First off, this is not a Volumio problem. One of my Volumio systems is connected to an old Tivoli Model One radio via the AUX connector. The sound is great, except for a prominent buzz. I tried upgrading the RCA to 3.5mm jack cable and swapped the (official) Pi PSU - no improvement. Any tips?
Pi 3B, Volumio 3.182, HiFiBerry DAC+ Pro.
I don’t think there is a easy fix. the Null level of both units are not equal, causing a buzz. Even if you would power then by the same PSU it will remain. using a BT transmitter can do the trick as this will be an galvanic separator between them. (not sure how to proper translate this one…)
had a simular issue with 2 DAC’s on a cheap RCA switch. Couldn’t get it solved. Well I got it solved by using the pre-amp part of the NAD C740, but that was a bit to much pre-amp’s in the system. So replaced one DAC and pre-amp with a NAD D3045. 
This might spur me into and earlier idea to retrofit the Tivoli with a Pi, DAC and amp and strip everything else out except the speaker. Maybe put a little screen where the tuning knob is.
Sound like o nice project.
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