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

[Ingo Debus]

> Am 19.03.2017 um 14:32 schrieb sleepy_dog at gmx.de:
> 
> 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.

To which ground did you connect the audio shield? Audio signal ground or USB ground? Or are these connected in the switch box? I’d keep analog and digital ground separate wherever possible.
Also, does the switch box have a metal case? If yes, to which ground is the case connected?

Ingo