The PPIMV (Post-PI-Master-Volume) thread.

[danman]

@bmwfreq...you can replace the nfb resistor with a potentiometer of a lower value than the resistor. Wire it as a variable resistor and not a voltage divider. This will allow you to dial in a bit of extra nfb voltage when you turn the ppimv down. You will want to add a small resistor in series with the pot to prevent you from dialing in too much nfb. From memory I believe that I have used a 10k (linear) pot with a 5k series resistor with good results.

[novosibir]

The NFB resistor value has to be at least twice of the resistor's value below the slope R to ground, otherwise the poweramp will oscillate

[djseban]

Hey there,

In my 2203 I recently moved PPIMV to the front (left from Presence pot above the filter cap with 56K resistors on it), but since this I experience problems with oscillation - when I slowly increase PPIMV pot, the amp starts to do these windy noises/crackling and sometimes pops. Usually I don't go above 25-30%. Does anyone have idea on what might be the culprit? I use very fine shielded wire. I also tried relocating the PPIMV pot to the back - problem totally gone, dimed the pot without an issue. I'd be grateful for any insight on this. I thought about interference with filter cap and presence pot, but I am not sure, maybe the wires going to swampers (5k6) on EL34s are too long?

[danman]

The wire leading from the board to the presence pot can cause problems in a stock amp if it's not routed properly. It may be that your PPIMV pot being so close to the presence pot is causing the issue. It's weird since you did use shielded wire but that's the only thing that I can think of.

[ESR]

Hey there,

In my 2203 I recently moved PPIMV to the front (left from Presence pot above the filter cap with 56K resistors on it), but since this I experience problems with oscillation - when I slowly increase PPIMV pot, the amp starts to do these windy noises/crackling and sometimes pops. Usually I don't go above 25-30%. Does anyone have idea on what might be the culprit? I use very fine shielded wire. I also tried relocating the PPIMV pot to the back - problem totally gone, dimed the pot without an issue. I'd be grateful for any insight on this. I thought about interference with filter cap and presence pot, but I am not sure, maybe the wires going to swampers (5k6) on EL34s are too long?

I hope Danman's response it helpful to you. On a different note, I've been curious for a while about how a PPIMV would sound on a Master Volume circuit. So, am I correct in assuming that you have your PPIMV wired in as an *additional* master on top of the original stock master on the front panel? In other words, you've got a preamp gain knob, an original master volume control, AND a PPIMV... yes?

So, if this is the case, I've always thought that you can use the preamp gain knob to adjust how much "sizzle" you want, while using the "stock" master volume to slam your phase inverter with the full onslaught of the preamp... then use your PPIMV to dictate overall volume output of the amp...

Is that how yours works? Good results? Details?

[novosibir]

@ djseban - you haven't specified, what kind of PPIMV you've installed, what pot value.
Keep in mind, that a PPIMV's pot value of 2*500K is much more sensitive and hence much more prone for cross talk or picking up stray fields than a 2*250K pot like i.e. the LarMar PPIMV

And yes, keep an eye to the routing of the violet Presence wire and keep it far away of the PPIMV's wiper and their cable as well as far away of the OT's primaries.