The mixer I've got (Behringer x1204usb) has issues on 5/6 and 7/8 channels, to be no precise in the section closest to jack sockets.
The problem occurs only when gain switch is on -10 and TS jack inserted. In any other condition it seems working fine.
When I connected to that line input socket an audio source (it was guitar multi-effect pedal), the sound was distorted, but mixer didn't clipped. Switch was on -10 position, as +4 was quite.
What I did:
1) traced components and drew a diagram
2) desoldered in+ in- out lead from pcb, the voltage was 2.1 2.9 14.2 V respectively.
3) desoldered those leads of another op amp in mic section. The results were the same. Therefore, that's probably normal operation.
What is the culprit?
PS: Could you please recommend any articles/books on op amps, where their operation are described in detail?