Cap change effect question

[ninemag]

Can someone give me an idea of the result when changing the value of C12 on the attached schematic? Its a BEOD clone from pedalpcb-
Someone recommended changing the 120pf to something like a 1n or 1N5?
Thanks for the help!

[Betty Wont]

Someone suggested you change it, but didn't say why? You can always socket and see what it does.

[ninemag]

Trying to tame the squeal-

[Scruffie]

It'll remove a lot of high end, probably smooth out the clipping quite a bit too.

[midwayfair]

The 120p straddles the output and (negative) input pins of the op amp stage. A small impedance for a frequency between those two pins (negative feedback) will result in less amplification of that frequency. A large impedance will result in a lot of gain for that frequency.

Resistors impede all frequencies essentially equally, but capacitors impede lower frequencies more than they impede high frequencies, and eventually level off and allow all frequencies above a certain amount though. (Resistors also pass DC but capacitors block DC and only pass AC.) The exact frequencies they cut will depend on the resistance involved in the RC (resistor--capacitor) filter they form.

Anyway, 120pF is presenting essentially no impedance to very high frequencies in the feedback loop, and was likely intended largely to just cut frequencies above what can be reproduced by an amplifier (or heard by a human). If you increase the size of the capacitor to 1nF (=1000 pf, almost 10x larger), you will lower the frequencies cut by, I think, a little over 3 octaves. This is almost certain to make the pedal sound darker and suppress some of the distortion harmonics. Whether or not that's a good thing depends entirely on your tastes. However, if it's to prevent a problem like "the pedal squeals when I turn the gain up," chances are there's a different/better solution that doesn't change the tone.

[BoleBezKontrole]

Did you try inserting an inverting buffer in front? Something like the SHO. I think it could help by making the input and output out of phase to stop constructive interference.

[Betty Wont]

Or try troubleshooting the squeal? It shouldn't be doing that in the first place.