Odd (I think) noise issue

Read everything I can about noise, but can’t get my head around this.

I have 4 Pis
Three of them have DAC+ on running the most excellent Volumio
One of them doesn’t as it’s a dedicate media server. This one runs Raspbian

I have a 6 channel Rotel amp (3 stereo pairs)

The idea is that each Pi feeds a pair of channels on the amp, and that drives a pair of speakers in a different zone of the house

The issue comes when I start using the same PSU. I’ve tried with an Anker USB supply (60w) and a 10A total 5v supply. The symptoms are the same

Whenever there are two (or more) Pis with the DAC+ connected to the Amp and the same PSU, there’s the faint background static odd noise.

If I move the Pi’s with the DACs onto individual supplies (even if I leave one of them on the same supply as the Raspbian box) the noise goes.

If all the Pis are connected to the same PSU but only one is connected to the Amp, there’s no noise.

The noise is present even if the OS isn’t booted. WiFi or wired.

We’ve earthed the amp, made no difference
Split the screen on the phono. No difference
Used different sockets for the amp and the PSU. No difference.

Bit stumped, really.

Any ideas (I’ve got some ground loop isolators on order but I don’t really like them and don’t have a lot of faith)



For future readers, ground loop isolators from Amazon seem to have pretty much eliminated the issue.