Simple Attenuators - Design And Testing

Gene Ballzz

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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
 

JohnH

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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?
 

luxspring

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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
 

JohnH

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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

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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.
 

Gene Ballzz

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@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
 

luxspring

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I
@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
 

JohnH

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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

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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

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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!
 

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