Simple Attenuators - Design And Testing

JohnH

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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

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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

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Tristan

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

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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

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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

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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

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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:

 

telesto

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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

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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

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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

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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.
 

ZackPlonk

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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:
Screen Shot 2022-05-02 at 18.15.08.png
 

Gene Ballzz

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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
 

ZackPlonk

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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
 

Browneyesound

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(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?
 

Browneyesound

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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.

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.
 

JohnH

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Yes, a bypass if included, must never be operated except with amp off or on standby So not good for a footswitch.
 

Gene Ballzz

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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
 

stickyfinger

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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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