[SOLVED] SMSL Amp Buzzes when no Audio is Playing

[normand]

I tried 2 setups, no difference:

1 piZero W with miniboss DAC 
2 piB+ with Hifiberry DAC+ ADC PRO

In both situations, silence in audio track is dead silent, but when I stop Moode playback , and after a few seconds, the SMSL amp start a low volume buzz.

Is there a setting I overlooked that can stop it?  Ideally I keep the SMSL amp on all the time.

Perhaps playing a silent track loop whenever audio stops?

If I leave the amp powered on and disconnect the rca cables, it goes dead silent.  I also tried an audio isolator between DAC and AMP, that had no effect.

Edit: Solved - Not 100% sure but I removed a ground loop isolator and problem is gone.

[CallMeMike]

@normand

if you unplug any IN cables from your SMSL amp and there's no buzzing it means that your having the classic case of a 'ground' loop. The devices on their own have no fault as such. There are electronic countermeasures (pricey), cheap solutions used in automotive industry and some DIY solutions involving having a mains plug having just the EARTH wire connected (no NEUTRAL and LIVE wires) and experimentally connecting it to the GROUND of each device.

I suspect (correct me if I am wrong) that you use switch-mode power supplies to feed 5VDC to the RPi combos... try to make a test using linear PSU.

[DRONE7]

What Mike said and...
both the  miniboss DAC  and Hifiberry DAC+ ADC PRO operate as master...so try running from the Pi onboard audio out and see if the initial fault occurs.

Yes, the audio will be low quality but we are trying to ascertain what variable is in play here. Do that and report.

Bob.

[normand]

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 

[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 :-))