep analysis, feeding through an assumed output impedance of the amp (I based it on 20 Ohms measured from my VM, but also test over a range lower and higher).

I run two models in parallel, one directly into a circuit model of a speaker (based on Aikens model) and another via the attenuator.

In the above plot, you can see the cap and inductor C9 and L20 from design M3, which you can negate to test without for M2.

All attenuation stages are there but the last two are effectively switched off, using very low and very high R values. Then I look at the two output plots to see how they are the same shape.


----------



## Grey

JohnH said:


> hi @Grey
> 
> Here's a picture of the kind of model I use, I this case its in 5Spice. Im sure you would use a more powerful sim program but this works for me.
> 
> View attachment 79829
> 
> What I do is:
> 
> A frequency sweep analysis, feeding through an assumed output impedance of the amp (I based it on 20 Ohms measured from my VM, but also test over a range lower and higher).
> 
> I run two models in parallel, one directly into a circuit model of a speaker (based on Aikens model) and another via the attenuator.
> 
> In the above plot, you can see the cap and inductor C9 and L20 from design M3, which you can negate to test without for M2.
> 
> All attenuation stages are there but the last two are effectively switched off, using very low and very high R values. Then I look at the two output plots to see how they are the same shape.


Thanks. I'm going to see if I can knock this up in LTSpice. I've not done any work involving circuits for audio so I'm curious to see how the speaker comes out. I've done enough work on custom models that I think I can work something out that could be fairly accurate. Not to re-invent the wheel, I may do a bit of research first to see if it's been done somewhere before, which I'm sure it has. I'll post my work if it bears fruit so that others can have a go.

I do have one question though. On your schematics you show a representation of an audio jack in a format I'm not familiar with. What would be the pinouts for those? I get the sleeve part but the other two could be interchangeable in my mind. Are you using 3 terminal jacks for everything and just leaving one pin un-soldered or recommending others do so? The reason I ask is because the connections seem rather odd for the 4ohm input and the 16ohm and 8/4ohm outputs on m2v considering that speaker plugs are two terminal devices as far as I know. I have an Egnater tweaker 15 combo and the speaker plug is a two terminal arrangement. I guess what i'm trying to say is on the 16ohm output one terminal connects to R10 and the other to R11 and the sleeve to ground and this is confusing from my point of view.


----------



## JohnH

Hi @Grey , For most of the jacks on my diagrams, simple 2-wire jack sockets are all that are realy needed. But I still like to adopt the TRS stereo ones because the extra ring connection provides added physical grip.

But there are a few cases where I do use the extra connection. In the M2V its part of connecting in the added RL network that adusts the input impedance, and also on the output, the adjustment for use of 16ohm cabs. For this, Im using the basic barrel of a standard mono plug being pushed into the TRS socket to connect the ring terminal to ground via the plug barrel, and so make the added connections wanted. An alternative is to add more switches instead.


----------



## crashcarlson

For the "M2" design would I be correct in assuming that either of the outputs (Out 1 or Out 2) can be used with a 16 Ohm cab (I'm building the 16 Ohm version and the amp output will be at 16 ohm)?


----------



## JohnH

Yes Out1 and Out2 are shown as two sockets in parallel, so they are the same. 

Having two Outs could be useful if you want to plug in a second 16 Ohm cab as well, which works fine with this design, so you'd then use your 16ohm amp, 16 ohm attenuator driving two 16 cabs which is an 8 Ohm load.


----------



## Yamariv

Hi John, thank you for all your hard work on this attenuator! 

I just received all the parts to make a couple M2's and wanted to see if anyone has done a wiring layout for it by chance? I am getting a lot better with schematics but I'm a bit worried about messing something up. If there's a layout available that would be easier for my brain to compute. If not, I guess I'll be doing it schematic style, just thought I'd check first..

Cheers


----------



## JohnH

Hi @Yamariv

its really a good idea if you work out the wiring yourself on a sketch, taking into account the size of the parts and the case that you have. Then you will have thought it through and have something to work off. 

Every layout built so far has needed to be a bit different and all builds that I know of have been a success. I haven't wanted to post a wiring layout because then it becomes possible to try to build these without thinking it through and hence mistakes could be made. Interpreting the schematic into a wiring layout is the entry exam for using the design!

When working out a physical layout related to the schematic, in this case, it doesn't matter how the wiring goes so long as the correct lugs are connected. eg A to B to C is the same as B to A to C or A, B and C all to one point etc. (not true in an amp but perfectly ok here)

On the schematic, the switched stages are shown -7, -14 then -3.5, for a small electrical reason. You can follow that in terms of wire connections but physically, you probably want to place the switches 14, 7, 3.5 or 3.5, 7, 14

Think about how to place the jacks and switches in your case, so that it operates conveniently in your rig. 

Try to minimise lengths of wire runs where possible

Think about the order of wiring and how you will work on it and open and close the case

The case should not be connected to anything.

When its built, test it with a resistance setting of a multimeter before using an amp


----------



## Maurizio Minguzzi

Good morning thanks for the work you are doing and it looks great. I made the m-4 version with two inductors my idea is to add a control for the treble by inserting a capacitor with fc 1000 hz on r2. for more crystal clear sound. There are other changes to the scheme to achieve this


----------



## Yamariv

JohnH said:


> Hi @Yamariv
> 
> its really a good idea if you work out the wiring yourself on a sketch, taking into account the size of the parts and the case that you have. Then you will have thought it through and have something to work off.
> 
> Every layout built so far has needed to be a bit different and all builds that I know of have been a success. I haven't wanted to post a wiring layout because then it becomes possible to try to build these without thinking it through and hence mistakes could be made. Interpreting the schematic into a wiring layout is the entry exam for using the design!
> 
> When working out a physical layout related to the schematic, in this case, it doesn't matter how the wiring goes so long as the correct lugs are connected. eg A to B to C is the same as B to A to C or A, B and C all to one point etc. (not true in an amp but perfectly ok here)
> 
> On the schematic, the switched stages are shown -7, -14 then -3.5, for a small electrical reason. You can follow that in terms of wire connections but physically, you probably want to place the switches 14, 7, 3.5 or 3.5, 7, 14
> 
> Think about how to place the jacks and switches in your case, so that it operates conveniently in your rig.
> 
> Try to minimise lengths of wire runs where possible
> 
> Think about the order of wiring and how you will work on it and open and close the case
> 
> The case should not be connected to anything.
> 
> When its built, test it with a resistance setting of a multimeter before using an amp



Thanks John, makes perfect sense about the layout when you explain it that way! I was reviewing the schematic after your reply and it's looking much simpler than I remember, I should be fine! Fingers crossed! 

Quick (possibly dumb) question.. if my inductors have a plastic bobbin as a surround, is there any issue with me using a metal bolt to mount it if the inductor itself is not touching any part of the chassis? Seems obvious to me that a metal mounting nut would be ok but just wanted to make sure that I wasn't missing anything as I haven't worked with inductors before (and I don't want to buy a bunch of nylon bolts and nuts if I don't have to.. )


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## JohnH

hi @Maurizio Minguzzi , that's a great solid looking build!

That cap idea should work to add further treble - you'll need a bipolar capacitor and youd work it out so 1/2pi.f.C = R2 if you want to start to roll in at f = 1000 hz. Using nearest standard values, I make that 15uF for the 8 Ohm build and 8.2uF for the 16 Ohm version.

Quite a few attenuators have this to try to compensate for their inherent tone issues, which we don't have in this design.

But, my concern with doing that is that it will throw off the input impedance as seen my the amp, reducing it at high frequency. So be careful, probably OK but some amps might not like it. I have heard that switching in such a cap on some other designs can stress the tubes

I tested it in a Spice run, using an 8 Ohm version of Design M (I think M2 will be similar, also 16 Ohm ones will show similar curves).




The upper lines represent what the amp sees and the lower ones are at the speaker, in this case at -14db. Switching in the cap raises the response from the lowest black line up to the blue line, so you get a few db more treble. The amp then sees the green response instead of the upper black one (or red with full volume no attenuator). What's good is that although the green line drops, it doesn't dip lower than a the red/black ones at mid frequency, so it looks safe to try with care.


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## JohnH

Hi @Yamariv . Even if a steel nut and bolt is not contacting anything, it can still affect the inductance, raising its value. Its because steel is much more permeable to magnetic flux than is air. That's why transformers and larger inductors have iron cores.

I tested this will mine and was surprised to find a significant affect on my coil inductances even with just a small steel bolt.

As an alternative, nylon or stainless steel bolts are ok (stainless is generally non permeable), or zip ties (see Maurizio's above), or, I just used an adhesive and put my coils on the base.


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## Yamariv

JohnH said:


> Hi @Yamariv . Even if a steel nut and bolt is not contacting anything, it can still affect the inductance, raising its value. Its because steel is much more permeable to magnetic flux than is air. That's why transformers and larger inductors have iron cores.
> 
> I tested this will mine and was surprised to find a significant affect on my coil inductances even with just a small steel bolt.
> 
> As an alternative, nylon or stainless steel bolts are ok (stainless is generally non permeable), or zip ties (see Maurizio's above), or, I just used an adhesive and put my coils on the base.



Oh wow.. I'm glad I asked then! I thought it was a dumb question  So now to decide from all those options, I might just do some silicone adhesive right to the case.. Thanks again John, gonna start building soon


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## Maurizio Minguzzi

Thanks JohnH for the very accurate answer. Now I'm thinking of building the M3 version with two coils and a capacitor, these attenuators work really well, have a good day Maurizio


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## Mcentee2

JohnH said:


> Hi @Mcentee2
> 
> Good questions, and there's good news:
> 
> Provided that at least the first -7db stage is on, it should work fine with a four ohm load on the attenuator, and using the 8 Ohm amp tap. You checked the resistance seen by the amp and it still stays close to the 8 ohms that the amp expects. The attenuation levels stay close to the same but with about another -1to -2 db attenuation through the mids compared to bass and high treble. You may not be able to hear this difference, but if you wanted to compensate, a small reduction in amp resonance and presence should fix it.
> 
> For using different amp taps, weber uses just a parallel resistor, but if you do that you partly loose the benefits of the reactive input. I worked out an add-on comprising another coil and two resistors which you can put in series with the input to convert to 16 ohms or in parallel to convert to 4 ohms. But in either of these cases, all attenuations drop another -3 db.
> 
> The diagram below has values to take a base 8 Ohm M2 and use it with 16, 8 or 4 ohm taps, or to start with a 4 Ohm M2 and use 8, 4 or 2 ohm amp taps (useful for certain old Fenders)
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator M2v 200420
> 
> 
> 
> __ JohnH
> __ Sep 20, 2020



I knocked up that new pre stage over the weekend, and it works brilliantly, even with my slightly off resistors.

Can this stage be used just by itself as a -3db drop with no ill effects as I can see the R12 and inductor arrangement is slightly different to the main -7db stage?

I know my resistor lower ratings are ok.


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## JohnH

Yes i think that's ok to use it on its own with a speaker. It takes off -3db and the impedance changes x2 or x1/2 depending how its connected.


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## Mcentee2

JohnH said:


> Yes i think that's ok to use it on its own with a speaker. It takes off -3db and the impedance changes x2 or x1/2 depending how its connected.



Apologies though, I probably didn't think it through fully before asking the question!!

I suppose on its own it is an 8r load, it is only when in parallel/series with a simple 8r M2 does it do it's useful 4r/16r thing.

On its own, it still a useful thing to have, a 3db 8r stage, but I was probably asking the "wrong question" , doh!!

My actual unit doesn't match your calculated R12/R13 values exactly, so when I use just it and my 2x8r speakers (4r), I get 2.8r, which is a bit low to just use on its own as I don't have any 2r amp outputs.


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## crashcarlson

I feel kind of dumb here, but I'm hoping this is a simple issue that someone else has run into and knows a way around it.

There's a few places where I need to solder two wires to a single lug of a resistor. For example, on the M2 design, L1/R1/R2B all need to connect, either one lug of R2B or one lug of R1 will have two wires connected to it. But with the 18 gauge wire I'm using, there's no way I can fit both wires through the lug - it's just too small.

Do I have some oddball resistors? Are you drilling those solder lugs a little wider before installing them? Are you putting one wire through the lug and soldering it, and then soldering the other wire directly to that wire, instead of going through the lug (I'm sure that would work but I'd prefer to have a physical connection to the lug if possible)? 

Thanks!


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## JohnH

hi @crashcarlson , personally, I feel ok about my joints if I have good tinned ends on my wires and a good amount of contact, even if the wires aren't all through holes. Then melt with a little more solder to flow smoothly. I want my solder joints to look like the liquid-metal bad guys in Terminator 2!


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## crashcarlson

JohnH said:


> hi @crashcarlson , personally, I feel ok about my joints if I have good tinned ends on my wires and a good amount of contact, even if the wires aren't all through holes. Then melt with a little more solder to flow smoothly. I want my solder joints to look like the liquid-metal bad guys in Terminator 2!



Sounds good - they are SUPER close to fitting. In fact, the inductor is 19 ga. instead of my 18 ga. wire, and I was able to fit those both into one lug. I might try getting a small circular file and seeing if I can very slightly increase the diameter. But if that fails I'll just glob them all together with more solder and ensure it's all connected with a multimeter. Thanks for the feedback!


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## crashcarlson

@JohnH - I just wanted to say THANK YOU for all the time and effort you put forward on this project. I was able to get it all wrapped up and tested out today and I am VERY happy with it!

My scenario might be a little different than most. I have a Blackstar HT Club 50 Mk II 50 watt amp. I don't need to crank it to get OD, since it has it's own OD channel...but I don't really use that anyway. I use the clean channel and a few nice dirt pedals. The amp does have a power scale (either 5W or 50W), as well as a master volume. I found that my pedals sound MUCH better with the added headroom on the 50W mode, but even with the master way down it was still pretty loud. With the attenuator hooked up, even with just the first -7dB stage, I can get some AWESOME sounds without getting way too loud for the room!

Now that I know what I'm doing I think I'm going to make another one but make it a little cleaner and maybe do some decals instead of just writing on it.

I did have one question - if I did want to add an overall "bypass everything" switch so I could run it with no attenuation, would I just need another 5A/125V switch? I'm thinking I would then wire the signal (Tip) from the input to the COM on the switch, and then one side directly to the output jack, and the other side to the R2A/L1 components right? If I go that route are there any "gotchas" (like, don't switch to fully bypassed while the amp is on, etc.)?

Thanks again for everything - this design you've done here is really awesome!













Attenuator in action



__ crashcarlson
__ Oct 8, 2020


















Inside attenuator



__ crashcarlson
__ Oct 8, 2020


















Bottom of attenuator



__ crashcarlson
__ Oct 8, 2020


















Back of attenuator



__ crashcarlson
__ Oct 8, 2020


















Front of attenuator



__ crashcarlson
__ Oct 8, 2020


















Top of attenuator



__ crashcarlson
__ Oct 8, 2020


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## JohnH

Hi @crashcarlson , I really like that build! Good use of space in the case. I like the hand graphics too, which remind me of my old Crate PowerBlock amp.


*Bypass:*

You can have it if you want it. It should be a two pole switch. One common to the input, one to the output. In bypass mode, one set of outer lugs are connected together, totally disconnecting the attenuator parts from input or output. The other pair of outer lugs direct the signal into and out of the attenuator. Its similar to a basic true-bypass wiring on a stompbox.

I set the current M2 base design without that, to keep it simple, save a switch and save two sets of contacts in the signal path. I figure there's rarely a reason to keep switching from bypass to attenuated, other than just to try it.

But, my own one is Design M. For that one, I show a bypass switch that happens not right to the output but to just before the final stage which is -3.5db. This allows the -3.5 stage to work on its own, so every 3.5db increment from 0 to -31.5db is available. See post No.1. I just wanted to try it, and it works and sounds fine. But I haven't found it to really be any use to me! But if wanted, it could be done either way on Design M2.

But the watchits with either of those bypass options:

Without the bypass, its not sensitive to switching on the fly. But, if you switched to bypass during playing, unknown transients might get back to the amp.

Also, without bypass, its very tolerant of different cab Ohms , provided you use the right amp tap as designed. But if you use bypass, or that extra -3.5db only setting above, then cab and amp Ohms need to match.


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## crashcarlson

JohnH said:


> Without the bypass, its not sensitive to switching on the fly. But, if you switched to bypass during playing, unknown transients might get back to the amp.



When you say "during playing" do you mean actually switching it while you're playing guitar, or just having the amp on? For example, if I put the amp in standby would it be safe to switch to bypass, or would the only real safe way to do it be to switch between bypass when the amp is completely off?


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## Paul Buxton

Hi everyone, just joined the forum after finding this awesome thread!
There is so much information here it is mind blowing.

Like a lot of people when the pandemic started, I decided to pick up an instrument, after a few weeks playing a guitar I had kicking around in the garage (a birthday present from my teenage years), I decided I would like to build my own guitar from a kit. Which I did. That naturally led me to look at amps as well, and it wasn't too long before I ordered a tube amp kit and built that too 
But it turns out that the amp was a little too good and I couldn't crank it up enough to get some nice distortion without incuring the wrath of my girlfriend, so I then started to look at attenuators. I built an lpad based one, which lets me crank the amp up, but when using it with my neck p'ups it really sounds like I am under-water! So the search for better attenuation led me here!

Skimming through the thread I think I will be looking at the M2 design. Could anyone confirm that using the 8Ohm version with a single 8Ohm speaker is fine? I note the two outputs on the design and just want to check that it is not intended to run 16Ohms in parallel which would be equivelant to an 8 (I think trying to dredge up whatever electronics I can remember)?

Given the volume of information here would it be useful to put it somewhere more structured? I would be happy to setup a github repository to collate it...

Thanks!

Paul


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## JohnH

crashcarlson said:


> When you say "during playing" do you mean actually switching it while you're playing guitar, or just having the amp on? For example, if I put the amp in standby would it be safe to switch to bypass, or would the only real safe way to do it be to switch between bypass when the amp is completely off?



With the amp powered but on standby its totally safe to switch anything or reconfigure your speakers no problem. That's what I do if I want to unplug/remove or add the attenuator, or use its bypass switch, also if I'm going to the -3.5 setting that I have. 

What I would definitely not do is hit a chord and go to or from bypass. 

Between the lower attenuated settings eg at least -7db and more, it should be ok to switch on the fly to find the level that you want but I prefer to just mute the strings or amp volume while I do that switch change.


----------



## JohnH

hi @Paul Buxton , welcome! you have found the right attenuator!

The 8 ohm M2 design is intended for using the 8 Ohm tap from your amp with either a single 8 ohm speaker cab, or two of 16 Ohms, hence its two parallel output sockets. Its also safe to use it with 8 ohm amp and a single 16 Ohm speaker, and if you build the optional red coloured parts in the diagram with the third output, it just corrects the tone for that arrangement.

What amp do you have?

Yes there's a lot of info and most of it is a journey rather than a destination. I try to keep all the current key info in post 1, though its limited to 10000 characters so it gets squeezed


----------



## Yamariv

JohnH said:


> Hi @Yamariv . Even if a steel nut and bolt is not contacting anything, it can still affect the inductance, raising its value. Its because steel is much more permeable to magnetic flux than is air. That's why transformers and larger inductors have iron cores.
> 
> I tested this will mine and was surprised to find a significant affect on my coil inductances even with just a small steel bolt.
> 
> As an alternative, nylon or stainless steel bolts are ok (stainless is generally non permeable), or zip ties (see Maurizio's above), or, I just used an adhesive and put my coils on the base.



Got my chassis's drilled out and started painting them today..just about to make up some decals and clear coat tomorrow

One quick question about the M2 16ohm Version I'm building.. Are there any drawbacks to running my amp's 16 ohm output through the Attenuator into an 8 ohm cab? Others have said it's safe to do, are there any other drawbacks to doing that, most of my cabs are 8 ohm.. I'm now thinking the 8 ohm version is what I should have built but I have all the parts for the 16 so..


----------



## JohnH

16 ohm amp tap, 16 Ohm M2 and 8 ohm cab is totally safe, unless you have a bypass switch in it in which case the amp would obviously see your 8 ohm cab directly. If should also sound fine too, maybe a tiny tad brighter. So all fine really.


----------



## Paul Buxton

JohnH said:


> hi @Paul Buxton , welcome! you have found the right attenuator!
> 
> The 8 ohm M2 design is intended for using the 8 Ohm tap from your amp with either a single 8 ohm speaker cab, or two of 16 Ohms, hence its two parallel output sockets. Its also safe to use it with 8 ohm amp and a single 16 Ohm speaker, and if you build the optional red coloured parts in the diagram with the third output, it just corrects the tone for that arrangement.
> 
> What amp do you have?
> 
> Yes there's a lot of info and most of it is a journey rather than a destination. I try to keep all the current key info in post 1, though its limited to 10000 characters so it gets squeezed



Thanks, that makes sense.

The amp I will be using this on is the Bud Tube amp kit. From tubedepot.com


----------



## Yamariv

JohnH said:


> 16 ohm amp tap, 16 Ohm M2 and 8 ohm cab is totally safe, unless you have a bypass switch in it in which case the amp would obviously see your 8 ohm cab directly. If should also sound fine too, maybe a tiny tad brighter. So all fine really.


Aweome John, good to know!! Thanks again


----------



## crashcarlson

JohnH said:


> Also, without bypass, its very tolerant of different cab Ohms , provided you use the right amp tap as designed. But if you use bypass, or that extra -3.5db only setting above, then cab and amp Ohms need to match.



John - does this mean that if I use the bypass, that I can only use a SINGLE output instead of the two outputs? For example, if I have the 16 Ohm amp and 16 Ohm attenuator, but it's driving two 16 Ohm cabs, isn't that an 8 Ohm load? In which case that would be bad if I engaged the bypass since the Ohms wouldn't match, right? If that's the case I would think if I do the bypass build I should only have one output, and only hook it up to a 16 Ohm cab?

Thanks!


----------



## JohnH

You're right about what you have worked out. We have all these output and input options to provide versatility and solve problems, but with them you have to look out for a few situations where you have to use them in a certain way. You have to find the best mix for your use between versatility and foolproofness. Personally I find having the two parallel output to be useful sometimes, but there's no issues if I just use one cab. 

Its why I show the basic M2 with no bypass but with two or three outputs. In practice, for most players with most amps, it is simple and can solve almost any real problem. Using it can include not using it at all! for the same as full bypass. Another useful workaround is if you have the right amp taps, use it in parallel to the speaker as a load, using half the amp ohms tap - this takes off 3db. And also, with no bypass, theres no unsafe combinations of speakers plugged into it.


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## crashcarlson

JohnH said:


> You're right about what you have worked out. We have all these output and input options to provide versatility and solve problems, but with them you have to look out for a few situations where you have to use them in a certain way. You have to find the best mix for your use between versatility and foolproofness. Personally I find having the two parallel output to be useful sometimes, but there's no issues if I just use one cab.



Thanks John - I'll have to decide if I'd rather have the bypass or the option for two cabs....I rarely use more than one but it's just good to know my options. Appreciate your quick response!


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## 2L man

Use two cabs when you don't need to attenuate and one cab with attenuator.

Esa


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## Gene Ballzz

*K.I.S.S.* (Keep It Stupidly Simple), as the fewer options, bells and whistles, the less chance of operator error in it's usage! Also, simpler, quicker and cheaper to build/test/troubleshoot!
Just My $.02,
Gene


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## aceofbones

Quick question about the 8ohm M2. When placing my resistor order I couldn’t find a 5.6ohm for R8. I ordered a 5, .68, and 6ohm instead. Is the 6ohm close enough or should I run the 5 and .68 in series? Cheers


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## JohnH

It'll make no effect on tone if you use 6 instead of 5 6. It adds about 0.15 db to the attenuation of the last stage, so you'd be unlikely to notice. Another way to tweak it if you want to is a 82 Ohm in parallel with 6, easy to add later if you wanted to adjust. The added resistors can be lower power


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## aceofbones

JohnH said:


> It'll make no effect on tone if you use 6 instead of 5 6. It adds about 0.15 db to the attenuation of the last stage, so you'd be unlikely to notice. Another way to tweak it if you want to is a 82 Ohm in parallel with 6, easy to add later if you wanted to adjust. The added resistors can be lower power



Excellent, thank you. I was hoping that was the case. Not really sure how much extra real estate I have inside my enclosure until my jacks and enclosure show up.
Thanks again!


----------



## BrokenBones

I have a question before I solder this all up. I'm still learning so pardon my ignorance. Do connections 1,2,4 get wired together? Or, is the use of a different trace color there to differentiate between 2 different signal paths? Thanks in advance.


----------



## JohnH

I think you read my intentions right! When I show teo different colour wires crossing, they dont connect to each other. Good luck with the build!


----------



## m1nl

@JohnH, thank you for a great design! I've built the M2 version and it sounds great!





Since I'm using the attenuator with a custom 36W 1959SLP clone, EL84-based power section becomes a bit fizzy when pushed hard. It is most audible when I enable all attenuation levels. However, I do also use VVR to reduce power, so it's not a big deal 

I've already made a hole on the wrong side of the front panel (d'oh!) and I'm thinking to add a switchable Zobel filter in the attenuator to decrease the impedance for high / very-high frequencies. Do you have any experience with it? I thought to include either a 2.2uF + 10Ohm in series or use a DPDT switch and to switch between 2.2uF and 1uF - is it going to be an audible difference?

I'm going to make some Spice simulations to check how does it affect impedance tomorrow, but maybe you have any thoughts on this?

Greetings from Warsaw!


----------



## JohnH

Hi m1nl, thanks for posting Looks like a nice build!

I haven't tried adding caps to a real build but they are easy enough to test in Spice. Can you post a diagram of what you have in mind? Also, what's your power amp design? does it have an NFB loop? does it have a presence control?

Easiest way to tame hi end is to add a resistor to ground at the output of the attenuator = across the speaker), but I haven't needed to do that on the versions so far.


----------



## m1nl

Thanks, @JohnH!

The basic idea is to add a resistor+capacitor in parallel with the rest of attenuator:



It should decrease the impedance of the circuit above 3k and stop oscillations from mostly inductive load for upper frequencies. Since I assume, the power section is being overdriven, the signal is full of high-frequency harmonics, which further enhance the brightness and fizziness of EL84 (you can try to Google "18 watt fizz" to get a general idea of this issue). Some people use either a Zobel filter (in parallel with speaker) or a cap+resistor across primary winding of the OT to tame those frequencies.

My amp does have NFB and presence control and this effect decreases as I roll off the presence knob. The power amp is very similar to TMB design (I cannot post links yet, Google for "mhuss 36 tmb"), but I'm still experimenting with grid stoppers values. I'm adjusting 1959SLP preamp signal level with a trimpot to prevent the PI (based on 5751) and EL84 power section from being overdriven too much.


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## m1nl

Blue is the attenuator set to max attenuation level, green is attenuator with a 1u+20R Zobel filter (2u2+10R seemed to drastic for me). It seems that filter should be transparent for "normal" guitar frequencies with these values.



I'm going experiment and let you know about the results. Once again, thank you for a great design!


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## JohnH

it looks interesting, let us know if it make a difference. Another place you can experiment is by shunting the L1 coil, either fully or by a resistor. This will make the amp see more of a resistive load.


----------



## Eoin O'Dea

Hi JohnH. Many thanks for all the work you have done on Attenuators here in this forum. I have and old 1983 JMP 2203, which I'm going to fire up after 25years being idle. I will check Capacitors for ESR, and other normal amp checks first. It was a serious weapon of an amp, too loud for most gigs, but when run deafiningly loud, the crunch and harmonics were great. I'm interested in building the M2v attenuator. But want to do it with 100watt rating. To have that rating, is all I need to do is double wattage rating for resistors and increase inductor CSA from 18AWG to 15AWG?. What do you think? . Also with your drawings, which are great, the colours help alot, just some advice, add solid dots at T-joints or X joints, and put saddles or hop-overs where wires cross over but don't joint. Thankyou again, Eoin.


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## JohnH

Hi Eoin - thanks for your interest. M2v was the one where I added an extra front-end circuit to help it deal with different amp taps. Its based closely on M2, which is dedicated to one amp ohms. But both can work with different speaker Ohms. Just raising that since most folk could work with just one amp Ohms value, even with a few different amps and different speakers, using either an 8 or a 16 Ohm version of M2. 

M2v, in its 16-8-4 version is basically an 8 Ohm M2. The extra circuit adds another reactive stage at the front in either series or parallel, which takes half the power (hence another -3db drop) when you engage it. This will keep the tone consistent when using the different taps, and is better than some other popular commercial models which just slap a resistor or two in front.

Here's the two drawings:













Attenuator M2v 200420



__ JohnH
__ Sep 20, 2020


















Attenuator M2 200702



__ JohnH
__ Jul 1, 2020






The designs are fine, but M2 got a new better drawing, whereas the M2v drawing was made up out of parts! Thanks for your comments on the drawings. I just do them in MS Word, and making the loop-overs is too fiddly for me. I use the colours to separate wires and on the M2 one, I slipped a small white break between wires that cross. Ive tried dots for connections too, and sometimes they don't align properly. But, I don't do many of these, and the attenuators are quite simple diagrams so people who know enough to be using them don't seem to go wrong with them.

For your 100 watt versions, yes basically double all resistor watt ratings. Id suggest to use a series or parallel pair to double the 100W ones, and separate the two halves to spread the heat out. Also, consider the switch current ratings and the quality of the jacks. 100W at 8 ohms is 3.5Amps and youd want at least double that as a spec. As drawn, they don't have a full bypass, so you don't need to switch the full current, but it all has to go through jack contacts. In M2v, I used extra contacts on a stereo jack to engage the additional front end. Itd be more direct just using the 8 Ohm input.

If you use a 16 Ohm amp tap, there is less current flowing through contacts. A 16 Ohm M2 will also work fine with 8 Ohm cabs.

On the inductors, if you are doing a 16Ohm M2 build, I think its OK at 18 or 16 gage but Id go with 16 (or 15) if you are doing 8 ohm for M2 or M2v. All air core, no steel bolts.

My builds have all been for my 40 and 50W amps, and I just don't find that they get very hot. But at 100W, particularly one cranking at more than that, the physics says that they just have to get toasty! Ventilation from below and above and a big thick aluminium case with lots of holes is key. Could also consider a fan system. But if you have the resistors all bolted with thermal grease onto a good base, any heating takes quite a while to build up. With steady input, mine take at least a half hour continuous to get to a stable temperature.


----------



## Eoin O'Dea

Thanks John, as always for your Attenuator work and help. Once I get it built and tested will forward on some photos.

All the Best,
Eoin


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## Mcentee2

JohnH said:


> Hopefully it's a useful idea for those with multiple different amps. I dont think it should affect the tone significantly. Just have to take account that the added front end parts take half the power when they are running so hence the added -3db drop.



I ended up building a parallel add-on module and put it at the front of my M2 to make it into a 4r, and it works fine except for that added -3db, it just takes it too far for the first stage.

I more often than not just go back to using my 8r M2 with my 4r speaker load.

The above is still great for when the amp is really singing as a plexi should, but I've also got used to a simple volume control in the FX loop if I want the amp to be cleaner anyway, it works a treat for that purpose.

Just thought I would share - no questions this time from me


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## matttornado

Hi JohnH.

I built your attenuator this time last year with much success & love it. Is there way to add a tone control to cut some highs? if so, can you provide me with a drawing and how and where to install it? 

I built design M4 without the 3rd output that has the red dotted outline.

Thanks!


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## JohnH

hi @matttornado

To trim some highs at the speaker, you could try a cap and a resistor in series with each other, all in parallel with the speaker output. This could be controlled by a switch. If you get an on-off-on switch you could have two treble-cut settings plus off. 

This chart shows two recipes, a 25 Ohm in series with 2.5 microFarad cap, and also 10 Ohm in series with 5 microFarad: (these values assume an 8 Ohm build with 8 Ohm speaker. For 16 Ohm values, resistors are x2 and capacitors x1/2)




See the lower traces. The top of the three lower ones is no treble cut and you can see how it matches the full output shape but at lower level. 

Youd need to try a few in your build to hear them, I haven't done this myself. The caps need to be non-polarized.

These settings affect the speaker tone but not at the amp, so not the lineout if you have taken it from the front end. They should also work with any of the other attenuator designs on this thread.

Or, instead of any of that, with design M that you have, with two coils, you can try shorting-out L2 - which should reduce treble a bit, roughly similar to the middle blue trace above.


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## matttornado

excellent thanks!


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## bikescene

I recently heard about the attenuator projects in this thread and am looking into building an M2 attenuator in a Hammond 12"x8"x2" enclosure. I was jumping around the thread to absorb as much information as possible.

Could anyone comment on the apparent listening levels of 31.5dB of attenuation on a 50W Lead Spec Plexi circuit? I was thinking of adding an additional 7dB attenuation stage to the layout.


----------



## JohnH

Hi @bikescene , thanks for your interest. -31.5db is a power factor of about 1/1400 so nominally 50W becomes 35mW.

When I started on these I was surprised that this apparently very small power is quite usable in a quiet house. With my amps (50W VM), its like a low to moderate TV volume. But I never feel any need to go lower than that, and if I did id just use the amp controls and it would then be at such a low volume where getting 'killer' tone would be less important than just hearing it at all.

But your amp is probably a tad louder than my VM. If you are in a situation where you are having to deal with thin walls to neighbours, sleeping family members or other reasons why you just cant have any significant volume, there's no problem adding another stage, its just a switch and two resistors. Id agree that another -7db stage would be a good choice and I would put it after Stage 2 (though it doesn't matter much where). If those issues don't apply, then for any normal home use -31.5db is plenty quiet and you have all the small -3.5db steps up from there.

Another approach is a line out into a cab-sim or IR box, then use headphones.


----------



## foil1260

Hoping someone might be able to help me with some really basic questions. 

I recently finished an L-pad attenuator design, and really want to build the 16ohm M2 version from this thread. 

Some questions I have in looking over some of these other builds:

Will 5A 120v on-on toggle switches work for this? I've got two amps I'm planning on using this with: an ORI50C and an SV20C. I'm currently using the L-pad with both on the 5W setting, but I'd like to try this design for both on the 10W/20W setting. 

Does the inductor have directionality / polarity? I see that it has two leads, but I also wasn't sure if there's any difference between them (the leads), or any difference in the way the signal flows through the inductor. 

Would 18AWG speaker wire work for this? I've got a whole bunch from my previous L-pad attenuator build. 

What's the preferred method for mounting the inductor? I've seen a couple iterations, and it looks like it just has to be mounted on something non-conductive (not resting on the enclosure)? 

Thanks in advance to anyone that can assist with the questions.


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## JohnH

Hi @foil1260 I think that's all oK

The Origin is the 50W model? At 16 Ohms and nominal 50W, the full current is 1.8A so a 5A switch is OK. But if as in M2, you don't have a full bypass, then the current that gets switched is a lot less. And if you are using a lower-power amp mode its less again. The switches should have 3 or 6 lugs.

The inductor doesn't have a polarity. Mount it so the wires aren't pressing on the case. The key watchit is not to use a normal steel bolt which will mess with its inductance. Zip ties are good, or nylon or stainless bolts. Mine are glued down to the base of my box with silicone (its fine but I'm not proud of that part!)

18ga wire is ideal 

Good luck with the build!


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## 2L man

That switch rating is good for attenuators. Voltage and current stay lower even in first stage and switches are rated when those limits are on and they usually exceed 10 000 cycles. Also usually there is no need to play when you do selections. Inductor circuits can cause voltage spikes but they have resistors parallel which damp them.

These inductors do not have polarity.

AWG18 wire is thick enough to do connections but there comes some power loss although it is very little when compared to what in whole circuit happens 

Non magnetic mounting is better but a coil should be possible to mount using one screw in the middle. Even aluminium case change circuit but I can't say does if effect...

Esa


----------



## J.Wiens

JohnH said:


> Here's a new drawing of Attenuator M2, stripped back to the basics, in 8 and 16 Ohm versions.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator M2 200702
> 
> 
> 
> __ JohnH
> __ Jul 1, 2020



Hi @JohnH - first off, thank you so much for all the hard work you've put into this project. I am interested in building one of these and was hoping I could ask a clarifying question quickly. I am not well versed in electronics, so just wanted to double check before doing something potentially idiotic 

In my scenario, I have an amp in the form of an old CSA version JMP with 4 and 8 ohm outputs only. I'd love to use this amp with a 16 ohm cab, so I was contemplating the modification you made to the circuit for 8ohm amp / 16 ohm cab.

I noticed in your diagram that for the 16 ohm "output 3" option, R10 is connected to the Ring portion of the TRS jack. With a standard speaker cable, am I correct that this resistor is now in circuit via connection between Ring and Sleeve by a mono TS cable?

If so, if I was to create a simplified version of your attenuator with only one 16 ohm output, could I now do so like this?




Thank you so much for your time.


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## JohnH

Hi @J.Wiens That all looks fine based on what you describe.

There's a few things worth noting:

If you do this fixed 8 to 16 version, you could combine R1 and R10 to be just a 12 ohm instead.

This version loses a couple more db compared to the pure 8 or 16 versions. Not a big deal for most users, but -9 db which would then be the first step, will cut a 50W amp down to about 6w.

Id suggest to build the full 8Ohm M2 if you can. The adjustment for the 16 Ohm output is just a tonal tweak. Its perfectly fine to plug a 16 ohm speaker into the 8 ohm output, with a slightly mellower tone. You might like that option. And then, its all set up for a different cab, or another 16 cab to use two 16's as an 8 Ohm pair.


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## J.Wiens

Thanks @JohnH, appreciate the confirmation and insights!


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## Gene Ballzz

@JohnH 
I think you went into this a little bit, somewhere in this thread, but can you please refresh my memory on the nomenclature between different types of SPDT switches for the stages. I'm assuming "ON-OFF-ON" is a three position switch, but what is the difference between "ON-ON" and "ON-NONE-ON"?
Thanks For Helpin'
Gene


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## JohnH

Hi Gene, yes I think we have only had two-position switches here. They usually get called on-on if anything, meaning they connect to lugs both ways. So they are three lugs, or six if they have two poles. There are also on-off switches, which might just have two lugs.

The three-position switches come in a few arrangements. An On-off-on I think is the same as an on-none-on. the look the same as the two position on-on ones, but in the middle position nothing is connected to anything. 

Other ones you see include on-on-on. Typically its a two pole switch where in the middle position, one side switches up and one side down. Can use these in guitars to make a series/single/parallel switch.

Also, some switches have brackets like on-(on), meaning that the bracketed setting is momentary and springs back after releasing.

That's all I know!


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## Gene Ballzz

@JohnH ,
Didn't you lay out a 4Ω and/or 2Ω version, intended for a Fender Super Reverb? I've looked but didn't find it. If it is here and I just missed it, may not be a bad thing to put a link to it in *POST #1*, the next time you do an update?
Thanks Again,
Gene


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## Gene Ballzz

@JohnH 
Sorry to bug you! After looking again I found the related section of posts on pages 43 & 44.
Thanks For Patience,
Gene


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## JohnH

hi @Gene Ballzz ,yes indeed, this one:













Attenuator M2v 200420



__ JohnH
__ Sep 20, 2020






In its 8-4-2 version, its basically a 4 ohm build, with an extra 4 ohm module to add to or divide the front end. Im thinking it might only be used for that Super Reverb! (were there any other amps with just a 2ohm tap?.) Do you have such a beast?. 

I expect that for most, the 16-8-4 one would be more useful.

Id stick this in post 1, but there's a 10000 character limit per post, so everytime I add something I have to delete something!


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## okgb

I'll delete or modify this post, but is there a recommended source / part # for the inductors ?
Bypass switch if used should be a " shorting [ MBB ] " to not leave the amp with no load when switching? If someone points me to the correct page with inductor info I'll change this post thanks.


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## _Steve

Hi @JohnH sorry if this has already been asked. But if I need to build a 300W version, do I need to increase the L1 guage or change its value in anyway?


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## JohnH

okgb said:


> I'll delete or modify this post, but is there a recommended source / part # for the inductors ?
> Bypass switch if used should be a " shorting [ MBB ] " to not leave the amp with no load when switching? If someone points me to the correct page with inductor info I'll change this post thanks.



Hi @okgb - good questions (worth asking!) The inductors tend to come from places that sell cross-over components for speakers. In the US, Madisound has the goods. They do a range in 19 ga which is fine for the 50W versions. They do 16 ga too, which would be good for 100W builds. Where are you? Happy to dig deeper and pot a product.

For a bypass switch, yes I guess mbb is best, but its moot because I would never recommend switching a full bypass switch on the fly. Always put the amp into standby while you do it. That would apply to any attenuator. 



hi @_Steve , actually nobody has asked about doing a 300W version before! What would you be using it with? The power doesn't change the values but all power and current ratings and gages would go up for wire, coil, resistors and switches. And also jacks. It would depend if you really intendrd to push such an amp, and also what ohms its for. Also, it would need powered fan cooling and be built big and open to spread heat.

The power ratings per the diagrams are based on the nominal amp power, allowing that they may get pushed. On that basis, 50W ones seem to stay reasonably cool/not too hot, if built with good venting and heavy chassis, and by 100W, if cranked, more serious heat is created needing better active cooling.


----------



## _Steve

JohnH said:


> Hi @okgb
> hi @_Steve , actually nobody has asked about doing a 300W version before! What would you be using it with? The power doesn't change the values but all power and current ratings and gages would go up for wire, coil, resistors and switches. And also jacks. It would depend if you really intendrd to push such an amp, and also what ohms its for. Also, it would need powered fan cooling and be built big and open to spread heat.
> 
> The power ratings per the diagrams are based on the nominal amp power, allowing that they may get pushed. On that basis, 50W ones seem to stay reasonably cool/not too hot, if built with good venting and heavy chassis, and by 100W, if cranked, more serious heat is created needing better active cooling.



OK got it thanks, so same Henries, heavier guage.

I should have said 150W sorry. 300W was in my head as the rating of R1 needed to handle a 150W amp. Im currently building a 120W Hiwatt with 6xEL34, and in the future I want to play with KT88s in various Marshall builds. So having a decent rated attenuator will be needed even just for testing on the bench.

Do you think 14, or even 16AWG will be OK for (max) 150W?


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## JohnH

ok, that sounds more feasible! But what amp ohms will you build it for? if you can use a 16 Ohm tap, then there's less current than for 8 Ohms at the same power.


----------



## _Steve

JohnH said:


> ok, that sounds more feasible! But what amp ohms will you build it for? if you can use a 16 Ohm tap, then there's less current than for 8 Ohms at the same power.



Good point, however I think I need to at least accommodate 8ohms. Maybe I can do without 4ohms.

Can I use the standard EE guidelines for AWG vs current? Or is it different in inductors?


----------



## JohnH

ok the basic M2 can be built to any one given amp tap, then you can use it with speakers of that or x2 or x1/2. So an 8ohm build?

In that case you'll have about 4.5 Amps at 150W. So really good jacks needed. If you can live without full byoass switch I'd recommend it. on the inductor wire, id go bigger than EE ratings just so it doesn't heat up too much. id expect 16ga would be ok but 14 would be better. Id use 16 for hook up.


----------



## _Steve

JohnH said:


> ok the basic M2 can be built to any one given amp tap, then you can use it with speakers of that or x2 or x1/2. So an 8ohm build?
> 
> In that case you'll have about 4.5 Amps at 150W. So really good jacks needed. If you can live without full byoass switch I'd recommend it. on the inductor wire, id go bigger than EE ratings just so it doesn't heat up too much. id expect 16ga would be ok but 14 would be better. Id use 16 for hook up.



Perfect. Great info as always thanks!


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## stareIO

@JohnH thank you for this generous gift to the community, especially during these trying times! Will make using my old 50W Jubilee combo while housebound a lot more fun.

I'm putting a parts list together for a simple 1-in/1-out 8 ohm M2 and have sourced all of the 'important' bits (the aluminum-housed resistors and inductor) -- I'm having trouble finding recommendations for the 'basics' (jacks and switches).

For the jacks, I'm curious if I need, or if there's an advantage to, TRS versions vs. TS? Should they be switched or non-switched? I assume all plastic or nylon housings, threads, nuts, etc. (I'd seen this mentioned earlier in the thread)? I see several Neutrik, or Neutrik-Rean, parts that look like they would work but wanted to be sure (I'm in the US).

And for the toggle switches, should they be SPST or SPDT (was planning to go with 5A or greater current capacity as mentioned earlier in the thread)? How many lugs should be on the back of each?

Thanks for any help you (or others) could provide, and thanks again for a design blueprint with a lot of thought and care behind it. I've build a very simple (L pad-based) attenuator a few years ago but wasn't happy with the results (seemed to 'deaden' the sound), looking forward to something with more design/intelligence behind it (as well as a nice little winter project).

Cheers!


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## JohnH

hi @stareIO and thanks for posting

On the switches you need single-pole, double throw SPDT as a minimum, which means one central lug can connect to either of two outer lugs, so three lugs in a line. SPST would just have two lugs. But you can also get double pole double throw, DPDT which is two separate SPDT's next to each other , with 6 lugs. These are very common and not much more. They can have a small advantage if you link across horizontally each of the three pairs of lugs so the two sides work in parallel. This doubles the current capacity and adds reliability. Not essential though. 

For jacks, they should be fully insulated from the chassis, so likely to be plastic bodied. For what you are building, just simple mono jacks are OK. I like the TRS ones though because the extra ring connection adds grip to the plug - but im only buying generic cheap sockets. I use the ring terminal electrically for the extra 16 ohm output in an 8 ohm M2 and to control variable inputs on M2V, but with 8 ohms and one in one out, you don't need it. Neutric is a good brand. Actually, none of the designs need a switched jack but you can use one if that's what you have. 

Good luck with the build! Lets us know how it goes.


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## stareIO

Copy all of the above -- thanks for the much needed help! Will post back when I get everything further along. All the best to you! Essential thread!


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## Baby Thomas

Great point about using both sides of a DPDT switch for double the current, I did mine the same way. Also you have something like a fail-safe protection (if one of the contacts "dies").
For the jacks - Neutik all the way! But another good option is Pure Tone, the image explains it all:


----------



## TheOtherEric

Thanks @JohnH for all your hard work on this amazing project! Have you (or anyone) found any real benefits to using Mouser (1% tol) resistors over ebay/chinese (5% tol)? The Mouser (Ohmite/Vishay) total approx $50 for a basic 9-resistor M2 build, whereas the ebay parts run $20. (Adding $30 cost to a $75 build is a lot!) I know there's the lead time issue (3-5 days vs 3-5 weeks) and potential quality/reliability, but I'm wondering what impact 5% tolerances may have on sound quality. I don't know enough about the model's sensitivity to know how the tolerances might shift the performance curves nor about how a shifted curve might affect sound quality. 

I guess half the answer would be found in Monte Carlo simulations, ha ha, but let's not go down that road!


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## JohnH

Hi @Baby Thomas - I like the idea of those Puretone jacks with the doubled contacts, id definitely use them in guitars. But as shown, the jack barrel is connected to the body, which means for the attenuator, one of the OT taps will be live to the case - not good.

Hi @TheOtherEric - Ive just used the chinese ones, I think they are fine. The design is not critical for precise tolerances and 5% is perfectly OK. Actually, I have had to adapt the values used to work with available standard ranges anyway, which tend to be in about 20% increments. 

Within these kind of variations, you wont hear a difference in tone, and the amp doesn't care either. The ranges available from China are often not the same series of steps as in a standard resistor range, and may have gaps. This is the link Ive used:

https://www.ebay.com.au/itm/25-50-1...af460118a:g:EqkAAOSwzoJfh5k3&var=692191819131

Its from ebay.au, but Im sure you can find it. I haven't bought any more since covid, but I had several packages last year. He shipped within a day, and then you wait......
……….but they do arrive. That's the best ebay range Ive seen and gets most of the key ones, but the values aren't all as standard. eg You have to use 5 and 6 instead of 4.7 and 5.6. Also, there's a big gap around 33, 50, 75. 

If you sub different values within say 10% of the design, the only implication is slight unevenness in the available steps of attenuation - it still works and sounds fine. The key thing is the ratio between the resistors within each stage. I had some 5W and 10W ceramic resistors around and in one case I put a high value one in parallel with a power resistor to bring it down slightly - just because I could.


----------



## TheOtherEric

JohnH said:


> ... That's the best ebay range Ive seen and gets most of the key ones, but the values aren't all as standard. eg You have to use 5 and 6 instead of 4.7 and 5.6. Also, there's a big gap around 33, 50, 75.
> 
> If you sub different values within say 10% of the design, the only implication is slight unevenness in the available steps of attenuation - it still works and sounds fine. The key thing is the ratio between the resistors within each stage. I had some 5W and 10W ceramic resistors around and in one case I put a high value one in parallel with a power resistor to bring it down slightly - just because I could.


Thanks for the help, and for the points about R selection, which I was puzzling over since the ebay seller I found had no 75R or 12R for the -3.5dB stage, just 68 or 80R and 10 or 13R. Closest I can get to preserving the ratio is 80R and 13R I guess.


----------



## Gene Ballzz

@Baby Thomas & @JohnH 
If the below linked shoulder and washer set fit those *"PURE TONE"* jacks, they would work fine!

https://www.tubesandmore.com/products/washer-switchcraft-s1029-shoulder-insulating-panels
https://www.tubesandmore.com/products/washer-switchcraft-s1028-flat-insulating-panels

I will definitely look into those jacks for guitar use!

Thanks Folks,
Gene


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## TheOtherEric

Here's a little help for anyone contemplating the project, which I'm posting since I haven't come across detailed parts lists. Just got everything ordered for a basic M2, and the costs are quite a bit lower than expected! (Note I'm actually building two, 8R and a 16R, but I'm showing avg costs for one assembly to avoid confusion).

A. Parts Express
1. Inductors. Jantzen 18g $11.
2. Feet. 4-pack .69"D x .39"H, #260-7514. $1.25
3. Thermal paste, #NTE303 $3
4. 1/4” jacks. Neutrik #090-972 $1.12x3
5. Switches. SPDT $1.69x3=$5
6. Hookup Wire, 18g #101-729, $5.
Subtotal, Parts Express incl tax, shipping = $37

B. Other
7. Resistors. From eBay (socoolmart) incl ship, tax = $19. From Mouser = $53 (hint- select Mouser's category Wirewound Resistors - Chassis Mount).
8. Project box. From eBay 200x106x55mm incl ship,tax = $18 (link)
9. Screws and nuts (size M3) = $5

*TOTAL incl. tax, shipping = $79 *if eBay resistors. $113 if Mouser resistors.


----------



## foil1260

I've collected all the parts I think I need for the M2 design, but I'm a relative beginner in terms of wiring and I was hoping someone here might be gracious enough to have a look at this layout I had planned. Apologies that it's kind of a mess, I tried to make all the wires contrasting colors so they would be easier to track. 

When I first started on this project, I figured it would be easy enough with the diagram, but I soon realized I had questions about the order that things are wired in, or whether that actually matters. As an example, should lug 3 of the switch for stage 2 go to the lug on R4 first, then to the lug on R6, and then to lug 2 on the stage 2 switch? Or should they all meet together at lug 2 on the stage 2 switch? 

In any case, my attempt at laying everything out is in the diagram, and I appreciate any help.


----------



## JohnH

hi @foil1260
I can't see any errors in the wiring.

In these devices, it doesn't matter how you run the wires, so long as what needs to be connected is so. So a run of wire A to B to C is the same as B to A to C, or A, B and C all to one point. So you can pick wire runs for the shortest or neatest results.

When you have all the parts, you might lay them out and decide to amend the wiring due to the relative sizes and locations.

You can also consider whether wiring with no more that a couple of wires per lug is easier than with more and move a connection to the other end of a wire if that's easier. None of these changes would affect the schematic diagram. Note, while the above is a typical principle for many circuits, some need careful attention to where wires actually go physically, eg where there is mains involved or high-gain stages and also sensitive input stages. That applies in an amp but not in a separate attenuator.

Note that I laid out the three switched stages as I did, for some smallish electrical reasons so the stages go -7(reactive), -7, -14 then -3.5. But you may prefer to physically place the switches for the three switched stages in an order like -14, -7, -3.5. Then its like binary numbers, each one is twice the one to the right


----------



## JohnH

TheOtherEric said:


> Here's a little help for anyone contemplating the project, which I'm posting since I haven't come across detailed parts lists. Just got everything ordered for a basic M2, and the costs are quite a bit lower than expected! (Note I'm actually building two, 8R and a 16R, but I'm showing avg costs for one assembly to avoid confusion).
> 
> A. Parts Express
> 1. Inductors. Jantzen 18g $11.
> 2. Feet. 4-pack .69"D x .39"H, #260-7514. $1.25
> 3. Thermal paste, #NTE303 $3
> 4. 1/4” jacks. Neutrik #090-972 $1.12x3
> 5. Switches. DPDT 10A #299-554 $1.69x3=$5
> 6. Hookup Wire, 18g #101-729, $5.
> Subtotal, Parts Express incl tax, shipping = $37
> 
> B. Other
> 7. Resistors. From eBay (socoolmart) incl ship, tax = $19. From Mouser = $53 (hint- select Mouser's category Wirewound Resistors - Chassis Mount).
> 8. Project box. From eBay 200x106x55mm incl ship,tax = $18 (link)
> 
> *TOTAL incl. tax, shipping = $74* if eBay resistors. $108 if Mouser resistors.



Thanks for this list. 

Particularly, that socoolmart eBay link for the chinese resistors looks very useful. They appear to be the same as I was getting but, they have a better standard range of values across 100W, 50W and 25W ratings that hit all the values that we need.


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## steka

Ok, John you got me on the hook. Joined this board just to participate in this discussion - and i don‘t even own a Marshall amp (anymore or... yet?). 
Interested in the M2 wiring, but have a question or two. 


I can‘t seem to get an air core inductor at 0.9mH. I only have an option at 1 mH with a height of 18mm (this was referenced as the preferred size before). What would be the implication from that?
How can i test the circuit without risking my amp? Any possibility with a standard circuit tester? 
Wire diameter - what is recommended?


----------



## JohnH

Hi @steka, thanks for joining!

im assuming you are in the US? and interested in an 8 ohm version?

1mH is fine

Once its built, you can test it with a multimeter set to Ohms. Even a cheap one is fine for this. With a speaker plugged in, the resistance measured across the input jack, ie what the zmo sees, should measure between about 7 and 10 ohms in all settings

I use 18 gage for hookup wire.

good luck!


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## adew1

@JohnH Hi John, is there a recommended enclosure size for the M2, 16ohm version? Thanks!


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## JohnH

I used a die-cast aluminium box for mine 170x120 x 55, see in post 1

this one linked by @TheOtherEric looks good too. 200 x 106 x 55
https://m.ebay.com/itm/Aluminum-DIY...e&pageci=e3ef6270-4428-4e62-8ff3-d726f3db7500

Bigger and heavier is always better


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## TheOtherEric

Finished my 16ohm and 8ohm M2 attenuators, so here are some pics. I listed the parts in this post. Overall it was very time-consuming, but rewarding. I only spent an hour toying with it, on 2 amps (a dumble style and a 20W plexi style) and it performs well. Sounds pretty transparent up to -14 db total (i.e. only the -7 db switch on) but beyond that, it starts feeling muffled and I didn't feel I could compensate with amp settings. I don't know if it's inherent to the amps, my other attenuating devices (see below), or the attenuator design, but I definitely didn't like highly-attenuated settings. So I'll be going with -14 db. I think others may have posted similar results.

On my plexi-style, I'm getting best results using 4 forms of attenuation - (1) a PPIMV to take the edge off, (2) a Ceriatone Mini-Kleinulator fx loop buffer with Return around 3 of 10, (3) the amp's MV, and (4) the M2 attenuator. I need to experiment with taking the MV/PPIMV and loop buffer out (in order to rely on the attenuator more) so I'll report back if I find any improvement.

Thanks @JohnH and everyone else here for making this possible!

Edit: Yes I found improvement! See later posts. Nothing wrong with the attenuator, I just needed to keep jacking up the amp’s treble and find the system’s sweet spot, and I’m now using -21dB total.












AttenuatorM2-16Rd



__ TheOtherEric
__ Dec 16, 2020


















AttenuatorM2-16Re



__ TheOtherEric
__ Dec 16, 2020


















AttenuatorM2-16Rc



__ TheOtherEric
__ Dec 16, 2020


















AttenuatorM2-16Rb



__ TheOtherEric
__ Dec 16, 2020


















AttenuatorM2-16Ra



__ TheOtherEric
__ Dec 16, 2020


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## TheOtherEric

JohnH said:


> I used a die-cast aluminium box for mine 170x120 x 55, see in post 1
> 
> this one linked by @TheOtherEric looks good too. 200 x 106 x 55
> https://m.ebay.com/itm/Aluminum-DIY...e&pageci=e3ef6270-4428-4e62-8ff3-d726f3db7500
> 
> Bigger and heavier is always better


Indeed this enclosure worked great for the 16 ohm build (8R too). Hat tip to @Gene Ballzz for finding that. As you see in my pics above, I had to mount some stuff in the lid, no biggie. Mounting the two 50W resistors there also may help with heat dissipation since you're getting them away from the 100W R. And the inductor (for the 16R build only) is so tall that I couldn't put anything below it. Definitely a 3D puzzle. But a bigger enclosure starts looking unsightly IMHO. The split enclosure makes access to everything super easy.


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## JohnH

Hi @TheOtherEric

Thanks very much for those posts, that's an excellent build, and very useful report with useful clear photos. It looks like the layout design got some very careful thinking to get it in! I tbink for myself, next time I build one ill go bigger again, just for an easy life, especially if any extra features go in.

Im sure you'll be testing more with different settings etc, but it would be worth exploring those tone issues relating to high attenuation. Its not an issue thats been found before in the testing and analysis or reports from others. If you'd like to explore if we can work to make it better to meet your uses, there's a few things that could be done:

1. As a diagnostic, get some resistance measurements at each setting, disconnected from the amp. From the input, measure resistance seen from the amp, preferably with a 16Ohm speaker plugged in. From the output, no speaker or amp, measure resistance of the attenuator as seen by a speaker. I can tabulate what those readings should theoretically be.

2. Can you identify the frequency range of the dullness that you hear? ie it in the very high presence-type treble? or mids? or the bass as well etc?. If you can make clear recordings using a consistent loop at high attenuation, moderate attenuation and full volume, we can plot them and see the difference and then engineer how to work around them.

3. Many attenuators have switches to tweak the tone. These can be used either to compensate for inherent tone issues that some units have, or to adjust for hearing FM effects or user preferences and to adjust for other parts of the rig. If its treble, we can lift the treble using a cap in parallel with one of the series resistors. The most obvious candidate is in the-14db stage, being R6 (30ohm in a 16ohm build). The sort of size might be in the range 2 to 3 uF, not huge but should be bipolar.

4. Maybe a quick one, if you are able to test with either two 16 cabs or an 8 ohm cab, you should in theory, get a small lift in high treble and bass resonance.

5. An EQ box in the amp fx loop.

Just putting those thoughts out there, in case you'd like to explore them. But id quite understand if you prefer to keep it as it is.


----------



## TheOtherEric

Thanks @JohnH for your offer of help; I’m happy to take you up on it. I’ll get those resistance readings tomorrow. What’s the best kind of signal to make recordings of? I can play guitar thru a looper or use Audacity to make noise or test tones. I’ll then record with a Shure SM57.

I’m finding the same high end dullness (at high attenuation) in both my attenuators — (a) 16 ohm M2 thru my Dumble style amp and 16 ohm 1x12 cab and (b) 8 ohm M2 build thru my plexi style amp and 8 ohm 2x12 cab.


----------



## TheOtherEric

@JohnH by the way, when I built these I wanted them in order -3.5, -7, -14 and instead of just physically moving the switches, I actually changed the order of the stages in the circuit. Could that impact it’s performance?


----------



## JohnH

Hi @TheOtherEric 

I can check out that stage order in my excel file - it shouldn't make much difference but the -7 stage is the most optimized hence i put that nearer the front. But at say -21db, it stage 1 (-7db) followed by -14, whichever way you wire it.

It looks like you have everything needed to make some useful tests. Using much the same set up, what I do is:

Put a short riff of a few sec into the looper - something with some chords and lots of frequencies
Set the amp loud and crunchy to try to bring out the power tube drive
Close mic the cab (sm57 is great, I use a Rode M1, also a dynamic mic)
Record into Audacity at full volume, being sure there's no recording distortion 
Step down through the attenuation settings, making no change to the mic or amp. i generally keep the recording level the same too

Do frequency plots in Audacity and export them to a csv file
using excel, I subtract one plot from another to see the difference

All of that could also work with noise or sweep signal instead, but i find I learn more from something that I can listen too

Then, the proof is when you take the clips at the different volumes, use audacity to normalize them to the same volume, and then listen for tone differences, all at equal volume.

You can probably do all that but id be very happy to do the processing if you can post audio clips preferable in a .wav file


Also, given you have an 8 ohm 2x12, you might try that out of the 16 Ohm attenuator and 16 Ohm Dumbloid amp.

cheers
John


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## TheOtherEric

Hi @JohnH, I'll start by fixing my wiring so it's exactly like your plan, to avoid any suspicion there. Then I'll get some resistance readings and make some recordings! (although it may take some days or weeks due to holidays).


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## JohnH

ok great

I did actually just try swapping the order of stages in my analysis, and it made no difference, so no real need to rewire just yet.


----------



## TheOtherEric

I recorded some 20-sec loops on 3 attenuator settings, then normalized the data and exported the spectrums to Excel. The spectrums look pretty much the same between all the switches off (-7dB) and all on (-31.5 dB). See below pic. And the 3 clips sound identical after normalizing them and playing through headphones. So I guess that's great! Except it leaves me scratching my head as to why the -31.5dB doesn't sound good to me. Leading theories are (1) purely my imagination, (2) pushing the power tubes (using higher PPIMV) somehow degrades sound quality in a way the spectrum doesn't show, or (3) there's some attack or other time-based difference that doesn't show in the spectrums. I have some work to do. 

But it DOES look like the attenuator design is flawless, all thanks to the genius and hard work of JohnH. 

p.s. I haven't "fixed" my wiring to correspond exactly to the wiring diagram since apparently that's not needed.













Comparo-3AttenuatorSettings



__ TheOtherEric
__ Dec 17, 2020


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## Dogs of Doom

could you post the 3 clips?

to soundcloud or similar


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## JohnH

hi @TheOtherEric , great tests and thanks for posting. I'm very happy that the box is doing what it was intended to do, given such a careful build.

But, we can still work on addressing the fact that some more treble is wanted. Maybe its how the ear hears lower volumes differently, maybe its the way the amps respond differently if they are getting turned up to higher volume settings as attenuation is added. My amps, even with the attenuators, have a sweet spot for tone which is not full volume.

We can get a tone adjustment into the attenuator if you want to try it. There's the idea of a cap across R6, which will come into play when the -14 stage is engaged. And there's another idea too.....

This one is so simple there's no reason not to try it, no new parts needed. 

The way the tonal balance is controlled is with L1 being partly in series and partly in parallel with the signal through the use of R2A and R2B. The series path reduces treble and the parallel path increases it. So the tweak is to move one wire on R2B to make L1 fully in parallel. According to the numbers, this will add a consistent 2-3 db of high treble, without any other issues that I can see.

So on the diagram, take the left side of R2B and move it from the bottom of the coil to the top of the coil so it connects to the left side of R2A. R2A and R2B are now in parallel with each other, at about 10Ohms combined in the 8 Ohm version. L1 and R1 are in series with each other and in parallel with the input

If the change is found to be good after a temporary reversible wire-up, then this can be made into a switched option. 

Here it is, without switch:


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## TheOtherEric

Dogs of Doom said:


> could you post the 3 clips?
> 
> to soundcloud or similar


I uploaded 2 of the 3 clips to Soundcloud, but I'm struggling to get these links to display properly.

Again, these clips (-7 db All Off and -31.5db All On fully attenuated) are indistinguishable. Perhaps my next step will be to record some soloing, where dynamics are much more apparent, and do another comparison.


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## Dogs of Doom

all you have to do, is post the direct link to the clip. The site automatically embeds the player into the forum...

example, I posted these exact links below the image of them:


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## Dogs of Doom

anyway, thanks. I wanted to hear the -31.5 db. You said that it sounds bad to you?



TheOtherEric said:


> Except it leaves me scratching my head as to why the -31.5dB doesn't sound good to me. Leading theories are (1) purely my imagination, (2) pushing the power tubes (using higher PPIMV) somehow degrades sound quality in a way the spectrum doesn't show, or (3) there's some attack or other time-based difference that doesn't show in the spectrums. I have some work to do.


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## TheOtherEric

Dogs of Doom said:


> anyway, thanks. I wanted to hear the -31.5 db. You said that it sounds bad to you?


My displeasure was related to some soloing and heavier stuff. But for this comparison, I kept it short and simple, which maybe wasn't the right choice because even listening to it while it was being recorded, all three settings sounded the same, just more or less loud.


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## TheOtherEric

JohnH said:


> ... Maybe its how the ear hears lower volumes differently, *maybe its the way the amps respond differently if they are getting turned up to higher volume settings as attenuation is added. My amps, even with the attenuators, have a sweet spot for tone which is not full volume*...


I think you nailed it there. Playing again today, it feels again like my amps lose the sparkly high-end when they're cranked (highly attenuated), but I don't know enough about amp design to understand how/why (a NFB shortcoming?). So the game for all of us is to add treble/presence to retain sufficient sparkle at the highest attenuation settings. This doesn't show up in my comparo test above because that was all done cranked, where the sparkly high-end is already gone. *That test showed the M2 to be working properly; the problem is our amps! *

I've found a nice sweet spot at -21db total on this plexi-style amp, so I'm happy! I'll probably try that wiring tweak after the holidays, when I'll have time.


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## Gene Ballzz

@TheOtherEric
Might I make a suggestion for testing? First, find your ideal tone and sound* "sweet spot"* with each amp *"unattenuated" *(yes, it will likely be way too loud) and then start attenuating to see/hear what it does to your sound. I suggest this as opposed to simply *"cranking"* the amp under the assumption that fully *"cranked"* must be best. The main idea of using these attenuators (or any attenuator, for that matter) is to allow you to run your amp at it's optimum setting and then trim the volume to not kill children, and small pets at ten paces!

And by the way, those pics show some very nice work!

Just My $.02,
Gene


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## emulator

Hi all, I appreciate @JohnH hard work on this and everyone's input and tips! Just putting together a parts order to build an M2-8 and an M2v-8-4-2 (I have one of those pesky Super Reverbs here which I love the sound of but it is incredibly loud for my small guitar playing room) and wanted to share an inductor supplier in Canada in case anyone is looking. I can't post links yet so will just say they are called Solen and their domain is .ca
They are the Canadian importer for Solen Capacitors from France and have a good selection of audio capacitors in addition to the inductors I need for this project!


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## JohnH

Hi @emulator 
Thanks for that tip, it looks like a useful source! Here's the inductors link:

https://solen.ca/product-category/inductors/

Good luck with your builds.


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## Marshall50w

Why would you use an attenuator over say a pre amp, pedals or modeller ?
By using an attenuator are you putting a lot of wear and tear on the amp especially the valves ?


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## JohnH

Marshall50w said:


> Why would you use an attenuator over say a pre amp, pedals or modeller ?
> By using an attenuator are you putting a lot of wear and tear on the amp especially the valves ?



Well that's a good basic question that should be answered from time to time. 

There's nothing wrong with any of those other approaches if they work for what we want. But our tube amps often sound better when they are out of first gear and the power amp tubes are running freely, at which point a big amp is usually too loud and a small amp may be too loud for some settings. So attenuators let us set the amp for the tone that we want at the right volume for what we need. It doesn't always have to be full amp volume.


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## Gene Ballzz

emulator said:


> Hi all, I appreciate @JohnH hard work on this and everyone's input and tips! Just putting together a parts order to build an M2-8 and an M2v-8-4-2 (I have one of those pesky Super Reverbs here which I love the sound of but it is incredibly loud for my small guitar playing room) and wanted to share an inductor supplier in Canada in case anyone is looking. I can't post links yet so will just say they are called Solen and their domain is .ca
> They are the Canadian importer for Solen Capacitors from France and have a good selection of audio capacitors in addition to the inductors I need for this project!



So very kind and thoughtful of you to post links for our Canadian friends! On top of that,  to our wonderful forum!



Marshall50w said:


> Why would you use an attenuator over say a pre amp, pedals or modeller ?
> By using an attenuator are you putting a lot of wear and tear on the amp especially the valves ?




A very good question indeed!




JohnH said:


> Well that's a good basic question that should be answered from time to time.
> 
> There's nothing wrong with any of those other approaches if they work for what we want. But our tube amps often sound better when they are out of first gear and the power amp tubes are running freely, at which point a big amp is usually too loud and a small amp may be too loud for some settings. So attenuators let us set the amp for the tone that we want at the right volume for what we need. It doesn't always have to be full amp volume.



And @JohnH your reply is spot on the mark, as always! 

Thanks To All & Happy Holidays (whichever one you happen to celebrate ),
Gene


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## Marshall50w

JohnH said:


> Well that's a good basic question that should be answered from time to time.
> 
> There's nothing wrong with any of those other approaches if they work for what we want. But our tube amps often sound better when they are out of first gear and the power amp tubes are running freely, at which point a big amp is usually too loud and a small amp may be too loud for some settings. So attenuators let us set the amp for the tone that we want at the right volume for what we need. It doesn't always have to be full amp volume.



John H. Sounds reasonable. I have not used an attenuator but I guess I would use it to get an amp into slight break up, like I would in normal use and therefore I wouldn't be putting the valves under any more duress than normal usage. I like what I am reading about the Universal Audio Ox especially surrounding the recording flexibility/options. 
Does anyone have an opinion of how the Ox compares against other attenuators just on the attenuator side rather than all the 'bells and whistles' ?


----------



## emulator

Quick question regarding case/chassis. Is steel OK for a case or is it better to stick to aluminum? I already have two steel chassis boxes here that would do the trick nicely for my builds but can easily purchase something non-ferrous if required. Could not find specific information pertaining to this on a search but figured I should double check seeing all the warnings about steel mounting bolts for the inductors!


----------



## JohnH

emulator said:


> Quick question regarding case/chassis. Is steel OK for a case or is it better to stick to aluminum? I already have two steel chassis boxes here that would do the trick nicely for my builds but can easily purchase something non-ferrous if required. Could not find specific information pertaining to this on a search but figured I should double check seeing all the warnings about steel mounting bolts for the inductors!



Thanks for asking that. Yes a steel case could greatly change the inductor value - increasing it, unless the coil is mounted a significant distance away from it, which I cant imagine how to do. And its hard to test unless you can measure inductance. So yes probably best to go for Aluminium cases. They are also easier to work with and are better thermally since they are more much conductive as a material and probably thicker (that's why a stainless steel cooking pot has an aluminium base)


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## JohnH

Marshall50w said:


> John H. Sounds reasonable. I have not used an attenuator but I guess I would use it to get an amp into slight break up, like I would in normal use and therefore I wouldn't be putting the valves under any more duress than normal usage. I like what I am reading about the Universal Audio Ox especially surrounding the recording flexibility/options.
> Does anyone have an opinion of how the Ox compares against other attenuators just on the attenuator side rather than all the 'bells and whistles' ?



With an OX, you are looking at a unit that's about 20x a expensive as the ones we are building here! Of course, Im a bit biased and its not a unit that I would buy. But it seems popular and well regarded. Best to hear from anyone that actually knows this unit, but, I have some concerns.....

Heres a review:
https://www.musictech.net/reviews/u...aining both the tone and the dynamic response.

Out of that I pick up that, as a pure attenuator, there are only 6 stepped settings (0-5)?
Also, the bells/whistles sections rely on proprietary software that it seems is only Apple based?

Then I read about some measurements:
This from Mike Lind on TGP - see his attenuator plots.
https://www.thegearpage.net/board/index.php?threads/attenuators-and-load-boxes.1947804/

The OX shows the amp a reactive curve but it is very different to that of a speaker, particularly at low frequencies where it gets very high in the sub bass area.


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## Marshall50w

Thanks for the information and links. I have a number of amps that are difficult to enjoy in their 'full glory'. Having read up a little more, the BOSS Waza Tube Expander looks to be more what I am looking for. I will continue looking into this and apologies as I am likely to be on the wrong thread.


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## Paul Buxton

Finally got my implementation of this built. Sounds much better than the L-Pad Attenuator I put together previously. Thanks for the work in designing this!


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## JohnH

Hi @Paul Buxton , welcome to our forum and thanks for trying this. Id be interested to know which version did you build and what amp or amps are are you using with it?
cheers
John


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## TheOtherEric

I posted pics of my 16 ohm build, so here's pics of the 8 ohm build. The shorter inductor gave more room for a cleaner layout. And again, I didn't follow the suggested circuit, but instead re-ordered the stages -3.5, -7, -14. Of course, I could have followed the suggested circuit and simply moved the switches around, but whatever. JohnH confirmed this works fine, and my testing (previously posted) shows that too. 












AttenuatorM2-8R-a-resz



__ TheOtherEric
__ Dec 21, 2020


















AttenuatorM2-8R-b-resz



__ TheOtherEric
__ Dec 21, 2020


















AttenuatorM2-8R-c



__ TheOtherEric
__ Dec 21, 2020


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## Gene Ballzz

TheOtherEric said:


> I posted pics of my 16 ohm build, so here's pics of the 8 ohm build. The shorter inductor gave more room for a cleaner layout. And again, I didn't follow the suggested circuit, but instead re-ordered the stages -3.5, -7, -14. Of course, I could have followed the suggested circuit and simply moved the switches around, but whatever. JohnH confirmed this works fine, and my testing (previously posted) shows that too.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> AttenuatorM2-8R-a-resz
> 
> 
> 
> __ TheOtherEric
> __ Dec 21, 2020
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> AttenuatorM2-8R-b-resz
> 
> 
> 
> __ TheOtherEric
> __ Dec 21, 2020
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
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> AttenuatorM2-8R-c
> 
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> __ TheOtherEric
> __ Dec 21, 2020



Well, that's *"slicker n snot on a platter!" *
Nice Job!
Gene


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## TheOtherEric

Gene Ballzz said:


> Well, that's *"slicker n snot on a platter!" *
> Nice Job!
> Gene


ha ha thanks! First time I've ever tried to wire anything beyond simple guitar pots, so I'm happy it turned out decent.

And I'm loving this attenuator more and more every time I use it! Through my Dumble-style amp (Ceriatone) it's absolutely amazing. Getting that amp out of 1st gear is really quite essential, it seems.


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## JohnH

Agreed, that's a really nice looking clean job.


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## adew1

Apologies , as I am sure this has been asked before, but where are people sourcing their inductors from? Couldn't find anything suitable on Mouser or Digikey, although I am probably searching for the wrong thing.  
Thanks!


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## JohnH

hi @adew1 , generally the inductors come from places that sell crossover parts for loudspeakers. Madisound.com seems like a good source in the US. For up to 50w amps, they do a range in 19 gage, which works fine instead of 18 gage. Or they do a 16 gage range.

Or see @TheOtherEric made a parts list, see page 69


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## adew1

@JohnH Thanks, John. I'm in Europe, so I'll have to do a bit more searching.

For a 16ohm version, 18 gauge is ok, yes? Thanks again!


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## Gene Ballzz

@JohnH
I've got a short list of dumb questions, but a little info to share first. So far only one unit has actually been built, modified and re-modified throughout the evolution process. This unit has proven (to my ears and feel) to be absolutely stellar in every way, including minimal, if any tonal and/or response anomalies or degradations with a fairly wide variety of amps ranging from 5 watts to 40-ish watts, from minimal to maximum attenuation levels. Even using an 8Ω speaker with this 16Ω version has not exhibited any negative issues! Some amps have negative feedback and/or PRESENCE circuits and a couple do not. I'm still pondering the custom layout/packaging/configuration for each individual unit for mostly permanent mounting in several various amps. This first 16Ω, M-lite got built utilizing our initial inductor values of L1 = 0.8mH & L2 = 1.25mH, as opposed to the current optimums of 0.7mH & 1.1mH. I've also been too lazy to address the mounting with regular steel machine screws and steel washers, as shown in the pics of post #178. When I got the coils for the first build, I ordered two sets of the same values and it turns out that MADISOUND does not have a 1.1mH in the 19 gage configuration, only a choice of 1.0mH or 1.25mH for L2, in the mounting style I like. I've got enough other components to finish four builds of the 16Ω, M-lite, except only enough inductor coils (0.8mH & 1.25mH) for the first two,unless those values are just plain WRONG and may cause unforeseen issues. My goal here is for all four units to be standardized (component value wise) with the only differences being in the mounting and layout configuration.

Following that, here come the dumb questions:
A> What problems and/or issues are being created by using the inductor coil values listed above?
1> Do I need to find a source for a suitable 1.1mH and swap out what I've already used, or is the difference so minimal as to be of little consequence?
2> Should subsequent builds simply use what I've already been using?
3> If I need to swap out inductors and still want to use the products from MADISOUND, should I go with the 0.7mH for L1 and then choose either the 1.0mH or 1.25mH for L2?​B> How much and what kinds of problems are actually being caused by my current steel screw and washer mounting arrangement?
C> Is it possible that the value discrepancies, combined with my current mounting is actually contributing to this *"test mule"* unit being a *"one size fits all"* that operates nearly identically in all of my tested applications?
D> Bottom line, do I need to step back and re-think everything, or should I just go with what seems to have worked well, so far?​Dumb questions, I know, but I'd rather make any changes now, before gearing up and collecting the remaining required inductors!

It's also my hope that these dumb questions don't complicate or confuse this thread!

Thanks Again @JohnH & Happy Holidays,
Gene


----------



## JohnH

adew1 said:


> @JohnH Thanks, John. I'm in Europe, so I'll have to do a bit more searching.
> 
> For a 16ohm version, 18 gauge is ok, yes? Thanks again!



The wire gage is more about the power of the amp. So for up to a 50W amp (allowing that it might get driven to more power) I think 18 or 19 gauge is fine. For 100W amps id suggest 16 gauge.

I had a quick look for EU suppliers, and found this in the Netherlands, but Im sure there are lots of places:

https://www.soundimports.eu/en/crossover-components/coils/air-core-coils/?min=€0&max=€50&filter[]=641991&filter[]=628296

I focussed the link just on the 1.8mH range in 18 gauge, for a 16 Ohm M2 build. You'll see there, the two main styles of these coils. They have to be air core, ie there is no iron core, and they are either wound on a plastic bobbin, or wound just to themselves and secured with zip ties. Both are fine and you'll see some of each on this thread. I think the bobbin style is easier to mount, but don't use steel bolts. Tolerance is not critical, so its not worth paying extra for better than 5% tolerance.


----------



## JohnH

Gene Ballzz said:


> @JohnH
> I've got a short list of dumb questions, but a little info to share first. So far only one unit has actually been built, modified and re-modified throughout the evolution process. This unit has proven (to my ears and feel) to be absolutely stellar in every way, including minimal, if any tonal and/or response anomalies or degradations with a fairly wide variety of amps ranging from 5 watts to 40-ish watts, from minimal to maximum attenuation levels. Even using an 8Ω speaker with this 16Ω version has not exhibited any negative issues! Some amps have negative feedback and/or PRESENCE circuits and a couple do not. I'm still pondering the custom layout/packaging/configuration for each individual unit for mostly permanent mounting in several various amps. This first 16Ω, M-lite got built utilizing our initial inductor values of L1 = 0.8mH & L2 = 1.25mH, as opposed to the current optimums of 0.7mH & 1.1mH. I've also been too lazy to address the mounting with regular steel machine screws and steel washers, as shown in the pics of post #178. When I got the coils for the first build, I ordered two sets of the same values and it turns out that MADISOUND does not have a 1.1mH in the 19 gage configuration, only a choice of 1.0mH or 1.25mH for L2, in the mounting style I like. I've got enough other components to finish four builds of the 16Ω, M-lite, except only enough inductor coils (0.8mH & 1.25mH) for the first two,unless those values are just plain WRONG and may cause unforeseen issues. My goal here is for all four units to be standardized (component value wise) with the only differences being in the mounting and layout configuration.
> 
> Following that, here come the dumb questions:
> A> What problems and/or issues are being created by using the inductor coil values listed above?
> 1> Do I need to find a source for a suitable 1.1mH and swap out what I've already used, or is the difference so minimal as to be of little consequence?
> 2> Should subsequent builds simply use what I've already been using?
> 3> If I need to swap out inductors and still want to use the products from MADISOUND, should I go with the 0.7mH for L1 and then choose either the 1.0mH or 1.25mH for L2?​B> How much and what kinds of problems are actually being caused by my current steel screw and washer mounting arrangement?
> C> Is it possible that the value discrepancies, combined with my current mounting is actually contributing to this *"test mule"* unit being a *"one size fits all"* that operates nearly identically in all of my tested applications?
> D> Bottom line, do I need to step back and re-think everything, or should I just go with what seems to have worked well, so far?​Dumb questions, I know, but I'd rather make any changes now, before gearing up and collecting the remaining required inductors!
> 
> It's also my hope that these dumb questions don't complicate or confuse this thread!
> 
> Thanks Again @JohnH & Happy Holidays,
> Gene



First, to others reading this and enjoying the good attenuation, I want to put out a big THANKYOU to Gene. Gene was the first to trust, build and validate these designs in their evolved form, almost 2 years ago now. His experience as a professional musician, builder and general good bloke continues to be invaluable.

OK Gene....

You have the advantage of having built a working unit that you've tried across many amps and found to be good for your uses. So I can understand the advantages of keeping consistency.

On the M design (for others: this has two coils, see post 1), the differences between 1.25 and 0.8 vs 1.1 and 0.7 are minimal. It's the ratio between the coils that is more important, which is pretty close with both recipes.

I have a $50 multimeter that also measures inductance, so since I have that and wanted to test exactly what I worked out, I used it to unwind a pair of slightly higher coil values down to a target. Its pretty easy, I just clip the meter across the coil wires as supplied and start unwinding the outer turns, without cutting the wire (once the wire is off the coil and loose, it no longer contributes to inductance, even if it is still part of the circuit). Then, when I get to the value I want, I snip the wire, re-tin it and secure it.

The way in which the one design is found to work well with many different amps is a lucky bit of magic that is hard to explain (ok, I cant fully understand/explain it!), but it is apparent in the analysis results. I tested this with a very wide range of assumptions about the effective output resistance of the amp, and the attenuators (M or M2 ) track this from high bass and low mids up to very high treble. The bass resonant peak is more dependent on the attenuator and so in theory would vary more. But the basic values that we use give a very good result. In theory, the more complex version M3 with a bass resonant circuit should track the low bass better across different amps. But, using just design M (and M2 would be the same), I have tried to reveal any issues with inconsistency in the bass on my two very different amps, and cant find any audible issue at all.

Steel bolts etc - these are increasing the coil inductances in unknown ways. There's no risk to the amp, the impedance seen by the amp is not affected until the upper treble range. But what would it do? It might affect the tone of the attenuator if one coil is being changed by a greater % than the other. It might also affect something in the 'feel' or 'dynamics') of the response, as you dig in with a signal that is driven hard in the power stage and decays - subtle maybe?

How about two ways to check this out?: 

1. Play a while as it is, then take the bolts out and see if you can notice any difference. Try this with an amp with no NFB to expect the greatest possible effect. A more scientific version of that would be to set up a looped riff and record with a close mic to get two otherwise identical clips. 

2. I have a prototype M and I can go and measure a coil inductance, then slip an M3 bolt through it and remeasure. - Ill do this....


----------



## adew1

JohnH said:


> I had a quick look for EU suppliers, and found this in the Netherlands, but Im sure there are lots of places:
> 
> https://www.soundimports.eu/en/crossover-components/coils/air-core-coils/?min=€0&max=€50&filter[]=641991&filter[]=628296



@JohnH Thanks for the link, John. I also found this place, also in the Netherlands:
https://europe-audio.com/

Thanks for the info re wire gauge, etc. Very useful. Cheers!


----------



## JohnH

*Steel bolts*!

So I cracked open an early prototype, which is now disused. The coil in the picture is about 40mm diameter, and disconnected it measures 0.41mH.

I dropped the two bolts into it on its axis, that are shown laid on top.

The small bolt which is a 10mm M2.5, raised the inductance to about 0.44mH so about +7%. The larger bolt is 30mm M3, and it raised it to 0.59mH which is +44%.




So, if mounting can be by short steel bolts or screws, it doesnt have too much effect. But a full length bolt through the coil axis causes quite a significant increase.

Fixings without carbon steel wouldn't do this.

Then, I placed the coil over the steel blade of a wood saw, as if it was in a steel case. Inductance went up about 10%. Thats with steel just under one face, so maybe it would be 20% more in a full steel chassis.

Aluminium is better for this.


----------



## Gene Ballzz

@JohnH 
Thanks for that!  I don't have a meter that will measure inductance so, greatly appreciated. When I can sonically test it all, in all amps, I'll swap to some nylon or non-magnetic stainless bolts and see if there is any difference. If there are any sonic differences, I'll let you know. I will likey just stick with the 0.8mH & 1.25mH coils for the rest of my current builds. I've pretty much nailed down my external layouts/configurations for most convenient use. I guess its time to start *"strippin', drillin' & screwin'"*  along with a bit of solderin'!
Thanks Again! 
Gene


----------



## JohnH

adew1 said:


> @JohnH Thanks for the link, John. I also found this place, also in the Netherlands:
> https://europe-audio.com/
> 
> Thanks for the info re wire gauge, etc. Very useful. Cheers!



That looks like a great site! very well organised and comprehensive. I found this one for an 18 gauge 1.8mH:
https://europe-audio.com/Product.asp?mfr=Intertechnik&part=LU55/1.8/100&Product_ID=5316

….and there's also the Jantzen 'not on a bobbin' style


----------



## Gene Ballzz

JohnH said:


> That looks like a great site! very well organised and comprehensive. I found this one for an 18 gauge 1.8mH:
> https://europe-audio.com/Product.asp?mfr=Intertechnik&part=LU55/1.8/100&Product_ID=5316
> 
> ….and there's also the Jantzen 'not on a bobbin' style



@JohnH

The terminology in that EU link led me to a state side supplier. Not understanding a lot of the jargon, or even how to convert "u" Henry to "m" Henry leaves me at a loss to determine if they are a suitable source or not. They certainly offere a very wide range of products!

https://www.coilcraft.com

Here also is a link to the goggle search that comes back from the search criteria "air core coil"

air core coil

Just Lookin'
Gene


----------



## MikeWinterhalt

John, Thanks so much for sharing your journey in designing this. I just finished my build of the M2v. I'm just checking things before I try this thing out. I put the circuit in Micro-Cap Spice and calculated Zin, Zout. 

When doing the calculations, I assumed 20 ohm as Zout of the amp at 8 ohm tap. Should we assume 10 ohms at 4 ohm tap and 40 ohms at 16 ohm tap?


----------



## JohnH

Hi @MikeWinterhalt , those values in your table look about right.

For checking out the design and the expected tonal balance, I used 20 Ohms for the output impedance of an amp with an 8 Ohm tap, and x2 or x1/2 that for 16 and 4, as you noted. But that's just a median set of values and I also consider a lot higher and a lot lower because it varies according to the amp design. Also, those kind of values are based on what is seen at audio frequencies. If you are looking for ohms values to use as a dc resistance check before powering up, then the dc output resistance of output transformers would probably be a lot less.

Id say your circuit analysis is looking right, so if you run it without the assumptions about amps and speakers connected, it should give you some check values for dc resistance that you can measure on your completed build.

Or, if you want to check it out initially to make sure its safe for the amp, then if you measure the dc resistance at the input (no amp), with speakers connected, it should be around 4, 8 or 16 ohms (depending of which input setting), within about say 20%


----------



## JohnH

Gene Ballzz said:


> @JohnH
> 
> The terminology in that EU link led me to a state side supplier. Not understanding a lot of the jargon, or even how to convert "u" Henry to "m" Henry leaves me at a loss to determine if they are a suitable source or not. They certainly offere a very wide range of products!
> 
> https://www.coilcraft.com
> 
> Here also is a link to the goggle search that comes back from the search criteria "air core coil"
> 
> air core coil
> 
> Just Lookin'
> Gene



Hi Gene,

I think in electronics generally there are applications for inductors that are several orders of magnitude different from those that we need. Often they are about tuning and shunting and blocking at Mhz frequencies. When you see uH etc, that's a shortcut for a greek 'mu' symbol, denoting micro henries, ie 1/1000th of a milli Henry (mH). There's also nano-henries (nH) which is 1/1000000th of a mH, or a billionth of a Henry. I had a look through that coilcraft site but they all seem intended for those kind of applications, I didn't see anything for us there.

The ones we need always seem to be designed for audio speaker applications. Its good that we can draw on that reasonably common product group to find the bits we need for our rather uncommon application.

Also, its worth talking about why we need air cored, instead of coils wound on an iron core. I got that point from various online threads about reactive loads. When a coil with a core gets high current, it can saturate and this momentarily reduces the inductance. We don't want that since the whole idea is to get the unit doing what we want with high amp power!

There's another place where inductance comes into our guitar tone, in the pickups themselves. Thousands of turns of very fine wire on a core leads to very high inductance values, of several Henries. eg a PAF humbucker might be around 4H which is 4000mH.


----------



## Travis Knight

Hi, looking forward to building this; I like the idea of reactive vs resistive attenuation - seems a more "natural" simulation of speaker load to me. Though, I wonder if anyone has thoughts on how a home theater volume control might work for this application. Being an attenuator, wonder if there's a play? Something like the OSD SVC100 (sorry, first post, no links)


----------



## RoadShow

Hi Folks,

So, over the holidays I got my attenuator put together built for 8 ohm with the 16 ohm option using a 1.0mH coil. Trouble is, using it with an 18W Marshall it's almost complete attenuation using it in both 8 and 16 ohm set ups. I've looked this over and over and am stuck. Maybe a fresh set of eyes will see where I've gone wrong?

Attached is a picture of the layout and wiring, my marked up schematic, and a drawing I made for wiring it up.

With the switch in the 1-2 position, at the input I read 8.8 ohms when the plug is in the 8 ohm outputs reading 18.1 ohms, and I read 8.1 ohms at the input when the plug is in the 16 ohm output where I read 28.1

With the switch in the 2-3 position, at the input I read 16 ohms when the plug is in the 8 ohm outputs reading 25.3 ohms, and I read 13.3 ohms at the input when the plug is in the 16 ohm output where I read 32.9

Please open my eyes!! I hope my mistake isn't too dumb...


----------



## JohnH

Hi @RoadShow 

Mysterious! It looks to be well thought out and neatly built, and I'm not seeing the issue yet. 

If you could run some more resistance tests, with an 8 Ohm speaker plugged into Out1/2, and measure the input resistance in all settings. It should stay around 8 ohms within about 20% or so. Also check the switches are working, and that the 1-2-3 lugs are operating the way you expect. I had thought that all toggles were similar, until I found out that they aren't.

The jack sockets look to be metal, not plastic bodied, hence the 'ground' side, which usually is not a ground but is just one end of the OT output winding, may be live to the case. Unlikely to cause the issue, unless something else is touching it too. (which is why plastic bodied is better)

Also, I'm suspecting something about the jack connections, only because its hard to see them in the pics.

I'm assuming that your amp at full volume is seriously loud? with minimum -7db attenuation on this M2 design, 18W is still delivering about 3.5W to the speaker, which is usually still too loud. 

And just for others looking in, we have collectively built dozens of these now so we know that they work fine.


----------



## JohnH

Travis Knight said:


> Hi, looking forward to building this; I like the idea of reactive vs resistive attenuation - seems a more "natural" simulation of speaker load to me. Though, I wonder if anyone has thoughts on how a home theater volume control might work for this application. Being an attenuator, wonder if there's a play? Something like the OSD SVC100 (sorry, first post, no links)



Thanks for posting, I found the link to those units, designed for combining and controlling sets of ceiling mounted audio speakers. 

These kind of units are often put into attenuators, and they tend to be responsible for the way the tone drifts off and gets dull as you turn down. They are designed to pay attention to the impedance seen by the amp, but not so much to that seen by the speaker. Controlling the output impedance seen by the speaker is the key to this design and the main reason it stays so consistent at all volumes. So although it can 'work' I'm not recommending it. The small switched steps that we have are actually audibly very small, and generally there's no need to fine tune any further. With the switches and carefully judged component values for each stage, we can maintain the required control.

I did work out a version with a variable pot, trying to mitigate and control some of the disadvantages, but its not tested and will depend on the specific pot used.


----------



## RoadShow

JohnH said:


> Hi @RoadShow
> 
> Mysterious! It looks to be well thought out and neatly built, and I'm not seeing the issue yet.
> 
> If you could run some more resistance tests, with an 8 Ohm speaker plugged into Out1/2, and measure the input resistance in all settings. It should stay around 8 ohms within about 20% or so. Also check the switches are working, and that the 1-2-3 lugs are operating the way you expect. I had thought that all toggles were similar, until I found out that they aren't.
> 
> The jack sockets look to be metal, not plastic bodied, hence the 'ground' side, which usually is not a ground but is just one end of the OT output winding, may be live to the case. Unlikely to cause the issue, unless something else is touching it too. (which is why plastic bodied is better)
> 
> Also, I'm suspecting something about the jack connections, only because its hard to see them in the pics.
> 
> I'm assuming that your amp at full volume is seriously loud? with minimum -7db attenuation on this M2 design, 18W is still delivering about 3.5W to the speaker, which is usually still too loud.
> 
> And just for others looking in, we have collectively built dozens of these now so we know that they work fine.



@JohnH THX John!!

The switches verify good.

With an 8 ohm speaker plugged in the readings vary from 7.9-8.8 ohms. With a 16 ohm speaker plugged in the readings vary from 7.3-9.6

The big news is that when I removed the jacks from the housing and let them lay loose like the switches it all started to work!! I have plastic jacks on had so I'll be swapping those later today and buttoning it up.

I will say that the minimum -7db puts it at a good quiet level with the 18W'r and guitar turned all the way up. I'll try it out with my JTM45 later.

Thanks for the tip, that was it!! Reading this thread a couple times over the last year and following along I knew it had to be something I was doing.

Bill


----------



## JohnH

Hi @RoadShow

That's great news, and I'll be looking forward to hearing about how it goes with the JTM45.

Those new resistances are spot-on as expected. Im still a bit curious though, about what was happening with the current metal jacks. All the sleeve connections on the jacks are all connected to each other anyway, so if they were all connecting to the case as well that shouldn't have really changed anything, unless something else was connecting to something?


----------



## RoadShow

JohnH said:


> Hi @RoadShow
> 
> That's great news, and I'll be looking forward to hearing about how it goes with the JTM45.
> 
> Those new resistances are spot-on as expected. Im still a bit curious though, about what was happening with the current metal jacks. All the sleeve connections on the jacks are all connected to each other anyway, so if they were all connecting to the case as well that shouldn't have really changed anything, unless something else was connecting to something?



Hi @JohnH ,

OK, I hooked it up to the JTM45 with a 16ohm load and it works fantastic. IMO it serves itself better on something like that or bigger. I think for use on an 18W Marshall the default -7db is sufficient. With gain and volume all the way up it's barely audible in the next room.

Hmm, I'm thinking I could build just the front end with smaller power resistors for use with the 18W, maybe 25W resistors? All I would need is R1, R2A&B, and L1 and I'd have a mini attenuator.

If I built the 8ohm version with the 16ohm option would I use the same values for R10 & R11 of 10 & 68 ohms?

Thoughts?

Thx, Bill


----------



## Osman Qureshi

RoadShow said:


> Hi @JohnH ,
> 
> OK, I hooked it up to the JTM45 with a 16ohm load and it works fantastic. IMO it serves itself better on something like that or bigger. I think for use on an 18W Marshall the default -7db is sufficient. With gain and volume all the way up it's barely audible in the next room.
> 
> Hmm, I'm thinking I could build just the front end with smaller power resistors for use with the 18W, maybe 25W resistors? All I would need is R1, R2A&B, and L1 and I'd have a mini attenuator.
> 
> If I built the 8ohm version with the 16ohm option would I use the same values for R10 & R11 of 10 & 68 ohms?
> 
> Thoughts?
> 
> Thx, Bill



Hi all, 

I'm new to the forum. What a great community you all have here!

Before I start building a JohnH attenuator myself, I would also be interested in any design tweaks that would make it even more suited for a 20W amp. And does the attenuator work as a dummy load without a speaker attached (the DSL20HR requires a speaker attached to use its emulated out, so could I just attach the attenuator without the speaker)? 

By the way, I'm very impressed with the design of the attenuator--is there a name for it yet? I haven't seen a design with an inductor before--ingenious! With the MiniMASS now costing north of $170, I won't be surprised if clones start showing up on eBay, or even Alibaba.

Thanks, Osman


----------



## JohnH

RoadShow said:


> Hi @JohnH ,
> 
> OK, I hooked it up to the JTM45 with a 16ohm load and it works fantastic. IMO it serves itself better on something like that or bigger. I think for use on an 18W Marshall the default -7db is sufficient. With gain and volume all the way up it's barely audible in the next room.
> 
> Hmm, I'm thinking I could build just the front end with smaller power resistors for use with the 18W, maybe 25W resistors? All I would need is R1, R2A&B, and L1 and I'd have a mini attenuator.
> 
> If I built the 8ohm version with the 16ohm option would I use the same values for R10 & R11 of 10 & 68 ohms?
> 
> Thoughts?
> 
> Thx, Bill



Its great that its working. You can definitely just build one with the front end and leave off other stages as you wish. Component values for the resistors stay the same in terms of Ohms, but you can scale back the power ratings if you want to. 

R1 takes about 1/2 the total power, and I reckon on a factor of x3 to allow for overdriving the amp and imperfect cooling. On that basis with an 18W amp, R1 rating is 18 x 50% x 3 = 27W. If your 18W amp is heating up the current build so its reasonably warm, then maybe use 50W for R1 and 25W for R2 A and B. If its staying cool with your use, maybe you can use 25W far all. Or use all 25W and use two in parallel for R1 (eg two 30 Ohm in parallel to make a 15)

On the inductor, stick with 18 or 19 gage and the same inductance. 

Just a comment on volumes though, and id be interested in any further comment from others particularly @Gene Ballzz . At home, Im running with my VM2266, which is basically an updated JTM45 and puts out about 35W-40W from its KT66 output stage. I often run it at full -31.5 db attenuation at home, and its like moderate TV volume late at night - still very clear and loud enough. That's cutting 35W down to about 25mW, and I'm at about 6 on the amp volume! Now its much more fun at about -21db or -24db, but then it is filling the house with sound. The equivalent to those settings for an 18W amp would be just 3db less. I reckon for a small gig I might be at about -14db with this amp. Obviously it depends on lots of things like how hard you drive the amp, what the speakers are etc

Im sure we all have different perceptions about how loud is loud, but your rig as described seems like it might be being quieter than Id have expected overall. That's just a thought, no prob if its all working at your end.


----------



## Gene Ballzz

Osman Qureshi said:


> Hi all,
> 
> I'm new to the forum. What a great community you all have here!
> 
> Before I start building a JohnH attenuator myself, I would also be interested in any design tweaks that would make it even more suited for a 20W amp. And does the attenuator work as a dummy load without a speaker attached (the DSL20HR requires a speaker attached to use its emulated out, so could I just attach the attenuator without the speaker)?
> 
> By the way, I'm very impressed with the design of the attenuator--is there a name for it yet? I haven't seen a design with an inductor before--ingenious! With the MiniMASS now costing north of $170, I won't be surprised if clones start showing up on eBay, or even Alibaba.
> 
> Thanks, Osman



@Osman Qureshi 

I call it the JohnH Design, JohnH Attenuator or simply *"The JohnH."* 

I'm not certain that these will be appropriate for use as a *"dummy load"* but it is not required for your DSL20 to use the *"emulated out."* All you need to do do is turn the amp on and leave the *"LOW/STB/HIGH"* switch in the center *"standby"* position. The speaker (while still connected as a load) will not produce any sound, but the *"emulated out"* should function just fine!

And  To The Forum!
Gene


----------



## JohnH

Osman Qureshi said:


> Hi all,
> 
> I'm new to the forum. What a great community you all have here!
> 
> Before I start building a JohnH attenuator myself, I would also be interested in any design tweaks that would make it even more suited for a 20W amp. And does the attenuator work as a dummy load without a speaker attached (the DSL20HR requires a speaker attached to use its emulated out, so could I just attach the attenuator without the speaker)?
> 
> By the way, I'm very impressed with the design of the attenuator--is there a name for it yet? I haven't seen a design with an inductor before--ingenious! With the MiniMASS now costing north of $170, I won't be surprised if clones start showing up on eBay, or even Alibaba.
> 
> Thanks, Osman



Hi Osman and welcome to our thread. There's nothing particular that would make the design better for a 20W amp as compared to the base designs which target 50W amps. But if you want to, you could reduce the 100W and 50W resistors down to 50W and 25W respectively - with the same Ohm values.

Yes you can definitely use this as a dummy load with no speaker, you just set the attenuation to max. At this setting, the amp doesn't see much of the speaker anyway (about 0.1%!), its fully loaded by the attenuator. But, I see Gene has pointed out that you may not need to worry anyway, using your particular amp (but I know there's been a couple of versions of DSL20's, which may differ in this respect).

If silent/headphone playing/recording is a key thing that you need, you might consider adding a line-out. ie, its another output fed with a small signal via a resistor an a pot. From this you can go to an external cab-sim or IR box. Some guys have built this feature in - easy to add later if you save enough space. Note that a line out wont sound any good as a direct signal though, it needs the cab sim etc. Whether that's worth doing might depend on how good the amps own emulated out is.


----------



## Gene Ballzz

@JohnH
I use three amps, somewhat interchangeably, depending on my mood of the day and or which one is at the rehearsal space two houses away and which one happens to be sitting next to my desk in my home studio. These amps are a 13 watt-5E3 Tweed Deluxe-G12M25/75hz, 20 watt-DSL20CR-G12M25/75hz and a +30 watt-JTM30-G12M65/75hz. For similar band use volumes in my medium volume 3-piece, with the bass using an SVT/8x10, my settings (remembering that the default minimum attenuation level is -7db) for each amp end up generally at:
*>* Tweed Deluxe, either -7db or -10.5db
*>* DSL20CR, either -10.5db or -14db, although I've been know to drop it to only-7db, if feeling especialy frisky and/or having a single coil, Strat only night!
*>* JTM30, -14db or -17.5db, depending on several factors, but with the MASTER of the amp on ten, I sometimes go to -21db. It should be noted that although this amp is *"officially"* rated at 30 watts, I'm fairly certain that if I ever get around to measuring the output, it will end up easily at 40 watts and maybe a tad more!​Now when it comes to *"bedroom-ish"* use in my studio/office, I use two settings, *A -* one for wife sleeping in an adjacent room and *B -* Wife awake, but not wanting to drive her out of the house:
*>* Tweed Deluxe, _*A- *_-21db or -24.5db and *B-* -17.5db.
*> *DSL20CR, _*A- *_-24.5db or -28db and _*B- *_-21db
*> *JTM30, _*A- *_-31.5 db and _*B- *_-28db​And yes, there is certainly some loss of *"movement of air"* as the perceived volume goes down, but at these kinds of volume levels, it is fairly negligible. The dynamics and response of the amps, along with their overall tone and character, remain consistent throughout all settings, including harmonic overtones and feedback! These atteuators are the most liberating pieces of gear I've ever owned, in +50 years of _*"Screechin' & Squakin'!" *_As a guitarist, few things are as blissful as being able to dial your *"favorite amp of the moment"* to it's sweetest tonal and response spot and then use this attenuator to *"correct"* the actual listening volume to the most appropriate level!
Thanks Again To @JohnH !
Gene


----------



## Osman Qureshi

JohnH said:


> Hi Osman and welcome to our thread. There's nothing particular that would make the design better for a 20W amp as compared to the base designs which target 50W amps. But if you want to, you could reduce the 100W and 50W resistors down to 50W and 25W respectively - with the same Ohm values.
> 
> Yes you can definitely use this as a dummy load with no speaker, you just set the attenuation to max. At this setting, the amp doesn't see much of the speaker anyway (about 0.1%!), its fully loaded by the attenuator. But, I see Gene has pointed out that you may not need to worry anyway, using your particular amp (but I know there's been a couple of versions of DSL20's, which may differ in this respect).
> 
> If silent/headphone playing/recording is a key thing that you need, you might consider adding a line-out. ie, its another output fed with a small signal via a resistor an a pot. From this you can go to an external cab-sim or IR box. Some guys have built this feature in - easy to add later if you save enough space. Note that a line out wont sound any good as a direct signal though, it needs the cab sim etc. Whether that's worth doing might depend on how good the amps own emulated out is.


Thank you everyone for your quick replies. I spent the evening trying to source the parts for a 16 ohm model here in the UK. Looks like I won’t get them all for several a weeks. I’m looking forward to the project. I may even try a headphone out just for the challenge. By the way, I was wondering what would happen if I plugged a mix of a 16 ohm and an 8 ohm speaker in the speaker jacks? Will the amp still just see a 16-ohm load and will the two speakers be equally loud? Thanks again, and happy new year to all. Regards, Osman


----------



## TheOtherEric

RoadShow said:


> ... I think for use on an 18W Marshall the default -7db is sufficient. With gain and volume all the way up it's barely audible in the next room.
> ...


That doesn’t jibe with my experience at all. I have two 20W amps (Ceriatone OTS20 and Son of Yeti) and use a 16 ohm M2 in one, and 8 ohm M2 in the other. I run both at -21 dB total, and they both can still get REALLY loud, meaning ear-ringing after a few minutes, and very angry wife. I’d quantify it better if I knew how, ha ha.


Osman Qureshi said:


> ... I would also be interested in any design tweaks that would make it even more suited for a 20W amp. ...


I wouldn’t bother tweaking it for a 20W amp. Sure you could save, what $5 (?) by using a couple smaller resistors, but then it couldn’t be used later on a 50W amp. The design works great on my two 20W amps, and I can’t imagine what changes could improve it.


----------



## JohnH

Osman Qureshi said:


> Thank you everyone for your quick replies. I spent the evening trying to source the parts for a 16 ohm model here in the UK. Looks like I won’t get them all for several a weeks. I’m looking forward to the project. I may even try a headphone out just for the challenge. By the way, I was wondering what would happen if I plugged a mix of a 16 ohm and an 8 ohm speaker in the speaker jacks? Will the amp still just see a 16-ohm load and will the two speakers be equally loud? Thanks again, and happy new year to all. Regards, Osman



Hi Osman, 

With the 16 ohm M2 design, I'll be fine to run a 16 and an 8 ohm cab in parallel. This mixture equates to a 5.3 Ohm speaker load. But with the first fixed stage always engaged, the amp always sees at least 14 Ohms, which should be fine for using the 16 Ohm amp tap. As you increase attenuation further, the amp sees less and less of the speakers and so what the amp sees will rise to nearer the nominal 16 value.

But with those two speakers, the 8 ohm cab gets 2/3 of the power and the 16 Ohm cab gets 1/3. You may also find a slightly boosted high treble occuring. All fine and safe to try though. If you have a presence control, you might adjust it to taste.


----------



## paul-e-mann

@JohnH whats the latest revision of this attenuator?


----------



## JohnH

pedecamp said:


> @JohnH whats the latest revision of this attenuator?



hi pedecamp

The latest basic version is version M2, shown near the top of the first post It can be built for 8 or 16 ohm amp taps, with both ok for different speaker ohms From there, other versions can be assembled, such as M2v that deals with different amo taps, and also line-out or bypass could be added too.


----------



## paul-e-mann

JohnH said:


> hi pedecamp
> 
> The latest basic version is version M2, shown near the top of the first post It can be built for 8 or 16 ohm amp taps, with both ok for different speaker ohms From there, other versions can be assembled, such as M2v that deals with different amo taps, and also line-out or bypass could be added too.


Do you have a plan to make it foot switchable on and off?


----------



## Osman Qureshi

JohnH said:


> hi pedecamp
> 
> The latest basic version is version M2, shown near the top of the first post It can be built for 8 or 16 ohm amp taps, with both ok for different speaker ohms From there, other versions can be assembled, such as M2v that deals with different amo taps, and also line-out or bypass could be added too.


Hi all, where can I find the latest schematic for the line-out addition? Thanks, Osman


----------



## JohnH

Hi @Osman Qureshi

Theres a few who have tried this successfully, but i haven't drawn it up myself. Its basically a pot and a couple of resistors. I can work something out if you let me know the type of amp youd use and what the line out would be going into


----------



## JohnH

pedecamp said:


> Do you have a plan to make it foot switchable on and off?



Turning the whole attenuator on or off with a footswitch is not a good idea, since youd be using it to switch the full amp power while playing on the fly and it could make nasty OT-damaging spikes. But. given the first stage is always on, a foot-switcheable boost is possible, basically another stage, taken out to a footswitch like on a stomp box. Is that of any interest?


----------



## paul-e-mann

JohnH said:


> Turning the whole attenuator on or off with a footswitch is not a good idea, since youd be using it to switch the full amp power while playing on the fly and it could make nasty OT-damaging spikes. But. given the first stage is always on, a foot-switcheable boost is possible, basically another stage, taken out to a footswitch like on a stomp box. Is that of any interest?


Yes


----------



## Osman Qureshi

JohnH said:


> Hi @Osman Qureshi
> 
> Theres a few who have tried this successfully, but i haven't drawn it up myself. Its basically a pot and a couple of resistors. I can work something out if you let me know the type of amp youd use and what the line out would be going into


Hi John, thanks for you offer. My DSL20 has an emulated out, but I’d like to plug this amp directly into my MacBook so I can try other cab sims using the Amp Designer function in GarageBand. Naturally, plugging the amp’s speaker out directly into the Mac’s 3.5mm mic/headphone socket will fry the laptop, but a line-out from the attenuator with all that attenuation shouldn’t with the right resistors and pot. So that’s what I’m trying to do. Thanks, Osman


----------



## JohnH

ok great, I might try to rig something up equivalent here, then we'll have more confidence in values etc.

As an alternative, one thing I've been doing without having a dedicated line-out is to go from one of the attenuator speaker outs, into a line-in on a very small and cheap Behringer Xenix mixer to set levels, then from there to a laptop mic in.


----------



## Gene Ballzz

@pedecamp & @JohnH 
It is my considered opinion that attempting to turn this attenuator into a *"stomp box"* is a very *BAD* idea. While electrically it can be done, the act of running long speaker cables to and from the amp/speaker and pedal board is an invitation to step, kick or otherwise *"jostle/unplug/break"* the speaker connection, causing an intermittent or shorted connection to the amp, along with potentially disastrous consequences! It seems unnecessary to attemp to re-invent an already nicely rolling wheel!

Now on the other hand, providing a power supply to the attenuator to use a suitable relay in the unit, that could be foot switched, would be a *"mo betta"* approach!

Just My Thoughts,
Gene


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## JohnH

Hi Gene, I'm thinking of an in-between non-powered offering without a relay. What would your view be of this:?

The attenuator box with amp and speaker wires stays neatly at the amp as usual

Between the last attenuation stage and the speaker, a TRS socket is provided, with contacts that bypass it when no plug is inserted.

Plug in a lead, with a 3core cable (probably made from a 5A 3 core mains flex) to insert the footswitch.

The footswitch is in its own small floor box , with two resistors, probably designed to make another -7db stage to switch on or off

Id use a 3pole stompbox switch. These have an ac contact rating of 2A. Can run them all in parallel for reliability, or use two in parallel with the 3rd pole used to switch a battery powered indicator LED

With this, the attenuator stays safe and not stomped on or tripped over. If anything happens to the footswitch wire, at least Stage 1 of the attenuator is still there connected to the amp.

I tried before to work out relay switching, but there are so many different relay variations that to be sure a circuit worked it would need to be built tested and tweaked, but this non relay method is really the same circuit as we already know.

what say you?


----------



## Gene Ballzz

JohnH said:


> Hi Gene, I'm thinking of an in-between non-powered offering without a relay. What would your view be of this:?
> 
> The attenuator box with amp and speaker wires stays neatly at the amp as usual
> 
> Between the last attenuation stage and the speaker, a TRS socket is provided, with contacts that bypass it when no plug is inserted.
> 
> Plug in a lead, with a 3core cable (probably made from a 5A 3 core mains flex) to insert the footswitch.
> 
> The footswitch is in its own small floor box , with two resistors, probably designed to make another -7db stage to switch on or off
> 
> Id use a 3pole stompbox switch. These have an ac contact rating of 2A. Can run them all in parallel for reliability, or use two in parallel with the 3rd pole used to switch a battery powered indicator LED
> 
> With this, the attenuator stays safe and not stomped on or tripped over. If anything happens to the footswitch wire, at least Stage 1 of the attenuator is still there connected to the amp.
> 
> I tried before to work out relay switching, but there are so many different relay variations that to be sure a circuit worked it would need to be built tested and tweaked, but this non relay method is really the same circuit as we already know.
> 
> what say you?



Given that this arrangement is *"late"* enough in the attenuator circuit, it wouldn't likely present a *"dead short"* or completely *"open circuit"* to the amplifier in the case of a failure, I guess that could work OK. From an overly cautious stand point though, I'm still not a fan and not sure I would ever want such a thing connected to my amp! I do see the attraction of such a thing, however I believe that all connections bewtween an amp and it's speaker need to be rock solid and as well protected as possible and running an extra 20 ft to 30 ft pair of speaker level cables to a panel that getsrepatedly *"STOMPED"* on does not really qualify, in my estimate. Then there's the question of that cable length possibly requiring heavier gauge wire. I see far to many opportunities for operator error.

One of the things I really appreciate about the initial designs of this attenuator is that if well built, with solid solder connections, and even the addition of using DPDT switches (with the two sides jumpered) makes them generally not only bullet, but "BOMB" proof! And then leaving out a bypass switch helps make them mostly idiot proof. About the only mistake that could be made is ignoring and/or choosing the wrong impedance at the amp or speaker! It should be remembered that not all proficient guitarist are rocket surgeons or brain scientists! Some have a hard time even finding their car keys, even with them right in their hand! Also needing to be factored in is that at least a couple musician type folks have been rumored to consume copious amounts of alcohol and other stupidity inducing substances while plying their trade!   

This Is All Just My Opinion, Of Course,
Gene


----------



## JohnH

Well, with the footswitch idea, if it was to be done, I'd do it thussly, based on the M2 design:













M2 Footswitch 200103



__ JohnH
__ Jan 3, 2021






The idea is to add another switchable attenuation stage, which is on for rhythm, then switched off to add more volume.

This has one 3-core cable from main box to floor box. It uses a switched TRS jack at the main unit, which bypasses the footswitch connection when no plug is inserted.  The footswitch is a three pole one such as for typical stomp boxes and uses a double contact pair in parallel for redundancy. At the jack socket, the bypass contacts are 'crossed over' so that when there is no footswitch, then the two jack contacts connect to bypass the jack with a double parallel pair. 

The 3rd switch pole can run an LED as an indicator, and I suggest this could be just using a simple battery in the footswitch. Alternatively, it could have a 2.1mm barrel socket to receive power from a pedal-board daisy chain. The LED resistor controls its brightness, its value will depend on what battery voltage is used and the LED type. It might be around 2.2k 1/4W with a 9V supply. 

The resistor pair in the foot unit determines the switchable cut and boost. I reckon -7db would be a good level, so I suggest that the footswitch resistors are the same as those in the main Stage 2. But the values could be changed, to set any level of footswitch attenuation from 3.5 to 14db.

*Whats it good for?*

I think this footswitch system would be fine, but probably wouldn't want to go a long way with it though, maybe 10-15 feet.

And I don't think I'd really use it myself, just because when I want a boost then I also want to push the front end of the amp harder, so a normal OD pedal or a change at the guitar does what I need. This takes me from edge of breakup to full crunch.

But, say you are working with a fully saturated/distorted rhythm tone, and want to switch to a louder lead tone, then pushing the amp might sing a bit more but may be still the same volume. In this case this footswitch will give the volume boost. But, in these cases you may be on an amp that is much more heavily dependent on its preamp or on your distortion boxes. In that case, then the simple volume reducers in the loop, or a different pedal, might do what you want instead. 

Also should note: This system needs to have Stage 1 always on, as it is in the M2 design. So the loudest settings will be -7db or -14db controlled by the footswitch, then less loud stepping down as required as determined by the main switches. 

So there it is, it could be useful in some cases but think of other options too, and take note of what Gene says above.


----------



## RoadShow

TheOtherEric said:


> That doesn’t jibe with my experience at all. I have two 20W amps (Ceriatone OTS20 and Son of Yeti) and use a 16 ohm M2 in one, and 8 ohm M2 in the other. I run both at -21 dB total, and they both can still get REALLY loud, meaning ear-ringing after a few minutes, and very angry wife. I’d quantify it better if I knew how, ha ha.



@TheOtherEric,

Yeah, I get it. But if you go back on page 72 and look at the information I provided, please point out my error. The resistances check out and I made the swap to plastic jacks and with just the front default attenuation the Marshall SV20H is at let's say a normal conversation level which is what I was really after. That's with gain and all volume knobs including the guitar dime'd. Adding any further attenuation from any of the 3 switches makes it sub-whisper level. It works totally different on the JTM45.

Please point out my mistake, I've studied it over and over.


----------



## Gene Ballzz

@JohnH 
While that *"foot switched"* setup is certainly *"electronically/electrically"* feasible, you just gotta know that some innovative moron will try to use that *"foot switched"* stage as a *"stand alone"* device! Of course, it may not be too unsafe, *if* the resistors in it are of sufficient wattage. Then there's the question of capacitive, inductive and resistive issues that may be caused by having the negative of the *"send & return"* in that three wire cable line shared by one wire, while the *"send & return"* of the two positives each having their own wire. This would also likely change as cable length changes! While doable, I would never want the liability of providing such a thing for anyone! *IS THAT SMOKE I SMELL?*
Just Spitballin' Here!
Gene


----------



## JohnH

all cool @Gene Ballzz ! Everyone reading this should take note of your views.


----------



## JohnH

hi @RoadShow 

Just to try to get to the bottom of the volume discussion, Im just wondering about the 18W amp. How loud is it usually, if not attenuated? and how compared to the JTM45? 

So far as I know, a JTM45 puts out just under 40W RMS clean. If the 18W was doing 18W, then the difference in level between them is somewhere 3 to 3.5db if similarly cranked. This is a small step in volume, about the same as the smallest step available on the attenuator. Does that match up to what you experience? 

You could also check by measuring AC voltage at the amp output, ie at the input to the attenuator or across the speaker if no attenuator. (needs some care to clip test leads securely). Nominally, at 18W into 8 Ohms, you expect 12V ac at the output. A measurement made by a meter using a guitar or other signal will be much more variable, but you should get 'several' volts there. 

With the JTM45, which attenuation setting gets you down to about that nice conversation level of volume?


----------



## RoadShow

JohnH said:


> hi @RoadShow
> 
> Just to try to get to the bottom of the volume discussion, Im just wondering about the 18W amp. How loud is it usually, if not attenuated? and how compared to the JTM45?
> 
> So far as I know, a JTM45 puts out just under 40W RMS clean. If the 18W was doing 18W, then the difference in level between them is somewhere 3 to 3.5db if similarly cranked. This is a small step in volume, about the same as the smallest step available on the attenuator. Does that match up to what you experience?
> 
> You could also check by measuring AC voltage at the amp output, ie at the input to the attenuator or across the speaker if no attenuator. (needs some care to clip test leads securely). Nominally, at 18W into 8 Ohms, you expect 12V ac at the output. A measurement made by a meter using a guitar or other signal will be much more variable, but you should get 'several' volts there.
> 
> With the JTM45, which attenuation setting gets you down to about that nice conversation level of volume?



@JohnH

OK, this is interesting. After reading this reply I thought of something I didn't try before. I had tried the

 18W-8ohm out to attenuator with 8ohm speaker in 8ohm attenuator jack (no worky)
 JTM45-16ohm out to attenuator to 16 ohm speaker in 16 ohm attenuator jack (worked)

So, I swapped heads around and ran the 18W-16ohm out to the attenuator to 16 ohm speaker in 16ohm jack, BINGO, works as expected. This may only add confusion, but the 16 ohm plug in works as expected with the 18W for initial attenuation followed by additional attenuation with each switch.

If all of my ohm readings from the other day were more or less correct, does the issue using 8ohms make sense? It doesn't to me.

So I inspect everything, and embarrassingly find the cable to the 8ohm cabinet went flakey. Why it worked straight from the amp to speaker beats me, must be it gets bent just right.

Embarrassed but relieved and happy!!

Thanks for bearing with me guys...


----------



## JohnH

i guess there must be some issue either with the 8 ohm output tap on the 18W amp, or with the 8 Ohm attenuator.

Heres a couple more tests: 

This one is zero risk if you can do it:

Get a battery, any type, maybe a 9V one but a 1.5V will do fine, feed it through a resistance of about 100 Ohms - whatever you have, and feed that into the front of the 8 Ohm attenuation, with 8 ohm speaker plugged in. Now measure the voltage at the front end across teh attenuator input jack - with 9V and 100 Ohms it would be about 600 to 700 mV. At the attenuator output it would be about 18mV at max attenuation across the speaker, and increase by a factor of about x1.5 each time you reduce attenuation by 3.5db (eg 18, 27, 40 mV etc) up to about 300mV at -7db. Exact values depend on your battery and resistor but there should be a fairly smooth increase by approximately constant multiplication factors. This is basically a dc version of assessing the attenuation.

Do exactly the same with the same battery and resistor, in the 16 Ohm build and the voltages should all be greater, about 1.2V at the front, 33mV at the 16 Ohm speaker with max attenuation., etc

Second test:

Do you have an 8 Ohm out on the JTM45? can you try it with the 8 ohm attenuator and 8 cab?


----------



## Gene Ballzz

RoadShow said:


> @JohnH
> 
> OK, this is interesting. After reading this reply I thought of something I didn't try before. I had tried the
> 
> 18W-8ohm out to attenuator with 8ohm speaker in 8ohm attenuator jack (no worky)
> JTM45-16ohm out to attenuator to 16 ohm speaker in 16 ohm attenuator jack (worked)
> 
> So, I swapped heads around and ran the 18W-16ohm out to the attenuator to 16 ohm speaker in 16ohm jack, BINGO, works as expected. This may only add confusion, but the 16 ohm plug in works as expected with the 18W for initial attenuation followed by additional attenuation with each switch.
> 
> If all of my ohm readings from the other day were more or less correct, does the issue using 8ohms make sense? It doesn't to me.
> 
> So I inspect everything, and embarrassingly find the cable to the 8ohm cabinet went flakey. Why it worked straight from the amp to speaker beats me, must be it gets bent just right.
> 
> Embarrassed but relieved and happy!!
> 
> Thanks for bearing with me guys...



Are you using any shielded instrument cables as speaker cableS?
Just Askin'
Gene


----------



## RoadShow

@JohnH 

I think you missed the line where I found it was the cable. Works great now.



JohnH said:


> i guess there must be some issue either with the 8 ohm output tap on the 18W amp, or with the 8 Ohm attenuator.
> 
> Heres a couple more tests:
> 
> This one is zero risk if you can do it:
> 
> Get a battery, any type, maybe a 9V one but a 1.5V will do fine, feed it through a resistance of about 100 Ohms - whatever you have, and feed that into the front of the 8 Ohm attenuation, with 8 ohm speaker plugged in. Now measure the voltage at the front end across teh attenuator input jack - with 9V and 100 Ohms it would be about 600 to 700 mV. At the attenuator output it would be about 18mV at max attenuation across the speaker, and increase by a factor of about x1.5 each time you reduce attenuation by 3.5db (eg 18, 27, 40 mV etc) up to about 300mV at -7db. Exact values depend on your battery and resistor but there should be a fairly smooth increase by approximately constant multiplication factors. This is basically a dc version of assessing the attenuation.
> 
> Do exactly the same with the same battery and resistor, in the 16 Ohm build and the voltages should all be greater, about 1.2V at the front, 33mV at the 16 Ohm speaker with max attenuation., etc
> 
> Second test:
> 
> Do you have an 8 Ohm out on the JTM45? can you try it with the 8 ohm attenuator and 8 cab?


----------



## RoadShow

Gene Ballzz said:


> Are you using any shielded instrument cables as speaker cableS?
> Just Askin'
> Gene



Nope, 16 gauge dual conductor cable.


----------



## Yamariv

@JohnH

Ok, so I've had all the parts to build 2- M2 16ohm Attenuators for a while now and finally got to soldering tonight and actually finished one of them! So I just wanted to double check the readings I got out in the shop before I plug in one of my beloved amps into it! I'm scared but also excited to finally have an attenuator 

So I hooked up my test speakers which are 2-16ohm in Parallel which reads at 7ohms together (had a brain fart and forgot I was building a 16ohm setup and should have just plugged one of the 16ohm speakers direct..)
Anyway, so having basically an 8ohm load plugged into the back, I was getting between 15.8 and 18ohms at the input in any and all combinations of switching. I'm assuming that is good to go readings? Am I safe to test it on an amp?

Out of curiosity, if I have an 8 ohm speaker hooked up to an M2 16ohm, why does the amp still see 16ohms with this design? Kinda cool but I'm lost to why it does lol


----------



## JohnH

Hi @Yamariv

Great! those readings seem fine.

When you measure from the front end, like what the amp sees, its at least 80% the attenuator and hardly any of the speaker. And since your build is designed to use a 16 ohm amp tap, it stays within a couple of ohms of 16.

So should be ok. Just start with low amp volume and make sure its coming through to the speaker and that the switches are working to change the level as expected


----------



## Yamariv

JohnH said:


> Hi @Yamariv
> 
> Great! those readings seem fine.
> 
> When you measure from the front end, like what the amp sees, its at least 80% the attenuator and hardly any of the speaker. And since your build is designed to use a 16 ohm amp tap, it stays within a couple of ohms of 16.
> 
> So should be ok. Just start with low amp volume and make sure its coming through to the speaker and that the switches are working to change the level as expected



Awesome, thanks John! I'll update when I try it!


----------



## Osman Qureshi

JohnH said:


> ok great, I might try to rig something up equivalent here, then we'll have more confidence in values etc.
> 
> As an alternative, one thing I've been doing without having a dedicated line-out is to go from one of the attenuator speaker outs, into a line-in on a very small and cheap Behringer Xenix mixer to set levels, then from there to a laptop mic in.


Hi John, as I start building an 8-ohm M2 attenuator, I wanted to include “placeholder” line out for the future line out with cab sim that you are working on. When convenient, can you please check the attached drawing of the proposed placeholder line out to see if the connections are correct and the specifications of the resistor (R12) and the 10k potentiometer are also correct? And am I right to assume that given the inclusion of the potentiometer, the line out could also be used for headphones or even to connect into a mic in (like into an iRig Pro I/O)? Thanks, Osman


----------



## Gene Ballzz

@Osman Qureshi 
Might I politely suggest that if and when you (or anyone else) post any personal revisions of JohnH's original drawings, you include some sort of obvious identifier on said drawing, like *"Osram Version" ,"Modified by OQ"* or similar, to help avoid any confusion, down the road? I will leave it to @JohnH to comment in detail on your proposed line out!

I do however, like the idea of deriving the line out *"after"* the attenuation, instead of from the input, *"before"* the attenuation. My only concern might be that if fully attenuated, it may not leave enough signal strength to properly drive headphones. Maybe a "PRE/POST" switch to toggle between the output and just after the initial, first attenuation stage (just prior to R4) may be in order? I'll make no other comments, because of not knowing if such a *"line out"* may or may not have some sort of *"loading"* or other affect on the attenuation process.
Keep Up The Good Ideas & Work!
Gene


----------



## Osman Qureshi

Gene Ballzz said:


> @Osman Qureshi
> Might I politely suggest that if and when you (or anyone else) post any personal revisions of JohnH's original drawings, you include some sort of obvious identifier on said drawing, like *"Osram Version" ,"Modified by OQ"* or similar, to help avoid any confusion, down the road? I will leave it to @JohnH to comment in detail on your proposed line out!
> 
> I do however, like the idea of deriving the line out *"after"* the attenuation, instead of from the input, *"before"* the attenuation. My only concern might be that if fully attenuated, it may not leave enough signal strength to properly drive headphones. Maybe a "PRE/POST" switch to toggle between the output and just after the initial, first attenuation stage (just prior to R4) may be in order? I'll make no other comments, because of not knowing if such a *"line out"* may or may not have some sort of *"loading"* or other affect on the attenuation process.
> Keep Up The Good Ideas & Work!
> Gene


Makes a lot of sense is Gene. I should have put a warning. Thanks, Osman


----------



## JohnH

Hi @Osman Qureshi

Your line out could be OK, But the pot would have three connection lugs. The pot value is OK at 10K, but use a log pot for good control and use a 24mm diameter one for a bit more rating. The 2.2k resistor is fine but it needn't be more than 1/2W rating since there's very little current flowing through it.

Im still thinking about line-outs and plan to post soon - there are a lot of different uses and different levels needed so I'm trying to come up with recipes that would suit a wide range. Im thinking of post, pre, balanced, unbalanced, cabsim etc

What you show looks OK for a line out and should be adjustable down to a mic level with care. I dont think it will drive phones and in any case they will all sound bad until there is some form of cab sim, IR loader or post EQ is applied. But if you can get the signal into a PC, then all that is possible. 

In your case I think you have a laptop setup with just a combined headphone/mic socket? If I was you id get a very simple small mixer with USB out, like a Behringer Xenix 502 USB or similar, which gives you a bunch of useful mic and line ins and outs, plus a headphone out, level controls and a bit of EQ. I have an old non-usb version and its great. Its about $100. With it, I can get a clear recording directly out of the attenuator output speaker jack, and use the mixer to set the level.


----------



## Osman Qureshi

JohnH said:


> Hi @Osman Qureshi
> 
> Your line out could be OK, But the pot would have three connection lugs. The pot value is OK at 10K, but use a log pot for good control and use a 24mm diameter one for a bit more rating. The 2.2k resistor is fine but it needn't be more than 1/2W rating since there's very little current flowing through it.
> 
> Im still thinking about line-outs and plan to post soon - there are a lot of different uses and different levels needed so I'm trying to come up with recipes that would suit a wide range. Im thinking of post, pre, balanced, unbalanced, cabsim etc
> 
> What you show looks OK for a line out and should be adjustable down to a mic level with care. I dont think it will drive phones and in any case they will all sound bad until there is some form of cab sim, IR loader or post EQ is applied. But if you can get the signal into a PC, then all that is possible.
> 
> In your case I think you have a laptop setup with just a combined headphone/mic socket? If I was you id get a very simple small mixer with USB out, like a Behringer Xenix 502 USB or similar, which gives you a bunch of useful mic and line ins and outs, plus a headphone out, level controls and a bit of EQ. I have an old non-usb version and its great. Its about $100. With it, I can get a clear recording directly out of the attenuator output speaker jack, and use the mixer to set the level.


Thanks John. Great idea about the Behringer Q502USB. It retails for less than £50 here. But I think I'll wait for your design and be the first beta tester--or even the alpha tester. On the three connection lugs on the pot, one is input, one is output, and the third is ground, which I guess just gets connected to the ground line going back into the amp? Regards, Osman


----------



## acromarmot

Hi JohnH and others,

First of all :thank you thank you thank you!!

I'm currently building my second tube amp, based on a Friedman BE-100 with some minor changes and a different power amp [~45W out of 2xEL34]. For space reasons I designed a pcb rather than having it on turret or "handwired" (BTW today's Friedman's are not exactly handwired). But I don't think a well design board is necessarily worse than hand wired, same goes for the type of caps, etc...

As I'm looking for a good but simple yet not too expensive attenuator I stumbled across this forum.

I want to include the attenuator into the amp (as I don't like to play around with several boxes and cables to draw, I'm more the keep-it-simple approach).
For obvious reasons I'd include a bypass.

Using a small headshell space is an issue, too. So I'd love to use the design with just one coil if it's the same result as the two-coils-design. Read your post about the whole process but couldn't find the one-coil schematic, can you point me to it?

Did you wind the coil yourself or did you buy an existing one? How about the power dissipation of the resistors? I've seen the chart with the calculations and max 30W dissipation (@50W in), however still you guys use twice or even three times the power rated components. Did I miss something?
I don't want to include a fan (unless I'm forced to) and case mounting is possible but I 'm not sure how effective (it's not a real heat spreader/sink!)... Do you think it is a problem to go with 35-40W rated resistors when case mounted (heat vents provided as I already have them for the tubes)....

Thanks for your thoughts,

Cheers
Gunnar

Edit N°1: found the latest M2 schematic, will include the bypass myself

Edit N°2: will try to get the inductor from a speaker parts supply.

Edit N°3: to fit into my amp I'll probably create a small pcb with TO-247 housing resistors and contact foil instead of paste as well as a heat sink. I could provide kiCad project files in case someone's interested.


----------



## JohnH

hi @acromarmot , welcome to our thread. That sounds like a great project!

The one-coil design is called M2. It's posted in a few places but it's most easily found near the top of post 1 on page 1. (July 2020 update). It's performance is virtually identical to the previous two-coil version, M, also described in post 1.

You can certainly have a bypass switch, just be careful to not use it to switch 'on the fly', only change it when the amp is off or on standby. Maybe it could be visually separated from the other switches, by moving it away or with some markings? Treat changing it like changing a loudspeaker cable. The other stage switches in M2 you can change while running.

The power ratings I've been recommending allow for a 50W amp, overdriven by a further 50%, then with a factor of 2 on that. The rated values on power resistors are only valid with a particular area of very effective and infeasibly large heatsink. For what you're doing I'd keep with the stated ratings (eg R1 is 100W, R2A and B 50W and the others are 25W), and maybe fix them to something substantial and aluminium in your chassis, but maybe open not enclosed in a full case. With that, and with a 45 W amp, it shouldn't need a fan.

You can wind the coils yourself on a bobbin but you'd need an inductance-measuring meter to get it right. There are online inductance vs winding calculators but it'd be very unlikely be exact. These air coils are easily available all complete from places that sell loudspeaker components, where they are used in cross-over networks. We know of places in several countries, depending where you are.

Best wishes for your project.


----------



## acromarmot

JohnH said:


> hi @acromarmot , welcome to our thread. That sounds like a great project!
> 
> The one-coil design is called M2. It's posted in a few places but it's most easily found near the top of post 1 on page 1. (July 2020 update). It's performance is virtually identical to the previous two-coil version, M, also described in post 1.
> 
> You can certainly have a bypass switch, just be careful to not use it to switch 'on the fly', only change it when the amp is off or on standby. Maybe it could be visually separated from the other switches, by moving it away or with some markings? Treat changing it like changing a loudspeaker cable. The other stage switches in M2 you can change while running.
> 
> The power ratings I've been recommending allow for a 50W amp, overdriven by a further 50%, then with a factor of 2 on that. The rated values on power resistors are only valid with a particular area of very effective and infeasibly large heatsink. For what you're doing I'd keep with the stated ratings (eg R1 is 100W, R2A and B 50W and the others are 25W), and maybe fix them to something substantial and aluminium in your chassis, but maybe open not enclosed in a full case. With that, and with a 45 W amp, it shouldn't need a fan.
> 
> You can wind the coils yourself on a bobbin but you'd need an inductance-measuring meter to get it right. There are online inductance vs winding calculators but it'd be very unlikely be exact. These air coils are easily available all complete from places that sell loudspeaker components, where they are used in cross-over networks. We know of places in several countries, depending where you are.
> 
> Best wishes for your project.



Hi @JohnH,

thanks for your thoughts. Yeah, we're on the same page. Inductive meter I could use from 
my job but I'd buy the coils for laziness

No switching on the fly, that's for sure. Gonna put the switch to the rear close to the speakers selector.

In front of the tubes I have space left, ve ting directly to the top. I'll design the pcb to use a dedicated heat sink, as it's gonna pointing upwards like the tubes and transformers the heat ain't gonna ruin my amp caps. So that should work flawless...

As soon as the whole thing is done (currently I wait for the mainboard to arrive from etching) I'd maybe share the results here but up to now I don't have any recording equipment.... 

Cheers
Gunnar 

... sitting in Germany...


----------



## acromarmot

Hi again @JohnH,

I read somewhere you were addressing a little loss in treble and wanted to add a parallel cap to the later or last attenuation stage... Has this come to a solution yet?

btw, the loss of high frequencies with lower volumes has not so much to do with the non-linearity of our ears. This kills low end, i. e. when volume is down we cannot sense low frequency as before.

I guess(!) the loss in treble is a loss in harmonic distortion plus maybe a non-linear speaker behaviour or simply the fact the speaker is not moving enough.

Cheers 
acromarmot


----------



## TheOtherEric

acromarmot said:


> Hi again @JohnH,
> 
> I read somewhere you were addressing a little loss in treble and wanted to add a parallel cap to the later or last attenuation stage... Has this come to a solution yet?
> ...
> I guess(!) the loss in treble is a loss in harmonic distortion plus maybe a non-linear speaker behaviour or simply the fact the speaker is not moving enough.
> 
> Cheers
> acromarmot


Unless I missed something, I don’t think folks are finding this attenuator (unlike some others) to cause a loss of treble at higher attenuation. I’ve found that my amps lose treble at higher output (which seems to be normal) but that’s an amp-specific issue so I’m not sure I’d want to try compensating by monkeying with the attenuator design, although I guess it’s feasible.

Maybe there was a case of using the attenuator outside its intended use that was causing lost highs?


----------



## acromarmot

@JohnH, @TheOtherEric

OK, then most likely I misread sth. Nevermind.

Cheers
acromarmot

Edit: I re-read and it was @TheOtherEric having trouble with treble but what he detected to be caused by his amps.

@TheOtherEric : odd behaviour of your amps. Mine has some (other) issues but an amp losing treble when cranked sounds a bit strange to me. Usually an amp would respond with more harmonic distortion (overall distortion increasing) and thus enhancing treble, some even being sharp/causing hiss [therefore the anti-hiss-cap in the PI in some designs or the need to reduce presence pot when cranked]. As with more volume our ears are more sensitive to low frequencies this would in theory also be an explanation... But then the bass needs to be brutal and washed to interpret this as muffled.
Just because I'm curious and play around with amp designs... What amps are you using ('cause I wanna learn sth)? 

Cheers 
acromarmot


----------



## JohnH

This design is balanced so that tone stays constant all the way down, which is different to many other passive attenuators which often lose a bit of treble due to damping of the speaker at low volume, and so they need some treble adjustment. 

Of course some may wish to adjust treble though. A cap across the series resistor on a later stage would be OK, but personally Id never add this myself. The way I view it, the attenuator has a simple job of delivering the tone of a loud amp, at lower volume with no change, so I prefer to just let it keep it doing that.

If there is a presence control on the amp, that is likely to be the best place to get the kind of adjustment that might be wanted.


----------



## acromarmot

JohnH said:


> This design is balanced so that tone stays constant all the way down, which is different to many other passive attenuators which often lose a bit of treble due to damping of the speaker at low volume, and so they need some treble adjustment.
> 
> Of course some may wish to adjust treble though. A cap across the series resistor on a later stage would be OK, but personally Id never add this myself. The way I view it, the attenuator has a simple job of delivering the tone of a loud amp, at lower volume with no change, so I prefer to just let it keep it doing that.
> 
> If there is a presence control on the amp, that is likely to be the best place to get the kind of adjustment that might be wanted.


@JohnH yeah, nevermind. I also don't want to add a treble bypass cap since I found out @TheOtherEric has a problem with the Amp rather than with your attenuator.

Thx and good night!
acromarmot


----------



## TheOtherEric

acromarmot said:


> @JohnH, @TheOtherEric
> 
> OK, then most likely I misread sth. Nevermind.
> 
> Cheers
> acromarmot
> 
> Edit: I re-read and it was @TheOtherEric having trouble with treble but what he detected to be caused by his amps.
> 
> @TheOtherEric : odd behaviour of your amps. Mine has some (other) issues but an amp losing treble when cranked sounds a bit strange to me. Usually an amp would respond with more harmonic distortion (overall distortion increasing) and thus enhancing treble, some even being sharp/causing hiss [therefore the anti-hiss-cap in the PI in some designs or the need to reduce presence pot when cranked]. As with more volume our ears are more sensitive to low frequencies this would in theory also be an explanation... But then the bass needs to be brutal and washed to interpret this as muffled.
> Just because I'm curious and play around with amp designs... What amps are you using ('cause I wanna learn sth)?
> 
> Cheers
> acromarmot


To be clear, there’s nothing odd or wrong with my amps; the reduced treble at high output can be easily compensated for (by simply turning the EQ knobs) until up near really high output and max attenuation (which isn’t how I want to run them anyway). I wasn’t aware of that issue at first yet tried running max attenuation, hence my initial displeasure. The amps are both Ceriatones — Son of Yeti (hot-rodded Plexi) and OTS 20 (Dumble style). Both are 20W 2x6V6 and have Bright switches and Presence adjustments that solve the issue. Nik confirmed that it’s normal to need to add treble in general as amp output goes up. The amps really sing now.


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## Gene Ballzz

@JohnH ,
I'm knee deep in my second *"16Ω, M-lite"* build, custom tailored/configured for mounting in my DSL20CR. The intention is to have the switches accessible through the opening in the back of the cabinet, as indicated by the blue arrow, in the pic below. I'm including a full bypass switch, a facility for utilizing the attenuator in parallel (yes, I know amp output impedance must get switched for parallel use), and an additional output jack from the attenuator that shuts off the combo's internal speaker when plugging in the additional speaker.

I curretly have the enclosure all drilled and most components mounted, but before continuing, finalizing and wiring, I want to confrim a few items:
_*A> *_Bypass switch.
_*1> *_IIRC, the switch needs to be a DPDT that switches both of the positive wires asshown in the modified diagram?
_*2> *_Can I assume that only the two positive leads of the circuit need to be switched simultaneously for safe operation? The switch is two position ON/ON, DPDT, rated rated for 6A/125V-3A/250V.
​_*B> *_Parallel operation*.*
*1>*I have determined that the easiest and safest way to incorporate a parallel use option is to simply have two input jacks, wired in parallel. To utilize the option, you simply unplug the speaker form the output jack and plug it into the other parallel input jack. This allows for use with an amp that only has one speaker output with a switch for selecting the output impedance. Do you see any problems with this arrangement?
_*2> *_I'm still trying to work out how to use the switching functions of the jacks to facilitate the extension cabinet jack to also operate in parallel mode, without complicating things too much. This may not be a truly feasible option.​Here's a quick pic of the current build. I still need to mount the chokes and resistors on the lid and all the internal jacks will be on one end plate on the right. Just waiting to get all the rest done to choose my positioning. FWIW, I have drilled and tapped the box for #6-32 machine screws, to avoid nuts.

And in case no one has noticed, I put all of the first *"always active"* stage components on the *"lid"* portion, to allow that "lid" portion to be used as a stand alone, -7db unit. I think I've decided to standardize all my builds in this manner, allowing it to be somewhat modular. I also think it may facilitate being the basis of a starting *"platform"* and minimizing wiring long runs, etc.

Thanks Again!
Gene


----------



## JohnH

Hi Gene, all sounds like a good plan, no problems, just a few comments:

*Bypass switch* - I agree its a dpdt, 6 Amp at 125 Vac is fine, switching just the upper side of the circuit as you show. But, I reckon there may be a better way to wire it!:

Instead of on your diagram, a single blue wire taking the bypass from switch half A to B, have two wires each connecting from a pole on one half to the relevant outer lug on the other half. This way, in bypass mode, instead of two switches in series you have two in parallel, cutting any contact resistance and increasing reliability. On the diagram, this will look like two long blue wires, but really this is just local across the switch. There's no difference to contacts when the attenuator is engaged. No downsides or cost, just slightly better maybe! 

*Parallel inputs* - seems fine to me, if that works best with your set up. So you plug the speaker into the other input jack, with e the first being to the amp. Then set attenuation to max so its like a load box with o speaker of its own. Halve the amp output tap Ohms, and you have a handy -3db setting. 

*Parallel Outputs* - to get the action of combo speaker cutting out when you insert a cab, I assume you're using a switched jack where input wires come in from one side and the internal speaker is connected to to the connections across the other side of the jack, and when you push in the new plug it disconnects the internal and replaces it. If you also want the option of having the internal and external cab together in parallel, then that sounds like it could involve another non-switched jack, wired to a attenuator output. Plug into that and the combo speaker stays connected. Adjust amp tap if you are also in full bypass mode.


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## Gene Ballzz

@JohnH ,
Regarding the bypass, I'm not certain I understand the arrangement you're speaking of? A quick drawing may help?

I will not ever have any desire to use another speaker or cabinet paralleled with the internal open back speaker. Instead I am trying to visualize how to set up the two output jacks to still have the external jack turn off the internal, even in parallel mode. I'll need to draw it out and see if I can figure that out.

Thanks Again Sir! 
Gene


----------



## diego_cl

I've just found this thread and I'm very grateful, because I was looking for a quality DIY attenuator since THD Hotplate became a thing.

Would it be too much difference building one with cheap Aliexpress resistors instead of Arcol resistors from Mouser ???


Sorry, but I'm not allowed to post any links, because my account is new.


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## Gene Ballzz

diego_cl said:


> I've just found this thread and I'm very grateful, because I was looking for a quality DIY attenuator since THD Hotplate became a thing.
> 
> Would it be too much difference building one with cheap Aliexpress resistors instead of Arcol resistors from Mouser ???
> 
> 
> Sorry, but I'm not allowed to post any links, because my account is new.



*First,*  to the forum, as well as the liberation that this realy fantastically designed attenuator will provide you! What part of this crazy planet are you located in?

*Next,* many of us use the cheap crap from Chinesiawanoreanam and I get mine on ebay. I've been pleasantly surprised by the close tolerances! Make sure you use good switches and *ONLY* Cliff UK jacks! This is the best operating attenuator out there, and the fact that you can collect all the materials for $75 to $120 USD, as opposed to +$500 for anything else even comparable is just icing on the cake!  And yeah, I like my beer!

Just My $.02,
Gene


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## JohnH

I like beer too.

Yes the Chinese resistors work fine, they are simple and robust components, you just need patience to wait for the boat to arrive.

The key thing is to find the values that we need. Some of the ranges are comprehensive and others are more limited.


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> Regarding the bypass, I'm not certain I understand the arrangement you're speaking of? A quick drawing may help?
> 
> I will not ever have any desire to use another speaker or cabinet paralleled with the internal open back speaker. Instead I am trying to visualize how to set up the two output jacks to still have the external jack turn off the internal, even in parallel mode. I'll need to draw it out and see if I can figure that out.
> 
> Thanks Again Sir!
> Gene



Heres the other bypass idea:



On the parallel out jacks, just have two wired in parallel for running two cabs, but the combo speaker is wired off the switched lugs the other side of one of them. Then you can have all the options.


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## Gene Ballzz

@JohnH ,
Thanks for that! Now I see what you meant.

And while it makes no difference electrically, all the jacks will be next to each other, facilitating some wiring convenience.
Thanks Again!
Gene


----------



## Osman Qureshi

Gene Ballzz said:


> @JohnH ,
> Thanks for that! Now I see what you meant.
> 
> And while it makes no difference electrically, all the jacks will be next to each other, facilitating some wiring convenience.
> Thanks Again!
> Gene


Hi Gene, what is your preferred Chinese supplier for resistors, and if I may ask it here, what speaker do you prefer with your dsl20?Thanks, Osman


----------



## Gene Ballzz

I got many of them here:

https://www.ebay.com/usr/zhany-m?_trksid=p2057872.m2749.l2754

But I used a couple other vendors along the way. Unfortunately, ebay history doesn't go any farther back than 2019! Its a bit of a pain to find the best deal on all the values from just one place! If I can find an email, PayPal or ebay message with other vendors' names, I'll be sure to let you know.

The speaker is currently a 1969 dated, pre-Rola, G12M/25 watt/75Hz Greenback. That speaker will eventually need to go back into its original 1960B cabinet that I'm refurbishing. I'll likely go with a current production Greenback of same specs, although I've found the 65 watt Creamback to sound similar enough, so I may go with that for the flexibilty factor of swapping it around. I spend enough time playing that break-in shouldn't be too tough! 

Happy Buildin' & Attenuatin'
Gene


----------



## JohnH

This seller from china was found for us by @TheOtherEric . Here's a link for all the values at 25W

25W 0.1-100k Ohm Shell Power Aluminum Housed Case Wirewound Resistor New | eBay

That has the full range, but, today Im only seeing it at 25W not 50 or 100W.

Other typical sellers, such as Gene posted, do pretty well up to 33 Ohm, then go 50 and 75 and 80, so there's a few missing values particularly for the R2A and R2B on 16 Ohm design M2 which need to be around 39 and 47. Within each stage of the attenuator, you can vary values slightly but should keep ratios intact. Eg, instead of 39 and 47 each 50W, you could use the values from the 8 Ohm version with two in series, so 2x 22 and 2x 18 in series, all 25W, and get them all at the link above. Would still need to find R1.

Also, in a couple of cases, Ive taken a resistor which is a bit high and put it in parallel with a much higher value ceramic power resistor to tweak the value to where I wanted it.


----------



## Guitar-Rocker

Gene Ballzz said:


> @JohnH ,
> Regarding the bypass, I'm not certain I understand the arrangement you're speaking of? A quick drawing may help?
> 
> I will not ever have any desire to use another speaker or cabinet paralleled with the internal open back speaker. Instead I am trying to visualize how to set up the two output jacks to still have the external jack turn off the internal, even in parallel mode. I'll need to draw it out and see if I can figure that out.
> 
> Thanks Again Sir!
> Gene





Very nice layout Gene. Hopefully my try will end up nearly as nice


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## acromarmot

Hi @JohnH

I assume the ohm resistance of the coil is pretty irrelevant, right? As I'm doing it on a pcb I could use a smaller coil with core instead of the air coil? I'd choose something like 1mH / 5A or even more current to make sure the core ain't gonna saturate...

As resistors I'd use TO-220 thick layer resistors or alike. All values available but 5.6R and 18R. 18R I'd build out of 3*56 in parallel. 5.6R I'd replace with 5R, or should I fit in another 0.5R?

The 15R/100W I'd build from 3*5R/50W.
Edit: with 56/3=18,667 another 47/3=15,667 would even come closer to with the same amount of resistors. So I'd use that. 
What do you think?

Cheers
acromarmot


----------



## JohnH

acromarmot said:


> Hi @JohnH
> 
> I assume the ohm resistance of the coil is pretty irrelevant, right? As I'm doing it on a pcb I could use a smaller coil with core instead of the air coil? I'd choose something like 1mH / 5A or even more current to make sure the core ain't gonna saturate...
> 
> As resistors I'd use TO-220 thick layer resistors or alike. All values available but 5.6R and 18R. 18R I'd build out of 3*56 in parallel. 5.6R I'd replace with 5R, or should I fit in another 0.5R?
> 
> The 15R/100W I'd build from 3*5R/50W.
> Edit: with 56/3=18,667 another 47/3=15,667 would even come closer to with the same amount of resistors. So I'd use that.
> What do you think?
> 
> Cheers
> acromarmot



I also considered cored inductors and the thick film resistors. I didnt go there, which doesn't mean it wouldnt work, but just watch out.

The inductor is working in a similar way to those in loudspeaker crossovers, in terms of values, frequencies and power and these are generally air coil except for the subwoofers. Also, I read this thread about dummy loads, which you've probably seen:

Aiken's Reactive Dummy Load. | The Gear Page 

where air coils are used for the treble inductor - so I followed that advice to reduce variables. But if you are confident to work with saturation currents then I cant see a problem and it will be interesting to see how it goes - we havent tried this here. The DCR looks like it will be less than for an air coil, and thats fine. I allow for the DCR in my calcs (1/2 ohm or so), but less is not a problem. 

For R2A and R2B, if you are using three in parallel, id say 3x56 and 3x68 would be fine.
For the 5.6, 5 or 6 would work and sound fine but the evenness of attenuation steps may be a bit off 

Those thick film resistors - i just didnt want to tackle them. I looked at some specs. The rated power was only if they are kept at 25C. At 100C the ones I saw were at about 30% and zero% at 150C. So its all going to be about cooling and since the idea is to be compact, you may be thinking of small with less heatsink mass and area. Being small, theyll heat up quickly and if they go, they could hurt the amp. So just watch out, and good luck!


----------



## acromarmot

JohnH said:


> I also considered cored inductors and the thick film resistors. I didnt go there, which doesn't mean it wouldnt work, but just watch out.
> 
> The inductor is working in a similar way to those in loudspeaker crossovers, in terms of values, frequencies and power and these are generally air coil except for the subwoofers. Also, I read this thread about dummy loads, which you've probably seen:
> 
> Aiken's Reactive Dummy Load. | The Gear Page
> 
> where air coils are used for the treble inductor - so I followed that advice to reduce variables. But if you are confident to work with saturation currents then I cant see a problem and it will be interesting to see how it goes - we havent tried this here. The DCR looks like it will be less than for an air coil, and thats fine. I allow for the DCR in my calcs (1/2 ohm or so), but less is not a problem.
> 
> For R2A and R2B, if you are using three in parallel, id say 3x56 and 3x68 would be fine.
> For the 5.6, 5 or 6 would work and sound fine but the evenness of attenuation steps may be a bit off
> 
> Those thick film resistors - i just didnt want to tackle them. I looked at some specs. The rated power was only if they are kept at 25C. At 100C the ones I saw were at about 30% and zero% at 150C. So its all going to be about cooling and since the idea is to be compact, you may be thinking of small with less heatsink mass and area. Being small, theyll heat up quickly and if they go, they could hurt the amp. So just watch out, and good luck!


Hi @JohnH,

Yeah, Aiken's posts and his website were great help with various projects.
I might use air coils too if I find a way to fix them correctly on the pcb. I'm not a big fan of plastic bolts as I see them fail regularly at work when units are carried and tossed around (however with my amp that wouldn't be a big problem as it most likely will not leave the house).

Regarding the resistors I also worry a bit about the heat especially as I have to mount several on the same heat sink.
I think I'll try it with passive cooling on two heat sinks.
I'll then measure max temp with an IR-camera. If it's too hot and for a possible 100-version in the future I'd like to add a whisper fan. Via some diodes and supercap I could power it from the amps output (say once one activates the - 14dB switch) allowing for active cooling whithout ext PSU.

Ah, that's gonna be exiting 

Cheers 
acromarmot


----------



## matttornado

Gene Ballzz said:


> @Osman Qureshi
> Might I politely suggest that if and when you (or anyone else) post any personal revisions of JohnH's original drawings, you include some sort of obvious identifier on said drawing, like *"Osram Version" ,"Modified by OQ"* or similar, to help avoid any confusion, down the road? I will leave it to @JohnH to comment in detail on your proposed line out!
> 
> I do however, like the idea of deriving the line out *"after"* the attenuation, instead of from the input, *"before"* the attenuation. My only concern might be that if fully attenuated, it may not leave enough signal strength to properly drive headphones. Maybe a "PRE/POST" switch to toggle between the output and just after the initial, first attenuation stage (just prior to R4) may be in order? I'll make no other comments, because of not knowing if such a *"line out"* may or may not have some sort of *"loading"* or other affect on the attenuation process.
> Keep Up The Good Ideas & Work!
> Gene


----------



## acromarmot

acromarmot said:


> Hi @JohnH,
> 
> Yeah, Aiken's posts and his website were great help with various projects.
> I might use air coils too if I find a way to fix them correctly on the pcb. I'm not a big fan of plastic bolts as I see them fail regularly at work when units are carried and tossed around (however with my amp that wouldn't be a big problem as it most likely will not leave the house).
> 
> Regarding the resistors I also worry a bit about the heat especially as I have to mount several on the same heat sink.
> I think I'll try it with passive cooling on two heat sinks.
> I'll then measure max temp with an IR-camera. If it's too hot and for a possible 100-version in the future I'd like to add a whisper fan. Via some diodes and supercap I could power it from the amps output (say once one activates the - 14dB switch) allowing for active cooling whithout ext PSU.
> 
> Ah, that's gonna be exiting
> 
> Cheers
> acromarmot



OK, the risk (that it's not gonna work out) and the price (30€ for all the wound resistors 1% tolerance w/ all needed values directly available from a decent supplier compared to almost 70€ for the thick film ones) made me change my plans. I'll bolt them to a heatsink and the heat sink to the amp chassis and then some silicone wires and I'm done...

Seems the cheapest and most solid solution...


----------



## Yamariv

JohnH said:


> Hi @Yamariv
> 
> Great! those readings seem fine.
> 
> When you measure from the front end, like what the amp sees, its at least 80% the attenuator and hardly any of the speaker. And since your build is designed to use a 16 ohm amp tap, it stays within a couple of ohms of 16.
> 
> So should be ok. Just start with low amp volume and make sure its coming through to the speaker and that the switches are working to change the level as expected



@JohnH you are the Friggen MAN!! I got around to building my second attenuator and I just tried them both out this morning and they work amazing right out of the box, seems like I have all the switches working well which is rare for me first go! Lol!

On the sound note, this is the first time I've heard my little home brew 5 Watt EL84 Class A amp at full distortion and it sounds awesome! All the DB switches do and sound like they claim, very cool! I can get it down to whisper quiet. Haven't tried it on any bigger amps yet..

I've attached a pic of one of the attenuators from the outside but I won't post my wiring inside as it's a little bit embarrassing compared to the other M2's I've seen posted here. If I were to redo another one, I'd have taken more time on placement to ensure my wire runs are short. 

Anyway, here it is and thanks again John for designing this! Your effort and time is very much appreciated!


----------



## Yamariv

Just curious, what's the DB attenuation when no switches are activated on the M2?


----------



## TheOtherEric

Yamariv said:


> Just curious, what's the DB attenuation when no switches are activated on the M2?


It’s -7 db with no switches activated.


----------



## Gene Ballzz

@Yamariv 
Yes, @JohnH is definitely *"the Friggen MAN"* on this design! And @TheOtherEric is correct about the -7db. On the other hand, minimum attenuation can be cut in half (to -3.5db) if you set the unit for *"maximum attenuation/all stages activated" *and run it in parallel with the output from the amp to the speaker. The caveat here is that the amp needs to be set for half the impedance. For example: If you have a 16 ohm attenuator and a 16 ohm speaker, when in parallel the amp needs to be set/tapped for 8 ohm output. If the attenuator is an 8 ohm unit with an 8 ohm speaker the amp needs to be set for 4 ohm, if capable. Many amps are not set up to drive a 4 ohm load! This means that if your unit is an 8 ohm build, you cn not use it in parallel with an amp not capable of 4 ohm operation. This is *ONLY* for parallel operation! This is one of the reasons I've chosen to build all 16 ohm units, as amps with both 8 & 16 ohm settings are a bit more common than amps capable of 4, 8 & 16 ohms!

Then a question: What did you use for the labeling on your attenuator?

Nice Job!
Gene


----------



## JohnH

hi @Yamariv yes that does look nice! I like the red colour (assuming thats paint and its not overheating red-hot! LOL)

When your working compactly, and there's necessarily a whole bunch of wires, I also find it hard to keep it looking neat, even with careful planning. But if the solder joints are good, then its good.

Gene's pointed out a handy work-around if you want to reduce that -7db minimum. Mostly we dont need it though. The basic first stage value -7db is just one step of reduction, not that much audibly, even though its a factor of x 1/5th on power.


----------



## Yamariv

Gene Ballzz said:


> @Yamariv
> Yes, @JohnH is definitely *"the Friggen MAN"* on this design! And @TheOtherEric is correct about the -7db. On the other hand, minimum attenuation can be cut in half (to -3.5db) if you set the unit for *"maximum attenuation/all stages activated" *and run it in parallel with the output from the amp to the speaker. The caveat here is that the amp needs to be set for half the impedance. For example: If you have a 16 ohm attenuator and a 16 ohm speaker, when in parallel the amp needs to be set/tapped for 8 ohm output. If the attenuator is an 8 ohm unit with an 8 ohm speaker the amp needs to be set for 4 ohm, if capable. Many amps are not set up to drive a 4 ohm load! This means that if your unit is an 8 ohm build, you cn not use it in parallel with an amp not capable of 4 ohm operation. This is *ONLY* for parallel operation! This is one of the reasons I've chosen to build all 16 ohm units, as amps with both 8 & 16 ohm settings are a bit more common than amps capable of 4, 8 & 16 ohms!
> 
> Then a question: What did you use for the labeling on your attenuator?
> 
> Nice Job!
> Gene



Hey Gene, wow that parallel stuff sounds a little complicated, I'd probably end up screwing something up with that hook up! Lol.... I did notice a bit of an issue for my setup by building the 16ohm versions(wish I had done the 8ohm's). I can't use the attenuators for one of my amps that only has a 4 and 8ohm output tap  While using my M2 16ohms, is there any sneaky way to safely run an 8ohm output into an 8ohm cab? Probably a dumb question with an definite answer of "NO" but I figured I'd ask incase 

For the decals it's a few step process but works well once you figure out the system. First I printed onto decal sheets from my computer. You then have to use clear coat spray paint (lightly misting it over a couple of coats) to seal in the ink onto the paper. If you don't do this, the ink will run when you cut out the decal and put it in water and try to stick it on. After the decals are stuck on, let them dry then mist a few few coats of clearcoat over them so they get sealed in and won't rub off. 

Note** With the clear coat I've been using, you can never put a heavy coat ever!..If you do, it will make the decal's ink run.Also I've gotten bad paint reactions with the basecoat I'm drying to clear over if I put too much of a thick coat. Not sure if it's the brand of clear I've been using but it's a royal pain sometimes and I have messed up a few pain jobs


----------



## Yamariv

JohnH said:


> hi @Yamariv yes that does look nice! I like the red colour (assuming thats paint and its not overheating red-hot! LOL)
> 
> When your working compactly, and there's necessarily a whole bunch of wires, I also find it hard to keep it looking neat, even with careful planning. But if the solder joints are good, then its good.
> 
> Gene's pointed out a handy work-around if you want to reduce that -7db minimum. Mostly we dont need it though. The basic first stage value -7db is just one step of reduction, not that much audibly, even though its a factor of x 1/5th on power.



Haha, no it's definitely just red paint  I haven't had the chance to really get them cooking through a bigger amp yet but I'm sure it will stay relatively cool with all the holes I drilled and the large space underneath for airflow.

After building the first one, I was a bit worried as some of the lenghts of wire could have been shortened a bit. But now playing through it, I don't notice any hum or noise difference from the second one with a bit tighter wiring so it's all good! Solder joints are all definitely well done 

No worries about the 7db initial attenuation, it works perfect for me I was just curious what it was. No need to cut it in half at all for my purposes, I could just tell it was cutting some sound, just wasn't sure how much


----------



## JohnH

Hi @Yamariv ,

Short wire lengths and neat wire dress are always a good idea, but its not really so critical in these attenuators as separate units because the high currents in the circuit are nowhere near sensitive amp input circuitry like in an amp.

If you have a 16 Ohm attenuator and want to use it with an 8 Ohm amp tap, and don't want to risk the mismatch, we can put another 16 Ohm load in parallel with the input. This can be just a resistor (15 or 18 Ohm is fine), which is safe, but better as another stripped-down reactive stage which would also have another coil and a second resistor. I'll post more on that.


----------



## JohnH

*Variable amp Ohms*

This is about how to adapt a given M or M2 attenuator version to handle different amp taps. I did this version a few months ago, which turns an 8 Ohm M2 build into one that can also work with 16 or 4 ohm taps. It works by adding another 8 Ohm reactive load in series or in parallel at the front. This could be done with switches but it can also be neatly done with switched TRS jacks as shown below. It also shows values for taking a base 4 ohm version and adapting it to 8 ohms, or to 2 ohms (for certain old Fender amps).













Attenuator M2v 200420



__ JohnH
__ Sep 20, 2020






When the different ohms are used there's an added -3db cut, but tone stays the same.

The diagram doesn't directly cover the case of a 16 Ohm build, and converting it to 8 Ohms, but the same ideas apply, with all values x2. So you can take a 16 Ohm M2 build, and add the three extra front end parts and another jack. Now the new parts will be, using nearest common values:

*Variable input ohms, for 16/8 using a 16 Ohm M2:*
L2 = 1.2 mH
R12 = 15 Ohm
R13 = 47 Ohm

With this, the full M2V diagram would imply a 32 Ohm input option too, but there's no use for this, so just ignore the top left jack.

Instead of adding another jack, the three added parts could be engaged with another toggle switch.


----------



## diego_cl

Gene Ballzz said:


> *First,*  to the forum, as well as the liberation that this realy fantastically designed attenuator will provide you! What part of this crazy planet are you located in?



Thanks! I'm from Chile and I like beer too, so much that I brewed my own for a couple of years until my former beer buddy bought my brewing stuff and opened a small brewery

I've a Marshall MA50C (50 watts - ECC83/EL34), pretty cheap and bad quality amp. The dirty channel is unusable... a Boss DS1 would be warm, smooth, creamy and all those buzzwords compared to this cringe sounding channel.

But the clean channel sounds nice at edge of breakup, just some harsh overtones when using single coils that are tamed when using humbuckers. So I'm gonna build one of this attenuators, because with this amp, edge of break up is on vol 6 with humbuckers and on vol 9 with single coils, while edge of good-neigbourhood is at vol 3.

I've a lot of questions I want to settle before buying parts.
A FAQ and a BOM would be nice for this proyect, because 76 pages is a lot to study while the project, despite being incredible, is a very simple design (just like the title of this thread)


----------



## diego_cl

I'd be very gratefull if someone help me clearing some doubts!

Does the inductor design matter?? or only the values matter (gauge and inductance)?
I've seen different designs on aliexpresss and there is some cost differences depending on the design.
I don' get why the inductor with the plastic cover is cheaper than the inductor with the plastic zip ties; both are from the same seller and I don't know if the enamel is the same with both designs.




For attenuating a 16 ohm combo... the M2 8 ohm version, built with the 16 ohm optional output would work just as good as the M2 16 ohm version, but 3dB quieter?
If so... the only advantage of building the 16 ohm version (instead of M2 8 ohm with 16 ohm output) is fewer parts and more volume?

The biggest aluminum case I can get is a 1590D (7.38 x 4.7 x 2.05" or 119 x 187 x 52 mm).
I saw jejj21 used a 1590D case for a lower wattage amp and he was very kind sharing his layout. Would the mass of this case be big enough as a heatsink for a 50 watts amp?

I can't get a 5R7 resistor. How does the sound change replacing it with a 6R resistor? Would the change be audible?

Jacks:
- Why do they need to be isolated?
- Attenuator M2 (except output 3): Why stereo jacks? Because they have more grip and mass for heat dissipation?


----------



## JohnH

hi @diego_cl ,

Those coils, if both from the same wire, inductance and also given they are both air cored (not iron or ferrite core), should work the same for us.

That case should be OK, its about the same size as what I used. The key thing about the case is having enough space to fit it in and wire it easily. Youll have to plan it out when you get the parts. The mass of the case should be fine

If you cant get a 5.6 resistor, you can use a 6 instead. There would be no difference in the tone, just a very small difference in the attenuation of that stage. I did this, and then just to adjust it I put a 5W 82Ohm ceramic resistor in parallel with it. Not really needed though, but I wanted it as close as possible for my testing.

With the jacks, plastic bodied (or fully isolated) is best. There is no real ground, the input wires from the amp are two output windings of the OT, usually neither is ground, so we dont want them live to the case although it would probably still work in most cases.

Yes the stereo jacks are just my preference based on the ones i buy. We dont have to use the ring connection for the main input and outputs on M2. Its used in the extra 16 ohm output on an 8 Ohm M2, and also for the different input options on M2v, see yesterdays post.

Not many people need to use a 4 Ohm cab or amp, and most amps have different output taps so if its a choice between an 8 ohm or a 16 Ohm version and you have both at the amp, Id build whichever suits your main cab or speaker.


----------



## Yamariv

JohnH said:


> Hi @Yamariv ,
> 
> Short wire lengths and neat wire dress are always a good idea, but its not really so critical in these attenuators as separate units because the high currents in the circuit are nowhere near sensitive amp input circuitry like in an amp.
> 
> If you have a 16 Ohm attenuator and want to use it with an 8 Ohm amp tap, and don't want to risk the mismatch, we can put another 16 Ohm load in parallel with the input. This can be just a resistor (15 or 18 Ohm is fine), which is safe, but better as another stripped-down reactive stage which would also have another coil and a second resistor. I'll post more on that.



Hey John!

Very cool updated M2 layout you just posted, looks like it's gonna be the future all in one Attenuator! I had a few more hours to play with my M2 16ohm and again I must say it sounds awesome, I've never heard my 5E3 clone sound so good! I have a great master volume on it but when comparing the master vs fully cranked there's a BIG difference! Cranked sounds ridiculous through the attenuator! 

So, I was looking last night at ordering all the parts to make a couple 8ohm M2 versions so I can run my one amp and any future 8ohm out only amps and was thinking. Could I modify M2 16ohms versions to have a 8ohm option without building a whole new attenuator. I see you alluded to it on a previous post but do you know what parts I need to add and what coil I'd have to order to do this? 

Thanks again


----------



## chas.wahl

JohnH said:


> With the jacks, plastic bodied (or fully isolated) is best. There is no real ground, the input wires from the amp are two output windings of the OT, usually neither is ground, so we dont want them live to the case although it would probably still work in most cases.
> 
> Yes the stereo jacks are just my preference based on the ones i buy. We dont have to use the ring connection for the main input and outputs on M2. Its used in the extra 16 ohm output on an 8 Ohm M2, and also for the different input options on M2v, see yesterdays post.



@JohnH: Having designed an attenuator box around your M2 design, and then found the M2v by way of the TDPRI forum, I have a couple questions:

1. Your diagram shows an instance of "no join here", but it seems to me that there are several locations where wiring is crossing without joining. I've pointed the likely ones out (along with that one noted) on this JPG with red circles, and put a couple question marks by another juncture; could you please confirm that the circles are "no join heres" and the question marks are a bona fide connection? (The latter appears to me to be necesary to include the resistor/switch network for increasing attenuation.)




2. The use of Cliff or Rean plastic-bodied jacks, and your convention for showing connections to them confuse me a bit. Those are naturally isolated from a chassis, unlike the Switchcraft metal ones I typically use (for which non-conductve bushing/washer are needed). I presume that, even though you've shown connections to elements shaped like an arrow (which in a Fender schematic, for instance, would represent a "shunt" or "switch" that is normally closed until the plug is inserted), those are actually the plug's connection to tip, sleeve, or for a stereo plug the ring. For the M2 circuit (not the one I've posted here, but its predecessor) the tip and sleeve would be the only connections used at both input and output, correct, and the jacks are presumed to be isolated from the chassis entirely, so that, as you say, the attenuator box is not "live" in any way. That is, there is no difference between the diagram's "ground", the sleeves of both input and output, and the connections you indicate nearest the sleeves at the jacks in the diagrams -- they're connected as part of the jack.




For the M2v variant, however, it looks like there's a difference between the jack connection nearest the sleeve and the sleeve itself at the 4 ohm input and the 16 ohm output (where those sleeves are shown connected to all the other input and output jacks, as well as the "ground" of the of the resistor network), implying that both a stereo jack and a stereo plug are required. I am not sure that a stereo *plug* is, in fact, necessary in either case, since the circuit itself does not appear to use the sleeves for anything except continuation of the "ground". It seems to me that it doesn't matter if the sleeves shown for the 4 ohm input and 16 ohm output are connected to anything at all; or if one simply used the sleeve (isolated from the chassis) in each instance, as the connection to the circuit, and omit connection of the sleeve to the other jack sleeves. Put another way, it seems to me that the diagram would be much easier to follow if you consistently used the connection at each jack closest to the sleeve (representing a connection to the _sleeve of an isolated mono *plug*_) and never showed the jack sleeve connected to anything at all. If I am in error, please explain. Here is an example of what I'm suggesting (with an additional "no join here"):




Does that represent a correct understanding of the intent of your diagram?

Thanks,
-- 
chas.wahl

PS: What I mean by a jack in a Fender schematic:


----------



## JohnH

Yamariv said:


> So, I was looking last night at ordering all the parts to make a couple 8ohm M2 versions so I can run my one amp and any future 8ohm out only amps and was thinking. Could I modify M2 16ohms versions to have a 8ohm option without building a whole new attenuator. I see you alluded to it on a previous post but do you know what parts I need to add and what coil I'd have to order to do this?
> 
> Thanks again



Sure, I listed the added parts to make an 8 Ohm input for a 16 Ohm M2 in post 1509 above. You can just put a single-pole switch to connect it in parallel to the input for 8 ohm use.


Hi @chas.wahl you are correct in interpreting my diagrams. I don't have any proper software for drawing these, so the diagrams are done in Word, which can get fiddly for making them look like normal conventions. In general if i show two wires the same colour meeting, then they join, if they cross and are different colours, then they don't join. That M2v was put together out of earlier diagrams and needs a redraw before I put it on post 1, but the info is OK.

I think for most users, the basic M2 version is best, its the simplest to build with the fewest parts. Most users would be able to pick 16 Ohm or 8 Ohm to best suit their amps or amps. But having variable input ohms could be a useful workaround for some, at the expense of another -3db so you are then at -10db as the loudest attenuated level.


----------



## Yamariv

Hey John, sorry I misunderstood your original reply in the post. So after re-reading it several times (brain is not always the best at technical..) you're not saying I need a new reactive load circuit to add to my M2 16ohm to run an 8ohm amp out then right? You're saying just literally at another 15-18ohm resistor in parallel with the input with an On/Off(?) style switch and that's it? It will run through the reactor load and sound just as good and be safe too?

If that's the case then i'll definitely do that! Since I'm not very technical, any chance I could bother you do draw a quick pencil schematic of the input circuit and the switch just so I don't mess something up. I've pretty much got it I think but schematic drawing is not my thing yet..I'm working on it though..


----------



## JohnH

hi @Yamariv

Its safe with just a resistor, but it could spoil some of the reactive tone since it then becomes 50% just a resistive attenuator. So I'm suggesting three added parts

*Variable input ohms, for 16/8 using a 16 Ohm M2:*
L2 = 1.2 mH 18 gage aircoil
R12 = 15 Ohm (100W)
R13 = 47 Ohm (25W)

Bitsleftover from the TGP forum is also interested in this, and he made this diagram to show the added parts switched in:




This is his thread on TGP. Its a very nice build, which adds a switcheable Lpad, with some thoughtful impressions about how that performs:

https://www.tdpri.com/threads/johndh-attenuator-build.1060083/


----------



## Gene Ballzz

WOW! 
As I look at the evolution of pics of others' execution, I'm kinda embarrassed to post my most recent. Especially compared to the gorgeous, "factory produced" looking piece by Bitsleftover that @JohnH linked over at TDPRI!  Even though it seems he should have included some sort of venting? I will however bite the bullet and get some pics up later!

And @Yamariv , yours looks great, but I hate dealing with water slide decals. I will likely practice my engraving skills and do my best!

Great Work Folks!
Gene


----------



## chas.wahl

No big deal, but the thread/build by Bitsleftover with L-pad is on TDPRI, not TGP. Link is correct, of course.

Whoa! Using Word to draw all of these is quite an achievement. It might be easier to use any PDF program that does "annotation" decently -- even the free version of Acrobat DC does well enough for that. A lot of people seem to like using DIY Layout Creator (DIYLC), though for me, that's just another program to learn.

I tend to create and markup projects using AutoCAD -- can import and scale images into it, and then draw vector stuff over, including "blocks" for canned symbols.


----------



## Gene Ballzz

While I don't expect the P-Touch labels to last very long, I needed something on there for reference here. As you can see on the IN/OUT labeling, my engraving skils are not the best. Still waiting on my supply of nylon screws to arrive and on the last pic, you'll see a little rubber *"test"* grommet, still waiting on my supply of them. My plan for mounting into my DSL20CR, is to use 2 1/2" screws, coming though from the top and they will go through the grommets (top & bottom) as well as having a sleeve of shrink tubing inside as a failsafe means of insulation. FWIW, the two *"IN"* jacks are actually wired in parallel, so that I can easily run the attenuator and speaker in parallel, by simply pulling the speaker plug out of the *"OUT" *jack and plugging it to the parallel *"IN"* jack, even if the amp doesn't have parallel speaker outputs. The unit operates flawlessly! All input impedance readings, with a 16Ω speaker are within less than 15% of 16 ohms (in a 15.7 ohm to 18 ohm range) and about .2Ω different across the board, than another identical build. With an 8 ohm speaker, all readings are within .3 ohms, except for the first reactive stage by itself, which is about 1.4 ohms lower than with a 16 ohm speaker.

I was a little annoyed by the slight mounting distance difference in the single oddball resistor that I got from MOUSER, to avoid the possible *"multi month"* slow boat from Chinesiawanoreanam frustration!

I also endeavored to make our good friend @JohnH happier by increasing my vent holes from 3/16" to 1/4" and adding 23 of them on the sides! 

Thanx 4 Lookin' Especially If You Don't Laugh To Hard! 
Gene


----------



## diego_cl

Thank you @JohnH, you're very generous with your knowledge and your time 

Is there any "agreement" on where to place the parts and ventilation holes for better air flow and heat dissipation?


----------



## JohnH

hi @diego_cl 

You'll see a few versions on the many different builds in this thread. I like to do it like this:

For M2, most of the power and heat is in R1 R2A and R2B, so I'd make sure they are reasonably central., if possible 
Vents above and below, well distributed for good thermal-driven air flow. 
I mount these hottest parts in the box part, with this then being on top with the flat case lid being the base, with feet to raise it (see mine in post 1)

But its not too critical as to exact specific locations. Main thing is a good thick case, thermal paste and bolts for the resistors, plus the vents


----------



## Yamariv

JohnH said:


> hi @Yamariv
> 
> Its safe with just a resistor, but it could spoil some of the reactive tone since it then becomes 50% just a resistive attenuator. So I'm suggesting three added parts
> 
> *Variable input ohms, for 16/8 using a 16 Ohm M2:*
> L2 = 1.2 mH 18 gage aircoil
> R12 = 15 Ohm (100W)
> R13 = 47 Ohm (25W)
> 
> Bitsleftover from the TGP forum is also interested in this, and he made this diagram to show the added parts switched in:
> 
> View attachment 84421
> 
> 
> This is his thread on TGP. Its a very nice build, which adds a switcheable Lpad, with some thoughtful impressions about how that performs:
> 
> https://www.tdpri.com/threads/johndh-attenuator-build.1060083/



Thanks John! I was a worried about having to add another aircoil to my build to retrofit as I don't think I have room. I think my best option, is to build an M2 8Ohm version and I'd be covered for my 8ohm out amp. Thanks again for all your help! I'm loving my attenuators, a whole new world has opened up


----------



## Guitar-Rocker

Nice job Gene !


----------



## Gene Ballzz

@JohnH 
So, I find myself needing to build an 8 ohm unit for my soon to be new Mea Boogie Fillmore 25! I'm torn between the 8 ohm M2 and the 8 ohm M-lite. For the M2, my preferred vendor (Madisound) for chokes, only offers 0.85mH and 1.0mH, not the specified 0.9mH. All parts for the M-lite are available to spec. Is there any benefit of one over the other? And if I wanted to do the M2, which of the two available values I mentioned would be best? Or would I just be better off with the M-lite? Of course, they do offer the 0.90mH choke in 16 ga, if I want to go that way.
Thanks,
Gene


----------



## JohnH

Hi Gene, all of those options would be fine. If I was picking one for general advice to others, it would probably be the M2 with 0.85mH. Its a more compact design than M, and 0.85 is very close to 0.9. But I doubt you'd notice any difference with 1mH. Once the values are close, its not like there is a super critical point where all the planets align and the sky goes black.

But M-lite is fine too!, and at 8 ohms its effectively the same as the 2019 build that I play every day

But, for you, lets apply a bit more to the choice, for this very cool amp. What do you know, or can you predict, about this amp and its use in your rig? ie:

amp - tube type? does it use NFB in the power amp? does it have a presence control?
speakers - what will you use it with, or might in the future?
Apart from 8 ohms, might you use it with 4 or 16 ohm cab/speakers?
what level or range of attenuation would you expect to be the most important for you with this amp? (If thinking about this based on use of other amps, allow that x2 on power is a 3db change)


----------



## Gene Ballzz

Well @JohnH ,
6V6 tubes at supposedly 18 watts "clean" and 23 watts "fully driven" what ever that really means. Same wattage range of 14 to 20 watts as I'm wont to use these days. Will likely use it most often with its Made For Mesa Boogie Celestion Custom 90 or a 65 watt Creamback. It does have a "Presence" control on each channel, but I'm not sure if Boogie uses negative feedback from the output transformer or as an NFB loop earlier in the circuit. I expect to usually be in the -3.5db (parallel mode) to -10db range. Although for personal bedroom level practice, I tend to prefer using what ever my current performance rig is, cranked to its sweet spot and then use the attenuator to keep the love of my life happy! This helps me get my effects levels and settings right (mostly echo/delay) and this kind of use is another place where these attenuators really shine. I can get all of the harmonic overtones and feedbacks that I could achieve un-attenuated at much lower volume levels, due to these fantastic attenuators!

Here's a link to the amp. You might want to check out the manual at the Mesa Boogie website. Very cool features and if it can actually do Marshall sounds (like most if other Boogies can't), it may be the Holy Grail!

https://www.sweetwater.com/store/de...-25-18-23-watt-1x12-inch-2-channel-tube-combo

Thank You Sir,
Gene


----------



## JohnH

Hi @Gene Ballzz , that looks to be a very well considered amp, it should be a killer!

Not everything is known, but inferring, it has a Celestion speaker, so let's assume its inductance isn't far off other Celestions. It's got Presence, and its described as in the Poweramp stage, so assumed basically normal, with NFB, and it should be a perfect place to tweak the attenuated tone.

Given the key use is not much attenuation a bit of the speaker reactance should still reach the amp, so if you go M2, (I hope you do!) then the 0.85mH is a great choice.

If you might like to try with a 16Ohm speaker, id highly recommend building in the 16 out. Its zero risk, you can use it or use the other outputs, but it just gets the tone back on track with the higher ohm speaker. You can also run 2x8 ohm = 4 ohm of course, just as the 16 attenuators work well with 8 ohm speakers.
cheers J


----------



## mike_lawyer

Anyone have a good source for 25W resistors? I am having a hard time tracking down ones for 4.7 ohms on Ebay from China.

Second question - what heat paste is everyone using to mount the resistors?

Third question - is there a good source in the US for the screws to mount the resistors? I am wondering what size to use. Thanks.

Mike


----------



## JohnH

hi @mike_lawyer , welcome to our project!

For the 4.7 resistor, you can use 5 instead and its available on a few chinese ebay stores such as this:
25/50/100W 0.01-5k Ohm Watt Shell Power Aluminum Housed Case Wirewound Resistor | eBay

For the thermal paste, not very critical what type but important to have it. Ive used either a white grease, or a grease with aluminum in it. A place that fixes pcs will have some.

The screws I use for resistors are 10mm M3, steel with nuts. They seem good with the chinese resistors. (but the coil should avoid normal steel bolts or it will throw off the inductance. Nylon or stainless bolts, or zip ties, or a dab of silicone and have it sit on the base is good.)


----------



## Osman Qureshi

mike_lawyer said:


> Anyone have a good source for 25W resistors? I am having a hard time tracking down ones for 4.7 ohms on Ebay from China.
> 
> Second question - what heat paste is everyone using to mount the resistors?
> 
> Third question - is there a good source in the US for the screws to mount the resistors? I am wondering what size to use. Thanks.
> 
> Mike


https://www.ebay.co.uk/itm/25W-0-1-...var=671208183431&_trksid=p2057872.m2749.l2649


----------



## JohnH

*Schematics software *

I've just been monkeying around with some software for drawing schematics. My diagrams to date have just been hand-carved out of MS Word, so I've been looking for something a bit more mission-specific. I've tried Tinycad, which is a free open-source download and works quite well, and also Scheme-it which is an online app based at Digi-key (you have to sign up to save diagrams).

So here's some versions of the Attenuator M2 diagram. No design changes here, just playing with graphics:

This is from Tinycad:




Here it is from Digi-key Scheme-it:




And here it is tricked-out with some colours and added info, using Word:




Tinycad was easy to use, and you can work offline, but I like the clarity of the Scheme-it linework and its also user friendly.


----------



## Gene Ballzz

I like the look/graphic of the switches better on the Scheme-it, but the overall look is cleaner and a bit more standard looking with the Tinycad. Which ever is easier for you and allows you to do what you want it to do would be the best! Though I must say that your diagrams have always looked pretty great to me!
Just Sayin'
Gene


----------



## mike_lawyer

For the air core resistor, does anyone have a good source in the US? I just need to source that part. Thanks!


----------



## JohnH

hi @mike_lawyer , madisound seems good. What amp power are you building for? they do a range in 19 gage which should be good for a 50W amp.


----------



## mike_lawyer

I am looking to make one for a 50 watt plexi (1987). Sounds like 19 gauge should work, I will check it out.


----------



## JohnH

ok, in 19 ga, i think they do a 0.85mH send a 1mH. 0.85mH is fine instead of 0.9. Or, if you are building to 16 ohms, see if they do a 1.8mH


----------



## TonyK

Hi all, especially @JohnH who is still totally astounding in his commitment to helping others! Coming back here after a month or two out, I feel very much like I'm entering a "Welcome Back My Friends to the Thread That Never Ends": Fantastic!!!

I've tried the search function within the thread and not had too much luck, so I'll put my idea/questions out there... My early build of M1 is still going great, and although I did try using it on smaller amps, I find that because of how good it is, it is simply more fun to use it on my 60W HotRod DeVille! Oh for what it's worth, I ended up buying a couple of those mini Vox MV50 amps and the attenuator works just fine on those as well, even though they are basically solid state (with a NuTube pre-amp tube).

Anyway, the only time I find that I don't like the attenuated tone is when I need truly bedroom level volume (+/- 70db), nothing to do with the attenuator and everything to do with the amount of air being pushed! So I've been thinking of modifying my unit to allow for a Line Out, with the idea of running that into an IR box, perhaps something as simple and relatively inexpensive as the Mooer Radar, in order to actually divert the sound through smaller studio speakers which probably will "sound" "better" than a large guitar speaker cab.

But this brings up the question of how to modify the unit so that it effectively becomes a Load-Box so I can disconnect the cab speaker totally. This is essentially what I've tried searching for but can't find the best (I'm sure they're there) earlier posts. Same for the possibility that the actual Line-Out add-on has been perfected somewhere within!

Finally, I note that JohnH has recently mentioned that he, too, is thinking about IRs, so I will probably wait to hear his thoughts before actually investing in a dedicated IR box. I'm basically trying to avoid using my computer for this application, by going direct from valve amp to attenuator to IR-something-or-other to powered monitor speakers for live use.

So sorry if this has been properly addressed above in which case perhaps someone can help direct me to its location


----------



## mike_lawyer

Will using a 16 AWG at 0.9 for the coil work? I see Madisound has them. Would that be too large for most enclosures?


----------



## JohnH

hi @TonyK , nice to see you again

There has been a few line outs on this thread, and ive been mulling over how best to do them. One one hand, many different ways would work ok, but what is best? Many different uses, different signal levels in and out, different input impedances, balanced or unbalanced?

For a simple one, Im thinking a pot in series with a resistor, and use a log pot to give good sensitivity at low level. Heres one that I was discussing with bitslover on the TDPRI forum (snipped from his diagram):



Ive seen versions where the resistor is higher. use a full size pot and about 1/2 w resistor

It could go across the amp input if you are running with no speaker. Or put it at the speaker to pick up a bit more of the speaker influence.

If no speaker, set the attenuator to max attenuation and then it is a good load box for the amp.


But the one Im thinking of will have a balanced option, but i haven't tested yet.

Mooer radar looks to be a great little unit, Im also working up an analogue cab-sim, just to see what it will be like.


----------



## JohnH

mike_lawyer said:


> Will using a 16 AWG at 0.9 for the coil work? I see Madisound has them. Would that be too large for most enclosures?



That would be fine, obviously its bigger and heavier than the 19 gage though. id use that if it needed to run with 100W amps, otherwise, Id be happy with their 0.85mH 19 gage.

Are you building the 8 Ohm version?


----------



## mike_lawyer

JohnH said:


> That would be fine, obviously its bigger and heavier than the 19 gage though. id use that if it needed to run with 100W amps, otherwise, Id be happy with their 0.85mH 19 gage.
> 
> Are you building the 8 Ohm version?



Yes, building the 8 ohm version. I have the case and most of the resistors. Just need to get the jacks, switches, and air core resistor.


----------



## TheOtherEric

@TonyK if your premise is that your amp sounds different at speaking volume versus screaming-loud, I’d encourage you to test that assumption before spending time and money building stuff. Back around page 69 or 70 I posted some spectrums and sound clips played thru a looper (recorded with SM57) at REALLY loud then max attenuated speaking-volume (without touching any amp controls). They were indistinguishable. Led me to tentatively conclude that the whole “pushing air” argument is bogus. You should try it too.


----------



## Gene Ballzz

TheOtherEric said:


> @TonyK if your premise is that your amp sounds different at speaking volume versus screaming-loud, I’d encourage you to test that assumption before spending time and money building stuff. Back around page 69 or 70 I posted some spectrums and sound clips played thru a looper (recorded with SM57) at REALLY loud then max attenuated speaking-volume (without touching any amp controls). They were indistinguishable. Led me to tentatively conclude that the whole “pushing air” argument is bogus. You should try it too.



While I often play at near bedroom volumes through either one of my main amps, using a 16Ω, 25 watt/75hZ G12M greenback in their open back combo cabs, I've found them to sound much bigger and better through the exact same speakers in my 4x12 1960B Marshall cabinet from '72. I'm not sure if its because of it being closed back or simply because of 4 speakers. Its not actually any louder, just more full sounding! Of course, I've been playing through this particular cabinet since 1974, it's my favorite, so I'm kinda partial to and very familiar with how it sounds!
Just My Take On It,
Gene


----------



## acromarmot

JohnH said:


> *Schematics software *
> 
> I've just been monkeying around with some software for drawing schematics. My diagrams to date have just been hand-carved out of MS Word, so I've been looking for something a bit more mission-specific. I've tried Tinycad, which is a free open-source download and works quite well, and also Scheme-it which is an online app based at Digi-key (you have to sign up to save diagrams).
> 
> So here's some versions of the Attenuator M2 diagram. No design changes here, just playing with graphics:
> 
> This is from Tinycad:
> 
> View attachment 84960
> 
> 
> Here it is from Digi-key Scheme-it:
> 
> View attachment 84961
> 
> 
> And here it is tricked-out with some colours and added info, using Word:
> 
> View attachment 84967
> 
> 
> Tinycad was easy to use, and you can work offline, but I like the clarity of the Scheme-it linework and its also user friendly.


Hi @JohnH,

You may wanna look into KiCad. It's a free electronic design suite, from schematic drawing, some simulation tools to pcb design. 
It might be a big over the top just for the attenuator drawings but is a great help if projects expand and somehow end up with a pcb or alike. 

Cheers and thanks for the great efforts and work!


----------



## Timonew

Many thanks to JohnH for the design.

I’ve enjoyed reading the thread - lots of good tips throughout - thanks to Gene and LordoftheLivingRoom.


I’ve kept my M2 8 Ohm build as simple as possible with plenty of space in my recycled bits case (cheers Pete). I bought DPDT switches with large contacts to make the soldering easier.


The inductor came from qtasystems.co.uk and after an email confirmation and PayPal payment it arrived well before the resistors from Mouser!


After checking the input resistance with the speaker plugged in was around 8 Ohms in all switch positions I tried the attenuator today with my 18 W combo (ampmaker) and it works! First impressions are it’s excellent and I’m going to be able to enjoy using the amp much more now.


----------



## JohnH

hi @Timonew , thanks for posting. That looks really well thought out with a very clean layout. I like the heatsink behind R1, which is where the greatest heat would be, plus it looks badass!


----------



## Gene Ballzz

@Timonew
Yessiree! Very nice and tidy indeed!
Great Work! 
Gene


----------



## TonyK

Hi @JohnH, sorry for the delayed response, we've been struggling with some immediate effects of the changing climate here - floods unlike any for about 40-60 years! Luckily our house didn't get flooded but most of our property is only just beginning to dry out 

Anyway, thanks very much for your thoughts. So if I understand you correctly, the 50W version of M1 should be adequate to simply disconnect the speaker from the attenuator box if I then use a Line Out to go somewhere else? That is very encouraging. As things stand, this would be with my 60W Deville. But I'm beginning to think that after 20 odd years of enjoying that amp, it's time either for me to give up guitar as the sound and feel no longer make me feel "good", or perhaps it's time to simply get an amp that is significantly "better", perhaps a Deluxe Reverb or more expensive boutique flavour of such, or I've even been toying with the idea of a Vintage Modern 50. But in any case, it sounds as though the solution you are beginning to outline would be just fine with these kind of alternatives. 

So a quick question. Running through the attenuator at maximum attenuation without a speaker ... what actually would pass through a Line Out? Would it essentially be the same sonic characteristics as the amp at real volume (with appropriate IRs of course), or would be more likely a watered-down version of such?

Good luck with your cab-sim project and thanks a bunch!


----------



## TonyK

TheOtherEric said:


> @TonyK if your premise is that your amp sounds different at speaking volume versus screaming-loud, I’d encourage you to test that assumption before spending time and money building stuff. Back around page 69 or 70 I posted some spectrums and sound clips played thru a looper (recorded with SM57) at REALLY loud then max attenuated speaking-volume (without touching any amp controls). They were indistinguishable. Led me to tentatively conclude that the whole “pushing air” argument is bogus. You should try it too.


Hi Eric, quite some time ago I actually recorded some clips using a looper while simply going through the different attenuation levels. When I normalised the recordings they were almost indistinguishable. So I think that's what you're getting at.

I guess what I'm saying is what many of us already feel, is that once the volume has dropped significantly, the feeling/depth/fullness/pleasure/[insert any other subjective adjectives] becomes different. And in my case, it's less pleasurable so I try to practice with a fairly loud volume, typically with SPL in the 80-90db range. That's what I was alluding to, but it is possible that you are referring to something different again.

As @Gene Ballzz says, there is something about certain speaker cabs that makes us "feel" like we're hearing something "better". In his case a lovely 4x12, but in my case generally the 4x10 of my Deville, or an oversized 1x12 cab loaded with an ET65. But neither feels very fulfilling at very low volume even though the actual tone perceived is very similar to the unattenuated sound. 

It was my understanding, and perhaps I'm wrong, that one of the pluses of using amps such as the Yamaha THR, or powered monitors like the Genelecs, is that these very small speakers are somehow designed to be far more pleasing without the SPL to reinforce the sound. That's what I was considering. But as I mentioned in my response to @JohnH, the reasons for my misgivings could indeed be something quite apart from all of this. with a tiny bit more detail, although I've been playing for well over 40 years, I have a neurological disorder that has significantly cramped my playing technique and finesse. So I adapt by looking for different songs to play, focussing on tone (obviously very technique-based) rather than fast or flashy type leads and melodies. Recently I decided to learn John Mayer's "Slow Dancing" for this reason, and no matter which amp I use in my collection, I am very dissatisfied with the tone that I'm getting (particularly Deville 4x10 with a G&L Legacy). So either it's me, or perhaps a "sweeter" amp might help, and perhaps with lower volumes as per my original question/idea!

Sorry to digress


----------



## JohnH

hi @TonyK , thanks for your comments above. I hope your place drys out!

So, attenuator at max attenuation, no speaker and line-out, you take the line out from the amp out = attenuator input, and dial down the line-out pot to a level to suit your connected device's input . 

The amp is seeing a pretty good electrical facsimile of a real speaker, and hence passing this on to the line-out. There's a couple of provisos:

Real speakers create a very local resonant peak in the bass at between 80 and 110 hz, for open backed and closed back cabs respectively. When we use our M or M2 attenuators with a speaker, the speaker creates this itself at the attenuator output. But these designs don't create this peak at the amp, and so its obviously not there when running with no speaker. This is not a problem at all. This peak is only relevant if you play, in a closed cab, around low A and a note or two either side. With open back, it would only affect low E, maybe up to G. Without the peak, you theoretically get a more even bass response, and feel free to add a bit of bass somewhere in the amp or downstream if you wish. 

But if you are not needing to run silent, you can keep the cab in and take a signal from there after the attenuator. It is probably already at a good level with little more reduction needed. I think a switch for line-out pre or post attenuator would be good. 

From about 150hz and above, our design is a very close electrical match to a real cab, its as close as any $1000 active unit that you can buy, and better than some, because it closely tracks the response of a real measured cab. What it misses is the acoustic response of the speaker, and any room and mic effects, which is where an IR box would come in. Something like a Mooer radar looks like a good small fun option for this. 

My cab sim ideas have gotten as far as making some raw recordings loaded by the attenuator only, twisting them with an EQ curve in software so their response matches a miced recording, and working out a network of components that will reproduce that in real time. I haven't tested the real version yet but from these sims, it sounds like the results will be very clear and direct sounding, great for tracking or PA, maybe add a little reverb to it. There are commercial all analogue cabsim boxes that do this too, such as a Hughs and Kettner Redbox.

Vintage Modern

If you think that you might like one of these, then you definitely need one! The combo is a heavy mutha though, but excellent in every way.


----------



## Gene Ballzz

@JohnH 
Do I recall, somewhere earlier in this thread, that you may have mentioned that the two choke M & M-lite designs do a slightly better job of mimicking and/or tracking that *"local resonant peak"* that you're speaking of? And would that mean that the "two choke" desigm might lend itself better to using as a load box, with line out functionality? All I kow is that my 16Ω, M-lite units sound absolutely fabulous in every sitution I've tried/used them in, with multiple amplifiers and speaker configurations. While I can't speak for the 8Ω units, M-2 and/or multi impedance designs, if they are even close to my M-lite units, they would compete quite well and even surpass the performance of most, if not all of the currently, commercially available offerings at and above the $500 price range! I would think that the multi input impedance/multi output impedance units couls easily sell in the $600 to $800 range, especially if combined with a selectable pre/post attenuator line out. The fact that they require no power supply is a huge plus! 
Just Curious?
Gene


----------



## JohnH

hi Gene

The M and M lite versions that you and Tony have done have virtually identical performance to the M2 (if anything, M2 may actually be a tad better). None of them try to make a bass rssonant circuit, leaving that to be done by the real speaker.

There is the M3 design that has that bass circuit, but the benefit looks to be minimal, at the cost of a lot of extra $ to do it right.


----------



## Gene Ballzz

Thanks for that clarification @JohnH ! So maybe still plugging an actual speaker into the attenuator and maybe putting that speaker into an isolation box or out in another room (even at bedroom volumes) may be beneficial? The isolation box could be much simpler than one intended to be used for miking purposes. Simply using the speaker for it's resonant peak response? Of course, that extra *"speaker in an isolation box"* would likely cost more and be much more *"messy & cumbersome"* than the extra *"bass circuit"* you mentioned!
Thanks John,
Gene


----------



## JohnH

The bass circuit is just two extra parts added to an M2. For an 8 Ohm base, its a 180uF cap (or two at 90uF in parallel), and a 9mH iron core inductor. At 16 Ohms, they become 90uF and 18mH, which is probably less $. The big Kahuna there is the cap which needs to be bipolar with high ripple current spec. The best are polypropylene. Those parts can add up to another $100. Id like to try it to test it. Note its not feasible on an M or M-lite, since it would then need two sets of caps and extra coil instead of one

If its use is primarily as a load box, then it may be of value to add those. But without actually trying it, Ive tried to find any tonal problem with not having it, and I just cant find it, even when really trying to provoke it with deep loud notes from Drop D up.


----------



## Gene Ballzz

@JohnH

Well, with the first consideration that all of our ears are subject to some version/degree of the anomalies of the Fletcher munson Curve. Once we get volumes down to the *"TRUE"* bedroom levels of babies sleeping in the next room, all bets for realistically full range sound and tonal/dynamic response are off! Coming really close at those levels is quite an accomplishment. Now, with these attenuators, just bringing the volume up by 3 - 7 or 10db from those "bedroom" levels can be a very "recordable" sound with plenty of natural dynamics at a volume that may wake the babies in the next room, but not likely elicit calls to the cops!

Now, if I recall correctly, the whole impetus of this somewqhat long thread was to generally tame (not eliminate/mute) the output wattage/volume of a fairly cranked tube amp, without losing it's tone and dynamic response. And it accomplishes this specific task nearly flawlessly, except at *"babies in the next room"* volumes! I kinda think that moving into the realm of being a silent load box/direct box for recording and/or headphone playing is a bit contrary to the whole original point. Its not that those functions and features are not really cool things, but controlling the speaker volume of a great amp while retaining its sound and character seems to be the most important point here.

This attenuator design is one of the greatest tools that a true tube amp aficionado could ever dream of. I can achieve all the same harmonic overtone feedback and response at speaking volume as I can at *"I can't hear a word your saying"* volumes! This is a massive accomplishment and evolution in the universe of playing electric guitar! The liberation of being able to play through a 30-35 watt, JTM45 (or similar), cranked to its sweet glory spot, with all its inherent character, in a situation and at a volume that calls for the volume level of a Tweed Champ is simply mind boggling!

Sure, adding on the bells and whistles can be fantastic, but I'm just urging us to not lose sight of the *VERY REAL* benefits provided by this design for real gigging players. These JohnH units are by far the best and most useful pieces of gear I've ever owned in my +50 years of Screechin' & Squawkin' with a guitar!

Another benefit I derive is being able to bring my full performce rig/setup into my personal, semi lower volume practice space and set it all up for performance levels and simply knock the volume/sound prssure level down as required. This affords me the luxury of rehearsing all of the intended feedbacks, squeals and howls required to achieve my style of said Screechin' & Squawkin'! They don't call me "The Ballzz" without reason!

There It Is, And There Ya Have It!
Gene


----------



## Osman Qureshi

JohnH said:


> hi @TonyK , thanks for your comments above. I hope your place drys out!
> 
> So, attenuator at max attenuation, no speaker and line-out, you take the line out from the amp out = attenuator input, and dial down the line-out pot to a level to suit your connected device's input .
> 
> The amp is seeing a pretty good electrical facsimile of a real speaker, and hence passing this on to the line-out. There's a couple of provisos:
> 
> Real speakers create a very local resonant peak in the bass at between 80 and 110 hz, for open backed and closed back cabs respectively. When we use our M or M2 attenuators with a speaker, the speaker creates this itself at the attenuator output. But these designs don't create this peak at the amp, and so its obviously not there when running with no speaker. This is not a problem at all. This peak is only relevant if you play, in a closed cab, around low A and a note or two either side. With open back, it would only affect low E, maybe up to G. Without the peak, you theoretically get a more even bass response, and feel free to add a bit of bass somewhere in the amp or downstream if you wish.
> 
> But if you are not needing to run silent, you can keep the cab in and take a signal from there after the attenuator. It is probably already at a good level with little more reduction needed. I think a switch for line-out pre or post attenuator would be good.
> 
> From about 150hz and above, our design is a very close electrical match to a real cab, its as close as any $1000 active unit that you can buy, and better than some, because it closely tracks the response of a real measured cab. What it misses is the acoustic response of the speaker, and any room and mic effects, which is where an IR box would come in. Something like a Mooer radar looks like a good small fun option for this.
> 
> My cab sim ideas have gotten as far as making some raw recordings loaded by the attenuator only, twisting them with an EQ curve in software so their response matches a miced recording, and working out a network of components that will reproduce that in real time. I haven't tested the real version yet but from these sims, it sounds like the results will be very clear and direct sounding, great for tracking or PA, maybe add a little reverb to it. There are commercial all analogue cabsim boxes that do this too, such as a Hughs and Kettner Redbox.
> 
> Vintage Modern
> 
> If you think that you might like one of these, then you definitely need one! The combo is a heavy mutha though, but excellent in every way.



Hi John, the parts have arrived finally and I’m about to start building the M2 including a line out with a pre- and post-attenuation switch. But before I start, I have three probably stupid questions. Can you please clarify for me (i) that the pre line out would be a parallel connection at the attenuator’s input jack, (ii) the post line out would be a parallel connection at that attenuator’s output jacks, and (iii) with full attenuation engaged both the pre and post line outs can safely go through a 2.2k ohm 1 watt resistor with a 10k ohm 0.25 watt 2.4cm log potentiometer in series? (By the way, I’m also going to leave space to a perfboard inside the attenuator and space for a couple of switches on the attenuator’s face to accommodate your future passive cab sim design.) Thanks in advance.


----------



## dougie tamson

The last items I need to build John's attenuator are the inductors, Instead of buying I made my own 
First I 3d printed a bobbin, I used '3d Builder' feee app from Microsoft, the whole thing is made up of a series of cylinders of various sizes, added to and subtracted from each other. I included 4 off-set screw holes to mount it.
The bobbins are made of ABS (same stuff Lego bricks are made of).

I then wound the 1.8mH coil with the following spec:
1mm wire, 278 turns, internal diameter is 15mm, 40mm OD, 20mm bobbin height.

I'll wind the 0.9mH one next on the empty bobbin, this will be the same spec but 215 turns.

Pic of empty bobbin and a completed wound inductor attached.

Doug


----------



## JohnH

Osman Qureshi said:


> Hi John, the parts have arrived finally and I’m about to start building the M2 including a line out with a pre- and post-attenuation switch. But before I start, I have three probably stupid questions. Can you please clarify for me (i) that the pre line out would be a parallel connection at the attenuator’s input jack, (ii) the post line out would be a parallel connection at that attenuator’s output jacks, and (iii) with full attenuation engaged both the pre and post line outs can safely go through a 2.2k ohm 1 watt resistor with a 10k ohm 0.25 watt 2.4cm log potentiometer in series? (By the way, I’m also going to leave space to a perfboard inside the attenuator and space for a couple of switches on the attenuator’s face to accommodate your future passive cab sim design.) Thanks in advance.



Great that all sounds fine. That line-out recipe is intended to be very versatile in terms of giving whatever signal level you need, from a wide range of amp powers. There may be times when you'll have to set it very low to suit your next device without overloading it, but also if you put it after the attenuator, with speaker attached, you should get a good signal even at max attenuation, if you turn the knob up high. Once you have tested it with how you want to use it, you might want a different resistor for the front end connection, but this is good to start.

On the component powers, it depends. But lets take a worst case, say you are driving an amp at 100W, and if this was a 16 ohm attenuator. (is it 8 or 16?). The signal voltage at the amp output = attenuator input would be (100 x 16)^0.5 = 40V. The power taken into your line-out circuit of 2.2k+10k would be 40^2/12200 = 0.13W. Of this, 10/12.2 x 0.13 = 0.11W goes into the pot. So there's still a good factor of more than 2 against its 0.25W spec.

Good luck with the build!


----------



## JohnH

dougie tamson said:


> The last items I need to build John's attenuator are the inductors, Instead of buying I made my own
> First I 3d printed a bobbin, I used '3d Builder' feee app from Microsoft, the whole thing is made up of a series of cylinders of various sizes, added to and subtracted from each other. I included 4 off-set screw holes to mount it.
> The bobbins are made of ABS (same stuff Lego bricks are made of).
> 
> I then wound the 1.8mH coil with the following spec:
> 1mm wire, 278 turns, internal diameter is 15mm, 40mm OD, 20mm bobbin height.
> 
> I'll wind the 0.9mH one next on the empty bobbin, this will be the same spec but 215 turns.
> 
> Pic of empty bobbin and a completed wound inductor attached.
> 
> Doug
> 
> View attachment 85337




Hi dougie, thanks for posting that, its awesome work!

Are you able to measure your inductance or is it based on a calculation?

i like the standoffs for the screws. Ive been thinking we should generally space the inductors off the case (eddy currents, more on that to follow soon) and this does that.


----------



## dougie tamson

JohnH said:


> Hi dougie, thanks for posting that, its awesome work!
> 
> Are you able to measure your inductance or is it based on a calculation?
> 
> i like the standoffs for the screws. Ive been thinking we should generally space the inductors off the case (eddy currents, more on that to follow soon) and this does that.



Hi John, my meter doesn't do inductance. These are calculated values but audio specific, site I used diyaudioandvideo dot com Calculator/AirCoreInductorDesigner/

Size wize they are similar to the dimentions of the ones for sale.

For some future research I was thinking of including an option to have a sliding or threaded iron bolt to adjust the inductance up/down by screwing it in and out to taste.

Now I can start building 

Doug


----------



## dougie tamson

Here are the calculated values for winding the coils for an M2v when using my 3d printed bobbin:

For the 16/8/4Ω version:

L1
*Inductance* 0.91 mH
*DC Resistance* 0.36 Ohms
*Wire Gauge* 18 AWG
*Wire Diameter* 40.3 mils (1 mil = .001 in)
*Coil Length* 0.8 in
*Coil Inner Diameter* 0.6 in
*Coil Outer Diameter* 1.49 in
*Average Turn Diameter* 0.99 in
*Wire Length* 55.99 feet
*Copper Weight* 0.28 pounds
*Turns* 215
*Levels* 10.83
*Turns/Level* 19.85
L2
*Inductance* 0.61 mH
*DC Resistance* 0.29 Ohms
*Wire Gauge* 18 AWG
*Wire Diameter* 40.3 mils (1 mil = .001 in)
*Coil Length* 0.8 in
*Coil Inner Diameter* 0.6 in
*Coil Outer Diameter* 1.41 in
*Average Turn Diameter* 0.93 in
*Wire Length* 45.24 feet
*Copper Weight* 0.22 pounds
*Turns* 185
*Levels* 9.32
*Turns/Level* 19.85


For the 8/4/2Ω version:
L1
*Inductance* 0.45 mH
*DC Resistance* 0.25 Ohms
*Wire Gauge* 18 AWG
*Wire Diameter* 40.3 mils (1 mil = .001 in)
*Coil Length* 0.8 in
*Coil Inner Diameter* 0.6 in
*Coil Outer Diameter* 1.33 in
*Average Turn Diameter* 0.9 in
*Wire Length* 38.93 feet
*Copper Weight* 0.19 pounds
*Turns* 166
*Levels* 8.36
*Turns/Level* 19.85
L2
*Inductance* 0.3 mH
*DC Resistance* 0.2 Ohms
*Wire Gauge* 18 AWG
*Wire Diameter* 40.3 mils (1 mil = .001 in)
*Coil Length* 0.8 in
*Coil Inner Diameter* 0.6 in
*Coil Outer Diameter* 1.24 in
*Average Turn Diameter* 0.85 in
*Wire Length* 31.47 feet
*Copper Weight* 0.15 pounds
*Turns* 142
*Levels* 7.15
*Turns/Level* 19.85


Doug


----------



## dougie tamson

Here's a zip of the 3d model I created to make the bobbin, result is better when printed in 2 parts then glued together (with acetone = nail varnish remover).


----------



## dougie tamson

Deep dive...

http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html


----------



## JohnH

Thanks for those references. 

When I last got a meter, I got one with inductance, it was about $50, for good amateur quality. It works well enough to show that inductors that I bought at a certain value and % range are within spec, which gives me confidence in both the meter and in the coil. 

It would be very interesting to compare calculated vs measured. Its also interesting to see how things like steel bolts increase the value. The next thing I want to test with it is how close proximity to an aluminium panel can reduce the inductance (and add resistance) by eddy effects.


----------



## dougie tamson

My main take-away from that reference was the cross sectional shape of the windings to give the most effiicient inductance/weight of copper, if you look at the commercially available coils,the coil cross section is mostly square.


----------



## JohnH

yes, I just learnt that too from your link!


----------



## dougie tamson

JohnH said:


> yes, I just learnt that too from your link!


Would be helpfull if you could post the dimensions (in mm) of the copper windings for your coils?
The wire gauge, internal diameter (ie 1st coil winding), outside diameter (last coil) and the height/thickness excluding the plastic.

I may revise my bobbin, the one I made might be a bit closer to the one on the right of the diagram in the reference!

Doug


----------



## Osman Qureshi

JohnH said:


> Great that all sounds fine. That line-out recipe is intended to be very versatile in terms of giving whatever signal level you need, from a wide range of amp powers. There may be times when you'll have to set it very low to suit your next device without overloading it, but also if you put it after the attenuator, with speaker attached, you should get a good signal even at max attenuation, if you turn the knob up high. Once you have tested it with how you want to use it, you might want a different resistor for the front end connection, but this is good to start.
> 
> On the component powers, it depends. But lets take a worst case, say you are driving an amp at 100W, and if this was a 16 ohm attenuator. (is it 8 or 16?). The signal voltage at the amp output = attenuator input would be (100 x 16)^0.5 = 40V. The power taken into your line-out circuit of 2.2k+10k would be 40^2/12200 = 0.13W. Of this, 10/12.2 x 0.13 = 0.11W goes into the pot. So there's still a good factor of more than 2 against its 0.25W spec.
> 
> Good luck with the build!



Thanks John. It’ll be an 8 ohm build so the 0.25 watt pot should be more than sufficient. I won’t get to building it until the weekend. Will revert with results.


----------



## JohnH

dougie tamson said:


> Would be helpfull if you could post the dimensions (in mm) of the copper windings for your coils?
> The wire gauge, internal diameter (ie 1st coil winding), outside diameter (last coil) and the height/thickness excluding the plastic.
> 
> I may revise my bobbin, the one I made might be a bit closer to the one on the right of the diagram in the reference!
> 
> Doug



Sure, I'll get a couple of examples to compare with in the next day or two.


----------



## JohnH

*Coil tests*

hi @dougie tamson

I don't have a 0.9mH coil around to be a direct match. But I have a 0.4mH one, which would serve as a way of comparison of theory vs actual inductance.

in free air, it measures 0.40mH, which is what it was bought at.

wire gauge 18
inner diameter = 15.5mm
outer diameter = 31.5mm
height of coil = 20mm
Measured resistance 0.2 to 0.3 Ohm (after allowing for meter leads)

There's roughly, 9-10 winding layers with about 20-22 turns per layer

I checked on that calculator:
Air Core Inductor Designer / Calculator (diyaudioandvideo.com)

Putting in the bobbin inner diameter, height and wire gauge, I get this:




The overall dimensions, once converted mm to inches, check out very closely, but i think im seeing more turns though. DCR is consistent but i can only measure to the nearest 0.1 Ohm at best, so not accurate.

Then i checked how it might change due to things through it or near it,

A 25mm M3 bolt right through it increased L to 0.6mH, This is due to high magnetic permeability of steel.

Placing it close to an aluminium box face reduced inductance reading from 0.4 to 0.39mH. With the bolt, 0.6mH was reduced to 0.57mH. This is an eddy current effect. Its like a transformer with a single turn secondary, or like an induction cook top. A current is induced in the aluminium plate that magnetically opposes that in the coil. I think this shifts the phase, making it less inductive and more resistive, hence absorbing energy. This effect may vary with frequency, which is not captured by my meter test.


----------



## dougie tamson

JohnH said:


> *Coil tests*
> 
> hi @dougie tamson
> 
> I don't have a 0.9mH coil around to be a direct match. But I have a 0.4mH one, which would serve as a way of comparison of theory vs actual inductance.
> 
> in free air, it measures 0.40mH, which is what it was bought at.
> 
> wire gauge 18
> inner diameter = 15.5mm
> outer diameter = 31.5mm
> height of coil = 20mm
> Measured resistance 0.2 to 0.3 Ohm (after allowing for meter leads)
> 
> There's roughly, 9-10 winding layers with about 20-22 turns per layer
> 
> I checked on that calculator:
> Air Core Inductor Designer / Calculator (diyaudioandvideo.com)
> 
> Putting in the bobbin inner diameter, height and wire gauge, I get this:
> 
> View attachment 85408
> 
> 
> The overall dimensions, once converted mm to inches, check out very closely, but i think im seeing more turns though. DCR is consistent but i can only measure to the nearest 0.1 Ohm at best, so not accurate.
> 
> Then i checked how it might change due to things through it or near it,
> 
> A 25mm M3 bolt right through it increased L to 0.6mH, This is due to high magnetic permeability of steel.
> 
> Placing it close to an aluminium box face reduced inductance reading from 0.4 to 0.39mH. With the bolt, 0.6mH was reduced to 0.57mH. This is an eddy current effect. Its like a transformer with a single turn secondary, or like an induction cook top. A current is induced in the aluminium plate that magnetically opposes that in the coil. I think this shifts the phase, making it less inductive and more resistive, hence absorbing energy. This effect may vary with frequency, which is not captured by my meter test.



Excellent info, thanks John, just what I needed.

I'll measure the 1mm wire I bought with a micrometer, I'm assuming the 1mm spec is for the copper only and doesn't include the insulation layer. I might need to add small amount of space to my bobbin to allow for this.
Even if it's only 0.05mm, after 20 turns that's an extra 1mm of space required to fit them all in.

Reminds me of the first guitar I built, when building the neck (a tele copy) I used a nice sharp pencil to mark the lines to saw the fret slots, after I glued the fret board to the neck and strung it up, the intonation was off and beyond any adjustments at he saddle. My mistake was measuring each successive fret cut line from the previous, even with a sharp pencil to mark the line, there was enough error in the pencil line width compounded 22 times for a 22 fret neck that the scale was way off between the open and 12th fret. Solution was to use a fine craft knife to mark the lines and mark with reference to the nut... I removed the fret board and made a new one.

Doug


----------



## acromarmot

Hi @JohnH @Gene Ballzz @TheOtherEric and others!

As mentioned earlier I'm trying to include the attenuator in my current amp-build. As tube positioning, housing etc. has already been done long before I came across this thread I have a hard time to place the coil.

Do you have any input on that?

Thx in advance!


----------



## acromarmot

Obviously I cannot add photos here. Dunno why but here're some...

http://cloud.ibak.de/index.php/s/TW496XmMlzlEwbO

Cheers 
acromarmot


----------



## ThePanda

dougie tamson said:


> The last items I need to build John's attenuator are the inductors, Instead of buying I made my own
> First I 3d printed a bobbin, I used '3d Builder' feee app from Microsoft, the whole thing is made up of a series of cylinders of various sizes, added to and subtracted from each other. I included 4 off-set screw holes to mount it.
> The bobbins are made of ABS (same stuff Lego bricks are made of).
> 
> I then wound the 1.8mH coil with the following spec:
> 1mm wire, 278 turns, internal diameter is 15mm, 40mm OD, 20mm bobbin height.
> 
> I'll wind the 0.9mH one next on the empty bobbin, this will be the same spec but 215 turns.
> 
> Pic of empty bobbin and a completed wound inductor attached.
> 
> Doug
> 
> View attachment 85337


That is really cool. What did you use the wind the coils?

I just realized this is my first post.

Hello everyone. I was lurking here and learned a lot from this thread.


----------



## JohnH

acromarmot said:


> Hi @JohnH @Gene Ballzz @TheOtherEric and others!
> 
> As mentioned earlier I'm trying to include the attenuator in my current amp-build. As tube positioning, housing etc. has already been done long before I came across this thread I have a hard time to place the coil.
> 
> Do you have any input on that?
> 
> Thx in advance!



Its hard to make a clear suggestion. Things to watch out for include that it probably sends out a field of magnetism when running, which could put feedback into input circuitry, This would be greatest on the coil axis. Id be inclined to try mounting it at high level, from the upper part of the heatsink fins, as high as possible above the small transformer, with its axis horizontal, maybe using zip ties?


----------



## dougie tamson

ThePanda said:


> That is really cool. What did you use the wind the coils?



This low cost winder:
https://www.ebay.co.uk/itm/Manual-C...rentrq:92ab37ff1770a9e423170f7bfff3b07c|iid:1


----------



## TonyK

JohnH said:


> Great that all sounds fine. That line-out recipe is intended to be very versatile in terms of giving whatever signal level you need, from a wide range of amp powers. There may be times when you'll have to set it very low to suit your next device without overloading it, but also if you put it after the attenuator, with speaker attached, you should get a good signal even at max attenuation, if you turn the knob up high. Once you have tested it with how you want to use it, you might want a different resistor for the front end connection, but this is good to start.
> 
> On the component powers, it depends. But lets take a worst case, say you are driving an amp at 100W, and if this was a 16 ohm attenuator. (is it 8 or 16?). The signal voltage at the amp output = attenuator input would be (100 x 16)^0.5 = 40V. The power taken into your line-out circuit of 2.2k+10k would be 40^2/12200 = 0.13W. Of this, 10/12.2 x 0.13 = 0.11W goes into the pot. So there's still a good factor of more than 2 against its 0.25W spec.
> 
> Good luck with the build!


Hi @JohnH, Thanks so much for your clarifications. I'm in no rush to do this yet, but have noted your thoughts and once finished with some house revoking, I'll do this as a little side project. 

Oh, the day after you commented on my idea of getting a Vintage Modern, one came up for sale in a city 200km from me. I called to go see and buy about an hour after the ad was posted, but the amp had already gone! Of course it was in very good condition and being sold for only 200 euros! That would have been an outstanding buy. Next time...


----------



## mountainhick

I just found the forum searching on DIY attenuators, having need for one and the skills to assemble, but lacking design chops. 

WHAT A GREAT THREAD!

I have some parts and am wondering If can get by for a while building a resistive version with what I have prior to going to reactive at a later date.

I am concerned about power handling. The amp is 85W full up, but I can knock it down to 2 of the tubes for ~40W. I also don't need to crank it for the sound I want, maybe master output at 3-5. So question about a possible alternative resistive box:

I don't need small dose attenuation, rather a larger drop in db. So, how would a primary stage with 200W of resistors into a 100W L-pad with capacitor bleed work out? Would the primary stage act as a soak in this setup before the pad?


----------



## JohnH

Hi @mountainhick , thanks for your message. L pad attenuators, although common, and they do work, will suck down your treble tone at low volume, unlike what we are doing here. What you propose will work, but our design will work better!.

If you are OK with your amp at 40W, you can build design M2 in whatever ohms you need and it will bring your amp down to late-night home volume with no tonal change and full responsive feel. You can build it a a pure resistive version too, which will get the basic tone but lose the feel. But the reactive coil is only about $10 to buy.


----------



## mountainhick

Thanks John. No argument there! Just that I have some parts and am functionally a cheapskate.

Re: the L-pad, I'd add a bleed cap across the L-pad, but then i reckon treble would be sucked off by the primary stage. And then, with 8ohm amp out and speaker, Would the impedance of the first stage plus 8 ohm L-pad stacked create impedance mismatch?

Hmm, I do also have a 6.8mh 18-19g coil. Not quite the specified value, and unsure about that gauge. Would that work in a pinch for 40W?


----------



## Gene Ballzz

@mountainhick
First,  to a great forum!

Next, bite the bullet for a couple of pennies and build it as designed. I stronly recommend that you don't attempt to reinvent an already nicely rolling wheel. If you don't build as designed, you will not likely be happy with the results, thus making it not worth your time and effort!

With that said, as designed this attenuator is by far the best and most useful piece of gear I've ever owned, in my 50+ years of Screechin' & Squawkin' with guitars and amps!  Every other attenuator I've tried (and I've tried MANY) was just a disappointment, by comparison! I would build it for 100 watts in your preferred impedance. All brand new parts, sourced diligently/intelligently, should come in at around $75 or less. Resistors from "chinesiawanoreanam" (ebay), eclosure of your choice (likely ebay again), Madisound for your coil (if in The Used To Be United States) and switches from Parts Express! If you want to be "frugal" on shipping, you could even get the coil from Parts Express, but the mounting is less convenient! Heat sinking to an aluminum enclosure, as well as liberal ventilation is absolutely crucial, especially considering the wattage you need to slough off/tame!

Just Sayin'
Gene


----------



## mountainhick

Thanks @Gene Ballzz, that's a heck of an endorsement!

Alrighty then. Whattheheck is chiesiawanoreanam?

BTW, sounds like we are contemporaries. Though by the list of your gear, you kept with it through the years. I'm on a comeback, have hearing loss, and the big ol amp at 1 on the output is so unsatisfying.


----------



## Dogs of Doom

mountainhick said:


> Alrighty then. Whattheheck is chiesiawanoreanam?


breakdown:

Chinese
Asia
Taiwan
Korea
Viet Nam

all lumped into one...

I made green the connection, some letters overlap...


----------



## mountainhick

Dogs of Doom said:


> breakdown:
> 
> Chinese
> Asia
> Taiwan
> Korea
> Viet Nam
> 
> all lumped into one...
> 
> I made green the connection, some letters overlap...



Thanks


----------



## Gene Ballzz

@Dogs of Doom 
You're quite a fart smeller and you get the gist, although more specifically, without overlap:
china
indonesia
malaysia
taiwan
korea
viet nam
But that's just me picking nits, as its all the same crap anyway!  And please understand that none of that is intended as ethnic slurring, simply referring to the manufacturing juggernaut where almost all cheap and cheesy global goods are produced! And as is common, its *"all about the $$$$"* and not about the people!

And on certain occasions, we could throw in "pan" and "land" for japan and thailand! Though I usually leave them out because japan actually makes some very good products, while thailand makes almost nothing.

And @mountainhick 
I've not updated my gear list in quite a while. But yeah, once I get gear, I rarely let it go, unless it turns out to be junk and then I usaully can't get rid of it, without taking a financial loss!

Just Bustin' Them Cojones!
Gene


----------



## Gene Ballzz

And yeah, I've actually been toying with the idea of building a 100 watt attenuator, so that I can actually use the Laney AOR Pro Tube Lead 100 that I picked up for pennies at a garage sale, along with a nicely matched 300 watt Peavey 412MS cabinet with Sheffield speakers. This amp sounds amazing and even the Master volume works quite well. Unfortunately, the head alone weighs more than any of my combos!  VERY impractical! I'm just gonna sell it!

Just Blastin'
Gene


----------



## JohnH

Hey Gene, I reckon that if wanted to, you could fire up that 100W amp briefly with a 50W build at 16Ohms, maybe at less than max crankage to begin. There's enough electrical capacity for 100W, it's all about heat, and that takes a few minutes to build up. If you can touch the resistors then they're not too hot.


----------



## paradox419

Just finished ordering parts to build the 16 ohm M-Lite. Resistors will be on a boat until almost April. I should have paid more attention when checking out


----------



## Gene Ballzz

paradox419 said:


> Just finished ordering parts to build the 16 ohm M-Lite. Resistors will be on a boat until almost April. I should have paid more attention when checking out



Its been my experience that they actually show up at least a little bit earlier than promised. I got my fingers crossed fer ya!
Just Solderin'
Gene


----------



## mountainhick

Alright, I am trying to order resistors for a 100W version with doubled wattage, and not finding all the needed R values. Looks like I'll have to halve resistor values for doubling up in series or double values if pairing resistors in parallel.

Is there any difference in power management/equal heat dissipation with resistors in series vs parallel? It would be easier if i can use either scheme for doubling for wattage.

And like Paradox419, I am finding a wild range of delivery date estimates on ebay, so trying to keep delivery estimate to April. Other listings pad the delivery estimate all the way out to June, so part of the challenge is to find values that will work from this vendor.

By combining in series or parallel I can get pretty close, but I've noticed that a couple of the paired combos leave little wiggle room, like R2A and B, 18 1nd 22 ohms respectively, so shoot for closest value even if R2A and B end up both 20 ohms, or shoot for the closest ratio at about 0.8 like 20 and 25 ohm?


----------



## JohnH

Hi @mountainhick 

If you're doubling, series or parallel is fine to get the right combination.

Are you building for 8 or for 16 ohms amp?

If you can share a link to where you are buying I'll work out an option or two.


----------



## tmingle

I'm probably going to start another build for my 40CR. What is the most recent version for 16ohm? 
I may try to incorporate this into the build. It has 2 8ohm transformers that I will wire in series for 16ohm. It will come after the 1st stage of attenuation.


----------



## JohnH

Hi @tmingle 
The latest is design M2, see near the top of post 1, with all the values. You can add things to it such as a line'out if you want one or an extra front end if you want to work with different amp taps (not needed for a dsl tnough)

That tranny/pad device looks interesting but I don't know to incorporate it or how it will affect the tone, except that it may well head south!


----------



## tmingle

JohnH said:


> Hi @tmingle
> The latest is design M2, see near the top of post 1, with all the values. You can add things to it such as a line'out if you want one or an extra front end if you want to work with different amp taps (not needed for a dsl tnough)
> 
> That tranny/pad device looks interesting but I don't know to incorporate it or how it will affect the tone, except that it may well head south!


It is actually a wall mount speaker volume. I got it for $5 at a Radio Shack store closing. It is designed for 8ohm stereo. I connected the transformers in series & measured about 15ohms. I will probably hook it up & try it on the earlier version I built a couple years ago 1st. The knob is A 10 position switch & not a potentiometer. The transformers actually have taps similar to the Marshall Power Brake. I will absolutely connect it after at least 1 stage of resistive attenuation to be sure. I've wasted $5 on less!!!


----------



## JohnH

good idea to rig it up as a test. Sounds like it will be safe. The risk is dull tone at low volume.


----------



## JohnH

hi @mountainhick , Ive been looking back again at how to work with the slightly different ranges available on ebay for resistors.

On the current diagrams, ive settled mainly on the standard series of steps in values that you find in all normal resistors. These go:

1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, 10 etc in factors of 10. The only one I add to that is 30 Ohms, which is key in the 16 ohm versions but doesn't seem hard to find. Can also make it from two 15's in series

But to get a huge $ reduction by going to ebay, the typical Chinese ranges come up differently, and not always the same.

Out of the values that we use, in the 8 ohm builds, the 18 and 22 may not be available, but you might find 20 and 25 for R2A and R2B. These two resistors are key to the tonal balance and need to be maintained in proportion. I ran some numbers. 20 and 25 is good, but with an overall loss of about another -0.5db. So stage 1 goes from just under 7 db to just over 7 db. Not a big deal really, but it annoys my OCD tendency. If you are making them out of two parts you can add two in series, like a an 8+10 and an 10+12. If however, you use two 20s for both R2A and R2B, the treble tone shifts off a bit by about 0.5db. Not a big deal really. But this design is based on getting every component relationship as perfect as possible to maintain tone, and that just makes it a tad less so!

Another thing you can do is to put a much larger value, but smaller powered ceramic resistor in parallel. eg, a 10W 180 Ohm across a 20 and across a 25 will make an 18 and a 22.

The other resistors downstream affect the evenness of the attenuation steps but not much the tone. 4.7 or 5 and 5.6 or 6 make a bit of difference of about 0.5db max. Youd unlikley to hear the difference, but I tweaked my 5 and 6 values down with 5W ceramic resistors in parallel. 82 Ohm is good for that. 

If you build at 100W, all power ratings should be x2 (or a pair, eg, two 100W 30 Ohm in parallel to make a 200W 15 Ohm pair for R1)


----------



## Gene Ballzz

Yes @JohnH , part of the reason I've been sticking to advising the two coil, M & M-lite is because of sporadic availability of those somewhat oddball resistor values for R2A & R2B on the M2. The 56R was unusual enough, though only having to source one value from a separate vendor wasn't too bad. Having to use several different sources, the shipping could add up quickly. You can get almost anything you want from Mouser or DigKey, but the price is much higher and many are Made In Mexico, with a different mounting footprint, and that's a whole different annoyance! On the other hand, once we get into the 100 watt versions, pairing more standard various resistance values to achieve the desired resistance and wattage requirements may be a little simpler?
I'm Not Sure Though?
Gene


----------



## mountainhick

JohnH said:


> hi @mountainhick , Ive been looking back again at how to work with the slightly different ranges available on ebay for resistors.
> 
> On the current diagrams, ive settled mainly on the standard series of steps in values that you find in all normal resistors. These go:
> 
> 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, 10 etc in factors of 10. The only one I add to that is 30 Ohms, which is key in the 16 ohm versions but doesn't seem hard to find. Can also make it from two 15's in series
> 
> But to get a huge $ reduction by going to ebay, the typical Chinese ranges come up differently, and not always the same.
> 
> Out of the values that we use, in the 8 ohm builds, the 18 and 22 may not be available, but you might find 20 and 25 for R2A and R2B. These two resistors are key to the tonal balance and need to be maintained in proportion. I ran some numbers. 20 and 25 is good, but with an overall loss of about another -0.5db. So stage 1 goes from just under 7 db to just over 7 db. Not a big deal really, but it annoys my OCD tendency. If you are making them out of two parts you can add two in series, like a an 8+10 and an 10+12. If however, you use two 20s for both R2A and R2B, the treble tone shifts off a bit by about 0.5db. Not a big deal really. But this design is based on getting every component relationship as perfect as possible to maintain tone, and that just makes it a tad less so!
> 
> Another thing you can do is to put a much larger value, but smaller powered ceramic resistor in parallel. eg, a 10W 180 Ohm across a 20 and across a 25 will make an 18 and a 22.
> 
> The other resistors downstream affect the evenness of the attenuation steps but not much the tone. 4.7 or 5 and 5.6 or 6 make a bit of difference of about 0.5db max. Youd unlikley to hear the difference, but I tweaked my 5 and 6 values down with 5W ceramic resistors in parallel. 82 Ohm is good for that.
> 
> If you build at 100W, all power ratings should be x2 (or a pair, eg, two 100W 30 Ohm in parallel to make a 200W 15 Ohm pair for R1)




Thanks again John!

At this point I've found singles or parallel combos of resistors for all values except the 18 ohm R2B, so with the considerations you've shared here, I'll pursue a possible series combo for R2B, or just go with the 20/25 pairing. For my needs, I think bumping up the amp's output level wouldn't be any problem to compensate if I need a little more volume around -7.5db.

And figured I'd share the links for resistors I'll be mixing and matching from for my 100W version:

https://www.ebay.com/itm/100W-0-1-1...991960?hash=item4b8c283318:g:K9QAAOSw-IlgGonS

https://www.ebay.com/itm/1-20-pcs-2...ewound-Resistor/183814586154?var=691442833313


----------



## TheOtherEric

Interesting that nailing the values for R2A and R2B is so important. Seeing that Mouser has the exact values desired and delivers super-fast makes using them a lot easier. Granted that puts the build cost near $100 instead of $70, but $100 is still quite a bargain for an attenuator that works so well!


----------



## mountainhick

TheOtherEric said:


> Interesting that nailing the values for R2A and R2B is so important. Seeing that Mouser has the exact values desired and delivers super-fast makes using them a lot easier. Granted that puts the build cost near $100 instead of $70, but $100 is still quite a bargain for an attenuator that works so well!



Thanks for that. Good reminder, I've ordered from Mouser before. I'll have a look at their pricing. It may be worth it for that one value as I need to order some other parts as well for other projects.


----------



## mountainhick

OK did the deed. Mouser is too pricey for my taste, all from ebay. Here's the breakdown:

R1: 15ohm 200W, from 2x 30ohm 100W in parallel
R2A: 22ohm 100W
R2B: 18ohm 100W, from 2x 9ohm 50W in series
R3,6: 15 ohm 50W
R4: 10 ohm 50W
R5: 4.7ohm 50W, actual 4.5ohm from 2x 9ohm 25W in parallel
R7: 33ohm 50W
R8: 5.6ohm 50W, actual 5.5 ohm from 2x11 ohm 25W in parallel


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## matttornado

Hi JohnH

As you know, I biult your 100 watt version back on page 28. How would i add a simple tone control? Same as a guitar?

It would be added at the attenuator's speaker output.


----------



## Osman Qureshi

Attached are photos of my M2 build. Works as advertised. I heard the clean channel on my DSL20 break up for the first time in my apartment. There’s not much to say about the awesome M2 design, so I’ll comment to some of my additions.

The line out is a 2.2KΩ 1W resistor and 10KΩ log (audio) 1/4W potentiometer in series. There is a pre- and post-attenuation switch. Switched to pre, it works quite well on my 32Ω headphones. The line-out is wired such that it won’t work in bypass mode. The perfboard is there for the future addition of a passive cab sim. The three resistors and the 1uF cap on the perfboard are there to switch to a lower mic-level signal and to block noisy DC voltage from seeping in from my old DI box. The objective of building the line out was to record on my Mac using GarageBand’s cab sims. Yes, the DSL20 has an emulated out, but I wanted an unemulated signal that I could use with GarageBand’s several cab sim options. I haven’t spent much time on the line out yet but will update on how well it works for recording later.

The switch in the back is there for a totally clean bypass, with the attenuator completely isolated. I have not noticed any difference in volume and tone between the isolation switch tuned on and off.

The ventilation holes on the top are not large but with the holes on the side and the larger ones on the bottom, the unit remains cool (I went with form over function vis-à-vis ventilation). With the DSL20 in 20W mode, fully cranked, the unit barely even gets warm. I put all the resistors, except R3, R5 and R7, on the “ceiling” of the box; the only warmth seems to come from R1 (don’t forget the heat sink paste like I did at first. Just for kicks, I briefly tested it on a 100W modelling amp, fully cranked, and it got hot fairly quickly.

The lettering on the box uses water slide decals printed on a laser printer, then slid on to the Hammond box, and then finally 10 light coats of clear automobile lacquer sprayed on with sufficient drying time between each coat. The last step is key to making the lettering permanent.

Some observations: The attenuation from Stage 4 is very slight—at least with a 20W amp—so much so that I thought that I hadn’t connected it correctly given my poor soldering skills, so I wonder if it is even needed. Overall, it’s a fairly easy project for someone who doesn’t normally build things and is a novice at soldering, especially if you don’t include the perfboard in your build. Also, I used 18 AWG wire, but given the size of the box (Hammond model 1550E) and the addition of the perfboard and line out switches and pot, I probably should have used more flexible 20 AWG wire or a bigger box. But it works really well so far with a 20W amp. Thanks JohnH and everyone else involved in the design and testing.


----------



## JohnH

matttornado said:


> Hi JohnH
> 
> As you know, I biult your 100 watt version back on page 28. How would i add a simple tone control? Same as a guitar?
> 
> It would be added at the attenuator's speaker output.



Hi Matt, that was a nice chance to look back at your build, with that big-a$$ resistor soaking up your 100W Plexi! How's it worked out for you after having it for a while? Did you do a fan in the end? Also interested if you built the line out that you were noting there.

On tone control, what sort of tone change would you be seeking? is it a treble cut as on a guitar? A cap and resistor in series, across the output might do it and I could run my sim to make a suggestion. If it had a pot, it would have to be very high power rating. A simpler version would be to get an on-off-on toggle to give you two settings and off. Also, yours is a 16 Ohm build but are you using a 16 Ohm cab? or an 8Ohm?


----------



## JohnH

hi @Osman Qureshi , that looks great, very nice job! thanks for the very helpful write up and pictures too.

I think that box is exactly the one I used in post 1. Its a good solid unit, and Ill build my next one in a bigger version of it. I think you were right to make use of 18 gage wire though, despite its stiffness. 

Your heat observations match others and its interesting to compare with Matts above. The temperatures seem to take off once you get past 50W, but below that it just gets warm.

id like to know more about how the line out is working. The values for you keep most of the available signal in case you need it, but it could be reduced with different values.


----------



## Gene Ballzz

@Osman Qureshi 
*First,* Excellent job on a gordeous build. So much better looking than any I've produced!  Am I correct in assuming that your "Stage 4" is wired as a -3.15db stage that can be added to any combination of the other stages, as well as being able to be used as a stand alone, resistive only stage?

*Next,* It sure is nice being able to finally open up that CLASSIC GAIN channel of the DSL20, isn't it? Before these attenuators, it definitely was not my *"go to"* channel, but now it's my default, fully cranked and the ULTRA GAIN is just used for a bit of fun, now and then! And yeah, even after a couple hours of full volume, 20 watt mode use, my mounted attenuator only gets barely warm. My other electrically identical attenuator does however, get mighty toasty with my JTM30! Although that particular unit is my "updated" first build with a few less and smaller vent holes than later units! I'm planning on opening up the existing vent holes and drilling a few more to see if it helps. I'll have a closer to accurate wattage measurement of that JTM30 in the next day or so. I'm really betting its in excess of 40 watts!

Again, Nice Job! Enjoy!
Gene


----------



## Gene Ballzz

@JohnH
Well, I've confirmed that my "little" JTM30 is in excess of 36 watts, dead nuts clean and approaching 53 watts when fully blazing, and I often use it that way! That likely explains my heating of my attenuator!
Some Pondering To Do Here?
Gene


----------



## JohnH

hi @Gene Ballzz 
It's a cookin' little amp, if not smokin'!

That spec you just measured on the JTM is exactly what they say about the 50W Vintage Modern which is my main squeeze. I've looped that at maximum settings and tried to heat up the attenuator but I just can't get my current one hotter than a warm Australian summers day. Maybe Im not getting that power. But the case is really thick and heavy and drilled extensively. From photos it's the same case as Osman used above.

But, how hot is too hot? I reckon if you can touch R1 for 1 second then its under 100C, ie less than boiling point.


----------



## matttornado

JohnH said:


> Hi Matt, that was a nice chance to look back at your build, with that big-a$$ resistor soaking up your 100W Plexi! How's it worked out for you after having it for a while? Did you do a fan in the end? Also interested if you built the line out that you were noting there.
> 
> On tone control, what sort of tone change would you be seeking? is it a treble cut as on a guitar? A cap and resistor in series, across the output might do it and I could run my sim to make a suggestion. If it had a pot, it would have to be very high power rating. A simpler version would be to get an on-off-on toggle to give you two settings and off. Also, yours is a 16 Ohm build but are you using a 16 Ohm cab? or an 8Ohm?


----------



## matttornado

Hi JohnH.

The attenuator been working great! I added a fan with an on/off toggle switch along with a power supply.

I did add the line out like the diagram I posted but the ground must be to the speaker ground, not the chassis ground like the diagram shows (If I remember correctly). Yes I'm looking to cut some treble as I turn the attenuator volume up. 

Thanks!


----------



## JohnH

matttornado said:


> Hi JohnH.
> 
> The attenuator been working great! I added a fan with an on/off toggle switch along with a power supply.
> 
> I did add the line out like the diagram I posted but the ground must be to the speaker ground, not the chassis ground like the diagram shows (If I remember correctly). Yes I'm looking to cut some treble as I turn the attenuator volume up.
> 
> Thanks!



Hi Matt

I did some runs. My SPICE model is set up for 8 ohms, so this plot is for 8 Ohms:




This relates to having a capacitor and a resistor in series, from hot to cold across the output jack. For this 8 ohm assessment, the cap is 10uF. The top line is the full tone, and you can see the usual rise in treble. With a 10 ohm resistor plus the cap, we get the red line, which exactly flattens the rise. Other resistors are plotted from 6 (lowest curve) to 22 (highest)

For your 16 Ohm build, the same curves will happen with half the cap and twice the resistor, ie 5 or 4.7uF cap and resistor from 12 to 44. (or 47 or 50, whichever you can find)

It assumes that the reactive stage 1 is always on

The caps need to be non-polarized, either electrolytic or polypropylene, rated for at least 50 Volts to go with your 100W amp. the resistors should be 25W or more. 

it may be hard to know how to relate the change that you want to how these curves look, so a bit of testing may be needed with different values.


----------



## Osman Qureshi

JohnH said:


> hi @Osman Qureshi , that looks great, very nice job! thanks for the very helpful write up and pictures too.
> 
> I think that box is exactly the one I used in post 1. Its a good solid unit, and Ill build my next one in a bigger version of it. I think you were right to make use of 18 gage wire though, despite its stiffness.
> 
> Your heat observations match others and its interesting to compare with Matts above. The temperatures seem to take off once you get past 50W, but below that it just gets warm.
> 
> id like to know more about how the line out is working. The values for you keep most of the available signal in case you need it, but it could be reduced with different values.



Hi John, 

I finally got the chance to record using the my M2 build's line out connected to a DSL20HR. I didn't connect the line out directly to my MacBook because I didn't have a Y-splitter that splits the headphone and the mic connection of the headset socket. Instead, I connected the post-attenuation line out to my iRig Pro, which did the A/D conversion and then fed the digitized signal into my Mac's USB port. On GarageBand I used the Amp Designer function that lets you pick a combination of amp, cab, and recording mic. I picked the clean Transparent Preamp, and then tested out various cab and mic combinations. It all worked as expected. With GarageBand's cab and mic sims, the sound was good for at-home recording purposes. There's also an option in GarageBand to not pick a cab and hear the direct signal, which I tried, and as expected, it was terrible. I also compared the emulated out from the DSL20 to the various cab sims in GarangeBand, and I think the DSL's cab sim is closest to GarageBand's Vintage British 4x12 option with the Ribbon 121 mic. GarageBand also let's you virtually move the mic around with reference to the speaker, but I just went with the default position. 

I also tried all of the above with my M2's pre-attenuation line out; other than more volume, I really didn't notice any difference (but my hearing is not the best and I'm not a tone connoisseur at all). The best use of the pre-attenuation line out would be for it to go through a passive cab sim to play with headphones (it has more than sufficient volume for headphones).

The mic-level I put into my M2 build with the three resistors and the cap was pretty useless in the end. I think I'll get rid of it and use the switch and perfboard for the passive cab sim that you're developing.

Happy to answer any other questions and run any tests. 

Thanks.


----------



## Osman Qureshi

Gene Ballzz said:


> @Osman Qureshi
> *First,* Excellent job on a gordeous build. So much better looking than any I've produced!  Am I correct in assuming that your "Stage 4" is wired as a -3.15db stage that can be added to any combination of the other stages, as well as being able to be used as a stand alone, resistive only stage?
> 
> *Next,* It sure is nice being able to finally open up that CLASSIC GAIN channel of the DSL20, isn't it? Before these attenuators, it definitely was not my *"go to"* channel, but now it's my default, fully cranked and the ULTRA GAIN is just used for a bit of fun, now and then! And yeah, even after a couple hours of full volume, 20 watt mode use, my mounted attenuator only gets barely warm. My other electrically identical attenuator does however, get mighty toasty with my JTM30! Although that particular unit is my "updated" first build with a few less and smaller vent holes than later units! I'm planning on opening up the existing vent holes and drilling a few more to see if it helps. I'll have a closer to accurate wattage measurement of that JTM30 in the next day or so. I'm really betting its in excess of 40 watts!
> 
> Again, Nice Job! Enjoy!
> Gene



Thanks Gene, I spent some time making it look pretty and labeling it to make using it self-explanatory for others, like my kids. On your question regarding Stage 4, my build is to the exact specs laid out on page one of this thread, so I suppose my Stage 4 can be added to any combination of the other stages (?), although I haven't tried it yet. (My only add-ons to the standard 8Ω M2 design are the two bypass switches, one at the amp-in socket and the other at the speaker-out socket. I also added a line out that's switchable between amp-in and speaker-out sockets, and a perfboard for a future cab sim circuit). Regards, Osman


----------



## matttornado

JohnH said:


> Hi Matt
> 
> I did some runs. My SPICE model is set up for 8 ohms, so this plot is for 8 Ohms:
> 
> View attachment 86075
> 
> 
> This relates to having a capacitor and a resistor in series, from hot to cold across the output jack. For this 8 ohm assessment, the cap is 10uF. The top line is the full tone, and you can see the usual rise in treble. With a 10 ohm resistor plus the cap, we get the red line, which exactly flattens the rise. Other resistors are plotted from 6 (lowest curve) to 22 (highest)
> 
> For your 16 Ohm build, the same curves will happen with half the cap and twice the resistor, ie 5 or 4.7uF cap and resistor from 12 to 44. (or 47 or 50, whichever you can find)
> 
> It assumes that the reactive stage 1 is always on
> 
> The caps need to be non-polarized, either electrolytic or polypropylene, rated for at least 50 Volts to go with your 100W amp. the resistors should be 25W or more.
> 
> it may be hard to know how to relate the change that you want to how these curves look, so a bit of testing may be needed with different values.



Thanks JohnH! So If I want to add a pot to control treble or tone, would I add a cap & resistor to the the pot? How would that work?


----------



## JohnH

matttornado said:


> Thanks JohnH! So If I want to add a pot to control treble or tone, would I add a cap & resistor to the the pot? How would that work?



Let's say you wanted to sweep through those tone curves. You'd get a small resistor (like 12 for you )and a high power pot (using 2 lugs) and tbe cap, all in series. Maybe 100 ohm pot. But youd want a switch too to get it all out of circuit.

I think a better way might be a rotary switch with 6 steps. One is no tone reduction, and the other 5 step down in even increments following those curves. A series of resistors in a chain, plus the cap.


----------



## Timonew

I’ve been using my first M2 8 Ohm 50W attenuator a lot with my Tweed Champ style amp so I’ve built another one for it.

I mounted all the Rs on a thick piece of aluminium and then into a aluminium case made from a shelf from Pete’s greenhouse. The connections use solid core wire - this made the soldering much easier. The reactive stage resistors are scaled back for an attenuator that handles 25W.

At home I’m typically using 10.5, 14, 17.5 or 21dB of attenuation even on this 4W amp. It all works perfectly with all the amp settings - eg NFB off and extra preamp treble boost switched in. The loudspeaker is a 12” Eminence GA-SC64

Thank you @JohnH for sharing your design, I’m playing my guitar much more now.


----------



## JohnH

A very nice clean looking build @Timonew I like the side vent slot idea.


----------



## Gene Ballzz

@Timonew 
Very nice job. I like the solid wire concept and agree that it's likely a bit easier to manipulate and work with. And yes, these attenuator designs are the greatest thing since sliced bread! And as you've obviously determined, economical enough to build a unit for each amp you own!

And @JohnH 
I've determined that when using one of my units with my 50-ish watt JTM30, if I leave the lid open, the hottest resistors do still get fairly hot, but even after an hour or more of pounding at the the amp's fully cranked sweet spot I can still touch them for a while without discomfort. So the unit I build/dedicate for this amp will simply be more openly ventilated and I should be good to go. I'm really hoping to avoid all the hassles and pitfalls of adding a fan! My experience does certainly demonstrate that a cranked up 50 watt amp is the absoulte limit for these units!

Enjoy,
Gene


----------



## Timonew

JohnH said:


> A very nice clean looking build @Timonew I like the side vent slot idea.


Thanks, and if it gets hot with a more powerful amp I could put the case on its side - with a chimney effect!


----------



## Gene Ballzz

Timonew said:


> Thanks, and if it gets hot with a more powerful amp I could put the case on its side - with a chimney effect!



Or drill some "decorative" holes in the top and bottom?

Just A Thought,
Gene


----------



## tmingle

Timonew said:


> I’ve been using my first M2 8 Ohm 50W attenuator a lot with my Tweed Champ style amp so I’ve built another one for it.
> 
> I mounted all the Rs on a thick piece of aluminium and then into a aluminium case made from a shelf from Pete’s greenhouse. The connections use solid core wire - this made the soldering much easier. The reactive stage resistors are scaled back for an attenuator that handles 25W.
> 
> At home I’m typically using 10.5, 14, 17.5 or 21dB of attenuation even on this 4W amp. It all works perfectly with all the amp settings - eg NFB off and extra preamp treble boost switched in. The loudspeaker is a 12” Eminence GA-SC64
> 
> Thank you @JohnH for sharing your design, I’m playing my guitar much more now.
> View attachment 86418
> View attachment 86419


Did you wind your own inductor?


----------



## Timonew

tmingle said:


> Did you wind your own inductor?


The inductor came from qtasystems.co.uk - great service from them.


----------



## paradox419

Parts were delivered today. This thing is everything it's claimed to be. My only change is going to be adding a bypass switch. You guys did an awesome job. Thank you.


----------



## JohnH

*Component value substitutions
*
I've been doing a bit of OCD R&D. The base design, being M2, has component values shown on the diagram taken from standard values supplied by Arcol. If you are pony-ing up for brand-name resistors, particularly since you can probably get them with fast shipping, then these values apply.

The most common ranges for resistors, in general, go 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, then multiples of 10, 100 etc times those. In the base designs for 8Ohm and 16Ohm builds of M2, I also use 30 and 75, which seem to be also available from Arcol suppliers. No changes are proposed to the base design:













Attenuator M2 200702



__ JohnH
__ Jul 1, 2020






But a lot of us are saving much $ (but with long shipping) buying resistors on ebay from China, and these work fine but generally don't come in quite the same range.

What you can find from a given ebay seller may vary, but a fairly common selection that Ive seen and bought from include:

1, 1.5, 2, 3, 3.5, 5, 6, 8, 10, 12, 15, 18, 20, 25, 27, 28, 30, 33, 50, 75, 80, 100, 120, 150, 200, 250, 300 etc

Here’s the link I look at: (Ebay from Australia)

25/50/100W 0.1-5000 Ohm Watt Shell Power Aluminum Housed Case Wirewound Resistor | eBay

They sell 25, 50 and 100W. Other links will sell similar ranges, but not always the same

So mostly pretty good, but there's a few notable differences for our design, particularly at 4.7, 5.6, 22, 39, 47 and 68 which are hard or not possible to obtain. (have seen the 22 at 25W though)

So how to adapt while keeping the design as optimal as possible? This design is based on keeping very close control of tonal balance and attenuation steps at each switched setting, and to do that, a series of very small decisions are made about optimum component values in each stage. Having worked those out, then we pick the nearest combinations that keep the balance using the ranges available. If we get close, then its close enough. So having decided on the best Ohm value to spec, then a tolerance of 5% is ok (no need to spec 1% tolerance). The ebay resistors that I've bought are within that tolerance.

We need to consider each attenuation stage, the values for the 8 and 16 Ohm builds and maybe also how best to do a 100W build based on the values required.


*Stage 1 - Main Reactive Stage - 7db*

This is the most important stage that sets everything up and receives the amp signal, absorbing most of it and setting tonal balance.

*R1* = readily available in all ranges at 15 Ohm or 30 Ohm for the 8 and 16 builds, at 100W for a 50w amp.

If going for a unit to run with a 100W amp, I suggest to use two of 100W in capacity and spread them apart as mounted on the case.

So for an 8 Ohm 100W build, use two 100W 30 ohm in parallel to get 200W 15 Ohms. If its a 100W build for 16 Ohms, use two 100W 15 Ohm in series to get 200W 30 Ohms.


*R2A and R2B for 8 Ohm build*

These should be about 20% different in values for tonal balance, its somewhat critical and the base 22 (R2A) and 18 (R2B) is close to ideal. But 22 is usually not available from ebay. If its not, I'm now suggesting to use R2A = 20 and R2B = 18. The result is very close. Mathematically the 22 and 18 is better but you'd likely never hear it. Another recipe that is OK is 25 and 20, at the expense of a 1/2 db. 


*R2A and R2B for 16 Ohm build*

This pair is harder to find on ebay and the standard 47 and 39 per the base spec aren't in the available ranges. There are big holes in the usual ebay ranges from 33 to 50 to 75. My suggestion is to combine two in series. eg R2A = 25+20 = 45 and R2B = 18+20 = 38. Each of these could be half the rating needed for R2A or R2B, ie 25W for a 50W build. This could be neat since they would then match the size of all the other resistors apart from R1.


*Stage 2 -7db*

Values needed for this are available in all ranges


*Stage 3 -14db 
*
This includes a 4.7 Ohm for R5 the 8 ohm build. The perfect value of this turns out to be about 4.8 Ohms, and so instead of 4.7, 5 ohms from ebay is a good choice, almost as close. It corresponds to 10 Ohm as shown for R5 in the 16 ohm versions.

R6 is available as spec'ed in all ranges
*

Stage 4 -3.5db*

For the 8 Ohm build this includes 5.6 Ohm shown for R8 combined with 33 for R7. R8 is ideally around 5.3 so if 5.6 is not available, it is best substituted by 5 from ebay rather than 6.

The 16 Ohm version uses 75 and 12 here, which are available in all ranges.


That covers the main attenuation stages.


*Out 3 for the 8 Ohm build into a 16 Ohm cab*

R10, is spec'ed at 68 but 75 is OK too.
R11, if used is 10 Ohms so easy to find


*Inductor L1*

for 8 Ohm build, use 0.9mH, if available, or 0.85mH (1mH is OK)
for 16 Ohm build, use 1.8mH

Wire gage use 18 for 50W (19 gage from Madisound in the US is fine too). Use 16 gage for 100W.


----------



## tmingle

I am considering these for my next build. 

https://www.mouser.com/ProductDetai...=sGAEpiMZZMtlubZbdhIBIAkLiGKeVnd8E4X/S9lpCvc=

I just need to think how to best mount them. I am seriously considering making them part of the amps. I now have 2 DSL40's, a 40C & 40CR & have no worries about modding them. Actually, these mods(provide they are done neatly) may actually increase any resale value.


----------



## Gene Ballzz

They're gonna have a pretty large footprint!
Just Food 4 Thought?
Gene


----------



## tmingle

Gene Ballzz said:


> They're gonna have a pretty large footprint!
> Just Food 4 Thought?
> Gene


Yes, they do. I was thinking about laying them out on an aluminum plate & mounting it inside the amp.
I think about a lot of stuff though.


----------



## matttornado

Here's my 300 watt / 30 ohm resistor for my 100 watt build. Mounted on an aluminum plate along wit the others.

FYI: I got the large 300 watt R on ebay for about 30 bucks I think?


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## JohnH

I just put up this new M2 diagram version in Post 1, to include the chinese resistor substitutions as discussed above. No changes to the base design.













M2-210318



__ JohnH
__ Mar 17, 2021


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## Marshall Arts

JohnH: I'll humbly take my hat off for hours of very interesting read. Thank you! 

If you don't mind, I might ask you a thing or two later on as I plan on making my own attenuator soon.


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## JohnH

hi @Marshall Arts 
Thanks, and you are very welcome.


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## Mjh36

Hey I'm about to order supplies for the M2 8 ohm version. I've read this thread a few times and it's fantastic. I have questions because the "switched jacks" concept eludes me still even though I try to read about it. I understand it for switched DC jacks in pedals, but in amps I don't. I don't even know how to phrase questions lol! I did read that you don't need them in this attenuator build, but I'm also about to kit together a 20w amp head...

So in general, can someone give me info or a link on *when* and *why* I would use switched jacks in an amp build? I'm trying to understand it in terms of an amp's "speaker out" being "switched" for safety reasons if a cabinet is unplugged so it doesn't hurt your amp. I thought I read that somewhere. For some reason I can't grasp this subject. Where does the signal go when the jack is unplugged and is a "closed circuit" I guess? Thanks for this thread and trying to understand my question!


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## JohnH

Mjh36 said:


> Hey I'm about to order supplies for the M2 8 ohm version. I've read this thread a few times and it's fantastic. I have questions because the "switched jacks" concept eludes me still even though I try to read about it. I understand it for switched DC jacks in pedals, but in amps I don't. I don't even know how to phrase questions lol! I did read that you don't need them in this attenuator build, but I'm also about to kit together a 20w amp head...
> 
> So in general, can someone give me info or a link on *when* and *why* I would use switched jacks in an amp build? I'm trying to understand it in terms of an amp's "speaker out" being "switched" for safety reasons if a cabinet is unplugged so it doesn't hurt your amp. I thought I read that somewhere. For some reason I can't grasp this subject. Where does the signal go when the jack is unplugged and is a "closed circuit" I guess? Thanks for this thread and trying to understand my question!



Its true that on the main designs on this thread, im not actually using the switches on switched jacks, although some diagrams show them. I do use the ring connection on TRS ie 'stereo' jacks in some cases though, typically to ground something when a mono jack plug is pushed in.

Switched jacks can be useful to create a closed contact when there is no plug. eg, to ground an input when there's nothing connected, to reduce noise, or at the output, connect a dummy load if there is no speaker plugged in. Also. in home and practice gear, a headphone socket uses a switched jack to send the signal on to the speaker, when phones aren't plugged in. In these cases, the switch contacts are part of the same circuit that the plug is connecting to. But there are other switched jacks where pushing in a plug operates a switch that is completely independent, as so can do something on another part of the circuit.


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## Gene Ballzz

Hey Folks,
FWIW, I just got some nice WEICO brand, pre-tinned, 18 gauge wire from AES/CE DIST. After stripping, it can get gently flattened to easily allow two wires into the lug holes on those cheap resistors from chinesiawanoreanam! Pretty nice stuff and not terribly expensive.

https://www.tubesandmore.com/products/wire-weico-18awg-stranded-top-coat-pre-tinned-600v-50-feet

And @Mjh36 , as @JohnH alluded to, there are multiple types of switching jacks, for multiple purposes, some "normally open" and some "normally closed." The CLIFF UK style is pretty straight forward and only available one way (if switched version), but Switchcraft and others offer multiple options. When planning switching jack use, you need to REALLY wrap your head around what is going on, as quite often using one switch contact for one thing can change the operation of one and/or another of the switches. One way to think of a switched CLIFF style jack is is that each pair of contacts is actually two switches. One connection is the plug contacting the spring loaded blade and the other is that spring loaded blade getting disconnected from it's related lug. It can get get a little confusing, at best!

Just Sayin'
Gene


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## dougie tamson

So finally got the M2 8ohm version built and running some tests, sounding sweet  will upload some samples to Soundcloud next week.

I used my 3d printed bobbin to wind L1 and used wire gauge for 50W amps. For the 100W capable coil I’ll print a bigger bobbin.

I added an extra heat sink over R1 and also a 12cm fan with an external mains power pack that has 3,5,7.5,9 and 12v dc switch which allows speed control for the fan.

I’ve run it with a 20W JCA22H head, the power to the fan was pulled, not needed, didn’t get warm.

I tried it also with a 4 x 6L6CG 120W amp with 2 valves pulled from the output stage so 60Watt (set the 4/8/16ohm switch to 4 on the amp and attached an 8ohm speaker load. I run this at full volume and full attenuation, got a bit warmer but was ok, I’ll take some temperature readings before and after next test.

Another test to do is to run the big amp with EL34 valves (there is a switch to select either type). This will give 50W with 2 valves pulled or 100W with the full set of 4.

I used a bigger case, 190x190x65cm, to take the fan. There is still room inside to double up the 15R 100W R1 as 2 x 30R in parallel giving 15R @ 200 watts.

I’ll get the new bobbin for L1 with heavier gauge wire done this week.








Doug.


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## JohnH

Hi @dougie tamson , great stuff and thanks for posting. That's a serious looking piece of kit!

On heat and fans, I reckon that with the thick AL box and ventilation then we are ok up to around 50W without fan, and at a 100W one might be warranted depending on how hard it's driven.


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## dougie tamson

JohnH said:


> Hi @dougie tamson , great stuff and thanks for posting. That's a serious looking piece of kit!
> 
> On heat and fans, I reckon that with the thick AL box and ventilation then we are ok up to around 50W without fan, and at a 100W one might be warranted depending on how hard it's driven.



Big thanks to John for his design.

Another plus for the bigger case was I drilled 2.5mm holes in the case then cut a tread with a M3 tap, I used the screws from the switches I bought, dual pole dual throw, each came with 3mm threaded terminals and 6 x 3mm screws, to fix the resistors to the case. I soldered the dual poles together with wire used for 32 amp UK mains socket runs (also used for all the internal wiring).

I was thinking that any heat build up might throw the resistance values off so went for the belt and braces approach.

Doug.


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## dougie tamson

Some sound clips from todays testing:
Strat PU position middle/neck position, middle has magnet poles reversed mod.

Clean channel gain maxed then master volume increasing with successive attenuation, ending up with the amp gain and master vol maxed out.


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## JohnH

Those clips sound great. I like them all, but you can hear how the amp is adding those power stage harmonics in the later clips, but other than that the tone is staying consistent. Its a good demo of why we might want an attenuator. The clean clip is a great example of fully clean too though.

if you're up for doing clips, and have a tolerant family, the other type of clip test is where you record at full volume unattenuated, again at -7db and then at max attenuation, all with amp settings the same. Probably have to set recording settings appropriate for each and then normalise the recording levels downstream. Then the ideal is that all clips sound the same and any differences are just due to the attenuator. BTW, what type of amp are you using? It definitely sounds to be of a design where the power stage can affect the tone and that it is very clearly heard. We haven't had many clip tests like that but there have been a couple.

Its an interesting point about how much do the resistance values vary with temperature. i went looking for data sheets and found this fairly comprehensive set from Arcol:

Microsoft Word - frontpagetemplate.doc (spiratronics.com) But i cant interpret the answer to that question from what I see there. But Im thinking that they must be at least somewhat stable over a reasonable temp range. its not too critical for the design. 

I saw a few other things there though. One thing is did see was inductance values on standard resistors, which are typical a few micoHenries (say for a 10 Ohm 100W), so less that 1% of the inductances that are involved in a speaker and so can be ignored. Another interesting factoid was how to derate resistors if you use a heatsink but no heatsink compound. its not as much as Id expected, just 15%, but id still use it. 

Of course, whether or not those data are anything like what applies to Chinese resistors is unknown, but im not too worried.


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## dougie tamson

JohnH said:


> Those clips sound great. I like them all, but you can hear how the amp is adding those power stage harmonics in the later clips, but other than that the tone is staying consistent. Its a good demo of why we might want an attenuator. The clean clip is a great example of fully clean too though.
> 
> if you're up for doing clips, and have a tolerant family, the other type of clip test is where you record at full volume unattenuated, again at -7db and then at max attenuation, all with amp settings the same. Probably have to set recording settings appropriate for each and then normalise the recording levels downstream. Then the ideal is that all clips sound the same and any differences are just due to the attenuator. BTW, what type of amp are you using? It definitely sounds to be of a design where the power stage can affect the tone and that it is very clearly heard. We haven't had many clip tests like that but there have been a couple.
> 
> Its an interesting point about how much do the resistance values vary with temperature. i went looking for data sheets and found this fairly comprehensive set from Arcol:
> 
> Microsoft Word - frontpagetemplate.doc (spiratronics.com) But i cant interpret the answer to that question from what I see there. But Im thinking that they must be at least somewhat stable over a reasonable temp range. its not too critical for the design.
> 
> I saw a few other things there though. One thing is did see was inductance values on standard resistors, which are typical a few micoHenries (say for a 10 Ohm 100W), so less that 1% of the inductances that are involved in a speaker and so can be ignored. Another interesting factoid was how to derate resistors if you use a heatsink but no heatsink compound. its not as much as Id expected, just 15%, but id still use it.
> 
> Of course, whether or not those data are anything like what applies to Chinese resistors is unknown, but im not too worried.



I had to mess about with the gain on the mic input to get the levels close to each other. I'll do another test with the EL34 valves in it, might be interesting.

I got the amp cheap on the local adds here in Edinburgh, £120 I think, it had the foot switch plug (mini 5 pin DIN) snapped off with bits still in the socket. I replaced it with a full size 5 pin DIN socket and plug. It has other issues too, random channel switching and the reverb only works on the clean channel, not looked into that yet.
It's a Bugera 333xl with 2 x 12" drivers, a lot of amp for the money. I was going to use it for parts but when I heard the clean channel it sounded really good so I decided to fix it up. 1st thing I did was to spray the control knobs matt black, they were awful chrome plastic, very cheap looking. The crunch channel is quite noisy, the lead channel is not bad.

I'll wait till the wife is out before I record the stock clean channel with the same volume I recorded the 1st clip.


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## TheOtherEric

@dougie tamson Yes, do that experiment John suggests— it’s quite an eye opener. I posted my results of that back around p. 68-70. Taught me 1) this attenuator is amazing, and 2) the idea that speakers “just need to move air” is silliness, unless maybe the goal is speaker cone distortion (no thanks).


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## TheOtherEric

Has anyone successfully included a Line Out jack? Seems to me you'd also need a pot to control the level, like Bray does in his Line Out Box below. I'd like to do this to run a signal into a reverb pedal then run that to another amp, the "wet" amp, like in the pic. Bray's box is only $50, but would be nice to have integrated into this attenuator.


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## JohnH

Hi @TheOtherEric , yes there's been a few line-outs. Basically they are a resistor and a pot in series, from hot to cold/ground. Output is taken from tbe pot centre lug and cold.

Most have done this at the amp output=attenuator input. You can also do it at the attenuator output, and pick up a bit of the speakers character including bass resonance.

The values of pot and resistor may depend on how powerful is the amp and how loud you run it, and also what kind of line-out signal you need. Id suggest a 5k or 10k full size log pot with a resistor in series somewhere 2k to 10k.

Other features that could apply might be ground lift and/or a DI transformer, depending if your cold connections get back to an overall ground, or are they floating.


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## TheOtherEric

Thanks @JohnH for the thoughtful response. So the parts would be cheap but likely require experimentation. In the spirit of this thread, I should buy some parts, ... but I’ll probably just buy Bray’s unit and transfer the guts to my M2 lol


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## Shipwreck0316

Hi all! Wanted to begin my question(s) by saying this is an awesome thread and it’s nice to see so many helpful people coming together to help the guitar community as a whole! Special thanks to JohnH for this awesome design! I’ve been looking over the latest and some older schematics of the attenuators design and stock it seems awesome. There’s just a couple features I’d like to add. The first is a headphone out jack with a pot for an output control. I know some people were throwing ideas out there, but has anyone implemented one of the ideas with success yet? Pics would be great! My second feature would be an impedance selector switch from 8R to 16R. If this was covered in on of the previous 82 pages I apologize in advance lol. Lastly, this is more me being lazy, but has anyone come up with an official Bill of Materials and layout/wiring diagram for this build? I see there’s several schematics, but I’m not sure what I’d want to use for an enclosure. It would have to fit all the stock options plus the headphone out and extra bits for the switchable two impedance values. I’d also like mine to be fan cooled. Any ideas would be appreciated. I’m probably looking to do a Mouser order her soon anyways so I’d probably get most, if not all materials from them. Thanks!

Edit: I’m an idiot. Correct me if I’m wrong but the two different outputs are for 8R or 16R output impedance if I’m reading this schematic correctly.


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## JohnH

Hi Shipwreck0316, welcome to our thread,

On the headphone jack, there's a few things that make that more complex than you might want. The job of the attenuator as drawn, is to take a tube amp output and reduce it to drive a guitar speaker. The tone of what you then hear is dependent on the amp, the attenuator and the speaker. The speaker is a huge part of that tone, particularly due to its non-electrical acoustic properties which smooth out the tone, shape it and cut out the treble above 4-5kHz. The raw tone of the electrical signal coming out of the amp or the attenuator, is very harsh without that acoustic filtering from the speaker.

Its easy enough to get a simple line-out signal using a pot and a resistor and a few guys have done this here. To use this with phones, recording, or direct to full-range speakers, shaping of the tone is needed either by an analogue cab-sim circuit, or better, an Impulse Response device, and this then needs to go to a headphone amp, or something that has one. That's all very achievable now, with devices such as Mooer Radar and simple mixers with headphone out and USB connection to a computer.

On impedances, the basic M2 design on page 1 can be built with values for being driven either from an 8 Ohm amp tap, or from a 16 Ohm tap and you have to chose which one to build to suit your amp. Either of those can then drive a 16 Ohm speaker or an 8 Ohm one. So you pick the version to suit your use, and Id suggest if you have the choice, build the Ohms version that matches amp, attenuator and speaker for your main most important use.

But if you need to be able to run from different amp taps, because you have multiple amps which don't share a common ohms, then there's an add-on front end that can provide variable input ohms too, either 8/16 or 4/8/16.

On wiring diagrams, I don't post them. Reading the schematic and converting it to a wiring layout is an important stage in understanding how it goes together, and its also the entrance exam for building this and then being able to troubleshoot if needed. Its not a complex circuit but I don't want it to look like anyone who has never understood a circuit can just build it paint-by-numbers (which Im assuming wouldn't apply to yourself anyway). Plus, based on the parts bought and the users wishes, they are all a bit different. Based on all this, we now have dozens of builds on this thread and every one of them that I know of has been a success.

On component sources, it seems to usually require about three orders, for resistors (if you want to save by getting from China on ebay), the coil, and the other bits.

If you want to try it, the starting point is, what amps to use it with, and what power and amp output taps? and what cab/speaker ohms? That will also determine about cooling - fans come into play if you want to drive an amp hard that is > than about 50W, but below that I reckon with the right case and ventilation its not needed


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## Shipwreck0316

@JohnH 
Thanks for the fast reply! I'm going to omit the headphone jack for the first variant. I understand about the wiring diagram. It makes sense considering everyone is sourcing their own enclosures, parts, etc that vary is size and shape to some degree. I'm good on that. I've built a few amp clones and about 30 effects pedals over the past 6 or 7 years. I think I'd like to have multiple impedance input options as I have several amps and don't mind the increased cost. Is the schematic I attached the most recent version of the M2v? I believe this is the version I'd want to build if I'm doing multiple impedance ins and outs right? My last couple of questions is regarding parts. Is it important to use the "marshall style" switched jacks which have plastic casings and are isolated from the unit enclosure? Do you recommend shying away from "open style" or Lumberg style jacks? Lastly have you experimented with air core inductors vs toroidal inductors? Thanks again for all your time!


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## JohnH

Yes that M2v is up to date. It shows the base resistor values from the Branded/Standard/Arcol range. On the latest M2 drawing, there is a table of substitutions from eBay, and these can also apply here.

With M2v, its core is an 8 Ohm M2, then the extra front end is switched in, in series or parallel if you want 4 or 16, and when these are used, there's another -3db taken off the power. So then the minimum attenuation is -10db, ie 1/10th power. Its not that big of a step but if your use is to just take a few dbs off, you might consider this. 

If you just want 8/16 front end options but dont need 4, you could wire in the added front end just using a switch instead of extra jacks. This could also lead to another variant option, where the core is a 16Ohm M2, and you switch in the extra stage with values x2, to absorb the -3db and bring in down to 8 Ohms. Then your 16 setting will be the base one.

Jacks - I recommend the plastic Cliff-style jacks, to keep the case fully out of circuit. On many amps, the output is just two taps on the winding, neither of which is ground. So although the diagrams look like the lower 'cold' wire is some type of ground, its not really one. I also like to use stereo TRS jacks for everything. They grip the plugs better. In two places on the diagrams, I use the insertion of a mono plug to provide a cold connection to the sockets ring terminal to engage a connection. Ive been buying TRS switched sockets, but strictly I'm not using the switched parts so far, just the added ring connection.

For the coils, air-core is the go. While designing this I read a few threads elsewhere about reactive load boxes, particularly by Aiken and his design. A reactive load box, even though its not a full attenuator, is a closely related design task. The advice there is that with a cored inductor, if it saturates it loses its inductance, and so air core is used because these can't saturate. The main purpose of the inductor coil in our design is for when the amp is driving hard, so if it loses its inductance then, there'd be no point in having it. I haven't tested the difference though.


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## Shipwreck0316

@JohnH 

Again thank you for the fast reply! All great information! I did have a couple more quick questions. Have you made 100W versions of the M2v? Would I just double all the power ratings of all the components and use 14 or 16 wire? Third question is about a built in fan. Is it even necessary with the resistors mounted to an aluminum case with thermal paste? The added cost, parts not to mention requiring power would probably not be worth it, but my OCD made me ask  My last question is about adding a switch to the M2v to bypass the unit completely. I have seen a user in this thread build an M2 with a 125V/6A toggle switch right off the input jack controlling the bypass. I think to implement a bypass switch on the M2v would require a 3 pole switch (one for each input jack)? Thanks again!


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## JohnH

Can you list your amps that you want to use, with power and available taps, and also the ohms of speakers/cabs that youd use (including in combos if you have them), and if you want to use them in pairs, or singles. And what uses? (current or possible future), like headphones, direct recording, home practice, studio, loud rehearsals or gigging etc, ?

We have enough ideas on this thread to solve almost any range of use scenarios, once it's decided.

Then we can address the other questions more clearly


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## Shipwreck0316

@JohnH 
I have 8 and 16 ohm cabs and amp outs for the most part. The attenuators would mostly be used for home playing at reasonable volumes.


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## JohnH

ok, how about the amp power, how many Watts are they?


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## Shipwreck0316

@JohnH 

I have a 100W, an 85W, a 50W, a 30W, two 20W, two 15W, two 5W and several 1W/<1W


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## JohnH

Thanks, looks like you might be looking for quite a versatile attenuator set up then.

*Fans and cooling*

Ive not built one with a fan myself. I use heavy Hammond aluminium cases (about 170 x 120 x 50), with large vent holes below and above, and this deals well with my amps which are both about 40W, I don't have a 100W to test with. Mine get warm at most, but others report that they get fairly hot with this kind of power, which is all OK. But the physics of heat says that cranking a 100W amp should start to get into the too-hot range, and many other attenuator or reactive load devices use fan cooling if they seek to deal with the big amps. Running the fans can be based on feeding in some dc using a wall wart to run something like a 12V computer fan, or a mains powered one, or more elegantly, rectifying and smoothing a little of the input power to drive the fan. That's the nicest idea since there are no added external connections. The fan will turn if there's enough power coming in, which is when it might be needed. 

But to make that work needs some experimenting since it would depend on the efficiency and resistance of the fan. A fairly sensitive efficient one would be best so that the power taken is small and it doesn't throw off the balances of impedance. Examples of such a circuit can be found in online schematics for things like the old Marshall Power Brake and the Suhr reactive load box. But I don't think the exact values can be just pulled from that info for use because they will depend on the specific fan model and how hard it needs to run.

If I was building one for 100W, id be tempted to start with a bigger case again, probably about 250 long, split all the resistors in stage 1 into two parts and spread them out across the case, then do lots of vents in the base raised up on feet and in the top. maybe then a fan would not be needed, but that's not proven. 

Of course, it depends on the how hard the amp needs to be run.

*
Bypass switches*

No problem having these, but, consider do you really need one? If you want to run at full power, just dont use the attenuator. So Ive stopped showing them on the diagrams. It makes for a simpler build, and there is less risk of getting something hooked up wrong or switching it under power which would be bad for the amp. But if you need one you can. On post 1, see the M design where I have a bypass switch and also, the -3.5db stage can run on its own. That same switching can go with M2 also to get 0, -3.5, -7, -10.5 db etc, or just a simple bypass switch to go 0, -7, -10.5..etc. 

(neat workaround if you just want to take a hair off the volume: use 1/2 the ohms on the amp, plug speaker direct into the amp and use the attenuator in parallel with it set to max attenuation. Then you have a -3db set up) 

With variable input, if you can forgo having a 4 ohm option, then you can do the 8/16 input change with a switch instead of with jacks, then with a single input jack, you can use a dpdt for bypass. 

*
What to build?
*
Looks like you have a heap of amps, and Im guessing you often build/acquire new ones. If you're building just one attenuator, Id guess the bigger amps already have 8 and 16 outputs, and they'd have plenty of power to afford a -10db reduction for use at home. But the smaller amps, even the 1W's might benefit from attenuation for some uses sometimes? If they are with only specific ohms taps, maybe they should determine whether an M2v is based on a 8 (as drawn) or a 16 core build. If you build 2, maybe different ratings and ohms?


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## Shipwreck0316

@JohnH 

Thanks for all the info! After reading your above reply I'm thinking I wouldn't need a 4ohm jack. Are you saying there's a configuration allowing one universal input and one DPDT switch can change that single input between 8ohm and 16ohm? That would be the way to go if so. Do you have a schematic for that configuration? If not I could probably figure it out. I'll also be trying to keep the part count down and not do a bypass switch or fan cooled for this first build. I can always build a second version later on right!


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## Gene Ballzz

Shipwreck0316 said:


> @JohnH
> 
> Thanks for all the info! After reading your above reply I'm thinking I wouldn't need a 4ohm jack. Are you saying there's a configuration allowing one universal input and one DPDT switch can change that single input between 8ohm and 16ohm? That would be the way to go if so. Do you have a schematic for that configuration? If not I could probably figure it out. I'll also be trying to keep the part count down and not do a bypass switch or fan cooled for this first build. I can always build a second version later on right!



Those are some great thoughts, along with the ones @JohnH has shared!  These attenuators are simple and inexpensive enough to build that this is my considered opinion: *"If multiple options are required and/or desired, it is likely best to build multiple units! Too many options, switches, bells & whistles provide greater opportunities for operator error!"*

FWIW, I made *"parallel mode"* available by simply installing an extra jack in parallel with the input jack. No switch. This affords me to use *"parallel mode"* with an amp that only has one output jack for my needed/desired impedance. Very unlikely to *"accidentally"* access this mode!
And  to the forum and thread!

Just My ,
Gene


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## Shipwreck0316

@Gene Ballzz

I agree 100% that avoiding unnecessary switches, jacks, and other parts is best. Not only does it increase the cost, but also the construction and operation complexity.

Quick question. If I was two make two versions, say a 100W and a 50W version, would I use the 150% rule on the component power ratings (ie 75W rated for a 50W build and 150W rated for a 100W build). Generally I'm not dimeing my amp so "technically" 50W components would work for a 50W amp 99.9% of the time, but overbuild is my last name and overkill is my middle name


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## JohnH

Shipwreck0316 said:


> @JohnH
> 
> Thanks for all the info! After reading your above reply I'm thinking I wouldn't need a 4ohm jack. Are you saying there's a configuration allowing one universal input and one DPDT switch can change that single input between 8ohm and 16ohm? That would be the way to go if so. Do you have a schematic for that configuration? If not I could probably figure it out. I'll also be trying to keep the part count down and not do a bypass switch or fan cooled for this first build. I can always build a second version later on right!



I dont have a diagram to hand for an 8/16 switched input. But its easy enough and there's two ways:

1. Starting with the M2v diagram above, its based on an 8 Ohm M2, and there are the yellow resistors R12 and R13, plus coil L2. These parts provide another 8 Ohm reactive load. So for this case, 'white-out' the 4 and 8 inputs and just use the 16 input, where the input signal goes trough the added 8 ohm load, then through the 8 ohm attenuator, so the amp sees 16.

Then use a switch to bypass R12,13 and L2 and its back to being an 8 Ohm input. 

2. Instead, start with a 16 Ohm M2, and add the L2 R12 and R13, with all values x2 as near as possible, so R12 =15 and r13 = 47 or 50 or a 20 and a 25 in series, L2 = 1.2mH. Put one end of that circuit to cold and use the switch to connect the other end to hot. Now you have a 16 Ohm attenuator in parallel with a 16 load so the amp sees 8.

If the added circuit is used at high power, it will heat up so the main resistor has a high rating, 100W for a 50 W amp and Id use two in series or parallel at 100W if its for a 100W amp.


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## JohnH

Shipwreck0316 said:


> @Gene Ballzz
> 
> I agree 100% that avoiding unnecessary switches, jacks, and other parts is best. Not only does it increase the cost, but also the construction and operation complexity.
> 
> Quick question. If I was two make two versions, say a 100W and a 50W version, would I use the 150% rule on the component power ratings (ie 75W rated for a 50W build and 150W rated for a 100W build). Generally I'm not dimeing my amp so "technically" 50W components would work for a 50W amp 99.9% of the time, but overbuild is my last name and overkill is my middle name



The ratings I listed are at least x3 on expected power, and are intended to work with a 50W amp, overdriven. So there's a basic x2 factor on calculated power, plus another x1.5 included. So they should be fine if bolted to a thick aluminium chassis using thermal grease, if your'e using with 50W amps. Go up x2 with 100W amps (plus get coils wound from 16 gage). Also, if you use it with a bigger amp but not flat-out, and the temperatures on the resistors can still be touched for a second or two, then its OK.


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## Shipwreck0316

@JohnH 

I understand how to simplify the M2v using one input jack (16 ohm) and a toggle switch to change between 8/16 ohm modes. What has me scratching my head a bit is how the R12, R13 and L2 section doesn't affect the other two inputs since it appears to be in parallel with the 8 ohm load. Does it have to do with using a switched jack? Also I'm having a hard time sourcing the inductors. Where do most people go to purchase L1 and L2?


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## JohnH

R12, R13 and L2 fofm a chain that is only connected permanently at one end, so until the other end is connected to something, it's doing nothing to the signal. The 8 ohm input doesnt connect to the floating end, so its not in play. The 16 ohm input connected its hot to the loose end, forcing all signal through it so it's impedance becomes part of the load. For 4 ohms, the loose end goes to the ring terminal of tbe jack, which grounds it only when a plug is inserted. Only one input is used at a time.

The coils are generally found where speaker parts are sold, since they are used in crossover networks. In the US, Madisound.com have them in 19 gage (ok for the 50W) and 16 gage (recommended for 100W). A wide range of values are available, but if you can't see 0.9mH, then 0.85mH is fine. We also know of places in the EU, UK and Australia.


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## Shipwreck0316

@JohnH 

Thanks for the tips. I ordered the parts and they should be here tomorrow. Build pics to come


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## iefes

Hi everybody, I came to the MarshallForum after I stumbled upon information about this gorgeous attenuator design by John and have since read quite a lot in this topic. I've built some amps so far and always wanted to find a nice attenuator design that wasn't too complex. I was about to build an attenuator based on Ken Fishers Airbrake but then got hooked by this thread here. 

I've got two questions before I start buying parts and building. 

I've got a large inductor laying around in my parts bin which has an inductance of 1.5 mH and will take 100W with ease. I'd love to use this but would prefer to build the 8 Ohm version. From a Spice simulation I ran, I can see that the impedance increase at higher frequencies starts a little "earlier" than with the 0.9mH inductor. Do you think this will be audible at all? Have you tried different inductances in an actual built setup?

Even though I find the design very elegant, I kind of don't like that it needs so many separate switches. I know this is nitpicking, but I was trying to wrap my head around other (not necessarily better) options. Here, each resistive stage has the shunt resistor in front of the series resistor, which is the same principle in the airbrake design. If I get this right, every stage is set up so that the impedance "as seen in both directions" is a constant 8 Ohms, no (for the 8 Ohm version)? Wouldn't it be possible to use an 8 Ohm L-Pad wired backwards instead of several resistive stages? If the wiper lug of the L-Pad was used as the input, the topology of shunt -> series resistors should be kept the same. This way it would be possible to control the volume by only one potentiometer and the impedance should still stay the same over the rotation. I'm not really sure if the L-Pad will provide a constant 8 Ohm when wired backwards, but if this was the case, it should work, don't you think? Like this there would be an initial 7dB reduction from the reactive stage and from there the Volume could be variably attenuated down to 0. 

That's just an idea so I'd be happy to hear your opinions about it. 

I'm looking forward to your thoughts!


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## JohnH

hi @iefes and welcome to our thread,

On inductors, although its not so critical that everything needs to be within 1%, the value does make an audible difference and the values posted are the sweet spot, within about 10%. My early versions which had two inductors (still a good scheme) started with two equal inductors and i wasn't hearing the benefit of them. Then I did more maths and @Gene Ballzz built one and it came to life with about a 40:60 ratio between two coils. The current versions use one coil and get the same performance, balanced by two resistors. Out of all that, the conclusion is that the values make a difference and its better to use the best values. The coil is not expensive and suitable ones are easily available in most places.

The coil should be 18 gage wire, air cored. This is to stop saturation and keep a low resistance. So its not maxed out from a power handling standpoint, its also about its resistance 

If your 1.5mH coil is 18 gage or thicker and is air cored, you can probably unwind it down to 0.9mH, if you have a way of working that out or of measuring inductance.

On the switching, Ive also thought a lot about using pots and l-pads, either way round. You could make something but its not going to keep the consistency we need. L pads are designed to maintain something not too far from 8 Ohms, as seen from the amp, and with quite a large margin, but they take no care at all of the impedance seen by the speaker. Turn them around and the problem is reversed. Also, in this design our target is 8 ohms seen by the amp but about 18-20 ohms seen by the speaker.

The best version of a rotary control is probably a rotary switch, if you can get one with good enough current handling. You can do a version where there's a fixed reactive stage, a toggled -7db stage (or a different value), then a rotary that does 0, -3.5, -7, -10.5, -14db. 

But, the toggles are very simple and very robust and reliable, working on a binary system where each provides x2 the attenuation. And the switched steps of -3.5db are audibly very small. You might consider if you want to engage 3.5db more, or not, but its unlikely that you'd need to split it any finer.


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## Gene Ballzz

Very well put sir!  While not as elegant as could be, the multiple toggle system is very reliable. I have searched for rotary switches with 10 positions and enough poles to handle the task of providing every switched/switchable level from Bypass to -31.5db in -3.5db increments. Capable units, with enough current capacity cost as much if not more than the rest of the attenuator's bill of materials! The most current listed build seems the best so far, with alternative values, etc, is in post #1165.
C'est La Vie?
Gene


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## iefes

Hi @JohnH and @Gene Ballzz ,

thank you very much for sharing your thoughts! With respect to the inductor, I see that it would make sense to get the right value. I mean it probably depends on the impedance of the actual speaker you want to replicate, but I guess your design is very well tested and therefore I will try to use these values. An impedance rise starting at slightly lower frequencies might correspond to a more pronounced treble roll off? Can it be put that way or is this a over-simplification? However, I'll try to find out how I can measure the inductance of my coil and maybe try to remove a few windings from it to bring it down to 0.9mH. That's really a good idea, I didn't think about it before. Thanks for that! 

I was playing a bit more using LTSpice last night and came to the same conclusion regarding the L-Pad. I simulated a backwards mounted L-Pad but it didn't give the desired results. I also simulated T-pads and various versions of cascaded L-Pads and such but none of the approaches seemed to yield good results. 
I totally agree that using separate switches gives the most reliable and flexible solution, so that's definitely the best way to go. I too think that steps of 3.5dB are fine enough, for me steps of 6dB would be probably sufficient. 
How would you wire the rotary that you suggested? You would have to switch between different stages rather than opening and closing switches in a layout of cascaded stages like in your single switch design, right? 

Why did you choose to let the speaker see 18 - 20 ohms? I may have missed a post where you explained this, so if it exists, a link to that post would be appreciated. 

I might as well think about a combination of your design and something along the lines of an Airbrake for the higher levels of attenuation, at whose supposedly the speaker doesn't interact so much with the amp anymore (?). I have two large wirewound potentiometers laying around anyway (50 Ohms, 1 of them 100W the other 200W) which could come in handy for this task. 

Thanks heaps for your thoughts! This was very helpful. I'll definitely let you know when I have built the attenuator  
Cheers


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## JohnH

The inductance of the coil is based on matching measured impedance curves of typical Celestion speakers, which are all about the same for 12" types

A rotary version would be based on making one rotary stage, but using two poles to adjust the values, like this:













Attenuator M2 Rotary 201219



__ JohnH
__ Dec 21, 2019






The 18-20 Ohms is based on measuring the actual equivalent output impedance of my amps. Some are lower and some are higher but this is a good mid range. Plus, due to some magic in the maths which Ive seen but is hard to explain, if you take these values as a basis, but use it with amps of different effective output impedances, it tracks the results very well. This may be why this attenuator has proven to work well with a wide range of different amps.

You can add as many stages as you like, but just make sure that the speaker always sees that consistent output impedance if you want consistent tone.


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## iefes

@JohnH thanks a lot for this diagram! I'll wrap my head around the different options and keep in touch  I'm actually leaning towards the standard 8 Ohm version at the moment. 

Thanks a lot so far!


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## ichocobo

Hi @JohnH,
Just a few words to thank you for your GREAT, AWESOME, ABSOLUTELY SHOCKING attenuator design.
I've Built the M2 and was surprised how easy was to build. And tone is flawless once plugged in (vs Two Notes Torpedo Captor -20db setting on the SV20).

I have to confess that I'm still suspicious something eventually will go very wrong, simply because I can't believe it works so good with such little effort... (on my side, of course)!


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## tschrama

JohnH said:


> ...The 18-20 Ohms is based on measuring the actual equivalent output impedance of my amps. Some are lower and some are higher but this is a good mid range. Plus, due to some magic in the maths which Ive seen but is hard to explain, if you take these values as a basis, but use it with amps of different effective output impedances, it tracks the results very well. This may be why this attenuator has proven to work well with a wide range of different amps.....




My hesitation with this kind of attenuators is summarized in the following hints:

Do you want to emulate the output impedance of an guitar amp? Closed-Loop or Open-Loop? Imagine what happens to the output impedance of a clipping power amp. Or should an attenuators aim for presenting a speaker load to a power amp and feed the resulting voltage waveform to the actual speaker without interaction, thus presenting the speaker a close to zero source impedance.


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## JohnH

ichocobo said:


> Hi @JohnH,
> Just a few words to thank you for your GREAT, AWESOME, ABSOLUTELY SHOCKING attenuator design.
> I've Built the M2 and was surprised how easy was to build. And tone is flawless once plugged in (vs Two Notes Torpedo Captor -20db setting on the SV20).
> 
> I have to confess that I'm still suspicious something eventually will go very wrong, simply because I can't believe it works so good with such little effort... (on my side, of course)!



Great! thanks for the feedback.

There's nothing to worry about, it's brick-solid and very simple robust technolgy. The trick was in working out what it needed to do, and optimising the maths to make it happen.


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## JohnH

tschrama said:


> My hesitation with this kind of attenuators is summarized in the following hints:
> 
> Do you want to emulate the output impedance of an guitar amp? Closed-Loop or Open-Loop? Imagine what happens to the output impedance of a clipping power amp. Or should an attenuators aim for presenting a speaker load to a power amp and feed the resulting voltage waveform to the actual speaker without interaction, thus presenting the speaker a close to zero source impedance.



What we know is, it works, based on showing the speaker a average value of an amp at small signals. What seems to happen is that as the amp changes its output impedance with different feedback, or harder driving, the attenuator responds to change its frequency response to suit due to the way the reactive front end is wired . This is demonstrated in analysis, but I can't explain it in words.


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## JohnH

hi @tschrama , further to the above, if you look at post 915, 8th May 2020, on page 46. It explores how the design adapts to different amp output impedances. Before that, there's also a couple of posts about different design options on page 27.


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## mike_lawyer

Got my resistors today from a slow boat, so I am going to start my build soon. Looking forward to starting the build!


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## mike_lawyer

Any store other than Madisound recommended for obtaining the air core inductor? Their website lists the 0.85 19AWG inductor as out of stock. Thanks!


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## JohnH

Hi Mike, these things are best found where loudspeaker components are sold. Im in Australia and I buy them here, but I see Parts Express in the US have this:

Jantzen Audio 0.90mH 18 AWG Air Core Inductor Crossover Coil (parts-express.com)

That one is of the type where its wound on itself without a bobbin. So you put something insulating under it, it could be wood, ideally a few mm thick to reduce eddy currents, then zip tie to the base of your case. Avoid steel screws. 

also , I think Madisound do a 16 gage range, but there'll be many other places since some are from main manufacturers and some are wound by small makers.


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## mike_lawyer

Thanks, I will have to check that out. Hopefully Madisound will also get more in stock.


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## mike_lawyer

Any comparisons of this attenuator with PPIMV? I imagine the attenuator would stay truer to the sound because you would have no loss of negative feedback. 

I am thinking of building a 50W Plexi with PPIMV that would allow some nice tweaking in conjunction with this attenuator.


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## JohnH

Sure, and they'd work well together.

I designed it initially around my Vintage Modern, which is basically a jtm45 with an extra stage and a ppimv. it gets a bit shrill with mv less than about 4, and with the attenuator it can be turned up to find any sweet spot at any volume.


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## Shipwreck0316

@JohnH I drew up a schematic for a switched 8/16 ohm input version of the M2v. Does this look correct? I’m wanting the input to be 8 or 16 ohms controlled with a SPDT switch on the front panel of the unit. I also want to maintain the 3 outputs of the standard M2v schematic. I went off is the 8ohm 50W version schematic you drew a few months back. Thanks!


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## JohnH

hi @Shipwreck0316 

Thanks for your diagram. It looks all correct, but here's a couple of suggestions.

For this design, you're basically building an 8 Ohm attenuator, then switching in an extra load in series to make it suit 16 ohm at the amp. I'd suggest to do that by fully hard-wiring the extra part, and use the spot switch to bypass it. It shouldn't make any difference but if it got switched while running, then there's maybe less of a risk of transients? But really, it should not be switched while running so that's probably moot.

Also, there's two ways to do this. The other way is to start with a 16 ohm M2, and switch in another 16 load in parallel to make 8. The 16 ohm ones do a good job with 8 or 16 speakers, so you'd not need the third output.

In any case, any of these designs are safe with any speaker, the third output is just a tonal tweak for using a 16 cab with 8 ohm attenuator.

But here's a thing to consider:
when you run the M2, either type, with the added load circuit and maybe the matching speaker, you lose some dB's from the available loudest attenuated setting. So you might consider whether 8 or 16 is your most important use, particularly if you want to get quite loud, and use that as the basis for the core of the build.


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## Shipwreck0316

Most of my cabs are 8 ohm that’s why I opted to do an 8ohm build with the added outputs for flexibility. What on the schematic I drew would you change with that in mind?


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## Shipwreck0316

Most of my cabs are 8 ohm that’s why I opted to do an 8ohm build with the added outputs for flexibility. What on the schematic I drew would you change with that in mind?


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## JohnH

Great, then I'd keep it as you have it, provided you can also usually use an 8 ohm amp tap? Maybe do the 8/16 ,switch as I suggested in my 2nd paragraph.

With that, you can also plug two 8 ohm cabs into out 1 and 2, for a 4 ohm load if you want to.


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## Shipwreck0316

Could you sketch me up a schematic for the changes you’re suggesting. I guess I’m not quite understanding.


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## JohnH

ok I will


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## JohnH

Turns out to be very simple, just put a wire link from lug 1 to 2 on the 8/16 switch:


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## Shipwreck0316

@JohnH oh I see now what you mean. Thank you for the clarification!


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## Shipwreck0316

@JohnH just for clarification, jumping the SW1 pole to the 16ohm throw essentially converts the unit to 16ohm by default then bypasses the 8ohm series for a 8ohm impedance?


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## JohnH

yes pretty much thats right


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## dbishopbliss

New to the forum. Been reading through the posts but there are a lot of 'em so sorry if I missed something. Is there a BOM that someone has put together that links to readily available parts (e.g. Mouser)? Including things like heat sinks, enclosures, switches, jacks, etc.

I know you can get parts cheaper from Ebay, etc but since its like $30 and not like $300, I'm willing to skip two pizzas so I know exactly what I'm getting.


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## JohnH

Hi @dbishopbliss thanks for your message.

There's quite a few aspects whereby different builds end up different, due to parts choices, ohm ratings, power and location, so we don't have a standard parts list. It's better to think it through fior yourself, but all the key info is in the M2 diagram in post 1. You will probably need 2 or 3 parts orders, plus some hardware-store stuff like paint, solder etc 

Typically, places like Mouser may sell you the switches and jacks, and expensive resisistors if you want them to, but not the air-core inductor which comes from speaker component suppliers. 

If you get a thick die-cast Al enclosure such as by Hammond, and drill it for venting, and bolt the resistors to that using thermal grease, then a separate heatsink is not needed.

Use plastic jacks such a Cliff or similar

Switches should be rated for at least 5A at 125V.


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## TheOtherEric

dbishopbliss said:


> New to the forum. Been reading through the posts but there are a lot of 'em so sorry if I missed something. Is there a BOM that someone has put together that links to readily available parts (e.g. Mouser)? Including things like heat sinks, enclosures, switches, jacks, etc.
> 
> I know you can get parts cheaper from Ebay, etc but since its like $30 and not like $300, I'm willing to skip two pizzas so I know exactly what I'm getting.


I posted a detailed parts list on page 69, and final pics on p. 70.


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## dbishopbliss

TheOtherEric said:


> I posted a detailed parts list on page 69, and final pics on p. 70.


Got it. Can you share which SPDT switches you used. Somewhere in this thread a rating was specified but I'm having trouble finding it now. I'm wondering if these push button switches at Parts Express would work (#060-958, the forum won't let me include a link in my post yet. I'm too new). Switch rating: 1.5~250V AC/DC up to 5A.

Then maybe I could use a couple of batteries so they would glow. Totally not necessary.


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## iefes

Hi! I finally had the time to build the attenuator. A great piece of gear, thanks a lot for all the information on this thread and all the experimenting which went into this project!

I built the M2 8 Ohm version using large robust APEM switches, including a bypass switch. Enclosure is a Hammond 1590E. Pictures are attached. The wiring could have been done nicer but it does the job. 

Until now, I only tested it at very low volumes and only for a few minutes but my first impression is very positive.

I was wondering if somebody uses it as a load box as well? I was thinking about adding another 8 Ohm resistor which could be switched in after the reactive stage for load-box purposes. It would be great to also include a line-out to go for recording with IRs. Should be fairly simple using a series resistor and a pot. Maybe this way the attenuator could replace my Torpedo Captor and work as a multi-function device for me. I can imagine using a high quality transformer like a Jensen JT-11-FL for a balanced line-out for example (any experience with this?).
Would you suggest to go with the two inductor version in this case to also include the low-frequency resonance? Or should the single inductor variant deliver the same results?

That's also the reason why I didn't include more holes for ventilation yet. I might need some space for the line-out parts.

Again, thank you very much for the help here!


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## JohnH

dbishopbliss said:


> Got it. Can you share which SPDT switches you used. Somewhere in this thread a rating was specified but I'm having trouble finding it now. I'm wondering if these push button switches at Parts Express would work (#060-958, the forum won't let me include a link in my post yet. I'm too new). Switch rating: 1.5~250V AC/DC up to 5A.
> 
> Then maybe I could use a couple of batteries so they would glow. Totally not necessary.



That push switch looks like it will work fine. I reckon 5A at 125V is the switch spec to meet, which it does. 

If youre building an 8 ohm M2 for 50W, then the nominal current coming in is 2.5A, but unless you are adding a full bypass, only about half that gets through to the switches, so it gives a good margin. 16 Ohm amps send out less current. 

If I build another one myself, Im thinking to go go back to mini-toggles, which can often get a 125V/5A rating. There's a bit of a fiddle being small, but they have a nice positive action. Plus, for added reliability and rating with a DPDT switch, you can link the contacts in parallel to create one parallel-pair SPDT out of a DPDT.


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## TheOtherEric

dbishopbliss said:


> Got it. Can you share which SPDT switches you used. Somewhere in this thread a rating was specified but I'm having trouble finding it now. I'm wondering if these push button switches at Parts Express would work (#060-958, the forum won't let me include a link in my post yet. I'm too new). Switch rating: 1.5~250V AC/DC up to 5A.
> 
> Then maybe I could use a couple of batteries so they would glow. Totally not necessary.


I used big heavy duty switches because I like the size and the lugs are big and easy to solder heavy wire to. Mine were from Amazon, On-On 15amp from “Apiele”, $10 for 3, link but Parts Express should have something similar.


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## JohnH

iefes said:


> Hi! I finally had the time to build the attenuator. A great piece of gear, thanks a lot for all the information on this thread and all the experimenting which went into this project!
> 
> I built the M2 8 Ohm version using large robust APEM switches, including a bypass switch. Enclosure is a Hammond 1590E. Pictures are attached. The wiring could have been done nicer but it does the job.
> 
> Until now, I only tested it at very low volumes and only for a few minutes but my first impression is very positive.
> 
> I was wondering if somebody uses it as a load box as well? I was thinking about adding another 8 Ohm resistor which could be switched in after the reactive stage for load-box purposes. It would be great to also include a line-out to go for recording with IRs. Should be fairly simple using a series resistor and a pot. Maybe this way the attenuator could replace my Torpedo Captor and work as a multi-function device for me. I can imagine using a high quality transformer like a Jensen JT-11-FL for a balanced line-out for example (any experience with this?).
> Would you suggest to go with the two inductor version in this case to also include the low-frequency resonance? Or should the single inductor variant deliver the same results?
> 
> That's also the reason why I didn't include more holes for ventilation yet. I might need some space for the line-out parts.
> 
> Again, thank you very much for the help here!
> 
> View attachment 89761
> View attachment 89762
> View attachment 89763
> View attachment 89764
> View attachment 89765
> View attachment 89766



That's come out looking very nice indeed!

Its the same case as I used for my M, yours is much neater though. One thing about that case that I'm sure you also had to take into account of is the inner flange in the middle of the long sides. In my case, i was going at 90 degrees to yours with switches and jacks laid out on those faces.

Earlier we were talking about your 1.5mH inductor. Did you try unwinding it down to the 0.9mH value?

I also see you have metal jacks. I normally suggest plastic bodied, like Cliff jacks, so the case is out of circuit. It's probably not a problem, more a 'watch-it', but in most amps, the two wires that come out of the speaker jacks are both just taps on the OT, and have voltages relative to each other, maybe neither grounded. However there have been other builds using metal jacks with no problem reported.

On the line-out/load-box applications:
We have had a few of these added successfully. Usually its just a pot and series resistor. if you look on Weber Mass schematics, that's all he does. The values will depend of your amp, how hard you run it and what you need to connect to. I'd use a full-sized 24mm pot, about 5k and a series resistor of a few k selected to suit your needs. I see Suhr uses that plus a transformer, and I saw a schematic posted - i guess that would be best, but a lot of units don't seem to use the tranny.

For our design, here's a few points:

1. its already a decent reactive load, safe for the amp. Just set it to max attenuation, then whether or not there is a speaker or added load connected or not is almost unknown to the amp.
2. I've had good results, with no dedicated line out, using one of the spare speaker jacks to feed into the line-in on a mixer. At high attenuation, the signal is then down to around line level in terms of voltage swing, within range of being controlled at the mixer.
3. As you have noted, M2 doesn't have a dedicated bass resonance circuit, and instead lets this develop at the speaker instead. If you are OK with a speaker running, you can take a line-out from the end of the circuit, across the speaker and you'll pick up some of the natural speaker influence that an amp would see.
4. But, a few guys here have had good success with line-out from the front of the circuit. The only thing you might find is a slight desire to tweak bass somewhere, in the amp, or down stream in your IR, recording or PA system.

The bass resonance that occurs on tube amps is IMO, a quirky side effect that happens due to high output impedance. Its not really fundamental to the character of the tone, and only occurs at a few of the very lowest guitar notes. Its nowhere near as important as the general rise at higher frequency.

I've tried to provoke a recording into demonstrating what might be missing by not having the bass resonance in M or M2 designs, with low drop D playing and pushing the amp, and I just cant find any audible tonal difference compared to full unattenuated volume.

But we do have circuit M3 worked out, which needs another big coil, and particularly a big low-loss bipolar cap. It would double the $ for parts though, and these are bulky too. I'd try without!


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## iefes

Hi John, thanks so much for your elaborate response! 

You're right, the inner flange on the long sides of the enclosure was a bit in the way for some mounting ideas I had. However, I really like those Hammond boxes and I got two of these for relatively cheap, so that's why I went for it. 

I was able to measure the inductance using the cheap Chinese component-tester I had laying around and unwound the inductor to 0.9mH. I don't know how accurate the device is but at least it gave consistent results. 
Regarding the metal jacks, I have used what I had. I also have some isolated jacks but not three of the same type (and I simply forgot to order some new ones). So as long as I don't run into trouble I'll keep it like this. Actually, I saw some commercial attenuators also using non-isolated jacks, so I guess it'll be fine. 

Thanks for your tips regarding the line-out option. I saw that R + pot on many designs and I'll probably try this approach as well. I will put it on the input of the attenuator to get all of the amp response, especially since I will mostly use it when recording silently or to play over headphones (with computer + IR). The transformer can still be added in case I find I use it very often and would like better isolation etc. 

I may however include a dedicated 8 Ohm resistor to be switched in when used as a load. This would just be a safety feature for me, because those switches at the front are easily flipped and I just would like to avoid running my amps full bore into the inappropriately set attenuator when being inattentive. However, where would you suggest to put the dummy resistor? At "the end" of the attenuator, i.e., at the output? Or would it make a difference to put it directly behind the reactive load? In this case the connection to the attenuation stages downstream would have to be switched open, no? 

Once I have all this included I may do a few comparison recordings against the Torpedo Captor. Maybe this will help to find out if a bass resonance circuit as in M3 is an even better option for load-box purposes. 

I'll keep you posted. 

Thanks! And if there's any more comments on the line-out feature, feel free to post!


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## iefes

Quick update: Tonight I installed a 10k 2W resistor in series with a 10k pot (only had a 16mm on hand but will change for a 24mm after I did some testing) and tapped the signal from the pot off into the interface --> garageband with Torpedo Wall of Sound. Signal is tapped directly at the input jack of the attenuator. I hooked up an 8 Ohm dummy resistor at the output. This way I could switch between the purely resistive and reactive loads and could hear the difference on my headphones.

Reactive definitely sounds brighter/livelier. I didn't expect such a difference. I mean if I wouldn't have heard the tone with the reactive load in direct comparison I probably wouldn't notice that something is missing, but with the reactive load it does sound really different in the highs/presence. I couldn't hear a difference when switching in the subsequent attenuation stages though.

Will do more testing but didn't have more time today.

Thanks again, so much fun to play with this!


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## JohnH

Hi @iefes ,on the dummy load resistor:

Personally, I wouldn't see a need for it and would prefer the simplicity of being able to get a dummy load setting with no added parts and switches. But if it was added, Id put it at the speaker end of the attenuator, then it wont matter how the switches are set.


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## iefes

Yep, simplicity is a good point. I think I'll leave it like it is for now. I can easily attach my 8 Ohm 150W dummy resistor (wired in an enclosure with jacks) and have the option to switch between reactive and resistive loads by using the bypass switch. In the reactive case the dummy resistor will sit at the end of the attenuator-chain and when bypassed, the dummy will take up all the power.

I build tube amps from time to time (currently working on a ~40W SLO and a Jim Kelley clone for a friend) so the option to switch between resistive and reactive loads may be nice for testing purposes (check voltages, adjust bias, frequency sweeps etc.). 
And then there's this great attenuator! I've played it a little more today and was really happy with the sound. I like it a lot more than the Weber MiniMass I had tested. I'll have to do a comparison to the TAD Silencer of a friend of mine at some point. 
I may have to add that I'm not really a Marshall player. Most of the time I play vintage Fender style amps and that's what I used to test the Attenuator. I tried the Blackface AB763 Channel and the 6G3 Channel of my Hybrid-Deluxe build and got really nice results with both of them. I tried it with and without negative feedback and liked the response with a bit of NFB better, but I don't think this has to do with the attenuator.


----------



## JohnH

Just another option to condider:
If you want to create a resistive load for testing or comparison, you can just shunt the coil with a switch. It doesn't change the basic resistance values seen by amp or speaker except for a part of the resistance of the coil itself


----------



## Mjh36

Thank you JohnH and GeneBallz for answering my question about switched jacks on Page 82. I think I get the concept finally. Recently got my parts for the build so I just have a few more questions.

With switched jacks on the attenuator I can:

1. Ground the input jack hot lug to reduce noise etc.?
2. Ground the hot lugs of the speaker out jacks AND line out jack when no cable is plugged in? Just for whatever reason, safety, etc.?
3. Basically it's ok to ground every jack tip when nothing is plugged in to that particular jack? Not necessary but doesn't hurt?

For example, say I use the amp > attenuator input > line out with full attenuation and no speaker plugged in. The 8 and 16 ohm jack tips will be grounded. All good?

While it seems unnecessary, I still want to do it and just want to make sure I'm not messing something up. I think it doesn't matter in this case if a jack tip is grounded or not, because nothing would be plugged into that jack anyway? I also think I worry too much! 

Other than that I hope to post pics soon!


----------



## JohnH

Hi @Mjh36 

On page 82, we were talking generally about switched jacks and their uses, which were mainly about amps. 

But for the attenuator, there's no need to ground the tips in any of the attenuator jacks, it's not needed and not really desirable.

But in any case, good luck with the build and we'll look forward to seeing it!


----------



## Gene Ballzz

Mjh36 said:


> Thank you JohnH and GeneBallz for answering my question about switched jacks on Page 82. I think I get the concept finally. Recently got my parts for the build so I just have a few more questions.
> 
> With switched jacks on the attenuator I can:
> 
> 1. Ground the input jack hot lug to reduce noise etc.?
> 2. Ground the hot lugs of the speaker out jacks AND line out jack when no cable is plugged in? Just for whatever reason, safety, etc.?
> 3. Basically it's ok to ground every jack tip when nothing is plugged in to that particular jack? Not necessary but doesn't hurt?
> 
> For example, say I use the amp > attenuator input > line out with full attenuation and no speaker plugged in. The 8 and 16 ohm jack tips will be grounded. All good?
> 
> While it seems unnecessary, I still want to do it and just want to make sure I'm not messing something up. I think it doesn't matter in this case if a jack tip is grounded or not, because nothing would be plugged into that jack anyway?



The whole attenuator circuit is still connected to the jacks, even with nothing plugged into/out of those jacks. Connecting the jacks' positives to negative creates a dead short! Remember, there is no actual ground, per se. And according to design, the jacks are best being Cliff UK style and isolated from the the chassis of the attenuator.

Best To Ya,
Gene


----------



## iefes

I would like to stress that it is probably best not to switch any of the tips to ground when nothing is plugged in.
1.) It does not have any impact on the noise as there's no active high impedance circuitry involved in the circuit.
2.) If used as a load you don't want the output jack's tip to be grounded. It won't be a problem as long as all stages are switched in, if you switch one of them off for whatever reason, in place of the 8 ohm speaker there'll be a short between the hot and ground wires which could lead to a lower reflected impedance and thus high current in your amp.
3.) You could ground the line out signal but it's not necessary, there won't be any effect on noise at all.

Regarding 2.) it's not a big issue, however, I would try to keep sources of errors to a minimum. In case you forget to switch on all the subsequent resistive stages and have the output jack grounded (no speaker cable plugged in), your amp will see an impedance of 6 Ohm instead of 8 (simulated with LTSPice for the 8 Ohm Version of the attenuator). That's not too much of an issue, however, it doesn't have any advantage, so I wouldn't do it.

Simply put: Don't switch tips to ground on unused jacks.

Edit: Sorry for the long post, I probably over-discussed this slightly


----------



## Mjh36

iefes said:


> I would like to stress that it is probably best not to switch any of the tips to ground when nothing is plugged in.
> 1.) It does not have any impact on the noise as there's no active high impedance circuitry involved in the circuit.
> 2.) If used as a load you don't want the output jack's tip to be grounded. It won't be a problem as long as all stages are switched in, if you switch one of them off for whatever reason, in place of the 8 ohm speaker there'll be a short between the hot and ground wires which could lead to a lower reflected impedance and thus high current in your amp.
> 3.) You could ground the line out signal but it's not necessary, there won't be any effect on noise at all.
> 
> Regarding 2.) it's not a big issue, however, I would try to keep sources of errors to a minimum. In case you forget to switch on all the subsequent resistive stages and have the output jack grounded (no speaker cable plugged in), your amp will see an impedance of 6 Ohm instead of 8 (simulated with LTSPice for the 8 Ohm Version of the attenuator). That's not too much of an issue, however, it doesn't have any advantage, so I wouldn't do it.
> 
> Simply put: Don't switch tips to ground on unused jacks.
> 
> Edit: Sorry for the long post, I probably over-discussed this slightly


Thank you I appreciate every bit of it. Still learning a lot, and I'm in that phase of knowing a little, yet wanting to implement everything in every build. Always feeling I need to build stuff extra bulletproof. So I'll maybe do the switched to ground input jack on the amp build when I start it. But for the attenuator I'll just keep it stock and simple.


----------



## iefes

Yes, on an amps input it is totally recommended. Otherwise you'll have a floating high impedance node picking up all the noise when no cable is plugged in.

Good luck with the build!


----------



## dbishopbliss

JohnH said:


> The bass resonance that occurs on tube amps is IMO, a quirky side effect that happens due to high output impedance. Its not really fundamental to the character of the tone, and only occurs at a few of the very lowest guitar notes. Its nowhere near as important as the general rise at higher frequency.
> 
> I've tried to provoke a recording into demonstrating what might be missing by not having the bass resonance in M or M2 designs, with low drop D playing and pushing the amp, and I just cant find any audible tonal difference compared to full unattenuated volume.
> 
> But we do have circuit M3 worked out, which needs another big coil, and particularly a big low-loss bipolar cap. It would double the $ for parts though, and these are bulky too. I'd try without!


Has anyone actually built the M3 design for comparison to the M2? Would a 100uf Motor Run capacitor work? I have one that I used in the power supply of a tube amp I built but its sitting around now.


----------



## JohnH

dbishopbliss said:


> Has anyone actually built the M3 design for comparison to the M2? Would a 100uf Motor Run capacitor work? I have one that I used in the power supply of a tube amp I built but its sitting around now.



i don't know of any builds of M3, and I haven't done it myself. If i did, Id use a big polypropylene cap, or maybe two in parallel, to get a bipolar cap with very low losses. Those motor caps might work OK though, so long as they are bipolar (I think they usually are) - so try at risk! 100uF would be a good value for a 16 ohm version.

M3 is theoretically better at capturing the bass resonance and showing it to the amp. But in all the testing Ive done without it, Ive not seen or heard any measured or audible issues with not having it. i currently think that it is not essential for capturing the main character of the amp tone and dynamic response. That resonance is more of a quirky outcome of speaker and amp design, which only affects a few of the very lowest notes. In M2, we capture it with the real speaker response, though the amp doesn't see it.

If you try it for science, Id be very interested particularly if you can configure it so the bass circuit can be shunted, which will convert M3 to M2.


----------



## dbishopbliss

JohnH said:


> i don't know of any builds of M3, and I haven't done it myself. If i did, Id use a big polypropylene cap, or maybe two in parallel, to get a bipolar cap with very low losses. Those motor caps might work OK though, so long as they are bipolar (I think they usually are) - so try at risk! 100uF would be a good value for a 16 ohm version.
> 
> M3 is theoretically better at capturing the bass resonance and showing it to the amp. But in all the testing Ive done without it, Ive not seen or heard any measured or audible issues with not having it. i currently think that it is not essential for capturing the main character of the amp tone and dynamic response. That resonance is more of a quirky outcome of speaker and amp design, which only affects a few of the very lowest notes. In M2, we capture it with the real speaker response, though the amp doesn't see it.
> 
> If you try it for science, Id be very interested particularly if you can configure it so the bass circuit can be shunted, which will convert M3 to M2.


I am building an attenuator for 8 ohms so I guess that motor run won't work for me. They are not polarized so I guess that makes them bipolar.

I was just looking on Parts Express and saw they have a bipolar 200uF/100V electrolytic cap for $2.35. I'm not sure if it qualifies as having "very low losses" but it is intended for use in crossovers so I suspect it would be appropriate for these purposes. The DF is 5%. You could also get two Audyn 100uF caps for under $33. The DF for these is 0.0003% so I suspect that is "very low". I think I will go ahead and give the electrolytic cap a try since I can get that and the inductor for under $20 shipped. Please confirm that 9mH is the correct value for L2.

Speaking of shipping. I decided to buy my power resistors off of the Ebay vendor mentioned earlier in this thread (zhany-m). I just got the tracking information and it says the estimated delivery is August 15. Does it really take 10 weeks to ship resistors from China to the US? If so, I may have to go ahead and buy the more expensive resistors from Mouser and just build a second one when they arrive.


----------



## JohnH

Yes 200uF and 9mH should be right for the resonance circuit in an 8 Ohm M3. It gives a resonance in circuit at about 120Hz, similar to greenbacks in a 1960A closed cab. Open-back cabs are more like 80 Hz, needing larger values if you want to match them.

In the attenuator, the amp will see this resonant circuit, and so will a line out if you take it from the input. A speaker attached to the attenuator output will do its own thing though, responding with its own natural resonant frequency, as it does in M2.

I honestly dont know about the minimum rating spec on the caps. I looked at this older thread about load boxes, where they use the big poly caps for the resonance:

Aiken's Reactive Dummy Load. | The Gear Page

In the load-box circuits, almost the entire signal is going back and forth through the cap. In our circuit, there'd be somewhere in the range 1.5 to 2A of reversing current through the cap, from a 50W amp input. So the cap will heat up based on the 5% DF factor, which implies a internal series resistance of around 0.5 Ohm in the cap. (as I understand it, Im happy to be corrected by anyone who knows more).This also takes a bit off the resonant peak. But I reckon it will probably work for a careful test, but id suggest to monitor the cap for over-heating.


----------



## dbishopbliss

I went ahead and ordered the 200uF cap and 9mH choke. I also ordered resistors from Mouser. I have a gig coming up and I would like to be able to turn up my amp without overpowering the rest of the band or the audience. I will just make a second and third attenuator when the parts arrive from China. This will allow me to do a side by side comparison to see if I can hear any difference.


----------



## musicheals

Hi, 
I'm new at this forum and I came across this thread. I have to say, it is such a pleasure to find so many good information here. Especially about John's attenuator, which I heard only good responses. Of course I am thinking to build one. I want to build the M lite for 8 ohms. In the past I made a air break, which is a good one to my opinion. But I'm sure, John's is much better. So here's my question:
I got a ferrite ring coil with 10mH (1.32mm wire diameter). My idea is, to unwind it, until I can measure 0.5mH. I would like to use the unwinded wire for winding the 0.33mH on the same ferrite core. Is this possible or do I need a seperate air coil?

Thanks a lot and kind regards
musicheals


----------



## JohnH

Hi @musicheals , thanks for your interest in these attenuators.

There's a couple of aspects where what you describe may work, but is outside of what we know does work.

The main coil is designed to be based on an air core, so that it can never saturate. If it does then it could lose its inductance, right at the time where we want it when driving hard. Nothing bad would happen, but it might not sound as good. 

Also, with M and M lite designs, which have two coils, putting them on one core will couple them, changing their performance in ways I can't predict.

M-lite was a simple version of M. But you could also just build M2 which can be even simpler and with the same performance, using just one coil. You can leave off the 3rd output jack and associated parts if you don't want it. For an 8Ohm build, the coil is 0.9mH. If you want to try your coil, you could unwind it to that value, and see. You'd probably never know if its different from an air-core one though, since the difference is probably a subtle effect and you wouldn't be able to A/B them.


----------



## musicheals

Thanks a lot John for your quick reply!
So I think it's best, not to use the core. I got a few empty plastic bobbins and I will use the wire of this big coil (10mH) and wind it myself by hand. This will be fun I think 

Thanks a lot 
musicheals


----------



## JohnH

That's great, very handy if you have a meter to measure inductance. I've got L and C ranges on mine and although its only a $50 unit, it's very useful for checking coils and pickups.

Your wire sounds like a good thickness, maybe 16ga? If you go to places like Madisound.com you can see the type of diameters and final ohms that they get for equivalent coils, then you can check there's enough wire in your donor coil. There's also a few online calculators for multilayer coils, which could help with a check on wire length needed.


----------



## musicheals

....when I look at the gauge/diameter list, it should be gauge 17. Is threre an advantage for having a stronger wire?

Yes, it's very comfortable to have a multimeter that measures L and C. Once I made a coil for a wah wah and it gave a great result. 
Next I will order the resistors, if possible from one seller (if I will find one in China maybe)

Thanks a lot 
musicheals


----------



## musicheals

@JohnH 
Is there a way, to use your M2 also as a dummy for recording?
If yes, where is the best place, to put in a line out?

Thanks a lot
musicheals


----------



## JohnH

musicheals said:


> @JohnH
> Is there a way, to use your M2 also as a dummy for recording?
> If yes, where is the best place, to put in a line out?
> 
> Thanks a lot
> musicheals



Sure, if you want to use it with no speaker, just set if to max attenuation and connect it to the amp. The amp is loaded fine, since at that setting it sees hardly any of the speaker anyway. You can put your ljne-out circuit across the input.

If you want to keep a speaker running though to monitor while you record, you can take a line out from the speaker end, maybe using the second speaker output. You may find that the signal level there is already at a good level to go to a mixer. An advantage of that way is you pick up some more of the real speaker character.

Both ways have been tested. The best values for a pot with series resistor are around 5 to 10k each, depending on what you are using.

Best to experiment to see what works best for your set-up.


----------



## musicheals

That's great! I thought I have to build me reactive load.
Today I will order all parts. When I finished your attenuator, I will review and send some photos...

Thanks a lot John 
musicheals


----------



## dbishopbliss

I finished assembling my M2 yesterday. I used the same enclosure as Gene Ballzz found back on page 75. One big difference, I installed the jacks on one end and the switches on the other end. I did NOT drill any additional holes for ventilation. I wanted to see how hot it would get. 

My amp is a 1962 Sivertone 1482 with circuit tweaks I made. It is similar to a Tweed Deluxe in terms of tube complement and power. Generally, I set my amp at about 2 and my bandmates complain it is too loud. It is louder than the drummer and louder than the other guitar player with an AC30 (he has a master volume). I usually have to turn it down so low that it seems to lose all character and sounds really thin... I think everyone knows that spot on their amps controls. 

The attenuator worked great. I had the default -7dB engaged plus the -3.5dB. So 10.5dB was perfect. I did not notice any loss in response or frequencies, just a slightly lower volume, BUT it did not have that thin sound that it would get if I simply turned it dow. After 3 hours of band practice today I can report the enclosure was cold. I kept checking it every couple of minutes, just in case. Seriously, not warm in the slightest. Again, this enclosure has no holes for ventilation. So, if you have a 20-watt amp that you don't run dimed you might be able to able to save yourself some complexity.


----------



## JohnH

That's great to hear, and congrats for getting it together.

You observations on heating make sense.

On the temperatures, I think the relationship with power input is different to that for perceived volume vs power, and that's consistent with the physics. When I'm feeding in about 30W I can make it warm, with continuous thrashing, but that's not my playing style! But I think a cranked 50W needs all the passive cooling it can get, and at 100W, id reckon some active cooling or much bigger build may be needed.


----------



## dbishopbliss

For those that like photos (like me), here are the glamor shots.

The 100W 15R resistor (R1) was much larger than I expected. I tried to mount it as centrally as I could on the top of the enclosure so it would be able to dissipate heat efficiently. I have to put slightly off center so I could mount the L1 inductor next to it. The resistor is mounted with 4-40 bolt and locking nuts using thermal paste (the white goop). The inductor is mounted with hot glue.

The rest of the resistors were mounted to the bottom. Again, I located the 50W resistors centrally so they would have more surface area to dissipate heat. I spread out the remaining resistors as much as I could. I only used one screw per resistor. I figured they will not have any stress on them. The locking nuts held them securely and should not loosen over time.

I used DPDT switches for no particular reason other than someone said they used the same ones for an attenuator on Parts Express. On one hand they are pretty large and probably overkill. On the other hand, bridging across both polls made it easy to solder the connections from the resistors.

I cut and soldered the wires before putting on the end-plates with the switches and jacks. I probably could have mounted the plates on first and then cut the wires to exact size. Initially, I was thinking I could leave them long in case I needed to take the plates off but in retrospect I don't think I will ever do that. Doesn't impact performance in any way so I'm not going to worry about it now.

Another thing I think I would do is change the order of the switches so that the values were sequential: -14dB -7dB -3.5dB. Just makes things easier to remember.

I wish I could find an enclosure like this one that was slightly larger. There is not enough room to build an M3 with the additional inductor and capacitor. I have a hammond enclosure that is 5x7x3 that should work fine but it doesn't look as nice. I have some heat sinks laying around that are pretty big. I might mount them to the sides of the enclosure so the attenuator can handle a larger amp.


----------



## JohnH

hi @dbishopbliss , thanks for posting the build pics, it looks very effective!

Those Arcol 100W resistors look like they use a fair amount of real estate, with the flanges on each side. The eBay ones just have one flange, more compact but less heat transfer.

One thing I can mention: in the circuit, the stages go -7db reactive, -7, -14, -3.5. There are minor electrical reasons for that. but physically, if you wish, you can mount the switches in ascending order of value, if that appeals.

I'm glad it's working well and letting you crank up to the sweet spot!


----------



## Mark Soltow

A big thank you for posting up the M2 circuit.

Big day today as I finished construction and tried it out. Not to mention my first post on the Marshall Forum. (Edit--sorry about double posting the pictures)

This took a little longer than I thought, due to the amateur metal fabrication. I wanted something a little heftier than the project boxes most people use. Wiring went pretty slow too, 18g stranded was a pain with the little holes in the Arco resistors. Not having any amps over 15 W, I used lower rated resistors.

Anyway, tried it out with a project amp (6V6 push pull) and it sounds....attenuated. Never used an attenuator of any kind before so didn't really know what to expect.

Playing at 60 dB SPL is a different sensory experience than playing at 85-90 dB SPL. Once you get past that and focus on what you're playing, it's actually pretty cool.

Well worth it in my opinion.






,


----------



## JohnH

hi @Mark Soltow , thanks for posting. It looks to be an awesome build, nice to have plenty of space to work in!

That system for mounting the coil looks ideal for the bobbin-less type, being zip-tied to a perf-board, which is then bolted to the case with a stand-off gap. That avoids inductance and loss issues with both the bolts and the case.


----------



## dbishopbliss

Mark Soltow said:


> This took a little longer than I thought, due to the amateur metal fabrication. I wanted something a little heftier than the project boxes most people use.


That certainly looks hefty! I thought about doing something similar. I bet the aluminum cost as much as all the other parts.


----------



## JohnH

*Double Trouble! Attenuator M2x2
*
Here's a new variant based on M2. It's aimed at getting a more versatile option for a unit to work across different amp and cab impedances.

*Previously: 
*
Designs M and M2 were based on a fixed amp impedance tap, and then work with various cab ohms.
M2V added an extra stage, to convert to x2 or x1/2 impedance at the front, at the expense of soaking up an extra 3db of power.

*Attenuator M2x2*

I wanted to see if a box can be made that works natively at 8 or 16 ohms, with no added losses, as well as convert to different cab ohms.

It's based on two 8 Ohm M2 circuits:




They are arranged symmetrically to make a mirror image at top and bottom. With both engaged, the two 8 Ohm circuits make a 16 Ohm one, with no quirks.

The stage switches S1, S2 and S3 are now DPDT (each with A and B poles), to operate on both halves at once. But mostly, this type of switch is what we use anyway since they are common units.

S5A and S5B is a DPDT that shunts the lower circuit, giving effectively an 8 Ohm M2, which is how the diagram is shown above.

S4A and S4B is another DPDT to reduce damping and adjust impedance, to lift and correct the tone when using a 16 cab with and 8 Ohm tap.

But whether or not S4 is engaged, it's safe to put any 4, 8 or 16 cab into the output, using either 8 or 16 input.

All the values of each half are as M2, with the lower circuit references R101 instead of R1 etc. All the 100 series resistors can be half the power rating of the upper ones, since they always share the work. L101 should be the same as L1 ie 0.9mH 18 gage air cored.

So although there's more wiring, its not hard to use and the extra parts are actually only a fairly small part of the total cost.

There's also an even more versatile one possible, with the same two circuits, that adds a native 4 Ohm front end by putting the two circuits in parallel. But this needs a 4-pole switch for S5, with high current, and I figure that most amps have at least an 8 or a 16 tap.

I worked this out mainly just as a puzzle to see where it might lead. Maybe it will be useful for some, though for most, a basic M2 will do the business.

If anyone is tempted to try it, the way it works is fully proven based on previous designs, so no issues there. In this case it becomes essential (instead of just advisable) to use insulated jacks eg (Cliff type), and it'll definitely need a bigger case.


----------



## dbishopbliss

JohnH said:


> But whether or not S5 is engaged, it's safe to put any 4, 8 or 16 cab into the output, using either 8 or 16 input.



Please comment on what happens if you use a 4 or 16 ohm cab with an 8 ohm M2. Is it safe? I'm guessing it is sort of similar to mismatching a speaker cab so probably safe unless your amp is pushing extremes. What impact would you expect it to have on the response and/or sound?


----------



## JohnH

It's definitely safe for the amp. Given the first reactive stage is fixed, the nominal ohms seen by the amp stays close enough to 8. The greatest variation is if all other stages are off. Then the ohms seen by the amp are about 7.3, 8.3 and 9.7 for a 4, 8 and 16 Ohm cab.

The tone is closest to normal with an 8 ohm cab. With a 4 ohm cab, it's a tiny tad brighter, easily compensated in the amp, with treble or presence, if you want to. A 16 ohm cab is a bit more muted in the high treble, and its a more noticeable change. That's why the 8 Ohm M2 circuit has a special 16 ohm out, that adjusts for this.


----------



## iefes

JohnH said:


> Here's a new variant based on M2. It's aimed at getting a more versatile option for a unit to work across different amp and cab impedances.



Hi John,

great you're never getting tired of working out further evolutionary stages of the attenuator. Even though for me that's not the most elegant approach as it involves many parts, it's a really nice idea and it's good to know it would work like this. It's great your publishing all those ideas, a lot of stuff for us to learn and think through and eventually all these ideas can be used, combined etc. to work out suitable designs for different needs! 

So thanks for your ongoing research on this!


----------



## musicheals

musicheals said:


> That's great! I thought I have to build me reactive load.
> Today I will order all parts. When I finished your attenuator, I will review and send some photos...
> 
> Thanks a lot John
> musicheals


I got all the parts now. I didn't order in China. I bought at my long time supplier Reichelt in Germany. It was no big difference in price. Only the big guy was around 7€. I attach a picture...

Thanks
musicheals


----------



## JohnH

musicheals said:


> I got all the parts now. I didn't order in China. I bought at my long time supplier Reichelt in Germany. It was no big difference in price. Only the big guy was around 7€. I attach a picture...
> 
> Thanks
> musicheals



Thanks, and that does look like a great place to buy from in Europe. eg Heres a page of their metal case resistors:

Wirewound resistors in metal housing at reichelt elektronik

I saw suitable coils there too. Good luck with your build.


----------



## musicheals

Thanks a lot John! The coil I bought there also. It was a 1mH and I unwinded it until it was exactly 0.9mH


----------



## hoost

Hi John,

Thanks for all of your hard work and contributions on this and the various other projects you've come up with for us guitarists (I've built and installed your buffer in one of my guitars).

I built what I think is the M1 version of this attenuator a couple of years ago and it works well.

I've since acquired a Tone King Sky King which has two built in attenuators and I really like having them on board and I like the functionality of the rotary switch.

I'm working on building a single channel deluxe reverb in a princeton reverb chassis, and I would like to include this attenuator on a rotary switch as a sort of master volume.

I've reviewed the new M2 design as well as your previous design which incorporates a rotary switch, and come up with the following:






My goal is to have a six position rotary switch with the following db reductions: 0, 7, 14, 21, 28, 35

Am I on the right track here? Do the values look right?

Thanks again for all of your help and all of your contributions.

Sincerely,

hoost


----------



## musicheals

@JohnH 
I finished the M2 today. Tomorrow I will test. 
I attach some photos...


----------



## musicheals

Here the inside


----------



## JohnH

hoost said:


> Hi John,
> 
> Thanks for all of your hard work and contributions on this and the various other projects you've come up with for us guitarists (I've built and installed your buffer in one of my guitars).
> 
> I built what I think is the M1 version of this attenuator a couple of years ago and it works well.
> 
> I've since acquired a Tone King Sky King which has two built in attenuators and I really like having them on board and I like the functionality of the rotary switch.
> 
> I'm working on building a single channel deluxe reverb in a princeton reverb chassis, and I would like to include this attenuator on a rotary switch as a sort of master volume.
> 
> I've reviewed the new M2 design as well as your previous design which incorporates a rotary switch, and come up with the following:
> 
> 
> 
> 
> 
> 
> My goal is to have a six position rotary switch with the following db reductions: 0, 7, 14, 21, 28, 35
> 
> Am I on the right track here? Do the values look right?
> 
> Thanks again for all of your help and all of your contributions.
> 
> Sincerely,
> 
> hoost



Hi hoost, thanks for your interest and good to hear the M that you built before worked out well.

Doing these with a rotary as you describe is trickier than you might expect and i dont think that diagram will work rightly. To make these designs work its essential to keep control of both input and output impedances, for the safety of the amp and the consistency of the tone. It looks like the sketch is based around 8 Ohms, but I see that at minimum setting, the amp will see the 3.75 Ohm resistor, and output impedance varies widely.

The other issue is that if the switch is dealing with the full output as there, it needs to be rated for the full current from the amp which is several amps. Its very hard to find rotaries to do that.

Using the toggle switches allows a wide variation of settings, with plenty of switch rating, and each setting optimized for consistent results.


----------



## Mjh36

Adding mine to the pile. Thanks to all the heavy hitters here, I've learned a lot. 50w 8 0hm M2 w/ additional 16 Ohm out, 10k line out pot, Hammond 1590F enclosure. I went with Mouser for the enclosure and resistors, PartsExpress for the coil as mentioned in earlier posts. The switches are 15 Amps from Tayda, so a little big but that's ok. Various gauges of wire for fun, 14,16,18 awg.

I don't have a tube amp yet but it sure looks nice! It'll be paired with a 20w Jet City style amp eventually. I got what looked like normal readings when using a speaker and measuring the input. It read between about 8 to 10.5 Ohms in all the positions. I might add lights... I messed up on two holes on top so I added umbrellas for cooling. I may have a couple more questions but I'll put that in another post. This is in the top ten for the greatest threads on the entire internet.


----------



## musicheals

Today I tested with a Vibrolux reverb, Princeton reverb, a JCM 800 and a 20 watt Marshall....
No change in tone and feel at all. The attenuation steps are very practical and need no change. Well done John and thank you so much! My air brake will get retired. 
Next I will try a foot switchable attenuation for stage use.

musicheals


----------



## hoost

JohnH said:


> Hi hoost, thanks for your interest and good to hear the M that you built before worked out well.
> 
> Doing these with a rotary as you describe is trickier than you might expect and i dont think that diagram will work rightly. To make these designs work its essential to keep control of both input and output impedances, for the safety of the amp and the consistency of the tone. It looks like the sketch is based around 8 Ohms, but I see that at minimum setting, the amp will see the 3.75 Ohm resistor, and output impedance varies widely.
> 
> The other issue is that if the switch is dealing with the full output as there, it needs to be rated for the full current from the amp which is several amps. Its very hard to find rotaries to do that.
> 
> Using the toggle switches allows a wide variation of settings, with plenty of switch rating, and each setting optimized for consistent results.



Hi John,

Thanks for your reply!

As I have it now, the 3.75 ohm resistor would be seen in the maximum attenuation setting.

The resistors on the left are the "parallel resistors", with the top most setting being 0 db attenuation.

The resistors on the right are the "series resistors".

So the first stage switches in the reactive load stage as in your M2 design, only on the series side.

The following four stages are based around the -7db stage in the M2.

Position 1 (0 db): no resistors engaged
Position 2 (-7 db): M2 reactive load stage on the series side of the switch
Position 3: (- 14 db) M2 reactive load + 10 ohms in series tip to tip | 15 ohms in parallel to ground
Position 4: (-21 db) M2 reactive load + 10 ohms + 10 ohms series | 7.5 ohms in parallel to ground
Position 5: (-28 db) M2 reactive load + 10 ohms + 10 ohms + 10 ohms series | 5 ohms in parallel to ground
Position 6: (-35 db) M2 reactive load + 10 ohms * 4 series | 3.75 ohms in parallel to ground

Does that make sense?

Thanks!

hoost


----------



## hoost

JohnH said:


> Hi hoost, thanks for your interest and good to hear the M that you built before worked out well.
> 
> Doing these with a rotary as you describe is trickier than you might expect and i dont think that diagram will work rightly. To make these designs work its essential to keep control of both input and output impedances, for the safety of the amp and the consistency of the tone. It looks like the sketch is based around 8 Ohms, but I see that at minimum setting, the amp will see the 3.75 Ohm resistor, and output impedance varies widely.
> 
> The other issue is that if the switch is dealing with the full output as there, it needs to be rated for the full current from the amp which is several amps. Its very hard to find rotaries to do that.
> 
> Using the toggle switches allows a wide variation of settings, with plenty of switch rating, and each setting optimized for consistent results.



Also, regarding the switch, I'm not sure if I'm doing the calculation perfectly, but I figured 22W at worst case 4 ohms is a maximum current of 2.35A, so if I got a switch that can handle 3A it should be able to handle the load. 

Does that sound right?

They're not cheap but they are available and for me the convenience of having everything in one package as well as the aesthetics are worth the price.

Thanks again!


----------



## JohnH

Hi @hoost 

If you have a 3A switch with the right poles, and its just for a 22W amp then yes that's fine.

But on the wiring, on my diagrams, the -7db reactive stage is always first in line after the amp, before the switched stages. So the amp never sees the low parallel resistors directly and always sits reasonably close to the ohms that it expects. 

Also, the basic two-resistor arrangement of each stage works best at -7db, and works well at 1/2x or 2x that if its after Stage 1. Outside of that range the tone starts to drift away from being an tone match.

I did have a go at a rotary version, which used rotary for a 0 to -15db range, and another fixed stage:













Attenuator M2 Rotary 201219



__ JohnH
__ Dec 21, 2019


----------



## JohnH

musicheals said:


> Today I tested with a Vibrolux reverb, Princeton reverb, a JCM 800 and a 20 watt Marshall....
> No change in tone and feel at all. The attenuation steps are very practical and need no change. Well done John and thank you so much! My air brake will get retired.
> Next I will try a foot switchable attenuation for stage use.
> 
> musicheals



That's great news! In one session you have tested the design with most of the key amp types!


----------



## hoost

JohnH said:


> Hi @hoost
> 
> If you have a 3A switch with the right poles, and its just for a 22W amp then yes that's fine.
> 
> But on the wiring, on my diagrams, the -7db reactive stage is always first in line after the amp, before the switched stages. So the amp never sees the low parallel resistors directly and always sits reasonably close to the ohms that it expects.
> 
> Also, the basic two-resistor arrangement of each stage works best at -7db, and works well at 1/2x or 2x that if its after Stage 1. Outside of that range the tone starts to drift away from being an tone match.
> 
> I did have a go at a rotary version, which used rotary for a 0 to -15db range, and another fixed stage:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Attenuator M2 Rotary 201219
> 
> 
> 
> __ JohnH
> __ Dec 21, 2019



Hi John,

This is what I modeled the concept on, but instead of having stage 1 fixed, I moved it between pins 1 and 2 on the series side of the rotary switch and attempted to balance out the remaining stages for an additional -7 db each. 

I'm just starting to wrap my head around the math of the additional resistive stages so I may need to tweak them a bit, but the M2 -7 db stage was using 10 ohms in series and 15 ohms in parallel, so that's what I targeted.






Thanks,

hoost


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## JohnO9

Hi John,
In my search for a DIY attenuator I came to this thread somewhere around page 82. Reading it completely, it looked perfect to build. I've just finished an M2 version and it works well - maintains tone with attenuation with my Tweed Deluxe and varients.
The case is made around a heatsink from Parts Express, where I also got the switches and inductor. Aluminum L pieces form the chassis, with a slot at the bottom for air flow. The inductor is mounted on a plastic piece that came with the heatsink. The resistors are from Digikey, and vinyl graphics from IndCom.
Thanks for this project. JohnO


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## Gene Ballzz

JohnO9 said:


> Hi John,
> In my search for a DIY attenuator I came to this thread somewhere around page 82. Reading it completely, it looked perfect to build. I've just finished an M2 version and it works well - maintains tone with attenuation with my Tweed Deluxe and varients.
> The case is made around a heatsink from Parts Express, where I also got the switches and inductor. Aluminum L pieces form the chassis, with a slot at the bottom for air flow. The inductor is mounted on a plastic piece that came with the heatsink. The resistors are from Digikey, and vinyl graphics from IndCom.
> Thanks for this project. JohnO



Looks great and  to this fantastic forum! Yes, these attenuators are the holy grail of the whole genre!
Just Sayin'
Gene


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## JohnH

hi @JohnO9 , thanks for posting. It looks badass with that heatsink and i like the rocker switches, much sleeker than the usual toggles.


----------



## hoost

Okay, I've taken another stab at the rotary switch concept.

My goal being to get the following attenuation levels on a rotary switch to be built into an amp (similar to my Tone King Sky King with Ironman Attenuator):

0 db
-7 db
-14 db
-21 db
-28 db
-35 db

In this layout I'm proposing using a 6 position, 4 pole rotary switch.

This would allow me (hopefully) to utilize JohnH's already proven -7db and -14db attenuation stages.

See any issues with the layout?






The biggest issue aside from the layout will be the cost and availability of a rotary switch with an adequate number of poles and a sufficient current rating for the amp's load.

In doing some digging, there are many folks at the amp garage who were building the air brake attenuator using a Lorin rotary switch (CK1455) with no issues for use with amps at least as large as 50W; this switch is rated for only 150mA.

I am eyeing a 6 position, 2 pole, 2 deck rotary switch made by Grayhill, which is rated for 1A. They have a shorting version (44D30-02-2-AJS) and a non-shorting version (44A30-02-2-06N). Both retail for around $60. Any thoughts on which is preferable? I would think the shorting version, but I haven't fully considered the risks.

Thanks!


----------



## JohnH

hi @hoost

You are the best to judge what risks you want to take with switch ratings for your gear, and I don't know enough to advise on if ratings can be exceeded. It could be said that switch ratings are based on actually switching those amps and volts under load, whereas the sensible attenuator user will only move the switches when not under load (at least, not playing at that moment). But then, what if you strummed a mighty power chord and then dialed in your volume while it rang out? If contact failure leads to an open circuit under high output, out pops the output tubes with possible OT damage (and ive done that!)

Your switch diagram looks ok to me and seems to get the connections as intended. I think make before break is the way to go.

The switch that you linked looks to be 2-pole though. What you are finding is why I haven't gone very far with rotary versions of these units, since they are way simpler with the separate two-position switches. You quickly get used to it and you get more different settings available too.


----------



## hoost

JohnH said:


> hi @hoost
> 
> You are the best to judge what risks you want to take with switch ratings for your gear, and I don't know enough to advise on if ratings can be exceeded. It could be said that switch ratings are based on actually switching those amps and volts under load, whereas the sensible attenuator user will only move the switches when not under load (at least, not playing at that moment). But then, what if you strummed a mighty power chord and then dialed in your volume while it rang out? If contact failure leads to an open circuit under high output, out pops the output tubes with possible OT damage (and ive done that!)
> 
> Your switch diagram looks ok to me and seems to get the connections as intended. I think make before break is the way to go.
> 
> The switch that you linked looks to be 2-pole though. What you are finding is why I haven't gone very far with rotary versions of these units, since they are way simpler with the separate two-position switches. You quickly get used to it and you get more different settings available too.



Thanks John.

The 44D30-02-2-AJS switch linked above is two poles per deck, with two decks, so I think it should work.

For those looking in the future, here is another option with a 5A current rating:

44A45-04-1-06S

Mouser doesn't stock that part so I'm not sure on lead time or minimum order quantity.

I do have the multi-switched version M as a standalone unit and it does work well.

After having the Tone King, I'm hung up on the rotary switch concept built into the amp (I'm getting ready to build a single channel deluxe reverb with no tremolo in a Princeton chassis), so I think I'm going to push forward with the idea.

Thanks again for all of the help, and all of your great contributions to the community. As I mentioned before, I recently built and installer your buffer in my PRS McCarty and I love it, too. Thank you.


----------



## Gene Ballzz

@hoost 
An observation and/or suggestion, without analyzing your switch wiring (I'm horrible with switch configurations/interactions) or doing the math. If you take the 0db option off of the rotary and control that via a "bypass" toggle switch of high enough current rating (4 to 5 Amps) the current requirement for all the subsequent stages on the rotary drops dramatically. Make it so that the first, initial reactive stage is always activated, anytime the unit is not in "bypass" mode. That also takes away the need for two switch positions. Likely an easier to find switch. Then add a -3.5db stage that can be added on to any/all of the other stages, via a fairly low current rated toggle, to get those in between settings. Trust me when I tell you that -3.5db up/down fine tuning helps get to the best sweet spots!
Just My Considered Experience!
Gene


----------



## hoost

Gene Ballzz said:


> @hoost
> An observation and/or suggestion, without analyzing your switch wiring (I'm horrible with switch configurations/interactions) or doing the math. If you take the 0db option off of the rotary and control that via a "bypass" toggle switch of high enough current rating (4 to 5 Amps) the current requirement for all the subsequent stages on the rotary drops dramatically. Make it so that the first, initial reactive stage is always activated, anytime the unit is not in "bypass" mode. That also takes away the need for two switch positions. Likely an easier to find switch. Then add a -3.5db stage that can be added on to any/all of the other stages, via a fairly low current rated toggle, to get those in between settings. Trust me when I tell you that -3.5db up/down fine tuning helps get to the best sweet spots!
> Just My Considered Experience!
> Gene



Thanks Gene. I will stew on the idea for a bit.

What I've found with my Sky King is I'm usually either on the -15db or the -24db setting. Rarely will I go to the -36db. If I'm going to play that quietly I just pop on my Waza air headphones. That being a 40W amp, and this amp I'm planning to build being a 22W, it might indeed make sense to have the 3.5db increment, but I might go something like 0, 7, 10.5, 14, 17.5, 21.

I think at this point I might just have to build it and see what works for me. I can always swap resistor "attenuation stages" around until I get it dialed in.

Doing the bypass switching as you describe would certainly work, but I'm not too hung up on the cost of the switch and the cleanliness and simplicity of the interface is worth the cost to me. If I burn up the switch and some tubes and transformers I might change my mind pretty quickly though!

Thanks!


----------



## JohnH

All ok, but i have just a couple more thoughts:

The idea of having at least the bypass function as a separate switch seems like a very sensible and practical feature. The 'bypassed' or 'not-bypassed' change is where the greatest care is needed to ensure everything is connected up, under control and your signal zero, to avoid nasty transients. Shorting contacts (make-before-break) should be fine but a bit of dirt or aging contacts and you could start to get the nastinesses on every switch move. But if you get reactive stage 1 fully engaged before the rotary its all a lot safer, and you are less likely to go to that bypass mode inadvertently. 

And then you get two more benefits:

The max current through the rotary switch is about halved and actually, at 22w and 8 Ohms, its halving from 1.7A to 0.85A so that switch is then working within spec.
Plus, you have an extra switch position for an attenuated setting. To make the most of that feature, we can change the attenuation values of each stage to make them closer, see as follows:

On the basic designs, the three resistive stages each have two resistors. In theory, the ideal db value for these, based on this design is the -7db stage, which is why this setting is used in stage 1 and also 2. But, for stage 2 , 3 and 4, they also work fine either side of that in the range from about -3db to -15db provided reactive Stage 1 is first. M and M2 therefore use -3.5, -7 and -14db. 

But, its pretty easy to work out sets of values for resistive stages anywhere within the range. This, together with the spare knob position and a choice that you don't really need to go to such a low setting, could allow you to set closer spacings between attenuation levels.

eg, you could use 4db steps and have:
-0db Fully bypassed
-7db Reactive Stage 1, then:
-11, -15, -19, -23, -27 db 

or 5 db steps:
-0db Fully bypassed
-7db Reactive Stage 1, then:
-12, -17, -23, -28, -33 db

If any of that is of interest, Id be happy to run numbers for it.


----------



## hoost

JohnH said:


> All ok, but i have just a couple more thoughts:
> 
> The idea of having at least the bypass function as a separate switch seems like a very sensible and practical feature. The 'bypassed' or 'not-bypassed' change is where the greatest care is needed to ensure everything is connected up, under control and your signal zero, to avoid nasty transients. Shorting contacts (make-before-break) should be fine but a bit of dirt or aging contacts and you could start to get the nastinesses on every switch move. But if you get reactive stage 1 fully engaged before the rotary its all a lot safer, and you are less likely to go to that bypass mode inadvertently.
> 
> And then you get two more benefits:
> 
> The max current through the rotary switch is about halved and actually, at 22w and 8 Ohms, its halving from 1.7A to 0.85A so that switch is then working within spec.
> Plus, you have an extra switch position for an attenuated setting. To make the most of that feature, we can change the attenuation values of each stage to make them closer, see as follows:
> 
> On the basic designs, the three resistive stages each have two resistors. In theory, the ideal db value for these, based on this design is the -7db stage, which is why this setting is used in stage 1 and also 2. But, for stage 2 , 3 and 4, they also work fine either side of that in the range from about -3db to -15db provided reactive Stage 1 is first. M and M2 therefore use -3.5, -7 and -14db.
> 
> But, its pretty easy to work out sets of values for resistive stages anywhere within the range. This, together with the spare knob position and a choice that you don't really need to go to such a low setting, could allow you to set closer spacings between attenuation levels.
> 
> eg, you could use 4db steps and have:
> -0db Fully bypassed
> -7db Reactive Stage 1, then:
> -11, -15, -19, -23, -27 db
> 
> or 5 db steps:
> -0db Fully bypassed
> -7db Reactive Stage 1, then:
> -12, -17, -23, -28, -33 db
> 
> If any of that is of interest, Id be happy to run numbers for it.



Hi John,

I'm still kicking around the idea of a bypass switch and I do think it has a lot of practical benefits. Maybe I'd put the toggle above the rotary switch kind of how Dumble does some switches above the pots.

If it's not too much trouble to run the numbers for the 4 and 5 db steps, I'd certainly appreciate it, and I would imagine there may be some others who come along later who might find it useful.

I think I'd be inclined to start with the 4 db steps, although it's hard to know until you crank it up!

Thanks again!


----------



## JohnH

No problem, here they are:




To get the 4db steps, you'd need one -4db stage and two at -8db. Or for 5db steps, a 5 and two 10's.

The values are shown above. If you cant get a 6 Ohm, get 5.6.

The plots are based on the 4 db steps, including with stage 1 being the -7db reactive stage, followed by the resistor stages. Its running the whole circuit analysis including a speaker sim model.

Red represents the full unattenuated speaker response, then you can see the calculated attenuated plots below it, in fairly even steps each close to 4db. At right, all the plots are normalised to the same volume so you can see the consistency of the predicted tones. The 5 and 10 db values produce an equivalent pair of plots.


----------



## hoost

JohnH said:


> No problem, here they are:
> 
> View attachment 90920
> 
> 
> To get the 4db steps, you'd need one -4db stage and two at -8db. Or for 5db steps, a 5 and two 10's.
> 
> The values are shown above. If you cant get a 6 Ohm, get 5.6.
> 
> The plots are based on the 4 db steps, including with stage 1 being the -7db reactive stage, followed by the resistor stages. Its running the whole circuit analysis including a speaker sim model.
> 
> Red represents the full unattenuated speaker response, then you can see the calculated attenuated plots below it, in fairly even steps each close to 4db. At right, all the plots are normalised to the same volume so you can see the consistency of the predicted tones. The 5 and 10 db values produce an equivalent pair of plots.



Excellent!

Thanks so much for all of your help John. I really appreciate it.

It will probably take a few months to get the amp design reviewed and built, but I will report back when everything is up and running with how it works.

Thanks!


----------



## JohnH

hi @hoost

One more idea, in case it helps in due course:

With a toggle bypass, fixed reactive stage, then three more switched stages and a 6position switch, that probably needs 3 switch poles. If you have the 4pole switch, all good. But if it's easier or more economical to use a two pole 6 position, then it can be done, in a couple of ways. It needs some more resistors though. I'll post more if it's of interest to anyone.


----------



## dbishopbliss

I'm just about to finish assembling what might be the first M3 build. I noticed the diagram says R1 is 12 ohms while R1 is 15 ohms on the M2. The diagram also says that R2B is 15 ohms, but there is a note in the comments that says 18 is better. Did I miss a note somewhere or do I need to order a 12R resistor?


----------



## JohnH

hi @dbishopbliss 

Well that's exciting news! Yes I'd say you are the first known builder if an M3, though we can't tell who else builds these without posting.

The best optimised values for new designs sometimes take a few iterations to get to. Where it's at now, after tweaking the design a few times, all the values for M3 would be the same as M2, plus the extra coil and cap. But if you have values from an earlier diagram of mine, then they should work fine too. Generally, there is a fairly broad sweet spot around the best sets of component values.


----------



## dbishopbliss

Very early and subjective thoughts... the levels for each position are pretty much the same when I switch back and forth between my M2 and M3. Its hard to say exactly because I have to turn off the amp, unplug it, etc. But I think this is basically expected.

The bigger question is sound. They are VERY similar but I think there is a slight difference - could be wishful thinking. Then again I am comparing Arcol resistors from Mouser to the Ebay Chinese supplier resistors. The values aren't exactly the same but I used parallel resistors to get as close as I could to the Arcol values. 

It does seem there is a little more top-end in the M3 and a little more complexity to the sound if that makes sense. Again, very very subtle and maybe wishful thinking. 

Another difference between the builds is that I have L1 off of the chassis with a 1/4" piece of wood. Could it be that there is less inductance allowing more highs through?

I think what I really need to do is open up the chassis and jumper over the bass resonance components while I'm playing to see if I hear a difference.


----------



## JohnH

hi @dbishopbliss 

Thanks very much for posting those thoughtful observations, and I see no surprises there!

The quality of resistors, if there is any, shouldn't make any significant difference to tone if the values work out the same

L1 and its proximity to the case would be likely (so far as i can think it through) to increase inductance if its a steel case and reduce it if aluminium, and in both cases add resistance to it. (effects of eddy currents in both and permeability in the case of steel)

The M3 bass circuit should have its greatest influence when playing notes that are driven hard, around its resonant frequency, ie around low A on a guitar and a few notes either side. Depending on the amp, you might hear a small level difference, but more likely a difference in the harmonics of those bass notes, and the fading of those harmonics as the notes decay. 

I find it easier to A/B compare tones using a looped riff, and a miced recording.

All really interesting and many thanks again!


----------



## BrokenBones

So i've been on sabbatical from this thread and I noticed the circuit has underwent some changes. I've already bought the components for Design M-4 just haven't built it yet. Is there much of a difference between the two designs to justify buying all new parts?


----------



## JohnH

hi @BrokenBones 

Can you let me know which diagram or post number you are referencing?
cheers
John


----------



## BrokenBones




----------



## JohnH

Thanks yes that's the first fully resolved version, I just call it M.

its totally fine, it the one i built myself and i still use it every day. The main later version is M2 and it has virtually identical performance, but a simpler build since it only needs one coil. M2 needed a bit more thinking and some harder math to solve it, but it doesn't work any better than M.


----------



## dbishopbliss

Here are the glamour shots of the M3. I think she's the sexiest attenuator so far.

Love the way the push button switches (060-958) look and they are very smooth and have a solid feel when they are engaged. They require a 16mm hole, I had a 16.5mm drill bit. Worked out fine.

The enclosure came from china and cost twice as much as the enclosure I used for my M2, but then again look at that front panel.

I used disconnects (095-286) for the wires to the switches. This made connecting them very easy. The gauge is exactly what is required for the switch (had them in a drawer from when I was connecting tweeters on an audio speaker project).

I used an non-polarized electrolytic capacitor for C1 (027-366) and an I Core inductor for L2 (266-578). The electrolytic capacitor is significantly cheaper than a film capacitor. This capacitor is intended to be used in speaker crossovers so I can't imagine that a film capacitor would sound significantly different.

I have had a bit more time comparing the M2 and M3 and there is just the slightest difference. Hard to say exactly what it is but it seems like this is a little more complexity in the sound. As an anology, its sort of like when you have a 2x12 cabinet that has two of the same speaker. Then you change one of them. The sound is still basically the same but there are different tones that weren't there before. It's like that but even more subtle.

One more thing... You might be able to make out that I have extra resistors in there labeled 5a, 5b and 8a, 8b. This is because the Chinese resistors did not have the exact values so I used 8 and 11 ohm resistors to give me 4.63 ohms and 9 and 15 ohm resistors for 5.63 ohms. Probably not necessary but I wanted the values to be close to the 4.7 and 5.6 ohm resistors I used in my M2.













M3 Front Panel Close Up



__ dbishopbliss
__ Jul 6, 2021


















M3 Front Panel



__ dbishopbliss
__ Jul 6, 2021


















M3 Rear Panel



__ dbishopbliss
__ Jul 6, 2021


















M3 Internal wiring



__ dbishopbliss
__ Jul 6, 2021


----------



## JohnH

Thanks, yes that does look bad-ass! and thanks for the comments on tone.0l


----------



## Gene Ballzz

dbishopbliss said:


> Here are the glamour shots of the M3. I think she's the sexiest attenuator so far.
> 
> Love the way the push button switches (060-958) look and they are very smooth and have a solid feel when they are engaged. They require a 16mm hole, I had a 16.5mm drill bit. Worked out fine.
> 
> The enclosure came from china and cost twice as much as the enclosure I used for my M2, but then again look at that front panel.
> 
> I used disconnects (095-286) for the wires to the switches. This made connecting them very easy. The gauge is exactly what is required for the switch (had them in a drawer from when I was connecting tweeters on an audio speaker project).
> 
> I used an non-polarized electrolytic capacitor for C1 (027-366) and an I Core inductor for L2 (266-578). The electrolytic capacitor is significantly cheaper than a film capacitor. This capacitor is intended to be used in speaker crossovers so I can't imagine that a film capacitor would sound significantly different.
> 
> I have had a bit more time comparing the M2 and M3 and there is just the slightest difference. Hard to say exactly what it is but it seems like this is a little more complexity in the sound. As an anology, its sort of like when you have a 2x12 cabinet that has two of the same speaker. Then you change one of them. The sound is still basically the same but there are different tones that weren't there before. It's like that but even more subtle.
> 
> One more thing... You might be able to make out that I have extra resistors in there labeled 5a, 5b and 8a, 8b. This is because the Chinese resistors did not have the exact values so I used 8 and 11 ohm resistors to give me 4.63 ohms and 9 and 15 ohm resistors for 5.63 ohms. Probably not necessary but I wanted the values to be close to the 4.7 and 5.6 ohm resistors I used in my M2.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M3 Front Panel Close Up
> 
> 
> 
> __ dbishopbliss
> __ Jul 6, 2021
> 
> 
> 
> 
> 
> 
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That looks fantastic sitting on that SilverTone cabinet. Very nice build, although I see no ventilation? The fact that I recognize that cabinet has me seriously dating myself!  

Great Work!
Gene


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## dbishopbliss

Gene Ballzz said:


> That looks fantastic sitting on that Silvertone cabinet. Very nice build, although I see no ventilation? The fact that I recognize that cabinet has me seriously dating myself!
> 
> Great Work!
> Gene



I have been using the attenuator with a Vox AC15 and a Silvertone 1482 and monitoring the heat but I have not found the enclosure or resistors to get warm at all so I have not included any ventilation holes. Perhaps if I had higher power amps then I might have a need but for my amps it doesn't seem necessary.

With regard to the Silvertone cabinet... that's a good catch, but its actually not original at all. I rebuilt the cardboard enclosure out of wood/plywood. I milled the trim to match the angles of the original and then I covered the amp in "blackened tuna" tolex. That's the closest covering I could find to the original. The grill cloth is not even close to the original, but I like it. Its actually Ampeg grill cloth installed back-side out.


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## abzic

Hi

I am happy to share that I built my attenuator based on the info on this thread, and it was a total success! Bought everything from China, total cost for two units was well below $100. I am using my attenuators with a Fender DRRI (22w), Fender Bassbreaker 007 (7w), Ampeg GVT15H (15w) and EVH 5150 iii LBX (15w). 

Tone is super and it runs totally cool. Another big plus I found out is the dramatic reduction in the base hum, actually I can not hear any hum at all when using the higher attenuation values. This is very welcome in a recording situation. 

Attached is a picture with a tube screamer on top of the attenuators. Units feature two switches, the one to the left bypasses the attenuation, the other flips attenuation between 14db and 28db.

Thank you all for contribution on this thread and specially to JohnH.


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## JohnH

hi @abzic thanks for posting. A stripped down unit with just a couple of settings can be a very useful thing, and a neat simple build. And suppressing residual hiss and hum is another good purpose for an attenuator, for when you need to be at very low volume.


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## Northtone

abzic said:


> Hi
> 
> I am happy to share that I built my attenuator based on the info on this thread, and it was a total success! Bought everything from China, total cost for two units was well below $100. I am using my attenuators with a Fender DRRI (22w), Fender Bassbreaker 007 (7w), Ampeg GVT15H (15w) and EVH 5150 iii LBX (15w).
> 
> Tone is super and it runs totally cool. Another big plus I found out is the dramatic reduction in the base hum, actually I can not hear any hum at all when using the higher attenuation values. This is very welcome in a recording situation.
> 
> Attached is a picture with a tube screamer on top of the attenuators. Units feature two switches, the one to the left bypasses the attenuation, the other flips attenuation between 14db and 28db.
> 
> Thank you all for contribution on this thread and specially to JohnH.



That looks really nice! I am planning to build an attenuator as well. Could you share where you sourced your parts?


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## _Steve

abzic said:


> Hi
> 
> I am happy to share that I built my attenuator based on the info on this thread, and it was a total success! Bought everything from China, total cost for two units was well below $100. I am using my attenuators with a Fender DRRI (22w), Fender Bassbreaker 007 (7w), Ampeg GVT15H (15w) and EVH 5150 iii LBX (15w).
> 
> Tone is super and it runs totally cool. Another big plus I found out is the dramatic reduction in the base hum, actually I can not hear any hum at all when using the higher attenuation values. This is very welcome in a recording situation.
> 
> Attached is a picture with a tube screamer on top of the attenuators. Units feature two switches, the one to the left bypasses the attenuation, the other flips attenuation between 14db and 28db.
> 
> Thank you all for contribution on this thread and specially to JohnH.



Looks great! How did you manage to squeeze everything in there?


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## abzic

_Steve said:


> Looks great! How did you manage to squeeze everything in there?


Hi... Thank you for your comment. I used resistors with power rating adjusted for a 25w amp, this results in resistors with reduced size. Also, I only used the resistors required for 28db and 14db. See attached image.


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## abzic

Northtone said:


> That looks really nice! I am planning to build an attenuator as well. Could you share where you sourced your parts?


Hi... Thank you for your comment. I bought parts at AliExpress, it is a well established marketplace from China, not too different to eBay. I live in Mexico and it took about two weeks having the parts delivered. Options at AliExpress are plenty, and I had to hand pick parts from several different sellers, ask for shipping cost adjusts, etc. If you are willing to put the time looking for the best deals you can get a low total price. If you live in USA I don't think it's worth the hassle and would buy locally instead.


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## abzic

JohnH said:


> hi @abzic thanks for posting. A stripped down unit with just a couple of settings can be a very useful thing, and a neat simple build. And suppressing residual hiss and hum is another good purpose for an attenuator, for when you need to be at very low volume.


Very true!


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## abzic

JohnH said:


> hi @abzic thanks for posting. A stripped down unit with just a couple of settings can be a very useful thing, and a neat simple build. And suppressing residual hiss and hum is another good purpose for an attenuator, for when you need to be at very low volume.


This has probably been covered before, but just in case, I want to comment that this attenuator complemented with a DI box (like the wirldwind director) and a software cabinet emulator with impulse responses (like two notes wall of sound) makes a killer solution for quietly recording a tube amp. I am using it this way and I can't be happier, consistent fantastic results everytime with no fuss. I have tried other setups for recording, including miking my cabs, and lots of different software/hardware amp modellers. With modellers, at least in my opinion, either the tone is not there or requires endless tweaking to get it, not to talk about the playing feel. Miking the cab has also proved difficult for me, among other reasons because of my room is far from adequate and my limited skills.


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## dbishopbliss

I built a third attenuator for a friend that does not want it (he has an Ox Box). I was thinking of listing it on Reverb with a link back to this thread for reference. Are there any rules against doing that?


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## Dogs of Doom

dbishopbliss said:


> I built a third attenuator for a friend that does not want it (he has an Ox Box). I was thinking of listing it on Reverb with a link back to this thread for reference. Are there any rules against doing that?


no...

you can also list it in the classifieds here...


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## Gene Ballzz

dbishopbliss said:


> I built a third attenuator for a friend that does not want it (he has an Ox Box). I was thinking of listing it on Reverb with a link back to this thread for reference. Are there any rules against doing that?



Which version did you build? Pics would be nice? Price?
Just Curious?
Gene


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## JohnH

dbishopbliss said:


> I built a third attenuator for a friend that does not want it (he has an Ox Box). I was thinking of listing it on Reverb with a link back to this thread for reference. Are there any rules against doing that?



Ive no problem with that.

But, just putting this out there in case it is possible in due course: Id be very interested to hear any simple sound recording that you could make with A/B changes either between your M3 build with the resonance circuit, and with that bypassed so its an M2. And/or, any such comparison involving your friends Ox!


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## dbishopbliss

JohnH said:


> Ive no problem with that.
> 
> But, just putting this out there in case it is possible in due course: Id be very interested to hear any simple sound recording that you could make with A/B changes either between your M3 build with the resonance circuit, and with that bypassed so its an M2. And/or, any such comparison involving your friends Ox!


I will try to get some recordings soon. I also have a CaptorX and a Jet City attenuator (somewhere). The CaptorX works great for recording but the 20dB attenuator is too much for band practice. I generally use 10 or 14dB on the M2. I got the Jet City as a Christmas present years ago. I didn't care for the sound back then. Will be interesting to compare to the M2.


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## ThePanda

Prototyping M2 before I spend money on a chassis. Going with the Hammond 1590D in black.

** sorry for the size... I didn't know Imgur would do this... **


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## dbishopbliss

ThePanda said:


> Prototyping M2 before I spend money on a chassis. Going with the Hammond 1590D in black.


One thing I found making my second attenuator is that you can minimize the number of separate wires required with some creative positioning of the resistors. For example you can connect groups of resistors like this:

EDIT: Text layout removed because the spaces were not maintained. Refer to the screen shot below for the proper layout.




Search back a page or two and you will see a photo of my M3 layout that does this. I used parallel resistors for R5 and R8 to get closer to the values in the design because the ebay vendor did not carry the exact values, buy you should get the idea.


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## JohnH

dbishopbliss said:


> One thing I found making my second attenuator is that you can minimize the number of separate wires required with some creative positioning of the resistors. For example you can connect groups of resistors like this:
> 
> +--R8--+
> |
> +--R4--+--R6--+
> |
> +-----R2B-----+
> |
> +-----R2A-----+
> 
> 
> 
> +--R5--+--R7--+
> |
> +--R3--+
> 
> Search back a page or two and you will see a photo of my M3 layout that does this. I used parallel resistors for R5 and R8 to get closer to the values in the design because the ebay vendor did not carry the exact values, buy you should get the idea.



For others - It's definitely worth planning out positions and wiring runs to minimise wire-tangle, for a simpler and cleaner build. The schematics show what is connected to what but they needent be followed for specific wire runs. eg, take R4 and R6, connected to each other, and also to two switches. i would do a link switch to switch, one R4 to R6 then a single link from those switch lugs to the resistors. Also consider that it is usually easier if only one or two wires need to meet at a given lug.

dishopbliss - I can see when i quote your message above, that your text diagram is right as you intended, but when see it in my browser (Edge) within the post it's coming out wrongly, wires shifted etc. Could I please ask that it be replaced with a screen shot or something fixed? Its just that it could be a cause of confusion


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## JohnH

I think it should look like this:


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## JohnH

Hi @ThePanda , i just noticed on your parts photo. It looks like you are going for the 8 Ohm M2 with 3 output jacks. If so, Output 3 for use with a 16 ohm cab is intended to be a stereo jack with 3 lugs. When you plug in a normal mono plug, the ring gets shunted to ground, which brings in a resistor to adjust tone and impedance. its only an issue if you use a 16 cab with it so no biggie. Good luck with the build.


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## Gene Ballzz

Fairly early on in this thread, @JohnH mentioned preferring to use STEREO/TRS Cliff UK jacks throughout, even when only MONO/TS is needed, due to more solidly gripping /supporting the plug when inserted. Upon judicious consideration I agree completely!  I'll even add the observations that it conveniently leaves the builder some switching options (should they be experimental modders, etc) and the fact that the cost difference is negligible. Heck, I've even seen sales that sell the STEREO/TRS for less. CLIFF UK is the important part, as the knockoffs are really terrible. I hear the REAN isn't bad, bur have little experience with them, here in the U.S. 

And again, as has been previously advised, keep the attenuator circuitry isolated from the chassis, by NOT using stadard SWITCHCRAFT units!

Just Kibitzin' 
Gene


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## dbishopbliss

JohnH said:


> I think it should look like this:
> 
> View attachment 92064


That is what I typed but it seems that the extra spaces are being collapsed in the HTML. I guess that is why you don't try to draw with text.


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## ThePanda

JohnH said:


> Hi @ThePanda , i just noticed on your parts photo. It looks like you are going for the 8 Ohm M2 with 3 output jacks. If so, Output 3 for use with a 16 ohm cab is intended to be a stereo jack with 3 lugs. When you plug in a normal mono plug, the ring gets shunted to ground, which brings in a resistor to adjust tone and impedance. its only an issue if you use a 16 cab with it so no biggie. Good luck with the build.


Thanks! I went ahead and ordered some stereo jacks (and chassis feet) that are coming today!

I was doing some research on the 16 Ohm Stereo Cab and don't really understand how it is used. 

How is that used exactly?


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## JohnH

stereo cabs can either be in mono at 16ohms, or as two separate 8ohm pairs, depending on how you use the switch and jacks. I haven't really figured it out, but I font see them as much use. Best to just had it wired simply 

Not related to the attenuator though. The 'stereo' jacks are used for other reasons.


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## ThePanda

This is what I came up with following you guys advice. I may play with some more before I start drilling.

The bottom plate to be the is supposed to be the bottom and the switches/jacks come out the side.


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## JohnH

Hi @ThePanda 

it looks like you are using the same box that I used, which is a good solid one, great for spreading out heat.

My own build is on post 1, and its the earlier design M with two coils, but really works the same. My thinking is the 'lid' of the box becomes the base. I fixed the coils to that and everything else, switches jacks and resistors go into the body of the case, so the resistors end up on top. Plenty of large holes in base and top too for ventilation

Theres lots of ways to lay it all out though


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## Blaoskaak

I am looking for a attenuator for my sv20h. Now i use a harley benton cheap one and i hate the sound of my sv20h. When i played it without attenuator and blew my ears up, the amp sounds amazing! So i've been looking on google and everytime i'll end up on this topic. So it must be good. The problem is that i can't read diagrams, but i can solder. 

Is there a summary somewhere of what parts you need and a roadmap? If this is there then I will probably figure it out and not have to spend 100s of euros for an attenuator.

Thanks!


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## neikeel

Currently planning 300w version on thermal pcb directly mounted to large heat sink, will be stealing basic layout from you John. Hopefully pics to follow.


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## JohnH

neikeel said:


> Currently planning 300w version on thermal pcb directly mounted to large heat sink, will be stealing basic layout from you John. Hopefully pics to follow.



Well that's cool! or maybe blisteringly hot! Will you use a fan?

I'm very pleased that such a respected member as yourself is trying this design and I'll be most interested in what you put together and your impressions of it.


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## JohnH

Blaoskaak said:


> I am looking for a attenuator for my sv20h. Now i use a harley benton cheap one and i hate the sound of my sv20h. When i played it without attenuator and blew my ears up, the amp sounds amazing! So i've been looking on google and everytime i'll end up on this topic. So it must be good. The problem is that i can't read diagrams, but i can solder.
> 
> Is there a summary somewhere of what parts you need and a roadmap? If this is there then I will probably figure it out and not have to spend 100s of euros for an attenuator.
> 
> Thanks!



My recommendation would be that to build this, you need to get to a point where the diagram makes sense to you and you can relate it to a physical build layout. This is the only way that you can embrace what you are putting together and own it, and also check it. Its also the reason that after a couple of years on this thread , every build that I know of has been a success for those who have built them. Its actually a simple circuit, much simpler than an amp, so Im sure you can get your head around it.


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## neikeel

My son and I are plotting as we speak. I think a fan will almost certainly be necessary but there are some monster heat sinks out there to use, using metal base pcb and surface mount 50w resistors saves space so you can have lots of options. Need to study inductor types too.
This:





and this:




He is working on some telecom gear at the moment (I'm not signed up to know exactly what!) but has access to some decent programmes and kit. Ideally to be 16, 8 and 4 ohm switchable to handle his SVT and current 300w bass project, maybe incorporate the Valvewizard line out.
But, you are right, removing heat is the issue with those loads, I've managed to get my Hotplate untouchable with a 68 SL at full tilt for a couple of hours so melted solder is not an option!


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## Blaoskaak

JohnH said:


> My recommendation would be that to build this, you need to get to a point where the diagram makes sense to you and you can relate it to a physical build layout. This is the only way that you can embrace what you are putting together and own it, and also check it. Its also the reason that after a couple of years on this thread , every build that I know of has been a success for those who have built them. Its actually a simple circuit, much simpler than an amp, so Im sure you can get your head around it.



Ok, thanks for your input. I am just gonna order the parts and do some research on the internet how the diagrams work. Is de diagram on page 1 the only thing you need to build m2?


What i understand is that i take the value 8 or 16 ohm. Depends on speaker cab? I have 2x 16ohm. That makes 8. So i only take the resistors values fron 8 ohm?


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## JohnH

Yes the M2 diagram shows it all. The most important thing to match is the amp output ohms. That determines whether to build 8 or 16 ohms, or maybe you have both on your amp. Ideally your speakers match that too anyway, If your main set up is with 2 x16 speakers, and if the amp has an 8 out, then build the 8 ohm version.

eg, if you have 8 and 16 outputs, or your amp, and if you usually use 2x16ohm cabs, build the 8 version 

You can use it with different cab ohms too, so long as it matches the amp output


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## johan.b

Great work..
... just wondering (and to dim myself to do this kind of math  )... is there any reason i can't replace R3/4 onwards with an commercial L-pad, for stepless attenuation beyong the 7db insertion loss?
J


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## Gene Ballzz

@JohnH can explain it much better, but from all that I can gather, it is the balance/ratio between the series and parallel resistance that is critical to proper operation and preservation of tone and tactile response. If you read the full thread (well worth the time, and the chaff is fairly easy to skip past) you will see that several folks have tried making this a continuously variable device. The success level has been varied and limited, at best. With that said, the -3.5db steps are audibly small enough to make the need for continously variable operation mostly superfluous. 

Just My -3.5db,
Gene


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## Blaoskaak

Thanks! What kind of switch should i use? Cant find it in the diagram. On/off 2 pin 125ac 5? and the jack, 

RS PRO 6.35 mm Panel Mount Stereo Jack Socket, 3Pole 5A 

will these work?


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## LesPaul_Marshall_Rocker78

Hi there, I´m new here 

I first have to say a MASSIVE THANKS thanks to JohnH for starting this 
Fantastic tread and a wealth of information!!! 


I Really like my Marshall Tones, espesially Those Classic 60`s & 70`s Classic Rock Tones,
I have a small collection of tube amps and Really like the Plexi-style/JMP/800 Tones!! 

So I realized I had to buy a Attenuator / Loadbox ...
After a TON of Research.... I ended up Buying a Universal Audio OX-Loadbox And used as a Loadbox (the software side of it..) it sounds quite good, BUT I have to say the Attenuator portion of it was quite disapointing??  First though I have to say I DO think it preserves/maintains the feel of the playing at all Attenuator volumes but the (Attenuator)sound/tone.... I am not too impressed... 

Soo I am back in the market again for a great sounding ATTENUATOR which do NOT color the sound too much??? 
The old celestions especially the Pulsonic greenbacks lose their character
with the UAD OX It adds too much bass and removes the topend I feel!??
In MY opinion it only sounds good between setting switch to 4-5 on the Attenuator 
and values 1,2, and to certain degree also `3` sounds waaay `too bassy lossy of the topend/precence 
`it almost kills the prerola/pulsonic sound` at low volumes.. 

Also The problem this Attenuator has is `I Do not Really know` what attenuation level I NEED as the UAD OX does not say which soundlevels are at which db-level attenuation???


I have been considering these:
Tone King Ironman II (100watt)
Mike Lind - Power-Plant 100watt Attenuator 16-Ohm version
There was also a suggestion to run/modify the OX to use a different MARSHALL-Friendly Loadbox such as the Suhr Reactive Load in paralell bypassing the OX `bathtub`load curve ??

and perhaps maybe as a last resort (Boss Tube Amp expander)


Can you please share some insight ?? Thanks in advance 

My Amps :
Marshall JMP 2104 78/80s - 50watt - 8ohm combo converted to headshell
Marshall JCM-900 90`s - 50watt 8ohm combo- EL34 Dual Hi-Gain Reverb 
Bugera 1960 infinum - 100watt - 1959 Plexi `clone style` head
Trinity Amps (Handwired) 18watt(1974`ish EL84 - Marshall Plexi style head with powerscaling
Fender - Superchamp X2 (6v6 powered Hybrid amp)
Fender - Rumble 40 Bass-amp (solidstate)
Digidesign/Avid - Eleven Rack
Kemper profiling amp (unpowered toaster)
(And a also a Sennheiser E906 microphone for guitar KPA Profiling/recording )

My speaker cabs :
72/73 Marshall 4x12 1935 - `checkerboard cloth`4x12 (birch) 25w 55hz pulsonic-cones
LOPO cabs `vintage style Bluesbreaker salt&pepper cloth` (before Mojotone buyout) Oversize (birch) 1x12 - 73 Greyback 55hz 25watt pulsonic-cone
Marshall JCM900 combo cab 1x12(birch) Replaced by Celestion old 80`s G12-65 - 8Ohm
I prefer the sound/feel of 16Ohm I think..

Guitars of use :
2011 Gibson SG 60s Tribute 2 P90 pickups, 
1982 Les Paul Std. upgraded 2 PAF(Humbucker) style pickups,
2015 Squier Classic Vibe 60 (sss) Stratocaster 
2016 Squire Matt Freeman sig. 70s bass guitar 
& GFS/Slick SL60(Ash/maple) - LP Special type converted to a single P90 LP Jr. type guitar



- - - -
Btw pardon the long tread and, please excuse bad `grammar` English is not my native language...


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## donwagar

Blaoskaak said:


> Thanks! What kind of switch should i use? Cant find it in the diagram. On/off 2 pin 125ac 5? and the jack,
> RS PRO 6.35 mm Panel Mount Stereo Jack Socket, 3Pole 5A
> 
> will these work?



I haven't started soldering mine together yet, so it's possible I'm mistaken, but, I think you need ON-ON switches SPDT - I thought those would be a bit harder to solder, so I went with DPDT. And I don't think you need stereo jacks, just 2 pole.


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## Blaoskaak

donwagar said:


> I haven't started soldering mine together yet, so it's possible I'm mistaken, but, I think you need ON-ON switches SPDT - I thought those would be a bit harder to solder, so I went with DPDT. And I don't think you need stereo jacks, just 2 pole.



Thanks alot! i did read a few pages back that @JohnH said that you should use 3 pole for the 16ohm input. Don't know if it still counts. i did orded both. they are not so expensive. Is on/on not always on then?

Thanks again!


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## donwagar

Sorry, yes you're right, if you are adding Out 3 (16 ohm cab with an 8 ohm attenuator) then you do need a stereo jack for it. I'm not using Out 3, so I never noticed that.

Someone will correct me if I'm wrong on the switch: I think with the switch in one position, you're sending the circuit through the resistors, with it in the other position the circuit has to continue on and bypass the resistors, so On-On.


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## Blaoskaak

donwagar said:


> Sorry, yes you're right, if you are adding Out 3 (16 ohm cab with an 8 ohm attenuator) then you do need a stereo jack for it. I'm not using Out 3, so I never noticed that.
> 
> Someone will correct me if I'm wrong on the switch: I think with the switch in one position, you're sending the circuit through the resistors, with it in the other position the circuit has to continue on and bypass the resistors, so On-On.



No problem, i probably will not use it too, but you'll never know. i ordered on-on switch, we will see what happens. Thanks!


----------



## JohnH

Blaoskaak said:


> Thanks! What kind of switch should i use? Cant find it in the diagram. On/off 2 pin 125ac 5? and the jack,
> 
> RS PRO 6.35 mm Panel Mount Stereo Jack Socket, 3Pole 5A
> 
> will these work?



That RS jack is perfect. id use it for all the jacks. Only Out3 uses the middle 'ring' connection, but with three contacts, they grip the jack plugs nice and firmly so you get a good solid and consistent feel when plugging in. Also, it declares that it is rated at 5A which is good. many others look similar but dont specify this.

Switches need to to be at least 'single pole double throw' or SPDT, which have three lugs. this means that the middle 'common' or ''pole' lug can connect to either of two other lugs. Now take a look back at the diagram and see the symbol for each switch? you should be able to see how this relates. The switch has two positions, and this is often referred to as 'on-on' 

Current rating needs to be at least 5A at 125V

But often its easier to find 'double pole double throw' DPDT. This is effectively two separate not connected switches next to each other, with 6 lugs. You can use these and just use one side.

If you find a switch, post a link if youd like it checked. You also need the coil. We have had some good EU sources posted in recent pages.


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## JohnH

hi @LesPaul_Marshall_Rocker78 welcome to our thread.

If i was you with what you have i would do as i did and build what we do! 

But, for a commercial unit in the form of a passive attenuator, we hear great things about the Ironman designs. but personally, Id love to buy one from Mike Lind. He's a very cool guy who posts on TGP, and while developing his design, he generously shared a lot of very helpful testing data, which was a great help to me. i dont know exactly how his works, not the same as ours here, but it sounds to be right. I think he builds them to order. 

But if you are up for a build project, stick around here!


----------



## JohnH

johan.b said:


> Great work..
> ... just wondering (and to dim myself to do this kind of math  )... is there any reason i can't replace R3/4 onwards with an commercial L-pad, for stepless attenuation beyong the 7db insertion loss?
> J



Best not to do that. Attenuators that use Lpads generically tend to damp the tone, losing treble, at more than a few db of attenuation. Its easily understood from the low output impedance of an Lpad as its turned down. Our design keeps much better control through a series of small optimised steps.


----------



## Blaoskaak

JohnH said:


> Yes the M2 diagram shows it all. The most important thing to match is the amp output ohms. That determines whether to build 8 or 16 ohms, or maybe you have both on your amp. Ideally your speakers match that too anyway, If your main set up is with 2 x16 speakers, and if the amp has an 8 out, then build the 8 ohm version.
> 
> eg, if you have 8 and 16 outputs, or your amp, and if you usually use 2x16ohm cabs, build the 8 version
> 
> You can use it with different cab ohms too, so long as it matches the amp output





JohnH said:


> That RS jack is perfect. id use it for all the jacks. Only Out3 uses the middle 'ring' connection, but with three contacts, they grip the jack plugs nice and firmly so you get a good solid and consistent feel when plugging in. Also, it declares that it is rated at 5A which is good. many others look similar but dont specify this.
> 
> Switches need to to be at least 'single pole double throw' or SPDT, which have three lugs. this means that the middle 'common' or ''pole' lug can connect to either of two other lugs. Now take a look back at the diagram and see the symbol for each switch? you should be able to see how this relates. The switch has two positions, and this is often referred to as 'on-on'
> 
> Current rating needs to be at least 5A at 125V
> 
> But often its easier to find 'double pole double throw' DPDT. This is effectively two separate not connected switches next to each other, with 6 lugs. You can use these and just use one side.
> 
> If you find a switch, post a link if youd like it checked. You also need the coil. We have had some good EU sources posted in recent pages.



Thanks alot! This was very helpfull! I am starting to understand the diagram. Is it not so difficult.

I ordered this coil:
https://europe-audio.com/Product.asp?mfr=Jantzen Audio&part=0001969&Product_ID=9631

I think it is the good one. Can you check please?


----------



## Blaoskaak

JohnH said:


> Yes the M2 diagram shows it all. The most important thing to match is the amp output ohms. That determines whether to build 8 or 16 ohms, or maybe you have both on your amp. Ideally your speakers match that too anyway, If your main set up is with 2 x16 speakers, and if the amp has an 8 out, then build the 8 ohm version.
> 
> eg, if you have 8 and 16 outputs, or your amp, and if you usually use 2x16ohm cabs, build the 8 version
> 
> You can use it with different cab ohms too, so long as it matches the amp output





JohnH said:


> That RS jack is perfect. id use it for all the jacks. Only Out3 uses the middle 'ring' connection, but with three contacts, they grip the jack plugs nice and firmly so you get a good solid and consistent feel when plugging in. Also, it declares that it is rated at 5A which is good. many others look similar but dont specify this.
> 
> Switches need to to be at least 'single pole double throw' or SPDT, which have three lugs. this means that the middle 'common' or ''pole' lug can connect to either of two other lugs. Now take a look back at the diagram and see the symbol for each switch? you should be able to see how this relates. The switch has two positions, and this is often referred to as 'on-on'
> 
> Current rating needs to be at least 5A at 125V
> 
> But often its easier to find 'double pole double throw' DPDT. This is effectively two separate not connected switches next to each other, with 6 lugs. You can use these and just use one side.
> 
> If you find a switch, post a link if youd like it checked. You also need the coil. We have had some good EU sources posted in recent pages.



Thanks alot! This was very helpfull! I am starting to understand the diagram. Is it not so difficult.

I ordered this coil:
https://europe-audio.com/Product.asp?mfr=Jantzen Audio&part=0001969&Product_ID=9631

And this switch

https://www.conrad.nl/p/miyama-ms-5...-125-vac-6-a-1x-aanaan-continu-1-stuks-703010


----------



## JohnH

The coil looks fine

The switch is hard for me to read about, but there's an English data sheet there which says it is a 3-position switch. It's described as on-off-on, which means that in the centre position, nothing is connected, which isn't what we want. We need a 2position switch on-on There will be a version that does that, since it's a simpler design


----------



## matttornado

Hi JohnH. I built your 16 ohm attenuator way back so can you please refresh my memeory?
I built a 16 ohm attenuator to match my 16 ohm 4x12. I now have a a 2x12 8 ohm cab as well.

I can use my 8 ohm cab instead of the 16, right?

thanks!


----------



## JohnH

hi @matttornado , great to hear from you again

Yes you can run the 8 ohm cab out of the 16 ohm attenuator, and you'll still plug into the 16 ohm amp jack . Of course, if you have a full bypass switch, you'd best not engage it unless you reset the amp to match the cab


----------



## LesPaul_Marshall_Rocker78

Hi again, Thanks for the Welcome!! 
Today I HAD to find out something!!?
(As a starting point for further Reference...)
- - - - - -

Soo I pluged in the (NMB plexi) 100watt SuperLead 1959 clone(Bugera 1960 Infinum) **I COMPLETLY DISENGADED THE PPIMV MASTER** ( =Should be Bonkers LOOOUD....!!!!!)
*
My Cab of choice for these Attenuation/volume tests :*
* old 16-ohm(15-ohm?)original 73' 4x12 "Marshall 1935" Greenback Pulsonic cone 25w 55hz cab (large checkerboard cloth)

** THEN I put the UAD OX-Attenuator/Load* in between speaker/amp "to limit the crazy speaker volume"

**Amp settings**
I set the 100w SuperLead amp-controls to ***EVH CRANKED!!!***(All knobs on full)
Presence : 10
Treble : 10
Middle : 10
Bass : 10
Volume -1 (gain) : 10
Volume -2 (gain) : 10
- - - - - -

**UAD OX Load box settings**
For FINAL volume control, I set the OX Attenuator Like this:
First Knob to 1: (Highest Attenuation)
Then Knob to 2: (Second Highest Attenuation)
And last Knob to 3: (Medium Attenuation, "getting" quite loud...)
Then I varied OX knob and amp knobs...

*.......*tweaking*......*
Then last I tweaked (varied settings) on all knobs *trying to keep the OX* at certain volume levels "as detailed above"

----------------------

Testing Results ...
**With UAD OX-Attenuator/Load knob # 1 :*
In my opinion it sounds "too low" when FULLY *CRANKED* (all knobs on 10 including the two Superlead (gain)volumes)

**With UAD OX-Attenuator/Load knob # 2 : *
Getting better, Amp breaths a bit better now, and sounds less congested.. but still far away from ?Loud? and almost "bedrom friendly" IMO ?

**With UAD OX-Attenuator/Load knob # 3 :*
Waay better, starts to sound like a proper TUBE(valve)-AMP now... Amp-EQ is also starting to be more useable/responsive!
The extreme exessive Bass at low volumes and loss of highend/precence is going away a little now..


***** OX Attenuator knob at #3 *****
FULLY **CRANKED Amp (all knobs at 10, including Amp Volume 1 & 2 )**
AT OX-knob #3(medium Attenuation) is becoming Loud TV/very Loud bedroom ??!
"I STOPPED" (I don't need gigging/consert levels LOL) at this "Louder setting" and tweaked madly on all possible settings back and forth between the UAD OX Attenuator knob settings 1-3


- - - - - - - - - - -
*MY CONCLUSION :*
Amp does not NEED position 1
At Max Attenuation it needs 2 perhaps??
I think I could get away with an Attenuator that attenuates down to a MAXIMUM 2 - 2.5 ??? On the OX Load.. when it is FULLY CRANKED....

PS ! PS !  *Rabbit hole caveat* your ears close to the 412(as with a "on axis" micked up amp) sounds wayyy better even at lower attenuations thats a bit annoying  hmmmm....




-----------------------------------------------
Regards
"LesPaul Rocker"


----------



## Blaoskaak

JohnH said:


> The coil looks fine
> 
> The switch is hard for me to read about, but there's an English data sheet there which says it is a 3-position switch. It's described as on-off-on, which means that in the centre position, nothing is connected, which isn't what we want. We need a 2position switch on-on There will be a version that does that, since it's a simpler design



Yeah, now i see it. On the page from the switch it says on/on. In the documentation on/off/on. Can i still use it when i dont attach middle or use only upper and middle? Otherwise i have to order some new one. That isn't a big problem too.

I'm glad the coil is the right one!


----------



## matttornado

JohnH said:


> hi @matttornado , great to hear from you again
> 
> Yes you can run the 8 ohm cab out of the 16 ohm attenuator, and you'll still plug into the 16 ohm amp jack . Of course, if you have a full bypass switch, you'd best not engage it unless you reset the amp to match the cab


awesome thanks!


----------



## JohnH

Blaoskaak said:


> Yeah, now i see it. On the page from the switch it says on/on. In the documentation on/off/on. Can i still use it when i dont attach middle or use only upper and middle? Otherwise i have to order some new one. That isn't a big problem too.
> 
> I'm glad the coil is the right one!



Definitely don't use on-off-on switches for this. The disconnected middle position can cause nasty transients.


----------



## Blaoskaak

Thanks alot!


----------



## JohnH

JohnH said:


> hi @LesPaul_Marshall_Rocker78 welcome to our thread.
> 
> If i was you with what you have i would do as i did and build what we do!
> 
> But, for a commercial unit in the form of a passive attenuator, we hear great things about the Ironman designs. but personally, Id love to buy one from Mike Lind. He's a very cool guy who posts on TGP, and while developing his design, he generously shared a lot of very helpful testing data, which was a great help to me. i dont know exactly how his works, not the same as ours here, but it sounds to be right. I think he builds them to order.
> 
> But if you are up for a build project, stick around here!



Further on this, I had a read-up on the Ironman II. While it is evidently a great product, I have some questions/concerns:

Here's a page from its manual:



If I've understood it right, you get 3 ranges, solo, hi, and lo. Each separated by 3 db. Within each range, you get three steps of -4db, then two at -10 db. -10db is quite a big step, and to find places in between, it seems like you have to jump back and forth between the three ranges. Seems like a fiddle?

It offers the solo setting as a footswitchable way to get a change of level for a solo boost, even from 0db fully bypassed going down. But this implies switching and unswitching reactive circuitry on the fly, and this can create transients, which can cause damage. So the manual states that the solo and other controls should not be switched unless you damp the strings to stop the signal. This does not seem like a reliable procedure to require when under pressure, launching from loud chugging power chords into a face-melting solo at a gig.

I checked out the impedance curve, measured by Mike Lind on its smaller brother, the Ironman II mini. It doesn't look much like that of a real speaker.



Just some watch-its...


----------



## LesPaul_Marshall_Rocker78

Thanks so much! JohnH  I rely on my Marshall brothers for valuable info! 

It's just a pity Mike Lind don't have "any" video/recorded test-clips of his attenuator for his showing what it sounds like at all no quick and dirty iphone clips or anything??  at lot of people have waited roughly 2-3years to hear just something...

--------------
I have once again started reading "this forum thread" from the humble-beginning to sort my head/mind a little bit, after my recent testing session with Bugera "Superlead" I became a little bit wiser as what I probably want !!?? 

If I build "the simplest version" with one stage -14 or -18 desibel "reduction"
say a Resistive 16ohm or 8ohm (prefer 16ohms)
What do you think would be the result compared to feel/sound of the OX ? 

At this stage of the Tone-Journey I feel we are "on our own" and probably have to build something.....

I have 1 year back in 1994(we had 3 year education plan for "94") as Electric/electronics education(basic priciples) of 3 year-school and used "the two other years" to do my dream ocupation draughtsman today i am Cad-designer(no engineer though)

I am not an Idiot when it comes to safety, But I would never dare doing a tube-amp(thats why I purchased a built Trinity I considered Metroamp JTM45 kit before that..) I am not a great solderer but I think I can manage ?(But I dont know if I have created a Cold solder or not??) I have a hard time understanding and calculating circuits though .... (Mathmatics is quite my nemesis..)

Question:
Would you guys help me understand it? 
I sort of remember the symbols and I even have "2 multimeters"(cheaper ones)..
I ALSO have a quite good "genuine" Japaneese soldering station Hako FX888 70watt (adjustable watts/heat) 

Problem is though getting suitable parts for build the attenuator case/box ?

Here in Norway I can get Arcol wire-wound-Resisors(would prefer High Quality build..even if more expensive..(there is Arcols in the OX) quite easily it seems probably some other small parts also, but that case/enclosure though


----------



## JohnH

Hi @LesPaul_Marshall_Rocker78

I reckon you have what you need to do this if you want to. It's actually a simple circuit, with nice big robust parts. There's nothing that wasn't invented at least 100 years ago, using theory from the 19th Century!

Arcol are perfect for the resistors. If you build the pure resistive version, you can expect it to be tonally at least as good or better than other resistive attenuators.

But seriously, Id recommend doing the reactive one based on design M2 (or part of it). It's no more complicated really. You can order the coil that it needs from various places in EU, and there are links in recent pages. Just have to interpret the diagram. You could build one stage, though it can be a bit tricky to decide what level to go to, and in any case multiple settings are useful just for different uses and even different times of day.

Units like OX and other high-end ones pack a heap of added functionality, effects loops, IR loaders, reamping ect. Our design is just an attenuator. It just takes a loud amp and reduces the volume. But, after a couple of years with this design, I reckon it does that with virtually no difference in tone or feel or dynamics . That's based on testing, analysis and listening. I've never compared it to an OX or other similar but I compare it the original sound of the amp and speaker, which is the best comparison really.

For the case you can can buy one. I used a Hammond die-cast one. All it needs is drilling, which with careful marking out isn't too hard to do with care.

Happy to discuss further!


----------



## CO_Hoya

Hi John,

First, thank you very much for starting and maintaining this thread. I've been needing an attenuator for years and I think you've finally motivated me into action. You've been very generous with your time and knowledge.

Also, kudos to the many people here who have posted their projects resulting from this thread, letting the rest of us learn from you.


Beyond purchasing a pile of power resistors via eBay, I'm still in the planning stages, figuring on a 4/8/16 ohm version.

I've put together a proposed circuit for my own version, which I'm calling the M3V - this is the full main reactive load to preserve a bit more of the bass resonance, as well as the ability to switch between inputs via a 4P3T rotary switch (C&K A40305RNZQ).




The C&K rotary is 125VAC-rated for 5A continuous, 2.5A switched. Per Randall Aiken, this *should* be enough in most cases with the understanding that the user would never switch the position while the amp is powered. The unattenuated signal from the amp always passes thru 2 conductors within the switch - after one stage of attenuation, I assume the load would be reduced enough that it could then pass thru a single switch conductor.


If you don't mind, I'd like to ask a couple of questions:
1. Do you see any issues with the proposed circuit? I've also sketched this for a 3P3T slider switch but that circuit is still half-baked and a bit messy, while the rotary switch seems straight-forward.
2. My attenuator would be primarily (exclusively?) for open-backed cabs, 1x12 (e.g. Fender Deluxe Reverb). Is that frequency curve different enough that I should consider some changes to the component values that you spec?


_As I'm new here, I've had to scrub out embedded links - hopefully you can find that rotary switch via Google._


----------



## JohnH

hi @CO_Hoya welcome to our forum and to our thread. Thanks for your drawing. It looks like Digikey Schem-it? i use that too.

Looks like you are on the right lines, and a few observations may help.

The bass resonance circuit (C1 and L3 above) is not so important in this design a it is in some others. Only one person i know of has built design M3 with that part and that's @dbishopbliss , who might like to comment further here on the difference it makes. His review on builds of M2 and M3 are on page 89. 
If you dont have the bass circuit, you still get the bass peak because in this design, it is generated by the speaker itself. What you might not get is the added bass harmonics as the amp responds non-linearly to the resonance when driven hard. Instead, the bass may be a very slightly cleaner and more solid. 

Ive not built M3 myself but I did some tests trying to provoke any difference between the bass tone of the full unattenuated speaker, and the attenuated. i couldn't hear any difference driving very hard with drop-D tuning trying to hit those frequencies on my VM2266c amp. Relevant to your set up, this is also a 2x12 open back combo, and it probably has a resonance at about 80hz instead of about 110hz on a closed-back.

The bass circuit may have more need if you use a line out into an IR system, but you can also probably just add a bit of EQ if needed, and a number of builders here have used M and M2 designs with line-outs.

But if you do have the bass circuit, and if the input ohms options are important, then note that the added front end with L2 does not add a share of the bass resonance. Still it will work fine as described above, but with half of whatever the bass circuit does. We could work out another L and C for it if wanted.

if you want to target open-backed combos, then the bass resonance should be lowered using larger values. i could work something out if you like. Personally, I don't think the bass circuit its really needed since it cant do anything much except to the every lowest notes on the 6th string!

The switch sounds fine and its good that it lists the 5A carry/non-switched rating. I guess most switches would have that added capacity when not operated, but its nice to see it declared. Also of interest is whether its shorting or non-shorting. Shorting would be preferred, if only for the accidental case of moving the switch in use.

On the output side, see the red parts on the M2 design. These correct the tone when 16 ohm cabs are used on the basically 8 ohm circuit. Safe to use without but with a bit less high treble. 

Last thing I have for now: the idea of making the switched conversion by adding L2/R12/R13 inherently uses up 50% of the power ie another -3db reduction of max volume level. That may be significant if you need low attenuation settings since he least attenuation is then -10db. On page 87 posted a full double circuit that allows different ohm settings with no such extra loss. You could instead, just consider building two dedicated attenuators. Or, if you need an overall -3db reduction, use it as a load box in parallel with the speaker, and use a lower amp tap.

Good luck with the build!


----------



## Jordan Prysmiki

Hey guys, was there a -1, -7, -7, -14, -3.5 version?

It's the addition of the "-1" that differs from the main Model M2 that I'm after.


----------



## JohnH

Jordan Prysmiki said:


> Hey guys, was there a -1, -7, -7, -14, -3.5 version?
> 
> It's the addition of the "-1" that differs from the main Model M2 that I'm after.



Hi Jordan, we dont have anything drawn with a -1db stage. That's a very small step, so for most purposes it would generally not be needed. Even the -3.5db is quite small, and most players are finding they don't want to subdivide it further. But, if you can describe what you have in mind and why, and what you want to use it for, lets discuss, its probably possible. It's significant as to whether or not you need this stage to run on its own, or only in combination with other stages.


----------



## Jordan Prysmiki

Hey John. Thanks for responding.
OK so it's the -3.5db I'm after! I built mine with the -7,-7,-14 thinking I would not need the -3.5 originally. In wanting to build my second one for another amp and cab I wrongfully remembered a -1db option instead of the -3.5db. So I should be good to go now!


----------



## JohnH

All good! when you have that -3.5db stage, in any of the designs, you can then control the switches to get every even step in small, equal -3.5db increments from min to max.


----------



## Blaoskaak

Who knows where i can get a aircore in europe? I ordered one, but they are scammers. 

I searched on ebay, but they don't have 0.9. Can i get 1.0 too?


----------



## CO_Hoya

Hi John,

Thanks for the reply, responses are in-line.


JohnH said:


> hi @CO_Hoya welcome to our forum and to our thread. Thanks for your drawing. It looks like Digikey Schem-it? i use that too.



I discovered Schem-it thanks to this thread, in spite of being a regular Digikey shopper. It's a bit quirky, but good enough for this sort of work.



> Looks like you are on the right lines, and a few observations may help.
> 
> The bass resonance circuit (C1 and L3 above) is not so important in this design a it is in some others. Only one person i know of has built design M3 with that part and that's @dbishopbliss , who might like to comment further here on the difference it makes. His review on builds of M2 and M3 are on page 89.
> If you dont have the bass circuit, you still get the bass peak because in this design, it is generated by the speaker itself. What you might not get is the added bass harmonics as the amp responds non-linearly to the resonance when driven hard. Instead, the bass may be a very slightly cleaner and more solid.
> 
> Ive not built M3 myself but I did some tests trying to provoke any difference between the bass tone of the full unattenuated speaker, and the attenuated. i couldn't hear any difference driving very hard with drop-D tuning trying to hit those frequencies on my VM2266c amp. Relevant to your set up, this is also a 2x12 open back combo, and it probably has a resonance at about 80hz instead of about 110hz on a closed-back.
> 
> The bass circuit may have more need if you use a line out into an IR system, but you can also probably just add a bit of EQ if needed, and a number of builders here have used M and M2 designs with line-outs.


Understand on all points above. I have room for one additional toggle in my current layout and so I've been toying with the idea of incorporating the bass circuit and then being able to bypass it (although I may go a different way with that toggle). I figure that I'd only ever build one (or two) of these units, so why not go all out with the "Cadillac" design.

More practically, I'd at least like to design a layout that can accept those additional components down the road if desired.



> But if you do have the bass circuit, and if the input ohms options are important, then note that the added front end with L2 does not add a share of the bass resonance. Still it will work fine as described above, but with half of whatever the bass circuit does. We could work out another L and C for it if wanted.


I think having bass resonance on both reactive loads may be overkill, even for me. I've two 8-ohm amps (and one 4-ohm) and so I'd primarily be using the attenuator in the 8-ohm position with the bass circuit designed for that, which would be fine for me.



> if you want to target open-backed combos, then the bass resonance should be lowered using larger values. i could work something out if you like. Personally, I don't think the bass circuit its really needed since it cant do anything much except to the every lowest notes on the 6th string!


I think I'd want to target something around 90 Hz. I can work out an LC pair that would give me that resonant frequency (e.g. 10 mH + 300 or 320 uF), but this would provide a different L/C ratio than yours and its not clear to me how important that ratio is.

I'm finding that there is a big jump up in price once you get above 10mH so I'm trying to design around that if possible.



> The switch sounds fine and its good that it lists the 5A carry/non-switched rating. I guess most switches would have that added capacity when not operated, but its nice to see it declared. Also of interest is whether its shorting or non-shorting. Shorting would be preferred, if only for the accidental case of moving the switch in use.


Yeah, you got me on that one - that's a non-shorting switch, which isn't great.

I can find only one available version of that switch that's both shorting and has solder lugs (p/n A40303RSZG), and I'm only seeing that from a single vendor with a min qty of 2. Since that version is double the price, that becomes an expensive "upgrade" to the inputs. Alternatively, there is a thru-hole shorting version (p/n A40303RSMCGK) but that would take some delicate soldering skills to use.



> On the output side, see the red parts on the M2 design. These correct the tone when 16 ohm cabs are used on the basically 8 ohm circuit. Safe to use without but with a bit less high treble.


Thanks, but I don't have a 16-ohm cab so don't see the need. I did consider building a 2/4/8 ohm version, and then I would use that circuit for the extra output.



> Last thing I have for now: the idea of making the switched conversion by adding L2/R12/R13 inherently uses up 50% of the power ie another -3db reduction of max volume level. That may be significant if you need low attenuation settings since he least attenuation is then -10db. On page 87 posted a full double circuit that allows different ohm settings with no such extra loss. You could instead, just consider building two dedicated attenuators. Or, if you need an overall -3db reduction, use it as a load box in parallel with the speaker, and use a lower amp tap.


Honestly, I'm guessing that even -7dB would be too little for my application, which is to get bedroom volumes with 1x12 combos.



> Good luck with the build!


Cheers!


----------



## JohnH

Blaoskaak said:


> Who knows where i can get a aircore in europe? I ordered one, but they are scammers.
> 
> I searched on ebay, but they don't have 0.9. Can i get 1.0 too?



The value for the 0.9mH coil is fine in the range about 0.8mH to 1mH with only a very small difference in theory

These people sell 0.82mH and 1.0mH You should get 1mm wire which is also called 18 gage

VIS SP 5013: VISATON SP coil - 0.82 mH - 1.0 mm at reichelt elektronik


----------



## JohnH

CO_Hoya said:


> Hi John,
> 
> Thanks for the reply, responses are in-line.
> 
> 
> I discovered Schem-it thanks to this thread, in spite of being a regular Digikey shopper. It's a bit quirky, but good enough for this sort of work.
> 
> 
> Understand on all points above. I have room for one additional toggle in my current layout and so I've been toying with the idea of incorporating the bass circuit and then being able to bypass it (although I may go a different way with that toggle). I figure that I'd only ever build one (or two) of these units, so why not go all out with the "Cadillac" design.
> 
> More practically, I'd at least like to design a layout that can accept those additional components down the road if desired.
> 
> 
> I think having bass resonance on both reactive loads may be overkill, even for me. I've two 8-ohm amps (and one 4-ohm) and so I'd primarily be using the attenuator in the 8-ohm position with the bass circuit designed for that, which would be fine for me.
> 
> 
> I think I'd want to target something around 90 Hz. I can work out an LC pair that would give me that resonant frequency (e.g. 10 mH + 300 or 320 uF), but this would provide a different L/C ratio than yours and its not clear to me how important that ratio is.
> 
> I'm finding that there is a big jump up in price once you get above 10mH so I'm trying to design around that if possible.
> 
> 
> Yeah, you got me on that one - that's a non-shorting switch, which isn't great.
> 
> I can find only one available version of that switch that's both shorting and has solder lugs (p/n A40303RSZG), and I'm only seeing that from a single vendor with a min qty of 2. Since that version is double the price, that becomes an expensive "upgrade" to the inputs. Alternatively, there is a thru-hole shorting version (p/n A40303RSMCGK) but that would take some delicate soldering skills to use.
> 
> 
> Thanks, but I don't have a 16-ohm cab so don't see the need. I did consider building a 2/4/8 ohm version, and then I would use that circuit for the extra output.
> 
> 
> Honestly, I'm guessing that even -7dB would be too little for my application, which is to get bedroom volumes with 1x12 combos.
> 
> 
> Cheers!



Ill have a look at values for a 90hz resonance. When I looked at it before, it was getting good bipolar capacitors with high current and low losses was a big cost factor.


----------



## CO_Hoya

JohnH said:


> Ill have a look at values for a 90hz resonance. When I looked at it before, it was getting good bipolar capacitors with high current and low losses was a big cost factor.


Thanks.

The caps I'm considering are the non-polar electrolytics offered by Parts-Express.com. Those are spec'ed with a max DF = 5%, which gives an ESR = 0.26 ohm at 300 uF / 1 kHz, if I did the math correctly.

Most are out of stock at the moment, but I'm a long way off from purchasing.

On the other hand, bumping up to 12-15mH may not be as expensive as I thought.


----------



## aceofbones

I’m just about ready to wire my M2 up and had a question about wire gauge. I know you’ve said 18awg, but would 20awg solid core be fine? As far as I can tell, 20awg 80*C 1000v solid core wire should be good up to 10amps. I’m asking because I have a ton of this wire, but I’m out of 18.


----------



## Gene Ballzz

aceofbones said:


> I’m just about ready to wire my M2 up and had a question about wire gauge. I know you’ve said 18awg, but would 20awg solid core be fine? As far as I can tell, 20awg 80*C 1000v solid core wire should be good up to 10amps. I’m asking because I have a ton of this wire, but I’m out of 18.




What is the maximum wattage amp you plan to use the attenuator with?
Just Askin'
Gene


----------



## JohnH

I guess it's OK, particularly if it's for 50W amp or less, 

You want to account for the difference, you could double up on the wires from the input jack. After those, the current splits and divides into different paths.


----------



## aceofbones

Sorry, I should’ve mentioned that. I’m building the 50watt version of M2 and my amp is a Trainwreck Express clone so it’s 36 watts. I do have a few short chunks of 18g I could use from the input if thats the only spot that’s a concern. Otherwise I can just grab a spool of 18g for the whole thing.


----------



## Blaoskaak

I've been busy today with the m2 attenuator. i have a few questions.

1. I accidently ordered very small switch. i can't get the wire in the hole from the pin, can i thin the wire and use it then? the wire is 16 gauge.

2. i'm a bit confused where i can attach the wire from r4, r6 and r8. is it possible to just solder 3 wires inbetween r4, r6 and r8 and then to the switch?

3. i ordered the wrong air core i think. it's a 9ohn instead of 9mhz. What's gonna happen now?


I attached pictures. if someone could help me, that would be great!


----------



## JohnH

On the wire gages, I've used 18 gage stranded and it 'looks' right to me. But if you do a reasonably well laid-out build, I cant see a real problem with 20 gage for the hook up wire. Lets say you used 3' of it in the build (hopefully much less than that!), the total resistance of that in 18 gage is 0.019 Ohm while in 20 gage is 0.03 Ohm, so not significant as to the performance of it, and I agree the current is ok for that wire too. Id still use the 18 or 2x20 from the input jack though.

But another factor could be the difference of solid vs stranded. Solid is great for laying out neatly within a fixed case. My version has a few components on the lid and the others in the base, so I preferred stranded to let it be flexible for assembling and disassembling.

@Gene Ballzz has worked on far more gear than I, so we should defer to his counsel if he has a further comment or different view.


----------



## JohnH

Blaoskaak said:


> I've been busy today with the m2 attenuator. i have a few questions.
> 
> 1. I accidently ordered very small switch. i can't get the wire in the hole from the pin, can i thin the wire and use it then? the wire is 16 gauge.
> 
> 2. i'm a bit confused where i can attach the wire from r4, r6 and r8. is it possible to just solder 3 wires inbetween r4, r6 and r8 and then to the switch?
> 
> 3. i ordered the wrong air core i think. it's a 9ohn instead of 9mhz. What's gonna happen now?
> 
> 
> I attached pictures. if someone could help me, that would be great!



Thanks for the pictures, which come out nice and clear.

On the soldering of wires etc, the real work is in having a smooth, shiny flow of solder between the wire and lug, which should contact each other as you make the joint. Having the wire actually go through the lug is good, but secondary to the flow of the solder. i think what I would try would be to seperate the ends of the strands into two bunches, twist each one, put one through the switch lug and wrap the other around the lug, then solder it all. (the ideal solder joint looks like the liquid metal guy in Terminator 2)

The schematic shows what is connected to what, but it doesn't show an exact layout of wires, because in general circuits, it doesn't matter. eg, take R4, R6 and the two switches. The diagram shows which lugs are joined. But you should place the wires to join those lugs by the shortest /neatest route. This may be, for example, R4 to R6, then a wire between the switch lugs, then one to join the resistors to the switches, using one lug at each end.

The coil you have is 2.7mH (milli Henry, a unit of inductance). It's too big, and although in theory you can unwind it to reduce, without a meter that measures this its too hard to get right. So I think you need a new coil.


----------



## Blaoskaak

JohnH said:


> Thanks for the pictures, which come out nice and clear.
> 
> On the soldering of wires etc, the real work is in having a smooth, shiny flow of solder between the wire and lug, which should contact each other as you make the joint. Having the wire actually go through the lug is good, but secondary to the flow of the solder. i think what I would try would be to seperate the ends of the strands into two bunches, twist each one, put one through the switch lug and wrap the other around the lug, then solder it all. (the ideal solder joint looks like the liquid metal guy in Terminator 2)
> 
> The schematic shows what is connected to what, but it doesn't show an exact layout of wires, because in general circuits, it doesn't matter. eg, take R4, R6 and the two switches. The diagram shows which lugs are joined. But you should place the wires to join those lugs by the shortest /neatest route. This may be, for example, R4 to R6, then a wire between the switch lugs, then one to join the resistors to the switches, using one lug at each end.
> 
> The coil you have is 2.7mH (milli Henry, a unit of inductance). It's too big, and although in theory you can unwind it to reduce, without a meter that measures this its too hard to get right. So I think you need a new coil.



Thanks again for the help! So it doesn't really matter where i place the extra wire? Best would be from switch right to the lug. So r4,r6 lug has to wires that goes to the switch. So i has the shortest path.

I have a multimeter, i can try to unwind it. I really want to use the attenuator today. I've been waiting al week to get the part. I'm feeling like a Child that is getting a present. Lol

Does it work with the wrong coil and replacement it until i get the new good one?

And the switch that i have? Is it too small? Or can i still use it. My 16 gauge wire is too thick and can't get it in eye from the pin. Is it possible to make thenwire a bit thinner and use it then?


----------



## JohnH

Best not to rush!

Very few meters can measure inductance, so it's unlikely yours does unless it is unusual. Without this you can't adjust the coil.

But you can get ready, and also, you must do a full check on the units resistance before you try it with an amp.

Until you get the right coil, you can just bypass the coil with a wire, shorting it out. Then, you have quite a good resistive attenuator, virtually equivalent to as described early in this thread. When you put the new coil on, then you have the full thing.

Check all connections. It is very easy to get wiring wrong and only you can check it.

Plug a speaker into the output of the attenuator.

Without connecting to the amp, measure resistance at the input, across the input cable that would go to the amp. It should be between 8 and 10 ohms in every setting of the switches, check them all.

If it all checks out, then you can try carefully with an amp, starting at low volume. If you don't get sound when turned up very slightly, switch off! don't turn up further.

good luck!


----------



## Blaoskaak

JohnH said:


> Best not to rush!
> 
> Very few meters can measure inductance, so it's unlikely yours does unless it is unusual. Without this you can't adjust the coil.
> 
> But you can get ready, and also, you must do a full check on the units resistance before you try it with an amp.
> 
> Until you get the right coil, you can just bypass the coil with a wire, shorting it out. Then, you have quite a good resistive attenuator, virtually equivalent to as described early in this thread. When you put the new coil on, then you have the full thing.
> 
> Check all connections. It is very easy to get wiring wrong and only you can check it.
> 
> Plug a speaker into the output of the attenuator.
> 
> Without connecting to the amp, measure resistance at the input, across the input cable that would go to the amp. It should be between 8 and 10 ohms in every setting of the switches, check them all.
> 
> If it all checks out, then you can try carefully with an amp, starting at low volume. If you don't get sound when turned up very slightly, switch off! don't turn up further.
> 
> good luck!



Allright, thanks again! I got a new coil. It is 1.0mh instead of 0.9. I dont think that is gonna be a problem? I also got new switches 10a on 250v. Do now put everyyhing together. Will let you know the outcome!

Edit: ok. I put everything together, but ingot no sound at all. Maybe i did something wrong with the jack. I don't know.



 

 

 

 



Edit: it looks like that r1 has to be connected to the aircoil too. I don't have that. Only 2a and 2b are connected to air coil. Do i have to attach 2 wires from r1. One wire to input jack and 1 wire to air coil


----------



## JohnH

Blaoskaak said:


> Allright, thanks again! I got a new coil. It is 1.0mh instead of 0.9. I dont think that is gonna be a problem? I also got new switches 10a on 250v. Do now put everyyhing together. Will let you know the outcome!
> 
> Edit: ok. I put everything together, but ingot no sound at all. Maybe i did something wrong with the jack. I don't know.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Edit: it looks like that r1 has to be connected to the aircoil too. I don't have that. Only 2a and 2b are connected to air coil. Do i have to attach 2 wires from r1. One wire to input jack and 1 wire to air coil



I don't think you did what was in my last post! Yes R1 has to be connected. Did you do the resistance checks I noted above? 

I suggest to remove the attenuator until it is complete and checked, and put the speaker back directly to the amp and check it works and has not been damaged.


----------



## aceofbones

Thanks John,
I’ll use 18g from the input and go with the 20g solid afterwards. All of my parts fit in the lower portion of my case, so I don’t need to worry about wires moving while opening. Everything will be in a fixed position and I much prefer working with solid wire in that situation. I’ll post some pictures once I finish it up. Thanks again.


----------



## JohnH

*Attenuator M3, with bass resonance circuit
*
Since there has been some interest in the M3 variant, as described recently by @dbishopbliss and @CO_Hoya, I thought I'd draw it up again and also run some numbers around investigating component types and values.

M3 is the same as M2, with an extra resonant cap and inductor circuit inline with the main L1 coil. Its purpose is to show the amp a peak in the bass impedance, that may result in it responding a bit differently at low bass frequencies, possibly nearer to that of a directly connected speaker. It doesn't make a big difference to the resulting attenuated tone, and M2 is still a very good and more simple/economical design. But it may also make it a bit more accurate in the low bass when used as a load box for IR processing, and also adapting to different amps.













M3 210822



__ JohnH
__ Aug 22, 2021






The extra coil is wound on an iron core and the cap is a large bipolar. The values shown are targeting the response on a closed back G12M 4x12 cab, with a bass resonance at about 120hZ.













M3 C1 And L2 Options 210822



__ JohnH
__ Aug 21, 2021






On these calculated graphs, the red curve is the db response of a cab directly connected, the blue is the attenuated output and green is what the amp sees from the attenuator. The ideal is that they all match. The calcs were done at -14db attenuation, and then normalized for level so they can be compared.

Top left (chart A) shows without the bass resonance ie, as M2. What's good about it is that even without the added parts, the red and blue match well. But the green signal as seen by the amp has hardly any bass peak. This may mean that the amp doesn't get drawn into creating some extra bass harmonics if it's being driven hard. This is likely to be quite a subtle effect though.

Chart B shows the new components with coil resistance and capacitor losses based on an 18 gage L2 coil and a bipolar electrolytic cap (dissipation factor DF=0.05). Green now has most of the peak that red and blue show. This may be enough and is reasonably economical. Take care that such a cap has enough current rating though, since it will heat up and is passing a lot of power.

Chart C is the same values but with a lower loss cap (polypropylene) and a thicker coil with less resistance. You can see the green peak being a bit nearer to the red.

Chart D is with everything shifted down from about a 120hz peak to an 85-90hz peak, more relating to an open backed cab instead of closed back. The suggested values for C1 and L2 are shown.

I think it's interesting. I'm quite happy with my attenuator without this addition, but its an option if wanted.


----------



## Blaoskaak

JohnH said:


> I don't think you did what was in my last post! Yes R1 has to be connected. Did you do the resistance checks I noted above?
> 
> I suggest to remove the attenuator until it is complete and checked, and put the speaker back directly to the amp and check it works and has not been damaged.



Honestly i didn't. I tried it, but i have no idea how i do it. I put the multimeter at 20k ohms and put 1 pin from the multimeter to input jack wire and then i follow the wire, but i get no readings. I conntected r1 to r2b , and removed 1 cable from r1 to the other side and i disconnected r1 and 2b on one side.


----------



## JohnH

Blaoskaak said:


> Honestly i didn't. I tried it, but i have no idea how i do it. I put the multimeter at 20k ohms and put 1 pin from the multimeter to input jack wire and then i follow the wire, but i get no readings. I conntected r1 to r2b , and removed 1 cable from r1 to the other side and i disconnected r1 and 2b on one side.



Lets pause and review carefully . We can work this out and check it. Im happy to help. Ill send you a private message so we can discuss together.


----------



## JohnH

Just giving us a small bump back to our usual place near the front of the workbench! Also, congrats to @Blaoskaak who got his M2 running successfully.

Here's one thing I'm very happy about: 93 pages in and countless builds later, every single build based on this thread that has gone ahead and been reported here, has been a success.


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## nzingler24

Hi there im just ordering the parts required, one of the resistors is out of stock HS2575R F they do however have HS2575R J - (i believe its 5% tolerance as opposed to 1%) would this still be ok? Thanks


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## JohnH

hi @nzingler24 , thanks for your post. The 75 Ohm is for R10 on an 8 ohm M2? 5% tolerance is fine. In fact, if you look at the latest diagram at the top of post 1, I've got two sets of values, depending on available ranges, with R10 at 68 or 75 Ohms. 

The ranges I picked from have values spaced at about 20%, and I picked the best combinations from those ranges. But in general, 5% is good enough accuracy for any of them.


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## nzingler24

Thanks for the quick reply @JohnH this is for R7 on the M2 16 ohm build, i hope iom reading that correct as to what i need, also is there a range reuqired for the switches in terms of amps volts etc. 

Im looking at cliff uk for the jacks as im based in uk, which do you think seems most suitable, not sure of the differences of S1 S2 S4, thanks again


----------



## donwagar

With JohnH answering a lot of questions and pointing me in the right direction, I finally finished mine for my SV20C. It works great.

I do own an old THD Hotplate, the M2 is better.

I made a couple of changes, I only wanted it for gigs and practices, not intended for home use. Because the SV20 has a 5 Watt setting, I left the -14dB Stage out. Also, I wanted a switchable volume cut between rhythm and solo. I was using an EQ in the loop to accomplish this, but that also cuts gain, I didn't want that. JohnH came up with a footswitchable Stage (I went with -3.5 dB). Note: I'm still waiting on the resistors to finish this, they should arrive later this week. 

At JohnH's suggestion, last night at practice I switched in the extra -7dB, and switched the amp from 5 W to 20W. The volume was the same, but, at least in my mind, the tone was better.

As far as I can tell, it is totally transparent. And I will have many volume options to choose from: the amp has 5W or 20W, the attenuator has -7 (base), -7 and -3.5 (switchable) and -3.5 footswitchable. All without killing the tone of the amp. 

Also at gigs we mic everything, so I do have to control my volume no matter how big a room we are in. 

I generally run jumpered at about 8 on High Treble and 4 on Normal.

Sorry if the pics are too big, I am a bit, er, computer challenged.


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## JohnH

Hi @donwagar , thanks for following this through and for trusting the design. Your thoughtful feedback is very valuable and it shows, together with other reports, that these attenuators are a good option for the Marshall Studios, particularly the SV20's where there is no MV, and the tone you want is up high.

The observation about using the 20W and 5W settings is very interesting, and it seems that they may be slightly different in tone or response. So having the option to choose either while still getting to the volume level that you need is a plus.

One thing that I find intriguing is how our design adapts to different amps. The Studio amps with their EL34's have a very different output stage to the VM and DSL combos that I design and test with. But when I run my maths using different amp and speaker assumptions, you can see that it tracks the full volume response of whatever amp and speaker is used.

it looks like a nice simple build and it will be interesting to see how the footswitch works out.

For others, Don made the choice to leave out the -14db stage based on being familiar with how the amp will be used. But if you aren't sure what settings will be needed, or if you sometimes want to run it in your apartment at TV volume, then that -14 stage will be useful. My VM combo, although nominally about 2x the power of the SV, is probably not really much different the way I run it. i quite often use the full -31db if there are others in the house.


----------



## MarkS2

Thanks for sharing this and being so helpful to everyone! Fantastic work.

Does anyone have any reservations or caution about doing a simple bypass switch like the top of the following?


----------



## JohnH

nzingler24 said:


> Thanks for the quick reply @JohnH this is for R7 on the M2 16 ohm build, i hope iom reading that correct as to what i need, also is there a range reuqired for the switches in terms of amps volts etc.
> 
> Im looking at cliff uk for the jacks as im based in uk, which do you think seems most suitable, not sure of the differences of S1 S2 S4, thanks again



Oh yes, R7! 75 Ohms for a 16 Ohm M2, and 5% tolerance is fine, even if 5% off it makes less than 0.1db or 0.1Ohm difference overall.

I had a look at the Cliff specs, which i wasn't familiar with in detail. Actually they are all good in terms of that they will work fine in this design. The key thing about all of them is they are plastic bodied, so the case in not connected to the circuit, also they spec a rating of 5Amps and are high quality.

But, ideally, we want panel mounted, so solder lugs with a hole for the wire, instead of smaller PCB lugs. So I think that rules out S1 types which look to be small bodied and just for PCB (but if that's what you had you could use them). Not sure what the difference is with S4, maybe the nut style? still OK.

i don't think we need gold contacts

S2 looks fine. For your 16 Ohm M2 build, you only strictly need the simplest with two contacts. But I highly recommend getting the stereo type. The added spring contact grips the plug better even though you wont use it. (it does get used in the 8 ohm version). Many jacks also come with switched contacts, which connect across the switch and disconnect when you plug in. Again, all fine to use but make sure you use the lugs that are connected to the plug when inserted. (In the foot-switched version that Don is building, we do use the switch contacts)

On switches, most use a full-sized toggles either SPDT or DPDT in which case you can just use one side, or bridge the two sides in parallel to get more redundancy. Most switches are specced for mains switching with current ratings using 125V or 250V. Our voltages are much less, so we can use the 125V ratings. Minimum current rating should be 5A.


----------



## JohnH

MarkS2 said:


> Thanks for sharing this and being so helpful to everyone! Fantastic work.
> 
> Does anyone have any reservations or caution about doing a simple bypass switch like the top of the following?



Hi Mark and welcome to our thread. Yes you can have a bypass, but not as you show, which shunts the top of the circuit leaving the R3, R5 and R7 resistors still in circuit.

Bypass circuits need two poles to disconnect at both ends. I see yours starts the bypass after the first stage, so you would still get the first -7db reactive stage in circuit. That's all fine (with a two-pole bypass switch) if that's what you are wanting. May I ask what you want to use the bypass for? i have a bypass on my build (design M on post 1), but I took it off the ongoing current base design M2, since it makes it simpler and less subject to errors in use.


----------



## MarkS2

JohnH said:


> Hi Mark and welcome to our thread. Yes you can have a bypass, but not as you show, which shunts the top of the circuit leaving the R3, R5 and R7 resistors still in circuit.



Sorry for the false start and waste of time. This is what I get for trying to bang out a quick question with a distracting toddler on hand.

What I meant was to connect the left side to the input tip. i.e. a single pole switch connecting/disconnecting from input tip to output tip.



> May I ask what you want to use the bypass for?



I'm simply trying to be able to restore full volume without unplugging and plugging cables. Would the direct input-to-output tip be the best bet for this? If not can you tell me a bit more about what you had in mind? Or alternatively why I don't really need it? EDIT: I took a closer look at the original M circuit and yes, I see what you mean. I'm simply going to bypass from input tip to output tip with a DPDT. 

Thanks again!


----------



## Bearzooka

Hello Marshall Forum! Quick question: On the M2, instead of using 3 separate switches could you use a rotary switch instead? Thanks!


----------



## JohnH

MarkS2 said:


> Sorry for the false start and waste of time. This is what I get for trying to bang out a quick question with a distracting toddler on hand.
> 
> What I meant was to connect the left side to the input tip. i.e. a single pole switch connecting/disconnecting from input tip to output tip.
> 
> 
> 
> I'm simply trying to be able to restore full volume without unplugging and plugging cables. Would the direct input-to-output tip be the best bet for this? If not can you tell me a bit more about what you had in mind? Or alternatively why I don't really need it? EDIT: I took a closer look at the original M circuit and yes, I see what you mean. I'm simply going to bypass from input tip to output tip with a DPDT.
> 
> Thanks again!



OK yes, like this:



Things to watch out for:

Not operating the bypass while the amp is on, since the full power is being switched, so you'd have thte amp off or on standby
Also, M2 is able to be used for different cab Ohms, provided the attenuator ohms matches the amp. But in bypass mode, a match of amp and speaker is needed and also the Out3 if you have it, is not in the bypass loop as shown, so you need a load in Out 1 or 2 

Bypass could be useful if your power levels are such that you often want to go from attenuated to not attenuated, or if you are building it into a head or combo as a fixed unit. But usually, a given set up such as a home studio, will always need the attenuator, or always not need it in which case you dont need it then.


----------



## JohnH

Bearzooka said:


> Hello Marshall Forum! Quick question: On the M2, instead of using 3 separate switches could you use a rotary switch instead? Thanks!



in principal yes, but getting a robust rotary with enough current capacity and enough poles and switch positions can be difficult and expensive. The three toggles are cheap, simple to wire up and reliable and together they give you 8 even steps.


----------



## Bearzooka

JohnH said:


> in principal yes, but getting a robust rotary with enough current capacity and enough poles and switch positions can be difficult and expensive. The three toggles are cheap, simple to wire up and reliable and together they give you 8 even steps.



Thanks for your reply JohnH. What kind of switches would you recommend (Volts, Amps, Watts)?


----------



## JohnH

Bearzooka said:


> Thanks for your reply JohnH. What kind of switches would you recommend (Volts, Amps, Watts)?



Actually we had this earlier today!

On switches, most use a full-sized toggles either SPDT or DPDT in which case you can just use one side, or bridge the two sides in parallel to get more redundancy. Most switches are specced for mains switching with current ratings using 125V or 250V. Our voltages are much less, so we can use the 125V ratings. Minimum current rating should be 5A.


----------



## Bearzooka

Much obliged.


----------



## Graham G

Hi JohnH, I've read quite a lot of the thread & although i'm capable of building on of these I have no circuit reading or technical knowledge at all, I would need a paint by numbers or detailed to pics to build, is anything like this available in the thread ?, or is it not for dim wits .
Just in case it can be done by a dim wit, I have a HF Modded ORI50 (8 ohm speaker) which I only use for gigs, the problem is I don't know how much attenuation I will need, so I suppose a Variable knob would work best, as I guide my ORI20 & DSL20 are not loud enough so I suspect half way between the 20 & the 50.
If this is not possible i'd just like to say that people like yourself, that give up so much knowledge & time for free is generous to the extreme.


----------



## JohnH

Hi Graham, thanks for your interest. I've actually made a point of not posting a pictorial paint-by-numbers guide. The reason is, it's really important to be able to relate the diagram to the build and have a sense of how its all connected, and to be able to review and check it, and test it with a multimeter. It's almost impossible to trouble shoot over the internet, and the only person who can verify a build is the one who built it. Also, all builds come out a bit different due to their parts and the needs of their owner.

It's actually a simple circuit with robust parts. If you can figure out how to wire it, and are proficient with a soldering iron and drill, then im happy to discuss. But reading the diagram is the entry exam !


----------



## MarkS2

JohnH said:


> OK yes, like this:
> View attachment 93963
> 
> 
> Things to watch out for:
> 
> Not operating the bypass while the amp is on, since the full power is being switched, so you'd have thte amp off or on standby
> Also, M2 is able to be used for different cab Ohms, provided the attenuator ohms matches the amp. But in bypass mode, a match of amp and speaker is needed and also the Out3 if you have it, is not in the bypass loop as shown, so you need a load in Out 1 or 2



Good info thanks. I'm omitting out2 and out3. Just a single output for 8ohms only.



> Bypass could be useful if your power levels are such that you often want to go from attenuated to not attenuated


Exactly. I'm going to be trying this on a 15W that's just on the border of too loud.


----------



## Gene Ballzz

@donwagar & @JohnH 
So cool to see another one done. Don's usage experience mirrors and confirms my own regarding amps with multiple wattage settings, especially concerning the use of this attenuator design. The amp will nearly always sound better at it's full wattage/power setting and attenuated to suit, as opposed to a lower wattage/power setting with less attenuation to achieve similar sound pressure levels. This is not always the case with other attenuators!

I am however, going to pick a few nits here.
*>* I'm not familiar enough with that style/brand/model of jacks, but if they are not fully isolated from the chassis/frame/enclosure, they should be. Particularly in the case where you will be sending AC voltage through a wire, to a pedal that gets stepped on by a human who is solidly connected to ground/earth through that human's guitar/cable etc! This in addition to a few other anomalies that have popped up throughout this thread with non-isolated jacks. If the jacks actually facilitate isolation, "never mind!"
*>* Never been a real fan of push on/blade connectors, especially in something that may get bounced around in a gig-bag, cable box etc.​Other than those couple nits, looks great and really glad you've discovered how well it actually works!
Nice Job!
Gene


----------



## Graham G

JohnH said:


> Hi Graham, thanks for your interest. I've actually made a point of not posting a pictorial paint-by-numbers guide. The reason is, it's really important to be able to relate the diagram to the build and have a sense of how its all connected, and to be able to review and check it, and test it with a multimeter. It's almost impossible to trouble shoot over the internet, and the only person who can verify a build is the one who built it. Also, all builds come out a bit different due to their parts and the needs of their owner.
> 
> It's actually a simple circuit with robust parts. If you can figure out how to wire it, and are proficient with a soldering iron and drill, then im happy to discuss. But reading the diagram is the entry exam !



Thanks John, so if I only need an 8ohm out, which is the simplest diagram/version to get to grips with, on the plus side I do have a decent iron/solder & a multi meter & I have a lot of experience building hi-fi amps including PCB stuffing, so maybe I should start to understand in a small way how what i'm doing


----------



## donwagar

Gene Ballzz said:


> @donwagar & @JohnH
> 
> *>* I'm not familiar enough with that style/brand/model of jacks, but if they are not fully isolated from the chassis/frame/enclosure, they should be. Particularly in the case where you will be sending AC voltage through a wire, to a pedal that gets stepped on by a human who is solidly connected to ground/earth through that human's guitar/cable etc! This in addition to a few other anomalies that have popped up throughout this thread with non-isolated jacks. If the jacks actually facilitate isolation, "never mind!"
> Gene​


​Thanks Gene.

I am a pack rat (I prefer that to 'hoarder' or 'junk collector'). Those jacks are, I think, these:

https://www.markertek.com/product/s...gqabVIx9myr2XmDcnFvHCwhQiSuea8WhoCDk0QAvD_BwE

They are stereo switching from a Mackie circuit board I picked up years ago (had 50 of them on the board, I think I paid $10 for it), so I've used them anywhere I need a jack. I don't think they are isolated, it's not a big deal to switch them if you think it's warranted.


----------



## MarkS2

Any issues or concerns about switching the 2-4 stages on and off while powered up?


----------



## Gene Ballzz

Two important suggestions to all!
*A>* It is my considered opinion that we should all firmly and truly embrace the title of this thread: *"Simple Attenuators - Design And Testing!" *While many bells, whistles and/or additions are certainly possible and almost endless, the simplest form and usage of this design performs so flawlessly at the task that many folks have been dreaming of since the first guitarist playing gloriously into a cranked amplifier was asked/told to _*"TURN THAT DAMNED THING DOWN" *_that some of those additions may end up compromising (or at the very least *"complicating"*) the whole build, simplicity, durability and longevity of the finished product! The plain, unadorned unit does exactly what it was initially intended to do: "Allow the use of an amplifier, cranked up into it's glorious (yet usually way to loud) sweet spot, and then have that overall volume tamed to reasonable levels *"WITHOUT"* compromising tone and tactile response/feel. There is a lot to be said for "*K*eeping *I*t *S*tupidly _*S*_imple!"
*B>* While I realize this is @JohnH 's thread, as an avid fan, contributor and participant I humbly beg anyone who modifies anyof John's drawings, please, *PLEASE,* *PLEASE* clearly notate on that drawing that it has been modified/edited from the original! Who knows what kind of achiving, condensing, etc may happen to this thread in the future and it would be sad to see erroneous and sometimes incorrect diagrams to be saved and passed on!​I've said it before and I'll say it again: _*"I've built multiples of the M-Lite and have found them to be the greatest and most liberating pieces of gear I've ever owned, in my +50 years of "Screechin' & Squawkin'" with guitars through amplifiers!"*_

Thanx @JohnH & All,
Gene


----------



## donwagar

MarkS2 said:


> Any issues or concerns about switching the 2-4 stages on and off while powered up?



Someone that actually knows what they are talking about will probably chime in. But I don't think so, I do that when I'm setting my volume, also I'm building a -3.5dB footswitch to cut volume on the fly at gigs.


----------



## JohnH

Thanks @Gene Ballzz and Hi @donwagar 

Gene makes a good point about the jack insulation, that I should have picked up when you showed me your jacks before and i apologize. I agree that yours are not isolated, since the metal threaded part connects to the plug barrel and its also what is contacting the case. This puts the attenuator case connected to the 'cold' wire of your cable from the amp. This is not usually a good ground, and may be just one end of the OT output winding. Im not sure if this will really cause a problem, but Ive been advising to use the plastic bodied Cliff jacks which keep the case not involved in the circuit.

I had a look at pictures online of Weber Mass attenuators, which clearly use non-isolated open jacks, but he may have an isolating washer/bush each side of them. Gene, if you still have your Weber, maybe you could take a look? eg On the front panel, is there a white isolating washer under the head of the jack nut? 

But, even non-isolated, it still works, as you have proven. Your jacks loo fairly tigt next to the switches, and lugs are at the back, so changing them may be tricky?

But for your footswitch jack, i think it should have isolation (since the footswitch box is more likely to meet something else on the floor). You can use the Cliff type with 6 lugs that I have a small photo of on the footswitched diagram. 

To isolate a non-isolated jack, it needs a plastic washer both sides and something around the threads as it passes through the chassis.


----------



## donwagar

Thanks JohnH. I checked my local shop, they carry Neutrik NYS219s in stock. They look right, so I'll replace all the jacks when I finish the footpedal. Easy fix. I'm sure I have room on the footswitch jack, if the amp and speaker jacks are tight I can look into isolating the existing jacks.


----------



## JohnH

donwagar said:


> Someone that actually knows what they are talking about will probably chime in. But I don't think so, I do that when I'm setting my volume, also I'm building a -3.5dB footswitch to cut volume on the fly at gigs.



Hi @donwagar and @MarkS2 , yes I agree, the always-on first stage, together with the continuous non-switched connections through R4, R6 and R8 through to the speaker, plus the fact that you are only flicking one switch out of three at any given instant of time, protects the amp from seeing an open-circuit transient even though the toggle switches are likely to be non-shorting.

This is very different to the full bypass switch, which for an instant, leaves the amp fully open with no load which may make high transients unless the amp is off or on standby.


----------



## JohnH

Graham G said:


> Thanks John, so if I only need an 8ohm out, which is the simplest diagram/version to get to grips with, on the plus side I do have a decent iron/solder & a multi meter & I have a lot of experience building hi-fi amps including PCB stuffing, so maybe I should start to understand in a small way how what i'm doing



That sounds like you could get your head around this! if you sketch out the parts to scale, then you can draw a wiring diagram for yourself and check it three times to make sure things are connected where they should be.


----------



## nzingler24

Simple Attenuators - Design And Testing

Thanks @JohnH all parts have been ordered now


----------



## MarkS2

JohnH said:


> Hi @donwagar and @MarkS2 , yes I agree, the always-on first stage, together with the continuous non-switched connections through R4, R6 and R8 through to the speaker, plus the fact that you are only flicking one switch out of three at any given instant of time, protects the amp from seeing an open-circuit transient even though the toggle switches are likely to be non-shorting.
> 
> This is very different to the full bypass switch, which for an instant, leaves the amp fully open with no load which may make high transients unless the amp is off or on standby.



Thanks for the confirmation. That makes a lot of sense.

I finished putting mine together tonight, and will hook it up, take some measurements and test it out tomorrow!


----------



## JohnH

hi Mark, that's a nice open build, and very neat graphics on it.


----------



## Graham G

That build of Marks attenuator looks exactly the spec. I want/need for my ORI50 with 8ohm Speaker & I can easily follow what he's done to build one myself, regardless of this excellent work Mark.


----------



## MarkS2

JohnH said:


> hi Mark, that's a nice open build, and very neat graphics on it.



Thanks! I tested it out this morning and it works perfectly! What a difference for getting better sounds out of this amp.

By the way it's got a cover, it just wasn't in that photo. On that note, the sheet metal is pretty thin so I don't know how it'd behave thermally for higher wattages (I'm running 15W), but I left the R1 on the back face with room for a heat sink which could be a later upgrade.

Edit: also by the way, those graphics were applied simply with a piece of cardstock run through my laser printer, then attached with some spray adhesive. Quick and dirty but highly effective!



Graham G said:


> That build of Marks attenuator looks exactly the spec. I want/need for my ORI50 with 8ohm Speaker & I can easily follow what he's done to build one myself, regardless of this excellent work Mark.



If you're looking at that pic, 2 things to be aware of: I omitted the extra jacks, and I have a bypass switch wired like JohnH sketched up in this post #1866


----------



## dbishopbliss

A little background... I have an amp with two speaker outputs - one for 8 ohms and the other for 4 ohms. It is wired so that if you plug in two 8 ohm speakers it will parallel the two cabinets for a 4 ohm load.

Right now, I have an M2 and a M3 for 8 ohm loads. I have used both plugged into the amp. It seems to work. Any reason you can think that I should not do this?

If not, I think I'm going to build a dual M2. That is, use DPDT switches with connections for M2 on one side and the other M2 on the other side. 

Either that, or is there a 4 ohm design (I didn't see one but the thread is long)? Maybe I could build it so that I could plug both speakers into the attenuator and they would be paralleled and then one cable to the amp. That saves me the cost of the resistors and an inductor. 

On other hand, the attenuator would be less useful. I could always use just one side for 8 ohm loads.


----------



## JohnH

dbishopbliss said:


> A little background... I have an amp with two speaker outputs - one for 8 ohms and the other for 4 ohms. It is wired so that if you plug in two 8 ohm speakers it will parallel the two cabinets for a 4 ohm load.
> 
> Right now, I have an M2 and a M3 for 8 ohm loads. I have used both plugged into the amp. It seems to work. Any reason you can think that I should not do this?
> 
> If not, I think I'm going to build a dual M2. That is, use DPDT switches with connections for M2 on one side and the other M2 on the other side.
> 
> Either that, or is there a 4 ohm design (I didn't see one but the thread is long)? Maybe I could build it so that I could plug both speakers into the attenuator and they would be paralleled and then one cable to the amp. That saves me the cost of the resistors and an inductor.
> 
> On other hand, the attenuator would be less useful. I could always use just one side for 8 ohm loads.



I reckon all of that would work!

You can plug your two attenuators into the amp for a combined 4 ohm load, (and set the levels of each speaker differently if you want to)

You can use just one M2 or M3 into an 8 Ohm amp output, then use it with one cab, or two 8 ohm cabs for a 4 Ohm load out of the attenuator but still using the 8 ohm amp tap

You can build a 4 Ohm version. All resistors and inductors are half their value as compared to an 8 Ohm build. But if you were wanting an M3, you would also double the capacitor, probably with several in parallel. Power ratings of resistors stay the same


----------



## JohnH

MarkS2 said:


> Thanks! I tested it out this morning and it works perfectly! What a difference for getting better sounds out of this amp.
> 
> By the way it's got a cover, it just wasn't in that photo. On that note, the sheet metal is pretty thin so I don't know how it'd behave thermally for higher wattages (I'm running 15W), but I left the R1 on the back face with room for a heat sink which could be a later upgrade.
> 
> Edit: also by the way, those graphics were applied simply with a piece of cardstock run through my laser printer, then attached with some spray adhesive. Quick and dirty but highly effective!
> 
> 
> 
> If you're looking at that pic, 2 things to be aware of: I omitted the extra jacks, and I have a bypass switch wired like JohnH sketched up in this post #1866
> 
> View attachment 94066



Looks great and great news that its working fine. With all the space in the box and a 15W amp, Im guessing it warms up just mildly?

Since Graham G and others may be referring to it, I've got a couple of general points:

In my schematic, I show the order of stages as -7db reactive, -7db, -14db, -3.5db then to the output. There are small electrical reasons for that choice of order. The -7 stages are the best balance of impedances so i put them both at the front. It actually works pretty fine with any order, but I slightly prefer it that way. It also allows, with different switch wiring, to use -3.5db on its own, which I show on design M (post 1) and in my own build, but I took it off M2 and M3 for simplification, since its rarely needed.

But anyway getting to the point: You can wire it per the diagram, but its probably more logical to physically place the switches with -7 in the middle, eg -14, -7.-3.5. Then its like binary numbers, where each digit is 2x the previous.

On choice of cases, steel vs aluminium: Steel is much stiffer and stronger, but cant be cast easily for cases, so its made by folding. Hence steel cases are much thinner than aluminum ones. At the same time, aluminium as a material is about 5x as thermally conductive as steel (stainless steel pots often have an aluminium base). So an AL case (or heatsink), being thicker and more conductive, is usually better when heat dissipation is needed for a high power.

Thermal grease under the resistors can help to spread heat due to high power.

There's a couple of watch-it's with mounting the inductor. Some air-coil inductors are on a plastic bobbin with a bolt hole. Using a normal steel bolt through this hole can add 40-50% to its inductance, so better to use a plastic or stainless steel bolt, or another method like zip ties or silicone, as on this build.

Also, having the coil close to the chassis can increase inductance if its steel, or decrease it if aluminium due to two competing effects of magnetic permeability and eddy currents, Something non metallic to space the coil a few mm (say 5mm) off the case avoids this. That being said, Ive only understood those points aver time, and my build works just fine without accounting for them!

Jacks - should be plastic (as you have here) so insulated from the case, eg Cliff type. I like to show at least two output jacks in parallel and I use them both for one of my rigs which is a combo on a cab. But the main reason is that when I'm plugging it all together, I know that the single jack is the input and the group of jacks is the output, even if i get my right and left mixed up!


----------



## dbishopbliss

JohnH said:


> You can use just one M2 or M3 into an 8 Ohm amp output, then use it with one cab, or two 8 ohm cabs for a 4 Ohm load out of the attenuator but still using the 8 ohm amp tap


Am I understanding this correctly? I can plug an 8 ohm M2 into the 8 ohm output of my amp and then plug a 4 ohm load (two 8 ohm speakers in parallel) into the M2??? I thought the speaker load was part of the load the amplifier sees.

It would certainly make things easier if all I had to do was add another speaker jack in parallel to the M2.


----------



## JohnH

dbishopbliss said:


> Am I understanding this correctly? I can plug an 8 ohm M2 into the 8 ohm output of my amp and then plug a 4 ohm load (two 8 ohm speakers in parallel) into the M2??? I thought the speaker load was part of the load the amplifier sees.
> 
> It would certainly make things easier if all I had to do was add another speaker jack in parallel to the M2.



Yes indeed. The attenuator between the speaker and the amp dilutes the effect of the speaker impedance from reaching the amp. The greatest difference is if all the switched stages are switched off. Then with an 8 Ohm speaker, the impedance seen by the amp (at 440 hz) is about 8.6 ohm. If you change to a 4 Ohm speaker pair this only drops to 7.6 Ohms, a negligible change.

If you switch on more stages, then the difference is even less. At max attenuation, you can pull the speaker out and use it as a load box. M2 and M3 are safe with any speaker ohms, if you match the attenuator to the amp. There's a very slight tonal change in theory, with about 1db more presence and resonance, probably not an issue and easily adjustable at the amp if it is. 

Conversely, you can run a 16 Ohm cab from an 8 Ohm attenuator and its safe. The tonal change is now a reduction in presence and resonance, and its a bit more, nearer 2db which I can hear. So for this use, i have the added optional output 3 which corrects this tone change, and I use this feature myself sometimes.

if you try them with the 4 ohm cab combo, and you notice and want to adjust for that very slight tonal change within the build rather than elsewhere , its an easy addition with a resistor and a switch or a switched jack.


----------



## Graham G

I had decided that because I don't have the technical knowledge required to follow Johns schematics etc., ( I do have the building ability to build one), i'd have to buy an off the shelf box, but it would have to be at the budget end of the market, at 76 I'm past buying expensive equipment, I may get the chance to only use it once , so after reading some reasonable reviews(& some negative ones) I bought a Bugera PS1 & used it at Saturdays gig, it's going back Today no more comment needed.
So i'll have to try to get to grips with the schematics & measuring etc & build one, wish me luck, i'll need it .


----------



## JohnH

Hi @Graham G , way to go! keep in touch, I'm happy to discuss each step. What type of amp and speaker is it for? After that, the next step is to identify the parts to buy, and then lay them out to plan the wiring.


----------



## Graham G

Hi John, Origin 50C(with HF ORI20 Mod,which I did myself) Celestion Redback 8ohm, only used for gigs(no home use).
I've started to draw out an M2 circuit, I only need 1 8ohm output, so I'm trying to draw it out with just an input & 1 output(8ohm),I also only need Band level reduction.


----------



## Graham G

Hi, attached is my drawing of the schematic, "if" this is correct I could follow this to start my build, please don't laugh too much, there's a first time for everything .
If the drawing makes sense i'll post my interpretation of what I think is going on in the schematic.

https://i.imgur.com/37hN2Dn.jpg


----------



## JohnH

Hi @Graham G , neat drawing and everything is right, except at switch 2, the left lug has ended up touching the 90 degree corner of the red wire so it looks connected to it. It should be like switch 1 or 3 which I expect was the intention.

As sketched, the switches have the poles connected to the left lug and all the attenuation stages are working. Now imagine them all to the right. In that setting, R3, R5 and R7 only have their lower ends connected, the top ends are floating, so they aren't active. R2, R4 and R6 are all bypassed by their switches, so they are also inactive. So like that, only the first fixed stage is working, for minimum attenuation of -7db.

Next job, order the parts and make a sketch to scale, to work out where you will physically place the parts. This is easier when you have the parts to hand. For an easy life, Id suggest to get a box that is bigger than you might really need. Mine on post 1, which has a few more parts, was a squeeze! Though it would be an easier fit with what you are building. 

There can be a mental leap involved in going from the schematic to a more pictorial wiring diagram. While you can lay it out very close to how the schematic looks, you will probably shuffle the parts to make them fit better, and run the wires to give the shortest runs and neatest wiring. Wires will go between the lugs of the components rather than joining each other in mid-wire. So long as the correct lugs are connected to each other, it doesn't really matter the wiring route between the parts. Some circuits are more critical for this (eg within an amp), but not this one, so there is much flexibility.


----------



## Graham G

JohnH said:


> Hi @Graham G , neat drawing and everything is right, except at switch 2, the left lug has ended up touching the 90 degree corner of the red wire so it looks connected to it. It should be like switch 1 or 3 which I expect was the intention.
> 
> As sketched, the switches have the poles connected to the left lug and all the attenuation stages are working. Now imagine them all to the right. In that setting, R3, R5 and R7 only have their lower ends connected, the top ends are floating, so they aren't active. R2, R4 and R6 are all bypassed by their switches, so they are also inactive. So like that, only the first fixed stage is working, for minimum attenuation of -7db.
> 
> Next job, order the parts and make a sketch to scale, to work out where you will physically place the parts. This is easier when you have the parts to hand. For an easy life, Id suggest to get a box that is bigger than you might really need. Mine on post 1, which has a few more parts, was a squeeze! Though it would be an easier fit with what you are building.
> 
> There can be a mental leap involved in going from the schematic to a more pictorial wiring diagram. While you can lay it out very close to how the schematic looks, you will probably shuffle the parts to make them fit better, and run the wires to give the shortest runs and neatest wiring. Wires will go between the lugs of the components rather than joining each other in mid-wire. So long as the correct lugs are connected to each other, it doesn't really matter the wiring route between the parts. Some circuits are more critical for this (eg within an amp), but not this one, so there is much flexibility.



Thanks John, Yes I drew it out by hand & the connection you mentioned is meant to be seperated,
So there is no actual positive negative as such ? & what wire/cable would you advise, as in AWG stranded, solid core etc ?.
Am I reading it correct in that the Reactive Stage(not sure what that means) is always connected(so -7db) & the other 3 stages are switched in or out? as needed.
By the way i'm in the UK.
Thanks for your help.
Cheers, Graham.


----------



## JohnH

Hi Graham
Id suggest 18awg wire. Solid is good if all your connections are between items fixed to the chassis, or stranded if some are on the lid, as in my build. Stranded for all is ok too.

Yes there's not really a ground, although the lower part of the diagram looks ground-ish, it may not be, it's all really just connected to two taps coming off the amp output tranny. So best to use the plastic Cliff jacks so the case is not part of the circuit.

Yes stage 1 is always on, and sets everything up. It's reactive because it has the coil, and it's impedance changes with frequency. Another way to say it is resistors have resistance and inductors and capacitors have reactance, changing with frequency and shifted in phase.


----------



## Blaoskaak

JohnH said:


> Just giving us a small bump back to our usual place near the front of the workbench! Also, congrats to @Blaoskaak who got his M2 running successfully.
> 
> Here's one thing I'm very happy about: 93 pages in and countless builds later, every single build based on this thread that has gone ahead and been reported here, has been a success.



Thanks! I'm very very happy with the attenuator! It really sounds amazing. In fact, it sounds so good that I took the pedals out in between too.

I can shoot some pics when i'm home for the one's who are interested.

Thank you @JohnH too! Without your help it would never worked.


----------



## Graham G

Hi guys,i'm UK based & i'm struggling to source the L1 inductor spec'd in the M2 attenuator, it's probably not helping that I don't really understand the spec ratings , CPC are showing Visaton (german) conductors but not the spec on Johns list, can anyone help ? Cheers.

Can I also ask if there's a preference or spec for the 3 switches.


----------



## JohnH

Hi Graham

The inductor tends to come from places that sell crossover components for loudspeakers. There are two styles, either wound on a plastic bobbin, or wound just on itself and secured with zip ties and both are fine. The wire gage should be 18 awg, which is also 1mm diameter. 

heres one in EU for the 0.9mH inductance:
Looking for a Dayton Audio LW18-90 crossover coil? - SoundImports

Some ranges don't have the 0.9mH, but in truth, the difference that you will hear is negligible between 0.82mH and 1mH, which opens up some more options, eg:

Visaton SP Air-Core Coils – Impact Audio
product 5013 or 5014 looks fine

there are heaps of hifi component places in the UK, so you may find other options.

In mounting, dont use a normal steel bolt, and best to add about a 5mm plastic or wood spacer against the metal case , to stop it affecting the value.

Switches will be two position, and may be called on-on. Definitely not on-off-on which is three position. Rating should be 5A amps at 125V or better.

Most use standard toggle switches which mount into a 10 or 12mm hole. there are also mini-toggles that go into a 6.5mm hole, which can still have that rating but they are more of a fiddle to wire up with the fairly thick wire we use.


----------



## Graham G

Thanks John, thanks for the link, I now realise that these are the type of coils/components used in passive x-overs in Hi Fi speakers 
really appreciate the info & help.


----------



## Timonew

Hi @Graham G , I’m in the UK and I bought my inductor for 8Ohm M2 from


https://www.qtasystems.co.uk/loudspeaker-components/inductors.htm


Part No ACC391

Air Core Inductor, Type 3, 0.91mH, 1.0mm Wire, DCR 0.49 Ohms. 5% Tolerance


To order you have to email your request and use PayPal to pay.

It’s easier than it sounds! Cost was £14.12 including delivery.


----------



## Graham G

Timonew said:


> Hi @Graham G , I’m in the UK and I bought my inductor for 8Ohm M2 from
> 
> 
> https://www.qtasystems.co.uk/loudspeaker-components/inductors.htm
> 
> 
> Part No ACC391
> 
> Air Core Inductor, Type 3, 0.91mH, 1.0mm Wire, DCR 0.49 Ohms. 5% Tolerance
> 
> 
> To order you have to email your request and use PayPal to pay.
> 
> It’s easier than it sounds! Cost was £14.12 including delivery.



Thanks Timonew, i'll order it now.


----------



## donwagar

Footswitch -3.5dB Stage






Someone on one of the SV20 threads mentioned it might be good to have a footswitchable attenuator to cut just the volume (without losing tone). I thought that was a brilliant idea. An easy way to go from rhythm to solo volumes.

I was just starting to plan to build an M2, so I asked JohnH if it was possible. He already had a schematic for it.

I finally finished it, it works great. (I still have to collect the parts and add an LED to it).

Because the SV20 already has a 5W switch, and because I'm not planning on using it at home (I keep it in our practice space, only use it for practices and gigs), I left the -14dB Stage out of mine.

So it has -7 (reactive), -7 and -3.5 switchable, and -3.5 on the footswitch. My amp is the SV20C combo, so one speaker, so I only used one speaker jack.

I think this attenuator is fantastic, anyone that is reasonably handy really should build one.

[Gene noticed the original jacks I used weren't isolated, thanks for that, fixed now]


----------



## Rvenn

Hi Everyone, AWESOME thread thanks, extremely interesting & buckets of great info & build inspiration!

One question I don't think has been asked:
I've got a Dayton 0.9mH air core inductor kicking around that's 20awg and 0.71 Ohm, Would this be ok to use in L1 or should I really get a 18Awg inductor? (which are closer to 0.49ohm) 

I only plan on running a 15W or 20W amp through the attenuator at most, but cant ascertain if the lower resistance value is important

Cheers


----------



## JohnH

Hi @Rvenn , welcome to our thread and thanks for your interest in this design. 

A 20awg coil will probably work fine, but it will get hotter and push the ohms seen by the amp slightly. Try it if you like, but I think 18awg is the best type.


----------



## zfrittz6

Hello, a crazy idea, if in the 8 Ohm output of the transformer of an amplifier we put an 8 ohm load, and then we make a third winding of, let's say 0.1 ohm, and we connect an 8 ohm speaker ...
What impedance would the outlet valves see?
Would the sound of the amp change?
If the impedance is not greatly affected, the transformer could be modified to have various powers, from 0w to 1 w to play at home, and since it does not have resistors or anything to attenuate the signal, the sound should be the same as the original.
Greetings


----------



## matttornado

donwagar said:


> Footswitch -3.5dB Stage
> 
> 
> 
> 
> 
> 
> Someone on one of the SV20 threads mentioned it might be good to have a footswitchable attenuator to cut just the volume (without losing tone). I thought that was a brilliant idea. An easy way to go from rhythm to solo volumes.
> 
> I was just starting to plan to build an M2, so I asked JohnH if it was possible. He already had a schematic for it.
> 
> I finally finished it, it works great. (I still have to collect the parts and add an LED to it).
> 
> Because the SV20 already has a 5W switch, and because I'm not planning on using it at home (I keep it in our practice space, only use it for practices and gigs), I left the -14dB Stage out of mine.
> 
> So it has -7 (reactive), -7 and -3.5 switchable, and -3.5 on the footswitch. My amp is the SV20C combo, so one speaker, so I only used one speaker jack.
> 
> I think this attenuator is fantastic, anyone that is reasonably handy really should build one.
> 
> [Gene noticed the original jacks I used weren't isolated, thanks for that, fixed now]



That's awesome! I was just thinking of that the other day. I'm interested in doing that as well. Does the speaker signal run through the footswitch? That would be bad news for sure. 

Can someone repost the schematic showing the footswitch idea?

thanks!!!


----------



## matttornado

For a -3.5db footswitch, do you just simply replace the -3.5 db toggle switch on the attenuator with a 1/4" female jack and run a cord to a typical footswitch?


----------



## JohnH

matttornado said:


> That's awesome! I was just thinking of that the other day. I'm interested in doing that as well. Does the speaker signal run through the footswitch? That would be bad news for sure.
> 
> Can someone repost the schematic showing the footswitch idea?
> 
> thanks!!!



This was the diagram:













M2 Footswitch 200103



__ JohnH
__ Jan 3, 2021






The signal does go out to the footswitch and back, via a special 3-core cable made using 5A mains flex and stereo jacks. But It's not the whole current, it's already attenuated. For a typical 50W 8ohm build, it's about 1.1A max. On the footswitch, two of the three poles are used in parallel to get plenty of current rating. And a typical 3pdt such as you build into a stomp box, has a rating of 4A at 125V per contact anyway.

If the footswitch is not plugged in, then the switched jacks sends the signal through to the speaker 

I think this idea is ok provided the first reactive stage is always on, so not using it for a full bypass.


----------



## JohnH

zfrittz6 said:


> Hello, a crazy idea, if in the 8 Ohm output of the transformer of an amplifier we put an 8 ohm load, and then we make a third winding of, let's say 0.1 ohm, and we connect an 8 ohm speaker ...
> What impedance would the outlet valves see?
> Would the sound of the amp change?
> If the impedance is not greatly affected, the transformer could be modified to have various powers, from 0w to 1 w to play at home, and since it does not have resistors or anything to attenuate the signal, the sound should be the same as the original.
> Greetings



Not sure, but I think it might work. But to sound right, I'd say that the load would need to be a properly designed reactive load rather than just a resistor

The idea could maybe be adapted to work with amps as they are.

The following is pure speculation, don't try it unless you understand, accept and agree!

Studio amps, current DSLs and many classic amps have a full group of output taps, 4, 8 and 16. If you plug a 16 cab
into the 4 tap, it's giving 1/4 of the load the amp needs. 

Then, use a reactive load worked out for 8 ohms x 4/3 and put it in the 8ohm tap, to give the other 3/4 of the load. The amp is now fully and properly loaded by reactive loads, and the actual speaker is drawing 1/4 the power, a -6db attenuation.

Maybe?


----------



## zfrittz6

Hello, I am sorry I do not know how to calculate a reactive load, if you have any scheme of some to use it, I could test my theory.


----------



## JohnH

zfrittz6 said:


> Hello, I am sorry I do not know how to calculate a reactive load, if you have any scheme of some to use it, I could test my theory.



This is the classic thread about reactive load boxes:
Aiken's Reactive Dummy Load. | The Gear Page

You can also use our attenuator on this thread as a reactive load (switch to full attenuation and no speaker in it).

For your idea, adding a new winding to an OT sounds complicated? I don't know.


----------



## zfrittz6

Thanks for the link,
winding a transformer is not complicated, all the power supply and audio output transformers for the amplifiers that I do myself.


----------



## JohnH

zfrittz6 said:


> Thanks for the link,
> winding a transformer is not complicated, all the power supply and audio output transformers for the amplifiers that I do myself.



Cool! well good luck! it sounds like an interesting idea.


----------



## diego_cl

Hi
Is there any downside of using switches like these?


----------



## matttornado

JohnH said:


> This was the diagram:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Footswitch 200103
> 
> 
> 
> __ JohnH
> __ Jan 3, 2021
> 
> 
> 
> 
> 
> 
> The signal does go out to the footswitch and back, via a special 3-core cable made using 5A mains flex and stereo jacks. But It's not the whole current, it's already attenuated. For a typical 50W 8ohm build, it's about 1.1A max. On the footswitch, two of the three poles are used in parallel to get plenty of current rating. And a typical 3pdt such as you build into a stomp box, has a rating of 4A at 125V per contact anyway.
> 
> If the footswitch is not plugged in, then the switched jacks sends the signal through to the speaker
> 
> I think this idea is ok provided the first reactive stage is always on, so not using it for a full bypass.


Thank you so much!


----------



## matttornado

JohnH said:


> This was the diagram:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Footswitch 200103
> 
> 
> 
> __ JohnH
> __ Jan 3, 2021
> 
> 
> 
> 
> 
> 
> The signal does go out to the footswitch and back, via a special 3-core cable made using 5A mains flex and stereo jacks. But It's not the whole current, it's already attenuated. For a typical 50W 8ohm build, it's about 1.1A max. On the footswitch, two of the three poles are used in parallel to get plenty of current rating. And a typical 3pdt such as you build into a stomp box, has a rating of 4A at 125V per contact anyway.
> 
> If the footswitch is not plugged in, then the switched jacks sends the signal through to the speaker
> 
> I think this idea is ok provided the first reactive stage is always on, so not using it for a full bypass.


Hi John. Is there a way to do it so i can leave the resistors in the attenuator?


----------



## Northtone

JohnH said:


> This is the classic thread about reactive load boxes: [link removed]
> 
> You can also use our attenuator on this thread as a reactive load (switch to full attenuation and no speaker in it).



Hi John, this sounds interesting.
So, to use the attenuator as a reactive load, I would need a line-out. 
I can follow the post from the user matttornado, right (p. 41, post 814 - unfortunately I can't add links yet)?


----------



## JohnH

diego_cl said:


> Hi
> Is there any downside of using switches like these?
> 
> View attachment 94694



I think they look fine, the right contacts and plenty of current capacity. You just have to work a line of neat rectangular holes in your chassis, but the switch looks like it overlaps the edges a bit to hide any small inconsistencies.


----------



## JohnH

Hi @Northtone , This was the post with @matttornado 's line out (and i agree with the comments):




matttornado said:


> Here's the line out that I used if anyone is interested. Please note that on the drawing it shows that lug 3 is soldered to the pot's case. That must NOT be soldered or else you'll get weird ground loop noise. I hooked it up at the Input of the attenuator so its seeing 100% amplifier signal. Works well here or after the reactive stage. its says to use a Linear pot which I am using now but it is suggested that an audio pot be used instead. I will eventually try an audio taper type.
> 
> View attachment 68290


----------



## JohnH

matttornado said:


> Hi John. Is there a way to do it so i can leave the resistors in the attenuator?



Probably. The switch for each stage has three lugs and you could send it out with a similar 3 core cable that I was showing, using 5A flex. Youd build an extra set of resistors in the main box, and use a switched jack so its bypassed when you don't use the footswitch. 

The advantage of what I was showing was that you just have to add the jack insert in the main unit.

I think the footswitched stage, wherever it is, is best as an additional stage so that you still have access to all the other stages and db settings

Or, set up a relay system triggered by dc - Then the speaker signals aren't going out to the floor box. Untested though, Id worry about dc pulses being picked up by other cables.


----------



## ThePanda

JohnH said:


> Hi @Northtone , This was the post with @matttornado 's line out (and i agree with the comments):


Just a quick few questions.

What do you do with lug 3 of the pot? Is it not used? And what would the benefit be to have this Line Out be after stage 1 instead of seeing full power?

If there is a significant difference I was thinking of adding a switch to go from 'full' to 'stage 1'.

Thank you!


----------



## JohnH

Just to note that this isn't our diagram, so Id have drawn it differently, but its basically ok. The right lug of the pot needs to go to the 'cold' connection, ie the wire that goes across the base of our diagrams, and joins the jack outer barrels. We don't really have a ground to the chassis.


----------



## matttornado

Hi John. I built the M4 version without output 3 in dotted red lines.

I'm willing to add a footswitch with resistors so I need to:

1. add an additional stage after the -3.5db?

2. for a solo boost would I want a 7db or 3.5 db stage?

3. what values would I use for the footswitch? I have a 16 ohm build.

4. Do I simply just Follow the diagram above and add 7 db footswitch stage with same values I already used with the 1st 7 db stage? 

I really like this because I always struggle with a good volume boost for solos. This is my ticket!!!!

Any help or additonal diagrams would be super appreciative. Once Iwrap my head around this, I'll be good to go. I looked at the diagram with the footswitch and that raised these questions.

Thanks!!!!


----------



## matttornado

ThePanda said:


> Just a quick few questions.
> 
> What do you do with lug 3 of the pot? Is it not used? And what would the benefit be to have this Line Out be after stage 1 instead of seeing full power?
> 
> If there is a significant difference I was thinking of adding a switch to go from 'full' to 'stage 1'.
> 
> Thank you!



FYI: From my experience - I tried the line out at the input of the attenuator (100% amp power) and at the end of the attenuator (attenuated output) & it works best at the attenuator input at 100% because this way you'll always have enough line out signal. When I had the line out hooked up at the attenuated levels, my line level was really weak when heavily attenuated.


----------



## ThePanda

matttornado said:


> FYI: From my experience - I tried the line out at the input of the attenuator (100% amp power) and at the end of the attenuator (attenuated output) & it works best at the attenuator input at 100% because this way you'll always have enough line out signal. When I had the line out hooked up at the attenuated levels, my line level was really weak when heavily attenuated.



That makes sense I will put it at the input. 

What are you using to capture your line out?

Just ordered the parts this morning.


----------



## matttornado

ThePanda said:


> That makes sense I will put it at the input.
> 
> What are you using to capture your line out?
> 
> Just ordered the parts this morning.


I run my line out into the FX return on my DSL40CR


----------



## JohnH

matttornado said:


> Hi John. I built the M4 version without output 3 in dotted red lines.
> 
> I'm willing to add a footswitch with resistors so I need to:
> 
> 1. add an additional stage after the -3.5db?
> 
> 2. for a solo boost would I want a 7db or 3.5 db stage?
> 
> 3. what values would I use for the footswitch? I have a 16 ohm build.
> 
> 4. Do I simply just Follow the diagram above and add 7 db footswitch stage with same values I already used with the 1st 7 db stage?
> 
> I really like this because I always struggle with a good volume boost for solos. This is my ticket!!!!
> 
> Any help or additonal diagrams would be super appreciative. Once Iwrap my head around this, I'll be good to go. I looked at the diagram with the footswitch and that raised these questions.
> 
> Thanks!!!!
> 
> View attachment 94754



Hi Matt, I had a look back at your build pics, which look very classy indeed

I note that its a 16 Ohm build. I'm not seeing a bypass switch either? is that the case? if so that helps and makes the addition of the footswitch more fool-proof. I can help with diagrams.

Also, what are the usual levels of attenuation that you use?

As for the footswitch db change, Don found that a -3.5db step worked best for him. Before that, i would have guessed -7db. Values for these would be the same as in the main box. It would be a good idea to try with the levels of change that you can get with the current switched stages to judge this for your own use. If you decide that something in between would be most ideal, its easy to work out values for that too. It is also possible to add another switch into the footswitched box to make its value changeable.


----------



## matttornado

JohnH said:


> Hi Matt, I had a look back at your build pics, which look very classy indeed
> 
> I note that its a 16 Ohm build. I'm not seeing a bypass switch either? is that the case? if so that helps and makes the addition of the footswitch more fool-proof. I can help with diagrams.
> 
> Also, what are the usual levels of attenuation that you use?
> 
> As for the footswitch db change, Don found that a -3.5db step worked best for him. Before that, i would have guessed -7db. Values for these would be the same as in the main box. It would be a good idea to try with the levels of change that you can get with the current switched stages to judge this for your own use. If you decide that something in between would be most ideal, its easy to work out values for that too. It is also possible to add another switch into the footswitched box to make its value changeable.



Hi John! yes NO bypass switch so my amp constantly sees that huge 300 watt Resistor.

I don't remember what level I had my attenuator set at the last time I gigged with it to be honest, sorry about that. I'd like to start with a -3.5db stage for now and maybe tweak later?

I pretty much understand your footswitch diagram except for the added Footswitch female jack at the attenaator.

I can't quite make out how it is wired.
Are the female jacks switched? I would imagine the one at the attenuator would have to be so it can work without the footswich connected. Not sure about the one at the footswitch.

I might hard wire the cord at the footswitch with a strain relief like a typical amp footswitch. I might use my footswitch that came with my DSL! HAHA!

When I get this all together and working I will post many pics!

Thank you so much!


----------



## JohnH

Yes the jack at the attenuator is a switched stereo jack with 3 pairs of lugs. When there's no plug, the main contacts each connect over to the respective lug on the other side With a plug inserted, this contact is opened. It's the kind of socket that might usually be used to cut out a pair of speakers on a stereo when you plug in headphones.


----------



## matttornado

JohnH said:


> Yes the jack at the attenuator is a switched stereo jack with 3 pairs of lugs. When there's no plug, the main contacts each connect over to the respective lug on the other side With a plug inserted, this contact is opened. It's the kind of socket that might usually be used to cut out a pair of speakers on a stereo when you plug in headphones.



Thanks! I don't understand how that gets wired at the attenuator yet. not sure I understand the connection lines on the diagram.


----------



## JohnH

I'll draw something in the next couple of days


----------



## matttornado

Can I get away with using 25 watt resistors in the footswitch since it will be so far down the attenuation chain? I believe I have 100 watt resistors for the 3.5 db stage inside the attenuator.


----------



## JohnH

matttornado said:


> Can I get away with using 25 watt resistors in the footswitch since it will be so far down the attenuation chain? I believe I have 100 watt resistors for the 3.5 db stage inside the attenuator.



On the M version that you have, the full diagram lets the -3.5db stage run on its own, so it would then take a lot of power and hence high ratings. But with yours, I think the -7db stage at least, is always on. So if you also have a -3.5 stage after it then, then according to numbers, and based on 100W coming in (i think you have a 100W head?) then the 75 Ohm will get 6W and the 12 Ohm will get 10W. My rule of thumb is x3 on that for the component rating. So based on that, 25W is just below for the 12 Ohm, but almost certainly not a problem, so long as its bolted with thermal grease etc to the aluminium box.


----------



## Graham G

John, to avoid using a steel Bolt to fasten the Air Core to the case(aluminium), would it be OK to use hot melt glue on the plastic spool to attach to the case, or not advisable ?.
Thanks.


----------



## JohnH

Hi @Graham G , hot melting glue may not be the best, since it could melt it it gets hot. A dab of silicone would be better, or zip ties. or a non steel bolt


----------



## matttornado

Hi John. Looking at the footswitch diagram....I'm confused as to why a stereo jack is used and not mono switched jack? 

Thanks!


----------



## Gene Ballzz

@matttornado 
Although there may be an instance or two (in this very lengthy thread) that actually use the ring for switching, @JohnH described early on that he likes the "stereo" jacks for the extra sturdiness and support they provide Even when the ring terminals are not connected to anything. Please remind me what post number range includes pics of your build?

Ain't this attenuator design just the greatest thing since sliced bread?  

Quieter In Oh Wellia,
Gene


----------



## matttornado

Gene Ballzz said:


> @matttornado
> Although there may be an instance or two (in this very lengthy thread) that actually use the ring for switching, @JohnH described early on that he likes the "stereo" jacks for the extra sturdiness and support they provide Even when the ring terminals are not connected to anything. Please remind me what post number range includes pics of your build?
> 
> Ain't this attenuator design just the greatest thing since sliced bread?
> 
> Quieter In Oh Wellia,
> Gene


It sure is, Gene! This attenuator is coolest and best thing I ever built.


----------



## matttornado

Gene Ballzz said:


> @matttornado
> Although there may be an instance or two (in this very lengthy thread) that actually use the ring for switching, @JohnH described early on that he likes the "stereo" jacks for the extra sturdiness and support they provide Even when the ring terminals are not connected to anything. Please remind me what post number range includes pics of your build?
> 
> Ain't this attenuator design just the greatest thing since sliced bread?
> 
> Quieter In Oh Wellia,
> Gene



I start chiming in with questions on page 20 & then start posting pics of my build on page 28. I'm slow builder though. lol


----------



## matttornado

So the ring and tip are tied together at the attenuator's FS stereo switched jack but separated at the footswitch's stereo jack?


----------



## donwagar

matttornado said:


> So the ring and tip are tied together at the attenuator's FS stereo switched jack but separated at the footswitch's stereo jack?



Yes, when the plug is out at the attenuator, the ring and tip then connect via the switched terminals. Then the attenuator can work without the footswitch.


----------



## ThePanda

Can someone check my work. Adding 5k pot and resistor for the line out.


----------



## JohnH

The ring terminal and switch contacts are used at the footswitch jack at the main box because there are three wires that need connecting, being cold, and each side of the hot being output from the amp, and also onward to the speaker. Diagram soon...

The other place I use the ring terminal is for Out 3 on the 8 ohm designs.


----------



## JohnH

Here's a new version of the footswitched diagram, coloured to show how the jack is hooked up. In the inset picture, the main contacts are on the far side. The pink and red wires cross over , and youd wire them underneath but I showed it over the top to make it clear













M2 Footswitch 210923



__ JohnH
__ Sep 22, 2021


----------



## JohnH

ThePanda said:


> Can someone check my work. Adding 5k pot and resistor for the line out.



This looks ok. See how it goes, the best values may depend on how loud the amp is and what you are driving the line-out into. If you need more output level, the 470 could increase to 1k or 2.2k


----------



## matttornado

excellent! thanks for clearing that up and making a new chart. that diagram alone is priceless!!!!!


----------



## Graham G

JohnH said:


> Hi @Graham G , hot melting glue may not be the best, since it could melt it it gets hot. A dab of silicone would be better, or zip ties. or a non steel bolt



Thanks i'll avoid the the Hot Melt , the last of my parts should be arriving Today, this may be already in the thread but I've missed it, but are there any meter measurements/tests I can do to check I have the circuit & switch wiring correct ?.


----------



## JohnH

Yes, and I id advise that too!

Best test is to check resistance seen by the amp. Plug speaker into attenuator, and a cable into the input, but no amp. Measure resistance across thd input cable in every switch setting. For the 8 Ohm build, it should be between about 7 and 10 ohms in every setting. x2 for the 16 ohm ones.


----------



## Graham G

Thanks John, will do, the switches (On -On) are going to be the last items to arrive, so i'll see if I can figure out the contacts wiring before I start asking for help .


----------



## matttornado

JohnH said:


> This was the diagram:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Footswitch 200103
> 
> 
> 
> __ JohnH
> __ Jan 3, 2021
> 
> 
> 
> 
> 
> 
> The signal does go out to the footswitch and back, via a special 3-core cable made using 5A mains flex and stereo jacks. But It's not the whole current, it's already attenuated. For a typical 50W 8ohm build, it's about 1.1A max. On the footswitch, two of the three poles are used in parallel to get plenty of current rating. And a typical 3pdt such as you build into a stomp box, has a rating of 4A at 125V per contact anyway.
> 
> If the footswitch is not plugged in, then the switched jacks sends the signal through to the speaker
> 
> I think this idea is ok provided the first reactive stage is always on, so not using it for a full bypass.



can a 3 prong A/C extension cord be used?


donwagar said:


> Footswitch -3.5dB Stage
> 
> 
> 
> 
> 
> 
> Someone on one of the SV20 threads mentioned it might be good to have a footswitchable attenuator to cut just the volume (without losing tone). I thought that was a brilliant idea. An easy way to go from rhythm to solo volumes.
> 
> I was just starting to plan to build an M2, so I asked JohnH if it was possible. He already had a schematic for it.
> 
> I finally finished it, it works great. (I still have to collect the parts and add an LED to it).
> 
> Because the SV20 already has a 5W switch, and because I'm not planning on using it at home (I keep it in our practice space, only use it for practices and gigs), I left the -14dB Stage out of mine.
> 
> So it has -7 (reactive), -7 and -3.5 switchable, and -3.5 on the footswitch. My amp is the SV20C combo, so one speaker, so I only used one speaker jack.
> 
> I think this attenuator is fantastic, anyone that is reasonably handy really should build one.
> 
> [Gene noticed the original jacks I used weren't isolated, thanks for that, fixed now]


where did you get your cable for the footswitch? that’s all i need and then i’ll be ready to build.
thanks!


----------



## JohnH

matttornado said:


> can a 3 prong A/C extension cord be used?
> 
> where did you get your cable for the footswitch? that’s all i need and then i’ll be ready to build.
> thanks!



In principle, yes that's fine as a cable type, you get three reasonably thick stranded cores in a non-screened cable, equivalent to a speaker cable with an extra core. The main issue is its thick enough, but not to get one too thick that it cant squeeze into a jack plug barrel (you still need to use a jack socket and plug). I've got some three-core 5A flex and that's what Id use if I built this myself.


----------



## donwagar

matttornado said:


> can a 3 prong A/C extension cord be used?
> 
> where did you get your cable for the footswitch? that’s all i need and then i’ll be ready to build.
> thanks!



the one I used came from Amazon. It's 18AWG 3 conductor 25'.

here's the cable on Amazon US
https://www.amazon.com/18AWG-UL2464...rds=18+awg+3+conductor&qid=1632453951&sr=8-33


----------



## Graham G

As I suspected i'm struggling with working out the switch , 
the switch I have ordered is a DP On-On with 2 rows of contacts, top & bottom contacts labelled On On with no label on Centre contact, see pic below.
I have no pin continuity with the toggle up(or off) but with the Toggle in the down(or on)I do have continuity between Top(on) pin & the centre the centre pin, but I have no continuity with the Bottom (on) pin in any toggle position, i'm a bit lost, I'm not sure in Johns original drawing(which I copied in my Hand sketch) which are the On-On ar centre pins as per my switch.
Or do I have the wrong type of switch ?
Cheers, Graham.


----------



## JohnH

It's a good idea that you are testing the switches with a meter set to ohms

So the lugs are in a 2x3 pattern. Arranging that pattern vertically, so that the lever flicks up and down, then use just one side ie one group of three. Labelled 1, 2, 3 as you sketched, we expect as follows:

Lever Up: 2 is connected to 3, 1 is disconnected
Lever down: 2 is connected to 1, 3 is disconnected 

Ie, usually, Lever up makes the middle lug connect to the lowest lug.

All of that applies to the left and to the right sets of three. You can ignore say, the left three, since we only use one pole.

Does that work?


----------



## Graham G

John, once again thanks for your time & help, yes on a continuity test the switch works as you have said, my issue is I can't work out which wire connects to which lug.
If I take the First switch I have the following wires to connect, if i'm reading your diagram correctly,i.e.
1 wire from front of Resistor 4(or rear of 2A) to Switch.
1 wire from rear of Resistor 4 to Switch.
1 wire from top of Resistor 3 to Switch.
I can't work out which wire goes to which Lug of the switch, I can follow it fine on your diagram but the lugs are in a different pattern, I know i'm not quite technical enough to understand how the circuit works, but i'm ok doing the work.

Edited to add, i'm trying to figure this out & I've come up with this, using Switch 1 as an example.
Front of Resistor 4 to Centre lug.
Rear of Resistor 4 to Bottom lug.
Front of resistor 3 to Top lug.
Anywhere near ? .


----------



## matttornado

donwagar said:


> the one I used came from Amazon. It's 18AWG 3 conductor 25'.
> 
> here's the cable on Amazon US
> https://www.amazon.com/18AWG-UL2464...rds=18+awg+3+conductor&qid=1632453951&sr=8-33


thank you!!!


----------



## JohnH

Hi Graham, yes I think that is correct and it will work. All the switches can be similar. 

It actually implies a choice though, as to whether when you flick a switch up, it gets louder or quieter. The circuit works fine either way, and the difference is whether you swap the outer lugs as compared to how you listed them. 

Your switches connect down when the lever is up, as do most toggles (except for the ones that I bought, which caught me out!). In the down switch position as you describe, R3 is connected and R4 is not shorted-out, so this stage is attenuating. Flick the switch up and its not attenuating so its louder.

That's the way i think is most logical, switches up is louder.

There's a UK/US cultural issue here though! In the UK and here in Aus, you flick a light switch down to turn it on, but in the US you flick it up. But how does this relate to our design? its a free choice but i did mine to work the same way that you describe. The most important thing is that you do them all the same.


----------



## Graham G

Thanks John I really appreciate you helping me to get the attenuator built, what time is it in Australia now ? early hours in the Morning ?.
I was going to make a joke about your US comment, but that could come over wrong in print .
One last question well maybe 2 , is it ok to do switching with the Amp at stage Volumes & do you have a Donate button ?, if not maybe it would be a good idea, for members to show some appreciation for all your help & knowledge ?.
Thanks again.


----------



## JohnH

Hi @Graham G , Aus is 9 hours ahead of the UK right now, and thanks for that idea.

I'm happy that the M2 design is safe to switch on the fly while the amp is powered up. You'll likely pause your playing while you flick switches, but I think there is minimal risk anyway. If there was an issue, it would be due to transients generated while switches move from one setting to the next. But our design protects the amp from those because the first stage between amp and switches is always connected, providing a consistent load. Also, there is a constant path through R2, R4, R6 and R8 to the speaker, independent of switches, and you can only flick one switch in any given millisecond.

We don't have a full bypass switch shown on M2, but some add them and that's all fine. But if this is installed, it should not be moved unless the amp is on standby or off.


----------



## Graham G

My M2 measures correct at the input Jack all around 8ohm give or take a couple of decimal points,but I have no sound at all if I crank the Amp volume I get a very low "chirping" sound.
I don't think it's my wiring because the ohms measure correctly, in my ultra non tech head I think it may be the Jacks, i'm getting no bleep continuity across the grounds on the lead Jacks, is there a way I can bleep out the jack sockets to check if they're correct, of course it may be something else, but I always look for simple 1st & then my mistakes 2nd .
Cheers.


----------



## JohnH

Is the amp ok if you plug the speaker in normally?


----------



## Graham G

Thanks, John.Yes I checked the Amp first & then tested the M2 ,in the instructions(somewhere in the thread) it stated not to use switched jack sockets & gave a Cliff part number, which I ordered but they may not have sent the same part(thet switched a couple of resistors from Arcol to Dale, without notice, i'll try & find the bag & check the part number, should I get continuity(bleep) across the Jack covers(steel) ?.


----------



## JohnH

Best to measure ohms rather than just a beeping continuity tester, do you have a multimeter? Also, best to measure across the jack plug that would go to the amp, so it's testing the jack wiring and cable too. Happy to look at photos if you like.

Best not to try with the amp until it's all proven.


----------



## Graham G

Thanks John, it is working they had shipped switched Jack sockets, I switched the wires to the contacts that worked when the switches where open & it works great  .
Do you think it is ok to use the M2 with this "bodge" until I get the correct sockets next week ?,a gig Tonight & I would love to try it. 
Cheers, Graham. 
Once again thanks for your considerable help


----------



## JohnH

I think so, the switched connections don't cause a problem if you don't use them. Just use the lugs that connect to the plug when pushed in.


----------



## Graham G

Yes it's all done as you suggest, looking forward to using it Tonight, it's just about the quietest gig we play at


----------



## ThePanda

Drilled holes and test fit


----------



## Graham G

I used the M2 at last nights gig & it really does a great job of maintaining a cranked Amp sound at really low volume, the pub we played always insists on low volume & last night was even more difficult than usual, there was only a handful of people in the pub(unusual) & without the M2 I would have been struggling with my sound/volume all night. It took me a few numbers to get the balance between cranking the Amp & settling on the best -db cut to use, I ending up just using the default setting with the other stages off, just for reference I use a HF Modded Origin 50.
So the M2 worked really well & I can't really see ever needing more volume cut than just the default setting, which leads me on to ask if it would be possible to adapt the M2 to have a -3.5 default setting & then 3 stages of -3.5db switches.


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## matttornado

Graham G said:


> I used the M2 at last nights gig & it really does a great job of maintaining a cranked Amp sound at really low volume, the pub we played always insists on low volume & last night was even more difficult than usual, there was only a handful of people in the pub(unusual) & without the M2 I would have been struggling with my sound/volume all night. It took me a few numbers to get the balance between cranking the Amp & settling on the best -db cut to use, I ending up just using the default setting with the other stages off, just for reference I use a HF Modded Origin 50.
> So the M2 worked really well & I can't really see ever needing more volume cut than just the default setting, which leads me on to ask if it would be possible to adapt the M2 to have a -3.5 default setting & then 3 stages of -3.5db switches.


very cool. I built the 100 watt version and with a superlead turned up to 6 -8, the default setting is still too loud from my experience so far. I still have to attenuate a little. i have a fan cooling it as well and that keeps it at Room temp!!!! I kid you not. sometimes i forget the fan and after a few hours, the attenuator surface reaches about 150F. that’s cool by that type of standard.


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## matttornado

ThePanda said:


> Drilled holes and test fit


looks great! don’t forget thermal grease under the resistors.....just in case.


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## ThePanda

matttornado said:


> looks great! don’t forget thermal grease under the resistors.....just in case.



Thank you! Good call will do!

Does anyone know what voltage/amps come off the input jack? 

I am thinking of routing a hole for a 40mm case fan that runs 5V


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## JohnH

Hi @ThePanda

That's heading towards being a really nice well thought out build.

On fans etc, Id suggest that if its for a 100W amp, yes its a good idea. If its for 50W or less, and with that thick Hammond case (my fav too!), you'll be OK if you drill a pattern of decent sized holes in the top and the bottom. See my build on post 1, my holes are maybe 10mm. I've tested by cranking my 50W VM through it. With all the weight and thick aluminium that it has, it takes about 40min at continuous max power to arrive.

what power of amp will you use?

You can run a fan powered by the amp using a bridge rectifier, resistor and cap, and it will start to turn as the power gets high, which is what you want. Make sure there are holes for air inlet as well as for the fan mounting. 50W at 8 ohms is nominally 2.5A and 20V. But you need a fan that takes only a very small amount of current or else it will affect the tone. Definitely possible but it will need experimenting and also be dependent on the fan design. Ideally you'd like it spinning when the power gets up but not at its full speed, so it stays very quiet. Or you can use a fan that runs with its own power supply input, as Matt did. But your build is looking nice an neat, and may get crowded it you add a fan and other parts.

Two more watch-its relating to the jacks, which look to be the right type:

These jacks have 6 lugs each. Make sure you wire up to the side that has the main contacts, not the side with the switched contacts. Might be obvious but that has happened, and you end up with no sound and worse, no load on the amp! Only Out3 uses the middle ring connection.

You have the four main jacks for in and out in a nice neat even row. I suggest that you add something very obvious to identify for yourself, which one is the input jack! (I can suffer brain fade while wiring up for a session)

Good luck, its gonna be great!

When its built, before connecting to the amp, plug a speaker in, plug your amp lead into the input and measure resistance across the lead at the plug that will go to the amp. It should be between 7 and 10 ohms in all settings, after deducting for meter leads. When you try with the amp, keep amp volume low and make sure you get some sound. If you get no sound, don't turn it up, but recheck connections, ohms etc.


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## JohnH

Graham G said:


> I used the M2 at last nights gig & it really does a great job of maintaining a cranked Amp sound at really low volume, the pub we played always insists on low volume & last night was even more difficult than usual, there was only a handful of people in the pub(unusual) & without the M2 I would have been struggling with my sound/volume all night. It took me a few numbers to get the balance between cranking the Amp & settling on the best -db cut to use, I ending up just using the default setting with the other stages off, just for reference I use a HF Modded Origin 50.
> So the M2 worked really well & I can't really see ever needing more volume cut than just the default setting, which leads me on to ask if it would be possible to adapt the M2 to have a -3.5 default setting & then 3 stages of -3.5db switches.



Hi Graham, that's great news and congrats on getting it together.

Im not surprised it takes a while to find the best settings for it with your rig. Ive not tried one, but the Origin 50 specs talk about its own three power settings of 50W, 10W and 5W, plus it has a master volume. So you get to pick settings for all of those, before you decide on attenuator settings. Which amp power setting do you prefer? If its the full 50W and max on the MV, and at -7db on the attenuator, then nominally you have 10W, which would usually be pretty loud in a quiet pub (and it seems the amp has this setting itself anyway).

Its very interesting to get feedback on how this all works best for different users and different amps. Some people mic the amps, some don't, some use the very lowest volumes late at night, for some its only for gigging, and it depends on heaps of choices.

With regard to other attenuator setting designs, there are a few possibilities:

I cant do a switched version that starts at -3.5db ad then goes down in further switched steps. The maths required to keep tone and impedance consistent drifts away too much. One of the jobs of stage 1 is to separate the switched stages from the amp, for which some dbs are needed.

But, in a design based on just the lower attenuations, you can:
1. Have all switches doing -3.5db with stage 1 remaining at -7db. Then you just switch on as many stages as you want and it doesn't matter which ones
2. There is a design that is possible and should work well, where the idea in 1 above is squeezed up a bit, so you have a -6db first stage, and the added stages could all be -3db each, or all -3.5db, or all another value. Its calculated but not tested
3. If you've settled on an attenuation value, and would like a dead-simple box, you can make a single fixed stage, no switches, one jack in and one out. it could be designed for any one given setting, eg -6db, or -7db, or more, or even less since its optimized for just the one setting.


(There's another work-around, though I think it doesn't work with your rig. It gives a -3db attenuation, ie half power. You plug the speaker direct to the amp, and plug the attenuator as a load-box without a speaker, set to max attenuation, in parallel. Then you set the amp to lower ohms, eg 4 ohms with an 8 ohm speaker and attenuator. I think that's the bit that doesn't work with your 8 ohm cab and the Origin.)


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## Graham G

Hi, John. thanks again for the info, i'm holding off on any decision about alterations to the M2 'till I can test it a bit more at our practice space.
I've had a listen to a live playback of our Gig(which is a crappy ipad stream) & the Guitar still sounds fairly loud,all though I didn't get any hassle from the owner about my volume(for the 1st time ever).
With reference to your post above, am I reading it correctly that with my ORI50 the default -7db is giving about 10W from my Amp ? & that a -3.5bd would be about 25W or so ?, in my head 25W is what I feel would be about right for me, but after listening to the playback i'm now not sure.
The odd thing is I also have a DSL20, which is too quiet for most of our gigs, but is just about usable at Saturdays gig, I don't use it much because it's either to Clean on the Green CH. or too Distorted(for want of a better word) on the red Ch.
Sorry for rabbiting on, i'll check out the M2 some more & then get back.
Once again thanks for your help.


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## matttornado

ThePanda said:


> Thank you! Good call will do!
> 
> Does anyone know what voltage/amps come off the input jack?
> 
> I am thinking of routing a hole for a 40mm case fan that runs 5V



I ended up using a 14 volt DC ( I think) power supply along with a switch & LED to power a DC 14 volt fan. I tried & failed miserably at trying to power a fan with the attenuator's electricity.


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## matttornado

I made this volume level chart in case anyone is interested. Please let me know if I missed a step or if you see any errors. Thanks


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## aceofbones

Finally finished my build, mostly. I do have to redo my input and outputs, totally forgot about the switched jacks so may have screwed that up. Anyways, I haven't tested it yet but this is where I'm at. Thanks John!


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## JohnH

hi @aceofbones , very nice indeed! I particularly like the neat dressing of the wiring, the 'bus' connections between jacks, and the elegant looking case (your design?), with the funky retro dymo labels.

I think the jacks are all wired fine, given that you connect across the switches, and we aren't using the switches in this design.

Speaker plugged in, and test input resistances as would be seen by the amp, 7 to 10 ohms for an 8 ohm build.


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## JohnH

hi @matttornado , table looks find to me. On building, I suggest to lay the switches out on the front panel 14, 7, 3.5, so its more like binary numbers. It doesn't have to be the same as the electrical order.

And thankyou for taking us onto page 100!


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## aceofbones

JohnH said:


> hi @aceofbones , very nice indeed! I particularly like the neat dressing of the wiring, the 'bus' connections between jacks, and the elegant looking case (your design?), with the funky retro dymo labels.
> 
> I think the jacks are all wired fine, given that you connect across the switches, and we aren't using the switches in this design.
> 
> Speaker plugged in, and test input resistances as would be seen by the amp, 7 to 10 ohms for an 8 ohm build.



Thanks very much John. Ok, glad the jacks are good to go. I thought they were ok like that, but then saw your warning about the switched jack wiring within the last couple of pages so thought I would double check. I found the case on AliExpress and thought it would provide good cooling having the air gap below and it came vented on either side. I just had to drill the holes in the top. It cost as much as all the other parts combined, but I figured it was worth it. 
I’m just finishing up my amp build, but I will report back after I have that fired up and have the attenuator working with it. Cheers.


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## JohnH

hi @matttornado , thanks again for the table idea. I thought it would be interesting to add up the db's for each setting on an M2, and also show the actual nomjnal amp power that you end up with, based on 50W coming in. Its amazing how little power is delivered at max attenuation, and yet this setting can be still quite useful late at night.


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## mike58

Please, post a link for that case, it’s really ubercool. Sort of retro sci-fi with a touch of laboratory thrown in


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## auflauf

Hi!
I am deeply impressed by the expertise and experience gathered here. Maybe my questions have been answered in this thread, I apologize for that, but please forgive me for not reading through all 99 pages. My amp is only a Harley Benton Tube15 (Monoprice Stage Right in the US) with two EL 84 valves. It is said to be a descendant of the Laney CUB 12 which's first stage is said to be loosely based on a Marshall design - if that could serve as a ticket in this forum?

I want to build a simplistic reactive attenuator with a fixed value of eg 20db attenuation and one single switch to toggle attenuation/bypass. If I understand it correctly, the second coil as in M3 or the combination of parallel and serial inductors as in M-lite is not worth the effort, and M2 appears to be the sonically best (or the most cost effective?) iteration of John Hewitts design - is that the consensus here? I would then settle with schematic M2 as from 2. July 2020 and 18. March 2021, omitting all resistors except R1, R2a and R2b.

May someone help me out with these questions:
What is the formula to calculate R1, R2a, R2b and L1 for a given attenuation for an 8 Ohms speaker? I understand it is a loaded voltage divider, but my results don't add up.

Did someone try a "bright cap" of eg 10µF or 4.7µF parallel to R2a?

As shorting switches are hard to find, does a simple non-shorting switch which may leave the amp output open for milliseconds harm the amp?

Finally: db attenuation in this context refers to power, not to voltage? Half the power is -3db, half the voltage is -6 db, half the perceived loudness is -10db. Thus for half the perceived loudness how much calculated attenuation do I need? 31% of voltage or 10% power?

Any help is much appreciated, thanks!


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## aceofbones

@mike58 Here you go…
https://a.aliexpress.com/_mLadpLc

@JohnH I just tested resistance as you suggested and it ranged from 8.1-9.2 ohms depending on switch combinations. Looks like it’s good to go. Thanks again!


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## JohnH

Hi @auflauf and welcome to our thead. Thanks for your interest in this design. 

Yes M2 is the best base design. The M and M-lite versions in the early pages work just as well, but not better. M2 simplifies the build with the same performance. M3 is more accurate in the very low bass, in theory, but it really makes minimal audible difference and makes no difference at all except when you hit the low frets on the low strings.

There's no simple formula for the component values, since with the resistive and reactive components, its too complex to solve directly. Instead, its a step by step iterative process using complex numbers. i do it on a spreadsheet which is about 100x100 cells, which also has to model the speaker. If you want to investigate yourself, Id suggests using a Spice program.

But, the first stage is highly optimised for several parameters, even if it looks simple. So it's not easy to vary it. If you want to build a fixed version, Id suggest to build stage 1 as it is, then add one or two other fixed stages to it.

But id also ask, do you really know what attenuation you want? It may not be obvious what you need until you try it. It depends on your own percepetions and also how you use your amp. Happy to discuss.

If you are building with an overall bypass switch, id recommend that you never operate the switch unless the amp is on bypass or off. Hence shorting or non-shorting doesn't matter. I think most, or maybe all toggles are non-shorting. 

On other attenuator designs, the treble capacitors try to compensate for loss of treble due to the attenuator design, typically because an L pad is used that damps the speaker at low settings. We dont have that issue, so we don't need this feature as a work-around. If you want to try this just for tonal adjustment thoug, id suggest to do it on a later stage, such as R4. I dont think anyone has done this so far though.


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## auflauf

Thank you for the kind reply. Concerning the attenuation: I play the Tube 15 in a rented flat, gain at 4-5, volume at 1 to 2 or gain at 1.5, volume at 3. Even this is sometimes a little loud during the day and definitely too loud during the night. I guess I would like to attenuate the perceived loudness to a quarter to two thirds of the original. Desired sound should range from clean to a little crunchy and sometimes singing overdrive - classic late 60s blues and bluesrock, ... I liked a silverface fender champ and a modeled VOX AC 30. I'm just dipping my toe in the water with the 'modest' Tube 15, my first valve amp.

Concerning the schematics: got it and I am even more impressed. And you are right, the resistors are way, way cheaper than the coils, so they are not that much of a factor.


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## JohnH

Those Tube 15's sound like a great entry way to try tubes. its still hard to really relate db settings to perceived volume though. Some people say that -10db is like half volume audibly, but its a big loose variable. If your are not sure, the switched stages help you explore and select what you need. The individual increments of -3.5db are very small steps. If you go to post 1, there are some sound clips stepping up through the stages to full volume.

The coil that you need for M2 is about $10 to $15 usually.


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## auflauf

Just as a ballpark figure, I guess 0, -10, -20, -30 db would be fine. If the reactive stage would have -8 db, then one -8db and one -16db switchable resistive stage would allow -8, -16, -24, -32 db.

Does it make a difference if the reactive stage had eg -20db without a resistive stage or -8db reactive plus -12 db resistive?

What is your experience and opinion with loudness (the change in frequency sensitivity of the ear with the acoustic power)? Given that your design is sonically very transparent, is there any value in a "bright cap" parallel to R2a? I guess a switchable capacitor ruins/alters the carefully fine-tuned combination of resistive and inductive loads? The same question for a "bass boost coil": where could one put that in order to have an increasing effect with increasing attenuation? Serial or parallel to R2a? Elsewhere?


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## JohnH

To be honest I see no real value in adding treble bleed caps to this, and so far in 100 pages, I don't think anyone has wanted to add one either!. I think that the Fletcher-Munson hearing effects get blamed for a lot of issues with attenuators when it is mostly about the design of them. I play mine loud and also quiet. I find if I am playing loudly, and them have to go right down to very low volume, it sounds muted and dull. But if I start in a quiet environment, and begin at low volume, it sounds fine and clear and full. Our ears adapt all the time. On the attenuator, the real tone is virtually identical at all volumes. 

On the bass cap and coil shown in the M3 design, it doesn't actually relate to the level of bass that you hear. There is still a bass resonance generated by the speaker interacting with the attenuator, in a similar way that it would usually interact with the amp. The only difference is, the amp doesn't see this so in theory doesn't respond in quite the same way. Actually, what you get is a very slightly cleaner more solid bass, which is not a bad thing at all. 

Based on the above and what you describe, I'd suggest to build the M2 as it is, but leave out the last -3.5db stage. This will give you a good range of -7db steps. Ive had that arrangement in an earlier build, and its quite good when you aren't needing to match into a band mix. And everything is worked out ready to go. Even without that -3.5db stage it'll get you down to about the same volume with your 15W amp as I get to with my VM, which is about a 35-40W output really.


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## Graham G

JohnH said:


> Hi Graham, that's great news and congrats on getting it together.
> 
> (There's another work-around, though I think it doesn't work with your rig. It gives a -3db attenuation, ie half power. You plug the speaker direct to the amp, and plug the attenuator as a load-box without a speaker, set to max attenuation, in parallel. Then you set the amp to lower ohms, eg 4 ohms with an 8 ohm speaker and attenuator. I think that's the bit that doesn't work with your 8 ohm cab and the Origin.)



Hi John, could I ask a couple of questions about the above "option",
Could this work by using a 16ohm out(& Speaker) & then using the 8ohm out for the M2 ?.
Or does the Amp need an ohms switch ?, the ORI50 has a 16ohm out & 2 8ohm outs, but no adjustable ohms switch, I also believe it to be Class 2 wiring, I don't know if that makes any difference.
I still have the original 16ohm Speaker.
Thanks again.


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## JohnH

Hi Graham, yes I see how the Origin is configured looking at the manual, with three output jacks and no switch.

What you describe above would put 2x the load on the amp compared to what is intended, since it'd be getting a full 16 Ohm load and a full 8 Ohm load. The idea I posted above would work if you were using a 16 speaker and also had built the M2 for 16 ohms. Or with your gear, it would work if the amp had a pair of 4 Ohm taps.

But, if you want a half power, -3db set up there is a way, that requires a bit of wiring. Basically youd set the M2 at max attenuation with no speaker, and wire it in series (not parallel) with the 8 ohm speaker. Now you'd have two 8 Ohm things in series to make a correct 16 Ohm load, and plug that into the 16 Ohm amp output. That could be done as a switch addition to the attenuator, but Id suggest it may be more reliable in the form of a specially wired cable, with three jack plugs for amp, M2 and cab. The change to his arrangement would need to be while the amp was off or on standby.

The theory is all good, but the risk, in my case, would be my errors in plugging/switching it in.


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## Graham G

JohnH said:


> Hi Graham, yes I see how the Origin is configured looking at the manual, with three output jacks and no switch.
> 
> What you describe above would put 2x the load on the amp compared to what is intended, since it'd be getting a full 16 Ohm load and a full 8 Ohm load. The idea I posted above would work if you were using a 16 speaker and also had built the M2 for 16 ohms. Or with your gear, it would work if the amp had a pair of 4 Ohm taps.
> 
> But, if you want a half power, -3db set up there is a way, that requires a bit of wiring. Basically youd set the M2 at max attenuation with no speaker, and wire it in series (not parallel) with the 8 ohm speaker. Now you'd have two 8 Ohm things in series to make a correct 16 Ohm load, and plug that into the 16 Ohm amp output. That could be done as a switch addition to the attenuator, but Id suggest it may be more reliable in the form of a specially wired cable, with three jack plugs for amp, M2 and cab. The change to his arrangement would need to be while the amp was off or on standby.
> 
> The theory is all good, but the risk, in my case, would be my errors in plugging/switching it in.



Ok so as a "quick fix" solution, for my 8ohm M2 & 8ohm Speaker wired as in my sketch ?
Must all switches be left "ON" & cannot be altered ?, so the -3.5db stage could not be used ?

So the correct(long Term) solution is a 16ohm Speaker & reconfig my M2 for 16ohms ?.
I'm sorry to be a pain in the arse, but I hadn't realised that -7db would cut my Amp to 10W.
Regards, Graham.


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## JohnH

Graham G said:


> Ok so as a "quick fix" solution, for my 8ohm M2 & 8ohm Speaker wired as in my sketch ?
> Must all switches be left "ON" & cannot be altered ?, so the -3.5db stage could not be used ?
> 
> So the correct(long Term) solution is a 16ohm Speaker & reconfig my M2 for 16ohms ?.
> I'm sorry to be a pain in the arse, but I hadn't realised that -7db would cut my Amp to 10W.
> Regards, Graham.



I'm happy to do you a sketch of this (its not quite as yours). Once its done, it should work fine, no need to rebuild anything and it should sound right too. But its a one step fix for getting a small -3db cut. The attenuator doesn't control the level, and has to be set at max. Then to go to lower settings, youd replug the wires back to the normal way.

If you try this and it turns out to be just right, then we could maybe consider other ways to implement this setting. 

btw. Are you running a combo or a head? It affects whether the speaker end of this is a plug or an inline socket.


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## Graham G

JohnH said:


> I'm happy to do you a sketch of this (its not quite as yours). Once its done, it should work fine, no need to rebuild anything and it should sound right too. But its a one step fix for getting a small -3db cut. The attenuator doesn't control the level, and has to be set at max. Then to go to lower settings, youd replug the wires back to the normal way.
> 
> If you try this and it turns out to be just right, then we could maybe consider other ways to implement this setting.
> 
> btw. Are you running a combo or a head? It affects whether the speaker end of this is a plug or an inline socket.



Hi John, if you can do me a sketch that would be great & yes I have a Combo, so the Jack lead is hard wired to the speaker.


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## matttornado

I finished the footswitch today. @JohnH I'm measureing 75.5 ohms resistance. Is that correct?


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## _Steve

JohnH said:


> hi @matttornado , thanks again for the table idea. I thought it would be interesting to add up the db's for each setting on an M2, and also show the actual nomjnal amp power that you end up with, based on 50W coming in. Its amazing how little power is delivered at max attenuation, and yet this setting can be still quite useful late at night.
> 
> View attachment 95261



Wow those numbers are astonishing. 0.89 watts sounds a lot louder than I thought! Thanks for posting it.


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## JohnH

And with that, we are at 2000 posts!


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## auflauf

Concerning ergonomics, would you prefer to switch attenuation on by switching a toggle up or down? Consensus appears to be the activation of a function by moving a lever up/forward. Thus activating an attenuator should require the lever in the active, up position. On the other hand, down means low - imagine a linear potentiometer beneath a toggle attenuator switch in a studio. Logic requires both down for low loudness. Does it makes a difference how the user understands the function of a toggle? Eg "does that switch activates an attenuator or a 'volume booster?'"
Granted, this isn't the most important question in music, but I like self-explanatory gear and I hate it when i have to reserve a part of my small brain for the operation of unnecessarily stupid devices, hence the question.

My personal preference is to mark "0db" up and "-something" down.


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## JohnH

auflauf said:


> Concerning ergonomics, would you prefer to switch attenuation on by switching a toggle up or down? Consensus appears to be the activation of a function by moving a lever up/forward. Thus activating an attenuator should require the lever in the active, up position. On the other hand, down means low - imagine a linear potentiometer beneath a toggle attenuator switch in a studio. Logic requires both down for low loudness. Does it makes a difference how the user understands the function of a toggle? Eg "does that switch activates an attenuator or a 'volume booster?'"
> Granted, this isn't the most important question in music, but I like self-explanatory gear and I hate it when i have to reserve a part of my small brain for the operation of unnecessarily stupid devices, hence the question.
> 
> My personal preference is to mark "0db" up and "-something" down.



I agree with that, and its how I built my ones. I think in this passive circuit that has the job of reducing volumes, 'on' and 'off' in terms of operation of reduction stages is not as relevant as relating the switches logically to the end result of what they do. In english language, we talk about 'turning the volume up' to make it louder. And in any audio or PA gear, volume controls particularly slider ones, go up to make the signal louder. So i put my switch levers up when the relevant stage is not attenuating, ie it is louder.


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## JohnH

matttornado said:


> I finished the footswitch today. @JohnH I'm measureing 75.5 ohms resistance. Is that correct?
> 
> View attachment 95387
> View attachment 95388



Assuming you have a 75 and a 12 ohm resistor in there? The wiring looks OK to me. The meter test looks like two wires (tip and ring?) are clipped to the red lead and the other to the barrel? If so, then yes youd read the 75 ohm resistor. 

With switch set to attenuating, you'd expect:
barrel to tip: 87 Ohm
barrel to ring: 75 Ohm
ring to tip: 12 Ohm

With switch set to off, not attenuating:
barrel to tip: infinite
barrel to ring: infinite
tip to ring: zero 

When you wire in the socket to the main box, its important to get the tip and ring the right way round to match the footswitch end - scope for SNAFU!


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## JohnH

Graham G said:


> Hi John, if you can do me a sketch that would be great & yes I have a Combo, so the Jack lead is hard wired to the speaker.



Here is my suggestion, for your combo speaker, attenuator and amp. This should produce a 16 Ohm nominal load with your 8 ohm speaker and and 8 Ohm M2.




The two jack plugs are interchangeable. If you trace the signal flow, it goes amp hot, to attenuator to speaker and back to amp cold, any way you do it.

i show it wired up with each wire from a tip to a barrel lug. This diagram also works if you had a head, in which case you'd have three plugs. Wired this way, they are all interchangeable, no scope for plugging it in wrong. It can also be used to connect two 8 ohm cabs to make a 16 ohm load.

Important to test this with meter before trying with the amp. Plug in the attenuator set to max attenuation and also the speaker. Measure resistance across the plug that will go to the amp. You should read about 6-8 (speaker) plus 8-9 (M2) = 14 to 17 ohms. 

Then try with the amp, using the 16 ohm amp out. As before, you should get sound even at very low amp volume, but don't turn up amp further unless this is OK.


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## Graham G

Thanks John, 
Edited to add, so when you said my sketch was "not quite correct" it was totally wrong .


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## matttornado

JohnH said:


> Assuming you have a 75 and a 12 ohm resistor in there? The wiring looks OK to me. The meter test looks like two wires (tip and ring?) are clipped to the red lead and the other to the barrel? If so, then yes youd read the 75 ohm resistor.
> 
> With switch set to attenuating, you'd expect:
> barrel to tip: 87 Ohm
> barrel to ring: 75 Ohm
> ring to tip: 12 Ohm
> 
> With switch set to off, not attenuating:
> barrel to tip: infinite
> barrel to ring: infinite
> tip to ring: zero
> 
> When you wire in the socket to the main box, its important to get the tip and ring the right way round to match the footswitch end - scope for SNAFU!


Hi John. yes they are the measurements I get. Thanks for verifiying that for me. 
You said make sure the ring and tip are the right way around to match the footswitch at the attenuator's jack but according to your drawing, they are tied together in aX pattern aren't they?


----------



## JohnH

The X is just a way to combine the contacts in parallel for greater reliability when the footswitch is not connected. But, all I mean is, just follow the diagram which I think is ok.


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## Graham G

JohnH said:


> Here is my suggestion, for your combo speaker, attenuator and amp. This should produce a 16 Ohm nominal load with your 8 ohm speaker and and 8 Ohm M2.
> 
> View attachment 95391
> 
> 
> The two jack plugs are interchangeable. If you trace the signal flow, it goes amp hot, to attenuator to speaker and back to amp cold, any way you do it.
> 
> i show it wired up with each wire from a tip to a barrel lug. This diagram also works if you had a head, in which case you'd have three plugs. Wired this way, they are all interchangeable, no scope for plugging it in wrong. It can also be used to connect two 8 ohm cabs to make a 16 ohm load.
> 
> Important to test this with meter before trying with the amp. Plug in the attenuator set to max attenuation and also the speaker. Measure resistance across the plug that will go to the amp. You should read about 6-8 (speaker) plus 8-9 (M2) = 14 to 17 ohms.
> 
> Then try with the amp, using the 16 ohm amp out. As before, you should get sound even at very low amp volume, but don't turn up amp further unless this is OK.



Hi, John I'm 4-5 ohm at both the M2 & the line socket , at plug in it reads 8 ohm then immediately drops to 4-5ohm, (i'm using a jack socket until I can source an in-line socket).
I'm plugging the Speaker Jack into the in-line Jack Socket, then I Jack into the M2 input & the other Jack into the 16ohm Amp output .
Sorry to once again be a pain.


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## matttornado

JohnH said:


> The X is just a way to combine the contacts in parallel for greater reliability when the footswitch is not connected. But, all I mean is, just follow the diagram which I think is ok.


You are the MAN!


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## matttornado

JohnH said:


> hi @matttornado , thanks again for the table idea. I thought it would be interesting to add up the db's for each setting on an M2, and also show the actual nomjnal amp power that you end up with, based on 50W coming in. Its amazing how little power is delivered at max attenuation, and yet this setting can be still quite useful late at night.
> 
> View attachment 95261


So with my 100 watt Superlead cranked, it's putting out well over 100 watts, right? With my 16 ohm build, How much in wattage do you think my giant 300 watt / 30 ohm R1 is lowering the wattage before entering the switched stages? 40 -50 watts maybe?


----------



## JohnH

Graham G said:


> Hi, John I'm 4-5 ohm at both the M2 & the line socket , at plug in it reads 8 ohm then immediately drops to 4-5ohm, (i'm using a jack socket until I can source an in-line socket).
> I'm plugging the Speaker Jack into the in-line Jack Socket, then I Jack into the M2 input & the other Jack into the 16ohm Amp output .
> Sorry to once again be a pain.



Sounds odd! But the intended reading around 16 ohm is with the amp not connected at all, just measuring through this new lead with the speaker and M2 connected. Is that the case?


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## Graham G

Sorry John, I had the Jack connected to the Amp(not switched on), the readings are around 8ohm, just measuring the lead .


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## JohnH

matttornado said:


> So with my 100 watt Superlead cranked, it's putting out well over 100 watts, right? With my 16 ohm build, How much in wattage do you think my giant 300 watt / 30 ohm R1 is lowering the wattage before entering the switched stages? 40 -50 watts maybe?



R1 takes just over half the total power, and overall, the stage 1 takes 80% of the total, With some cranking, it might be 100W in the first stage.


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## JohnH

Graham G said:


> Sorry John, I had the Jack connected to the Amp(not switched on), the readings are around 8ohm, just measuring the lead .



If the cab and the attenuator are connected to the lead, and no amp, you should get the 16 ohm approx, measured at the plug that will go to the amp. So something is not right if it's reading 8 ohm


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## Gene Ballzz

JohnH said:


> If the cab and the attenuator are connected to the lead, and no amp, you should get the 16 ohm approx, measured at the plug that will go to the amp. So something is not right if it's reading 8 ohm



Notice that Graham G said: "I had the Jack connected to the Amp(not switched on), the readings are around 8ohm, just measuring the lead." With it plugged into the amp (even though amp is not on) his readings of the attenuator/speaker combination will not be correct!
Unless I'm Missing Something?
Gene


----------



## Graham G

JohnH said:


> Sounds odd! But the intended reading around 16 ohm is with the amp not connected at all, just measuring through this new lead with the speaker and M2 connected. Is that the case?





JohnH said:


> If the cab and the attenuator are connected to the lead, and no amp, you should get the 16 ohm approx, measured at the plug that will go to the amp. So something is not right if it's reading 8 ohm



Hi John, I've only just read your reply, yes I am getting 16ohm at the Jack that goes to the Amp & 8ohm at the M2, I don't know where I measured the second 8ohm I mentioned earlier .
In the meantime I have taken the Amp & M2 to band practice & it works great, I can now crank the Amp, but with attenuator a much more manageable stage volume & the Amp retains all of it's dynamics feel & welly, sounds great, once again thanks for your help. Oh am I correct in thinking the M2 is running nothing like as Hot ?(it's freezing in our practice shed).


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## Graham G

Gene Ballzz said:


> Notice that Graham G said: "I had the Jack connected to the Amp(not switched on), the readings are around 8ohm, just measuring the lead." With it plugged into the amp (even though amp is not on) his readings of the attenuator/speaker combination will not be correct!
> Unless I'm Missing Something?
> Gene



Hi Gene,i had just been posting & hadn't seen your comment, just to confirm, I am getting 8ohm at the M2 & 16ohm at the Jack that goes to the Amp .


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## JohnH

Hi Graham, all good, it seems to be fine then. Do you think this -3db work-round will be helpful overall?

On heat, i think the way these work is more dependent on power than is the actual volume - different physics at play. For example consider 25W (a cranked SV20?), 50W and 100W. There's only a 3db step between each one, which is a small step of volume. But on temperatures, in a thick case, 25W gets warm, 50W can get hot if you thrash continuously but not excessively so, and 100W can get way hot and a fan may be needed. Also, it takes quite a while for the temperature to change as it heats up the case, so the power that heats it up is averaged over maybe 30 minutes.


----------



## Graham G

JohnH said:


> Hi Graham, all good, it seems to be fine then. Do you think this -3db work-round will be helpful overall?
> 
> On heat, i think the way these work is more dependent on power than is the actual volume - different physics at play. For example consider 25W (a cranked SV20?), 50W and 100W. There's only a 3db step between each one, which is a small step of volume. But on temperatures, in a thick case, 25W gets warm, 50W can get hot if you thrash continuously but not excessively so, and 100W can get way hot and a fan may be needed. Also, it takes quite a while for the temperature to change as it heats up the case, so the power that heats it up is averaged over maybe 30 minutes.



Hi John, based on yesterdays brief practice session, I think it will be exactly what I need forJ my pub gigs, but i'm at the Merseycats Jam night Tonight(which will be a good test & then Friday we're doing an actual gig so by Saturday i'll have a really good idea if the fixed Stage solution is all I need, right now I think it will be.
I'm thinking of making it a permanent fixture in the back of my Amp, the case I've used is a tiny bit too deep, but I think i'll be able to coax it in there with a big hammer .
I'm also thinking about "hard wiring" inside instead of the y shaped lead, so could I ask a couple of questions ?
My reading of the box I've built now is i'm not using the switching(permanently on) & i'm not using the original speaker out Jack, so could I hard wire the circuit in the following way ?
wire resistors 3-5-7 to 4-6-8 ?, it looks to me like this would work to by pass the switches in the On position.
Also could I disconnect the leads from resistors 7-8 from the original speaker Jack(not now used) & then use the jack for my internal Y wiring ?.
I'll report back on Saturday after my gigs.
Thanks John. How's the D button idea coming along ? .


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## JohnH

ok great. Once you've concluded, you might be interested in putting together a simple, smaller, single box to do the same. Without all the attenuation settings, it can be distilled down to:

Output from amp 16ohms
standard speaker cable
box with one jack in, one jack out and three components (two resistors and a coil)
plug 8 ohm speaker into that. 

Power is reduced -3db to 25W, as with the 3-way cable system.


----------



## Graham G

JohnH said:


> ok great. Once you've concluded, you might be interested in putting together a simple, smaller, single box to do the same. Without all the attenuation settings, it can be distilled down to:
> 
> Output from amp 16ohms
> standard speaker cable
> box with one jack in, one jack out and three components (two resistors and a coil)
> plug 8 ohm speaker into that.
> 
> Power is reduced -3db to 25W, as with the 3-way cable system.



That sounds great, i'll report back on Saturday


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## matttornado

I added the switched stereo jack to the attenuator. Not as neat as I'd like but it was a tight squeeze. I am making the cord now.


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## matttornado

I tried the footswitch addition yesterday for about a half hour and it worked great! I can't wait to use it at one of our gigs. I think the 3.5 db boost will be enough but I'll find out soon.

Thanks again @JohnH!


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## Graham G

I used the M2 on Thursday(jam night) & Friday nights(gig) running it with a wired fixed -3.5db reduction that John did me a drawing for & it sounded really Great the M2 keeps all dynamics & welly in tact at about half power.
My Amps a HF modded ORI50C & to be honest I just can't believe how good the combination of the Mod & the M2 sounds, the M2 allowed me to run the Gain & the Volume right in the sweet spot & it's the best on stage(pub band stage) sound I've had for years & I've had some very good & expensive Amps.
In an ideal world i'd like to have an M2 that could switch from this fixed state to go up & down by about 10w, but I think from Johns comments that's not possible, but sometimes you just can't have everything & so i'm really happy with this set up.


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## JohnH

Following on from what Graham is finding, I think there are several ways to split the first step of attenuation, to have a built-in -3 or -3.5db setting. A new idea: This can include making the first stage switchable between -3.5 and -7. I'll post more on that. 

There are also ways to make a fixed -3 or -3.5, using the M2 as a load box, or making a much stripped down fixed load.

But for most users, the basic M2 does what's needed and is simple and versatile.


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## JohnH

I've worked out a way, with one more switch, to make the first reactive stage switchable between-7db as usual, and -3.5db. This means that when just a small reduction is needed, that step is available ie, 50W goes down to about 20W. The tone looks like it will be correct at -3.5 db. It'll be safe to switch other switches too from there, but slightly less 'reactive'. Best to start with -7db at stage 1 when lower settings are needed.


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## matttornado

I tried my 3.5 db footswitch the other night at an outside party we played. It worked very well. There were no delay when turning off the footswitch stage and the footswitch enclosure just got a little warm. I'm not sure if 3.5 db is enough of a volume boost or not for doing leads and solos. I think I need to try it a few more times, especially indoors before I can decide. Would 7db be too much of a volume boost? 

Could there be a stage in between 3.5 & 7 db?


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## JohnH

matttornado said:


> I tried my 3.5 db footswitch the other night at an outside party we played. It worked very well. There were no delay when turning off the footswitch stage and the footswitch enclosure just got a little warm. I'm not sure if 3.5 db is enough of a volume boost or not for doing leads and solos. I think I need to try it a few more times, especially indoors before I can decide. Would 7db be too much of a volume boost?
> 
> Could there be a stage in between 3.5 & 7 db?



Thats great news! 
In principle, the footswitch stage can be any amount of dB's in the range. It's easy to work them out if you want an in between one.

Also, It would be possible to add another switch to the footswitch box to change the footswitch dB's.

But I'd suggest to experiment with the main box switches, particularly the -7 db switch, to help decide on values.


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## auflauf

Back again and I am completely blown away. Some nice person on this forum lend me a beautifully built M2. So I tried it and my results range from very good to quite ok. An attenuation of -7db sounds fine. -10db starts to sound a bit muddy, - 21db more so and so on. This is fine on its own right, attenuators are said to behave like this. The M2-design is most probably one of the better ones as it sounds quite nice for me down to -14db.

So I expected to find a loss of high frequencies with progressing attenuation. I checked the frequency response with a crude setup:

Harley Benton Tube15, semi-open combo, back removed for better access to the wires, positioned arbitrarily on a shelf in a corner
Behringer ECM8000 mearsurment microphone positioned in 1m distance in front of the speaker's center
noname 15€ el-cheapo USB-interface
REW Room Equalization Wizard 5.20.3
output from USB-interface to amp input jack
all curves are smoothened by 1/6oct
These are the results: the first image shows frequency response curves without and with a different amount of attenuation. What appears to be too good to be true: they are perfectly parallel to each other. This is exemplified in the second image: the curve of the frequency response measured with -10db has been added 10.5db in the software, the same has been performed for the -21db measurement. They fit perfectly well to the 0db-curve. The low frequencies appear to rise with more attenuation, but this results from the noise floor which is not affected by the attenuation and hence is relatively louder with more attenuation.






Thus, the attenuator works absolutely perfect. The audible result is good, but not perfect for me. I would like to add eg +5 to +7db to the heights. This might be due to Fletcher-Munson or to Robinson-Dadson, but the effect is so marked that I doubt that.

Reduced distortions and second/third harmonics with increasing attenuation might be the reason, in other words an amplifier-speaker-interaction. But I would have expected to see the loss of harmonics causing a loss of heights in the frequency sweep above.

Next I will look at the spectrum of a chord and if there is a loss of harmonics visible.

For now, I am deeply impressed by your attenuator and I have stuff to think about...


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## JohnH

hi @auflauf , thankyou very much for these tests! Id like to check a couple of things about them:

Are these tests using a frequency sweep input? (ie not a guitar signal)
How hard was the amp working in terms of its output power?

Some comments:

The Tube15 amp has a 12" Celestion speaker, which generally all have about the same coil inductance, which is what the M and M2 designs are based on. So the parallel curves at each attenuation, from low-mids up are as expected
Even the -31db curve, is following the others well above about 300hz.

With different amps, Id expect some variation in the bass, since we do not include a bass resonance circuit in M2

On your traces, there's a bit of a peak around 80hz, which is usually the resonant frequency of a 12" speaker in an open cab

i find that the low attenuation settings sound fine to my ears if you start from a condition of quietness, but when you come down from high volume then they can sound more muted and dull. We could investigate how to lift the high frequencies if needed based on user preferences.

The curves are building up a bass increase at lower attenuations, apparently quite high as you get down to the lowest settings. We haven't seen this in the couple of tests done before. I wonder why? the Tube 15 is likely to have different output characteristics to my VM, which could explain, except that an such difference for that reason would be likely to occur between 0 and -7, and then stay fairly constant instead of building up increasingly at lower settings. As you noted above, it is maybe a noise-floor issue.

My own tests are posted in Post 1 with sound clips, here are the charts again:













Attenuator M Frequency Plots 190302



__ JohnH
__ Mar 1, 2019






I think the differences to yours are:
Its a looped guitar riff, at high power with distortion. The mic is placed close and its a dynamic vocal mic.
I made the comparisons by subtracting the trace at -7db from the others, hence to show ideally a flat line on each top trace for comparison
Its based on design M, which has a different front end but its electrical performance is almost identical to M2

Thanks again!


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## JohnH

hi @auflauf , if you are doing some testing to look for harmonics, here are some things to watch and listen out for:

In principle, we expect the harmonic response to be quite close to the full unattenuated amp on account of the reactive coil, except possibly around the frequencies of the low bass speaker resonance. At this frequency, M2 does not show the amp an impedance that follows this, instead it remains flat. This may leave the bass with slightly less harmonics, or a different amplitude, if you play notes at high power around that frequency. In an open back 12" cab, we expect that around low E. Ive tried to find any such difference, exploring with Drop D tuning, but I cant hear them at all with my amp. 

We have design M3 which includes a resonant circuit, that was built by @dbishopbliss, but there was not much difference noted. It would be very interesting to know your results on this using M2, for low bass notes and for higher frequencies, using your amp.


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## auflauf

Hi John! Thanks for your reply!
The volume setting was quite low and the amp definitely playing clean clean, thus no distortion coming from the amp.

The mic appears to be well suited for the job, published frequency response curves for it are really flat. 1 m distance is standard for speaker measurements, so this should be fine. The setup was crude, amp was not standing free, but as the bass was fine for me, I did not care.

The low frequency hump is definitely the noise floor. The USB sound interface is a 15€ piece of the size of a thumb drive. There are cables convoluted, humm etc. There are two pendulum clocks in the room with a half-second pendulum and a three-quarter pendulum. I don't know what the software makes of these peaks... maybe I repeat the measurement without them. Anyhow, a low frequency rise should not make the sound dull, thus I just disregard the bass.

Actually the otherwise fine REW-software only accepts frequency sweeps as output signals. I will have to tinker with one computer playing a recorded guitar chord and one computer do the recording with eg Audacity.

I definitely support the idea of including some kind of tone control to an attenuator. Ideal would be a high frequency rise adjustable in db and slope. The general "bad" reputation of attenuators appears to come solely from the perceived "dull" sound. this could be remedied by a beefy high boost of any sort.

Have to rush to work, Australia is preparing for end of work. Envious.


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## JohnH

Thanks again

Lots of simple attenuators have a bypass cap to add treble. Generally, these are to compensate for dullness caused by an Lpad control damping the speaker, even though Fletcher Munson is blamed! As you've seen, this design does not have such a technical problem. But there could still be a user wish to boost treble. I'll look at it again. Need to be cautious though because adding the treble boost messes with impedances in a passive circuit like this.

btw, When I do these tests with guitar, I use a delay pedal set as a looper to make the input, then I mic into a mixer, then into Audacity


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## iefes

Really nice investigation auflauf, thanks for these tests! I'm glad you like my attenuator and I'm happy it does exactly what it is supposed to do. However, I'm wondering if you can really rule out that the low frequency increase at high attenuation levels (e.g., green line in the normalized graph) comes only from the noise floor. An increase around 200 Hz could make the amp sound more bassy and drown the high frequency content, thus make it sound duller. That's just an idea though. 

It's cool how your test shows that a perfect attenuator is not always the perfect thing soundwise. Apparently, it highly depends on the amp + speaker being used and on your own preferences. Did you try to position the amp at different places and listening to it from different angles while playing? I don't know how different speakers behave at different volume levels but I can imagine that different speakers distribute the sound "unevenly" depending on the loudness. 

However, I think these are great tests and these show really nicely that the attenuator is almost perfect on its own, but may be even improved in flexibility by adding some kind of a treble-bleed. 
Actually, some attenuation in treble is caused by the inductance and the corresponding rise in impedance at high frequencies. Couldn't we just use a switch-resistor network to bypass the inductor partly, so it contributes less to the impedance curve? In this way no capacitors would be needed. 

I might test this in LTSpice when I have time. But I bet JohnH will be faster


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## JohnH

Actually, the inductor doesn't really control the tone, that is mainly determined by the resistor network The inductor controls the dynamics, how it responds to different input levels, and also adjusts to different amps.

If you short out the inductor completely then the result is a resistive attenuator, pretty much exactly the same as on page 4 ,3 years ago 

The M2 design is set to try to minimise differences in tone at different attenuation levels.

I agree wuth @auflauf about the bass results.

Ive also run a few more calculated tests. If a treble lift is desired, i think it is safe to try a capacitor across R2A It needs to be bipolar, with high current capacity, like a crossover capacitor in a speaker. 10 uF (in an 8 ohm M2) gives a few db lift in treble. 3uF adds just a bit of extra presence. Try with caution, not tested yet I wouldn't use more than 10uF because it affects input impedance at high frequency


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## dbishopbliss

JohnH said:


> hi @auflauf ,
> We have design M3 which includes a resonant circuit, that was built by @dbishopbliss, but there was not much difference noted. It would be very interesting to know your results on this using M2, for low bass notes and for higher frequencies, using your amp.


Let me know if you want to measure my M3. I have a gig that I need it for in 2 weeks but after that I could send it to you as long as you promise to send it back.


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## auflauf

An update and I am even more confused.
First, the noise floor. No input at the amp, gain 2, vol1. This is in concordance with the previous results showing a relative increase in lows with lower sound levels



Then an B7 chord played clean, recorded directly into the usb soundcard, spectrum analysis with audacity. No amp involved here:



Here is the recording of the amp playing the input of above file, no attenuation:



And here the spectrum of the recording of the attenuated amp



There is something missing ... this reflects the sound impression: dull

And here white noise played over the amp, no attenuation:



White noise played over the amp with -20db:



Conclusion:
1. There is a loss of heights with attenuation visible at the second, fourth and so on harmonics of the clean chord and at the white noise recording.

2. This is conform with the broad consensus of an attenuated amp sound.
3. This contradicts the measurements posted 4th of oct above. They have been produced with a frequency sweep.
4. The frequency sweep results did not go well with my expectations.
5. Perhaps absolute sound level and thus the speaker behavior plays an important role. Measurement setup was not very constant, calibrated etc. Overall level was rather low.

So, anyone with a brilliant explanation, please step forward.


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## JohnH

I think its actually working fine. When you look at plots from a music signal, you just have to look at the peaks, everything else is noise.

Here is a copy of the full volume signal at the top and the -21db reduction below. I traced a dark blue line to join the peaks at the top, and also two horizontal lines 20 db apart. On the lower picture, I did the same in red. Then I brought the group of red lines up to the top chart, and adjusted by stretching vertically so the db scales were the same. You can see that the lines are quite close, and it was done roughly.

Above about 3khz, the trace is below the level of the recording of the attenuated plot.


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## auflauf

John, you are right. I imported the audio files from the recording at 0db and at -20db into REW and did a spectral analysis and smoothened rather drastically (1/6 oct). Then I superimposed the curves. Result: no discernible loss of heights. The 3-4db difference of the curves might arise from the sloppy and complicated workflow (mic sensitivity and output level set with non-calibrated sliders of the PC operating system, recording with a small program "Piezo", importing mp3 into Audacity, converting to wav, importing wav into REW).




Still, the perceived dullness remains.

So:
Attenuation level with the M2 attenuator apparently has no influence on frequency response if tested via
-frequency sweeps (post of 4. Oct 21)
-white noise (above)
-clean bright chords (above)

The frequency response tested included the chain of amplifier (input to end stage), speaker, cabinet and room.

What has not been tested was the influence of the attenuator on the guitar (highly unlikely) and the influence of level/attenuator/attenuation on human perception.
I guess it's psychoacustics, all the more as the sound level at the recording was rather low and in no way comparable to a rehersal room or a gig. Referring to a crude estimate from iPhone apps, the level of the chord playback was in the range from 65 to 75db unattenuated, thus 45 to 55db attenuated, which is lower than normal conversation level.


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## JohnH

That all looks right, thanks for following through.

I cases where a perceived loss of treble at very low volume becomes something that a user wants to adjust for, I think the capacitor across R2A is an option and easy to add. As I noted above, a 3 to 10 uF bipolar cap could be tried, subject to testing.


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## auflauf

dbishopbliss said:


> Let me know if you want to measure my M3. I have a gig that I need it for in 2 weeks but after that I could send it to you as long as you promise to send it back.


Hello dbidshopbliss! If you are asking me: thank you for the offer. That would be very interesting. But I guess detecting differences between the M2 and M3 design requires a/b listening comparisons as well as more reliable measurements than mine done so far. It’s iefes‘ M2, so maybe the att willl be back at the owner in the coming weeks. And if both designs don‘t hamper the frequency response, what to look for and how to measure that? Amp-speaker interaction, dynamics, response to transients, all that quantitatively...I am not negative here, but this may require more than just semi-automatically measuring FR. I am having a few days of now, but will be busy the coming weeks. let‘s discuss this when you did your gig, ok?
Are you located in Germany in order to avoid international shipping?


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## JohnH

If you are up for more tests, there's more that might be investigated with the M2. It depends if you can get loud and drive the the amp hard, at least for the duration if one loud strum! , without overloading your recording system.

The reason is that the output impedance of a valve amp changes dynamically when the power amp section drives hard. The ability of the attenuator to respond to this change is the main reason for a reactive design. My tests are done at high amp volume, and it would be very interesting to compare with the different amp that you are using.


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## dbishopbliss

auflauf said:


> Hello dbidshopbliss! If you are asking me: thank you for the offer...
> Are you located in Germany in order to avoid international shipping?


I'm in the US so probably not cost efficient to send to DE. I built an M2 and an M3... it might be more effective to help me figure out how to make the measurements you did so that I can post them.


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## JohnH

dbishopbliss said:


> I'm in the US so probably not cost efficient to send to DE. I built an M2 and an M3... it might be more effective to help me figure out how to make the measurements you did so that I can post them.



It would be great to get some readings on your two boxes. We can do useful science in a range of different ways depending on what facilities you have. @auflauf is using some special tools and producing very nice plots. i might see what i can do with that REW software, which looks to be free in is basic version.

But, you can also work with just general musician gear. The reason is, we don't really need to know about absolute values of frequency response and SPL's etc, everything we'd like to know is relative, looking for the changes at different settings and comparing apples to apples, rather than accurately measuring a specific apple.

Here's how I do it.

Set up the amp, and close-mic it as if you were recording a track. Set it loud, so the power amp stage is driving
Record a few seconds of playing. make sure the strings are ringing cleanly. Open chords are best with all the strings, or a sequence of notes at each semitone up the first few frets on the low strings to explore bass response.
I use a delay pedal in my loop, set as a looper. Its very important to have consistent input as you explore settings, even though its not too important exactly what that input is. 
Then with this annoying loop playing, record into the computer. I use a dynamic mic into a small mixer, then into Audacity.
Start at full volume, no attenuator, recording as high as you can but no clipping
Keep everything in place and the loop running, put the amp to bypass and put in the attenuator
record, stepping down through the attenuation settings

At that point, you could post a wav file and Id be very happy to post process.

But, eg with Audacity, you can then select a part, and do a frequency response plot, and export that to a csv file and so into excel. These plots are very spiky, so I smooth using a moving maximum to find the greatest level within a few steps either side of the frequency

Then plot results

For your M2 and M3 comparison, it will be interesting to compare particularly the low bass notes around 100 hz.


If you have means of doing a frequency sweep or a white noise input, that is also very good and tests other aspects. Probably keep that one at a clean level of amp drive. You can download wav files of these and play them from a phone or mp3 player.

If you have different amps, interesting to compare them, particularly if they have different output stages.


If anyone else who has built one of these would like to try something like this, Id be very grateful and it would be a big help to this project.


----------



## auflauf

Hello!
My setup isn‘t really sophisticated. The only non-ordinary piece is the relatively cheap measurement microphone. I bought it for testing and adjusting room response for my Hi-Fi-chain (which I never did). As John said: as we don‘t require absolute, but relative measurements, any decent microphone will do.
The free REW-software offers all features you might desire: frequency response as posted, elegant possibilities to smoothen the curves, waterfall analysis and so on. Room analysis is only one of the features. It might seem overwhelming at first, but once you understood where to start and where the preferences/options (divided into genreal and measurement-related, called „control“), its really easy. You just have to play around a bit to get acquainted at first. If you want to analyze the spectrum of a guitar signal or white noise instead of a frequency sweep, you have to offer them to REW as wav files. 

Edit 15.Oct: 

If you want to do frequency sweep measurements (this is recommended for room analysis, it probably won't harm for frequency response measurements as I did): get REW, its free.


*Mic, sound interface*

Get a microphone. You may even try a headset's mic, but sensitivity for low frequencies may be low. For a start, you may even try the built-in mic of a laptop.
Plug it into the combined headset jack at your PC/laptop. 
If you are using a dynamic mic or a measurement mic, you may need an USB interface audio to USB. Min costed less than 20$. You may need a phantom power supply for electret microphones. I think a laptop plug provides phantom voltage, but (IIRC) it's suited for headsets, not for standard mics/measurement mics. I needed a phantom voltage source, it costed me 20-30$
Position the microphone in front of the amps speaker. Standard for hifi speakers is 1m. Standard for room measurements is your listening position. Standard for miking an amp is very close. As long as you keep the distance constant and ≤1m, it probably doesn't matter
Connect the output of the usb interface (or your headphone output) to the guitar amp)
I did not have success with splitters which go from combined mic/headphone jack to separate mic and phones jacks. There are two different connection schemes for mobile phones and... it was a headache, so I prefer the cheap usb interface.


*REW, setup*
Open REW (I now refer to the Mac version, the windows version may be similar).
Preferences in the upper right corner of the std. window: select the usb device or whatever you want as your input mic. Select your output to the amp. 
Slide mci sensitivity and output volume up high in REW and in the operating system. They are synchronized in a Mac and probably in a Windows device too.
Preferences/lower part "Levels". Select "main speaker", not "subwoofer".
Click "Measurements" in the upper left corner of the main window.
Name your measurment session.
Start to end 20-20000Hz is fine. 
Length of the sweep 512k or 1M will do. For increased precision, you may select repetitions. I didn't
No need for acoustic timing reference, but it doesn't hurt.
Check levels until you and REW are satisfied

*REW, measurements*
You may do all your measurements at low bedroom levels as I did and additionally at higher levels as John did.
Start measurement. You can then repeat measurements at different attenuation levels, they appear as different curves in one session.
if you want to analyze guitar chords, white noise etc, which is not built into REW, you need to play them and record them via some other software. I used a small music chain as source and recorded via audacity. You then convert/store the recordings as wav-files. You open them in REW. Their analysis is the same as written below for the built in frequency sweeps in REW

*REW, analysis*
Click "all spl" in the window showing the measurement results. You can deselect curves in the lower pane. 
Apply smoothing (menu strip at the top: "graph"), 1/12 may be a compromise.
use "controls" in the upper right corner of the results window: play with trace arithmetics etc, there you can add or subtract db in order to move curves up or down in the graph. Smoothing via menus/graph affects all curves, using "control" affects only the selected curve. 
play with all the options in the measurement results window, eg. distortion which shows you among others the noise floor.
I exported the result graphs via screenshots. This is not the most standardized way, but I didn't find pixel graph export.


That‘s it, feel free to ask. To quote Nike: Just do it.


----------



## Graham G

JohnH said:


> ok great. Once you've concluded, you might be interested in putting together a simple, smaller, single box to do the same. Without all the attenuation settings, it can be distilled down to:
> 
> Output from amp 16ohms
> standard speaker cable
> box with one jack in, one jack out and three components (two resistors and a coil)
> plug 8 ohm speaker into that.
> 
> Power is reduced -3db to 25W, as with the 3-way cable system.



Hi, John if you have the time I want to build the simpler Attenuator that you've described above, the M2 I've built & rewired to a fixed output then used as a load box is an ideal gigging volume for me & it allows me to get my Amp(HF modded ORI50) in it's "sweet spot" & for me the Amp retains all it's "tone & feel" without providing everybody with ear plugs.
I'd like to thank you again,the M2 as i'm using it has helped(along with the Headfirst Mods), to give me the best gigging sound I've had for years, maybe ever(& ever is a long time for me) with the possible exception of the days when I used to use 2 amps.
Thanks, Graham.


----------



## JohnH

Graham G said:


> Hi, John if you have the time I want to build the simpler Attenuator that you've described above, the M2 I've built & rewired to a fixed output then used as a load box is an ideal gigging volume for me & it allows me to get my Amp(HF modded ORI50) in it's "sweet spot" & for me the Amp retains all it's "tone & feel" without providing everybody with ear plugs.
> I'd like to thank you again,the M2 as i'm using it has helped(along with the Headfirst Mods), to give me the best gigging sound I've had for years, maybe ever(& ever is a long time for me) with the possible exception of the days when I used to use 2 amps.
> Thanks, Graham.



Sure no problem! Since we discussed before, I think there's more than one option. 

1. The simplest is, you use your 8 ohm cab, plug it into this new box with three components in it, and plug that into the 16 Ohm amp tap. Its equivalent to how you've been using the M2 with the series lead.

2. There's also one with 4 components, where you use an 8 ohm cab and the 8 ohm amp tap.

3. Finally, a bit of reconfiguring, you can convert the full M2 to make the first stage switchable between 3.5 and 7 db, then switch the other stages off. Then you have all your settings in one box.

I reckon option 1 is simplest, unless you prefer 2. Either way, given you've figured out what you need, it will be a very simple reliable fixed unit.


----------



## Graham G

JohnH said:


> Sure no problem! Since we discussed before, I think there's more than one option.
> 
> 1. The simplest is, you use your 8 ohm cab, plug it into this new box with three components in it, and plug that into the 16 Ohm amp tap. Its equivalent to how you've been using the M2 with the series lead.
> 
> 2. There's also one with 4 components, where you use an 8 ohm cab and the 8 ohm amp tap.
> 
> 3. Finally, a bit of reconfiguring, you can convert the full M2 to make the first stage switchable between 3.5 and 7 db, then switch the other stages off. Then you have all your settings in one box.
> 
> I reckon option 1 is simplest, unless you prefer 2. Either way, given you've figured out what you need, it will be a very simple reliable fixed unit.


.

Thanks John, i will be happy with the Option 1 fixed box, i only ever use the Amp for gigs(or practice) & really don't need it any lower than the way i'm using it now, could i ask a question, a Guitarist asked me at last nights gig could the way i'm using the M2 work for cutting down a 100w Amp ? & if so would it be about half the volume(as in the 50w) or would it be a different calculation ?.


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## JohnH

Hi @Graham G , ok option 1 then

With the 100W amp, yes the same system could work, but would need to be rated for the higher power. 100W will go down to 50W. But, I think most people would perceive that as a much smaller reduction in volume than 50%. While all the electrical numbers have very well defined meanings, perceived volume is a much more subjective parameter. I have heard it said that a 1/10th factor on power is about a halving of volume., ie -10db. Personally, I find our -7db setting, which is 1/5th power, is a good downwatd step, taking 100W to 20W.


----------



## JohnH

Hi @Graham G , here is the stripped back design for the series load box.




It will absorb half the amp power (mostly in R2), so in theory your 50W amp will send 25W through to the speaker.

It's likely to heat up, so a good aluminium box, thermal paste, ventilation, feet, etc are needed. The usual watch-its about mounting the coil apply - no steel bolts, raise it a bit off the case.

So as with your recent use of the M2 in this way, the box is intended to be similar to an 8 Ohm speaker, in series with a real 8 ohm speaker, to add up to a 16 Ohm load and you use the 16 Ohm amp tap.

There's a nice bonus with such a simple design. Although the diagram shows the 8 Ohm resistor on the speaker side, its actually reversible. Either jack can be the input or the output, so long as its connecting to the 16 Ohm amp tap and the 8 ohm speaker.


----------



## Graham G

JohnH said:


> Hi @Graham G , here is the stripped back design for the series load box.
> 
> View attachment 96202
> 
> 
> It will absorb half the amp power (mostly in R2), so in theory your 50W amp will send 25W through to the speaker.
> 
> It's likely to heat up, so a good aluminium box, thermal paste, ventilation, feet, etc are needed. The usual watch-its about mounting the coil apply - no steel bolts, raise it a bit off the case.
> 
> So as with your recent use of the M2 in this way, the box is intended to be similar to an 8 Ohm speaker, in series with a real 8 ohm speaker, to add up to a 16 Ohm load and you use the 16 Ohm amp tap.
> 
> There's a nice bonus with such a simple design. Although the diagram shows the 8 Ohm resistor on the speaker side, its actually reversible. Either jack can be the input or the output, so long as its connecting to the 16 Ohm amp tap and the 8 ohm speaker.



John this is great , i'll order the components & build it this week .
Any progress on that Button we've discussed 
Regards, Graham.


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## fperezroig

Hello! First of all thank you for such a great design and comprehensive thread. This is a really big contribution to the diy community. I'm about to order the components and build the M2 version of the attenuator but am still a little confused about the way I could connect a 4ohm cab to it. 

Could I just halve the resistors and inductor values of the 8ohm version, and then connect the 4ohm amp tap through the attenuator to the 4ohm cab? What should be the values for R10 and R11 to get a third output for 8ohm cab with the proposed 4ohm version ?

Kind regards.
Fran


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## diego_cl

I'm currently building the M2 (16 ohms version) attenuator.

I can't understand how the 2nd speaker output works on the 16 ohms version.

With the 8 ohms version I suppose it works like this:

*Input: 8 ohms amp tap*
Output Speaker 1: 8 ohms speaker
Output Speaker 2: disconnected
Output Speaker 3: disconnected

*Input: 8 ohms amp tap*
Output Speaker 1: 16 ohms speaker
Output Speaker 2: 16 ohms speaker
Output Speaker 3: disconnected

*Input: 8 ohms amp tap*
Output Speaker 1: disconnected
Output Speaker 2: disconnected
Output Speaker 3: 16 ohms speaker

- Is the 2nd output in parallel?
- What's the use of the 2nd speaker output on the M2 16 ohms version?

With the 16 ohms version I suppose it works like this:

Input: 16 ohms amp tap
Output Speaker 1: 16 ohm speaker
Output Speaker 2: disconnected

Input: 16 ohms amp tap
Output Speaker 1: *32 ohms *speakers ???
Output Speaker 2: *32 ohms *speakers ???

I suppose that the 16 ohms version will work with a 8 ohms speaker as long as the reactive load is running, but what is the best use of its 2nd output?


----------



## JohnH

fperezroig said:


> Hello! First of all thank you for such a great design and comprehensive thread. This is a really big contribution to the diy community. I'm about to order the components and build the M2 version of the attenuator but am still a little confused about the way I could connect a 4ohm cab to it.
> 
> Could I just halve the resistors and inductor values of the 8ohm version, and then connect the 4ohm amp tap through the attenuator to the 4ohm cab? What should be the values for R10 and R11 to get a third output for 8ohm cab with the proposed 4ohm version ?
> 
> Kind regards.
> Fran



Hi Fran, thanks for your interest in this design.

The attenuator is safe to use with different cab Ohms, so long as the amp tap Ohms matches the design.

With a 4 ohm cab you can either build an 8 Ohm M2 and use it from an 8 Ohm amp tap and plug the 4 Ohm cab into it. Or, halve all the values and then you have a 4 Ohm M2 to use with a 4 Ohm amp and cab.

If you use the 4 cab with 8 M2, you lose a little more volume, about 2db more reduction at each setting, also the tone is very slightly brighter, which if you notice it at all, can be adjusted for at the amp.

But I think that if there is one main intended use for the unit, then its often best to build for that, and the flexibility for other cabs is a bonus. When halving all the resistor values and the inductor, sometimes there is not an exact value available in a common range. Try o keep the ratios between the resistors at each stage close to the design. eg, for the first stage, using standard values, R1=8.2, R2A=12, R2B=10. These are all a bit higher than 1/2x, but in proportion.


----------



## JohnH

hi @diego_cl , thanks for your message. 

See above to Fran. The two outputs Out 1 and Out 2 are wired in parallel. With a 16 Ohm M2, they let you use it for two 16 Ohm cabs to make an 8 Ohm load if you want to. Still run it from a 16 Ohm amp tap though.


----------



## dbishopbliss

auflauf said:


> Hello!
> My setup isn‘t really sophisticated. The only non-ordinary piece is the relatively cheap measurement microphone. I bought it for testing and adjusting room response for my Hi-Fi-chain (which I never did). As John said: as we don‘t require absolute, but relative measurements, any decent microphone will do.
> The free REW-software offers all features you might desire: frequency response as posted, elegant possibilities to smoothen the curves, waterfall analysis and so on. Room analysis is only one of the features. It might seem overwhelming at first, but once you understood where to start and where the preferences/options (divided into genreal and measurement-related, called „control“), its really easy. You just have to play around a bit to get acquainted at first. If you want to analyze the spectrum of a guitar signal or white noise instead of a frequency sweep, you have to offer them to REW as wav files.
> 
> Edit 15.Oct:
> 
> If you want to do frequency sweep measurements (this is recommended for room analysis, it probably won't harm for frequency response measurements as I did): get REW, its free.
> 
> 
> *Mic, sound interface*
> 
> Get a microphone. You may even try a headset's mic, but sensitivity for low frequencies may be low. For a start, you may even try the built-in mic of a laptop.
> Plug it into the combined headset jack at your PC/laptop.
> If you are using a dynamic mic or a measurement mic, you may need an USB interface audio to USB. Min costed less than 20$. You may need a phantom power supply for electret microphones. I think a laptop plug provides phantom voltage, but (IIRC) it's suited for headsets, not for standard mics/measurement mics. I needed a phantom voltage source, it costed me 20-30$
> Position the microphone in front of the amps speaker. Standard for hifi speakers is 1m. Standard for room measurements is your listening position. Standard for miking an amp is very close. As long as you keep the distance constant and ≤1m, it probably doesn't matter
> Connect the output of the usb interface (or your headphone output) to the guitar amp)
> I did not have success with splitters which go from combined mic/headphone jack to separate mic and phones jacks. There are two different connection schemes for mobile phones and... it was a headache, so I prefer the cheap usb interface.
> 
> *REW, setup*
> Open REW (I now refer to the Mac version, the windows version may be similar).
> Preferences in the upper right corner of the std. window: select the usb device or whatever you want as your input mic. Select your output to the amp.
> Slide mci sensitivity and output volume up high in REW and in the operating system. They are synchronized in a Mac and probably in a Windows device too.
> Preferences/lower part "Levels". Select "main speaker", not "subwoofer".
> Click "Measurements" in the upper left corner of the main window.
> Name your measurment session.
> Start to end 20-20000Hz is fine.
> Length of the sweep 512k or 1M will do. For increased precision, you may select repetitions. I didn't
> No need for acoustic timing reference, but it doesn't hurt.
> Check levels until you and REW are satisfied
> 
> *REW, measurements*
> You may do all your measurements at low bedroom levels as I did and additionally at higher levels as John did.
> Start measurement. You can then repeat measurements at different attenuation levels, they appear as different curves in one session.
> if you want to analyze guitar chords, white noise etc, which is not built into REW, you need to play them and record them via some other software. I used a small music chain as source and recorded via audacity. You then convert/store the recordings as wav-files. You open them in REW. Their analysis is the same as written below for the built in frequency sweeps in REW
> 
> *REW, analysis*
> Click "all spl" in the window showing the measurement results. You can deselect curves in the lower pane.
> Apply smoothing (menu strip at the top: "graph"), 1/12 may be a compromise.
> use "controls" in the upper right corner of the results window: play with trace arithmetics etc, there you can add or subtract db in order to move curves up or down in the graph. Smoothing via menus/graph affects all curves, using "control" affects only the selected curve.
> play with all the options in the measurement results window, eg. distortion which shows you among others the noise floor.
> I exported the result graphs via screenshots. This is not the most standardized way, but I didn't find pixel graph export.
> 
> 
> That‘s it, feel free to ask. To quote Nike: Just do it.


My M3 is at the rehearsal space so I will need to pick it up on Saturday. I have an audio interface and one of those electet microphones from Parts Express somewhere. I will see what I can figure out.


----------



## dbishopbliss

JohnH said:


> Thats great news!
> In principle, the footswitch stage can be any amount of dB's in the range. It's easy to work them out if you want an in between one.
> 
> Also, It would be possible to add another switch to the footswitch box to change the footswitch dB's.
> 
> But I'd suggest to experiment with the main box switches, particularly the -7 db switch, to help decide on values.


I haven't been keeping up with the entire thread much since I finished mine, but I was just thinking it would be nice to "boost" the volume of my amp as opposed to boosting the signal going into my amp and the footpedal idea seems like it will meet my needs. 

My amp is already pretty dirty (Supro Coronado Clone) and adding a boost pedal really just makes things more dirty and compressed as opposed to bringing up the volume.

Could there be different footpedals that just get plugged in? That is... could I make a 3.5dB pedal and a 7dB pedal and then just use the one I feel like using depending upon the gig?


----------



## JohnH

dbishopbliss said:


> I haven't been keeping up with the entire thread much since I finished mine, but I was just thinking it would be nice to "boost" the volume of my amp as opposed to boosting the signal going into my amp and the footpedal idea seems like it will meet my needs.
> 
> My amp is already pretty dirty (Supro Coronado Clone) and adding a boost pedal really just makes things more dirty and compressed as opposed to bringing up the volume.
> 
> Could there be different footpedals that just get plugged in? That is... could I make a 3.5dB pedal and a 7dB pedal and then just use the one I feel like using depending upon the gig?



Yes no problem. You can use the resistor values for the 3.5 or 7 db stages in the footswitches.

Or, you can also make an in-between value, like -5db (if anyone wants that I'll work it out)

Or, in one footswitch, you could add another -3.5 stage and another switch (could be a second footswitch? or maybe a toggle or slide?) so the overall footswitch box is selectable to go 0db to -3.5db, or 0db to -7db.


----------



## fperezroig

Thank you John, I will halve all values and build a 4ohm version then. In order to use the M2 4 ohm using the 4ohm ot and an 8ohm cab, could I add R10=34ohm and R11=5ohm? Can’t wait to order and star building this! Best regards


----------



## JohnH

fperezroig said:


> Thank you John, I will halve all values and build a 4ohm version then. In order to use the M2 4 ohm using the 4ohm ot and an 8ohm cab, could I add R10=34ohm and R11=5ohm? Can’t wait to order and star building this! Best regards



That's the right idea, but you can go to the nearest standard values eg 33 and 4.7 if they dont have a 5


Here's a suggestion for a full set for a 4 Ohm M2, assuming its from a brand make like Arcol:

R1 8 or 8.2
R2A 12
R2B 10 
R3 8 or 8.2
R4 5.6
R5 2.7
R6 8 or 8.2
R7 18
R8 2.7
R9 not used
R10 33
R11 4.7

It also works fine shipping from China, much cheaper but much longer wait and a different range of values. Im happy to review if you like


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## fperezroig

Thanks, I’m ordering the resistors from a big supplier, and they offer all these values. Hope to get this finished ASAP!


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## auflauf

I just measured treble caps in the M2 attenuator.

My copy of an attenuator was lend to me by iefes (thanks again) and he allowed me to open it and to clamp some caps on. His attenuator is extremely well built, hats off, really!

I tested caps at three positions:
A. parallel to all resistors from input signal to output signal
B. parallel to the voltage divisor resistor R2A
C. parallel to the chain of resistors after the voltage divisor, that is after R2a to output signal, thus bridging R4 to R8

Levels tested were 0db, -10db and -20db with cap values of 10µF and 4.7µF

As I was a bit nervous about my provisorily clamped-on caps while plugged in a running tube amp, I hurried and only measured at 0db, -10db, -20db, but not at -30db and did only a very short listening test at -20db of the 4.7µF cap parallel to R2A (B.) versus no cap.

Results
Listening test
The cap made the sound markedly brighter, the effect was even a bit too much for my taste and I preferred the non-cap-version. The sound characteristic was a bit harsh, edgy as the distorted harmonics were risen. As I played a bit louder, at living-room-level instead of bedroom level as before, I guess it really depends on the listening level more than anything else. I could imagine to like that effect if playing at lower levels or at -30db.

Frequency response measurmenents
First a confirmation that the frequency response is really independent of the attenuation level



The effect of different cap positions:
A. The cap parallel to input-output causes an extreme treble boost up to 17db. As one would expect, the effect depends on the attenuation level as low resistance values just bypass the cap. The effect is not useful for our intentions.




B. The cap parallel to the voltage divisor resistor R2A gives a marked treble boost independent of the attenuation level, again this is as I would expect because the cap is parallel to a constant resistive value.



C. The cap parallel to the chain of resistors after the voltage divisor, from the end of R2A to output signal, gives less effect than pos. B above, but it increases with attenuation and produces some kind of level-dependent loudness which might be desirable.



The 4.7µF starts boosting at higher frequencies than the 10µF cap. The latter can even be regarded (and might sound like) as a bass attenuator. Judging by the frequency response graph, 4.7 µF may probably the highest suitable capacity, less may be even better.



Bottom line: I like the M2 att. and I think I may build one. This would allow to play my Harley Benton Tube 15 at home just at the edge of distorsion. That means Gain 3-4, Vol 2. This may require -10db during the day and -20db in the evening and gain 6-7 and Vol 5 would require -30 db. Switchable attenuation levels should be 0db/bypass, -10db and -20db and a slight (adjustable?) treble boost with a switchable 4.7µF or 2µF cap in pos. B or C might be just the icing on the cake for late evening playing.

Thanks John and iefes


----------



## JohnH

Hi @auflauf , A great study as usual!

Those measured plots are quite close to the equivalent in my SPICE analysis within about a db, once you allow that yours are measured off the speaker and so include the speaker response, while mine are just the electrical signal.

If you do this, definitely across R2A is the right place.

You can further adjust the effcct by adding a resistor in series with the cap. The lower traces on this graph are at -14db, a 4.7uF, 4.7uF in series with 12 Ohm (red trace) and no cap (lowest trace):




This one is similar using a 2.7uF cap and 22 Ohm:



BTW the upper curves are the signal at the amp, Full speaker aed also with the attenuator and added cap. You can see how the rise as seen by the amp drops at high frequency when the cap is switched in.


----------



## CO_Hoya

I finally got around to wiring up (and troubleshooting) my version of John’s attenuator (I’m calling mine the M3V version).

Glamour shot:





With all of the bells and whistles, it comes in at a svelte 5.5 lbs (2.5 kg), and would make a good blunt weapon in a pinch.

I’ve only tried it for a few minutes strumming some chords to confirm basic function, so I can’t contribute much to this thread in terms of the technical discussion above. I can say that the tone stays fairly well-preserved by my crap ears down until max atten (-31.5 dB). And at that level, my semi-hollow body was probably louder than the amp itself.

I haven’t had a chance to try the line out, or taste test with the bass resonance circuit in and out (which ended up using 10mH / 300 uF for L and C). I expect at some point the bass LC will be hard-wired and I’ll repurpose that toggle or hole.

Cheers to John and others for this thread. It was a fun project.


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## JohnH

Hi @CO_Hoya , thanks for posting that. It looks Bad@$$! 
Looking forward to hearing more about it in due course.


----------



## dbishopbliss

auflauf said:


> ... a slight (adjustable?) treble boost with a switchable 4.7µF or 2µF cap in pos. B or C might be just the icing on the cake for late evening playing.


Being able to add a little sparkle for the late night heavy attenuation is a great feature. This may have to be included in my next build. I have the parts for another M2 and I'm pretty sure I have some 2uF caps in my drawer. I have a neighbor that wants one so I will give him my first M2 and keep the M2+ for me.


----------



## CO_Hoya

JohnH said:


> Hi @CO_Hoya , thanks for posting that. It looks Bad@$$!
> Looking forward to hearing more about it in due course.


Thanks, John.

I have no update other than to post a few more pics showing off my work.

Here are the guts as I was trying to figure out wire routing - you can see how I engineered myself into a mess which made circuit-tracing during troubleshooting a chore. I'd build into a bigger enclosure if I did it again.





I milled some vents into the bottom of the box, not very well. There was a bit too much flex in the extrusion for that, and so I tried to clean up as well as I could. Thankfully, normally these are hidden from view. You can also just see the rear heatsink.





Here's the rear - not very interesting. Note the failed attempt at routing a slot, which caused me to switch to a row of holes instead. Sigh.





And here it is sitting on an amp to give some sense of scale.


----------



## JohnH

Hi @CO_Hoya , I reckon you did an outstanding job getting that all together, especially given all the added features you got in there, and the fact that everyone's build is basically a prototype with respect to the layout and construction.

I've also learned to wish I used a bigger case , even though they can be built compactly.

Yours is the second M3 that we know of, after @dbishopbliss 's build. There's another may coming sometime soon on the TDPRI forum , where Bitsleftover is converting an M2 to an M3.

The big question that we'd be very interested in your feedback on is, to what extent does the resonant circuit in M3 make a difference, as compared to M2?

If it makes a difference, it is most likely to be noticeable when using the line-out to record, or feed a PA, or re-amp back into a cab. The latter is where Bitsleftover was finding a drop in bass since the solid-state re-amping doesnt let the speaker develop its own resonance, as M2 does when used passively.

https://www.tdpri.com/threads/johndh-attenuator-build.1060083/page-6#post-11002872

Also, with M3 used passively as an attenuator, you may get a few more harmonics on bass notes when driving the amp hard.


----------



## diego_cl

I'll ruin the glamor of that beautiful M3V with my next question...

Is there any problem placing the inductor pressed against a foam? (like static electricity)

I didn't have nylon bolts, so I used some Old Spice deodorant parts 
Tried it just as a test, but it worked way better than I expected ... I'm just worried about that black foam


----------



## JohnH

I'd never have thought of that! seems like a cool (and dry!) idea. I don't see any electrical issues with the foam. If the coil got very hot it could be an issue maybe, but usually the coil may get a bit warm at most, even if the resistors heat up more


----------



## malibu91

Hi All,

I have spent the last week going through every page of this thread and am exhausted but excited to start my build.

Firstly a great thanks to JohnH. My friend you are worth your weight in gold.
I was close to pulling the trigger on a Tone King Ironman mini but couldn't justify the crazy price.

I am going to build an M2 with bypass to tame my massive Princeton Reverb '65 Reissue.

I am in Sydney and have ordered a Hammond 1590DBK case, resistors, and coil and am getting my shopping list for Jaycar as we speak.

I just had one or two questions around a couple of items if you could be so kind as to offer some advice.


*1/ Toggle switches* 
All DPDT. Which would you would recommend for the 4 x switches?

Option 1:
https://www.jaycar.com.au/dpdt-stan...2609d7c5ba833535597056cb270436&sort=relevance






Option 2:
https://www.jaycar.com.au/dpdt-mini...2609d7c5ba833535597056cb270436&sort=relevance







Option 3:
https://www.jaycar.com.au/dpdt-flat...2609d7c5ba833535597056cb270436&sort=relevance







*2/ Thermal compound*

How many grams is enough?
Link to Jaycar options is below
https://www.jaycar.com.au/search?te...SRFToken=a6cd3d46-5e81-4aa4-bff5-f8c85dfed4b5


*3/ Hookup wire*

Will this gauge work OK. I am intending to buy a metre or so in a few colours
https://www.jaycar.com.au/red-heavy...os=5&queryId=db9880586b6243de1df02a7ff407bf3b

I think that's about it. Again I really appreciate all the solid work that has gone into this attenuator!

Cheers
Jules


----------



## JohnH

Hi @malibu91

I also based in Sydney (I live in Wilton south of Cambelltown). So Ive been through Jaycar to get everything that they have applicable to our project

Ive tried all those switches. My own build uses the Option 1 standard toggles. Its OK but I dont like them much, they feel clunky and Ive had one fail internally for no good reason. My next build will use the dpdt mini-toggles with the flat levers, option 3 on your list. Although these are a small units and mount in just a 6.5mm hole, they have a nice solid but well engineered feel. They have surprisingly high current rating, 5A at 125V. The lugs are small though, and you probably cant get the 18 gage wire through them, so have to do a good close contact and well-flowed solder joint. I used them in a earlier prototype. 

On the heatsink paste, just one of the small 1.5g or 3g tubes will be fine. Its just a very thin smear that is needed, across the face contact. Actually you wont get much heat with the 15W amp.

That hookup wire is fine.

Get the plastic stereo jacks. Jaycar also has the Aluminium diecast cases. Also 10mm M3 nuts and bolts to mount the resistors 

Where did you order your coil? 

Good luck with your build and I hope you like it!


----------



## malibu91

Many thanks for the advice John.

Looks like it’s gonna be some warm days coming up for us in Sydney so I’m looking forward to that!

I’m in the Hills District myself

I’m pretty competent with a soldering iron, so I’m comfortable soldering onto small terminals on the suggested toggle switch.

I managed to pick up a coil from Wagner locally on eBay. Hopefully it comes on some sort of spool, otherwise I’ll knock something up and use a non steel bolt.





The box and resistors I ordered from RS Australia, everything else I’ll get from Jaycar or Bunnings including the plastic stereo jacks.

Now I have all the information I need I can’t wait to get cracking and finally crank up my Princeton.

Thanks again
Jules


----------



## Gene Ballzz

@malibu91 & @JohnH ,
Ya know, at face value it seems almost comical to think about and hear phrases like _*"I need I can’t wait to get cracking and finally crank up my Princeton." *_What makes it truly funny is how well these units actually work for dialing in/taming even little amps like this into their sweet spots without ending up in jail or divorce court, as well as finding the perfect volume blend for use with a band! These attenuators are the greatest thing since sliced bread!  

Just My  & Thanks Again To JohnH,
Gene


----------



## malibu91

I was hoping somebody would enjoy that comment! 

Don’t worry Princetons can get plenty loud when pushed just ask Mike Campbell. 

Mine’s a lounge lizard and never gets past 3. Thus this attenuator should be very, very good fun for me.

My Mesa/Boogie 295 Stereo Simulclass has been banished to the garage lol. It has little in the way of manners.


----------



## Gene Ballzz

malibu91 said:


> I was hoping somebody would enjoy that comment!
> 
> Don’t worry Princetons can get plenty loud when pushed just ask Mike Campbell.
> 
> Mine’s a lounge lizard and never gets past 3. Thus this attenuator should be very, very good fun for me.
> 
> My Mesa/Boogie 295 Stereo Simulclass has been banished to the garage lol. It has little in the way of manners.



And speaking of manners, where are mine? Forgetting to  you to this forum and this fantastic thread is not typical of folks around here! Kudos for reading the whole thread. While there's a little chaff here and there, the thread contains some fabulous info, along with some gorgeous builds, like that recent great looking one by @CO_Hoya ! Hats off to him on that one!

This is a really fun place with a lot of truly knowledgeable folks. There's even a really nice gentleman who was involved in the building of the very first Marshalls and some that currently work and design there that stop by occasionally!  Once you get this attenuator under your belt, some of us will likely get you to build your amp, maybe even from scratch! 
Just Sayin'
Gene


----------



## telesto

Hi John and all, 
I haven't looked at this thread in a year or two, good to see it alive and kickin  And nice to see someone else using REW. Cool tool. I just discovered it recently and was playing around with it testing some attenuator designs I'm working on, and figured I'd test it on some others as well. I did impedance plot comparisons of John's staged-pad style design vs. a rotary Lpad (*note: I didn't use any inductors, to keep it purely resistive) using the freq sweep generator and default -6dBFS setting in REW. I measured the voltage output to the speaker of John's, and then dialed in the same for the Lpad, so they both had exactly the same output to the speaker. Speaker was a 12" Randall, but that doesn't matter.

At -14dB John's has a higher and better defined bass-peak, and there is a noticeable increase in the high-freqs over 4kHz. I would guess the higher-highs and lower-lows is the reason it has a little more "rounded" sound, and the Lpad having the lower bass-peak and lower highs, has a more mid-range focus (or a little more "flat"). Altho to be fair, the plots aren't too far from each other.




Testing at -21dB, they both start getting flattened, altho John's keeps a better bass-peak and slightly higher response over 10kHz. But from other testings, I see most attenuators start going flat around mid-20dBs and below. Once the signal get's so heavily attenuated, I don't think any design is going to save it, it just starts going to a flat line in terms of impedance plots, and sound to an extent as well ...well ok, not too bad, I play mine at bedroom levels and am fine with it, but at that point, it might be a good idea to look into Aiken's reactive-load box and play into headphones if you're a golden-ear kinda guy 



Looking back at the very first page of John's SPICE simulations, the plots all seem to look exactly the same down to -30dB, which makes me wonder is that really accurate after looking at real-world measurements (?). I ran my measurements twice to make sure I didn't make a mistake, there were some small variations, but generally they looked similar. Smoothing and zooming in could also change plot appearances a little, but the general trend was always the same. If anyone else wants to run some impedance plots in REW for the full spectrum of 7dB+7dB+etc down to -30dB, it would be interesting to see and compare with my measurements. Also with the inductor in and out to compare the effects. I didn't use the inductor and just used a series of two and three 7dB stages.

For the Lpad vs stepped-bridge attenuator discussion, I think there is always a trade-off. The bridged design gives a slight advantage with the better freq response, but the you loose the flexability and simplicity of the Lpad design. And vice-versa, Lpad lets you dial in exact level of attenuation, but then you lose a little in the high/low ends. You can pick one, but not both 

As for the filter cap thing, I think it's generally a bad idea in any design. It boosts the highs artificially, but not the bass-hump, and just makes things worse IMO.


----------



## malibu91

Thanks for that Gene,

I'll admit I have been wanting to build a 5e3 for some time, but haven't been able to find a readily available kit locally.

Cheers
Jules


----------



## JohnH

hi @telesto , thanks for your investigations. 

Id be interested to know more about how you do those tests?. If they just measure impedance as seen by the amp, and its a purely resistive version, then yes they will stay at close to nominal 8 ohms, getting flatter as you attenuate more and the input is seeing less and less of the speaker. But this doesn't tell you what the speaker is seeing. Better to run a signal through a valve amp and measure signal at the speaker terminals as a voltage trace, and hence db's. Then you should see a more consistent and realistic treble rise and bass peak. Most probably, that from the Lpad will flatten as you turn down, whereras that from our design does not. Better again is to mic the amp and record that if possible.

I do my design based on SPICE and also a spreadsheet file where Ive coded in all the maths. But we have several real-world recorded tests that show that tone is consistent down to very low volumes and matches the sims.

In terms of the reactive vs resistive differences, a good resistive version of our design can still give a fairly accurate response at the speaker, but adding the coil in design M2 will give better response to level changes and dynamics from the amp, at frequencies above about 150hz, and also make it better to adapt to many different amps. Add the second coil and capacitor from design M3 and this will apply down to bass frequencies. This bass end resonance at the front end makes not much difference when used as an attenuator, since the real speaker can do this resonance itself. But it may help when used as a load box with no speaker, at which point the unit is working very similarly to Aikens design.


----------



## telesto

Hi John, thanks for the reply. I'm doing the impedance plot measurement as given by REW's websites directions






It works by doing a freq sweep (20Hz-20kHZ) sending a small audio signal (ie: voltage) to the speaker, and measures the current from the parallel "sense" resistor, and then uses these measured V and I values to derive the impedance of the speaker at every freq step along the way. So it does a "round trip" measurement which should include "reverse impedance" of what the speaker sees, ie: it records the speakers reaction. (Oh, ignore the y-axis "ohms" on the plot, it's not important, and they were slightly different for each, I had to move the plots up/down to align them exactly.)

I have a Scarlett 2i2 and use the headphone out with a 100R resistor. The default setting in REW is a 214mV output signal (which is then less after the sense resistor), which is audible on my 12" Randall speaker (ie: TV level volume or so). When the signal is attenuated to -14dB and -21dB the signal is only a few millivolts, which is barely audible. I don't know if the low signal would have an impact on the plots? In theory, I think it shouldn't matter? If you play a 100W or 1W amp into an attenuator, it should still have the same impedance reaction at different freqs? 

There is an option with REW setup to connect a power amp and run the test thru that, but then you have to put a zener diode clamp on the sound-card input so you don't blow it up with the high voltage. 
Your comment about playing thru a tube amp is interesting, altho the REW impedance test does based on a voltage/current measurement, if the voltage is coming from a test generator or a tube amp shouldn't matter (?) Altho it could be an interesting experiment. I'd really be glad to see more people trying and testing with it to be able to compare results. I'm not an expert in this and am learning myself


----------



## RJB

Hi John,

First of all thanks for this post and your great work!

Since I have tinnitus I play at very low volume levels. My 15W Fender bluesbraker JR4 is over-sensitive at the volume-pots at low levels and impossible to 'push' without getting way too loud. So I am looking forward to build your 8 ohm M2 for it. Whilst I am on it, I also will build a 16 ohm version for my DSL5. Just for the fun of it, and also to be able to push it more (at higher volumes in 5Watt mode it get's way too loud, and even the 1/0.5W mode it can get too loud).

I've read the most pages of this post. If I underdstand correct the reactieve -7db stage is fixed, and the other stages can be added on or bypassed as liked, so you can make all kind of combinations. I think for the 15W Fender this is good.

For the 16 Ohm version for the DSL5 I'm thinking of replacing the resistive -7db and -14db stages, with two extra -3.5db stages. This way I could toggle from -7db to -17,5db which feels more applicable for the DSL5. Do you think this assumption is correct?

Of course I will share my experience with both smaller amps after builing and using the attenuators to help others again.

Best regards,
Reinout


----------



## JohnH

hi @RJB
Thanks for your interest. You can mix and match the resistive stages freely if it better meets your needs.

But, if you build them as drawn, you can still find every increment of -3.5db from -7 to -31.5 db. So its more versatile with the greater range.

When I started on these, I was surprised to find how much attenuation I wanted, for times when I needed to play quietly. I often use -31 db, with about a 35w amp. That would be about -23db taken off a 5W amp.


----------



## RJB

JohnH said:


> hi @RJB
> But, if you build them as drawn, you can still find every increment of -3.5db from -7 to -31.5 db. So its more versatile with the greater range.
> 
> When I started on these, I was surprised to find how much attenuation I wanted, for times when I needed to play quietly. I often use -31 db, with about a 35w amp. That would be about -23db taken off a 5W amp.



Thxs!! I see what you mean; there is indeed no need for extra -3.5db stages, since it is all covered. A real smart-design you made  !!
I will build the full M2 for both amps and share my results when ready!!


----------



## Gene Ballzz

RJB said:


> Thxs!! I see what you mean; there is indeed no need for extra -3.5db stages, since it is all covered. A real smart-design you made  !!
> I will build the full M2 for both amps and share my results when ready!!



*First >*  to the forum!
*And Next >* I fully expect it to take a little while for you to get back to us to "share your results" as I'm betting you will be busy for quite awhile enjoying the new found liberation these attenuators can provide!  Best gear I've ever added to my arsenal in +55 years of "Screechin' & Squawkin'" with guitars through amps!
Enjoy & Another Special Thanks To @JohnH ,
Gene


----------



## RJB

Gene Ballzz said:


> to the forum!
> @JohnH ,
> Gene



Thank you Gene! I was able to order all items yesterday for the 2 attenuators here in France. I really look forward to the build (and playing  ) !!


----------



## jenfi

Hi!

I have a marshall 20W SV20H, and i have a headroom problem for the solo at reherseal volume...
I use a -6db Bridged-T attenuator, and the sound is pretty good for me, and i can play with the "volume" at 8.
The problem is that i can't add more volume with a clean boost in the effects loop because the amp is already "cranked".

And i have found this very interested thread on this M attenuator with the footswitch that can provide me the volume i need to be heard when i play guitar hero's solo!

But i don't understand how the footswitch works. (-7db + -3.5db and when i activate the footswitch, it remains only -7db ?)

sorry, i'm french, and a newbbie in electronics.


----------



## JohnH

jenfi said:


> Hi!
> 
> I have a marshall 20W SV20H, and i have a headroom problem for the solo at reherseal volume...
> I use a -6db Bridged-T attenuator, and the sound is pretty good for me, and i can play with the "volume" at 8.
> The problem is that i can't add more volume with a clean boost in the effects loop because the amp is already "cranked".
> 
> And i have found this very interested thread on this M attenuator with the footswitch that can provide me the volume i need to be heard when i play guitar hero's solo!
> 
> But i don't understand how the footswitch works. (-7db + -3.5db and when i activate the footswitch, it remains only -7db ?)
> 
> sorry, i'm french, and a newbbie in electronics.



hi @jenfi 
There have been a few recent builds here with footswitches. It is done by adding another attenuation stage in the footswitch, which you then bypass to get the volume boost. So you need 2 attenuator stages. The first is always on for both your rhythm and your lead tone, the second you switch on and off to get down to the rhythm tone. 

You must never switch the entire signal while playing, since this could damage the amp. There must always be one attenuation stage always on after the amp.

A bridged T is not necessarily the best circuit for a single -6db stage. Could you post the design?. The -7db stage in our design just has two resistors and could be adapted to -6db. I suspect it probably has a higher output resistance that may give you a clearer tone, while still loading the amp correctly. But better would be our stage 1 reactive circuit with the coil.

An issue that you may have is that your rhythm tone will now be two stages, so less volume. But maybe you can adapt to that with other settings.


----------



## jenfi

Thx for your reply. 

-6 or - 7db is the max level of attenuation that I can have in order to have the sound I like at gig volume with my rock band.


----------



## telesto

Hi John,
Coming back to the inductor/capacitor question... I did a quick test of the M3 first stage only




The plots with the L1/L2/C1 in and out of the circuit:




The first thing I noticed, doing a constant 1khz test freq at 33mV, the output with the coils in gave 20mV out and with the coils removed it gave 15mV. So stage-1 with the coils is giving -4.3dB attenuation. Without it's -6.8dB. The plots above are "raw" exactly as measured, I didn't normalize anything for the power difference, so I can't do a one-to-one overlay. But eyeballing it doesn't look too bad, but not exactly keeping the faith of the original signal either. The bass hump and especially high-end look exaggerated. 

I used a .82mH coil instead of .9mH for the high-end, and a 7mH instead of 9mH for the bass-hump, so there's a slight variation, but shouldn't make too much difference. 

Anyway, just thought I'd share...


----------



## JohnH

Thanks again, It looks mostly right to me. The design is based on a 4x12 cab with G12M speakers. These have an impedance of about 20 Ohms at 5khz, close to your measurement.

At 200 hz you have 10 ohms, and this is also as calculated. It is a little wider peak than the real speaker, due to losses in the coil and cap and that depends on the type and rating of the components used.

In the bass you have two peaks overlapping. One is at just over 90hz and that is your actual speaker resonance coming through. Above that is one created by L2 and C1. The calcs were based on about 110 to 120 hz for this. If the peaks aligned, you would get a sharper peak as expected

With guitar signals, its best to judge the overall level of attenuation at about 400 to 500 hz. By 1000 hz, a real speaker is beginning to rise. 

How about a test just on your speaker itself? Then we can compare to the attenuator results.


----------



## JohnH

jenfi said:


> Thx for your reply.
> 
> -6 or - 7db is the max level of attenuation that I can have in order to have the sound I like at gig volume with my rock band.



I that's for your rhythm tone, maybe two 3.5 stages could work to give you a lead boost when you switch one off. Or if its your lead tone, you could reduce from there.


----------



## jenfi

Yes, 2 3.5 stages could be a solution.

Does the second stage could be replaced by a system which would allow the boost to be adjusted more finely? A LPad?

Thanks


----------



## donwagar

jenfi said:


> I have a marshall 20W SV20H, and i have a headroom problem for the solo at reherseal volume...



Hi,
are you playing in 5W or 20W mode? Are you jumpering the channels? Do you mic your amp at gigs?

I have an SV20C, I run it at 20W jumpered with high treble about 8 and normal about 6. We do mic our amps for gigs.

I built one of JohnH's M2 Attenuators with the footswitch. I find the -3.5dB in the footswitch perfect for the boost I want, but I also ride the volume controls on my guitar. Maybe some people would want a bigger jump on the footswitch, but not me. I think this is the perfect solution. I've quit using pedals, I just get the tone from the amp.

Maybe JohnH could give you a plan for a reactive load in the -3.5 range, then with the footswitch (at -3.5) you'd be about -7 until you kick the footswitch. For me, I think that would be too loud. You could add a couple more switched stages though, so you can adjust between practice and gigs.

I didn't put the -14 stage in mine, it has -7dB (reactive built in), -7 and -3.5 switchable, and -3.5 on the footswitch.


----------



## jenfi

I play in 20w mode, i used to jumped the channel, but yesterday, i do not, and the sound was pretty good only with the treble channel.
i play with a two notes Torpedo captorX for gigs, and with my cab (2x12 V30) at rehersal.
at this moment i play with a -6db bridgeT atténiator and it's perfect. I have try -9db and the volume is too low.
For sure, -7db will be a good value. 
I'm just afraid that -3.5db when i use the footswitch will be too much loud for the solo.


----------



## donwagar

I'd suggest you ask JohnH to design a schematic with -3.5 as the reactive load, and use -3.5 in the footswitch, but also add a switchable -3.5 and maybe even a -7 switchable in the box. Then you have a ton of options. I don't think you'll find switching off the -3.5 to be too loud.

These amps sound fantastic when they are allowed to breathe.


----------



## jenfi

@JohnH, Is it possible to have a schematics with a -3.5 reactive load and another -3.5db stage for the footwsitch ?

Thanks a lot


----------



## telesto

JohnH said:


> Thanks again, It looks mostly right to me. The design is based on a 4x12 cab with G12M speakers. These have an impedance of about 20 Ohms at 5khz, close to your measurement.
> 
> At 200 hz you have 10 ohms, and this is also as calculated. It is a little wider peak than the real speaker, due to losses in the coil and cap and that depends on the type and rating of the components used.
> 
> In the bass you have two peaks overlapping. One is at just over 90hz and that is your actual speaker resonance coming through. Above that is one created by L2 and C1. The calcs were based on about 110 to 120 hz for this. If the peaks aligned, you would get a sharper peak as expected
> 
> With guitar signals, its best to judge the overall level of attenuation at about 400 to 500 hz. By 1000 hz, a real speaker is beginning to rise.
> 
> How about a test just on your speaker itself? Then we can compare to the attenuator results.


Ok, so with this M3, the output will then be a blend of the original speaker and the "modelled" one? The two bass-peaks had me scratching my head for a second, but now makes sense.

I run a 1 khz ref signal tone before i test different attenuators, and measure the volts coming in and out of the attenuator, just to make sure I'm comparing apples and apples. When I saw M3 had higher voltage out, then I knew I was not comparing the same attenuation values. The freq is not important, I think 1khz was default in REW. (Oh, I just noticed I used a 220uF cap, not 200, just to be accurate, if you are going to compare with SPICE)

Here's the original speaker, a 12" 8ohm Randall. I'm not sure what model, and too lazy to look right now


----------



## diego_cl

Hi! I just finished my M2 build and I'm happy to say it lives up to expectations.

Could someone please tell me if the switches should be wired like this???



I did it wrong the first time, but then I wired it like that and it worked beautifully. Just one problem: my M2 gets too hot with my Marshall MA50C when pushing the volume at 75% or more while boosting the signal with pedals.

This stuff sounds so much better than my Harley Benton attenuator and its treble hungry L-Pad, but I miss the cooling method though.

I didn't want to see the nuts from the exterior, so I made threaded holes, trimmed down the bolts and sanded the case all the way to 3500 grit.
Then I gave a try to my black gilding paper, but I failed... I should have used a spray can in the first place.





Here's a picture from the guts. Forgive me my filthy solder joints, but my 7 USD unbranded 65W soldering iron is too unreliable... it made me lose 3 switches while fixing the wiring. I need something decent like my low wattage Goot, but it's too weak for this Job. The tinned tip from the unbranded iron gets burned after a few seconds, it doesn't look shinny after pressing it against brass wool... maybe it gets too hot, but I really don't know the reason. 



Thank you John !!!


----------



## JohnH

Hi @diego_cl , that looks really good. The rocker switches look great.

I agree with how you annotated the switch connections, with 'on' meaning that stage is attenuating.

As to heat, your amp may be pushing out quite a lot more than the 50W when its overdriven, and these things do heat up. More vent holes can help, particularly in the base since it looks like you have the box lid underneath, with nothing mounted to it so you could take it away and drill it. Holes in the base help air to vent out through the top. More vents in the top maybe, but I wouldn't go there and mess up the nice look.

Then, if it doesn't have them already, get some feet to lift up the base

But, I wonder how hot it really is? Standards for touchable items are about 60 degrees C I think. But electrically and functionally it can go hotter. Does it get too hot to touch? A metal kettle of boiling water at 100C will instantly burn your finger. Can you touch the hottest parts (near R1) for a second or two?


----------



## diego_cl

Thank you @JohnH !

I didn't take pictures of that, but I already added rubber feets and made holes in the base (the same pattern and location of the holes on the top). I thinking about drilling more holes on both sides and maybe using taller feets.

It gets hot in about 5-10 minutes of heavy playing. It's barely touchable, like holding a cup of boiling water; pretty uncomfortable, but the hands can take it. I'm afraid I could melt the inductor's enamel. 

At the time I remove the lid for checking the inside, the resistors doesn't feel as hot as the case and the inductor is just warm.

At this stage I'm worry about cranking the amp for a long period of time without improving the cooling first. But it sounds too good, especially for under 100 bucks!! I thought it couldn't be done for cheap after reading that the parts that make up the reactive load of the Weber mass attenuator are too expensive.

Cranking the amp at bedroom level is priceless. I wanted to do that since I knew about THD Hot Plate in the early 2000's.

John, you are the best


----------



## diego_cl

One more thing: the bottom lid doesn't get hot at all... would be a good idea to use thermal paste between the box and the lid?? Has anyone done this?


----------



## JohnH

I'd guess there's not much to be gained with thermal paste on the lid edge, unless the case is hot right down to the edge?

Would there be depth enough to add a thin computer fan somewhere? Other than that, its drilling of holes, even some more large ones in the top, and the base.

I'm glad it sounds good!


----------



## vqgnb

JohnH said:


> Thanks again, It looks mostly right to me. The design is based on a 4x12 cab with G12M speakers. These have an impedance of about 20 Ohms at 5khz.



What is a V30 like, has anyone tried it and is it worth it to model different speakers?

I think that's what might be going on in the universal audio OX box, since they don't allow third party IR's they know what the IR is supposed to represent and they model the impedance of the speaker that would match the IR. (This is speculation)

How does this attenuator (m3) sound when you use the line out, does it suffer the same "flat" sound as any attenuator on a high setting?

Is it possible to use the attenuator at a fairly low level of attenuation connected to a cab while using the line out as well? (like in a W/D/W setup)


----------



## diego_cl

JohnH said:


> I'd guess there's not much to be gained with thermal paste on the lid edge, unless the case is hot right down to the edge?
> 
> Would there be depth enough to add a thin computer fan somewhere? Other than that, its drilling of holes, even some more large ones in the top, and the base.
> 
> I'm glad it sounds good!



Maybe I could place a tiny fan between the plugs, but first I think I should balance the heat screwing some resistors to the lid. I didn't consider the heat factor when placing the parts inside.

I'll try drilling more holes first, then rearranging the resistors and as a last resource, installing a fan.

I don't know how it works, but the fan from the Harley Benton works only when it's needed and it uses the current from the amp tap. Tonewise, I don't know if it has any downside.


----------



## JohnH

vqgnb said:


> What is a V30 like, has anyone tried it and is it worth it to model different speakers?
> 
> I think that's what might be going on in the universal audio OX box, since they don't allow third party IR's they know what the IR is supposed to represent and they model the impedance of the speaker that would match the IR. (This is speculation)
> 
> How does this attenuator (m3) sound when you use the line out, does it suffer the same "flat" sound as any attenuator on a high setting?
> 
> Is it possible to use the attenuator at a fairly low level of attenuation connected to a cab while using the line out as well? (like in a W/D/W setup)



I have some V30's, and they work well with these designs. When driving a speaker, what you hear is the tone of the speaker itself. The assumptions about which speaker was used to work out the coils etc affects what the amp sees, and how it responds, but is not imposed on the tone of the speaker. Also, V30's seem to have similar electrical characteristics to the G12M, acoustically different but that's not relevant to what is modelled here.

I don't think anything specific is worth changing on the attenuator for V30's. But if you are using a line-out, You'll need a cab-sim or preferably an IR box and that is where the speaker type would be relevant.

One thing that can be relevant though is the resonant frequency which varies based on open or closed back cabs. In M3, I based it on closed back. Open back resonance is a lower frequency. But, If you are just attenuating a speaker, M2 is fine and the real speaker will handle the resonance. If you want a line out and also run a speaker, you can if you wish, take the signal from across the speaker and you will capture the speaker performance including its specific resonance. Its a lower level of signal but more than enough for a line out feed. I do this with my M version, which performs the same as M2 and does not have a resonant circuit


----------



## JohnH

diego_cl said:


> Maybe I could place a tiny fan between the plugs, but first I think I should balance the heat screwing some resistors to the lid. I didn't consider the heat factor when placing the parts inside.
> 
> I'll try drilling more holes first, then rearranging the resistors and as a last resource, installing a fan.
> 
> I don't know how it works, but the fan from the Harley Benton works only when it's needed and it uses the current from the amp tap. Tonewise, I don't know if it has any downside.



It'd be a shame to spoil your neat layout. Before diving in, how about a test where you leave the base off and turn it upside down, so fully open? see where the heat is and how hot it gets. R1 should be the hottest.

You can in principle drive a small low-powered fan by rectifying some of the amp power. Marshall Powerbrakes had that. But its not possible to design it unless you can test it with the specific parts.


----------



## malibu91

Whilst I await 2 pesky parts to arrive from OS, is there an option to run the M2 with a 4 Ohm speaker as found in some Fender amps?
I'm going to have 2 x speaker outs and rather than run them both for a 8 Ohm load, I was figuring I maybe could have a mix of 8 Ohm and 4 Ohm options.

Cheers
Jules


----------



## JohnH

malibu91 said:


> Whilst I await 2 pesky parts to arrive from OS, is there an option to run the M2 with a 4 Ohm speaker as found in some Fender amps?
> I'm going to have 2 x speaker outs and rather than run them both for a 8 Ohm load, I was figuring I maybe could have a mix of 8 Ohm and 4 Ohm options.
> 
> Cheers
> Jules



Provided you match the ohm version of the attenuator to the amp, you can use different speakers. Eg with an 8 ohm M2, and 8 Ohm amp output, you could run an 8, two 8's or a 4, or one or two 16's.

The 4ohm or 2x8 ohm will sound about 1db brighter 

A 16 ohm will sound about 1.5 db less highs more mids, unless you build in output 3. All safe anyway though.


----------



## malibu91

JohnH said:


> Provided you match the ohm version of the attenuator to the amp, you can use different speakers. Eg with an 8 ohm M2, and 8 Ohm amp output, you could run an 8, two 8's or a 4, or one or two 16's.
> 
> The 4ohm or 2x8 ohm will sound about 1db brighter
> 
> A 16 ohm will sound about 1.5 db less highs more mids, unless you build in output 3. All safe anyway though.



OK That makes sense. 
So an M2 is all good for either an 8 Ohm, or 16 Ohm amp as we have 2 sets of values established for the resistors.
However, if one wanted to build an M2 for a 4 Ohm amp, then it would mean a new build due to resistor values needing changing.

Thanks again John!


----------



## JohnH

You can build M2 for one of 16, 8 or 4 ohms on the amp. Basically, the 4 ohm values are 1/2 x the 8 ohm ones. But i have them worked out, adjusted to use the best nearest standard values.


----------



## VintageCharlie

@JohnH, saw this project only yesterday and am really impressed by the work behind it. I have not managed to read through all the pages so excuse me, if i am repeating a question, but is there a feasible way how to implement this with a rotary switch instead of the bat switches, or are there reasons why this won't work?

And one more question - i gather that the M3 version is the most up-to date, but there is only one inductor listed in the table (0.9mH), though the schematic shows 2 inductors.


----------



## JohnH

hi @VintageCharlie , welcome to our thread.

Rotary switches get hard to find, big and expensive ! We need to switch quite a high current. The three toggles together give 8 settings, but an 8 way rotary to replace them with the current rating needed is hard to obtain.

M3 is the same as M2, with an extra bass resonant circuit. For attenuating a speaker, its not really needed, M2 sounds fine and the speaker itself does it. It could be better when running it as a load box with no speaker The coils and cap are at the top of this diagram:













M3 210822



__ JohnH
__ Aug 22, 2021


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## Graham G

JohnH said:


> Hi @Graham G , here is the stripped back design for the series load box.
> 
> View attachment 96202
> 
> 
> It will absorb half the amp power (mostly in R2), so in theory your 50W amp will send 25W through to the speaker.
> 
> It's likely to heat up, so a good aluminium box, thermal paste, ventilation, feet, etc are needed. The usual watch-its about mounting the coil apply - no steel bolts, raise it a bit off the case.
> 
> So as with your recent use of the M2 in this way, the box is intended to be similar to an 8 Ohm speaker, in series with a real 8 ohm speaker, to add up to a 16 Ohm load and you use the 16 Ohm amp tap.
> 
> There's a nice bonus with such a simple design. Although the diagram shows the 8 Ohm resistor on the speaker side, its actually reversible. Either jack can be the input or the output, so long as its connecting to the 16 Ohm amp tap and the 8 ohm speaker.



Hi John, i'm only just about to order the components to build the series load box & have another question if you don't mind, would it be possible to add a 16ohm output along with 8 ohm out, or could you tell me the values i would need to build a separate 16ohm series box .
Also i went to Band practice yesterday & forgot to take the M2, i don't know how any gigging Guitar player could use a 50W amp without one of these, it reminded me how much better the Amp is with the control of the M2.


----------



## JohnH

Hi @Graham G 

The series box takes your 8ohm cab, adds another 8 ohm reactive load in series to make a 16 ohm.
If you had this box with a 16ohm output, what amp ohms would it be plugged into? 

If its 16ohms, then its a different thing, not a series box, more like stage 1 of the full attenuator. Best to do a separate box for that.


----------



## Graham G

JohnH said:


> Hi @Graham G
> 
> The series box takes your 8ohm cab, adds another 8 ohm reactive load in series to make a 16 ohm.
> If you had this box with a 16ohm output, what amp ohms would it be plugged into?
> 
> If its 16ohms, then its a different thing, not a series box, more like stage 1 of the full attenuator. Best to do a separate box for that.



Hi John, I use an 8ohm Redback in my ORI50 Combo,using the M2 as a load box from the 16ohm output from the Amp wired as you instructed me, but I may re-fit the standard 16ohm Celestion Midnight back in the Amp to save weight(i'm old ) & would then need a Box for that 16ohm speaker, but would only still need a stripped down 1//2 power box, if that's possible with 16ohm ?.
Thanks for replying.
Graham.​


----------



## JohnH

Sure its possible, i can work something out.


----------



## Graham G

JohnH said:


> Sure its possible, i can work something out.



Thanks,  Graham.


----------



## VintageCharlie

JohnH said:


> hi @VintageCharlie , welcome to our thread.
> 
> Rotary switches get hard to find, big and expensive ! We need to switch quite a high current. The three toggles together give 8 settings, but an 8 way rotary to replace them with the current rating needed is hard to obtain.
> 
> M3 is the same as M2, with an extra bass resonant circuit. For attenuating a speaker, its not really needed, M2 sounds fine and the speaker itself does it. It could be better when running it as a load box with no speaker The coils and cap are at the top of this diagram:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M3 210822
> 
> 
> 
> __ JohnH
> __ Aug 22, 2021



I would also want to use it as a load box occasionally. What needs to be added or changed, to be able to safely remove the cab and operate it as a load box?


----------



## JohnH

VintageCharlie said:


> I would also want to use it as a load box occasionally. What needs to be added or changed, to be able to safely remove the cab and operate it as a load box?



Nothing to change. You just set it to max attenuation and remove the speaker. At this setting, the amp sees virtually none of the speaker anyway.

You can add a pot and resistor to control your line output level


----------



## JohnH

Graham G said:


> Hi John, I use an 8ohm Redback in my ORI50 Combo,using the M2 as a load box from the 16ohm output from the Amp wired as you instructed me, but I may re-fit the standard 16ohm Celestion Midnight back in the Amp to save weight(i'm old ) & would then need a Box for that 16ohm speaker, but would only still need a stripped down 1//2 power box, if that's possible with 16ohm ?.
> Thanks for replying.
> Graham.​



Here it is!




This will take about 3.3db off, using a 16 ohm amp and 16 ohm speaker.

Depending where you buy, R3 can be either 30 or 33, virtually no difference.


----------



## Graham G

JohnH said:


> Here it is!
> 
> View attachment 97364
> 
> 
> This will take about 3.3db off, using a 16 ohm amp and 16 ohm speaker.
> 
> Depending where you buy, R3 can be either 30 or 33, virtually no difference.



Thanks (again) for the circuit, i'll get the components ordered for both the 8 & 16 ohm boxes & get them built ASAP.


----------



## diego_cl

For a Fender amp like the custom vibrolux, with 4 ohms output tap and 4 ohms speaker arrangement, should I build the M2 attenuator with the inductor and resistors of half the impedance of the 8 ohms version?

Like this??


----------



## JohnH

hi @diego_cl That's the right idea. I worked out a set of values for 4 Ohm, optimised onto standard resistor values. Ill post them tommorow.


----------



## diego_cl

thanks!!


----------



## JohnH

Here's my suggestions for 4 Ohm, to keep everything even and in proportion but based on standard values:

R1 8.2
R2A 12
R2B 10
R3 8.2
R4 5.6
R5 2.7
R6 8.2
R7 18
R8 2.7
R10 27
R11 4.7

L1 0.45mH or 0.5mH

If you are getting from a different range, try to keep the proportions within each stage close to that of the 8 Ohm set

This is the set of frequency response- attenuations that are predicted for those 4 ohm values:


----------



## diego_cl

Soundwise... is there any downside oversizing the inductor's AWG?
For example using an AWG 15 inductor for a 100w build


----------



## RJB

Hi John,

I've build the 8ohm version today for my 15W Fender Blues Junior 4, and it is awsome! I'm going to put it in the cabinet, so I could even do the lay-out close to the schematic.

The sound is magnificent. For the first time I am able to turn the amp up, and able to have volume control at low volumes! A whole new world of sound. BIG THANK YOU!

One question: If I max out volume and master, with all stages 'on' for max dampning, the sound is still pretty loud (to my ever-tinnitus-ringing ears); round 80 to 84 db measured with my iPhone at 1 meter. 

Could this be correct? I measured the stages with the speaker connected, and it seems ok (round 7.7 ohm only inductive stage, with other stages engaged between 8.2 en 8.6ohm). All resitors become hand-warm, R1 the warmest.

Again: I am super happy, and if there is no obvious error I will replace the 3.5db with an other -7 or -14 stage. Just for the (rare) moments I want to max out the amp ;-)
Regards,
Reinout


----------



## JohnH

diego_cl said:


> Soundwise... is there any downside oversizing the inductor's AWG?
> For example using an AWG 15 inductor for a 100w build



All fine, its better. With thicker wire there's less resistance and the coil will heat up less and have more mass and volume to disdipate heat. No change to tone.

For 100W, use lots of space, lots of holes in the base and top. Some have added a fan for 100W.


----------



## JohnH

RJB said:


> Hi John,
> 
> I've build the 8ohm version today for my 15W Fender Blues Junior 4, and it is awsome! I'm going to put it in the cabinet, so I could even do the lay-out close to the schematic.
> 
> The sound is magnificent. For the first time I am able to turn the amp up, and able to have volume control at low volumes! A whole new world of sound. BIG THANK YOU!
> 
> One question: If I max out volume and master, with all stages 'on' for max dampning, the sound is still pretty loud (to my ever-tinnitus-ringing ears); round 80 to 84 db measured with my iPhone at 1 meter.
> 
> Could this be correct? I measured the stages with the speaker connected, and it seems ok (round 7.7 ohm only inductive stage, with other stages engaged between 8.2 en 8.6ohm). All resitors become hand-warm, R1 the warmest.
> 
> Again: I am super happy, and if there is no obvious error I will replace the 3.5db with an other -7 or -14 stage. Just for the (rare) moments I want to max out the amp ;-)
> Regards,
> Reinout
> 
> View attachment 97575



That's great news! thanks for posting.

Those db measurements could be right:

Depending on just how much power the amp puts out overdriven, maybe 50% more, and also on the speaker. 

eg, maybe the 15W amp puts out an extra 50% when maxed That's 22.5W or 13db more than 1W. If you had a speaker with a sensitivity of 98db at 1m, it would do 111db at that power. Run it through the attenuator at -31db and we arrive at 80db.

If you want to go lower, changing the 3.5 stage to 7 is just one extra of the smallest steps, and you lose that fine adjustment. It might be better to add in another additional -7 stage instead?


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## RJB

JohnH said:


> If you want to go lower, changing the 3.5 stage to 7 is just one extra of the smallest steps, and you lose that fine adjustment. It might be better to add in another additional -7 stage instead?



Thank you! I follow your advise and add a -7. I have also plenty of room left on the alu plate. Whilst waiting for the resistors i’ll build the 16 ohm version for my DSL5CR. I’ll again share my experience with both of them!


----------



## RJB

Finished today the 16 ohm ‘low power’ version for my DSL5CR. It works perfect. 

The sound is way more ‘linear’ than with the 15W Fender BluesJr. With the Fender the attenuator opens up a whole new world of sounds and depth that normally can’t be produced without getting loud to very (very)loud. The little Marshall is way better setup for doing its thing at ‘living-room volumes’. Of course it also lacks a real end-stage with the EL84 as the Fender has.

On the Fender the attenuator is an absolute ‘must have’ if you want to enjoy all the range of the amp at lower volumes. For the DSL5CR it gives a little extra ‘depth and room’ since you can crank the amp a bit more, especially when you raised the input with an overdrive. A nice to have- and a fun project to build in an afternoon. Since you can make it with all 5W components (10W for R1, but they stay all cold), it won’t cost you more than 15 Euro. 

Again thanks to John; it really is a great design and with the Marshal you can hear very well the sound stays the same and there is no tone-change (at the same output volume) at all!


----------



## diego_cl

JohnH said:


> Hi @diego_cl , that looks really good. The rocker switches look great.
> 
> I agree with how you annotated the switch connections, with 'on' meaning that stage is attenuating.
> 
> As to heat, your amp may be pushing out quite a lot more than the 50W when its overdriven, and these things do heat up. More vent holes can help, particularly in the base since it looks like you have the box lid underneath, with nothing mounted to it so you could take it away and drill it. Holes in the base help air to vent out through the top. More vents in the top maybe, but I wouldn't go there and mess up the nice look.
> 
> Then, if it doesn't have them already, get some feet to lift up the base
> 
> But, I wonder how hot it really is? Standards for touchable items are about 60 degrees C I think. But electrically and functionally it can go hotter. Does it get too hot to touch? A metal kettle of boiling water at 100C will instantly burn your finger. Can you touch the hottest parts (near R1) for a second or two?



Today the amp was punished for 45 minutes in a sunny day. The case was very hot where the R1 is bolted on the inner side. I could touch it for 5-6 seconds without hurting my skin. I'll drill more holes in the base and sidewalls.


----------



## JohnH

Hi @RJB , thanks for that feedback, and I like the mini version! DSLs of all types get more of their tone from the preamp, so the benefits of the attenuator are more subtle, as compared to just turning down the MV and in comparison to the Fender. 

Hi @diego_cl , that sounds like it was a good workout for it, and since you could still touch at R1, even though it felt hot, it means it wasn't too hot. A bit more venting, as high as you can, plus in the base will help air to flow through


----------



## Echelon

Hi all,

I've stumbled upon this thread a couple of weeks ago and I'm currently in the process of building my own. Thank you, John and everyone who contributed for sharing their knowledge and experience.

I'm going for a 16ohm version. This will be paired with a 50W Ceriatone JM50 (Dumble clone for clean tones). In the future I might be getting a high gain amp like the PRS MT15. I recently built a 2x12 oversized open-back Dumble style cab with Celestion G12-65s. I'll probably make another 2x12 closed-back cab for the high gain amp. All of these amps and cabs are rated for 16 ohms.

FYI: (I can't post links yet, but you can replace the [dot] with ".")

I already obtained the 1.8mH inductor and the enclosure, currently trying to order the remaining parts. The enclosure will be a modified chassis which is usually made for hifi gear: audiophonics[dot]fr/en/mini-dissipante-2u/hifi-2000-minidissipante-chassis-heatsink-2u-200x250mm-10mm-black-front-panel-p-10680.html
Internal dimensions are 200x250x80mm. The heatsinks should also work nicely. The 2mm bottom plate will be replaced by a 5mm one.

Here's the (unfinished) 3D mockup that I made, still need to play around with the layout:



I have some initial questions:
- R7 is rated at 75ohms for the 16ohm version. This is 33ohms for the 8ohm version. Is there a reason that this is more than twice the amount? I also found 68ohm, which is closer. Not sure which one to get.
- I like the idea of the high end cap that @auflauf tested. I found a 3.3uF cap, it's between 2.2uF and 4.7uF so should be a nice middle ground. soundimports[dot]eu/nl/jantzen-audio-001-0242.html However, I'm not really sure how to wire this up though. Is this correct?







- Would this Neutrik jack do?: neutrik[dot]com/product/nmj6hf-s I think it's a switching jack with 6 lugs.

I'm still not sure whether I want to build the M2 or M3 version. I don't mind spending a bit extra to get more defined/accurate bass, as I will be playing in tuned down settings. I'm not sure how much difference this would make, perhaps @CO_Hoya or other M3 builders could shed some light on this.

I was planning on getting these parts if I were to go the M3 route:
- 100uF cap: soundimports[dot]eu/nl/jantzen-audio-001-0290.html
- 18mH inductor: soundimports[dot]eu/nl/mundorf-bs140-18.html. They also have a C-coil: soundimports[dot]eu/nl/jantzen-audio-000-6595.html Might be overkill perhaps? And is 18mH still the value to go for?

I will be putting the 1.8mH coil on a raised (20mm) acrylic plateau. This is based on the following testing:
troelsgravesen[dot]dk/coils.htm

Thanks!


----------



## JohnH

Hi @Echelon , thanks for your post and the interesting links. I can see that you're going for a very carefully planned 'high end' build and I reckon it will work out well.

As a general comment, I appreciate the care that you are using in selecting parts. Obviously, a guitar amp system as different needs with respect to tolerances etc than does a hifi system. Also, I like to try to find the best values for every part of the design, even if individually they make a small difference, but together this approach maintains the consistency at each setting 

The case looks great, the best thing is it has some thick aluminium and is a fairly generous size.

R7 works with R8 to set up the -3.5db stage. There is unlikely to be any noticeable difference between 75 or 68 at R7. But, in the analysis, if you change from 75 to 68, you get 0.2db more attenuation, 0.06db less high treble and 0.09db less bass resonance, the max difference seen at the amp is about 0.1Ohm. None of this is of any significance! So its fine to buy either, from the range available to you. If both are available id use 75, it is in closer proportion to R8 which is 12.

Adding the cap, try it if you like, but this design doesn't need it in the same way as others, which end to get dull at high attenuation. Switch it across R2A. Personally, I wouldn't add it since it changes what the amp sees and how it responds. I use the amp controls to adjust tone. The cap type looks ok though.

I haven't built an M3 myself. But i compare the tone of M and M2 to the full volume tone and cant find any difference at all. But there may be a few extra low bass harmonics with M3, and it may adjust to different amps a bit more closely. If you add it, I hope you will add a switch so you can try with and without. Id like to know! I think the resonant part of M3 may be more important if you use it as a load box with no speaker. 

On the jacks, obviously Neutrik is a great brand, and they look to be the right type. I think stereo jacks are good just to grip the plug better. You will not be using the switch lugs with your build, so if you get them, make sure you use the main tip and barrel lugs. But I see that Neutrik is only rating the jacks at 3A, whereas Cliff rates similar jacks at 5A. I dont know if there a real difference, 3A is OK. 50W into 16 Ohms is 1.8A. Id be a bit more worried with a 100W amp.

If you do M3, that cap and the Mundorf coil look good, wound with 15awg. You should get very low losses which is the key to getting the best peak. The values 100uF and 18mH are aimed at matching a closed back A cab with a resonance around 115-120hz. If you wanted open back, Youd target a lower resonance. But its a really subtle difference because your cab will sound like your cab anyway, even if the amp thinks its driving a slightly different cab.

Raising the coil, its a good idea. He used a 15mm thickness of Al to test. With my case at maybe 2-3mm, I found less raising is fine.

I think thats all, good luck with your build and I look forward to hearing more about it!


----------



## CO_Hoya

Since @Echelon paged me, I thought I'd jump in to the convo.

My understanding of this project / circuit is that the M3 version is unnecessary unless the user is looking to use a line-out from the attenuator. That was my intention, at least.

Updating the status of my version:
I tried to do some testing with REW last week but I couldn't seem to get the output plots to look like what was shown several pages back. I only spent an hour on it, so I'm sure it's just user error. This was using the amp speaker output and a USB mic my sons have for online games, and the mic freq response may be the issue. I couldn't find anything like a cal file for that mic online.

I have a proper mic somewhere but I don't think it will provide line-level without a preamp which I do not have handy, and then I'd probably need an SPL meter as well.

I was short one cable to connect the line-out from the attenuator back to the PC sound card, but that is now ordered. I'm hoping to be able to spend some time getting REW to work and provide some test results before the end of the year.


----------



## Gene Ballzz

@Echelon  to this great forum!

Very nice layout and a excellent enclosure, although it is considerably larger than really necessary and seems pretty pricey! Also the front panel may be a bit thick for mounting of many standard switch styles? On the other hand, should you decide to upgrade it to a 100 watt version, there is plenty of real estate to easily accommodate such!

Enjoy the build and please keep us posted on your progress, including any difficulties and solutions to them.

Build On!
Gene


----------



## Echelon

Thanks for the reply @JohnH you've made a lot of things clear for me now.
I'll go for the 75ohm as per the file, this will also keep things consistent with the other builds.
I'll leave out the high-end cap for now, perhaps I will do some testing later if I end up purchasing it anyway.
Perhaps I won't be getting switching jacks then, to keep things fool-proof. I'll have a look at the Cliffs as I can get either.
Regarding the low end inductor, would 15mH instead of 18mH be a better match for open-back? I plan on getting a closed-back cab for overdriven tunes though. But maybe it works out as I will be playing down-tuned songs on that.

Hi @CO_Hoya, thanks for chiming in. I'm not planning on using a line-out (for now), so perhaps the M2-version might be the way to go.
Did you hear a difference when switching the bass reflex on/off when playing through speakers? If there was no perceived difference, then the M2 would make more sense for me.

Thanks for the warm welcome @Gene Ballzz. Yes, the case is a bit overkill, but I don't mind the size. I measured the real estate on top of my amp and it should fit nicely. The Ox box and Waza craft attenuators are comparable in size, I think. I see the case as a project in itself and am also planning to add some engraving and led indicators. I'm an industrial designer so the design is half the fun for me haha. The front panel is quite thick indeed, but I'll have it machined so it will accommodate the components.
Even though this won't be a "budget" build, it will still be a lot more cost-effective than the above-mentioned attenuators. And I get to learn a whole lot of new things in the process 
It will be a slow build, but I will gladly share my progress here!


----------



## JohnH

hi @Echelon ,
If you were trying to get a lower resonance, you'd need a larger cap and larger inductor value. The resonant frequency is 
F = 1/ (2pi.sqrt (L.C))

If you do use an open back, the tone you get has the resonance of that real cab in any case, M2 or M3. Its just the response in terms of over-driven bass harmonics will be as for the closed back. It'd be a very subtle change, given that even bypassing the bass circuit makes hardly any difference. 

I think if you build it as drawn, but with a switch to bypass the resonant circuit, then you'll have plenty to explore. Or, just do M2 at least to start with.

An open cab might have a resonance at about 80hz. So its not even relevant until you are on the last few lowest notes on your low E string. And they will come through in any case, but the real character of the tone, for which a reactive circuit helps, is in the high mids and highs, covered by the small coil and the way its set up in the circuit.


----------



## Gene Ballzz

@JohnH ,
There is so much great info contained in this thread, you could write, publish and sell a book!  Lots of those nit-picky little details that we anal retentive nut bags tend to really thrive on!

I do a lot of advertising of your design across the innernest. While I let them know that all the necessary info for a build is in the first post, I strongly encourage all to take the time to read the whole thread, for the amazing knowledge and understanding that can be gleaned from it! Your unending and relentless mission of making tiny improvements, along with helping others turn this design into a *"Swiss Army Knife"* of simple, yet effective attenuation is utterly amazing and greatly appreciated! As always *THANK YOU SIR!*
U B Da Man!
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> There is so much great info contained in this thread, you could write, publish and sell a book!  Lots of those nit-picky little details that we anal retentive nut bags tend to really thrive on!
> 
> I do a lot of advertising of your design across the innernest. While I let them know that all the necessary info for a build is in the first post, I strongly encourage all to take the time to read the whole thread, for the amazing knowledge and understanding that can be gleaned from it! Your unending and relentless mission of making tiny improvements, along with helping others turn this design into a *"Swiss Army Knife"* of simple, yet effective attenuation is utterly amazing and greatly appreciated! As always *THANK YOU SIR!*
> U B Da Man!
> Gene



Thanks Gene, I would never have carried on if you hadn't been the first to notice that it worked, and so I dug into how it worked!

I could indeed write it all up, and I'm thinking....
The first post, as with any post, is limited in max length. (10000 characters, and its at 999x). And we are at 108 pages, this is post 2143. There's a heap of stuff, but a lot of repeats, and some fluff too. 

So I was thinking of writing it all up, soup to nuts for everything pertinent, with charts and diagrams etc, then make it available as a pdf online in some way, for a few $. Even though the whole thread can stay public, I reckon there'd be appeal in having an updated concise version.

Does anyone have a view on that?


----------



## Echelon

Hi @JohnH, I'll probably start out with the M2 as that seems like the more versatile version. Thanks again for the advice.

A pdf with all the information neatly in one place sounds like a great idea. I've worked my way through all the pages, but I'd definitely find that handy and would more than gladly pay for it. It would only be fair to get some $ back for all the time and effort you've put into this.


----------



## Gene Ballzz

Echelon said:


> Hi @JohnH, I'll probably start out with the M2 as that seems like the more versatile version. Thanks again for the advice.
> 
> A pdf with all the information neatly in one place sounds like a great idea. I've worked my way through all the pages, but I'd definitely find that handy and would more than gladly pay for it. It would only be fair to get some $ back for all the time and effort you've put into this.



My good friend @JohnH ^^^^^^^ What He Said! ^^^^^^^

I think a good format might be to have a "Foreword" detailing some of the finer points of your initial inspirations and testing. Then make a separate "chapter" for each differnt model/design, including the different impedance choices, along with the testing results. And then a final "chapter" detailing the various modifications and additions and how to best implement them! Then maybe even a section with pics of others' builds, along with the builder's descriptions and comments?

I'll Buy The First One Off The Press!
Gene


----------



## Stephen H

Mr H, I must thank you from the heart for your many hours of effort designing and testing and sharing so that we can spend only a little time and effort to produce an amazingly versatile, useful and great sounding and feeling unit. I finished mine last night and was able to give it a really good workout pretty much most of the day today. It's a real pleasure to hear the whole amp working as it should and not be limited to pre-amp distortion. my ppimv is redundant. 

Many thanks! (probably some from my housemates too!)


----------



## diego_cl

I've just built an Aion FX Refractor (klon clone) and the instructions manual (PDF) was very good. If you're willing to prepare a PDF for sale, maybe you would like to take some ideas from there. General Guitar Gadgets have also very good documentation for people like me that doesn't understand about electronics.

It took me a long time to understand this thread before I was able to buy the parts needed for my first M2. It would have been a lot easier having a PDF with just the right information, bill of materials, wiring diagrams and some pictures.

Besides the PDF for sale, I think this project deserves something like a Paypal donate button.


I've found some good and inexpensive sellers on Aliexpress for the inductors, 1590D enclosure and for the 25W, 50W and 100W resistors that I like to share with you guys. Some of them are at sale right now.

Thank you John for this awesome circuit!
Best regards.


----------



## CO_Hoya

Hi @Echelon, sorry I didn’t see this sooner.

I don’t perceive a difference through the speaker with the bass resonance on/off. I’d like to spend the time to actually test that though as well as through the line out, but I will need some more time to figure out REW.

Sounds like you are going with the M2 design, which seems most sensible.



Echelon said:


> Hi @CO_Hoya, thanks for chiming in. I'm not planning on using a line-out (for now), so perhaps the M2-version might be the way to go.
> Did you hear a difference when switching the bass reflex on/off when playing through speakers? If there was no perceived difference, then the M2 would make more sense for me.


----------



## Gene Ballzz

All of the ones I've built so far have been the M-lite, as when I purchased my coils, was fairly ealy in the life of this thread and I ordered enough of all components for three builds. The next one will be an M-2. It's gonna be a bit of a challenge, as it needs to fit in a fairly small amp head cabinet. My eventual goal is to have one mounted in each amp I own! I think the most practical will be to mount it to or make it as part of the removeable rear panel of the cabinet. I've found a piece of 2 or 3 mm thick, perforated aluminum plate that I could mount it all on and then simply leave it open on the inside of the cabinet?
Just Plannin'
Gene


----------



## Tatzmann

If your doing a pdf me thinks it would be
nice to have a pictured step-by-step
build instruction of the basic design,
lot of beginners need pictures additional
to the schematic to get them going.


----------



## diego_cl

Has anyone tried amphenol jacks?

According to digikey, contacts are made of tinned brass and it's rated to support up to 10A. 

Product:
https://tubedepot.com/products/amphenol-acjs-ihs-stereo-1-4-jack

Specs:
https://www.digikey.com/en/products/detail/amphenol-sine-systems-corp/ACJS-IHS/10443421


----------



## JohnH

Echelon said:


> Hi @JohnH, I'll probably start out with the M2 as that seems like the more versatile version. Thanks again for the advice.
> 
> A pdf with all the information neatly in one place sounds like a great idea. I've worked my way through all the pages, but I'd definitely find that handy and would more than gladly pay for it. It would only be fair to get some $ back for all the time and effort you've put into this.





Gene Ballzz said:


> My good friend @JohnH ^^^^^^^ What He Said! ^^^^^^^
> 
> I think a good format might be to have a "Foreword" detailing some of the finer points of your initial inspirations and testing. Then make a separate "chapter" for each differnt model/design, including the different impedance choices, along with the testing results. And then a final "chapter" detailing the various modifications and additions and how to best implement them! Then maybe even a section with pics of others' builds, along with the builder's descriptions and comments?
> 
> I'll Buy The First One Off The Press!
> Gene





Tatzmann said:


> If your doing a pdf me thinks it would be
> nice to have a pictured step-by-step
> build instruction of the basic design,
> lot of beginners need pictures additional
> to the schematic to get them going.



Thanks guys, I think Ill do it! and thanks for the suggestions.

Things that I'm trying to figure out are mainly based on how to actually make it available. I don't think it'll be high volume, so cant afford to spend much on hosting it somewhere if I want to keep a few $. It would need to be such that it can be paid for and sent to anywhere, so I may have to use Paypal or similar. Also need to consider how to discourage a pdf being freely posted elsewhere. Ebay does not allow such virtual goods as a pdf or ebook, but some do use it to sell things like pdf scans on old knitting patterns and car manuals. 

I have some web space and i can write simple web pages. So maybe another way is to write it all there and find a way to control access to the inner pages.

On content, yes I probably would add a build layout for a specific simple version of an M2. Id probably build a new one in order to do it and present it. But Ive been cautious about this before because reading the schematic and interpreting it into a wiring diagram has been the entrance exam for building the design. Only those who have passed this test have built it, and so each one has been a success.


----------



## JohnH

CO_Hoya said:


> Hi @Echelon, sorry I didn’t see this sooner.
> 
> I don’t perceive a difference through the speaker with the bass resonance on/off. I’d like to spend the time to actually test that though as well as through the line out, but I will need some more time to figure out REW.
> 
> Sounds like you are going with the M2 design, which seems most sensible.



Thanks, Ill be very interested in any insights you can gain from your testing. Differences if any, will be most apparent when driving low notes hard, like around low A 110hz and a few semitones each side.


----------



## JohnH

diego_cl said:


> I've just built an Aion FX Refractor (klon clone) and the instructions manual (PDF) was very good. If you're willing to prepare a PDF for sale, maybe you would like to take some ideas from there. General Guitar Gadgets have also very good documentation for people like me that doesn't understand about electronics.
> 
> It took me a long time to understand this thread before I was able to buy the parts needed for my first M2. It would have been a lot easier having a PDF with just the right information, bill of materials, wiring diagrams and some pictures.
> 
> Besides the PDF for sale, I think this project deserves something like a Paypal donate button.
> 
> 
> I've found some good and inexpensive sellers on Aliexpress for the inductors, 1590D enclosure and for the 25W, 50W and 100W resistors that I like to share with you guys. Some of them are at sale right now.
> 
> Thank you John for this awesome circuit!
> Best regards.



Thanks for those Aliexpress links. They look to be very comprehensive ranges. If you buy a bunch, does shipping cost come out reasonable?


----------



## JohnH

diego_cl said:


> Has anyone tried amphenol jacks?
> 
> According to digikey, contacts are made of tinned brass and it's rated to support up to 10A.
> 
> Product:
> https://tubedepot.com/products/amphenol-acjs-ihs-stereo-1-4-jack
> 
> Specs:
> https://www.digikey.com/en/products/detail/amphenol-sine-systems-corp/ACJS-IHS/10443421



That's good info thanks. i have a few of their guitar cords and they are very high quality. 10A rating on jacks would deal very comfortably with even 100W amps and low ohms.


----------



## Gene Ballzz

@JohnH
Retaining at least some semblance of an "entrance exam" approach is likely a very good idea. Some of the resistor connections to the switches are not really quite as obvious as they might seem. Even though I'm at least "marginally" savvy, if you recall, I needed to contact you through emails to go over some stuff. You really helped me out on avoiding multiple long back and forth wire runs that were more easily accomplished elsewise. What can get tricky is that each switch has two lugs that need to connect to another switch, as well as one lug of a resistor. Where and how to physically make those junctions, in the most convenient way, with the least amount of wire can be a little confusing. I have an idea for a layout diagram format that might make it a bit more clear. It'll be a couple/few days for me to get it to you and I'll send it directly to your email (for your perusal), rather than muddying any waters by posting it here. Even properly laying out a "bypass" was a bit more daunting than it would seem. Some of the physical layout is a bit like playing three dimensional chess, as solving one routing issue needs to consider the not so obvious (and sometimes unintended & detrimental) interaction of components with each other.

The subject of how to distribute and minmize free sharing between folks and also collect the funds is much trickier indeed! I have no suggetions on that!

Thanks Again, As Always!
Gene


----------



## VintageCharlie

What should be the rating for the C1 cap for a 50w amp and what would it be for a 100w amp?

I am planning to build the M3 version.

I am also wondering, if an 8ohm version with the added output 3 for the 16 ohm output would sound the same, as i when i wopuld build a straight 16ohm version or is there a compromise?

Thanks!


----------



## VintageCharlie

Parts availability looks OK apart from the 18mH iron core inductor. I'm in EUrope and the only option i can find is a 64 EUR (about 70 USD) inductor, which is a bit insane - half the price of what people seem to pay for the whole project parts wise.
Do you guys have any hints towards more budget friendly options?

This isn't an issue for the 8R version - there's good options for the 9mH inductor. You can also buy a different higher value and unwind it a bit until you have the right value.


----------



## JohnH

hi @VintageCharlie , thanks for your messages. Looks like you're planning for an M3 build? What kind of uses and amps etc do you have in mind? For most uses, M2 works really well and doesn't use the bass resonant circuit.


----------



## Gene Ballzz

@VintageCharlie 
The best sources for inductor/choke coils is places that sell parts/components for build speaker cabinets and crossovers! Usually much cheaper than from an electronics supply like Mouser! And I agree with @JohnH that the M2 should make you more than happy!
Gene


----------



## malibu91

Hi All,

Well my parts finally arrived and I was able to assemble and test my new M2 8 Ohm JohnH attenuator over the weekend. I did a bit of a custom build in that all switches are on the front (bypass and 3 x attenuation steps).

I also wired up the switches in what I felt was a more logical order that being from least attenuated to most from left to right.

I used a small Hammond enclosure which came already painted black which fit everything in nicely. I also used a “special sauce” component to hold the coil together.

What do I think of the results?

I’m absolutely thrilled with the build. This thing does everything I hoped and more. It’s PERFECT!

My Princeton BFRI now lives on 5-7 on the volume instead of 2. It’s opened the amp up completely and I’m now able to get the Tube Screamer working right where it should be on the edge of breakup.

John, so many thanks for the time and effort you have put into this project. This thing is just brilliant and I’m so happy!

Pictures of the build are below.


----------



## Gene Ballzz

malibu91 said:


> Hi All,
> 
> Well my parts finally arrived and I was able to assemble and test my new M2 8 Ohm JohnH attenuator over the weekend. I did a bit of a custom build in that all switches are on the front (bypass and 3 x attenuation steps).
> 
> I also wired up the switches in what I felt was a more logical order that being from least attenuated to most from left to right.
> 
> I used a small Hammond enclosure which came already painted black which fit everything in nicely. I also used a “special sauce” component to hold the coil together.
> 
> What do I think of the results?
> 
> I’m absolutely thrilled with the build. This thing does everything I hoped and more. It’s PERFECT!
> 
> My Princeton BFRI now lives on 5-7 on the volume instead of 2. It’s opened the amp up completely and I’m now able to get the Tube Screamer working right where it should be on the edge of breakup.
> 
> John, so many thanks for the time and effort you have put into this project. This thing is just brilliant and I’m so happy!
> 
> Pictures of the build are below.
> 
> View attachment 98632
> View attachment 98633
> View attachment 98634
> View attachment 98635
> View attachment 98638
> View attachment 98639
> View attachment 98640
> View attachment 98641
> View attachment 98642



WOW!  Another picture perfectly executed build! Fantastic layout!  Now if the amp had more than twice that wattage, some exra holes for "flow through" ventilation between the resistors, or on the ends/sides might be warranted. On the other hand, highly I doubt that Princeton or even an amp up to nearly 30 watts will ever get it hot or even wrm enough to ever worry about.

The fact that it's performance works as intended, pleases you and even exceeds expectations speaks volumes for the design and pretty much echos my assessment of the @JohnH design! Its the reason I've already finished two of them and am working on three and four to be custom configured/housed/sized for installation in each amp above 5 watts that I use/own! 

Great Build & Thanks For Sharin'
Gene


----------



## malibu91

Most welcome Gene. I decided to not drill any additional holes until I ran it for a while.
It barely even gets warm with the amp cranked, so I have left it nice and clean.

It is intuitive to use and has a permanent home next to the amp.
The enclosure I used could easily be mounted into the cabinet of the Princeton if I ever wanted to.

It's really allowed this lounge room amp to be opened up to deliver it's inherent tone.
Best of all with premium parts, the build came to only around AU$200 (US$140). 

I was close to pulling the trigger on an Ironman Mini for AU$700 until I found this thread!


----------



## Gene Ballzz

@malibu91 
If a little heat ever starts to become an issue, you may think about puttin the feet on the face that you now consider the top and flip it over.  Then it accomplishes at least three things:
A> Allows the heat to escape through the top, reducing the heat buildup a bit.
B> Hides the majority of the screw/bolt heads, as well as any vent holes you may eventually drill around the resistors.​*And* last but not least:
C> Showcases that nice, attractive drill job on the cover! Heck you could even leave the feet on both face, if you are so inclined! ​Just some observations and totally/easily doable, given that you haven't chosen to label/engrave anything on the front or rear yet. I think someone on another website, somebody labeled their build both directions to have it not matter which side was up or down! 
Again, Great Build!
Gene


----------



## JohnH

hi @malibu91 , yes that really is a beautiful piece of work. Many thanks for showing it to us and for trusting the design. I'm glad you like how it sounds!


----------



## diego_cl

JohnH said:


> Thanks for those Aliexpress links. They look to be very comprehensive ranges. If you buy a bunch, does shipping cost come out reasonable?



You can simulate the shipping price on the website. The shipping cost to my location was fair; 6 USD for 1 coil, 8 USD for 2. Very impressive, considering that only took 2 weeks to arrive and it came in a very well protected box. If I buy something locally, the delivery price is not much cheaper (except for food).

I looked everywhere and this store had the best prices. But I don't know how good is the tolerance of the coils, because my DMM doesn't measure inductance and I don't know how to measure that with my DMM not having this feature.

On the other hand, the resistors I bought were all within 5% of tolerance. I had tried another seller before, but it was a disaster. When looking for good Aliexpress sellers, there's a lot of trial and error. But there are some pretty good sellers in there. I love the "Donlis" brand, they make amazing pickups and they are very affordable. Just beware of brass covers and ceramic magnets (they make both quality and cheap stuff).

I'm on the way of building 3 more M2 attenuators (and I think my last ones).


----------



## diego_cl

malibu91 said:


> Hi All,
> 
> Well my parts finally arrived and I was able to assemble and test my new M2 8 Ohm JohnH attenuator over the weekend. I did a bit of a custom build in that all switches are on the front (bypass and 3 x attenuation steps).
> 
> I also wired up the switches in what I felt was a more logical order that being from least attenuated to most from left to right.
> 
> I used a small Hammond enclosure which came already painted black which fit everything in nicely. I also used a “special sauce” component to hold the coil together.
> 
> What do I think of the results?
> 
> I’m absolutely thrilled with the build. This thing does everything I hoped and more. It’s PERFECT!
> 
> My Princeton BFRI now lives on 5-7 on the volume instead of 2. It’s opened the amp up completely and I’m now able to get the Tube Screamer working right where it should be on the edge of breakup.
> 
> John, so many thanks for the time and effort you have put into this project. This thing is just brilliant and I’m so happy!
> 
> Pictures of the build are below.
> 
> View attachment 98632
> View attachment 98633
> View attachment 98634
> View attachment 98635
> View attachment 98638
> View attachment 98639
> View attachment 98640
> View attachment 98641
> View attachment 98642




 looks very nice, both the amp and the attenuator

Did you use a metal bolt for fastening the inductor? I thought you shouldn't do that, but I'm not sure why.


----------



## JohnH

diego_cl said:


> looks very nice, both the amp and the attenuator
> 
> Did you use a metal bolt for fastening the inductor? I thought you shouldn't do that, but I'm not sure why.



Yes that's true, for steel bolts. The ferrous content acts like a core to increase inductance higher than spec. I have inductance ranges on my meter and Ive seen 30 to 40% added due to a bolt. Stainless steel is ok, or aluminium, brass or plastic/nylon. 

The other watchit is not to put the coil in close contact with the case, or else eddy currents are induced. This effect adds damping (effectively like resistance for ac) and reduces inductance. Its most noticeable (as a measurement) with aluminium cases due to their high conductivity (which is a good thing thermally) and being non-ferrous, you don't get this second effect hidden by the first effect. A few mm is enough.

None of htis is a show stopper, and my early versions did not take these things into account. But having optimised the value of the coil, might as well avoid effects that mess with it.


----------



## malibu91

diego_cl said:


> looks very nice, both the amp and the attenuator
> 
> Did you use a metal bolt for fastening the inductor? I thought you shouldn't do that, but I'm not sure why.


Hi,

I used stainless steel to avoid this issue.

Cheers
Jules


----------



## Gene Ballzz

JohnH said:


> Yes that's true, for steel bolts. The ferrous content acts like a core to increase inductance higher than spec. I have inductance ranges on my meter and Ive seen 30 to 40% added due to a bolt. Stainless steel is ok, or aluminium, brass or plastic/nylon.
> 
> The other watchit is not to put the coil in close contact with the case, or else eddy currents are induced. This effect adds damping (effectively like resistance for ac) and reduces inductance. Its most noticeable (as a measurement) with aluminium cases due to their high conductivity (which is a good thing thermally) and being non-ferrous, you don't get this second effect hidden by the first effect. A few mm is enough.
> 
> None of htis is a show stopper, and my early versions did not take these things into account. But having optimised the value of the coil, might as well avoid effects that mess with it.



That's really interesting info @JohnH ! On my builds (so far) I've only used the bobbin face of the coils to insulate/isolate from the case. I might try opening one of them up and using 2-3mm thick disks made of luan plywood or plastic, to increase the isolation. I'll let you know if I notice any difference!
Thanks, As Always!
Gene


----------



## JohnH

Gene Ballzz said:


> That's really interesting info @JohnH ! On my builds (so far) I've only used the bobbin face of the coils to insulate/isolate from the case. I might try opening one of them up and using 2-3mm thick disks made of luan plywood or plastic, to increase the isolation. I'll let you know if I notice any difference!
> Thanks, As Always!
> Gene
> 
> View attachment 98736



To be honest, mines the same as what you have done! I really doubt that you'd identify an audible difference just due to this, except the case under the coil will be a tad cooler due to reducing the induction-cooker effect. I see it as one of the tiny OCD details that, if we take care of them all without going overboard, they may add up to making the whole thing a tad better.

Another thing I was thinking about, from your post earlier: If you are building one into a head, so close to active circuitry, will there be beam of magnetism coming out of the coil axis? so best not to aim it at V1!


----------



## Gene Ballzz

JohnH said:


> To be honest, mines the same as what you have done! I really doubt that you'd identify an audible difference just due to this, except the case under the coil will be a tad cooler due to reducing the induction-cooker effect. I see it as one of the tiny OCD details that, if we take care of them all without going overboard, they may add up to making the whole thing a tad better.
> 
> Another thing I was thinking about, from your post earlier: If you are building one into a head, so close to active circuitry, will there be beam of magnetism coming out of the coil axis? so best not to aim it at V1!



OCD guitarists?  You must be joking!  

The one I'm building into the amp will keep the single coil (gonna be my first M2 build) as far to the opposite end of the transformers as possible. A "guesstimate" tells me that it will likely end up being even a little bit farther from the transformers than setting it on top of the amp. Once done, I'll try it with the rear panel (attenuator attached to it) removed and then mounted, to see if I can discern any noticeable difference! The amp will only be 15-ish watts, so I can likely cheat a little on all the wattage ratings of the resistors except in the first stage, keeping the footprint to a minimum, if needed.
Thanks Again!
Gene


----------



## TheOtherEric

I’ve been away for some months but just wanted to give another THANK YOU to @JohnH for this amazing attenuator. I just got a Trainwreck Express clone, and that amp is unusable without an attenuator because for overdrive, you have to crank its single volume knob to near max output (35W+). So I can use its Volume knob as a Gain knob, and the attenuator switches become the master volume. The sound is amazing!! Thanks John.


----------



## Gene Ballzz

Always great to hear of another cappy hamper with the @JohnH attenuator! 
Thanks For Shain'
Gene


----------



## VintageCharlie

JohnH said:


> hi @VintageCharlie , thanks for your messages. Looks like you're planning for an M3 build? What kind of uses and amps etc do you have in mind? For most uses, M2 works really well and doesn't use the bass resonant circuit.



Yes, i am after the M3 build, as i will use it also as a reactive load for silent recording. I guess there the bass resonance circuit would make a difference? I have a suhr reactive load - it works great for that, but i want one unit that can also attenuate if needed, not just function as a load box.

Amps are JTM50, AC30, DR504, etc.

I guess i just have to bite the bullet on tge expensive coil, if i want the M3 in 16ohm. 8 ohm would be way cheaper, but i gather tgat many amps supposedly sound better at 16ohms.

JohnH, what would the C1 capacitor spec have to be for running 50w amps (often fully dimed)?

And i know ibalready touched this, but just in case i manage to find it - what current should the rotary switch have to be able to handle, if i wanted to substitute that instead of the switches?


----------



## VintageCharlie

Gene Ballzz said:


> @VintageCharlie
> The best sources for inductor/choke coils is places that sell parts/components for build speaker cabinets and crossovers! Usually much cheaper than from an electronics supply like Mouser! And I agree with @JohnH that the M2 should make you more than happy!
> Gene



Thanks for the hint. Yes, i already am fishing at places like that, but the L2 coil for a 16r version is insanely expensive everywhere. Oh well, i guess i have to bite the bullet.


----------



## JohnH

These 20mH Erse coils look good, maybe not too expensive, but if you can find them (on back order according to this Parts Express link)

Wire Gauge, Inductance (parts-express.cohttps://www.parts-express.com/speaker-components/crossover-components/crossover-inductors/solid-core-inductor-crossover-coils/wiregauge/16,18?inductance=15.83to20.00m)

If you want to replace the three main switches with one rotary. then youd need 8 positions. If as drawn, they come after the first stage, then they are handling nominally 10W out of the 50W. At 16 Ohm this is 0.8A. Id look for 1.5A at 125V for a spec, make before break. But really, wiring that up will be a tangle of wires, and the three toggles are simple and reliable and if you place them on your panel in order from smallest to largest db's then its quite intuitive, and you don't need to adjust them all the time.

Spec for the cap Id advise a 100uF film/polypropylene one, or a group that adds up to this in parallel, for low losses. Its not just about current rating but preserving the resonant peak. If you do that, it will have plenty of rating. Or, not as good, bipolar electrolytics, maybe 100V 2.5A ripple current rating but you lose a bit more of the peak. Almost your entire signal is running through the cap.

Another thing to consider. Do you really need silent conditions? If you can still have the speaker running at max attenuation, then it can create its own peak resonance with no bass circuit, and take the line out from across that. The signal level there at the end of the circuit at max attenuation is quite low, and Ive taken that direct into my mixer line in. Youd just need to add IR or cab sim.


----------



## Traynor

JohnH said:


> Here's my suggestions for 4 Ohm, to keep everything even and in proportion but based on standard values:
> 
> R1 8.2
> R2A 12
> R2B 10
> R3 8.2
> R4 5.6
> R5 2.7
> R6 8.2
> R7 18
> R8 2.7
> R10 27
> R11 4.7
> 
> L1 0.45mH or 0.5mH
> 
> If you are getting from a different range, try to keep the proportions within each stage close to that of the 8 Ohm set
> 
> This is the set of frequency response- attenuations that are predicted for those 4 ohm values:
> 
> View attachment 97569


Would the R10 and R11 create an 8 Ohm tap in this 4 Ohm set up. Thanks


----------



## JohnH

Traynor said:


> Would the R10 and R11 create an 8 Ohm tap in this 4 Ohm set up. Thanks



Yes, you could then use an 8 Ohm speaker with the 4 ohm attenuator and using a 4 ohm tap on the amp. In this arrangement, the tone is very well controlled but you lose about 2db more volume at each setting. So the first step changes from -7db to -9db. That's only an issue if you need to play at around that level, which is still loud on most amps.


----------



## TomBallarino

Hey,

just wanted to add another build to the impressive list on this forum. My version of this design (thanks to everybody involved!) is a M3 version with the option to short the reactive (bass-resonance) part.
I designed the aluminum chassis (4mm thickness) and had it laser-cut and bent, cost about ~$30 per piece. Then I powder-coated it (first try, apart from the inside it went okay-ish) and engraved the markings on a laser-engraver (I f-d up the backside).
As I need this either for silent recording or for rather loud live gigs, I opted for a -7/-3.5/-1.75 dB switching, with the option to switch two -1.75dB stages via a microcontroller (MIDI). This is needed because I play in 2-guitar-player band, and we both have our amps heavily saturated and both need an option to increase volume for solo play. I did not test this in a live situation yet, let's keep our fingers crossed we can start to play gigs again in spring. 
The capacitor is a foil-HiFI-type from China, I did some simulations for 4xEL34 Marshall-type amps (which put out wayyy more than a 100Watts if dimed) and decided to opt for safety. I will propably never record such an amp in this situation, but it is good to know I could. Thermally the 4mm aluminum chassis works pretty good, for a test I did input ~72 Watts/50 Hz for about 30 minutes, which did raise the body temperature of the whole thing to about 65° celsius, with the curve still looking upwards. With the aid of a small fan which is powered by the amp itself, the body temperature did settle at about 50° celsius.
The attenuator/load box is mostly used with 2xKT66 (think JTM-45) amps, or 50 watt plexis. For recording I go from the line-out into a DI box and into an interface. I use Ownhamm IRs or Two Notes Wall-of-sound for cab simulation, so far I like the IRs better. I guess I will never mike a cabinet for recording again!

Thanks again!

.


----------



## JohnH

Hi @TomBallarino , thanks for posting that. It looks really great and you have pushed the design forward in a few areas. So Id love to ask a few questions, which will help others:

How are you finding the results from it so far? 

I'd really like hear about your impression of the difference between having the bass resonance circuit in or out, (ie keeping L1 in but shorting L2 and C1)

Whats your line-out sub circuit?

What values did you work out for the -1.75 stages?

What fan did you use and what's the circuit for it?

thanks agsin for posting!


----------



## TomBallarino

JohnH said:


> Hi @TomBallarino , thanks for posting that. It looks really great and you have pushed the design forward in a few areas. So Id love to ask a few questions, which will help others:
> 
> How are you finding the results from it so far?
> 
> I'd really like hear about your impression of the difference between having the bass resonance circuit in or out, (ie keeping L1 in but shorting L2 and C1)
> 
> Whats your line-out sub circuit?
> 
> What values did you work out for the -1.75 stages?
> 
> What fan did you use and what's the circuit for it?
> 
> thanks agsin for posting!



Hi, 
of course, here are (some) answers:
I did not use the device in attenuation mode yet (at least not with proper volume levels, just "bedroom"-tests), so I cannot say anything about the efficiency of the resonance circuit. I am using it as a load box for recording most of the time, and I have to say that even in this mode the type of cabinet simulation may change the sound more than switching the resonance circuit, but as soon as I did some more tests I will report my findings.
The interesting part of the schematic is this:



I used a 40x20mm low noise fan (Noctua NF-A4x20 FLX), and took the circuit from a drawing of the Suhr reactive load if I remember correctly. Both fan supply and lineout are tapped directly off the amp output as seen in schematic.

I made some measurements with the Arta LIMP software to confirm that everything works as expected, here's a chart with everything set to full attenuation, and the resonance circuit not shorted (=active):


I was a little suprised by the height of the resonance peak, in my simulations (which included any real parasitic resistances, capacitances that I could get my hands on) the peak was higher.

I simulated the whole circuit in ngspice to get values for max power, max currents etc, which, of course, just confirmed your calculations!
As seen in the schematic I used 68/3 ohm resistors for the -1.75 stage. I am not sure how much of a boost is needed, I will report that too when I test in practice

Tom


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## JohnH

Thanks @TomBallarino 

That's very helpful indeed.

I agree with the values for the -1.75db stage

The fan system looks nice and simple. You have a set of components there that you have found to work, with a specific fan that they work with. We have never had that info here before so now you have made a reference that others might try (at their own risk of course!). I've not tried a fan system myself, so have never been able to advise on specific components. If you found a need to spin the fan faster, a bridge rectifier of 4 diodes might do it instead of a single one. But there's no need to drive it any harder than needed to start it spinning when at very high power. Your box is nice and heavy, and as you found, it takes a long time to heat up. That means that where the temperature ends up in practice is based on an average power over about say 30 minutes. 

Your line-out looks fine too. Quite a lot of voltage reduction with 47k and 5k. So those finding a need for more signal, maybe when using a smaller amp, might use a smaller value than 47k. 

Here's my plots. representing what you have, set at -14db: 

This one is the impedance. Red is based on a fit to a real measured 4x12 cab, and blue is the calculated impedance with that speaker after the attenuator, set to -14db:




The real speaker has a huge peak, but the attenuator tracks it very closely except at that very specific frequency. I think that's fine and is close over a broad range and matching the exact peak is neither necessary nor desirable.

I calculated with a low loss film cap (dissipation factor 0.001 at 100hz), an L2 coil of 0.4 Ohm (15 or 16 gage). I used a 9mH, which shifts the peak frequency a tad but not the height. My calculated impedances are just a bit higher than your measured ones, maybe more like your calculated values? This is apparent both at resonance, and at say 5kHz I wonder if the fan system, taking a bit of power, is making that small difference? Not a big deal though.

Here's the calculated signals, at a real speaker (red), at the amp driving the attenuator (green - equivalent to the lineout position), and at the attenuated speaker.


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## Gene Ballzz

@JohnH 
I find it so hard to believe that folks actually come here and then end up buying a Bugera PS1 or Weber MiniMass! Of course, than they stae as a supposed fact that attenuators simply suck tone! Oh well, you can lead a horse to water, but.......
Great Stuff!
Gene


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## TomBallarino

I agree @JohnH , the line out resistor value 47k is a little bit on the high side. It was calculated for my DI box input (max +10dbU) and a 100W amp (so calculating with 130W spikes), and the pot fully open. This is a little bit of overkill, as you can always turn down the pot a little bit when really driving the load box with such a big amp.

With my component values I simulated the resonance peak to max out at around 24 Ohm:



As everything else (e.g. output levels at various settings) was nearly exactly as predicted through simulations this puzzles me a little, but if it sounds good?

I am thinking of building a M2 version without line out for live purposes as the current version is a little bit on the heavy side, and as you and some others have noticed, the bass resonance does not do very much soundwise when used as an attenuator. But I will do some tests first with the full M3 version.

Tom


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## JohnH

My calcs are very close to yours. But, i reckon the fan circuit could make the difference. The fan resistor is 82, so I'm guessing maybe about 100 ohm total for the whole fan circuit?. 24 in parallel with 100 is 19, close to the measurement. Could that be right? I think LCR circuits are just resistive at resonance, so maybe that simple math could be about right.


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## TomBallarino

JohnH said:


> My calcs are very close to yours. But, i reckon the fan circuit could make the difference. The fan resistor is 82, so I'm guessing maybe about 100 ohm total for the whole fan circuit?. 24 in parallel with 100 is 19, close to the measurement. Could that be right? I think LCR circuits are just resistive at resonance, so maybe that simple math could be about right.


I did the same impedance measurement with (green) and without (yellow) the fan connected:


It shows some difference, as expected. The measurement was done with about 1.5W@8Ohm, and the fan was therefore not turning. I expect slightly different absolute values with the fan connected and actually working, but I do not have the equipment to test that.


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## Gene Ballzz

Hey @JohnH & All Others,
Having built two of these already and at least two or three more in the planning stages, it has become evident that the most time consuming and pain in the butt parts of the process is the drilling for ventilation! Does anyone know of a good source for "pre-ventilated" project boxes for our purposes? In various sizes/shapes, etc. One like this seems pretty close, but is kinda pricey, not really the best size (for me at least) and possibly not availble globally:

https://www.ebay.co.uk/itm/273942089907?hash=item3fc83890b3:g:pFgAAOSwBGJeykJs




Shipping is also outrageous and the IEC connector is not necessary, although may be handy for fan power, but......

Watcha Think?
Gene


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## Tone_Chaser

I built the M1 for a Trainwreck Rocket clone I built because most of the time I couldn't get it loud enough to get it to edge of breakup. The attenuator worked great and it really works great with my drive pedals.

I'm building a high gain amp now that has a good master volume so I don't need an attenuator for it, but I'm getting involved with a project mainly as the vocalist but will be playing some guitar. They use processors instead of amps on stage. Could I use the simple 3dB circuit as a reactive load and not use a cabinet without damaging the amp? I'm building a line out into the amp so I could use an IR loader for speaker simulation.


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## JohnH

hi @Gene Ballzz
That looks to be a nice case. I found it also on eBay here, for the same equivalent price, which is a lot once its doubled by shipping. What I find tricky with the drilling is getting all the resistor screw holes right. Despite my best efforts or marking and centre punching there's always a couple that get ugly. For venting I use use a couple of lines of big holes. According to the physics of fluid flow, one 10mm hole is worth more than 4x a 5mm. I recently inherited a big floor-mounted drill press, so I'm hoping that will help reduce my workshop issues.

On that case it looks like it could all come apart for assembly. It would be great if all the resistors could mount to the side heatsinks. This would be good for heat control, and easy to wire up on the flat, before sssembling the case panels.

For other cases, I can buy Hammond-type diecast aluminium ones from a local shop (Jaycar), and that's the best option for me although theres no pre-venting. Id always want aluminium for conductivity.


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## JohnH

Tone_Chaser said:


> I built the M1 for a Trainwreck Rocket clone I built because most of the time I couldn't get it loud enough to get it to edge of breakup. The attenuator worked great and it really works great with my drive pedals.
> 
> I'm building a high gain amp now that has a good master volume so I don't need an attenuator for it, but I'm getting involved with a project mainly as the vocalist but will be playing some guitar. They use processors instead of amps on stage. Could I use the simple 3dB circuit as a reactive load and not use a cabinet without damaging the amp? I'm building a line out into the amp so I could use an IR loader for speaker simulation.



If you already have an M or an M2, you can use that as a load box if its the right ohms for your new amp. Just set it to max attenuation and no speaker. The amp doesn't know. You can add a line out circuit at the front, or you may well find that the signal from the attenuator output is at a good level to go to a mixer or IR. If you want a simpler dedicated box to do that with no switching, you could build that, I can help.

What all of those load-box scenarios don't reproduce is the bass resonance peak. So you may want to turn up bass somewhere. Its not yet concluded if this is really important or not (it only affects a few low notes), but in any case its all safe for the amp.


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## Tone_Chaser

JohnH said:


> If you already have an M or an M2, you can use that as a load box if its the right ohms for your new amp. Just set it to max attenuation and no speaker. The amp doesn't know. You can add a line out circuit at the front, or you may well find that the signal from the attenuator output is at a good level to go to a mixer or IR. If you want a simpler dedicated box to do that with no switching, you could build that, I can help.
> 
> What all of those load-box scenarios don't reproduce is the bass resonance peak. So you may want to turn up bass somewhere. Its not yet concluded if this is really important or not (it only affects a few low notes), but in any case its all safe for the amp.



I'm going to mount the one I built in the combo box so I'm going to build a dedicated unit for this. I'm building a line out in the amp, so just need something to take the load off the amp. Could build it into the amp as well but a stand alone unit is fine too.


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## Gene Ballzz

Tone_Chaser said:


> I'm going to mount the one I built in the combo box so I'm going to build a dedicated unit for this. I'm building a line out in the amp, so just need something to take the load off the amp. Could build it into the amp as well but a stand alone unit is fine too.



Building that into the amp (as a "dummy" load) might get a little dicey, due to the generated heat!
Just Something To Think About?
Gene


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## JohnH

Tone_Chaser said:


> I'm going to mount the one I built in the combo box so I'm going to build a dedicated unit for this. I'm building a line out in the amp, so just need something to take the load off the amp. Could build it into the amp as well but a stand alone unit is fine too.



This is about the simplest possible reactive load:




This one is for 8 Ohms. If you want 16 Ohms, double the values and pick the nearest standard values (ie 47 Ohm, 15 Ohm and 1.2mH). The resistors need to mount to a heatsink or aluminium chassis. The power ratings shown are probably good for about a 40W amp max. If you want to crank a more powerful amp, then Id go for a higher rating. A good way to get 200W for R2 would be two 15 Ohm 100W in parallel.

This circuit will do a very good job of tracking a real speaker from low mids up, and see if you want to add a bit of bass EQ to compensate for a bass peak at 100 to 120 Hz, which this does not do.


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## Mcentee2

Hello all, been away for a while enjoying my M2 build, can't believe how long it's been!!

Am building another amp now so am also pairing it with a new M2 - this one will have the full 100w R1, a complete bypass toggle as well as the other 3 toggles.

I have a larger Hammond box, painted grey this time, left over from another project so already has some air holes, lol!

Main question for posting now, is to ask if there had been any particular advice or gotchas learnt about layout?

Last time I just crammed all the resistors in more or less in circuit order.

Has anyone come up with any sort of "best practice" layout ?


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## JohnH

hi @Mcentee2 , great to see you again. I don't think there's one definitive layout yet, But I like the idea of using a larger box, for an easier build. How big is your Hammond box?

I think with those, its good if the box lid becomes the base, and most parts end up on the inside of what is then the upper surface. The coil too, if you have a neat way of fixing it, spaced a few mm from the aluminium, without a steel bolt involved. For those reasons, I think another good option is to let the coil be on the base, zipp-tied or siliconed down with a spacer.

Im planning to build a new one and write it up. But there are seversl great builds on recent pages. eg, by @malibu91 , on page 109.


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## Mcentee2

JohnH said:


> hi @Mcentee2 , great to see you again. I don't think there's one definitive layout yet, But I like the idea of using a larger box, for an easier build. How big is your Hammond box?
> 
> I think with those, its good if the box lid becomes the base, and most parts end up on the inside of what is then the upper surface. The coil too, if you have a neat way of fixing it, spaced a few mm from the aluminium, without a steel bolt involved. For those reasons, I think another good option is to let the coil be on the base, zipp-tied or siliconed down with a spacer.
> 
> Im planning to build a new one and write it up. But there are seversl great builds on recent pages. eg, by @malibu91 , on page 109.



Thanks John, I have read back a few pages and looks like these builds are going from strength to strength with your constant help and advice, so thankyou again!!

My box is pictured below, 22cm X 14.5cm X 6cm

So it should be large enough for good spacing, although I have yet to get all my resistors together to place mark them.

And yes, the intention is to fasten everything to the "top" of the enclosure, the flat lid currently has 4 rubber feet so will stay as the bottom 

@malibu91 that's a lovely looking internal layout. I see you kept all the ground sides together in one place, and also you moved the stages around for a better spread of heat from your most used resistor combinations 

My first thought was to somehow split up the R1 and R2 a/b group around the box still somehow minimising the wire runs.


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## Tone_Chaser

JohnH said:


> This is about the simplest possible reactive load:
> 
> View attachment 99292
> 
> 
> This one is for 8 Ohms. If you want 16 Ohms, double the values and pick the nearest standard values (ie 47 Ohm, 15 Ohm and 1.2mH). The resistors need to mount to a heatsink or aluminium chassis. The power ratings shown are probably good for about a 40W amp max. If you want to crank a more powerful amp, then Id go for a higher rating. A good way to get 200W for R2 would be two 15 Ohm 100W in parallel.
> 
> This circuit will do a very good job of tracking a real speaker from low mids up, and see if you want to add a bit of bass EQ to compensate for a bass peak at 100 to 120 Hz, which this does not do.



Thanks! That is perfect. Building a 20W amp so that is well within the parameters. 8 or 16 would be fine too.


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## Gene Ballzz

@Mcentee2 & @Tone_Chaser
The layout by @malibu91 in post #2161/page #109 is absolutely stellar and well addresses the length of wire runs. I think the longest runs are created by including the BYPASS switch, so without a BYPASS, it would be even more clean and neat. IMHO, it's one of the better layouts for an M2 I've seen yet! He did add the BYPASS and delete the OUT 3 jack. Be forewarned though that in his first pic, there appears to be two #3 resistors and that his wired pic shows that the resistors got moved around a bit. To anyone going this route, see my comments after that #2161 for improved cooling.

For those with 16 ohm amp output capability, its my considered opinion/experience that the 16 ohm version offers a bit more flexibility in the use of these units, over an 8 ohm version.
> First, it most easily allows the use of an 8 ohm speaker without messing with anything, tonally or impedance wise. and while the 8 ohm unit is still "safe" for the amp, with a 16 ohm speaker, it is not as sonically happy as a 16 ohm with and 8 ohm speaker.
> Next, it facilitates "parallel" use at 8 ohm amp output, if your amp also has that 8 ohm option. If your amp has no 4 ohm option, you can't use an 8 ohm unit in parallel. What the "parallel" use provides is a stand alone -3.5 db attenuation, while still incorporating the reactive function.
> Most Marshall amps offer at least 8 & 16 ohm operation andonly "some offer an additional 4 ohm output. There are also a few that have only 16 ohm output!​@JohnH has graciously designed work around options for multiple input and output impedances. The only downsides are that these "work arounds" increase the number of components involved, complexity of the build and the margin of error in both the build process and use of the unit.

Bottom line is my suggestion to be realistic about what useage this unit will actually see and build it that way. If you REALLY need it to be a "Swiss Army Knife" so be it, but the builder/user had better know his stuff and be able to pay close attention to details. On the other hand, the simple units are cheap and easy enough to build, that extra options may be better accomplished by simply building a couple or several different units.

Just My $.02,
Gene


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## Gene Ballzz

And oh, FWIW, I'm working on a layout/wiring diagram for an M2, very similar to the one by @malibu91 , but with the BYPASS switch deleted. I will submit it directly to @JohnH to allow him to choose whether it gets posted or not.
Cheers! 
Gene


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## Josh Blake

Wow this forum is amazing, im hoping to build a design for my amp, Im looking to have a headphone out as well as line out, becuase of the headphone out i want this section to have a cab sim, I was just wondering what was the point of m3 vs m2 does the m3 have a cab simulation as you keep mentioning it is based on a 60s marshall cab? also i have seem some attenuators have a bright switch as the high ends go with there design, is this something that would be benifitial for this design?


----------



## Mcentee2

malibu91 said:


> Hi All,
> 
> Well my parts finally arrived and I was able to assemble and test my new M2 8 Ohm JohnH attenuator over the weekend. I did a bit of a custom build in that all switches are on the front (bypass and 3 x attenuation steps).
> 
> I also wired up the switches in what I felt was a more logical order that being from least attenuated to most from left to right.
> 
> I used a small Hammond enclosure which came already painted black which fit everything in nicely. I also used a “special sauce” component to hold the coil together.
> 
> What do I think of the results?
> 
> I’m absolutely thrilled with the build. This thing does everything I hoped and more. It’s PERFECT!
> 
> My Princeton BFRI now lives on 5-7 on the volume instead of 2. It’s opened the amp up completely and I’m now able to get the Tube Screamer working right where it should be on the edge of breakup.
> 
> John, so many thanks for the time and effort you have put into this project. This thing is just brilliant and I’m so happy!
> 
> Pictures of the build are below.
> 
> View attachment 98632
> View attachment 98633
> View attachment 98634
> View attachment 98635
> View attachment 98638
> View attachment 98639
> View attachment 98640
> View attachment 98641
> View attachment 98642




Hi, as per posts above, you have a good layout I think 

However, now I'm looking a lot closer, I am having trouble following your switching scheme ?


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## JohnH

hi @Josh Blake , thanks for your message.


What these designs do is to reduce the electrical power that reaches a speaker, while still maintaining the electrical interactions that theռ amp sees due to driving a speaker, and lets the speaker behave as if its driven from an amp. That way, speaker and amp both behave and sound as normal, but at lower volume.

But what the attenuator doesn't do is replicate the acoustic response of a speaker. There's no need to if we still have an actual speaker involved. But with headphones you do need something to provide that response and an IR device would do it, or a cab sim. This needs to be a separate item such as a Mooer Radar or similar.


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## Mcentee2

Got all the parts together now, inductor coil took a while to get through the seasonal post so I'm a bit delayed!

Layout isn't too crammed in. Still deciding where I best to put the IN/OUT jacks - spread on both sides at the back as in the photo, or both moved across to the side away from the big R1.

I might even put the bypass toggle at the back as well to shorten those wires overall.

Otherwise looking ok so far.


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## JohnH

hi @Mcentee2 , yes that is looking clean. What is the size of that case? i think, that if you want a bypass switch, then on the back is a very good place because it discourages from using it as a control on-the-fly, while the other switches you can flick in use. Shouldn't operate the bypass switch unless the amp is off or on standby.

On jacks, i like to separate input and output, and also have two output jacks. Sometimes its useful but its also a really good visual and tactile clue as to which are input and which are output when wiring-up under pressure. If you take two jacks, you can rotate and engage them together as a soldered pair, and then drill a second hole to mount them as a unit.

So long as nothing touches R1, I don't think there's a problem being close to it.


----------



## Mcentee2

JohnH said:


> hi @Mcentee2 , yes that is looking clean. What is the size of that case? i think, that if you want a bypass switch, then on the back is a very good place because it discourages from using it as a control on-the-fly, while the other switches you can flick in use. Shouldn't operate the bypass switch unless the amp is off or on standby.
> 
> On jacks, i like to separate input and output, and also have two output jacks. Sometimes its useful but its also a really good visual and tactile clue as to which are input and which are output when wiring-up under pressure. If you take two jacks, you can rotate and engage them together as a soldered pair, and then drill a second hole to mount them as a unit.
> 
> So long as nothing touches R1, I don't think there's a problem being close to it.



Tha is John, the box measures 22cm X 14.5cm X 6cm

It did have a univibe in it based on R G Keene's circuit, but that had long gone into another smaller box.

This one has plenty of room and metal surface area so am hoping it runs fairly cool. It is intended for a new JTM50 build and will probably be running at -14 or -21db a lot of the time!!

I do need to get another output Jack, as I Ieft that off my BoM when ordering, but yes, my other M2 has two outs.

The main on/off is settled now to be at the back.


----------



## Gene Ballzz

Well folks, here is a nice layout for a 16 ohm, M2 that minimizes the length of all wire runs except two. Being 16 ohm, it doesn't need an "Out 3." Simply refer to the diagram for the M2 that @JohnH so graciously provided in post #1 of this monster thread, for component values. This layout was created for use with a Hammond 1590D (add BK to the end of the part number for black) box, or larger. Click the link below for dimensions:

https://www.hammfg.com/files/parts/pdf/1590DBK.pdf

It is intended for all components to be in the box and use the removeable lid as the top. You can flip it over, if you like and swap the R7 & R8 resistors with R5 & R6. It uses the logic of having the lowest attenuation switch on the left and the highest on the right. It will also work well for the 8ohm M2, as that coil should be smaller, although you might feel the need to add the extra "Out 3" and resistors for use with a 16 ohm speaker. There should be plenty of room for those resistors, if you put them or R1 on the wall.

How the switches function can be a bit confusing, but if you look just at the -14db switch and it's associated R5 & R6 resistors and how the lower lugs of the switch pass the signal on to the next switch.

R2A & R2B are mounted to the walls of the box and with my coil source, the 1.8mh unit is 54mm in diameter. If your chosen coil is larger, you could mount R1 to the wall also and then maybe move R1A to the floor of the box. It will all depend on the physical size of your coil.

It was drawn to scale, considering all Ohmite/Arcol resitors, but the ones many are getting from Chinesiawanoreanam take up even a little less real estate.

And by the way, if not obvious, the switches are intended to be mounted in the wall, which actually gives a bit more "wiggle" room than the drawing shows.




Happy Attenuatin'
Gene


----------



## Mcentee2

Gene Ballzz said:


> Well folks, here is a nice layout for a 16 ohm, M2 that minimizes the length of all wire runs except two. Being 16 ohm, it doesn't need an "Out 3." Simply refer to the diagram for the M2 that @JohnH so graciously provided in post #1 of this monster thread, for component values. This layout was created for use with a Hammond 1590D (add BK to the end of the part number for black) box, or larger. Click the link below for dimensions:
> 
> https://www.hammfg.com/files/parts/pdf/1590DBK.pdf
> 
> It is intended for all components to be in the box and use the removeable lid as the top. You can flip it over, if you like and swap the R7 & R8 resistors with R5 & R6. It uses the logic of having the lowest attenuation switch on the left and the highest on the right. It will also work well for the 8ohm M2, as that coil should be smaller, although you might feel the need to add the extra "Out 3" and resistors for use with a 16 ohm speaker. There should be plenty of room for those resistors, if you put them or R1 on the wall.
> 
> How the switches function can be a bit confusing, but if you look just at the -14db switch and it's associated R5 & R6 resistors and how the lower lugs of the switch pass the signal on to the next switch.
> 
> R2A & R2B are mounted to the walls of the box and with my coil source, the 1.8mh unit is 54mm in diameter. If your chosen coil is larger, you could mount R1 to the wall also and then maybe move R1A to the floor of the box. It will all depend on the physical size of your coil.
> 
> It was drawn to scale, considering all Ohmite/Arcol resitors, but the ones many are getting from Chinesiawanoreanam take up even a little less real estate.
> 
> And by the way, if not obvious, the switches are intended to be mounted in the wall, which actually gives a bit more "wiggle" room than the drawing shows.
> 
> View attachment 99623
> 
> 
> Happy Attenuatin'
> Gene




Thanks Gene, just in time for me as it has cleared up a couple of brain farts I made on my drawings !!


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## Marcomel79

Hey @JohnH, on saturday you suggested i build your attenuator instead of modifying my amp. Well, you neednt say anymore. These came yesterday. I would like to thank @Gene Ballzz, which was very gracious to take A LOT of his time to help this guy out with layout and many questions, for me to build your 16Ohm M2 attenuator. Really appreciated sir!


Im waiting for the enclosure, the cliff jacks and the inductor coil. I have the three mini switches and might implement the full bypass down the road.

And a massive thanks to you John, for being so kind to share your work FREE OF CHARGE.

Marco


----------



## Mcentee2

Ok, all the bits installed ready for some soldering after I have drawn it all out as per Gene's switch layout 

Having a bit more room weirdly makes it all look a little uneven and less elegant, bit it will all be on the inside away from eyes


----------



## JohnH

Hi @Gene Ballzz , thanks for the layout and I'm sure it will be helpful. It looks right to me!

A couple of things for others to note:

Now there's a layout to look at, its tempting just to follow it without needing to figure out what is going on. But, do not just do that. Instead, use it as a tool to help understand the schematic and relate the parts layout to the diagram. That way you can understand more and can troubleshoot your build.

If anyone compares Gene's layout to the schematic, you may spot that Gene swapped the electrical order of the -7 and -14 stages to make a neater wiring, while providing a logical order on the front panel. We checked this in the analysis and it works fine with no consequences.

Before connecting the attenuator to an amp, it is essential to test it fully using a multimeter. Plug in a speaker to the output. Plug in a cord to the input (but no amp). Measure resistance at every switch setting, with leads across tip and barrel of the jack plug that will go into the amp. It should be between about 7 and 10 ohms (8 ohm build), or 14 and 20 ohms (16 Ohm build), after subtracting for the meter leads. 

If this is your first build, or like me, you are a bit ham fisted or cant see as well as you'd like to, a slightly larger case for a few more $ is well worth it to let you spread the parts out more. See how @Mcentee2 is doing it above. Genes layout is still a great reference though. It will also allow the small resistors to spread out, so the nut/bolts to fix them aren't under the solder tag from another. A larger Hammond case of the same basic type is referenced 1550. 

The layout is based on branded Arcol resistors, or similar. The 100W Arcol one for R1 is a big mutha because it has two flanges. An alternative is two at 50W in series or parallel to get the value needed. Two 30's in parallel is 15, if you are doing an 8 Ohm build. Two 15's in series is 30, for a 16 Ohm build. 

Happy Xmas and happy building!


----------



## Gene Ballzz

@JohnH 
I'm curious which "1550" box you are referring to and what the "alphabet soup" following the "1550" in the part number is. Please click the below link to notice that "1550" is simply a "series" and available in many different sizes. Given that you mentioned a "slightly larger case" can I assume you are referring to the "1550G" or 1550H"? Of course, "BK" at the end of the "alphabet soup" in the part number is for black painted!

https://www.hammfg.com/electronics/small-case/diecast/1550

Thanks & Merry Christmas To All,
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH
> I'm curious which "1550" box you are referring to and what the "alphabet soup" following the "1550" in the part number is. Please click the below link to notice that "1550" is simply a "series" and available in many different sizes. Given that you mentioned a "slightly larger case" can I assume you are referring to the "1550G" or 1550H"? Of course, "BK" at the end of the "alphabet soup" in the part number is for black painted!
> 
> https://www.hammfg.com/electronics/small-case/diecast/1550
> 
> Thanks & Merry Christmas To All,
> Gene



Yes good point! I'm thinking 1550G or 1550GBK for black:

1550G.pdf (hammfg.com)

I can buy one that size locally, so that'll be my next one. Ive been using the unpainted ones, then when I've done the drilling and committed all the machine shop sins, I can use wire wool and wet/dry followed by a rattle can to get a good finish, including into the vent holes too.


----------



## brad Messier

Good Day to all. I am sure the answer is here somewhere... but so many posts now. 
Webber 5F1 clone, so right around 5 watts power, going to a 4 ohm Webber 10A125-4 speaker.
What changes to the M2 version would be needed for this setup, if possible? This is a crazy in depth thread, love it! Thanks in advance.

Brad


----------



## Mcentee2

All done, and works a treat. Used up some leftover ends of wire so it looks a bit colourful  as usual some of the soldering looks a bit untidy, am improving each time though!!

Must say -31.5db is awfully quiet...


----------



## JohnH

brad Messier said:


> Good Day to all. I am sure the answer is here somewhere... but so many posts now.
> Webber 5F1 clone, so right around 5 watts power, going to a 4 ohm Webber 10A125-4 speaker.
> What changes to the M2 version would be needed for this setup, if possible? This is a crazy in depth thread, love it! Thanks in advance.
> 
> Brad



Hi Brad and welcome to our thread. The main thing would be to confirm that the amp is based on a 4 ohm output, which I assume it is if that's the the speaker Ohms. If so, you can build M2, with all values x 1/2 (compared to the8 Ohm one) to make a 4 ohm version. i can find a list of best standard values for that. If you have an 8 Ohm tap, you can also use an 8 Ohm M2 and its fine to run it with an 8 or a 4 Ohm speaker.

You can also reduce power ratings on resistors if you wish, or keep it at 50W spec for use with other amps.

On the M2 diagram, you may not want Out 3, and for this amp, you could choose to omit the -14db stage if you wanted.


----------



## JohnH

Hi @Mcentee2 , thanks for posting. Yes -31.5db can get quiet, but what amp are you using it with? I use it quite often with my 50W VM, though I always like a few more db's if I can


----------



## Mcentee2

JohnH said:


> Hi @Mcentee2 , thanks for posting. Yes -31.5db can get quiet, but what amp are you using it with? I use it quite often with my 50W VM, though I always like a few more db's if I can



At the moment with my Tweed based 2x6v6 putting out about 15w.

Plan is to use it with a new JTM50 Black Flag build, where it will be more at home!!


----------



## JohnH

It should be just right for that. I reckon the -31.5db will be right for the quietest you'll need; and it will find the perfect volume that you want somewhere in the range. The bigger case will also help with heat, if you're cranking it.


----------



## Markus Bender

Hi people,
I recently found this thread and after a while of reading through all the posts, I would like to contribute.
First John, thanks for developing this attenuator, for all that incredible work and for sharing this. And all the others for their comments. This is extremely useful, also educative and understandable for an "medium-level-skilled electronic nerd" like me.
Attached is a schematic I have drawn out from my Rivera RockCrusher. I hope is might also interesting for the community. No guarantee for correctness...
Eventually for comparison with the JohnH Attenuator. This one works vice versa, first resistive attenuation, followed by the reactive network. To my rude understanding this doesn't matter as the output drive of a tube amplifier acts as a current source and therefore the ordering of the resistive / reactive part should have the same result.
Just found that S1a/b is not labelled, this is of course the attenuation / bypass switch.

All the best,
Markus


----------



## JohnH

Thanks @Markus Bender , that's very interesting indeed. Based on that, it looks to be just a simple resistive attenuator with some tone shaping. Once turned down, the amp sees almost none of that reactance. Here's a quote from their site:

_"We built into the RockCrusher a reactive load network for proper speaker-amplifier interaction. Rather than a purely resistive load like many of the other power attenuators on the market, the Rock Crusher ensures that the amplifier and speaker see each other in a proper relationship of impedance and inductive/capacitive reactance. This translates to a superior tone, regardless of level." 
_
After looking at your diagram, I find those statements to be disappointing and inaccurate!


----------



## Marcomel79

So yesterday i got my Hammond box to build @JohnH's attenuator and today i was playing around to see the best layout for my 16Ohm build, based on Gene's diagram. I couldnt find the 50W 39Ohm resistor so i ordered a 20+18Ohm. Both 50W! Oh well.... Anyway, im still waiting for the inductor coil from China, hopefully it will be here soon...

So this is the first layout i tried, so as to maximize the space for the coil




And then i trid this one. There should be plenty of space for the coil.





By the way, that 100W 30Ohm resistor is just massive! I have some nice heatsink from an old power supply i no longer use which might come useful behind that big mutha.


----------



## JohnH

I've also been thinking about how best to lay out the parts, so its easy to follow and easy to see and wire up.

Here is a reasonably to-scale M2 layout, in a Hammond 1550G case:













M2 Layout And Schematic211231



__ JohnH
__ Dec 30, 2021





(diagram edited 31/12/21)

It's basically an 8 Ohm M2, including the extra Out 3 for 16 Ohm output (which I find useful), and a basic line-out. There's a few new wrinkles, so I put the schematic and parts table on the diagram.

With long shipping times form China, more people seem to be using branded resistors such as Arcol. As noted above, the 100W ones get huge. So this uses a pair of 50W resistors for R1 (ie R1A and R1B). This may give a more compact and neater arrangement. Two 15's in series to make 30, or two 30's in parallel make 15. I separated the two R1A and R1B resistors to spread out the heat.

I moved the order of stages to go -14, -7, -3.5 db so that the logical front panel order matches the electrical and internal order. This is the same as Gene did above, and has been found to make no difference technically.

If you get jacks with 3x2 legs, then its important to connect to the primary lugs, not just to the switched ones, which are not used here. So to guard against error, the lugs are all shown linked across the jack so there's no risk. I show the jacks all in line, for ease of drawing. But what I actually build in mine is to put Out 1 and Out 2 vertically aligned, and you can rotate and engage/solder the lugs together as a preassembled unit - see inset diagram.

The case size is 222 x 146 and allows a fairly spacious layout, or by squeezing it up more, more add-on features.

The extra output and the line-out seem to be the most useful add-ons. I'd recommend against adding too many extras, but there are several others that have been developed in this thread:


options for* variable input impedance* (useful if you have several amps with no output tap values in common. But note you can already use different speaker ohms in the base designs. Also, you can easily build a couple of these units at different ohms to cover your options. Most ways to convert to different input ohms on one unit take off some more power, or get complex)

*switching to reduce attenuation of Stage 1*, to get a -3.5db setting (this can be useful if you want to have just a slight reduction. But with most rigs, even -7db is still loud. And, as a work around, you can set the unit as a load box in parallel with a speaker, to take off just 3 db at half the ohms amp tap)

*foot-switching attenuation stages* (useful if you want a lead/rhythm change, with no added drive, and can afford a few extra db's reduction.)

*bass resonance circuit* (M3 design - tests so far suggest that this makes very little difference to perceived tone when using a speaker, but may be desirable for when used as a load box with no speaker. The added cap and coil to convert an M2 to an M3 are relatively bulky and expensive. Note that when you run with a speaker, it will naturally make is own bass resonance without this circuit.)

*full bypass switch* (IMO, very few players really need this, unless you are setting levels with no or very little attenuation. After a few tests, most will either use the attenuator to attenuate, or not use it if its not wanted. A full bypass switch is handling the full amp power, so the chances of setting it up or operating it wrongly are significant)

*fan cooling* (the designs listed here, in a thick AL box are good for up to about a 50W amp with no fan. A few running amps at 100W have used them however)
Note that with the *line-out* option, even if you don't use the pot, you may well find that the signal level at the final attenuated speaker output is at a good level to feed into a mixer line-in, with no dedicated line-out. In any case, an IR loader or cab-sim will be needed to replicate a credible speaker tone.


----------



## brad Messier

JohnH said:


> Hi Brad and welcome to our thread. The main thing would be to confirm that the amp is based on a 4 ohm output, which I assume it is if that's the the speaker Ohms. If so, you can build M2, with all values x 1/2 (compared to the8 Ohm one) to make a 4 ohm version. i can find a list of best standard values for that. If you have an 8 Ohm tap, you can also use an 8 Ohm M2 and its fine to run it with an 8 or a 4 Ohm speaker.
> 
> You can also reduce power ratings on resistors if you wish, or keep it at 50W spec for use with other amps.
> 
> On the M2 diagram, you may not want Out 3, and for this amp, you could choose to omit the -14db stage if you wanted.



Thanks for all the info. That new layout looks great BTW! So I can drop all of the resistor values by half, and go down to a 25W rating pretty safely. 
Should I drop the value of the inductor coil down to .5mH? Looks like I have a couple of options .55, .5, and .45 in 20 AWG coils. 
I am planning on dropping the Out 3, no real need. Any benefit to keeping the two outputs? I don't see any real need for two any time soon.
I will drop R5 and R6 from your older diagram and omit the -14db stage. 
If I want to add in the line out do you suggest any adjustment to R12 and R13 values? 

Thanks again, can't wait to get building!


----------



## JohnH

brad Messier said:


> Thanks for all the info. That new layout looks great BTW! So I can drop all of the resistor values by half, and go down to a 25W rating pretty safely.
> Should I drop the value of the inductor coil down to .5mH? Looks like I have a couple of options .55, .5, and .45 in 20 AWG coils.
> I am planning on dropping the Out 3, no real need. Any benefit to keeping the two outputs? I don't see any real need for two any time soon.
> I will drop R5 and R6 from your older diagram and omit the -14db stage.
> If I want to add in the line out do you suggest any adjustment to R12 and R13 values?
> 
> Thanks again, can't wait to get building!



Hi Brad,

For a low power, 4 Ohm build, id suggest as follows:

R1: just use one, as with earlier diagrams, 8 Ohms (or 8.2)
R2A: 12
R2B: 10
L1: 0.45mH (0.5mH is OK), use 18 gage for lower resistance:
R3: 8 (or 8.2)
R4: 5.6
R5, R6 - omitted
R7: 18
R8: 2.7

If you are doing the line-out, the output voltage will be fairly low since its 4 Ohms and low power. so i suggest to reduce R13 from 5k to say 1.5k

Sure just 1 output is fine, unless you want to try different cabs. If you built Out 3, it would be for using an 8 Ohm cab (resistors 27 and 4.7), but no need if you don't want that.

Good luck with your build.


----------



## Marcomel79

Hey @JohnH, what do you think of this layout? Will the 47Ohm resistor get too hot to be standing right under the input/output jacks (top left)?



Initially i was gonna order the resistors from China but after shipping and import tax costs (i live in Norway), i decided to buy locally for actually less money. I bought all 1% tolerance. The coil was 1mm, which is the closest to 18awg.

That will be the last to arrive...look forward to put it together and try it.

Thank you so much for sharing this FREE OF CHARGE!

Marco


----------



## JohnH

hi @Marcomel79 , I think it will be ok. R2A and R2B don't get very hot really. I'd suggest to think through your wiring and access to solder lugs before committing though!


----------



## Marcomel79

JohnH said:


> hi @Marcomel79 , I think it will be ok. R2A and R2B don't get very hot really. I'd suggest to think through your wiring and access to solder lugs before committing though!


I will be sure to check it three times! Thank you for the prompt answer John


----------



## bikescene

I am finally going to build an attenuator based on the 50W M2 design after a long bout of procrastination. I will be using 100W, 50W, and 25W Arcol resistors. Do you guys have any tips on applying the optimal amount of thermal paste onto resistors? I’ve only ever used thermal paste on older PC builds, not heat sink resistors.

I thought I saw pictures in this thread showing the application of paste onto the resistors, but I haven’t been able to find them again.

I’ve drilled out a Hammond 12”x8”x2” enclosure that I had laying around. I just received a Dayton 0.9mH 18AWG inductor from Parts Express. I am just waiting on some nylon hardware before I proceed.


----------



## Mcentee2

bikescene said:


> I am finally going to build an attenuator based on the 50W M2 design after a long bout of procrastination. I will be using 100W, 50W, and 25W Arcol resistors. Do you guys have any tips on applying the optimal amount of thermal paste onto resistors? I’ve only ever used thermal paste on older PC builds, not heat sink resistors.
> 
> I thought I saw pictures in this thread showing the application of paste onto the resistors, but I haven’t been able to find them again.
> 
> I’ve drilled out a Hammond 12”x8”x2” enclosure that I had laying around. I just received a Dayton 0.9mH 18AWG inductor from Parts Express. I am just waiting on some nylon hardware before I proceed.




Good luck with it all, a bit of careful planning goes a long way I think!

I used some grey computer CPU paste, can't recall the brand at the moment, and basically just squeezed the tube and applied it to the base of the resistor and swirled it round roughly so it was present across the whole surface, still blobby though.

Then just bolted the resistor to the enclosure. Whilst tightening it all up the paste is squeezed and spreads out somewhat to leave a residual thin layer between resistor and enclosure.

Of course though, you end up with a fair bit of paste squeezed out of the sides to somehow mop up with a cloth or wipe, and it can get quite messy as it's sort of sticky floppy stuff.

I prefer having too much and messy, than not putting enough on.


----------



## bikescene

Mcentee2 said:


> Good luck with it all, a bit of careful planning goes a long way I think!
> 
> I used some grey computer CPU paste, can't recall the brand at the moment, and basically just squeezed the tube and applied it to the base of the resistor and swirled it round roughly so it was present across the whole surface, still blobby though.
> 
> Then just bolted the resistor to the enclosure. Whilst tightening it all up the paste is squeezed and spreads out somewhat to leave a residual thin layer between resistor and enclosure.
> 
> Of course though, you end up with a fair bit of paste squeezed out of the sides to somehow mop up with a cloth or wipe, and it can get quite messy as it's sort of sticky floppy stuff.
> 
> I prefer having too much and messy, than not putting enough on.



Thanks. I was trying to Google general tips on applying the paste and was having no luck. I'll need to make some cut outs that are the same sizes as these resistors and experiment with the amount.

I 've got the chassis all drilled out already. I need to double check my planned wiring before I start, since I'm deviating a bit. I will be adding an additional fixed 7dB stage since I will only be using the attenuator at home.


----------



## JohnH

Hi @bikescene , welcome to our thread. It looks like you are doing some good careful planning.

For the thermal paste, the ideal is to end up with the thinnest possible layer that covers the full base of the resistor. I smear it thinly then as I place the resistor, move the resistor a little side to side while pressing, then do up bolts. It all helps though it's not as critical as for a CPU. 

You could practice on one, then pull the resistor off to see how complete the coverage is. A thinner layer is better than a thick one. The best paste is that with the highest conductivity, I think that's usually the grey stuff that has aluminium in it.


----------



## diego_cl

bikescene said:


> I am finally going to build an attenuator based on the 50W M2 design after a long bout of procrastination. I will be using 100W, 50W, and 25W Arcol resistors. Do you guys have any tips on applying the optimal amount of thermal paste onto resistors? I’ve only ever used thermal paste on older PC builds, not heat sink resistors.
> 
> I thought I saw pictures in this thread showing the application of paste onto the resistors, but I haven’t been able to find them again.
> 
> I’ve drilled out a Hammond 12”x8”x2” enclosure that I had laying around. I just received a Dayton 0.9mH 18AWG inductor from Parts Express. I am just waiting on some nylon hardware before I proceed.



I've no idea if I'm doing it correctly, but this is how I do it. I know it works, because the attenuator I made before goes very hot with a 50W amp (see video):
https://imgur.com/EL3ICMA

I spread it like peanut butter with a palette knife, not too thick, but also not too thin... I try to cover the base of the resistor with just enough paste to hide the aluminum. The bolts make the rest. Then I use a q-tip for cleaning, sometimes with isopropyl alcohol if it's too messy.

This is my M2 tool kit:





- Thread cutting tap (M3)
- 3/32 drill bit
- Step drill bit (one of the best purchase I've ever made on Aliexpress)
- Cheap thermal grease from Aliexpress
- Stainless steel allen screws (M3, 5mm)












The 3/32 drill bit works great with the M3 tap and with the step drill bit.

The allen screws are great when the resistors are screwed sideways. Last time I had a lot of trouble driving phillips screws without enough space for the screwdriver. Besides, this screws are stainless steel and they don't break inside the threads like the ones I used before. It's a real pain in the ass to use a screw extractor and I had to do that like 3 times... didn't imagine this fragile aluminum case was able to break cheap screws so easily.

Right now I'm on my second build and this time I'm making sure the case has enough ventilation. Drilling is by far the most laborious job from the build, but this time it's going faster and better than the last time. I'm using a printed pattern, a punch tool, a 3/32 drill bit, a step drill bit and 3-in-one oil for protecting the drill bits. The holes ended up better aligned than last time.


----------



## bikescene

Thanks @JohnH and @diego_cl . It’s been very helpful to get some advice. The video was the perfect visual aid. I just mounted a few of the smaller 25W resistors after absorbing the feedback. I’ll move onto the larger resistors when I get the time.


----------



## Gene Ballzz

fitz288 said:


> HNCD!
> But the AX is supposed to have greenbacks, were you surprised by the made in UK?





bikescene said:


> Thanks. I was trying to Google general tips on applying the paste and was having no luck. I'll need to make some cut outs that are the same sizes as these resistors and experiment with the amount.
> 
> I 've got the chassis all drilled out already. I need to double check my planned wiring before I start, since I'm deviating a bit. I will be adding an additional fixed 7dB stage since I will only be using the attenuator at home.




While your drawing is well done, it does not clearly illustrate three lugs on each switch. Can you confirm they are actually three lug, SPDT, ON-ON switches?

Thanks,
Gene


----------



## Marcomel79

Hi @JohnH, last night i finished your 16 Ohm attenuator. The coil hasnt arrived yet, so @Gene Ballzz suggested i use it without in the meantime. It reads 16Ohm on the dot!

I only had a chance to try it on my DSL5C and it sounds amazing. Put the amp on its sweet spot and now i have all these different levels of attenuations to play with! I cant imagine how much better it will be when the coil will be in place, im looking forward to it.

Here are some pictures of the build:






The lugs of the mini toggle switches were really tiny so i came up with this solution by cutting the sides of the lugs, allowing the buss wire to be soldered in place. This way there would be more space for the 18awg wire to be soldered, following Gene's layout. A bit extra work but i totally enjoyed the build.








A massive, massive thanks to you John for sharing your work and being so helpful answering whatever questions we throw at you. I take my hat off to you sir.
Couldnt be happier with the result!

Ill post an update when the coil arrives.

Marco


----------



## JohnH

Hi @Marcomel79 , that's a very nice build! And a clever way with the switches. I've also come to the view that those mini-switches are a good option, if you can deal with their small size.

The basic tone of what you have so far is probably very good. The coil may make it match a tad better. But mainly, listen for it's effect when you drive the power hard, you may get more dynamics and sizzle, varrying as the notes decay. It's a subtle effect, but if you get it, you'll know!


----------



## Marcomel79

JohnH said:


> Hi @Marcomel79 , that's a very nice build! And a clever way with the switches. I've also come to the view that those mini-switches are a good option, if you can deal with their small size.
> 
> The basic tone of what you have so far is probably very good. The coil may make it match a tad better. But mainly, listen for it's effect when you drive the power hard, you may get more dynamics and sizzle, varrying as the notes decay. It's a subtle effect, but if you get it, you'll know!


It sounds amazing already John! Like i said, i cant imagine its gonna be better with coil since it sounds SO good already without ! The DSL5C comes with a 5 to 0.5W power reduction switch which i never use because it sounds horrible. THIS is what Marshall should use!


----------



## bikescene

Gene Ballzz said:


> While your drawing is well done, it does not clearly illustrate three lugs on each switch. Can you confirm they are actually three lug, SPDT, ON-ON switches?
> 
> Thanks,
> Gene



I am using Carling 2FB53-73 SPDT On-On switches. I was originally only using the drawing to lay out components as they will be oriented. I quickly added some wiring afterwards, so I’ll have a visual reference to keep track of my wiring. I do see how it looks a bit incomplete without a good view of all 3 lugs of the switch.


----------



## Gene Ballzz

@Marcomel79
I think you're going to find that all tone is well retained at all attenuation settings with what you currently have. What will come back, when you're coil arrives, is the tactile feel and response, making it all seem even a bit more natural. There is little as pleasing as enjoying your amp at it's "Sweet Spot" without having family and friends angrily screaming *"TURN IT DOWN!" *It will be interesting to see how you finish up the external cosmetics!
Great Job On Another Succesful Build & Happy New Year!
Gene


----------



## Marcomel79

Gene Ballzz said:


> @Marcomel79
> I think you're going to find that all tone is well retained at all attenuation settings with what you currently have. What will come back, when you're coil arrives, is the tactile feel and response, making it all seem even a bit more natural. There is little as pleasing as enjoying your amp at it's "Sweet Spot" without having family and friends angrily screaming *"TURN IT DOWN!" *It will be interesting to see how you finish up the external cosmetics!
> Great Job On Another Succesful Build & Happy New Year!
> Gene


Gene, im beside myself! I tried it today with my superlead clone and its amazing! I can now have the high treble channel on 6/7 without the wife filing for divorce!
I was playing for an hour earlier and the box barely got warm. Also, i have not drilled the lid vents yet... i cannot recommend it enough. I might build one each for a couple friends of mine. 

Look forward to getting and installing the coil. Also thank you so much for all the help, this was a team effort!

Happy new year to you too!


----------



## Gene Ballzz

Marcomel79 said:


> Gene, im beside myself! I tried it today with my superlead clone and its amazing! I can now have the high treble channel on 6/7 without the wife filing for divorce!
> I was playing for an hour earlier and the box barely got warm. Also, i have not drilled the lid vents yet... i cannot recommend it enough. I might build one each for a couple friends of mine.
> 
> Look forward to getting and installing the coil. Also thank you so much for all the help, this was a team effort!
> 
> Happy new year to you too!



Yeah, these things are cheap and easy enough to build (especially after the first one) that I'm working towards having one to install in each and every amp I own. The physical shape and size may need a little tweaking for each amp, especially the head models. The most tedious part of the build is ventilation drilling! I'm trying to envision a more convenient solution! Maybe using two panels of perforated aluminum or stainless sheet and cutting the top and bottom to mount that perforated sheet to, from the inside? Something like this:

https://www.amazon.com/Stainless-Pe...ocphy=9030801&hvtargid=pla-669006187633&psc=1

OR THIS:

https://www.metalsdepot.com/aluminu...DCogjQhPTLU_F8QsOPP8D1GqeA8TB2IBoCsooQAvD_BwE

AND A GOGGLE SEARCH:

perforated aluminum sheet

The extruded aluminum boxes I've previously used, as well as the die-cast boxes from Hammond are fairly easily worked with a router, so you could easily cut a rounded corner, rectangular hole, router/round the edges over and mount the panel from the inside, on top and bottom. It would also provide a wide array of mounting holes for the components and have the screw heads recessed in those cutouts! It would still provide the heat sinking of attaching to the aluminum, as well as having massive amounts of flow through, convection ventilation! I'll try a probably crude drawing, when I get the chance!

Just Thinkin'
Gene


----------



## diego_cl

Gene Ballzz said:


> Yeah, these things are cheap and easy enough to build (especially after the first one) that I'm working towards having one to install in each and every amp I own. The physical shape and size may need a little tweaking for each amp, especially the head models. The most tedious part of the build is ventilation drilling! I'm trying to envision a more convenient solution! Maybe using two panels of perforated aluminum or stainless sheet and cutting the top and bottom to mount that perforated sheet to, from the inside? Something like this:
> 
> https://www.amazon.com/Stainless-Pe...ocphy=9030801&hvtargid=pla-669006187633&psc=1
> 
> OR THIS:
> 
> https://www.metalsdepot.com/aluminu...DCogjQhPTLU_F8QsOPP8D1GqeA8TB2IBoCsooQAvD_BwE
> 
> AND A GOGGLE SEARCH:
> 
> perforated aluminum sheet
> 
> The extruded aluminum boxes I've previously used, as well as the die-cast boxes from Hammond are fairly easily worked with a router, so you could easily cut a rounded cornoer, rectangular hole, router/round the edges over and mount the panel from the inside, on top and bottom. It would also provide a wide array of mounting holes for the components and have the screw heads recessed in those cutouts! It would still provide the heat sinking of attaching to the aluminum, as well as having massive amounts of flow through, convection ventilation! I'll try a probably crude drawing, when I get the chance!
> 
> Just Thinkin'
> Gene


I've been thinking about something similar, because drilling is too much work.

For the hammond cases I think one alternative could be to use a jigsaw and cut out material from the sides of the case. Another option could be using longer bolts with a spacer, so there's a gap between the case and the cover. But I think both options would look terrible. 

I want to build a 100W attenuator using a thick sheet of aluminum (6mm) as a heatsink. And I want to place that sheet inside a case made of bent steel and wood. I don't have a hammond case big enough for that job.


----------



## Amped eMind

This thread is pure gold, as is the heart of John, Gene and all the numerous and generous posters who contribute their knowledge and ideas to the benefit of the community!!!

I read the whole thread a while back before subscribing and probably will skim through again in the near future. I got the parts for an M3 build (go big or go home?) but have to get through a number of unrelated projects first so it is still a few weeks out. 

Here's wishing you all a wonderful New Year!


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## XTRXTR

@Gene Ballzz I have not built one of these, I have read much of this thread over time and I know the design is proved out. Its the final things like layout and materials that are still being tweaked and developed for a variety of specific use types. What about an actual heat sink design. I am not a metallurgist but I think the volume and density of the heat sink make a difference. Plainly I mean the more metal used the less air flow is required. I would have to double check some thermodynamics to verify that absolutely. If its true a design where the heat sink has enough volume that you could rout out the inside enough for the RL layout and then use a simple box as a cover that only houses the jacks and switches on the walls. Or just place the layout directly on the flat side of the heat sink and cover with the box. Hmm you may have to use blade connectors to remove or replace the box. Well I'm just throwing out ideas.

@JohnH This is a really cool device and I like that everyone can do their own layout to fit the need.


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## Gene Ballzz

@XTRXTR 
I'd love to see some layman's results of your thermodynaic research and I'll bet others would too! I'm gonna make a guess that it will actually come down to a balancing act. Meaning that as the heat sinking increases the need for ventilation decreases and vice/versa, but only to a certain point. The other part of that guess is that just one or the other will be lacking, to some degree? I think one important factor is to keep the unit as compact as possible, while avoiding the inconvenience of providing power for a fan. Keeping the price down is also a big consideration!

And yes indeed, the drilling, while fairly simple and easy is a time consuming, tedious, *ROYAL* *P.I.T.A*.

Keep On Thinkin'
Gene


----------



## XTRXTR

There are people that have made Play stations without a fan because they designed the chassis itself to be a heat sink. 

The resistor sizes are determined by the dissipation needed, so the increased size of the heat sink could offset the size of the resistor power rating a little. Anyway, just trying to throw in some ideas.


----------



## JohnH

I ran some tests on my one, soon after I built it. Mine is in a Hammond=style 1590 case (see post 1). I'm into the thermodynamics based on my day job, so it was another interesting aspect for me to over-analyse!

My test was to feed in dc , since that way I knew from voltage and resistance exactly what the power was, which came out at 7.2W continuous. It started to heat up and I made measurements over time with an infra-red thermometer. As heavy as it was, it took a long time to heat up. It went from 21C to 28C in 20 minutes. It was at 31C in about 40 minutes, and after 75 minutes got to about 33C. Id expect that with more power, it would take a similarly long tome o reach max temperature, (which would be higher of course)

So nothing happens fast with a thick Al case, and the power producing heat can be considered as an average over maybe 20 to 30 minutes.

Next, to understand the thermodynamics: There are 4 basic mechanisms in play:

*Conductivity*: This is the parameter defining heat flow through the solid case. Al is many times better than any steel and the thicker and more continuous the metal is, the easier the heat flows away from the heat source and spreads out through the case. The thermal paste bridges a potential bottle neck in the flow. (btw, id not be too keen on bolting the stage 1 resistors to a perforated sheet, they are not very thick and the flow of heat through the material is constrained by all the holes)

*Thermal capacity*: Determines how fast it heats up with a given heat input. The slow temperature response is due to the high thermal capacity of the relatively heavy case and resistors.

That leaves 2 other parameters which set the cooling, and they are *radiation* and *convection*. Both are dependent on the heated surface area. Convection is cooling due to the flow of fluid (air in this case) and the temperature difference between object and air. Radiation is electromagnetic radiation through space. It depends (very interestingly!) on the 4th power of absolute temperature, and radiative cooling is the balance between heat radiated out and that received in from the surroundings.

So, all of that has known formulae applicable, and i had a go at assessing it. For my test, based on what I could work out I figured that about 60% of the cooling was by radiation and 40% by convection.

We can use such a test to calibrate the maths. Ive never been able to get my attenuator really hot, running probably at around 20-30W on my 50W VM, but semi clean and not a continuous thrash. Lets say over a decent session, my average power is 25W, then Id predict about 60C, which is fine. But if you drive to 50W on average, Id predict 90C, which is too hot to touch. Hence possible use of fans for use with larger amps, which can increase convective cooling by a factor of 2.5 to 3.


----------



## Gene Ballzz

@JohnH
Thanks for sharing that! So my take away is that about 75°C is the absolute maximum we could tolerate seeing, over extended periods of use? And it would likely take close to an hour or so to get there, pounding steadily at 45 to 50 watts, with a well ventilated 3mm thick case? And if it levels off at that 70-75 degrees, we're probably OK? the wire I use is rated for 105°C and the switches are rated for 85°C. I'd guess that is pushing the limits a bit!
Just Spitballin'?
Gene


----------



## JohnH

I reckon that's about right. I don't think we are close to electrical problems there, but you don't want to get burnt! As a reference, a metal pot of boiling water at 100C is too hot to touch even momentarily. I reckon if you can touch the hottest parts for a couple of seconds, then its ok.


----------



## JohnH

i looked back again at my heat tests and worked a bit more excel....

I think for my tests, its more like 60% convective 40% radiative

Anyway, here are numbers for anyone who cares to pick through it:




At left is my test. The heat is not the same all over the case, so I found the most reasonable set of numbers as an average, noting that all the hot bits are on the case base, if I assumed the base (=top in my case) is at a constant temp and I ignore the lid area.

I also found online, that a standard for the max temp of hot things hat might get touched is 60C. So feeding that in, I can estimate the power that can be dissipated steady-state at 60C. Assuming a dark case, and 20C ambient temp, its about 600W per sqm of the case effective area. Add a fan to improve convection, then its then about 1100W per sqm.

Then I can estimate how much power from an amp will create that. And then what amp will provide that power? The lowest rows above take the average heat power and estimate the amp spec by dividing by a factor to do with how much of the time the amp is driving over say 30 minutes (I put 80%). Then, how hard is the amp driving during that time? from say 80% of max for cleanish/edge of breakup, to 120% for full powerstage overdrive.

So, with no fans, I get to about a 50W amp for a 1590 case if you play cleanish, or 30W if driven. About 50% more for a larger 1550 case, and add a fan and it suggests around a 100W amp driven, using a 1550 case. 

All of the above is speculative and guesswork and the parameters are just assumptions based on minimal data, though using some science. Really its just to explore the parameters.

Does anyone have any anecdotal descriptions of attenuator hottness vs amp power, case size and playing style?


----------



## donwagar

on the low end of the scale, mine is the 50W M2 16 ohm, can't remember the case size (it's at our rehearsal space so I can't measure it) but it's a tight fit, and I didn't have room for a lot of vent holes. Running my SV20 on 20W with almost everything dimed (normal channel on about 5 and high treble on about 8, tone stack all in the 8 range. At gigs, I do 3 one hour sets. It barely gets warm.

And it sounds fantastic.


----------



## diego_cl

M2 50W with chinese 1590D case (2.6mm thick) and resistors (1.9mm thick at its base), both from Aliexpress. Amp is a Marshall MA50C (50W).

It doesn't care if volume is at 6 or 12, but after 20 minutes it gets at least over 65°C if it's distorting (with or without booster pedals). Edge of break up makes the case hot, but not over 65°C.

I'm worried about the inductor enamel. I have to improve the vent holes and I want to put a dedicated heat sink over R1.

I don't have an IR thermometer. I'm just estimating the temperature by using my experience with measured hot water.


----------



## JohnH

Thanks @donwagar and @diego_cl both those descriptions fit in pretty well with my theoretical hack described above.


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## Gene Ballzz

Hey Folks,
I just thought of a stupidly simple upgrade for the cosmetic aestheics of our builds. Given the thickness of the enclosures we use, simply utilize flat head machine screws/bolts for mounting the resistors and counter sink them into the case for a much cleaner look!

Simply Screwin'
Gene


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## Marcomel79

Gene Ballzz said:


> Hey Folks,
> I just thought of a stupidly simple upgrade for the cosmetic aestheics of our builds. Given the thickness of the enclosures we use, simply utilize flat head machine screws/bolts for mounting the resistors and counter sink them into the case for a much cleaner look!
> 
> Simply Screwin'
> Gene
> View attachment 100126


Thats a very good idea Gene!! I purposely left my screws at the bottom so i wouldnt see them. But if it cant be helped and they are visible, thats a very neat idea.


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## Gene Ballzz

I have been drilling and tapping the holes in the boxes and then cutting the screws to length, but this is a quite tedious and often "iffy" process! Sometimes fitting a nut on the inside may be a little tough on some resistor brands, but it seems that the holes in the resistors could get redrilled to a slightly larger size, for wiggle room. Alternatively, if the holes are a consistent size, simply tap threads into the resistors, avoiding nuts altogether? I don't know why I didn't think of this sooner!

Of course, its kinda hard to avoid *"nuts"* like me, in this thread! 

Still Screwin'
Gene


----------



## Amped eMind

Speaking of methods of attachment of the resistors, do you guys see many cons on using rivets instead of screws and nuts? Rivets are easy and fast to install and provide a rather neat look (though maybe not as neat as flush screw heads as Gene proposes).

The resistors are not likely to need to be removed and the rivets could always be drilled out if the need really arose.


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## Gene Ballzz

@Amped eMind
I see no issue with pop rivets, as long as they actually clamp the resistor firmly to the case, instead of leaving it "standing off" a little bit, as I've sometimes seen rivets do! I would think it most appropriate to firmly clamp each resistor in place before applying the rivets and then make sure the rivet head falls out or gets knoked out, so that it doesn't end up inadvertently falling out to short out the circuitry?
Just Clampin'
Gene


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## Amped eMind

Yes, I'll make sure the resistors are held firmly in place and that the pop rivets are tight after they snap. Thanks!


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## diego_cl

Gene Ballzz said:


> I have been drilling and tapping the holes in the boxes and then cutting the screws to length, but this is a quite tedious and often "iffy" process! Sometimes fitting a nut on the inside may be a little tough on some resistor brands, but it seems that the holes in the resistors could get redrilled to a slightly larger size, for wiggle room. Alternatively, if the holes are a consistent size, simply tap threads into the resistors, avoiding nuts altogether? I don't know why I didn't think of this sooner!
> 
> Of course, its kinda hard to avoid *"nuts"* like me, in this thread!
> 
> Still Screwin'
> Gene



With my first build I tapped the threads on the case and used some very soft screws that were very easy to cut almost flush with a fret cutter.

With my current build I tapped the threads using a cordless drill and then fasten the resistors with 5mm SS allen screws. They where 0.4mm too long, but I didn't had to cut them, because the sanding prior to painting was enough. This method was very fast and better looking, but I kind of stripped a thread because of not doing it by hand. Luckily the other 19 threaded holes were so tight that it would be easier to break the allen wrench rather than stripping the threads.

I really like these M3 screws (M4 were too thick for the resistors)

Edit: pictures of the M3 screws on both sides.


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## Gene Ballzz

Its really inspiring to see all the tips, inventive suggestions and comments!  Lots of really great folks!  I think it would be fantastic if everyone who has built one (or more) of these great attenuators were to PayPal/send @JohnH a $10 or $20 bill for all his efforts! He certainly deserves it!  This is by far, one of the best DIY threads anywhere on the innernest!
I Love This Place!
Gene


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## bikescene

I have a question about the order of the stages in the M2 design. Is the input/output impedance relationship not maintained if the resistive 7dB and 14dB stages (2 and 3) are swapped in order and both are engaged? I came across some posts early on in the thread that alluded to electrical reasons for the resistive stages to be -7, -14, and -3.5 dB respectively.


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## Marcomel79

diego_cl said:


> (M4 were too thick for the resistors)



Except for my 100W 30Ohm one!


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## diego_cl

Does anyone know an easy and inexpensive way for labeling the enclosure?

This method looks cool, but only if you leave the enclosure unpainted:


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## Gene Ballzz

bikescene said:


> I have a question about the order of the stages in the M2 design. Is the input/output impedance relationship not maintained if the resistive 7dB and 14dB stages (2 and 3) are swapped in order and both are engaged? I came across some posts early on in the thread that alluded to electrical reasons for the resistive stages to be -7, -14, and -3.5 dB respectively.



I think @JohnH found some small theoretical, electrical differences, but he said he felt they were minimal enough to be of little or no consequence. Those of us who've swaped the order seem to have discovered no real world. operational anomalies! It is my "theoretic" observation that the order of stages and resistor number has evolved mostly to retain some semblance of consistancy from the earliest designs to the later versions, with their added stages, etc? I can only imagine what a *P.I.T.A.* it would be for John to go back and change the numbering of all his earlier posts to new numbering of components and order of stages!

I hope John will correct me, if I am mistaken?

Just Thankin,
Gene


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## JohnH

bikescene said:


> I have a question about the order of the stages in the M2 design. Is the input/output impedance relationship not maintained if the resistive 7dB and 14dB stages (2 and 3) are swapped in order and both are engaged? I came across some posts early on in the thread that alluded to electrical reasons for the resistive stages to be -7, -14, and -3.5 dB respectively.



Good question! have a 'brownie point' for diligent study !

Yes I did put that. One of the key design principles with this attenuator is to keep both input and output impedance as consistent as possible. Ideally this applies to each separate stage, so that it is always maintained no matter which ones are switched on. To make an attenuation stage with a particular attenuation and particular input and output impedance (different values), you need 3 resistors in either a 'Pi' or a 'T' arrangement. But with the values I used, based on measurements of my amp, at about -7db, the leading resistor becomes zero (T pattern), so there's just two. This was the basis of Stage 1 and 2, and it made sense to put them first. Following stages are never first in line, hence so long as they have the right output impedance, their input impedance can vary since only a small part of that difference reaches the amp. So they can also be just 2 resistors. 

Ive got analysis set up that will take any version of these designs, and crunch the numbers to run them with any speaker and amp assumption in all possible switch combinations. Based on this i found that there were actually negligible to zero real differences between having the switched stages -14, -7 and -3.5, as compared to -7, -14, -3.5

There's no real need to mount switches physically in the same order as they are wired, but if you do its neater. So the layouts on p 111 and 112 are on that basis.


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## Gene Ballzz

A small burr like this can work, if you have a REALLY steady hand. A less than steady hand produces "free hand" results, like in the bottom pic! Functional, but less than elegant! I've not yet tried a punch set. There are some who have made their own water slide decals and buried them under clear lacquer, or such. Others have simply printed on either self stick paper or used card stock and glued it on.
Another optin may be to go to a shop that makes trophies and such and see how much they want to do engrave it professionally? Given the rather low price of trophies, ............? Or a silk screen shop?

Just Labelin'?
Gene



https://www.amazon.com/Dremel-107-E...ocphy=9030801&hvtargid=pla-433019193279&psc=1


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## JohnH

Interesting discussion on bolts etc, and rivets!

Personally I quite like the industrial/steam-punk look of vent holes and shiny bolt heads on top. And I think I prefer if the case is 'upside down' at least for Hammond types, since they are slightly tapering, and it gets the hot bits up in the air. Bolts with nuts also allow max wiggle room for getting it to fit, with two clearance holes in play.

But pop-rivets are an interesting idea! We used them when I was studying my engineering, to build test structures for the lab. They do work well if you get it right. They need exactly the right hole in the right place. You can work from the outside, and if you start with good contact, then the fixing of the rivet adds positive compression. It was a good enough joint for Spitfires, Mustangs and Messersmitts to fly around without falling apart. Add some logos to your box to match your favorite airforce? Polished aluminium would look cool, but actually be the worst for heat transfer (OK for 20W studio people) . How about some curvaceous nose-art from a USAAF B17?, Or a big red star from a Soviet Mig15?


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## Amped eMind

Thanks @JohnH . As you note, the precision needed for the holes and the lack of wiggle room for the rivets is a concern but if I find it to be a problem, it won't be too late to switch to bolts.

It's funny that you mention the industrial/steampunk look as it is the main vibe I am going for (even though the rivets used then were hammered and not blind aka POP rivets).


----------



## Amped eMind

@diego_cl For the labeling, I will use a cheap set of metal letter punches and paint to fill the stamping and bring out the letters.

The spacing and imperfect alignment of this method is right between industrial and DIY so it may or may not fit in with the desired look and feel of your own project. It is definitely not a professional commercial look.


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## diego_cl

JohnH said:


> Interesting discussion on bolts etc, and rivets!
> 
> Polished aluminium would look cool, but actually be the worst for heat transfer (OK for 20W studio people) .


Could you please elaborate a bit more about that please??


----------



## sah

Whew, I started at the beginning of this thread and have finally caught up to present-day! Thanks @JohnH for working out these designs, and for sharing so much interesting info about how and why they work.

I'm putting together some parts orders to build a couple variations and experiment with some possibilities. I've been building tube amps (just for fun), but I can't really turn up even a 5W amp in my apartment, so I only get to hear them at their best when I can take them somewhere more volume-friendly — not great for learning how different circuit variations affect them at gig volumes. With a good 50W attenuator I could maybe even experiment with some Marshall-style circuits!

Anyway, the above is just to say that half the fun for me is experimenting and learning more about what's going on in a good-sounding amp. 

To start with, I'm thinking I'll build a switchable combined Resistive/M2/M3 so I can easily compare all the main variations with all other parts identical, for science. (So, one switch shorts around L2/C1, and a second switch shorts around L1.)

I also definitely want a good 4ohm build to use with my 5F1/5F2A Tweed Champ/Princeton style amp that has only a 4ohm output. I play that one a lot and I'm excited to be able to get it cooking at home. I haven't decided yet whether this should be a separate, dedicated build or just a 4ohm input on one of the 4/8/16 designs. Any thoughts on how much difference it makes? I'm worried a minimum of -10db may be a lot for a 5W amp, even in an apartment.


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## Gene Ballzz

sah said:


> Whew, I started at the beginning of this thread and have finally caught up to present-day! Thanks @JohnH for working out these designs, and for sharing so much interesting info about how and why they work.
> 
> I'm putting together some parts orders to build a couple variations and experiment with some possibilities. I've been building tube amps (just for fun), but I can't really turn up even a 5W amp in my apartment, so I only get to hear them at their best when I can take them somewhere more volume-friendly — not great for learning how different circuit variations affect them at gig volumes. With a good 50W attenuator I could maybe even experiment with some Marshall-style circuits!
> 
> Anyway, the above is just to say that half the fun for me is experimenting and learning more about what's going on in a good-sounding amp.
> 
> To start with, I'm thinking I'll build a switchable combined Resistive/M2/M3 so I can easily compare all the main variations with all other parts identical, for science. (So, one switch shorts around L2/C1, and a second switch shorts around L1.)
> 
> I also definitely want a good 4ohm build to use with my 5F1/5F2A Tweed Champ/Princeton style amp that has only a 4ohm output. I play that one a lot and I'm excited to be able to get it cooking at home. I haven't decided yet whether this should be a separate, dedicated build or just a 4ohm input on one of the 4/8/16 designs. Any thoughts on how much difference it makes? I'm worried a minimum of -10db may be a lot for a 5W amp, even in an apartment.



First> to the forum!
Next> These are cheap and easy enough to build, so why not build one for each variation you expect to encounter, or like me, simply build one to install in each amp?
Just Thinkin'
Gene


----------



## JohnH

diego_cl said:


> Could you please elaborate a bit more about that please??



Sure thing! The colour and finish affects the radiative part of the heat transfer, ie, that which would occur through a vacuum with no air involved. This part, in a system with no fans and of a dark colour, I reckon to be about 40% of the total. 

The key parameter is called emissivity. The best colour for maximising heat transfer is perfectly black, with an emissivity of 1.0. Most paints are in the range around 0.7. A bright white paint can be 0.4 or 0.5. But the least of all is a perfect mirror finish, which is 0.0. This does not absorb, nor emit any radiation. So the polished aluminium is not good!

This is why, if your Grandma polishes her silver teapot, her tea will stay hotter, because Grandma knows all about thermodynamics and the importance of emissivity.


----------



## JohnH

Hi @sah , welcome to our thread.

That sounds all good. Without exactly knowing what will suit you best, here are some comments:

If you attenuate a 5W amp by 10db, you have 0.5W. That's still a noisy amp to play in an apartment, and I reckon you may still use a few steps less than that at home. For myself, playing pleasantly loudly in the house, enough to spoil the TV in the next room, I think I'm using about 0.2W. Where there could be an issue though would be if you wanted just a bit less than the 5W, for a jam.

Next comment, I'm really happy to hear about your idea to switch out the reactive parts to explore the differences. That will be very interesting. The resonant part, if built, is potentially the biggest most expensive part. When you also add the extra front end to convert input impedances, if you do the one with one extra coil and two resistors, it's converting the impedance very well from high bass up, but doesn't convert the bass resonance, so you'd only have a half of that resonance in the 4ohm setting. All safe and will sound fine , but just FYI. 

The resonance circuit, from testing to date, doesn't seem to make much difference when using guitar speaker, which develops its own resonance in this design. But it may make a difference if run as a load box for direct recording.

Apart from that 4 ohm build, do you have other amps in mind that only have one output tap ohms? Ie, if you built an 8, or a 16, would that potentislly deal with all your other amps? or do you need 8 and 16 inputs in some way?


----------



## sah

Aside from the 4 ohm, all of my other amps at the moment have an 8 ohm output. The one borderline case is my AC15, which normally uses a 16 ohm greenback. I could use its 8 ohm tap and convert to 16 at the attenuator output, but it would add some variables to any tests I might want to do with that one. (Or I could use an external 8 ohm speaker for testing, it just won't quite make the sound I'm used to from that amp.)

I imagine eventually I'll want to take Gene's advice and build a few different units tailored to specific purposes, but for exploration I think something based around 8 ohms with a lot of switchable options is going to be the quickest and cheapest way to go.


----------



## bikescene

Thanks @Gene Ballzz and @JohnH for answering my questions yesterday. I was able to finish mounting all the components and wiring it up. I have the switchable stages set up as -14, -7, and -3.5 dB. I added an additional fixed -7dB stage before the switchable stages. I only got to play through the attenuator briefly with a 22W amp last night, but the volume reduction was a very welcome addition. I will try my 50W Plexi clone for a bit tonight.

I just saw the post about aluminum and emissivity. I'll look into a black painted enclosure if I do another build. I only get to play my amps for about 20-30 minutes at a time, so I am hoping that I am not stressing the big resistors too bad.


----------



## JohnH

bikescene said:


> Thanks @Gene Ballzz and @JohnH for answering my questions yesterday. I was able to finish mounting all the components and wiring it up. I have the switchable stages set up as -14, -7, and -3.5 dB. I added an additional fixed -7dB stage before the switchable stages. I only got to play through the attenuator briefly with a 22W amp last night, but the volume reduction was a very welcome addition. I will try my 50W Plexi clone for a bit tonight.
> 
> I just saw the post about aluminum and emissivity. I'll look into a black painted enclosure if I do another build. I only get to play my amps for about 20-30 minutes at a time, so I am hoping that I am not stressing the big resistors too bad.



It looks great, thanks for posting. Very nice wiring! Are you finding you need that extra stage? On heat, if you can touch R1 for a couple of seconds then it's not too hot, and I wouldn't think it would have any problems with 22W anyway.

My ramblings about radiation relate to a range of colours from deep black 1.0 to fully polished mirror-like 0.0. Dull or raw metal finishes are quite high, like typical paints. Not as good as black for emitting heat, but not so far behind.


----------



## Gene Ballzz

@JohnH 
I enjoy your *"ramblings!"*  What is your guesstimate of the radiation properties of the satin black anodized, extruded aluminum cases I've used in the past? At least I think they are are extruded and anodized? I guess I'm wondering if lack of good radiation qualities might be part of the source of why they heat up so much with my 50 watt amp?

https://www.ebay.com/itm/142615103868

Just Curious,
Gene


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## JohnH

Gene Ballzz said:


> @JohnH
> I enjoy your *"ramblings!"*  What is your guesstimate of the radiation properties of the satin black anodized, extruded aluminum cases I've used in the past? At least I think they are are extruded and anodized? I guess I'm wondering if lack of good radiation qualities might be part of the source of why they heat up so much with my 50 watt amp?
> 
> https://www.ebay.com/itm/142615103868
> 
> Just Curious,
> Gene



Hi Gene, that black finish looks black enough, and the surface area of the box is not far off that of a Hammond 1590. When you run that 50W amp, what's your opinion of how far into power-stage overdrive its going? ie, estimate of real power coming out while playing? 

Another thing i see is that it looks to be made from separate panels, at least for top and sides. If the main parts are on the top (or base) I reckon there may be imperfect thermal coupling from top to sides, hence the sides are less effective and they provide a lot of added area. Hammond cases that I use are one piece top/sides plus a base. When it heats up, can you feel that the heat is evenly distributed? or just mainly on one surface?


----------



## Gene Ballzz

So @JohnH 
The pics in post #178/page #9 and post #1484/page #75 give a clearer view of how the top half of the enclosure has a rail with an "innie" on one side and a rail with an "outie" on the other side. These fit into opposite "innies/outies" on the bottom half of the enclosure. I'm wondering if some thermal, heat sink paste on those "rails" might improve the thermal conductivity between the two halves, in any appreciable manner?
Just Askin'?
Gene


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## JohnH

yes i see what you mean now. Maybe a bit but maybe not so much! Depends how tight the gaps are and also what kind of power is being developed.

Ive just being running another measured heat test:




What I did, with my 50W VM, was to loop a continuous heavily distorted riff, turned up to 10, ran it through the attenuator, but with no speaker just as a load. No pauses, set breaks or quiet intros! That is the orange line above and it records the max temperature on the case top above R1. No speaker noise, but the OT was humming and rattling!. In 15 minutes it had risen from 24C to 55C. Then I turned down to 6, which is where it sounds best and left it to settle down for the rest of 60 minutes total. It cooled down and settled at 39C max. 

On the graph, the blue line was the previous test with a constant 7.2W fed in, which gained 12C over time. I used that factored and shifted to judge where the full volume test was headed had it been left to run, which looks to be somewhere north of 80C! - see the grey line.

Back to todays test, once it had settled after 60 minutes, I took a range of spot temps around and under the case. Everywhere was warm, but not at the max temp , usually a few C less. I doubt much heat is lost directly through the base, although holes in the base are good for venting and hence cooling other surfaces.

Looking at the various surfaces and the relative temperatures they achieved, I figured out that the cooling of the whole thing, with its various temperatures, is about 60% to 65% of what it would be if all the surfaces were at the same max temperature and all in free air. 

Back figuring all that, in a practical case of heavily overdriving an amp but, in a real context with gaps and breaks ect, and targeting 60-65C max It looks like a 1590 case (170x120x55) will handle a 30-35W amp (amp spec based on max RMS). Gene - that eBay case is about the same parameters, setting aside its different joining. 1550 222 x146x51 would do similar for a 45-50W amp. 

Cleanish, more varied playing, maybe all x 1.5 on amp spec


----------



## Gene Ballzz

@JohnH 
That tracks pretty well, considering my JTM30 is 36-ish watts clean and tops out at nearly 53 watts at full tilt! Given that this amp only really gives up it's goods with channel and master volumes maxed, and in any typical song, I'll average 30% to 40% cranking at that 53 watts and the rest of the song at likely 40 to 45 watts for "edge of breakup" with an occasional clean-ish spot, here & there. I'll give my infrared thermometer a try and see what it tells me. I'm going to guess at my normal use, it'll be at that 55°C or a little above. If it doesn't creep up past 60°C, I'm likely pretty OK?
Thanks For All You Do! 
Gene


----------



## JohnH

Great! I'm glad you've got one of those! I got one for a uni project where we were measuring solar heating of steelwork.

Yes that will be a great test, and I agree that if you stay below 60 C then all is good. Mine got to 55C before if wound it back. At that time, I could comfortably touch for 2-3 seconds, but not more.

For yours, I reckon the definitive result is at the end of a long set, since they take a while to heat up.


----------



## Barnsley Boy

@JohnH & @Gene Ballzz,

Just switching over from the other thread to this one! Many thanks for the swift response to my queries. I'll continue reading through the rest of this thread while I am waiting for my parts to arrive. There is some really interesting stuff on here - if Tim Berners-Lee had invented the interweb just for this forum and particularly this thread, he could rest easy!! 

In terms of the case temperature when you are really pushing the amp, I wonder if fitting a simple grill thermometer to the front panel (or top if layout suits) would work? Something like this:






If it works, it would give at least an indication of how hot things are getting, as a warning to not touch. Perhaps a physical connection to the casing of R1 as this is going to get the hottest? My initial searches brought me to something like this:






But if you're pushing 350 degrees, I'm guessing that your amp is probably toast, and the fire brigade would be on the way!

I'm searching around for a suitable enclosure which incorporates some form of in built ventilation and heat sink. There are some really cool looking ones from China which are pretty pricey, but if you consider how much you can pay for a known brand decent reactive attenuator, then maybe adding the cost of a decent box is worth it.

Some truly great work here - BIG RESPECT!


----------



## brad Messier

Slowly gathering all the components for my build, working on squeezing everything into an enclosure. Any suggestions on mounting the coil? I have seen to use nylon or stainless bolts and to avoid direct contact with the case. Do I need to keep the coil isolated from the other parts as well? I have seen other builders using plastics to hold the coil in place, no danger of this coil getting hot? Thanks!

Brad


----------



## AtomicRob

I'm building a 16 ohm "100W" version of the M2 circuit and I decided to set it up in LTSpice and look at the dissipation of the resistors. I set the power level at ~170W RMS output of a superlead. Of course it depends on which stages are switched in, but the list below was what I found worst-case across all the possible switch positions. (Edit again! I refined my model a bit and got better numbers. Total adds up to more than 170 because this is the worst case for all states...) This was at 100hz. Dissipation is somewhat lower at higher frequencies but not much.

 Pmax(W)
R1 90 
R2a 30 
R2b 36 
R3 16 
R4 11 
R5 21 
R6 3.3 
R7 8 
R8 12 

The resistor power ratings depend a lot on the heat sink and how hot you want it to get. You could just double the ratings of the 50W version and be ok, but R4, R6, R7, and maybe R8 would still be ok with 25W. And R5 is kind of borderline at 50W, 75W would be better.

Luckily I had an awesome heat sink chassis in my junk box - it's from a big old Targa car amplifier with fins all on the top and sides, probably weighs 5lbs. Shown below you can sort of see one of the 100w resistors mounted there for scale.

(Pro tip, old car amps usually have great heat sinks for projects like this...)


----------



## JohnH

AtomicRob said:


> I'm building a 16 ohm "100W" version of the M2 circuit and I decided to set it up in LTSpice and look at the dissipation of the resistors. I set the power level at ~170W RMS output of a superlead. Of course it depends on which stages are switched in, but the list below was what I found worst-case across all the possible switch positions. (Edit, actually I need to double check whether these are RMS or peak values... these numbers might be ~1.4x too high, can't remember how I had configured it...)
> R1: 134W
> R2a: 75W
> R2b: 61W
> R3: 28W
> R4: 25W
> R5: 33W (with only stage 1 and 3)
> R6: 7W (with only stage 1 and 3)
> R7: 14W (with only stage 1 and 4)
> R8: 38W (with only stage 1 and 4)
> 
> I'm also considering whether stage 2 needs to be switched. If it's always on, the dissipation on R5,6,7,8 is much lower, like 8W max and you can get away with smaller parts.
> 
> So my thought now is to use two 100W resistors in series
> for R1, 100W for R2a/R2b, 50W for R3/4, and 25W for the rest, with stage 2 always on. This doesn't have a huge safety margin but I'm hoping that really good heat sinks, plus the fact that guitar is not like a constant 170WRMS signal will mean that it won't get as hot as predicted by the Arcol datasheets (around 60C or 140F.)
> 
> Luckily I had an awesome heat sink chassis in my junk box - it's from a big old Targa car amplifier with fins all on the top and sides, probably weighs 5lbs or more. Shown below you can sort of see one of the 100w resistors mounted there for scale.
> 
> (Pro tip, old car amps usually have great heat sinks for projects like this...)
> 
> View attachment 100276



Thanks for posting! That's quite a best if it's putting out 170W. I reckon there is that 1/1.4 factor to take out. Otherwise there's more heat in stage 1 than the total amp power.

In post 1, I also have a table of resistor power numbers. I based mine on nominally 50W, then allowed a factor x3. But, taking R1 for example, your value x1/1.4 matches with mine x170/50. R2 from that original design, split to become R2A and R2B, for about the same total power.

Those heatsinks look b@d@$$!, but have you considered using a fan with it?


----------



## JohnH

brad Messier said:


> Slowly gathering all the components for my build, working on squeezing everything into an enclosure. Any suggestions on mounting the coil? I have seen to use nylon or stainless bolts and to avoid direct contact with the case. Do I need to keep the coil isolated from the other parts as well? I have seen other builders using plastics to hold the coil in place, no danger of this coil getting hot? Thanks!
> 
> Brad



Yes, best to use stainless or nylon bolts, or zip ties yo avoid messing with the inductor value. A spacer off the case, (to avoid eddy losses) could be plastic or wood. My current thinking (this week) is to fix the switches jacks and resistors to the base of the case, but placed upside down with the resistors under the top. Then put the coil on the lid which is now the base. Needs longer leads but It means it's fixings aren't seen and it's not on the top panel in case it gets hot.


----------



## sah

While I wait for parts to trickle in, I've started trying to measure the output impedance of some amps, in hopes of someday better understanding how it affects attenuators.

First up, I measured my 5F1 champ clone, which has a handy switch to disconnect the stock negative feedback loop. All measurements were done with a 1kHz signal and a clean volume level that wasn't touched between measurements.

In the stock config, with negative feedback:
4 ohm load: 0.27VAC
8 ohm load: 0.34VAC
If I did my algebra right, that works out to a 3 ohm output impedance.

With negative feedback disconnected:
4 ohm load: 0.51VAC
8 ohm load: 0.86VAC
Which I think makes a 17 ohm output impedance.

Lower values than what we've seen from other amps. I'm guessing that may have to do with the 4 ohm output tap. Does my math look right?


----------



## JohnH

Barnsley Boy said:


> @JohnH & @Gene Ballzz,
> 
> Just switching over from the other thread to this one! Many thanks for the swift response to my queries. I'll continue reading through the rest of this thread while I am waiting for my parts to arrive. There is some really interesting stuff on here - if Tim Berners-Lee had invented the interweb just for this forum and particularly this thread, he could rest easy!!
> 
> In terms of the case temperature when you are really pushing the amp, I wonder if fitting a simple grill thermometer to the front panel (or top if layout suits) would work? Something like this:
> 
> 
> 
> 
> 
> 
> If it works, it would give at least an indication of how hot things are getting, as a warning to not touch. Perhaps a physical connection to the casing of R1 as this is going to get the hottest? My initial searches brought me to something like this:
> 
> 
> 
> 
> 
> 
> But if you're pushing 350 degrees, I'm guessing that your amp is probably toast, and the fire brigade would be on the way!
> 
> I'm searching around for a suitable enclosure which incorporates some form of in built ventilation and heat sink. There are some really cool looking ones from China which are pretty pricey, but if you consider how much you can pay for a known brand decent reactive attenuator, then maybe adding the cost of a decent box is worth it.
> 
> Some truly great work here - BIG RESPECT!



I hadn't thought of that gadget on our design!

I think at 50W, it's on the boundary between needing a fan of not, dependent how hard it's driven. Or, a big slab of heatsink across the top surface could help, if the resistors are directly under and with good contact and thermal grease. Maybe use countersunk screws from resistors to case, and/or drill the underside of the heatsink a few mm to make sure no protrufing bolt-heads prevent contact from heatsink to top. 

A heatsink with fins and its own thick base will add thermal mass, spread heat out and also dissipate it.


----------



## JohnH

sah said:


> While I wait for parts to trickle in, I've started trying to measure the output impedance of some amps, in hopes of someday better understanding how it affects attenuators.
> 
> First up, I measured my 5F1 champ clone, which has a handy switch to disconnect the stock negative feedback loop. All measurements were done with a 1kHz signal and a clean volume level that wasn't touched between measurements.
> 
> In the stock config, with negative feedback:
> 4 ohm load: 0.27VAC
> 8 ohm load: 0.34VAC
> If I did my algebra right, that works out to a 3 ohm output impedance.
> 
> With negative feedback disconnected:
> 4 ohm load: 0.51VAC
> 8 ohm load: 0.86VAC
> Which I think makes a 17 ohm output impedance.
> 
> Lower values than what we've seen from other amps. I'm guessing that may have to do with the 4 ohm output tap. Does my math look right?



Yes I agree with those numbers. My VM measures about 20 ohm that way, but its an 8 ohm tap and with kt66 tubes. One of the mysteries of this design is that the tonal response adapts to these changes over most of the tonal range. You can see it happening in the maths, but its hard to explain. But it means that this attenuator sounds right for a wide range of very different amps.


----------



## Gene Ballzz

@Barnsley Boy 
In a different "Family Group Photo" thread you posted a stripped down version diagram of an 8Ω, M2.

https://www.marshallforum.com/threads/nad-family-group-photo.124511/

You deleted not only the 16 ohm Out 3, but as well, the extra parallel speaker out jack. I highly recommend keeping that extra parallel jack, especially given your allusion to always using two cabinets with your amp/rig. This would allow you to plug your speakers directly to the attenuator, rather than depending on the sometimes wonky parallel or thru jacks of some speaker cabinets. Just a friendly suggestion, as it's your time, your money, your rig and ultimately, your choice!

I look forward to seeing your build.

Just Sayin'
Gene


----------



## AtomicRob

JohnH said:


> Thanks for posting! That's quite a best if it's putting out 170W. I reckon there is that 1/1.4 factor to take out. Otherwise there's more heat in stage 1 than the total amp power.
> 
> In post 1, I also have a table of resistor power numbers. I based mine on nominally 50W, then allowed a factor x3. But, taking R1 for example, your value x1/1.4 matches with mine x170/50. R2 from that original design, split to become R2A and R2B, for about the same total power.
> 
> Those heatsinks look b@d@$$!, but have you considered using a fan with it?


John, thanks I fixed the numbers in my post - it was actually off by 1/SQRT(2) on both voltage and current so off by a factor of 2 overall. Now when I add up the results with all stages on, it's dissipating less than 170W total, as would be expected. The 170W RMS is based on what Marshall claims in the spec sheets based on 10% distortion.

I do have various small fans which would fit that case but I'm hoping it won't be necessary - it's such a massive chunk of heat sink.


----------



## Barnsley Boy

Gene Ballzz said:


> @Barnsley Boy
> In a different "Family Group Photo" thread you posted a stripped down version diagram of an 8Ω, M2.
> 
> https://www.marshallforum.com/threads/nad-family-group-photo.124511/
> 
> You deleted not only the 16 ohm Out 3, but as well, the extra parallel speaker out jack. I highly recommend keeping that extra parallel jack, especially given your allusion to always using two cabinets with your amp/rig. This would allow you to plug your speakers directly to the attenuator, rather than depending on the sometimes wonky parallel or thru jacks of some speaker cabinets. Just a friendly suggestion, as it's your time, your money, your rig and ultimately, your choice!
> 
> I look forward to seeing your build.
> 
> Just Sayin'
> Gene



Gene,

The JCM-900 that I have is a combo, so I will be putting the attenuator directly between the amp out and the speakers. I'm unlikely to run another cab - sadly the arrival of a 4 x 12 cab might be a step too far for "she who must be obeyed"! But in hindsight, and for the price of another jack, I may as well include the additional one. Resistors arrived today, the Inductor is being wound at the moment and will be here middle of next week once the coil locking compound has set and it has been tested. I'm just modelling the build in sketchup to make sure that it will look neat and tidy in the enclosure, and everything fits. I've found some pre-modelled power resistors on warehouse, which I've scaled to the correct sizes, and I'm in the midst of re-creating the Hammond 1455T2201BK which is looking favourite at the moment to house it all. The CAD drawing has been most helpful in that respect. The ventilation holes are going to be laborious, but may be a good excuse for me to procure a drill press and a stepped drill bit! If I can get away with pop riveting the resistors into place, I may go down that route. I've found a digital high temperature thermometer which I am really tempted to incorporate into the front panel so I can check that it's the right temperature to toast my crumpets!

I'm well excited about getting started on this, but planning is everything!


----------



## Nortally

I have two combo amps, 50 watt with a 16 ohm speaker & 22 watt with an 8 ohm speaker. Would I need to build two attenuators, or can I somehow modify the M2 spec to take either amp input? I'm OK keeping the master volumes at 4 or below.


----------



## JohnH

Nortally said:


> I have two combo amps, 50 watt with a 16 ohm speaker & 22 watt with an 8 ohm speaker. Would I need to build two attenuators, or can I somehow modify the M2 spec to take either amp input? I'm OK keeping the master volumes at 4 or below.



Hi Nortally, welcome to our thread!

The base design of the attenuators can deal with different speaker ohms so long as they match the amp ohms. What ohm outputs do your amps have? Then we can discuss the options to best suit your use.


----------



## Nortally

JohnH said:


> Hi Nortally, welcome to our thread!
> 
> The base design of the attenuators can deal with different speaker ohms so long as they match the amp ohms. What ohm outputs do your amps have? Then we can discuss the options to best suit your use.


The 50 W combo has 1 speaker jack labeled 16 ohms and 2 labeled 8 ohms. So I'm guessing that it "likes" to deliver 16 ohms but will deliver 8 ohms if I use a single 8 ohm output. If true, I've answered my own question ;-)

The 22 W combo is the same but one 8 ohm and two 4 ohm speaker outs.

FYI, the 50 W amp is a Silkyn Super 50, the 22 W amp is a Mesa Boogie Studio .22. I'm playing the Mesa through a friend's Boss Waza Tube Expander right now & just loving it, want to have a substitute lined up for when that loan expires. I totally recommend the Tube Expander, but it's too pricey for my needs.


----------



## Gene Ballzz

@Nortally 

Sounds like the unit most suited to your needs would be an 8Ω version of the M2, including the extra Out 3 for the 16 ohm speaker.

FWIW, the Silkyn Super 50 seems that can run one 16 ohm speaker/cabinet from the 16 ohm jack or either one 8 ohm speaker/cab or two 16 ohm speakers/cabs from those two 8 ohm jacks.

If you build the 8 ohm M2, you should run it from one of the 8 ohm jacks and plug your 16 ohm speaker out of the Out 3 jack of the attenuator. Should you chose to use two 16 ohm speakers/cabs, run them out of the parallel Out 1 & Out 2. The impedance of the speaker you use won' dramtically affect the impedance of the input of the attenuator, as that impedance is most set by the first "always on" attenuation stage.

Looking Forward To Your Build!
Gene


----------



## JohnH

Yes, I agree with Gene. An 8 Ohm M2 with all the Outs.

The 16 Ohm output 3, adds a tonal adjustment to make 16 Ohm speakers sound right in the 8 Ohm attenuator. I use this too on my build. Its actually perfectly safe to use the 16 speaker in Out 1 or 2 though - it may be a tad more mid focussed with less treble, but you can try it either way.

Your amps sound to be great units. And if you can keep borrowing that Boss TAE for a bit longer, it sets you up for the ultimate David vs Goliath face off: M2 vs TAE. TAE has incredible versatility, but, how do they compare just for attenuating when using the guitar speaker? It would be very interesting to hear about your considered opinion. Im sure there's some advantage to be had by spending 10x the $!


----------



## bikescene

brad Messier said:


> Slowly gathering all the components for my build, working on squeezing everything into an enclosure. Any suggestions on mounting the coil? I have seen to use nylon or stainless bolts and to avoid direct contact with the case. Do I need to keep the coil isolated from the other parts as well? I have seen other builders using plastics to hold the coil in place, no danger of this coil getting hot? Thanks!
> 
> Brad



I spent a little time trying to figure out how to mount the inductor too.

Some inductors already come with non-metallic supports. I’ve seen some in spindles. Some others already come with zip ties from the factory. The Dayton brand inductor I used came with two zip ties. I added two more zip ties so that I had one every 90 degrees, and the inductor was nicely offset from the enclosure.

I bought a 2” acrylic disk off eBay and drilled a hole in the middle to fit a nylon bolt.


----------



## Gene Ballzz

You can make your own acrylic discs with a piece of plexiglas/lexan or similar and a hole saw of the diameter you need. They are available in all sizes, at multiple qualities and price points and even at Harbor Freght!

https://www.harborfreight.com/1-in-2-12-in-carbon-steel-hole-saw-set-10-pc-57523.html

After you make it, you have the benefit of a hole already drilled in the center. Put one under and one above the coil and use a nylon, stainless steel or aluminum bolt to secure it. I'm not really sold on the stainless stuff I find though, as it still appears to be magnetic!

Just Cuttin'
Gene


----------



## sah

JohnH said:


> One of the mysteries of this design is that the tonal response adapts to these changes over most of the tonal range. You can see it happening in the maths, but its hard to explain. But it means that this attenuator sounds right for a wide range of very different amps.



Do you know whether this effect depends on the speaker at all, or if it's caused entirely by L1?

Beyond making the design adaptable to a variety of amps, it may also be an important effect for amps with no negative feedback (or, with presence turned up). Doing some more experiments, it looks like on my Champ the output impedance rises with frequency when NFB is disconnected (I tried 440hz, 1kHz, and 2kHz — it rises from about 10 to 17 to 22 ohms across that range). When NFB is engaged it's fairly constant across signal frequencies, which makes sense since the NFB adjusts the signal as needed to get the output voltage where it's supposed to be.

Another effect I noticed, which I think you've mentioned, is that output impedance seems to drop as I drive the amp into distortion.


----------



## Barnsley Boy

Gene Ballzz said:


> Well folks, here is a nice layout for a 16 ohm, M2 that minimizes the length of all wire runs except two. Being 16 ohm, it doesn't need an "Out 3." Simply refer to the diagram for the M2 that @JohnH so graciously provided in post #1 of this monster thread, for component values. This layout was created for use with a Hammond 1590D (add BK to the end of the part number for black) box, or larger. Click the link below for dimensions:
> 
> https://www.hammfg.com/files/parts/pdf/1590DBK.pdf
> 
> It is intended for all components to be in the box and use the removeable lid as the top. You can flip it over, if you like and swap the R7 & R8 resistors with R5 & R6. It uses the logic of having the lowest attenuation switch on the left and the highest on the right. It will also work well for the 8ohm M2, as that coil should be smaller, although you might feel the need to add the extra "Out 3" and resistors for use with a 16 ohm speaker. There should be plenty of room for those resistors, if you put them or R1 on the wall.
> 
> How the switches function can be a bit confusing, but if you look just at the -14db switch and it's associated R5 & R6 resistors and how the lower lugs of the switch pass the signal on to the next switch.
> 
> R2A & R2B are mounted to the walls of the box and with my coil source, the 1.8mh unit is 54mm in diameter. If your chosen coil is larger, you could mount R1 to the wall also and then maybe move R1A to the floor of the box. It will all depend on the physical size of your coil.
> 
> It was drawn to scale, considering all Ohmite/Arcol resitors, but the ones many are getting from Chinesiawanoreanam take up even a little less real estate.
> 
> And by the way, if not obvious, the switches are intended to be mounted in the wall, which actually gives a bit more "wiggle" room than the drawing shows.
> 
> View attachment 99623
> 
> 
> Happy Attenuatin'
> Gene




@Gene Ballzz 

A slight tweak to your layout Gene. I took @JohnH 's M2 version, re-ordered the stages so that the switching is in ascending order as per your scale drawing, and annotated for an 8 ohm variant. I was just moving the reactive stage around in my sketchup model to see which orientation would best suit my case, and I spotted what I think is a slight error. I think that the red wire from R2A and R3A should go to the -3.5 db switch, and the black output wire from the -14 db should go to the output jacks. I have drawn what I believe is the correct version:




Do you concur? or am I being a numpty?


----------



## JohnH

sah said:


> Do you know whether this effect depends on the speaker at all, or if it's caused entirely by L1?
> 
> Beyond making the design adaptable to a variety of amps, it may also be an important effect for amps with no negative feedback (or, with presence turned up). Doing some more experiments, it looks like on my Champ the output impedance rises with frequency when NFB is disconnected (I tried 440hz, 1kHz, and 2kHz — it rises from about 10 to 17 to 22 ohms across that range). When NFB is engaged it's fairly constant across signal frequencies, which makes sense since the NFB adjusts the signal as needed to get the output voltage where it's supposed to be.
> 
> Another effect I noticed, which I think you've mentioned, is that output impedance seems to drop as I drive the amp into distortion.



All true, I get the same observations. My two valve amps are a VM2266 which has NFB with a presence control and KT66's, and I have a DSL401 with EL84's and no NFB. The output impedance deduced for that one is very high, 50 or 60 Ohms! The maths at that point gets difficult to accurately calculate since it's very sensitive to the readings.

But, The attenuator deals with all of that. Although my main amp (VM) has output z of about 20 ohm at most settings, and that's my main reference for design, I check the results at much higher and lower (like 5 to 50 from an 8ohm tap). 

Once the amp is driven hard and saturating, those parts of its cycle are equivalent to much lower impedance. I don't model a full output stage though, but making these assumptions based on the tests seems to give results that lead to a reasonable understanding and a good design.


----------



## Gene Ballzz

Barnsley Boy said:


> @Gene Ballzz
> 
> A slight tweak to your layout Gene. I took @JohnH 's M2 version, re-ordered the stages so that the switching is in ascending order as per your scale drawing, and annotated for an 8 ohm variant. I was just moving the reactive stage around in my sketchup model to see which orientation would best suit my case, and I spotted what I think is a slight error. I think that the red wire from R2A and R3A should go to the -3.5 db switch, and the black output wire from the -14 db should go to the output jacks. I have drawn what I believe is the correct version:
> 
> View attachment 100373
> 
> 
> Do you concur? or am I being a numpty?





JohnH said:


> All true, I get the same observations. My two valve amps are a VM2266 which has NFB with a presence control and KT66's, and I have a DSL401 with EL84's and no NFB. The output impedance deduced for that one is very high, 50 or 60 Ohms! The maths at that point gets difficult to accurately calculate since it's very sensitive to the readings.
> 
> But, The attenuator deals with all of that. Although my main amp (VM) has output z of about 20 ohm at most settings, and that's my main reference for design, I check the results at much higher and lower (like 5 to 50 from an 8ohm tap).
> 
> Once the amp is driven hard and saturating, those parts of its cycle are equivalent to much lower impedance. I don't model a full output stage though, but making these assumptions based on the tests seems to give results that lead to a reasonable understanding and a good design.



To add to what @JohnH said above, he mentioned earlier that the wiring order of the different stages presents little or no real consequences. As long as the reactive stage is always first and each stage's resistor pairs are kept together!

Happy Building!
Gene


----------



## Barnsley Boy

Gene Ballzz said:


> To add to what @JohnH said above, he mentioned earlier that the wiring order of the different stages presents little or no real consequences. As long as the reactive stage is always first and each stage's resistor pairs are kept together!
> 
> Happy Building!
> Gene



Ok, that's cool, I'll follow your layout then, as it seems to be the most frugal in terms of wire length!

I'm just having a fiddle with some perforated 1.5mm aluminium sheet to see what it looks like to replace the enclosure lid:




10mm holes @ 13mm pitch - plenty of ventilation, but the hole layout looks messy at the edges. I may have to revert back to my original idea - drilling the holes myself.

.... Am I overthinking this? or have I got too much time on my hands!!


----------



## colchar

Gene Ballzz said:


> @JohnH
> I find it so hard to believe that folks actually come here and then end up buying a Bugera PS1 or Weber MiniMass! Of course, than they stae as a supposed fact that attenuators simply suck tone! Oh well, you can lead a horse to water, but.......
> Great Stuff!
> Gene




Many of us are incapable of building the various units described in this thread. If someone here was to make some and sell them I'm sure there would be buyers. But barring that, people will turn to commercially available units.


----------



## Gene Ballzz

colchar said:


> Many of us are incapable of building the various units described in this thread. If someone here was to make some and sell them I'm sure there would be buyers. But barring that, people will turn to commercially available units.



I've considered offering an "assembly service" but the legal liabilities (if used improperly) and possible/probable complaints about cosmetics has steered me clear of that idea. I'd build one for a friend, with possibly the understanding that it might still look somewhat "home made" along with a promise to understand how to use it properly and do so! I'd estimate the time and labor involved to be worth about +/- $150.

On the other hand myself and others seem to be demonstrating how really simple it is to build, in the hopes of encouraging others to learn the fairly simple skills required. It's really just the use of some simple hand tools, a drill and a soldering iron, not rocket surgery or brain science! Having a simple multi-meter for testing is also important! Even if soldering and using a ruler seems like voo-doo to some folks, almost everyone has a friend who has the skills!

Just My Take On It,
Gene


----------



## JohnH

Gene Ballzz said:


> I've considered offering an "assembly service" but the legal liabilities (if used improperly) and possible/probable complaints about cosmetics has steered me clear of that idea. I'd build one for a friend, with possibly the understanding that it might still look somewhat "home made" along with a promise to understand how to use it properly and do so! I'd estimate the time and labor involved to be worth about +/- $150.
> 
> On the other hand myself and others seem to be demonstrating how really simple it is to build, in the hopes of encouraging others to learn the fairly simple skills required. It's really just the use of some simple hand tools, a drill and a soldering iron, not rocket surgery or brain science! Having a simple multi-meter for testing is also important! Even if soldering and using a ruler seems like voo-doo to some folks, almost everyone has a friend who has the skills!
> 
> Just My Take On It,
> Gene



I also agree with this. In addition, just personally, although I can analyse these things inside-out and upside-down, and can build functional prototypes, I don't have the time, set-up and dexterity to build them such that I would want to sell them.

But, in the works soon; I'm working on a condensed pdf with all the key info, which I'll sell for a few $ to help those who may want it.


----------



## JohnH

Barnsley Boy said:


> Ok, that's cool, I'll follow your layout then, as it seems to be the most frugal in terms of wire length!
> 
> I'm just having a fiddle with some perforated 1.5mm aluminium sheet to see what it looks like to replace the enclosure lid:
> 
> 
> View attachment 100382
> 
> 10mm holes @ 13mm pitch - plenty of ventilation, but the hole layout looks messy at the edges. I may have to revert back to my original idea - drilling the holes myself.
> 
> .... Am I overthinking this? or have I got too much time on my hands!!



The diagrams look so much more awesome in perspective!

On that perf Al sheet. I think it should be good for the top venting as you are using, (but not for fixing parts to.)

I see what you mean about a ragged edge though. There's probably no neat way to align the hole pattern with the case size. What if you cut out almost all of the existing lid, leaving just a 'picture frsme' including the screw holes, then use that to frame and clamp down on a wide piece if that perf Sheet?


----------



## Gene Ballzz

JohnH said:


> The diagrams look so much more awesome in perspective!
> 
> On that perf Al sheet. I think it should be good for the top venting as you are using, (but not for fixing parts to.)
> 
> I see what you mean about a ragged edge though. There's probably no neat way to align the hole pattern with the case size. What if you cut out almost all of the existing lid, leaving just a 'picture frsme' including the screw holes, then use that to frame and clamp down on a wide piece if that perf Sheet?



Cut out the lid leaving the "picture frame" as mentiond and fasten the perforated sheet to the inside of the lid. And again, don't mount the components to the perf. Although given that the coil doesn't need heat sinking, it would likely be OK to mount that to the perf, if the room was needed in the case body for some specific build, with added components.
Just Brainstormin'
Gene


----------



## StingRay85

Anyone tried building a 200W(+) version already?


----------



## JohnH

StingRay85 said:


> Anyone tried building a 200W(+) version already?



Not that I know of, but ok in principle with uprated component power ratings, heavy case and a very good fan system. I'd suggest doing it as a 16ohm version if possible to reduce current


----------



## endofall

Hey guys! I have read several pages of this huge thread and was wondering if some one can help me. Most of what is being talked about is over my head a good bit. I have a clone of a 1959 super lead and it is beyond loud and would like to attempt to build one of these attenuators for it. I see most examples used in this thread are for 50 watt amps, could someone tell me what parts/resistors etc I would need to build one for a 100 watt amp? Thanks for any help


----------



## Gene Ballzz

endofall said:


> Hey guys! I have read several pages of this huge thread and was wondering if some one can help me. Most of what is being talked about is over my head a good bit. I have a clone of a 1959 super lead and it is beyond loud and would like to attempt to build one of these attenuators for it. I see most examples used in this thread are for 50 watt amps, could someone tell me what parts/resistors etc I would need to build one for a 100 watt amp? Thanks for any help



@endofall
You can skim past the *"over my head"* stuff, technical jargon and even a couple few derailments from folks who wanted to re-invent the wheel, and not miss the important stuff that pertains to us layman. Basically, if you digest the first post and then skip to page #110, you'll be fine, although there is a lot of good info in between and as mentioned, its fairly easy to skim past the chaff without missing anything!

For a 100 watt build, you simply need to double the wattage ratings of all resistors and "I believe" make sure the coli you use is made of #16 gauge wire! I don't recall if the amperage ratings for the switches need to be bumped up, as the ones suggested for the 50 watt units are already fairly over specced and robust, although the higher the amperage rating the better.

Rest assured that @JohnH will likely stop by shortly, as he follows this thread fairly religiously and his support is far beyond what most commercially entities would consider sufficient!

Knowing what speakers you plan to use and the intended use/environment for your rig might help us point you to the best model for your needs?

FWIW, I am a happy builder and user and rabidly avid proponent of this attenuator design. depending on you skill levels, I suggest avoiding to many of the added bells and whistles that have been developed and just stick to a basic, simple unit that fits your situation!

Also understand that a lot of that "over your head" stuff was simply rigorous testing results to confirm proper operation early on, but you can have confidence that many of these units have been built and widely used in the real world and they are hands down, the best passive units that could be had!

Happy Attenuatin'
Gene


----------



## JohnH

Hi @endofall
I agree with Gene, and yes basically for a 100W amp you use the same Ohms values, and x2 the power ratings. Use 16gage or 15gage for the coil.

In addition, if you want to crank it up you'll need to put extra attention into dealing with heat dissipation. A large thick aluminium case, drilled for ventilation is needed. But also, it's likely that a fan will be needed for cooling. There have been a couple of such builds by others.

The fan can be powered from an external supply, or run from some rectified power from the amp. The latter is the nicest idea but needs some experiment to find the right fan to draw the right amount of power to spin the fan when power gets up, but not too much as to affect the tone response.

For a fan circuit, see @TomBallarino 's build at the end of page 109, with more info on p110.


----------



## AtomicRob

endofall said:


> Hey guys! I have read several pages of this huge thread and was wondering if some one can help me. Most of what is being talked about is over my head a good bit. I have a clone of a 1959 super lead and it is beyond loud and would like to attempt to build one of these attenuators for it. I see most examples used in this thread are for 50 watt amps, could someone tell me what parts/resistors etc I would need to build one for a 100 watt amp? Thanks for any help


I'm building one now but I might downsize the resistors in Stage 3/4 to 25w because I'm considering leaving Stage 2 always on, which decreases the max possible dissipation on stage 3/4. If I'm using an attenuator I'm usually taking it way down to a low level anyway. For R1 it's easier/cheaper to use two 100W resistors in series or parallel instead of a 200W. (The 200W Arcol resistors are like $40, but 100W are like $11...)

I agree with what's already been said about using a good heat sink. Cast Hammond-style project boxes look really good and are easy to work with, but aren't heat sinks... For 100W you'll really need a proper heat sink enclosure or at least replace the hammond lid with a heat sink from somebody like these guys: https://www.heatsinkusa.com/. If you look at the Arcol data sheet for example, the 100W rating is based on a minimum 1.0 °C/W rated heat sink. This 8" wide heat sink cut to 6" long (so, 8"x6") would be rated 0.45 °C/W which means approximatly it would rise 45°C when dissipating 100W. The 1959 superlead is more like 170W so if you're dissipating most of that 170W you'll want a good heat sink, and expect it to get pretty darn hot.

I'd also recommend sticking with legit resistors from Arcol, Vishay/Dale, etc from a reputable distributor. For me personally anyway I'm not comfortable using the $2 ebay cheapie resistors when it might cost me my amp if a resistor melts down. The extra ~$50? is worth the peace of mind IMHO that the parts will perform to spec.


----------



## Gene Ballzz

AtomicRob said:


> I'm building one now but I might downsize the resistors in Stage 3/4 to 25w because I'm considering leaving Stage 2 always on, which decreases the max possible dissipation on stage 3/4. If I'm using an attenuator I'm usually taking it way down to a low level anyway. For R1 it's easier/cheaper to use two 100W resistors in series or parallel instead of a 200W. (The 200W Arcol resistors are like $40, but 100W are like $11...)
> 
> I agree with what's already been said about using a good heat sink. Cast Hammond-style project boxes look really good and are easy to work with, but aren't heat sinks... For 100W you'll really need a proper heat sink enclosure or at least replace the hammond lid with a heat sink from somebody like these guys: https://www.heatsinkusa.com/. If you look at the Arcol data sheet for example, the 100W rating is based on a minimum 1.0 °C/W rated heat sink. This 8" wide heat sink cut to 6" long (so, 8"x6") would be rated 0.45 °C/W which means approximatly it would rise 45°C when dissipating 100W. The 1959 superlead is more like 170W so if you're dissipating most of that 170W you'll want a good heat sink, and expect it to get pretty darn hot.
> 
> I'd also recommend sticking with legit resistors from Arcol, Vishay/Dale, etc from a reputable distributor. For me personally anyway I'm not comfortable using the $2 ebay cheapie resistors when it might cost me my amp if a resistor melts down. The extra ~$50? is worth the peace of mind IMHO that the parts will perform to spec.



@AtomicRob 
Those are some well informed and wise research, observations and decisions! Thanks for sharing them.

Of course, as always,  or it didn't happen! 

Happy Buildin' & Attenuatin'! 
Gene


----------



## JohnH

Hi @AtomicRob , just a comment on having stage 2always on: That sounds like it would prevent you from having access to every 3.5db increment, hence gaps in the range? Or are you adding a second fixed 'stage 2' ?


----------



## AtomicRob

JohnH said:


> Hi @AtomicRob , just a comment on having stage 2always on: That sounds like it would prevent you from having access to every 3.5db increment, hence gaps in the range? Or are you adding a second fixed 'stage 2' ?


Yes it would have gaps - I’m still trying to decide whether I need all the flexibility or if I want just a couple levels, like only -14db and -31.5db. I suspect I’ll probably end up leaving it always in the same setting and never using any of the switches, problem is I won’t know what setting I prefer until I test! I’ll probably just do some hard wired tests before deciding which stages to make switched.


----------



## JohnH

Yes that's the thing, its hard to know which couple of settings will be the keepers. And it could change in different circumstances, or over an evening where you feel like turning up the wick a little more, (and then a bit more again). 

For me at home, Id say my most used settings are -31.5, and then switching the -7 and -3.5 stages to get -24.5 or -21. In an earlier build, I didn't have the -3.5 stage and that was ok for home, but when playing with others I find its useful to match levels, and also to get all the stages on to reduce level when needed at home. Another factor is that my amps have Master Volume controls, and while its much nicer to be able to turn them up and attenuate, the MV's are fine for the small changes if needed. But with NMV amps, the attenuator is the main source of volume change at a given tone.

For what you're doing, if you decided to keep all the switched options, you could consider another hard-wired -7db stage, which combined with Stage 1 brings your nominally 100W amp down to equivalent to a 4W amp for home studio use 
(-7db is 1/5th power. 100W /5 /5 = 4W )


----------



## len_zwerf

Thank you very, very much for the design.
I've been struggling to get a good 'late night' tone. I have a Champ micro with an EF80 output tube which outputs about 1 watt and I've modded my diy Madamp m15 to use EF80 tubes instead of the EL84. In push pull the EF80 do about 2 watt.I've changed the master volume to a post phase inverter version, but at the lowest setting it gets fizzy. They both are too loud when pushed. I have a Koch Dummybox but it sucks tone, even at the lowest setting.

First I've built an 8 ohm M version without the -14 and 3,5 stage and no switches, just to try. The coils for that one were easier to find I used 0.33 and 0.47 mH coils. Close enough, I hoped.

It's messy but it worked. I'm searching for a good case. I used double sided tape and with max 3 watt of power, heat won't be an issue.












8ohm-attenuator



__ len_zwerf
__ Jan 16, 2022






I was very happy! In combination with the PPI master I could finally push the Madamp.

I have a nice 8ohm Jensen and a 16 Ohm Greenback so I decided to make the 16 ohm version with switches but without the 3,5 db stage. I couldn't find the proper coils so I used a 1 and a 0.68 mH instead of 1,1 an 0,7 mH.The PPI master is ideal for fine-tuning the final volume. Couldn't decide about a case so I put it in a messed up amp chassis. Before I used it I checked if nothing made a short but I need to fix some potential disasters before I use it again. ADHD is a bitch if you need patience.













16ohm-attenuator



__ len_zwerf
__ Jan 16, 2022






So how does it sound? Tonight I participated in a backing track challenge at TDPRI, not my best work but I don' think it sounds attenuated. Recorded at 80db-ish measured by my phone app.


So again thank you for making my life better!


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## diego_cl

Thanks again @JohnH for sharing your knowledge and being so noble-minded. I finished my second M2 build this weekend and I'm very happy about how it turned out.

This time I made a better job on the cooling than my previous build, by enlarging the vent holes, moving the small resistors to the base and placing R1, R2A and R2B farther way from each other... to the detriment of adding cable length, solderless connections and many hours of wiring.

Fortunately you can't see the inefficient placement of parts from the outside.

I failed a couple of times trying to label the case using the tools I have at home. Finally the solution was easier than I thought: printed paper underneath clear coats. It doesn't look factory made, but at least it doesn't look like it was made by a toddler. I should have used glossy paper instead of matte, because the matte paper absorbed some of the acrylic enamel and the white darkened a bit.

Using a deluxe reverb with 6L6 tubes, cranked up to eleven and pushed hard with a pedalboard, it barely reached 40°C at R1.

I should test it with my 50W Marshall that I use with a another M2 build; I wanna know if it does a better job handling the heat or if the extra hassle was a waste of effort.

Quoting @Gene Ballzz, "Of course, as always,  or it didn't happen! 
"

Best wishes for this year that has just begun


----------



## Gene Ballzz

diego_cl said:


> Thanks again @JohnH for sharing your knowledge and being so noble-minded. I finished my second M2 build this weekend and I'm very happy about how it turned out.
> 
> This time I made a better job on the cooling than my previous build, by enlarging the vent holes, moving the small resistors to the base and placing R1, R2A and R2B farther way from each other... to the detriment of adding cable length, solderless connections and many hours of wiring.
> 
> Fortunately you can't see the inefficient placement of parts from the outside.
> 
> I failed a couple of times trying to label the case using the tools I have at home. Finally the solution was easier than I thought: printed paper underneath clear coats. It doesn't look factory made, but at least it doesn't look like it was made by a toddler. I should have used glossy paper instead of matte, because the matte paper absorbed some of the acrylic enamel and the white darkened a bit.
> 
> Using a deluxe reverb with 6L6 tubes, cranked up to eleven and pushed hard with a pedalboard, it barely reached 40°C at R1.
> 
> I should test it with my 50W Marshall that I use with a another M2 build; I wanna know if it does a better job handling the heat or if the extra hassle was a waste of effort.
> 
> Quoting @Gene Ballzz, "Of course, as always,  or it didn't happen!
> "
> 
> Best wishes for this year that has just begun
> 
> View attachment 100866
> View attachment 100867
> View attachment 100868
> View attachment 100869
> View attachment 100870
> View attachment 100871
> View attachment 100872



 IS RIGHT!   

Another gorgeous and successful build! Nice job! 

I'm curious, does a faint hint of the Old Spice aroma waft around as the unit warms up? 

Noice!
Gene


----------



## diego_cl

Gene Ballzz said:


> IS RIGHT!
> 
> Another gorgeous and successful build! Nice job!
> 
> I'm curious, does a faint hint of the Old Spice aroma waft around as the unit warms up?
> 
> Noice!
> Gene



That's a very perceptive question. 
Most of the scent faded away during the build, specially after the paint job. Maybe it will break free if I remove the lid, but I hope I won't find out, because closing the case was not a good experience... just thinking about it makes me cringe.


----------



## Guitar-Rocker

Does anyone have a tested method for clean up/removing the excess spluge of thermal paste that mashes out from under the resistors when tightening them to the base?


----------



## BlueX

diego_cl said:


> I finished my second M2 build this weekend and I'm very happy about how it turned out.



It looks good! One question: What are the dimensions of the box you used? (for possible future project)


----------



## Gene Ballzz

Terry @Guitar-Rocker ,
I just recently recall a post where somebody described a Q-Tip and ?? It may have been alcohol ot naphtha? I'll look, but I'm not sure where it is in this labyrinth of a thread! I do believe it was in the past 10 pages, or so!
Just Splugin'
Gene


----------



## diego_cl

Guitar-Rocker said:


> Does anyone have a tested method for clean up/removing the excess spluge of thermal paste that mashes out from under the resistors when tightening them to the base?





Gene Ballzz said:


> Terry @Guitar-Rocker ,
> I just recently recall a post where somebody described a Q-Tip and ??


In deed, many q-tips and isopropyl alcohol. The cleaning was more tedious than i thought, but I regret not cleaning my previous build, because my hands got dirty the whole time and also it was a magnet for gilding paper particles.




BlueX said:


> It looks good! One question: What are the dimensions of the box you used? (for possible future project)


It's a 1590D from Aliexpress, the dimensions are 188x119x56mm. I've bought 3 cases from them. Actually 4, because they sent me one of them with the wrong dimensions, but they gave me a refund later. Pretty decent seller.


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## BlueX

diego_cl said:


> It's a 1590D from Aliexpress, the dimensions are 188x119x56mm.



Thanks! Looks a good size, with some air between components. I'll save this info for later.


----------



## Barnsley Boy

Probably a question for @JohnH and @Gene Ballzz:

I am mid build, and the DPDT mini toggles are causing some problems. I've decided to wire everything up with 18 AWG silicone flexible wire - all colour coded, so I can keep track of things. From Gene's layout which I am working to, his suggestion is that the DPDT offers increased current capacity if a bare buss wire is used as a jumper between adjacent terminals. I'm worried that I might overheat the terminals whilst soldering and melt the plastic switch, and as they are pretty small and my hands ain't as steady as they used to be, it's a disaster waiting to happen!

I have some 2.2mm female spade connectors which I'm thinking of using instead of soldering straight to the terminals. That way I could always disconnect the front panel if I need to, or re-use the switches if I ever decide to change things around.

The question is, can I get away with just using one half of the DPDT? They are rated at 5A @ 120V AC, or 2A @ 250V AC.


----------



## Gene Ballzz

Barnsley Boy said:


> Probably a question for @JohnH and @Gene Ballzz:
> 
> I am mid build, and the DPDT mini toggles are causing some problems. I've decided to wire everything up with 18 AWG silicone flexible wire - all colour coded, so I can keep track of things. From Gene's layout which I am working to, his suggestion is that the DPDT offers increased current capacity if a bare buss wire is used as a jumper between adjacent terminals. I'm worried that I might overheat the terminals whilst soldering and melt the plastic switch, and as they are pretty small and my hands ain't as steady as they used to be, it's a disaster waiting to happen!
> 
> I have some 2.2mm female spade connectors which I'm thinking of using instead of soldering straight to the terminals. That way I could always disconnect the front panel if I need to, or re-use the switches if I ever decide to change things around.
> 
> The question is, can I get away with just using one half of the DPDT? They are rated at 5A @ 120V AC, or 2A @ 250V AC.
> 
> 
> View attachment 100916



Not a fan of those tiny spade connectors, especially in such critical situations. For the buss wire, you don't need to use the "big honkin'" stuff that @Marcomel79 used. #18 gauge is fine and solders right quick. How up to snuff are your solder and iron/tip maintenance procedures? Makes a huge difference in torching components or not! A dirty tip is the mortal enemy of good soldering!

Here's a lengthy, but fairly accurate description of how I do it that I posted on another website. Others may have their own favorite ways of achieving the same results.
_
"Ya know, to me it's hilarious that many guitarists can sit and work/practice on a specific lick for hours on end, yet refuse to get a good soldering tool and spend an hour "practicing" how to *"PROPERLY"* use it! The main trick to good soldering is the proper and simple maintenance of the iron! Following is a novel I wrote in another thread:

First, whenever soldering to the back of a pot that has never seen solder before, it really helps to either scrape, file or sand a portion of the surface to get to "clean" metal, as there may be oils, glaze, release agents and/or other contaminants or finish on the metal to reduce corrosion or aid the manufacturing process. Cleaning this area "before" applying solder will enhance the heat transfer of your iron AND the adhesion of some fresh solder.

Next, read, digest and follow my verbose suggestions below and amaze/impress your friends with the quality of your new soldering skills!


Proper care, etiquette and maintenance are as important, if not more so, than the iron itself!

*A)* Get a soldering iron/station that has the option of various tip sizes, along with a small assortment (2-4) of sizes of tips. Use the smaller for "fine" work and the larger for heavy stuff, like the back of pots, larger audio speaker connectors, etc. Right tool for the job. A stand is also good to have, if you don't get a station with one built in.
*B) *Get a small sponge. The ones made specifically for soldering iron use are best and keep it damp to wet while soldering. Clean it by running under water and squeezing it out a few times before each soldering session. And, oh yeah, have a trash can handy!
*C)* Before first use, heat iron up and wipe it on the sponge, then liberally apply solder to the tip and shake the excess off into the trash can. I call it the rattlesnake shake! Wipe tip off on the sponge and reapply solder. The iron is now ready to either be turned off/stored or sit there heated up and ready for use. "NEVER" let a soldering iron sit, while on or in storage without having had a fresh cleaning and coating of solder! The coating protects and seals the tip from oxidation and that oxidation (as well as a dirty tip) is the enemy of good, efficient heat transfer.
*D)* Now that your iron is heated, prepped and sitting happily in it's stand, get you wires/work arranged the way you want them. Pull the iron from it's stand and do the rattlesnake shake into the trash can, wipe tip on sponge, and dab a touch of solder onto the part of the tip that you want to use, apply that tip to the joint and then apply solder to the joint.
*E)* When a suitable amount of solder has flowed into the joint, pull the solder away and then pull the iron away, "WITHOUT" disturbing or jostling the wires.
*F)* If more joints need to be soldered, repeat "D" & "E". If done or more prep is needed for the next joint, do step "C"!
*G)* When you're done, do step "C" and store your iron/station, ready for it's next adventure.
*H)* FWIW, each wire should get "pre-tinned" before use. This is simply heating the wire enough to apply a bit of solder and then shaking off the excess. If ya can't shake it (not enough length or room to do so), just a little dab of solder will do.
I realize this sounds like a lot of details, but religious adherence to these steps will get you well on your way to successful soldering that you can be proud of, as well as good longevity of your soldering iron! Once you develop it as a "HABIT" it's not as complicated as it sounds, though the details and sequence of events is fairly important!

Oh yeah, a couple more points. Once your tip no longer likes to accept solder and provide (when wiped) a nice shiny coating, has a lot of black deposits on the working surface or becomes heavily oxidized, its time to replace it. And most tips won't give much satisfaction from "sanding" to a fresh surface, especially the more "high-tech" temperature sensing/controlled units!

This is the one I've been using almost daily for over 10 years, for multiple repairs and amplifier builds, and a wide variety of tips are easily available:
_
_https://www.amazon.com/Weller-WES51...9143216&sr=8-2&keywords=weller+soldering+iron_
_

This one saves a few bucks, but is just "so-so" and if you think you will use it more than a couple times a year, bite the bullet and get a good one:
_
_https://www.amazon.com/Weller-WLC10...9143216&sr=8-1&keywords=weller+soldering+iron_
_
And while free standing "pencil on a cord" units can be handy for a "quick, mobile" toolbox, they just don't have the quality to make them a good value."_


Just My $.02 & Likely Worth Even Less!
Gene

Quote Reply
Report


----------



## diego_cl

Barnsley Boy said:


> Probably a question for @JohnH and @Gene Ballzz:
> 
> I am mid build, and the DPDT mini toggles are causing some problems. I've decided to wire everything up with 18 AWG silicone flexible wire - all colour coded, so I can keep track of things. From Gene's layout which I am working to, his suggestion is that the DPDT offers increased current capacity if a bare buss wire is used as a jumper between adjacent terminals. I'm worried that I might overheat the terminals whilst soldering and melt the plastic switch, and as they are pretty small and my hands ain't as steady as they used to be, it's a disaster waiting to happen!
> 
> I have some 2.2mm female spade connectors which I'm thinking of using instead of soldering straight to the terminals. That way I could always disconnect the front panel if I need to, or re-use the switches if I ever decide to change things around.
> 
> The question is, can I get away with just using one half of the DPDT? They are rated at 5A @ 120V AC, or 2A @ 250V AC.
> 
> 
> View attachment 100916



Soldering the wires directly to the switches is the best option, give it a try.

At first I was afraid of the lack of space for welding the DPDT mini switches, so I made some kind of jumper with a lug for soldering the wire.

Then I realized I was bitching needlessly... I just had to pre tin the switch pins and the tip of the wires. I used my crocodile clamps for placing the wires touching both connectors. Then I just had to touch the pins for a couple of seconds with the soldering iron. Very easy, I just needed one hand.


----------



## JohnH

My contribution to the suggestions above is to put wires on the switches before putting them in the case, so they can be held in a small clamp or vice with excellent accessibility. Tin the wire ends and the switch lugs separately, and then lay the end of the wire across two lugs and melt them on. Then you have the switches done, with flying leads and no more work to them.

The idea of doubling up the switch poles as we suggest is nice to have, adding redundancy and reliability. Using just one side is ok though. With our base design (no full bypass switch), the max power flowing through the switch contacts is 1/5 of the amp power, given stage 1 is -7db. Take a 50W amp that happens to push out say 70W. That becomes 14W at the first switch. Say its 8 ohms, then current is 1.3 Amps (power = I^2.R)


----------



## Barnsley Boy

That's great, thanks for the replies! Having slept on it, and taking on board your comments/ observations, I'm going to ditch the spade connector idea. @Gene Ballzz thanks for the tips (forgive the pun) on soldering technique. I have a soldering station with digital readout, and religiously keep the tip cleaned and tinned. If I had the money, the WES51 is a great iron, but at the moment I'll stick with my Chinese Amazon special, which seems to do the job ok. I'm using leaded solder, 63:37 Tin to Lead ratio, with 1.8% flux. I tend to switch the iron off between joints so that it is not left hot for long periods of time. I'm not sure if this is correct, whether I should leave it running, or maybe notch down the temperature whilst it is idle? Also, what temperature do you usually operate your iron at? I know for the back of pots it needs to be quite high, but for general wire soldering I set mine at about 360-370. I appreciate that there isn't a one size fits all, but searching on the interweb just causes more confusion!

I'd presume that the jumpers don't actually have to be the same gauge as the rest of the wire that I'm using? I'm using stranded 18 AWG for all connecting wires, which is nice and flexible and less susceptible to breakage. For the jumpers I have some single core cloth covered wire that I use for internal guitar wiring. Without the insulation it fits nicely through the switch terminal holes, and because it is thinner than the 18 AWG, it will heat up quicker, so less time for the iron to wreak havoc!

I have a couple of helping hands to keep things still while soldering, but because of the enclosure that I am using I think it is easier to solder the jumpers and connections between switches whilst they are mounted on the panel. Can't wait to finish work, so I can get the soldering iron out!!

It goes without saying, but I'm going to say it anyway, your help and guidance is very much appreciated.


----------



## matttornado

Hello Everyone. I noticed a few pages back you guys were discussing temperatures. I superglued a thermocouple on the giant 30 ohm / 250 watt power resister I use for the first stage (R1). I cranked my 100 watt Superlead up to 10 with the attenuator at MAX attenuation.

I then jammed for about an hour while recording (logging) the resistor's temp every minute with my Fluke 54IIB meter. This was done prior to adding a fan so it would be interesting to remeasure wit hte fan on but the attenuator barely gets warm with the fan running.


----------



## ThePanda

Can someone check my wiring?
This is Attenuator M2.


----------



## brad Messier

Good day to all. Thanks for all the input. Got my unit put together, managed (just) to squeeze it all in a pretty small box. Ended up countersinking holes for all my screws and having the bolts on the inside. Looks good, but man were some of those M3 nuts a bit hard to get seated in my tightly spaced enclosure. I based the layout on Gene's that is posted here on page 111. I dropped the -14 stages, one output at 4 OHMs and added in a line out. So far so good. 
Then I plugged it in... no signal to my speaker. To my surprise the line out was working fine, I was able to get a signal out to my computer. Any thoughts on trouble shooting? My signal to the output is currently coming from the -3.5 switch, on the "top" lug, and that is tied in to one end of R8 at the switch. No effect noticeable when I throw the switches. I am guessing I have the switch wiring wrong, but not sure where to start.


----------



## JohnH

Hi @brad Messier , thanks for posting. An important test is to test resistance as seen by the amp. 

Plug speaker into attenuator 
Plug a cord into the attenuator input, but without the amp
Measure resistance across the jack plug that would go to the amp. For 4Ohm version, we expect 3.5 to 5 Ohms in all settings. (amp is not connected in this test)


----------



## Gene Ballzz

Hey @JohnH ,
I think I'm gonna start a new thread, simply for completed builds. I'll ask for only those who have finished a build, with pics to post, and direct those with questions to this thread. It might be kinda fun to actually know how many completed builds there are out there?
Watcha Think?
Gene


----------



## AtomicRob

Barnsley Boy said:


> The question is, can I get away with just using one half of the DPDT? They are rated at 5A @ 120V AC, or 2A @ 250V AC.


The switches for the stages won't exceed 5A (or 120VAC) so using SPDT or half of the DPDT would be fine. Using ohms law, I = sqrt(P/R), so for a nominally 50W amp which puts out like 90W RMS, if you're building the 8 ohm version that's 3.4A, the 16 ohm version sees 2.3 amps. For 100W amps, at max 170W RMS output, you get 4.6A at 8 ohms, 3.3A at 16 ohms. That 4.6A is uncomfortably close to the limit. BUT the stages are all after the initial -7db fixed stage so the power is actually reduced to 1/5 of that and the current is worst case like 2A.

The bypass switch sees the full signal so I'd recommend a 10A rated switch for that one if you're building an 8 ohm version or especially a 100W version. I'm building a 100W right now and I'm using Carling 2M1 series for the stages and G series for the bypass.

BTW you can't really double the rating of a DPDT by ganging the poles - a 5A rated DPDT can't switch 10A on a single circuit. The reason is that mechanically the two poles don't switch at the exact same time, so one pole or the other will always see the full current initially when switching. You might get away with it if you never switched it under load - and that may be true for an attenuator - but still it's not a safe way to build.


----------



## brad Messier

JohnH said:


> Hi @brad Messier , thanks for posting. An important test is to test resistance as seen by the amp.
> 
> Plug speaker into attenuator
> Plug a cord into the attenuator input, but without the amp
> Measure resistance across the jack plug that would go to the amp. For 4Ohm version, we expect 3.5 to 5 Ohms in all settings. (amp is not connected in this test)



Well something is clearly off, my resistance reading was 14, 19 and 21 depending on the switch positions! Please forgive my crude drawing, but this is the layout I ended up with.


----------



## Gene Ballzz

@brad Messier ,
Look at R1, and the input negative! That should be connecting to the negative of the speaker out and by proxy to R7 & R3. I think the rest is correct.
Let us know?
Gene


----------



## Guitar-Rocker

Hi all, Built my first John H. It's a M-Lite. I have meter tested the finished unit, will tone test it this weekend. I'm pretty happy with most everything except the printed tags, but they will keep me straight for now.


----------



## Gene Ballzz

@Guitar-Rocker 
NOICE!
Gene


----------



## JohnH

Hi @brad Messier . The switches look a bit odd too where you connect to each side of the two-pole switches. The basic design only uses one switch pole per switch, so when we use two poles, they are ganged together with links across each pair of lugs, as in Gene's or my diagrams.

Definitely don't test with the amp until all confirmed by resistance tests!


----------



## JohnH

Gene Ballzz said:


> Hey @JohnH ,
> I think I'm gonna start a new thread, simply for completed builds. I'll ask for only those who have finished a build, with pics to post, and direct those with questions to this thread. It might be kinda fun to actually know how many completed builds there are out there?
> Watcha Think?
> Gene


'

Sure! it would be nice to have a more condensed gallery. I bet there'll be random questions on it too! But happy if you'd like to manage that. 

I lost count years ago on how many of these we have seen. I love seeing all the builds. I'm guessing about one new build every 3 pages or so?


----------



## Gene Ballzz

Yeah @JohnH ,
Off the top of my head I was guessing somewhere roughly between 35 & 50. And then there were a few less vocal folks who likely built and never showed off their work! And then likely some that came, saw, built, never posted and are happy users! Ya just gotta bet that some manufacturer will glom on, take the design, patent it, package it and then try to sue you for infringing on it!   This design is just too good to be kept a secret for much longer!
Just Teasin'
Gene


----------



## JohnH

Yes indeed! Not worried about anyone else patenting it, they can't. It's well published right here as 'prior art' with dates etc.


----------



## junk notes

Guitar-Rocker said:


> Hi all, Built my first John H. It's a M-Lite.


----------



## Barnsley Boy

AtomicRob said:


> BTW you can't really double the rating of a DPDT by ganging the poles - a 5A rated DPDT can't switch 10A on a single circuit. The reason is that mechanically the two poles don't switch at the exact same time, so one pole or the other will always see the full current initially when switching. You might get away with it if you never switched it under load - and that may be true for an attenuator - but still it's not a safe way to build.



Understood! 





Next time, I'll probably use SPDT chunkier toggle switches. These mini ones are ok for guitar pickup switching options, but I think maybe something more industrial could be on the cards for the next build!

After faffing around with soldering bits of jumper wire between terminals and then bending the wires into something half decent to solder to, I came to the conclusion that I am better off using spade connectors after all. Rather than crimping the wires onto the connector, I have soldered them in place. The connectors can then be pushed onto the switch terminals. Connection between adjacent poles is achieved using a small piece of wire bent into a u-shape, laid into the trough of the connector. Flowing some solder into the trough then connects it all together. I've used heat shrink to tidy it all up. The only disadvantage of this is that the whole assembly becomes quite bulky, but as the case I'm using is fairly spacious, It isn't an issue.




@JohnH & @Gene Ballzz - good idea on the completed build thread. During my planning stage I started to look back on some of the previous builds to see how other people had tackled things. 118 pages (and counting) is a lot to get through. I even toyed with ploughing through and compiling them myself into a single document.

I think this thread is possibly one of the most important on the whole forum for users of valve amps. Is there any way that this could be stickied (I believe that is the correct terminology), so that it is easily accessible?


----------



## brad Messier

Gene Ballzz said:


> @brad Messier ,
> Look at R1, and the input negative! That should be connecting to the negative of the speaker out and by proxy to R7 & R3. I think the rest is correct.
> Let us know?
> Gene



OK, I think that has taken care of the issues. I tied the - end of R1 in to R7 and R3 via a jumper from the input ground over to the output ground. I was looking at this from a guitar/pedal perspective and figured a ground is a ground... did not understand those negative ends needed to be tied together, I separated them to keep the wiring clean.

Switch wise, I did jump the poles of the switches together, for clarity I omitted this from my illustration.

New resistance readings, 5.3 6.6 and 8.8 with various switches engaged. Clear to plug it back in?


----------



## JohnH

brad Messier said:


> OK, I think that has taken care of the issues. I tied the - end of R1 in to R7 and R3 via a jumper from the input ground over to the output ground. I was looking at this from a guitar/pedal perspective and figured a ground is a ground... did not understand those negative ends needed to be tied together, I separated them to keep the wiring clean.
> 
> Switch wise, I did jump the poles of the switches together, for clarity I omitted this from my illustration.
> 
> New resistance readings, 5.3 6.6 and 8.8 with various switches engaged. Clear to plug it back in?



Still maybe some gremlin I think! A 4ohm M2 shouldn't go as high as 8.8. But, the measurements should be with the speaker plugged into the output. So maybe that accounts for it?


----------



## Gene Ballzz

@brad Messier ,
Where are you deriving the "-/negative" for the line out? I see from your pics that you are using isolated Cliff style jacks for all else, but the line out appears to have it's "-/negative" tied to the chassis, but nothing else is! Please note that I am avoiding the term "ground" as nothing here is actually ground, simply +plus and -minus, and all should be isolated from the chassis!

Most of the one or few issues that have come up over the course of this thread seem to have centered around allowing the case/chassis to become any part of this circuit! While it kinda doesn't make any sense that the chassis/case can't be used, most folks' issues have been cured by make sure all circuitry electrically "floats" inside the chassis!

Again, Please Let Us Know?
Gene


----------



## brad Messier

Gene Ballzz said:


> @brad Messier ,
> Where are you deriving the "-/negative" for the line out? I see from your pics that you are using isolated Cliff style jacks for all else, but the line out appears to have it's "-/negative" tied to the chassis, but nothing else is! Please note that I am avoiding the term "ground" as nothing here is actually ground, simply +plus and -minus, and all should be isolated from the chassis!
> 
> Most of the one or few issues that have come up over the course of this thread seem to have centered around allowing the case/chassis to become any part of this circuit! While it kinda doesn't make any sense that the chassis/case can't be used, most folks' issues have been cured by make sure all circuitry electrically "floats" inside the chassis!
> 
> Again, Please Let Us Know?
> Gene



Ok, seems to be in order now. I had a Switchcraft input jack in there due to space issues. Swapped that out for a slightly modified Cliff jack and got the negative side tied together and isolated from the case. Added jumpers to the jacks themselves, I have not worked with this style jack before not exactly Resistance now checks out, I am seeing 3-5 Ohms resistance to the output tip with the speaker plugged in. Reading a steady 5 Ohms on the - side of the circuit. I will have some time tomorrow to give it a full test. Not the neatest build of all time, but things are really squeezed in there tight!


----------



## JohnH

Hi @brad Messier , yes that sounds to be better now.


----------



## JohnH

*M2 Gig
*
This is a possible new 'Variant of Interest'. Not sure of it will go 'viral'!

Its intended to address a issue that has come up a few times, where the attenuator is intended only for small volume reductions for gigging or loud jamming, using low or medium powered amps. It focusses on providing well-balanced tones with reductions of 3.5 or 7 db, taking say a 20W amp down to 9W or 4W. It's basically Stage 1 from the M2 design, with a DPDT switch to provide the extra -3.5db setting. (The base M2 starts at -7db)













M2 Stage1 3.5-7 220124



__ JohnH
__ Jan 24, 2022






In the drawing, the twi parts of the switch are shown at the -3.5db setting.

Values are as follows, for use at 8 ohms or 16 Ohms. Power rating for resistors are 50W min for 50W amps.

R1A 27 47
R1B 33 82
R2A 22 47
R2B 18 39
R2C 10 15

L2 0.9mH 1.8mH

Its also possible to build this as Stage 1 into a full M2 design. Can post if interested, there are a couple of other tweaks needed in that case.


----------



## Emiel

JohnH said:


> *M2 Gig
> *
> This is a possible new 'Variant of Interest'. Not sure of it will go 'viral'!
> 
> Its intended to address a issue that has come up a few times, where the attenuator is intended only for small volume reductions for gigging or loud jamming, using low or medium powered amps. It focusses on providing well-balanced tones with reductions of 3.5 or 7 db, taking say a 20W amp down to 9W or 4W. It's basically Stage 1 from the M2 design, with a DPDT switch to provide the extra -3.5db setting. (The base M2 starts at -7db)
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Stage1 3.5-7 220124
> 
> 
> 
> __ JohnH
> __ Jan 24, 2022
> 
> 
> 
> 
> 
> 
> In the drawing, the switches are shown at the -3.5db setting.
> 
> Values are as follows, for use at 8 ohms or 16 Ohms. Power rating for resistors are 50W min for 50W amps.
> 
> R1A 27 47
> R1B 33 82
> R2A 22 47
> R2B 18 39
> R2C 10 15
> 
> L2 0.9mH 1.8mH
> 
> Its also possible to build this as Stage 1 into a full M2 design. Can post if interested, there are a couple of other tweaks needed in that case.



Using this as Stage 1 into a Full M2 design would have my vote!


----------



## JohnH

hi @Emiel , no problem! I'll put that one up next.


----------



## Gene Ballzz

Hey @JohnH ,
That looks sweet! Just lookin' & thinkin', would it not be possible to change the switching around a bit, with a three position switch to allow for -3.5db, -7db and -10.5db?
Just Curious?
Gene


----------



## JohnH

Gene Ballzz said:


> Hey @JohnH ,
> That looks sweet! Just lookin' & thinkin', would it not be possible to change the switching around a bit, with a three position switch to allow for -3.5db, -7db and -10.5db?
> Just Curious?
> Gene



I think so, but I think it either needs a 3 pole rotary, or , it could be done with a 4pdt on-on-on mini-toggle.

Either way, I think it would need the extra two resistors to add the usual -3.5db stage after stage 1, to combine with -7db to make -10.5db in the third switch position


----------



## Gene Ballzz

@JohnH ,
Yeah, I see, and by the time we go there, might as well build a full M2 unit! I must add though, that my method of running parallel for just a -3.5db, reactive cut works pretty well and is fairly/semi fool proof. Having it all on switches is better for the masses, using the 86 principle! 
Keep On Keepin' On,
Gene


----------



## JohnH

sure I agree. This idea came from a PM convo with @Emiel , who has an amp with only an 8 ohm tap


----------



## Joe Patti

I created an account here just so I could thank everyone who helped develop these designs.

I decided to shoot for absolute over-build and doubled the wattage of everything so I don't have to worry about heat, and so far it's handling my SLO 100 with ease. The fan and Heatsink were added cause I had them on hand and I figured the cooler the better.


----------



## JohnH

Hi @Joe Patti, that's an awesome unit! Do you find the fan makes a big difference? also, how hard do you like to drive that amp? Cooling on these for the 100W amps is an ongoing study.


----------



## Joe Patti

Hey @JohnH

I started by running this unit with just the fan as I was waiting for the thermal epoxy to arrive for mounting the heatsinks. The fan by itself moves enough air through the enclosure to cool it down pretty quickly after a 30 minute jam session, but the box still heated up to what I would call an uncomfortable temperature with the SLO masters on about 7.

After adding the heatsinks, I plugged the unit into my KT66 powered JTM45 which, even with the amp dimed didn't make the load box enclosure warm. I'll have another opportunity to try it with the SLO tonight in my practice space and will report back how things go, but so far the heatsinks have done the most to keep the heat off the resistors.


----------



## Barnsley Boy

Evening chaps,

Finally finished and tested my 8 ohm M2. Wow! that's all I'm going to say! @JohnH thank-you so much for the design and @Gene Ballzz many thanks for rescuing me from making a dire attenuator mistake, and diverting me to this thread!

Holes yet to be drilled in the lid, but I'm very happy about how it has turned out.


----------



## Joe Patti

I swear, I have never heard this amp until I got to turn it up past halfway today...

I built this Hiwatt DR103 clone last year during peak introvert season and it turned out great, but I have never had the opportunity to really hear it opened up cause it is just so much loud. Today during a break I plugged it into my sealed 2x10 loaded with Greenbacks and it was quite a sound to behold.

The box was still getting pretty hot as I was hitting chords and pushing the amp into a fair bit of both pre and power amp saturation. Settings in the picture are what I was running.

I would feel comfortable running this for a while under observation just to ensure it doesn't go too nuclear, but the heat seems fine. I can still touch the top and sides of the box without pulling away, but I'll get a temperature reading on it at some point just to see where it's at under a load like this.


----------



## Barnsley Boy

@Joe Patti I'm lovin the build. I'm now toying with the idea of a tastefully designed heatsink bolted to my lid!


@JohnH - I'm gonna sound like a right lightweight here, but the JCM900 is still too loud for me to fully crank, so I could do with adding another switched stage. Is this feasible? and would it just be a case of replicating and adding onto the end of the signal chain?

So adding another -14db cut would just add another R5 and R6? Physics lessons were a long time back - I'm sure it's a simple ohms law calculation, but my head's still spinning with the opportunities this is opening up!


----------



## Barnsley Boy

BTW - awesome Hiwatt clone!


----------



## Joe Patti

Barnsley Boy said:


> BTW - awesome Hiwatt clone!



Thanks a lot @Barnsley Boy!

Here's a few pictures from the end of the Hiwatt build if you're interested. Pretty proud of this one.


----------



## Barnsley Boy

That is a lovely build - was it a kit? I'd love to tackle something like that - maybe a lower power combo version.


----------



## JohnH

Great projects everyone!

@Barnsley Boy , yes you can add another stage. You can use another of any of the resistor stages, and you can insert it anywhere after Stage 1.


----------



## Joe Patti

Barnsley Boy said:


> That is a lovely build - was it a kit? I'd love to tackle something like that - maybe a lower power combo version.



Yes, it was a Mojotone kit. They actually added a 50W version a la DR504 shortly after I did this one but their wait times for unassembled kits is up to 8 weeks last I looked at 'em, so be mindful of that if you look into it.

I generally prefer to source my own parts instead of kits, but the mojo kit was the easiest way to source a chassis and faceplates that wasn't from the UK. Shipping on stuff was killer so I put the project off until I found the Mojo kit.


----------



## Barnsley Boy

JohnH said:


> Great projects everyone!
> 
> @Barnsley Boy , yes you can add another stage. You can use another of any of the resistor stages, and you can insert it anywhere after Stage 1.



I'm gonna squeeze another one into the case. Good job I left a space for an additional switch! If I were to go the whole hog, what values would a -28db stage be? That would give me a good range of options and more importantly save the wear and tear on my eardrums (not from the amp..... from the wife!!!)


----------



## JohnH

I think with another-28 db, you need 3 resistors. I can work that out (what's ohms are you using?) but really that's very very low with all the other stages. But I suppose it's the next logical step, and it'd give you all the next steps like -35, -38.5 etc


----------



## Barnsley Boy

With all 4 stages engaged, I have 7+3.5+7+14= 31.5db cut. If there was a -28db cut, yes it's a massive drop (-59.5db total), but it's worth it to open up the other switching options to get intermediate values. I'm anticipating that I'll find a suitable sweet spot and maybe reconfigure the unit to suit.



JohnH said:


> I can work that out (what's ohms are you using?) but really that's very very low with all the other stages.



Not sure what you mean by "what's ohms are you using?"?


----------



## Marcomel79

Gene Ballzz said:


> Hey @JohnH ,
> I think I'm gonna start a new thread, simply for completed builds. I'll ask for only those who have finished a build, with pics to post, and direct those with questions to this thread. It might be kinda fun to actually know how many completed builds there are out there?
> Watcha Think?
> Gene


Very good idea Gene! I see from tracking that my coil has arrived in Oslo today, so im guessing ill be adding it before the weekend....


----------



## JohnH

hi @Barnsley Boy , what I meant was, which amp tap are you using 8 or 16 ohms? = is it an 8 or a 16 ohm version of the M2?


----------



## JohnH

JohnH said:


> *M2 Gig
> *
> This is a possible new 'Variant of Interest'. Not sure of it will go 'viral'!
> 
> Its intended to address a issue that has come up a few times, where the attenuator is intended only for small volume reductions for gigging or loud jamming, using low or medium powered amps. It focusses on providing well-balanced tones with reductions of 3.5 or 7 db, taking say a 20W amp down to 9W or 4W. It's basically Stage 1 from the M2 design, with a DPDT switch to provide the extra -3.5db setting. (The base M2 starts at -7db)
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Stage1 3.5-7 220124
> 
> 
> 
> __ JohnH
> __ Jan 24, 2022
> 
> 
> 
> 
> 
> 
> In the drawing, the twi parts of the switch are shown at the -3.5db setting.
> 
> Values are as follows, for use at 8 ohms or 16 Ohms. Power rating for resistors are 50W min for 50W amps.
> 
> R1A 27 47
> R1B 33 82
> R2A 22 47
> R2B 18 39
> R2C 10 15
> 
> L2 0.9mH 1.8mH
> 
> Its also possible to build this as Stage 1 into a full M2 design. Can post if interested, there are a couple of other tweaks needed in that case.



Here's the M2 with a switchable 3.5/7 db stage 1, as part of a full circuit:













M2 With 70-35 220126



__ JohnH
__ Jan 26, 2022






Stage 1 is as quoted above
Stage 3 and 4 are the usual values for M2 -7dB and -3.5dB stages
Stage 2 is the -14db stage, and it needs to become a three resistor version to maintain correct amp load, since it is less separated from the amp if Stage 1 is at -3.5dB. It needs both sides of a dpdt switch to engage and disengage it.

Values for Stage 2 (8 ohm build then 16 Ohm):
R5 5.6 12
R6A 3.3 6.8
R6B 15 33

Also, for the 8 ohm version if building Out 3,
R10 = 39 (small change from 56)
R11 = 10

These additions allow the -3.5db setting to be made in Stage 1, for a small volume reduction. The intent is that the following stages are off in that case.

If you want to use -7dB or more attenuation, switch Stage 1 to -7dB and it becomes as per the normal M2 circuit. But, with the changes above, its still safe to set Stage 1 to -3.5dB and engage following stages - the sound will be fine, but a bit less 'reactive'. Try it!


----------



## JohnH

Hi @Barnsley Boy 



JohnH said:


> I think with another-28 db, you need 3 resistors. I can work that out (what's ohms are you using?) but really that's very very low with all the other stages. But I suppose it's the next logical step, and it'd give you all the next steps like -35, -38.5 etc



This is what a -28db stage looks like:




To switch it, see my previous post above, where a similar three resistor arrangement is used in Stage 2 of an M2.

Values for this as a -28db stage are as follows (8 ohm build then 16 Ohm): Power rating is for a 50W amp.

RA 6.8 12 25W
RB 1.2 2.7 5W
RC 18 39 1W

Alternatively, you could just build another -14dB stage, to get overlap between ranges.


----------



## Barnsley Boy

That is awesome @JohnH - you are a true Jedi when it comes to the dark power of the attenuator!!!




I think I'll add another -14db stage and see how that works out. It's an 8 ohm M2 BTW!


----------



## JohnH

Barnsley Boy said:


> That is awesome @JohnH - you are a true Jedi when it comes to the dark power of the attenuator!!!
> 
> View attachment 101311
> 
> 
> I think I'll add another -14db stage and see how that works out. It's an 8 ohm M2 BTW!



"If embrace the Power of the Amp you wish, then Attenuate you must ..."


----------



## Barnsley Boy

Probably a real rookie question, but for the 8 ohm -28db stage, would the 18 ohm 1w resistor be a normal metal film resistor as opposed to one of the big chunky wire wound ones?


----------



## JohnH

Barnsley Boy said:


> Probably a real rookie question, but for the 8 ohm -28db stage, would the 18 ohm 1w resistor be a normal metal film resistor as opposed to one of the big chunky wire wound ones?



Yes in fact, the power dissipated in that resistor is way less, like less than 0.1W


----------



## Barnsley Boy

Nice one @JohnH that will save some space! Many thanks. Just one more question, and I'll leave you in peace, The power rating specified for all of these components I would presume are the minimum values? If I can only get hold of a 10w 1.2 ohm can I use that? Supply problems seem to be an issue at the moment!


----------



## JohnH

Yes,no problem with using higher rating. But are you going to do another -14 stage instead of a -28? it affects the power ratings.


----------



## waltschwarzkopf

JohnH said:


> This was originally the first post, on 22/10/2017, moved here to allow my July 2019 summary in post 1
> 
> *Schematic*
> 
> 
> I've tried a few recipes, but the current one that I've been using these past few months has three fixed resistances, combined to in theory take -7db off the input power, which is a x0.2 power reduction, so 40W becomes 8W. Each resistance is made up of several 5W power resistors. With 40W from the amp and an 8Ohm load, each resistor is dissipating between 2W and 4W.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> The idea of the three resistances is to not only match the Ohms as seen by the amp, but also to control the impedance as seen by the speaker. This gives the speaker a bit of damping but not too much, as when directly connected to a tube amp. This is different to an SS amp which has super-low output impedance, which creates high damping and suppresses some of the speakers natural response.



Why did you use an Pi-pad instead of an L-Pad?


----------



## Barnsley Boy

I can get all the correct values for the -28 option, so I'm now considering incorporating that as my final stage. I'm just trying to cover bases if components are unavailable. In general, I'm sticking to the tried and tested values! If it ain't broke etc. etc.


----------



## JohnH

waltschwarzkopf said:


> Why did you use an Pi-pad instead of an L-Pad?



L-pads as used in most simple attenuators, will damp down your tone as you reduce volume more than a small amount. It's because at higher attenuation, they show the speaker a low output resistance. This stops the natural resonance and treble rise of the speaker from developing, as they do with the highish output impedance of a tube amp.

To get a better resistive attenuator, you need to maintain control of both output impedance and input impedance, combined with the desired reduction. Typically this needs a T or Pi arrangement, or a chain of them as in our designs here.

Then, to get the right dynamics and feel, the amp has to see not just a nominal resistive load, but a load that varies with frequency, as it does when driving a speaker directly. For that reason we have reactive parts.

Getting all that together across a wide range of levels is what our attenuators achieve, better than most other designs.


----------



## michiel

Hi John and others,
I have been reading this topic for a while and I made several months ago the 8 ohm M2 version. Perfect for me. I think this was already asked before, but can it work also as a load? Just connect it to my 50 watt tube amp with no speaker? If possible, does it matter which stages are swithed on? 
Second question: Although my version works good I measuren the resistance on the speaker side. It ranges between 13.7 and 18 ohms. Is that logical. The amp side measures between 9 and 12 ohms.
Thankx.


----------



## JohnH

hi @michiel , thanks for posting. Yes you can use it as a load with no speaker. Just switch all stages on to max attenuation.

On the resistance readings, measure the input with a speaker plugged to the output. Then it should read 7 to 10 Ohms at the input in all settings. Is that what you did? If si, 12 seems high, would be ok if there was no speaker.

Measured from the output, was the amp plugged to the input?


----------



## michiel

Hi John,
I measured with nothing attached. So I remeasured the values as described. It all fits within your given limits. Thanx.


----------



## botacco

Does anybody created a shopping list for the needed components from any European store? I am getting crazy finding the inductors for a reasonable price in Finland. Only find very expensive "audiophile" ones (starting from 25 euro each)


----------



## Barnsley Boy

Hi @botacco

I got mine from QTA systems in the UK. Might be worth dropping them an e-mail to see if they can ship one out to you at a reasonable price


----------



## Marcomel79

botacco said:


> Does anybody created a shopping list for the needed components from any European store? I am getting crazy finding the inductors for a reasonable price in Finland. Only find very expensive "audiophile" ones (starting from 25 euro each)


.


----------



## Marcomel79

So, after waiting 5 weeks, my inductor coil has finally arrived. Wow, if i loved the attenuator before, i love it even more now. The feeling and response has improved and it sounds so natural, even when most attenuated.

Once again @john, thank you so much for sharing this little jewel. I cant recommend it enough. Here are some pics of the finished build.
Also a huge thanks to @Gene Ballzz for helping me with the build!


----------



## Gene Ballzz

Absolutely STELLAR!  Looks rock solid/bullet proof. And yes indeed, that coil *DOES* make a difference! Should be no cooling issues.
Great Job & Thanx 4 Sharon, She's A Good Girl, In All The Best, Bad Ways! 
Gene


----------



## Barnsley Boy

@Marcomel79 Lovely neat job. My soldering iron is barely cool, and I'm planning my next build. That's some serious gauge of bare copper wire. I'm tempted to go down the same route with the next one. It's got that "hard-wired" look about it, and it looks to be quicker and easier to get some solid connections. I've got some bigger SPDT "chunk-mongers"
instead of the mini-toggles:




Finding a suitable size (and cheap) enclosure is the thing!

Well done though, good job fella!


----------



## Marcomel79

Barnsley Boy said:


> @Marcomel79 Lovely neat job. My soldering iron is barely cool, and I'm planning my next build. That's some serious gauge of bare copper wire. I'm tempted to go down the same route with the next one. It's got that "hard-wired" look about it, and it looks to be quicker and easier to get some solid connections. I've got some bigger SPDT "chunk-mongers"
> instead of the mini-toggles:
> 
> View attachment 101391
> 
> 
> Finding a suitable size (and cheap) enclosure is the thing!
> 
> Well done though, good job fella!


Thanks you @Barnsley Boy, i think i might have used 16awg wire thats why it didnt fit in the lugs of the mini switches, and so i had to improvise... those big toggle switches look nice and sturdy although they may take a big chunk of real estate in your build.

I think hammond makes the best value for your money enclosures. Good luck with your next build. I cant believe how good this thing sounds!


----------



## Dogs of Doom

@Gene Ballzz started a thread:

Completed JohnH Attenuators?

for finished units.

One thing I might add, is that you might be a little more technical in there. Like list the parts, how many watts the unit is & what load variables, etc.

Everyone's units will probably have a little variance. But, giving people understanding to what you built, how you built it, & the end spec, will add a lot, IMO...


----------



## Gene Ballzz

@Dogs of Doom 
You make a very good point! On the other hand, all the technical info is already located here and my main impetus was to get a general count and simple showcase of successful builds? I was not necessarily intending another lengthy thread with all the nuts and bolts details that already exist here. If one were to read through this lengthy thread, I'm guessing 35 to 50 builds, not counting those that have likely been built without any thread participation?
Thanks For The Comment & Thoughts,
Gene


----------



## lordjester

Hi John,
first, thanks a lot for your incredible work. I live in germany and just wanted to order the parts to build the M2. Now, it seems to be impossible, to get the right air coil here. I can find 0,82 mH or 1 mH, both AWG 18. So, does this change any of the resistor values? I want to build the 8 Ohm version!

Thanks, Lothar


----------



## brad Messier

I have been using my 4ohms M2 modified unit for about 2 weeks now- it is GREAT! Full disclosure, I also built a L-pad design attenuator with a pretty basic design. Main feature on that one is the knob. I do like the ability to make volume adjustments with the knob. But tone wise it is not even close! L-pad had quite a bit of speaker distortion, especially when pushed by humbuckers. No such issues with the M2 unit. My 5f1 clone is not heating things up at all, barely warm to the touch after an hour or so of playing. Glad I put in the line out, have been able to run signal into my computer no problems, love it. Thanks again for all the help and inspiration. My guitars and amp will get a lot more attention now and I have learned a lot!


----------



## JohnH

Hi @brad Messier , that's great and thanks for the update!


----------



## JohnH

lordjester said:


> Hi John,
> first, thanks a lot for your incredible work. I live in germany and just wanted to order the parts to build the M2. Now, it seems to be impossible, to get the right air coil here. I can find 0,82 mH or 1 mH, both AWG 18. So, does this change any of the resistor values? I want to build the 8 Ohm version!
> 
> Thanks, Lothar



hi Lothar and welcome to our thread.

For the inductor coil, where I put 0.9mH on the M2 diagram, the range 0.82 or 0.9 or 1.0 is really a sweet-spot where it doesn't make any significant difference that you could hear, ie a small fraction of a db. So if you can't find 0.9, either 0.82mH or 1.0mH is OK. Its not very critical in this range.

If you'd like a technical basis for choosing (even though you wont hear it!), I'd pick 0.82 if your main use is at low attenuation, like -7db or -10.5db (small amp at home or medium amp at rehearsal etc). If you will most commonly run at more than -15db attenuation, then Id pick 1.0mH. The reason for the difference is the bit of inductance from the real speaker that reaches the amp at low attenuation.

But, back to your actual question none of that affects the resistor values.


----------



## TomBallarino

lordjester said:


> Hi John,
> first, thanks a lot for your incredible work. I live in germany and just wanted to order the parts to build the M2. Now, it seems to be impossible, to get the right air coil here. I can find 0,82 mH or 1 mH, both AWG 18. So, does this change any of the resistor values? I want to build the 8 Ohm version!
> 
> Thanks, Lothar



Hey Lothar,

try reichelt.de : VIS SP 5013 from a company called visaton. I bought mine there and they work great,

Tom

EDIT: Sorry, I understood that you CAN'T find these values..


----------



## BlueX

botacco said:


> Does anybody created a shopping list for the needed components from any European store? I am getting crazy finding the inductors for a reasonable price in Finland. Only find very expensive "audiophile" ones (starting from 25 euro each)



I'm also looking into this, from Sweden. Let's see if we can get a vendor list together, at least.


----------



## BlueX

I made a parts list for M2, including Out 3, for sourcing components (coil and resistors) in Europe. Switches, jacks, or wires not included.

Air core coil in the region of 0,9 mH was not easy to find, but a Danish webshop (netsound.dk) for speakers and accessories had one (0,9 mH, 0,39 Ohm, AWG 17 or 1,2 mm wire). That might work. Seems like this webshop ships outside DK.

For the resistors I checked elfa.se (part of Distrelec). They are usually well stocked.


----------



## BlueX

Some questions (8 Ohm M2, with Out 3)

Diagram seems to show input/output jacks with ground/ring/tip (stereo), but the ring not connencted. Can I use ground/tip (mono) instrument jacks?

Out 1 and Out 2 (8 or 16 Ohm): Is this to connect either 1x8 Ohm, or 2x16 Ohm (as on most amps)? Then I guess Out 3 would be for 1x16 Ohm?

Does M2 have four different "modes": Stage 1, Stage 2, Stage 3, and Stage 4 (S1, S2, S3, S4)? Would "bypass" be a fifth stage (Stage 0)?

Should the stages be used separately from each other, or can you activate two or three modes at the same time?

In the diagram, are the switches for S2, S3, and S4 drawn in the activated or non-activated position? (as I understand ON/ON switches should be used)

(Edit: Multi-layer rotational switches are really expensive. I'll stick with toggle switches.)


I'm planning to build an 8 Ohm M2/Out3 attenuator. I've read the first post, and browsed this thread. However, I could not figure out the above topics. Appreciate input on this, and also want to thank @JohnH for all work you put into this!


----------



## JohnH

Hi @BlueX , thanks for the Q's

_1. Diagram seems to show input/output jacks with ground/ring/tip (stereo), but the ring not connencted. Can I use ground/tip (mono) instrument jacks?
_
Yes you can, except if you do Out3, it uses the ring terminal. But, having tried both, we find that the stereo jacks with their extra lug grip the plugs much more nicely, so I recommend them for all.


_2. Out 1 and Out 2 (8 or 16 Ohm): Is this to connect either 1x8 Ohm, or 2x16 Ohm (as on most amps)? Then I guess Out 3 would be for 1x16 Ohm?_

Yes Out 3 is for 16 ohm speakers in an 8 Ohm M2. Its a tonal adjustment to make them sounlld more correctly. You can put a 16 speaker into Out 1 or 2 and its totally safe. But you find a bit more mids and less treble and resonance. If you build it, try it both ways.


_3. Does M2 have four different "modes": Stage 1, Stage 2, Stage 3, and Stage 4 (S1, S2, S3, S4)? Would "bypass" be a fifth stage (Stage 0)?_

They are stages of attenuation. Stage 1 is always on, and you can switch 2, 3 and 4 on or off in any combination while in use. With stage 1 fixed and -14, -7 and -3.5 stages switched, you can find any increment of 3.5 between -7 and -31.5db. 

We dont show a full bypass switch on the current diagrams. There's no fundamental problem with having it, but it needs a lot more care because it is fully interrupting the entire power of the amp. It should never be operated unless the amp is off or on standby. Very few players really need a bypass switch because if you dont want the attenuator, then you can wire up without it! So I omitted it for safety. But some might need one, for a fixed installation.


_4. Should the stages be used separately from each other, or can you activate two or three modes at the same time?_

See 3


_5. In the diagram, are the switches for S2, S3, and S4 drawn in the "On" or Off" position?_

The M2 diagram in post 1 is at max attenuation, all stages actively reducing level. Imagine moving a switch, and you can see that it will disconnect one resistor to ground, and bypass another, to negate that stage.

_
(Edit: Multi-layer rotational switches are really expensive. I'll stick with toggle switches.)_

Yes they are! much cleaner and simpler to get three toggles that give you 8 settings with simple DIY wiring and high current capacity. Once you have tested, you tend to know where to set the switches and just leave them set there.


----------



## BlueX

JohnH said:


> Yes you can, except if you do Out3, it uses the ring terminal. But, having tried both, we find that the stereo jacks with their extra lug grip the plugs much more nicely, so I recommend them for all.



Thanks for very clear answers, @JohnH !

Still one question on the stereo jack (for Out 3): Does the "ring lug" connect to ground on the speaker cable? In other words, is R10 connected to ground?


----------



## JohnH

hi @BlueX , Yes R10 is grounded via the jack when you insert a plug into Out 3


----------



## Gene Ballzz

BlueX said:


> Thanks for very clear answers, @JohnH !
> 
> Still one question on the stereo jack (for Out 3): Does the "ring lug" connect to ground on the speaker cable? In other words, is R10 connected to ground?





JohnH said:


> hi @BlueX , Yes R10 is grounded via the jack when you insert a plug into Out 3



As a quick reminder to all. We need to be careful thinking and speaking of *"ground"* on these units, as no conductor should ever make contact with the chassis. This includes avoiding the use of Switchcraft type jacks, unless they have the appropriate "isolating" washer sets on them. It would likely be a good idea to completely avoid the use of the term *"ground"* and instead refer to +/positive & -/negative, in relation to the jacks!

While I'm not sure that anyone has been able to explain exactly why, the only failures/snafus/complications with these units have surrounded the use of jacks *"grounded/connected"* to the chassis. We need to remember that we are dealing with AC voltage and current here and that even the -/negative is actually +/positive around 50% of the time and vice versa!

The term *"ground"* should only be used to refer to an actual connection to *"Earth/Ground"* which may or may not even happen at the amp's output jacks!

And while I'm not by any means and Electrical Engineer, I *HAVE* been *ZAPPED *by a speaker cable at a Holiday Inn!   

Just Sayin'
Gene


----------



## BlueX

I think the correct terms for tele plugs are: Tip/Ring/Sleeve, TRS (and not Tip/Ring/Ground). Good to point out if Sleeve should be separate from chassis.



Gene Ballzz said:


> avoiding the use of Switchcraft type jacks



Could you recommend any type/brand? I have Switchcraft jacks on my parts list:
https://www.elfa.se/sv/paneljack-ve...q=&pos=1&origPos=5&origPageSize=50&track=true


----------



## JohnH

With those ones, the barrel is connecting to the case. So for us, one end of your output transformer is live the the attenuator case! Not good. So use the plastic ones as on Marshall amps. The classic brand is Cliff, or similar.


----------



## Gene Ballzz

Use either these jacks, we prefer the TRS/stereo for the added support:

https://www.tubesandmore.com/products/14-jack-cliff-solder-lugs-switched

And be careful that they are actually CLIFF UK, as there have been some really crappy knockoffs labeled simply Cliff, in recent years!

Or get these isolation washer sets for switchcraft jacks, although I kind of view them as a band aid type fix:

https://tubedepot.com/products/isol...6V6yH1j0l8QbDo65wH6E1zaHNOrpcrusaAhXjEALw_wcB

I vastly prefer the CLIFF UK style units for this build! I think the similar Neutrik and REAN units are up there in quality also.

Just My Take On It,
Gene


----------



## BlueX

JohnH said:


> With those ones, the barrel is connecting to the case. So for us, one end of your output transformer is live the the attenuator case! Not good. So use the plastic ones as on Marshall amps. The classic brand is Cliff, or similar.





Gene Ballzz said:


> Use either these jacks, we prefer the TRS/stereo for the added support:
> 
> https://www.tubesandmore.com/products/14-jack-cliff-solder-lugs-switched
> 
> And be careful that they are actually CLIFF UK, as there have been some really crappy knockoffs labeled simply Cliff, in recent years!



Thanks for pointing this out! Better to make the mistakes before ordering components.

I made a new parts list (version 2, see enclosure), for anyone in Europe (or elsewhere) interested in sourcing this.

Jacks (5A rated, isolated plastic housing, stereo TRS. Hope I get proper Cliff products): https://www.digikey.se/en/products/detail/cliff-electronic-components-ltd/CL12207A/13663628
Switches (5A/125VAC rated, ON-ON, Single-Pole/Double-Throw (SPDT)): https://www.digikey.se/en/products/detail/nidec-copal-electronics/ET105D12-Z/5086742
Resistors now from Digi-Key. They offer free shipping to Europe on orders already from about EUR 50. Let's see when I get the customs invoice also.

Found a Swedish supplier of air core coils (hifikit.se): Jantzen coil 0,8 mH, 1,4 mm wire (think that is gauge 15 or 16). They also have 1,0 mH (1,4 mm), but not 0,9 mH: https://www.hifikit.se/komponenter/filterkomponenter/spolar/spole-luftlindad-o1-4-mm-trad.html


----------



## Barnsley Boy

JohnH said:


> Hi @Barnsley Boy
> 
> 
> 
> This is what a -28db stage looks like:
> 
> View attachment 101309
> 
> 
> To switch it, see my previous post above, where a similar three resistor arrangement is used in Stage 2 of an M2.
> 
> Values for this as a -28db stage are as follows (8 ohm build then 16 Ohm): Power rating is for a 50W amp.
> 
> RA 6.8 12 25W
> RB 1.2 2.7 5W
> RC 18 39 1W
> 
> Alternatively, you could just build another -14dB stage, to get overlap between ranges.




@JohnH I'm just waiting for the 6.8 ohm 25w resistor to arrive (hopefully in time for some weekend soldering shenanigans).
Thought I had better check, The best I could do on the 1.2 ohm 5w is a wire wound ceramic box resistor:




I'm guessing that one of these would be ok?


----------



## JohnH

Barnsley Boy said:


> @JohnH I'm just waiting for the 6.8 ohm 25w resistor to arrive (hopefully in time for some weekend soldering shenanigans).
> Thought I had better check, The best I could do on the 1.2 ohm 5w is a wire wound ceramic box resistor:
> 
> View attachment 101681
> 
> 
> I'm guessing that one of these would be ok?



Yes that's fine, I'd do the same.


----------



## matttornado

Hello John.

So to be clear, with a 16 ohm build, either a single 16 ohm cab, two 16 ohm cabs ( 8 ohms total) or a single 8 ohm cab an be used, correct? The attenuator doesn't care about the speaker load, it's whats going into it from the amp that matters, right?

I'm about to get a second 16ohm 4x12 so I'm just checking. I probably asked this before sorry. 

I been setting my amp to 8 ohms going into the 16 ohm attenuator because I like the sound better. I always done this going into a 16 ohm cab. That's ok too, right?


----------



## BlueX

Gene Ballzz said:


> And be careful that they are actually CLIFF UK, as there have been some really crappy knockoffs labeled simply Cliff, in recent years!




@Gene Ballzz , do you see anything strange with these jacks (labeled CLIFF UK)? Price was about USD 3 each at Digi-Key.

All resistors from Digi-Key measured well within tolerance. I placed the order this Wednesday, and got all goods (no back-orders) today Friday, to Sweden with free shipping (from US). First time I used this supplier, and I'm not disappointed.


----------



## Gene Ballzz

BlueX said:


> @Gene Ballzz , do you see anything strange with these jacks (labeled CLIFF UK)? Price was about USD 3 each at Digi-Key.
> 
> All resistors from Digi-Key measured well within tolerance. I placed the order this Wednesday, and got all goods (no back-orders) today Friday, to Sweden with free shipping (from US). First time I used this supplier, and I'm not disappointed.
> 
> View attachment 101819




Nothing at all wrong with those jacks. They're even the higher priced "gold" contacts! While the tolerances of the resistors are not critical, closer is always "Mo Betta!" It's good to know that the free shipping came through for you and that DigiKey promptly ships to the EU. This will be handy to know, for others over there!
Thanx 4 Sharin',
Gene


----------



## JohnH

matttornado said:


> Hello John.
> 
> So to be clear, with a 16 ohm build, either a single 16 ohm cab, two 16 ohm cabs ( 8 ohms total) or a single 8 ohm cab an be used, correct? The attenuator doesn't care about the speaker load, it's whats going into it from the amp that matters, right?
> 
> I'm about to get a second 16ohm 4x12 so I'm just checking. I probably asked this before sorry.
> 
> I been setting my amp to 8 ohms going into the 16 ohm attenuator because I like the sound better. I always done this going into a 16 ohm cab. That's ok too, right?


Hi Matt

Yes you can take the 16 Ohm amp tap into the 16 Ohm attenuator, and run it with a one or two 16 ohm cabs,, or an 8 Ohm cab. Its totally safe because the amp sees close enough to 16 Ohms all the time. And the tone is very close too. In theory, you may hear a tad more presence when using 8 or 2x16 cabs, or you may not notice it. easily adjusted for if needed. 

If you are using an 8 Ohm tap into a 16 Ohm cab or 16 Ohm attenuator, then you have a mismatch. The attenuator wont mind but you have to be happy that it is OK for your amp, and that is your call. Personally I prefer not to do that but there are many who will have different opinions.


----------



## JohnH

hi @BlueX , ,yes I agree those jacks look great and will do any position in any of our diagrams on this thread.

This is a good time to point out a 'trap for young players' , for others reading this:
Those jacks have 6 lugs, and mostly we are only using the main contacts that connect to the plug when its inserted. We have had a couple of instances where builders have wired to the other switched lugs instead, which are not connected when a plug is in, and then the amp sees no load! It is easy to get left and right swapped if you don't observe the jack rightly.

So on Genes layout on p110, and mine soon after, we show wires across both sides of the jack, so this problems cant occur.


----------



## BlueX

JohnH said:


> So on Genes layout on p110, and mine soon after,




It's on p111 now, at least on my browser, but thanks for point this out. Valuable info during building and testing!


----------



## sah

I finally finished the main part of my build. I'm super impressed so far. It sounds great!

What I've built is basically a 4/8/16 M2v 50W with a few extras to enable easy experimentation: a bypass switch, a switch that replaces L1 with a short to get the 8 ohm resistive design, and an insert jack for an external (bulky!) L2/C1 unit to get an M3 (the jack is between L1 and R1, and shorts when disconnected). My L2 is on backorder, so that piece will come later.

Compares very well against the Weber MicroMASS I use with my 4 ohm 5W amps. I can dial in a good tone with the MicroMASS, but the M2v hits the ideal I'm aiming for or maybe slightly improves on it, with no dialing. I'll probably wind up putting together a little 4 ohm, 5W version of an M2 to build into my 5F1 Champ.

With my 15-22W amps, I find 28dB attenuation is about right for daytime apartment use. For late night playing I wind up fully engaging everything and still being a little careful with the volume. I could maybe use an extra 7dB or so.

Playing around with the L1 bypass (in an 8 ohm setup, so the other inductor doesn't come into it), I think I agree that it makes a small but significant difference that is a little bit hard to pinpoint. I think it's only certain frequencies and only noticeable when I dig in, but the result is that I enjoy playing more when L1 is engaged.

Not the prettiest build this thread has seen by a long shot — you can definitely tell I don't have access to a drill press. But, it's all working and provides a lot of flexibility. Probably doesn't matter much, but I tried to keep the inductors as far apart as I could, and to avoid pointing the bulk of the coils straight at much of anything.

Still to come: the external M3 add-on, a line out, and some recordings to compare different configurations!


----------



## JohnH

hi @sah Thanks very much for posting and that's an awesome project to put all that together! Your impressions and testing will be a very valuable bench mark that will help this design move further forward. I'll be very interested in anything you post about how it it working out in its many settings. Not many builders will need all those features but having them in your one main unit allows valid comparisons to be made.

So far, it all seems as expected. In particular, your description of the resistive/reactive switch. For small signals and very clean tones, it's the balance of the resistive parts that mainly control the tone, although the coil does help to control the tone across different amps. But as you note, it's when you dig in at high power that the reactive elements make the notes sizzle more by drawing more power from the amp.


----------



## sah

I edited my previous post for posterity because I realized I somehow made two different mistakes when I reported the attenuation I'd been using. -28dB (not 10!) is my idea of the right level for daytime apartment playing with a 15-22W amp.


----------



## JohnH

sah said:


> I edited my previous post for posterity because I realized I somehow made two different mistakes when I reported the attenuation I'd been using. -28dB (not 10!) is my idea of the right level for daytime apartment playing with a 15-22W amp.


I find about the same with my rig, when there are others who I don't want to disturb too much. My amp is probably one step more power than that, so I use one click more attenuation at -31db


----------



## mike_lawyer

Hey guys, I have been tied up with different projects but now have all my supplies and am ready to go. I have an air core resistor but no great way to hold it to the chassis, like a nylon spool. Does anyone have any ideas?

Also, following up on the previous post, if one wanted to add one more attenuation stage, what would the resistor values be?


----------



## JohnH

hi @mike_lawyer , If your coil is the type that has no spool, you can mount it with zip ties to the base of your chassis. Put a spacer under it, maybe wood, a few mm (say 5 or more) thick for insulation and to reduce induction from coil to case.

If you want more attenuation, you can add another stage using the same values as the current ones. You can chose whether it's an extra -14, -7 or -3.5 stage. -7 db is a factor of 1/5th on power. -14db is 1/25th


----------



## ozz007

Hi guys! 
I'm new here and this is my first post. Uff!! I just finished reading *all* this amazing and very instructive thread. I'm definitely going to build an M2. Thanks very much especially John!


----------



## Gene Ballzz

ozz007 said:


> Hi guys!
> I'm new here and this is my first post. Uff!! I just finished reading *all* this amazing and very instructive thread. I'm definitely going to build an M2. Thanks very much especially John!



@ozz007 ,
*FIRST>*  to the forum!
*NEXT> *That was a very wise, though tedious and lengthy move to read this monstrous thread! Now you have a good overview of the different options, why they exist and a good selection of choices for how you want to do your build, as well as parts sourcing, etc, ad nauseam!

If you enjoy the sound and feel of a well cranked tube amp, I'm betting you will find this unit to be the most liberating and often used piece of gear you've ever owned!

Happy Building,
Gene


----------



## BlueX

JohnH said:


> Wire for hookup and also the winding of air-cored inductors should be 18 gage for 50W attenuators, and this is also OK for a 100W one, if built to the 16 Ohm values.



A question on wire gauge: In the conversion tables I can find, 18 gauge corresponds to diameter 0.0403", or 1,024 mm. This would mean 0,82 mm2 cross-section area. When I source electrical wire in Europe, wire size is usually given as cross-section area in mm2.

Anyone who can comment if I have made the right selection with 0,82 mm2?


----------



## Gene Ballzz

@BlueX

Does this help ya out?








If not, heres a link to a goggle list of multiple charts, giving various comparisons:

stranded wire gauge sizes

Just Attenuatin'
Gene


----------



## BlueX

Gene Ballzz said:


> Does this help ya out?



Thanks @Gene Ballzz ! Yes it does, to some extent.

Seems like cross-section for AWG 18 varies depening on stranding, from 0,82 to 0,96 mm2 (or from 100 to 117%). The wire I have planned to use is solid, and measures 1,1 mm diameter. That's 0,95 mm2, and should be OK.


----------



## JohnH

I think you are ok with all that. I've been assuming 18 gage is 1mm diameter.


----------



## BlueX

JohnH said:


> I think you are ok with all that. I've been assuming 18 gage is 1mm diameter.



Thanks! I thought it could be of general interest to have the "SI" dimensions also.


----------



## mike_lawyer

BlueX said:


> Thanks! I thought it could be of general interest to have the "SI" dimensions also.


I have two cliff jacks, but they have 4 solder lugs instead of 6. Does it make any difference for this build?


----------



## JohnH

mike_lawyer said:


> I have two cliff jacks, but they have 4 solder lugs instead of 6. Does it make any difference for this build?


Sounds like a mono jack. That'll work for everything except if you are building an 8 Ihm M2, and you want to include Out 3 which is for a 16 Ohm speaker. 

Also, the stereo jacks just feel better, they grip firmer due to the extra spring contact, even if it's not connected.


----------



## NoNaMe

The simplest attenuator for Effects loop. Marshall MA100C. Works perfect!


----------



## Barnsley Boy

NoNaMe said:


> The simplest attenuator for Effects loop. Marshall MA100C. Works perfect!
> 
> View attachment 102109



Ummmm!! ..... not really! That if I am very much not mistaken is a volume control that sits between the pre and power amp stage. I have a Donner Emo amp controller (effectively the same thing) that I use with my Peavey Classic VTX. It's the only way I can get it anywhere near domestic playing levels.

The VTX is my next candidate for one of @JohnH 's super duper attenuators. @NoNaMe please do yourself and your lovely Marshall a favour and get building!!!


----------



## JohnH

NoNaMe said:


> The simplest attenuator for Effects loop. Marshall MA100C. Works perfect!
> 
> View attachment 102109


That's a neat version of how to put a volume control in the FX loop. I wonder if the spacing between jacks is consistent enough for it to fit across a range of models? 

That sort of simple device will let you reduce the volume if you don't have a master volume control, or as an alternative to it. Your poweramp stage will stay clean and you find your tones in the preamp or with pedals. But as BB notes, what it cant give you is the distinctive tone, and dynamic feel, of a power amp running hard at high power. For that the power amp actually has to be working hard. Its that which our attenuator is designed to provide, at any volume that you want.

Your MA is likely to to respond well to that.


----------



## mike_lawyer

In terms of switches, what amp and voltage ratings are fine for the DPDT switches? I have some rather large switches on hand but they are harder to throw, I'd like to get some mini toggles instead.


----------



## Barnsley Boy

Afternoon all,

The soldering iron has been unsheathed again, with an additional -28db stage added to my 8 ohm M2, and replacement of mini-toggle switches with some serious chunk-mongers. Pics will be posted on the completed build thread in due course, but at the moment I'm struggling getting the -28db to work. 

For me to get it clear in my head, I have to draw it out so that it makes sense. 

@JohnH 's M2 design schematic, with the stages re-ordered in ascending order, and additional stage tagged onto the end:






To make sure I understood the switching for that last stage, I extracted it and "exploded" the DPDT switch:





Finally, I incorporated into my colour coded pictorial layout, loosely based around @Gene Ballzz 's really helpful layout diagram, but adjusted to suit the layout in my enclosure:





Note - stage 2, 3 and 4 have been drawn as SPST switches to make it a bit easier to trace the signal path, and to reflect my latest modifications.
When I throw the switch to engage stage 5, it cuts off the sound. Disengaging, and the other stages work perfectly (as they did previously before I started tinkering).

@JohnH and @Gene Ballzz can you see any mistake that I may have made?


----------



## JohnH

hi @Barnsley Boy , the schematic bits look ok. On the wiring diagram, the -28 stage is first after stage 1 and that's ok too. I think RA and RC are swapped. Coming from the amp side, the signal should go through RA (6.8) first. But that swap shouldn't cut out all sound. I may be missing something, or there could be something in the build not on the diagram.


----------



## Barnsley Boy

Hi @JohnH .... Ah, you may have a point!!




I think maybe it should look like the image above. I'll fiddle around with it tomorrow and see if it makes a difference. The cutting out bit may just be a bit of dodgy soldering on my part.

Am I correct in assuming that connecting a multimeter across one of the output jacks and switching through each of the combinations from -7db up to -59.5 (GULP) db will give me a progressively higher resistance reading? I've worked out that it will give 16 combinations in -3.5db steps? Should the difference in resistance be the same for each step? I'm thinking that it would be a good measure of checking that everything is working correctly.

Thanks for coming back to me on this, your time and effort is very much appreciated


----------



## Lancer X

Hi @JohnH , @Gene Ballzz and attenuation crew. Thanks for sharing your amazing work with with the community!

My questions will require some setup. Like @len_zwerf , who posted a few pages back, my most recent project was a micro amp. In my case, Rob Robinette's Deluxe Micro Mod design, which is a 1.0-2.0W single-ended class A version of a 5E3 Tweed Deluxe. I built it in a Vibro Champ combo cab (open-backed ) with a 10" Weber 10A100T alnico speaker (8 ohm). Rob's design does include a switchable negative feedback loop option, but of course the default Tweed setting does not.

The point of this amp build was to simplify my practice rig. Previously, I had built a full-sized 5E3 and ended up getting a Weber MiniMASS since a Tweed Deluxe is *not *family-friendly once you get it singing. Instead, this little Deluxe Micro is only 1W and offers a Master volume. Still, I would like to keep the power section grooving without ticking everyone off, but even this little amp can still make about 95 dB. Your M2 seems like just the ticket.


*Question #1* - if my goal is limiting to ~60-70 dB with the master dimed, which M2 stages do you recommend that I include? I assume that your -7 or -14 dB references are relative to a 50W amp, right??

*Question #2* - could I order say 10W and 5W power rating resistors, since this amp is only 2W max?

*Question #3* - would anyone be interested in assisting me to tweak the M2 design to model the reactive response of an alnico speaker in an open-backed cab? _(Or perhaps that exercise isn't necessary??)_ If so, I'd propose a 12" Jensen P12R 8 ohm as our model - Jensen publishes the impedance response curve on the product page. Should be broadly applicable for vintage "American" amps, and useful info for the "rest of us" who aren't rocking Marshalls. (Expecting that '65+ Fender-style designs probably track pretty well with your existing ceramic Greenback model.) Maybe this is a deep endeavor - I don't want to presume? But willing to lend my effort if doable and of interest.


----------



## JohnH

Hi @Lancer X , welcome to our world!

Taking on your numbers at face value, if the dimed amp can do 95db, and you might like to hear that as low as 60db, then that's a reduction of -35dB. The full M2 is -31.5db, but I'm sure that that will be enough. But you might not want to leave any stages out, just in case you need them sometimes. I reckon you'll mostly use less attenuation though.

Yes you can scale down the power rating of the resistors. For a nominally 50W amp, R1 is spec'ed at 100W rating, ie 2x the amp power. It takes about 30% of the power of the amp, which gives is a factor of x3 for a case-mounted resistor. You might chose to try the ceramic block resistors, for which Id give a bigger factor. say 5. 2W x 30% x 5 = 3W. So you could go 5W or 10W.

Id still suggest to use the same 18 or 19 gage coil. Obviously it can handle a lot more power, but we need it also to be low resistance too. 

On the Jensen speaker, actually, no need to change anything if you are building the M2 design, and will be using that speaker with the attenuator. The coil in conjunction with the circuitry around it is modelling the rise in impedance at higher frequency, That Jensen is within about 10% of the Celestion G12M measurements in terms of high frequency impedance. At the low end, the resonance and the influence of the cab type is not modelled, because your real speaker will do that itself. Instead, the attenuator is trying to feed a signal to the speaker like that of an amp. so the speaker can do its own thing. Feed it into a closed back cab instead and that is what it will sound like.

We do however, also have design M3, which does indeed model the bass resonance. this is mainly for when it is used as a loadbox with no cab. In that case, a resonance could be matched to the Jensen. But with a cab, this makes a very slight difference, only if you play the low bass notes, and generally we have found its not really needed of you are using a guitar cab.


----------



## JohnH

Barnsley Boy said:


> Hi @JohnH .... Ah, you may have a point!!
> 
> View attachment 102194
> 
> 
> I think maybe it should look like the image above. I'll fiddle around with it tomorrow and see if it makes a difference. The cutting out bit may just be a bit of dodgy soldering on my part.
> 
> Am I correct in assuming that connecting a multimeter across one of the output jacks and switching through each of the combinations from -7db up to -59.5 (GULP) db will give me a progressively higher resistance reading? I've worked out that it will give 16 combinations in -3.5db steps? Should the difference in resistance be the same for each step? I'm thinking that it would be a good measure of checking that everything is working correctly.
> 
> Thanks for coming back to me on this, your time and effort is very much appreciated


Diagram looks better....

The resistance readings shouldn't continue to increase at higher settings. The idea is that they stay much the same.

With no amp connected, but speaker plugged in, the reading across a cord into the input (ie where the amp should go) should be 7 to 10 ohms in all settings, after subtracting for the meter leads. With the new -28db stage, id should be even closer, like 8 or 9 ohms.


----------



## Barnsley Boy

Nice one @JohnH . Fingers crossed, I should be up and running tonight!


----------



## Gene Ballzz

Lancer X said:


> Hi @JohnH , @Gene Ballzz and attenuation crew. Thanks for sharing your amazing work with with the community!
> 
> My questions will require some setup. Like @len_zwerf , who posted a few pages back, my most recent project was a micro amp. In my case, Rob Robinette's Deluxe Micro Mod design, which is a 1.0-2.0W single-ended class A version of a 5E3 Tweed Deluxe. I built it in a Vibro Champ combo cab (open-backed ) with a 10" Weber 10A100T alnico speaker (8 ohm). Rob's design does include a switchable negative feedback loop option, but of course the default Tweed setting does not.
> 
> The point of this amp build was to simplify my practice rig. Previously, I had built a full-sized 5E3 and ended up getting a Weber MiniMASS since a Tweed Deluxe is *not *family-friendly once you get it singing. Instead, this little Deluxe Micro is only 1W and offers a Master volume. Still, I would like to keep the power section grooving without ticking everyone off, but even this little amp can still make about 95 dB. Your M2 seems like just the ticket.
> 
> 
> *Question #1* - if my goal is limiting to ~60-70 dB with the master dimed, which M2 stages do you recommend that I include? I assume that your -7 or -14 dB references are relative to a 50W amp, right??
> 
> *Question #2* - could I order say 10W and 5W power rating resistors, since this amp is only 2W max?
> 
> *Question #3* - would anyone be interested in assisting me to tweak the M2 design to model the reactive response of an alnico speaker in an open-backed cab? _(Or perhaps that exercise isn't necessary??)_ If so, I'd propose a 12" Jensen P12R 8 ohm as our model - Jensen publishes the impedance response curve on the product page. Should be broadly applicable for vintage "American" amps, and useful info for the "rest of us" who aren't rocking Marshalls. (Expecting that '65+ Fender-style designs probably track pretty well with your existing ceramic Greenback model.) Maybe this is a deep endeavor - I don't want to presume? But willing to lend my effort if doable and of interest.



@Lancer X ,
*First>*  to the forum!

*Next>* I strongly recommend that you build the full blown, full wattage version of the M2. Once you plug your *"full sized"* 5E3 into the M2, your Weber MiniMass will collect dust! You may find that your Deluxe Micro also collects dust, as two triodes in parallel for the power section simply do not have the dynamics and overall sound of two 6V6s in push/pull.

While the Weber is a nice, convenient unit for some things (4/8/16 ohm settings) and just shaving *"a little off the top" *volume wise, it won't even come close to holding a candle to the performance of the @JohnH unit! I too own a MiniMass and will never get rid of it, but.......... Trust me when I state that you will not miss (or need) that Treble compensation switch of the MiniMass. Any initial, perceived cost, complexity and/or size savings of building a* "trimmed down"* version will end up, IME, being a waste of the extra effort! The M2 will quickly become the most used piece of gear in your arsenal and you will want to use it with any other amp you happen to acquire! The only part you might want to consider *"trimming down" *could be the extra, compensated 16 ohm output jack, if building an 8 ohm unit.

*Also>* Can we assume that both of the mentioned amps have an 8 ohm output tap and an 8 ohm speaker?

Happy Attenuatin'
Gene


----------



## Gene Ballzz

@JohnH ,
Has it ever been determined whether or not the 8 ohm M2 is as friendly to a 4 ohm load as the 16 ohm version is to an 8 ohm load?
Just Curious?
Gene


----------



## Lancer X

JohnH said:


> Hi @Lancer X , welcome to our world!
> 
> Taking on your numbers at face value, if the dimed amp can do 95db, and you might like to hear that as low as 60db, then that's a reduction of -35dB. The full M2 is -31.5db, but I'm sure that that will be enough. But you might not want to leave any stages out, just in case you need them sometimes. I reckon you'll mostly use less attenuation though.



*Thank you so much* for your advice, response, and generosity @JohnH and @Gene Ballzz !!

RE my 95 dB number, that was a quick back-of-envelope estimate using an online SPL calculator and the sensitivity of a Jensen P10R (Weber doesn't publish specs, which is super annoying). Calculator says 92-94 dB from clean to gritty (1.0-1.5W output). Please do correct me if my SPL assumptions are faulty!

I hadn't understood that the dB attenuation numbers were universal regardless of amp wattage - thanks. I will look up the theory/math on that topic.

One last question if you'd be so kind: from what I'm reading here, sounds like that at this low wattage a simple aluminum Hammond chassis should suffice as a heat sink?? How about if I build an M2 for my full-size Tweed Deluxe (15W, No NFB)?

Thanks again - so excited to build this critter!


----------



## Gene Ballzz

A Hammond 1590D will work well, but @JohnH prefers the slightly larger 1550G. These are the fairly thick die cast boxes, not the thin aluminum sheet metal units! I implore you to build it as a well ventilated tank and you will never have a problem!
Just Sayin'
Gene


----------



## Lancer X

Gene Ballzz said:


> @Lancer X ,
> *First>*  to the forum!



Thank you, sir! I arrived here specifically Googling for reactive attenuators. Lucky me!! 



Gene Ballzz said:


> *Next>* I strongly recommend that you build the full blown, full wattage version of the M2. Once you plug your *"full sized"* 5E3 into the M2, your Weber MiniMass will collect dust! You may find that your Deluxe Micro also collects dust, as two triodes in parallel for the power section simply do not have the dynamics and overall sound of two 6V6s in push/pull.



I may build one dedicated to the big 5E3, which I am selling, as an added feature. Likewise for the Deluxe Micro, integrating them into the back of the combo cabs. The 5E3 was my first build, so still jonzing for more soldering.

As such, I plan to build Rob Robinette's 2W Tweed 5F6A Bassman Micro with the proceeds of the 5E3 sale. Specifically, he also designed a variant with two EF80 pentodes in push-pull. 99% of my playing for the foreseeable future will be at home, and this little gal will be absolutely perfect for my needs. So, I think I'm right with you re my direction.

_That said, you'd be surprised how lovely this Deluxe Micro sounds with just that little 12AU7. While it likely differs a bit from a 5E3 (also lacks the cathodyne phase inverter of course), there's something special about single-ended Class A tone._



Gene Ballzz said:


> *Also>* Can we assume that both of the mentioned amps have an 8 ohm output tap and an 8 ohm speaker?



Yep, precisely!


----------



## Lancer X

Gene Ballzz said:


> A Hammond 1590D will work well, but @JohnH prefers the slightly larger 1550G. These are the fairly thick die cast boxes, not the thin aluminum sheet metal units! I implore you to build it as a well ventilated tank and you will never have a problem!
> Just Sayin'
> Gene



At the moment, I am chasing these Micro amps, so my application is a 2W bedroom amp. @JohnH confirms that I can get away with 5-10W resistors, and I'd like to go in that direction. (I'm not keeping the 5E3 regardless.) I want this attenuator to fit into, and be at least semi-permanently integrated in the amp chassis _(edit: oops - I meant inside the amp *cabinet,* not in the chassis)_. I know it's OCD of me, but the Weber MiniMASS sitting on top of my Deluxe always bugged me.

Given the 2W amp output and a M2 with 5-10W resistors, do you think that a ventilated sheet metal style chassis would offer sufficient cooling?


----------



## Gene Ballzz

Here is one of my M-Lite versions installed in my DSL20CR:


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> Has it ever been determined whether or not the 8 ohm M2 is as friendly to a 4 ohm load as the 16 ohm version is to an 8 ohm load?
> Just Curious?
> Gene


Yes indeed, it should be much the same. In theory a tad more high treble but unlikely to be noticed or easily adjusted for at the amp.

So an 8 Ohm M2 with all the Outs, is good for 16, 2x16, 8, 2x8 and 4 Ohm cabs.


----------



## JohnH

Hi @Lancer X , dB numbers are ratios expressed on a log scsle. So anytime you halve the power, that's -3db. 1/4 power is -6db. 1/10th power is -10db. Our first stage is -7db so that's 1/5th power. etc

At 2W from the amp, there's really not much power. So I think any sensible mounting will be fine.

For your building into the amp chassis, which I haven't tried, I'd watch out for placement of parts particularly the coil. Maybe test before committing. The coil may produce a tendency to induce signals into any input wiring nearby, particularly on its axis. And mount it with a spacer to the chassis, using non-ferrous bolts or other means such a zip ties.


----------



## Lancer X

@Gene Ballzz , I love your attenuator peeking out of the bottom back vent hole. That's a handy match up.



JohnH said:


> Hi @Lancer X , dB numbers are ratios expressed on a log scsle. So anytime you halve the power, that's -3db. 1/4 power is -6db. 1/10th power is -10db. Our first stage is -7db so that's 1/5th power. etc



Ahh, gotcha. As a scientist, you'd think I would have understood that unit better... That's why this new hobby is fun - learning!



JohnH said:


> At 2W from the amp, there's really not much power. So I think any sensible mounting will be fine.
> 
> For your building into the amp chassis, which I haven't tried, I'd watch out for placement of parts particularly the coil. Maybe test before committing. The coil may produce a tendency to induce signals into any input wiring nearby, particularly on its axis. And mount it with a spacer to the chassis, using non-ferrous bolts of other means such a zip ties.



Sorry, I misspoke! I intend to mount the M2 in its own metal enclosure inside the amp *cabinet* - outside of and separate from the amp chassis.

I have a lot of free space on the transformer side of the cab (furthest from the input section). Hopefully the metal enclosure will help with shielding too. Will try to use a box small enough that I can reposition the mount if needed.

Just to make sure I understood you correctly, ideally I want to point the torroid axis of rotation away from the input section? In other words, align the axis of the coil in the same orientation as the speaker cone axis?

Thanks again!! Time to place a Mouser order!


----------



## JohnH

Yes sounds OK! What does @Gene Ballzz think on these placement issues?


----------



## Lancer X

JohnH said:


> Hi @Lancer X , welcome to our world!
> 
> Yes you can scale down the power rating of the resistors. For a nominally 50W amp, R1 is spec'ed at 100W rating, ie 2x the amp power. It takes about 30% of the power of the amp, which gives is a factor of x3 for a case-mounted resistor. You might chose to try the ceramic block resistors, for which Id give a bigger factor. say 5. 2W x 30% x 5 = 3W. So you could go 5W or 10W.



Sorry, @JohnH - seeing now that this info was in your page 1 master post. Thanks for your patience.


----------



## Lancer X

Gene Ballzz said:


> @Lancer X ,
> *Next>* I strongly recommend that you build the full blown, full wattage version of the M2. Once you plug your *"full sized"* 5E3 into the M2, your Weber MiniMass will collect dust!





Gene Ballzz said:


> A Hammond 1590D will work well, but @JohnH prefers the slightly larger 1550G. These are the fairly thick die cast boxes, not the thin aluminum sheet metal units! I implore you to build it as a well ventilated tank and you will never have a problem!
> Just Sayin'
> Gene



You know what - you've convinced me. 

A 1590D will fit in my Deluxe Micro cabinet, so I'm gonna just build the 50W M2, with bypass I think. (Out of curiosity, I priced out resistors for a 25W model, and it was only $8 less.)

If I build the Bassman Micro as a piggyback head as planned, I can stuff the M2 in the speaker cab.


----------



## Barnsley Boy

I'm rarely speechless, but .......................


----------



## Barnsley Boy

................................


----------



## Barnsley Boy

...... come on man, get a grip on yourself!

It's all up and running and working correctly, and I have to say it is awesome. I have knocked my JCM 900 down from the beast that it was, to a purring kitten (with large teeth and claws). Thank-you @JohnH , @Gene Ballzz and all of the various contributors along the way - this is something very, very special.



JohnH said:


> Yes indeed, it should be much the same. In theory a tad more high treble but unlikely to be noticed or easily adjusted for at the amp.
> 
> So an 8 Ohm M2 with all the Outs, is good for 16, 2x16, 8, 2x8 and 4 Ohm cabs.



Now this has also got my interest too. Effectively John, what you are saying is my 8ohm M2 is also useable with my Peavey Classic VT and VTX (both of which run a 4ohm speaker load? If that is the case, happy days - I can just replicate the build with probably a few cosmetic tweaks and a good dose of "lessons learnt" from my first one. One thing though, the VTX is 1 65 watt amp, is there enough redundancy in the power resistors to accommodate this? or would I need to build a 100 watt 8ohm variant?


----------



## Barnsley Boy

Completed build just posted on @Gene Ballzz 's Completed Attenuators thread if anyone is interested.
​


----------



## JohnH

Hi @Barnsley Boy , great news on getting that going! 

Here's a maths puzzle, for anyone to try: If BB is running his 50W amp, at the max attenuation of -31.5db, plus the added -28db stage, how many Watts are reaching the speaker?

With all the different cab options that the 8Ohm M2 can do, just note that it has to always run from an 8Ohm tap on the amp. If that leaves out some amp/cab combinations that you need, a front end can be added to convert.


----------



## BlueX

JohnH said:


> Here's a maths puzzle, for anyone to try: If BB is running his 50W amp, at the max attenuation of -31.5db, plus the added -28db stage, how many Watts are reaching the speaker?


-3 dB is half the power (1/2), -6 dB one quarter (1/4 = 1/2 * 1/2), etc.
(-31,5) + (-28) = -59,5, or almost -60 dB. That equals -3 * 20, which would be (1/2)^20 = 9,5 E-7 = 0,00000095
50W * 9,5 E-7 = 4,8 E-5 W = 0,000048 W = 0,048 mW = 48 microW


----------



## Barnsley Boy

48 micro-watts, that's not very rock and roll at all!!!
Jimi would be spinning in his grave if he knew!


----------



## JohnH

Pretty close! The full logarithmic formula goes:

power ratio = P1/P2 = 10^(decibels/10)

or, decibels = 10.log(P1/P2)

In this case
P1/P2 = 10 ^(-59.5/10) = 0.00000112

power is therefore 50W x 0.00000112 W =* 56 microW*

I expect you're finding a setting to use quite a few dB's louder, but if you engage this full attenuation, can you still hear it?


----------



## Lancer X

@JohnH , @Gene Ballzz , and crew:

Here's a link to an interesting post about placement and orientation of a pair of air core induction coils versus its effect on measured inductance. Likewise, they discuss placement close to 15mm thick aluminum panels, which seems to have a major effect on inductance.

Now of course you guys have been building in ~2mm thick Al enclosures all along, and you're happy with the curves that your attenuator designs are generating, so this is largely academic. Thought it might be helpful for future designs though, since you're probably not getting the exact inductance value that you planned for the circuit.

Cheers!


----------



## Lancer X

@Gene Ballzz , I see that you're placing your attenuator flat in the bottom of your speaker cabinet. I assume this means you've never experienced any noise associated with the field generated by the inductance coil?

I want to place my M2 box in my amp cab. @JohnH was concerned about EMI from the inductor, and was asking your advice about placement.

Per my quick research, it seems that the strongest fields are generated in the direction of the axis coil (of course, since this is how electromagnets work). If you aren't suffering any noise from your coils aimed up towards your amp chassis, then I should be in good shape for pretty much any well-placed location.

My chassis doesn't have a slot in the bottom like yours (see below), so I may need to stand my box up on end. Was concerned about blasting my input section with noise.


----------



## Gene Ballzz

@Lancer X ,
Kudos on being so actively participatory as a new member! Great info on the coil proximity/orientation testing!

I've been using the 19 gauge from Madisound and @JohnH determined it would be fine for a 50 watt unit. All my units so far have been M-Lite and 16 ohm, requiring two coils. I believe Parts-Express has a fairly wide selection of Dayton, Jantzen and other brands, although none are bobbin wound, which I would prefer. I've since decided that cutting appropriate sized discs out of Lexan (or similar), wood or corian, with a hole saw makes for nice mounting. 

As for location, I've moved my units around, before mounting in both my combos (5E3 & DSL20) and found no audibly discernible artifacts, anywhere. In the 5E3 Deluxe (similar cabinet to yours) I simply mounted it in the bottom (feet for stand offs, of course) and while I can't see the switches or labeling, I know what's what and I just reach down in to switch it! What is that amp, anyway? Cool color for the Tweed!

Just Attenuatin'
Gene


----------



## JohnH

Lancer X said:


> @JohnH , @Gene Ballzz , and crew:
> 
> Here's a link to an interesting post about placement and orientation of a pair of air core induction coils versus its effect on measured inductance. Likewise, they discuss placement close to 15mm thick aluminum panels, which seems to have a major effect on inductance.
> 
> Now of course you guys have been building in ~2mm thick Al enclosures all along, and you're happy with the curves that your attenuator designs are generating, so this is largely academic. Thought it might be helpful for future designs though, since you're probably not getting the exact inductance value that you planned for the circuit.
> 
> Cheers!


Thanks for posting that. Yes it is interesting. I think I've seen something from that study before, though not that page.

It led me to try some tests of my own, using parts nearer to what we are using. I have a spare 0.4mH coil, and a lid from a Hammond box. In air it measures 0.40 mH using my meter. When it's placed down flat onto the lid, it read 0.40 or 0.39, like it's in between. That's as close as I can read it with the meter that I have. It shows that it's not a big effect though in this case. Lifting a few mm returned the full value. My coil is on a bobbin, so it has a spacer built in. Others are wound without a bobbin. I think a few mm spacer is worth having.

The Web page is using 3.3mH and 15mm, so as seen, there's a lot more effect then.

But what makes a huge difference is any steel bolt. My coil was about 25mm thick. A small 10mm bolt on-axis raised its value from 0.40mH to 0.44mH. I've tested bolts right through that can add 50% or more to coil inductance.


----------



## Lancer X

Thanks, guys! I ended up ordering a Dayton Audio coil from Parts Express. Will try to create a little spacer to place it centrally in the box. 

Kinda makes sense that you aren’t getting any exterior EMI effects if the field is that interactive with the aluminum box. 



Gene Ballzz said:


> @Lancer X ,
> What is that amp, anyway? Cool color for the Tweed!



The pic is my Deluxe Micro - built in a Blackface Vibro Champ cab. 

That tweed is actually Tolex! I wanted grey fabric tweed but couldn’t find it. The pattern is really indistinguishable from lacquered fabric from more than a foot or two away.

I wanted to make a crossover look between Blackface and Tweed cabinets. It’ll prob be done today - still fiddling with resistors to get the B+ voltages just right. I’ll post a glamour shot when it’s done.


----------



## Barnsley Boy

JohnH said:


> Pretty close! The full logarithmic formula goes:
> 
> power ratio = P1/P2 = 10^(decibels/10)
> 
> or, decibels = 10.log(P1/P2)
> 
> In this case
> P1/P2 = 10 ^(-59.5/10) = 0.00000112
> 
> power is therefore 50W x 0.00000112 W =* 56 microW*
> 
> I expect you're finding a setting to use quite a few dB's louder, but if you engage this full attenuation, can you still hear it?



@JohnH ...... a slight tweak to the calculation

10 ^(-59.5/10) = 891250.9381. 
Assuming P1 is the unattenuated power (in my case 50w), then the calculation should read:

P2 = 50/891250.9381 = 56 microwatts.

Just out of interest, I populated the table below showing the 17 increments of reduction (assuming the first one is with no attenuator connected) and the associated power reduction.




It's fascinating firstly how quickly the power drops, and secondly how bl**dy loud even 0.4 of a watt is!

With all stages engaged, what I would class as DEFCON 5, I would say that it is at a level where you could almost play the amp with everything cranked and it wouldn't wake a sleeping baby. Things get a lot hairier very quickly when the switches get flipped. It would be great to play at a volume more suited to the amp, and push the speakers to get that proper Marshall goodness, but without a soundproof enclosure and a decent pair of ear defenders, I'm going to have to wait!

My ears have never been the same since a Godfathers St. Valentines Day Masacre gig 5 years back. Tinnitus is a terrible thing, the ringing sound never stops!


----------



## JohnH

Hi @Barnsley Boy , thanks for the table! Isn't human hearing amazing that it's possible to hear that power level?


----------



## Lancer X

@Gene Ballzz we'll call this pretty much finished. (Switch 2 changes cathode bypass and cap options for V1A, and Switch 3 adds negative feedback and elimination of the bypass cap.)

Just need to relabel the controls: Input, Bright/Normal, Volume, Tone, Master, Voicing (Lead/Clean/Tweed), and Gain (Lo/Mid/Hi). Bought white-on-black tape for Susan's label machine, but we moved recently and it's still hiding at the moment... 




Sorry to hijack - back to attenuatin'...


----------



## Gene Ballzz

Lancer X said:


> @Gene Ballzz we'll call this pretty much finished. May tweak the B+2 voltage again later, but will try it a while like this for now. (Switch 2 changes cathode bypass and cap options for V1A, and Switch 3 adds negative feedback and elimination of the bypass cap.)
> 
> Just need to relabel the controls: Input, Bright/Normal, Volume, Tone, Master, Voicing (Lead/Clean/Tweed), and Gain (Lo/Mid/Hi). Bought white-on-black tape for Susan's label machine, but we moved recently and it's still hiding at the moment...
> 
> View attachment 102365
> 
> 
> Sorry to hijack - back to attenuatin'...



So COOL! 

Who made the "Lancer" logo plate? I'd love to have one, different logo, of course! 

Thanx 4 Sharon, She's A GOOD Girl, In The Worst Ways! 
Gene


----------



## Lancer X

Gene Ballzz said:


> Who made the "Lancer" logo plate? I'd love to have one, different logo, of course!



This Etsy shop. They use a "Fender" font, and these are cut from stainless. It looks really nice:








Custom Metal Guitar Amp Badge Custom Metal Name Badge - Etsy


This Hooks & Fixtures item by DroplightIndustries has 225 favorites from Etsy shoppers. Ships from Chalfont, PA. Listed on Dec 30, 2022




www.etsy.com





I have a cool idea for my final "company" logo, but that reveal has to wait for the Bassman Micro build...


----------



## Gene Ballzz

Lancer X said:


> This Etsy shop. They use a "Fender" font, and these are cut from stainless. It looks really nice:
> 
> 
> 
> 
> 
> 
> 
> 
> Custom Metal Guitar Amp Badge Custom Metal Name Badge - Etsy
> 
> 
> This Hooks & Fixtures item by DroplightIndustries has 225 favorites from Etsy shoppers. Ships from Chalfont, PA. Listed on Dec 30, 2022
> 
> 
> 
> 
> www.etsy.com
> 
> 
> 
> 
> 
> I have a cool idea for my final "company" logo, but that reveal has to wait for the Bassman Micro build...



Thanks for that,
Gene


----------



## XTRXTR

JohnH said:


> Here's a maths puzzle, for anyone to try: If BB is running his 50W amp, at the max attenuation of -31.5db, plus the added -28db stage, how many Watts are reaching the speaker?


I did this without using any log calculation. 

I could do the log but I thought this was a good enough approximation and it follows the idea that for every -3db you get half of the power. 31.5db/3 is 10 with a remaining 1.5 or its half way to 11 times or 10.5.
50x.5^10.5 = 34.5 milli-watts
and now we do a similar thing with the additional attenuation of -28db 28/3=9.33
the final equation then
50x.5^(10.5+9.33) = .000053523-->54 micro-watts


----------



## XTRXTR

I think my calculation is slightly off because -3db isn't exactly or precisely half but is close at 0.501.
50*.501^(10.5+9.33) = 0.000055687-->56 micro-watts


----------



## JohnH

XTRXTR said:


> I think my calculation is slightly off because -3db isn't exactly or precisely half but is close at 0.501.
> 50*.501^(10.5+9.33) = 0.000055687-->56 micro-watts


All good! yes -10 db is exactly 1/10th power, per the log definition. The other simple values that I listed are very close but not absolutely exact.


----------



## ibmorjamn

Barnsley Boy said:


> @JohnH ...... a slight tweak to the calculation
> 
> 10 ^(-59.5/10) = 891250.9381.
> Assuming P1 is the unattenuated power (in my case 50w), then the calculation should read:
> 
> P2 = 50/891250.9381 = 56 microwatts.
> 
> Just out of interest, I populated the table below showing the 17 increments of reduction (assuming the first one is with no attenuator connected) and the associated power reduction.
> 
> View attachment 102341
> 
> 
> It's fascinating firstly how quickly the power drops, and secondly how bl**dy loud even 0.4 of a watt is!
> 
> With all stages engaged, what I would class as DEFCON 5, I would say that it is at a level where you could almost play the amp with everything cranked and it wouldn't wake a sleeping baby. Things get a lot hairier very quickly when the switches get flipped. It would be great to play at a volume more suited to the amp, and push the speakers to get that proper Marshall goodness, but without a soundproof enclosure and a decent pair of ear defenders, I'm going to have to wait!
> 
> My ears have never been the same since a Godfathers St. Valentines Day Masacre gig 5 years back. Tinnitus is a terrible thing, the ringing sound never stops!


Yes, this
“


Barnsley Boy said:


> Tinnitus is a terrible thing, the ringing sound never stops!


----------



## luxspring

Hello.
New member who joined after reading about the JohnH attenuator project while researching on Google.
I have read a fair amount on the pages but not had the time to manage all 125 !!

The background to getting involved is that I have a friend who asked me if I would be able to build him an attenuator and the JohnH sounds amazing.
I have limited electronic skills but I have built a couple of amps from kits so I am keen to have a go.

The M2 seems to be the one most suited to his needs so that is the one I will try to build.
One thing I would like to know before I proceed is whether the M2 in the standard config will allow him to play his Vox AC30TB at bedroom levels.

Many thanks in advance for any help and advice, I'm really looking forward to the challenge.
Cheers


----------



## JohnH

hi @luxspring and welcome to our thread. Yes an M2 should be ideal for an AC30. It's rated for 50W amps. It should be able to set any level down to about a low to moderate TV volume with max amp power.

A key thing to check first is what ohms value to build to match the amp and speakers. I think AC30's usually have two 8 ohm speakerinvited in series so 16 ohms, but this needs to be checked.

On page 111, @Gene Ballzz posted a helpful wiring diagram. If you follow it, be sure to trace through it in relation to the schematic on post 1. It will help you check the build and adapt it to the case that you have.


----------



## Splawn

My low watt M-Lite for my 2.5 watt LTP P-P EF80 Plexi/2204. I divided all the wattages by 10. Cost was about $25.


----------



## Guitar-Rocker

Question. On a M-Lite, can the always "on" -dB cut of -7dB be reduced to a -3.5dB always on cut?


----------



## luxspring

JohnH said:


> hi @luxspring and welcome to our thread. Yes an M2 should be ideal for an AC30. It's rated for 50W amps. It should be able to set any level down to about a low to moderate TV volume with max amp power.
> 
> A key thing to check first is what ohms value to build to match the amp and speakers. I think AC30's usually have two 8 ohm speakerinvited in series so 16 ohms, but this needs to be checked.
> 
> On page 111, @Gene Ballzz posted a helpful wiring diagram. If you follow it, be sure to trace through it in relation to the schematic on post 1. It will help you check the build and adapt it to the case that you have.


Hi John, and many thanks for answering personally. It’s greatly appreciated.
Great tip re the layout by Gene 

You are correct re the Vox, it is 16 ohm, however I will build the 8ohm version with the extra Jack for a 16ohm cab as I know he has other amps he may use it on which are 8 ohm.

A question re the switches. I am in Europe with 230v mains. What amp rating do the switches need to be ?

Many thanks


----------



## JohnH

luxspring said:


> Hi John, and many thanks for answering personally. It’s greatly appreciated.
> Great tip re the layout by Gene
> 
> You are correct re the Vox, it is 16 ohm, however I will build the 8ohm version with the extra Jack for a 16ohm cab as I know he has other amps he may use it on which are 8 ohm.
> 
> A question re the switches. I am in Europe with 230v mains. What amp rating do the switches need to be ?
> 
> Many thanks


That's OK but need to check that the AC30 has an 8 ohm output option that is reasonably accessible - I'm not sure. The tap schematics show that it probably has an 8 ohm tap, but it may be configured to only work with another cab as an extension to the internals as well rather than as a standalone . Another small thing to note, when you use the 16 ohm out on an 8 ohm M2, there's about another 2db overall reduction. If the AC30 is the main use, I'd suggest to build it at 16 Ohms. In any case, need to match the attenuator to the amp output ohms.

On switches, only relatively low voltages reach the switches , like 20-30V max. Switches are generally rated at mains voltages 120 or 240V. So you can look at the specs for 120V for switches since we are not actually switching mains, for which we want 5 Amps ac current. Most toggles will do this.


----------



## JohnH

Guitar-Rocker said:


> Question. On a M-Lite, can the always "on" -dB cut of -7dB be reduced to a -3.5dB always on cut?


Not directly. But there are a few ways to get a 3db or 3.5db setting. What do you intend this for? Do you have an M-lite? if not I suggest to build an M2, its simpler.

With any of them, you can get a -3db setting by using it in parallel with the speakers, setting it to max attenuation, and setting the amp to half the ohms. It works if you have the right output taps and there's nothing to change on the attenuator

It is possible to switch the M2 front end down to -3.5db, then if you want -7 or more, switch it back to standard setting. Its more complex with the older M and M-lite designs since they have two coils. You can also have a dedicated -3.5 box.


----------



## Barnsley Boy

Hi @luxspring, welcome to this great community!

Just to put my 50p in the slot based on my experience building an 8 ohm M2:
1 - Buy the biggest enclosure that you can, a bit more air circulation isn't a bad thing, and some decent working space to see what goes where
2 - I had great success pop riveting the resistors to the case
3 - Good tools to have - step drill bit and an automatic centre punch.
4 - If your friend really wants to play his 50 watt Vox at bedroom levels, add the -28db stage to the shopping list
(it opens up a few more volume control options)
5 - Planning is everything, leave a bit of room in case you want to add something else (I had to jiggle things about a bit to add the -28db stage.
6 - @Gene Ballzz 's layout on page 111 is a fantastic starting point, don't be afraid to draw it out yourself so that you can get to grips with which wire goes where
7 - I found the mini toggles a bit too fiddly to work with, so opted for some big ones and spade connectors

Finally, post pics of the completed build on (you guessed it) the completed build page!

N-Joy!


----------



## Guitar-Rocker

I built the M-Lite. When trying it out yesterday -14dB cut was the most I would ever want out of the 14watt amp that I built the Lite for. I was wondering if I could revise it to always a -3.5dB when not in bypass, then have the three switchable stage options as 3.5 each also?


----------



## luxspring

JohnH said:


> That's OK but need to check that the AC30 has an 8 ohm output option that is reasonably accessible - I'm not sure. The tap schematics show that it probably has an 8 ohm tap, but it may be configured to only work with another cab as an extension to the internals as well rather than as a standalone . Another small thing to note, when you use the 16 ohm out on an 8 ohm M2, there's about another 2db overall reduction. If the AC30 is the main use, I'd suggest to build it at 16 Ohms. In any case, need to match the attenuator to the amp output ohms.
> 
> On switches, only relatively low voltages reach the switches , like 20-30V max. Switches are generally rated at mains voltages 120 or 240V. So you can look at the specs for 120V for switches since we are not actually switching mains, for which we want 5 Amps ac current. Most toggles will do this.



Good points John.
I will do a bit more digging to get the exact spec of the AC30.
And as I haven't completed the parts order, I will hang back in case I need to build the 16Ohm version

If I do build the 16Ohm version, can I connect an 8ohm amp as well ?


----------



## luxspring

Barnsley Boy said:


> Hi @luxspring, welcome to this great community!
> 
> Just to put my 50p in the slot based on my experience building an 8 ohm M2:
> 1 - Buy the biggest enclosure that you can, a bit more air circulation isn't a bad thing, and some decent working space to see what goes where
> 2 - I had great success pop riveting the resistors to the case
> 3 - Good tools to have - step drill bit and an automatic centre punch.
> 4 - If your friend really wants to play his 50 watt Vox at bedroom levels, add the -28db stage to the shopping list
> (it opens up a few more volume control options)
> 5 - Planning is everything, leave a bit of room in case you want to add something else (I had to jiggle things about a bit to add the -28db stage.
> 6 - @Gene Ballzz 's layout on page 111 is a fantastic starting point, don't be afraid to draw it out yourself so that you can get to grips with which wire goes where
> 7 - I found the mini toggles a bit too fiddly to work with, so opted for some big ones and spade connectors
> 
> Finally, post pics of the completed build on (you guessed it) the completed build page!
> 
> N-Joy!


Thanks a lot for the info Barnsley Bioy, very much appreciated.
I have a Hammond 1550G in my cart at the moment. Do you think that is big enough ?
Is the -28DB stage on any of John's schematics ?

Cheers


----------



## JohnH

hi @luxspring . There's variant called M2V that adds three extra front end parts that are switched in to convert between 8 and 16 ohms. It works fine but loses another 3db. 

Also, most amps have a 16 and an 8 output available to match to the amp.

But in many cases, given that these units are pretty simple and economical to build, it's best to keep each one as simple as possible dedicated to one use and just build another.


----------



## JohnH

Guitar-Rocker said:


> I built the M-Lite. When trying it out yesterday -14dB cut was the most I would ever want out of the 14watt amp that I built the Lite for. I was wondering if I could revise it to always a -3.5dB when not in bypass, then have the three switchable stage options as 3.5 each also?


I'm sorry but I don't think so. It's OK to have a different combination of switched stages , eg three of -3.5. But, in a multi-stage unit, based on my designs, the first stage has to be -7db. It has an important job of setting up what the amp sees, and separating it from the speaker and following stages.

With M2, there can be a switch on the first stage to reduce to -3.5, which is designed to be used only if later stages are off (to keep tone consistent, it's not a safety issue though) . I dont have a design where stage 1 is always -3.5 followed by other stages.


----------



## Amped eMind

Sort of an aside question here...

I've seen it mentioned a few times in this thread (most recently by @Barnsley Boy ) that a step drill bit is desirable. I mostly do woodworking instead of metal work and I don't really get its usefulness unless you're enlarging an existing hole or you are drilling a large diameter.

I have a small drill press that will be used for most of the holes in the top and bottom of the case but not the sides: how useful is a step bit when compared to a regular one? Can it still be used handheld or just with the press?

Thanks!


----------



## Barnsley Boy

Hi @Amped eMind 

The enclosure that you have in your cart is the same size as mine, so should be perfect (I went for a 1455). In terms of the additional -28db stage, Here is what I worked to, and I know that it works:




In terms of the step drill, I found it really useful and a lot more accurate to drill the holes. Stick some masking tape down, use the centre punch, then drill a small pilot hole and gradually enlarge it using the step drill to the desired diameter. If you try to drill a 10mm hole through aluminium I found it took ages and there was the risk of the drill wandering a bit. They are relatively cheap, and I'm sure you will find a use for it on some other DIY project.
Having a drill press will be loads easier, particularly when drilling the ventilation holes.

Hope that helps.


----------



## Amped eMind

Thanks for the info on the step drill @Barnsley Boy , much appreciated!

Let me tag @luxspring here as he was the one asking about the enclosure size and -28dB stage.


----------



## JohnH

A drill press is great to have. I dont have a step drill so for bigger holes such as jacks, full-size toggles and vents, I do masking tape, marking, centre punch, about a 5mm drill and then the final drill.

On the extra -28 db stage. @Barnsley Boy is the first to have it! Its all fine and it does let you go to very low volumes at home. But all the other builds in 2500 posts have been fine with the base designs. which go down to -31.5 db. It depends on the amp and the need for very low volume.


----------



## luxspring

JohnH said:


> hi @luxspring . There's variant called M2V that adds three extra front end parts that are switched in to convert between 8 and 16 ohms. It works fine but loses another 3db.
> 
> Also, most amps have a 16 and an 8 output available to match to the amp.
> 
> But in many cases, given that these units are pretty simple and economical to build, it's best to keep each one as simple as possible dedicated to one use and just build another.


Hi John, and thanks again for the clarifications.
Sorry for the dumb beginners question but what do you mean when you say it loses another 3db ?
Do you mean even when only using stage 1 or are you implying that 3db is not a huge difference in volume ?
Also, why are there extra parts for the 8 and 16ohm when you already have that on the M2 ?
Perhaps I am getting mixed up between the amp and cab jacks !!

Many thanks again.


----------



## JohnH

luxspring said:


> Hi John, and thanks again for the clarifications.
> Sorry for the dumb beginners question but what do you mean when you say it loses another 3db ?
> Do you mean even when only using stage 1 or are you implying that 3db is not a huge difference in volume ?
> Also, why are there extra parts for the 8 and 16ohm when you already have that on the M2 ?
> Perhaps I am getting mixed up between the amp and cab jacks !!
> 
> Many thanks again.


hi @luxspring 

The basic designs provide a minimum attenuation of -7db, then steps of -3,5db below that. If you take an 8Ohm M2 and add parts to make it work for an 16Ohm amp output tap, or a 16 Ohm M2 and convert it to work at 8 Ohm, the front end parts added absorb half the power. So instead of -7db minimum, the lowest attenuation is -10db. Its one more small step. It affects some users which are either using a small amp or who want to have just a small amount of attenuation.

An 8 Ohm M2 with its three output jacks will work with 8 or 16 Ohm speakers (or 4 ohm speakers) but it must be used with an 8 Ohm output from the amp, unless it has the extra bits from M2V.


----------



## Barnsley Boy

Amped eMind said:


> Let me tag @luxspring here as he was the one asking about the enclosure size and -28dB stage.


Doh! it had been the end of a long day yesterday when I posted the reply!


----------



## Mcentee2

Hope someone can chip in, as am trying to track back in the thread about the Line Out Freq response.

Both my M2 have line outs on the speaker output, and I know this is a raw feed without the speaker response applied.

Well, tbh even that is my assumption, because it is a small feed off the speaker so it's definitely got the amp side Freq content that the real speaker impedance is reacting with.

Now, I am not feeding this as a "dry" signal anywhere for audio output but into a wet only reverb (Hall of Fame) and then into a stereo mixer and out to frfr monitors, so the eq only needs to be rough and ready.

I am trying to work out if I really ought to eq before the reverb to tame the Freq response, and if so what areas to boost/cut.

My gut feel is to match a rough speaker response curve, which is basically a low end narrow boost and a ruse after 1k-5k then hard drop.

But that doesn't sound right when I try it, too trebly and bassy!!

Am I missing something obvious ?


----------



## luxspring

Barnsley Boy said:


> Doh! it had been the end of a long day yesterday when I posted the reply!


No problem Barnsley Boy, I saw it anyway and knew it was for me 
Much appreciated


----------



## luxspring

JohnH said:


> hi @luxspring
> 
> The basic designs provide a minimum attenuation of -7db, then steps of -3,5db below that. If you take an 8Ohm M2 and add parts to make it work for an 16Ohm amp output tap, or a 16 Ohm M2 and convert it to work at 8 Ohm, the front end parts added absorb half the power. So instead of -7db minimum, the lowest attenuation is -10db. Its one more small step. It affects some users which are either using a small amp or who want to have just a small amount of attenuation.
> 
> An 8 Ohm M2 with its three output jacks will work with 8 or 16 Ohm speakers (or 4 ohm speakers) but it must be used with an 8 Ohm output from the amp, unless it has the extra bits from M2V.


Great John, perfectly clear now


----------



## Gene Ballzz

While a "step-bit" can be quite handy for much of the drilling, it will not provide the needed precision for drilling and tapping threads for the resistor mounting holes into the case, as I am wont to do! On the other hand, for my next build, I may tap the threads into the resistors themselves! I hate monkeying with nuts, unless they are mine, of course! 
We'll See How It Works?
Gene


----------



## Amped eMind

Mcentee2 said:


> Hope someone can chip in, as am trying to track back in the thread about the Line Out Freq response.
> 
> Both my M2 have line outs on the speaker output, and I know this is a raw feed without the speaker response applied.
> 
> Well, tbh even that is my assumption, because it is a small feed off the speaker so it's definitely got the amp side Freq content that the real speaker impedance is reacting with.
> 
> Now, I am not feeding this as a "dry" signal anywhere for audio output but into a wet only reverb (Hall of Fame) and then into a stereo mixer and out to frfr monitors, so the eq only needs to be rough and ready.
> 
> I am trying to work out if I really ought to eq before the reverb to tame the Freq response, and if so what areas to boost/cut.
> 
> My gut feel is to match a rough speaker response curve, which is basically a low end narrow boost and a ruse after 1k-5k then hard drop.
> 
> But that doesn't sound right when I try it, too trebly and bassy!!
> 
> Am I missing something obvious ?


The speakers mostly roll off the bass and the highs. Make sure you use the speaker frequency response curve and not its impedance curve.

And I agree with you that a rough EQ should be adequate to feed the effects.


----------



## Kewl ToyZ

I've bookmarked this well written post. I have my Marshall's single and stereo chorus, and a few Fender M80's single and stereo chorus. The attenuation design here gives me insight into what I always wished I could do to fix the low volume output of the Fenders. They are simply way too loud. At 1.5 volume they are at such high decibel levels they can only be used in a live setting. I'm going to have to look at this in detail a bit longer to see if I can even consider this approach with a solid state.


----------



## Flowboy

I must thank JohnH for his work on this / these attenuators & for following up throughout the thread. Bloody fantastic work! I've only read a few pages & frankly don't have time to read them all. 126 pages, are you kidding? 
I have that many other projects on the go that I will be unlikely to get round to making one of the designs any time soon but it's great to know its available & road tested. 
I fitted an Eminence Maverick attenuating speaker some years back, to my Pro Reverb for the same reason. I know there's a lot of opinion about these & the pro.. but my hearing's not that great that I can tell, or care any more... It does the job of not annoying the wife / neighbours while sounding ok. Only trouble with the speaker is it's a small pain to adjust - faffing around in the back of the amp. Something sitting on the amp or fixed somewhere accessible for quick switching has got to be better & leaves you with your original or fancy speakers, rather than sacrificing one of them to be an attenuator.


----------



## JohnH

Hi @Flowboy , thanks for joining our thread. If you might be interested, most of it is summarised in post 1, then jump to page 111 and onwards, where Gene posted a useful layout diagram that several have referenced for recent builds. Then if its something that you might like to try, check back here and we can discuss how to best suit your uses.


----------



## Mcentee2

Mcentee2 said:


> Hope someone can chip in, as am trying to track back in the thread about the Line Out Freq response.
> 
> Both my M2 have line outs on the speaker output, and I know this is a raw feed without the speaker response applied.
> 
> Well, tbh even that is my assumption, because it is a small feed off the speaker so it's definitely got the amp side Freq content that the real speaker impedance is reacting with.
> 
> Now, I am not feeding this as a "dry" signal anywhere for audio output but into a wet only reverb (Hall of Fame) and then into a stereo mixer and out to frfr monitors, so the eq only needs to be rough and ready.
> 
> I am trying to work out if I really ought to eq before the reverb to tame the Freq response, and if so what areas to boost/cut.
> 
> My gut feel is to match a rough speaker response curve, which is basically a low end narrow boost and a ruse after 1k-5k then hard drop.
> 
> But that doesn't sound right when I try it, too trebly and bassy!!
> 
> Am I missing something obvious ?





Amped eMind said:


> The speakers mostly roll off the bass and the highs. Make sure you use the speaker frequency response curve and not its impedance curve.
> 
> And I agree with you that a rough EQ should be adequate to feed the effects.



DOH!! Spot on, I have been absent mindedly thinking of the impedance curves !

Many thanks


----------



## JohnH

Mcentee2 said:


> DOH!! Spot on, I have been absent mindedly thinking of the impedance curves !
> 
> Many thanks



I did some work on this, to find out what would be a good EQ curve to apply to the direct out from an M or M2, and turn it into a reasonable cab-sim signal.

Here's where I got to:




What I did was to set up a broad strummed signal with lots of frequencies in a loop. I recorded the direct out across the attenuated speaker and also recorded a mic'ed sound, all into Audacity. Then I exported the two normalized frequency plots into Excel, and subtracted them. That's the orange line and in principle, that would be the EQ curve to convert from direct to close mic'ed. Then I was playing around with some circuit designs to try to capture that in a physical piece of kit and ended up with the red curve. I then applied that curve back to the direct line out recording in Audacity. It sounded pretty reasonable, obviously not quite the same as the mic'ed signal. 

I reckon that red curve is the sort of EQ that might work for what you want, though I've not built the actual physical circuit that I was designing. 

Likely a better solution would be a simple IR box like a Mooer Radar


----------



## mike_lawyer

I have all of the components set in my box, about to begin the wiring. Maybe a silly question, but the air core inductor has a lead that is on the outside of the ring of wire, and a lead that goes to the middle of the ring of wire. Does it make any difference which lead is wired to the input jack and which one goes to the resistors?


----------



## JohnH

mike_lawyer said:


> I have all of the components set in my box, about to begin the wiring. Maybe a silly question, but the air core inductor has a lead that is on the outside of the ring of wire, and a lead that goes to the middle of the ring of wire. Does it make any difference which lead is wired to the input jack and which one goes to the resistors?



Hi Mike, its a good question but no it doesn't matter. The coil is not directional.


----------



## Mcentee2

JohnH said:


> I did some work on this, to find out what would be a good EQ curve to apply to the direct out from an M or M2, and turn it into a reasonable cab-sim signal.
> 
> Here's where I got to:
> 
> View attachment 102603
> 
> 
> What I did was to set up a broad strummed signal with lots of frequencies in a loop. I recorded the direct out across the attenuated speaker and also recorded a mic'ed sound, all into Audacity. Then I exported the two normalized frequency plots into Excel, and subtracted them. That's the orange line and in principle, that would be the EQ curve to convert from direct to close mic'ed. Then I was playing around with some circuit designs to try to capture that in a physical piece of kit and ended up with the red curve. I then applied that curve back to the direct line out recording in Audacity. It sounded pretty reasonable, obviously not quite the same as the mic'ed signal.
> 
> I reckon that red curve is the sort of EQ that might work for what you want, though I've not built the actual physical circuit that I was designing.
> 
> Likely a better solution would be a simple IR box like a Mooer Radar



Many thanks! I thought there was a chance you had looked at this. I dug out my old captures, but none if these were from a line out.

Looks straightforward enough to feed a reverb


----------



## Lancer X

And so it begins…




Accidentally bought one resistor with big doofy leads on it…


----------



## mike_lawyer

Ok, I have run into some problems. I finished all the soldering, ran through the schematic and it looks like I had everything wired correctly. The problem is with the first 7 db switch, it doesn't do anything when I flip the switch. The 14 db and 3.5 db switches seem to work fine.

When wiring, is everyone running two wires to the center lug of the DPDT switches, or just one? I am running a wire to the lug of the first resistor, then a wire to the center of the DPDT switches. I wouldn't think it would make a difference, but maybe I wired it up wrong.

I have tried to upload photos of my build, but it says the uploaded photos are too large.


----------



## JohnH

hi Mike - Have you done resistance tests before testing with the amp?


----------



## Gene Ballzz

mike_lawyer said:


> Ok, I have run into some problems. I finished all the soldering, ran through the schematic and it looks like I had everything wired correctly. The problem is with the first 7 db switch, it doesn't do anything when I flip the switch. The 14 db and 3.5 db switches seem to work fine.
> 
> When wiring, is everyone running two wires to the center lug of the DPDT switches, or just one? I am running a wire to the lug of the first resistor, then a wire to the center of the DPDT switches. I wouldn't think it would make a difference, but maybe I wired it up wrong.
> 
> I have tried to upload photos of my build, but it says the uploaded photos are too large.



Most comfusers provide a function/option to resize your photos. I usually choose to make a copy or duplicate photo and then resize that, to retain the original. You will normally find the option by right clicking the pic, for a contextual/drop down menu.
We'll Get You Sorted,
Gene


----------



## mike_lawyer

The wiring diagram on page 123 with the 28 db reduction, is that the way everything should be wired on the M2? I just saw that wiring diagram, it is slightly different than I have it wired up. Might be what is causing the problem.


----------



## Lancer X

Pro tip for aspiring builders: apply your thermal compound *before* fastening down all your resistors, kids!


----------



## mike_lawyer

[URL="https://www.marshallforum.com/media/albums/attenuator-photos.1443/"]

[/URL]


----------



## mike_lawyer




----------



## JohnH

When wiring up, Genes diagram on p111 is very good. But, its important to trace it through relative to the schematic for yourself and dont try with the amp until the checks and resistance tests are done. For this, plug in speaker, plug in cord to input without amp and measure across the jack that would go to the amp. It should be 7 to10 ohms in every setting, for an 8 Ohm M2 , x2 for a 16 Ohm build


----------



## mike_lawyer

Thanks, I am going to try and wire it up like Gene outlined it. Any tips for making bus wires across the DPTP switch pads?


----------



## Gene Ballzz

mike_lawyer said:


> Thanks, I am going to try and wire it up like Gene outlined it. Any tips for making bus wires across the DPTP switch pads?



I use #18 gauge solid buss wire or simply strip the wire long that I'm using to the resistors, tin it snd put it across the two lugs. The solid buss wire works best for me! That way I can simply solder my wires to that jumper instead of trying to get tinned, stranded wire through those tiny lugs!

HTH,
Gene


----------



## IvanMan

Hello. I've build in the past a complete resistive attenuator (with my own calculations) but with a no-consistent frequency response.
Now I've found your project and I'm very interested to applicate it, but (if possible) re-using the high power resistor I bought.
But they are difference by my available resistor and the suggested value in your M2 project, for example R1 --> 33 Ohm vs. 30 Ohm, R2B --> 33 or 47 Ohm vs 39 Ohm, , R3 --> 33 Ohm vs 30 Ohm, R4= 22 Ohm vs. 20 Ohm, etc.
The questions are:
1) Is there a tolerance in the suggested value or
2) Are there formulas or and Excel spreadsheet to calculate different couple of value for series / parallel and match my available resistor
3) May be the use of my resistor is limited to pure resistor part, and for the reactive part I try to combine my resistor to obtain the suggested values?

Another question is about the inductor: is it possible to diy. In the past I realized (with success) a couple of output transformers. Someone can help me in finding instructions to build the inductor(s)

Many thanks and compliments for the project. Very, very interesting

PS1 I attach a couple of picture of my previous realization. Please note the aluminium part I used to dissipate - it is very efficient also without drilling holes
PS2 Only for information the resistor I've at home are 50W, with the following value / q.ty:


0,011​0,221​0,471​0,821​1,202​1,505​2,206​12,005​18,001​22,002​24,001​33,004​36,001​47,002​68,003​100,002​150,001​470,001​750,002​


----------



## Barnsley Boy

mike_lawyer said:


> The wiring diagram on page 123 with the 28 db reduction, is that the way everything should be wired on the M2? I just saw that wiring diagram, it is slightly different than I have it wired up. Might be what is causing the problem.


The correct one is on page 126 - I know that one definitely works!


----------



## Barnsley Boy

JohnH said:


> With all the different cab options that the 8Ohm M2 can do, just note that it has to always run from an 8Ohm tap on the amp. If that leaves out some amp/cab combinations that you need, a front end can be added to convert.


Hi @JohnH,

Rookie question, but I'll ask it anyway, below are pictures of the back of my VTX:









Would I be able to use my 8 ohm M2 for this amp? Or will I need to build a 4 ohm version? I was a bit confused about what you meant when you said about running from an 8 ohm tap. My JCM900 has an 8/16 ohm switch on the back which I would presume selects which tap to use. The VTX only has the speaker jacks as indicated.

Also, could I get away with a 50 watt M2? or would I need a 100 watt version to accommodate the 65 watt power output? The amp is incredibly loud, and as I've mentioned previously, can only be used at home with a volume control in the FX loop. It would be good to do away with this and warm up the valves to really get it going using the attenuator!


----------



## velvet_man

This thread has so much info in it, it's hard to know which is the most current. Can someone point me to the most recent/best schematic for maximum attenuation into a 16 ohm cab?

EDIT:

I should note I only use this amp for band rehearsals and gigs, so it's never used at home (i.e. I don't need "bedroom" volume, just not destroying my eardrums volume).


----------



## JohnH

Barnsley Boy said:


> Hi @JohnH,
> 
> Rookie question, but I'll ask it anyway, below are pictures of the back of my VTX:
> 
> View attachment 102731
> 
> 
> 
> View attachment 102732
> 
> 
> 
> Would I be able to use my 8 ohm M2 for this amp? Or will I need to build a 4 ohm version? I was a bit confused about what you meant when you said about running from an 8 ohm tap. My JCM900 has an 8/16 ohm switch on the back which I would presume selects which tap to use. The VTX only has the speaker jacks as indicated.
> 
> Also, could I get away with a 50 watt M2? or would I need a 100 watt version to accommodate the 65 watt power output? The amp is incredibly loud, and as I've mentioned previously, can only be used at home with a volume control in the FX loop. It would be good to do away with this and warm up the valves to really get it going using the attenuator!



It looks like it needs a 4 Ohm load (and reading the manual, does it also have a 2 Ohm option?). So an 8 Ohm M2 is not right. But if you add three parts switched in at the front end (a coil and two resistors) you can convert the input from 8 to 4. These parts will absorb 1/2 the power, and they could be a separate box. That will take the heat off the main unit. 



https://www.marshallforum.com/media/attenuator-m2v-200420.11998/full?d=1600635241



The values are on the top line of the table. This one does 16-8-4, but you could simplify just to have 8 and 4 and a switch to connect the new parts in parallel with the input.


----------



## JohnH

Hi @IvanMan , welcome to our thread.

The values are very carefully optimised to maintain tonal consistency at all settings and provide equal steps. I suggest 5% tolerance is OK. Maybe you could get some higher value and lower powered resistors to put in parallel with what you have to bring down the values? 

Designing this needs close control of both input and output impedance at each setting, taking into account the amp and the speaker as a reactive load. Its too complex for a formula, I do it using a huge spreadsheet that plots every setting, and also SPICE models.

I guess you can wind the coil, if you can keep it tight and have a way to measure it afterwards. Personally I wouldn't trust formulas for inductance except as an approximation.

Good luck!


----------



## BlueX

Here’s my take on the M2/8Ohm/Out3. Circuit and specifications according to John's post #1 in this thread. I borrowed part of Gene’s layout in post #2207 (p 111), to have R3-R8 placed in the same order as the “descending” switches.

The bottom of the “box” is made from 1,5 mm thick, perforated aluminum sheet (5 mm holes). Hope that will be sufficient heat sink. Resistors mounted flat to the sheet, with thermal compound in between. The sheet is reinforced with 12x12x2 mm aluminum profiles. Coil strapped with plastic cable ties to a piece of wood. Top cover is a black aluminum mesh (for covering vent holes, etc.). Ventilation should be sufficient. Dimensions are 300x140x80 mm.

I did the following tests on the unit (test report enclosed), if anyone’s interested. I found a bad solder joint!

No connection between chassis or any part of the circuit
Continuity within each sub-circuit
Resistance between different lugs on in- and output. Interesting thing: Resistance increases as more attenuation is added, until the last step when it decreases slightly.

I got problems with the “Cliff UK” speaker jacks. For two of them I get consistent readings when measuring on the solder lugs and measuring with a plug. However, for the other two I get OL (overload) for the plug, but reasonable reading at the lugs. For one of the bad jacks I can get consistent value for the plug, if I press the plug sideways. However, once I release the pressure reading goes back to OL.

I cannot trust these jacks, and will replace all four. Consequently, I have not yet played with this unit. Found the following two types instead, and will try to source them (in stereo/TRS configuration). Any comments on speaker jacks? Is it possible to use “switched” jacks?

https://www.parts-express.com/Switchcraft-Z15J-1-4-High-Power-Speaker-Jack-093-1056

https://www.parts-express.com/Neutrik-NMJ6HC-S-1-4-Stereo-Phone-Jack-Switched-090-976

I hope that @JohnH and Marshall Forum consider this fair use of names. I want to give credit. Finally, thanks to John and Gene for all work and support!


----------



## Lancer X

So I’m happily wiring away when I realize I’m maybe making a mistake. See black wires in pic 1 below. Should I avoid routing wires closely in parallel like this? Am I creating a crosstalk risk?

Trying to be neat and tidy, but now second guessing myself. 





Here an example of where I’m only crossing wires perpendicularly. Better?


----------



## Barnsley Boy

Hi @JohnH ,

Been busy at work and pondering on the 4 ohm additional box, so haven't had a chance to respond back. I've drawn out what I think it should look like if I were to use an extension box to convert the 8 ohm to a 4 ohm unit. Effectively this entails adding another input jack which the box would connect to putting the components in parallel with the input from the amp. Below are diagrams showing operation as an 8 ohm unit, then a 4 ohm unit. Could you run your expert eye over these to check that I have interpreted the requirements correctly?

I like to draw these things out so I can get a good understanding of which wire goes where, and hopefully if I am correct, in some small way it may help others out .

If I were to build another 4 ohm unit, just for my Peavey (and including the -28db stage) would it be a case of putting all the components as drawn above in one box i.e. using using 2 coils? or would I be able to build another M2 with just a single coil, and change the resistor values to suit? If so, could you advise on what the component part values would be, and would the 50w version handle my 65w Peavey? One last question, I'm not sure that I understand the rationale behind the 2 output jacks. does this allow me to plug two 8 ohm speakers in? Or if the extension box is connected, two 4 ohm speakers?

The more that I delve into this massive post, the more it blows my mind!!!! Keep up the good work.


----------



## Gene Ballzz

BlueX said:


> Here’s my take on the M2/8Ohm/Out3. Circuit and specifications according to John's post #1 in this thread. I borrowed part of Gene’s layout in post #2207 (p 111), to have R3-R8 placed in the same order as the “descending” switches.
> 
> The bottom of the “box” is made from 1,5 mm thick, perforated aluminum sheet (5 mm holes). Hope that will be sufficient heat sink. Resistors mounted flat to the sheet, with thermal compound in between. The sheet is reinforced with 12x12x2 mm aluminum profiles. Coil strapped with plastic cable ties to a piece of wood. Top cover is a black aluminum mesh (for covering vent holes, etc.). Ventilation should be sufficient. Dimensions are 300x140x80 mm.
> 
> I did the following tests on the unit (test report enclosed), if anyone’s interested. I found a bad solder joint!
> 
> No connection between chassis or any part of the circuit
> Continuity within each sub-circuit
> Resistance between different lugs on in- and output. Interesting thing: Resistance increases as more attenuation is added, until the last step when it decreases slightly.
> 
> I got problems with the “Cliff UK” speaker jacks. For two of them I get consistent readings when measuring on the solder lugs and measuring with a plug. However, for the other two I get OL (overload) for the plug, but reasonable reading at the lugs. For one of the bad jacks I can get consistent value for the plug, if I press the plug sideways. However, once I release the pressure reading goes back to OL.
> 
> I cannot trust these jacks, and will replace all four. Consequently, I have not yet played with this unit. Found the following two types instead, and will try to source them (in stereo/TRS configuration). Any comments on speaker jacks? Is it possible to use “switched” jacks?
> 
> https://www.parts-express.com/Switchcraft-Z15J-1-4-High-Power-Speaker-Jack-093-1056
> 
> https://www.parts-express.com/Neutrik-NMJ6HC-S-1-4-Stereo-Phone-Jack-Switched-090-976
> 
> I hope that @JohnH and Marshall Forum consider this fair use of names. I want to give credit. Finally, thanks to John and Gene for all work and support!
> 
> View attachment 103015
> View attachment 103016
> View attachment 103017
> View attachment 103018
> View attachment 103019
> View attachment 103020
> View attachment 103021
> View attachment 103022
> View attachment 103023



*FIRST>* Nice build so far!
*NEXT>* Avoid that linked Switchcraft jack, as it will connect the "-/negative" of the circuit to the chassis.
*THEN>* @JohnH did an analysis a few pages back concerning the effectiveness of that very nice type of "perforated" sheet and determined it's heat sink value to be not great. Depending on the amp wattage, you may be OK.
*FINALLY>* I'm quite surprised that those CLIFF jacks don't seem to be working well! Are they old/salvaged/used? Or did you overheat and melt them during soldering? These jacks are known to be fairly rock solid and rarely exhibit issues. It may be an issue with the way you are testing. The switching functions in the jacks can sometimes give unexpected results, especially in combination with the switching in/out and bypassing of components! The best way to test is to:
*A>* Plug your speaker out of the appropriate output jack with a proper speaker cable.
*B>* Plug either a speaker cable (or bare, unwired plug) into the "Amp in" jack. Do not connect to the amp!
*C> *Put your meter test leads across the positive and negative connections of the cable or plug that is plugged into the "Amp in" jack and note/write down the readings for each and every switch combination. If possible, I would try this first with an 8 ohm speaker (or two 16 ohm speakers) plugged out of each (both) of the "Speaker out" "1 X 8 Ohm or 2 x 16 ohm" jacks and then try the same with a 16 ohm speaker plugged out of the "1 x 16 Ohm" jack. All readings should stay in a range of 1 to 3 ohms, surrounding or near 8 ohms.

Please Keep Us Posted?
Gene


----------



## mike_lawyer

Okay, I rewired the attenuator as shown above, and both the first switch and second switch work perfectly. However, the -3.5 db switch makes no difference. I am thinking that the DPDT switches I ordered off Amazon might be very fragile with soldering. Does anyone have a source for high-quality DPDT switches that will hold up well? I would like to use mini DPDT switches for space.


----------



## BlueX

Gene Ballzz said:


> *FIRST>* Nice build so far!
> *NEXT>* Avoid that linked Switchcraft jack, as it will connect the "-/negative" of the circuit to the chassis.
> *THEN>* @JohnH did an analysis a few pages back concerning the effectiveness of that very nice type of "perforated" sheet and determined it's heat sink value to be not great. Depending on the amp wattage, you may be OK.
> *FINALLY>* I'm quite surprised that those CLIFF jacks don't seem to be working well!


Thanks Gene, and thanks for explaining the function of that jack!
In the sheet I'm using there are two 5 mm holes in a square of 14x8 mm (112 mm2), i.e. 2xPix(5/2)¨2=39 mm2 or about 35% of the contact surface is gone. I will use this M2 with a SV20H so I guess it will be OK. I missed John's analysis (quite a lot of info in this thread).
The CLiff jacks were brand new, without any traces of use, when I got them. Overheating is always a risk, but I cannot see any damages. Maybe the contact lugs lost their spring properties? I will do some trouble-shooting and share the results. Could be of interest to others.


----------



## Lancer X

BlueX said:


> The CLiff jacks were brand new, without any traces of use, when I got them.



I had heard of some people receiving counterfeit Cliff ones. Any chance of that?


----------



## JohnH

Lancer X said:


> So I’m happily wiring away when I realize I’m maybe making a mistake. See black wires in pic 1 below. Should I avoid routing wires closely in parallel like this? Am I creating a crosstalk risk?
> 
> Trying to be neat and tidy, but now second guessing myself.
> 
> 
> View attachment 103029
> 
> 
> Here an example of where I’m only crossing wires perpendicularly. Better?
> 
> View attachment 103030



I don't think there's any issues there. It's all low impedance output wiring, there are no sensitive input stages nearby. So a nice neat build is all it needs to be.


----------



## JohnH

mike_lawyer said:


> Okay, I rewired the attenuator as shown above, and both the first switch and second switch work perfectly. However, the -3.5 db switch makes no difference. I am thinking that the DPDT switches I ordered off Amazon might be very fragile with soldering. Does anyone have a source for high-quality DPDT switches that will hold up well? I would like to use mini DPDT switches for space.



Hi Mike I've never had an issue with mini switches, but I'm buying locally and I don't know what brand. Can you test with an ohm meter setting directly onto the lugs, in circuit, to see if the contacts work?


----------



## JohnH

Barnsley Boy said:


> Hi @JohnH ,
> 
> Been busy at work and pondering on the 4 ohm additional box, so haven't had a chance to respond back. I've drawn out what I think it should look like if I were to use an extension box to convert the 8 ohm to a 4 ohm unit. Effectively this entails adding another input jack which the box would connect to putting the components in parallel with the input from the amp. Below are diagrams showing operation as an 8 ohm unit, then a 4 ohm unit. Could you run your expert eye over these to check that I have interpreted the requirements correctly?
> 
> I like to draw these things out so I can get a good understanding of which wire goes where, and hopefully if I am correct, in some small way it may help others out .
> 
> If I were to build another 4 ohm unit, just for my Peavey (and including the -28db stage) would it be a case of putting all the components as drawn above in one box i.e. using using 2 coils? or would I be able to build another M2 with just a single coil, and change the resistor values to suit? If so, could you advise on what the component part values would be, and would the 50w version handle my 65w Peavey? One last question, I'm not sure that I understand the rationale behind the 2 output jacks. does this allow me to plug two 8 ohm speakers in? Or if the extension box is connected, two 4 ohm speakers?
> 
> The more that I delve into this massive post, the more it blows my mind!!!! Keep up the good work.
> 
> 
> View attachment 103031
> 
> 
> 
> 
> View attachment 103032



I like the idea of the separate box, it's a convenient addition and it spreads the heat. But if you want a dedicated 4 ohm M2, basically all values halve I have an optimised set of nearest standard values if you want them.


----------



## BlueX

Lancer X said:


> I had heard of some people receiving counterfeit Cliff ones. Any chance of that?



Could be, but difficult to say. They are properly branded, look well made, and were supplied by what seems like a professional company ( in business since 1972). I will have a closer look at them.


----------



## Barnsley Boy

JohnH said:


> I like the idea of the separate box, it's a convenient addition and it spreads the heat. But if you want a dedicated 4 ohm M2, basically all values halve I have an optimised set of nearest standard values if you want them.


Hi @JohnH, if you have a set of values that would be great as I'm not sure whether I should be rounding up or down!

I'm guessing that L1 would be a 0.45mH/ 18awg coil? Closest that I can get is:

0.47mH, 1.0mm Wire, DCR 0.30 Ohms
 Is that close enough?


----------



## mike_lawyer

Success! I got everything to work, all switches work well. I am contemplating adding the -28 db option as well.

This unit sounds great and it was a really fun project. Thanks to Gene and all the contributors for a great design, this thing sounds awesome!


----------



## JohnH

hi @Barnsley Boy , this post has the 4 ohm values: post #2127



JohnH said:


> Here's my suggestions for 4 Ohm, to keep everything even and in proportion but based on standard values:
> 
> R1 8.2
> R2A 12
> R2B 10
> R3 8.2
> R4 5.6
> R5 2.7
> R6 8.2
> R7 18
> R8 2.7
> R10 27
> R11 4.7
> 
> L1 0.45mH or 0.5mH
> 
> If you are getting from a different range, try to keep the proportions within each stage close to that of the 8 Ohm set
> 
> This is the set of frequency response- attenuations that are predicted for those 4 ohm values:
> 
> View attachment 97569


----------



## Barnsley Boy

JohnH said:


> hi @Barnsley Boy , this post has the 4 ohm values: post #2127


Hi @JohnH , 
Many thanks for that. Incredible to think that this thread has 2555 posts!!! From small acorns, large oak trees grow.
So, adding my -28db stage, am I correct to just halve the values? If so, the closest I can get (according to my diagram) are:

R9A - 25w, 3.3 ohm
R9B - 5w, either 0.62 or 0.56 ohm
R9C - 1w, 8.2 or 10 ohm

Could you advise which would be the correct values to use?

I'm torn between building the 4ohm extension box, or building a new 4ohm M2 (mainly because it is so bl**dy good). I'll need to check back on the previous posts to brush up on the theory and evolution of dropping the L2 inductor in favour of a single L1. I did read about it, but I was so busy lapping up all the good stuff in the thread that it probably went in one ear and out the other!


----------



## JohnH

hi @Barnsley Boy ok, I can run numbers on that -28db stage, to find the best values for a 4ohm build.

On how to get the 4 ohm, could you describe how you'll use it some more? is it an important use that you'll want often? or just occasional? Also, what was the power of the 4ohm amp? Will you need loud but attenuated settings, like a loud jam or gig? or just home /studio use with low volume?


----------



## JohnH

hi @Barnsley Boy , I reckon the best matching set of values for a -28db stage at 4 Ohms is* 3.3, 0.56 and 10 ohms*


----------



## Barnsley Boy

JohnH said:


> hi @Barnsley Boy ok, I can run numbers on that -28db stage, to find the best values for a 4ohm build.
> 
> On how to get the 4 ohm, could you describe how you'll use it some more? is it an important use that you'll want often? or just occasional? Also, what was the power of the 4ohm amp? Will you need loud but attenuated settings, like a loud jam or gig? or just home /studio use with low volume?


Hi @JohnH ,
I am currently a home user with aspirations to get back into a bit of recording so low volume is a key consideration. If we don't get burnt to a crisp over the coming days/ weeks/ months, I've been contemplating getting a band together or at least looking for a couple of like minded jam buddies.

The Peavey Classic VTX is a 65 watt amp, and is incredibly loud, hence my temporary fix of using a volume control pedal in the fx loop. At the moment it's the only way to be able to use the lead channel and tweak the saturation control. The M2 with -28db stage is absolutely perfect for my JCM900 - full attenuation on gives me exactly what I want and does a fantastic job. If I can get the same results for the Peavey, then the jobs a good 'un!!

Thanks for the resistor values, I shall mull over the options and start assembling components!


----------



## JohnH

Maybe you could try the simple 3-part addition for the 8 ohm M2 to work at 4ohm. It's a lot less to build than another full one. Then, you could decide if you'd like a separate 4ohm unit?


----------



## Barnsley Boy

JohnH said:


> Maybe you could try the simple 3-part addition for the 8 ohm M2 to work at 4ohm. It's a lot less to build than another full one. Then, you could decide if you'd like a separate 4ohm unit?


I'm thinking that is the best bet. I'm just about to place my order for the components! I'll report back once it is built and tested.


----------



## IvanMan

JohnH said:


> Hi @IvanMan , welcome to our thread.
> 
> The values are very carefully optimised to maintain tonal consistency at all settings and provide equal steps. I suggest 5% tolerance is OK. Maybe you could get some higher value and lower powered resistors to put in parallel with what you have to bring down the values?
> 
> Designing this needs close control of both input and output impedance at each setting, taking into account the amp and the speaker as a reactive load. Its too complex for a formula, I do it using a huge spreadsheet that plots every setting, and also SPICE models.
> 
> I guess you can wind the coil, if you can keep it tight and have a way to measure it afterwards. Personally I wouldn't trust formulas for inductance except as an approximation.
> 
> Good luck!


Hi JohnH, and thanks for your reply.
After some calculations I find series/parallel combination of the resistences I've already bought (used for my previous project) to match the requested values for 16 Ohm M2. My only concern is: using two (and in one case 3 resistor in parallel) may cause a change in tone / performance of the M2?
For the coil I find here in Italy a producer of HiFi component that have the 1.8mH / 18Awh(1mm) in catalogue for 10Euro. One problem solved  
After your opinion I'll start with the realization.


----------



## JohnH

If you can get close to the specified R values, with a series or parallel combination of any kind, then it should make no difference to the tone compared to using a single resistor


----------



## donwagar

I'm just starting a second one for a friend, a standard M2 8 ohm.

Thanks Gene for the wiring diagram on pg 111, that's awesome.

Question: what gauge Bus Bar wire are you guys using? 18? The heaviest solid core wire I have on hand is 20 ga.


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## JohnH

hi @donwagar , I had some bare runs between lugs to connect on mine. I used what I had which was 0.7mm, but with two runs of it in parallel which adds up to the same area as 18ga . It was just my work-around to get my prototype built but it was easy to bend and fit through small switch lugs.


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## donwagar

JohnH said:


> hi @donwagar I used what I had which was 0.7mm, but with two runs of it in parallel which adds up to the same area as 18ga .


Thanks John,
great idea, saves me a drive across the city.


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## Lancer X

Finally completed my M2 build. Works perfectly and sounds great! Much less high end loss than my Weber MiniMASS. This is clearly a superior design. Hooked it up to my Tweed Deluxe, and at max attenuation with the amp dimed was at a mild conversational volume with no loss of character. Power tube overdrive in full effect. Excellent.

Build thread part one here, and part two here.

Thanks so much out to @JohnH , @Gene Ballzz , and everyone else contributing here!


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## JohnH

Hi @Lancer X , thanks very much for trusting this design and for building it so nicely. I'm very happy that it's working well in your rig.


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## BlueX

I've got problems with my M2. After completing the soldering I didn't get proper connection or consistent Ohm readings on inputs and outputs. De-soldered the jacks, and did some measurements.

When I measure on both lugs for each connection (Tip, Ring, and Sleeve, respectively), there is continuity (low resistance) without any plug in the jack. However, when I insert the plug there is no continuity (OL on resistance) between lugs at each connection. This is the same for all jacks and connections, also Ring on input and Out 1 and Out 2 which I haven't soldered. See pictures.

Anyone knows if this is normal, or if I got jacks with some kind of switch function?

EDIT (I forgot to add the previous test): When I measure with a plug inserted (Sleeve connected to black input on DMM) there is only one of the corresponding lugs that have contact with the plug sleeve. See the last two pictures.


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## JohnH

Hi @BlueX , is see you have the switched jacks with 6 lugs. That's all fine but are you connecting to the correct lugs? A basic M2 only uses one side of the jack, being the contacts that connect to the plug when its pushed in. If in doubt, you can bridge across each jack , as on Genes diagram on p111 and mine on the next page.

Important to make sure you are getting 7 to 10 Ohms (for an 8 ohm M2) at the far end of a cord (ie where it will go into the amp) plugged into the input, with a speaker in the output.


----------



## BlueX

JohnH said:


> Hi @BlueX , is see you have the switched jacks with 6 lugs. That's all fine but are you connecting to the correct lugs? A basic M2 only uses one side of the jack, being the contacts that connect to the plug when its pushed in. If in doubt, you can bridge across each jack , as on Genes diagram on p111 and mine on the next page.



Thanks @JohnH , that clarifies the situation! I will keep these jacks, bridge both lugs, and check for proper connection.


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## BlueX

I resoldered the jacks and now my M2 works properly. Consistent repeated readings between In/Tip and Out1/Tip, Out2/Tip, and Out3/Tip, for the different switch settings. Speaker cable in Amp In sees between 7,4 and 8,4 Ohm with one 8 Ohm cab connected.

Did some playing too, through my SV20H, and it sounds just great. It's a real pleasure to hear the tubes scream, at moderate listening levels. Thanks for all support, and many thanks for designing this great piece of gear!


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## JohnH

hi @BlueX , that's great news and congrats for getting it together and working. Thanks for posting.


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## Gene Ballzz

Yes, more evidence that simple "spot testing" of different internal portions of this design can produce unexpected results, due to less than obvious interactions of multiple components. The real litmus test is with a speaker (of known, proper impedance) plugged into the output jack and a cable or bare connector measured while plugged into the input but not into the amp. Even testing continuity at lugs that are connected by a simple wire won't necessarily provide accurate results. Test as per @JohnH and the above suggetions and if not correct, you need to physically trace, inspect and "proofread" your work!

As a random example: Testing for continuity of the wire/solder joints between R1 and R5, at the resistor lugs may or may not show continuity, even if one or both of those solder joints is cold or otherwise not well connected! Again, this was just one random example, but continuity tests are not appropriate here!

And yes, congrats again to @BlueX for getting this up and running!

Just Connectin'
Gene


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## russbryant

John, Thanks for all the work you have done on this. Haven't bought a tube amp yet but starting to look around for one. I may just go ahead and build one while waiting. Did you ever test on the Crate SS amp? Wondering how it went as I have an Orange Micro Terror that puts out 20 watts of loud.

Thanks,
Russ


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## JohnH

russbryant said:


> John, Thanks for all the work you have done on this. Haven't bought a tube amp yet but starting to look around for one. I may just go ahead and build one while waiting. Did you ever test on the Crate SS amp? Wondering how it went as I have an Orange Micro Terror that puts out 20 watts of loud.
> 
> Thanks,
> Russ



Hi Russ, thanks for joining our thread.

Actually, I dont think I ever did try it with my old Crate PowerBlock amp. There's no reason why it wouldn't work as a test bed for building an attenuator though. It is likley to have a different tone though due to the very different output stage. Ongoing, it probably wouldn't really be useful except as a test until you get a tube amp;


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## russbryant

Thanks for the reply John. I found this thread a few weeks ago and have finally read through it all. Will be building it at some point. I used to build HI-Fi speakers a few years back. Got started learning guitar a couple of years ago after retiring. Still a beginner but getting better. Have a background in EE and software.


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## IvanMan

JohnH said:


> If you can get close to the specified R values, with a series or parallel combination of any kind, then it should make no difference to the tone compared to using a single resistor


Here is my M2 16 Ohm attenuator, using serial / parallel resistor to obtain the requested value.
Unfortunately the user (my son, the family guitarist!) is not so satisfied.
We tested it with a 100W Marshall Plexi head, and the Marshall 1960 cab.
He says that mid-high frequencies are lost and also the sound is poor of harmonics.
Do you think that something in the realization may be the reason?
Is there the possibility to make some tests to verifiy the frequency response of the attenuator (I've a signal generator, oscilloscope, and other stuff).
Many thanks

PS You can see in the picture that the jacks are not Cliff type, but they are insulated ones.


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## Gene Ballzz

IvanMan said:


> Here is my M2 16 Ohm attenuator, using serial / parallel resistor to obtain the requested value.
> Unfortunately the user (my son, the family guitarist!) is not so satisfied.
> We tested it with a 100W Marshall Plexi head, and the Marshall 1960 cab.
> He says that mid-high frequencies are lost and also the sound is poor of harmonics.
> Do you think that something in the realization may be the reason?
> Is there the possibility to make some tests to verifiy the frequency response of the attenuator (I've a signal generator, oscilloscope, and other stuff).
> Many thanks
> 
> PS You can see in the picture that the jacks are not Cliff type, but they are insulated ones.



Nice work! Is that a steel chassis?
Just Askin'
Gene


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## JohnH

hi @IvanMan , congrats on getting it together but sorry to hear that it doesn't seem to sound right.

Id like to help understand the issue, and there are a number of tests that you can do. What we know from testing and evaluation on the previous builds is that the M and M2 designs make very little difference to tone, at any setting. 

That being said, a truely loud amp will sound different to an attenuated one. Even with a perfect attenuator, there will be somewhat different speaker response, acoustic feedback to the guitar, and differences in hearing and personal experience. We need to accept that these may be a factor, although my belief now after a few years on this is that they get blamed for more issues than they really cause.

So back to testing the attenuator. Its ideal goal is a simple one: At any attenuated volume, and at any amp setting, or any input to the amp, the measured response at the speaker should match that with no attenuator (ie speaker direct to amp), but at a lower volume. So, if you record at full and attenuated settings, normalize them so they are at the same max volume, you should hear no difference in the recording, and also see no difference in a frequency response trace. This design gets very close to that ideal, and its been confirmed by independent tests with different amps.

On yours, I suggest first to do some resistance tests to confirm the wiring and also the resistor combinations that you used.

1. If you unplug everything and switch all switches to not attenuating, then there are no paths through the circuit except around L1, R2A and R2B. So if you measure across each of the groups that you assembled to match the values in the design, you should be able to check each one with no rewiring to check if they are close. Measure across R2A (22 Ohm) and you are reading in combination with L1 and R2B, and you should get very close to 10 Ohm.

2. The most important check before trying with an amp: Amp disconnected, speaker plugged into the attenuator, measure across a lead plugged into the attenuator input (where the amp would go), and in every setting, it should read 7 to 10 ohms for an 8 Ohm M2. 

When noting resistances, subtract for the meter leads and any zero error when touching meter leads together.

3. Frequency tests: I like to do these by close micing the speaker, even though this adds the variable of the speaker response at different levels. I set up to run a short loud riff that pushes the amp output, and record that via a mixer into my pc, where I use Audacity. Need to be sure that the mic, mixer and pc can deal with the loudest sound with no added recording distortion. Then I use a delay pedal to set up a looped riff, just a few seconds is fine, and record that.

Then, without stopping the loop or moving the mic or any control, *switch amp to standby (important)* and put the attenuator in, and reengage the amp. Now record the same loop at the attenuated setting. I normalise the recordings and listen. Then I make frequency plots and export to excel to overlay them. They should be parallel.

You can see and hear my version of this in Post 1 (Design M but it works the same). There are some small ripples between full volume and the first setting, but all attenuated responses are very close and they all sound very similar.

4. The alternative to that is to feed in a frequency sweep. That might give a cleaner plot, though harder to evaluate as a guitar sound. You might also do that set so the amp stays clean for this test.

You could try all that at just one key setting, or work through the different steps as I did.


----------



## IvanMan

Gene Ballzz said:


> Nice work! Is that a steel chassis?
> Just Askin'
> Gene


----------



## IvanMan

No, it's aluminium. In the attachment you can see the external, with the wings that help dissipation (all the resistor are mounted on the ceiling).
PS The picture is my old realization, now internally I changed to M2


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## lordjester

Hi John,
I finally build an M2, with 3,5/7 dB Stage and a bypass switch. I did a few tests and I´m really impressed, it works perfect! I have tried a few attentuators over the yeras, my first one must be around 1982, a Tom Scholz Power Soak. It cuts treble, from the moment it is one, as far as I remember. I tried Weber Mini Mass, I sent it back immediately, I tried Toneking Ironman, same. I liked the UAD OX Box, but it is too expensive for my taste, because I really don`t need the speaker simulation stuff...I like the THD Hotplate, although most people don`t, as long as you don`t use the Pot. Up to minus 16dB without the pot, it works not bad. But the M2 is really much more natural to me and I probably will use it all the time. I think about building a road version, because on a regular gig, I problaby don`t need the extreme attenuation. So it would be nice to have small box, with maybe only one or two stages.
Thanks a lot for sharing your knockledge, I appreciate it alot!!


----------



## JohnH

Hi @lordjester , thanks for posting and congrats on getting it all together. I'm really happy it works well, and I see it has several of the extra features such as 3rd output for 16 ohm, line-out, and 3.5/7 db front end. Ill be interested in your comments on how those are, in due course. Thanks also for your comments in relation to the other attenuators, which are very encouraging for what we are doing here!


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## lordjester

I really wanted to build an "all options version", everything works fine. I did not really check out the 16Ohm output and the line out, yet. 
For live playing, I would not need all this options. I probably like to build a smaller version, only input and one output, no bypass, no line out, without the -14db stage, and maybe even with only -3,5 reactive stage, instead of the the switching version from 3,5 to 7dB.


----------



## JohnH

I thought Id put up something to try to explain what the switchable -3.5/-7db front end is expected to do.

These are analysis results, not physical testing, but so far of these designs, the calcs have given a fairly good prediction

*Red* is an unattenuated 4x12 cab, *Blue* is what we expect from the same cab after the attenuator, and *Green* is what the amp sees feeding into the attenuator. All traces a matched for volume so they can be compared for tone. Ideally they would all overlay, though with M2, we don't show the amp the low resonance peak.





The upper trace is an M2 running at -14db, using the basic -7db Stage 1 and another -7db stage. It gives a very good tonal match from Red to Blue, as it does at other settings too, and above the low bass, Green follows as well so the amp is responding accurately.

The Middle trace is just Stage 1, now set to -3.5db. It also does a good job of following the intended curves.

The Lower trace is with stage 1 set to -3.5, then a -7 and -3.5 stage to make -14db, (as in the top trace). Still Red and Blue are quite close, but the green trace is fallen off the ideal. Basically the amp is not seeing a full reactive load. It will still work OK, and with clean sounds it will likely sound fine, but the extra dynamics coming from a pushed amp may be diminished. Compare this to the top trace, which is a better looking version of -14 db.

What's happening is that at -3.5db, some extra reactance from the speaker is reaching through to the amp, giving it about the right amount, whereas with stage 1 at -3.5, and then more following attenuation, this doesn't come through.

I don't know if these differences can be heard, but Id expect that they sometimes could be. So, it shows how the intent of adding the optional switching to make -3.5db at the Stage 1 was really just for getting that one extra setting of -3.5, instead of starting at -7db. At -7db and below, I was expecting it to be a bit better when set to the base setting with a -7db Stage 1, although it can be tried either way.


----------



## Gene Ballzz

So @JohnH , is the signal you are virtually applying for your sim testing considered to be white noise, pink noise, or……….?
Just Curious?
Gene


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## JohnH

Gene Ballzz said:


> So @JohnH , is the signal you are virtually applying for your sim testing considered to be white noise, pink noise, or……….?
> Just Curious?
> Gene



Its basically a frequency sweep, done in a humongous spreadsheet, but the result is based on the same type of assumptions as would go into a SPICE model. It is based on each frequency being the same amplitude. So its an informative design tool for comparing options, but doesn't capture the relative levels of typical real music signals


----------



## TomBallarino

JohnH said:


> Hi @TomBallarino , thanks for posting that. It looks really great and you have pushed the design forward in a few areas. So Id love to ask a few questions, which will help others:
> 
> How are you finding the results from it so far?
> 
> I'd really like hear about your impression of the difference between having the bass resonance circuit in or out, (ie keeping L1 in but shorting L2 and C1)
> 
> Whats your line-out sub circuit?
> 
> What values did you work out for the -1.75 stages?
> 
> What fan did you use and what's the circuit for it?
> 
> thanks agsin for posting!


Hi @JohnH , 

I promised I will return back when I used my build in a real-life situation, which I did last weekend. I played a live-gig with my version of the M3 (with option to short out L2/C1), and a controllable -3,5dB solo boost 
The boost worked very well and did what it should do, the volume bump was exactly what I needed for soling.
While sound check my bass player and I switched back and forth the "reactive" switch which shorts the extra L2/C1, and could not agree if or what did actually change in the sound on the stage, so the differences are really (as many have predicted/noticed) very subtle. So for live-purposes I will build the M2 version, as the extra inductor and capacitor (when done in specs, e.g. foil capacitor) are a heavy addition to the device.

Amp was a JCM800-2204 clone with KT66 output tubes and the master pretty wide open.


----------



## JohnH

Thanks @TomBallarino , that's great feedback. It looks like the extra resonance circuit in M3 is not needed then, unless perhaps when used for silent recording with no real speaker.


----------



## EC Strat

JohnH said:


> Thanks @TomBallarino , that's great feedback. It looks like the extra resonance circuit in M3 is not needed then, unless perhaps when used for silent recording with no real speaker.


JohnH,

Forgive me as i'm REALLY late to the game here and just now jumping onboard the gravy train. I have a JTM 45 RI and i'm interested in building your M3 design - is there a list of parts that i need to get started that i can order from? I'm at square 1 here on this. 

I've gotten pretty good at soldering electronics in my guitars and i can follow a schematic reasonably well but i've never tackled anything like this before.


----------



## JohnH

Hi @EC Strat and welcome to our thread. There has been a few parts lists through the thread, but most builders buy from the suppliers that work best for them depending where they are, and so the ordering list ends up a bit different. Also whether to get brand-name resistors shipped quickly, or order from eBay in China, which are fine but take a long time,

A few things to be decided:

1. What ohms value from the amp do you want to use? you can build these for 4 or 8 or 16 ohms. (its also possible to add a few parts to deal with different input ohms, but best to pick one)
2. What speaker ohms do you want to use? You can use 4, 8, or 16 with an 8 ohm build, or 8 or 16 with a 16 ohm build. Its best to match for your main use.
3. What is the power of the largest amp that you might want? I think a JTM45 is close to the VM that I have, and 50W spec is good on that
4. Our basic design is M2, and it works great for any use where you are playing through a guitar speaker. M3 adds two more parts, to capture the bass resonance at the amp. But, as described by Tom above and others, it doesn't make a significant difference when used through a cab because the design is set up so cabs develop their own natural resonance. It could be useful if you want to do silent recording with no cab and a line-out into a cab-sim or IR box. The parts to go from M2 to M3 are quite bulky and expensive, so its a choice to be made. Given some planning, you can add these later to an M2 to make it M3, there's no other changes. So, is silent recording through a Line-out something youd like to have? Do you need a line out?
5. What will be the use of the attenuator? If its for home or studio, the base designs work well, which start at -7db which is a factor of 1/5 on power. A 50W amp becomes 10W, or less. A few builders have wanted a smaller reduction for loud gigs, so we can add a switch to provide -3,5db setting.
6. You can build in a socket to add a footswitch to make a level change for solos, if needed.

What do you think about all of that? The best advice is to build it as simple as possible to meet your needs. Its not expensive and keeping it simple makes for a simple and reliable wire-up and easy use.


----------



## Gene Ballzz

@JohnH beat me to it, almost word for word! John and I go back & forth, round & round on the following opinion: If you have 16 ohm taps on the amps you plan to use, I recommend the 16 ohm version. The 16 ohm M2 exhibits less compromise when using an 8 ohm cabinet that the 8 ohm unit does with a 16 ohm cabinet, if that makes any sense. It also allows you to run the unit at full attenuation in parallel with the speaker/cabinet (with amp tap set to 8 ohm) for a -3.5db cut. If you want to run parallel with an 8 ohm unit, your amp needs to have a 4 ohm output tap available! While there are design work arounds and add ons to get around these points, I will reiterate John's comment: *"The best advice is to build it as simple as possible to meet your needs. Its not expensive and keeping it simple makes for a simple and reliable wire-up and easy use."*

I will add that good heat sinking of resistors and good flow thru, convection ventilation is crucial for use with any amp above 20 watts! Also, all metal used in construction should be aluminum. Isolated, CLIFF UK style jacks are also important, to ensure that the enclosure/chassis does not ever become part of the circuit, as there is no ground,* per se,* simply positive and negative.

Have A Fun Build!
Gene


----------



## EC Strat

JohnH said:


> Hi @EC Strat and welcome to our thread. There has been a few parts lists through the thread, but most builders buy from the suppliers that work best for them depending where they are, and so the ordering list ends up a bit different. Also whether to get brand-name resistors shipped quickly, or order from eBay in China, which are fine but take a long time,
> 
> A few things to be decided:
> 
> 1. What ohms value from the amp do you want to use? you can build these for 4 or 8 or 16 ohms. (its also possible to add a few parts to deal with different input ohms, but best to pick one)
> 2. What speaker ohms do you want to use? You can use 4, 8, or 16 with an 8 ohm build, or 8 or 16 with a 16 ohm build. Its best to match for your main use.
> 3. What is the power of the largest amp that you might want? I think a JTM45 is close to the VM that I have, and 50W spec is good on that
> 4. Our basic design is M2, and it works great for any use where you are playing through a guitar speaker. M3 adds two more parts, to capture the bass resonance at the amp. But, as described by Tom above and others, it doesn't make a significant difference when used through a cab because the design is set up so cabs develop their own natural resonance. It could be useful if you want to do silent recording with no cab and a line-out into a cab-sim or IR box. The parts to go from M2 to M3 are quite bulky and expensive, so its a choice to be made. Given some planning, you can add these later to an M2 to make it M3, there's no other changes. So, is silent recording through a Line-out something youd like to have? Do you need a line out?
> 5. What will be the use of the attenuator? If its for home or studio, the base designs work well, which start at -7db which is a factor of 1/5 on power. A 50W amp becomes 10W, or less. A few builders have wanted a smaller reduction for loud gigs, so we can add a switch to provide -3,5db setting.
> 6. You can build in a socket to add a footswitch to make a level change for solos, if needed.
> 
> What do you think about all of that? The best advice is to build it as simple as possible to meet your needs. Its not expensive and keeping it simple makes for a simple and reliable wire-up and easy use.


johnH - thank you for the lengthy reply!

1. 4 and 8 Ohms
2. I use two cabinets, both 8 ohm cabs wired in parallel. so at home, the amp sees 4 ohms and gigs 8 ohms ( i only use 1 cab for gigs. - too much shit to carry). Would want a 4 and 8 ohm switch
3. JTM 45 i believe is a 30 watter
4. M2 design will be perfect
5. home and gigs - would like a - 3 and 5 db, the 7 db and i think i saw one of yours have a -14 db as well? Question, can you run the attenuator will all of the switches thrown in series for a total of -31 db?
6. this would be cool to add and i would like to add it.

What are the parts that i'll need and which suppliers do you tend to use? I'm thinking i'll just hit the mojotone website and go shopping. they've already got everything at a one stop shop.


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## JohnH

hi @EC Strat

Looks like it'll be an 8 Ohm M2 built to 50W then. This will work fine with 8 or 4 ohm loads, so one or two 8 ohm cabs. It has twin parallel outputs but no switch is needed for that. THe base M2 design also has a 3rd output, for using a 16 ohm cab with an 8 ohm M2, so you can ignore those parts if you dont want them. 

The basic design starts with always on -7db, then switchable stages of -14, -7 and -3.5db. You can put those on or off in any combination, to get every 3.5db step from -7 down to -31.5 db. Post 1 on page 1 has some clips demonstrating. 

3.5db is a very small step, so its a pretty fine level of adjustment provided. With your home and 4 ohm set up, I would think that running all that at -7db would be fine and you might well want more attenuation from the added stages. 

For your gigs, it depends how you set up, how loud you get, do you mic the amps? etc. If you build the basic M2 starting at -7db, theres a neat work around that gives you an extra -3 db option. You use your amps 4 Ohm taps, put one 8 ohm cab in and in the other amp output socket, plug in the attenuator set to max attenuation with no cab. Now the box acts just as a load, taking half the power. There's no added parts for this. 

Or, you can add a few parts to get a native -3.5db option in the box

On parts, if you go as above, minimal parts, Gene has a great layout on page 111 which you could use. A few others have tried it before,

You can get most of what you need from a major electronics supplier like Mouser or Digikey. The coil comes from places that sell loudspeaker parts, eg Madisound.com and Parts express in the US. is that where you are?

Can discuss more later


----------



## Gene Ballzz

@EC Strat ,
MojoTone does not offer the aluminum case/heat sinkable resistors required for this build. Mouser is a good source, but DigiKey appears to even ship free to the EU, as well as US, if you order enough stuff, like the case also. Air Core Inductor coils in the US can be obtained from Madisound and/or PartsExpress. Parts and enclosure sourcing discussion starts in earnest around page #110 of this thread! A Hammong 1590D or larger is the minimum size for convenient fitment, especially if using the commonly/readily US/EU available resistors, but the cheaper resistors from chinesiawanoreanam have a smaller footprint and work OK, if willing to wait for them! Don't skimp on the rating of the switches. Minis are OK, but the larger full size ones from Carling have bigger lugs and are a bit easier to be neat with.
Happy Buildin'
Gene


----------



## EC Strat

Gene Ballzz said:


> @EC Strat ,
> MojoTone does not offer the aluminum case/heat sinkable resistors required for this build. Mouser is a good source, but DigiKey appears to even ship free to the EU, as well as US, if you order enough stuff, like the case also. Air Core Inductor coils in the US can be obtained from Madisound and/or PartsExpress. Parts and enclosure sourcing discussion starts in earnest around page #110 of this thread! A Hammong 1590D or larger is the minimum size for convenient fitment, especially if using the commonly/readily US/EU available resistors, but the cheaper resistors from chinesiawanoreanam have a smaller footprint and work OK, if willing to wait for them! Don't skimp on the rating of the switches. Minis are OK, but the larger full size ones from Carling have bigger lugs and are a bit easier to be neat with.
> Happy Buildin'
> Gene


OK so i just ordered all the parts. Had to go to a few different websites as some of the resistors were on backorder at Digikey - Mouser had those items for me. I ordered the Hammond 1590D Black aluminum box and got the .9mH 18 awg inductor air coil from parts express - the coil looks red but it says it's highly refined copper ?? Hope its the right one

Looks Ike I still need to order 2 out jacks and 1 in and a foot switch - I’m assuming the foot switch needs an in-Jack also? Where are good websites for those?

Deeply appreciate your help Gene and JohnH


----------



## donwagar

The red coating on the coil is lacquer I think. Sand off the ends to connect them.

I bought my jacks from Mouser, I used 532-ACJS-PHOP (Mouser Canada, but I assume Mouser US uses the same numbers).

You'll want: 2 (speakers), 1 (from amp), and 2 for the footswitch, so 5.

I used a standard guitar pedal size aluminum box for the footswitch. I added an LED to it so I installed a power supply jack rather than use a battery.


----------



## JohnH

hi @EC Strat 

If you're doing the footswitch , then it needs a couple of resistors too. It amounts to another external attenuation stage. 3.5db seems to be the favoured spec. But its not part of the basic design. A few have been built and are reported to work well, out of many dozens of the basic M2's. I suggest to make space for a jack, but consider not wiring it in in the first build, in order to test the main parts. The jack for the footswitch needs to be a switched stereo jack, with 6 lugs, Cliff type


----------



## diego_cl

donwagar said:


> I bought my jacks from Mouser, I used 532-ACJS-PHOP (Mouser Canada, but I assume Mouser US uses the same numbers).



I'm planning to build a 100W M2, so I would like to know if I should give it a try to the Amphenol Jacks or stick with Cliff UK.

I bought some Amphenol jacks from Tubedepot with solder lugs along with some tubes, back in november. But I think those jacks from Mouser "532-ACJS-PHOP" are Amphenol with pins for pcb mounting.
TubeDepot never sent them to me due to tubes shortage. I had to cancel the tubes so they could ship me the jacks alone (bad experience). I'm still waiting (overseas shipping).

What jacks are tougher, Amphenol or Cliff UK??
According to datasheets, Amphenol hold up to 10A and +1.000 cycles, while Cliff UK hold up to 5A and +10.000 cycles, but I don't know if datasheets are trustworthy when comparing 2 different brands.


----------



## EC Strat

donwagar said:


> The red coating on the coil is lacquer I think. Sand off the ends to connect them.
> 
> I bought my jacks from Mouser, I used 532-ACJS-PHOP (Mouser Canada, but I assume Mouser US uses the same numbers).
> 
> You'll want: 2 (speakers), 1 (from amp), and 2 for the footswitch, so 5.
> 
> I used a standard guitar pedal size aluminum box for the footswitch. I added an LED to it so I installed a power supply jack rather than use a battery.


.


----------



## EC Strat

diego_cl said:


> I'm planning to build a 100W M2, so I would like to know if I should give it a try to the Amphenol Jacks or stick with Cliff UK.
> 
> I bought some Amphenol jacks from Tubedepot with solder lugs along with some tubes, back in november. But I think those jacks from Mouser "532-ACJS-PHOP" are Amphenol with pins for pcb mounting.
> TubeDepot never sent them to me due to tubes shortage. I had to cancel the tubes so they could ship me the jacks alone (bad experience). I'm still waiting (overseas shipping).
> 
> What jacks are tougher, Amphenol or Cliff UK??
> According to datasheets, Amphenol hold up to 10A and +1.000 cycles, while Cliff UK hold up to 5A and +10.000 cycles, but I don't know if datasheets are trustworthy when comparing 2 different brands.


thank you for linking those items - saved me a ton of time


----------



## Gene Ballzz

Yeah, the Amphenol are at least a sturdy/robust as the CLIFF UK, but you probably don't want to deal with the added hassle of using the PCB mount units. A good source for switches and jacks:



https://www.tubesandmore.com



And FWIW, knowing what part of the planet yer at can help with sourcing suggestions!

Best To Ya,
Gene


----------



## EC Strat

Gene Ballzz said:


> Yeah, the Amphenol are at least a sturdy/robust as the CLIFF UK, but you probably don't want to deal with the added hassle of using the PCB mount units. A good source for switches and jacks:
> 
> 
> 
> https://www.tubesandmore.com
> 
> 
> 
> And FWIW, knowing what part of the planet yer at can help with sourcing suggestions!
> 
> Best To Ya,
> Gene


Oh sorry - I’m in the USA


----------



## donwagar

I don't know if this was mentioned, if I recall correctly, you need stereo (TRS) jacks for the footswitch. I just order all of them as TRS.

Edit: I looked back, yes John did mention this.


----------



## Paul L Ewing

I wouldn't even think about making anything for an attenuator since a small error or something breaking down can toast your amp. Any repair will I am sure exceed the $100 USD I paid for the Bugera PS-1 Power Soak that Mike Soldano designed. I have been using it for years with zero problems. At the price, performance, heavy build and reliability it is a no-brainer to buy.


----------



## Graham G

Paul L Ewing said:


> I wouldn't even think about making anything for an attenuator since a small error or something breaking down can toast your amp. Any repair will I am sure exceed the $100 USD I paid for the Bugera PS-1 Power Soak that Mike Soldano designed. I have been using it for years with zero problems. At the price, performance, heavy build and reliability it is a no-brainer to buy.


Please don't think i'm trying to start an argument or trying to be a smart arse, but before i built Johns attenuator i thought pretty much the same as you, so i bought a PS1 & did a gig with it & didn't think the Amp sounded that great with the PS1(maybe a bit dead sounding), so i decided to build the M2 & to me the PS1 sounds "shit"(for want of a better word) in comparison(i think i commented so in a post), if i was you i'd build an M2(or have a tech build it if you prefer) & do a comparison, i suspect you may well think the same.
Cheers, Graham.


----------



## diego_cl

Paul L Ewing said:


> I wouldn't even think about making anything for an attenuator since a small error or something breaking down can toast your amp. Any repair will I am sure exceed the $100 USD I paid for the Bugera PS-1 Power Soak that Mike Soldano designed. I have been using it for years with zero problems. At the price, performance, heavy build and reliability it is a no-brainer to buy.


I had a Harley Benton PA-100 (I think it's a clone of the Bugera PS-1) and the attenuation pot was like a low pass filter... the attenuation muffled the tone. A 50W Marshall sounded like a subwoofer if cranked at bedroom level, pretty unusable. With a 22W Deluxe Reverb it wasn't nearly as bad, the tone suck was notorious but pretty acceptable. 

The M2 attenuator is so much better, especially when attenuating a lot of dB. Anyway a quiet 12" speaker isn't inspiring, but that's not the attenuator fault.


----------



## Gene Ballzz

Paul L Ewing said:


> I wouldn't even think about making anything for an attenuator since a small error or something breaking down can toast your amp. Any repair will I am sure exceed the $100 USD I paid for the Bugera PS-1 Power Soak that Mike Soldano designed. I have been using it for years with zero problems. At the price, performance, heavy build and reliability it is a no-brainer to buy.



This project is definitely *NOT* for those with poor soldering skills and/or are not good at following instructions!  Of course, that would also apply to making one's own speaker cable, rewiring one's speaker cabinet, etc, ad nauseam! The trick is (much like making sure you stay in your own lane on the road, to avoid fatality) to not make those small errors, check your work for confirmation and use quality components and procedures. If one lacks confidence in those skills, they should certainly *NOT* tackle this build or benefit from the enjoyment of the results! This project is intended for those somewhat skilled *"do it yourselfers"* who understand that paying attention to details and testing is the key to any successful outcome!

Most errors and accidents are the result of not paying attention! For example: If a person falls fatally off a cliff while walking with their face and attention buried in a smart phone, where does the fault lie? Is it with the cliff? No. Is it with the smart phone? No! Is it with……? 

With that said and what @Graham G and @diego_cl said above, this attenuator design absolutely blows away any and all of the cheaper units, including the Bugera, Weber, etc, in terms of performance, tonal retention, response and feel.

You are of course, entitled to your opinion, and so am I. My opinion is that my skills _(I build amplifiers and have been doing a wide variety of electronics repairs for +50 years)_ are quite likely at least a small step above those of the forced child laborers that are the probable assemblers of the Bugera units!

Of course, I also understand the propensity some have for piddling in others' bowls of Corn Flakes! That's OK too, as long as they're not MY Corn Flakes, as the consequences may be even less pleasurable than amplifier failure! 

Enjoy Your PS1 & Your Opinion, 
Gene


----------



## Point 2 Point

I lost track of this thread, glad you bumped it up.
As I recall the simple attenuator was / has been redesigned along the way, what is the latest design?
Page # 27?


----------



## Freddy G

Gene Ballzz said:


> This project is definitely *NOT* for those with poor soldering skills and/or are not good at following instructions!  Of course, that would also apply to making one's own speaker cable, rewiring one's speaker cabinet, etc, ad nauseam! The trick is (much like making sure you stay in your own lane on the road, to avoid fatality) to not make those small errors, check your work for confirmation and use quality components and procedures. If one lacks confidence in those skills, they should certainly *NOT* tackle this build or benefit from the enjoyment of the results! This project is intended for those somewhat skilled *"do it yourselfers"* who understand that paying attention to details and testing is the key to any successful outcome!
> 
> Most errors and accidents are the result of not paying attention! For example: If a person falls fatally off a cliff while walking with their face and attention buried in a smart phone, where does the fault lie? Is it with the cliff? No. Is it with the smart phone? No! Is it with……?
> 
> With that said and what @Graham G and @diego_cl said above, this attenuator design absolutely blows away any and all of the cheaper units, including the Bugera, Weber, etc, in terms of performance, tonal retention, response and feel.
> 
> You are of course, entitled to your opinion, and so am I. My opinion is that my skills _(I build amplifiers and have been doing a wide variety of electronics repairs for +50 years)_ are quite likely at least a small step above those of the forced child laborers that are the probable assemblers of the Bugera units!
> 
> Of course, I also understand the propensity some have for piddling in others' bowls of Corn Flakes! That's OK too, as long as they're not MY Corn Flakes, as the consequences may be even less pleasurable than amplifier failure!
> 
> Enjoy Your PS1 & Your Opinion,
> Gene


I would like to add that there are not many attenuators I've met that didn't change the sound for the worse. So for me this DIY (I made two of these attenuators for my stereo rig) was worth it, whatever the cost. Because it is astonishing how well it works while keeping the sound AND feel of the amp in tact 100%.


----------



## Gene Ballzz

Freddy G said:


> I would like to add that there are not many attenuators I've met that didn't change the sound for the worse. So for me this DIY (I made two of these attenuators for my stereo rig) was worth it, whatever the cost. Because it is astonishing how well it works while keeping the sound AND feel of the amp in tact 100%.



The fact that *"Ears Extraordinare" * @Freddy G has seen fit to grace us with his presence on this subject, should be no small testament to the stellar performance this design exhibits! For those who aren't aware, Freddy is one of the finest sound engineers on the planet, excellent luthier/guitar builder, proficient guitarist and all around great guy!

Thanks Freddy,
Gene


----------



## JohnH

Point 2 Point said:


> I lost track of this thread, glad you bumped it up.
> As I recall the simple attenuator was / has been redesigned along the way, what is the latest design?
> Page # 27?


Post 1 on Page 1 has the latest basic schematic for the M2, which has been updated a few times. its probably due for another update, but its still right for the best base design that we have. If you want to read further, probably skip forward to about page 110. There are wiring layouts on pages 111 and 112, and you can read about a few options and variants from there.

Here's my view on who should build these:

Its not a complicated thing, actually its a very simple circuit with no complicated or active parts, nice big robust components and entirely passive. But, its going to be a key element at the end of a valuable amplifier and so it needs to be done correctly. This means it has to be wired right with the right parts, and the soldering and general construction has to be effective. It doesn't have to be beautiful (but many of them are), but good soldering with the right tools and procedure is neat and well 'flowed'. Its a craft skill that needs to be developed by practice and preferably by experience before tackling the attenuator.

There are quite a lot of wires between parts. You need a pictorial wiring diagram as a visual reference to work off. But you also need to be able to follow the flow of the circuit and for that you need to be able to understand the schematic (ie as on Post 1). My view is that, even though its not essential to understand the electronic theory, a builder of this attenuator should take the time to look at the schematic and understand what it is showing, and then relate that to a wiring layout. The wiring can be based on a layout you make yourself, or, see those on p111 and p112. With those in place , then you can trace your work, check it and correct it and troubleshoot it if needed

For a long time a resisted having the wiring diagrams here. The schematic was then like an 'entrance exam'. Those who can read it are likely to be able to follow through to a successful build. Wiring it up just from a wiring diagram 'paint by numbers' style without relating it to what is being connected to what risks an error going undetected. 

Genes wiring diagram on p111 is great and its helped a few builds since. I see it best as a way of helping relate the build layout to the schematic, to step-up the understanding, rather than a lowering of the bar to avoid needing to understand.

Finally, before trying with an amp, it is entirely essential to run some multimeter tests to check resistances.


All that being said, we have maybe 50 or so successful and well-liked builds on this thread, and no amp damage reported to date! For those who can tackle the build, and have time and interest to do so, you can build a much better sounding and more robust attenuator than you can buy for the $.


----------



## EC Strat

JohnH said:


> Post 1 on Page 1 has the latest basic schematic for the M2, which has been updated a few times. its probably due for another update, but its still right for the best base design that we have. If you want to read further, probably skip forward to about page 110. There are wiring layouts on pages 111 and 112, and you can read about a few options and variants from there.
> 
> Here's my view on who should build these:
> 
> Its not a complicated thing, actually its a very simple circuit with no complicated or active parts, nice big robust components and entirely passive. But, its going to be a key element at the end of a valuable amplifier and so it needs to be done correctly. This means it has to be wired right with the right parts, and the soldering and general construction has to be effective. It doesn't have to be beautiful (but many of them are), but good soldering with the right tools and procedure is neat and well 'flowed'. Its a craft skill that needs to be developed by practice and preferably by experience before tackling the attenuator.
> 
> There are quite a lot of wires between parts. You need a pictorial wiring diagram as a visual reference to work off. But you also need to be able to follow the flow of the circuit and for that you need to be able to understand the schematic (ie as on Post 1). My view is that, even though its not essential to understand the electronic theory, a builder of this attenuator should take the time to look at the schematic and understand what it is showing, and then relate that to a wiring layout. The wiring can be based on a layout you make yourself, or, see those on p111 and p112. With those in place , then you can trace your work, check it and correct it and troubleshoot it if needed
> 
> For a long time a resisted having the wiring diagrams here. The schematic was then like an 'entrance exam'. Those who can read it are likely to be able to follow through to a successful build. Wiring it up just from a wiring diagram 'paint by numbers' style without relating it to what is being connected to what risks an error going undetected.
> 
> Genes wiring diagram on p111 is great and its helped a few builds since. I see it best as a way of helping relate the build layout to the schematic, to step-up the understanding, rather than a lowering of the bar to avoid needing to understand.
> 
> Finally, before trying with an amp, it is entirely essential to run some multimeter tests to check resistances.
> 
> 
> All that being said, we have maybe 50 or so successful and well-liked builds on this thread, and no amp damage reported to date! For those who can tackle the build, and have time and interest to do so, you can build a much better sounding and more robust attenuator than you can buy for the $.


My biggest unknown is how I’m going to drill holes in the case - other than that, I’m good to go with the schematic


----------



## diego_cl

EC Strat said:


> My biggest unknown is how I’m going to drill holes in the case - other than that, I’m good to go with the schematic


For me drilling the case was the most laborious part of the build. I had better and faster results using a punch tool with a hammer, a drill bit and a step drill bit.

The punch tool allowed me to use the drill by hand without the need of my shitty portable drill guide. I used that guide before and the holes ended up crooked and it was way too time consuming.

Using a heavy drill bit at once was also a bad idea, because it was slow, imprecise and it shaved the holes in an ugly way.

The notch made by the punch tool allowed me to use a drill by hand without wandering around the surface. The drill bit had to be big enough for the step drill bit and for not flexing with compression, but small enough so it would drill fast. The step drill bit saved me a lot of time. The one thing I didn't like about the step drill bit is the lack of control over the bevels of every hole. 

I wish I have a drill press though.


----------



## JohnH

EC Strat said:


> My biggest unknown is how I’m going to drill holes in the case - other than that, I’m good to go with the schematic


Aluminium drills pretty good with normal hand tools. Gene and a few others have superior workshop skills. But I do OK with a careful mark out onto masking tape, very small pilot holes, then two or three sizes of drill to get up to large diameters such as for Jacks or if you have large toggles. A bench vice or equivalent clamping is a must-have and if you have access to a drill-press it is much better but its possible with a hand-held cordless.

I like to buy unpainted cases - then when I've drilled I can clean up my workshop sins and do a black rattle can finish, including into the raw edges of vent holes.


----------



## Barnsley Boy

Paul L Ewing said:


> I wouldn't even think about making anything for an attenuator since a small error or something breaking down can toast your amp. Any repair will I am sure exceed the $100 USD I paid for the Bugera PS-1 Power Soak that Mike Soldano designed. I have been using it for years with zero problems. At the price, performance, heavy build and reliability it is a no-brainer to buy.


I have previously owned a Jettenuator (same as PS-1) which I used to tame my lovely Cornford Roadhouse running into a 2 x 12 with Greenbacks. The amp was a beast. If only @JohnH had come up with the M2 design back then, I would never have sold the Roadhouse. I almost made the same mistake again with my newly aquired JCM900, but thankfully @Gene Ballzz pointed me in the right direction. If you can solder, drill holes and follow instructions as said previously you can build something that blows an L-pad based resistive out of the water. 

@Paul L Ewing it really is a great bit of kit. Lots of the builds are crammed into some quite small enclosures which can be challenging if soldering is not your strong point. Also using mini toggles is a bit fiddly. Failing that, buy all the bits and bung a mate who's handy with a soldering iron a few quid/ bucks/ euros/ rubles to do it for you. You won't be sorry .... trust me!


----------



## Lancer X

EC Strat said:


> My biggest unknown is how I’m going to drill holes in the case - other than that, I’m good to go with the schematic



Even a cheap set of step drill bits will make this project (and any future amp chassis mods you tackle) so much easier. As @JohnH said, aluminum is also much, much easier to modify than steel.

A drill press is recommended. I hand-drilled mine, and it made for a pretty physical evening of drill-wrangling.


----------



## EC Strat

Lancer X said:


> Even a cheap set of step drill bits will make this project (and any future amp chassis mods you tackle) so much easier. As @JohnH said, aluminum is also much, much easier to modify than steel.
> 
> A drill press is recommended. I hand-drilled mine, and it made for a pretty physical evening of drill-wrangling.


Appreciate the advice

BTW What sized drill bits did you use? Looks like 1/8” bit should do the trick


----------



## EC Strat

Well, parts have all arrived. Got the resistors labeled

@JohnH - I’d like to do a bypass switch - is there a schematic showing that?


----------



## JohnH

Hi @EC Strat , Some do build these in. Can you remind me how many output sockets are you using, ie just one or two in parallel or are you doing the output 3 as well (makes bypass more tricky)

Also, do you really need a bypass? because with a bypass engaged, you could instead just not use an attenuator. After a few tests you are likely to find that just one or two settings are all you use. Unless your use involves often adjusting between attenuated or not, then maybe you dont need one? A bypass switch is switching the full power and current of the amp, and must never be operated unless the amp is off or on standby, whereas with the other switches they can be operated on the fly.


----------



## EC Strat

JohnH said:


> Hi @EC Strat , Some do build these in. Can you remind me how many output sockets are you using, ie just one or two in parallel or are you doing the output 3 as well (makes bypass more tricky)
> 
> Also, do you really need a bypass? because with a bypass engaged, you could instead just not use an attenuator. After a few tests you are likely to find that just one or two settings are all you use. Unless your use involves often adjusting between attenuated or not, then maybe you dont need one? A bypass switch is switching the full power and current of the amp, and must never be operated unless the amp is off or on standby, whereas with the other switches they can be operated on the fly.


I’ll be switching from attenuated to non-attenuated. I like to use pedals and the JTM 45 clean mostly often. But I want an attenuator so I can use the amp as intended when I’d I like to 

I’m planning on 2 output jacks in parallel


----------



## EC Strat

Been working on this and planning it most of the day today. Here’s the fruits of the labor thus far: I’m going to pre-install the jumpers prior to installing the jacks and pots tomorrow

@JohnH one of the resistors is covering up a heat sink on the bottom Should I drill more? I have 8 up top

Well now it won’t let me attach a pic


----------



## EC Strat

Ah here we go -


----------



## JohnH

Hi @EC Strat

If you can get another hole or two in the base it would be good, so long as it doesn't hit anything.

Here's a good way to do a bypass on M2. Ignore the Out3 wiring in red.




The looping green and blue wires give a double set contacts in parallel when bypassed. The inset picture shows how to wire it on a DPDT switch.


----------



## EC Strat

JohnH said:


> Hi @EC Strat
> 
> If you can get another hole or two in the base it would be good, so long as it doesn't hit anything.
> 
> Here's a good way to do a bypass on M2. Ignore the Out3 wiring in red.
> 
> View attachment 105014
> 
> 
> The looping green and blue wires give a double set contacts in parallel when bypassed. The inset picture shows how to wire it on a DPDT switch.


Thank you for posting the schematic. I’ll print it out 

Btw - what would happen if I did say turn on the bypass while the amp was on / engaged? Ka-boom?


----------



## ToneWitch

EC Strat said:


> Thank you for posting the schematic. I’ll print it out
> 
> Btw - what would happen if I did say turn on the bypass while the amp was on / engaged? Ka-boom?


Yes, don’t ask me how I know lol


----------



## EC Strat

ToneWitch said:


> Yes, don’t ask me how I know lol


Sometimes experience is the best teacher LOLOL


----------



## JohnH

As you go through a full bypass, there could be a nasty transient when the contacts change where the amp is momentarily open-circuit, particularly with toggles, which are break-before-make. Also due to the speaker coil being suddenly connected. This may be bad for the amp. So to use a bypass safely, better to avoid that risk.

This is not really different to any other passive attenuator with bypass. Tone King Ironman for example, tells you to mute the strings, even though they offer it on a footswitch so you might more likely to be in mid riff when you did it!

Another watchit is, while an 8 Ohm M2 can work well with 8 or 4 ohm speakers, or 16 (whether or not you use the Out3 circuit which is just a tone correction), if you do a bypass, then you have to match speakers to amp.

I do have one on my build, and I have never used it except for testing. I could see it'd be useful sometimes though, eg for an attenuator built into an amp. 

A suggestion would be to make it distinct from the others, maybe around the back if the main switches are on the front, and/or, a different style of switch.


----------



## EC Strat

JohnH said:


> As you go through a full bypass, there could be a nasty transient when the contacts change where the amp is momentarily open-circuit, particularly with toggles, which are break-before-make. Also due to the speaker coil being suddenly connected. This may be bad for the amp. So to use a bypass safely, better to avoid that risk.
> 
> This is not really different to any other passive attenuator with bypass. Tone King Ironman for example, tells you to mute the strings, even though they offer it on a footswitch so you might more likely to be in mid riff when you did it!
> 
> Another watchit is, while an 8 Ohm M2 can work well with 8 or 4 ohm speakers, or 16 (whether or not you use the Out3 circuit which is just a tone correction), if you do a bypass, then you have to match speakers to amp.
> 
> I do have one on my build, and I have never used it except for testing. I could see it'd be useful sometimes though, eg for an attenuator built into an amp.
> 
> A suggestion would be to make it distinct from the others, maybe around the back if the main switches are on the front, and/or, a different style of switch.


thanks, yes i put it in the back. I'm about halfway through the wiring schematic. I had to redrill a few holes bc the pots / jacks were too low and contacting the resistors. So, it's a bit swiss cheese like but hey the more heat sinks the better i suppose.

VERY tough to get the switches lined up all nice and near. The hand drill really drifts a bit 

I'm using 14 AWG wire and bare copper wire for the jumpers. 

that 1590D box is really tiny! Had to get creative and redrill some holes on the sides etc for the resistors. If i ever do another one, i'll look for the next size up probably. taking me longer than i thought it would, but i'm not rushing and going slowly, check connections with the multimeter as i go and verfiy where the wires should be running before i get too far down the road. Learned the hard way on that one with some partscaster builds and pickups / pots installs. 

Deeply appreciate your mentorship! This has been a fun project and i'm sure it will be rewarding in the end


----------



## Gene Ballzz

@EC Strat ,
Just to clarify some terminology to avoid confusion. The holes drilled in the box are *"vents"* that aid cooling through convection/air movement not *"heat sinks."* The *"heat sink"* is actually the thick aluminum box that the resistors fasten to and it aids cooling through* "conduction"* of heat by the resistors being securely bolted to and held in contact with it, with thermal heat sink paste/compound/grease in between them. You *ARE *using *"thermal heat sink paste/compound/grease"* for assembly, correct?

thermal heat sink paste



Just Checkin'
Gene


----------



## EC Strat

Gene Ballzz said:


> @EC Strat ,
> Just to clarify some terminology to avoid confusion. The holes drilled in the box are *"vents"* that aid cooling through convection/air movement not *"heat sinks."* The *"heat sink"* is actually the thick aluminum box that the resistors fasten to and it aids cooling through* "conduction"* of heat by the resistors being securely bolted to and held in contact with it, with thermal heat sink paste/compound/grease in between them. You *ARE *using *"thermal heat sink paste/compound/grease"* for assembly, correct?
> 
> thermal heat sink paste
> 
> 
> 
> Just Checkin'
> Gene



Great catch Gene - had no idea I needed to use it. I’ll pick up a tube at Home Depot and apply it 

I should basically apply it to the underside of each resistor? 

Thanks again


----------



## JohnH

Yes, and the ideal is the thinnest layer that covers the whole surface. It's good to di a practice or two on a spare surface. Smear it over the resistor and squish it down, moving it side to side. Then pull it up and see how it went.


----------



## Jason deBroux

ToneWitch said:


> Yes, don’t ask me how I know lol


How do you know !!? 
You know all of us guitar amp/electronics nerds love stories about how to make errr, I mean how things blow up.


----------



## tfletchii

Is there a wiring diagram for wiring the -3.5 stage on its own?
I see it mentioned and drawn in the original M attenuator but I'm a novice and I'm not following things clearly. I have the bypass wired and working well. I've built the M3 version and really enjoying it with my plexi and my BF princeton. The jump to -7db though is a bit much with the princeton and I think being able to use the -3.5 stage independently would be a great addition for me. 
Thanks John, the attenuator has made using my amps at there sweet spot on gigs so much easier!


----------



## JohnH

tfletchii said:


> Is there a wiring diagram for wiring the -3.5 stage on its own?
> I see it mentioned and drawn in the original M attenuator but I'm a novice and I'm not following things clearly. I have the bypass wired and working well. I've built the M3 version and really enjoying it with my plexi and my BF princeton. The jump to -7db though is a bit much with the princeton and I think being able to use the -3.5 stage independently would be a great addition for me.
> Thanks John, the attenuator has made using my amps at there sweet spot on gigs so much easier!


Hi @tfletchii and welcome. Im very happy to hear that your M3 is working well. What specs did you build to, eg power and 8 or 16 Ohm?. Also how many outputs did you have?

There have been a few builders who have wanted something inbetween 0 and -7db, mainly when gigging with medium or lower powered amps where you want to keep most of the volume.

The way I showed in in Post 1 for design M can still apply to M2 or M3, and it works ok but you mayhave to get new larger resistors for the -3.5 stage since it may take most of the power, and it has some watch-its when using different speaker ohms. Plus, its just a resistive stage, not reactive although you get some speaker reactance coming through. But there are two other ways developed since, which are probably better:

1. Easiest, if it works for you, nothing to build!

Set the attenuator to max attenuation with no speaker, and plug it into the amp in parallel with your speaker, using a second output jack on the amp. Set your amp to half the speaker ohms. Now the attenuator takes half the amp power acting as a load box, and the speaker gets half. This is an attenuation of -3db. It should work perfectly with an M3, if you have the right amp outputs,

2. You can add a switch that turns the -7db Stage 1 to -3,5db. Use this when you want this setting and other switches are off. You can also add the other stages in too, but the tone may be very slightly better in that case if you revert to -7db in the first stage if possible. Here's the schematic. Can you follow it?: If its of interest I can provide values to suit your build.









M2 With 70-35 220126







www.marshallforum.com





Its for an M2, but with M3, it would be similar with the extra coil and cap in series with L1, as in the base M3 design.


----------



## tfletchii

JohnH said:


> Hi @tfletchii and welcome. Im very happy to hear that your M3 is working well. What specs did you build to, eg power and 8 or 16 Ohm?. Also how many outputs did you have?
> 
> There have been a few builders who have wanted something inbetween 0 and -7db, mainly when gigging with medium or lower powered amps where you want to keep most of the volume.
> 
> The way I showed in in Post 1 for design M can still apply to M2 or M3, and it works ok but you mayhave to get new larger resistors for the -3.5 stage since it may take most of the power, and it has some watch-its when using different speaker ohms. Plus, its just a resistive stage, not reactive although you get some speaker reactance coming through. But there are two other ways developed since, which are probably better:
> 
> 1. Easiest, if it works for you, nothing to build!
> 
> Set the attenuator to max attenuation with no speaker, and plug it into the amp in parallel with your speaker, using a second output jack on the amp. Set your amp to half the speaker ohms. Now the attenuator takes half the amp power acting as a load box, and the speaker gets half. This is an attenuation of -3db. It should work perfectly with an M3, if you have the right amp outputs,
> 
> 2. You can add a switch that turns the -7db Stage 1 to -3,5db. Use this when you want this setting and other switches are off. You can also add the other stages in too, but the tone may be very slightly better in that case if you revert to -7db in the first stage if possible. Here's the schematic. Can you follow it?: If its of interest I can provide values to suit your build.
> 
> 
> Its for an M2, but with M3, it would be similar with the extra coil and cap in series with L1, as in the base M3 design.


Hey John, option 1 won't work for me because I modded my princeton and put a mid knob where the extra speaker jack was located but option two sounds perfect and its what i was hoping since this option keeps the reactive load. I'll give it a go and report back. Thanks!


----------



## JohnH

hi @tfletchii , that's great. Have you got the values for the extra 2 resistors?


----------



## tfletchii

JohnH said:


> hi @tfletchii , that's great. Have you got the values for the extra 2 resistors?


No, I just sat down to take a good look at it. I would love your suggestions. At first I thought it would be as simple as taking the values from R7 & R8 and increasing the wattage but it doesn't look like thats the case.


----------



## JohnH

Values for the amended Stage 1 are in post 2359 on page 118, presented there as a standalone -3.5db/7db reactive stage, for either 8 or 16 ohm builds.

On page 120 post 2382 you can see the full circuit with i as i posted yesterday. You can see how stage 2 is now with 3 resistors. But if you don't use the stages 2, 3 and 4 unless stage 1 is at 7db (which i think will be better anyway) then you can keep them as they are. 

But you could also consider just building the one stage on page 118, which might be a nice simple version for your gigs.


----------



## tfletchii

JohnH said:


> Values for the amended Stage 1 are in post 2359 on page 118, presented there as a standalone -3.5db/7db reactive stage, for either 8 or 16 ohm builds.
> 
> On page 120 post 2382 you can see the full circuit with i as i posted yesterday. You can see how stage 2 is now with 3 resistors. But if you don't use the stages 2, 3 and 4 unless stage 1 is at 7db (which i think will be better anyway) then you can keep them as they are.
> 
> But you could also consider just building the one stage on page 118, which might be a nice simple version for your gigs.


Hey John, thanks again for all the help. I'm about to order the needed resistors but should I get 100w for the R1 values? I saw in your post that 50w should be fine but is that still ok if I'm using the attenuator with my plexi


----------



## JohnH

hi @tfletchii , They can be 50W because when in -7.5db mode, what was a single 100w resistor for R1in the base M2 is now shared by two 50W in parallel, and in -3.5db mode, the first stage takes less power. All of that is for a 50w amp. Does that cover yours?


----------



## vintageman

Hi John, I found this forum and I read almost the whole thread. Thank you very much to you and others involved for this great project. 

I have been longing for a good attenuator and I would like to build an M2 version. I would like an attenuator for the combo, which is a copy of the Princeton Reverb amp with a power of 15W (2x 6V6 tube). The speaker is a Weber California 12 (80w power for more headroom sound from amp). I would like an attenuator for playing in the bedroom or recording at night. I would like to achieve attenuation eg from 15W to 7W, 3W, 1W, 0.5W. 

I have a few questions. When I look at the drawing "J Hewitt 31/12/2021", is it possible to cut the resistors by half for my amp 15W? So it would be R1A and R1B at 25W or use one R1 15R 50W. R2A and R2B 25W, R3-R11 then 15W? R13 must be 1W or half (0.5W)? I'm from Europe, so I'll probably order from tme.eu, where some resistors have only a tolerance of 5%. Will it have a major impact on the circuit ?


----------



## JohnH

hi @vintageman , welcome to our thread. Yes if you are just using a 15W amp, you can halve all the power rating specs on the resistors, but still use case-mounted ones screwed to the case and with thermal grease. 

Also, keep the coil as wound with 18gage wire. 

5% spec is fine for the resistors. 

Good luck!


----------



## vintageman

JohnH said:


> hi @vintageman , welcome to our thread. Yes if you are just using a 15W amp, you can halve all the power rating specs on the resistors, but still use case-mounted ones screwed to the case and with thermal grease.
> 
> Also, keep the coil as wound with 18gage wire.
> 
> 5% spec is fine for the resistors.
> 
> Good luck!


Hi John, thanks for your prompt reply. For my low power amp it will probably be good to use stage1 -3.5dB / -7dB. Is it possible to make an attenuator with three stages? I don't think -14dB is needed for my 15W amp. Maybe to completely mute the speaker and use a line out ?

Stage1 -3.5dB / -7dB 
Stage2 -7dB 
Stage3 -3.5dB

I apologize for my questions, I want to find out more information before I start building.


----------



## BlueX

vintageman said:


> For my low power amp it will probably be good to use stage1 -3.5dB / -7dB. Is it possible to make an attenuator with three stages? I don't think -14dB is needed for my 15W amp. Maybe to completely mute the speaker and use a line out ?



I use my M2 attenuator, built with all three steps (-3,5, -7, and -14 dB), together with my 20W SV20H. When all three steps are "on" I would still disturb a person trying to sleep in the next room. Possible to speak normally where I'm playing though. Maybe that can give some hint on "effectiveness".


----------



## vintageman

BlueX said:


> I use my M2 attenuator, built with all three steps (-3,5, -7, and -14 dB), together with my 20W SV20H. When all three steps are "on" I would still disturb a person trying to sleep in the next room. Possible to speak normally where I'm playing though. Maybe that can give some hint on "effectiveness".


Thanks for the feedback. Then the M2 attenuator with all three steps will be suitable for me as well.


----------



## JohnH

Yes I use mine often at -31.5 db, with my VM. It's nice and clear in the room, and the volume is not even maxed at the amp. On paper, my VM is about 38W, which is just 4db louder than an equivalent 15W amp. 

So if you're playing at home, I'd suggest that -14 db stage will still be useful sometimes, even if you switch one or two of the others off.


----------



## donwagar

For my SV20 I left the -14 stage out, but I built it for gigs and practices. (And I have the additional -3.5 in a footswitch). For home use I'd keep the -14 stage.


----------



## russbryant

Hi John,

I posted a short while ago and mentioned that I would be building an M2 after I get a tube amp. Well I got it on Sunday. A second hand Marshall DSL40CR. I just about have the workshop whipped into shape after moving from Arizona to Virginia so I'll be ordering parts soon.

Thanks,
Russ in SW Va


----------



## JohnH

hi @russbryant , I reckon that's a great amp. Good luck with that and the M2 project!


----------



## Lancer X

Curious if any of you have considered a modification such that the attenuation levels could be hot-switched without a resulting "pop" or other effect detrimental to the speaker health? Maybe something like a high pass voltage divider on the output, or the like? I'm an amp electronics beginner, so please forgive my sophomoric stab at a solution.

Commercial attenuators with dial controls seem to be okay to hot-adjust. (Or am I mistaken about that? I certainly used my Minimass that way.) Perhaps the problem is only due to the discreet jumps in switched attenuation levels?


----------



## lordjester

I don`t have any switching noises with mine, no pops, nothing, not even with the bypass switch....


----------



## donwagar

lordjester said:


> I don`t have any switching noises with mine, no pops, nothing, not even with the bypass switch....



Neither do I, and I use the -3.5dB footswitch too.


----------



## BlueX

Lancer X said:


> attenuation levels could be hot-switched without a resulting "pop"





lordjester said:


> I don`t have any switching noises with mine, no pops, nothing, not even with the bypass switch....





donwagar said:


> Neither do I, and I use the -3.5dB footswitch too.



Same answer here, no switching noise. Could the problem be with your switches?


----------



## JohnH

hi @Lancer X , it's ok to operate the main stage switches -14, -7.5 and -3.5db 'on the fly'. There is enough permanent continuity through the circuit to control this when any one switch is operated in a particular moment of time. The foot switch option is the same and it's also true of the -7/-3 5 switch on stage 1 shown recently. 

Thd only switch I think should not be operated unless the amp is off or on bypass is the 'full bypass' switch, if you have one. It's a good reason not to have one IMO, so I don't draw it on the base diagrams. I don't think the bypass switches on commercial units work any differently really. The issue is fly-back voltages back into the amp. I know enough to be aware of a possible risk but not enough to say it isn't one!


----------



## tfletchii

JohnH said:


> hi @tfletchii , They can be 50W because when in -7.5db mode, what was a single 100w resistor for R1in the base M2 is now shared by two 50W in parallel, and in -3.5db mode, the first stage takes less power. All of that is for a 50w amp. Does that cover yours?


Hey John,
I wired everything up and it is working as planned. For playing at home this switch is unnecessary but I think it will be a great addition for those of us using your attenuator out on gigs. I'm gonna play a few dates with it and report back. If I actually find it as useful as I think I will probably just build one of the single stage attenuators on page 118 , with a bypass switch because it would be so compact.


----------



## john_skaar

Hi @JohnH ,

Great information here! Quite a few pages to trawl though in hope of finding an answer so thought I'd just ask straight up:

I'm about to order an air core from a supplier in the UK. I've narrowed it down to 2 options and want to check if the resistance in the specification will make a difference. There's a 21 day lead time (built to order) so I don't want to order the wrong one! 

The two options are:
1) Jantzen Air Core Coil 1.8mH AWG18 *0.74 Ohm* (part no. 000-1131)
2) Jantzen Air Core Coil 1.8mH AWG18 *0.70 Ohm* (part no. 000-1942)

The only other differences between the two are the dimensions (one is taller/thinner and the other is shorter/wider) and a resulting 20g difference in weight, but this would only add a practical factor in deciding what enclosure I need.

The question is, would a small difference of 0.04 Ohms matter, or should the resistance be something completely different which makes neither of these suitable?

Thanks in advance!
John S


----------



## JohnH

hi @john_skaar , welcome to our thread.

Either of those coils looks to be fine, the ohms are as expected and the 0.04 Ohms difference makes no difference at all. (but in general, less ohms is better for the coil, but those two are so similar that there's no point in using Ohms as a decision factor between them). BTW, the local supplier here in Sydney would sell one at 0.8Ohm, also fine.


----------



## Barnsley Boy

JohnH said:


> Maybe you could try the simple 3-part addition for the 8 ohm M2 to work at 4ohm. It's a lot less to build than another full one. Then, you could decide if you'd like a separate 4ohm unit?


Hi @JohnH 
Eventually got round to building my extension box to convert the 8ohm M2 into a 4ohm for my Peavey. I've plugged it all in, and just wanted to check what resistance I should be measuring at the output jack. I don't want to blow anything up .... them valves are gonna be like gold dust! I have the speakers attached, and I'm measuring 17.8ohms with full attenuation engaged, and 12ohms with all the switches disengaged. I'll post some pics of the "son of" on the completed attenuator thread. I managed to find a nice little inexpensive enclosure on Amazon that has a similar look to the main unit.

Keep up the good work!


----------



## JohnH

Hi @Barnsley Boy ,

The most important check is the ohms that the amp sees. So you have an 8Ohm M2, with an added front end circuit that adds another 8ohm reactivevload in parallel. Is that right?

1. 8 Ohm speaker plugged in, no amp connected, extra front end engaged, you should get around 3.5 to 5 Ohms measured across the jack that will go to the amp. With a 4 ohm speaker, not much different but slightly less.

2. Measured at the attenuator output, no speaker and no amp, I'd expect around 18 Ohms at max attenuation and about 15 at -7db, with the extra front end on.

Do those check out?


----------



## Barnsley Boy

Nice one @JohnH . 

Under criteria 1: 4 ohm speaker load with the added front end, at max attenuation 18 ohm, with -7db attenuation 12 ohm

Under criterial 2: I've got 17.7 ohms at max attenuation, and 15.5 ohms at -7db

Not sure why the first set of readings is so high, but the 2nd set seem to be close enough. I'm gonna connect it all up and give it a gentle test run. Just need to make sure I flick the the right switches (best case scenario I get some beautiful power tube based overdrive, worst case it's good bye Putin!)


----------



## JohnH

@Barnsley Boy , hang on! if its supposed to be going into a 4 ohm amp and its reading that high st thd front then its not right! Seems maybe the new circuit is in series I stead of parallel.


----------



## Barnsley Boy

JohnH said:


> @Barnsley Boy , hang on! if its supposed to be going into a 4 ohm amp and its reading that high st thd front then its not right! Seems maybe the new circuit is in series I stead of parallel.


Looks like it could be a schoolboy error on my part John, I'll open her up and do some further investigations! Many thanks!


----------



## Barnsley Boy

Problem traced to user error! I'd plugged the speaker into the input socket instead of the output socket. I'm now getting 3.4 ohms, so all good to go.

DOH!!!!!


----------



## andreio

@JohnH thank you so much for posting the schematics.
I only have rudimentary knowledge of physics so I would like to ask you if this attenuator can also work as a dummy load when set to maximum attenuation levels.
Btw, here's my attempt. Unfortunate placement of the 50w resistors as they get a lot hotter than the big one and the top plate is thiner:



It sounds great !!!


----------



## JohnH

Hi @andreio , thanks for posting and welcome to our thread. I glad your build is working. Yes you can use it as a dummy load, all switches to max attenuation. At that level, the speaker is taking only about 1/1400 of your amp power, so it its not there at all the amp doesn't feel any difference and so you can unplug the speaker.

If parts are getting hot, a couple of suggestions:
Are you using thermal paste under the resistors? I don't see any scraps of it oozing out. If not, it will help a lot to suck the heat out of the resistors into the case. A very thin layer is all that's needed. At this stage you could just do this to Stage 1

Some vent holes where possible in the lid, and also in the base. I see some small holes, but if you can safely drill them a lot bigger - mine are 8 or 10 mm.


----------



## JohnH

mrjones2004x said:


> Cab is 8ohms. 2 x 16 ohms speakers wired together.
> Thanks so far. Don’t go to any trouble over this.





Barnsley Boy said:


> Problem traced to user error! I'd plugged the speaker into the input socket instead of the output socket. I'm now getting 3.4 ohms, so all good to go.
> 
> DOH!!!!!


Is it going now? I think you are first, leading the way with this front-end gadget! You may need to turn up volume on the switches a couple of steps to get your usual volume since the added box for 4 ohms is taking some power.


----------



## Barnsley Boy

Hi @JohnH I've tried it with both of my Peavey's and everything works fine. I need to spend a bit more time (preferrably when there's no one in the house) with the amp up at higher volumes, but I can report back that it sounds just as good as the "un-front-ended" version. The VT sounds great, the VTX has got a bit of what I can only describe as fizzle or fly trapped in a net curtain sound as the notes decay. I'm not sure if this would warrant a change of power tubes. I got the amp off ebay for the ridiculous sum of £77 (or 44 of your Australian Dollar units!). As I don't know much about the history of the amp apart from some tell-tale signs of extreme tube heat on the adjacent panel, I'd hazzard a guess that it has been used at some pretty high volumes. It may be prudent for me to replace the 6L6's whil I can still get hold of some.

Could this extension box become a "thing" I wonder? Modular attenuation stages - add another stage or 2 if you change to a more powerful amp!! I really liked the idea of the switchable 3.5db boost extension box on a previous post.


----------



## Barnsley Boy

Scorching photo:


----------



## JohnH

Hi @Barnsley Boy , thanks its great its working!

There's now quite a range of add-ons to a basic M2 design, which can be added, or not, to suit needs. I think simple is best, and to add just the gizmo's that are necessary. Here's what I recall from the thread so far, starting with a basic M2 as shown in Post 1.

1. 3rd output to use 16ohm speakers in an 8 ohm M2, so it then works with 4, 8 or 16 ohm speakers.

2. Line-out for direct recording or PA (needs a cab-sim or IR box)

3. Resonant front end (design M3), for possibly more accurate response when used as a load. Consensus so far is that this is not needed (makes no audible difference) when just running to a cab.

4. Footswitch stage, a switchable extra -3.5db stage seems preferred.

5. Switch Stage 1 between -7 and -3.5db, for loud but just a small reduction

6. Full bypass, particularly for fixed or built-in versions

7. Switchable or plug-in-able extra front end to convert 16ohm input to 8, or 8 to 4., or 8 to 16 etc

8. Extra attenuation stages to get really low volume

9. Fewer attenuation stages for simplicity on small amps, or for gigging when very low volume is not needed.

10. Dedicated single-stage fixed version.

11. Extra power rating and fan for running big amps

12. Cocktail umbrellas for efficient cooling and to celebrate the true legacy of _*Rock.*_

Any other tweaks that have been tried, or wished for?

cheers
John


----------



## Barnsley Boy

No. 12 - my personal favourite


----------



## john_skaar

JohnH said:


> hi @john_skaar , welcome to our thread.
> 
> Either of those coils looks to be fine, the ohms are as expected and the 0.04 Ohms difference makes no difference at all. (but in general, less ohms is better for the coil, but those two are so similar that there's no point in using Ohms as a decision factor between them). BTW, the local supplier here in Sydney would sell one at 0.8Ohm, also fine.



Thanks for the info John!

I've ordered the 0.70 ohm version, hopefully it should be with me sooner than 3 weeks!

Keep up this gold mine of information!


----------



## Barnsley Boy

For anyone in the UK, a good source of air core inductors can be found here:









Audio Inductors - Air Core


Audio inductors, air core. We supply an extensive range of audiophile quality air core loudspeaker crossover inductors made by Intertechnik / Audyn and Monacor. These high end components are manufactured in Germany to tight tolerances of 1%, 2% or 3% using high purity OFC wire.




willys-hifi.com





and here:






Loudspeaker Crossover Inductors and Coil Bobbins


Loudspeaker Crossover Inductors and Bobbins. Ferrite and Air Cored Inductors, Choice of Bobbins sizes, Wire Sizes and Custom Inductances




www.qtasystems.co.uk





as @Gene Ballzz would say ......... "just inducting"!


----------



## vintageman

Hi John, I want to buy components for the M2 attenuator from tme.eu. Unfortunately they do not have an 18R (R2B) resistor available. Can I use 15R and 2R7 resistors in series ?


----------



## JohnH

hi @vintageman , yes that's ok. There are also ways to put two in parallel as an alternative. eg 22 and 100, or 20 and 180 to get 18


----------



## lordjester

Just finished a small version of the M2, 3,5/7dB switch plus 3,5dB and 7dB stages. It is small, light - just perfect for gigs - I probably don`t need more options. And it works perfectly! Thanks again John!


----------



## vintageman

lordjester said:


> Just finished a small version of the M2, 3,5/7dB switch plus 3,5dB and 7dB stages. It is small, light - just perfect for gigs - I probably don`t need more options. And it works perfectly! Thanks again John!


Hi lordjester, can you please provide a schematic for M2 3.5/7 with additional stage 3.5dB and 7dB? Scheme M2 with 3.5/7 switch what was presented here had the following stage 14dB.


----------



## vintageman

Hi John, what SW are you using to design the layout of the Hammond box?


----------



## lordjester

S


vintageman said:


> Hi lordjester, can you please provide a schematic for M2 3.5/7 with additional stage 3.5dB and 7dB? Scheme M2 with 3.5/7 switch what was presented here had the following stage 14dB.


Sorry, I don‘t have one. I just used the schematic of the M2 with 3,5/7 switch, but leaving out the 14 db stage and all other options like 16 Ohm Output and line out.


----------



## JohnH

lordjester said:


> S
> 
> Sorry, I don‘t have one. I just used the schematic of the M2 with 3,5/7 switch, but leaving out the 14 db stage and all other options like 16 Ohm Output and line out.


Yes all good. Just to confirm, you can add and delete stages no problem to suit what you need.


----------



## JohnH

vintageman said:


> Hi John, what SW are you using to design the layout of the Hammond box?


I do most of my diagrams in MSWord, it's more powerful than most people use, but you can make simple shapes and then combine them into groups and copy them etc. I think on p112, my diagram started from a screen shot of the box scaled to suit. 

For schematics, I can use the same, but in the last few months I've been using Scheme-it, which is at Digikey. It's a free web based software on their site and does a good job for whst I need.


----------



## vintageman

tristanc said:


> Hi John, just a quick note to say thanks for all your work on this. I've just today finished your version M, before your recent updates (already had the parts). Works really well so far - great being able to blend between pre- and power-amp distortion easily.
> 
> I reduced the resistor wattages as my amps are 18W and 2W... And got the closest inductor I could for L2 (0.47). Other than that, it's stock in a Hammond enclosure.
> 
> View attachment 62590
> 
> 
> View attachment 62588
> 
> 
> View attachment 62589
> 
> 
> Tristan





JohnH said:


> I do most of my diagrams in MSWord, it's more powerful than most people use, but you can make simple shapes and then combine them into groups and copy them etc. I think on p112, my diagram started from a screen shot of the box scaled to suit.
> 
> For schematics, I can use the same, but in the last few months I've been using Scheme-it, which is at Digikey. It's a free web based software on their site and does a good job for whst I need.


Thanks for the information, John. I want to create a modified version of M2 for my low power Fender amp. Then I would like to share my project here on the forum.


----------



## vintageman

lordjester said:


> S
> 
> Sorry, I don‘t have one. I just used the schematic of the M2 with 3,5/7 switch, but leaving out the 14 db stage and all other options like 16 Ohm Output and line out.


If I omit 2stage 14dB in this version, then am I not allowed to use 1stage 3.5dB and the next stage at the same time? For this reason, stage2 14dB has been modified, right?


----------



## JohnH

You'll have a 3.5/7 db stage, plus another 3.5 and a 7db, for a total range ftom -3.5 to -17.5db. You can set them in any order, on or off freely. 

The only thing I've noted is that unless you just want -3.5db, it's probably better to use -7db at stage 1., then engage the other stages as you need them. It's safe whatever way you switch, but in theory the tone is most consistent with -7 or less, if Stage 1 is at -7db. Or it may make no difference.!


----------



## vintageman

JohnH said:


> You'll have a 3.5/7 db stage, plus another 3.5 and a 7db, for a total range ftom -3.5 to -17.5db. You can set them in any order, on or off freely.
> 
> The only thing I've noted is that unless you just want -3.5db, it's probably better to use -7db at stage 1., then engage the other stages as you need them. It's safe whatever way you switch, but in theory the tone is most consistent with -7 or less, if Stage 1 is at -7db. Or it may make no difference.!


OK. Thanks for the detailed information.


----------



## telesto

botacco said:


> Does anybody created a shopping list for the needed components from any European store? I am getting crazy finding the inductors for a reasonable price in Finland. Only find very expensive "audiophile" ones (starting from 25 euro each)


Sorry for the late reply, but if you didn't find anything, try these guys over in Estonia, prices and selection are pretty good:





__





Fidelity components shop - Fidelity Components Shop


Fidelity audio components Mundorf, DH-Labs / Silversonic, Seas, Jantzen, RIKE Audio




www.audiohobby.eu


----------



## telesto

JohnH said:


> L-pads as used in most simple attenuators, will damp down your tone as you reduce volume more than a small amount. It's because at higher attenuation, they show the speaker a low output resistance. This stops the natural resonance and treble rise of the speaker from developing, as they do with the highish output impedance of a tube amp.
> 
> To get a better resistive attenuator, you need to maintain control of both output impedance and input impedance, combined with the desired reduction. Typically this needs a T or Pi arrangement, or a chain of them as in our designs here.
> 
> Then, to get the right dynamics and feel, the amp has to see not just a nominal resistive load, but a load that varies with frequency, as it does when driving a speaker directly. For that reason we have reactive parts.
> 
> Getting all that together across a wide range of levels is what our attenuators achieve, better than most other designs.


I've been playing around with L-pad designs, and what's interesting is that most L-pad guitar attenuators are deployed in the way you wrote, that as the signal gets more attenuated, the speaker (output impedance) sees a very small impedance from the amp, and this causes the speaker response to become flat. This type is also described by Aiken in the quote below taken from his FAQ page (Type 1)

But what is also interesting, is that it's possible to design an L-pad with the opposite effect, that the speaker sees MORE impedance as the signal is attenuated, which then gives a natural boost to highs/lows at low volumes (Type 2). With some caveats of being careful that the output impedance doesn't get too high. Which brings me to my question of "how high is too high?" (could be a good name for a song too, BTW  ) I know in this thread 18 or 20 ohms was established as the target for amp output impedance, based on measurements. But I also recall that some amps with or without (?) NFB/presence can show much higher impedance (?) So I'm curious now, for a heavily attenuated signal, say -30db, what would a good target value be for output impedance? Aiken didn't mention, but I guess some testing could shed some light on it... I'm testing one now that has output impedance around 200 ohms at about 30db attenuation, and it sounds pretty good to my ears. 




> Q: Can you elaborate a bit more about L-pads as an attenuator?
> A: There are two ways to make an L-type attenuator:
> 
> *(1) Match the impedance of the L-pad in the direction of the series arm.* This is the "traditional" L-pad configuration. In this case, the input impedance stays constant, and the output impedance gets lower as you increase the attenuation, down to a theoretical minimum of zero ohms. This configuration sounds like crap for a guitar amp, because the decreasing output impedance increases the damping factor and removes all "tubeyness" from the tone at lower volumes, because the amp no longer reacts to the variations in speaker impedance.
> *
> (2) Match the impedance of the L-pad in the direction of the shunt arm. * In this case, the input impedance also stays constant, but the output impedance gets higher as you increase the attenuation. This is great for guitar amps, up to a point. The increasing output impedance lowers the damping factor, which enhances the interaction between the amp and the speaker, giving a natural bass and treble boost as you increase the attenuation (sort of a "built-in" Fletcher-Munson effect compensation!). The problem is that it gets to be too much, and you end up with too much bass and treble boost and "hangover effects" as it was called in the old days of audio, and you get a flubby, fizzy tone at high levels of attenuation. This can be alleviated by limiting the maximum output impedance with an additional shunt resistor at the output. I used this method of attenuation on my second-generation Invader and Tomcat amplifiers, which sounded pretty good all the way down to whisper levels with the VAR control. It was designed to correctly match the 4, 8, or 16 ohm settings of the rear-panel impedance switch. The first generation pf the Invader used a reactive load, and the second generation was purely resistive, but I used a modified L-type matched in the direction of the shunt arm. It sounded a bit different than the reactive attenuator version, but I think I preferred it, as it was a bit smoother.
> 
> The Airbrake is basically a type 2 L-pad, but it is made with a fixed resistor in the shunt arm instead of the tapped/variable resistor normally required to keep the input impedance constant. This was likely done to save money and labor, because you don't need a tapped resistor and you don't have to wire it to another gang on the rotary switch. Because of this, the input impedance does not stay constant with variations in attenuation, as you can see in the graph shown here.
> 
> You can also design a "T-type" attenuator that matches the impedances in both directions. This will give a more constant tone with changes in attenuation, without the extra increase in bass and treble as the attenuation increases. While this will give a more "accurate" tone, as can be shown by recording and playing back at a normalized volume, it may not be as pleasing to the ear because it doesn't automatically compensate for the loss of bass and treble with volume reduction as the L-type does, but it also won't get as flubby and fizzy, either. A properly-compensated type 2 L-attenuator is probably the best sounding, but it takes a bit more circuitry to tailor the output impedance for best tone at all volumes..


----------



## JohnH

hi @telesto , these are all key points and figuring them out was the most important thing i learned in developing the designs on this thread.

A good starting point for thinking about a target output impedance is to think about the speaker impedance, and its frequency response. Guitar speakers have an acoustic response that falls off sharply above about 4khz. So I take 5kHz as an upper benchmark frequency for checking the electrical response of the system since above that its much less significntt acoustically . At 5khz, a typical 12 inch 8 ohm Celestion has an impedance of about 20 Ohms, and its mostly inductive. That means that if you feed it via a 20 Ohm resistive output impedance, then 5Khz will start to be the roll-over frequency of the output electrical signal, which is all fine since above that its less important. And it doesn't make much difference between models, or even across brands.

And 20 Ohms as an amp output impedance is a good midrange value, and happens to match my VM.
But amps with no NFB can be a lot higher, as you note. Keeping 5Khz as the reference, there's about another 1.3db of rise available if amp output impedance is 50 Ohm and 2.3db at 200 Ohm. These represent quite small lifts in the high treble, adding some more presence and I expect that is what you are hearing. Or, with a different amp with say, a 10 Ohm output impedance, there's about 1.4db less at 5 khz. At lower mid/treble frequencies where the bulk of the amp tone exists, everything is much closer.

So I think that if you want to get in the right range across various amps, with a simple reverse L pad, 20 Ohms output impedance is a good starting point for an 8 Ohm system, but you can experiment, so long as the amp sees the right load (basic Airbrakes take not much care of this)

BUT, with our reactive design, there is an added bonus because those differences in high treble get partly compensated for . ie, if the attenuator designed around 20 ohm output Z is used with amps of less or more output impedance, the differences in tone compared to full volume are much less than noted above. its really hard to figure out and explain but its all there in the maths and in the measurements. I think this is part of the reason that the M series designs have proven to be surprisingly effective across a wide range of very different amps .

I agree with Aikens points about L and T pads and his insightful descriptions were very helpful as a starting point. You can in principle, design a 3-resistor T or Pi attenuator stage with any Input Z, Output Z and Attenuation, so long as you target enough attenuation so one resistor doesn't become an impossible negative value. My key stage is the first -7db stage. And at somewhere around that level, using values that I'm targeting, a T pad becomes such that the leading resistor is zero and we have a Type-2 L pad that hits all the marks.


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## ZackPlonk

Hi,

I was wondering, could the stages after the reactive element in the M2 design be replaced by a rotary L-Pad attenuator such as a Monacor AT-62SK.
It would simplify the build and operation quite a bit. You'd no longer switch in stages but rather use the rotary knob to add additional attenuation after the initial reactive stage...

(The thread is quite long and I tried to see if this was discussed earlier. I didn't find anything. Apologies if I overlooked it and this was indeed covered before)

Cheers,
Lars


----------



## JohnH

Hi @ZackPlonk , I've worked on that too, but you can't keep enough control over the essential parameters, over a wide range, with any practical rotary knob. Need to control input and output impedances and keep them consistent. Our fixed stages do that as that's why the tone on an M2 is more consistent down to very low levels levels as compared to other designs.


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## ZackPlonk

Thanks for the clarification, John and thanks for sharing your work!

What should work though is to wire up a 8x3 rotary switch according to the stage-switch positions, That way, I'd still get a rotary control. Wiring is a little more complex but I think I personally would like that user-interface better...
Something like this:


----------



## Gene Ballzz

ZackPlonk said:


> Thanks for the clarification, John and thanks for sharing your work!
> 
> What should work though is to wire up a 8x3 rotary switch according to the stage-switch positions, That way, I'd still get a rotary control. Wiring is a little more complex but I think I personally would like that user-interface better...
> Something like this:
> View attachment 106956



Yes, *BUT!* Our research has found that a genuinely suitable switch is prohibitively expensive. Once you get one of these built, as suggested, and start using it, you will find that *A)* Continuously variable operation is not really needed, as each -3.5db step is so sonically small *AND*, *B)* Use of the switches and their various combinations becomes intuitively, second nature!

Re-invention of this wheel is not necessary and/or worth the effort for the results!

Just Attenuatin'
Gene


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## ZackPlonk

Gene Ballzz said:


> Yes, *BUT!* Our research has found that a genuinely suitable switch is prohibitively expensive.


I should have checked that first. Rotary switches with a suitable current rating (5A) do indeed cost north of 75 Euros. Not worth it.

Cheers!
Lars


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## Browneyesound

ZackPlonk said:


> (The thread is quite long and I tried to see if this was discussed earlier. I didn't find anything. Apologies if I overlooked it and this was indeed covered before)
> 
> Cheers,
> Lars


Yeah,
Is there a cliff notes version, or a link to the latest versions of this?


----------



## JohnH

Browneyesound said:


> Yeah,
> Is there a cliff notes version, or a link to the latest versions of this?


Thanks for your interest. Post no. 1 has the latest basic version (M2) and general info


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## Browneyesound

JohnH said:


> Hi @EC Strat
> Also, do you really need a bypass? because with a bypass engaged, you could instead just not use an attenuator. After a few tests you are likely to find that just one or two settings are all you use. Unless your use involves often adjusting between attenuated or not, then maybe you dont need one? A bypass switch is switching the full power and current of the amp, and must never be operated unless the amp is off or on standby, whereas with the other switches they can be operated on the fly.





JohnH said:


> Thanks for your interest. Post no. 1 has the latest basic version (M2) and general info


Thanks. That older post answered a question I had. Ideally, I’d like to have a foot switchable bypass along with a footswitchable attenuation level, but it looks no like for a footswitchable solution it should not include a bypass.


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## JohnH

Yes, a bypass if included, must never be operated except with amp off or on standby So not good for a footswitch.


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## Gene Ballzz

*PSA:*

I just made an interesting discovery! The holes in the lugs of genuine "CARLING" branded mini switches are just a tad bit larger than those on the cheaper brand (mostly Taiwan made) switches available through other vendors, like Parts Express. This makes for vastly easier wire connections and seems well worth the extra couple $, overall!
Just Sharin'
Gene


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## donwagar

Gene Ballzz said:


> *PSA:*
> 
> I just made an interesting discovery! The holes in the lugs of genuine "CARLING" branded mini switches are just a tad bit larger than those on the cheaper brand
> Gene


That's awesome. Thanks!


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## stickyfinger

Hi guys, I will be building a M2 at some point and am looking to make one for my 5watt DSl5c one switch to drop 10db and another switch to drop total 20db. 20db should be ruffly in the TV range or bedoom vol. 

I have a Rockcrusher and it works great for my 50 watt Marshalls but am curious about this JohnH designs. Ill be honest I haven't had time to read all the pages.. got up to page 10 so please bear with me as some of my questions may have already been talked about.

A few pages in Gene was taliking a bout a Split Load attenuator. I have thought about this design for years now. A single coil per -3dB setting. Effectively your attenuated signal gets dumped thinking its seeing a speaker but a small portion passes straight to the speaker. This in theory would be the best in tone but would be expensive to build. Haven't a clue on cost but probably cheaper than any normal commercial reactive attenuator. 

The Faustine Phantom attenuator about 10 years ago was regarded as the best attenuator out there. They were expensive and expensive to make due to the Electroswitch cost but would be interesting to see a M2 with a single rotary control.


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## JohnH

Hi @stickyfinger 

On rotary controls, I did figure something out in December 2019. See post 634 on page 32

https://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/page-32 dated 21 December 2019.




A fixed -7db stage, then another switched stage and a rotary with 6 positions giving -3db steps. If anyone is interested, please read the post about it above.

Never tested but it follows all the design principles. The issues with it compared to a standard M2 are, there's more parts needed, and finding a good enough rotary switch with the right current rating. By limiting the rotary to 6 positions and using just 2 poles, it should open up the selection. It may be good for small amps. Lets say its a 5W amp . After Stage 1, there's about 1W, which into 16 ohms is 0.25A or 0.35A with 8 Ohms. The switch spec should be x2 that, at 125V.

Some might prefer the variable control of a pot or L-pad. But with these, its not feasible to control the resistance values properly to maintain tone as volume is reduced. This is evidenced by most commercial units using Lpads, which sound muffled as you turn down. Also, looking at the green resistors above, you can see that they are a chain of specific values designed for the relevant attenuation step, and that amounts to an overall taper across the range that isn't found in any standard pot

Overall, I put this up again for discussion but really, the standard M2 and its variants is a lot simpler and once you get a feel for what each switch does, it works intuitively. Plus, once the right volume is found, you just leave the attenuator set and get on with playing!


----------



## JohnH

hi again @stickyfinger 
Interesting about the other attenuators and discussions that you referred to above. Just wanted to note that with the M series designs, even though only the first stage has reactive components, that's actually enough to show the amp the right impedance curve and they are balanced so that the following stages can just be resistive through to the speaker. What the speaker sees is then something like an amp output impedance,

That Faustine unit looks like a beast!


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## Gene Ballzz

@JohnH &All,

I just wired up my first 8 ohm, M2 attenuator. Actually it was a rewire for another member (I won't mention his name) whose soldering was poor, at best and had burned up a couple of the switches, etc.

While a 16 ohm unit still sounds fine on a 16 ohm amp tap, into an 8 ohm speaker, works well and is perfectly safe, the exact same amp and speaker sound noticeably better and more dynamically responsive with the dedicated 8 ohm, M2, out of the 8 ohm amp tap. I just ordered the parts for an 8 ohm build!

I thought you'd all like to know?

Simply Sharin'
Gene


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## donwagar

By now, you would think this thread would be a sticky.


----------



## Gene Ballzz

donwagar said:


> By now, you would think this thread would be a sticky.



If I recall correctly, some of the moderators, etc, here may not be real fans and/or subscribers to attenuator usage. There are many folks who believe no attenuator is really very good or useful!
Just My ,
Gene


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## JohnH

Gene Ballzz said:


> @JohnH &All,
> 
> I just wired up my first 8 ohm, M2 attenuator. Actually it was a rewire for another member (I won't mention his name) whose soldering was poor, at best and had burned up a couple of the switches, etc.
> 
> While a 16 ohm unit still sounds fine on a 16 ohm amp tap, into an 8 ohm speaker, works well and is perfectly safe, the exact same amp and speaker sound noticeably better and more dynamically responsive with the dedicated 8 ohm, M2, out of the 8 ohm amp tap. I just ordered the parts for an 8 ohm build!
> 
> I thought you'd all like to know?
> 
> Simply Sharin'
> Gene


HI Gene, thanks and yes that is very interesting. Based on the numbers that I run, an 8 ohm speaker out of a 16 Ohm attenuator is in theory equivalent to about 1 to 1.5 db less mid volume as compared to presence and resonance frequencies, if set the same. So it could be that. Or, it could be slight difference between M and M2 designs, or just the amp sounding different on different taps.

It supports the idea that the best version to build is where amp, attenuator and speaker all match Ohms for the main rig in use. 

A slight tone tweak on the 16 Ohm build to make the theoretical tone into an 8 speaker match better would be a 39 or 56 Ohm resistor switched in parallel across the speaker.

But, there's magic involved in these things and the theory only captures some if it!


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## JohnH

donwagar said:


> By now, you would think this thread would be a sticky.


I have asked for that before, but no reply was received


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## Gene Ballzz

JohnH said:


> HI Gene, thanks and yes that is very interesting. Based on the numbers that I run, an 8 ohm speaker out of a 16 Ohm attenuator is in theory equivalent to about 1 to 1.5 db less mid volume as compared to presence and resonance frequencies, if set the same. So it could be that. Or, it could be slight difference between M and M2 designs, or just the amp sounding different on different taps.
> 
> It supports the idea that the best version to build is where amp, attenuator and speaker all match Ohms for the main rig in use.
> 
> A slight tone tweak on the 16 Ohm build to make the theoretical tone into an 8 speaker match better would be a 39 or 56 Ohm resistor switched in parallel across the speaker.
> 
> But, there's magic involved in these things and the theory only captures some if it!



Yes indeed John, it appears that it is in the mids that a difference is heard. I'm surprised that in the 1db to 1.5db range it would be so audible, but you know us tone chasers! What is subtle to others is a whole universe of difference to some! That and the fact that the mid range is where we shape most of our tone, etc.

For example, there are several signature, feedback notes (from specific songs) that I seek to achieve with certain guitars and no pedals. Certain speaker setups and responses make them difficult to achieve, while others just tend to do them naturally. Certain tonal differences can make or break such nuances!

Thanks Again,
Gene


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## Lancer X

JohnH said:


> hi @Lancer X , it's ok to operate the main stage switches -14, -7.5 and -3.5db 'on the fly'. There is enough permanent continuity through the circuit to control this when any one switch is operated in a particular moment of time. The foot switch option is the same and it's also true of the -7/-3 5 switch on stage 1 shown recently.
> 
> Thd only switch I think should not be operated unless the amp is off or on bypass is the 'full bypass' switch, if you have one. It's a good reason not to have one IMO, so I don't draw it on the base diagrams. I don't think the bypass switches on commercial units work any differently really. The issue is fly-back voltages back into the amp. I know enough to be aware of a possible risk but not enough to say it isn't one!



Oh! Cool, thanks everyone. I must have been thinking of a message where someone cautioned against hot-switching the bypass.

Glad to hear this. I'm thinking about building this circuit right into my next amp build, so very cool that hot switching is okay.


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## mountainhick

Never mind... It took a while but I found the answer to my question.


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## BuckTennington

I love you for designing this and I want to make this however I have no idea where to start on this thread to build it. I’m having trouble following this thread.


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## Gene Ballzz

BuckTennington said:


> I love you for designing this and I want to make this however I have no idea where to start on this thread to build it. I’m having trouble following this thread.




*FIRST:*  to the forum!
*NEXT:* If you share what amp(s) you want o use it with and what bells and/or whistles you want to add, we can help you. And yes, this thread is pretty lengthy and cumbersome, although well worth the read, once skimming past some of the inevitable chaff. Lots of great testing and technical info as well as modifactions and additions to the original design, which by the way works fabulously in it's bare baones basic form, with very economical aspects. Depending on parts/components choice and sourcing, the price ranges from $75 to $150 for a basic unit. Obviously, that prices goes up a little, depending on what feature you may want to add. The price to build a basic unit is
*THEN:* All of the well updated component value and electrical info is in post #1 of this thread, along with links and/or page numbers to lead you to ther related info. Actual layout diagrams start on about page #110 or #111.
Let Us Know?
Gene


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## stickyfinger

Any way I can make a M2 that would work with a 16ohm amp and use a 8 ohm speaker?
I have a fully broken in 8 ohm Greenback that I would like to install in my DSL 5C that only has a 16 ohm tap. 
Thanks for the help!


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## JohnH

stickyfinger said:


> Any way I can make a M2 that would work with a 16ohm amp and use a 8 ohm speaker?
> I have a fully broken in 8 ohm Greenback that I would like to install in my DSL 5C that only has a 16 ohm tap.
> Thanks for the help!


Yes a 16ohm M2 can use an 8 Ohm speaker with it. This is because the first -7db stage is always engaged, so the amp doesn't see the speaker directly. But it can't convert 16 to 8 as an unattanuated setting.


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## DeluxeReverb

BuckTennington said:


> I love you for designing this and I want to make this however I have no idea where to start on this thread to build it. I’m having trouble following this thread.


I was in a similar situation as you. I heard about this attenuator while researching other products such as the Weber MiniMass, but I was intimidated by the length of the thread. However, the first posts by @JohnH have been updated to include the latest circuit design and parts specs.

You’ll need to balance your budget with your intended build timeline, and be prepared to order parts from multiple suppliers, but your efforts will be well rewarded. In my case, I ordered from 3 US suppliers that I’ve used before, and while not the cheapest, they got me the parts quickly. I could’ve saved money if my timeline was more relaxed and if I’d used eBay sellers. 

The updated circuit diagrams at the very front of the thread, combined with the layout diagrams on pg 111 (thereabouts) are the gold here. The rest is interesting - at least to me - but not essential.

DR


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## JuliusJ

Hello folks, this is my first post. JohnH, congratulations on this super project ! And Hello Gene Ballzz, i will greet you first ! Briefly, i have been learning tube amp technology for three decades. I am familiar with LTSpice, a great program. Years ago i was an avid reader at AX84 and the Music Electronics Forum. I think i need to inform the folks at MEF of the JohnH design !

I have been following this thread for months and have some things to contribute. When i write "you" i am mostly referring to JohnH. Sorry it is long but not too technical. I will be posting tech oriented stuff and observations.

1) I went to bed the other night with the first stage of the M2 design firmly in mind and went through all the circuit paths in both directions. The triangle with the shunt looked like a yield roadway sign, new and strange to me. Flatten out out the inductor horizontally ! Shortly, i got it ! I thought, eureka ! This stage has the same structure as a Bridged T attenuator. That is what i think has been derived here with a lot of time and hard work. The inductor is the left side of the T. R2A is the bridge resistor. R2B is the right side of the T. R1 is the shunt. Simple and intriguing. Maybe you knew this, but i wanted everyone to see this now, firstly because i thought it would save you the work and trouble of coming up with custom formulas since you mentioned you were looking into doing Delta Y transforms. The properties and formulas for the bridged T are well known. The formulas can be adapted for use with the inductor i am sure. My first thought about this structure was to try swapping the inductor position with R2B and see what the result is if any different or usefull. I believe the inductor is currently in the best position as we want the input impedance to be low and the output impedance to be high and close or equal to the output impedance of the amp. Second though, try an inductor on both sides of the T, test, and then parallel a resistor across both of them, and test. This looks more balanced, and food for thought. There are other attenuators we can look at like the balanced designs and figure out where to place the inductor(s), but those look more complicated.

2) Four attenuator units can be built and wired in series parallel like a 4x12 cabinet for use and analysis. This might help soak up the power for higher wattage amps, at -3db each unit, set to full load, this will give a -12db starting point, please confirm. Put that in parallel with another attenuator and speaker for fine loudness control.

Values for 32 ohm and 64 ohm attenuator units can be derived easily. Please see the excellent calculator here:

k7mem "dot" com/Res_Attenuator.html

I used this calculator and easily derived all the resistor values listed for the M2. Since we want to match the amp output impedance, select Match Z1, very important ! For an 8 ohm tap, i used 18.6 ohms as the source impedance Z1 and derived the same resistor values listed for design M2. For a 16 ohm tap i used 37.2 ohms as the amp output impedance to get your numbers. The calculator also shows what standard values come close. Please try out to confirm i am doing it right. it's fun and addictive ! I think this calculator is a miracle, just what i was looking for last night, and i am so glad i did not give up searching. Just what we need for our purposes. Custom attenuation values can be found.

3) The type 2 L pad you discovered at the beginning is also quite a eureka. At first i did not think using the online calculator would give me the correct results, because it is the regular series + shunt voltage divider, in which the input impedance is the sum of the two resistors, and the output impedance is the parallel formula. I am looking to confirm the input and output impedance formulas for the shunt + series divider arrangement. I think it might be just the opposite of the series + shunt voltage divider.

Some notes:

4) Looking back at Design G post #111 we can derive the bridged T structure by combining L1 and L2, and eliminating R2.

5) On the Aiken design a noted error, R11 at 16 ohms was copied on post #92. i noticed that was changed to 30 ohms, on post #489, good.

Ok that's it for now, thanks and talk to you all later.


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## JohnH

Hi @JuliusJ , welcome to our thread and thanks for all your thoughts!. That attenuator calc page is very smart. the only other ones I've seen just seem to do matched impedances.

At this point I do it all on a huge excel sheet, everything is coded into it, including the speaker complex impedance vs frequency. To test a new design, i put in the values I want to try and hit a button, and then it whirs its macros for a minute or so and plots every frequency at every attenuation step with 4, 8 and 16 ohm speakers. The Star/delta transforms are built in too. So that's all pretty handy! I have smaller routines for working out T groups to use for each stage.

But really - the thinking and research went something like this:

Aiken shows us how a speaker impedance can be modelled out of analyzable components - and Mike Lind posted an accurate measured plot to work to
An amp has a higher output Z that I thought, and it is not too different to being as if its resistive at most levels (a discussion I had on TGP)
Amp needs to see the right reactive load to respond correctly, and speaker needs to see a highish resistive load for the same reason
But, if the attenuator shows the amp an inductance that is all in series, then that inductance develops most of the treble, leaving a dull sound to go through. But if its all in parallel then the attenuator sees too bright a tonal balance. So Stage 1is a mix of inductance in series and parallel, which started with two inductors in Design M, but then developed into one combined inductor, in a series/parallel position for Design M2. M2 creates a tricky circuit to calculate using voltage dividers, but the maths of the Star/Delta transform turns it back into a simpler to calculate arrangement 
After Stage 1, the amp is seeing the right load but the signal through intermediate stages is almost at nil phase change, and can travel happily through the series of resistive stages before interacting with the complex speaker impedance


----------



## stickyfinger

JohnH said:


> Yes a 16ohm M2 can use an 8 Ohm speaker with it. This is because the first -7db stage is always engaged, so the amp doesn't see the speaker directly. But it can't convert 16 to 8 as an unattanuated setting.


Thanks John! I really appreciate all the time and effort you put into this design and helping all of us. 

I have a Rockcrusher I use for my big Marshalls and every now and then I forget to switch the ohm setting on the unit. 8ohm head going into 16 ohm attenuator and then into a 8 ohm cab and the tone changes and vol drops and I usually notice it right away. 

I assume I can expect the tone to change using a 16 ohm M2 into a 8 ohm speaker?


----------



## JohnH

Hi @stickyfinger , if you use an 8Ohm speaker in a 16Ohm M2, the amp is fine and sees almost no difference, the speaker tone will have about 1 to 1,5db more resonance and presence, in theory., very much like turning up those two controls , if you had them. A small change that you may or may not hear. 

Going the other way, a 16 speaker into an 8 ohm M2, there's a bit more of a difference in reduction of those frequencies. I can hear it, though its still safe for the amp . So for the 8 Ohm design I added Out 3 which compensates for this small tone change.


----------



## Browneyesound

So for a 100W version of the M2, you should double the watt rating of the resistors?


----------



## JohnH

Browneyesound said:


> So for a 100W version of the M2, you should double the watt rating of the resistors?


Yes that's right. To get a 200W rating at R1, you can consider two at 100W in parallel or in series, with ohm values x2 or x1/2 respectively, so that the combined values is right. You can also then consider spreading these two apart to help with heat.

If you plan to crank a 100W amp for more than a few minutes, then getting rid of heat becomes a key issue. Vent holes and a big, thick aluminium case with good ventilation holes on top and in the base (with feet to raise the base), help a lot. Several of the 100W builds have included a fan too.


----------



## Dblgun

Hey John, it is great to see that you and others have continued to tweak this to get the best from an evolving layout. With all those building examples of the attenuator and the massive spreadsheet you must be constantly be updating have come up with a chart of impedance check values for the different units. I wonder if some are completing these builds and are experiencing attenuation once implemented. I have looked through the thread and did not see any mention of measurements of values after the build and prior to using. Although to be honest, this thread is massive and I could have easily missed it!


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## matttornado

Browneyesound said:


> So for a 100W version of the M2, you should double the watt rating of the resistors?


I used a giant 300 watt resistor for the always on 1st stage. That’s for my cranked on 10 100 watt Marshall Superlead. It gets pretty hot but still well within its tolarence.
I also added a fan i can turn on to cool it off if i need to.


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## Gene Ballzz

Dblgun said:


> Hey John, it is great to see that you and others have continued to tweak this to get the best from an evolving layout. With all those building examples of the attenuator and the massive spreadsheet you must be constantly be updating have come up with a chart of impedance check values for the different units. I wonder if some are completing these builds and are experiencing attenuation once implemented. I have looked through the thread and did not see any mention of measurements of values after the build and prior to using. Although to be honest, this thread is massive and I could have easily missed it!



The most meaningful test is conducted with a speaker (of the intended impedance of the unit) plugged into the output and either a speaker cable or bare plug into the input and measure the ohms across that input, with all switch combinations. The resistance should not change more than an ohm or so from the impedance the amp is intended to see. Additional teting can include plugging different impedance speakers to the output and noting that the resistance at the input is not far off from that with the intended impedance speaker. Really difficult to post exact readings to expect, due to the possible stack up (or not) of 10%-ish tolerances of the resistors used.

With that said, these units can be *"safely" *used (no bypass) with almost any mismatch on the output side, but the tone/response/feel will likely be compromised from the designed intentions. Only the player's perception and opinion can determine if that level of compromise is acceptable, or not! This design, when used as designed, is the absolute best I've found, in +40 years of passive attenuator experimentation! It's a shame that building them commercially would likey be losing proposition, *UNLESS* someone already has an existing business, of producing kindred consumer products! It's such a *"niche"* market that a bare bones, basic M2 would need to sell for +/-$500, just to keep the lights on and the bills paid, and at that price, not many would get sold!

On the other hand, a really smart guy might think about offering an *"assembly service"* that included the end user signing some sort of *"no whining"* waiver to absolve the assembler of liability and detailing that the assembly was done as a favor and that payment would be considered a gift? That *"service"* would likely be be commensurate with a *"gift"* in the neighborhood of $150-$225, depending on any added bells/whistles and the detail level of labeling and other cosmetic considerations.

,,,?
Gene


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## JohnH

hi @Dblgun , as Gene noted, a simple resistance check across the jack plug that will go into the amp, with speaker plugged into the attenuator, is the best check that it's all safely wired. This needs to be in the range 7 to 10 Ohms for an 8 Ohm build. 

After getting it going, it's possible to confirm the attenuation at each setting by using a consistent signal or loop through the amp and micing the cab. I did this in post 1 and it confirmed that the stages were all working, to provided -3 5db steps. 

There could also be a way of doing that with a dc input from a power supply, a dummy load, and voltage measurements at the output.


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## Dblgun

Gene Ballzz said:


> The most meaningful test is conducted with a speaker (of the intended impedance of the unit) plugged into the output and either a speaker cable or bare plug into the input and measure the ohms across that input, with all switch combinations. The resistance should not change more than an ohm or so from the impedance the amp is intended to see. Additional teting can include plugging different impedance speakers to the output and noting that the resistance at the input is not far off from that with the intended impedance speaker. Really difficult to post exact readings to expect, due to the possible stack up (or not) of 10%-ish tolerances of the resistors used.
> 
> With that said, these units can be *"safely" *used (no bypass) with almost any mismatch on the output side, but the tone/response/feel will likely be compromised from the designed intentions. Only the player's perception and opinion can determine if that level of compromise is acceptable, or not! This design, when used as designed, is the absolute best I've found, in +40 years of passive attenuator experimentation! It's a shame that building them commercially would likey be losing proposition, *UNLESS* someone already has an existing business, of producing kindred consumer products! It's such a *"niche"* market that a bare bones, basic M2 would need to sell for +/-$500, just to keep the lights on and the bills paid, and at that price, not many would get sold!
> 
> On the other hand, a really smart guy might think about offering an *"assembly service"* that included the end user signing some sort of *"no whining"* waiver to absolve the assembler of liability and detailing that the assembly was done as a favor and that payment would be considered a gift? That *"service"* would likely be be commensurate with a *"gift"* in the neighborhood of $150-$225, depending on any added bells/whistles and the detail level of labeling and other cosmetic considerations.
> 
> ,,,?
> Gene





JohnH said:


> hi @Dblgun , as Gene noted, a simple resistance check across the jack plug that will go into the amp, with speaker plugged into the attenuator, is the best check that it's all safely wired. This needs to be in the range 7 to 10 Ohms for an 8 Ohm build.
> 
> After getting it going, it's possible to confirm the attenuation at each setting by using a consistent signal or loop through the amp and micing the cab. I did this in post 1 and it confirmed that the stages were all working, to provided -3 5db steps.
> 
> There could also be a way of doing that with a dc input from a power supply, a dummy load, and voltage measurements at the output.


This is great information and will benefit all that visit this thread and/or build a attenuator. Thanks to both of you.


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## chas.wahl

Hello again @JohnH,
I've bought all the resistors and inductors for a M2v version of your attenuator, but I'm still a bit stuck in terms of my understanding of the proper wiring for the input and output jacks. Back on page #76 of this thread, I asked some questions, which you graciously answered, but I'm still left wondering how I should proceed if I want to use open-frame Switchcraft 12 and 12A jacks (isolated from the conductive enclosure with plastic washers, I fully understand the reason for the isolation). Please see the attached image of the M2v design, for reference:




After poring over this awhile, I'm pretty confident that I understand what to do circuiting-wise in the grayed-out area where the M2v is pretty similar to the M2. It's the input area and the output jacks where I'd like some confirmation of what the design intent is.

On the left, if I understand things correctly, the jacks are set up so that if one plugs into the 4 ohm jack, the amp signal sees the network comprised of L2, R12 and R13 as a *load in *_*parallel* _with the loading between there and the output jacks at lower right, and this works because the ring contact of the jack is "switched" to the sleeve contact when the plug is inserted. If one plugs into the 16 ohm jack, then the L2, R12 and R13 network is, instead, put *in series with* the loading that follows it. And when using the 8-ohm input, that L2/R12/R13 network is bypassed. Have I understood things correctly?

With respect to the input jacks themselves, it seems to me that as long as they're all isolated from the enclosure, only the 4 ohm jack needs to be a TRS (stereo) jack -- correct?

And with respect to output jacks:
Even though the two 8/4 ohm jacks are shown as "switching" type, I suspect that the ring-to-sleeve contact is not strictly necessary to accomplish the circuit's requir2ements; that is, the blue lines and the ring contact are not really necessary, because they simply duplicate the ground that's shown connecting to the sleeve contacts of those two jacks.

BUT, I'm not sure about the 16 ohm jack. Is there a need to isolate R10 and it's connection to the input side of R2B from the continuous ground connecting all the sleeves of the output jacks, except when one plugs into the 16 ohm jack? If there is, then that should be a stereo jack with the same ring-to-sleeve "switching". If not, then that could be a mono jack as well, with the output side of R10 simply connected to the 16 ohm jack's sleeve.

Three more questions occur to me:
First, has anyone developed a schematic for using a single input jack with switching that puts the L2/R12/R13 network in series, in parallel, or disconnects it? And if so, what's the nature of that switch? An SPDT on-off-on, or a rotary switch like an impedance selector for an amp output?

Secondly, if one is content to have one output jack for 8/4 ohms (the difference being the selection of parallel connection of the L2/R12/R13 network, or its disconnection) then is there any problem omitting the 2nd (redundant) one?

And finally, could the attenuator have only one output jack, if one were to provide a double pole on-off switch to connect the output sides of R10 and R11 to that jack when 16 ohms is being used?

Thanks very much for all you've done here!
--
chas.wahl


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## JohnH

Hi @chas.wahl , thanks for your message. I think that everything you say is right, so thats easy!

On jacks, if you are using the open type and insulating, then it would need isolating washers with a bush around the shaft to stop that contacting the case. 

The only jacks that really need to be TRS are the 16 ohm output jack and also the 4 Ohm input jack. Although the diagram shows switched jacks, the switching is not used. But, at least with the plastic Cliff-type jacks that I use, I like TRS for everything because with the extra sprung contact, they grip the plugs more consistently and firmly.

I think its good to have two output jacks at least for the 8 and 4 ohms, since that allows 2x16 or 2x8 cab pairs. Apart for that, you can do it all with a switch and one jack. 

At the input, I have thought about a single jack with switch instead of the group of three. But any switch needs to carry a high current so it rules out most rotaries. With a dpdt three-position toggle switch, I think it can be done with an on-on-on type, but I've only seen these as a mini-switch. If you dont want all three input options, then a simple toggle or slide switch would do it.


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## Gene Ballzz

There are reasons why this design is centered around the suggested components.
*1> *To avoid the kinds of confusion encountered by @chas.wahl 
*2>* CLIFF UK jacks provide superior contacts compared to Switchcraft types. I won't go into all the details, but trust me on this!
*3>* It helps standardize the build process, as well making the troubleshooting assistance simpler.
*4> *And more……...
Simply Attenuatin'
Gene


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## chas.wahl

JohnH said:


> Hi @chas.wahl , thanks for your message. I think that everything you say is right, so thats easy!
> 
> On jacks, if you are using the open type and insulating, then it would need isolating washers with a bush around the shaft to stop that contacting the case.
> 
> The only jacks that really need to be TRS are the 16 ohm output jack and also the 4 Ohm input jack.


Thanks are to you @JohnH ! I'm much more confident I understand things now. I'll consider the Cliff jacks, but really prefer Switchcraft, even when I have to isolate them using washers (which is mostly). And I understand the problem(s) with switching vs multiple jacks -- didn't think that through very well.


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## AtomicRob

Gene Ballzz said:


> *2>* CLIFF UK jacks provide superior contacts compared to Switchcraft types. I won't go into all the details, but trust me on this!


OK this one I gotta ask about, please do go into the details! I have always used Switchcraft jacks for speaker connections because all the Switchcraft jacks I've seen anyway seem to hold the plug much tighter than a Cliff jack. I have no idea about the electrical connection but physically it's tight in there. When I really want to be sure a plug won't get pulled out, I use a Switchcraft. I use Cliff on the front panel for guitar inputs because I want the cable to pull out easily if I trip on the cord or something.


----------



## Gene Ballzz

AtomicRob said:


> OK this one I gotta ask about, please do go into the details! I have always used Switchcraft jacks for speaker connections because all the Switchcraft jacks I've seen anyway seem to hold the plug much tighter than a Cliff jack. I have no idea about the electrical connection but physically it's tight in there. When I really want to be sure a plug won't get pulled out, I use a Switchcraft. I use Cliff on the front panel for guitar inputs because I want the cable to pull out easily if I trip on the cord or something.



The key part of your reply is: "all the Switchcraft jacks I've seen anyway '*seem'* to hold the plug much tighter than a Cliff jack." The only force causing contacy for the sleeve on an open Switchcraft is the spring of the tip "cocking/skewing" the sleeve in it's bore to make its contact. If you fiddle with it enough, and in just the right directions and situations (even on a brand new jack), you can actually cause the sleeve to intermittently lose it's connection. This is often exacerbated by cheap, undersized plugs from questionable origins! This can be somewhat remedied by using a Switchcraft stereo/TRS jack, which creates force from the ring spring at 90 degrees to the tip spring. This affect can also be enhanced by using that same stereo/TRS jack and tying the lugs of the ring and sleeve together.

As far as the CLIFF UK jack feeling less solid, this is why @JohnH and many others recommend using stereo/TRS jacks even for mono/TS connections, as it feels more solid. It also helps to make sure they are genuine CLIFF UK or Neutrik/REAN jacks, instead of cheap foreign knock-offs. It also doesn't hurt to tie the ring and sleeve together on these types of jacks.

An interesting take on the downsides of Switchcraft jacks can be found here:









Pure Tone Technologies


Home of the revolutionary Pure Tone Output Jack




puretonetechnologies.com





Their tonal improvement claims may leave a bit to be desired, but at least their jacks have actual spring contacts connecting to the sleeve of the plug, as do the Cliff style jacks, simply done a different way!

And yeah, I realize this can seem like really "nit-picky" stuff, but real world experiences and testing show these issues to be real! I even suspect that some output transformer failures may have actually been caused by poor, undiagnosed bad connections at Switchcraft jacks, caused by the weight or tension of a speaker cable, *"IN JUST THE RIGHT '(WRONG)' DIRECTION!"*

Just Jackin'
Gene


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## lwgramith

Hey all, I’m a first-time poster, but I’ve been following this thread for years. Thanks so much to John, Gene, and everyone else who has contributed to this design and to this enjoyable thread.

Long story short, I purchased materials to build an 8ohm version of the M-Lite back in early 2020. I wanted to be able to take a bit off of my 2W home-built amp (essentially an AX84 October) so that I could crank the power section of the amp from my location in the basement without aggravating the wife upstairs. I put the project on hold when the pandemic hit and just got around to building it.

So here’s my dilemma: when I fired it up for a test run with my sacrificial Bassbreaker with matching 8ohm speaker, the amp was so quiet that I could hear the strings over the speaker. Even with the gain and master volume knobs cranked, I got an extremely quiet (but pleasing) sound. This was the case on all of the attenuation settings, including the -7dB “always-on” setting (minimum attenuation for the M-Lite). From what I’ve read in this thread, that is far more attenuation than should be happening. With only the reactive stage engaged, my Bassbreaker should still be able to put out quite a bit of volume. I tried to search the thread for other people with this problem, but I didn’t find anything. Has anyone experienced this problem? Did I misunderstand the application of this device for lower-wattage amps?

Some other details that might be pertinent:

1) I can detect a change in volume when each of the switches is engaged/disengaged.
2) The resistance test -- measuring across the jack of a cable that goes into the attenuator input while an 8ohm speaker is plugged into an attenuator output -- showed between 6.9r and 8.8r across all settings. This was close enough to the 7-10ohm ideal range for me to feel comfortable firing it up.
3) Due to sourcing problems back in 2020, I used a 22r rather than 27r resistor for R9.
4) For the same reason, L1 is 0.4mH and L2 is 0.6mH. It is my understanding that the most important thing is maintaining a particular ratio between the two, and this 2:3 ratio is close to the ideal.
5) The attenuator is built to handle a power rating of 25W.
6) Back in 2020, I designed my own layout (photo below) that in retrospect could be improved significantly. However, I’ve double-checked both that the layout matches the schematic and that the actual build matches the layout. It’s possible I’ve missed something here, but I’m not seeing it.



Layout:


Note: the T, B, L, and R markings near components refer to top, bottom, left, and right side of the schematic.



Some guts shots:







Thanks all!


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## Dee436

I am interested in building one of these but have a question. If I double the power rating on the resistors, ie build a 100w version, can I only use it with 100w amps, or could I also use it with my 50w


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## Gene Ballzz

@lwgramith ,
The problems you describe are indeed strange. When I can devote some uninterrupted, quality time and patience to pour over your diagram and pics I will do so. This kind of "on line" troubleshooting is fairly tedious and time consuming (all one color wire makes it even harder), but I will do my best, when I can. While wire termination and soldering leaves a bit to be desired, it looks generally adequate! Some pics with the tape removed (especially from the input jack) might be helpful.

Just curious as to which model Bassbreaker you are using?

Just Askin'
Gene


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## JohnH

Dee436 said:


> I am interested in building one of these but have a question. If I double the power rating on the resistors, ie build a 100w version, can I only use it with 100w amps, or could I also use it with my 50w


Hi Dee436, there are no problems using it with lower rated amps. If you are building for a 100W amp, it may get very hot even with up-rated parts. Use a large thick aluminium box with plenty of ventilation. Some have added fans to 100W builds.


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## Dee436

Thanks John

looking forward to building and using


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## Glennjeff

Where were you folks in 1980 - never could get those big resistor T's to sound like anything useful. Will be building a M3 with 3.5 in footswitch for home studio use with 6V6 PP, EL84 PP ( ~20 W 8 Ohm ) . Thanks all for the thread.


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## JohnH

hj @lwgramith , a curious issue, and domegimes its hard to trace, especially remotely.

What I can do is work out a few other resistance checks that you can make, with a fully disconnected unit, eg from tip of Input to tip of output, and also across tip and sleeve of output, in different settings. Easy to measure from outside and it nay give a clue....


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## JohnH

hi @lwgramith , further on these tests:

.....these would be with no amp or speaker, but with leads plugged in and measuring to the far end of leads.

All switches to max volume = min attenuation

*Test 1*
If you measure tip to tip from input to output, with M-lite, you should read just L1 (in parallel with resistors but that'll make very little difference to resistance) plus R2. So for an 8 Ohm build that would be about 10.4 Ohms.

*Test 2*
Now using just the input lead, measure tip to sleeve That should be L1 plus L2 plus R1. About 15.9 Ohms

Those readings could be higher by about a half Ohm due to leads and contact resistance, they could also vary slightly due to resistor tolerance, coil specs and meter accuracy.

These tests focus on the max volume -7db setting, which you noted is seeming quieter than expected


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## lwgramith

@Gene Ballzz ,
Thanks for taking a look! You’re right that the wire termination and soldering is rough. It was my first soldering since the pandemic. I should have practiced a bit to knock off the rust before jumping into a real project, but my impatience won out. The Bassbreaker I have is the 15W head, purchased, I believe, in 2017 or 2018. (Tried to link, but I don't have the 5 required posts to do so. It's model #2263000000 on the Fender website.) I’ve attached some photos of the attenuation box without the tape, special focus on the input jack. Thank you again for any time you can/do spend on this.

Thank you, @JohnH ! I ran the tests, and I’m getting measurements that differ significantly from the expected results. Both tests come back at 6.5/6.6 Ohms.

Out of curiosity, I also re-ran the pre-startup resistance test and discovered some interesting things. My measurements no longer come in at the 6.9-8.8 range I had before my initial startup. Instead, I’m getting 6.4/6.5 on all switch combinations that have stage 4 _disengaged_ and 8.2/8.3 on all combinations that have stage 4 _engaged_. Stage 4 is the switch on the left-hand side in the photos. I'm not sure if any of this is related to my problem. It just seemed odd that the readings were no longer showing a minimum of 6.9 as when I originally tested and that the stage 4 switch is the only one that impacts the measurements. I used the same speaker and speaker cable for all of tests (when applicable). 

I plan to reexamine the connections very closely when I can to see if a connection failed. Clearly something is wrong with the reactive stage, but it's not immediately apparent to me what is happening. I’ll be thankful if you can make sense of these measurements.

Photos without tape:


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## JohnH

I can't figure out how if might occur, but those two additional tests which are both giving 6.5 or 6.6, that's as if your R1 and R2 are being connected in parallel somehow. eg, a short to ground at the right side of R2, hence meeting the lower side of R1.


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## gbarker7815

This is a great resource. Looking forward to putting together a build soon. In sourcing the parts, for the resistors, does it make a difference if the tolerance is 1% or 5%? I've been trying to find all matching 1% Arcol ones, but for a few I can only find in 5%.


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## Gert-Jan van der Heiden

gbarker7815 said:


> This is a great resource. Looking forward to putting together a build soon. In sourcing the parts, for the resistors, does it make a difference if the tolerance is 1% or 5%? I've been trying to find all matching 1% Arcol ones, but for a few I can only find in 5%.


The alternative values are way out of 1%. Based on that alone I would see 5% is perfectly fine. Don't think speakers are that precise impedance either.


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## Gert-Jan van der Heiden

I think it would be safe to connect a headphone (left and right in parallel) with all stages attenuating in design M2 or am I missing something? Without those stages my ears _and_ headphones will be gone haha


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## Gert-Jan van der Heiden

at lwgramith :
Not the problem your facing right now, but your speakers are connected so that only 1 of them is connected if you're using Cliff's. Not sure that is your intention.


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## JohnH

Hi @gbarker7815 , welcome to our thread. 5% tolerance is fine for any of the resistors. 

For the base set of values, I selected from the most common ranges of resistors that have steps of about 20%, eg 10, 12, 15 etc. That shows how 5% is accurate enough if that is what you can get.

The most important values are in stage 1, which is what the amp sees and also the ratio of R2A and R2B sets up the tonal balance at all settings. Elsewhere, the ratio within each stage sets the attenuation level, and so the consistency of steps as you move across switch settings.


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## JohnH

hi @Gert-Jan van der Heiden , thanks for joining in. 

Yes It would be safe to plug in headphones at max attenuation. But it may sound bad since there's no treble roll off or other tone shaping due to a guitar speaker. It really needs at least a cab-sim circuit, or better, an IR device, before headphones.


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## Gene Ballzz

@lwgramith ,
I may be able to help, but you need to help me, help you!
*First>* Where are you located on this planet? If in North America, may we could talk on the phone?
*Next>* Communicating via email may make the whole process easier. It will be much easier to send pics and edits of pics, back & forth. If you PM/Conversation me your email, I'll share my email and phone with you. 
*Then>* If you can snap a duplicate pic of the first one in Post # 2,741, but with wires moved around/aside so that I can see the component labeling for identification and which wires go where, that would aid my assistance. I have a couple sneaking suspicions of a couple of improper connections, but don't want to send you off chasing your tail, if I'm wrong. If I'm correct, you may only need to move and/or replace a couple wires.
*Finally, For Now>* If my suspicions are incorrect, you may need to de-wire most of it and mostly do it over again.
Let Me Know?
Gene


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## lwgramith

Thank you @JohnH and @Gert-Jan van der Heiden for the insights. Gert, I’m glad you caught that. I probably won’t ever use it with more than one speaker, but it’s good to fix the wiring now just in case.

@Gene Ballzz, I am in the Eastern time zone of the US. I’ll PM you my contact info and the requested pictures sometime today. Thanks!


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## lwgramith

@Gene Ballzz , I have the pictures ready to send, but I can't find how to PM you. I figured there would be a button for it if I visited your profile, but nothing. Perhaps I haven't unlocked this feature yet given my low post count?


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## Gene Ballzz

The button is labeled as "Start conversation" after you single click my user name in any post, instead of PM/Private Message. The problem is that I've not found a way to attach pics in a "conversation." That's another reason that doing emails may be easier. I'm gonna send you a "conversation" and the alert will show up near the upper right corner of the page!
HTH,
Gene


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## Gene Ballzz

@lwgramith 
I believe I have all components identified, via comparison between your diagram and pics. Of special concern is the jumble of wires between R3, R4, R5 & R6 and where they go to & from?
Gene


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## JohnH

hi @lwgramith , you're in good hands with Gene. If a problem does not quickly become apparent, I'd suggest to do a little temporary rewiring and one more set of tests.

Test 3
check that everything that should be 'grounded' (ie connected to the sleeve lugs of the jacks, though it's not a real ground) is so. So resistance checks between input sleeve and output sleeve, and from each of them to the 'ground' side of R1, R3, R5 and R7. All of these should be very low resistance, determined only by lead and connection resistance and the meter All less than about a half Ohm in practice, and zero in theory.

Then, if still hunting for issues, I'd suggest to unwire the tip connection at the output jack, disconnect R2 from R3 and the first switch, and wire directly from R2 to the output this should bypass any effect from the switched stages and give just stage 1 at -7db. Check input resistance before testing, which should be a full sound, pretty loud, but a step down from max amp volume


----------



## Gene Ballzz

@JohnH ,
That's a pretty slick and fairly simple, yet logical testing/toubleshootong procedure. I can see how one could add and test each stage in the same manner, consequtively, after first wiring back in the stage 2 switch and R4 and then the disconnected other end of R4 to the output. Then simply rinse and repeat for the next two stages, at R6 and finally at R8.
Thanks John,
Gene


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## lwgramith

Quick update on my attenuator. I spoke with Gene yesterday to do some troubleshooting. He had me shift some wires around so I could get better photos for him. We talked for a bit but weren’t able to identify a problem that would cause my issues.

Then something weird happened. Gene left me with a few suggestions for better jack wiring, as well as some tests to run as he continued to look over the photos I had sent. Well, while I was running those tests, I just happened to measure for resistance from sleeve to tip of the input and it came in at 16.1 -- the number John said I should expect for a properly wired attenuator. I then reran the other tests John described, and everything came out right. After that, I plugged the attenuator in for a test run and it worked!

This thing is awesome! Such a great range of attenuation! I’ve never been able to drive the power sections of my amps hard due to my living arrangements, but now I can. It’s like I now have a whole new set of amps!

I guess my task is to figure out what changed between then and now. My suspicion is that I had either a bad joint or a stray wire strand that was “resolved” when I moved the wires around to take new pictures for Gene. I’ll be rewiring my jacks in line with Gene’s suggestions, so I will probably reflow the solder joints to ensure proper connections. I'll also do a very, very close examination for stray wire strands. Hopefully that makes it durable in the long run.

Anyways, just want to say thanks to @Gert-Jan van der Heiden , @JohnH , and especially @Gene Ballzz for giving their time to my project. This is such a great community.


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## Gene Ballzz

@lwgramith 
While you could certainly connect your meter as described by @JohnH and jostle the wires around to "pinpoint" any errant connection(s) and/or short(s), I highly recommend simply "shaotgunning" and closely inspecting all solder joints! Really happy that you don't need to completely dismantle and start over. Also happy that you have generally found the anomaly!
Happy Attenuatin'
Gene


----------



## JohnH

Hi @lwgramith , Im very happy that you got it going! My hunch from the various readings is it was some kind of short to ground, maybe between stages 1 and 2.


----------



## Gene Ballzz

Hey @JohnH ,
What do think of this enclosure?









Pedal enclosure box- 6 1/2 X 6 X1 7/8 inches- .08 aluminum | eBay


Find many great new & used options and get the best deals for Pedal enclosure box- 6 1/2 X 6 X1 7/8 inches- .08 aluminum at the best online prices at eBay! Free shipping for many products!



www.ebay.com










I would think that six, 3/8" - 10mm in the top and six more, in between components, on the bottom would provide sufficient cooling? Looks quite easy to work with and can be easily painted or powder coated to taste. Seems easier than working down inside the Hammond style boxes? Might provide a cosmetically pleasing end product!
Watcha Think?
Gene


----------



## Gene Ballzz

And then one other set of P.I.T.A. questions. On an 8 ohm M2, if the only choice for the 0.9mh coil is 0.8mh or 1.0mh, what would be the better choice and what are the downsides of going in either direction from the specced 0.9mh?
Thanks @JohnH ,
Gene


----------



## JohnH

Hi @Gene Ballzz , I think the case looks good for the basic versions, given some planning . Would it need some components on the top and on the bottom in order to fit? It's not huge in surface real-estate.

On the coils, I think both those values are within range of the sweet spot sych that there's not much difference. There'd be no real difference in basic clean tonal balance (which is set by the resistors), and maybe a tad difference in the reactive harmonics when you drive the output.

For me, I'd pick the 1.0mH, since my meter has an inductance range and I can use it to unwind them a few turns. Ive done this before, and if you keep the meter running as you unwind, (not cutting the wire yet), you can watch the Henry's reduce as each turn comes off and so is removed from the coil. Then trim and resecure the rest of the coil. But even if not, I think I'd still pick 1.0.


----------



## Gene Ballzz

@JohnH ,
Thanks for that reply. Kinda what I figured. The current search for inductor coils, especially 0.9mH is pretty daunting. Most are either #20 gauge, #14 gauge (pricey) or aren't even secured wit zip ties! I really dislike the zip tied units anyway, for multiple picayune reasons, but one is that they simply look cheesy! I really like the mounting convenience and potential extra sturdiness of bobbin wound units. 

On another note, the #20 gauge units "claim" to be good for 200 watts, but it seems that would defeat the whole purpose of wiring these units with #18 gauge wire? Although, I guess that since full power signal does not really pass through that coil, maybe #20 gauge is sufficient for a 50 watt build? Maybe not!

And given that I don't have an accurate means of testing inductance, might you suggest a "guesstimate" number of turns to remove from this Madisound #19 gauge, 1.0mH unit, to get it closer to the 0.9mH ballpark?

Thanks Aagin John,
Gene


----------



## JohnH

ok, I can have a go at working that out. Another reason not to go down to 20ga is to control resistance, which we don't really want in the coils.


----------



## Gene Ballzz

I had no idea that the small value difference in resistance is critical, although I can see how the difference between 0.48 or .50 and 0.67 or 0.73 is in the neighborhood of 40% or more! Thanks for that reminder. Can I assume that the lower the DCR the better, or more inconsequential it becomes? For example, a #14 gauge, 0.9mH is like 0.23 DCR.

Of course, this would be "electrically" perfect,









Home


Jantzen Audio 0.90mH 18 AWG Air Core Inductor Crossover CoilThe Jantzen Audio 18 gauge air core inductors are precision wound with high-purity copper wire to a ± 3% tolerance. A special Jantzen high-temperature baking process bonds the wires throughout the inductor, eliminating the possibility...



www.parts-express.com





but then there's the conundrum of securely mounting it to avoid damage or ugliness! Yeah, I know, zip ties and a couple disks to squeeze it between, but…….., I'm not really a fan of* "squeezing"* these coils, given the relative *"fragility"* of the varnish coating and the fact that these units might get bounced around in gig bags, while still hot from use, etc! I really want to enhance the cosmetics a bit, with flush, countersunk mounting screws and certainly no zip ties protruding through the outside of the enclosure! I'm guessing the Madisound #19 gauge, 1.0mH, on its bobbin, unwound a bit is the best solution? Of course, *"engineering"* a suitable *"bobbin"* might be an option, but rather tedious and time consuming, at best!

Gene


----------



## Gene Ballzz

Hey @JohnH ,
I just accidentally sent you an email from my junk account and I'm going to send it again from my real email, just in case you think the first was SPAM!
Thanks Again,
Gene


----------



## Gert-Jan van der Heiden

I'm building a M2 design. Does it make sense to swap the order of the switches into stage 3, then stage 2 followed by stage 4. So you get -14db, -7db and -3,5 db?


----------



## Gert-Jan van der Heiden

Gene Ballzz said:


> Hey @JohnH ,
> What do think of this enclosure?


I know, I know, I'm not John. Like the shape, but the joint between the front and the top cover has a gap. If it bothers you you could add some moulding. I haven't bothered to convert the imperial measurements. The 165 mm length doesn't allow for much play. I used a longer one I had for another project, but that one is a bit to long, it doesn't quite fit on the top of my JCM800 head. There isn't much room on the top.


----------



## Gene Ballzz

Gert-Jan van der Heiden said:


> I know, I know, I'm not John. Like the shape, but the joint between the front and the top cover has a gap. If it bothers you you could add some moulding. I haven't bothered to convert the imperial measurements. The 165 mm length doesn't allow for much play. I used a longer one I had for another project, but that one is a bit to long, it doesn't quite fit on the top of my JCM800 head. There isn't much room on the top.



I did a full mockup using those dimensions and everything fit wonderfully, even using the huge ARCOL/OHMITE 100 watt resistor for R1. The gap did not concern me, but I had hoped to get him to do two more identical rows of vents on the top. Unfortunately, these boxes are no longer available and will not ever be again, unless at a *much* higher price!

Oh Well,
Gene


----------



## BlueX

Gert-Jan van der Heiden said:


> I'm building a M2 design. Does it make sense to swap the order of the switches into stage 3, then stage 2 followed by stage 4. So you get -14db, -7db and -3,5 db?


Maybe post # 2,207 on page 111 (!) answers your question:





Simple Attenuators - Design And Testing


Wow this forum is amazing, im hoping to build a design for my amp, Im looking to have a headphone out as well as line out, becuase of the headphone out i want this section to have a cab sim, I was just wondering what was the point of m3 vs m2 does the m3 have a cab simulation as you keep...




www.marshallforum.com


----------



## stickyfinger

Gene Ballzz said:


> I did a full mockup using those dimensions and everything fit wonderfully, even using the huge ARCOL/OHMITE 100 watt resistor for R1. The gap did not concern me, but I had hoped to get him to do two more identical rows of vents on the top. Unfortunately, these boxes are no longer available and will not ever be again, unless at a *much* higher price!
> 
> Oh Well,
> Gene


Crap that was actually the best looking enclosure i had found after searching for a few weeks even before you linked it. I originally wanted to make a tube preamp out of it.


----------



## Gert-Jan van der Heiden

Yeeey, it works!



And it's guts:


Rubbed the paint off where the resistors are mounted with thermal grease. I didn't want the paint to thermal isolate. I intended to make slots in the back, by "connecting" the holes, but sticked with the holes. 3D printed a little mounting thingy for the coil. I used the colors to keep track what I'm doing. It get's warm, but not hot after playing it on over 8 at volume level. Something like red = hot = in, orange is a bit cooler etc. Used different colors for each stage. I didn't connect ground to the chassis.

Edit: Thank you @JohnH


----------



## Gene Ballzz

Gert-Jan van der Heiden said:


> Yeeey, it works!
> 
> View attachment 110301
> 
> And it's guts:
> View attachment 110302
> 
> Rubbed the paint off where the resistors are mounted with thermal grease. I didn't want the paint to thermal isolate. I intended to make slots in the back, by "connecting" the holes, but sticked with the holes. 3D printed a little mounting thingy for the coil. I used the colors to keep track what I'm doing. It get's warm, but not hot after playing it on over 8 at volume level. Something like red = hot = in, orange is a bit cooler etc. Used different colors for each stage. I didn't connect ground to the chassis.
> 
> Edit: Thank you @JohnH



Great use of space! Nice, neat and tidy layout. I'd expect the need for more/better ventilation, but if it doesn't get actually "HOT" with your amp suitably cranked to it's sweet spot, you're good to go! Ain't it liberating to be able to get great sounds that don't elicit screams of "TURN IT DOWN?"
Nice Job! 
Gene


----------



## Gert-Jan van der Heiden

@Gene Ballzz thank you. I’ve heard “turn it down” quite a few times in my live  Really nice to hear it roar again, but not that loud. Like a tiny tiger kitten haha. I like the sound. It’s awesome.


----------



## JohnH

Hi @Gert-Jan van der Heiden . That looks like a really nice build, clean and simple and I like the 3d coil-mount gadget.

Just on the order of stages, yes you can reorder them and it makes no significant difference. A little history....

At the start of the thread, you can see the previous design M, which had two coils, but otherwise works like M2. In both these, the -7db stages (the reactive Stage 1 and the switched -7db) are the most perfect ones that have the ideal input and output impedances. The -14 and -3.5db stages have the right output impedance but are allowed to have a somewhat different input impedance because the amp generally never sees them directly, only via Stage 1. This allows them to remain as nice simple 2-resistor stages that can be switched with a single pole.

But, with M I wanted to explore getting a dedicated -3.5db setting, to fill the gap between full power and -7db. The -3.5db stage works quite well on its own, so in M, I put that last and the switching allows that you can use it on its own. I works fine, you can see and hear results in Post 1. I put the -7 stage first in line, since it is more ideal than the -14 stage. So I had -7 (reactive), -7, -14 then -3.5. That was the electrical order, but the physical order can be different, and like with binary numbers, -14, -7, -3.5 is probably most logical, as you look at the switches on the panel.

Now we have other better ways to get a -3.5 or -3 db setting if needed, plus, its a setting that is rarely used since its only very slightly less volume than full power. So for the base M2 designs, its not shown. But I kept the order of stages just for consistency.

But with further analysis, I found it makes really no difference what order the stages are wired up after Stage 1, and of course in any case they all have to work after stage 1 without the other two. So for easier wiring and understanding, Im now showing them -7(reactive) -14, -7, -3.5 in diagrams that Im doing. Im keeping the R numbers though, so R3, R4 etc is consistent through the thread.


----------



## JohnH

Gene Ballzz said:


> I did a full mockup using those dimensions and everything fit wonderfully, even using the huge ARCOL/OHMITE 100 watt resistor for R1. The gap did not concern me, but I had hoped to get him to do two more identical rows of vents on the top. Unfortunately, these boxes are no longer available and will not ever be again, unless at a *much* higher price!
> 
> Oh Well,
> Gene


That's a shame! I would have looked very nice

Im happy with the Hammond styles for mine, and the box 'lid' is the base which looks a bit better i think due to the slight taper of the sides. These boxes also give you continuous Aluminium for what is then the top and sides, to get max conduction and cooling to resistors mounted there. I put the coil on the lid (ie final base), so if I ended up with anything zip tied or with an unusual screw, its underneath so not seen. Also, its weight presses down, so on my prototypes, once Id figured out that steel screws are not good with the coils I actually just used dabs of silicone to stick the coils down. Perfectly safe for my use, except maybe not ideal for rattling around in a gig bag!


----------



## Gert-Jan van der Heiden

JohnH said:


> Hi @Gert-Jan van der Heiden . That looks like a really nice build, clean and simple and I like the 3d coil-mount gadget.
> 
> Just on the order of stages, yes you can reorder them and it makes no significant difference. A little history....
> 
> At the start of the thread, you can see the previous design M, which had two coils, but otherwise works like M2. In both these, the -7db stages (the reactive Stage 1 and the switched -7db) are the most perfect ones that have the ideal input and output impedances. The -14 and -3.5db stages have the right output impedance but are allowed to have a somewhat different input impedance because the amp generally never sees them directly, only via Stage 1. This allows them to remain as nice simple 2-resistor stages that can be switched with a single pole.
> 
> But, with M I wanted to explore getting a dedicated -3.5db setting, to fill the gap between full power and -7db. The -3.5db stage works quite well on its own, so in M, I put that last and the switching allows that you can use it on its own. I works fine, you can see and hear results in Post 1. I put the -7 stage first in line, since it is more ideal than the -14 stage. So I had -7 (reactive), -7, -14 then -3.5. That was the electrical order, but the physical order can be different, and like with binary numbers, -14, -7, -3.5 is probably most logical, as you look at the switches on the panel.
> 
> Now we have other better ways to get a -3.5 or -3 db setting if needed, plus, its a setting that is rarely used since its only very slightly less volume than full power. So for the base M2 designs, its not shown. But I kept the order of stages just for consistency.
> 
> But with further analysis, I found it makes really no difference what order the stages are wired up after Stage 1, and of course in any case they all have to work after stage 1 without the other two. So for easier wiring and understanding, Im now showing them -7(reactive) -14, -7, -3.5 in diagrams that Im doing. Im keeping the R numbers though, so R3, R4 etc is consistent through the thread.


Thank you. Like the idea of keeping the reference numbers the same.


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## Gene Ballzz

@JohnH ,
Given your relative expertise in cooling, is the die cast material of the Hammond boxes as good of a heat sink/thermally conductive as extruded aluminum? Also, how much better or worse for cooling, is it to have all the resistors mounted to the top, vs to the bottom of the unit! I definitely prefer the Hammond boxes having the lid on the bottom, as the other way gives it an odd "keystone" look! Also the screws holding the lid are handy for also mounting the feet, if they can be found with a small enough diameter!
Just Pickin' Nits, As I'm Wont To Do,
Gene


----------



## JohnH

Aluminium is all good, whether its die-cast or extruded. Its about 5x as conductive as steel, and probably 2x as thick in a given application, so 10x better. So between those two AL cases, it all comes down to the thickness of the material and the surface area. My belief is that the conductivity across a junction say box to lid is not as good as just within the material, so on a Hammond, best to fix hot parts to the larger section. Also, the top of the attenuator will get better air cooling than the base. But you have to enjoy seeing all the screws etc. - I quite like the Steam-Punk vibe of the screw heads!


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## Gene Ballzz

@JohnH 
Another dumb question from the peanut gallery, that may have already been answered? For the R8 in an 8Ω M2, 5.6Ω is called for. If my only choices were 5Ω & 6Ω what would be the better choice and what would be the consequences, compared to the specced 5.6Ω? Would it also require a different value for R7?
Thanks For Helpin'
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH
> Another dumb question from the peanut gallery, that may have already been answered? For the R8 in an 8Ω M2, 5.6Ω is called for. If my only choices were 5Ω & 6Ω what would be the better choice and what would be the consequences, compared to the specced 5.6Ω? Would it also require a different value for R7?
> Thanks For Helpin'
> Gene



Hi Gene, I looked at that before, to try to work out a set of values to buy from typical Chinese sellers - see the first diagram. I went for 5 Ohm at R8 and the diagram shows R7 staying at 33 Ohm. Theres no tonal consequence (less than 0.1db at any frequency) but it shifts the attenuation down by about 0.15db. When you step through the range, then the series of increments available at each step, intended each to be -3.5db, then vary from about -3.3db to -3.7db. With the specced values the range is about -3.5 to -3.6 db, so a tad more consistent.

If you change R7 from 33 to 30, with R8 at 5, then its back to virtually the same as specced.


----------



## Gene Ballzz

JohnH said:


> Hi Gene, I looked at that before, to try to work out a set of values to buy from typical Chinese sellers - see the first diagram. I went for 5 Ohm at R8 and the diagram shows R7 staying at 33 Ohm. Theres no tonal consequence (less than 0.1db at any frequency) but it shifts the attenuation down by about 0.15db. When you step through the range, then the series of increments available at each step, intended each to be -3.5db, then vary from about -3.3db to -3.7db. With the specced values the range is about -3.5 to -3.6 db, so a tad more consistent.
> 
> If you change R7 from 33 to 30, with R8 at 5, then its back to virtually the same as specced.



Thanks for that info. R7=30Ω & R8=5Ω. Makes for "Mo Betta" ordering, availability, etc!
Thanks Again,
Gene


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## JohnH

Following on from above, here's a diagram for those who love diagrams:





This the calculated response of a basic 8 Ohm M2, driven from an 8 ohm amp tap (based on my VM), stepping through all the attenuation steps from full volume down to nominally -31 db. 'db inc' is the difference from the previous step, to show consistency, 'R440' is the impedance at 440 hz, and 'bass rise' and '5khz rise' show the output peaks at low and high frequencies, relative to mid frequencies. 

The thick red lines are the response of a full-volume G12M closed back cab, modelled to match measured impedances (data from Mike Lind - TGP) 

The central plots show frequency response of the signals reaching the cab, on the right side is what the amp sees. In this model, there's no bass resonance circuit, so you can see that diminishing from the point of view of the amp, but its there in the speaker output because its generated by the speaker itself.

The 16 Ohm data at the top (blue background) adopt Output 3, which is tonally compensated for 16 Ohms in an 8 Ohm M2
The central band in green is 8 ohm for speaker, amp and attenuator
The lower band in yellow is for a 4 Ohm cab into an 8 Ohm M2. This has no specific extra compensation and you can see a db or so of extra high treble and a couple of db of extra bass peak (its possible to add a couple of parts to compensate if wanted)

The whole thing is from a giant excel sheet that runs a macro to step through all the variations and paste them onto this chart. It takes a minute or so to run a new set.

I use this system to check out variations in the design and to keep close control of all parameters, to ensure that at every setting, tone is maintained accurately. By being very picky with optimizing each of the numbers, I can control small changes in the design to a level which cant be heard, so that when all are combined, it all sounds as consistent and as seamless as possible.


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## Gene Ballzz

@JohnH
What is row #1, that is greyed out? Is that parallel function?
Just Askin'?
Gene


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## JohnH

Gene Ballzz said:


> @JohnH
> What is row #1, that is greyed out? Is that parallel function?
> Just Askin'?
> Gene



That's the -3.5db resistive stage working on its own, without Stage 1, as it did in Design M shown on post 1. It works pretty well, but I stopped promoting it since it creates wiring fiddle and a few quirks when using a 16 ohm cab with 8 ohm attenuator (in that case, the amp has to be set at 16 too) It's still in my excel file though.


----------



## Gene Ballzz

@JohnH 
Please confirm or deny an assumption I have, concerning the 16Ω, "Out 3" an 8Ω, M2 attenuator. My "assumption" is that the switching function of that jack need not be used to introduce and remove R10 from the circuit?, Instead that switching function is accomplished by the sleeve of a Tip/Sleeve plug connecting the Ring to the Sleeve of the Tip/Ring/Sleeve jack and vice/versa when removing said plug?

This brings to mind a somewhat related question. Is there a similarly simple solution for correcting the small tonal anomalies of using an 8Ω speaker with a 16Ω, M2? Or does that get more complicated?

Thanks John, As Always,
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH
> Please confirm or deny an assumption I have, concerning the 16Ω, "Out 3" an 8Ω, M2 attenuator. My "assumption" is that the switching function of that jack need not be used to introduce and remove R10 from the circuit?, Instead that switching function is accomplished by the sleeve of a Tip/Sleeve plug connecting the Ring to the Sleeve of the Tip/Ring/Sleeve jack and vice/versa when removing said plug?
> 
> This brings to mind a somewhat related question. Is there a similarly simple solution for correcting the small tonal anomalies of using an 8Ω speaker with a 16Ω, M2? Or does that get more complicated?
> 
> Thanks John, As Always,
> Gene




All true, the only extra thing used is the ring connection, which gets grounded by the sleeve of the incoming plug. No switched contacts are involved and need not be provided when buying the jacks.

Yes there is an even simpler adjustment if you want to further tweak the 16 Ohm build for 8 Ohm. The difference in tone is due to the 8 ohm speaker seeing a higher output impedance from the 16 ohm M2 than it would get from an 8 Ohm M2. The fix would be to add a 39 Ohm resistor in parallel across the speaker. This should trim back the treble and bass resonance in proportion to the mids. It could be added as a switch, or using a second jack, wire tip and sleeve direct to the other jack and put the 39 Ohm one end to tip and other end to the new ring lug so it goes to ground when the plug is inserted.

Youd have two jacks, one dedicated to 16 Ohm (or a brighter output for 8 ohm), and the other for corrected 8 Ohm. Or put a 16 cab into each and it will correct the pair for the 8 Ohm total.

But Id really like it if you could test this with a lash-up of some kind first, just temporarily wire the resistor across the 8 cab and see if it does as predicted?

The ultimate version of all this would be the 8 Ohm M2, with just two parallel jacks, and a switch to adjust tone for 16/8/4 cabs.


----------



## Gene Ballzz

Ha Ha John! This debate between us over 8Ω vs 16Ω will likely continue forever!  My stance is that in my universe, 16Ω is the more prevalent cabinet and/or speaker. Also, the 16Ω unit allows for more flexibility (especially with the 8Ω tweak you just mentioned) for use in parallel, as many amps do not have a 4Ω tap. Can I also assume that the 16Ω tweak for an 8Ω M2, as well as the above mentioned 8Ω tweak for a 16Ω M2 do not affect the impedance seen by the amp, in any appreciable way?

I must say though, that I just recently revisited your design incorporating both an 8Ω and 16Ω M2 combined into one unit! I'll need to pour over it a bit more, but at face value, it is quite intriguing! Might be the way to move forward, should this ever become any sort of commercial enterprize! The added components are not very expensive and it would not require a very much larger enclosure!

On the other hand, it sure is nice having a friend to sensibly , politely and logically debate such picayune details with!  Maybe someday we'll actually at least get to speak with each other on the phone, although I think I'd truly enjoy, even more, having a cold one with you in some seedy gin mill with a cool band playing! 

Again, Thank You!
Gene


----------



## JohnH

Hi Gene, yes all the ohms as seen by the amp stay within reasonable range with tbe various tweaks. For 8Ohm versions, I try to keep what the amp sees between 7 and 10 Ohms at any setting. You can see that in tbe charts I posted yesterday, on the 'R440' column.

I tend to focus on the 8Ohm versions since that's what I need to cover both my rigs, which are both based on 2x12. But it looks like most tube Marshall combos come with a 16 ohm speaker but do Fenders usually come with 8?

That double-barrelled 8/16 version is appealing, since the main switches and jacks can be the same as a basic version. It can be made either with two 8ohm circuits or 2x16, and it'd work 'natively' at either 8 or 16 Ohms in either case.

cheers
John


----------



## Gene Ballzz

@JohnH 
I get your whole "8Ω thang" but was simply bustin'yer cojones. And yeah, Fenders are a different story, although my 5E3 clone has a selectable 4/8/16Ω output transformer! Some Bassman and Super Revereb amps were actually designed for optimal use at 2Ω. In case you hadn't noticed, the one nice thing about the updated software at theis forum, is that you can now use the Ω omega symbol more than two or three times without getting an error message! One big downside is that the "outline" format has gone away! It's funny though, as many other websites that use otherwise identical software still have the outline available!  My guess is when it comes to renewal/upgrade time, there is a selectable grocery style list to choose from. At least they did not remove some of our coolest emojis! Although this one  and this one will likely be the first to go, given the nature of the "PC" vortex that our world is irretrievably spiraling into!

Outline Wishin'
Gene


----------



## Gene Ballzz

JohnH said:


> All true, the only extra thing used is the ring connection, which gets grounded by the sleeve of the incoming plug. No switched contacts are involved and need not be provided when buying the jacks.
> 
> Yes there is an even simpler adjustment if you want to further tweak the 16 Ohm build for 8 Ohm. The difference in tone is due to the 8 ohm speaker seeing a higher output impedance from the 16 ohm M2 than it would get from an 8 Ohm M2. The fix would be to add a 39 Ohm resistor in parallel across the speaker. This should trim back the treble and bass resonance in proportion to the mids. It could be added as a switch, or using a second jack, wire tip and sleeve direct to the other jack and put the 39 Ohm one end to tip and other end to the new ring lug so it goes to ground when the plug is inserted.
> 
> Youd have two jacks, one dedicated to 16 Ohm (or a brighter output for 8 ohm), and the other for corrected 8 Ohm. Or put a 16 cab into each and it will correct the pair for the 8 Ohm total.
> 
> But Id really like it if you could test this with a lash-up of some kind first, just temporarily wire the resistor across the 8 cab and see if it does as predicted?
> 
> The ultimate version of all this would be the 8 Ohm M2, with just two parallel jacks, and a switch to adjust tone for 16/8/4 cabs.



@JohnH 
While at face value, I really like the idea of putting the output speaker impedance/tone compensation on a switch, some concern comes to mind. Let's say for an example that an 8Ω M2 is connected to a 50 watt amp with a 16Ω speaker, in compensated mode. What would be the ramifications if a bypass switch was included? If bypass was activated, might not the 25 watt resistors R10 & R11 go up in smoke? Same question for a 16Ω M2 with a bypass and 8Ω compensation? I'm simply trying to allow for unintended and/or unexpected circumstances. Remembering of course, that guitarists are only a step or so away from being drummers! 
Thanks,
Gene


----------



## JohnH

hi @Gene Ballzz , I think the resistors and the compensating components like R10 and R11 could be made to be OK. They are not taking a lot of power, really just tweaking things. The worst outcome could be with a bypass switch, if a guitarist lets a drummer set up their amp. If say an 8 ohm amp and 8 ohm M2 is wired to a 16 cab, but the bypass is engaged, then you have a 16 cab straight into a 8 ohm amp. But It'd bypass everything so no resistors would be involved. And, I doubt if any other attenuators can do much about that either.

Another issue with a 4/8/16 switch on the output is, I think its tricky to work out the switching with a simple 2-pole switch. I might have another look at that though.


----------



## XTRXTR

A DPDT switch can be wired for a triple select but you must find one that can handle the current which is likely large.


----------



## ebswift

All my parts came from Aliexpress fairly quickly, I got the inductor from Soundlabs Group and the switches from Jaycar. The case was some old specialised computer unit which is fairly large for the job and should be able to handle the heat very well. End support panels were 3D printed with heat thread inserts, and 1mm aluminium folded over the top and screwed on. All the holes for the resistors were tapped (also used thermal paste), and the inductor has a 3D printed 10mm base screwed on with nylon screws. This is a basic 16 Ohm M2, I haven't tested the line out yet. I haven't done much testing, but when I feel the case it remained cold to the touch.

I made this for my Vadis 600R, a 60W Aussie amp made in about 1965 with a 4x12 cab stuffed with Rola 12PEG's that I bought as 'electronics' non-working really cheap, but I put the effort in and got her singing again. Interestingly, cranked through the M2 this amp doesn't really seem to break up, it seems to just run clean. Anyway, thanks John for the design effort and all the followup info you have provided through the forum.

The vinyl didn't like me turning screws onto it as you can see in the closeups, but it looks pretty good from a few feet back. If the OCD catches up with me I might re-print them some day.
















One thing of interest is that my amp had some low level hissing, and a hum that came along with the reverb dial, but the hiss is 99% gone (and the hum 100% gone) with the attenuator hooked up, even though it used to be there with the volume turned right down low.


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## JohnH

thanks @ebswift , it looks great. Nice to see another build down here in Oz and its useful to note the coil supplier here too. I was buying from Queensland Speaker Repair but they stopped selling the coils. The other one I've noted is Wagner online in Sydney. I like the finned case.

Suppressing the output hiss and hum on a powerful amp is a nice extra benefit of attenuation.


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## Gert-Jan van der Heiden

@ebswift I had the same thing with the reverb tank on my Marshall JCM800 2205. But I had to dail everything to a ridiculous low level


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## Gert-Jan van der Heiden

The M2 design should have an attenuation of a little over 30dB. So the power gain should be roughly 10^(-30/10) = 1m. Resulting in an attenuated output of 50 mW. Okay, with a lot of distortion maybe 100mW.

Mine doesn't sound like that, it is still pretty loud when I crank the volume of the amp up. (Marshall JCM800, 2205, built in '86).

Is this suppose to be like that? I checked with the schematics multiple times. Measured to check what I expect with the switches on and off.


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## JohnH

Hi @Gert-Jan van der Heiden

It's surprising how loud a very small amount of power can sound through a guitar cab. I quite often play my amp set at about 6 or 7 , attenuated to -31.5db. I reckon my amp might be putting out about 20-30W, and the power reduction at -31.5 db is about 1/1400, so I'm getting maybe 14 - 20 mW!. It sounds about TV volume, and mixes well with unamplified vocal or acoustic guitar.

There's a few more checks you can do:

1. Set up some kind of consistent signal into the amp (could be a riff in a looper, or a steady tone). Then mic the amp and record into a computer. Set the levels so the recording is not distorting at full power, then keep everything set and step down through the attenuation. If you want to start this test at full volume, you'll need to switch to standby to engage the attenuator.

I did this test in post 1, and you can hear and see the results. The dB's were correlating well with expectations.

2. You could run a sound level meter app on a phone and note the changes in sound dB's between settings. I haven't tried this, but note that a phone is unlikely to cope with a full volume amp nearby. But you could try it with amp volume set low, just to see what the attenuator does.

3. You could use a meter to read a.c. Volts across your speaker. The voltage ratio is the square root of the power ratio. Lets say you tried this test between the -7db and -31.5db attenuator settings. That's a power ratio of 1/280 so the voltage ratio at the speaker is about 1/17


----------



## Gene Ballzz

@Gert-Jan van der Heiden

I've looked long and hard at your build pics and don't see anything glaringly obvious. Nice work, BTW. I've got a dumb question, as 'm not familiar with those particular switches, did you confirm, with a meter, which lugs did what "before" installing them? I have encountered some SPDT switches that the center lug is *NOT* actually the common lug. If operation/lug positions were different than expected/assumed, you'd be chasing your troubleshooting tail forever!
Just Thinkin'
Gene


----------



## Gert-Jan van der Heiden

Gene Ballzz said:


> @Gert-Jan van der Heiden
> 
> I've looked long and hard at your build pics and don't see anything glaringly obvious. Nice work, BTW. I've got a dumb question, as 'm not familiar with those particular switches, did you confirm, with a meter, which lugs did what "before" installing them? I have encountered some SPDT switches that the center lug is *NOT* actually the common lug. If operation/lug positions were different than expected/assumed, you'd be chasing your troubleshooting tail forever!
> Just Thinkin'
> Gene


Thank you Gene for taking your time to check this. Actually, I did make a mistake, I swapped the values of resistor R4 and R6. I have a 16 ohm version, so in my case it was 30 and 20 ohm. Not that drastic change. 

I did check the switches to confirm the common connection, as the data sheets where not that clear. I had to do some assumptions on the part numbers. No nice part number built up documentation.

I also checked by simple hooking up a multimeter to check the resistance. Al nicely around 16 ohms, not matter what switches where on or off. Also tried 2 multimeters and they both couldn't cope the 16 ohm input of the cabinet. 8 went fine. Not sure what that was about. I also didn't dive into the industry standard of measuring impedance of a speaker, I can imagine this is measured at some AC frequency, not DC.

Also checked the air coil, it is 0.7 ohms as it suppose to be, so now shorts in the coil other than the long wire in a loop.

I haven't checked the amp itself. No idea if it's biased right or that the plate voltages are correct. The inside and outside look really original. Even the big caps, and they measure fine. ESR value is very good. Don't know what it is, but I like it when you see these old stickers with names on them. Or the original stamps, you really can feel someone worked on this back in the days. Same thing when working on my car from '69. Yes, and I'm sorry they're old haha, if "only" from '86.


----------



## Gert-Jan van der Heiden

JohnH said:


> Hi @Gert-Jan van der Heiden
> 
> It's surprising how loud a very small amount of power can sound through a guitar cab. I quite often play my amp set at about 6 or 7 , attenuated to -31.5db. I reckon my amp might be putting out about 20-30W, and the power reduction at -31.5 db is about 1/1400, so I'm getting maybe 14 - 20 mW!. It sounds about TV volume, and mixes well with unamplified vocal or acoustic guitar.
> 
> There's a few more checks you can do:
> 
> 1. Set up some kind of consistent signal into the amp (could be a riff in a looper, or a steady tone). Then mic the amp and record into a computer. Set the levels so the recording is not distorting at full power, then keep everything set and step down through the attenuation. If you want to start this test at full volume, you'll need to switch to standby to engage the attenuator.
> 
> I did this test in post 1, and you can hear and see the results. The dB's were correlating well with expectations.
> 
> 2. You could run a sound level meter app on a phone and note the changes in sound dB's between settings. I haven't tried this, but note that a phone is unlikely to cope with a full volume amp nearby. But you could try it with amp volume set low, just to see what the attenuator does.
> 
> 3. You could use a meter to read a.c. Volts across your speaker. The voltage ratio is the square root of the power ratio. Lets say you tried this test between the -7db and -31.5db attenuator settings. That's a power ratio of 1/280 so the voltage ratio at the speaker is about 1/17


ok, that is an extensive reply! Thank you very much. I don't own a mic believe it or not, apart from the one built-in my macbook. I just recently hooked up my Marshall and guitar again, after many years of dust collection. I have invested in a wah pedal, looper, audio interface, midi keyboard, bunch of cables, tools for setting up my old guitar. So a bit strange, such a setup and no mic.
I have this amp for a long time. In the first years I used it at my parents house at rural environment. So, hearing this amp again at some decent volume (on the knobs) brings back nice memories of that sound. Also, I totally get what people say that you can feel the air moving.

I have my amp in the living room, and all walls are made of thick brickwork with a layer of plaster over them. So, the sound isn't absorbed anywhere. Basically, my room is a big cabinet too. I didn't knew that a cabinet could get so loud with so little power. So again I learned something new here. Now I should learn how to play a guitar... This attenuator surely helps a lot with that.

I think I should try #3 to check. I do have an oscilloscope but no function gen. Maybe I can generate a sine wave gen with an STM32. I better start looking for my ear protection, because a sine wave that loud isn't gonna be a pleasant journey haha.

I am aware of ground loop hazard with the scope. Your compatriot Dave from the EEVBlog taught me that. As the chassis of the amp is connected to mains earth and ground. So, I should use no other voltage ref than ground and I should be good.

But I really should start with the amp. See if it is biased ok and the voltages are correct. That's good anyway, as I'm cranking it up now. I did check the electrolytic caps, and replaced most of them.

I do think the attenuator is working just fine. No earth leakages or shorts, all resistors measure just fine, resistance over the resistors are correct, switches short out the correct places, and it is actually attenuating a lot.

I have trouble searching through this thread only, I use google for that (using site:....). But could I add another stage? I haven't figured out how you managed to maintain the impedance of 16 ohms the same. Very clever circuit sir.


----------



## Gert-Jan van der Heiden

okay, did the test with pleasing results.

I used an Arduino due as function gen, not ideal but hey. Added a little circuit: dc filter cap, bleed resistor, rc filter and then a voltage divider. The rc filter is to make the sinus a bit cleaner. I found a little Arduino script with a function gen and controlled it with a serial terminal on my mac.



I dragged everything to my little workshop, so nobody in the house has to listen to my loud beeps.




I used the scope to see there is no distortion. And 2 voltmeters to measure input and output. I measured at 3 frequencies, 500Hz, 1kHz and 4kHz. I had the readjust the output voltage (AC) from the amp at each frequency and stage / switch change on the attenuator. (In order to compare -7dB and -31.5dB). It is really attenuating a lot. The scope is on 20V/div on both channels:



Now "zoomed in" on 1V/div on second (speaker) channel:


I also used this:



As always, measurements, in practice things turn out a but different, but close enough to verify it functions as designed:



1/17 is about 0,059. Note that the output voltage of the amp is not as constant with different settings and the meter was jumping around a bit. I very very very tiny turn (tapped it) on the volume knob changed a lot. So, not very precise, but enough to tell verify it is in the ballpark


----------



## JohnH

hi @Gert-Jan van der Heiden 

That's perfect! Even the figure of 1/17 that I wrote, is just a simplification and the actual value is 1/16.78 which is 0.060. So that's exactly your value at 4000hz and the others are within a decibel which is still a great result over the range. 

Some small variations like that are explained by differences in the amps and the different output impedances they have at various frequencies, whereas my numbers are based on a constant value. 

All good!


----------



## RedHouse59

Hey JohnH, very sorry to be "_one of those guys_" (who skipped over stuff) but I made it to page 77 of this thread and decided it might just be better to ask a question.

I'm building your M2 version (16-Ohm input type with single inductor) as that would suit most of my gear.

I ordered enough parts to build 2 attenuators and for the second one I want to use it with a unique speaker cab I have that is 4-Ohms, it's an "Iso-Cab" (with internal mic) used as DI box.

My question is what values of resistors would I need to use/change in your basic M2/16-Ohm Input schem to operate with 4-Ohm speaker out as the norm?


----------



## JohnH

RedHouse59 said:


> Hey JohnH, very sorry to be "_one of those guys_" (who skipped over stuff) but I made it to page 77 of this thread and decided it might just be better to ask a question.
> 
> I'm building your M2 version (16-Ohm input type with single inductor) as that would suit most of my gear.
> 
> I ordered enough parts to build 2 attenuators and for the second one I want to use it with a unique speaker cab I have that is 4-Ohms, it's an "Iso-Cab" (with internal mic) used as DI box.
> 
> My question is what values of resistors would I need to use/change in your basic M2/16-Ohm Input schem to operate with 4-Ohm speaker out as the norm?



Hi Redhouse59, thanks for joining our thread.

That's a good question to ask, which has not come up before.

There's a few ways to use the attenuator to convert between ohms values between amp and cab. It all can work simply enough if you have the first -7db reactive Stage 1 always on, without adding a bypass switch (ie as on the base diagram). In this case, the amp mostly sees Stage 1.

Your amps are 16 Ohm, so that's what the attenuator needs to show to the amp, within a small margin

If you just use the basic 16 Ohm M2, into a 16 Ohm amp and a 4 Ohm cab, its actually safe, The amp sees somewhere between 14 and 18 Ohms in all settings. What diverts a bit is the tone, which will become more prominent in the high presence treble and bass resonance. This may be OK, or desirable, or not so. The difference is very much the same as changing from an amp with negative feedback, to one with no NFB. It comes from a 4 Ohm speaker expecting to be fed from an output with 1/4 of the impedance of that of a similar 16 ohm speaker.

Generally we have discussed this difference across a 1 step change, such as 16 to 8 or 8 to 4. At this change, the tone doesn't change much and is usually acceptable. But it can be adjusted for. From 16 to 4, there will be a bigger difference.

Id suggest to keep everything the same as the basic 16 Ohm version, but add a switch with a resistor across the speaker output. So, one end of the resistor goes to the 'cold' output the other to the switch, and the switch to the 'hot' output. Id suggest a value of 15 Ohm. For a 50W amp, it can be like the other small resistors, ie 25W rating, or even 10W. This will lower the apparent output impedance of the attenuator, correcting the speaker tone, but the amp will hardly know.

You could hard-wire it instead, but if you have it switched, you can try the different tones and see which sounds best..

One thing to note. When connected this way with the 16 to 4 conversion , all the attenuation levels are increased, by about -5db. So the Stage 1 value changes from -7 db to -12 db. Id expect that you'll still have plenty of volume available to record an Iso cab though.


----------



## RedHouse59

JohnH said:


> Hi Redhouse59, thanks for joining our thread.
> 
> That's a good question to ask, which has not come up before.
> 
> There's a few ways to use the attenuator to convert between ohms values between amp and cab. It all can work simply enough if you have the first -7db reactive Stage 1 always on, without adding a bypass switch (ie as on the base diagram). In this case, the amp mostly sees Stage 1.
> 
> Your amps are 16 Ohm, so that's what the attenuator needs to show to the amp, within a small margin
> 
> If you just use the basic 16 Ohm M2, into a 16 Ohm amp and a 4 Ohm cab, its actually safe, The amp sees somewhere between 14 and 18 Ohms in all settings. What diverts a bit is the tone, which will become more prominent in the high presence treble and bass resonance. This may be OK, or desirable, or not so. The difference is very much the same as changing from an amp with negative feedback, to one with no NFB. It comes from a 4 Ohm speaker expecting to be fed from an output with 1/4 of the impedance of that of a similar 16 ohm speaker.
> 
> Generally we have discussed this difference across a 1 step change, such as 16 to 8 or 8 to 4. At this change, the tone doesn't change much and is usually acceptable. But it can be adjusted for. From 16 to 4, there will be a bigger difference.
> 
> Id suggest to keep everything the same as the basic 16 Ohm version, but add a switch with a resistor across the speaker output. So, one end of the resistor goes to the 'cold' output the other to the switch, and the switch to the 'hot' output. Id suggest a value of 15 Ohm. For a 50W amp, it can be like the other small resistors, ie 25W rating, or even 10W. This will lower the apparent output impedance of the attenuator, correcting the speaker tone, but the amp will hardly know.
> 
> You could hard-wire it instead, but if you have it switched, you can try the different tones and see which sounds best..
> 
> One thing to note. When connected this way with the 16 to 4 conversion , all the attenuation levels are increased, by about -5db. So the Stage 1 value changes from -7 db to -12 db. Id expect that you'll still have plenty of volume available to record an Iso cab though.



Perfect, ...thank you John.


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## RedHouse59

Just to confirm, ...like this?:
(the added resistor and switch in GRAY color)


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## JohnH

hi @RedHouse59 , yes that looks fine.


----------



## Gene Ballzz

@JohnH & All,

I'm looking for some opinions on some standardization for packaging/layout, as I'm getting ready to build a few of these attenuators. I'm using the Hammond 1590DE boxes at a little less than 8" X a little less than 5". I'm curious as to what most folks see as most convenient for the switches and jacks. Are they better along the short sides or the long sides, as per the layout drawings I provided in post #2,207 on page #111? Obviously the switches and jacks will be opposite each other. They will fit well either way. I'm kinda leaning in one direction, but I'll wait to share that, until after I get a few opinions.

TIA,
Gene


----------



## Gert-Jan van der Heiden

@Gene Ballzz The box is nice and spacious. I think the switches look best on the long side. I also prefer to align the switches to the right, looks better somehow. But that is taste of course. The advantage of this orientation is that the attenuator may fit better on your amp (or not). Just put the box on your amps first.

I don’t like the little solder logs on the switches on the post. I used 1,5mm2 wire, maybe a little bit to much… But anyway, you may want to use thick wire. I=sqrt(P/R). For a 50w / 16ohm version that’s about 1.8A. After stage 1 the amperage through the wires has dropped.

I didn’t use those isolated shrinking thingies. In my country there called shrinking socks (if I translate literally). The reason is that it is easier to service. But at the switches I used these connectors, to prevent shorts, as they are close to the resistors. Also easier to take apart.

The coil is flat. I placed it up right, because of space, but also interference with the amp. But with the metal box that idea might be a little over the top. (Faraday).

I also drawn the schematics on a paper with the values and resistor refs. I numbered the “nets” for easy reference. I then made a wiring diagram and added those net list names to the diagram. Made it easier for me.

I’ve drawn a wiring diagram for the one you pointed out


I drawn it to get an idea of the wring. The pinkish coloured wires go to the switches. Looks pretty tidy this layout of conponents.

Hope this is to some use for you.


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## JohnH

I also favour putting switches and jacks on the long faces. A more spacious panel layout. Also, the unit is then narrower front to back, so they sit on my combos better on the rear side of the top handles without resting on the handle.


----------



## junk notes

Thanks to your one-man-marketing-machine @Gene Ballzz I am intrigued to build your attenuator design.
As everyone else, I do like having the option of two channels as on some reamping units.
I recently made the decision to cut off the straight ends off one end of the speaker cable and solder on 90s. I was temporarily using 90 adapters for a trial run.
Less gravity pulling down the back of my attenuators.


----------



## Browneyesound

I was thinking of making one that has switches and switched output jacks at each attenuation stage so a cable and box with a remote foot switch can be plugged into any of the attenuation stages to get a remote volume attenuation at the desired level. Is this doable?

The other question I had was there was a post a while back talking about the need to have the input (?) jack grounded to prevent oscillation from occurring when touching the unit. Is this correct? And all the other jacks should be not connected to the chassis?


----------



## JohnH

hi @Browneyesound
IMO the best way to add a footswitch is via one TRS jack. You make a three cored cable out of 5A main flex.









M2 Footswitch 210923







www.marshallforum.com





That one has a single value of attenuation in the foot unit. A couple of guys here found that -3.5 db is a good value for that. But another switch or two could be added at the foot unit to change its value.

On the jack grounding, for basic attenuator use, I think all jacks should be Cliff type fully insulated. So the case is not connected to anything. If there's a line-out, then its possible thst the case could then be grounded to the line out ground, not definite, it'd depend on the overall rig set up.


----------



## Gene Ballzz

Gert-Jan van der Heiden said:


> @Gene Ballzz The box is nice and spacious. I think the switches look best on the long side. I also prefer to align the switches to the right, looks better somehow. But that is taste of course. The advantage of this orientation is that the attenuator may fit better on your amp (or not). Just put the box on your amps first.
> 
> I don’t like the little solder logs on the switches on the post. I used 1,5mm2 wire, maybe a little bit to much… But anyway, you may want to use thick wire. I=sqrt(P/R). For a 50w / 16ohm version that’s about 1.8A. After stage 1 the amperage through the wires has dropped.
> 
> I didn’t use those isolated shrinking thingies. In my country there called shrinking socks (if I translate literally). The reason is that it is easier to service. But at the switches I used these connectors, to prevent shorts, as they are close to the resistors. Also easier to take apart.
> 
> The coil is flat. I placed it up right, because of space, but also interference with the amp. But with the metal box that idea might be a little over the top. (Faraday).
> 
> I also drawn the schematics on a paper with the values and resistor refs. I numbered the “nets” for easy reference. I then made a wiring diagram and added those net list names to the diagram. Made it easier for me.
> 
> I’ve drawn a wiring diagram for the one you pointed out
> View attachment 110780
> 
> I drawn it to get an idea of the wring. The pinkish coloured wires go to the switches. Looks pretty tidy this layout of conponents.
> 
> Hope this is to some use for you.



Be very careful and attentive when using* "right angle"* plugs, especially for speaker cables. The Switchcraft 226 series has the sleeve/barrel only "pressed" into what appears to be a die-cast plug body and can become loose, causing intermittent connections! Many other brands are constructed in a similar manner.




I tracked down this malady/anomaly on a few of my guitar/instrument cables after chasing my tail in suspicion of the jacks in my guitars! Annoying for an instrument cable, but *VERY BAD* for speaker connections!

Upon contacting Switchcraft, they admitted to being aware of the problem and sent me a pile of twenty *"New & Improved"* plugs that they claimed had corrected the issue! I was still able to "loosen" them, through not very many repeated wiggles and/or flexes!

I'm not saying that we shouldn't use this style of plug, but we should be aware of the possible issues and keep a close eye on them, as they age! I won't even get started on the pitfalls and shortcomings of Switchcraft style "open" jacks and their points of possible connection failure!

Thanks also for the comments on attenuator jack and switch placement.

Just A PSA,
Gene


----------



## Gene Ballzz

JohnH said:


> hi @Browneyesound
> IMO the best way to add a footswitch is via one TRS jack. You make a three cored cable out of 5A main flex.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Footswitch 210923
> 
> 
> 
> 
> 
> 
> 
> www.marshallforum.com
> 
> 
> 
> 
> 
> That one has a single value of attenuation in the foot unit. A couple of guys here found that -3.5 db is a good value for that. But another switch or two could be added at the foot unit to change its value.
> 
> On the jack grounding, for basic attenuator use, I think all jacks should be Cliff type fully insulated. So the case is not connected to anything. If there's a line-out, then its possible thst the case could then be grounded to the line out ground, not definite, it'd depend on the overall rig set up.



And @JohnH ,
I too have noticed the anomaly of getting a squeal when passing my hand (or guitar) near my attenuators or even touching them with my hand. While earth grounding may* "cure" *that specific problem, I'm not savvy enough as an electrical engineer to know or understand any other issues that approach may introduce? I can only guess and/or surmise at what forces may be in play here! I am however, somewhat aware of the premise that a *"Faraday Cage" *only functions as such when it is properly grounded? It seems that when NOT grounded, it simply acts somewhat as an antenna? There certainly are fair amounts of voltage, current and magnetic fields contained within the boxes!

On the other hand, addressing this issue may be somewhat "picking nits" and a bit similar to telling a doctor that my head hurts after banging it repeatedly into a wall, and the doctor's response nd solution is to simply *"Avoid banging my head into a wall!"*

Thanks, As Always,
Gene


----------



## RedHouse59

aceofbones said:


> Finally finished my build, mostly. I do have to redo my input and outputs, totally forgot about the switched jacks so may have screwed that up. Anyways, I haven't tested it yet but this is where I'm at. Thanks John!
> View attachment 95258
> View attachment 95259
> View attachment 95260



Where did you find a nice box like this?


----------



## Gene Ballzz

@aceofbones provided the link for multiple sizes and options, in the "Completed JohnH Attenuators?" thread:









11.42US $ |PC Box Custom Extrusion Housing Sheet Metal Bending Electronics Aluminum Case P01 133.4*55mm|Wire Junction Boxes| - AliExpress


Smarter Shopping, Better Living! Aliexpress.com




www.aliexpress.com





You'd still need to drill the top for ventilation and the "feet" appear to be metal? They sure do look nice though!

Happy Buildin'
Gene


----------



## JohnH

Gene Ballzz said:


> And @JohnH ,
> I too have noticed the anomaly of getting a squeal when passing my hand (or guitar) near my attenuators or even touching them with my hand. While earth grounding may* "cure" *that specific problem, I'm not savvy enough as an electrical engineer to know or understand any other issues that approach may introduce? I can only guess and/or surmise at what forces may be in play here! I am however, somewhat aware of the premise that a *"Faraday Cage" *only functions as such when it is properly grounded? It seems that when NOT grounded, it simply acts somewhat as an antenna? There certainly are fair amounts of voltage, current and magnetic fields contained within the boxes!
> 
> On the other hand, addressing this issue may be somewhat "picking nits" and a bit similar to telling a doctor that my head hurts after banging it repeatedly into a wall, and the doctor's response nd solution is to simply *"Avoid banging my head into a wall!"*
> 
> Thanks, As Always,
> Gene



Hi Gene, thats a curious one and I hadn't come across it before relative to the attenuators. I just did some tests and I couldn't reproduce it. I was running at max power and gain, touching the strings or not, and touching, or just being close to the attenuator. What sort of setup and settings do you run when it happens?

Im not across all the science of screening etc. But, Ive screened some guitars, and it helps cut out 'spiky' RF buzz from fluoro lights etc, if the screen is grounded, but not the 50 or 60hz hum. Ive read that to suppress audio frequencies by screening needs a 'battleship' thickness of grounded metal!. Also, Ive been working with the notion that the 'cold' lead that comes out of the amp for the speaker, may or may not be a ground, and I assume its probably not. What's your experience of that based on circuits that you've worked with?

The whole output of the amp, through unshielded speaker leads is unscreened anyway. But the part in the attenuator that may create the most field would be the coil., as would the speaker. On mine, its down at the base of the attenuator.


----------



## Gene Ballzz

JohnH said:


> Hi Gene, thats a curious one and I hadn't come across it before relative to the attenuators. I just did some tests and I couldn't reproduce it. I was running at max power and gain, touching the strings or not, and touching, or just being close to the attenuator. What sort of setup and settings do you run when it happens?
> 
> Im not across all the science of screening etc. But, Ive screened some guitars, and it helps cut out 'spiky' RF buzz from fluoro lights etc, if the screen is grounded, but not the 50 or 60hz hum. Ive read that to suppress audio frequencies by screening needs a 'battleship' thickness of grounded metal!. Also, Ive been working with the notion that the 'cold' lead that comes out of the amp for the speaker, may or may not be a ground, and I assume its probably not. What's your experience of that based on circuits that you've worked with?
> 
> The whole output of the amp, through unshielded speaker leads is unscreened anyway. But the part in the attenuator that may create the most field would be the coil., as would the speaker. On mine, its down at the base of the attenuator.



All the ones I've built for myself so far, have been M-Lite models, with both coils mounted on the top. My current build is an 8Ω M2. I'll let you know if it exhibits the same anomaly.

As for screening, I know that shielding guitar cavities does absolutely nothing until it is grounded. Additionally, the shielding plate or screen covering the open part, component side of an amplifier chassis doesn't do it's job unless making proper contact with the actual amp chassis. While I don't know much of the technical details, I do know the above through experience and experimentation. I do know that some amps have both the +positive & -negative speaker output leads sort of "floating" and mostly isolated from the rest of the circuit while other amps connect the -negative lead directly to amp chassis ground. In either case, I'm not sure that I would want the speaker output -negative doing double duty for providing a shield ground! I'm fairly certain though, that providing a chassis ground lug on the amp and a similar lug on the attenuator housing (given that the attenuator circuit electrically "floats" inside the the housing) and connecting them with croc clips and wire would not cause any issue and may fix the mentioned issue. Or not! I'll do some tests and let you know!
Thanks Again,
Gene


----------



## RedHouse59

JohnH said:


> ... Also, Ive been working with the notion that the 'cold' lead that comes out of the amp for the speaker, may or may not be a ground, and I assume its probably not....



Most of the "_usual suspects_" of old style tube amps (Fender, Marshall, etc) the OT's secondary side "common" lead/terminal is grounded, to both circuit and chassis ground.

Attaching a couple of schematics for refence.


----------



## narnodel

Hi JohnH, I'm new on this forum, but I wanted to take some time to thank you for sharing your attenuators plans.

I have just built a 4ohms version for my Fender Twin of your M2 attenuator, I have just divided the values by two.
I have only build the first -7dB reactive stage, since I wanted high quality components (arcol aluminium resistors and Jantzen air coil), the complete M2 would have been way too expensive !

The sound is absolutely perfect ! I have already bought the Torpedo Captor 4ohms, but I wasn't satisfied with the crushed sound coming with the fixed -20dB.
Now I can play on my twin at 4-5 without having my ears bleeding thanks to the -7dB, and I re-discover my real amp sound with only 3 resistors and a coil !

Thank you so much for being such a passionate and generous electronics-musician.

Greetings from France,

Arnaud


----------



## JohnH

Hi @narnodel , thankyou for those kind comments and I'm very happy that it's working well for you.


----------



## JohnH

RedHouse59 said:


> Most of the "_usual suspects_" of old style tube amps (Fender, Marshall, etc) the OT's secondary side "common" lead/terminal is grounded, to both circuit and chassis ground.
> 
> Attaching a couple of schematics for refence.



Thanks for posting those. maybe 'one lead grounded' is the more common case for the speaker outputs? I looked at a couple of newer designs too.


----------



## Gert-Jan van der Heiden

This might be a safety feature. On the primary side of the output transformer, there's deadly voltage. If for some reason (fault) this arcs through, at least it has a path to divert.

I don't see any audio reason to do this. If the leads pick up some electro magnetic garbage, it won't be enough to move the speakers. Speaker cables aren't shielded anyway.


----------



## Gert-Jan van der Heiden

Gene Ballzz said:


> All the ones I've built for myself so far, have been M-Lite models, with both coils mounted on the top. My current build is an 8Ω M2. I'll let you know if it exhibits the same anomaly.
> 
> As for screening, I know that shielding guitar cavities does absolutely nothing until it is grounded. Additionally, the shielding plate or screen covering the open part, component side of an amplifier chassis doesn't do it's job unless making proper contact with the actual amp chassis. While I don't know much of the technical details, I do know the above through experience and experimentation. I do know that some amps have both the +positive & -negative speaker output leads sort of "floating" and mostly isolated from the rest of the circuit while other amps connect the -negative lead directly to amp chassis ground. In either case, I'm not sure that I would want the speaker output -negative doing double duty for providing a shield ground! I'm fairly certain though, that providing a chassis ground lug on the amp and a similar lug on the attenuator housing (given that the attenuator circuit electrically "floats" inside the the housing) and connecting them with croc clips and wire would not cause any issue and may fix the mentioned issue. Or not! I'll do some tests and let you know!
> Thanks Again,
> Gene


If you try to understand the cage of Faraday and grounding etc, you enter a whole world in electrical engineering and physics. But in general, tying the shielding to ground makes it way more effective. (No potential difference between the shield and signal path).


----------



## Bearzooka

Hi, I've started gathering parts for the M2, 16 Ohms. I've got a couple questions:
1. Is there a specific type of wire I need for connecting the resistors and switches?
2. I'd like to hardwire the attenuator to my amp, no jacks. I am guessing I need to use speaker wire to do this, correct?
3. What kind of switch should I use for a full bypass switch? (I know DPDT, but what ratings? current, voltage, etc.)
4. Anyone know where I can find some big chunky switches with solder lugs for the -7db, -14db, -3.5db switches?
Thanks!


----------



## Gene Ballzz

Bearzooka said:


> Hi, I've started gathering parts for the M2, 16 Ohms. I've got a couple questions:
> 1. Is there a specific type of wire I need for connecting the resistors and switches?
> 2. I'd like to hardwire the attenuator to my amp, no jacks. I am guessing I need to use speaker wire to do this, correct?
> 3. What kind of switch should I use for a full bypass switch? (I know DPDT, but what ratings? current, voltage, etc.)
> 4. Anyone know where I can find some big chunky switches with solder lugs for the -7db, -14db, -3.5db switches?
> Thanks!



I'm not sure I understand what you mean and/or the purpose of endeavoring *"to hardwire the attenuator to my amp!"* What kind of amp are we talking about? Are you planning to* "mount"* the attenuator inside the actual amp chassis?

These switches will do well for all purposes! Can be obtained in short or long bat! For -3.5db, -7db & -14db use, simply tie both sides of the switch together.



https://www.tubesandmore.com/products/switch-carling-toggle-dpdt-short-bat



Just Askin'?
Gene


----------



## JohnH

hi @Bearzooka , welcome to our thread.

The Carling switches Gene pointed to are ideal for a 50 W amp at 8 or 16 Ohms (and at 16 ohms, they'd do 100W too). In general, switches usually come with current ratings for 250V or 125V, and since our voltages are lower, we can use the 125V specs. The Carling notes 125V and 6A. The current we need is:

Current = (Power / R) ^ 0.5 , and then double that for the switch spec.

Wire can be 18 awg. This can be insulated stranded wire, or solid (if wires will not need to flex), or bare solid for short runs between parts.


----------



## Gene Ballzz

JohnH said:


> hi @Bearzooka , welcome to our thread.
> 
> The Carling switches Gene pointed to are ideal for a 50 W amp at 8 or 16 Ohms (and at 16 ohms, they'd do 100W too). In general, switches usually come with current ratings for 250V or 125V, and since our voltages are lower, we can use the 125V specs. The Carling notes 125V and 6A. The current we need is:
> 
> Current = (Power / R) ^ 0.5 , and then double that for the switch spec.
> 
> Wire can be 18 awg. This can be insulated stranded wire, or solid (if wires will not need to flex), or bare solid for short runs between parts.



And yeah, the lugs are big enough to accept at least two #18 awg wires!
Simply Attenuatin'
Gene


----------



## Gene Ballzz

Hey All,
Some questions about fan cooling these attenuator units. Some folks have posted about using the energy dissipated in these units to power a cooling fan. I have concerns about the constant and repeated on/off/on cycling of said fans! I'd like to take this concept a step further, if not too complicated for some of the electrical engineers here abouts! How about, instead of powering the fan directly from the dissipated energy, we use that energy to charge an onboard battery and use that battery to power the fan? How difficult and/or complicated might this be? Understanding, of course, that the available voltage and current for the charging circuit will be constantly fluctuating?
Just Askin'?
Gene


----------



## JohnH

It's a good question about the fans. There are several commercial designs that do this, eg Marshall Powerbrake. Typically the DC fan is fed from a bridge rectifier, smoothed by a capacitor. Then all that is fed via a resistor from the hot input. There may be a zener diode to to limit max volts to tbe fan.

I think the only way to design this is with a specific fan component to test with. It should be one that runs on as low a current as possible. This will allow the right resistor to he found that will get the fan starting to turn at say 10 or 20W input, and also judge the loading effect of the fan circuit on tone response.

The fan is really always on, but it starts to turn, and then run faster, as more power comes in. But setting it right for the specific fan and power input is the trick.


----------



## Gene Ballzz

JohnH said:


> It's a good question about the fans. There are several commercial designs that do this, eg Marshall Powerbrake. Typically the DC fan is fed from a bridge rectifier, smoothed by a capacitor. Then all that is fed via a resistor from the hot input. There may be a zener diode to to limit max volts to tbe fan.
> 
> I think the only way to design this is with a specific fan component to test with. It should be one that runs on as low a current as possible. This will allow the right resistor to he found that will get the fan starting to turn at say 10 or 20W input, and also judge the loading effect of the fan circuit on tone response.
> 
> The fan is really always on, but it starts to turn, and then run faster, as more power comes in. But setting it right for the specific fan and power input is the trick.



Thanks @JohnH for that. I was trying to pick others' brains also, as alluded to in my recent email to you. Wouldn't a battery, recharged by the amp, provide for more consistent and reliable operation of the fan? The fan could even be switched on/off by a temperature controlled switch? I know nothing about batteries and chargers, other than what the manufactureers and retailers of such products tell me! Just spitballin' on plans for a 100 watt build!

It'll be nice to "ZOOM" with you, when we can, but I've got family comin' in this weekend, so….. maybe next week?

Thanks Again John,
Gene


----------



## tometome

Hello! This is my first post after stumbling across this incredible thread!... such amazing collaborative work! I've just built my first tube amp and was looking for an attenuator project and all the online recommendations strongly suggest the M2.. my first hurdle is trying to find the inductor! im in Melbourne/australia and would appreciate any leads anyone might have..? local or online etc.. Cheers, Tom


----------



## JohnH

hi @tometome welcome to our thread. I'm in Sydney, so I watch out for suppliers here in Oz . The place I have in mind for a coil for my next one would be wagneronline, here in Sydney They do a range of air-cored coils on plastic bobbins






INDUCTORS 1mm (18AWG) AIR-CORE 200Wrms


High power up to 200 Watts RMS, quality, pure copper, solid core wire with insulation coating. Wound on a plastic bobbin with an empty core for better acoustic properties. Solder tipped ends. DAI-CHI PURE COPPER




www.wagneronline.com.au


----------



## Gene Ballzz

tometome said:


> Hello! This is my first post after stumbling across this incredible thread!... such amazing collaborative work! I've just built my first tube amp and was looking for an attenuator project and all the online recommendations strongly suggest the M2.. my first hurdle is trying to find the inductor! im in Melbourne/australia and would appreciate any leads anyone might have..? local or online etc.. Cheers, Tom



Tom, first,  to the forum and this thread!

Then, I swear to you that once you get this unit built, it will be the most used and liberating piece of gear you've ever owned! @JohnH really hit it out of the park with this research and design! My sincerest suggestion is to keep it simple, with probably no more additions than tonal correction/compensation for a different impedance speaker output.

One of the really great things about this design is that keeping it simple also keeps it inexpensive and easy to build! That allows you to simply build another with slightly different features and or base impedance! Then you have two!  To me, it seems more useful to have a couple/few units, as opposed to one big honkin' *"Swiss Army Knife"* unit! Ther are multiple benefits to this approach.

Enjoy,
Gene


----------



## Eric2001

Hello , 

I'm new here and I want to thank you all and especially JohnH for this great work. I'm amazed by the lenght of this thread . 

I've finished the 8 Ohm version with a line out. It works realy great with my 18 Watt full tube Amp . I can now enjoy a fully cranked amp !! 

I'm sharing some pictures of my version.
Please note that the resistors and the coil are mounted on the upper side of the box ( I reused the thermal paste that I use on my computer builds). 

The budget of this build is around 85 € (in France) . 

I'm sharing also the LTSpice model of the build using Ducan's amps model of the Celestion G12T-75 (my speaker) . FYI, I've (re)done the AC Analysis and I (re)found the same charts (db output) people already shared .

Thanks again for everything 

Keep up the good work !

Eric


----------



## paul-e-mann

Gene Ballzz said:


> Tom, first,  to the forum and this thread!
> 
> Then, I swear to you that once you get this unit built, it will be the most used and liberating piece of gear you've ever owned! @JohnH really hit it out of the park with this research and design! My sincerest suggestion is to keep it simple, with probably no more additions than tonal correction/compensation for a different impedance speaker output.
> 
> One of the really great things about this design is that keeping it simple also keeps it inexpensive and easy to build! That allows you to simply build another with slightly different features and or base impedance! Then you have two!  To me, it seems more useful to have a couple/few units, as opposed to one big honkin' *"Swiss Army Knife"* unit! Ther are multiple benefits to this approach.
> 
> Enjoy,
> Gene


Gene has anybody built one that can switch between 8 and 16 ohms?

Also if I built 2 and went through an AB pedal for 2 different volumes, will this work?


----------



## Gene Ballzz

Eric2001 said:


> Hello ,
> 
> I'm new here and I want to thank you all and especially JohnH for this great work. I'm amazed by the lenght of this thread .
> 
> I've finished the 8 Ohm version with a line out. It works realy great with my 18 Watt full tube Amp . I can now enjoy a fully cranked amp !!
> 
> I'm sharing some pictures of my version.
> Please note that the resistors and the coil are mounted on the upper side of the box ( I reused the thermal paste that I use on my computer builds).
> 
> The budget of this build is around 85 € (in France) .
> 
> I'm sharing also the LTSpice model of the build using Ducan's amps model of the Celestion G12T-75 (my speaker) . FYI, I've (re)done the AC Analysis and I (re)found the same charts (db output) people already shared .
> 
> Thanks again for everything
> 
> Keep up the good work !
> 
> Eric



Very nice build! Thanks for sharing. Can you also share what product/process you used for the labeling?
Enjoy,
Gene


----------



## Gene Ballzz

paul-e-mann said:


> Gene has anybody built one that can switch between 8 and 16 ohms?
> 
> Also if I built 2 and went through an AB pedal for 2 different volumes, will this work?



> When you ask * "switch between 8 and 16 ohms?" *are you referring to input or speaker output impedance?

> Hot switching from one unit to another would not be recommended, for several reasons! There is, however, a diagram for having a foot switchable switchable stage (in whatever db you choose) somewhere in this thread. It requires a three wire cable, with TRS connectors. I'll see if I can get the time to find it. It should be a little bit before page #111.

Happily Attenuatin'
Gene


----------



## Eric2001

Gene Ballzz said:


> Very nice build! Thanks for sharing. Can you also share what product/process you used for the labeling?
> Enjoy,
> Gene


I have made severals pedals before and for now my process is :
1) Design the layout with Inkscape , free software (for the attenuator, it was the simplest have done) 
2) Print on normal paper to check alignement ( holes/frame)
3) Print on transparent stickers ( inkjet printing) 
4) Let dry the stickers ( 12 to 24 hours, but careful , it's never "really" dry) 
5) Stick the sticker avoiding bubbles and avoiding spreading ink with your fingers : most delicate part 
6) Several thin layers of clearcoat ( 3, 4 .. ) 
7) Let dry 24 hours

Some people apply some clearcoat before and after step 5. I've tried, but I was not that happy with the result.


----------



## paul-e-mann

Gene Ballzz said:


> > When you ask * "switch between 8 and 16 ohms?" *are you referring to input or speaker output impedance?
> 
> > Hot switching from one unit to another would not be recommended, for several reasons! There is, however, a diagram for having a foot switchable switchable stage (in whatever db you choose) somewhere in this thread. It requires a three wire cable, with TRS connectors. I'll see if I can get the time to find it. It should be a little bit before page #111.
> 
> Happily Attenuatin'
> Gene


Thanks Gene. I have 8 ohm 2x12's and 16 ohm 1x12's. Whichever cab I use I switch the amp to match, therefore I want to be able to switch the attenuator to match.

I want to be able to switch between attenuation volumes for lead volume boost.


----------



## tometome

Thanks for that John! I’ve had a look and the closest inductor options are .82mh or 1.0mh… which would be the better option. 

A couple of other questions.. I’m assuming the switches are SPST on-on? Should I be on the look out for specific voltage ratings? 

And finally, do you have a good recommendation for sourcing everything, except the inductors , from the one supplier? (Reduce postage costs etc..). 

Thanks again,
Tom


----------



## tometome

Gene Ballzz said:


> Tom, first,  to the forum and this thread!
> 
> Then, I swear to you that once you get this unit built, it will be the most used and liberating piece of gear you've ever owned! @JohnH really hit it out of the park with this research and design! My sincerest suggestion is to keep it simple, with probably no more additions than tonal correction/compensation for a different impedance speaker output.
> 
> One of the really great things about this design is that keeping it simple also keeps it inexpensive and easy to build! That allows you to simply build another with slightly different features and or base impedance! Then you have two!  To me, it seems more useful to have a couple/few units, as opposed to one big honkin' *"Swiss Army Knife"* unit! Ther are multiple benefits to this approach.
> 
> Enjoy,
> Gene


Hey Gene, thanks for the warm welcome! Yes, that’s exactly what I intend to do.. hopefully I can get the parts order in and get started!
Cheers
Tom


----------



## Gene Ballzz

Switches need to be SPDT or many folks use DPDT two position, ON/ON and tie the two sides together for more capacity and redundancy. I really prefer the CARLING stuff, but am not sure of availability in your locale. I prefer this larger model from CARLING, as the lugs are just big enough to accept two #18 gauge wires each, available in short bat or long bat:



https://www.tubesandmore.com/products/switch-carling-toggle-dpdt-short-bat



OR THIS IN SPDT:



https://www.tubesandmore.com/products/switch-carling-toggle-spdt-2-position-fender



AND THIS FOR YOUR FOOT SWITCH:



https://www.tubesandmore.com/products/footswitch-carling-dpdt-side-solder-lugs




Watch for a PM/Conversation from me!

Just Helpin'
Gene


----------



## JohnH

tometome said:


> Thanks for that John! I’ve had a look and the closest inductor options are .82mh or 1.0mh… which would be the better option.
> 
> A couple of other questions.. I’m assuming the switches are SPST on-on? Should I be on the look out for specific voltage ratings?
> 
> And finally, do you have a good recommendation for sourcing everything, except the inductors , from the one supplier? (Reduce postage costs etc..).
> 
> Thanks again,
> Tom


If you have a Jaycar nearby, you can buy alot of what you need there, including case, feet, screws (10mm M3 works with the resistors), thermal grease, wire, jacks, solder and switches. I don't actually love the switches they sell, except for the flat-toggle mini switches. Their jacks are fine , get the plastic stereo jacks for all, which grip the plug nicely. They dont do the resistors though. 

For budget control (but no time control) you can order resistors from eBay in china, as I've done. Use the values at the bottom of the M2 table since you generally cant get all the standard values. But if you want branded ones, you could go to one of the big online suppliers and get jacks and switches too (eg Digikey or Mouser), and Jaycar for the rest.


----------



## tometome

JohnH said:


> If you have a Jaycar nearby, you can buy alot of what you need there, including case, feet, screws (10mm M3 works with the resistors), thermal grease, wire, jacks, solder and switches. I don't actually love the switches they sell, except for the flat-toggle mini switches. Their jacks are fine , get the plastic stereo jacks for all, which grip the plug nicely. They dont do the resistors though.
> 
> For budget control (but no time control) you can order resistors from eBay in china, as I've done. Use the values at the bottom of the M2 table since you generally cant get all the standard values. But if you want branded ones, you could go to one of the big online suppliers and get jacks and switches too (eg Digikey or Mouser), and Jaycar for the rest.


Thank you John and Gene for your really helpful replies!!

Just wanting some guidance on the inductor. The link you provided John lists either the .82mh and the 1mh.. which would be better suited?? Cheers!


----------



## JohnH

hi @tometome , sorry I forgot about that question. I suggest to get the 1.0mH inductor.


----------



## tometome

JohnH said:


> hi @tometome , sorry I forgot about that question. I suggest to get the 1.0mH inductor.


Thanks John.. all resistors and inductor ordered.. the rest bill grab locally from jaycar!


----------



## Gene Ballzz

tometome said:


> Thanks John.. all resistors and inductor ordered.. the rest bill grab locally from jaycar!



The source I use didn't have a .9mH, but offered to *"unwind"* a 1.0mH to my spec for $3. It's worth asking. 
Just Coilin'
Gene


----------



## xiaozi0624

JohnH said:


> I ran some analysis and the impedance options look like they'll work well and consistently. I think the best values will be, for an 8-4-2 input range as follows:
> 
> A 4Ohm resistor which should be sized with a rating of 1.5x the amp power (it takes half the power, then give it a x3 margin), in series with....
> 
> ....An 0.25mH aircore inductor, 18 Gage or thicker, bypassed by a 10 Ohm resistor., probably 25W rating for a 50W amp.
> 
> But I'll draw it.


hi I want to know if I use a Jensen p12q 8ohm speaker with my amp, what the value has to change with m2 for one coil version to get the right curve?


----------



## JohnH

Hi @tometome , 

You order sounds good. 

Actually, I plan to do some more testing soon, so Ive just ordered a 1mH coil from Wagneronline. I got the 18ga one wound on a plastic bobbin. I have a meter that can measure inductance, so I will unwind it to 0.9mH (not that it'll make much difference really - its just a bit closer to optimum) . Then, Ill report on exactly what length of wire needs to be removed and also compare to results from online inductance calculators. This info will let you match what I did if you want to, and also help advise for others too. Its quite ok to wire up with the full 1mH though, this is just a very small difference. Ill be away next week so it'll be a couple of weeks.

Also, since I paid shipping, I got caps and coil so I can rig up and test the resonant circuit that converts M2 to M3 - just in the interests of science and curiosity. 


hi @xiaozi0624 

I looked at the P12Q specs and they are very close to the Celestion data that I used. Plus, on this attenuator, the electrical speaker response is generated mainly by the speaker itself, so I suggest to keep the M2 design for 8 Ohms as it is - there are no values that need changing.


----------



## xiaozi0624

JohnH said:


> Hi @tometome ,
> 
> You order sounds good.
> 
> Actually, I plan to do some more testing soon, so Ive just ordered a 1mH coil from Wagneronline. I got the 18ga one wound on a plastic bobbin. I have a meter that can measure inductance, so I will unwind it to 0.9mH (not that it'll make much difference really - its just a bit closer to optimum) . Then, Ill report on exactly what length of wire needs to be removed and also compare to results from online inductance calculators. This info will let you match what I did if you want to, and also help advise for others too. Its quite ok to wire up with the full 1mH though, this is just a very small difference. Ill be away next week so it'll be a couple of weeks.
> 
> Also, since I paid shipping, I got caps and coil so I can rig up and test the resonant circuit that converts M2 to M3 - just in the interests of science and curiosity.
> 
> 
> hi @xiaozi0624
> 
> I looked at the P12Q specs and they are very close to the Celestion data that I used. Plus, on this attenuator, the electrical speaker response is generated mainly by the speaker itself, so I suggest to keep the M2 design for 8 Ohms as it is - there are no values that need changing.


thank you
Another question is if I want to do a m3 version with bass boost,I can’t get a bp200uf cap,can I use 5 mundorf bp 47uf 70vdc/50vac caps in parallel?


----------



## tometome

JohnH said:


> Hi @tometome ,
> 
> You order sounds good.
> 
> Actually, I plan to do some more testing soon, so Ive just ordered a 1mH coil from Wagneronline. I got the 18ga one wound on a plastic bobbin. I have a meter that can measure inductance, so I will unwind it to 0.9mH (not that it'll make much difference really - its just a bit closer to optimum) . Then, Ill report on exactly what length of wire needs to be removed and also compare to results from online inductance calculators. This info will let you match what I did if you want to, and also help advise for others too. Its quite ok to wire up with the full 1mH though, this is just a very small difference. Ill be away next week so it'll be a couple of weeks.
> 
> Also, since I paid shipping, I got caps and coil so I can rig up and test the resonant circuit that converts M2 to M3 - just in the interests of science and curiosity.
> 
> 
> hi @xiaozi0624
> 
> I looked at the P12Q specs and they are very close to the Celestion data that I used. Plus, on this attenuator, the electrical speaker response is generated mainly by the speaker itself, so I suggest to keep the M2 design for 8 Ohms as it is - there are no values that need changing.



Hey @JohnH .. that sounds great! The inductor arrived today. I bought the same one you described. My resistors are due early next week so I’ll get to work once I’ve got everything.. I can always pull out the inductor anyway if I manage to build the attenuator before you get a chance to run your tests.


----------



## JohnH

hi @xiaozi0624 

Those caps look to be good units, with reasonably low dissipation, but I'm not sure if they are intended for this high-current use. They might be fine, but I'm just not sure. Its an area outside what I know. As I noted in my message above, I've just ordered some caps. The ones I'm getting are intended for cross-over networks (similar power levels to us), and are 100V rated. I'm only getting them for testing though. I want to see how they heat up, and rig up an M3 to test what it does.

But I'd ask this: If you build M3, what will you use it for? What we know from builds tested so far is that for just attenuating a guitar speaker, M2 works perfectly fine and everyone likes it. It doesn't have the resonant circuit because it doesn't need it, unlike other designs. Where it may be of benefit is if you want to use it as a load box, to output to an IR unit or cab sim for recording or PA. There, it may be useful but I think there are other ways to control bass too. That's the sort of thing I plan to test.

In any case, if you build with plenty of space, you can do M2 and then add the bass circuit later, there's no change to other components.


----------



## JohnH

paul-e-mann said:


> Thanks Gene. I have 8 ohm 2x12's and 16 ohm 1x12's. Whichever cab I use I switch the amp to match, therefore I want to be able to switch the attenuator to match.
> 
> I want to be able to switch between attenuation volumes for lead volume boost.


Just wanted to note that if you build an M2 at either 8 or 16 Ohms and use that corresponding amp tap, you can still use either an 8 or 16 ohm cab. The amp sees what it expects with no missmatch. The only issue is if you need just a little attenuation, then you lose 2 or 3 more dB's in the conversion so your loudest attenuated level is a step lower.


----------



## xiaozi0624

JohnH said:


> hi @xiaozi0624
> 
> Those caps look to be good units, with reasonably low dissipation, but I'm not sure if they are intended for this high-current use. They might be fine, but I'm just not sure. Its an area outside what I know. As I noted in my message above, I've just ordered some caps. The ones I'm getting are intended for cross-over networks (similar power levels to us), and are 100V rated. I'm only getting them for testing though. I want to see how they heat up, and rig up an M3 to test what it does.
> 
> But I'd ask this: If you build M3, what will you use it for? What we know from builds tested so far is that for just attenuating a guitar speaker, M2 works perfectly fine and everyone likes it. It doesn't have the resonant circuit because it doesn't need it, unlike other designs. Where it may be of benefit is if you want to use it as a load box, to output to an IR unit or cab sim for recording or PA. There, it may be useful but I think there are other ways to control bass too. That's the sort of thing I plan to test.
> 
> In any case, if you build with plenty of space, you can do M2 and then add the bass circuit later, there's no change to other components.


hi
i want to use it for a ir cab into my daw,or i want to use my fender blues deluxe amp as a bass amp,this amp's is very close to tweed bassman sound wise,so i think the bass boost is gonna help it a lot for those,but if i buy those mundrof caps is gonna cost me a lot,and i do not know how good the tone i am gonna get,so i might not to do the bass boost one,and another question is can i turn this box into max pad state（-31db）and use the load box's speaker output send it to my line in of my audio interface? since is got 31db max pad,is it safe to do that way?because i dont know how to wire a line out jack to the circuit because my english is not that good,and i do not see a drawing photo with line out for the m2 circuit.

and i think the bass boost might be very helpful for the ir cab,i use the daw to make music and i think all the load box right now in the market,especially for clean tone, do not have a very good tone.
i want to ask is there a better way for ir use to get the more real feel of the bass notes for a low cost?
but anyway, i m not building this yet, but i already bought some parts for this build, and i think i am gonna use some copper heat sink and wood box for the build and try to make it to get a good looking.
so i think im gonna try the m2 lite because it simple to build,if there is a better way for low cost to get the bass boost done i think i am gonna try to add it,i think a lot of people use the load box to the daw with the ir.


----------



## xiaozi0624

xiaozi0624 said:


> hi
> i want to use it for a ir cab into my daw,or i want to use my fender blues deluxe amp as a bass amp,this amp's is very close to tweed bassman sound wise,so i think the bass boost is gonna help it a lot for those,but if i buy those mundrof caps the is gonna cost me a lot,and i do not know how good the tone i am gonna get,so i might not to do the bass boost one,and another question is can i turn this box into max pad state（-31db）and use the load box's speaker output send it to my line in of my audio interface? since is got 31db max pad,is it safe to do that way?because i dont know how to wire a line out jack to the circuit because my english is not that good,and i do not see a drawing photo with line out for the m2 circuit.
> 
> and i think the bass boost might be very helpful for the ir cab,i use the daw to make music and i think all the load box right now in the market,especially for clean tone, do not have a very good tone.
> i want to ask is there a better way for ir use to get the more real feel of the bass notes for a low cost?
> but anyway, i m not building this yet, but i already bought some parts for this build, and i think i am gonna use some copper heat sink and wood box for the build and try to make it to get a good looking.
> so i think im gonna try the m2 lite because it simple to build,if there is a better way for low cost to get the bass boost done i think i am gonna try to add it,i think a lot of people use the load box to the daw with the ir.


----------



## Dee436

JohnH said:


> Just wanted to note that if you build an M2 at either 8 or 16 Ohms and use that corresponding amp tap, you can still use either an 8 or 16 ohm cab. The amp sees what it expects with no missmatch. The only issue is if you need just a little attenuation, then you lose 2 or 3 more dB's in the conversion so your loudest attenuated level is a step lower.


Hi John

I take it from this reply that with the 16 ohm M2 that I have built, if I set my amp at 16 ohms, I could run into a full stack i.e 2 16 ohm 4x12 cabs. I did run a simulation in everycircuit and no matter what is added to the outputs the amp always sees around 16 ohms

darren


----------



## Gene Ballzz

Dee436 said:


> Hi John
> 
> I take it from this reply that with the 16 ohm M2 that I have built, if I set my amp at 16 ohms, I could run into a full stack i.e 2 16 ohm 4x12 cabs. I did run a simulation in everycircuit and no matter what is added to the outputs the amp always sees around 16 ohms
> 
> darren



@Dee436 ,
As you've already surmised, an 8Ω speaker load is totally safe using a 16Ω M2 out of the 16Ω tap of your amp. The only down side is that the 8Ω load will have slightly accentuated high and lows, compared to a portion of the upper mids, or, depending how you want to think of it, a slight loss of those mids. For my frame of reference, that exact portion of the mid range is exactly what I'd rather see left alone, or even slightly exaggerated! @JohnH figured out an easy compensation (post #2786) in the form of a 39Ω resistor in parallel with + & - on the speaker out. He described an elegant way to do it, by placing the resistor across the tip and ring of an extra TRS jack that only activates that resistor when that jack gets plugged out of. A great benefit here is that if you have only two jacks, it leaves one of them stock/standard for 16Ω use and both jacks become 8Ω compensated when using that 8Ω jack, either together with the 16Ω jack or by itself. Simple, easy, peasy!
Hope This Helps?
Gene


----------



## Dee436

Gene Ballzz said:


> @Dee436 ,
> As you've already surmised, an 8Ω speaker load is totally safe using a 16Ω M2 out of the 16Ω tap of your amp. The only down side is that the 8Ω load will have slightly accentuated high and lows, compared to a portion of the upper mids, or, depending how you want to think of it, a slight loss of those mids. For my frame of reference, that exact portion of the mid range is exactly what I'd rather see left alone, or even slightly exaggerated! @JohnH figured out an easy compensation (post #2786) in the form of a 39Ω resistor in parallel with + & - on the speaker out. He described an elegant way to do it, by placing the resistor across the tip and ring of an extra TRS jack that only activates that resistor when that jack gets plugged out of. A great benefit here is that if you have only two jacks, it leaves one of them stock/standard for 16Ω use and both jacks become 8Ω compensated when using that 8Ω jack, either together with the 16Ω jack or by itself. Simple, easy, peasy!
> Hope This Helps?
> Gene


Hi Gene

Yes that makes sense. I have read every post but was planning just the single 4x12 originally, so would not have registered the extra info at the time. Will take a look at the post you referenced now.
thanks for the help


----------



## hoolahoop

Anyone in the UK want to collaborate on sourcing parts, etc? 
(thanks everyone for this thoroughly inspiring thread)


----------



## JohnH

hi @xiaozi0624 
On page 112 I drew a layout and schematic for an M2 with line-out, which might help. But you can also take a line out just from the attenuator output, either with a pot to control it, or directly provided you set the attenuation level to a suitable level for your DAW. Ive done this successfully too. If you can do that while also having a speaker connected to another jack output of the M2, the real speaker will shape the output including the bass resonance. 

The best values for the line-out circuit depend on what its going into and the power of your amp. But the values I put are a good start.


----------



## BlueX

hoolahoop said:


> Anyone in the UK want to collaborate on sourcing parts, etc?
> (thanks everyone for this thoroughly inspiring thread)


Why do you want to collaborate on sourcing?

I got free shipping at DigiKey, from US to Europe, on the parts for one M2 attenuator.


----------



## xiaozi0624

JohnH said:


> hi @xiaozi0624
> On page 112 I drew a layout and schematic for an M2 with line-out, which might help. But you can also take a line out just from the attenuator output, either with a pot to control it, or directly provided you set the attenuation level to a suitable level for your DAW. Ive done this successfully too. If you can do that while also having a speaker connected to another jack output of the M2, the real speaker will shape the output including the bass resonance.
> 
> The best values for the line-out circuit depend on what its going into and the power of your amp. But the values I put are a good start.


ok
thank you very much
one more question 
if i want to add a fan to the m2 lite
how to wire the circuit?


----------



## paul-e-mann

JohnH said:


> Just wanted to note that if you build an M2 at either 8 or 16 Ohms and use that corresponding amp tap, you can still use either an 8 or 16 ohm cab. The amp sees what it expects with no missmatch. The only issue is if you need just a little attenuation, then you lose 2 or 3 more dB's in the conversion so your loudest attenuated level is a step lower.


I thought everything had to match or you risk damage somewhere along the line? I mostly use my 8 ohm 2x12's but do have a 16 ohm 1x12 I'd like to use on occasion.


----------



## xiaozi0624

JohnH said:


> hi @xiaozi0624
> On page 112 I drew a layout and schematic for an M2 with line-out, which might help. But you can also take a line out just from the attenuator output, either with a pot to control it, or directly provided you set the attenuation level to a suitable level for your DAW. Ive done this successfully too. If you can do that while also having a speaker connected to another jack output of the M2, the real speaker will shape the output including the bass resonance.
> 
> The best values for the line-out circuit depend on what its going into and the power of your amp. But the values I put are a good start.


things is getting more and more clear for me
i read 140+ pages for 2 days now
it is a lot information
my eyes is getting tired 
i think i m gonna build the m2 lite first
if this thing getting hot i m gonna try to build a fan into this box
i want to thank you for post this diy load box project
cant wait to get the job done with my amp crank to max with the attenuator hhhhhhhhh


----------



## hoolahoop

BlueX said:


> Why do you want to collaborate on sourcing?


sorry i could have been clearer.
This will be quite ambitious for me. not just in sourcing components. I thought if someone else is considering a build, but also lacking a bit of confidence, sharing progress might be helpful.. There seems to me to be some benefit knowing that someone else is doing the same build with the same components. And might keep the noise down from me on here. It does sound like a total noob approach though!


----------



## Gene Ballzz

xiaozi0624 said:


> ok
> thank you very much
> one more question
> if i want to add a fan to the m2 lite
> how to wire the circuit?



It should be noted (as per post #1 of this lengthy thread) that the M-Lite and the M2 are significantly different in the front end of the circuit!

Adding a fan can be as simple as allowing the space for it and supplying power for it. You could complicate things by attempting to use the power dissipated by the circuit, or much more simply by using a wall wart or AC to power it.

Getting all your ducks in a row during the planning stages is the key to success here!

Just My ,
Gene


----------



## BlueX

hoolahoop said:


> sorry i could have been clearer.
> This will be quite ambitious for me. not just in sourcing components. I thought if someone else is considering a build, but also lacking a bit of confidence, sharing progress might be helpful.. There seems to me to be some benefit knowing that someone else is doing the same build with the same components. And might keep the noise down from me on here. It does sound like a total noob approach though!


Ah, yeah that makes sense. I thought you just wanted to save on shipping cost.

I'm in Sweden so I'm not a candidate here. Good idea otherwise.


----------



## xiaozi0624

Gene Ballzz said:


> It should be noted (as per post #1 of this lengthy thread) that the M-Lite and the M2 are significantly different in the front end of the circuit!
> 
> Adding a fan can be as simple as allowing the space for it and supplying power for it. You could complicate things by attempting to use the power dissipated by the circuit, or much more simply by using a wall wart or AC to power it.
> 
> Getting all your ducks in a row during the planning stages is the key to success here!
> 
> Just My ,
> Gene


how to wire the fan by using the amp's out to drive it?


----------



## JohnH

In principal you feed a rectifier circuit, that feeds a dc fan. Search online for rhe Marshall Powerbrake circuit. But in practice it would take ecoeriment to get the right values and components that work for you. So I wouldn't launch right into it yet.


----------



## PelliX

Hi gentlemen,

do we by any chance have an up-to-date BOM for an M2 ideally for a 200W max capacity circuit (thus easily capable of dealing with a 100W amp)? Ideally, I'm looking for a one-stop shop; case, jacks, resistors, switches, the works. Not actually going to build this for myself (though I will be testing the unit  ).

Thanks in advance.


----------



## Gene Ballzz

PelliX said:


> Hi gentlemen,
> 
> do we by any chance have an up-to-date BOM for an M2 ideally for a 200W max capacity circuit (thus easily capable of dealing with a 100W amp)? Ideally, I'm looking for a one-stop shop; case, jacks, resistors, switches, the works. Not actually going to build this for myself (though I will be testing the unit  ).
> 
> Thanks in advance.



Good question sir! I've not yet built a 100 watt unit. The resistors, coils, etc, are easy, but the enclosure and cooling/fan particulars are yet to be determined! Though I have no need for it for myself, a 100 watt should be one of my next projects, after I finish the few 50 watters that I'm doing for friends! What amp out/attenuator input impedance are you thinking of?

FWIW, design, layout and planning of the enclosure and cosmetics can be a real time sucker, while the actual assembly and wiring is a piece of cake! A quick tip here is that if you are good with drawing programs, like Sketchup, paint, etc, do your precise layout there and then print out a drilling template on peel and stick project paper and stick it to the enclosure. This makes the drilling much quicker and more accurate. Then there's the searching, choosing sourcing of fasteners, finish and labeling!

I plan to soon share some step by step pics of my current builds, to share some ideas!

Another tip is that if you think it's even possible that you'll build any more than one or two, you'll see significant savings by buying the resistors in bulk lots of 5 or 10, from a couple of the chinesiawanoreanam suppliers! I'm not sure what country you're in, but even though DigiKey and Mouser have almost all you need, their prices can get pretty high! Another quick clue is to make sure that all resistors of any given wattage be the same brand, as the mounting centers can be a little different, making accurate layout and drilling a real challenge!

You can PM/Conversation me for some suggested suppliers.

Just Helpin'
Gene


----------



## Gene Ballzz

As promised, here's the beginning pics of my most recent M2 builds. I think I've worked out a nice layout and packaging standard! While they may not look "factory" made, there will be no protruding screws/heads and labeling will be durable, though not professionally screen printed, yet! This is all quite time consumptive, but the countless hours spent on drilling templates will eventually save a bunch of time and aggravation going forward.

Some pics of after drilling and before and after powder coat. I still need to apply labels , clear over them and install the grating on the large, bottom vent holes! And then the actual fun part of assembly and wiring and *"especially"* testing can begin! I really love the *"testing" *part, as it requires untolled hours of guitar playing, through my favorite amps,
at cranked volumes!

Thanks For Watching,
Gene


----------



## Gene Ballzz

FWIW,
I'm likely going to rethink, revise and resize the large vent holes in the bottom lid, to accommodate a standardly available fans! For anyone who likes this (when its done), I can provide drilling templates, either through the physical mail or via pdf files! These builds are actually quite simple and easy, but the planning, hardware/component sourcing and layouts are massively time consuming!

Ideally, I'd like to figure out a way to power the fan(s) with a rechargeable battery or batteries, utilizing the spent energy from the amp for recharging! *I HATE WALL WARTS!* That would require the assistance of someone more well versed in electrical engineering than me. I'm just a nuts and bolts, soldering monkey!

Simply Attenuatin'
Gene


----------



## Manfred22

Hi everyone,
here I post my just completed version of the M2-attenuator which I use with a self-made 18W TMB.
It works very well and affects the sound only marginally. I will add a stomp box with a 1PDT switch to switch-off the -3.5dB or the -7dB stage as boost. 
At 18 W input the cast aluminum housing gets only hand warm even without ventilation holes. 
Many thanks to John and to all of you for your great input on this topic. 
Manfred, Hamburg, Germany


----------



## PelliX

Gene Ballzz said:


> Good question sir! I've not yet built a 100 watt unit. The resistors, coils, etc, are easy, but the enclosure and cooling/fan particulars are yet to be determined! Though I have no need for it for myself, a 100 watt should be one of my next projects, after I finish the few 50 watters that I'm doing for friends! What amp out/attenuator input impedance are you thinking of?



Yup, this is on request for someone else, too. It's not a challenging project for me, really, but indeed - it helps to get everything ordered in one go. If there's a BOM available, I'd give it a once-over and perhaps run through some options with the customer. In this case it's for any 100W cranked Marshall that might come his way, so it needs to withstand the equivalent of say 150W RMS. I overspec for that kind of stuff and stated 200W as a theoretical max. That should cover most of anything provided it's not 10/10 for hours on end.



Gene Ballzz said:


> FWIW, design, layout and planning of the enclosure and cosmetics can be a real time sucker, while the actual assembly and wiring is a piece of cake!



Totally, enclosures always cost me more time than the circuit itself with this kind of stuff. So true...



Gene Ballzz said:


> A quick tip here is that if you are good with drawing programs, like Sketchup, paint, etc, do your precise layout there and then print out a drilling template on peel and stick project paper and stick it to the enclosure. This makes the drilling much quicker and more accurate. Then there's the searching, choosing sourcing of fasteners, finish and labeling!



Yup, paper and masking tape have also often been close friends of mine...



Gene Ballzz said:


> I plan to soon share some step by step pics of my current builds, to share some ideas!



Always welcome 



Gene Ballzz said:


> Another tip is that if you think it's even possible that you'll build any more than one or two, you'll see significant savings by buying the resistors in bulk lots of 5 or 10, from a couple of the chinesiawanoreanam suppliers! I'm not sure what country you're in, but even though DigiKey and Mouser have almost all you need, their prices can get pretty high! Another quick clue is to make sure that all resistors of any given wattage be the same brand, as the mounting centers can be a little different, making accurate layout and drilling a real challenge!



Yup, I was fully intended on ordering twice the quantity of those just to have some on stock. DigiKey/Mouser, local retailer or eBay-from-China - up to the customer. I would provide a price indication for the options readily available. Up to them in the end - I don't mind.



Gene Ballzz said:


> You can PM/Conversation me for some suggested suppliers.
> 
> Just Helpin'
> Gene



If you have the BOM and layout (just the circuit schematic would do) of your last build... that would be appreciated


----------



## PelliX

Gene Ballzz said:


> FWIW,
> I'm likely going to rethink, revise and resize the large vent holes in the bottom lid, to accommodate a standardly available fans! For anyone who likes this (when its done), I can provide drilling templates, either through the physical mail or via pdf files! These builds are actually quite simple and easy, but the planning, hardware/component sourcing and layouts are massively time consuming!
> 
> Ideally, I'd like to figure out a way to power the fan(s) with a rechargeable battery or batteries, utilizing the spent energy from the amp for recharging! *I HATE WALL WARTS!* That would require the assistance of someone more well versed in electrical engineering than me. I'm just a nuts and bolts, soldering monkey!
> 
> Simply Attenuatin'
> Gene



Well, what some attenuators do is just tap off some of the current that's otherwise being dissipated as heat, rectify it to DC and drive a fan. A couple of large caps for smoothing would be good, but that should do the trick. As for recharging... sure, it's possible, too. What kind of batteries are we talking about here, 4x AA for something like 5-6V? It gets a bit more complex if you don't want to overcharge batteries, etc. Still not rocket science, but a circuit that cuts out when the charging (consumption) of the batteries drops. I'd probably go for DC power supply for the fan (though I understand your hatred) and otherwise some caps instead of batteries just to keep it running. I'm sure now someone is going to come along and add a thermistor to regulate the fan to only operate when the chassis temp rises above a given threshold, etc 

My advice would be, and I quote a certain Mr Einstein, "Make it as simple as possible - but no simpler than that".... Also take into consideration that the device will witness some 'intense' internal temps. Putting batteries in there... that is to say, in the device that is preventing your amp from destroying its OT... up to you.


----------



## Gene Ballzz

PelliX said:


> Well, what some attenuators do is just tap off some of the current that's otherwise being dissipated as heat, rectify it to DC and drive a fan. A couple of large caps for smoothing would be good, but that should do the trick. As for recharging... sure, it's possible, too. What kind of batteries are we talking about here, 4x AA for something like 5-6V? It gets a bit more complex if you don't want to overcharge batteries, etc. Still not rocket science, but a circuit that cuts out when the charging (consumption) of the batteries drops. I'd probably go for DC power supply for the fan (though I understand your hatred) and otherwise some caps instead of batteries just to keep it running. I'm sure now someone is going to come along and add a thermistor to regulate the fan to only operate when the chassis temp rises above a given threshold, etc
> 
> My advice would be, and I quote a certain Mr Einstein, "Make it as simple as possible - but no simpler than that".... Also take into consideration that the device will witness some 'intense' internal temps. Putting batteries in there... that is to say, in the device that is preventing your amp from destroying its OT... up to you.



All valid points, right there, my friend! My impetus was to simplify operation, although *NOT* at the sacrifice of reliability and/or safety! But then there's the lack of robustness, durability and reliability of those ^%%$ sucking *WALL WARTS, *and the wires/plugs that connect them!*  *And they eat up at least two spaces on a power strip, etc, ad nauseam!* Hate *is too kind of a word to describe my feelings about them! I'd rather run an IEC cable to the unit, but there's a pretty pi$$ poor selection of small AC fans, at reasonable prices!

Thanks For Sharon,
Gene


----------



## PelliX

Gene Ballzz said:


> But then there's the lack of robustness, durability and reliability of those ^%%$ sucking *WALL WARTS, *and the wires/plugs that connect them!**And they eat up at least two spaces on a power strip, etc, ad nauseam!



Yes, but if the fan stops, nothing bad happens right away.... A warning LED if the fan stops spinning should be easy enough to implement, but simplicity is often the best design. As for the power, why not do what I do for amps; use your pedal power supply? A 12V DC fan on 9V runs fast enough in general to get the job done, but a little quieter. True, on an attenuator this is hardly a required feature...

Also, a 15cm male to female mains extension cable is easy to make and you can regain your 'stolen' sockets. Not pretty, but functional, cheap and easy. Just snippin'.



Gene Ballzz said:


> *Hate *is too kind of a word to describe my feelings about them! I'd rather run an IEC cable to the unit, but there's a pretty pi$$ poor selection of small AC fans, at reasonable prices!



Indeed, and do you want to introduce mains AC into the device attached to your amp's OT? I think a benign branch off the pedal power supply for such a purpose is preferrable, or drive it off the amp's output as discussed. There are existing schematics for that, as it happens. I think that Harley Benton 250W job that came out recently does exactly that.



Gene Ballzz said:


> Thanks For Sharon,
> Gene



She was dynamite, you're welcome to her!


----------



## colchar

This thread is amazing. Even for someone like me who is functionally illiterate when it comes to electronics, it is amazing to read through all of this and to see the collaborative efforts and the community spirit (becoming much more rare these days!).

I am going to have a chat with someone here about one of these units, but wanted to ask a couple of quick questions here before speaking to him in person (it will help make me sound less of an idiot and he won't have to give me the 'For Dummies' explanations if I can tell him exactly what I want).

I only play at home, and never record so I only need an attenuator with no other functionality. 

1) Is it possible to get a unit that works with more than one impedance? I am currently using a Hughes & Kettner 2x12 with V30s, which is a 16ohm cab. But I might want another cab at some point so was wondering if I could have that factored into a build now, or if it would necessitate another attenuator later?

2) I am currently using an Orange AD30 (I also have one vintage and one modern Traynor combos) and am very happy with the amp. It has master volumes, but I want to be able to push the power tubes to open the amp up. I cannot ever see myself diming the thing, it would likely only be played at 50%-75% of its volume. But I will be buying another head at some point so wanted to factor that into the attenuator build. I am currently thinking of grabbing an SC20 or SV20, but am open to other Marshall heads - if a 50 watt JMP, Vintage Modern, or JCM800 was to come along I'd snap them up. Then again, with a good attenuator maybe a Class 5 would be enough since I'd be able to turn it up nice and loud while controlling it with the attenuator. 

But basically what I want to ask here is whether a 50 watt version of the attenuator will be enough for the amps I have mentioned? Even if I pick up one of the Marshall 50 watters mentioned above I cannot see myself diming it, and would only play it at 50%-75% of its volume.

3) Since I know bugger all about electronics, is there anything else I should be thinking about or is everything pretty much covered?

Thanks in advance.


----------



## PelliX

colchar said:


> 1) Is it possible to get a unit that works with more than one impedance? I am currently using a Hughes & Kettner 2x12 with V30s, which is a 16ohm cab. But I might want another cab at some point so was wondering if I could have that factored into a build now, or if it would necessitate another attenuator later?



Multiple options. You could have one with multiple impedence circuits, but you could also abstract the cab's impedence entirely and not let the amp ever notice a difference. Perhaps have an 8 and a 16 Ohm input - maybe just one of the two for simplicity. Then you could in reason attach any impedence cab. If you don't go absolutely insane with it you might just simply end up with a bit less volume than expected. (Simple version).



colchar said:


> 2) I am currently using an Orange AD30 (I also have one vintage and one modern Traynor combos) and am very happy with the amp. It has master volumes, but I want to be able to push the power tubes to open the amp up. I cannot ever see myself diming the thing, it would likely only be played at 50%-75% of its volume. But I will be buying another head at some point so wanted to factor that into the attenuator build. I am currently thinking of grabbing an SC20 or SV20, but am open to other Marshall heads - if a 50 watt JMP, Vintage Modern, or JCM800 was to come along I'd snap them up. Then again, with a good attenuator maybe a Class 5 would be enough since I'd be able to turn it up nice and loud while controlling it with the attenuator.
> 
> But basically what I want to ask here is whether a 50 watt version of the attenuator will be enough for the amps I have mentioned? Even if I pick up one of the Marshall 50 watters mentioned above I cannot see myself diming it, and would only play it at 50%-75% of its volume.



Hmmm, I'd go over spec. What if a 100-watter comes your way and you need to tickle its sweet spots? 100W rating for anything up to 50W and 200W rating for the 100W high gain amps... also depends on how much attenuation you want to apply of course. The more 'drop' you introduce, the more power is 'soaked up' and must thus be dissipated. Technically that would imply that a 10W attenuator would suffice for 10% attenuation on a 100W amp, but that's not how it works. The safety margin of a factor of 2 is really not as overkill as it might appear.



colchar said:


> 3) Since I know bugger all about electronics, is there anything else I should be thinking about or is everything pretty much covered?



A bypass switch seems a welcome option in most scenarios, perhaps?


----------



## Gene Ballzz

colchar said:


> This thread is amazing. Even for someone like me who is functionally illiterate when it comes to electronics, it is amazing to read through all of this and to see the collaborative efforts and the community spirit (becoming much more rare these days!).
> 
> I am going to have a chat with someone here about one of these units, but wanted to ask a couple of quick questions here before speaking to him in person (it will help make me sound less of an idiot and he won't have to give me the 'For Dummies' explanations if I can tell him exactly what I want).
> 
> I only play at home, and never record so I only need an attenuator with no other functionality.
> 
> 1) Is it possible to get a unit that works with more than one impedance? I am currently using a Hughes & Kettner 2x12 with V30s, which is a 16ohm cab. But I might want another cab at some point so was wondering if I could have that factored into a build now, or if it would necessitate another attenuator later?
> 
> 2) I am currently using an Orange AD30 (I also have one vintage and one modern Traynor combos) and am very happy with the amp. It has master volumes, but I want to be able to push the power tubes to open the amp up. I cannot ever see myself diming the thing, it would likely only be played at 50%-75% of its volume. But I will be buying another head at some point so wanted to factor that into the attenuator build. I am currently thinking of grabbing an SC20 or SV20, but am open to other Marshall heads - if a 50 watt JMP, Vintage Modern, or JCM800 was to come along I'd snap them up. Then again, with a good attenuator maybe a Class 5 would be enough since I'd be able to turn it up nice and loud while controlling it with the attenuator.
> 
> But basically what I want to ask here is whether a 50 watt version of the attenuator will be enough for the amps I have mentioned? Even if I pick up one of the Marshall 50 watters mentioned above I cannot see myself diming it, and would only play it at 50%-75% of its volume.
> 
> 3) Since I know bugger all about electronics, is there anything else I should be thinking about or is everything pretty much covered?
> 
> Thanks in advance.



*FIRST>* Concerning attenuator input/amplifier output impedance. It is totally doable to have multiple options here, but it gets kinda complicated, not so much in the building, but in usage. Current designs start with a base impedance (16Ω, 8Ω or 4Ω) and then use resistors and an added coil for modifying that input impedance. The down side is that deviation from the *"base"* impedance increases attenuation, by a couple/few db. It also opens the door for operator error in showing your amp's output the wrong impedance. My recommendation is choosing and sticking with one specific input impedance! In your case, I'd likely suggest a 16Ω based unit.
*NEXT>* As far as output impedance, the amp will only *"see" *generally the impedance of the attenuator and using different impedance speakers will only affect the input in minuscule amounts. Totally safe for the amp. The tone will be slightly affected, however, when deviating from the *"base"* impedance. Both the 16Ω and 8Ω designs incorporate extra, gently compensated outputs, to preserve the tone.
*FINALLY>* Of important note, whenever using a speaker load that is different than the input impedance, a bypass switch should never be activated! Also important to remember that a bypass switch should never be toggled without first turning the amp off or into standby mode!

Hope This Helps?
Gene


----------



## donwagar

colchar said:


> I am going to have a chat with someone here about one of these units, but wanted to ask a couple of quick questions here before speaking to him in person (it will help make me sound less of an idiot and he won't have to give me the 'For Dummies' explanations if I can tell him exactly what I want).



When I built mine, I had to ask John a lot of dumb questions. He was excellent in explaining stuff that I didn't understand. Don't ever feel bad about asking.

As I understand it, someone will correct me if I'm wrong, lol, match the ohms of the attenuator to the head. Protect the OT. Don't worry about the speaker impedance. If all your heads have a 16 ohm out, I'd go for that as Gene suggested above.

50 watt is probably good. If you do someday buy a JTM 50 or 100 Watt, well, they are a different deal IMO, double spec a new attenuator for them (by that I mean if it's a 50W JTM buy a 100W attenuator).

When you say a vintage Traynor, you aren't talking about a YBA-1 are you? Even though they are rated, I think, about 40W, I'm sure dimed they push way more than that. If so, you might want to order a 100W right off the bat.

BTW I played through a YBA-1 one night, dead stock, attenuated and dimed. It was glorious.


----------



## Gene Ballzz

There are no *"dumb questions"* when researching an investment of time, $$$, and/or labor for a device to allow cranked use of an amp (without vaporizing small pets and children, at 10 paces) and properly protecting your amp while doing it! These attenuators are the coolest thing since sliced bread, for those of us who love that "cranked amp" sound, without compromises!
Simple Attenuatin'
Gene


----------



## JohnH

hi @colchar , all good advice posted above!
If you can identify an amp output impedance that will work across the amps that you have or might like, then you can build an M2 for that, and the attenuator can let you run either 8 or 16 Ohm cabs safely, and with good tone. If you like heads, I think most of them will drive 16 or 8. My Vintage Modern is a combo, which only does 8 or 4, so I build 8 for myself since it works for both my amps.

Good luck and keep calling in here!


----------



## Gene Ballzz

Hi @JohnH ,
I searched, page by page for your "dual" design and somehow couldn't find it! Do you recall page or post number?
Thanks,
Gene


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## JohnH

Hi Gene, no prob, I'll post the double-barrel one again when I get on a computer.


----------



## JohnH

Hj @Gene Ballzz Here's the double version, described in post #1731 on page 87




There's a couple of ways to do this, depending whether each half is an 8 or a 16 ohm circuit.(8 ohm halves shown here) It's also possible to do 4, 8 and 16 out of two 8 ohm circuits, but switches with the right poles and rating are rare/expensive.


----------



## colchar

donwagar said:


> When I built mine, I had to ask John a lot of dumb questions. He was excellent in explaining stuff that I didn't understand. Don't ever feel bad about asking.
> 
> As I understand it, someone will correct me if I'm wrong, lol, match the ohms of the attenuator to the head. Protect the OT. Don't worry about the speaker impedance. If all your heads have a 16 ohm out, I'd go for that as Gene suggested above.
> 
> 50 watt is probably good. If you do someday buy a JTM 50 or 100 Watt, well, they are a different deal IMO, double spec a new attenuator for them (by that I mean if it's a 50W JTM buy a 100W attenuator).
> 
> When you say a vintage Traynor, you aren't talking about a YBA-1 are you? Even though they are rated, I think, about 40W, I'm sure dimed they push way more than that. If so, you might want to order a 100W right off the bat.
> 
> BTW I played through a YBA-1 one night, dead stock, attenuated and dimed. It was glorious.




No, I don't have a YBA-1. I have a YGL 3, which was their version of a Twin. Actually, it was known as the Twin Reverb killer.

And yeah, the wattage on vintage Traynors was at best a guess most of the time as models like the YBA-1 were waaaaaaaay louder than their listed wattage would indicate.


----------



## colchar

JohnH said:


> hi @colchar , all good advice posted above!
> If you can identify an amp output impedance that will work across the amps that you have or might like, then you can build an M2 for that, and the attenuator can let you run either 8 or 16 Ohm cabs safely, and with good tone. If you like heads, I think most of them will drive 16 or 8. My Vintage Modern is a combo, which only does 8 or 4, so I build 8 for myself since it works for both my amps.
> 
> Good luck and keep calling in here!




I looked into it today and all of the amps I mentioned in my previous post are 16ohm, and they can also be run at 8ohm (not sure about the Class 5 on that one but I would likely go for one of the other amps anyway). So a 16ohm verison of the attenuator would work fine for me.

I probably should have checked that before my previous post.

I was also wondering if this attenuator is resistive or reactive? Not that it really matters as I don't know enough about this stuff to have an opinion either way and will trust the expertise of everyone else in this thread, I am just trying to learn a wee bit more about attenuators as I go along here.


----------



## Gene Ballzz

colchar said:


> I looked into it today and all of the amps I mentioned in my previous post are 16ohm, and they can also be run at 8ohm (not sure about the Class 5 on that one but I would likely go for one of the other amps anyway). So a 16ohm verison of the attenuator would work fine for me.
> 
> I probably should have checked that before my previous post.
> 
> I was also wondering if this attenuator is resistive or reactive? Not that it really matters as I don't know enough about this stuff to have an opinion either way and will trust the expertise of everyone else in this thread, I am just trying to learn a wee bit more about attenuators as I go along here.



These units are both resistive and reactive. The reactive part surrounds the choke coil in the always on first stage of -7db, as well as how the speaker "reacts" to the unit!

A 16Ω based unit is excellent for your situation! Should you at some time find that the *"always on"* -7db stage is too much volume reduction, you can simply set/use the amp for 8Ω operation and use the attenuator (all stages active) in parallel with the 16Ω speaker load, for a -3.5db cut, while retaining the reactive nature of the attenuator circuit. FWIW, I've rarely found -3.5db to be preferable to -7db, except for testing and experimentation. Although, when using this attenuator with my Class5 or 13 watt 5E3, it *CAN* be a toss up! The designed -3.5db steps are really quite small and although audible, they are not drastic, from *"one to the other!" *(A little Twilight Zone reference/quote there, for those so inclined.) 
Simply Buildin'
Gene


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## PelliX

JohnH said:


> Hj @Gene Ballzz Here's the double version, described in post #1731 on page 87
> 
> View attachment 112001
> 
> 
> There's a couple of ways to do this, depending whether each half is an 8 or a 16 ohm circuit.(8 ohm halves shown here) It's also possible to do 4, 8 and 16 out of two 8 ohm circuits, but switches with the right poles and rating are rare/expensive.



... is it just me, or should this be called the Hewittzer?


----------



## Gene Ballzz

PelliX said:


> ... is it just me, or should this be called the Hewittzer?



That may well be an appropriate model name for a +100 watt, "double barrel" with all the bells and whistles,, multiple imedance in/out, line out, headphone out, fans, etc, ad nauseam……. 
Thanks For Playin' Along Here!
Gene


----------



## JohnH

PelliX said:


> ... is it just me, or should this be called the Hewittzer



LOL, yeah maybe but...really just *NO!*


----------



## Gene Ballzz

JohnH said:


> LOL, yeah maybe but...really just *NO!*



Yeah, upon further consideration, maybe naming an attenuator as something that explodes and/or blows things up is less than ideal! 

Still Kickin'
Gene


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## colchar

One more question (yeah, I know I could search through 14762 pages and find the info, but I've got a rye & coke in my hand, it ain't my first tonight, and I just don't feel like doing that search!!!) - do these attenuators come down in steps or do they have dials like a volume pot so that they can be dialed in?


----------



## PelliX

colchar said:


> One more question (yeah, I know I could search through 14762 pages and find the info, but I've got a rye & coke in my hand, it ain't my first tonight, and I just don't feel like doing that search!!!) - do these attenuators come down in steps or do they have dials like a volume pot so that they can be dialed in?



Take your pick, but generally a knob is included in the design. Although the knob works in a similar fashion to a regular pot, it's called a rheostat or L-pad. Same concept, beefier.


----------



## Gene Ballzz

PelliX said:


> Take your pick, but generally a knob is included in the design. Although the knob works in a similar fashion to a regular pot, it's called a rheostat or L-pad. Same concept, beefier.



Sorry Sir,
But those are the crappy attenuators that don't pay attention to the critical balance of series and parallel resistance in the attenuator and have very little in common with the @JohnH design and generally sound like DOO-DOO and destroy the natural feel and response of the amp! And while yes, the JohnH principal could certainly be developed into a unit with a "continuous knob" control, the technical logistics would be very complicated and cost prohibitive. I swear to all that is holy in rock/guitar/music, the small steps of this attenuator design present no realistic issue! The standard M2 design provides ten small steps between full amp volume and babby sleeping in the next room volume! If retaining all tone, sound, dynamics, feel and response, with truly continuously variable volume control, one needs to step up to a rea-amping unit, like a Fryette, UA Ox, etc, along with the price and operating nuances! I'll take the JohnH all day, every day!

And @colchar , you really should give me a call! Real, human to human conversations can cover a lot more ground than typed messages on a discussin board and in a shorter time frame!

Also Just Sippin'
Gene


----------



## PelliX

With all due respect, a rheostat is simply a variable resistor. I see your concern with regards to the notion of just having a 'zero to full' attenuation range on the rheostat, but I've encorporated them for the 'fine control' in designs without issues. Will there be a tonal response difference across the dial? Sure, to a degree, depending on the rest of the circuit. 

Or am I missing something here?


----------



## tometome

Hey! Pleased to announce a successful M2 build that works brilliantly!! Thank you so much for sharing this design.. I ended up using a combination of 18 and 20 gusge wire for now however will require it w 18 gusge once it’s built into the speaker box I’m planning on making.


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## JohnH

PelliX said:


> With all due respect, a rheostat is simply a variable resistor. I see your concern with regards to the notion of just having a 'zero to full' attenuation range on the rheostat, but I've encorporated them for the 'fine control' in designs without issues. Will there be a tonal response difference across the dial? Sure, to a degree, depending on the rest of the circuit.
> 
> Or am I missing something here?



Our designs use switched stages, for a few reasons:

Passive attenuator designs based on pots or Lpads may or may not sound good enough or acceptable, but as you note, there typically is indeed some tonal difference across the dial. In the basic examples it sucks! But M2 and its related variations go for zero tonal difference across the full attenuation range. As a criterion for this, a close-miced cab at full volume or at any attenuated level, recorded and the recordings normalised, sounds and measures as identical. and can be demonstrated by listening, measuring or circuit analysis and all these have been confirmed by others. The results typically achieve this within about a dB or less. This doesn't set out to account for cone distortion or hearing FM effects, but these get blamed too often for what is really sub-optimal design.

So, having worked out what the target is, testing by circuit analysis leads to values for what the ideal sets of resistors need to be to maintain not only tbe input impedance but also the output impedance (which is key!) of the attenuator. There is no simple pot or Lpad (or pot with other resistors) that can do this across a wide sweep. So we end up with sets of values each specifically optimised for one setting.

Hence switches, and then given the conclusion that only switched settings will nail the desired results at every setting, we use a series of toggles because they are simple, robust and economical. Three toggles, each controlling stages of attenuation x2 the previous, gives 8 closely-spaced settings.

Set the amp volume for best tone, then step it down with the attenuator to get the right volume, and then forget the attenustor and play!


----------



## colchar

Gene Ballzz said:


> Sorry Sir,
> But those are the crappy attenuators that don't pay attention to the critical balance of series and parallel resistance in the attenuator and have very little in common with the @JohnH design and generally sound like DOO-DOO and destroy the natural feel and response of the amp! And while yes, the JohnH principal could certainly be developed into a unit with a "continuous knob" control, the technical logistics would be very complicated and cost prohibitive. I swear to all that is holy in rock/guitar/music, the small steps of this attenuator design present no realistic issue! The standard M2 design provides ten small steps between full amp volume and babby sleeping in the next room volume! If retaining all tone, sound, dynamics, feel and response, with truly continuously variable volume control, one needs to step up to a rea-amping unit, like a Fryette, UA Ox, etc, along with the price and operating nuances! I'll take the JohnH all day, every day!
> 
> And @colchar , you really should give me a call! Real, human to human conversations can cover a lot more ground than typed messages on a discussin board and in a shorter time frame!
> 
> Also Just Sippin'
> Gene




Yes, I do need to give you a call. I've just been busier than expected with work so have been posting here when I've taken a moment or two off from grading papers (and yes, I was drinking last night while grading - if you read the papers that I had to read you would be drinking too!).

But I am about to more or less say fuck work because it is a long weekend. I'll get some work done so that I am not even more behind next week (I am already playing catch-up) but the majority of the weekend is mine so I will call you.


----------



## Gene Ballzz

So here is my latest iteration of layout and packaging. It is ready to wire up, test and be done! Labeling is still a thorn in my side and I'm still looking for a more elegant means that I'm capable of pulling off. The P-Touch labels are fine and hold up well, but they still look kinda janky, to me! I'll be talking to a screen printer today, just for poops & giggles! Note that the big honkin' vent holes in the bottom are sized and mounting holes drilled to accept a standard 50mm fan or even four of them!

Also the last box I posted, ended up as a sacrificial lamb, as I learned the hard lesson that regular, clear spray paint does not adhere well to powder coat!

Still Buildin'
Gene


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## C-Grin

Looks amazing two thumbs up


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## JohnH

Hi @Gene Ballzz , that looks really great! (I think I'd keep wanting to turn it over to admire the grills!)

And I see it's getting the neat 8-16 tweak on the output with the 39ohm. I'll get a schematic diagram to show that.


----------



## Gene Ballzz

Another M2/16Ω done! Still waiting for some labels for the side, that will say: *The JohnH* on one side and *M2/16ohms* on the other side! The fan is not mounted/included in this build, it is merely there to demonstrate the fit! I think in the future, I will drill an extra hole for a wall wart input and simply put a plastic/rubber plug in it, if no fan installed! Any comments criticism of the cosmetics and/or layout?

Thanks Folks & Especially @JohnH .
Gene


----------



## JohnH

Looks like a great bit of product design and production engineering! 

What's your coil-mounting arrangement? Any space to offset it a few mm up off the base?

Also, since you've got a credible looking fan, shall we try to take the next step with developing an amp-driven self-powered arrangement? To do this, I'd be interested to know what's its dc ohms is, and what dc voltage does it start to spin, and what voltage is needed to spin it moderately but not at max speed. Could rig it up to some AA batteries or wall warts or whatever you have to try different volt levels. When it's spinning nicely at whatever voltage that is, can you measure the dc current through it?

Given all that info, I can work out a first-pass at how to wire it up with a rectifier and resistor/ cap arrangement, then see if the load that it puts on the amp is enough to change our tones.


----------



## Gene Ballzz

JohnH said:


> Looks like a great bit of product design and production engineering!
> 
> What's your coil-mounting arrangement? Any space to offset it a few mm up off the base?
> 
> Also, since you've got a credible looking fan, shall we try to take the next step with developing an amp-driven self-powered arrangement? To do this, I'd be interested to know what's its dc ohms is, and what dc voltage does it start to spin, and what voltage is needed to spin it moderately but not at max speed. Could rig it up to some AA batteries or wall warts or whatever you have to try different volt levels. When it's spinning nicely at whatever voltage that is, can you measure the dc current through it?
> 
> Given all that info, I can work out a first-pass at how to wire it up with a rectifier and resistor/ cap arrangement, then see if the load that it puts on the amp is enough to change our tones.



I tried it with a fresh 9 volt battery. While it moves a little air and is "whisper" quiet, I don't think we'd want it spinning any slower. I had some used batteries and anything below 7.8 vots, the fan stops! The current rating listed on the fan is 0.08 amps at 12 volts. If you like, I can hook up that fresh battery, leave it running and see how long it lasts? That might give us some kind of ballpark guesstimate?

Don't know if that helps?

As for standing off the coli, the thickness of the end plates of the bobbin is 2.25mm. I've not been able to find any flat head aluminum or nylon screws longer than 1.5 inches. On top of that, all the stainless stuf I find is either listed as "mildly magnetic" and any I put my hands on are certainly magnetic! That's in 18-8, 316 and black oxide stainless! I've currently used 1.5 inch nylon with aluminum nut and not even enough room for a washer! And to get that 1.5 inch, I had to bite the bullet and go with flat blade screw head! Still on the hunt! FWIW, the base is die cast aluminum and the gutter grating is also aluminum. The screws holding the grate are plain jane steel! I didn't expect that to be an issue? I can run out and acquire some stainless screws, nuts and washers, but besides being kinda pricey, they'll be "mildly magnetic" also!

Thanks Again John,
Gene


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## JohnH

ok sounds fine. No issue with steel for the screws for other than the coil and you have that covered.

Can you check resistance of the fan as measured by a meter?


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## yogi.1026

Hi Gene, For the labelling google water slide decals for pedal box building they will give you some ideas. I think I did my design in Gimp, print onto water slide sticker, I used basic cannon desktop. spray with couple coats of clear to seal the text then wet and apply. Finish with some clear , jobs done!


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## Gene Ballzz

yogi.1026 said:


> Hi Gene, For the labelling google water slide decals for pedal box building they will give you some ideas. I think I did my design in Gimp, print onto water slide sticker, I used basic cannon desktop. spray with couple coats of clear to seal the text then wet and apply. Finish with some clear , jobs done!



I've not had great luck with making my own waterslide decals, especially if they can't get buried under clear, after application. The downside here is that I've not yet found a clear that will adhere to powder coat. I've yet to try lacquer, but I'm not very confident. I'll do a test piece and maybe I'll need to reinvent my wheel! I really hate to go away from the powder coat, as it is so rock solid and durable! I do have a couple references for screen printers who may be able to help me!

Yours looks so nice that I specifically recall it! How did you accurately cut the square holes for the switches

Thanks,
Gene


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## Gene Ballzz

PelliX said:


> With all due respect, a rheostat is simply a variable resistor. I see your concern with regards to the notion of just having a 'zero to full' attenuation range on the rheostat, but I've encorporated them for the 'fine control' in designs without issues. Will there be a tonal response difference across the dial? Sure, to a degree, depending on the rest of the circuit.
> 
> Or am I missing something here?



To expound on what @JohnH said in post #2,902, there *IS *something you seem to be missing here! If the main and only goal is to continuously/variably control the volume of a cranked amp, many of the currenttly marketed units will certainly accomplish that! If however, you want to control that volume, *"while still retaining the natural nuances of tone, feel and response while doing so"* the JohnH is one of the very few answers to the equation, as that volume reduction is done in a different manner! Yes, there are are then "re-amping" units like the Power Station, Unleash, etc, but those are totally diffferent again in their approach, although most of them are truly stellar and work quite well, *AT A HEFTY PRICE!*

You're obviously a smart guy, know your stuff, enjoyable to banter with and I don't want to pick nits with you. Understand that I've tried nearly every passive attenuator design since they first came on the market, in the early '70s and every one was a total disappointment, until the JohnH, although some were worse or better than others!

The real downside (and altruistic upside) is that the JohnH design has been developed, tested and perfected in the public domain so that it connot really be patented and or sold to a maufacturing compny! There is no secure and/or ongoing "profit" to be made on this design, other than allowing others to share the enjoyment of the design. A few industrious individuals can certainly build some of these units and collect a fair and reasonable fee for their time, labor and investment of using their own money, up front, to collect, in bulk, the componenets needed! Because of this, I don't see these ever being available commercially, Unless some bean counters in a large company, with sharp pencils, can see a real profit!

For a home garage, under the radar built unit, I see a fair price for a simple, M2 attenuator being in the $300 range. To offer them commercially would bump that price to $500, or more, once you factor in insurance, licensing, taxes, advertising, book keeping and all the other sundries that go along with operating a real business! And then to make them available "Internationally" we get into being RoHS compliant, import/export fees, inspections, and on and on!

Yeah, manufacturing can be a messy business at best (when following all the rules), which is why most manufacturing has been moved to less scrupulous parts of the world with societies who could care less about rules, human rights, child labor, etc, ad nauseam!

Or Well?
Gene


----------



## PelliX

Gene Ballzz said:


> You're obviously a smart guy, know your stuff, enjoyable to banter with and I don't want to pick nits with you.



Likewise, and thank you!  A little picking of the nits can be great fun at times. Perhaps I should reduce the picked nits in this particular thread, though. 

Fully agree on the economics here, I'm also not looking into manufacturing. Did offer another forum member one the other day but at the price of the parts plus two tenners for the beers/effort.



Gene Ballzz said:


> If however, you want to control that volume, *"while still retaining the natural nuances of tone, feel and response while doing so"* the JohnH is one of the very few answers to the equation, as that volume reduction is done in a different manner!



True, though I insist that the Weber Mass 'concept' is a very good one, *too*. A coil is a coil and all that. I think where we might have got our wires crossed [see what I did there?] is that typically the off-the-shelf units offer a 0-100% or 50-100% attenuation level by means of the rheostat. Without some rather complex circuit design and bumping the cost of the BOM way up, that's never going to work very well, as you rightly point out. I was in fact more advocating the use of a rheostat as a fine adjustment, complementing the switches present in the M2 design, for example - not necessarily negating them. Naturally, one might sacrifice the balance of the circuit by using this feature, but turn the knob all the way and the rheostat merely becomes a resistor in the (multi-level switched) circuit. I've since then also considered that a fine grained control is hardly necessary as one's amp already features this.
I respect your practical expertise in the matter and appreciate you taking the time to clarify it further! 

EDIT: I'd also be interested in your technical opinion of the Weber's I mentioned. Are there parts of their design that you inherently disagree with? Have you tried any, and if so, how did they line up with other attenuators in your opinion?


----------



## JohnH

hi @PelliX , I can pick up the question about the Webers.

With a good circuit model of a full speaker, we can model most passive attenuators in Spice. Here's my Weber Mini-mass analysis:




On the schematic at the bottom, a full speaker is modelled, and at the top the mini-mass driving a speaker.

The upper plot shows the signal at the full speaker in red, and the other upper lines are what the amp sees driving the attenuator.

The lower lines show the attenuated output at two settings of -10dB and -25dB

The red curve is the ideal tonal reference without attenuator and being logarithmic dB plots, ideally all curves should be parallel to red.

See how on the top chart, the attenuated tones are losing its bass peak and its treble rise? This is the classic issue with attenuators that work with pots and lpads. As you turn down, the low-impedance output of a turned down pot is damping tbe response, leading to dull tone. 

The Weber is doing a reasonable attempt at showing the amp a reactive load, but tbe right response is not feeding through to the speaker.

Webers typically have a treble compensation switch. The lower chart shows that, and it does lift the treble back up towards where it should be. But it doesn't replace the bass peak, and it throws the response seen by the amp way off and so the amp can't respond as usual.

Our M2 is a lot closer to being consistent in all these areas. It doesn't need a treble work-around, and it let's the real speaker develop its own response by showing it a consistent and amp-like output impedance.


----------



## Barnsley Boy

Gene Ballzz said:


> Another M2/16Ω done! Still waiting for some labels for the side, that will say: *The JohnH* on one side and *M2/16ohms* on the other side! The fan is not mounted/included in this build, it is merely there to demonstrate the fit! I think in the future, I will drill an extra hole for a wall wart input and simply put a plastic/rubber plug in it, if no fan installed! Any comments criticism of the cosmetics and/or layout?
> 
> Thanks Folks & Especially @JohnH .
> Gene
> View attachment 112078
> View attachment 112079


@Gene Ballzz , Haven't swung by the forum for a while, but just had to say that is a cracking build. I would have been inclined to flip it upside down to show off those grilles - love that look. Is that plasterers expanded metal mesh? And how did you form the large holes? I have a hole coring bit, but I think it's more for wood so I'd be worried about totally mullering the metal.


----------



## Gene Ballzz

Barnsley Boy said:


> @Gene Ballzz , Haven't swung by the forum for a while, but just had to say that is a cracking build. I would have been inclined to flip it upside down to show off those grilles - love that look. Is that plasterers expanded metal mesh? And how did you form the large holes? I have a hole coring bit, but I think it's more for wood so I'd be worried about totally mullering the metal.



Thanks for the kind compliment! Coming from you, with your stellar builds it is truly appreciated!

The mesh is aluminum and I believe intended as rain gutter guard. The plaster and/or stucco stuff is steel and the edges of the diamonds are more sharp, as well as the diamonds being smaller.

The holes were made with one of these (any size you choose, but mine is 1 3/4 ") and the hole is perfect for a standard 50mm format fan:








And one of these is your best friend, when drilling/cutting any holes larger than 1/8" in aluminum:



https://www.amazon.com/gp/product/B01L2XR4P2/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1









I'd like to know how you neatly cut the square holes in your build?

Again, Thankin'
Gene


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## Barnsley Boy

I got the square/ rectangular hole using a dremel to cut out roughly, then some painstaking filing. It helps if the switch/ digital thermometer has a small lip around it to hide any rough edges. I did something similar recently on this to enlarge the bridge pickup hole to convert from open to covered pickups. Now that was a proper pain to get right. Wish I'd had the de-burring tool.


----------



## Gene Ballzz

Barnsley Boy said:


> I got the square/ rectangular hole using a dremel to cut out roughly, then some painstaking filing. It helps if the switch/ digital thermometer has a small lip around it to hide any rough edges. I did something similar recently on this to enlarge the bridge pickup hole to convert from open to covered pickups. Now that was a proper pain to get right. Wish I'd had the de-burring tool.



Yeah, that de-burring tool is the schnit! I've used one bit forever on aluminum and plastic. Just a little bit of steel chewed up two bits, in no time flat! Still worth it though, for what it does. It even gets right into the corners of square holes! One of the handiest tools I own, along with this:









Paolini Pocket Rules & PPR Rack-It


More videos are available in the Video tab below. In the hills of western North Carolina professional woodworker Greg Paolini builds one-of-a-kind commissioned furniture and exquisite custom kitchens. In his spare time, he designs tools to make woodworking faster, easier and more enjoyable...




www.woodpeck.com





I've only got a 6 inch in inches and yeah, it's a bit pricey, but also priceless in its function!

Enjoy,
Gene


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## Barnsley Boy

Oohh I like that a lot! One day, when I have my dream workshop maybe I could justify adding it to my fantasy list of tools!
...... but until then!


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## KIL0

Hey there,
i was looking for a 16 ohms dummy load for a fender hot rod deluxe to use with my torpedo live (which is 8 ohms only) when I stumbled across this design and this thread!

First of all I need to say that this design with all the simulations and support is absolute great work! I'm an electronics dev myself and I highly appreciate all the time and effort that has gone into this!

I wasn't able to get through all of the by now 146 pages of this thread and the search function didn't get me where I wanted so please forgive me if I address a yet solved issue but I would like to discuss the following three things:
- Has anyone used the M2 as a dummy load without a cab with something like a torpedo? I guess that due to the simulation results it should be fine to connect an 16 ohm resistor to an switched jack to connect if there is no cab plugged in
- Is there any thought about a 4 ohms version of the M2 box? I'm actually a bass player by trade and I'm thinking about to get an Ampeg SVT 2 Pro. This amp only got 4 ohms output matching (it also got a 2 ohms setting, but this is irrelevant for this issue) but for smaller venues I want to be able to connect an 8 ohms cab to it as well (this issue has been talked through before by example of an 16 ohms cab with a 8 ohms M2 i guess).
- Can anyone link me an air coil they used or can give me some detailed specs about them? I'm actually able to wrap them on a machine at my company's plant, maybe even with a 3D-printed base to fit the height into a 1HE rack housing. Or has somebody even made the air coil themselves? In this case I'd be glad to get some experiences! ,)

Best regards from Berlin, Germany


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## Gene Ballzz

@KILO

*First,*  to the forum!


That's a pretty ambitious number of watts to dissipate!  I'm guessing the unit, as well as the resistors involved in the first *"always on" *stage to be massive and fairly large (or numerous)! Cooling will be a primary concern. That SVT 2 Pro is likely capable of putting out in upwards of 500 watts, when really cookin'!

Good, Detailed Planning Is Critical!
Gene


----------



## JohnH

hi @KIL0 and welcome to our thread.

For the HRD, if you need a 16 Ohm load, you can build a 16 Ohm M2 and use it with either a 16 or 8 cab, or in principle, an 8 Ohm Torpedo. But when I looked up HRD's specs, It looked to have an 8 Ohm output rather than 16Ohm?, also ok with two cabs for a 4 Ohm load. But is yours a 16 Ohm version?

As a load box, it's safe to run an M2 with no speaker, so long as it's set to max attenuation. In fact, if you have the fixed Stage 1 plus at least the -14dB stage engaged, it's all fine with no cab.

I haven't tried IR loaders myself, but others have, I believe. If the line-out comes from the input, it captures the correct treble rise like a speaker, but there is not a bass-resonance peak there because M2 doesn't include it. So you might add a bit of low EQ to compensate. I tried some direct recordings and I found a bit of bass boost on the mixer, plus a sub 100hz cut did a good job.

But M2 gets the right response at the speaker, and its the real speaker doing it. So at the output, you do get the bass resonance plus treble rise, and its whatever is natural for the actual speaker. If you can keep a speaker plugged in, a line out from the output might be better.

M3 added a bass resonance circuit at the front end, but the parts to do it right get bulky and expensive (ferrite cored coil and big film caps). M2 works really well without this and I'm thinking it's not worth it.

You can build a 4 Ohm version, and basically coil and resistor values are x1/2 compared to 8 Ohm, then tweaked to fit with standard component values, ideally keeping ratios of values within each stage s consistent as possible. I have some values for a 4Ohm M2 if needed and others have built this.

But, if its really for a 500W amp, that is 5x anything attempted here before! I don't know how that'd look in practice, dealing with enormous heat.

Coils are easily bought. For the HRD, wind with 18 guage wire on an air-cored bobbin. They come from places that sell speaker cross-over components.

If you're winding one, you can start with an online calculator to work out turns etc based on bobbin dimensions.

I just bought a 1mH coil, which I plan to unwind to 0.9mH. I can measure this on my meter, and if you are in electronics, maybe you have access to inductance measurement gear too? In which case you can use an online calc to work it out reasonably close, and adjust with measurement.

With the online coil calculators, I find some of them don't provide a design that matches well with known real coils. But I'm finding this one is very close, (dimensions in inches though)









Air Core Inductor Designer / Calculator


Learn how to wind your own Inductor using thick copper wire for a low resistance coil.




www.diyaudioandvideo.com





For my unwinding, I'm going to work out how much I need to unwind to go from 1mH to 0.9mH, first based on the calc, and then in reality with the meter. 8 Ohm M2 is designed with a 0.9mH coil, but many suppliers don't list this. Actually, it's not super critical but might as well get it at the optimum if possible.

cheers
John


----------



## KIL0

Hey Gene,
Hey JohnH,

thanks for your reply!

@Gene Ballzz There is something with the output power specs with tube amps that I never really understood. The Amp is rated as 300 W so I knew, that there would be a bigger array of parts in the first stage but I didn't thought of more than double the load to dissipate!
But for sure I planned with heat sinks and active controlled cooling with temperature probing.

@JohnH Actually I wasn't able to check the exact output configuration on the HDR due to me not having it here, yet. But due to it beeing single 12" combo I thought it would be 16 ohms ^^
If the HDR does 8 ohms, I would not need the M2 for this situation at all. But otherwise I'd be interessted to check between running the torpedo in front or in the back of the M2. I also read about the other variants (M3, ect), the work that went into this is great!

For the SVT 2 Pro I guess it wouldn't be necessary to dissipate the whole 500 W due to I never think I'll be able to run this amp at more than 50% but I will be taking a look on how much bigger it will get for sure! On the other hand I first thought about implementing a power measurement with the fan controller for measuring the real amp output.

Regarding the coil: This calculator looks really good, the results contain all necessary information! I need to check what wire gauges we got around but around 200 turns with AWG 16 for 0.9 mH would be absolute easy to manufacture.
Measuring the coils should be no issue as well. I guess I should have access to a measuring device at the plant, otherwise I've seen that there are lots of projects on hackaday ect. around as small saturday evening builds.

Thanks a lot so far!

Regards


----------



## JohnH

I looked up that SVT! it's a BEAST! If you want to attenuate because of better tone up high, all the design theory should work but built with Stage 1 like an industrial substation. Speakon connectors instead of jacks, thick gauge for the coil and uprated resistors, probably divided in multiples, heatsink and fan cooling. It should work though.

(That assumes that, like a guitar amp, there are better tones at high volume, instead of just turning it down.)


----------



## KIL0

I calculated the coil and the resistors and if I use AWG10 in the coil this should work. If the amp really kicks 600 W (12,25 A @ 49V) AWG10 should be fine. For the resistors I would use the HS100 series from Arcol for the resistors, 10€ per 100 W of power consumption with 3 in parallel for R2A (33R) and R2B (22R) and for R1 maybe six 33R in parallel. This bank would have a size of around 300x200 mm without the air coil, so there would be lots of space for heat sinks and cooling fans.

@JohnH The main reason why I considered to build an attenuator for this amp was to be able to run it with 8 ohm cabs when not gigging with my own SVT 810 ,)
I guess I'll get the amp and start to measure out what currents I really get while playing and then think about getting this attenuator done. But in any case I'll go ahead and check if we got such thick wires for winding at the plant ^^

Regards


----------



## JohnH

*Unwinding an inductor coil: 1mH to 0.9mH *

Nothing better to do on a Saturday morning, so I decided to unwind an air-cored inductor.

The 8 Ohm version of the M2 attenuator calls for a 0.9mH inductor. But many standard ranges only offer 0.82mH or 1.0mH. To be honest, its not really critical, a 10% difference in this value doesn't make much difference - its still near enough to the sweet spot. But these coils are very simple things, and its easy to take a 1mH coil and take off the tape, unwind it a few turns, and then secure it again and then trim the wires. But how much wire to remove?

Ideally, if you have a meter that can read inductance, you can measure it and unwind with the meter on, until the reading is reduced as wanted. While doing this, don't cut the wire yet, just let it be free of the coil and then its not part of the inductance. This is what I have done before, since I have an inexpensive but very useful multimeter that does L and C and a few other extra useful things. After the right value is achieved, then secure the coil with tape, trim the lead and scrape/solder-tin the new wire end. 

But most meters don't have Inductance (L) ranges. So for this coil, destined for a future M2, I thought I'd study a bit deeper and note the outcome FYI. There are formulae for working out the inductance of a multilayer coil of given inner and outer diameters, and length of coil, and you can find these online as web apps, and also as mobile apps. So in principle, we can use one to work out the length of wire in a coil at one inductance value (given bobbin dims), and then at the reduced one and subtract to see how much wire to remove. I've been using this online one:









 Air Core Inductor Designer / Calculator


Learn how to wind your own Inductor using thick copper wire for a low resistance coil.




www.diyaudioandvideo.com





Unlike some others, I'm finding it does a good job of predicting a few real known coils.

Here is my coil, 1.0mH as bought from Wagneronline in Sydney. Its 18 gauge on a plastic bobbin, much as many others used i this thread. The brand is Dai-Chi:





The bobbin has an inner diameter of 25.0mm (ie the surface where the wire is wound onto it) and a length of 21.5mm between the end flanges. Interesting how the mounting hole is at the opposite end to the leads, which may be unusual but allows mounting with just a short screw.

The first measurement read 1.04 mH:




I cant tell whether the extra 4% is the coil or the meter, maybe some of each. I wouldn't expect this meter to be super-accurate, though it does seems to measure known close tolerance parts within their spec. So, I decided to target unwinding a little conservatively, to 0.92mH. 

Here's my results, both predicting how much to unwind using the web tool, and also what I actually did using the meter:




Based on my measured result and chosen target, the web app is saying to remove 1.2m, and in practice to get the result I removed 1.4m to get a 0.92 mH reading

The chart at left shows how the measured inductance reduced over thee steps of unwinding.

Had I not had the meter, and just assumed that the coil was 1mH and I was heading to 0.9mH, the app used this way would have predicted a 1.05m unwinding.

*Conclusion:*

Given we are making a small change to improve and the result doesn't have to be exact, given its acceptable with no change at all, using the web app to predict unwinding is ok. The stating point is to check that it reasonably predicts the basic coil. But having a meter that measures inductance is cool, and I like it!

Note the the specific results here are based on the geometry of my coil, and would be different if it was a different shape.


----------



## Chappy

*Effect of Screw (ferrous content) on Inductance*

I was curious as to how much a normal machine screw placed through the centre of the coil would affect the inductance in my attenuator build. Results from my spare 0.91mH coil from Solen Canada measured on an LRC bridge.

Air core only: 0.94mH Q 8.6
1 screw: *1.045mH Q 5.4*
2 screws: 1.135mH Q 4.4
3 screws: 1.225mH Q 4.1

I put my build together with one ferrous machine screw through the centre of the inductor to hold it in place. Looks like I need to find a nylon one and swap it out.


----------



## JohnH

Hi @Chappy , thanks for posting those, And I totally LOVE that piece of test gear! I'm thinking that didn't come from eBay? I'm remembering a Whearstone Bridge circuit from the high-school lab in the 70's, but with a reactive branch? Cool anyway!

Those inductance shifts match what I found too. And there's also an opposite gremlin to investigate, a reduction in L due to proximity to a metal chassis causing eddy currents. Both best avoided, and sadly they both degrade the Q, even though L might stay similar overall.


----------



## Chappy

JohnH said:


> Hi @Chappy , thanks for posting those, And I totally LOVE that piece of test gear! I'm thinking that didn't come from eBay? I'm remembering a Whearstone Bridge circuit from the high-school lab in the 70's, but with a reactive branch? Cool anyway!


John,

Actually it did come from Ebay. It is a US military AN/URM-90 test set. Built using top of the line components back in the late 50s. I had to wait quite a while for one to show up that was still in good condition. Fortunately I was the only bidder on this one, and I got it for a steal. I had to pay almost as much for the shipping as I did for the unit. I had to replace a few electrolytic caps that had dried up but otherwise it seems to work as advertised, giving very accurate results.

It is just as good, or better than the more popular General Radio bridges which show up quite often on Ebay. Also, at much lower cost as it is not as well known. Different types of bridges are switched into the circuit depending on the DUT. Wheatstone for R, De Santy for C, Maxwell or Hay for L. It uses a 0.1uF precision capacitor for reactance measurements. It has two internal DC voltages for resistance measurements, and a 1KHz sine signal for L, C, & AC resistance measurements. External signals can be used as well as external meters for showing the null.

Steve


----------



## Gene Ballzz

So folks,

Having just built three of these units, I tested them extensively, in a sonic manner, as in no SPL meter! One was an M2/16ohm with an extra, tonally compensated 8ohm out and the other two were M2/8ohm with an added, tonally compensated 16ohm out. One of the M2/8ohm units has a fan installed! These made six of these units I've built! The first three were prototypes for testing, layout and reworks.

The test subjects were my Marshall SV20H head, a 16Ω 1960B, 4x12 with 98db Greenbacks and a 1965B 4X10 with 94db G10L-35s. I recorded a 1 minute-ish, cranking rhythm loop and for each test (except stepping through the stages) I let the loop crank at full attenuation for at least an hour or more. It should be noted that comparing a 98db speakers loaded cabinet to a 94db speakers loaded cabinet complicated the comparison, just a little bit.

Here are the sonic and heat results:
> Both the 8Ω and 16Ω units performed flawlessly, with no tonal artifacts and/or losses at all attenuation levels. 
> On both units, the compensated out (for 16Ω on the 8Ω unit and 8Ω out on the 16Ω unit) retained all tone, but did indeed give just a tiny bit more attenuation (I'd guess around 1 or 2 db) than using a cabinet "native" to the unit's input impedance.
> All three units heated up pretty well (not burn your hand hot, but almost as hot as a fresh cup of coffee), but the one 8Ω unit with a fan, cooled down to just barely "luke warm" with the fan running!

My conclusion is that this attenuator design is "BY FAR" the best ever conceived, retaining all tone, feel and response throughout its range of operation! When properly ventilated and heat sinked, it requires no fan up to about 30 watts, fully cranked for long perioids of time. Anything above 35 watts and yup to 50 watts, fully cranked should utilize a fan!

Now, FWIW, the reason I've built/am building so many of these, is that every time I get one finished and folks hear/see it, someone wants to buy it! I use them all the time, so I need to have at least an 8Ω & 16Ω unit on hand, for my own use! Here ar pics of #4 & #5, and I'll get pics up of #6, once the fan guard/filters arrive and get installed! This is the formt I plan to use for all future 50 watt M2 builds, although, I've found some slightly more pleasing feet! Anyone interested in getting their hands on one of these, while avoiding the time/effort of building, can feel free to contact me privately, as custom features can be either added or deleted! I'm not a business and am not making these commercially, I simply want folks to be able to enjoy the liberation these units provide, for those not equipped to do a build themselves! They truly are the greatest thing since sliced bread!

Thanks 4 Looking!
Gene

M2/16Ω


----------



## JohnH

Hi Gene! I'm really delighted to see these and its great news to hear about the testing. They look really good and I'm very happy to have developed the design together with you! 

Thanks also to everyone who has trusted this design and provided encouragement and feedback.


----------



## EC Strat

Gene Ballzz said:


> So folks,
> 
> Having just built three of these units, I tested them extensively, in a sonic manner, as in no SPL meter! One was an M2/16ohm with an extra, tonally compensated 8ohm out and the other two were M2/8ohm with an added, tonally compensated 16ohm out. One of the M2/8ohm units has a fan installed! These made six of these units I've built! The first three were prototypes for testing, layout and reworks.
> 
> The test subjects were my Marshall SV20H head, a 16Ω 1960B, 4x12 with 98db Greenbacks and a 1965B 4X10 with 94db G10L-35s. I recorded a 1 minute-ish, cranking rhythm loop and for each test (except stepping through the stages) I let the loop crank at full attenuation for at least an hour or more. It should be noted that comparing a 98db speakers loaded cabinet to a 94db speakers loaded cabinet complicated the comparison, just a little bit.
> 
> Here are the sonic and heat results:
> > Both the 8Ω and 16Ω units performed flawlessly, with no tonal artifacts and/or losses at all attenuation levels.
> > On both units, the compensated out (for 16Ω on the 8Ω unit and 8Ω out on the 16Ω unit) retained all tone, but did indeed give just a tiny bit more attenuation (I'd guess around 1 or 2 db) than using a cabinet "native" to the unit's input impedance.
> > All three units heated up pretty well (not burn your hand hot, but almost as hot as a fresh cup of coffee), but the one 8Ω unit with a fan, cooled down to just barely "luke warm" with the fan running!
> 
> My conclusion is that this attenuator design is "BY FAR" the best ever conceived, retaining all tone, feel and response throughout its range of operation! When properly ventilated and heat sinked, it requires no fan up to about 30 watts, fully cranked for long perioids of time. Anything above 35 watts and yup to 50 watts, fully cranked should utilize a fan!
> 
> Now, FWIW, the reason I've built/am building so many of these, is that every time I get one finished and folks hear/see it, someone wants to buy it! I use them all the time, so I need to have at least an 8Ω & 16Ω unit on hand, for my own use! Here ar pics of #4 & #5, and I'll get pics up of #6, once the fan guard/filters arrive and get installed! This is the formt I plan to use for all future 50 watt M2 builds, although, I've found some slightly more pleasing feet! Anyone interested in getting their hands on one of these, while avoiding the time/effort of building, can feel free to contact me privately, as custom features can be either added or deleted! I'm not a business and am not making these commercially, I simply want folks to be able to enjoy the liberation these units provide, for those not equipped to do a build themselves! They truly are the greatest thing since sliced bread!
> 
> Thanks 4 Looking!
> Gene
> 
> M2/16Ω
> View attachment 112748
> 
> 
> 
> View attachment 112749
> View attachment 112750
> View attachment 112751


I'm loving the one you built for me! Completely agree, it's absolutely incredible.


----------



## Gene Ballzz

A new standard for my future builds. I'm including a fan that can either be used or not. Up to at least 35-40 watts, fully cranked is not really a problem for heat, but a 50 watt amp cranked for long periods definitely needs a bit of help from a fan! I figured the filters to be necessary to keep out the majority of gin mill smoke and dust, as that kinda goo is not conducive to good cooling!
Just Sharin'
Gene


----------



## Gene Ballzz

pietro.castelli said:


> Hi Folks,
> I’m very interested in the simple attenuator.
> I would really like to build one to tame the plexi 51 all access I’m finishing, but I’m afraid the 50w version will be struggling to deal with the amp.
> 
> Has anybody succesfully built a 100w version of the attenuator? What changes are required over the standard?
> 
> I apologize if this has already been discussed in the original thread, but the latter gee almost 150 pages long
> 
> Thanks to anybody willing to help!





JohnH said:


> Hi @pietro.castelli
> 
> For 100W amps, we double all resistor power ratings, get a coil based on 16 gage wire and add a fan. The simplest way to do that is a 12 Volt DC fan, and run it to a 9V wall wart. Use a thick aluminum diecast case and plenty of holes for venting. Please post in main thread if any questions.



@JohnH ,
Is it actually necessary to double the wattage of all the 25 watt resistors, for a 100 watt build, or is simply doubling the wattage of those in the first *"always on"* stage enough? I'm getting ready to design my first 100 watt unit, so………?
Thanks John,
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> Is it actually necessary to double the wattage of all the 25 watt resistors, for a 100 watt build, or is simply doubling the wattage of those in the first *"always on"* stage enough? I'm getting ready to design my first 100 watt unit, so………?
> Thanks John,
> Gene



Hi Gene. Some of the 25W ones could be OK for a 100W build. but not all IMO.

When i worked these out, I took the nominal power of the amp, assumed it was steady state and worked out how much goes to each resistor. Then I added 50% for overdriving the amp and then a factor of x2. This was for the M design but is hasn't changed really, especially for the later stages 2. 3 and 4. Design M had the -3.5 stage able to run on its own, but I also worked it out if not. See post #1

This was the table:





So, at 50W in, R4, R6 and R8 only got 4W max. Double that for 100W and then x3 you get 24W. by my method, so OK for a 25W resistor. But the others would go up to 50W rating on this basis. eg 7W x 2 x3 = 42W, so 50W rating.

At stage 1, Id use all 100W resistors, with two of them combining for R1. And wire them as spread out as possible.

All that may be over conservative, and probably the fan cooling and case size is more important than the power ratings, but I think its generally ok.


----------



## pietro.castelli

Thanks both for your precious help! This forum is absolutely great!

I’ve a couple additional questions, If I may:

-what’s the purpose of having out1 and out2 in the circuit? Just connecting two speakers in parallel?

-would it be theoretically possible to add one more resistive stage to achieve a total attenuation close to 37-38db?

Cheers

Pietro


----------



## JohnH

Hi Pietro

Yes Out 1 and Out 2 can be for two cabs, or a cab and an internal speaker. On mine, I have an 8 Ohm version and Out 1 and Out 2 are great for either one 8 ohm cab, or two 16 Ohms which equal to 8Ohm together.

You can add more stages, with no problem. They can be any value you want within the range -3.5db to -14db, and you can add more than one added stage if wanted. No tone is lost .


----------



## pietro.castelli

I'm shopping for the components, but I'm having trouble finding the correct inductor.

Would something like mouser 80-SCF25XV2402R006JV work?

Also for switches: are mouser 691-6FB53-73 ok?

For the enclose I was thinking an Hammond 1550J


----------



## BlueX

pietro.castelli said:


> but I'm having trouble finding the correct inductor.


I think you should have something like this: 0,9 mH, 18 AWG, air core coil (for the 8 Ohm M2 version)








Order the Jantzen Audio 000-1953 crossover coil


Buy the 000-1953 at SoundImports ✓ Same day shipping ✓ Low shipping rates




www.soundimports.eu





Switches look OK with SPDT ON-ON design and 10A rating at 250VAC (again for the normal M2 version).


----------



## matttornado

Gene Ballzz said:


> So folks,
> 
> Having just built three of these units, I tested them extensively, in a sonic manner, as in no SPL meter! One was an M2/16ohm with an extra, tonally compensated 8ohm out and the other two were M2/8ohm with an added, tonally compensated 16ohm out. One of the M2/8ohm units has a fan installed! These made six of these units I've built! The first three were prototypes for testing, layout and reworks.
> 
> The test subjects were my Marshall SV20H head, a 16Ω 1960B, 4x12 with 98db Greenbacks and a 1965B 4X10 with 94db G10L-35s. I recorded a 1 minute-ish, cranking rhythm loop and for each test (except stepping through the stages) I let the loop crank at full attenuation for at least an hour or more. It should be noted that comparing a 98db speakers loaded cabinet to a 94db speakers loaded cabinet complicated the comparison, just a little bit.
> 
> Here are the sonic and heat results:
> > Both the 8Ω and 16Ω units performed flawlessly, with no tonal artifacts and/or losses at all attenuation levels.
> > On both units, the compensated out (for 16Ω on the 8Ω unit and 8Ω out on the 16Ω unit) retained all tone, but did indeed give just a tiny bit more attenuation (I'd guess around 1 or 2 db) than using a cabinet "native" to the unit's input impedance.
> > All three units heated up pretty well (not burn your hand hot, but almost as hot as a fresh cup of coffee), but the one 8Ω unit with a fan, cooled down to just barely "luke warm" with the fan running!
> 
> My conclusion is that this attenuator design is "BY FAR" the best ever conceived, retaining all tone, feel and response throughout its range of operation! When properly ventilated and heat sinked, it requires no fan up to about 30 watts, fully cranked for long perioids of time. Anything above 35 watts and yup to 50 watts, fully cranked should utilize a fan!
> 
> Now, FWIW, the reason I've built/am building so many of these, is that every time I get one finished and folks hear/see it, someone wants to buy it! I use them all the time, so I need to have at least an 8Ω & 16Ω unit on hand, for my own use! Here ar pics of #4 & #5, and I'll get pics up of #6, once the fan guard/filters arrive and get installed! This is the formt I plan to use for all future 50 watt M2 builds, although, I've found some slightly more pleasing feet! Anyone interested in getting their hands on one of these, while avoiding the time/effort of building, can feel free to contact me privately, as custom features can be either added or deleted! I'm not a business and am not making these commercially, I simply want folks to be able to enjoy the liberation these units provide, for those not equipped to do a build themselves! They truly are the greatest thing since sliced bread!
> 
> Thanks 4 Looking!
> Gene
> 
> M2/16Ω
> 
> 
> 
> Looks amazing!!!! I'd like to re house mine a well. Ttrying to find the perfect enclosure as I have a giant 300 watt resistor taking on the load at all times.


----------



## Gene Ballzz

pietro.castelli said:


> Hammond 1550J




I'm getting ready to work up a layout and drilling template and the 1550J is exactly what I was looking at! FWIW, I do some cool templates that can be printed on peel & stick project paper and stuck to the box for center punching and drilling. It would give a look similar to the pics in posts #2931 & #2934 of this thread, labeling/art work is up to you. Saves a bunch of time and layout hassle. It may take up to a week for me to get the layout done! 

Knowing what part of the world you're in would help for parts sourcing suggestions! Also, reminding us of whether this is a 16Ω or 8Ω build would be helpful? Additionally, avoid the Vishay/Dale resistors from Mexico, as the mounting footprint is a little different than the TE and Ohmite/Arcol units. DigiKey currently has a better selection of "in stock" resistors available than Mouser, in TE Conectivity brand, and they ship almost anywhere for free, with a big enough order!







BlueX said:


> I think you should have something like this: 0,9 mH, 18 AWG, air core coil (for the 8 Ohm M2 version)
> 
> 
> 
> 
> 
> 
> 
> 
> Order the Jantzen Audio 000-1953 crossover coil
> 
> 
> Buy the 000-1953 at SoundImports ✓ Same day shipping ✓ Low shipping rates
> 
> 
> 
> 
> www.soundimports.eu
> 
> 
> 
> 
> 
> Switches look OK with SPDT ON-ON design and 10A rating at 250VAC (again for the normal M2 version).




Pietro is building a 100 watt (as my next build will be) so needs at least a 16 AWG coil. I think PartsExpress may be the most convenient source.






Home


In-stock Air Core Inductor Crossover Coils sold in various inductance, wire gauge, and brands like Dayton Audio, Jantzen Audio and ERSE Audio at Parts Express.



www.parts-express.com





I personally prefer the bobbin wound units from folks like Madisound, but they don't sell a 0.90mH and the 1.0mH that could get unwound a little bit, is currently out of stock!






16 AWG Madisound Sidewinder Inductors: Madisound Speaker Components, Inc.


Free support for loudspeaker projects, sourcing OEM speaker building supplies, and passive crossover design. We sell raw speaker drivers (tweeters, woofers, subwoofer, midrange drivers, full range drivers), speaker kits, amplifiers, capacitors, resistors, and inductors. Hard drawn copper for...




www.madisoundspeakerstore.com





Just Attenuatin'
Gene


----------



## Marcomel79

Wow its was about 116 pages when i was building my attenuator!! Things happen quickly here!


----------



## BlueX

pietro.castelli said:


> but I'm having trouble finding the correct inductor.





Gene Ballzz said:


> Pietro is building a 100 watt (as my next build will be) so needs at least a 16 AWG coil. I think PartsExpress may be the most convenient source.


Here's a 0,9 mH, 15 AWG, version, from another European web shop if that's more convenient.





Jantzen Audio 000-1657 Jantzen 0,900 mH air core coil Cu 1,4 (15AWG) Rdc 0,290 Ohm | Europe audio


High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.Modern self-supporting design, low tolerance and power handling dependent on the wire used, makes it the most versatile coil on th




europe-audio.com





Since different versions are now built, here is the Jantzen webpage with an Excel file you can download. There you can filter on inductance, AWG, etc., to find the right item.








Air Core Wire Coil - Jantzen-audio.com


High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.




www.jantzen-audio.com





I then just google "Jantzen <item no>" to find a web shop where it's available. Seems like Jantzen's hifi coils are available with many different models in Europe.


----------



## pietro.castelli

Gene Ballzz said:


> I'm getting ready to work up a layout and drilling template and the 1550J is exactly what I was looking at! FWIW, I do some cool templates that can be printed on peel & stick project paper and stuck to the box for center punching and drilling. It would give a look similar to the pics in posts #2931 & #2934 of this thread, labeling/art work is up to you. Saves a bunch of time and layout hassle. It may take up to a week for me to get the layout done!
> 
> Knowing what part of the world you're in would help for parts sourcing suggestions! Also, reminding us of whether this is a 16Ω or 8Ω build would be helpful? Additionally, avoid the Vishay/Dale resistors from Mexico, as the mounting footprint is a little different than the TE and Ohmite/Arcol units. DigiKey currently has a better selection of "in stock" resistors available than Mouser, in TE Conectivity brand, and they ship almost anywhere for free, with a big enough order!



Sweet! I would greatly appreciate the printable drilling layout when you are done! 

I’m based in Europe, so mouser is definitely convenient for me! Digikey is also fine.
I’d like to build the 8 ohm version with the additional 16 ohm output.

I’ve already identified all the necessary resistors from ohmite/arcol and they are sitting in my cart.
The 100w 18 ohm resistor unfortunately is out of stock at the moment, but I’ll wait a bit before ordering.

I just need to find alternatives for the switches as the one I’ve selected initially are very expensive!

I think I just miss the inductor but I’ll follow all the precious advices I’ve been given to source it!

Thanks so much Gene and also thanks to all the people who chimed in to help!


----------



## pietro.castelli

BlueX said:


> Here's a 0,9 mH, 15 AWG, version, from another European web shop if that's more convenient.
> 
> 
> 
> 
> 
> Jantzen Audio 000-1657 Jantzen 0,900 mH air core coil Cu 1,4 (15AWG) Rdc 0,290 Ohm | Europe audio
> 
> 
> High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.Modern self-supporting design, low tolerance and power handling dependent on the wire used, makes it the most versatile coil on th
> 
> 
> 
> 
> europe-audio.com
> 
> 
> 
> 
> 
> Since different versions are now built, here is the Jantzen webpage with an Excel file you can download. There you can filter on inductance, AWG, etc., to find the right item.
> 
> 
> 
> 
> 
> 
> 
> 
> Air Core Wire Coil - Jantzen-audio.com
> 
> 
> High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.
> 
> 
> 
> 
> www.jantzen-audio.com
> 
> 
> 
> 
> 
> I then just google "Jantzen <item no>" to find a web shop where it's available. Seems like Jantzen's hifi coils are available with many different models in Europe.



This is great! Thanks!


----------



## Gene Ballzz

pietro.castelli said:


> Sweet! I would greatly appreciate the printable drilling layout when you are done!
> 
> I’m based in Europe, so mouser is definitely convenient for me! Digikey is also fine.
> I’d like to build the 8 ohm version with the additional 16 ohm output.
> 
> I’ve already identified all the necessary resistors from ohmite/arcol and they are sitting in my cart.
> The 100w 18 ohm resistor unfortunately is out of stock at the moment, but I’ll wait a bit before ordering.
> 
> I just need to find alternatives for the switches as the one I’ve selected initially are very expensive!
> 
> I think I just miss the inductor but I’ll follow all the precious advices I’ve been given to source it!
> 
> Thanks so much Gene and also thanks to all the people who chimed in to help!



For all the 100 watt resistors, you may want to try flea-bay for some of the vendors from chinesiawanoreanam, as they have a smaller footprint and are consiederably less pricey. There is often quite a lag for shipping, but the other conveniences make it worth the wait! A bit of time spent can find the best vendors that won't "bone" you on prices/shipping fees! I'm assuming that you've already caught on that the 200 watt R1 is best tackled by two 100 watt, 30Ω resistors in parallel, for your 8Ω build?

Part of my layout research will surround which is bet for the 50 watt resistors, two 25 watt in series and/or parallel or single 50 watt units. With the templates I'll provide, it's your choice whether you countersink the mounting holes or leave them for protruding screw heads. Also, we'll need to share real email addresses, as we have no good way to share pdf, tiff or png files here.

Just Helpin'
Gene


----------



## lordjester

Hi Folks,

I´m still not sure about the impedance thing. I built a 8 Ohm M2 (with 16 Ohm Option). I´m normally using it with a Magnatone Varsity, 8Ohm speaker output with an 8 Ohm speaker. No problem so far. If I use a second speaker, 8 Ohm at the second 8 Ohm speaker output of the M2, the amp sees 4 Ohms? Right? So , how can I use a second 8 Ohm speaker with the M2. Any mods for this? thanks, Lothar


----------



## JohnH

scozz said:


> I think maybe you meant to say “famous” and not “infamous”, when describing Fender amps, but I could be wrong. Maybe you just don’t like Fender amps?





pietro.castelli said:


> Thanks both for your precious help! This forum is absolutely great!
> 
> I’ve a couple additional questions, If I may:
> 
> -what’s the purpose of having out1 and out2 in the circuit? Just connecting two speakers in parallel?
> 
> -would it be theoretically possible to add one more resistive stage to achieve a total attenuation close to 37-38db?
> 
> Cheers
> 
> Pietro





lordjester said:


> Hi Folks,
> 
> I´m still not sure about the impedance thing. I built a 8 Ohm M2 (with 16 Ohm Option). I´m normally using it with a Magnatone Varsity, 8Ohm speaker output with an 8 Ohm speaker. No problem so far. If I use a second speaker, 8 Ohm at the second 8 Ohm speaker output of the M2, the amp sees 4 Ohms? Right? So , how can I use a second 8 Ohm speaker with the M2. Any mods for this? thanks, Lothar


Hi Lothar,

You can do this with an M2. Plug both cabs into the two 8 Ohm outputs , and although the attenuator sees 4ohm, it still shows the amp an 8 0hm load because Stage 1 is still there, so its quite safe.

You may hear a very small tone difference at 4ohms, with about a db more treble and bass compared to mids . If you'd like to adjust for that slight difference, you can put a 39 Ohm resistor across the speaker output either via a switch or you can engage it using another jack. I suggest to try as it is first though .


----------



## lordjester

Thanks! Does it have to be 25 W ??


----------



## JohnH

hi @lordjester Actually, apologies but the right value for this 4ohm case is 20 or 22 Ohm . A 25w would be fine, 10w is also ok with a 50w amp.


----------



## MikeD2

JohnH said:


> I've also been thinking about how best to lay out the parts, so its easy to follow and easy to see and wire up.
> 
> Here is a reasonably to-scale M2 layout, in a Hammond 1550G case:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> M2 Layout And Schematic211231
> 
> 
> 
> __ JohnH
> __ Dec 30, 2021
> 
> 
> 
> 
> 
> (diagram edited 31/12/21)
> 
> It's basically an 8 Ohm M2, including the extra Out 3 for 16 Ohm output (which I find useful), and a basic line-out. There's a few new wrinkles, so I put the schematic and parts table on the diagram.
> 
> With long shipping times form China, more people seem to be using branded resistors such as Arcol. As noted above, the 100W ones get huge. So this uses a pair of 50W resistors for R1 (ie R1A and R1B). This may give a more compact and neater arrangement. Two 15's in series to make 30, or two 30's in parallel make 15. I separated the two R1A and R1B resistors to spread out the heat.
> 
> I moved the order of stages to go -14, -7, -3.5 db so that the logical front panel order matches the electrical and internal order. This is the same as Gene did above, and has been found to make no difference technically.
> 
> If you get jacks with 3x2 legs, then its important to connect to the primary lugs, not just to the switched ones, which are not used here. So to guard against error, the lugs are all shown linked across the jack so there's no risk. I show the jacks all in line, for ease of drawing. But what I actually build in mine is to put Out 1 and Out 2 vertically aligned, and you can rotate and engage/solder the lugs together as a preassembled unit - see inset diagram.
> 
> The case size is 222 x 146 and allows a fairly spacious layout, or by squeezing it up more, more add-on features.
> 
> The extra output and the line-out seem to be the most useful add-ons. I'd recommend against adding too many extras, but there are several others that have been developed in this thread:
> 
> 
> options for* variable input impedance* (useful if you have several amps with no output tap values in common. But note you can already use different speaker ohms in the base designs. Also, you can easily build a couple of these units at different ohms to cover your options. Most ways to convert to different input ohms on one unit take off some more power, or get complex)
> *switching to reduce attenuation of Stage 1*, to get a -3.5db setting (this can be useful if you want to have just a slight reduction. But with most rigs, even -7db is still loud. And, as a work around, you can set the unit as a load box in parallel with a speaker, to take off just 3 db at half the ohms amp tap)
> *foot-switching attenuation stages* (useful if you want a lead/rhythm change, with no added drive, and can afford a few extra db's reduction.)
> *bass resonance circuit* (M3 design - tests so far suggest that this makes very little difference to perceived tone when using a speaker, but may be desirable for when used as a load box with no speaker. The added cap and coil to convert an M2 to an M3 are relatively bulky and expensive. Note that when you run with a speaker, it will naturally make is own bass resonance without this circuit.)
> *full bypass switch* (IMO, very few players really need this, unless you are setting levels with no or very little attenuation. After a few tests, most will either use the attenuator to attenuate, or not use it if its not wanted. A full bypass switch is handling the full amp power, so the chances of setting it up or operating it wrongly are significant)
> *fan cooling* (the designs listed here, in a thick AL box are good for up to about a 50W amp with no fan. A few running amps at 100W have used them however)
> Note that with the *line-out* option, even if you don't use the pot, you may well find that the signal level at the final attenuated speaker output is at a good level to feed into a mixer line-in, with no dedicated line-out. In any case, an IR loader or cab-sim will be needed to replicate a credible speaker tone.


Hello everyone - 
New to the forum. Looking forward to the M2 build with the line-out option - gathering parts now. One question I had is: does it matter if the potentiometer is linear or log taper?
PS: Thanks to everyone on here, (especially John H, otherwise I's still be running my Ceriatone ODS at probably 40 watts at home - not a good idea).


----------



## JohnH

hi @MikeD2 and welcome to our thread.

Linear and log pots of the same value will give the same range. But log may offer more control to set the level that you need. Also given that you'll need to adjust the pot to suit the amp setting plus the input range of whatever you are connecting to. And, if the signal is too hot overall, the fixed resistor or resistors could be changed.


----------



## MikeD2

JohnH said:


> hi @MikeD2 and welcome to our thread.
> 
> Linear and log pots of the same value will give the same range. But log may offer more control to set the level that you need. Also given that you'll need to adjust the pot to suit the amp setting plus the input range of whatever you are connecting to. And, if the signal is too hot overall, the fixed resistor or resistors could be changed.


Thankyou John, appreciate your help! Off to AliExpress


----------



## GeorgeLG

Thanks for all the work done on this massive thread. I want to build an 8 ohm reactive load. Can someone point me to the place in this discussion where the best design is using the most readily available component values and a source for the high powered inductors and resistors? Dealing with chemo and I just don't have the energy to read this entire discussion. Thanks in advance,

George


----------



## JohnH

hi @GeorgeLG , welcome to our thread

Happy ro help! Maybe if you could describe some more about what you'd like it to do? Reactive Load boxes generally are to replace a guitar speaker and provide a line-out signal to go to a cab-sim, Impulse Response loader and/or mixer or recording software, ie an actual guitar speaker is not involved, except possibly after another power amp. It's focus is just to show the amp an impedance vs frequency curve similar to a guitar cab.

Our designs here are Attenuators, which also show the amp a reactive load, but then include circuitry to step that down to feed a guitar speaker. You can use them without a speaker, just as a load, but the design focus is based on use with a speaker and that's what they are optimised for. Post 1 of this thread has the basics covered, see design M2

A guitar speaker impedance changes with frequency It hits the nominal ohms value at around 400 or 500 hZ. Above that it rises due to the inductance of the voice coil . And below this, there is a strong bass peak due to resonance, at around 80 to 120 hz.

It's this low resonance that can be the main difference with loadboxes vs our designs. To properly create the bass resonance in a load box with no speaker needs a big cap and a large coil. The best reactive load boxes eg by Suhr, include this. Our M2 Attenuator design does not have this, instead, we let the real speaker develop it itself. This saves the most expensive parts, but it needs a speaker. This approach works very well in the attenuator.

We can use M2 as a loadbox, safely and effectively but since the lineout doesn't have a bass peak, a bit of bass compensation downstream may be needed. Or, we have M3 that has these parts.

The bass peak is very narrow, sharply around just a few low notes on a guitar. So it's actually not that significant for most of the guitars range 

So what's your intent?

1. You could build our M2 to use with a guitar speaker, with a line-out for when you don't use a cab, and maybe tweak the bass when you just use line-out.or,
2. Can add parts to make it M3, to show the amp a bass peak, or
3. As M2, but no speaker out just a load and add bass if needed (inexpensive and simple, just a few parts) or 
4. As 3, with bass peak circuit.

Related questions are, what amps do you want to use it with? (Power and output ohms), and where in the world are you wrt ordering parts in your region?

If you'd like to read up more just on reactive load boxes, this is a great thread on TGP, which helped a lot with our designs:

https://www.thegearpage.net/board/index.php?threads/aikens-reactive-dummy-load.1072793/

All the best!


----------



## redberon2003

Decided to build one of these in a 100w version using the minimum number of resistors instead of math because lazy………also wasn’t happy with the 200w 30ohm selection so I decided to just get a 300w

……….that 300w is a real honker lol


----------



## redberon2003

btw. If anyone has any enclosure ideas, I’m all ears.


----------



## JohnH

Hammond cases are good, get one that is generously sized (maybe 250mm length) and plan the layout carefully. For 100W, the concensus here is thst you need a fan. A brushless dc fan designed for 12v but running at 9v from a wall warts is a good option. And plenty of vent holes above and below


----------



## matttornado

redberon2003 said:


> Decided to build one of these in a 100w version using the minimum number of resistors instead of math because lazy………also wasn’t happy with the 200w 30ohm selection so I decided to just get a 300w
> 
> ……….that 300w is a real honker lol
> View attachment 113629


I use one of those 300 watt 30 ohms too. MASSIVE! It does the job well but does get hot when pushing a cranked 100 watt JMP through it! I added a fan with a switch to cool it down when needed. Probably don't need it but added the fan for a piece of mind.


----------



## emann

Hi to All,

first post here after being directed to this website from someone who* I thank so much!*

I wanted to check with anyone who has maybe used this attenuator with a fender 65 Deluxe Reverb Reissue and what results are achieved. I mainly use mine with a KingTone Duellist pedal and cannot get anywhere near the tone that I am after if not at high volumes. Any experiences here please?

I was advised that the build I would require for the 65 DRRI is the 8ohm M2 variant and so I found this thread #2223 on page 112 hoping that someone can confirm is the latest design please? I am no tech myself but maybe I can get all the required parts and find a local engineer who would be able to do the assembly for me.




Can I get confirmation that from the table above and for the 8ohm M2, the items I require are the ones in the row of the 8ohm model please? Could any one also maybe list other items I would need such as paste, type of bolts, type of input jacks, etc...bear with my ignorance at this stage but any help is appreciated.

Regards,
Emann


----------



## emann

pietro.castelli said:


> This is great! Thanks!


Hi,

noting you are preparing your first built and you are also from Europe. I started reading this thread and am about page 10 now!

If you are doing the8ohm M2 version, can I kindly ask you to list all the parts required for the assembly and respective suppliers in Europe please - I note you say that you have most of the items in the cart with a supplier so this would be very helpful for me.

Regards,
Emann


----------



## JohnH

Hi @emann , welcome to our thread. The diagram that you linked above is based on the latest design. Is your amp 22W with an 8 ohm speaker? If so the 8 ohm version with parts as listed should work very well. It has been tested with many different amps including a range of Fenders, Im not sure about your one myself though.

With all these builds, there is the core design and then then some options. That diagram features a line-out circuit and also extra jacks so that you can use a pair of 16Ohm cabs, or a single 16 ohm cab, still running the attenuator from the 8Ohm amp. You can decide if they are useful to you.

Jacks should be plastic Cliff type, and you should get 'stereo' (TRS = Tip, Ring, Sleeve). They grip better than mono jacks even if only used for mono. Bolts for mounting resistors are usually M3, I use 10mm long. You can countersunk or dome head. Everything else is shown but feel free to post here again.


----------



## Gene Ballzz

@emann 

To add to the comments from @JohnH , All wire, as well as the 0.90 mH inductor coil need to be minimum #18 gauge. Switches can be tough, as many have very small holes in the lugs. I have just started ordering switches from GC part number 35-130 for SPDT and 35-146 for DPDT, both rated at 20A for 125 volts. They have screw terminals, but once you remove the screws, you have big honkon' holes that will accept multiple wires, as needed. Many of us have been using DPDT switches and simply tying both sides together for added current capacity, as well as redundancy! I'm not certain that is necessary with these model switches!

In your situation, I'm going to bet that if you want to either use a different speaker, or add a speaker it would be another 8 ohm speaker. In that case, you'd want the extra speaker jack to give compensation for a 4ohm load. I don't think @JohnH has provided the single resistor value for that, or shown the rather simplified version of that part of the circuit. From our private communications, I'm also betting the line out that John showed may well be useful to you, although it will likely require some EQ treatment in you DAW.

Best To Ya,
Gene


----------



## BlueX

emann said:


> Hi,
> 
> noting you are preparing your first built and you are also from Europe. I started reading this thread and am about page 10 now!
> 
> If you are doing the8ohm M2 version, can I kindly ask you to list all the parts required for the assembly and respective suppliers in Europe please - I note you say that you have most of the items in the cart with a supplier so this would be very helpful for me.
> 
> Regards,
> Emann


Here are the parts I ordered for my M2 (8 Ohm in, 1x8/2x16 and 1x16 Ohm out, 50 W).
Jacks, switches, and resistors from Digi-Key, about EUR 75. Free shipping, within 48 hours, from US to Europe. Maybe not the cheapest, but I appreciate fast, and complete, delivery.

IndexQuantityPart NumberManufacturer Part NumberDescription1​4​3185-CL12207A-NDCL12207AS2 BBB SOLDER TAG GOLD ASSEMBLED2​3​563-1838-NDET105D12-ZSWITCH TOGGLE SPDT 5A 125V3​1​A102357-NDHSC10015RJRES CHAS MNT 15 OHM 5% 100W4​1​HS5022RJ-NDHS50 22R JRES CHAS MNT 22 OHM 5% 50W5​1​A132038-NDTHS5018RJRES CHAS MNT 18 OHM 5% 50W6​2​A131986-NDTHS2515RJRES CHAS MNT 15 OHM 5% 25W7​2​KAL25FB10R0-NDKAL25FB10R0RES CHAS MNT 10 OHM 1% 25W8​1​A102145-NDHSA254R7JRES CHAS MNT 4.7 OHM 5% 25W9​1​A132015-NDTHS255R6JRES CHAS MNT 5.6 OHM 5% 25W10​1​A102139-NDHSA2533RJRES CHAS MNT 33 OHM 5% 25W11​1​A102426-NDHSA2568RJRES CHAS MNT 68 OHM 5% 25W

Jantzen air core coil 0,9 mH, 18 awg: part numbers 000-1647, 000-1953, or 000-1969 (different sizes for the spooled coil). Google "Jantzen <part no>" to find best price and shipping rate to your location (and availability).

Distrelec is a big European supplier of parts for electrical and electronic applications, but they didn't have the jacks on stock when I ordered.


----------



## emann

JohnH said:


> Hi @emann , welcome to our thread. The diagram that you linked above is based on the latest design. Is your amp 22W with an 8 ohm speaker? If so the 8 ohm version with parts as listed should work very well. It has been tested with many different amps including a range of Fenders, Im not sure about your one myself though.
> 
> With all these builds, there is the core design and then then some options. That diagram features a line-out circuit and also extra jacks so that you can use a pair of 16Ohm cabs, or a single 16 ohm cab, still running the attenuator from the 8Ohm amp. You can decide if they are useful to you.
> 
> Jacks should be plastic Cliff type, and you should get 'stereo' (TRS = Tip, Ring, Sleeve). They grip better than mono jacks even if only used for mono. Bolts for mounting resistors are usually M3, I use 10mm long. You can countersunk or dome head. Everything else is shown but feel free to post here again.


Thanks to JohnH, Ballzz and BlueX.

With some assistance from work and being pointed to a good website, I maybe can get the items and have someone solder them in the enclosure.

I proceeded to prepare a table based on Bluex and hyperlinked the products I could find in order to check the cost as per below. The supplier Farnell seems to have everything in the list except for the resistor RES CHAS MNT 4.7Ohm 5% 25W and some other items that maybe I still need to understand how to search for them! Also some queries:

- Is there any possible replacement for RES CHAS MNT 4.7Ohm 5% 25W

- I am also not sure on R12 and R13 in the drawing as these were not included in the table of BlueX. Can you kindly advise me how to look for them in the Farnell website if possible to indicate to me the products that I need.

- Can you also maybe indicate the next larger hammond enclosure as I think I would go for something a bit larger than the 1550G - also from the same website please.

- I do not know how to search the item marked as L1 (inductor??) on the drawing, it also does not seem to be included in BlueX table. So same if you can assist with the Farnell website.

- Now, as regards to R1A and R1B, the table of BlueX shows only one resistor is required? However the drawing shows two resistors. So in this case same situation can you confirm that the type chosen in the table attached is correct and that two of these are required please.

-As for the thermal grease, is that a good product and is one quantity enough for this work.

- how are the resistance components fixed to the enclosure; do I need any particular fastener please..can you indicate if possible 

- what type of cable should I buy to solder the component - possible sending me specs or part numbers to be able to search the website

- is there any particular solder wire that I would require as well please.

Any assistance is appreciated in getting to finalise the complete parts list to check and compare some costs and also to ensure that I am gathering all the items that would be required.

Apologies for the long thread and number of questions but since I am new to this I would like to understand in as much detail as possible...Look forward to hearing from you!


IndexQuantityPart NumberManufacturer Part NumberDescriptionItem Part on DwgFarnell143185-CL12207A-NDCL12207AS2 BBB SOLDER TAG GOLD ASSEMBLEDPhone Audio Connector, 3 Contacts, Receptacle, 6.35 mm, Panel Mount, Gold Plated Contacts23563-1838-NDET105D12-ZSWITCH TOGGLE SPDT 5A 125VToggle Switch On-On DPDTPanel Mount 5A 15V31A102357-NDHSC10015RJRES CHAS MNT 15 OHM 5% 100WR1A,R1B??Resistor, Solder Lug, 15 ohm, HS Series, 100 W, ± 5%, Solder Lug, 1.9 kV41HS5022RJ-NDHS50 22R JRES CHAS MNT 22 OHM 5% 50WR2AResistor, Axial Leaded, 22 ohm, HS Series, 50 W, ± 5%, Solder Lug, 1.25 kV51A132038-NDTHS5018RJRES CHAS MNT 18 OHM 5% 50WR2BResistor, Axial Leaded, 18 ohm, THS, 50 W, ± 5%, Solder Lug, 1.25 kV62A131986-NDTHS2515RJRES CHAS MNT 15 OHM 5% 25WR3, R6Resistor, Axial Leaded, 15 ohm, HS Series, 25 W, ± 5%, Solder Lug, 550 V72KAL25FB10R0-NDKAL25FB10R0RES CHAS MNT 10 OHM 1% 25WR4,R11Resistor, Aluminium, 10 ohm, HS Series, 25 W, ± 1%, Solder Lug, 550 V81A102145-NDHSA254R7JRES CHAS MNT 4.7 OHM 5% 25WR591A132015-NDTHS255R6JRES CHAS MNT 5.6 OHM 5% 25WR8Resistor, Axial Leaded, 5.6 ohm, THS, 25 W, ± 5%, Solder Lug, 550 V101A102139-NDHSA2533RJRES CHAS MNT 33 OHM 5% 25WR7Resistor, Axial Leaded, 33 ohm, HS Series, 25 W, ± 5%, Solder Lug, 550 V111A102426-NDHSA2568RJRES CHAS MNT 68 OHM 5% 25WR10Resistor, Solder Lug, 68 ohm, HS Series, 25 W, ± 5%, Solder Lug, 550 VR12 ??R13 ??1THERMAL GREASEThermal Grease, Silicone-Based, Thermalcote™, Opaque White, Tube, 57g1CASE: HAMMOND 1550GMetal Enclosure, Shielded, Diecast, IP54, Small, Diecast Aluminium, 222 mm, 146 mm, 55 mm, IP544LEG SUPPORTS FOR CASEBumper / Feet, Stick On, Pack 24, Adhesive, 5.8 mm, 12.7 mm, Square, Black2INDUCTOR COILS??

Thanks.


----------



## BlueX

Note: The info I've provided is for the 50W version (not the 100W that some people are building)!



emann said:


> The supplier Farnell seems to have everything in the list except for the resistor RES CHAS MNT 4.7Ohm 5% 25W


Here's a 4,7 Ohm, 5%, 25W chassis mount resistor with solder lugs (HS25 4R7), at Farnell:


https://se.farnell.com/arcol/hs25-4r7-j/resistor-wirewound-4-7ohm-5-axial/dp/2478182





emann said:


> I do not know how to search the item marked as L1 (inductor??) on the drawing,


This is L1 (inductor):


BlueX said:


> Jantzen air core coil 0,9 mH, 18 awg: part numbers 000-1647, 000-1953, or 000-1969 (different sizes for the spooled coil).


and here is one web shop where you can order it:





Jantzen Audio 000-1647 Jantzen 0,900 mH air core coil Cu 1,0 (18AWG) Rdc 0,530 Ohm | Europe audio


High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.Modern self-supporting design, low tolerance and power handling dependent on the wire used, makes it the most versatile coil on th




europe-audio.com







emann said:


> as regards to R1A and R1B, the table of BlueX shows only one resistor is required? However the drawing shows two resistors.


This should be the 30 Ohm, 50W resistors you need for R1a and R1b:


https://se.farnell.com/vishay/rh05030r00fc02/wirewound-resistor-30-ohm-50w/dp/1063972





emann said:


> -As for the thermal grease, is that a good product and is one quantity enough for this work.
> 
> - how are the resistance components fixed to the enclosure; do I need any particular fastener please..can you indicate if possible


I can't find thermal grease in stock at Farnell, but any local store selling parts for electronic applications should have it. Smallest package is sufficient, you don't need much.

M3 threaded bolts/screws, nuts, and locking washers, to mount.



emann said:


> what type of cable should I buy to solder the component


18 AWG, or 0,82 mm2, solid hook-up wire:


https://se.farnell.com/w/c/cable-wire-cable-assemblies/hook-up-wire?wire-gauge=18awg&conductor-area-csa=0.82mm-&no-of-max-strands-x-strand-size=solid&range=inc-in-stock



I don't have Line Out function, so I don't know exact specifications for R12 or R13.


----------



## emann

Thanks a lot BlueX - I inserted some further comments in red below please:


BlueX said:


> Note: The info I've provided is for the 50W version (not the 100W that some people are building)! YES I believe this is the size of unit I need for the deluxe fender reverb reissue - the amp is 22W 8ohm. Maybe John or Ballzz can confirm this as well?





BlueX said:


> Here's a 4,7 Ohm, 5%, 25W chassis mount resistor with solder lugs (HS25 4R7), at Farnell:
> 
> 
> https://se.farnell.com/arcol/hs25-4r7-j/resistor-wirewound-4-7ohm-5-axial/dp/2478182
> 
> 
> GREAT thanks
> 
> 
> This is L1 (inductor):
> 
> and here is one web shop where you can order it:
> 
> 
> 
> 
> 
> Jantzen Audio 000-1647 Jantzen 0,900 mH air core coil Cu 1,0 (18AWG) Rdc 0,530 Ohm | Europe audio
> 
> 
> High-purity copper wire baked in perfect shape to create a distortion-free inductor, for all audio applications.Modern self-supporting design, low tolerance and power handling dependent on the wire used, makes it the most versatile coil on th
> 
> 
> 
> 
> europe-audio.com
> 
> 
> 
> 
> GREAT - Unfortunately this is not in stock at the supplier...I will try other sources. Would you maybe have other equivalents that I can search for please?
> 
> 
> This should be the 30 Ohm, 50W resistors you need for R1a and R1b:
> 
> 
> https://se.farnell.com/vishay/rh05030r00fc02/wirewound-resistor-30-ohm-50w/dp/1063972
> 
> 
> SO in that case I would not need this item then from your table Index 3: RES CHAS MNT 15 OHM 5% 100W? Can you kindly confirm?
> 
> I can't find thermal grease in stock at Farnell, but any local store selling parts for electronic applications should have it. Smallest package is sufficient, you don't need much. NOTED and I think I would get this locally then
> 
> M3 threaded bolts/screws, nuts, and locking washers, to mount. NOTED and thanks
> 
> 
> 18 AWG, or 0,82 mm2, solid hook-up wire:
> 
> 
> https://se.farnell.com/w/c/cable-wire-cable-assemblies/hook-up-wire?wire-gauge=18awg&conductor-area-csa=0.82mm-&no-of-max-strands-x-strand-size=solid&range=inc-in-stock
> 
> 
> NOTED and thanks
> 
> I don't have Line Out function, so I don't know exact specifications for R12 or R13. OK - will ask John or Ballzz for this information then hopefully they can find this in farnell as well.



THANKS a lot for your assistance! Can I ask you how is your unit performing and with what amp do you use it please?


----------



## BlueX

emann said:


> Can I ask you how is your unit performing and with what amp do you use it please?


I use my M2 together with my Marshall Studio VIntage 20H, a 20W amp without master volume. The M2 works really well, and I'm very satisfied. I can't hear any change in tone when attenuating at different levels (other than the actual change in sound volume). Full attenuation with M2 doesn't mean whispering quiet when the amp is turned up, though. The sound level is reduced, but I would still disturb someone trying to sleep in the next room.


----------



## Gene Ballzz

BlueX said:


> I use my M2 together with my Marshall Studio VIntage 20H, a 20W amp without master volume. The M2 works really well, and I'm very satisfied. I can't hear any change in tone when attenuating at different levels (other than the actual change in sound volume). Full attenuation with M2 doesn't mean whispering quiet when the amp is turned up, though. The sound level is reduced, but I would still disturb someone trying to sleep in the next room.



I have the same setup and totally agree! However, for my situation, it is right on the edge of being quiet enough for sleeping in the next room! Of course, my wife is a heavy sleeper and when I had children, we went out of the way to intentionally keep household noise levels *UP*, to avoid having light sleeping kids grow into light sleeping adults!
Just My $.02,
Gene


----------



## BlueX

Gene Ballzz said:


> However, for my situation, it is right on the edge of being quiet enough for sleeping in the next room!


Interesting! Could there be a difference in performance depending on e.g. tolerances for the resistors? My resistors measured within 2% of nominal. The coil is a 0,8 mH, instead of 0,9 mH.

We use the same amp (SV20H), and I play mine through the stock SV212 (V-Types) or another 2x12 cab with 16 Ohm Greenbacks.

Might be difference in perception, of course.


----------



## Gene Ballzz

BlueX said:


> Interesting! Could there be a difference in performance depending on e.g. tolerances for the resistors? My resistors measured within 2% of nominal. The coil is a 0,8 mH, instead of 0,9 mH.
> 
> We use the same amp (SV20H), and I play mine through the stock SV212 (V-Types) or another 2x12 cab with 16 Ohm Greenbacks.
> 
> Might be difference in perception, of course.



I typically use my 1965B cabinet, loaded with 94db sensitivity G10L-35s, as opposed to your 98 or more db V-Types. I'm betting the 4 or 5db difference in the speaker sensitivity makes the difference. My Greenback (98db) loaded 1960B is definitely louder!
Just Thinkin'
Gene


----------



## emann

i play in the room at the roof of the house, so hopefully it will not affect that much any one in the next room!

I also try to use my interface to record takes in my DAW and then improvise against them - that is why I am quite interested in the line out.

@Gene Ballzz: would it be possibe if I ask you to give some more information on the R12 and R13 of the diagram of JohnH in post 2961 above or if you can maybe indicate some websites or Farnell so that I know what I need to search please. I believe these are the two remaining items I need to check for now. From Farnell
https://export.farnell.com/bourns/p...5k ohm 24mm variable resistor (potentiometer)
R12:
POT, ROTARY, 5KOHM, 24MM, 15%
R13:
Through Hole Resistor, 5.6 kohm, MCF, 1 W, ± 5%, Axial Leaded, 500 V

For R12 I also found this from ebay:
https://www.ebay.co.uk/itm/124515353526

Can you advise if the above products are the correct ones please.

Thanks.


----------



## Gene Ballzz

@emann ,
Either of those potentiometers should be OKAY, but I'd try to find a 1 watt rated one, (instead of 500mw - 1/2 watt rating) if the price isn't too bad. I'd also likely bump up to 2 watt for R13, just because!

On another note, It's my considered opinion that on these units, a line out is better achieved with a unit like an H&K RedBox, or similar, to be placed either before or after the attenuator in the amp to speaker chain.

H&K Red box

Just My Thoughts,
Gene


----------



## JohnH

hi @emann , all good advice from Gene and BlueX above!

A few comments on the layout that youre using, related to questions:




*Sound levels*

On the attenuation levels generally, nearly all the main designs on this thread use the same steps and generally the range is from -7db down to -31db. Build details and resistor tolerances wont affect those and generally its a good range for most cases. We have a few ways of going to a bit less attenuation (-3 or -3.5dB), and they can be useful if you have small amps, or are working at gig level and need just a small reduction. At the quiet end -31db, with most speakers and and amps say 10-50W, you can fill your studio with clear sound at about the volume of a moderately loud TV. Youd hear it to some extent in the next room of most houses and apartments though.

But there's a huge variation based on how you run your amp, and how your speakers are, and how acceptable sound levels are in your situation. If you want to, you can add further attenuation stages, and the design still works fine.

*Resistor R1 , or R1A and R1B*

R1 is the component first in line that works the hardest to absorb the amps power, taking about 50% of it in most cases. The diagram in post 1 has this as a single 100W rated resistor for a 50W build, either 15 ohm for an 8 Ohm build or 30 Ohm for a 16 Ohm attenuator and this is what most people have used. But in the layout above, I split this into two 50W resistors, R1A and R1B. so that all of the first stage is consistently based on the same size component, plus, the two parts R1A and R1B can be separated, to help spread out heat. But either way is fine. If you do R1A and R1B, they are wired in parallel for an 8 Ohm design, two x 30Ohm in parallel equates to 15 Ohm . 

*Line Out *

The diagram shows a very simple line out that is intended to go to either a cab simulation box, or an impulse response loader, or to a mixer or software to do that. Those units will apply the acoustic response of a guitar speaker to the electrical response coming out of the amp, smoothing out the highs and shaping the curve. Ideal components for this wil vary a lot for different uses both with amp power, amp settings and input device that its feeding, I tried to pick values that will give a good range. If you build this, try it out and see if you can get a good clean signal to feed your next stage, If its too hot, we can adapt the values. Its intended to get a reasonable line-out signal from a smallish amp set not at max, or a larger amp set higher, if you turn the knob down. Line out levels are higher than mic levels though so it depends what it feeds into.

@Gene Ballzz Heres how I figured the power specs for the line-out on that diagram:

There's a 5k pot in series with a 5k resistor - total =10k. Lets say its a 16Ohm build running at 50W input since that highest voltage. Voltage coming in is nominally 28V. Total power is 28^2/10000 = 0.08W, or 0.04W each in the resistor and pot.

Those Red Boxes are the kind of thing I was thinking that the line-out could feed, and the pot should allow the levels to be set appropriately. I also see that a Red box can work off a full speaker signal, but without variable level control.

@emann , your amp is basically 8 ohms but works at 4 Ohms with another cab. As Gene noted, you might consider a small tone tweak for 4 Ohms use, and omit te 16 Ohm output (unless you have other uses in mind). Its very simple and i can draw it - get another resistor, 20 or 22Ohm, 25W if you might want this.


----------



## Gene Ballzz

@JohnH ,
Yeah, while there is not a variable level control, there is at least a 0db/-25db pad switch. The Red Box has been around, in several forms and iterations since the mid '80s and has been fairly well regarded for its operation.
Just Notin'
Gene


----------



## emann

Gene Ballzz said:


> @emann ,
> Either of those potentiometers should be OKAY, but I'd try to find a 1 watt rated one, (instead of 500mw - 1/2 watt rating) if the price isn't too bad. I'd also likely bump up to 2 watt for R13, just because!
> 
> On another note, It's my considered opinion that on these units, a line out is better achieved with a unit like an H&K RedBox, or similar, to be placed either before or after the attenuator in the amp to speaker chain.
> 
> H&K Red box
> 
> Just My Thoughts,
> Gene


Thanks Gene,

so as for the potentiometer, I managed to find this:
Rotary Potentiometer, Cermet, 5 kohm, 1 Turns, Linear, 1 W, ± 10%, 149
https://export.farnell.com/vishay/14910f0gjsx10502ka/potentiometer-1w-single-5k/dp/8557462
For R13, I managed to find this:
Through Hole Resistor, 4.7 kohm, MCF, 2 W, ± 5%, Axial Leaded, 500 V

As regards to the line out and as per our previous communication, you are right that I am most interested in this. I also value your consideration for the RedBox and already started reading about it. In the meantime I have some queries to JohnH in the next thread.

Kindly advise on the two components above please.


----------



## emann

Hi JohnH - thanks for the response...including some queries in red below please:



JohnH said:


> hi @emann , all good advice from Gene and BlueX above!
> 
> A few comments on the layout that youre using, related to questions:
> 
> View attachment 113887
> 
> 
> *Sound levels*
> 
> On the attenuation levels generally, nearly all the main designs on this thread use the same steps and generally the range is from -7db down to -31db. Build details and resistor tolerances wont affect those and generally its a good range for most cases. We have a few ways of going to a bit less attenuation (-3 or -3.5dB), and they can be useful if you have small amps, or are working at gig level and need just a small reduction. At the quiet end -31db, with most speakers and and amps say 10-50W, you can fill your studio with clear sound at about the volume of a moderately loud TV. Youd hear it to some extent in the next room of most houses and apartments though.
> 
> But there's a huge variation based on how you run your amp, and how your speakers are, and how acceptable sound levels are in your situation. If you want to, you can add further attenuation stages, and the design still works fine.


Thanks for the above. As per my first post, all this information is numbers for me...that is as far as I understand these electronics hehe!! So with regards to my 22W 8Ohm Amp and from reading this thread (I am at page 30 now!), I think it is fine for me that I can get the volume of a moderatly loud TV in the room at the roof of my house. I do not gig as I am only a home player and so I reckon I do not need -3.5dB. However, I kindly ask you, what does it involve if I would like to have also a first stage of -3.5dB and something more than -31dB at the other end of sound volume? would it be a completely different and much more complicated circuit then? If not I may consider the modification.



JohnH said:


> *Resistor R1 , or R1A and R1B*
> 
> R1 is the component first in line that works the hardest to absorb the amps power, taking about 50% of it in most cases. The diagram in post 1 has this as a single 100W rated resistor for a 50W build, either 15 ohm for an 8 Ohm build or 30 Ohm for a 16 Ohm attenuator and this is what most people have used. But in the layout above, I split this into two 50W resistors, R1A and R1B. so that all of the first stage is consistently based on the same size component, plus, the two parts R1A and R1B can be separated, to help spread out heat. But either way is fine. If you do R1A and R1B, they are wired in parallel for an 8 Ohm design, two x 30Ohm in parallel equates to 15 Ohm . In following the circuit, I would proceed with R1A and R1B. Just to confirm that this build of up to 50W should be fine for my Deluxe Reverb Reissue correct?
> 
> *Line Out*
> 
> The diagram shows a very simple line out that is intended to go to either a cab simulation box, or an impulse response loader, or to a mixer or software to do that. Those units will apply the acoustic response of a guitar speaker to the electrical response coming out of the amp, smoothing out the highs and shaping the curve. Ideal components for this wil vary a lot for different uses both with amp power, amp settings and input device that its feeding, I tried to pick values that will give a good range. If you build this, try it out and see if you can get a good clean signal to feed your next stage, If its too hot, we can adapt the values. Its intended to get a reasonable line-out signal from a smallish amp set not at max, or a larger amp set higher, if you turn the knob down. Line out levels are higher than mic levels though so it depends what it feeds into.


This is were I have the main clarification required. I start with taking the opinion of Gene and already looking in the Red Box. Now, I have a Radial J48 DI Box. I also have a Sono Audient interface to connect to my laptop in Reaper. With my acoustic amplifier (Fender Acoustansonic 30), I take a line out and feed this into the J48. From the J48 I use a XLR out to the Audient and the sound I get in Reaper is fantastic. I was thinking if I could do the same with the Deluxe Reverb - only that the line out I would get from the attenuator rather then directly from the amplifier. The other option is to connect directly in the input line of the Audient - then has the Two Tone Software with it and hence I can have cab sim from this software. Do you think this would work in any way. I am not sure what the amps or voltage of the line out from the attenuator to the J48 or the Audient would be however I can find the specs for these two units that I have and send you any information you would need. At the end, I definitely do not want to show any disrespect to your expertise in this area and I easily understand that what I am asking can be a load of crap hehehe!...but at least I can get an experienced opinion about this.


JohnH said:


> @Gene Ballzz Heres how I figured the power specs for the line-out on that diagram:
> 
> There's a 5k pot in series with a 5k resistor - total =10k. Lets say its a 16Ohm build running at 50W input since that highest voltage. Voltage coming in is nominally 28V. Total power is 28^2/10000 = 0.08W, or 0.04W each in the resistor and pot.
> 
> Those Red Boxes are the kind of thing I was thinking that the line-out could feed, and the pot should allow the levels to be set appropriately. I also see that a Red box can work off a full speaker signal, but without variable level control.
> 
> @emann , your amp is basically 8 ohms but works at 4 Ohms with another cab. As Gene noted, you might consider a small tone tweak for 4 Ohms use, and omit te 16 Ohm output (unless you have other uses in mind). Its very simple and i can draw it - get another resistor, 20 or 22Ohm, 25W if you might want this.  I am not sure to understand this.At the moment my only requirement is to connect the Deluxe Reverb. What would be beneficial for future use if I change the 16 Ohm to 4 Ohms please?


Thank you so much for your time - both you and Gene!


----------



## JohnH

Hi @emann , The 50W M2 that we are duacussing should easily deal with your 22W amp. My own build is similar in capability and ratings (earlier M design). I run it with my Vintage Modern which puts out about 35W, and it gets just warm.

Those extra attenuation steps (high and low), I doubt you'd need them for your use, but they could be added later if space is kept. It would involve two more switches and five resisistors, two extra for the added low-volume range and one removed and three added in stage 1 to make it switcheable -3.5 and -7. The rest of the circuit is unchanged But I'd strongly advise to just keep it simple.

It looks like you have some great interface gear! I'd take Gene's advice on this but it looks to me that the line-out from this M2 could feed into your Sono unit. There, you could engage a cab-sim in the Torpedo software, and probably not use its amp sims since you're running from a good real amp. Whether the DI box is needed between M2 and Sono, I don't know, but you can try it if needed.


----------



## chas.wahl

Hello again @JohnH,
I'm getting ready to build the M2v attenuator, which doesn't seem to be as popular (by "hits" on a Google search) as other variants of your design. The reason I want to do the "V" is because
• amps each have a multiple-secondary OT: 4, 8 or 16 Ω
• have a cab with dual speakers and wish to use the attenuator as just a single output, connected to a separate box to circuit them in parallel or series.

I'll never have an amp in the 50 W range; maybe 22–25 W at most, so I used your "Attenuator M Power 190331" chart to scale back the wattage (trying to keep the size of the box down to something reasonable), as follows:




The wattages in the right column are based on those "M Power" loads for a 50W amp, halved, and then tripled (safety factor) and rounded up to the nearest available wattage.

My intent is to use the attenuator as a "straight through" device: matching the impedance of the input used and the output. What I'm concerned about is whether, if using the 16 Ω input and output, the wattages should be bumped up to a higher level. I'm happy to buy different, more robust resistors, especially for the 10 ohmers, if you think I've erred on the side of "mingy". On the other hand, if I've misunderstood how the M2v design should be used properly, then please set me straight.
Thanks,
-- 
chas.wahl


----------



## JohnH

chas.wahl said:


> Hello again @JohnH,
> I'm getting ready to build the M2v attenuator, which doesn't seem to be as popular (by "hits" on a Google search) as other variants of your design. The reason I want to do the "V" is because
> • amps each have a multiple-secondary OT: 4, 8 or 16 Ω
> • have a cab with dual speakers and wish to use the attenuator as just a single output, connected to a separate box to circuit them in parallel or series.
> 
> I'll never have an amp in the 50 W range; maybe 22–25 W at most, so I used your "Attenuator M Power 190331" chart to scale back the wattage (trying to keep the size of the box down to something reasonable), as follows:
> 
> View attachment 113952
> 
> 
> The wattages in the right column are based on those "M Power" loads for a 50W amp, halved, and then tripled (safety factor) and rounded up to the nearest available wattage.
> 
> My intent is to use the attenuator as a "straight through" device: matching the impedance of the input used and the output. What I'm concerned about is whether, if using the 16 Ω input and output, the wattages should be bumped up to a higher level. I'm happy to buy different, more robust resistors, especially for the 10 ohmers, if you think I've erred on the side of "mingy". On the other hand, if I've misunderstood how the M2v design should be used properly, then please set me straight.
> Thanks,
> --
> chas.wahl



That all seems OK. The power ratings seem fine. And if you use the 16 or 4 Ohm input options, then the three extra parts engaged at the front will absorb half the power, which the ratings for them allow for. So in these cases, power taken by all the other parts reduce too, ie so no increase.

But you should note that by the same reasons, engaging the other Ohms options increases attenuation, by -3db at the front end and about another -2dB if you use the other output options. So you'd need to dial up the volume using the main switches. This all fine if you have a few dB to spare, but could be an issue if you need to run at just a small attenuation at a gig or loud rehearsal. 

If that's a problem, there's a recent tweak to provide a -3.5/-7 dB switch for stage 1, replacing the lost dB's if needed.

Another recent tweak, for 4Ohm use, a tone fix to make the balance of bass, mids and highs match the target better ( or else the 4 out has slightly less mids)


----------



## emann

JohnH said:


> Hi @emann , The 50W M2 that we are duacussing should easily deal with your 22W amp. My own build is similar in capability and ratings (earlier M design). I run it with my Vintage Modern which puts out about 35W, and it gets just warm.
> 
> Those extra attenuation steps (high and low), I doubt you'd need them for your use, but they could be added later if space is kept. It would involve two more switches and five resisistors, two extra for the added low-volume range and one removed and three added in stage 1 to make it switcheable -3.5 and -7. The rest of the circuit is unchanged But I'd strongly advise to just keep it simple.
> 
> It looks like you have some great interface gear! I'd take Gene's advice on this but it looks to me that the line-out from this M2 could feed into your Sono unit. There, you could engage a cab-sim in the Torpedo software, and probably not use its amp sims since you're running from a good real amp. Whether the DI box is needed between M2 and Sono, I don't know, but you can try it if needed.


Thanks. @JohnH and @Gene Ballzz


So for now I shall follow on with this design and finalise a parts list to send for some enquiries.

Just two questions:

1. Do you think the Hammond 1550G case would be enough if I would opt to include the below option of attenuation - if not can you maybe suggest a correctly sized enclosure for this.

2. Also - I should be meeting someone to assist me with the assembly. Can you indicate the tests that I need to ask him to do when the attenuator is completed. I want to be sure that I do not damage my deluxe reverb and would definitely ask him for the specific tests required and post the answers here before I would proceed to hook up the unit to the amplifier.

3. As regards to the inductor, I cannot find the exact 0.9mH 18awg air core coil. Can you let me know if either 0.90mH - 1.0mm Air Coil heat sealed or Jantzen Air-Coil 1.00mH 1.00mm is adequate please.
​
Regards.


----------



## chas.wahl

JohnH said:


> That all seems OK.  {wattages of resistors}


Great; thank you.


JohnH said:


> But you should note that by the same reasons, engaging the other Ohms options increases attenuation, by -3db at the front end and about another -2dB if you use the other output options. So you'd need to dial up the volume using the main switches. This all fine if you have a few dB to spare, but could be an issue if you need to run at just a small attenuation at a gig or loud rehearsal.


Well, this goes back to my question about my lack of understanding about how to use the attenuator properly. If I'll _always_ use an amp that has an 8 Ω output, is there _anything to be gained_ by building an attenuator that will accept 4 Ω or 16 Ω input from the amp? Rather than having multiple inputs, will the result sound pretty much the same if I plug an 8 Ω amp in, and connect to the attenuator either 8 Ω load (single 8 Ω speaker), 4 Ω (two 8 Ω speakers in parallel) or 16 Ω (two 8 Ω speakers in series, using the 16 Ω output with R10 and R11)? If that's the case, then maybe I should just build the M2 (no "v") and forget about the additional front end stage the M2v has, to accommodate 4 Ω and 16 Ω inputs.


JohnH said:


> If that's a problem, there's a recent tweak to provide a -3.5/-7 dB switch for stage 1, replacing the lost dB's if needed.


Found that here, if I understand correctly. That's a nice add. As I understand it, the necessary changes for conversion are:
Ditch the R1 that I have (15 Ω) and replace with R1A & R1B (at the same wattage as R1, presumably?)
Add R2C (R2A & R2B remain the same) -- same wattage as R2A?
Add a DPDT to switch Stage 1 from -7 dB to -3.5 dB
Add R6A to Stage 2 (R6 becomes R6B) -- again, same wattage as R6?
Change Stage 2 SPDT switch to DPDT

Also, I presume that unless I want the "line out" feature shown on that M2 diagram, the R12 and R13 at the front end are not necessary; correct?


JohnH said:


> Another recent tweak, for 4Ohm use, a tone fix to make the balance of bass, mids and highs match the target better ( or else the 4 out has slightly less mids)


If an 8 Ω amp is fine (see my first query above in this post) then there's no need for that; but I guess I'd like to know where that's documented, for the sake of interest.

I appreciate your help!
--
chas.wahl


----------



## Gene Ballzz

@chas.wahl ,

It seems that you may be overthinking this and possibly complicating the issue, maybe due to a small lack of understanding. My following response may be similarly complicated and overthought, though I will try to simplify and narrow things down a bit!

*>* From what I can gather so far, all of your amps are capable of multiple impedance outputs. If this is the actual case, simply choose the attenuator design that matches your most likely and most often used speaker impedance configuration, 8 ohm seems the most likely. Once you "hit upon" that perfect configuration, all other scenarios will probably get relegated to the historic bin of experimantation!

*>>* The best and most consistent performance of these units is achieved when using it in its "native" design impedance on both the input and output. There is really very little to be gained by having both multiple input and output impedances. Remembering what @JohnH said that compensating the "native" design's input impedance increases the overall attenuation by +/- 4db and tonally compensating the output impedance adds about another -2db of overall attenuation.

*>>>* It should also be noted that with any given input impedance design (let's say 8 ohm), any speaker impedance (4 ohm, 8 ohm or 16 ohm) is totally safe for the amp, but will be most natural sounding with its "native" output impedance speaker. A 16 ohm or 4 ohm speaker will simply have slightly more pronounced highs and lows and subdue the mids just a tad, until you add the small tonal compensation resistors.

*>>>>* Also your desire to "shrink" the size of the box by going to lower wattage resistors will net you a very minimal reduction in actual size, while ultimately limiting the use for any possible, future amp upgrades!

*#####* It is my sincerest and well considered suggestion that you simply go with the well proven, tried and true, M2/50 watt design, probably 8 ohm, with either a single compensated output for 4 ohm or 16 ohm or two for both! I'd also recommend bumping up @JohnH 's suggestion of the Hammond 1550G box, to the slightly taller 1550H. It allows a bit more "wiggle room" for layout! I'd also negate the line out, as there are multiple alternative options for achieving such, if it ever becomes a necessity!

In addition, knowing what part of the world you are located in would be helpful for component sourcing tips!

Simply Attenuatin'
Gene


----------



## JohnH

Hi @chas.wahl , yes it sounds like just an 8Ohm unit is fine for all your amps and you don't need th 'v' front end. At the output, a pair of output jacks will let you run one 8ohm cab or two 16's. Add the 16 Ohm out if you have any 16 Ohm cabs, since it'll make them sound better. The 4ohm out would be if you want to run a pair of 8 ohm cabs, or a 4 ohm cab. It's very simple but I haven't drawn it yet!

All these output options are just small tone tweaks. Not about mismatches or amp safety.

To get them all, I'm thinking a 16 out, a pair of outs for 8 or 4, and one just for 4 ohm. If you plug into the dedicated 4, or the 4 and one of the 8/4 pair, you get the 4 ohm tone tweak.

Another way is a 16 out, an 8/4 pair , and a switch to convert 8 to 4.


----------



## chas.wahl

Hello @JohnH, thanks for the above.

You and @Gene Ballzz appear to differ in advice regarding dedicated inputs for 4/8/16 Ω from the amp. At the expense of a bit more complication (and attenuation for 4 & 16), maybe I'll stick with the M2v. I can always ignore the lower- and higher-ohm inputs.

What I'd like to know is whether I can match the M2v front end network to the M2 "Gig" design with option for -3.5 or -7 dB attenuation at Stage 1 (but omitting the "line out" front end). I don't see why not, but since that's getting complicated for me, in my head, I will draw up what that looks like to me, and submit it for review (along with my own use in drawing up a layout).

I also don't quite understand the change to Stage 2 for the M2 "Gig" design. There's a new resistor in series right before the Stage 2 voltage divider, but the DPDT switch simply puts Stage 2 "in" or "bypassed" -- in other words, there's no real difference in Stage 2 depending on which option is switched in for Stage 1, it seems to me -- the resistor change is operative for use with either option (-3.5 or -7 dB). Am I correct about that?



I'd really like to see the "tweak" for 4 ohm output. I can't find mention or a diagram of it in this mega-thread, only mention that when using the 8/4 output, some mids are repressed (maybe so minor that one can't tell).

@Gene Ballzz, thank you kindly for your suggestions, but a) I will never have or play through an amp over 22–25 W in a way that requires attenuation, and b) I've _already bought_ all the resistors for an M2v, using the wattages as detailed in my post at the end of the previous page of this thread. So I've crossed the Rubicon on building a 25 W max variant. Plus, I want it to fit in the bottom of a Tweed Deluxe cabinet, along with a small box for doing the speaker switching and a "browner" box (Rob Robinette's "Buckminister") -- and two speakers! That's for storage -- I'd pull the boxes out in use -- but I live in a small apartment, where compactness is a requisite consideration for all "things".


----------



## emann

Hi @JohnH

is it possible you kindly check my queries of post 2982 above please. 

I have now found all the items from one supplier except for the L1 air coil and provided some different specs to the one in the schematic. Can you please advise if any of these models is correct so that I can order this as well please. I am not sure what is the difference between the 0.9mH and and 1mH; 18awg should be close to 1mm I believe.
Jantzen Air-Coil 1.00mH 1.0mm​JANTZEN AUDIO 4N Copper Air Core Wire Coil 18AWG 1mH​0,92 mH · 1,2 mm Jantzen Air Core Wire Coil​


Emann


----------



## GeorgeLG

@JohnH Thank you for your response. While starting this process I found a good deal on a Suhr reactive load so I bought it to start. My main objective is full power tone into the recording device/audio interface But at some point I also want to be able unload most of the power and still play through a proper guitar cabinet.


George


----------



## Gene Ballzz

chas.wahl said:


> Hello @JohnH, thanks for the above.
> 
> You and @Gene Ballzz appear to differ in advice regarding dedicated inputs for 4/8/16 Ω from the amp. At the expense of a bit more complication (and attenuation for 4 & 16), maybe I'll stick with the M2v. I can always ignore the lower- and higher-ohm inputs.
> 
> What I'd like to know is whether I can match the M2v front end network to the M2 "Gig" design with option for -3.5 or -7 dB attenuation at Stage 1 (but omitting the "line out" front end). I don't see why not, but since that's getting complicated for me, in my head, I will draw up what that looks like to me, and submit it for review (along with my own use in drawing up a layout).
> 
> I also don't quite understand the change to Stage 2 for the M2 "Gig" design. There's a new resistor in series right before the Stage 2 voltage divider, but the DPDT switch simply puts Stage 2 "in" or "bypassed" -- in other words, there's no real difference in Stage 2 depending on which option is switched in for Stage 1, it seems to me -- the resistor change is operative for use with either option (-3.5 or -7 dB). Am I correct about that?
> View attachment 114025
> 
> 
> I'd really like to see the "tweak" for 4 ohm output. I can't find mention or a diagram of it in this mega-thread, only mention that when using the 8/4 output, some mids are repressed (maybe so minor that one can't tell).
> 
> @Gene Ballzz, thank you kindly for your suggestions, but a) I will never have or play through an amp over 22–25 W in a way that requires attenuation, and b) I've _already bought_ all the resistors for an M2v, using the wattages as detailed in my post at the end of the previous page of this thread. So I've crossed the Rubicon on building a 25 W max variant. Plus, I want it to fit in the bottom of a Tweed Deluxe cabinet, along with a small box for doing the speaker switching and a "browner" box (Rob Robinette's "Buckminister") -- and two speakers! That's for storage -- I'd pull the boxes out in use -- but I live in a small apartment, where compactness is a requisite consideration for all "things".




I was simply considering your desire for conservation of real estate! I have one of my M2 builds semi-permanently mounted (via Velcro) in my 5E3 Tweed Deluxe and it only ever gets a little bit warm, even under long term, heavy use! It tames the beast quite well!




emann said:


> Hi @JohnH
> 
> is it possible you kindly check my queries of post 2982 above please.
> 
> I have now found all the items from one supplier except for the L1 air coil and provided some different specs to the one in the schematic. Can you please advise if any of these models is correct so that I can order this as well please. I am not sure what is the difference between the 0.9mH and and 1mH; 18awg should be close to 1mm I believe.
> Jantzen Air-Coil 1.00mH 1.0mm​JANTZEN AUDIO 4N Copper Air Core Wire Coil 18AWG 1mH​0,92 mH · 1,2 mm Jantzen Air Core Wire Coil​
> 
> 
> Emann



Yes, #18 gauge is 1.024mm. @JohnH did a test to determine how much wire needs to be unwound from a 1.0mH to make it be a 0.9mH, but I believe he also determined that the math tells him that the difference between the two values is mostly inconsequential in actual operation.

Simply Attenuatin'
Gene


----------



## emann

Gene Ballzz said:


> I was simply considering your desire for conservation of real estate! I have one of my M2 builds semi-permanently mounted (via Velcro) in my 5E3 Tweed Deluxe and it only ever gets a little bit warm, even under long term, heavy use! It tames the beast quite well!
> 
> 
> 
> 
> Yes, #18 gauge is 1.024mm. @JohnH did a test to determine how much wire needs to be unwound from a 1.0mH to make it be a 0.9mH, but I believe he also determined that the math tells him that the difference between the two values is mostly inconsequential in actual operation.
> 
> Simply Attenuatin'
> Gene


thanks a lot for this...hence i believe i can proceed with the full order.

reading through this thread I am almost inclined to try the project myself as it seems some of the persons who did had almost zero experience on such builds...i am quite hands on for woodwork and steel work but not on electronics...so at the moment I am thinking to give it a go myself.

At the end the help here is immense!!


----------



## gbarker7815

I tried to find an answer to my question in the thread, but not sure I found a clear answer. I'm building the 8 ohm version for a 100w (whatever the standard values work for on the 3/18/21 M2 map / value chart). I doubt I'll ever use more than a 50w amp with this, and am planning to use with a 20w Marshall SV20. 

I finally received all the parts today to wire everything up. I have 18 gauge wire, and believe it will be fine for everything except possibly the switches. Can I use smaller gauge wire on these, particularly the jumpers?


----------



## gbarker7815

Another newbie question....

When operating the unit, I assume you would need to operate the stages in order? Meaning that in order to activate stage 4, you would need to have stages 2 and 3 active, or would any combination work?


----------



## Gene Ballzz

All switch combinations work. For examples:

*A)* -7db is no extra switches/stages active, except for the first "always on" -7db, reactive stage.
*B)* Actvivating the -3.5db switch gives you total -10.5db.
*C)* Deactivating the -3.5db and activating the -7db switch gives -14db.
*D)* Adding the -3.5db to the already activated -7db switch gives -17.5db.
*E)* Deactivating -3.5db & -7db and activating the -14db gives -21db.
*F)* Adding -3.5db to the -14db gives -24.5db.
*G)* Deactivating -3.5db & adding the -7db to the -14db gives -28db
*H)* And finally, when all switches are activated, you get-31.5db!

Remembering, of course, that without a bypass switch, the -7db reactive stage is always active, whenever the attenuator is plugged in. That first, reactive stage is where most of the work of attenuation gets done and that's why its resistors are significantly more robust in wattage. And while an overall bypass switch is fairly easy to add, there are some caveats about it's safe use that *MUST* be strictly followed.
*FIRST)* A bypass switch must *NEVER* be toggled while the amp is in run mode!
*SECOND)* While a speaker that is mismatched to the input impedance is totally safe, while the attenuator is active, it is *NOT* safe to bypass the attenuator in that mismatched configuration, as that would allow the amplifier *see* that mismatched speaker load!

And oh yeah, regarding wire gauge. You should really use #18 gauge minimum for all. A pic or link to the switches you've chosen would help me make suggestions for how to best tackle this! One of the reasons many of us use DPDT swiches is for jumpering the two sides together and tha jumper provides an added place for an additional wire to be attached. The "mini" switches can be tricky! Depending on the brand, they will just barely accept an #18 wire!

Hope This Helps?
Gene


----------



## Gene Ballzz

emann said:


> thanks a lot for this...hence i believe i can proceed with the full order.
> 
> reading through this thread I am almost inclined to try the project myself as it seems some of the persons who did had almost zero experience on such builds...i am quite hands on for woodwork and steel work but not on electronics...so at the moment I am thinking to give it a go myself.
> 
> At the end the help here is immense!!



Develepoing good soldering technique is fairly simple and is a skill that will last a lifetime and open up many doors for a guitarist/music gear junkie! There are several spots where two wires need to make the same junction and that junction can be done either at the resistor lug or the switch lug, depending on several things. Either links to or pictures of your chosen components can help us help you!

This is not a difficult project, as long as you go slow and are fairly methodical! And we are here to help!

Just Sayin'
Gene


----------



## gbarker7815

Gene Ballzz said:


> All switch combinations work. For examples:
> 
> *A)* -7db is no extra switches/stages active, except for the first "always on" -7db, reactive stage.
> *B)* Actvivating the -3.5db switch gives you total -10.5db.
> *C)* Deactivating the -3.5db and activating the -7db switch gives -14db.
> *D)* Adding the -3.5db to the already activated -7db switch gives -17.5db.
> *E)* Deactivating -3.5db & -7db and activating the -14db gives -21db.
> *F)* Adding -3.5db to the -14db gives -24.5db.
> *G)* Deactivating -3.5db & adding the -7db to the -14db gives -28db
> *H)* And finally, when all switches are activated, you get-31.5db!
> 
> Remembering, of course, that without a bypass switch, the -7db reactive stage is always active, whenever the attenuator is plugged in. That first, reactive stage is where most of the work of attenuation gets done and that's why its resistors are significantly more robust in wattage. And while an overall bypass switch is fairly easy to add, there are some caveats about it's safe use that *MUST* be strictly followed.
> *FIRST)* A bypass switch must *NEVER* be toggled while the amp is in run mode!
> *SECOND)* While a speaker that is mismatched to the input impedance is totally safe, while the attenuator is active, it is *NOT* safe to bypass the attenuator in that mismatched configuration, as that would allow the amplifier *see* that mismatched speaker load!
> 
> And oh yeah, regarding wire gauge. You should really use #18 gauge minimum for all. A pic or link to the switches you've chosen would help me make suggestions for how to best tackle this! One of the reasons many of us use DPDT swiches is for jumpering the two sides together and tha jumper provides an added place for an additional wire to be attached. The "mini" switches can be tricky! Depending on the brand, they will just barely accept an #18 wire!
> 
> Hope This Helps?
> Gene


Thanks Gene, 

Very helpful information. 

The switches are mini DPDT. I believe I may have found a part number somewhere in this massive thread when I ordered everything a few months back. I can't remember. I'll give it a shot tonight after work. I didn't try to fit the wire through the lugs just yet, I just eyeballed it and it looked like I might have issues (the mail came in before I left for work, so I only had a few minutes to look).


----------



## emann

Gene Ballzz said:


> Develepoing good soldering technique is fairly simple and is a skill that will last a lifetime and open up many doors for a guitarist/music gear junkie! There are several spots where two wires need to make the same junction and that junction can be done either at the resistor lug or the switch lug, depending on several things. Either links to or pictures of your chosen components can help us help you!
> 
> This is not a difficult project, as long as you go slow and are fairly methodical! And we are here to help!
> 
> Just Sayin'
> Gene


Thanks @Gene.. for the encouraging words!

Tomorrow the order is off then!


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## JohnH

Further to Gene's explanation above, with each switched stage having x2 the attenuation of the previous, you can think of the settings like binary numbers and with the three basic switches in an M2, you can get 8 settings each spaced by one 'step' of -3.5dB. That all makes most sense if on the front panel, you place the switches -14, -7 then -3.5 dB. (the internal electrical order doesn't really matter but on recent diagrams I show it the same)

And before that, there's the fixed -7dB reactive stage. Let's say a stage that is attenuating is denoted '1', and if it's not, it's a '0'




Maybe that helps? If not, no problem!

That's for a basic M2. If you have a full bypass, then you get 0dB, which means you aren't using attenuation. A bypass may be useful in a situation where you need to move across small amounts of attenuation, so long as your amp is off or on standby when you operate it.

The extra switch on Stage 1 that Chas discusses above, turns Stage 1 from -7dB to -3.5 dB. This fills in the last gap in the sequence to allow just one very small step of -3.5dB total. The intent of this switch is only to provide that one extra -3.5dB setting. If you want -7dB or more, the design is best (in terms of showing the amp the 'correct' reactive load) if Stage 1 is at -7dB. But, given this extra switch, The intent is that it's safe to use it freely, try it with different switch settings to explore.

To that end, I added the extra resistor to Stage 2. This makes sure that the amp still sees close to an 8 Ohm load if Stage 1 is at -3.5dB and you choose to operate Stage 2. With the standard version of Stage 2 with two resistors and Stage 1 at -3 5, the amp sees a load that is a bit low, about 6.5Ohm. Not likely to cause a problem but I try to keep all the designs to very tight parameters so that overall, these units have very good consistency at all settings.

Chas, if you like the -3.5/-7 switch but don't want to change Stage 2, and follow the above, then its all fine. With Stage 1 at the usual -7dB , it separates the amp enough from the following stages so that amp load is consistent with the standard two-resistor following stages.

I see the -3 5/-7 switch as an extra that may be useful for just a few users.

Background theory;

Most of how these designs work is based on close control of the resistive stages, based on controlling three things, being input imoedance seen by the amp, the attenuation level, and also output impedance seen by the speaker (this is the key difference to most other designs). In principle, at each stage, you need three resistors to fully control these three things, in a ''T' or a 'Pi' arrangement. But, based on the target values, at about -7dB, one resistor becomes zero and we have just two. This is the basis for the -7dB stage, which is ideal, and it is also developed into the reactive front stage. The -14 and -3.5dB stages with two resistors, have the right attenuation and output resistance, but go a bit high or low at their front end. But they are always behind -7dB Stage 1, so the amp doesn't see much of this. But with Stage 1 set at -3.5, a bit more variation gets through to the amp, so Stage 2 is upgraded to 3 resistors for that use,


----------



## JohnH

Hi @gbarker7815 , on wire gauge, yes to 18 gauge as a spec for all. Thsts a 1mm diameter or equivalent stranded. I also have used a couple of other work-arounds too. Where I want to engage the hole through small lugs, I sometimes use 0.7mm solid but loop it back so two strands are engaged. One strand goes through, one is in contact and both well soldered. Or take a 18 guage stranded and twist it into two, one goes through, the other around.


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## chas.wahl

Thanks @JohnH !!
I like the "binary" explanation, and it could be a chart on top of the box, or a fold-out on the side. I've always just thought of it as "-7dB is the base, and then you add up whatever additional attenuation you want using the switches as marked".
And thank you also for explaining how Stage 2 works with Stage 1 in the -3.5 | -7 dB scheme.

I am working on a schematic for "22 W M2v Gig (4|8|16 Ω)". It will be a resistor-fest, jack-o-rama (5) and switch-travaganza (4), for sure. I'll try to make a chart like you have, indicating wattages for 50 W and resistances for 2|4|8 Ω.

I'm still interested to see the mids correction for 4 Ω output, as you have time and inclination.


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## JohnH

hi @chas.wahl
The tweak for 4 ohms on an 8 ohm M2, is related to one for 8 Ohms on a 16 Ohm M2. It's subtle, and has only been tried in the last few weeks following building and testing by Gene. Basically, you can plug a 8 cab into a 16ohm M2 , or a 4cab into a 8 M2 and it works safely and fine. But you get a bit more treble and low bass in proportion to mids, about a 1.5dB difference. It comes from the output impedance of the M2 bring based on matching at the native ohms. It's analogous, and technically very similar, to an amp with a bit less NFB, or with its presence and resonance dialed up.

So, it's ok as it is, but compensating is a very simple fix, putting a resistor across the output. It's 39 or 40 Ohms for adjusting a 16 Ohm M2 for 8 Ohms, or 20 or 22 Ohms for 4ohm can into an 8 ohm M2. It can be achieved using a switch, or a TRS jack. I'll draw both. Gene has confirmed that it corrects this tone change, so might as well have it available if wanted.

I'll draw it both ways soon.


----------



## Gene Ballzz

So @JohnH ,
Here's some questions to possibly make your head explode. Those tonal tweaks for the 4Ω and 16Ω speaker outs seem to be centered around a "natively" 8Ω build. How do those tweaks work out on an M2v, when the input is different than 8Ω? And then, what happens to that -3.5db, first stage option when used with other than 8Ω input?

And then it would seem that including a "bypass" switch would be a big* "NO - NO" *on an M2v, as an unwary user could potentially get themselves into trouble very easily?

Sincerely Curious?
Gene


----------



## emann

@ Gene Ballzz and JohnH

apologies to interrupt again on the subject of the inductor.

The inductor is really giving me trouble in sourcing the correct one. The ones that I asked you about before are now Out of Stock as well!

In my search for the correct type, should I only check for an air oil of 0.9mH and 18AWG only please?

I am finding two types available in Europe as at 0.9mH, 18AWG and the resistance is 0.48ohms, or 0.46ohms. Is there any particular requirement for the resistance or as long as I am within the 0.9mH 18AWG then I should be ok.

Thanks again for your patience and time to reply.

Regards.


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## JohnH

Hi @emann , no need to look at the resistance if the coil. It will generally be about 0.5Ohm, based on the wire size and the inductance, but it may vary slightly and that's OK. If you can't get 0.9mH, 18 gauge, get 1.0mH. If its on a plastic bobbin, I can help advise how to reduce by unwinding it, or you can just keep it as is and its not far off.


----------



## JohnH

Gene Ballzz said:


> So @JohnH ,
> Here's some questions to possibly make your head explode. Those tonal tweaks for the 4Ω and 16Ω speaker outs seem to be centered around a "natively" 8Ω build. How do those tweaks work out on an M2v, when the input is different than 8Ω? And then, what happens to that -3.5db, first stage option when used with other than 8Ω input?
> 
> And then it would seem that including a "bypass" switch would be a big* "NO - NO" *on an M2v, as an unwary user could potentially get themselves into trouble very easily?
> 
> Sincerely Curious?
> Gene



Hi Gene, I think the 3.5/7 switch is OK. I already ran the 4 and 16 ohm tweaks with the 3.5/7 switch added, through my mega-spreadsheet and with the few amendments noted it all seems like it works well. Tone corrections still work and ohms seen by the amp are within reasonable control. With the 'V' input mod I don't think that changes.

With the bypass switch, the V mod could be configured to do useful things in bypass mode, such as converting an 8ohm speaker to a 4 or 16 load without the attenuation stages. Just a -3dB reduction. But there could be some settings that could be not good in bypass, like if a 16 ohm amp and a 16 cab are plugged into the respective jacks in bypass mode, the amp might then actually see about x2 the expected 16Ohms even though a user might expect this to be a valid setting. Will think further on that....


----------



## emann

JohnH said:


> Hi @emann , no need to look at the resistance if the coil. It will generally be about 0.5Ohm, based on the wire size and the inductance, but it may vary slightly and that's OK. If you can't get 0.9mH, 18 gauge, get 1.0mH. If its on a plastic bobbin, I can help advise how to reduce by unwinding it, or you can just keep it as is and its not far off.


thank you so much for the reply!


----------



## KIL0

I'm back at the party, too. ,)

Got my SVT 2 Pro now so the project of building an M2 (or maybe even an M3) now starts to get real.
I actually had a very "me" idea (electronics dev) while thinking about the thermal situation with the *600 W* "heating" module: I took some minutes and calculated the parts needed as 0207 tht resistors.

The reason for this idea is about the cooling. When building the resistors from many smaller parts (several 0,6 W metal film resistors in parallel AND in series) I could use the space I planned more efficiently and optimize the air flow. I want to split the 4 Ohms monster into 4 identical parts in parallel (witch in essence are four 16 Ohms attenuators in parallel) and stack these pcbs with spacers in front of some 40mm fans.
The calculation I did ended up using 3.840 resistors soldered together (by hand on a piece of stripe board) so I thought again ^^

I then went one step further and calculated everything again for using 0603 smt resistors. These are rated with 0.1 Watts but are very cheap and can be placed automatically. I calculated everything with ordering the assembled boards at JLCPCB with aluminium core single layer pcb and I can get my four pcbs for about 140$ (actually the minimum order quantity is five, so I'll even have a single 16 Ohm pcb left at a single price of about 30$).
The pcb will have a size of about 240 x 180 mm and with stacking four high I should be able to fit it into an 1 U rack housing (with fans and stuff of course). On this space will be 5.952 smt resistors on one side of every pcb, summing up to 23.808 resistors in total. So the design maybe could take some evenings.

A side effect of splitting the resistors is that I could match the resistance very precise if needed so @JohnH if you got more specific resistance values besides the E12 scale I'd be happy to try ,)
I still need to check if I'll be winding just one coil or if I'm going to take the step of building 4 separate coils into the unit.

Any thoughts about this?


----------



## JohnH

Hi @KIL0 , welcome back. I'm happy to play along but I'm having trouble visualising what this will look like and also how you can get the 300W of heat out of it without meltdown! Can you offer a diagram or maybe one of something else similar? (BTW, I'm an engineer but I'm not in electronics myself, just an amateur nerd!)

I have some excel macros that home-in iteratively on the values for attenuation stages, which I'd be happy to re-run. There's not much benefit getting too specific with Ohm values. I find the standard E12 series hits the design intent pretty well, given that each one is a slightly fuzzy choice within a few % given that the design is intended to work for many amps. But no harm in exploring.

Here's the amp, for others to see:



Ampeg: Pro Series - SVT-2PRO



Would this build be for 2 Ohm or 4 Ohm?

If you want to really optimise, are you able to offer any info on the following specifics:

1 The impedance vs frequency curve of the cabs or similar, or if not, the coil inductance.?
2. The effective output impedance of the amp at mid frequencies, for small signals?

The last one, on my VM amp I tested as around 20 Ohms (8 ohm tap), and that's the basis of the design to date. It turns out that a design based on this works surprisingly well across many different amps. But your big bass amp may be in a different ballpark. The way to test is with a smallish consistent signal into a known resistive load, measure the ac output voltage, then repeat with a different load and solve some maths.


----------



## gbarker7815

I managed to finish my build today. It seems to work great.

One question I have, and I saw some mention of this but wasn’t sure if anyone had actually tried it, was adding a cap to boost the treble a little.

That’s the only problem I have with it, and it could be I am just used to hearing a lot more and can’t seem to dial it in while using this attenuator (I leave open the possibility that I could have screwed something up too).

Assuming the treble loss is normal, has anyone tried adding a cap? If so what value, and where would it be added? 

If it matters, I built the M2 8 ohm version and am using a Marshall SV20 amp.


----------



## chas.wahl

@john H: OK, when working on the schematic combining M2v and the "Gig" option, I've come up against this issue (raised, in my mind, by your post #1053):


JohnH said:


> Those added R10 and R11 parts are great if you want to run a 16 Ohm cab from an 8 Ohm attenuator. It just fixes input and output impedance so the amp keeps seeing not too much more than 8 ohms and the 16 ohm speaker sees the right amount to keep its treble and bass correct. Without them, a 16 ohm cab gets noticeably less high treble It all safe, and fine if you want a softer tone, But I like the clarity. It works with either M or M2. I use this feature to run a 16 ohm V30 cab from my VM combo which doesn't have a 16ohm tap.



The M2v schematic dated 2020-04-20 has both a 16 Ω input jack and the 16 Ω output jack with R10 & R11, while the M2 (2021-03-18) has only the latter. So if one wants to connect a multiple output impedance amp to a 16 ohm load through the M2v, should one use 8 Ω on the input (from the amp) going to 16 output jack on the attenuator? Does the M2v actually "double correct" if one puts 16 Ω in and hooks up to 16 Ω out?

If the correction is made by the _input_ network added to M2v, then could (or should) I omit the R10/R11/16-ohm jack and just use the "uncorrected" 8 Ω jack out for a pair of 8Ω speakers in series?

Thanks,
-- 
chas.wahl


----------



## Gene Ballzz

gbarker7815 said:


> View attachment 114181
> View attachment 114182
> 
> I managed to finish my build today. It seems to work great.
> 
> One question I have, and I saw some mention of this but wasn’t sure if anyone had actually tried it, was adding a cap to boost the treble a little.
> 
> That’s the only problem I have with it, and it could be I am just used to hearing a lot more and can’t seem to dial it in while using this attenuator (I leave open the possibility that I could have screwed something up too).
> 
> Assuming the treble loss is normal, has anyone tried adding a cap? If so what value, and where would it be added?
> 
> If it matters, I built the M2 8 ohm version and am using a Marshall SV20 amp.



Treble loss is not normal for this design! I'm suspecting something being a little bit off? A close up pic of the wiring side of the switches may help. I'm not seeing anythingn odd in the first, reactive stage. Did you run a resistive test at the input with only your 8 ohm speaker plugged into the output? I can see that you kept it simple, without the added 16 ohm out.

Just Sayin'
Gene


----------



## JohnH

hi @gbarker7815 , yes the M2 should be very consistent with tone of the amp when running at a given power. Maybe play it a bit longer. If previously you had to turn the amp down and now you can turn it up through the attenuator, then they'll be a difference in tone from the amp anyway. 

And some resistance measurements may help check for build gremlins. So unplug from amp, plug in the cab and measure between tip and barrel at the plug that would go to the amp. It should be between 7 and 10 ohms in all settings. Also, unplug the speaker, no amp, measure the resistance at the speaker output jack with all attenuation stages on, which should be around 18 or 19 Ohms


----------



## JohnH

chas.wahl said:


> @john H: OK, when working on the schematic combining M2v and the "Gig" option, I've come up against this issue (raised, in my mind, by your post #1053):
> 
> 
> The M2v schematic dated 2020-04-20 has both a 16 Ω input jack and the 16 Ω output jack with R10 & R11, while the M2 (2021-03-18) has only the latter. So if one wants to connect a multiple output impedance amp to a 16 ohm load through the M2v, should one use 8 Ω on the input (from the amp) going to 16 output jack on the attenuator? Does the M2v actually "double correct" if one puts 16 Ω in and hooks up to 16 Ω out?
> 
> If the correction is made by the _input_ network added to M2v, then could (or should) I omit the R10/R11/16-ohm jack and just use the "uncorrected" 8 Ω jack out for a pair of 8Ω speakers in series?
> 
> Thanks,
> --
> chas.wahl



Hi Chas
From what Ive understood of your amps, they all have a range of output taps available, including they all have 8 Ohms. If an 8 ohm cab is one of your main uses, then there seems no benefit in building the V input, just build an 8 Ohm M2 with the outputs to suit 8 Ohm (the main one) and also the 16 and 4 output and any other features that you need if they will be useful . M2V is really just an 8 Ohm M2, with conversion at the front to match amps that have to be 4 or 16 Ohms. But, there's no benefit in using the converted 4 or 16 input, if you can instead work with its natural 8Ohms. That applies whatever the cab ohms are. Its simpler and less extra power loss involved. So Id suggest to not build the V input, but maybe leave space if you think you may acquire an amp that does not have an 8 Ohm output.


----------



## KIL0

Hey @JohnH,

I modeled the pcb stack in cad for visualisation. I spaced the pcbs little more for using 80mm fans in a 2 U rack housing.



The layout on the pcb is as following:



Every single pcb could be used as a single 16 Ohms attenuator (coil, switches and connectors are missing of course) with about 150 Watts of power rating, so with four pcbs in parallel this build would have an impedance of 4 Ohms.
I want to get the pcbs as aluminium core so the whole pcb itself works as a heat sink.

For measuring the amp and the cab I'll think about how to set up my equipment.

Regards


----------



## donwagar

gbarker7815 said:


> I managed to finish my build today. It seems to work great.
> 
> One question I have, and I saw some mention of this but wasn’t sure if anyone had actually tried it, was adding a cap to boost the treble a little.
> 
> That’s the only problem I have with it, and it could be I am just used to hearing a lot more and can’t seem to dial it in while using this attenuator (I leave open the possibility that I could have screwed something up too).
> 
> Assuming the treble loss is normal, has anyone tried adding a cap? If so what value, and where would it be added?
> 
> If it matters, I built the M2 8 ohm version and am using a Marshall SV20 amp.



You didn't mention, or I missed it, are you using an 8 ohm speaker or a 16?

I built the M2 16 ohm for my SV20C as it has a 16 ohm speaker in it. I didn't notice any treble loss.


----------



## chas.wahl

JohnH said:


> Hi Chas
> From what Ive understood of your amps, they all have a range of output taps available, including they all have 8 Ohms. If an 8 ohm cab is one of your main uses, then there seems no benefit in building the V input, just build an 8 Ohm M2 with the outputs to suit 8 Ohm (the main one) and also the 16 and 4 output and any other features that you need if they will be useful . M2V is really just an 8 Ohm M2, with conversion at the front to match amps that have to be 4 or 16 Ohms. But, there's no benefit in using the converted 4 or 16 input, if you can instead work with its natural 8Ohms. That applies whatever the cab ohms are. Its simpler and less extra power loss involved. So Id suggest to not build the V input, but maybe leave space if you think you may acquire an amp that does not have an 8 Ohm output.


I'm still interested in building the M2v with the "Gig" feature. It's more things to play with; maybe I'll like 16 Ω input and 16 Ω output more than I like 8 Ω in and 16 Ω out. Or I could try 16Ω in and 8 Ω out too, so long as it doesn't blow up anything. It will confuse the heck out of whomever inherits this thing, though. I'd better write a manual.


----------



## donwagar

what is a Gig feature?


----------



## JohnH

donwagar said:


> what is a Gig feature?



H Don, Chas is referring to a recent option of an amendment to Stage 1 to fill in the space between -7db (the loudest attenuated setting on a basic M2), and zero dB, being either a bypass mode or just not using the attenuator. It's a switch that turns the front -7dB stage to -3.5dB. The intention is that it's used just for that one setting, and then if -7dB or more is needed, it's switched back to standard. (it's safe to use with more downstream, but the tone is optimised just for -3.5)

It involves an added dpdt switch and a couple more resistors.

I called it 'gig', because it's most important use may be for performers who need just a small amount of attenuation, or none at all, and want to quickly switch between 0, -3.5 or -7 in response to the outcome of a soundcheck or an annoyed sound guy!


----------



## JohnH

chas.wahl said:


> I'm still interested in building the M2v with the "Gig" feature. It's more things to play with; maybe I'll like 16 Ω input and 16 Ω output more than I like 8 Ω in and 16 Ω out. Or I could try 16Ω in and 8 Ω out too, so long as it doesn't blow up anything. It will confuse the heck out of whomever inherits this thing, though. I'd better write a manual.



OK! The main watch-it will be if you include a full bypass switch. With that, you generally need to match amp to speaker, even though the unit will have inputs and outputs labelled with different ohms. Also in bypass, should use just the main 8ohm output jacks, since the 16 gets an added resistor in series. See discussion above with Gene.


----------



## JohnH

KIL0 said:


> Hey @JohnH,
> 
> I modeled the pcb stack in cad for visualisation. I spaced the pcbs little more for using 80mm fans in a 2 U rack housing.
> View attachment 114205
> 
> 
> The layout on the pcb is as following:
> View attachment 114206
> 
> 
> Every single pcb could be used as a single 16 Ohms attenuator (coil, switches and connectors are missing of course) with about 150 Watts of power rating, so with four pcbs in parallel this build would have an impedance of 4 Ohms.
> I want to get the pcbs as aluminium core so the whole pcb itself works as a heat sink.
> 
> For measuring the amp and the cab I'll think about how to set up my equipment.
> 
> Regards



It's an awesome idea. I'd describe it as 'cool' too, except I'm sure it will get fairly warm! LOL.

So it's targeting 4 Ohm in total. How about the switching? will you run vertical connections at each switch lug connection so that you can control using simple spdt or dpdt switches? 
Will you omit a full bypass switch or will you need that? If you omitt full bypass, then it avoids some risks if operated while the amp is on. Also, all the switching and output circuit after Stage 1 then just has to deal with 60W RMS (plus overdrive margin)

For the coils, each based on a 16Ohm module, may be ok at 18 guage but it maybe wouldn't hurt to consider 16 guage.?


----------



## gbarker7815

donwagar said:


> You didn't mention, or I missed it, are you using an 8 ohm speaker or a 16?
> 
> I built the M2 16 ohm for my SV20C as it has a 16 ohm speaker in it. I didn't notice any treble los





JohnH said:


> hi @gbarker7815 , yes the M2 should be very consistent with tone of the amp when running at a given power. Maybe play it a bit longer. If previously you had to turn the amp down and now you can turn it up through the attenuator, then they'll be a difference in tone from the amp anyway.
> 
> And some resistance measurements may help check for build gremlins. So unplug from amp, plug in the cab and measure between tip and barrel at the plug that would go to the amp. It should be between 7 and 10 ohms in all settings. Also, unplug the speaker, no amp, measure the resistance at the speaker output jack with all attenuation stages on, which should be around 18 or 19 Ohms










I think it works fine. All the tests work out fine. 

I think I may have described the treble loss wrong. It isn’t anything drastic like using a Weber or L-Pad attenuator without a treble switch. 

I would say the thing for me is I’ve only got to play the amp once without an attenuator, and it was the day after I got it a few months back at a fiend’s house where volume didn’t matter. At home, I can’t even use the 5 watt mode without attenuation. It’s just too loud for the kids and neighborhood. I haven’t had a chance to gig with it yet.

So basically what I’ve done until I built this one, is I built a L-Pad attenuator. It is what it is. I was able to make it not terrible by also running a volume pot box I put in the effects loop. I dial the volume box to 50%, then use the L-Pad to taste with the treble boost on. Without using the volume box, it has this terrible unnatural “sizzle” sound when a chord rings out.

Anyways, I managed to dial in a sound I really like that way for the most part, and it is bright, but I prefer a bright signal. It’s really probably more like the way my amp is, but with a treble boost pedal on all the time.

I probably just need to play with the new attenuator more and try to get used to it, and maybe build a Rangemaster to go with it or something.


----------



## KIL0

@JohnH I actually found some 4PDT switches on Musikding for only 3,50€ so I'd go with these for the attenuation stages. I didn't think of a bypass switch but in fact I thought about an impedance switching thing (single, two parallel, four parallel)

The thing is that I put a single 0603 220R (only E12 in the office) to it's rated power limit with 4,8V (resuming in 22mA => 0,1 W) for more than 2 hours just with two wires soldered on and it wasn't even warm after that so I'm really looking forward to how warm this stack is gonna be xD

I placed the fans at the side of R1 and R2A/B so the biggest dT will be at the biggest wattage but if you have more detailed into about the power dissipation in each resistor I could finetune this design even further ,)
For now I planned to just check one single PCB with my lab power supply at about 150W for maybe four hours with a temperature probe mounted (or even a temperature controlled fan controller) to check how hot it's gonna get

The coils are actually an other story. I'll need to check what I can find at our labs for winding because I can not find wires with the diameters and the length I need. I was looking for AWG12 with at least 100 ft due to what I read on this thread so far.
I'd go for four coils with 1.8 mH but I'm not sure about the diameter of the wire.

Regards


----------



## donwagar

gbarker7815 said:


> Anyways, I managed to dial in a sound I really like that way for the most part, and it is bright, but I prefer a bright signal. It’s really probably more like the way my amp is, but with a treble boost pedal on all the time.
> 
> I probably just need to play with the new attenuator more and try to get used to it, and maybe build a Rangemaster to go with it or something.



Have you tried running it unjumpered, just plugged into the top of the High Treble? I see lots of guys prefer this.


----------



## gbarker7815

donwagar said:


> Have you tried running it unjumpered, just plugged into the top of the High Treble? I see lots of guys prefer this.


I have tried that. It did give a little bit more on the bright end. I’ve spent more time tonight playing around with settings and all, and it does kind of sound to me like there is a light blanket laying over the speakers, for lack of a better way to explain what I’m hearing. 

I wish I could play it without the attenuator to see how it is with the same settings. That might give me a better idea if I messed something up with the attenuator or not, or if I’m just too used to how I had it set up with the L-Pad / Volume box life support system.

Not sure I ever answered your original question, but I am running an 8 ohm 4x10 cab.


----------



## chas.wahl

JohnH said:


> OK! The main watch-it will be if you include a full bypass switch.


Nah, I know how to plug an amp directly into a speaker, and don't have any expectation of having to switch between attenuation and non-attenuation "on the fly".

I don't know how much use this might be to anyone but me, but I've made a trawl through this thread to find the records of resistor values (and wattages). Here it is:


And here are the links to the posts referenced:
_Post #1, p. 1_
_Post #549, p. 28_
_Post #686, p. 35_
_Post #2359, p.118_
_Post #2382, p.120_

I started with @JohnH's power dissipation table in Post #1 (below the various layouts), logged the revisions to values that I've found in the thread, and interpolated newer additions (along with their revisions). Why?_ Because I'm building M2v with the lower-dB "Gig" front end and Stage 2, and I'm trying to figure out what values and wattages of resistors I will need to replace _(already bought the ones for a plain M2v)_. _The colors indicate the posts where changes were made or resistor specs added (blue, red, purple) and some questions I have (green). The blueish strikethrough is simply identifying which variations I can ignore for my own build because the "M2v Gig" doesn't use them.

In particular, it would be nice if there were power dissipation figures (based on design for the 50 W design) for the items shown in green. @JohnH did recommend 50 W resistors for the R1x and R2x variants, but didn't mention wattages for the later R5x, R6x and R10 variations. If it's a simple "query existing modeling" exercise to have the actual dissipation figures, I'd be interested to see them. If, on the other hand, it's something that takes more time and iterative work to develop those dissipation figures, then I'd be interested (in "teach a person to fish") how that's done. I've never used a tool like LTSpice, but I guess it's freely available for hobbyists, and I would not be averse to learning how to use it.

Also, in the latest R10 modification for the Gig design, @JohnH mentions that the change to 39 Ω is a "small change from 56" -- but I can't find anyplace where R10 was ever indicated to be 56 Ω. So far as I can ascertain, it started at 75 (not apparently used in any of the current designs, then 68 and now (for the Gig mod) 39. I feel like I'm missing _something._

If anybody thinks this is useful as a comparative record, I'm well on the way to translating the whole thing into a (n Excel) spreadsheet, which I'm happy to post. If not, then I apologize for getting so far into the weeds on this!

Thanks,
--
chas.wahl


----------



## tfletchii

I've been using an M3 design combined with the amended Stage 1 in post 2359 on page 118 for -3.5db or -7db in the first stage. I'm finding for live use though I think I want to build a design with a group of 4 or more approximately 1.5db stages that i can "stack" as needed. I have a Dr Z airbrake that operates this way and I find it better suited for me but I prefer the tone of the M3 attenuator. That said I couldn't find a resistor value on the thread for a ~1.5db reduction. Any ideas or suggestions?


----------



## JohnH

tfletchii said:


> I've been using an M3 design combined with the amended Stage 1 in post 2359 on page 118 for -3.5db or -7db in the first stage. I'm finding for live use though I think I want to build a design with a group of 4 or more approximately 1.5db stages that i can "stack" as needed. I have a Dr Z airbrake that operates this way and I find it better suited for me but I prefer the tone of the M3 attenuator. That said I couldn't find a resistor value on the thread for a ~1.5db reduction. Any ideas or suggestions?



Hi tfletchi, I don't have a design for that right now but Ill have a think about it. So for this unit, what would be the full range steps that you'd be looking for? Its quite tricky to do those small steps and also the full range and keep the tone and response consistent, without it also being a complex switching arrangement or fiddly to use. My designs work differently to other attenuators, but It will be possible in some way though! How about the first stage which is -7db, gets split into 4 steps, instead of two as with what you have?


----------



## JohnH

chas.wahl said:


> Nah, I know how to plug an amp directly into a speaker, and don't have any expectation of having to switch between attenuation and non-attenuation "on the fly".
> 
> I don't know how much use this might be to anyone but me, but I've made a trawl through this thread to find the records of resistor values (and wattages). Here it is:
> View attachment 114308
> 
> And here are the links to the posts referenced:
> _Post #1, p. 1_
> _Post #549, p. 28_
> _Post #686, p. 35_
> _Post #2359, p.118_
> _Post #2382, p.120_
> 
> I started with @JohnH's power dissipation table in Post #1 (below the various layouts), logged the revisions to values that I've found in the thread, and interpolated newer additions (along with their revisions). Why?_ Because I'm building M2v with the lower-dB "Gig" front end and Stage 2, and I'm trying to figure out what values and wattages of resistors I will need to replace _(already bought the ones for a plain M2v)_. _The colors indicate the posts where changes were made or resistor specs added (blue, red, purple) and some questions I have (green). The blueish strikethrough is simply identifying which variations I can ignore for my own build because the "M2v Gig" doesn't use them.
> 
> In particular, it would be nice if there were power dissipation figures (based on design for the 50 W design) for the items shown in green. @JohnH did recommend 50 W resistors for the R1x and R2x variants, but didn't mention wattages for the later R5x, R6x and R10 variations. If it's a simple "query existing modeling" exercise to have the actual dissipation figures, I'd be interested to see them. If, on the other hand, it's something that takes more time and iterative work to develop those dissipation figures, then I'd be interested (in "teach a person to fish") how that's done. I've never used a tool like LTSpice, but I guess it's freely available for hobbyists, and I would not be averse to learning how to use it.
> 
> Also, in the latest R10 modification for the Gig design, @JohnH mentions that the change to 39 Ω is a "small change from 56" -- but I can't find anyplace where R10 was ever indicated to be 56 Ω. So far as I can ascertain, it started at 75 (not apparently used in any of the current designs, then 68 and now (for the Gig mod) 39. I feel like I'm missing _something._
> 
> If anybody thinks this is useful as a comparative record, I'm well on the way to translating the whole thing into a (n Excel) spreadsheet, which I'm happy to post. If not, then I apologize for getting so far into the weeds on this!
> 
> Thanks,
> --
> chas.wahl


hi Chas

Thanks for trawling through all that. You sound a bit frustrated - i can understand that! With this thread almost 5 years old, starting from a position of zero idea about what is possible, there have been a few changes as you have noticed. I try to keep things as consistent as I can. But sometimes I change my view about what is the best value for a part or realize something new or a new idea emerges. 

I think you are right about the 56 Ohm value and it hasn't been posted in a diagram, That one is about putting a resistor in parallel with R1 (which is 15 Ohm) to bring it down a bit to keep Ohms seen. by the amp nearer to nominal, and it also helps to keep treble tone correct. Being in parallel with 15 Ohms, the difference 56 to 68 to 75 is not huge.

I do most of my design on a massive spreadsheet coded with macros to step through all switch settings. This models the attenuation and tone based on complex numbers to track reactance and phase etc. But for power dissipation, I just do a simpler resistive calc, which is good enough in the midrange frequencies where most of the power is - basically a series of resistive voltage dividers, with macros to switch them on and off. Ill run this again as an update to bring power calcs up to date.


----------



## chas.wahl

@JohnH, I am sorry to hear that you think I sound frustrated -- it's not that at all. On most projects, I spend a lot more time planning than executing; maybe that's a fault, but I find it interesting, and a good way to learn.

I would just like to know what the recommended wattages are for the resistors denoted with ????* above (no, those are just question marks, not comic-book cursing!), so I can buy those and do my layout based on physical sizes.

Your patience with this project, and your commitment to it are exemplary.

Thanks,
--
chas.wahl


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## JohnH

hi @chas.wahl , all fine! I've got my power calcs almost done. I wanted to take some care over them since all the tweaks and variations affect how power is spread across the components. So I'll post as soon as I can
cheers
John


----------



## JohnH

Hi @chas.wahl

Here is a run through the power calcs for an M2, including the -3.5dB Stage 1 option. All switch combos are included and plus 4, 8 and 16 ohm speaker loads. It's an envelope of the most power for each component through all settings:




One thing that happens is that since stage 1 lets more power through when set to -3.5, if the following stages are used with it, they take more. Hence on that basis, and based on power x3, some of the resistors that were 25W go up to 50W. But, the intent of having that -3.dB option is only to use it on its own, and to switch down to -7 for more attenuation. If you follow that approach, then R3 to R8 can stay at 25W. Also, its all about heat and it takes quite a long time for things to heat up. So if you want keep those ones at 25W, I think its OK.

For the basic M2 shown on post 1, no changes are needed based on this and its all still fine and recommended. This one is just about exploring this -3.5dB front end option and see what it affects it has elsewhere.

The variable input Ohms V option is not shown here because it doesn't change any maximums, When used, it takes half the power so everything else gets less.


----------



## JohnH

To go with the diagrams above, here are calc results on frequency response for the same design, showing the attenuation steps with the -3.5dB front end just used for that one step:
e



From the results, tone is staying very consistent. The ohms seen by the amp is moving a bit further, since the -3.5dB Stage 1 is less able to control what the amp sees compared to at -7dB. At mid frequencies and max attenuated level, the amp is seeing from 6.6 Ohms with a 4 Ohm speaker, to 11.1 Ohms with a 16 Ohm speaker a swing of about +/- 30%. I think thats ok though.


----------



## tfletchii

JohnH said:


> Hi tfletchi, I don't have a design for that right now but Ill have a think about it. So for this unit, what would be the full range steps that you'd be looking for? Its quite tricky to do those small steps and also the full range and keep the tone and response consistent, without it also being a complex switching arrangement or fiddly to use. My designs work differently to other attenuators, but It will be possible in some way though! How about the first stage which is -7db, gets split into 4 steps, instead of two as with what you have?


Hey John, Yes that would be great. I currently have the -7db stage divided between two switchers. I would be more than willing to build one with just the first -7db stage split up into 4 levels.


----------



## chas.wahl

@JohnH, thank you so much for that information in post #3030! I've revised the numbers in my BOM. I have just a few questions:

I presume that if I build what's pictured, but with the 4|8|16 Ω front end network (the "v" variant instead of the "line out" indicated), the previously indicated values for that network are OK:
L2 0.6 mH 18 AWG
R12 8.2 Ω 100 W (for 50 W amp)
R13 22 Ω 50 W (ditto)
Correct?
For clarification: I'm talking about the "standard" 8 Ω "middle" input design.

The M2 design has 4.7 Ω for R5 and 5.6 Ω for R8; the M2v design has 4.7 Ω for both R5 and R8, and the M2 Gig design above has 5.6 Ω for both R5 and R8. Which value(s) do you recommend that I use, if I build the M2 Gig with the 3-input "v" front end? I realize that they're close, within 20%, but I figure there must have been a reason for the variations, when other values remained constant.

I assume that the 16 Ω output still has a -2 dB attenuation when using R10 + R11, correct?
Does the 4Ω output with R14 also have an associated attenuation?

Here is my BOM file, which I've sorted two ways: numerical order (as presented above, and also in "stage" order, since the most current approach is to put the stages behind the front end in -14 dB, -7 dB, -3.5 dB order (swapping the 7 dB 14 dB order seen in most of the diagrams/designs posted).

_EDIT: I've revised previous errors in the value of the 16 Ω build inductors, and to coordinate value of R13 with input by @JohnH below._







I used a lookup table in Excel to select wattage ratings available (based on the dissipation values provided by @JohnH tripled) from the two aluminum-clad resistor manufacturers whose products I've purchased: Ohmite and Vishay (Dale). And the last two columns on the right are my "downscaling" for an attenuator for amps with lower power -- 25 W max.

I will post the schematic I've worked up for combining the M2v with the M2 Gig front end as soon as I have double-checked all the component values and added the R14 resistor.

I guess I can't upload the native Excel files here, so if anyone would like to have either/both of those, just send me an email address in a "conversation", and I'll forward.


----------



## JohnH

Hi @chas.wahl 

The V compenents that you list:

L2 0.6 mH 18 AWG
R12 8.2 Ω 100 W (for 50 W amp)
R13 22 Ω 50 W (ditto)

These are all fine. R13, I showed as a 25W. The power through that is not easy to assess. Below about 500hz, it is almost fully shunted by the coil L2, then above 500hZ it starts to conduct until at 10kHz and above, its taking almost the full current. Taking a weighted average and working out power x 3, I get that a 25W is OK but no harm in a 50W.

R5 went up to 5.6 because that stage also gets the 3.3 Ohm R6A

R8 is slightly better at 5.6 Ohm.

On the table in my last post above, you can see all the attenuations for each setting at left. The 16 Ohm speaker set is being knocked down a bit more with these values, because R10 has been reduced to 39 (to control input ohms.) The difference to 8 Ohms is about one 3.5db step. The 4 Ohm results drop just under 3 db below the 8 Ohm results. If you let the treble rise just a little more by using a 27 ohm instead of 20 for R14, you can get about a half dB back, with about a half dB more treble relative to mids. Youd likely never hear those differences though.


----------



## chas.wahl

Thanks again @JohnH. I appreciate the answers and help.
-- 
chas.wahl


----------



## JohnH

Hi @chas.wahl , I just noticed that in your lists for 16 Ohm M2's, the inductors are 1/2 of those for 8 Ohm. They should be x2 instead ie 1.8mH for L1


----------



## redberon2003

Welp, took me quite a while, as it was my first real DIY project soldering wise. Gonna test it in the next couple days and triple check my work before I risk my amp on myself. Hahah. Here’s my 100 watt 16 ohm m2. Thanks again John.


----------



## chas.wahl

JohnH said:


> Hi @chas.wahl , I just noticed that in your lists for 16 Ohm M2's, the inductors are 1/2 of those for 8 Ohm. They should be x2 instead ie 1.8mH for L1


Whoops! I got confused, mixing and matching table values between M2 and M2v designs. Corrected and edited above.

Here is my attempt at a schematic for my own 25 W build: M2v Gig




Layout in box is next. Hoping to fit it all in a Hammond 1590DE, 200 x 120 x 64 mm.


----------



## JohnH

Thanks for posting! Check resistance at the input before testing with an amp. Plug a speaker in. Plug the lead in that will go to the amp (but no amp yet). Measure ohms across tip and barrel of the jack plug that will go to the amp, which should be between 14 and 20 in all settings.

I see there's a fan, but maybe not many vent holes? See how it goes. If it heats up hotter than you can touch for a few seconds, more holes will help the fan work. In your case, given the airflow is fan powered, you could add multiple large holes in the base, maybe at the end remote from the fan and add feet to the case to raise it.


----------



## MikeD2

JohnH said:


> Hi @chas.wahl
> 
> Here is a run through the power calcs for an M2, including the -3.5dB Stage 1 option. All switch combos are included and plus 4, 8 and 16 ohm speaker loads. It's an envelope of the most power for each component through all settings:
> 
> View attachment 114456
> 
> 
> One thing that happens is that since stage 1 lets more power through when set to -3.5, if the following stages are used with it, they take more. Hence on that basis, and based on power x3, some of the resistors that were 25W go up to 50W. But, the intent of having that -3.dB option is only to use it on its own, and to switch down to -7 for more attenuation. If you follow that approach, then R3 to R8 can stay at 25W. Also, its all about heat and it takes quite a long time for things to heat up. So if you want keep those ones at 25W, I think its OK.
> 
> For the basic M2 shown on post 1, no changes are needed based on this and its all still fine and recommended. This one is just about exploring this -3.5dB front end option and see what it affects it has elsewhere.
> 
> The variable input Ohms V option is not shown here because it doesn't change any maximums, When used, it takes half the power so everything else gets less.


Hi John - Quick question about the M2 build with the line out option - My circuit analysis is a bit rusty after forty years, but was wondering if it would be safer/better to move the resistive voltage divider (R12 / R13) to the 8 ohm output instead of the input? My concern is that if one or both of those components fails (in a short circuit manner) that the amplifier output transformer will be driving a short circuit - I am only ever going to be using a single 8 ohm output anyways. Thoughts?


----------



## 1esotericguy

Hi All. New person here. Got inspired to build one of these JOHNH M2 attenuators, because I want to justify buying a SV20H : ) 
- Did my component buy already. $129 delivered for everything including enclosure and thermal paste and some M3 screws. 
- I'm a pretty good solderer, planner, builder - but never learned to read schematics (even simple ones trip me up).
- Question: Did I interpret the schematic correctly, in the little cartoon wiring diagram I made (image attached)? 

The way I read the schematic:
- R4, R6. and R8 are supposed to be in series to the Output tip. 
- And then separately R3, R5, and R7 can cut in via the toggle switches....

Hopefully that's correct, and if yes, hopefully my little wiring diagram reflects that properly. 

- The other thing I wasn't sure of: I bought DPDT toggle switches because I saw them in other people's builds in the thread, but when I drew out the wiring - seems I'm only needing SPST functionality - am i nuts? 

Thanks. Looking forward to building this an playing with it. Not looking forward to drilling a million holes in thie hammond box with no drill press : )


----------



## JohnH

H @chas.wahl , your diagram looks all good. On jacks, I'd suggest two in parallel for the 8 ohm out, si that two 16 cabs can use them at 8 ohms. Also, if a 4 ohm load is based on two 8 ohm cabs, one can go in the 4 out and the other in an 8 out, and the net 4 ohm pair will get the tone adjustment. Or, you can still put them both in the 8 ohm pair, it's still safe and you get a slightly less damped tone, like turning up presence a tad.

Also, I use stereo jacks for everything, even where mono would be ok. The stereo ones grip the plugs better due to the extra sprung contact.


----------



## JohnH

MikeD2 said:


> Hi John - Quick question about the M2 build with the line out option - My circuit analysis is a bit rusty after forty years, but was wondering if it would be safer/better to move the resistive voltage divider (R12 / R13) to the 8 ohm output instead of the input? My concern is that if one or both of those components fails (in a short circuit manner) that the amplifier output transformer will be driving a short circuit - I am only ever going to be using a single 8 ohm output anyways. Thoughts?



Hi Mike, You can get a very good signal for a line out from the attenuator output, but to get the right tone it needs to have a speaker there too with its complex impedance vs frequency. Without a speaker it would be a flat response there. Also, it is at an attenuated level, but probably more than enough for line out.

For line out at the front, over-spec on the rating of the 5k resistor should offer good safety, and very little power is dissipated in such a high value resistance.


----------



## Gene Ballzz

1esotericguy said:


> Hi All. New person here. Got inspired to build one of these JOHNH M2 attenuators, because I want to justify buying a SV20H : )
> - Did my component buy already. $129 delivered for everything including enclosure and thermal paste and some M3 screws.
> - I'm a pretty good solderer, planner, builder - but never learned to read schematics (even simple ones trip me up).
> - Question: Did I interpret the schematic correctly, in the little cartoon wiring diagram I made (image attached)?
> 
> The way I read the schematic:
> - R4, R6. and R8 are supposed to be in series to the Output tip.
> - And then separately R3, R5, and R7 can cut in via the toggle switches....
> 
> Hopefully that's correct, and if yes, hopefully my little wiring diagram reflects that properly.
> 
> - The other thing I wasn't sure of: I bought DPDT toggle switches because I saw them in other people's builds in the thread, but when I drew out the wiring - seems I'm only needing SPST functionality - am i nuts?
> 
> Thanks. Looking forward to building this an playing with it. Not looking forward to drilling a million holes in thie hammond box with no drill press : )
> 
> View attachment 114581



*FIRST>*  to the forum!
*THEN>* You're not nuts at all (at least not that I know of), but you are somewhat incorrect!  The purpose of at least a SPDT switch is that as each of the odd numbered (R3, R5 & R7) resistors gets turned off, it respective mate (R4 for R3) needs to get bypassed/jumpered around by the other end/throw of the switch. The diagram in post # 2,207 on page # 111






Simple Attenuators - Design And Testing


Wow this forum is amazing, im hoping to build a design for my amp, Im looking to have a headphone out as well as line out, becuase of the headphone out i want this section to have a cab sim, I was just wondering what was the point of m3 vs m2 does the m3 have a cab simulation as you keep...




www.marshallforum.com





shows this.

The only reason many of us use the DPDT instead of SPDT is to tie the two sides of the switch together for increased current capacity and redundance and basically turning it into a SPDT.

The jumper that goes from the center lug of each switch to the end of the next switch is part of what accomplishes this. It seems confusing, until you get what is actually going on. It might help to think of the center lug of the switch as the input to that stage. In one position the "odd" number resistor is put into the circuit, while the "bypass/jumper" of it's "even" numbered mate is removed, allowing it to be now in the circuit. Then, with the switch in the other position. the "odd" number is taken out of the circuit and it's "even" mate is bypassed/jumpered around!

HTH,
Gene


----------



## 1esotericguy

Gene Ballzz said:


> The jumper that goes from the center lug of each switch to the end of the next switch is part of what accomplishes this. It seems confusing, until you get what is actually going on. It might help to think of the center lug of the switch as the input to that stage. In one position the "odd" number resistor is put into the circuit, while the "bypass/jumper" of it's "even" numbered mate is removed, allowing it to be now in the circuit. Then, with the switch in the other position. the "odd" number is taken out of the circuit and it's "even" mate is bypassed/jumpered around!
> 
> HTH,
> Gene



Wow - glad I asked. It wasn't clicking. But I think I got it now. Thanks. I took a do-over below. I think I comprehend it now after some staring. 

Question 2: Just making sure - since I'm building this as a 16ohm specific unit, to use this specifically with a 16ohm 2x12 cab - is there any reason to implement the 2 separate unique output jacks like shown in the schematic? I can't think of a speaker combo in paralel that nets me 16ohms, hence I deleted the other output jack in my little wiring diagram.... 

Is the two output jack option really for an 8Ohm build? (so one can plug a 16ohm cab in each jack, to equal an 8ohm speaker load)?


----------



## Gene Ballzz

@1esotericguy ,
Yeah, that's more like it! The two output jacks is to facilitate two 16 ohm speakers. While either an 8 ohm or 4 ohm speaker load on the attenuator is totally safe for the amp, there is a slight tonal compromise that is easily corrected for an 8 ohm load by adding one 25 watt, 39 ohm resistor at just one of those jacks. Be aware that it is critical to use all isolated jacks like the Cliff UK style and never use the case as part of the circuit! We prefer the TRS/stereo jacks, for multiple reasons. One is that it grips the plug more solidly and it also facilitates that added resistor, tonal tweak for the most common, alternate use of an 8 ohm speaker load. We can show you how to wire that, really simple. Also, any cost increase for the TRS over TS is minimal!

Once that first, "always active" reactive stage is in place, it mostly isolates the amp's output from any impedance changes down the line. This is why any normal speaker impedance load is safe and as your design goes, it is simply optimized for a 16 ohm speaker load, and the amp will always see a correct impedance load, +/- one or two ohms.

And yes, this attenuator design is the best sounding passive unit you can find!

HTH,
Gene


----------



## chas.wahl

JohnH said:


> H @chas.wahl , your diagram looks all good. On jacks, I'd suggest two in parallel for the 8 ohm out, si that two 16 cabs can use them at 8 ohms. Also, if a 4 ohm load is based on two 8 ohm cabs, one can go in the 4 out and the other in an 8 out, and the net 4 ohm pair will get the tone adjustment. Or, you can still put them both in the 8 ohm pair, it's still safe and you get a slightly less damped tone, like turning up presence a tad.
> 
> Also, I use stereo jacks for everything, even where mono would be ok. The stereo ones grip the plugs better due to the extra sprung contact.



Thanks for the review and your advice, and that's one way to do it, but there's no series option. I have an ingenious design (not mine, found online) for a separate box to manage _speaker connections_ from a single (whether attenuated or not) amp input. With 3 jacks on the input side, it will allow parallel, series, or single connections to two jacks on the output side.



No, no mixing of inputs from amps of differing output impedance; but you can't have everything. (As you are no doubt aware, jacks are a lot less expensive than switches, and they're not as prone to damage. I guess Big Telecom knew that from the outset.)

On jacks, I like Switchcraft, and have a bunch of them -- Fender vs Marshall persuasion I guess. I just use their isolation bushing (plastic with shoulder) and phenolic washer, which are thinner than the all-plastic isolators that are also available. Plus, I dislike the plastic nuts on Cliff jacks, and it's very hard to find them with chromed brass nuts (at least for me) in the US.

In the TMI department: If one wished to be super-cautious, OT protection resistors (high-Ohm, like 470R) might be added across the S and TS lugs of outputs J4 and J5. That way, plugging in and using inputs from the amp side without having a speaker connected doesn't leave the OT with an open secondary. Some people do this with output(s) at the amp. The usual way in an amp is to connect the resistor from T to S, which makes it a permanent load, in parallel with any connected speaker load. Adding the resistor from S to TS makes it operative only when the jack is not in use, but leaves the possibility that the circuit is open between that jack and the speaker, in which case there's no protection wherever the TS lug was used.

I guess my point is: anyone who wants to use anything other than a simple amp-to-speaker connection, whether attenuator or mix-and-match speaker connection selection, should think about all the possibilities for leaving the OT secondary open inadvertently (due to inattention or because someone trips over a cable).


----------



## redberon2003

JohnH said:


> Thanks for posting! Check resistance at the input before testing with an amp. Plug a speaker in. Plug the lead in that will go to the amp (but no amp yet). Measure ohms across tip and barrel of the jack plug that will go to the amp, which should be between 14 and 20 in all settings.
> 
> I see there's a fan, but maybe not many vent holes? See how it goes. If it heats up hotter than you can touch for a few seconds, more holes will help the fan work. In your case, given the airflow is fan powered, you could add multiple large holes in the base, maybe at the end remote from the fan and add feet to the case to raise it.



Resistance looks good and gonna go ahead and start off with my 30 watt head to test it. Probably be a while before I can get it hooked up to 100. But I like future proofing.

Id considered drilling more holes, but since the cooling is active and single direction I wanted to see how it’ll work. Leaving more metal in tact does create a larger heatsink and more surface area exposed (both good things) and may help with cooling, if it’s not working the drill comes back out though. When I can eventually set it up and crank 100 I’ll be sure to share my heat results.

Last question for you as well. I had it in my mind 18awg was good but looking back 16 may have been a better choice for 100 watts. My air coil is 16 AWG don’t know why I didn’t think about the rest of the wire.

Wondering if 18 and 100W is workable?

I’m figuring if anything I can re-do the input to the first set of resistors/air coil and everything down wind of that should be happy on 18AWG?


----------



## JohnH

Hi @redberon2003 
I think 18 gage is ok and a lot of Marshall cabs use it. 16 is good for your coil since it will heat up less. Also, the guage needed is related to current, and with 16Ohms, there's less current for a given power than at 8 Ohms.


----------



## 1esotericguy

Im sure this is answered in this thread somewhere…hoping someone knows the post or page number…

Question: is there a the test procedure after the build is complete, so that one can use a digital multimeter to validate the build prior to plugging in an amp. 

For example: do you just meter the input jack and one should see 16ohms at the input jack, and you know you’re build is correct? 

Thanks.


----------



## JohnH

1esotericguy said:


> Im sure this is answered in this thread somewhere…hoping someone knows the post or page number…
> 
> Question: is there a the test procedure after the build is complete, so that one can use a digital multimeter to validate the build prior to plugging in an amp.
> 
> For example: do you just meter the input jack and one should see 16ohms at the input jack, and you know you’re build is correct?
> 
> Thanks.



Basically yes, with a few comments:

Plug a speaker into the output, when you measure, since the speaker forms apart of the load
Plug a speaker cable into the input and make your measurements across the tip and barrel of the plug that will go into the amp, This way you test the leads and the jack wiring as well as the internals
With a 16 Ohm M2, you should read between 14 and 20 ohms in every switch setting, try them all
subtract the resistance of your meter leads (touch them together)

That's it. This wont test everything but it does check that the amp will be seeing a load that is OK. If it passes, then you can try with the amp, first at low volume and make sure sound comes through in all settings. If no sound, stop right there rather than turn the amp up high!


----------



## Mjh36

Howdy ya'll I'm back to build another. My first was an 8ohm 50w M2, this time around I'm going for an 8ohm 100w unit, possibly an M3. I want it beefed up. Few q's:

-- Does *L2* (if I go with the M3) also need to be 16awg or greater? I only found an 18awg 9.0mH, or there's a 15awg 10mH. I can unwind the 10mH I assume.

-- I plan on using a big case and parallel 300w 30ohm resistors for R1. Yup! Plus a 150w resistor for R2A. Do you think that'll be enough dissipation to avoid a fan? I'll have the room later if I have to add one though.

-- Are the below diagrams current for the M2 and M3 bass resonance coil/capacitor circuit? Just add the L2 coil and cap after L1 to the newer schematic yeah? Just don't know if I've missed a new diagram version. Thank you in advance. - Mitch


----------



## JohnH

hi @Mjh36

Those diagrams are still fine. Since then there's not been much new except for:

1. An added switch to lift the first stage up from -7 dB to -3.5 dB. That's maybe useful if you really need just a very small reduction, otherwise not needed.
2. A tweak to the tone if you want to run a 4 ohm cab, or two 8ohms. It just trims the bass and highs a tad to make this setting more consistent.

See post 3030 on page 152, which also has a full set of power calcs for 50w. x2 for 100W.

Not sure if you'll avoid needing a fan, it depends on how big and thick the case is and how hard you drive. But for 100w, definitely make provision for one with vents above and below. The easiest is a 12v dc brushless fan and run it to a 2.1mm centre-negative socket like for a stomp box. Then plug in a 9V wall-wart if needed.

For the bass circuit, those value are fine and I'd agree that thicker than 18gage would be best. The design is tuned to match a 1960A cab with greenbacks. If you use the 10mH, it'll be a bit more like a B cab, ie, it's still fine. I haven't built this bit myself, but the concensus is that it's of most use with a line-out circuit. For use just to attenuate a guitar cab, it's not really needed.

If you don't have the bass resonance circuit, you still get a good bass peak at the guitar cab because it is the real cab itself that creates it, interacting with the attenuator. This is the natural way that a cab interacts with an Amp too. With the added parts, the tone at the cab is not much different but you may get a few extra bass harmonics, and though a line-out, you get the peak whereas without it, you might add some bass downstream. Also, the bass peak is only relevant at all if you play very low guitar notes. With a closed back cab this is typically around low A and a semitone or two each side. This is where the M3 design is tuned to.With open back, it's more around low E

The biggest decision with the bass circuit is the caps. The best and most exoensive are big polypropylene ones which have very low losses. Or for much less $ and not as good, bipolar electrolytics with high ripple current and low dissipation factor. It's likely best as a group of them in parallel. I've bought some recently but I haven't tried them yet.


----------



## Mjh36

Cool thanks John that clears some things up. I did read around a little more and found that small deviations in mH in the inductors is ok, like you said 10mH is no big deal.

About steel laminate inductors... I remember air core and non-ferrite components are important. But the Aiken gearpage forum I noticed some pics with what look like some type of metal core inductors for L2. I haven't read much over there but just was browsing to see build pics.

Is that possible in this design as well? For example here's a steel laminate from Madisound ($40) and an air core from PartExpress ($118), as well as an Erse SuperQ ($22). This will help narrow down the enclosure options I have.

edit: Post #2180 bottom of page 109, @TomBallarino looks like he has a steel core L2. And amp powered fan whhhhhaaatt? Is that a custom circuit board? I'll probably go the easier route of just a DC wall wart if I need a fan but that is nice.









Steel Laminate 10.0 mH 15 AWG Inductors


Steel Laminate 10.0mH inductor with 15 AWG Copper Wire117mm x 46mm x 32mmDcr 0.38 For all available values, DCR, and size of coil, please visit the comparison page, http://www.madisound.com/manufacturers/madisound/inductors/sledgehammer.php.




www.madisoundspeakerstore.com













Home


Jantzen Audio 10mH 15 AWG Air Core Inductor Crossover CoilThe Jantzen Audio 15 gauge air core inductors offer a lower DCR alternative to the other Jantzen lines. They feature the same quality construction including high-purity copper wire wound to a ± 3% tolerance, and the special Jantzen...



www.parts-express.com










ERSE - Super Q


ERSE Super Q inductor. Premium high power audio grade steel core crossover coil with low resistance DCR and low distortion.




www.erseaudio.com


----------



## JohnH

hi @Mjh36 
Yes L2 can have a core, otherwise it gets to be too huge. I also used the Aiken design as a reference when figuring out what type of parts are needed.

Amp powered fans are definitely a possibility, and some like the old Marshall Powerbrake have them. But they'd have to be specifically designed for particular parts and uses, and checked that they don't suck tone. So there's testing needed and we can't post a generic design with confidence it will work well.


----------



## 1esotericguy

1esotericguy said:


> Wow - glad I asked. It wasn't clicking. But I think I got it now. Thanks. I took a do-over below. I think I comprehend it now after some staring.
> 
> Question 2: Just making sure - since I'm building this as a 16ohm specific unit, to use this specifically with a 16ohm 2x12 cab - is there any reason to implement the 2 separate unique output jacks like shown in the schematic? I can't think of a speaker combo in paralel that nets me 16ohms, hence I deleted the other output jack in my little wiring diagram....
> 
> Is the two output jack option really for an 8Ohm build? (so one can plug a 16ohm cab in each jack, to equal an 8ohm speaker load)?
> 
> View attachment 114597



Finished my build. Very fun and satisfying build. 

My best move was tapping the resistor mounting holes, so that I didn't have to fiddle with nuts. I ended up having to improvise to tap the holes but it worked fine. For anyone's future reference:

1. 7/64 drill bit (7/64 was the closest size I had to the hole)
2. I just used an M3 steel screw to tap the holes - the steel machine screw tapped into the Hammond box like butter.
3. (M3 - .5) x 5mm long button head cap screws are the perfect length to finish flush with the top of the enclosure when you screw through the resistor mounting holes from the inside of the box.

I had one wiring error, but I caught it when I metered prior to firing it up. Was getting 32 ohms or so with no switches engaged. Had the Output (+) connected to the wrong switch leg. Other than this one build error, it was an easy build. Plenty of room in a Hammond 1590D enclosure. 

Also, a step drill is critical to making this build easy. I didn't have to debur even 1 hole. 

Thanks John H for the awesome design.

Here's some pictures.


----------



## JohnH

Hi @1esotericguy , that looks great! Thanks for posting. Is it sounding good too?
Letting the screws tap themselves is smart. Did you have any problem getting the holes drilled accurately enough?


----------



## 1esotericguy

JohnH said:


> Hi @1esotericguy , that looks great! Thanks for posting. Is it sounding good too?
> Letting the screws tap themselves is smart. Did you have any problem getting the holes drilled accurately enough?



It's sounding really good. It's a cool design. I built a simpler L-PAD based design (it's the attenuator deign that one finds on Google if you search 'DIY Attenuator,') and it's OK and it worked - but this JohnH circuit is markedly better. Wish I had just built this one first.

To answer your question on getting the holes drilled accurately. No problem with accurate hole locating. I used an automatic spring loaded center punch to punch the hole locations. (sounds fancy, but they're inexpensive).
1. I just physically laid out all the resistors into the enclosure
2. Then I used the automatic center punch to punch right into the mounting holes on the feet of each resistor. One doesn't have to use a hammer or anything. Just push and the spring punches a dot. Can't miss. 
3. Then I used a 1/16 inch bit to drill each center punch from inside the enclosure to outside. The 1/16 bit sits right in the center punched 'dot,' so the hole is perfectly located in the center of the mounting hole. 
4. Then I flipped the entire box over and re-drilled each hole to the larger 7/64 inch, now drilling from outside to inside the box (this isn't even the right size bit for tapping an M3 thread, but it worked well)
5. Then I tapped from the outside towards the inside of the box (just to have a little more room to work)
6. The only problem I encountered was actually silly: I had an actual M3 tap and the correct wire size drill bit , but the tap oversized the first hole I did (probably a cheap tap). That's when I tried just using an M3 screw (instead of a real tap) and it worked great. Very silly that the wrong tool worked better than the right tool in this case.

Here's a picture of the automatic center punch. It would have been impossible IMO, to easily locate these holes accurately enough for tapped holes without this inexpensive tool:


----------



## Gene Ballzz

1esotericguy said:


> It's sounding really good. It's a cool design. I built a simpler L-PAD based design (it's the attenuator deign that one finds on Google if you search 'DIY Attenuator,') and it's OK and it worked - but this JohnH circuit is markedly better. Wish I had just built this one first.
> 
> To answer your question on getting the holes drilled accurately. No problem with accurate hole locating. I used an automatic spring loaded center punch to punch the hole locations. (sounds fancy, but they're inexpensive).
> 1. I just physically laid out all the resistors into the enclosure
> 2. Then I used the automatic center punch to punch right into the mounting holes on the feet of each resistor. One doesn't have to use a hammer or anything. Just push and the spring punches a dot. Can't miss.
> 3. Then I used a 1/16 inch bit to drill each center punch from inside the enclosure to outside. The 1/16 bit sits right in the center punched 'dot,' so the hole is perfectly located in the center of the mounting hole.
> 4. Then I flipped the entire box over and re-drilled each hole to the larger 7/64 inch, now drilling from outside to inside the box (this isn't even the right size bit for tapping an M3 thread, but it worked well)
> 5. Then I tapped from the outside towards the inside of the box (just to have a little more room to work)
> 6. The only problem I encountered was actually silly: I had an actual M3 tap and the correct wire size drill bit , but the tap oversized the first hole I did (probably a cheap tap). That's when I tried just using an M3 screw (instead of a real tap) and it worked great. Very silly that the wrong tool worked better than the right tool in this case.
> 
> Here's a picture of the automatic center punch. It would have been impossible IMO, to easily locate these holes accurately enough for tapped holes without this inexpensive tool:
> 
> View attachment 115282



One of my best friends in the shop. $3.99 at harborFreight!

Nice job! What amp is it intended for?

Again, Nice work!
Gene


----------



## 1esotericguy

Gene Ballzz said:


> One of my best friends in the shop. $3.99 at harborFreight!
> 
> Nice job! What amp is it intended for?
> 
> Again, Nice work!
> Gene


I’m keeping my eyes peeled for a used Sv20h, but right now I have the attenuator connected to a Ceriatone 18watt TMB kit (that wasn’t built by me). It’s a great amp. And the cab im using is a Sour Mash 2061cx style slant, 16ohm, 2x12 (the diagonal speaker layout one). It’s loaded with 2 70’s Celestion blackbacks.


----------



## Gene Ballzz

1esotericguy said:


> I’m keeping my eyes peeled for a used Sv20h, but right now I have the attenuator connected to a Ceriatone 18watt TMB kit (that wasn’t built by me). It’s a great amp. And the cab im using is a Sour Mash 2061cx style slant, 16ohm, 2x12 (the diagonal speaker layout one). It’s loaded with 2 70’s Celestion blackbacks.



NOICE!

If you go much larger in wattage, you may need a little more ventilation, especially around R1 and R2A. Under 25 or 30 watts, you should be fine! I'm so happy it works well for you. Mine are supremely liberating! My goal is to end up having one in each of several amps. The problem is that folks keep asking if I will sell one to them!

Nice layout and execution. I had been tapping some different boxes I had previously used, but have gone to using countersunk, SAE #4 screws, with nuts. I'm looking into using slightly larger countersunk screws and tapping the actual resistors themselves!

Great Job!
Gene


----------



## emann

Hi again,

in waiting for the parts to arrive (hopefully in approx 2 weeks I would have everything now!), I prepared a dwg to ensure a good layout of all the parts in the hammond 1550jbk enclosure that I ordered (against the 8ohm M2 layout schematic from a previous thread that i attached here). Since I play in a room at the roof of the house which gets quite hot in summer, I tried to space the components as much as possible for better venitlation hopefully. I plan to stick this to the box so that I have all required holes in place for drilling.






In reading through this thread (I am now at page 131 so getting there!!), i note that the majority of the builds have the components fitted to the enclosure box. Allow me a dumb question - is it not easier that I install the components on the lid instead? I mean like this I have everything at a flat surface for soldering and it should be easier rather then fiddling at the depth of the enclosure. Like this I would install all resistors and inductor on the lid and the cliff jacks, toggle switches and potentiometer on the sides of the enclosure.

Any thoughts on the general layout and the question above please?

Thanks to all.


----------



## JohnH

Hi @emann 

On layouts, you can do it anyway that works best for you. But here's a few things to think about:

The Hammond cases are slightly tapered and can look best if the lid (the widest part) is actually the base

The main box with sides is the largest continuous piece of metal, so if the resistors are fixed to it, and its also on tbe top, then you have the best heat dissipation with the max cooling using the largest surface.

There are quite a number of wires from switches and jacks to resistors, and switches will be on the box sides. So if switches jacks and resistors are all on the main box, then wiring runs are short and don't need to flex when opening the lid.

Id suggest that if you wish, you could put just the coil on the lid-base, since then it's weight presses it down, and there's only two wires to it to go up.

J


----------



## emann

thanks a lot JohnH for that - I understand better now the geometry and heat dissipation and hence will opt to install everything in the box and leave the lid base free.

Regards.


----------



## Gene Ballzz

@emann ,
*FIRST>* As per some of our previous, private email communications, please allow me to sincerely commend, compliment and congratulate you on the decision to dive in and tackle this yourself! The process will likely help you perfect some very useful skills that should serve you well, throughout your lifetime! If your solder skills are already pretty good, that is a plus! If not, the most important advice I always share is paying close attention to the maintainence of your soldering iron tip and keeping it clean! Here's a kinda lengthy tutorial, and whille it is quite specific, none of it is difficult and simply needs to be followed as a standard procedure for successful soldering. Post #21 of this thread over at TDPRI:






Yet Another Fezz Parka Mod Question


Ya know, to me it's hilarious that many guitarists can sit and work/practice on a specific lick for hours on end, yet refuse to get a good soldering tool and spend an hour "practicing" how to "PROPERLY" use it! The main trick to good soldering is the proper and simple maintenance of the iron...




www.tdpri.com





NEXT> Concerning putting the switches & jacks on the enclosure and the other components on the lid. While meticulous planning could make it work OK, the extra wire lengths could make it a real mess very quickly and the wire lengths would end up longer than they should be. On top of that, any future troubleshooting/wire tracing may be a bit tedious! Remember that each switch has three or four wires that need to go to the other components. Yes, you could make some of the wire junctions at the resistors, but that can be a bit fiddly, at best.

I suggest wiring the switches out of the box, leaving enough extra wire length to easily make it to the appropriate components. Some sort of bench vise is handy here!

All The Best & Again, Congratulations To You,
Gene


----------



## emann

@Gene Ballzz : I have to thank you Gene for your support and encouragement to tackle this circuit!

I put that tdpri thread on my watch list to ensure I go through it.

And I also have decided to place everything on the box - tomorrow I work again the layout as the dimensions of the box are a bit smaller than the lid so that I have this finalised and all ready to when the components start arriving.

Will keep you posted all here ...also I do not exclude some other technical help in our private communication when things start getting more complete!

Thanks!


----------



## Gene Ballzz

And don't forget to drill a healthy number of healthy sized holes in between those resistor locations, for ventilation/cooling!
Just Remindin'
Gene


----------



## Mjh36

Ok, almost have all my parts for an 8 ohm 100W M3.

I'm going to use 18awg hook-up wire. Should be ok? I've looked and tried to see other 100w attenuator pics to see what others used. I've just been on the fence about 16awg vs 18awg. The inductors will be 15awg so there's that. I know it's mentioned that Marshall uses 18awg in their cabinets which points to it's ok to use.

Also, I intend to try the self-powered fan mod posted by @rallye440 on Page 1 of the Completed Attenuators thread. I'll include his pics here. As far as power handling, what's the wattage for the 33 ohm resistor and voltage for the 470uf cap? Sorry I don't know how to calculate those things. Also rallye440, what gauge hookup wire did you use in your 100w build?


----------



## JohnH

hi @Mjh36
I think 18ga is ok for hookup, even at 100W and 8 ohms. Nominal current through it is 3.5 Amps. 16 is good for rhe coil, since there's a lot of wire on a small place in a coil and the lower resistance of 16ga will keep it cooler.

The greatest voltage at 100 W at 8 ohms is 28 V. So for the fan cap , given some is dropped by the resistor, I'd suggest 50 V spec minimum . I don't know about the 33 Ohm, and also I can't say if this will affect tone. Maybe @rallye440 can advise? It may depend on exactly what the model and spec of the fan is. it also depends on the ohms you are building to. How well it works can only be determined by testing.


----------



## DuaneOh

Hello All, this is my first post as I am inspired to build one of these attenuators. I am planning on building a simple M2 on the benchtop to start, then and add different variations to find what suits my needs before I put it into a box. I was looking for a bypass switch but did not find one (very long thread), so I came up with the following idea.

View attachment Simple Attenuator.png


My goal is that it could be hot switched. I would appreciate it if JohnH or someone else could let me know if this will work. 
Thanks for all the work that was done on this project! Duane.


----------



## JohnH

Hi @DuaneOh , and welcome to our thread. Thanks for your intertest.

I think your bypass diagram, with one switch pole on hot and the other on cold, probably works OK as compared to the more usual version where both are on hot. But I don't see an advantage to it, and also, I don't think it changes the requirement not to switch while running. Any bypass diagram still has to connect and reconnect the full power of the amp and so the amp needs to be off or on standby IMO.

Many commercial designs for passive attenuators also offer a full bypass function, and may or may not offer advice on how to use them. But I expect that really all are essentially the same and the issue is the risk of transients due to switching, causing damage to the amp output circuitry, combined with the risk of user error.

So we stopped showing bypass switches on the base diagrams, to head-off this issue except where builders chose to add it. Most users won't really need a bypass in practice, after initial exploring. Either a rig set-up will need some attenuation for a certain use, or it won't in which case the attenuator needn't be connected. There's a couple of likely reasons that a bypass could be useful though:

Initial testing to find out what the attenuator does
Use with small amps, or maybe larger amps in a gig, where it's not obvious yet whether attenuation is needed, or maybe the sound guy will want you to reduce volume. The bypass would allow a quick change, without replugging leads (which itself has some risk of user fumble)

All that being said, no problem at all having bypass, if its all understood and used safely. It can be a good idea to make it distinct from the other switches, eg put it around the back, or a different style of switch.

I reckon it's fine to switch any of the other switches on the fly, since so far as I could work out, operating any one switch , no matter what the others are doing, always leaves enough consistent connection during switching.

Thanks for your diagram, it's very clear! what do you use to draw it?

I saw one more thing on it, which may be reading-in too much or it may be intended:
The jacks look to imply an open type with the barrel screwed to the case, like normal Switchcraft. (and some of mine look the same). We recommend Cliff jacks or similar, plastic and fully insulated from the case, in case the OT cold output is not a ground. Also, they connect better with spring contacts for all connections. Plus, using a 'stereo' TRS style is better again, even though you may not use the ring lug, since the extra sprung connection helps to grip the plug better.

Good luck with your build!


----------



## DuaneOh

Thanks for your detailed reply. On the bypass switching I was thinking that it kept some load on the OT while switching so the transients would be reduced. Anyway, the bypass will be useful for testing and will probably not go into the final design. 

At work I have access to a schematic capture / pcb layout program called Target 3001, It's pretty good and I think not very expensive. It seems that most people over at el34world.com to use JSchem (jschem.bplaced.net) which is free.

Thanks for the heads-up on the jacks, and thanks again for your help.


----------



## chas.wahl

Not really my business, but I notice that @DuaneOh's attenuator schematic has an "earth" connection of the circuit. My understanding of all the advice I've seen here is that the enclosure, which should be aluminum for heat dissipation, should NOT be grounded electrically to the circuit in any way -- the attenuator circuit should float through it in isolation (just as the aluminum cladding of each of the power-absorbing resistors is isolated electrically, though not thermally, from the resistor element itself). The ground reference for the attenuator circuit comes from the amp via the negative side of the input jack, also isolated from the attenuator case -- @JohnH please confirm.
--
Charles Wahl


----------



## JohnH

chas.wahl said:


> Not really my business, but I notice that @DuaneOh's attenuator schematic has an "earth" connection of the circuit. My understanding of all the advice I've seen here is that the enclosure, which should be aluminum for heat dissipation, should NOT be grounded electrically to the circuit in any way -- the attenuator circuit should float through it in isolation (just as the aluminum cladding of each of the power-absorbing resistors is isolated electrically, though not thermally, from the resistor element itself. The ground reference for the attenuator circuit comes from the amp via the negative side of the input jack, also isolated from the attenuator case -- @JohnH please confirm.
> --
> Charles Wahl


I agree, noted in my reply three posts above. (But if a certain rig works better with a grounded case, it's then easy to add explicitly.)
J


----------



## emann

Hi again,

of all the parts required for the M2 circuit, I did not order the solid core copper 1mmsq wire thinking I would buy this locally.

It turns out to be very difficult to find this type of hook up wire. I have the option to use 0.5sqmm solid core copper wire. I thought maybe I can use this to solder the terminals of the toggle switches and the cliff jacks. 

Then to wire between the components, I would use 1sqmm wire but stranded not solid.

Is this ok please? If not can I go ahead to use 1sqmm stranded wire for the full circuit?

Lastly - any suppliers in europe you may have used to get some solid copper 1sqmm wire please.

Thanks.


----------



## Gene Ballzz

Stranded wire is fine, although you will find that "pre-tinned" is much more convenient and easier to work with. You did not specify what you chose for switches?

This wire is especially nice to work with, although you'll need to see if this style and/or brand is accessible and affordable in your location! I'm guessing that shipping from the US might be outrageous!



https://www.tubesandmore.com/products/wire-weico-18awg-stranded-top-coat-pre-tinned-600v-50-feet



Keep Us Posted,
Gene


----------



## emann

Gene Ballzz said:


> Stranded wire is fine, although you will find that "pre-tinned" is much more convenient and easier to work with. You did not specify what you chose for switches?
> 
> This wire is especially nice to work with, although you'll need to see if this style and/or brand is accessible and affordable in your location! I'm guessing that shipping from the US might be outrageous!
> 
> 
> 
> https://www.tubesandmore.com/products/wire-weico-18awg-stranded-top-coat-pre-tinned-600v-50-feet
> 
> 
> 
> Keep Us Posted,
> Gene


thanks to reply.

the switch ordered is:
https://export.farnell.com/alcoswit...toggle-switch-dpdt-5a-125vac-panel/dp/3133586

I checked the tubesandmore website and definitely it does not make any financial sense to buy from the US to here. 

In any case I will go with the 1sqmm stranded core. 

as regards to the wire between the terminals of the switchs and the jacks - do you think it is ok Gene to use the solid core 0.5sqmm wire or would you recommed still to use the stranded one please?

thanks.


----------



## Gene Ballzz

I strongly recommend the same size wire for everything!
Sincerely,
Gene


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## emann

ok...then 1sqmm stranded for everything it is!


----------



## esolate

Nice


----------



## Gene Ballzz

esolate said:


> Nice



 to the forum, and that's one heck of a unique first post!
Still Attenuatin'
Gene


----------



## sah

After a long while (and many happy hours playing through my M2!) I've come back around to try to complete the pluggable M3 add-on I had planned, to see how much difference it makes for recording direct from the attenuator output.

By the way, the reason this project sat for so long is that the inductor I originally ordered was back-ordered for many months until the supplier eventually gave up!

Anyway, I've found a couple interesting options are available now, but they don't come in 9mH and I don't know how that value was calculated. Here's one example: https://www.parts-express.com/Jantzen-6430-8.2mH-14-AWG-C-Coil-Toroidal-Inductor-255-832

So, I can get 8.2mH or 10mH. Will one of those values work well? I have an LCR meter, so another option might be to get the 10mH and try my luck removing the shrink wrap plastic layer and unwinding it until it measures 9mH. Any advice would be appreciated!


----------



## JohnH

Hi @sah , I'm really interested in what you find by this test.

The extra parts L2 and C1 are aimed at hitting the same bass resonant frequency as a typical cab and the values I listed were based around measurements by Mike Lind on TGP of a G12M 'A' cab. I reckon the best option will be to get the 10mH , which will bring the frequency down about 5% compared to 9mH, maybe similar to a B cab. So still a good value to use. And you can unwind it if wanted.

The proportion of L to C is all about getting the 'Q' shape of the peak close to as tested in the simulations, as well as getting the frequency as wanted. This leads to quite a big cap for the 8 Ohm version.

On using the line out and its position and the resonant circuit, my current expectations are:

If you take the line out from the attenuator output, you'll get a good response shape with bass peak and treble rise if you have a real speaker at the attenuator output too. Ie, it's attenuated but it's not a silent solution. This should work very well even just with M2, with no resonant circuit. But one of the principles of these designs is that the final tone at the ouyput is really determined by the real speaker. Thats how they track with different rigs so well. So a line out at the attenuator output with no speaker is likely to be flat with no treble rise nor bass peak. It could be a good basis for post processing though if that's taken into account.

A line out derived at the amp output will pick up the treble rise on M2, and also the bass peak on M3, whether or not a real speaker is connected (turn to max attenuation if not)

I hope that helps.


----------



## Gene Ballzz

@JohnH & All,
The wide variety of scenarios for desired *"line out"* use is why my preference would lean towards an outboard device like the H&K Red Box and/or a fairly wide choice of similar units, from Radial, Palmer, etc, ad nauseam. The Red Box (in several iterations) has been used with great results since the '80s and there are good reasons why they are still around!
> They can be used either "pre" or "post" attenuator.
> Many provide multiple output levels.
> Most provide multiple cab/speaker emulations.
> They convert speaker level signal to well tested, balanced XLR, mic level output.
> They don't appreciably alter the impedance of the speaker line they are drawn from.
> They don't require added real estate in the attenuator for the added components.









						Red Box 5
					

Technology of Tone




					hughes-and-kettner.com
				






http://hughes-and-kettner.com/wp-content/uploads/2017/01/Red_Box_5_BDA_1_2.pdf



It just seems like a more practical and versatile option to consider!

Simply My Honest Opinion,
Gene


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## sah

Okay, 10mH L2 is on the way (and I already have the huge 200uF capacitor). Will report back when I have my plug-in M3 module going!


----------



## Mjh36

@Gene Ballzz - I'm shopping for the bits for countersinking on this next build. You did that if I remember reading... if so what countersink bits did you use or any do you have advice on that?


----------



## Gene Ballzz

Mjh36 said:


> @Gene Ballzz - I'm shopping for the bits for countersinking on this next build. You did that if I remember reading... if so what countersink bits did you use or any do you have advice on that?



The source depends on whether you are using Metric, 3mm ( https://www.mcmaster.com ) or SAE #4 ( https://drillsandcutters.com ) screws, as the taper angle is different. Given that I'm building multiple units and the bit tips clog easily, I opted for SAE, as the bits are so much cheaper. I do, however, prefer the fit of the 3mm just a bit more! If the mounting hole locations of the resistors were more precisely consistent, I think I would opt for slightly larger screws that thread directly into the resistors, with no nuts needed, but…….? Given that even the higher priced units at DigiKey and Mouser have a +/- 0.3mm tolerance, its hard to depend on. I'm still pondering this one!
HTH,
Gene


----------



## Mjh36

Gene Ballzz said:


> The source depends on whether you are using Metric, 3mm ( https://www.mcmaster.com ) or SAE #4 ( https://drillsandcutters.com ) screws, as the taper angle is different. Given that I'm building multiple units and the bit tips clog easily, I opted for SAE, as the bits are so much cheaper. I do, however, prefer the fit of the 3mm just a bit more! If the mounting hole locations of the resistors were more precisely consistent, I think I would opt for slightly larger screws that thread directly into the resistors, with no nuts needed, but…….? Given that even the higher priced units at DigiKey and Mouser have a +/- 0.3mm tolerance, its hard to depend on. I'm still pondering this one!
> HTH,
> Gene


Thanks Gene, I may go with the ones at McMaster, though I was trying to avoid the extra cost for it's just one build. I do want metric and I'll end needing an M3, M4, M5, and M6 countersink bits. And the M5 is for only two screws to mount L2! But damn I like the look of black countersink screws a lot and I want the right fit for everything. That'll probably be the way I go unless there's some workaround.


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## chas.wahl

Dear @JohnH,

I'm trying to do a layout for my M2V Gig variant. It's got a lot of resistors, and several of them are associated, in Stage 1 and Stage 2, with the two inductors. I had assumed that I'd mount the inductors on the Hammond box lid, and most of the rest of the components on the (5-sided, much more surface area) other part of the box. But in order to maximize real estate, I have considered mounting resistors R13 (stage 1) and R2A, R2B and R2C on the lid as well. Can you tell me if that's a bad idea, due to proximity of those resistors to the inductors, for either heat or electromagnetic reasons?

See the attached graphics, showing the entire schematic, and a view of the lid (1st pass, could change things up depending on how the connections from these resistors to the rest of the circuit work out -- obviously transition of all wires between the lid and bottom of box [5 instead of 4 for just the inductors] should occur on one long side, for maintenance). Note that I've enclosed the components that I'm proposing go on the lid within a dashed line on the schematic.






Thanks!
--
chas.wahl


----------



## Gene Ballzz

@chas.wahl 
While @JohnH can answer more accurately and definitively than me, I don't believe there is any problem associated with the proximity of resistors and the coils. Nice layout and drawing and you have done well in minimizing the number of "floating" wires to the lid. If more real estate is needed in the box, you could also move R12 to the lid, without changing the wire count to the lid. Can I assume that the grouping on the lid is to leave room for a fan, or two, if necessary?
Nice Work!
Gene


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## JohnH

Hi @chas.wahl 
I agree, I don't see any issues with coil and resistor proximity. Putting things on the lid is fine if it works for you, and I see you are taking care of organising the wire layout so that you can work on it with lid off.

(On the lid drawing, I see inductors are written as 0.06 and 0.09 instead of 0.6 and 0.9)


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## MikeD2

Hey John - Finally finished my M2 build (had to add a fan because it was getting a wee bit hot with my Ceriatone 50 W ODS). Sounds great though!. Thank you for sharing your design and answering my questions .


----------



## chas.wahl

JohnH said:


> (On the lid drawing, I see inductors are written as 0.06 and 0.09 instead of 0.6 and 0.9)


Urk! my mistake. I guess it's obvious I don't have a lot of experience using inductors/chokes. I did buy the correct ones, though.

@Gene Ballzz: No plan for fan(s): I'm only building a 25 W attenuator, and none of my amps will be over 15-18 anyway. Lotsa holes is the direction I'm headed.

Thanks,
--
chas.wahl


----------



## ellipsis

Hi all,

This is my first post here; I joined the forum, as I suspect many have, simply to join this thread. At the risk of repeating the obvious, what a tremendous source of knowledge and help, in a mega-thread that appears to have avoided the trolls and rabbit holes that seem to infect so many similar threads and forums on the interwebs.

@JohnH thank you for starting this and continuing to provide "above and beyond" support, evolution and innovation, and @Gene Ballz for your consistent engagement and support as well.

I'm planning to embark on two projects: first, an M2 to use with my amp collection with actual cabs, and also an M3 to use as a silent load for direct recording using IRs.

I fully appreciate that the thrust of this mega-thread is "attenuators", but in the 90 or so pages that I've read so far, there has been some discussion of line outs and possible implementations when going speakerless, but I'm not sure I've seen discussions of the following - if I only wanted to use this design as a silent load with a line out, while keeping the impedance characteristics (and using M3 instead of M2, as there will be no speaker at the end of the chain, so I like to think that I'd like the bass impedance circuit), is there a single pair of series/parallel resistors after the -7db reactive stage that I could use to provide enough load to the amp for this purpose, without needing the options to switch between the various attenuation choices, not needed because there would be no speaker at the end of this chain?

Many thanks in advance for thoughts and guidance. I'm envisaging an 8ohm box, with amps maxing out at 22w-ish power.


----------



## JohnH

hi @ellipsis , welcome to our thread!

All that sounds fine and I'll write more later on the M3. For the M2 one, presumably all your amps have an 8 Ohm out? And what ohms of speakers are your cabs? some 8 and some 16? any 4 Ohms? Or might you wish to try pairs of cabs eg two 8 ohms to equal 4? 

Also, what kind of use is it for? will you be gigging/loud rehearsal with the attenuator? or is it just for home/studio use?

These questions relate to what extra options might be useful for you. Always best to keep it as simple as possible though!


----------



## Gene Ballzz

@JohnH ,
It appears that @ellipsis is speaking of creating a reactive 8 ohm load box, with a line out for a 20 t0 25 watt amp and no speaker at all!
Just My Understanding?
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> It appears that @ellipsis is speaking of creating a reactive 8 ohm load box, with a line out for a 20 t0 25 watt amp and no speaker at all!
> Just My Understanding?
> Gene


I think so, as one of the two projects. So Ill see how best to do that - its actually a simpler problem than making a load box and attenuator in one and getting them both right. There are more precedents too, diy online and as commercial units.


----------



## ellipsis

Wow, it's astounding to see how responsive the main protagonists of this thread continue to be. Many thanks @JohnH and @Gene Ballzz .

On my end, yes, two separate projects, a "regular" attenuator, and then a silent load box with no speaker attached.

For the attenuator, I'm thinking of doing a "standard" M2 @50 watts, 8 ohm. I have mostly low wattage amps (Swart AST Tremolo, Fargen Mini Plex mkII, Morgan Dual 20, a Champ clone I've built, plus an AB764 I'm building, currently on the bench), but my Acme Silvertone 1484 clone gets into mid-30 watt space. All of my amps have 8 ohm outputs, and my cabs are 8 ohm, but I could see using a couple paralleled for 4 ohms. Use is generally at home, including idle noodling, recording in the home studio and the odd rehearsal at home (moderated volumes, drummer uses an electronic kit). I don't play out very often, but when I have in recent years, it's been with a Kemper and now Quad Cortex which have captured my real amps (apologies to those on this thread that might frown upon such behavior). A decent attenuator might change that, but in any event it would be used either at smaller venues or through a PA.

For the silent load, I'm thinking about an 8 ohm M3 at "max attenuation" or reasonable attenuation, with a line out and no speaker attached. My hope is that since the attenuation level is fixed, this could be done with say a pair of resistors after the -7db reactive load instead of the ladder of resistor choices in an M2/M3 wired essentially with all of the switches on. The use case here is playing/recording when everyone else is asleep (with headphones from the DAW interface), and also one for my son for use in his dorm room at school.

Many thanks again for your suggestions.


----------



## Sparks910

@JohnH This thread is amazing and after building an M2 I'm so impressed with what this box can do. My only issue is mine actually MAKES sound! any idea as to why I can hear my 
guitar coming from the attenuator?! I only noticed this after adding the line out. My signal chain is Guitar>Amp> 8ohm out>Attenuator in and set to max attenuation> Line out>interface.
The unit when tested shows an 8.5ohm load so no problems there and I get great signal through to my interface from the DI out with no speaker connected for silent practice (although 
its quite flat as you mentioned even from the amp in)

If I turn the Amp master up I hear distorted guitar tones from the box same as if your running direct with no cab sim. This is with my studio monitors turned off!

Any idea whats going on here?


----------



## JohnH

Sparks910 said:


> @JohnH This thread is amazing and after building an M2 I'm so impressed with what this box can do. My only issue is mine actually MAKES sound! any idea as to why I can hear my
> guitar coming from the attenuator?! I only noticed this after adding the line out. My signal chain is Guitar>Amp> 8ohm out>Attenuator in and set to max attenuation> Line out>interface.
> The unit when tested shows an 8.5ohm load so no problems there and I get great signal through to my interface from the DI out with no speaker connected for silent practice (although
> its quite flat as you mentioned even from the amp in)
> 
> If I turn the Amp master up I hear distorted guitar tones from the box same as if your running direct with no cab sim. This is with my studio monitors turned off!
> 
> Any idea whats going on here?



Thanks for posting! I reckon that sound is probably coming from the coil, which in conjunction with various metal parts nearby, forms a kind of a not very good speaker when driven hard. It might be possible to reduce it by mounting the coil differently, but no need to unless its actually causing a disturbance. Ill see if I can get mine to do this at the weekend. I think Ive heard it do so, but only with extreme cranking way beyond how Id want to run my amp.


----------



## JohnH

Hi @ellipsis , 

To turn the M3 into just a load box with no speaker, it can be as simple as replacing the 2nd, 3rd and 4th stages with just one resistor of 8 or 8.2 Ohm from the end of stage 1 to ground. That is effectively what the stage 1 feeds into when all attenuation stages are engaged.

But given this now has just one job to do, instead of being both a load and working over a range of attenuation, the values can be adjusted to optimise it for this role. This will make a small improvement in the shape of the bass peak, but there's no added complications or cost, so might as well do it. Basically R2A and R2B will go up and the added resistor noted above will go down. I've started looking at this with the analysis and it seems all good and I'll provide some values in a day or two.

For your M2 attenuator box, a basic 8 ohm version sounds like it will be fine. There's one addition you can consider, to tweak an issue that Gene identified recently. If you want to run a 4Ohm cab or a pair of 8's on an 8Ohm M2, it's perfectly safe but you get a bit more presence and resonance. This can be adjusted for with one more output socket and one more resistor. That way, with the rest of M2 as in post 1, you can run from an 8ohm amp into any cab or cabs that are 16, 8 or 4 ohms and get the tone right. I'll draw it.


----------



## Sparks910

JohnH said:


> Thanks for posting! I reckon that sound is probably coming from the coil, which in conjunction with various metal parts nearby, forms a kind of a not very good speaker when driven hard. It might be possible to reduce it by mounting the coil differently, but no need to unless its actually causing a disturbance. Ill see if I can get mine to do this at the weekend. I think Ive heard it do so, but only with extreme cranking way beyond how Id want to run my amp.


Thanks JohnH. I thought maybe the coil but wanted to reassure myself there's nothing else going on. I have the coil mounted to a piece of 5mm thick PVC to isolate from the aluminium base and as I said it works perfectly. Just making sound is not something I was expecting! It's not loud enough to hear over speakers but when totally silent it becomes audible and yes this is when driving my 50w modded origin hard 6+ on the master volume.

Also I only had a 250k volume pot on hand for the DI level control. Ive wired it as per schematic with a 5k 1/2w resistor and it works well but will this cause a significant effect on the tone of the DI out?


----------



## ellipsis

That's great, @JohnH , looking forward to seeing the M2 attenuator and M3 silent load box tweaks as you've noted. 

I'm shopping for enclosures now. Does anyone know whether a painted enclosure would be materially worse for heat-sinking purposes than a natural one? E.g. natural vs. black in the Hammond 1590 series?


----------



## JohnH

ellipsis said:


> That's great, @JohnH , looking forward to seeing the M2 attenuator and M3 silent load box tweaks as you've noted.
> 
> I'm shopping for enclosures now. Does anyone know whether a painted enclosure would be materially worse for heat-sinking purposes than a natural one? E.g. natural vs. black in the Hammond 1590 series?


Black is the best colour for losing heat by radiation, but it probably doesn't make a lot of difference in this case. We have painted, pre-painted and raw on this thread.

But I prefer to buy unfinished aluminium boxes, that way I can do all my drilling and smooth it all down with wet/dry paper and wire wool, then fininish with a black rattle can, to get all the edges of vent holes painted too. No paint inside though.


----------



## SwedishWings

Are there any audio samples comparing this design with commercial attenuators available?

I must admit i did not read all 150+ pages in this thread, apologies!


----------



## JohnH

SwedishWings said:


> Are there any audio samples comparing this design with commercial attenuators available?
> 
> I must admit i did not read all 150+ pages in this thread, apologies!



I don't recall any A-B comparison recordings with other attenuators, although there are plenty of descriptions. 

But I think the best type of audio comparison for an attenuator, is to compare to the unattenuated tone of the same amp and speaker. Volume levels can be adjusted for, so that the tone can be compared. There's a few of these, and I did one early on and its linked near the bottom of post #1


----------



## Gene Ballzz

Sparks910 said:


> @JohnH This thread is amazing and after building an M2 I'm so impressed with what this box can do. My only issue is mine actually MAKES sound! any idea as to why I can hear my
> guitar coming from the attenuator?! I only noticed this after adding the line out. My signal chain is Guitar>Amp> 8ohm out>Attenuator in and set to max attenuation> Line out>interface.
> The unit when tested shows an 8.5ohm load so no problems there and I get great signal through to my interface from the DI out with no speaker connected for silent practice (although
> its quite flat as you mentioned even from the amp in)
> 
> If I turn the Amp master up I hear distorted guitar tones from the box same as if your running direct with no cab sim. This is with my studio monitors turned off!
> 
> Any idea whats going on here?





JohnH said:


> Thanks for posting! I reckon that sound is probably coming from the coil, which in conjunction with various metal parts nearby, forms a kind of a not very good speaker when driven hard. It might be possible to reduce it by mounting the coil differently, but no need to unless its actually causing a disturbance. Ill see if I can get mine to do this at the weekend. I think Ive heard it do so, but only with extreme cranking way beyond how Id want to run my amp.





Sparks910 said:


> Thanks JohnH. I thought maybe the coil but wanted to reassure myself there's nothing else going on. I have the coil mounted to a piece of 5mm thick PVC to isolate from the aluminium base and as I said it works perfectly. Just making sound is not something I was expecting! It's not loud enough to hear over speakers but when totally silent it becomes audible and yes this is when driving my 50w modded origin hard 6+ on the master volume.
> 
> Also I only had a 250k volume pot on hand for the DI level control. Ive wired it as per schematic with a 5k 1/2w resistor and it works well but will this cause a significant effect on the tone of the DI out?



The quoted observations lead me to suspect that this is the source of the anomaly I mentioned earlier of getting a "squeal" if my guitar is in very close proximity to the attenuator, when fully cranked up! I figured it would end up being something fairly simply and easily explained!  
Thanks Folks!
Gene


----------



## Gene Ballzz

ellipsis said:


> Wow, it's astounding to see how responsive the main protagonists of this thread continue to be. Many thanks @JohnH and @Gene Ballzz .
> 
> On my end, yes, two separate projects, a "regular" attenuator, and then a silent load box with no speaker attached.
> 
> For the attenuator, I'm thinking of doing a "standard" M2 @50 watts, 8 ohm. I have mostly low wattage amps (Swart AST Tremolo, Fargen Mini Plex mkII, Morgan Dual 20, a Champ clone I've built, plus an AB764 I'm building, currently on the bench), but my Acme Silvertone 1484 clone gets into mid-30 watt space. All of my amps have 8 ohm outputs, and my cabs are 8 ohm, but I could see using a couple paralleled for 4 ohms. Use is generally at home, including idle noodling, recording in the home studio and the odd rehearsal at home (moderated volumes, drummer uses an electronic kit). I don't play out very often, but when I have in recent years, it's been with a Kemper and now Quad Cortex which have captured my real amps (apologies to those on this thread that might frown upon such behavior). A decent attenuator might change that, but in any event it would be used either at smaller venues or through a PA.
> 
> For the silent load, I'm thinking about an 8 ohm M3 at "max attenuation" or reasonable attenuation, with a line out and no speaker attached. My hope is that since the attenuation level is fixed, this could be done with say a pair of resistors after the -7db reactive load instead of the ladder of resistor choices in an M2/M3 wired essentially with all of the switches on. The use case here is playing/recording when everyone else is asleep (with headphones from the DAW interface), and also one for my son for use in his dorm room at school.
> 
> Many thanks again for your suggestions.



@ellipsis ,

There is a very slick and easy way to wire the dedicated 4Ω output jack so that it becomes useable with one 4Ω cabinet by itself, yet also puts that jack and the "native" 8Ω jack in parallel to facilitate pluging an 8Ω cabinet into the 8Ω jack, along with another 8Ω cabinet into the 4Ω jack for tonally compensated 4Ω operation. This also leaves the 16Ω output jack untouched, for use only by itself! The best of many worlds!

This scenario (along with how the dedicated/compensated 16Ω jack gets wired) requires the use of stereo/TRS jacks for at least those two locations. For the 4Ω out, you simply place a resistor (20 ohm, I believe) betwen the tip and ring of the 4Ω jack, while wiring the rest of the jack (tip & sleeve) in parallel with the "native" 8Ω jack! As mentioned many times in this thread, @JohnH and I both recommend isolated CLIFF UK style, TRS/stereo jacks throughout this build, as even when not using/wiring the extra "RING" lug, they simply grip the plug much more securely! And FWIW, nearly every issue encountered throughout this thread has beenattributable to non-isolated, Switchcraft style jacks being used. I'm not sure that we've tracked down the "exact" reason this is so, but…….

And thanks for referring to us whackos as *"protagonists!"* I enjoy being viewed as a proactive, rebel type!

Happy Buildin' & Attenuatin'
Gene


----------



## ellipsis

Gene Ballzz said:


> For the 4Ω out, you simply place a resistor (20 ohm, I believe) betwen the tip and ring of the 4Ω jack, while wiring the rest of the jack (tip & sleeve) in parallel with the "native" 8Ω jack



Like this?




I'm not sure I understand what that would accomplish, but then the coffee hasn't kicked in this morning. Wouldn't this, in effect, place a 20 ohm resistor in parallel to whatever's plugged in to the 4 ohm output, decreasing impedance of a 4 ohm speaker to 3.3 and and a single 8 ohm speaker plugged in to the 4 ohm jack to 5.7? And with two 8 ohm speakers, the total impedance would be 3.3, no?

BTW re: isolated Cliff vs. non-isolated Switchcraft jacks, I'm totally on board and only use the Cliff types. Easier to control grounding strategies.


----------



## emann

Hi again... with an update - supposedly all components will be in hand this week.

So today I started working on the box and drilled the positions of the components. Next would be to drill the sides (but will wait to take measurements of the items themselves as soon as in hand). 




I will also complete a small dwg for the ventilation holes and post here for your recommendations this week.

As for the thermal paste, is it just a matter of applying some paste on the base of the components and then screw them in place or is there some other precaution to take please.


----------



## Gene Ballzz

ellipsis said:


> Like this?
> 
> View attachment 117717
> 
> 
> I'm not sure I understand what that would accomplish, but then the coffee hasn't kicked in this morning. Wouldn't this, in effect, place a 20 ohm resistor in parallel to whatever's plugged in to the 4 ohm output, decreasing impedance of a 4 ohm speaker to 3.3 and and a single 8 ohm speaker plugged in to the 4 ohm jack to 5.7? And with two 8 ohm speakers, the total impedance would be 3.3, no?
> 
> BTW re: isolated Cliff vs. non-isolated Switchcraft jacks, I'm totally on board and only use the Cliff types. Easier to control grounding strategies.



By itself, and out of context (as drawn) yes, but you need to look back at the earlier parts of the circuit and you will see that the overall resistance picture is a bit more complex than that.

Also, we would not necessarily intend *"a single 8 ohm speaker plugged in to the 4 ohm jack"* but if you found that you preferred the very slight tonal compensation of that configuration, it would be just fine and totally safe! Remembering that once the speaker out signal of the amp has gone through the first *"always on"* reactive stage, the speaker load's impedance has a minimal effect on the impedance that the amplifier sees! Of course this is the reason that a *"full bypass switch"* (if you include one) should *NEVER* be used in any configuration, other than the *"native"* 8Ω output scenario!

Additionally, utilizing either the 4Ω or 16Ω compensated outputs increases attenution by approximately -1db to -2db. It is not recommended to combine the 16Ω output with either the 4Ω and/or 8Ω output.

I Hope This Helps?
Gene


----------



## Gene Ballzz

@emann ,
If you are patient for few minutes, I'll post up a pdf drilling template that you can resize and print onto "peel & stick" project paper, allowing you to "stick" it onto the box for drilling! This is assuming you are using a Hammond, 1590DE box, 200.20mm (7.882") X 120.20mm (4.732")?
Just Helpin'
Gene


----------



## ellipsis

Gene Ballzz said:


> Remembering that once the speaker out signal of the amp has gone through the first *"always on"* reactive stage, the speaker load's impedance has a minimal effect on the impedance that the amplifier sees!



Aha, got it. So this is mainly a tone tweak to make a 4ohm speaker in a 4 ohm output sound more equivalent in an 8ohm attenuator?



Gene Ballzz said:


> Of course this is the reason that a *"full bypass switch"* (if you include one) should *NEVER* be used in any configuration, other than the *"native"* 8Ω output scenario!



Excellent point. I hadn't though about that, but of course makes perfect sense. Well noted.



Gene Ballzz said:


> I Hope This Helps?



Yes, very much so, thanks! 

I've nearly gathered the components for my first build, which will be an M2 (and will build an M3 silent load after I manage to put together the M2). I still need to order a "proper" coil - I've been able to already source a cheap and readily available 1mH Monacor air coil inductor, but at 0.85mm/20AWG that's underspec-ed for this project, right? It is, supposedly, rated for 100W, but I'd still have to unwind it and get it to 0.91mH (I have an LCR meter), so maybe I'll just sit tight and order something with the proper spec.

There's also the issue of hookup wire. I use 20 AWG solid core wire for my amp projects, and have plenty of it, but the consensus here is that 18AWG is the way to go? I'm building the 50W version.


----------



## Gene Ballzz

ellipsis said:


> Aha, got it. So this is mainly a tone tweak to make a 4ohm speaker in a 4 ohm output sound more equivalent in an 8ohm attenuator?
> 
> 
> 
> Excellent point. I hadn't though about that, but of course makes perfect sense. Well noted.
> 
> 
> 
> Yes, very much so, thanks!
> 
> I've nearly gathered the components for my first build, which will be an M2 (and will build an M3 silent load after I manage to put together the M2). I still need to order a "proper" coil - I've been able to already source a cheap and readily available 1mH Monacor air coil inductor, but at 0.85mm/20AWG that's underspec-ed for this project, right? It is, supposedly, rated for 100W, but I'd still have to unwind it and get it to 0.91mH (I have an LCR meter), so maybe I'll just sit tight and order something with the proper spec.
> 
> There's also the issue of hookup wire. I use 20 AWG solid core wire for my amp projects, and have plenty of it, but the consensus here is that 18AWG is the way to go? I'm building the 50W version.



Suggestions for component and materials sourcing is easier if we know your general location on this rock called "Planet Earth!"

You may be "OKAY" with 20AWG, if all intended amps are under 50 watts, but that is kinda pushing the limits on wire size! On the other hand, if all intended victims are under the 20 to 30 watt range, you should be fine! My sincerest question is: *"Why would you want to limit your future options by under sizing anything here?" *For that matter, if the physical size didn't become so cumbersome, 100 watt rated units would seem best, as the component costs don't increase by very much! But that's another whole story!

Let Us Know?
Gene


----------



## Gene Ballzz

@emann & All,
Attached is a pdf version of my working/drilling template for use with a Hammond 1590DE box, at 200.20mm (7.882") X 120.20mm (4.732") or larger. There are a few notes and disclaimers, should anyone choose to use it!

*A>* The actual printed size of the upper, resistor layout drawing must come out to exactly 196.69mm X 116.69mm, to the outside lines, for all the resistor mounting holes to be correct. The bottom lid drawing needs to measure (when printed), 200.20mm X 120.20mm.
*1>* If using the resistors available on fleabay, from "chinesiawanoreanam" the resistors will end up generally straight and square, although manufacturing tolerances may leave them ever so slightly skewed.
*2>* Name brand resistors from the real supply houses will still fit fine, but end up a bit more slanted/skewed, but again, manufacturing tolerances, etc. The larger format, 100 watt resistors from these supply houses will not work with this template. I strongly recommend the fleabay units, at least for the 100 watt resistors
*B>* The black dots, by themselves are intended for 3.5mm countersunk screws, but non-countersunk is OK. Countersink bits can bit pricey and fiddly to deal with, especially without a drill press, or similar
*C>* Pink circles are 6mm holes and blue circles are 8mm. They could all be 6mm, and will only reduce any convection cooling by a minimal amount.
*D>* The large holes in the corners of the bottom lid (with black dots around them) are intended for standard size 50mm X 10mm fans or as simple vents. Your choice as far as how many, if any, fans you use. These holes are cut with a hole saw to appeoximately 45mm.
*E>* The large circle in the center of the bottom lid is NOT a hole, but simply shows the location of the choke/inductor/coil.
*F>* It should be noted that this template is intended to be applied to the "outside" of the box (the resistors are on the inside), so the resistors will show opposite orientation, unless you flip the drawing horizontally.

I strongly recommend repeatedly printing out on plain paper, to check your sizing, etc, before wasting any of the much more expensive "peel & stick" paper!

Simply Attenuatin'
Gene


----------



## ellipsis

Gene Ballzz said:


> Suggestions for component and materials sourcing is easier if we know your general location on this rock called "Planet Earth!"



Yes, of course. These days I generally spend my time between London and Riga, but mostly London. I'll generally be building stuff in the UK (because I spend most of my time here), but using a lot of it in Riga, because that's where my studio and most of my gear is.

I've found this place (possibly via this thread) as a possible source for inductors in the UK:






Loudspeaker Crossover Inductors and Coil Bobbins


Loudspeaker Crossover Inductors and Bobbins. Ferrite and Air Cored Inductors, Choice of Bobbins sizes, Wire Sizes and Custom Inductances




www.qtasystems.co.uk





Any other suggestions most appreciated. For the possible benefit of others, in the UK I've been happy sourcing stuff from RS (uk.rs-online.com) for general electronic stuff and components (including Arcol resistors for an M2 build, already delivered), Mouser for relatively quick delivery of Hammond transformers (which are not so widely or immediately available in the UK), Modulus and Hot Rox for amp building stuff, and Hifi Collective for some components. But I haven't cracked "inductors" yet.



Gene Ballzz said:


> "Why would you want to limit your future options by under sizing anything here?"



In general I agree, and not a problem to buy some thicker wire, so I will. But specifically, I plan to build a few of these boxes (if successful with the first one), because there are a few places where I have playable amps. I expect my first effort to possibly wind up a bit ugly, so I'm planning to chuck the first one I build in the back of a low wattage combo, and it won't need any bells and whistles, nor the ability to extend up to 50 watts, much less 100. 

Thanks for all the suggestions and guidance!


----------



## Gene Ballzz

@emann & All,
And oh yeah, here's a generic, early layout pic. Your particular build may include up to two more 25 watt resistors, in the middle, between the 50 watt resistors!
Happy Attenuatin'
Gene


----------



## Gene Ballzz

ellipsis said:


> Yes, of course. These days I generally spend my time between London and Riga, but mostly London. I'll generally be building stuff in the UK (because I spend most of my time here), but using a lot of it in Riga, because that's where my studio and most of my gear is.
> 
> I've found this place (possibly via this thread) as a possible source for inductors in the UK:
> 
> 
> 
> 
> 
> 
> Loudspeaker Crossover Inductors and Coil Bobbins
> 
> 
> Loudspeaker Crossover Inductors and Bobbins. Ferrite and Air Cored Inductors, Choice of Bobbins sizes, Wire Sizes and Custom Inductances
> 
> 
> 
> 
> www.qtasystems.co.uk
> 
> 
> 
> 
> 
> Any other suggestions most appreciated. For the possible benefit of others, in the UK I've been happy sourcing stuff from RS (uk.rs-online.com) for general electronic stuff and components (including Arcol resistors for an M2 build, already delivered), Mouser for relatively quick delivery of Hammond transformers (which are not so widely or immediately available in the UK), Modulus and Hot Rox for amp building stuff, and Hifi Collective for some components. But I haven't cracked "inductors" yet.
> 
> 
> 
> In general I agree, and not a problem to buy some thicker wire, so I will. But specifically, I plan to build a few of these boxes (if successful with the first one), because there are a few places where I have playable amps. I expect my first effort to possibly wind up a bit ugly, so I'm planning to chuck the first one I build in the back of a low wattage combo, and it won't need any bells and whistles, nor the ability to extend up to 50 watts, much less 100.
> 
> Thanks for all the suggestions and guidance!



I hear ya! My first builds are in post #1 of this thread






Completed JohnH Attenuators?


Hey Fine Folks, The Simple Attenuator thread has become so lengthy that it is difficult to ascertain how many successfully completed builds there are of the fantastic @JohnH attenuator design. :thumbs: I'm starting this thread with the intention of showcasing all of these great builds. It is my...




www.marshallforum.com





And have evolved into what is shown in post #61 of the same thread. There have been additional "evolutions" including fan and hole size, spacing, etc, since that post. I've built more than a dozen of these things, and am just about ready to drill, mount and assemble my first 100 watt unit! Pics to follow!

And if you would like any tips (and pitfalls to avoid) feel free to contact me privately, through the conversation system and real email. We could even set up phone calls, via "whatsapp" if you're so inclined!

Enjoy!
Gene


----------



## JohnH

ellipsis said:


> Like this?
> 
> View attachment 117717
> 
> 
> I'm not sure I understand what that would accomplish, but then the coffee hasn't kicked in this morning. Wouldn't this, in effect, place a 20 ohm resistor in parallel to whatever's plugged in to the 4 ohm output, decreasing impedance of a 4 ohm speaker to 3.3 and and a single 8 ohm speaker plugged in to the 4 ohm jack to 5.7? And with two 8 ohm speakers, the total impedance would be 3.3, no?
> 
> BTW re: isolated Cliff vs. non-isolated Switchcraft jacks, I'm totally on board and only use the Cliff types. Easier to control grounding strategies.


Hi @ellipsis , very close, but swap the tip and ring lugs! Currently it shows the main hot connection going to the ring.

Next post is an update to the main M2 schematic...


----------



## JohnH

Ive been updating the M2 schematic, to catch up with basic tweaks over the last couple of years. No changes to the main ideas or values:




Ive shown the most basic stripped back version first, just so the main signal path can be easily seen. Then I added a line-out, and extra outputs to deal with different speakers. It includes a new output (called Out 4 above), for using speakers of lower Ohms than the attenuator build - Its a slight tonal tweak to address an issue identified by Gene, so that all tones are consistent.

The 8 ohm version is most awesome with 4 output jacks, and you can then get the tone optimized for 4, 8, 16, 2x8 or 2x16 cabs. The attenuator is running natively with 1x8 or 2x16 cabs, and Out 1 and Out 2 are paralleled to use with 2x16 (that's one of my main set ups on mine)

The 16 Ohm version can be stripped back to just two outs. Out 1 is for a 16 cab, and Out 4 (per the diagram) is for 8 ohms, or use both for 2x16

There are several other additions to the base circuit, discussed before, but this is to update for the most common uses.

Ill be updating Post 1 with this, in a day or two subject to any comments.


----------



## emann

Hi again. thanks for that pdf Gene however the box I opted for a slightly larger box to have more spacing as well as maybe some more ventilation since the room at the roof tends to get pretty warm during summer. The circuit I am doind is also a bit different then yours for my fender 65 deluxe reverb reissue as per below.

The grid below is the one I came up with for the ventilation holes - 8mm diameter. Do you think 8mm is an overkill and maybe 6mm will be better? Holes will be vertically on top of each other on box and lid.




curcuit I am building:





What do you think of this arrangement please?

Also re my previous question on the paste, is it just a matter of applying some paste on the base of the components and then screw them in place or is there some other precaution to take please.

Thanks to all.


----------



## SwedishWings

JohnH said:


> Ive been updating the M2 schematic, to catch up with basic tweaks over the last couple of years.


Thanks John!

Looking at the power ratings, it looks like most resistors are over-dimensioned (with 50W amp, basic version, 8Ω).

If i understand correct, worst case dissipation would be ruffly R1=25W, R2A=12W, R2B=10.2W, R5=6.5W, R6=0.9W, R3=5.2W, R4=2.4W, R7=2.8W and R8=2.8W.

Edit: corrected, I mixed up order of stages.

In free air with ambient temp up to 70C, a derating factor of 0.8 is common. What is the reason for the large derating, in particular for R2x through R8?


----------



## JohnH

SwedishWings said:


> Thanks John!
> 
> Looking at the power ratings, it looks like most resistors are over-dimensioned (with 50W amp, basic version, 8Ω).
> 
> If i understand correct, worst case dissipation would be ruffly R1=25W, R2A=5.8W, R2B=6W, R5=4.5W, R6=3.5W, R3=6.5W, R4=1W, R7=2.8W and R8=2.8W.
> 
> In free air with ambient temp up to 70C, a derating factor of 0.8 is common. What is the reason for the large derating, in particular for R2x through R8?



Well I'm not an electrical engineer, but I am an engineer in other fields, so I have a tendency to analyse stuff and to tend to be conservative! I work out the values for power rating by calculating power in every component at every switch setting. I run the numbers using the nominal amp power, then I add 50% for amp overdrive and x2 for safety and temperature control, so basically x3 total on nominal amp power. 

So take R1, on that basis based on 50W in, it dissipates 28W max, so x3 and the next common power rating is 100W. R2A and R2B are trickier to work out because their response is frequency dependent. So 50W for them is on the safe side. Of the others, some can get up to around 20W with that x3 factor included, so 25W is a good common value. Once we are at a 25W rating, it's a fairly compact unit so they might as well all be similar.

I also note that Arcol rates their values using a standard heatsink, which is 995cm.sq for the 100W. We don't get as much surface as that, but if I read it right, unmounted you just get 30W rating. Also, many of us buy the resistors unbranded from Asia, where those labelled as 100W are smaller. They work well, but don't want to push them too hard!




I'm pretty happy with the ratings that I put, but the units still get hot, especially at 50W or more, and fans are needed by the time you get to 100W amps.


----------



## ellipsis

JohnH said:


> Hi @ellipsis , very close, but swap the tip and ring lugs! Currently it shows the main hot connection going to the ring.



Thanks @JohnH , that's just me drawing the jack symbol incorrectly. I always assume tip is on the outside and ring in the middle. Just need to go back and redraw the position of the "little v things"


----------



## ellipsis

Gene Ballzz said:


> if you would like any tips (and pitfalls to avoid) feel free to contact me


Much appreciated @Gene Ballzz !


----------



## ellipsis

JohnH said:


> Ive been updating the M2 schematic, to catch up with basic tweaks over the last couple of years


This is great, many thanks @JohnH !

I see that this version now has the post-reactive stages in descending order of attenuation, -14, -7, -3.5. The previous versions had -7, -14, -3.5, and I seem to recall you saying somewhere on this thread that this previous order was slightly electrically better, but suggested to at least mount the switches on the panel in descending order. Any particular reason for this change, or does it simply make wiring cleaner and explanations easier, with no/minimal tonal difference?


----------



## SwedishWings

JohnH said:


> Well I'm not an electrical engineer, but I am an engineer in other fields, so I have a tendency to analyse stuff and to tend to be conservative! I work out the values for power rating by calculating power in every component at every switch setting. I run the numbers using the nominal amp power, then I add 50% for amp overdrive and x2 for safety and temperature control, so basically x3 total on nominal amp power.
> 
> So take R1, on that basis based on 50W in, it dissipates 28W max, so x3 and the next common power rating is 100W. R2A and R2B are trickier to work out because their response is frequency dependent. So 50W for them is on the safe side. Of the others, some can get up to around 20W with that x3 factor included, so 25W is a good common value. Once we are at a 25W rating, it's a fairly compact unit so they might as well all be similar.
> 
> I also note that Arcol rates their values using a standard heatsink, which is 995cm.sq for the 100W. We don't get as much surface as that, but if I read it right, unmounted you just get 30W rating. Also, many of us buy the resistors unbranded from Asia, where those labelled as 100W are smaller. They work well, but don't want to push them too hard!
> 
> View attachment 117823
> 
> 
> I'm pretty happy with the ratings that I put, but the units still get hot, especially at 50W or more, and fans are needed by the time you get to 100W amps.



Agreed that being conservative is mandatory. But I suspect derating for several resistors might be a waste of money. In the graph below: typical derating for wire wound aluminum housed resistors with limited heat sink gives a hint (ref is 900cm^2). Curve B is for Vishdale RHA series 25W size. Note that derating is 50% without heat sink. For R5, R6, R3, R4, R7 and R8 10W would suffice even in a small box.




I just ordered parts to wet theory in practice. To be sure, I ordered the cheapest Chinese stuff i could find  R5, R6, R3, R4, R7 and R8 are all 10W cement type that will us direct convection cooling only.


----------



## JohnH

Hi @SwedishWings , thanks for your assessments. It sounds like you are confident in this area and clear on what you are doing.

Ill just make a couple more points:
6 or 7 watts max in the smaller resistors, with 50W coming in, gets to around 10W if a 50W amp is driven hard, as some may wish to do Derating on a 25W resistor, assuming (conservatively) that heatsink is much less than the standard (so zero), could be 12.5W, which is a good spec in comparison to 10W and the heatsinking that is available helps. A 10W air-cooled ceramic inside a warm box is getting very hot!.
These resistors from china are just a dollar or two, so nothing really to save unless smallest size is important
When all the resistors are case mounted, theres a good amount of mass and everything heats up slowly, which averages out peaks very nicely over about 20-30 minutes. Air mounted resistors will heat quicker.

Just my thoughts and I wish you all the best for your build!


----------



## JohnH

*Load boxes and using M3 as a loadbox

Hi @ellipsis *

For a pure loadbox, no speaker, I dug into it again to explore what the best design could be.

The M3 attenuator works well as a loadbox, and as an attenuator. But if its just a loadbox, it can be simplified, and also optimized to make it a bit better for that one purpose.

So I came up with this just following my nose to get the best curve that I could. It ended up a bit different:




The curves are dB vs frequency plots. The blue one is from a more complex model that matches real 4x12 measurements very closely - and I use it for these calcs. The red is the circuit above, also allowing for losses in the coils and cap. Its a very close match. I let the width of the bass peak be a bit wider on the low side to cover a bit more range.


*TGP Loadbox Thread - Freeman and Aiken*

This kind of circuit has been posted before, and there is a classic thread on TGP by James Freeman, building on designs by Randall Aiken:






Aiken's Reactive Dummy Load.


I have been planing to build a good dummy load box to my tube amp with a line-out, then re-amp with a quality Solid-State Power Amp into my Guitar Cab. After some research Aikens design seems to load the Tube amp like a speaker would...



www.thegearpage.net





He comes up with something very similar and has used it for years.


*Use of an M3 Attenuator as a load box*

Now turning to the M3, when it's used as a load box, its also good, almost as close:





The same reference curve is now the orange one and green is M3 switched to full attenuation with no speaker. To optimise it for this use, I changed L1 slightly from 0.9mH to 1mH (will make negligible difference to the attenuator use) Its also a really close match except at the bass peak where its about a further dB down than the design shown above. Thats because as an attenuator, energy needs to be drawn off from the front end to feed the speaker, adding a bit more damping and supressing the peak. But this effect is localized just to when playing frequencies at the peak (around low A), over a one or two semitones range, and I dont think its a big issue.

I hope that's interesting or useful!


----------



## ellipsis

JohnH said:


> So I came up with this just following my nose to get the best curve that I could.



That's great, @JohnH , thanks so much for continuing to share your impressive work.

On most or all of this thread, the "benchmark" impedance curve is that of a closed-back 4x12 cabinet, right? And you optimize your circuits to match that curve as closely as possible. Do you have any idea what the impedance curve for something on the other end of the spectrum, say an open-backed 1x10 cab, might be, and how robust the attenuator designs here might be for non-4x12 scenarios?



JohnH said:


> there is a classic thread on TGP by James Freeman, building on designs by Randall Aiken:



Oh great, another deep rabbit hole to go down! ;-)



JohnH said:


> I hope that's interesting or useful!



Very much so!


----------



## JohnH

Hi @ellipsis

With regard to different cabs, and also different amps, I was surprised early on to find that having designed the attenuator circuit based on measurements from just my main amp (a VM combo), and tweaking a little based on my DSL401, it worked very well for many others as well. Quite honestly I haven't heard of a single build out of somewhere between 50 and 100 that I've read, that hasn't been reported as sounding good. So most of those will be with 12" speakers, some open and some closed back, but smaller combos like the Studio series have 10" open back, and there are several generically different output circuits out there with different tubes, different NFB and biasing arrangements.

So there's something lucky about this attenuator circuit to make the planets align. I can see it coming through in the maths but its still hard to rationalise. What I think is as follows:

There's two aspects to the performance that are key.

One is how the amp reacts to the attenuator, which affects dynamics and how the tone develops as you dig in hard, and as a note decays. This is all about the reactive Stage 1. The most important part of this is the treble rise determined by L1. This controls the feel and tone at the amp end, and if its right, then it covers all frequencies from low mids upwards Any 12" speaker seems to be well covered by the L1 value listed, whether its Celestion or others, and its not affected by open/closed back. I expect that for a 10", it might be a bit lower, but that turns out not to be very important for the attenuator use (but maybe in a load box scenario).

The bass rise is not represented at the amp in the M2 design, and that turns out to be totally fine for attenuator use because the real speaker does it itself. But it is important when used as a loadbox, unless some EQ adjustment is made downstream. An open backed cab will have a lower resonant frequency, close to that of a speaker in free air (which is what speaker data tells us). To adapt for that, L2 and C1 would be larger values. The difference might be around 80hZ instead of around 110hZ with closed back

The second key point is how the speaker reacts to the attenuator. One of the design parameters is the output impedance of the attenuator which needs to be kept consistent and fairly high, like a real amp. Then the speaker reacts as itself, so the sound you get is the resonance and treble rise of the real speaker, which will adapt to whether it is open or closed back etc a different speaker will sound different

When you investigate the signal when there are several stages of attenuation involved, you see the amp reacting to the reactive front end, but mid-way through the circuit, the response is almost flat in terms of tone, though it contains the dynamics from the amp. Then at the speaker, the speaker does its part to provide its own bass resonance and treble lift. This is why its not critical to change the attenuator design for different speakers 

There's one more trick: All of the design assumptions are based around starting with the effective output impedance of the amp, which varies with different amps and with frequency, and is dependent on NFB and resonance and presence controls, if they exist. I based the design on around 20 Ohms for an 8 ohm output, which I measured, but its just an assumption. But it turns out that with different amp output impedances, the reactive front end shapes the tone appropriately, so that you still get a really close match at the speaker. 

To summarize for your question - M2 and M3 are tonally very robust in terms of being used as attenuators and no changes are really needed for different speakers. As a load box with no speaker, M3 is better than M2 (or the dedicated load box as above), and the tone shape will be based on the design assumption for the front end, ie a closed back 12", unless its adapted.


----------



## Marcomel79

Hi @JohnH, i just wanted to ask you a question and i apologize if this has been asked before, but with more than 150 pages it would be impossible to find. I have built your M2 50W attenuator which works beautifully, again thank you so much for your work. Can i use it as a dummy load with all the attenuating stages engaged?

Marco


----------



## JohnH

Marcomel79 said:


> Hi @JohnH, i just wanted to ask you a question and i apologize if this has been asked before, but with more than 150 pages it would be impossible to find. I have built your M2 50W attenuator which works beautifully, again thank you so much for your work. Can i use it as a dummy load with all the attenuating stages engaged?
> 
> Marco


Yes you can and it will give the amp a safe load to run width. It will pick up the main treble rise, but since theres no bass circuit, it wont have the bass peak which only affects a few low guitar notes. Still useful though. If you are feeding into a mixer and cabsim or IR loader , you might want to add a touch of extra bass EQ. I get a good result via a mixer with some added bass then cut below 100 Hz


----------



## SwedishWings

JohnH said:


> Hi @ellipsis
> 
> There's one more trick: All of the design assumptions are based around starting with the effective output impedance of the amp, which varies with different amps and with frequency, and is dependent on NFB and resonance and presence controls, if they exist. I based the design on around 20 Ohms for an 8 ohm output, which I measured, but its just an assumption. But it turns out that with different amp output impedances, the reactive front end shapes the tone appropriately, so that you still get a really close match at the speaker.



Thanks, again, for your efforts. I'm still waiting for components and am truly excited to hear the results!

Meanwhile, an observation on output impedance from tube amps. I have measured this before on HiFi and guitar amps, and found that the output impedance is lower than what you found in your amps. I took the time to simulate the power amp of my home brew 50W (2xEL34, 8Ω output) guitar amp which i have valid data on (transformer inductance and DCR, etc).

The output impedance varies as expected depending on NFB. I also tested several other parameters like plate voltage, transformer inductance etc, but output impedance variance was small in comparison.

The presence control affect the feedback which affects the output impedance. The first graph is presence at zero and the second presence at max. The circuit for presence is from the Marshal plexi.







Not sure if this is of any interest, but figured someone might be interested.


----------



## jmerwin62

I have a totally nieve question. 

I've used an L-Pad attenuator before. It worked fine on my low power tube amp (Epiphone Valve Jr- I sold it a few years ago). It was limited to a specific impedence (8 or 16 Ohm I don't remember), but that's OK, just limits the amps that it can be used on, and there are other L-Pads for other impedences, so I could make an 8ohm and a 16ohm attenuator. The 50 Watt version of the L-Pad is $20 on amazon (see link below). There's a few DIY versions of this idea out there. Some add bright caps an other options if you're willing to add complexity and components.

It seems like the complexity of the M2 design is brought on by trying to vary the attenuation and hold(or select) the impedence. The design of the L-Pad seems to handle setting the impedence. 

The other part of the M2 secret sauce is the inductor.

So, couldn't I just place an inductor across the input on the L-Pad and see improved results over the L-Pad only design with significantly simpler design? I'll admit inductors and chokes still sort of baffle me. 

Or maybe a slightly more complex design that has R1, R2a, R2b, L1 and the L-pad?





https://www.amazon.com/Parts-Express-Speaker-L-Pad-Attenuator/dp/B0002KR1EQ/ref=sr_1_1?crid=8J0S0HH5YI4G&keywords=l-pad+attenuator&qid=1666894969&qu=eyJxc2MiOiIzLjQ0IiwicXNhIjoiMi4zOSIsInFzcCI6IjIuMjcifQ%3D%3D&sprefix=l-pad%2Caps%2C78&sr=8-1


----------



## Gene Ballzz

jmerwin62 said:


> I have a totally nieve question.
> 
> I've used an L-Pad attenuator before. It worked fine on my low power tube amp (Epiphone Valve Jr- I sold it a few years ago). It was limited to a specific impedence (8 or 16 Ohm I don't remember), but that's OK, just limits the amps that it can be used on, and there are other L-Pads for other impedences, so I could make an 8ohm and a 16ohm attenuator. The 50 Watt version of the L-Pad is $20 on amazon (see link below). There's a few DIY versions of this idea out there. Some add bright caps an other options if you're willing to add complexity and components.
> 
> It seems like the complexity of the M2 design is brought on by trying to vary the attenuation and hold(or select) the impedence. The design of the L-Pad seems to handle setting the impedence.
> 
> The other part of the M2 secret sauce is the inductor.
> 
> So, couldn't I just place an inductor across the input on the L-Pad and see improved results over the L-Pad only design with significantly simpler design? I'll admit inductors and chokes still sort of baffle me.
> 
> Or maybe a slightly more complex design that has R1, R2a, R2b, L1 and the L-pad?
> 
> 
> 
> 
> 
> https://www.amazon.com/Parts-Express-Speaker-L-Pad-Attenuator/dp/B0002KR1EQ/ref=sr_1_1?crid=8J0S0HH5YI4G&keywords=l-pad+attenuator&qid=1666894969&qu=eyJxc2MiOiIzLjQ0IiwicXNhIjoiMi4zOSIsInFzcCI6IjIuMjcifQ%3D%3D&sprefix=l-pad%2Caps%2C78&sr=8-1



*FIRST>*  to the forum!

*THEN>* One of the more important aspects of the @JohnH design is a specific and critical balance/ratio of series and parallel resistance, that simply can't be easily achieved with an L-Pad type design. While it could likely be accomplished, to do so would be far more complicated (and probably costly) than the M2 design! Throughout this thread there have been multiple endeavors to add a variable component to cover the "in between" gaps of the -3.5db steps. These attempts have yield varying results and been generally deemed as not worth the effort. The base version of the M2 works amazingly well with minimal loss of tone, dynamic and tactile feel and response. This design is by far the best passive design on the net, until possibly getting to the much higher priced units, like the full sized Tone King IRONMAN II, at +/- $800.

I will leave the technical discussion to @JohnH , who will likely chime in at some time soon.

Happy Attenuatin'


----------



## JohnH

Hi @jmerwin62 , thankyou for your post.

Most of the lower cost attenuators on the market use L-pads or similar, and as you note, they are usually very simple and there are several DIY versions out there. They may work acceptably depending on the uses and expectactations, but in general they can't fully preserve the tone and dynamics of the amp as they turn down. Typically they become to some extent, dull and flat sounding. It's not possible to properly control all the required parameters in these units. If it was, we would not have this thread.

Our designs aim higher, and the target is to fully match the amp tone and feel across a range of optimised steps, in a design that is still a simple DIY build. The switched steps allow optimisation at each stage, which is not possible in a single swept control.


----------



## JohnH

SwedishWings said:


> Thanks, again, for your efforts. I'm still waiting for components and am truly excited to hear the results!
> 
> Meanwhile, an observation on output impedance from tube amps. I have measured this before on HiFi and guitar amps, and found that the output impedance is lower than what you found in your amps. I took the time to simulate the power amp of my home brew 50W (2xEL34, 8Ω output) guitar amp which i have valid data on (transformer inductance and DCR, etc).
> 
> The output impedance varies as expected depending on NFB. I also tested several other parameters like plate voltage, transformer inductance etc, but output impedance variance was small in comparison.
> 
> The presence control affect the feedback which affects the output impedance. The first graph is presence at zero and the second presence at max. The circuit for presence is from the Marshal plexi.
> 
> View attachment 117943
> 
> 
> View attachment 117944
> 
> 
> Not sure if this is of any interest, but figured someone might be interested.



These are interesting results. I've seen numbers like these quoted from measurements on some Plexi or EL34 amps, but not modelled across such a sweep of frequencies and control positions. Is your model able to adapt to represent other amps? It would be very interesting to see results with say, the NFB taken out, or maybe a JTM45 circuit with KT66 tubes (it's the basis of my VM), or one with no NFB such as an AC30.


----------



## pietro.castelli

Question for @JhonH and whoever built the attenuator so far:
Should/could the sleeves of the input/output jacks be grounded to the metal enclosure?

Thanks!


----------



## JohnH

pietro.castelli said:


> Question for @JhonH and whoever built the attenuator so far:
> Should/could the sleeves of the input/output jacks be grounded to the metal enclosure?
> 
> Thanks!



Hi pietro, I reckon the case should not be grounded, and with the plastic Cliff jacks that we recommend, then the case is not in circuit at all.


----------



## Gene Ballzz

JohnH said:


> Hi pietro, I reckon the case should not be grounded, and with the plastic Cliff jacks that we recommend, then the case is not in circuit at all.




@JohnH ,

I have a *"proposed"* guess as to why we've encountered issues when the jacks get *"grounded/connected together" *through the metal enclosure. The speaker signal passing through this unit is AC and what we are considering to be *"negative"* (at the sleeves of the jacks) actually becomes *"positive"* for a portion of the full form of the wave cycle. Please correct my thought process, if I am full of Doo-Doo?

On another, yet possibly related note: I'm not sure *"phasing"* of the signal from stage to stage or input to output has ever been discussed?

Oh, and wait till you see Pietro's so far *STELLAR* 100 watt build! He's got some skills! He's been the *"guinea pig" *for my 100 watt drilling template and layout. It seems to work quite well! Now that he has "proven" it, I'd be happy to share with anyone interested! It is, however, centered around the use of the resistors from "chinesiawanoreanam!"

Thanks, As Always!
Gene


----------



## JohnH

*Post 1 updated*

I've just given the first post on page 1 an update do-over, with the latest M2 schematic and discussion. I've also kept the original M design there, and reposted my sound samples, now from Soundcloud. 

Nothing really new, but hopefully a bit clearer for new readers.

cheers
John


----------



## Gene Ballzz

JohnH said:


> *Post 1 updated*
> 
> I've just given the first post on page 1 an update do-over, with the latest M2 schematic and discussion. I've also kept the original M design there, and reposted my sound samples, now from Soundcloud.
> 
> Nothing really new, but hopefully a bit clearer for new readers.
> 
> cheers
> John



NOICE!


----------



## paul-e-mann

@JohnH has anybody figured out how to add an effects loop to your attenuator?


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## JohnH

paul-e-mann said:


> @JohnH has anybody figured out how to add an effects loop to your attenuator?



Thanks for your message. Effects loops for attenuators only work in designs with active re-amplification, but not with a purely passive attenuator such as our designs, which deal with speaker signals. If you take a line-out, that could go to an FX loop and then you need another power amp if you want to feed that to a speaker. Units like Boss TAE or Ox work that way..


----------



## JohnH

*M4 Attenuator - all the bells and whistles*

In Post 1, first page of this thread, you can see the latest basic design for an M2 attenuator, then with a few additions such as line-out and correction for different speaker Ohms.

But there's several other ideas that have been developed and tried over the last couple of years, so this diagram puts them together, and I'm calling it M4. Mostly, its M2 with a series of extras that can be added, or not, as wanted. I'm hoping that by showing the basic design and then the first couple of add ons in Post 1, it will help to follow what is added.




*Stage 1 - Bass resonance circuit*

This has been referred to as M3. An iron-cored coil L2 and capacitor C1 are added in series with the air-core inductor L1. This shows the amp the bass peak and makes it track the tone slightly more accurately. From a few builders, the concensus to date is that if just for attenuating a guitar speaker, the circuit works perfectly ok without this bass circuit, since the real speaker replicates the peak, interacting with the attenuator. But it may improve the response when using a line-out without a speaker. If it's included, the parts are fairly bulky and the best capacitor may be two or more in parallel, using large non-polarized film caps for very low dissipation.

Whether of not the bass circuit is used, no other parts are affected and it could be added later if wanted.


*Stage 1 -3.5/-7db switch*

The M2 provides attenuation from -7dB down to -31.5dB, in small equal steps of -3.5 dB. This will cover most needs, but for some, particularly those gigging or using small amps, a -3.5dB setting may be useful. So this switch changes resistances in Stage 1 to provide that option, with full reactive tone. The intention (for best tone) is that this is only used when that small -3,5dB setting is needed, and to go to the next step down, it is returned to -7dB as usual. However, it is safe to use it in any switch arrangement, such as -3.5 in stage 1 and other stages engaged too.


*Stage 2 - Three resistors*

Stage 2, which is -14dB, usually has just 2 resistors, but to go with the amended Stage 1 with the -3.5/-7dB switch above, it needs one more resistor to fix the correct impedance, since it can be electrically a bit closer to the amp when Stage 1 is set at -3.5.


*Line out *

For this diagram, I've shown a 1k pot with resistors, so that the output is nominally max 1/10th of the amp voltage. This should allow a good level to be found with most amps, depending how they are driven. It is intended to go into an unbalanced line-in on a mixer or similar. A cab-sim or IR box will be needed to get a reasonable speaker-like tone in a recording or PA feed. But you may not need a line out, some devices such as Redbox by H&K or Mooer Radar go between amp and load, with a direct pass through. You can also take a signal from a spare output on the attenuator, if a speaker is plugged in.

If using the line out with no speaker, set all attenuation stages to max. If you don't have the bass circuit discussed above, you might add a bit of low EQ down-stream, eg slight bass boost with cut below 100 hZ which is setting that I've found works quite well.


*Multiple outputs*

The basic circuit is optimised for speaker ohms = attenuator build = amp output. But so long as the attenuator version matches the amp, you can plug different speakers into it safely since the Ohm miss-match amp to speaker, is adequately corrected by the attenuator. But the base tone may be a bit bright or dark, so the added outputs correct for this. Otherwise, a 16 Ohm speaker into an 8 Ohm attenuator will be a bit dark, and a 4 Ohm speaker will be brighter.

Out 1 and Out 2 are in parallel, useful to plug two 16 ohm cabs into an 8 Ohm attenuator for an 8 Ohm load. Out 3 corrects for a single 16 Ohm cab in an 8 attenuator, and Out 4 is for speaker Ohms < attenuator. Its to do with the impedance and damping seen by the speaker. Just build what you need. eg, if you build for 16 Ohm, you just need Out 1 and Out 4 to deal with a 16 speaker, or an 8, or two 16's.


*Full bypass switch*

Many people may feel that this is needed but don't actually use it. ie, if you don't want to attenuate, then don't connect the attenuator. With a bypass switch, you need to be very careful to always match speaker to amp, and never operate it unless the amp is 'off' or on 'standby' . But it could be useful at a gig, where it becomes necessary to engage or disengage the attenuator later to control volume, without fully rewiring. Ive shown it here so that it should be safe with any of the outputs, but to get the attenuator fully out of circuit, use just out 1 or out 2.


*Footswitch*

Depending how you like to play, a small boost or cut for rhythm may be useful. This circuit puts another -3.5dB stage in a remote footswitch box. It is wired with a 3 core cable, using a TRS jack with switch contacts. The switch contacts for tip and ring bypass it when not plugged in and they are crossed over in parallel.

Also included is an LED, to light up in the louder setting. The switch can be a 3PDT, such as used on stompboxes. Two contacts in parallel give redundancy for the rmain signal. The cable could be made from 3 core 5 Amp mains flex, I suggest it is wired fully into the foot switch box so its not confused with any normal cable.


Overall, few will need all these features. There is a huge advantage in keeping builds as simple as possible, to build, and also in use. But I hope that some of this will be useful and interesting too!.


----------



## ellipsis

JohnH said:


> this diagram puts them together, and I'm calling it M4


@JohnH , this is fantastic. Thanks once again for putting in so much work on this and sharing.


----------



## ellipsis

After having the previous few weekends pre-empted by other obligations, I've finally put together my first M2 over the past 24 hours. I've done an 8 ohm M2 with bypass switch and line out, and it's going to live in my office/workshop, attached to both a 1 x 12 cab under the desk and an audio interface.



I'm quite happy with myself that I've apparently wired and soldered everything properly, because upon first try all of the switches and knobs and jacks do what they're supposed to, and the amp I used to test it didn't go up in smoke.

First impressions: I like it, a lot. I've always had a use for good attenuation, and I've owned various options over the years, and sold them because they weren't quite it - Dr. Z Brake Lite, Jet City Jettenuator, Swart Night Light Jr., Tone King Ironman II. This M2 really does what it's designed to do - delivers multiple usable playable volumes without sacrificing tone at the lower and lowest volume settings.

The line out works just fine as well. I used a 5k (well, 4.7k, since that's what I had available) resistor and a 5k pot, from a @JohnH schematic from a couple of weeks ago. The output is pretty hot, I don't get much past 9 o'clock on the pot before the signal is too much for the interface. With the cab disconnected and max attenuation (using it as a silent load), it sounded pretty good just slapping a generic cab sim on the signal in a DAW. I suspect it would sound much better if i put some thought into it (use a good plugin and tweak etc.). I must say it feels like there's "something" missing in the bass, but I don't know if I'm just telling myself that because I know that I want to build one of these boxes with the bass resonance circuit. A quick EQ-ing helped things a bit, and it'd probably get better if I spent more that 30 seconds on it, but I'll probably build an M3/M4 to satisfy my curiosity.

I still have a UAudio Ox, Two Notes Captor X and Fryette Power Station PS-2A strewn across the couple of places I live and have gear. The Ox and Captor X will be going up for sale, because the M2 has made them obsolete/superfluous. I am keeping the Fryette, though, because it is an excellent tool, and an entirely different level of functionality than the boxes on this thread - integrated tube power amp, fx loop etc.

Here's my new unit in its new home, in the upper right hand corner:



Once again, many thanks to @JohnH and @Gene Ballzz for all of their efforts here.


----------



## JohnH

Hi @ellipsis , congrats on getting that together. It looks like a great build and thanks for all that feedback! May I ask what amp you have tried with so far?


On line-out, You can cool it down with a resistor across the pot outer legs, and I might change the M2 diagram to show thst explicitly.



We are deeply honoured by your 'for-sale' list!


----------



## ellipsis

JohnH said:


> May I ask what amp you have tried with so far?



An Acme Silvertone 1484 - 2 x 5881 push-pull, which is the most powerful amp I have with me here in the UK. It also happens to be one of my favorite amps in general (https://www.acmeverb.com/home) and much better sounding and more reliable than the actual Silvertone 1484 I used to own and subsequently sold. 

As I may have mentioned earlier in this thread, my amp collection is generally low wattage (2 x 6V6, 2 x EL84, SE 6V6), so I wanted to first try the M2 with the amp with the most juice and see what happens. All good - after nearly an hour of playing the amp nearly dimed, I can't say the M2 was even particularly warm. I didn't want to drill holes in the top unless I needed to (I don't like the aesthetics), and apparently I don't need to. I do have a few holes in the side and in the base.


----------



## Gene Ballzz

ellipsis said:


> After having the previous few weekends pre-empted by other obligations, I've finally put together my first M2 over the past 24 hours. I've done an 8 ohm M2 with bypass switch and line out, and it's going to live in my office/workshop, attached to both a 1 x 12 cab under the desk and an audio interface.
> 
> View attachment 119528
> 
> I'm quite happy with myself that I've apparently wired and soldered everything properly, because upon first try all of the switches and knobs and jacks do what they're supposed to, and the amp I used to test it didn't go up in smoke.
> 
> First impressions: I like it, a lot. I've always had a use for good attenuation, and I've owned various options over the years, and sold them because they weren't quite it - Dr. Z Brake Lite, Jet City Jettenuator, Swart Night Light Jr., Tone King Ironman II. This M2 really does what it's designed to do - delivers multiple usable playable volumes without sacrificing tone at the lower and lowest volume settings.
> 
> The line out works just fine as well. I used a 5k (well, 4.7k, since that's what I had available) resistor and a 5k pot, from a @JohnH schematic from a couple of weeks ago. The output is pretty hot, I don't get much past 9 o'clock on the pot before the signal is too much for the interface. With the cab disconnected and max attenuation (using it as a silent load), it sounded pretty good just slapping a generic cab sim on the signal in a DAW. I suspect it would sound much better if i put some thought into it (use a good plugin and tweak etc.). I must say it feels like there's "something" missing in the bass, but I don't know if I'm just telling myself that because I know that I want to build one of these boxes with the bass resonance circuit. A quick EQ-ing helped things a bit, and it'd probably get better if I spent more that 30 seconds on it, but I'll probably build an M3/M4 to satisfy my curiosity.
> 
> I still have a UAudio Ox, Two Notes Captor X and Fryette Power Station PS-2A strewn across the couple of places I live and have gear. The Ox and Captor X will be going up for sale, because the M2 has made them obsolete/superfluous. I am keeping the Fryette, though, because it is an excellent tool, and an entirely different level of functionality than the boxes on this thread - integrated tube power amp, fx loop etc.
> 
> Here's my new unit in its new home, in the upper right hand corner:
> View attachment 119529
> 
> 
> Once again, many thanks to @JohnH and @Gene Ballzz for all of their efforts here.





JohnH said:


> Hi @ellipsis , congrats on getting that together. It looks like a great build and thanks for all that feedback! May I ask what amp you have tried with so far?
> 
> 
> On line-out, You can cool it down with a resistor across the pot outer legs, and I might change the M2 diagram to show thst explicitly.
> 
> 
> 
> We are deeply honoured by your 'for-sale' list!



Very nice job sir! And yes indeed, that *"for sale list"* speaks *VOLUMES* (pun definitely intended ) about John's design!

Just Noticin'
Gene


----------



## ellipsis

As I now consider building another attenuator, this time with the bass resonance circuit, can anyone enlighten me on why it would be preferable to use a huge and expensive film/polypropylene capacitor (or a pair of 100uF ones to get to a 200uF capacitance) instead of a seemingly more sensible bipolar electrolytic? Thanks in advance for any insight.


----------



## JohnH

You might be ok with the bipolar electros, or you might not. The resonance happens at about 110hZ, and above that frequency (ie almost the full spectrum), most of the amp power has to pass through the capacitor. Hence a low loss and high ripple current is needed or else the capacitor may heat up.

Electrolytics have a much higher dissipation factor. So apart from the heating and current issues, the Q of the bass resonance will be better.

But I'd suggest maybe a test. You could add the bass circuit as an external test to your current build. Just add it in series with L1. And since you can monitor, you could do this with the iron-core coil and a set of electrolytics in parallel. Try it with and without and you'll see if it's making a difference and how it heats up, If it does.

I thought about the big film caps from a thread on TGP on load boxes:






Aiken's Reactive Dummy Load.


I have been planing to build a good dummy load box to my tube amp with a line-out, then re-amp with a quality Solid-State Power Amp into my Guitar Cab. After some research Aikens design seems to load the Tube amp like a speaker would...



www.thegearpage.net


----------



## ellipsis

JohnH said:


> You could add the bass circuit as an external test to your current build


Many thanks @JohnH . I'm about a week away from taking delivery of the large inductor, and need to order the capacitors, but when these items arrive I'll give the bass circuit test a shot and report back.


----------



## luxspring

JohnH said:


> *Background*
> 
> Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve output stage develop, but without excessive volume.
> 
> The attenuator must present a load to the amp that is similar to a speaker and also maintain the tone as volume is reduced. It needs a consistent tonal and dynamic response from low attenuation, down to sub-bedroom level. This is where the simplest designs can be inferior, and the best commercial designs get expensive.
> 
> With feedback and testing by others I think we have a design that achieves this. For about $100-$120, anyone with workshop skills and the ability to follow a circuit schematic can build this. I want to thank everyone who has built one of these or contributed to this thread, with a special thankyou to @Gene Ballzz who was the first to see the potential and has been a great source of insight and practical help for everyone.
> 
> *An important point:*
> 
> Anyone who builds it does so at their own risk, and takes responsibility for working out their own wiring for their own private, non-commercial use, and completing it safely
> 
> 
> *Summary: November 2022*
> 
> This thread focusses on the design of reactive attenuators for DIY building using simple construction and inexpensive parts. It started with just a few resistors and then developed into multistage resistive and reactive designs. The latest designs work much better than I’d first expected and have been built and tested successfully by many others. The design continues to evolve but the main principles have been constant for several years. This 1st post shows the current basic design M2, and a few possible additions to it.
> 
> View attachment 118594
> 
> 
> It must be matched to the output tap of the amp, eg 8 Ohm or 16 Ohms. Component values for both are given, which differ by a factor of two:.
> 
> There are 4 attenuation stages, engaged or bypassed by switches.
> 
> Stage 1 is the key reactive stage and includes an inductor coil. This stage on its own, reduces power by a factor of 5, or -7db, reducing a 50W amp to 10W. The inductor coil is configured so that the impedance presented to the amp is similar to that of a real speaker (values based on various Celestions), particularly how impedance rises with frequency.
> 
> After Stage 1, three more stages are provided. These can be mixed and matched, but the design shown is based around additional -3.5db, -7db and -14db stages. By combining these switches in combination, and with Stage 1, reductions of up to -31.5db can be achieved in small, equal steps of -3.5db, at which point a 50W amp, at full power, is playing quietly at about 35mW.
> 
> On pages* 111* and* 112* are two construction layouts which may assist (designs are very close to above, not identical though). I recommend studying the diagrams above to understand the connections and then adapt the layouts to suit your needs. Further features can be added, discussed in the thread, such as bass resonance circuit, foot-switchable stage, variable input impedances, bypass switch etc.
> 
> If you'd like to see a schematic with most of these further add-ons, go to Page 158, post 3146 from 19/11/2022.
> 
> 
> *Component values and power ratings*
> 
> The table above shows the recomended power ratings for each resistor, based on up to 50W amps. The component ratings need to have a good margin above the actual power. I use a factor of at least 3 for case-mounted aluminium resistors, bolted (using thermal grease) to a heavy metal chassis or heatsink , and a factor of 5 or more for air-cooled resistors. These values fit with the spec in the schematic diagram above and also allow for overdrive of the amp.
> 
> Wire for hookup and also the winding of air-cored inductors should be 18 gage for 50W attenuators, and this is also OK for a 100W one, if built to the 16 Ohm values. For switches, use at least 5A rating (at 125V ac) for a 50W 8 Ohm build. The best jacks are plastic Cliff jacks, TRS (ie stereo type) which grip the plugs better than mono jacks.
> 
> 
> *Cooling*
> 
> With amps > 30W at high power, the unit will heat up as it dissipates power. A good size die-cast aluminium case is best. Once components are positioned, then a number of additional large vent holes should be drilled, in the top and in the base, with feet to raise up the base. This will help to promote good convective flow of air out through the top, replaced by cool air at the base. The best colours for cooling are dark. For amps more than 50W, a fan should be added
> 
> Mount the coil without using a ferrous bolt, with a few mm timber or plastic spacer off the case surface.
> 
> 
> 
> Thanks for reading. If you are interested, and since this is a long thread, I suggest to read this post, look at the layouts and the most recent few pages, then make a post yourself and we'll be happy to discuss what would work best for you.
> 
> For nice collection of completed builds, see this thread:
> 
> 
> 
> 
> 
> 
> Completed JohnH Attenuators?
> 
> 
> Hey Fine Folks, The Simple Attenuator thread has become so lengthy that it is difficult to ascertain how many successfully completed builds there are of the fantastic @JohnH attenuator design. :thumbs: I'm starting this thread with the intention of showcasing all of these great builds. It is my...
> 
> 
> 
> 
> www.marshallforum.com
> 
> 
> 
> 
> 
> 
> 
> 
> *Attenuator M ( January 2019)*
> 
> The following is the original reactive design M which I built myself, with sound samples. M and M2 are closely related and perform the same, although M has two coils. It also has a bypass switch and a -3.5dB setting.
> 
> The build was in a case 170 x 120 x 55mm of thick aluminium:
> 
> 
> 
> 
> 
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> AttenuatorM Outside 190217
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> 
> __ JohnH
> __ Feb 16, 2019
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> AttenuatorM Inside 190217
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> __ JohnH
> __ Feb 16, 2019
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> Attenuator M 190110
> 
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> __ JohnH
> __ Jan 10, 2019
> __ 1
> 
> 
> 
> 
> 
> 
> 
> *Simplified version M-Lite*
> 
> This was the same design, omitting the bypass switching and the 3rd output. Minimum attenuation is -7db. Its an easier build and it should still meet most needs.
> 
> 
> 
> 
> 
> 
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> 
> Attenuator M-lite
> 
> 
> 
> __ JohnH
> __ Aug 24, 2019
> 
> 
> 
> 
> 
> 
> 
> *Performance*
> 
> In the schematic above, there is a graph showing a calculated frequency response at each attenuation level from 0 to -31.5 db. These use a spreadsheet to calculate the signal at each stage of the circuit, as a series of voltage dividers, using complex number theory to assess magnitudes and phase angles. The speaker was represented, for analysis, by an equivalent load model, by Aiken:
> 
> http://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator
> 
> ...adjusted to match the measured performance of a G12M 4x12 cab. The plots are based on small signals, with the amp output impedance assumed to be 20 Ohms, for an 8 Ohm tap, based on measurements of my VM2266C. These calcs were used to adjust the values in the design.
> 
> 
> *Sound Samples*
> 
> The ideal is for volume to reduce, but with no change in tone or feel. This is best tested with a consistent loop, with attenuated sounds then normalised back to equal volume:
> 
> *Attenuator M: Max attenuation to non-attenuated:*
> 
> 
> *Attenuator M: Normalised:*
> 
> 
> It’s a simple looped riff, played twice at each attenuation setting from -31db up to full unattenuated in 3.5db steps. The second file is based on the same recording, with each stage normalised for volume so you can hear any differences in the tone.
> 
> My VM2266c amp was on LDR mode, body at 6, detail at 9, master vol at 6, tones and presence at 6, using my LP bridge pickup, miced off a speaker.
> 
> 
> 
> 
> 
> 
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> 
> 
> Attenuator M Frequency Plots 190302
> 
> 
> 
> __ JohnH
> __ Mar 1, 2019
> 
> 
> 
> 
> 
> 
> The plots are taken from the sound sample posted above. The lower set of data are the basic plots, from full volume down to -31db (db scale is arbitrary, but relative db's are right).
> 
> The upper plots are intended to show the differences between responses. I took the -7db recording as the base case, so this is shown as a flat line. The others are the various other settings, with the -7db trace subtracted. The ideal for these traces is therefore also a flat line. And for all the traces below -7db down to -31db, this is what is happening, there is virtually no further tonal change at all as you attenuate down as far as you want. It measures as consistent.



Hi guys. I’m sure this question has already been asked but is it safe to connect an 8ohm amp speaker output to a 16ohm version of the johnh attenuator ?
Thanks a lot
Paul


----------



## JohnH

luxspring said:


> Hi guys. I’m sure this question has already been asked but is it safe to connect an 8ohm amp speaker output to a 16ohm version of the johnh attenuator ?
> Thanks a lot
> Paul


EDIT: I may have miss-read the question - see posts below

Yes its perfectly safe with an M2, 3 or 4. The fixed stage 1 which is always on, separates the speaker enough from the amp so the amp still sees close enough to 16 Ohms.

The tone may be a little brighter, a bit like turning up a presence and also a resonance knob, if you had one. On page 1, the second diagram has an extra output jack Out 4 that compensates for this, But its only a tone tweak, no safety or miss-match problem. 

If however, you have a full bypass switch, then to use that, amp ohms must match speaker ohms.


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## Gene Ballzz

luxspring said:


> Hi guys. I’m sure this question has already been asked but is it safe to connect an 8ohm amp speaker output to a 16ohm version of the johnh attenuator ?
> Thanks a lot
> Paul





JohnH said:


> Yes its perfectly safe with an M2, 3 or 4. The fixed stage 1 which is always on, separates the speaker enough from the amp so the amp still sees close enough to 16 Ohms.
> 
> The tone may be a little brighter, a bit like turning up a presence and also a resonance knob, if you had one. On page 1, the second diagram has an extra output jack Out 4 that compensates for this, But its only a tone tweak, no safety or miss-match problem.
> 
> If however, you have a full bypass switch, then to use that, amp ohms must match speaker ohms.



@JohnH,

As the wording was a little odd, you may have misunderstood the question from @luxspring ? I think he was asking if it is OK to plug a 16 ohm attenuator out of an 8 ohm amplifier output. I don't think he meant using an 8 ohm speaker, although that may also be the case. I've re-read the question a couple of times and still am not quite sure?

I've noted that some folks get confused by the direction of signal flow, especially when dealing with a combo, as they assume the speaker to be part of the amp.

To be more clear, if the amplifier's "output" jack is only 8 ohm, this should only feed the "input" of an 8 ohm load (either attenuator or speaker), but the "output" of the attenuator cares a little less whether it is feeding a 4 ohm, 8 ohm or 16 ohm speaker load, except for the tonal aspects that you mentioned.

*AMPLIFIER OUTPUT>ATTENUATOR INPUT/ATTENUATOR OUTPUT> SPEAKER INPUT*

Simply Attenuatin'
Gene


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## luxspring

Gene Ballzz said:


> @JohnH,
> 
> As the wording was a little odd, you may have misunderstood the question from @luxspring ? I think he was asking if it is OK to plug a 16 ohm attenuator out of an 8 ohm amplifier output. I don't think he meant using an 8 ohm speaker, although that may also be the case. I've re-read the question a couple of times and still am not quite sure?
> 
> I've noted that some folks get confused by the direction of signal flow, especially when dealing with a combo, as they assume the speaker to be part of the amp.
> 
> To be more clear, if the amplifier's "output" jack is only 8 ohm, this should only feed the "input" of an 8 ohm load (either attenuator or speaker), but the "output" of the attenuator cares a little less whether it is feeding a 4 ohm, 8 ohm or 16 ohm speaker load, except for the tonal aspects that you mentioned.
> 
> *AMPLIFIER OUTPUT>ATTENUATOR INPUT/ATTENUATOR OUTPUT> SPEAKER INPUT*
> 
> Simply Attenuatin'
> Gene


Hi again guys. Gene nailed it there, so sorry for how I worded it. 

So in that case , why can we not do a one way mismatch with the Amp to attenuator connection like we can normally do with an amp direct to a speaker (no attenuator) ?
I.e. 8 ohm out of the amp direct to a 4ohm or 16ohm speaker.


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## JohnH

Thanks Gene - yes I see now!

@luxspring - you must indeed match the output of your amp to the ohms rating of the attenuator. If you missmatch, that is at your risk, but its not recommended


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## luxspring

JohnH said:


> Thanks Gene - yes I see now!
> 
> @luxspring - you must indeed match the output of your amp to the ohms rating of the attenuatoOR


Thanks for the reply John , so that is loud and clear then.
Which leads me to ask, is there any possibility of another version (M5?)with a selectable amp input impedance ? 

Thanks
Paul


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## Gene Ballzz

There are many folks who claim that a one step mismatch, in either direction is OK. Some even intentionally use it as a subtle nuance of tone chasing. And while it is certainly true that some amp designs are more *"forgiving"* than others, it is also true that some designs are much more *"sensitive"* to such. What I see as the best approach is to always seek to err on the side of caution and simply match output impedance with the load it is driving/feeding. In any case, a mismatch will always affect the tone, at least to some degree!

This does become difficult with certain Mesa Boogie amps (and others) that only have 8Ω & 4Ω outputs and one desires to use a 16Ω 4x12 box. Although Mesa Boogie is one of the manufacturers that seemingly claims in heir own literature to be more* "forgiving" *of such a higher impedance mismatch. On the other hand, any 16Ω, 4 speaker box can be easily rewired to 4Ω.
Just My $.02,
Gene


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## JohnH

Hi @luxspring

There's a load that you can add, to bring a 16ohm M2 down to 8Ohm. It reduces all settings by a further 3db since it takes half the power.

To help discuss, could you describe the amps and cabs that you'd want to use? power-ratings and Ohms. And how loud do you need to go?


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## luxspring

JohnH said:


> Hi @luxspring
> 
> There's a load that you can add, to bring a 16ohm M2 down to 8Ohm. It reduces all settings by a further 3db since it takes half the power.
> 
> To help discuss, could you describe the amps and cabs that you'd want to use? power-ratings and Ohms. And how loud do you need to go?


Hi again John and thanks again to you and Gene for the replies.
So a friend has requested me to build one for him and he has currently three amps he would like to use it with:
- Fender Twin with 4,8 and 16 ohm selectable 
- Vox Ac30TB with only 16 ohm selectable although OT has an unused 8 ohm tap. He prefers I don’t tamper with that though.
- AN other (don’t remember now) which means boy has an 8ohm output.

He doesn’t use any external cabs, they are all combos. He just wants to put the attenuator between between the amp and its internal speakers(s)

Volume wise he is usually gigging in small bars with a hard rock band.

Cheers
Paul


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## JohnH

Ok thanks, I reckon we need @Gene Ballzz to offer a considered opinion on how much power may be needed for those scenarios. It may well be best to build two simple M2's, one at 8 and one at 16 Ohm?

The issue is that when you use the attenuator with a circuit to convert the front from say 16 to 8 ohms, and another to convert back to 16, you can lose up to 3 db at each conversion. And the base M2 takes of 7db at stage 1 anyway. So you could be starting with -13 db, and a say 30W amp is now only sounding at about 1.5 W. Fine for home, but probably not for a hard-rock bar gig!

An M2 running at its intended ohms would be 6W with a 30W amp, which might be about right.

The latest diagram also has a -3.5dB setting too, 30W goes to 13W, or lower.


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## chocol8

I would suggest building two load boxes for him. One at 8 ohm and one at 16 for the Vox. You could put in a pair of main resistors to make a single box switchable, but it is going to be bigger, and unless you switch out other components, it will be a compromise. At that point you might as well just build two separate units.


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## Gene Ballzz

@JohnH & @luxspring ,
The question marks here surround:

*FIRST> Which Twin?* As thare have been at least four general incarnations, ranging form about 80 watts to a whopping 135 watts. There is the Tweed *Twin Amp* (80 watts), the '60s style (most prevalent) *Twin Reverb* (85 watts), late '80s/early '90s *"The Twin"* (100 watts) and an "Ultralinear" version, not sure of the nomencature, (135 watts). Research and memory tells me that the only version with truly "selectable" impedance is that '80s/'90s *"The Twin!" *This makes me suspect the 100 watt* "The Twin" *which is a more modern style amp.

*SECOND> *Is the other* "don't remember now" *amp?

*THIRD>* It should also be remembered that most (if not all) versions of the AC30TB do not have any negative feedback, for whatever that may mean?

We really need clearer descriptions to make the most accurate recommendations.

Wish I Could Help More?
Gene


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## luxspring

I


Gene Ballzz said:


> @JohnH & @luxspring ,
> The question marks here surround:
> 
> *FIRST> Which Twin?* As thare have been at least four general incarnations, ranging form about 80 watts to a whopping 135 watts. There is the Tweed *Twin Amp* (80 watts), the '60s style (most prevalent) *Twin Reverb* (85 watts), late '80s/early '90s *"The Twin"* (100 watts) and an "Ultralinear" version, not sure of the nomencature, (135 watts). Research and memory tells me that the only version with truly "selectable" impedance is that '80s/'90s *"The Twin!" *This makes me suspect the 100 watt* "The Twin" *which is a more modern style amp.
> 
> *SECOND> *Is the other* "don't remember now" *amp?
> 
> *THIRD>* It should also be remembered that most (if not all) versions of the AC30TB do not have any negative feedback, for whatever that may mean?
> 
> We really need clearer descriptions to make the most accurate recommendations.
> 
> Wish I Could Help More?
> Gene


Hi Gene.
It’s an Evil Twin. 100 watt with switch for 25 watt.
There is a switch for 4 8 or 16 ohm


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## JohnH

I looked up that Evil Twin. Since the intent is to run with the combo speakers, we need to know what they are, even though the amp itself can deal with various options. According to what I read, the speakers look to be two 8 Ohm in series = 16 Ohm. If that's right (need to be sure) then a 16 Ohm M2, rated at 100W, with a fan would deal with that one. An 8 Ohm build, with Out 3 for 16 Ohms should be OK too since theres plenty of power

AC30's generally also have 2x8 in series = 16 Ohm. One of my amps, an old DSLcombo has a very similar output stage with 4xEL84 and no NFB, so I know the attenuator design will work. 

What more can we know about the 8 Ohm mystery amp?


----------



## stickyfinger

I plan on building a low wattage version for my DSL5c similar to what 
RJB has done in this thread. https://www.marshallforum.com/threads/completed-johnh-attenuators.124991/page-2​The amp is used at low volume but sounds best when the master is up to 5 or 6. Amp will likly see max attenuation of 31db reduction.​​Im a bit confused as what wire I should use and what Inductor for this low wattage version? Maybe its no different?​Thanks!


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## JohnH

Hi @stickyfinger 

For lower watts for a DSL5, I recommend to keep with the inductor design spec, being based on 18 awg wire, or 19 awg is ok too (eg from Madisound if in the US). That's to get the same resistance as the design. Hook up wire can stay 18 awg too, but I dont think it would be a problem if you went down to 20 awg. Resistors will have the same Ohm values, but can scale down in power rating. 

Good luck!


----------



## Mave

Hi all, new to this thread (and forum!) 
Firstly, thanks JohnH for all the fantastic info in this thread. 

I've been reading through it, and doing sums, and managed to get the right results. So far so good. 

I'm planning to do a lowish power single impedence build for my deluxe reverb - yes, non-Marshall sacrilege!) 

I'm after a quick bit of feedback - I've ever up with a few inductors (blame DPD) - 0.8mH, 0.9mH, 1.5mH. Has anyone played with switching extra in and out in series? Is it worth the effort? 

Thanks, Mave


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## JohnH

Hi @Mave , welcome to our thread and thanks for your interest.

I'd suggest to just use the one inductor per the design. If you have 0.9mH wound with 18 or 19 gage, then that's the one for an 8 Ohm build. This value suits a wide range of rigs. Actually small changes in the value don't make a big change in the tone that you hear, but if you go as high as 1.5mH for an 8 Ohm, then its outside of what we know works well. Keeping it simple is a great idea!


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## Gene Ballzz

Mave said:


> Hi all, new to this thread (and forum!)
> Firstly, thanks JohnH for all the fantastic info in this thread.
> 
> I've been reading through it, and doing sums, and managed to get the right results. So far so good.
> 
> I'm planning to do a lowish power single impedence build for my deluxe reverb - yes, non-Marshall sacrilege!)
> 
> I'm after a quick bit of feedback - I've ever up with a few inductors (blame DPD) - 0.8mH, 0.9mH, 1.5mH. Has anyone played with switching extra in and out in series? Is it worth the effort?
> 
> Thanks, Mave



@Mave ,
*FIRST>*  to the forum!
*NEXT>* @JohnH has the capability of plugging all different values into a spreadsheet and has already optimized all the values for them to achieve the most natural response and tones. If you have some specific alterations/goals you are seeking to accomplish, @JohnH may be able to advise you, but simply and randomly experimenting as you suggest, will not likely be very productive. While actual, physical combinations may produce very slightly different results, the numbers that JohnH works with are pretty darned accurate!

Just Attenuatin'
Gene


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## Mave

Gene Ballzz said:


> @Mave ,
> *FIRST>*  to the forum!
> *NEXT>* @JohnH has the capability of plugging all different values into a spreadsheet and has already optimized all the values for them to achieve the most natural response and tones. If you have some specific alterations/goals you are seeking to accomplish, @JohnH may be able to advise you, but simply and randomly experimenting as you suggest, will not likely be very productive. While actual, physical combinations may produce very slightly different results, the numbers that JohnH works with are pretty darned accurate!
> 
> Just Attenuatin'


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## Mave

Thanks guys. I'll build with the 0.9 and see how it sounds


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## stickyfinger

stickyfinger said:


> I plan on building a low wattage version for my DSL5c similar to what
> RJB has done in this thread. https://www.marshallforum.com/threads/completed-johnh-attenuators.124991/page-2​The amp is used at low volume but sounds best when the master is up to 5 or 6. Amp will likly see max attenuation of 31db reduction.​​Im a bit confused as what wire I should use and what Inductor for this low wattage version? Maybe its no different?​Thanks!





JohnH said:


> Hi @stickyfinger
> 
> For lower watts for a DSL5, I recommend to keep with the inductor design spec, being based on 18 awg wire, or 19 awg is ok too (eg from Madisound if in the US). That's to get the same resistance as the design. Hook up wire can stay 18 awg too, but I dont think it would be a problem if you went down to 20 awg. Resistors will have the same Ohm values, but can scale down in power rating.
> 
> Good luck!


I got the inductor in the mail the other day but I'm still uncertain as to what sized resistors I should use to scale a M2 for 5 watts.
I want to keep this as small as possible. Metal oxide resistors and a few cement.
This is going in the back of a combo and will have plenty of air flow. Most likely a mesh cage type enclosure.
Thanks for the help!


----------



## JohnH

stickyfinger said:


> I got the inductor in the mail the other day but I'm still uncertain as to what sized resistors I should use to scale a M2 for 5 watts.
> I want to keep this as small as possible. Metal oxide resistors and a few cement.
> This is going in the back of a combo and will have plenty of air flow. Most likely a mesh cage type enclosure.
> Thanks for the help!


If its just for 5w amp, then based on the 50W design ,you could scale power ratings x 1/10. So 10W, 5W and 2.5W ratings, with good ventilation and space around each one


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## stickyfinger

JohnH said:


> If its just for 5w amp, then based on the 50W design ,you could scale power ratings x 1/10. So 10W, 5W and 2.5W ratings, with good ventilation and space around each one


Thanks and just to be clear metal oxide resistors are ok ?
I remember reading a post where you recommended much higher wattage resistors for air cooled resistors over the normal aluminum clad resistors. I looked and couldn't find that post. Thanks


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## JohnH

I haven't built one targeted at such low Watts, but if you get those resistors as above then there should still be at least a 3x safety factor with a basic M2, which given they are designed for air cooling, I figure should be OK but they may heat up and thats ok too. Nothing wrong in principle with metal oxide. If concerned, use greater ratings


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## emann

Mave said:


> Hi all, new to this thread (and forum!)
> Firstly, thanks JohnH for all the fantastic info in this thread.
> 
> I've been reading through it, and doing sums, and managed to get the right results. So far so good.
> 
> I'm planning to do a lowish power single impedence build for my deluxe reverb - yes, non-Marshall sacrilege!)
> 
> I'm after a quick bit of feedback - I've ever up with a few inductors (blame DPD) - 0.8mH, 0.9mH, 1.5mH. Has anyone played with switching extra in and out in series? Is it worth the effort?
> 
> Thanks, Mave


Hi Mave,

I am just putting the final touches on the M2 unit which I finished off some weeks ago - as soon as it is ready I will put a full thread on the other post Completed JohnH Attenuators.

I have used the components as specified in the schematic of John and believe me the amp is thanking me every day I switch it on to let it come alive and not just keeping the volume between 2 and ...I am only hearing the amp now for the past few days and coupled with my kingtone duellist and new fuzz v2 pedal it is just glorious. 

My suggestion would be to stick to the values specified for the deluxe reverb and you cannot go wrong.

Best - emann


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## Mave

emann said:


> Hi Mave,
> 
> I am just putting the final touches on the M2 unit which I finished off some weeks ago - as soon as it is ready I will put a full thread on the other post Completed JohnH Attenuators.
> 
> I have used the components as specified in the schematic of John and believe me the amp is thanking me every day I switch it on to let it come alive and not just keeping the volume between 2 and ...I am only hearing the amp now for the past few days and coupled with my kingtone duellist and new fuzz v2 pedal it is just glorious.
> 
> My suggestion would be to stick to the values specified for the deluxe reverb and you cannot go wrong.
> 
> Best - emann


Thanks for the feedback. I will indeed build my unit as per the values


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## sylaswojciechowski@gmail.

Hi, I am starting to gather components for an M4 version of this attenuator. Just a few questions about the schematic before I place some orders. 

Is stage 1 and 2 attenuation, as well as bypass going to be using 3 different DPST switches? So the 2 switches shown on stage 1, 2 and bypass are really just 1 DPDT switch (totaling 3 DPDT switches and 2 more SPDT for stage 3 & 4)? Hopefully I explained my question well enough, Im not sure how to word it.

Another question, can R10A, R10B, R9, and R11 as well as outputs 4, 3, and 2 all be safely removed without changing any other resistances for an 8ohm attenuator build for an 8 ohm speaker?


----------



## AtomicRob

sylaswojciechowski@gmail. said:


> Is stage 1 and 2 attenuation, as well as bypass going to be using 3 different DPST switches? So the 2 switches shown on stage 1, 2 and bypass are really just 1 DPDT switch (totaling 3 DPDT switches and 2 more SPDT for stage 3 & 4)? Hopefully I explained my question well enough, Im not sure how to word it.


The switches for each stage are SPDT nominally. Many folks are using a DPDT and bridging the two sides, which doesn't hurt anything but probably doesn't help either, since toggle switch ratings are for the whole switch (edit- for clarity, meaning not in parallel - a 5A DPDT can switch two 5A circuits but not one 10A circuit...) But DPDT switches seem to be more commonly in stock anyway, so why not.

I'm building a 16ohm 100W version, and I did a bunch of LTspice simulations to check current through the switches. With a 170W input signal like you might see from a "100W" Marshall amp, the bypass switch sees about 5A current worst case, and the three switches for the stages see about 2.5A max, usually much less, depending which stages are switched in. So I'm using a big Carling G series 10A rated switch for the bypass, and the smaller Carling 2M1 series mini toggles for the stages, which are rated 5A. (And if my ltspice calcs were wrong somehow and a mini toggle dies, it won't actually hurt anything. If the bypass switch dies, that would be bad... that one's more important to use a beefy switch!)


----------



## JohnH

hi @sylaswojciechowski@gmail. 

Welcome to our thread and thanks for your interest. All agreed on your points about switches and deleting the parts. Id be interested in how you'll use this version with its extra features (you are the first to reference the M4 design, though all the parts have been tried)


----------



## JohnH

Hi @AtomicRob 

My current understanding of switch ratings is that it is per contact, but that you cant combine in parallel to double ratings. Thats because the rating is determined by the effects of switching rather than continuous current. At the instant of switching, the two sides will change at fractionally different times. This link I found helpful:









DPDT rating combined?


https://www.utvinc.com/switches/utv-inc-back-lit-dual-blue-dpdt-on-off-on-vertical-turn-signal-switch/ I want to use a few of the switches in the link above. They are double pole double throw and r...




electronics.stackexchange.com





On the bypass switch, it should never be switched while running under load (for the sake of the amp), so that will also help to preserve the switch. I think if there is a DPDT, and we only need one side, paralleling is still a bit helpful, if not for doubling of capacity but for added redundancy.


----------



## AtomicRob

JohnH said:


> Hi @AtomicRob
> 
> My current understanding of switch ratings is that it is per contact, but that you cant combine in parallel to double ratings. Thats because the rating is determined by the effects of switching rather than continuous current. At the instant of switching, the two sides will change at fractionally different times. This link I found helpful:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> DPDT rating combined?
> 
> 
> https://www.utvinc.com/switches/utv-inc-back-lit-dual-blue-dpdt-on-off-on-vertical-turn-signal-switch/ I want to use a few of the switches in the link above. They are double pole double throw and r...
> 
> 
> 
> 
> electronics.stackexchange.com
> 
> 
> 
> 
> 
> On the bypass switch, it should never be switched while running under load (for the sake of the amp), so that will also help to preserve the switch. I think if there is a DPDT, and we only need one side, paralleling is still a bit helpful, if not for doubling of capacity but for added redundancy.


Yes, definitely true about switch timing - mechanical switches are all over the place with timing and bounce. I hadn’t thought about the unswitched parallel rating but sure enough in the data sheet Carling use the term “contact rating“ which does seem to imply that each set of contacts would be rated individually.


----------



## Mjh36

Back for the attack with another build, this one being a 100 watt M3 version. Took me a while and it came in well over-budget due to problems in the R&D department (i.e. my mistakes). My goal was to use a Hammond 1550N enclosure with countersunk screws. I ordered a bunch of screws and metric bits from McMaster-Carr and one of those Harbor Freight drill presses. I found that M6 countersink drill bits are huge and I was trying too hard to get the screws as flush as possible. So I messed up making holes too big on not one, but two 1550N enclosures. Also fitment, I thought I would have enough space for easy work with a 1550N, but I always misjudge that. So I ended up getting a Hammond 2U 19 inch rack enclosure which has plenty of space and used regular pan-head screws. Turned out great, it's huge and heavy but that's what I wanted.

I'm hoping the parallel 300 watt resistors will help with heat, but if need be I left a DC jack and switch as you can see. It's to test switching fan power between 9VDC or self-powered off the speaker tap. I have the couple components to wire it up with that schematic from a Two Notes design that's on here, it's the last thing to do. Another positive is the vent holes on top were predrilled and line up perfectly with a 40x40mm brushless fan to mount. Then I can switch between the two power options and see if there's tone suck at all with the self-powered option or if it even works because I don't know what I'm doing usually, just following schematics. But that's only if I need the fan, I'll leave it out if heat isn't an issue with this build.

Speaking of, I still haven't played through it. A year and a half later, two homemade attenuators and no tube amp yet. It's because I'm waiting on a Ceriatone AH-100 Deluxe and it's been about a year since I ordered it. I've seen comments of people that ordered before me that are still waiting, so I may have a few more months still. Then I can report back and talk about how it sounds with the bass resonance circuit and how it handles heat. Rock on ya'll.


----------



## Mjh36

I'll have to investigate. I'm taking readings at the attenuator input with a single 8 ohm load just now. When I have one 8 ohm speaker output in use, the input stays within 7-10 ohms across all switching. When I unplug the speaker and insert it into the second 8 ohm speaker output, the input readings eventually jump up to 16 ohms when all switches are engaged. I double checked the wiring I thought it looked good. It shouldn't matter which 8 ohm speaker output I'm using, if I'm using just one, it should still stay in the 7-10ohm range correct?


----------



## JohnH

Hi @Mjh36 

Nice looking build! Those readings where it's looking inconsistent on the second output match up with the numbers you'd get if the speaker was not connected. So maybe check the jack and its wiring? ef A classic issue ( I've done!) is to solder to the switched lugs instead of main lugs.


----------



## Mjh36

JohnH said:


> Hi @Mjh36
> 
> Nice looking build! Those readings where it's looking inconsistent on the second output match up with the numbers you'd get if the speaker was not connected. So maybe check the jack and its wiring? ef A classic issue ( I've done!) is to solder to the switched lugs instead of main lugs.


Thanks John, can't wait to put it to use. Yeah, I do have those Neutrik switched jacks and I tell ya that stuff is my Achilles Heel. I always have to re-learn the abc's of it. Ok so in the picture, I have the jacks marked. I have those two 8 ohms soldered straight across all solder lugs. Should I be linking just one hot lug to the next 8 ohm jack hot lug?


----------



## JohnH

That's odd! linking across all the lugs should work, and the good and problem 8 ohm jacks should both be good!

Maybe pull the cover of the plug back so you can test continuity from plug lugs to socket lugs?


----------



## Mjh36

Good call I'll do that tomorrow and check my cable, reflow some solder, change the jack if need be.
Edit as I'm typing this:
Just looked over at the desk, broken solder joint at what looks like the hot lug of problem jack, it's loose. Even slightly further investigation would have uncovered this before asking! I tell myself to check the easy stuff and still miss it, everytime. Should be the fix thanks again.


----------



## JohnH

@rowandg has kindly posted some very helpful sound clips in the 'completed builds' thread, using an M2 built by Gene:






Completed JohnH Attenuators?


Now, these I would not be embarrassed to have sitting on top of my amp, onstage! Although, these two were built for a couple friends! My sincerest thanks to @DeluxeReverb for the inspiration to switch over to countersunk metric screws, for resistor mounting and to @Dblgun for the "gutter grate"...




www.marshallforum.com





...and since Im a tragic engineering nerd, obviously Im gonna analyse the #$%! out of them!

Rowan made a clip repeated at a range of 6 attenuation settings from full bypass (no attenuation) down to -31.5db (max attenuation), all normalised for equal volume so we can compare the tone. I then brought them into Audacity and assembled frequency plots in Excel:




The lower spiky traces are all the plots on top of each other, with the full volume trace in thick red.

Above that, are flatish lines where the traces at full volume and attenuated are subtracted from each other, to show the difference. Ive also separated them vertically so they are clearer. The ideal response would be completely flat which would indicate no change in tone with frequency.

What we see is fairly flat consistent traces across the frequency range at all settings, except for about a 2 to 3dB bass bump below 100hZ. ie, the attenuated sound has a tad more low bass. This is pretty much what Id expect with an SV20 going into an M2 ad its because an SV20 has a lower effective output impedance than the VM that I used as a design basis. I think its not a bad thing though. Good to get a bit more thickness in the lows at low volume.

Thanks for the data Rowan!


----------



## Wizardof_Oz

Hi @JohnH, first of all thanks so much for this great thread and the continuous development over the past years! I already have a power soak in use with my DSL20 (SPL Cabulator, just resistors but no reactive parts) but with higher level of attenuation it sounds quite dull. So I came across this forum and appreciate all your effort and explanations  
I just need attenuation. So my starting point is the M2 8 Ohm variant but the more I read here, the more questions I got :-( 
I've spent several hours reading back and forth in this thread, so please forgive me if my questions were already explained somewhere...
- I found your drawings for the M2 (dated 31/12/2021) and M3 (dated 22/8/2021). They are pretty much the same except for the line out (which I don't need). BUT: the M3 has added an a second coil L2 plus C1 in parallel. Is there benefit vs. L1 only for simple attenuation? As well, R1 (15 Ohm) is substituted by R1A and R1B with 30 Ohm each in parallel. Is there a specfic reason or just an equivalent option?
- are air coils with plastic housing easier to mount? I read somewhere, that you recommend a 5mm spacer (no metal) between the coil and the housing. I guess this still applies, right?
- for curiosity in the M2 layout dated 31/12/2021: R12 (5k, 24mm) is a lin or log pot?
- I just need (for now) 4 or 8 Ohm Out (Out 1+2). That means, I can skip Out 3 plus R10/R11 plus wiring. Right?

I built my own guitar (SG with maple top), a 2x12 cab (Marshal 1936) with UK vintage 30, next is your attenuator (and then... own DIY amp). I will post pictures of the final assembly 
Cheers, Frank


----------



## JohnH

@Wizardof_Oz

Hi Frank, welcome to our thread and thanks for your interest in these designs.

BTW - Being of Oz, are you down here in Aus?

On the questions:

There's been a lot of M2's built, and a few M3's (where the second coil and cap get added), and all of them can have various add-ons. If you are just wanting to attenuate volume, and play through a guitar speaker, then the consensus is that the basic M2 is fine and there's no need to add the extra bass resonance circuit to make it M3. This has been demonstrated across a wide variety of different amps.

The extra parts for M3 may become of more significance if you use a line-out, which you note that you dont need. This shows the amp a bass peak, which changes the response slightly but only for a very few low notes. Without the bass circuit, the real speaker does pretty much the same thing by itself anyway.

In the last few weeks, Ive been updating and linking the first post. The most basic M2 is shown in the top diagram, then with a few extras. There is a link to page 158 where more extra features are added. Also links to the two build diagrams that you saw, and sound clips including the latest from Rowan above. BTW Im now calling the latest diagram M4.

It sounds like your basic use will be with an 8 Ohm cab? and then another to make 4 ohms? If that 4 Ohm use is important, you might consider the very simple addition of Out4 (a recent development), see the larger diagram on post 1. It just makes a small tone tweak for 4 ohm use. Without it, you can just use Out 1 and 2 and the tone with 4 Ohms will be a little brighter.

R1 takes the greatest share of power, so its rated higher. You can use one 100W resistor, or as in the diagram on p112, use two at 50W, which seemed to lead to a neater layout and better spread of heat. No real difference though.

For the air-cored coil L1, we have them on plastic bobbins (as I have used and also Gene), or wound on itself. You can mount with zip ties if needed. The spacer is recommended in any case, it just controls eddy current losses induced in the chassis.

People have found plastic or non-ferrous screws, zip ties, or in my case, I let the coil sit down onto the case lid (which becomes the attenuator base) via the spacer, fixed with dabs of silicone and a screw which I later remove.

For the line-out pot, log or lin is ok, it just changes the sweep, not the range.

You can omit Out 3 and associated resistors if you dont want a 16 Ohm output. I find this useful though with my DSL combo, which has a 16 Ohm speaker but I also use it with a 16 cab at 8 Ohms.


----------



## Wizardof_Oz

@JohnH 
Hi John, nah. Wish I was though. I used to live down under in my 20s for a year. Loved it, hopefully I can show the beauty of your country to my daughter sometime soon...

Yup, besides the DSL20 which I run with a 8Ohm cab, I own a Twin Reverb from 1977 (Ultralinear with 135W) that has two 8Ohm speakers in parallel (4Ohm cab output). The speakers (original Fender) sound great (even with heavy rock sounds), so I want to alternatively run my DSL20 trough them. So the first attenuator will be for the 20W amp with your suggested M2 ratings. 

As for the add-on on Out4, you mean R9, right? Yes, I will add this to my layout. But will leave out Out3 to keep it a simple as possible. I can always add it later if ever needed...

Having learned from this, I plan a high powered version for the Twin. Which ratings would you suggest? Do doubled values provide enough margin?

Cheers, Frank


----------



## Wizardof_Oz

@JohnH
btw: I came across the idea of bitsleftover in the tdpri forum to add an FX loop to the design. This would be an awesome add-on for amps that lack this feature (like old plexies or twin reverbs) to use modulation pedals in the signal chain.
As far as I understand, this is the same idea that the Freyette Power Station provides (with tube amplification of the signal out to a cab or sim). Could you possibly make a design draft how this can be realized? What kind of (pedal?) amplifier could be used for a good sound quality?


----------



## JohnH

hi @Wizardof_Oz

Ok I see where youre coming from now. The DSL and the M2 should be great for those uses, and a nice simple build.

That 135W Twin may be challenging! Happy to think about it, while you build the 50W unit, Does that Twin have any other output options or just 4 Ohm?. Theres no problem scaling ohms and coil values x1/2 to make a 4 Ohm version. But the power rating might depend on whether it really needs to be pushed hard, or is that big power output best treated as headroom and really its best tones are at more moderate power? in any case, parts would need to be uprated and a fan will be needed. A brushless dc fan in a well vented case gets rid of a lot of heat. It can be quiet too, if a 12V fan is run at 9V. But in theory, the current into 4 Ohms at 135W may be more than typical jacks are rated for!

On FX loops, I think youve realised that to do this then needs the line-out feature to another amp, since we cant insert FX in the speaker-level signal of a typical M2. So to change over fully to this mode of operation, its best done as a different design. Reactive load to line out to FX and cab sim, then to a different power amp. The reactive load could be a line-out from one of the attenuator designs (set as a load with no cab). But we can get a slightly better load with fewer parts if its a dedicated design.

But another version of this use keeps the attenuator and passive guitar speaker, and then adds the FX and separate amp as a second channel from a line-out to make a wet/dry rig. Given a speaker is still used on the attenuator, a line out derived at the speaker is a good option and picks up the electrical response of it (bass and treble rise)

Amps for this could be almost any amp with a neutral clean input. Ive rigged up something similar with my old Crate PowerBlock. Something like an EHX Magnum44 might also be good.

But with this load then re-amping scenario, its quite a different philosophy to the passive designs here and we are not in Kansas anymore!


----------



## Gene Ballzz

Wizardof_Oz said:


> @JohnH
> Hi John, nah. Wish I was though. I used to live down under in my 20s for a year. Loved it, hopefully I can show the beauty of your country to my daughter sometime soon...
> 
> Yup, besides the DSL20 which I run with a 8Ohm cab, I own a Twin Reverb from 1977 (Ultralinear with 135W) that has two 8Ohm speakers in parallel (4Ohm cab output). The speakers (original Fender) sound great (even with heavy rock sounds), so I want to alternatively run my DSL20 trough them. So the first attenuator will be for the 20W amp with your suggested M2 ratings.
> 
> As for the add-on on Out4, you mean R9, right? Yes, I will add this to my layout. But will leave out Out3 to keep it a simple as possible. I can always add it later if ever needed...
> 
> Having learned from this, I plan a high powered version for the Twin. Which ratings would you suggest? Do doubled values provide enough margin?
> 
> Cheers, Frank



 to the forum!

You seem to have a good handle on what's going on here! 

For your first, smaller iteration, I'd recommend an 8 ohm M2, just as you described, with #1, #2 & #4 outputs, leaving the option to add the #3, 16 ohm output, if desired. What may not be immediately evident is that there are a couple optional, additional benefits here, even when deleting the #3 output. Out #1 & #2 are in parallel, so you can plug an 8 ohm load out of either of them *OR* 1- 16 ohm load out of each of them for a total 8 ohm load. Additionally, as soon as you plug a load out of out #4, it parallels #4 with #1 & #2, allowing you to run either one 4 ohm load out of #4 *OR* an 8 ohm load out of each of #4 *AND* either or #1 & #2 for a 4 ohm total. A caveat here is to not use #1 & #2 & #4 together *AND* never use #1, #2 and/or #4 if/when using the #3, 16 ohm out, if you include it in the build.

If you'd like some drilling templates for resistors and cooling, for a box that ends up looking like the pics below, let me know!

Happy Building,
Gene


----------



## Wizardof_Oz

@JohnH and @Gene Ballzz 
Merry Christmas guys and thank you for the prompt replies despite the holiday season.

Yes John, it was obvious that the FX requires additional circuits. So, nothing for my first built
I already ordered parts, incl. a Hammond 1590DE housing.
Thank you, Gene. Very neat built! Would you have a drilling template for the 1590DE? (How do you come up with such huge drilling holes in a massive aluminum lid??). Do you always build the designs upside down or was this here due to the fan?
As for the resistors I have a question. I had difficulties to order certain types for the M2 for reasonable money (all are Arcol with 1% tolerance), so I have following variations:
R1A/R1B: instead of 2x 50W/30Ohms => 2x 50W/33Ohms
R1B: instead of 50W/18Ohms => 50W/22Ohms (even 50W/20Ohms wasn’t available)
R8: instead of 25W/5.6Ohms => 25W/4.7Ohms (even 25W/5Ohms wasn’t available)
Are these variations still in an acceptable range?
Cheers, Frank


----------



## Gene Ballzz

Wizardof_Oz said:


> @JohnH and @Gene Ballzz
> Merry Christmas guys and thank you for the prompt replies despite the holiday season.
> 
> Yes John, it was obvious that the FX requires additional circuits. So, nothing for my first built
> I already ordered parts, incl. a Hammond 1590DE housing.



Frank,
If your order hasn't yet shipped, you may want to consider cancelling the resistors. I've had the best luck with this vendor from "chinesiawanoreanam" off of flea-bay, although the wait is kinda long. The prices are good, quality sufficient, all values available and my templates surround their sizings.









coolcheapworld | eBay Stores


Welcome to my eBay Store. Please add me to your list of favorite sellers and visit often. Thank you for your business.



www.ebay.com








Wizardof_Oz said:


> Thank you, Gene. Very neat built! Would you have a drilling template for the 1590DE? (How do you come up with such huge drilling holes in a massive aluminum lid??). Do you always build the designs upside down or was this here due to the fan?



Yes, my templates utilize the 1590DE box, although will certainly work also on bigger boxes. Big holes are done with a hole saw, available in various sizes:






And as far as "upside down" if you look at the box, you'll notice that the lid dimensions are a little larger than the opposite face's dimensions, so if the lid is on top, it is somewhat "keystone" shaped and to me, less aesthectically pleasing! There are also some other benefits:

*>A* - Keeping all the switches, resistors and jacks attached to the main box body makes it so the only pair of flexible, pigtail leads of the circuit need go to the coil, and of course to the fan(s), if included. But the fan is not really part of the attenuator circuit.
*>B* - Having the resistors on the top aids the radiational cooling from the heat sink of the box.
*>C* - Mounting the coil to the lid frees up a lot of real estate in the main box. You could also mount one or a couple of closely related/wired resistors on the lid, if more real estate in the box is required for extra features. A bit of thought and planning goes a long way here.




Wizardof_Oz said:


> As for the resistors I have a question. I had difficulties to order certain types for the M2 for reasonable money (all are Arcol with 1% tolerance), so I have following variations:
> R1A/R1B: instead of 2x 50W/30Ohms => 2x 50W/33Ohms
> R1B: instead of 50W/18Ohms => 50W/22Ohms (even 50W/20Ohms wasn’t available)
> R8: instead of 25W/5.6Ohms => 25W/4.7Ohms (even 25W/5Ohms wasn’t available)
> Are these variations still in an acceptable range?
> Cheers, Frank



Also, my templates don't include the switch and jack mounting holes. Some folks prefer them on the ends of the box, other like them on the sides. I have genericized the layouts to accommodate multiple versions of this M2, that may have different numbers of jacks and switches. There is also a variance on sizes of these components and their mounting hole diameters. It should be noted that when placing said jacks and switches, it becomes similar to playing a game of three dimensional chess, for clearance with resistors and other things!

For more detailed questions, help and/or communication, that is best done through private email and possibly even a phone call through "whatsapp." PM me and we can swap contact info.

Stay In Touch?
Gene


----------



## Wizardof_Oz

@Gene Ballzz
Hey Gene, of course. Tried to send you a PM, but couldn't find "start a conversation" anywhere. Maybe you can help me out here...

I will use pretty much the same setup as shown in your pictures, except for the fan, jacks and the switches. They will be on the long sides (switches: front, jacks: rear).
Man, I didn't think about a hole saw. I've got some that can be used for softer metals. 

Shipping is already on the way, unfortunately...

As for the design, I will probably create a simple 3D drawing with SketchUp to see if there's any collisions. So I can re-use this for future projects with the same housing and even provide this if anyone is interested...

Cheers, Frank


----------



## JohnH

Hi @Wizardof_Oz

Just on the resistors, there's been a lot of fine tuning of the values in the design, and while none of them are mission-critical to 1%, (5% is fine) by the time values start to move away by 10% or 20%, some of the balances of ratios are getting out ot 'wack'. The circuit is based around a bunch of very small decisions about component values, each of which might only be worth a half a dB or so, but by addressing each one, the net result over several stages is kept consistent and close to optimum.

*R1*
But I do appreciate that the range of these power resistors from a given supplier is often not quite ideal. But its worth making an adjustment. The values you had to buy are larger than the spec, so Im suggesting to tweak them back down by piggy-backing a larger value resistor across them, but of lower power rating. It could be a ceramic resistor and take up no more space.

For R1, in the 8 Ohm build, the basic value is 15 Ohm at 100W, and you went for two in paralel at 50W. Usualy 30 Ohm is available, to get 15 in parallel, though its not in the most common ISO series. If you are getting 2 at 33 Ohms, the outcome would be Ohms seen by the amp is 1.5Ohm higher, and the circuit tends to end up a bit high anyway, 9 Ohms or more in an 8 Ohm build. So to get 2x33 down to 15 net, in theory put 165 Ohm across it. 150 would be a great value, net result 1/ (2/33 + 1/150) = 14.9. it will take 10% of the power in the group, so 10W would be OK.

Another way with R1 would be to swap one 33 for a 27, net result 14.85. The 27 takes a bit more power but still fine. 

*R2B*

This is the 18 Ohm value, and I assume this is the one you couldn't get? The ratio with R2A (22 Ohm) is important here to set the tonal balance coming out of Stage 1 . If they are the same, treble gets a bit too high. 100 Ohm in parallel with 22 will get 18, a 10W one would do it

*R8 *

Usually 5.6 is in the standard range. If you use 4.7 instead, then the attenuation step in the -3.5dB stage is a bit less. There'll be a slightly wider variation in the equalness of overall attenuation steps when using the various stages. With the base design, each step between settings, nominally of -3.5dB, is theoretically between -3.4dB and -3.7dB. With the final stage a bit different using a 4.7 Ohm for R8, the range of different steps is more like -3.2dB to -4dB. Not a big problem, and you may not notice it. There wouldn't be a significant tonal change


----------



## Gene Ballzz

Wizardof_Oz said:


> @Gene Ballzz
> Hey Gene, of course. Tried to send you a PM, but couldn't find "start a conversation" anywhere. Maybe you can help me out here...
> 
> I will use pretty much the same setup as shown in your pictures, except for the fan, jacks and the switches. They will be on the long sides (switches: front, jacks: rear).
> Man, I didn't think about a hole saw. I've got some that can be used for softer metals.
> 
> Shipping is already on the way, unfortunately...
> 
> As for the design, I will probably create a simple 3D drawing with SketchUp to see if there's any collisions. So I can re-use this for future projects with the same housing and even provide this if anyone is interested...
> 
> Cheers, Frank



Given that you're new, I don't think you can start a conversation yet, but I can send you one, with my contact info. Given that you are Sketchup savvy, I could even send you the Sketchup file (instead of a PDF), for you to slightly modify for your resisitors. One cool way to use the template is to first get it sized properly on plain paper and then print it onto peel & stick craft paper, apply it to the box and wail away!

Look For A Conversation?
Gene


----------



## Wizardof_Oz

@Gene Ballzz,
Hi Gene,
I just sent you an email…
Cheers Frank


----------



## JohnH

*Parts Lists and Parts Suppliers*

(Note: Ive been trying to streamline this thread recently, updating post 1 and also giving a series of links there to key posts in the thread and other relevant threads. One of the limitations in this forum is a limit of 10000 characters per post, which I'm always running up against in Post 1, so not everything can be in one post.)

This post is a place where I want to accumulate parts references and suppliers links, for M2 M3 and M4 attenuators, from wherever they can be obtained. Then Ill link this post back to post 1 so it can be found, and add to it over time.

Typically, its not quite possible to get everything from one place. eg, depending where you are, it may take an order to an audio parts specialist (for the coils and maybe jacks), and electronics retailer for switches and jacks, maybe the case and feet and a hardware suppler for paint, bolts. Power resistors ordered from Asia can be very good value and they are fine, given long shipping.

So, Ill add links that I know about below, but please remind me in a post or pm if you have any favourites that should be shared and Ill post them in here.

Just to get this started:

*For Australia*

I get a lot from Jaycar, either online or bricks and mortar shops:
https://www.jaycar.com.au/
Jaycar can cover a lot, and I get case, feet, wire, solder, nuts/bolts, jacks and switches there. There are several that I can go to in person too. They don't stock aluminium power resistors or coils of the types we need.

Wagner electronics will also cover the electrical basics, and have the right coils for M2, M3 and M4, and bipolar caps if you are doing M3 or M4





Wagner Online Electronic Stores | Wide range of electronic products, components and accessories


Wagner Electronics Online Store for Audio Visual Installation, Cellular Accessories, Electronic components, Batteries, Chargers, AC / DC Power Supplies, Lighting, Telephone accessories, Electronic Test Equipment, Tools, Audio Speakers, Car sound, Data Racks, Computer Accessories, Cat5/6 Cables...




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*Resistors from Asia*

Supply of asian power resistors can be via eBay. They tend to be much less $ than branded ones from Arcoll etc and quality is fine for this job, with tolerance usually at 5% which is ok too. Shipping is usually free but long time!

There are lots of sellers there, I think the trick is to go to their eBay stores and order from there, to acculmulate the various sizes needed into an order. Many of them just have a few values, but there are some that have all or most of the standard range. eg:








socoolmart | eBay Stores


Welcome to my eBay Store. Please add me to your list of favorite sellers and visit often. Thank you for your business.



www.ebay.com





This one is noted by Gene:








coolcheapworld | eBay Stores


Welcome to my eBay Store. Please add me to your list of favorite sellers and visit often. Thank you for your business.



www.ebay.com


----------



## Gene Ballzz

Hey @JohnH , didn't a dedicated 2 ohm unit get designed and built for use with a Super Reverb, along with it's 2 ohm speaker load? I searched, but couldn't find it!
Thanks Again Sir,
Gene


----------



## JohnH

Gene Ballzz said:


> Hey @JohnH , didn't a dedicated 2 ohm unit get designed and built for use with a Super Reverb, along with it's 2 ohm speaker load? I searched, but couldn't find it!
> Thanks Again Sir,
> Gene


Hi Gene

I dont recall a dedicated 2 ohm version, though there was a variable 2/4/8 one based on a 4 Ohm M2 core (some losses in the conversions). There could be a 2 Ohm one though. Dealing with high current through connections might be an issue.


----------



## Marcomel79

You guys truly are two gentlemen


----------



## Gene Ballzz

JohnH said:


> Hi Gene
> 
> I dont recall a dedicated 2 ohm version, though there was a variable 2/4/8 one based on a 4 Ohm M2 core (some losses in the conversions). There could be a 2 Ohm one though. Dealing with high current through connections might be an issue.



@JohnH 

When you say *"Dealing with high current through connections might be an issue"* are you referring to the need to upgrade the switches, or needing to increase wire gauge, reistor wattage, etc?

There are many Super Reverb owners who would truly love and appreciate the liberation that an M2 could provide! For instance, there is a member at Strat_Talk asking for assistance in building one. Component values would be very helpful?

Thanks Again & Happy Holidays!
Gene


----------



## Gene Ballzz

@JohnH ,

Dumb questions time! If incorporating a foot switchable stage, via a three wire cable & TRS connectors, what happens to the impedance that the amp sees in the following scenarios that could be caused by the cable getting damaged during a performance, being stepped on, yanked, etc?
*A>* Any and/or all of the three leads of the TRS get shorted to each other?
*B>* Any and/or all of the leads become open?
While I realize that most of these scenarios may cause a loss of sound at the speaker, my big concern is the impedance that the amp sees! I'm viewing this as an amplifier useage and safety concern. 
Thanks Again,
Gene


----------



## JohnH

HI Gene


Gene Ballzz said:


> @JohnH
> 
> When you say *"Dealing with high current through connections might be an issue"* are you referring to the need to upgrade the switches, or needing to increase wire gauge, reistor wattage, etc?
> 
> There are many Super Reverb owners who would truly love and appreciate the liberation that an M2 could provide! For instance, there is a member at Strat_Talk asking for assistance in building one. Component values would be very helpful?
> 
> Thanks Again & Happy Holidays!
> Gene


I guess its everything to do with current that would need to be considered. Im not sure what amp power can come from these 2 Ohm Fenders? If its like 135W from the big Twins, then into 2 Ohms, and without further overdrive allowance, that's a current of: 
sqrt (135 / 2) = 8.2 Amps

Wire for hook-up and coil could be upgraded to suit, as could the switches. But what about jacks? I think Cliff jacks are rated for 5 Amps. Id assume the amps use jack sockets as usual. I wonder if this issue is addressed, or is it found not to be a problem? or is the amp power a lot less?


----------



## Gene Ballzz

@JohnH ,
The Super Reverb is rated at 45 watts, with two 6L6s and tube rectified! Much like the '59 Tweed Bassman, although I'm not sure right now if the Bassman is rated for 2Ω or 4Ω.
Simply Sharin'
Gene


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> 
> Dumb questions time! If incorporating a foot switchable stage, via a three wire cable & TRS connectors, what happens to the impedance that the amp sees in the following scenarios that could be caused by the cable getting damaged during a performance, being stepped on, yanked, etc?
> *A>* Any and/or all of the three leads of the TRS get shorted to each other?
> *B>* Any and/or all of the leads become open?
> While I realize that most of these scenarios may cause a loss of sound at the speaker, my big concern is the impedance that the amp sees! I'm viewing this as an amplifier useage and safety concern.
> Thanks Again,
> Gene


 Its an important question, thanks for raising it again. I think its OK, but lets consider it and im interested in your views:

The most extreme thing that could happen to the footswitch circuit is either it gets fully shorted out or fully open circuit. And what that means to the impedance seen by the amp depends on how many attenuation stages there are engaged before it. When there's lots of attenuation before, hardly any of the footswitch condition is seen by the amp.

So the conditions to consider are with just Stage 1 on, -7db, and the footswitch is being used to change down to -10.5 dB. Quite a likely scenario.

In this case, if Stage 1 is there and suddenly there's an open circuit after it, then sound is cut off and the amp now sees just R1 and the coil, about 16 Ohm instead of 8 Ohm. There's no instant where the amp sees fully open. 

Or, if suddenly the footswitch gets shorted, then R2A and R2B get in parallel with R1 and the amp sees about 6 Ohm instead of 8 

If the footswitch jack gets pulled out, then it defaults to no footswitch and the signal goes straight through to the speaker and two jack contacts in parallel provide this with redundancy. I prefer that the cord is hard-wired (and gripped) to the footswitch box, so the same issues dont occur at that end. 

But there's one further outlier, if this is also combined with Stage 1 having a -3.5/-7db option as in my latest M4 diagram. In that case, an open circuit condition could be 27 Ohm


----------



## JohnH

Gene Ballzz said:


> @JohnH ,
> The Super Reverb is rated at 45 watts, with two 6L6s and tube rectified! Much like the '59 Tweed Bassman, although I'm not sure right now if the Bassman is rated for 2Ω or 4Ω.
> Simply Sharin'
> Gene


Thats much easier then! 45W and 2 Ohms is 4.7 Amps - just about OK!

The Ohm and mH values all factor down in proportion, but as usual, they then have to be stepped to the nearest available values while keeping their ratios to each other under control.

I haven't got the 4 Ohm or 2 Ohm values on the current drawings, but its easy to do if there's a project wanting such a build.


----------



## AtomicRob

For doing a foot switch wouldn't it really be easier/safer using a relay and not sending speaker level signal through the foot switch? Then you could use any old foot switch and cable. This would mean you'd need power but then you could also have an indicator LED for whatever you're switching, which IMHO is a requirement for anything used in live performance - so you can see at a glance whether it's on/off.


----------



## JohnH

hi @AtomicRob , thanks for your comment. I have the 'passive' footswitch on the latest M4 diagram and it has been used successfully by a few guys. I show a LED and battery circuit in the footswitch to give an indicator. Its pretty simple and can be added to any of the designs since only an added jack is needed in the main box

But a relay fed through power in the attenuator could be a step up, and itd be a good thing combined with the power also being used for a fan.


----------



## Gene Ballzz

AtomicRob said:


> For doing a foot switch wouldn't it really be easier/safer using a relay and not sending speaker level signal through the foot switch? Then you could use any old foot switch and cable. This would mean you'd need power but then you could also have an indicator LED for whatever you're switching, which IMHO is a requirement for anything used in live performance - so you can see at a glance whether it's on/off.





JohnH said:


> hi @AtomicRob , thanks for your comment. I have the 'passive' footswitch on the latest M4 diagram and it has been used successfully by a few guys. I show a LED and battery circuit in the footswitch to give an indicator. Its pretty simple and can be added to any of the designs since only an added jack is needed in the main box
> 
> But a relay fed through power in the attenuator could be a step up, and itd be a good thing combined with the power also being used for a fan.



That is some stellar spitballin' right there folks! The power is already there for the fan(s) on all my builds and hopefully the power requirements for relays may not be "mission critical" just like for the fans!

Great Thoughts!  
Gene


----------



## Gene Ballzz

JohnH said:


> Thats much easier then! 45W and 2 Ohms is 4.7 Amps - just about OK!
> 
> The Ohm and mH values all factor down in proportion, but as usual, they then have to be stepped to the nearest available values while keeping their ratios to each other under control.
> 
> I haven't got the 4 Ohm or 2 Ohm values on the current drawings, but its easy to do if there's a project wanting such a build.



@JohnH 
This guy at Strat_Talk is kinda chompin' at the bit and I'd love to help him get another of these units into circulation. The beauty here is that I found that the Bassman is indeed also 2 ohms, so that expands the coverage and usefulness of a 2 ohm unit!
Thanks Again,
Gene


----------



## AtomicRob

JohnH said:


> hi @AtomicRob , thanks for your comment. I have the 'passive' footswitch on the latest M4 diagram and it has been used successfully by a few guys. I show a LED and battery circuit in the footswitch to give an indicator. Its pretty simple and can be added to any of the designs since only an added jack is needed in the main box
> 
> But a relay fed through power in the attenuator could be a step up, and itd be a good thing combined with the power also being used for a fan.


Ah, you have the LED - I should have actually looked at the diagram before replying!  The new M4 diagram is great, maybe that one can also move to the front page. Or since we're spitballing here, I'd nominate that this project deserves it's own sub-forum under the Workbench so we can post questions in threads - it's honestly hard to find stuff within a 161-page thread!


----------



## RedHouse59

I would agree that a dedicated "Fender" variant would be most beneficial to your attenuator evolution JohnH.

Maybe an "M2F", with 2 and 4 Ohm (Amp inputs) and whatever outputs as needed/necessary would be good for the Fender folks, but keeping in mind most Fender users don't use the 16 Ohm Marshall type closed back (4-12) cabs because they change the tone and feel too much.

Many (most) vintage Fender amps do not have alternative outputs like modern stuff, most in the 35-55w region are 2 or 4 Ohm load OT's, with the 80-135w amps in the 4 or 8 Ohm load OT's. When I say 2 "or" 4 I mean either not both.
(Leo was big on wiring speakers in parallel)

As a side note, on the Fender "style" amps I build I always use multi-ohm OT's, but I also have vintage Fenders that have only 2 or 4 Ohm out's. (Blackface Deluxe and Twin Reverbs, Vibrolux Reverb, etc)

Last month on post 3,146 you posted a new _all-the-bells-n-whisles_ diagram with component chart, but alas ...(sigh)... no 2 and 4 Ohm variants listed.

(I'm not trying to increase your workload, just chiming-in with my 2¢ worth)


----------



## JohnH

hi @RedHouse59 

I'm actually tuning up my designs for 2 and 4 Ohms. I have posted for 4 Ohms before, and a way to convert down to 2, but not a dedicated 2 Ohm build. Since you're here, let me describe the options and get your opinion on behalf of the Fender community:

M2 attenuators have a core Ohms value that they are designed to work at. Typically, this has been 16 or 8. And by factoring component values (and allowing to step to the nearest actually available), 4 Ohm and also 2 Ohm can be derived.

Usually the best version to build is one to directly suit the most important use in terms of amp output tap and also speaker Ohms. So for use just with a 4 Ohm Fender amp with 4 Ohm cab, it should be a 4 Ohm build. Or a 2 Ohm one if that's the target use.

But from there, we can also work with different cab Ohms, by means of an extra output socket and another part or two. If you look on the second diagram of the main first post, you can see a version with four output sockets, which gives a good tone with 16, 8 or 4 Ohm cabs, with an 8 Ohm core build. But you lose about 2dB more power to convert the output from 8 to 16 or 4.

A similar diagram could work with a 4 Ohm core, with speakers converted for 8, 4 or 2 ohms, and the 2dB added drop applying with 8 or 2 Ohm cabs.

But that doesn't fix the input Ohms. To do that, we can add a group of three added parts to bring the 4 ohm core down to 2 Ohms, or up to 8 Ohms. But these conversions also use up power, this time an added -3dB reduction . Its all fine and actually such has been built and tested by others. But the thing is, if what you really need is a unit for a 2 Ohm Fender, and you build this convertable unit, the first, loudest attenuated level is -7 -2 -3 = -12dB. So take say a 40W amp, its loudest attenuated setting is now reduced -12dB at about 2.5W. Still loud at home, maybe OK for studio but seems to much of a reduction for gigging?. 

So, what do you think about a versatile convertible unit, as compared to dedicated, and simpler fixed Ohm boxes? I think it may be better to build more simpler ones!


----------



## Gene Ballzz

JohnH said:


> hi @RedHouse59
> 
> I'm actually tuning up my designs for 2 and 4 Ohms. I have posted for 4 Ohms before, and a way to convert down to 2, but not a dedicated 2 Ohm build. Since you're here, let me describe the options and get your opinion on behalf of the Fender community:
> 
> M2 attenuators have a core Ohms value that they are designed to work at. Typically, this has been 16 or 8. And by factoring component values (and allowing to step to the nearest actually available), 4 Ohm and also 2 Ohm can be derived.
> 
> Usually the best version to build is one to directly suit the most important use in terms of amp output tap and also speaker Ohms. So for use just with a 4 Ohm Fender amp with 4 Ohm cab, it should be a 4 Ohm build. Or a 2 Ohm one if that's the target use.
> 
> But from there, we can also work with different cab Ohms, by means of an extra output socket and another part or two. If you look on the second diagram of the main first post, you can see a version with four output sockets, which gives a good tone with 16, 8 or 4 Ohm cabs, with an 8 Ohm core build. But you lose about 2dB more power to convert the output from 8 to 16 or 4.
> 
> A similar diagram could work with a 4 Ohm core, with speakers converted for 8, 4 or 2 ohms, and the 2dB added drop applying with 8 or 2 Ohm cabs.
> 
> But that doesn't fix the input Ohms. To do that, we can add a group of three added parts to bring the 4 ohm core down to 2 Ohms, or up to 8 Ohms. But these conversions also use up power, this time an added -3dB reduction . Its all fine and actually such has been built and tested by others. But the thing is, if what you really need is a unit for a 2 Ohm Fender, and you build this convertable unit, the first, loudest attenuated level is -7 -2 -3 = -12dB. So take say a 40W amp, its loudest attenuated setting is now reduced -12dB at about 2.5W. Still loud at home, maybe OK for studio but seems to much of a reduction for gigging?.
> 
> So, what do you think about a versatile convertible unit, as compared to dedicated, and simpler fixed Ohm boxes? I think it may be better to build more simpler ones!



@JohnH ,
In this case I agree that *"Simplerer is Mo Betta!" *It seems a native, core, 2Ω input with possibly the option of an additional 4Ω ouput. We can call it *The Super JohnH.* It might be nice if the 4Ω jack is only 4Ω until the 2Ω jack gets used and then both jacks become 2Ω, paralleled. This would cover most Tweed Bassman amps and Super Reverbs, with at least a couple speaker impedance options. There are a few Fenders that prefer a 4Ω load, but I've seen few requests for that, yet. It seems that the real issue is when we combine multiple input imedances with multiple output impedances, so maybe skip the multiple input choices?
Simply Spitballin'
Gene


----------



## JohnH

Yes Okay, The diagram would be based on the second one on Post 1, with all values scaled down by about x1/4 to convert 8 Ohm core to 2 Ohm. It would need Out3 as the dedicated 4 Ohm output (which in the diagram is for 16 Ohms) and Out 1, or Out 1 and Out2 in parallel for 2 Ohms. 

I have the values now, Ill check again and post today or tomorrow


----------



## Gene Ballzz

JohnH said:


> Yes Okay, The diagram would be based on the second one on Post 1, with all values scaled down by about x1/4 to convert 8 Ohm core to 2 Ohm. It would need Out3 as the dedicated 4 Ohm output (which in the diagram is for 16 Ohms) and Out 1, or Out 1 and Out2 in parallel for 2 Ohms.
> 
> I have the values now, Ill check again and post today or tomorrow



Is "quartering" the resistor values of an 8Ω, M2 sufficient? As I notice, and am extrapolating, that the optimal values for an 8Ω compared to a 16Ω are not exactly half? I'm just guessing that the closer we get to a *"dead short"* (2 ohms), on the amp's output, the more critical specific values might be, from safety and dynamics standpoints? Super Reverbs and Tweed Bassmans are known to run pretty hard when at their *"sweet spots" *so getting this optimized may be beneficially critical. These particular amps are the specific Fenders that seem to run neck & neck with Marshalls for tone and dynamic response and being pushed to their limits. Most other Fender designs seem to be bit more forgiving. I do suspect though, that as time goes along, requests for taming Blackface and Silverface Bassmans will be forthcoming.

On a side note: One cool aspect of this design and DIY/custom build format is that we can provide " semi-generic" designs, as well as specifically targeting optimized designs for specific amps and applications. I am thrilled to have become at least some small part of this process! Designing an attenuator to nicely tame the venerated and well loved Super Reverb and Bassman amps, in their native impedances, would be a wonderful and laudible accomplishment!

Just Askin'?
Gene


----------



## JohnH

Hi Gene

So far as any theory I have goes, going from an 8 Ohm to a 2 Ohm build should be a x 1/4 factor in principle. I don't know of any reason to change that. But in practice, all the values are adapted to hit actual available values.

The ranges of values that I've been using are the most common series that goes 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, then factors of 10 on those. We also use 20 and 30 Ohms in the 16 Ohm builds, and 75, which seem to be findable too

But, when considering values for builds of 16, 8, 4 and 2, all in factors of 2 or 1/2, then the required values drift away from those simple factors since standard values x2 or x4 don't align with others in the range. eg R2A and R2B, at 18 and 22 for an 8 ohm build have to be 4.7 and 5.6 in a 2 Ohm build instead of 4.5 and 5.5. So I start with a best guess and then run the spreadsheet macros to check every setting. Sometimes I then tweak a value or two to make the steps and overall impedances balance as best they can.


----------



## RedHouse59

*Whoa there Gene*, slow down a minute, maybe you could take a _back seat_ on this one good sir?
(no disrespect intended)

When it comes to Tweed/Blackface/Silverface Fender amps the 2 ohm stuff is the *minority* so we DONT want a "2Ω input with possibly the option of an additional 4Ω output" check the chart below and see that there are really only four (4) models with 2 Ohm OTs, there are nineteen (*19*) models with *4 Ohm* OTs and twenty eight with 8 Ohm OTs.

John has already done nearly every permutation of 8/16-to-8/16 Ohm attenuator version already with the great help from you, but I have to STRONGLY disagree with the "_2Ω input with possibly the option of an additional 4Ω ouput_" thing you are chiming-in with, sorry.




All I was referring to (asking for?) was that John just expand on the new M4 with chart entries specifically calculated to include the 4 Ohm and 2 Ohm iterations so people can have that choice without doing all the halving and quartering values thing to get going.

IMHO most folks with Fenders don't run all the permutations of 16 and 8 ohm cabs that John and yourself have worked out so far with the Marshall oriented stuff already so we don't need another one with all that.

.....

John, and BTW got to say I'm not "a Fender guy" speaking on behalf of the Fender community, indeed I own more Marshall and Marshall'esque based amps than Fenders, but I work on all the old amps and have many customers with Fenders so that is my interest in having a M2-F or M4-F version of your designs.

If I may respond to the questions you asked me in post #3,222 above, I agree with all you said "_A similar diagram could work with a 4 Ohm core, with speakers converted for 8, 4 or 2 ohms, and the 2dB added drop applying with 8 or 2 Ohm cabs._" but I would encourage this not to be a _swiss-army-knife_ type combined thing, that only increases the build cost for people. You already have an 8 Ohm (input) version so we could drop that from the spec altogether and it would hurt no one.

I myself, like our good man Gene here, am more interested in builds particular to the amp they will be used for, and I intend to make several of these myself for my own personal stuff, but installed into the amp cabinets. As an amp tech I will likely build a one-size-fits-all or swiss-army-knife box to use in a studio environment when the customers amp is a wild-card just so I can cover the bases, but mainly I would encourage you to focus on the basic M2/M4 iterations designed specifically for a 4 Ohm (OT) and a 2 Ohm (OT) and add that to the latest-n-greatest graphic you post for cunsumption.

The reality of it all is that most Fender amps are combo amps, yes there are a few piggy back (heads/cabs) but the majority (by a long shot) are combo amps and following that _most_ Fender users don't run extension cabs unlike the Marshall type amp users, ya its weird, but it is what it is.

Thank you John (and Gene) for letting me bend your ear(s).


----------



## RedHouse59

If I could add one more thought for you John, I have noticed along this _long and winding road_ of the thread, we all tend to toggle/vacillate between two thought processes, and it leads to your having to re-design everything all the time.
(bless your heart)

1.) The optimized design which is best for a specific amp OT configuration which is best for the people who want to, or don't mind, building multiple units for their amps, but want it _optimized_. These folks usually want the _least parts count_ build that suits their target amp.
(this is me)

2.) The _swiss-army-knife_ (one-build-fits-all) design which is best for people who want to build only one unit (or maybe two) to suit a variety of amp situations. These folks are ok with opting for the additional parts, wiring, box size, and respective heat issues, that can target an array of different amps.

It seems to me it might be easiest for all if you separate these notions, have one main graphic/schem/chart addressing each native OT and maybe call it the "Basic" chart , much like your M4 seems to be (if it included 4 Ohm and 2 Ohm).

Then have an additional graphic/schem/chart breaking things out into the multi-solution, maybe call it a _swiss-army-knife_ type build. In that way people could reference the type build they are intested in more directly. I'm ok with schematics as I've been a electronics tech for 50 years, so I can break things out of complex schems, and add things into basic schems, but your average guy isnt so able to do this and that leads to all the people asking the same questions in a manner they can understand more easily, and you (and Gene) having to keep re-posting the answers, ad infinitum, ad nausium.

I took the liberty of photoshop'ing your M4 graphic ino these below, one is basic optimized attenuator with 2-through-16 Ohm values, the other your M4 "all in" with everything. People can see both, and recognize the differences easily.








(the 2 and 4 Ohm part values are not present)


----------



## Wizardof_Oz

Wizardof_Oz said:


> @JohnH and @Gene Ballzz
> Merry Christmas guys and thank you for the prompt replies despite the holiday season.
> 
> Yes John, it was obvious that the FX requires additional circuits. So, nothing for my first built
> I already ordered parts, incl. a Hammond 1590DE housing.
> Thank you, Gene. Very neat built! Would you have a drilling template for the 1590DE? (How do you come up with such huge drilling holes in a massive aluminum lid??). Do you always build the designs upside down or was this here due to the fan?
> As for the resistors I have a question. I had difficulties to order certain types for the M2 for reasonable money (all are Arcol with 1% tolerance), so I have following variations:
> R1A/R1B: instead of 2x 50W/30Ohms => 2x 50W/33Ohms
> R1B: instead of 50W/18Ohms => 50W/22Ohms (even 50W/20Ohms wasn’t available)
> R8: instead of 25W/5.6Ohms => 25W/4.7Ohms (even 25W/5Ohms wasn’t available)
> Are these variations still in an acceptable range?
> Cheers, Frank





JohnH said:


> Hi @Wizardof_Oz
> 
> Just on the resistors, there's been a lot of fine tuning of the values in the design, and while none of them are mission-critical to 1%, (5% is fine) by the time values start to move away by 10% or 20%, some of the balances of ratios are getting out ot 'wack'. The circuit is based around a bunch of very small decisions about component values, each of which might only be worth a half a dB or so, but by addressing each one, the net result over several stages is kept consistent and close to optimum.
> 
> *R1*
> But I do appreciate that the range of these power resistors from a given supplier is often not quite ideal. But its worth making an adjustment. The values you had to buy are larger than the spec, so Im suggesting to tweak them back down by piggy-backing a larger value resistor across them, but of lower power rating. It could be a ceramic resistor and take up no more space.
> 
> For R1, in the 8 Ohm build, the basic value is 15 Ohm at 100W, and you went for two in paralel at 50W. Usualy 30 Ohm is available, to get 15 in parallel, though its not in the most common ISO series. If you are getting 2 at 33 Ohms, the outcome would be Ohms seen by the amp is 1.5Ohm higher, and the circuit tends to end up a bit high anyway, 9 Ohms or more in an 8 Ohm build. So to get 2x33 down to 15 net, in theory put 165 Ohm across it. 150 would be a great value, net result 1/ (2/33 + 1/150) = 14.9. it will take 10% of the power in the group, so 10W would be OK.
> 
> Another way with R1 would be to swap one 33 for a 27, net result 14.85. The 27 takes a bit more power but still fine.
> 
> *R2B*
> 
> This is the 18 Ohm value, and I assume this is the one you couldn't get? The ratio with R2A (22 Ohm) is important here to set the tonal balance coming out of Stage 1 . If they are the same, treble gets a bit too high. 100 Ohm in parallel with 22 will get 18, a 10W one would do it
> 
> *R8 *
> 
> Usually 5.6 is in the standard range. If you use 4.7 instead, then the attenuation step in the -3.5dB stage is a bit less. There'll be a slightly wider variation in the equalness of overall attenuation steps when using the various stages. With the base design, each step between settings, nominally of -3.5dB, is theoretically between -3.4dB and -3.7dB. With the final stage a bit different using a 4.7 Ohm for R8, the range of different steps is more like -3.2dB to -4dB. Not a big problem, and you may not notice it. There wouldn't be a significant tonal change


@JohnH 
@Gene Ballzz 
Hey John and Gene,
before I start the assembly, would you be so kind to double check on my design and values real quick?


Cheers, Frank


----------



## Gene Ballzz

@RedHouse59 ,
Some very insightful and well considered observations, info and comments, right there and certainly no disrespect taken! Thank you for all of that!

My impetus here was that even though the few amp models that are intended for 2Ω operation are not high in number, especially the Super Reverb and Tweed Bassman seem to be the main models that many folks want to tame the volume of and there are certainl many of them out there in circulation. That, and the fact that have a fellow member at another forum who is specifically looking to source components for a dedicated 2Ω input build with a 2Ω speaker configuration.

I guess my questions are surrounding the optimal rounding up/down of alternative resistor values, especially within each stage's pair. For exampl R7 & R8, optimal would be 8.25R & 1.4R, but my choices are 7R/8R/10R & 1R/1.5R and I want to find the best pairing for consistency? I'm also trying to avoid stacking extra resisitors in series or parallel to achieve specific values. Real estate considerations, dontcha know? The math provides other discrepancies in the pairings for other stages and I'm endeavoring to minimize the "stack up" of those discrepancies. It would kinda suck for the -3.5db stage to actually be -2.5db or -4.5db and the switchable -7db stage to actually end up at -6db or -8db, etc, etc. Then there is also the consideration of any tonal anomalies that may occur thru upsetting the balance/ratio of series and parallel resistance?

Thanks Again,
Gene


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## JohnH

Wizardof_Oz said:


> @JohnH
> @Gene Ballzz
> Hey John and Gene,
> before I start the assembly, would you be so kind to double check on my design and values real quick?
> View attachment 122274
> 
> Cheers, Frank


Hi Frank, that all looks fine to me, given that you didn't need an output for 16Ohm so omitted Out 3 and its parts. Best wishes for the build !


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## RedHouse59

Gene Ballzz said:


> I guess my questions are surrounding the optimal rounding up/down of alternative resistor values, especially within each stage's pair. For exampl R7 & R8, optimal would be 8.25R & 1.4R, but my choices are 7R/8R/10R & 1R/1.5R and I want to find the best pairing for consistency? I'm also trying to avoid stacking extra resisitors in series or parallel to achieve specific values. Real estate considerations, dontcha know? The math provides other discrepancies in the pairings for other stages and I'm endeavoring to minimize the "stack up" of those discrepancies. It would kinda suck for the -3.5db stage to actually be -2.5db or -4.5db and the switchable -7db stage to actually end up at -6db or -8db, etc, etc. Then there is also the consideration of any tonal anomalies that may occur thru upsetting the balance/ratio of series and parallel resistance?
> 
> Thanks Again,
> Gene



Yes, me too.

In the graphics I photoshop'd above, I left the 2 and 4 ohm rows in the chart blank so I didn't skew whatever John (and you) come up with for those. As for me, I'm in the camp of having it optimized for best sound, less parts count, and like you I don't mind building them just for their particular target amp. I want my Marshall to sound like a Marshall and my Fender to sound like a Fender when cranked up ...and attenuated.


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## AtomicRob

JohnH said:


> The ranges of values that I've been using are the most common series that goes 1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, then factors of 10 on those. We also use 20 and 30 Ohms in the 16 Ohm builds, and 75, which seem to be findable too
> 
> But, when considering values for builds of 16, 8, 4 and 2, all in factors of 2 or 1/2, then the required values drift away from those simple factors since standard values x2 or x4 don't align with others in the range. eg R2A and R2B, at 18 and 22 for an 8 ohm build have to be 4.7 and 5.6 in a 2 Ohm build instead of 4.5 and 5.5. So I start with a best guess and then run the spreadsheet macros to check every setting. Sometimes I then tweak a value or two to make the steps and overall impedances balance as best they can.


I know it makes the project a bit less clean and simple but it's not too bad if we have to combine some values, like for example I wasn't able to find a 200W 30 ohm resistor for my build, not in stock anyway, so I got two 16 ohm 100W resistors in series, in parallel with a 470ohm to get 29.96 which I'm sure is close enough. Since it's a current divider the 470 will get less than 1/10th the dissipation, so a 10W part is fine for that. But... a little more messy to wire it all up.


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## JohnH

Hi @Gene Ballzz and @RedHouse59 

Thanks for all your input to the 2 and 4 Ohm scenarios.

After some thinking, I think the best starting point is the M2 as shown in Post 1, which comes as a very simple basic diagram, and also another with some output options. I think that's a better set of parameters than the bells and whistles M4, though elements of that could be adapted. See below for some comments on this.

Here is the table of values to go directly on the M2:





The values should be OK to find (from Asia, try the socoolmart link in my recent parts post - which has the 1.2 Ohm - otherwise, a piggy backed 5.6 on a 1.5 Ohm will get there, but we need something in between 1 and 1.5!). Or at higher price, all resistor values seem findable at Digikey or similar.

Everything is factored down in value, but im suggesting a thicker wire gauge for the inductor of the 2 Ohm

For these Fenders, looks like we are 95% dealing with open backed combos. Open backed cabs have lower resonances than closed back, and the bass resonance circuit as in M4, is probably even less important. Also, the higher currents involved with low Ohms may put more pressure on the footswitch diagram, and Im not ready to offer that at low ohms

With the values above, an M2 build can be targeted at 2 or 4 Ohms, and with Out 4, it should also drive cabs of 4 or 8 Ohms respectively, if thats wanted - which it might not be. Can also go down one step using Out 4, so a 4 Ohm Core could drive 2 Ohms (which it can anyway, this is just a tone tweak)

Any good?


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## Gene Ballzz

Yeah @JohnH ,
It seems the only value not available from china is the 3.9Ω in 25 watts.

Thanks For That!
Gene


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## JohnH

Hi @Gene Ballzz 
But our Asia links do seem to show 4.0 Ohms. That's only 2.5% away, which is within spec for what we need!


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## Gene Ballzz

If you approve of that difference, I'm good to go!
Thanks,
Gene


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## JohnH

Sure! I think 5% tolerance is fine on these, and 4 is only half that tolerance away from 3.9. Also, the ranges Im pickng from are generally in about + 20% steps anyway


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## RedHouse59

Thanks John, and Gene.


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