This is about the Keeley Compressor Plus pedal. I have 5 pedals from different brands, 3 of them does the same, but I need the Keeley the most.

It suffers from terrible intermodulation. Becomes audible when you play intervals around frets 20-24. No matter what interval you play, you hear a 3rd phantom note appearing, sometimes being distorted. The note is always lower than the fundamentals of the interval.

Here is an example, I picked in a manner to exaggerate the problem:

I hear the phantom note in the example as 540Hz. This was later confirmed in Spectralayers, it is there exatcly at 540Hz:



Beating outruled. f1 - f2 = fbeat I am fretting A#5 E6, That is 1318Hz - 932Hz = 386Hz.
Very likely intermodulation instead: 2f2-f1 = 1864-1318 = 546 Hz (540Hz in my case due to the intonation of the guitar).

Any advice where the intermodulation happens? How to solve the problem? Input gain is OK, measured, the intermodulation happens even if the input signal is as low as 5mv Vpp.

Here is the schematic:
https://aionfx.com/app/files/schematics/keeley-compressor-plus-trace-schematic.png

 

Sounds like a question for the professionals over at https://forum.pedalpcb.com/