Metropoulos Forum

This is used on lots of audiophile amps like TWrecks and Matchless amps. Has anyone tried this and if so how does it sound?

http://uploads.bestupload.com/redir/68659.bmp

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Haven't tried it, but it looks interesting. And makes sense.

george

I used it in a TWreck clone I just finished 2 days ago but I havnt opened the amp up yet so I dont really know how it sounds. Its definitely easier but I was hoping someone could give me some cons as compared to the 'Marshall' type PPIV masters that ground out the signal as opposed to this one which cancels out the two PI signals. One thing I have noticed at home so far is it doesnt cancel out all the signal. Turned down it still passes some signal through though its very low. Maybe a higher value pot, say 4M, would work better. One pro I see that I like is its very easy to turn off and out of the signal path. You just break the path at one side. SPST switch.

here is one PPIMV I got from Mark Huss on the JTM45 group forum here is the post.......

There are 2 pdf's for the PPIMV .... the rich mod and bruce
> collins' .... the difference is that bruce's has a resistor and cap
> after each pot (output) ..... is this required to keep the negative
> bias voltage off the pot? .... which one should I use? ..... thanks

Actually, I don't like either one. Bruce's will force the PI to try to
drive most of it's full output into ground at low MV settings, and
Rich's has the DC bias groing through the pot (which tends to cause
scratchy noises before long).

and

So the junction of the pots is going to ground and not the bias feed?
>

Yes, that's the reason for having four caps. Two keep out the PI DC
and two keep out the bias DC. The MV can then be hooked up like a
normal dual volume control.

here is the drawing he put up...


then I used the rich mod drawing and redid it for this mv

tooloud wrote:here is one PPIMV I got from Mark Huss on the JTM45 group forum here is the post.......

There are 2 pdf's for the PPIMV .... the rich mod and bruce
> collins' .... the difference is that bruce's has a resistor and cap
> after each pot (output) ..... is this required to keep the negative
> bias voltage off the pot? .... which one should I use? ..... thanks

Actually, I don't like either one. Bruce's will force the PI to try to
drive most of it's full output into ground at low MV settings, and
Rich's has the DC bias groing through the pot (which tends to cause
scratchy noises before long).

and

So the junction of the pots is going to ground and not the bias feed?
>

Yes, that's the reason for having four caps. Two keep out the PI DC
and two keep out the bias DC. The MV can then be hooked up like a
normal dual volume control.

Im quite familiar with those PPIVMVs tooloud. Ive read in TUT1 Oconnor thinks the Rich version is unsafe for the very reason you said with the bias running through the pot and he said that about the tendancy to get scratchy. Most people here seem to prefer the sound of that one though. Or maybe there hasnt been enough people here whove tried the one with the added caps. I was curious if anyones tried my version which achieves signal attenuation by phase cancelation by both PI signals which is a much more elegant master to have there but it for some reason may not sound as good as the others. I dont know.