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It's been a while since I've done analog circuit theory, and I'm trying to interpret the 2nd gain stage sub-circuit in this schematic for a guitar pedal:

schematic

I'm not sure if I'm approaching the circuit correctly, but I noted that if the magnitude of Vout is less than the forward-bias of the diode(s), this becomes a 2nd-order bandpass filter structure, where the high cutoff frequency is controlled by the gain potentiometer.

Does this mean that this is essentially just a bandpass filter that also clips the output?

This article kind of explains the sub-circuit from a different approach, and I can't tell if their analysis contradicts mine or not:

[...] there's a cap in series with the Rin resistor. This forms a high-pass filter with a corner frequency of ~72Hz, so all of your bass will pass through. Then there's a set of LEDs used for soft clipping and a 47pF cap to smooth out some of the treble content. Overall, this looks a lot like the tube screamer clipping section. The major difference is that it uses LEDs (a common TS mod) and the corner frequency on a Tube Screamer is much higher, cutting out a lot of bass (~720Hz).

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Does this mean that this is essentially just a bandpass filter that also clips the output?

Yes, its essentially doing this but with the gain of the opamp. So the output is clipped which is useful if the next stage (or ADC) has a smaller input voltage range.

enter image description here
Source: http://technlab.blogspot.com/2018/07/study-of-diode-as-clipper-clamper.html

The easiest thing to do would be to just simulate that stage in spice (or Lt spice) and do a DC operating point simulation. (I might add one later if I have time).

One way to analyze these is to do a regular gain analysis without the diodes and then draw the point where the diodes begin to turn on, and at that point the output will start to be clipped.

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  • \$\begingroup\$ Thanks! I actually just did the AC analysis in qucs and confirmed the frequency response was as expected, but I haven't done a DC point sim yet \$\endgroup\$
    – Nick Nagy
    Jan 25, 2021 at 22:00
  • \$\begingroup\$ Oh, if you do a DC sim you might want to short C8, just saw that. The DC will be centered around zero because of C8, but that will give you a better idea \$\endgroup\$
    – Voltage Spike
    Jan 25, 2021 at 22:07
  • \$\begingroup\$ In my AC sim the gain magnitude in the bandpass is 46.5 (Rgain + R10 / R9), even with the diodes in place. Is that just because clipping / non-linearities are ignored in AC sweep simulations? EDIT: just re-read the last sentence in your original answer which makes me think yes \$\endgroup\$
    – Nick Nagy
    Jan 25, 2021 at 22:31

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