Active filters – how does this one work?

[Tdub]

I'm tinkering with a Blues Pro overdrive I have laying around and I really need to get a bit more treble/top end presence out of the tone control.

He's used an interesting high pass filter on the second part of the op amp and I'm wondering if anyone can explain a) how this works and b) how do I tweak it to get more top end out of it?
In particular the two resistors and caps together are throwing me a bit - I would understand a bit more if it was as simple as a cap in parallel with a resistor in the ic loop but I'm not sure what the combination does?

Here's a schem (note that the second opamp has its + and - round the wrong way)

[samhay]

I am working from the assumption that the 2nd op-amp doesn't have it's input drawn the wrong way round, as it would be an oscillator if that were the case.

The tone control works against C7 - which could be connected to ground rather than VR - to give you a low pass filter with corner frequency from:
470R vs 22n = 15 kHz down to (470R + 10k) vs 22n = 690 Hz.

The tone control will not have much effect on the second op-amp, which is an inverting low-pass shelving* filter with a corner frequency of ~300 Hz.
That's why it sounds dark. If you decrease the 1.5n cap to somewhere in the 100-470p range, it should sound like a guitar effect. The 10p is just there for good behaviour.

*Shelving in that at low frequency you have a gain of -6.6(ish) and at high(er) frequencies you have a gain of -3.3(ish).
The peak gain of about 6.6 might be enough to clip the rails. If it sounds fizzy, I would increase R7 to 220k or 330k.

[Tdub]

I am working from the assumption that the 2nd op-amp doesn't have it's input drawn the wrong way round, as it would be an oscillator if that were the case.

The tone control works against C7 - which could be connected to ground rather than VR - to give you a low pass filter with corner frequency from:
470R vs 22n = 15 kHz down to (470R + 10k) vs 22n = 690 Hz.

The tone control will not have much effect on the second op-amp, which is an inverting low-pass shelving* filter with a corner frequency of ~300 Hz.
That's why it sounds dark. If you decrease the 1.5n cap to somewhere in the 100-470p range, it should sound like a guitar effect. The 10p is just there for good behaviour.

*Shelving in that at low frequency you have a gain of -6.6(ish) and at high(er) frequencies you have a gain of -3.3(ish).
The peak gain of about 6.6 might be enough to clip the rails. If it sounds fizzy, I would increase R7 to 220k or 330k.

Ha, whoops. My mistake - I got caught out by an earlier schematic that that issue, I thought it was this one.

This is awesome, thank you so much for that...and being gentle on my noobness!

[samhay]

No problem, and happy modding.