Simple Attenuators - Design And Testing

narnodel

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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

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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.
 
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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.
 
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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).
 
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Bearzooka

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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

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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.


Just Askin'?
Gene
 

JohnH

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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

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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

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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

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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

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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

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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
 

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Gene Ballzz

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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, :welcome: 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! :naughty: 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

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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
 

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paul-e-mann

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Tom, first, :welcome: 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! :naughty: 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?
 
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Gene Ballzz

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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
 
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Gene Ballzz

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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

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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.
 

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