An old Quad 34 preamp was picking up computer noises and hiss etc when connected up to a Raspberry Pi based audio streamer.
The preamp had bad earthing (so bad it would have failed a PAT test), burnt out tracks on the PCB and a snubber network across the input to the mains transformer that had self destructed.
After the above was repaired the unit worked properly.
Please contact me if you need some electronics or hifi repaired.