[SOLVED] SMSL Amp Buzzes when no Audio is Playing

[CallMeMike]

Got is solved somehow!

First off, I tried suggestion to connect audio out directly to pi audio, NO buzz.

I reconnected hifi berry DAC, still buzzes.  I then replaced the switching supply with linear supply, still buzzes.

I removed the ground isolator that may have been there from the beginning and buzzing stopped (not this one but similiar)

Whatever the cause it is dead silent now, which makes me 

Ah... an (unreported) automotive-sourced ground-loop defeater. It meant that the ground connection from the RPi to the amp was unnecessarily disrupted when the HAT was ON (unavoidable pun :-))