Simple Attenuators - Design And Testing

JohnH

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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.

Attenuator M2 Rotary 201219.gif

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

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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!
 

Gene Ballzz

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@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
 

Gene Ballzz

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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 :2c:,
Gene
 

JohnH

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@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!
 

Gene Ballzz

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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!:p 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
 

Lancer X

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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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