Home made Attenuator questions

[Regis]

bump for the bypass cap question

[Steve Conner]

There's not really any "correct" place to put it in this circuit, but between pins 2 and 3 doesn't seem any worse than any other place. The only drawback is that the bypass cap kind of shorts the amp to ground when the pot is turned near to minimum. For this reason (and to help the bypass cap generate more treble) you probably want to put, say, a 8 ohm resistor in series between the L-pad wiper (lug 2) and everything else.

[Regis]

There's not really any "correct" place to put it in this circuit, but between pins 2 and 3 doesn't seem any worse than any other place. The only drawback is that the bypass cap kind of shorts the amp to ground when the pot is turned near to minimum. For this reason (and to help the bypass cap generate more treble) you probably want to put, say, a 8 ohm resistor in series between the L-pad wiper (lug 2) and everything else.

Hi Steve, thanks for the reply. Is this what you mean?

Will the 8 ohm resistor affect the inpedance the amp sees? And I assume it will have to be a pretty hefty wattage too?

Thanks,

Regis

[hasserl]

I think you've got the L-pad connected incorrectly. Shouldn't the amp + go into the L-pad but not continue on to the speaker, then terminal #2 would go to the speaker + terminal, and the amp - terminal would connect to both the L-pad terminal 3 and the speaker - terminal. Basically the opposite of what you've got. The I'd put the by-pass cap between term's 1 & 2, just bridge it across the two.

Like this:

[Regis]

I think you've got the L-pad connected incorrectly. Shouldn't the amp + go into the L-pad but not continue on to the speaker, then terminal #2 would go to the speaker + terminal, and the amp - terminal would connect to both the L-pad terminal 3 and the speaker - terminal. Basically the opposite of what you've got. The I'd put the by-pass cap between term's 1 & 2, just bridge it across the two.

Like this:

Hasserl, thanks for the reply. I think you are right. Here is the schematic that came with the Lpad, I scanned it off the box:



That is what I followed to do my hookup. It's the same thing you diagrammed (thanks) but pin 1 an 3 are reversed. Will it work if I swap pins 1 and 3?
If I hook it up like your diagram I guess I'm worried that the direction of the pot will be reversed, that is, turning it UP will decrease the volume and turning it DOWN will increase it.

Also, what do you think of steves idea of hooking an 8 ohm resistor between pin 2 and everything else? Thanks for your help.

[hasserl]

Regis, I guess there's some confusion in that the schem's aren't really schem's, they're diagrams. And they don't show which direction the pot is facing, making it difficult to determine exactly what they want. They also don't indicate pos/neg on their drawing.

I'm just going from memory here, it's been a couple of years since I messed with one. But I do remember connecting it up as I've drawn. And in the case of my diagram the shaft would be pointing up, opposite the terminals.

With the way you had it drawn the by-pass cap would shunt hf away from the speaker and to ground. The way I have it drawn the bypass cap will direct hf past the L-Pad to the speaker, emphasizing the highs. I don't see any advantage to adding a series resistor, it would alter the impedance of the circuit, something I don't think is good. And it would need to be a rather high wattage resistor also, high enough to handle all the current flowing thru it, with reserve.

[dai h.]

there's a thread on the twreck (amp garage) site showing a layout for an Air Brake with parts values, etc. denoted:

http://ampgarage.com/forum/viewtopic...=1878&start=30

if I'm reading it right, it looks like it'd be easy to do the same thing since it's mainly just resistance/ors (and one cap for "Bedroom" mode). Basically seems to be two parallel Rs across hot and ground making up the load, with one R fixed and the other R made up of a variable R and the spk. load in series. I guess you could use a power rheostat in place of the fixed R w/taps as well to get increased control over the range also.

[Regis]

Hasserl, I breadboarded the circuit you supplied and it worked great. The Lpad gets pretty warm but I have an idea about that. I tested it with my BF Bandmaster and it sounded pretty good.

I used a 6.8mf bypass cap but I may experiment with that because it was a little fizzy.

Another thing is that this Lpad will totally shut off the signal when you dial it all the way down, I may have to figure out a stop of some kind to prevent that. Even though this Lpad is rated at 100 watts somehow I don't think it would stand up to a fully dimed Bassman at low settings. That's not what I was thinking about using it with anyway, I intended it for my Kay 720 which is around 20 watts.

Thanks for everyones help, I'll post pics and maybe clips when I get the whole thing done.

regis

[hasserl]

No, I'd highly recommend against using the L-Pad like this with your Bassman, you'll likely fry it and the output trans will follow. You started this thread saying you were building this for your Kay amp, which the L-Pad should be fine for. But not with the Bassman. It might do just fine, but it might not. And if it frys while under load the results could be really messy.

[bob p]

there's a thread on the twreck (amp garage) site showing a layout for an Air Brake with parts values, etc. denoted:

http://ampgarage.com/forum/viewtopic...=1878&start=30

The taps on my double-secret TW attenuator schematic are similar to that layout, but are at mildly different values: 4.3, 10.6 and 17.4.

If you wanted more adjustability, you could easily add additional taps on the 25 ohm 100watt variable resistor at virtually no cost.

[Analog Assassin]

The taps on my double-secret TW attenuator schematic are similar to that layout, but are at mildly different values: 4.3, 10.6 and 17.4.

If you wanted more adjustability, you could easily add additional taps on the 25 ohm 100watt variable resistor at virtually no cost.

I notice a lot of people use these attenuators for 8 and 16 ohm loads. What about amps like bassmans and super reverbs? Are they safe at 2 ohms? Does it matter to an attenuator like this?

[bob p]

A fellow ampager drew-up a schematic for a generic attenuator and passed it along to me several years ago on the condition that I would not reveal his identity. I didn't ask why, I just thanked him for the schematic and respected his wishes. The schematic been on my web space since 2002:

Generic Attenuator Schematic

As it turns out, this schematic turns out to be the exact same circuit as the circuit layout for the Airbrake that someone else has published recently at the Amp Garage. Dai provided a link to it a few posts earlier:

Airbrake Layout Diagram

While Ken Fisher was alive, many people in the amp building community showed their respect for his contriubutions by not "stealing" his designs, and by not openly publishing all of his schematics. Maybe that's because he was such a nice person who would share his knowledge with other amp builders who were lucky enough to talk to him on the phone.

I think that its a pity that as soon as Ken has passed away, things changed so dramatically. As one example, "Dave Funk" is now making claims on The Gear Page and on the Thunderfunk Web Site that he ghost-manufactured some of the Trainwreck amps for Ken. Its interesting that these sorts of claims would only be made after Ken's death. Perhaps the reputation of ghost-manufacturing Trainwrecks will further elevate Dave Funk's stature among builders now that there's nobody around to refute his claims.

As another example, some guy who posts on the Amp Garage has started selling Airbrake Attenuator kits. Unlike Komet Amps and Dz. Z, that guy probably won't be paying any royalties to Ken's mother.

I guess its inevitable that somebody else would try to pimp the design by selling kits as soon as he thinks that the designer is dead and there's nobody left to stop him. In light of that development, I guess that the schematic is no big secret anymore, so in the spirit of Fair Use, I've posted a link to the schematic as a PDF for those interested in the theoretical aspects of the design.

Looking at the schematic, its not at all complicated. To me, it looks like an example of the prototype circuit for a "minimum-loss pad-matching" attenuator. A similar schematic could be found in any radio electronics textbook. My copy of Reference Data for Radio Engineers, for example, lists that as one of the attenuator designs in the "Attenuators" chapter of the book.

...continued...

[bob p]

I notice a lot of people use these attenuators for 8 and 16 ohm loads. What about amps like bassmans and super reverbs? Are they safe at 2 ohms? Does it matter to an attenuator like this?

To answer Analog Assasin's question, I think that the reason that Ken would have chosen to use a circuit like that one for his Airbrake is becuase it is a simple design that is inexpensive to build. From a design standpoint its definitely not the best possible design for the application. A more compicated design that performs better impedance matching would undoubtedly be safer for the amp. How would it sound? That's an entirely different question.

Other attenuators like the Bridged-T and Bridged-H offer better impedance matching, but they are much more complicated when used in a ladder design. The parts count is very high and the cost is very expensive. For an example of the parts count on impedance-matched designs, check out the Attenuation Page at Adam's Amps. As you can see, the design becomes more complicated when the design requires constant load matching.

The other attenuator options, like ladders that use a series of symmetrical balanced/unbalanced stages for constant load matching have the drawback of being extremely lossy, with insertion losses starting at a minimum of 6 dB -- more attenuation that many guitarists are willing to accept.

The Airbrake circuit may not have the best impedance matching, but it has minimum insertion loss, its cheap to manufacture, and it sounds pretty good.

[stevem]

I saw a "AudioPlex VC 30W In-Wall Music Mono Volume Control" on e-bay. Can these be used as an attenuator for a guitar amp? I have a Fender SuperChamp DX that I need to quiet down a little for evening practicing.

Thanks,
SteveM

[Regis]

I saw a "AudioPlex VC 30W In-Wall Music Mono Volume Control" on e-bay. Can these be used as an attenuator for a guitar amp? I have a Fender SuperChamp DX that I need to quiet down a little for evening practicing.

Thanks,
SteveM

You would be better off building an attenuator like my Ampwell House, the audioplex wouldn't be high enough wattage.

Parts Express has the 100 watt attenuators I used to build the AH, and also here is a 4/8 ohm version I built as well using the stereo 8 ohm stereo L-Pad. The Parts express L-Pads are inexpensive and work fine for lower powered amps.