[sdiy] (OT) Preventing digital noise in audio out of a "2 PCs -> 1 audio amp" with USB box

[sleepy_dog at gmx.de]

Hey there,

take a look at this:
https://postimg.org/image/h6sd0ds6f/

I took one of those old school USB switch boxes, which use a mechanical 
rotary switch to select one of two connected computers to get the 
connected USB device.
2 PCs are connected, usually only one of them (PSU) active. The 
plugged-in USB device is a keyboard which in turn has a USB jack to 
which a mouse is connected.

To also be able to switch audio from those two PC's sound outputs to one 
audio amp*, I added some audio jacks and a 2x two-way switch. I did not 
have shielded audio wire, so I just wrapped some aluminium foil around 
the audio wires and grounded it.

Still, when I crank up the audio amplifier, digital noise is present.

Can I do anything to get rid of it?


Regards,
Steve

* (a good old technics stand-alone multiple-input amp - which has all 
the other inputs used already for other devices ;-) Which are *not* 
usually on at the same time, neither their PSUs)