Amp Chord 3350 schematic

From your description it sounds like the problem could be related to temperature. I’ve had luck using freeze spray to isolate problems in electronic equipment. Best wishes.

Thank you for your answer;

Yes,  Chord don't want to share the schema... 

The shema is for my technician. The problem is : my left speakers give some bad noises after 2 hours of use. If i turn off the amp some minutes and turn it back on, everything works normally again

Unfortunately that’s going to be difficult, I’m assuming you have contacted Chord. Some manufacturers are just not willing to share schematics. What is the problem?