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

Richie Burnett rburnett at richieburnett.co.uk
Sun Mar 19 21:00:35 CET 2017

Don't connect USB (digital) ground and audio (analogue) ground together because this will cause an earth loop. Then you will get digital noise from the internal workings of the computer appearing on the audio.


Sent from my Xperia SP on O2

---- sleepy_dog at gmx.de wrote ----

>the whole box is of steel, and the TRS and RCA jacks I had inevitably 
>conenct to the case, which in turn, apparently through the rotary 
>switch, connect the USB GND to the case as well, so it looks like I'm 
>stuck with that. The wire I wound around the aluminium foil to fasten it 
>and get contact, I soldered to one of the RCA jacks' GND connector.
>Oh. It could be that the other end of the DIY shielding connects to the 
>opposite wall of the case as well, by touching the audio 2-way switch's 
>case, which, I believe, is electrically connected to the threaded end 
>that is mounted through the front "wall" with electrical contact.
>All audio jacks are screwed into the metal case rather close to each 
>other, so I guess there should be very direct GND paths between those? 
>They would certainly not have to go through "USB territory", judging 
>from my naive understanding of such things.
>>> 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
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