When USB is connected, noise on main out

Dragonsf · Oct 2nd 2015

When the KPA is connected to my PC via USB, I can here some noise on the main output, while the input is quiet. The balanced output is not affected. Ideas?
(if I remove the USB cable, no noise at all, USB is direct connected to PC, no hub involved).

DamianGreda · Oct 2nd 2015

I think this is called EMI . Try to use bellow EMI suppresor(s) on your USB cable.
You don't hear this on symmetrical output because thats how it works - signals induced in cables are removed in differential op amp. [(+signal+EMInoise) - (-signal+EMInoise) ]= 2*signal without noise

[Blocked Image: http://hackedgadgets.com/wp-content/uploads/2014/09/How-Ferrite-Beads-Work-EMI-Suppression_4-500x240.jpg]

Dragonsf · Oct 3rd 2015

I tried putting an EMI suppressor on both ends of the USB cable, with no real satisfactory results. I'm getting better results now, when I put one of the suppressors on the cable to my tube power amp and pushing the 'ground lift button'. Now the noise is nearly gone (not completely).

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