Electronic – Odd distortion in op-amp circuit for audio amplification

Well you can modify the linear pot to get an almost logarithmic response by adding a resistor from the CCW side to the wiper. Because of its simplicity this might be worth a try. Here's what the idea looks like: -

For the pedal circuit you need this might be ideal. It's not perfect of course but it reduces the risk of buying linear pots because these can be made "almost logarithmic by adding one resistor.

One thing to remember is that if the design calls for a 1M pot and you have a 1M resistor across wiper to CCW, the end to end pot value will change as the wiper advance towards CW. This would probably mean that you should use a 2M pot and 2M resistor.

Some experimentation is required and if you could post a circuit I'm sure better advise could be given. Pictures gleened from here

EDIT - picture of circuit attached with my best guess at log/lin: -

First thing I notices is the time constant with your input capacitance and 1Mohm resistor - it's 47 seconds or equivalent to a high pass 3dB frequency of 0.0034 Hz.

This might take ages for your inputs to settle - make the input capacitor 47nF is my recommendation - this puts the high pass frequency at about 3.4Hz. Even a 10nF wouldn't be unreasonable (high pass of 16 Hz).

Your op-amp is a TL084 so I'm less worried about input current leakages causing offset voltage problems AND I'm happier with non-polarized caps at the inputs because if it is an electret mic and has a bias voltage on it, the electrolytic (have to be polarized correctly) input caps will become reverse biased and this may indeed be a possible reason why you see a problem.

Regarding your 50k pots, these will potentially give higher noise than say 5kpots and with 5k pots you can reduce R5, R10, R15 and R16 to something like 10ks also reducing noise. Noise is proportional to the square root of resistance so always best to go lower a bit if you can.

C7 and C12 are also problematic in the the signal they see may become reverse polarized. The time constants you are using around these circuits are very large given that those caps are 47uF - go for 1uF and non-polarized. U2 just appears to be a gain of ten stage so lose the centre biasing - 0V is centre and that is just fine assuming your Vcc and Vdd are largely equal and opposite.

You are using TL084 output amplifiers but this is really daft because you have an output pot that will likely introduce an impedance of about 25k (mid point). Just use pots and get rid of the extra 3 op-amps that feed the individual outputs. C9, C10 and C11 are not needed either or if you feel happier with them use non-polarized 1uF caps.