times it goes, sometimes he does not, noise

Do this for about 2 years now.
When switching occurs every now and then before that he did not "Starts". The Green Power LED will flash and the amplifier pulses a signal to the output, regardless of whether a signal is present or not.
I do not know why that makes it and how we can stop it.

The power supply grounds the case not (ground is missing).
Have always had an unpleasant reason hum.
Did the mass soldered to a ground wire and behold humming away.

Bask small, compact and extent quite well.
Greeting
Falko