I want to reduce the gain in my Peavey Rage 158.

[bclemons]

I have a Teal Stripe Peavey Rage 158. I'd like to reduce the amount of gain on the SuperSat channel. I have already removed the diodes from the feedback loop and it is still very saturated early on in the pot's range. The pot appears to decrease negative feedback in this circuit. Would it be a better idea to reduce the value of the pot with a resistor, or bleed off some signal to ground with a resistor between the op amp stages? I have included the schematic.

[DrGonz78]

Well I have a question to ask about this schematic too, which might help your quest. R1 and R2 have a 13v supply that drops some voltage down just before the inputs of pin 5 and pin 3 of the U1 op amp. My question is what is that bit of voltage doing to the gain or bias of the opamp? If you look at U1A it has 220k for R2 and U1b has 8.2k for R1. Does this effect the gain at all per stage?

On another note my first thought would be to adjust the values of R12 and R10 to reduce the gain of that stage. I would guess that bleeding signal to ground with voltage dividing would have more impact on gain than altering the value of the pot with a resistor. Although either approach seems like a good idea.

[Enzo]

Gonz, note the power supply is single ended, not split supply, so instead of +15 and -15 to the op amp ICs, we have +27v and ground. This means the inputs and outputs need to sit at about half that, so the +13v to the input is a reference voltage.

In an op amp stage, the feedback resistor ratio is what determines the gain, hence the gain pot is the feedback for the stage.

I don't see any diodes in the feedback loop, but I do see four of them used as limiters across the post control.

Sure you could tack a resistor parallel to R7 to lower the stage gain. try it, easy enough to clip it out if you don't like it. You could also try raising the resistance of R1.

Just my opinion, but I think I'd leave R12 alone. I mean, go ahead and experiment, no harm will come. But R10 and the post control form a voltage divider: 4.7k over 10k, so it reduces signal to the control by a third. if you increased R10 to 10k, it would reduce the signal by half instead. But that would be just the same as lowering the post control, I don't see it as changing the gaininess.

[bclemons]

Oh, I was thinking the diodes were part of a soft clipping circuit. Where, technically does the feedback loop end on that circuit?

[Enzo]

Oh they will clip if the signal grows past about 2v. But they are just across the volume control, the bottom end is ground when the channel is selected.

The feedback is local to the stage. Feedback is pin 1 to pin 2. So C7, R7, VR2, C10, C11.

C15 couples the stage to the following circuits.