Originally Posted By: Ryszard
Lawrie,
1. I should have spelled #3 above. I meant the video card (VPU), not CPU. IT's a Zotac 560 2-Gb card.

Ahh, I would have called that the GPU (Graphics Processor Unit).

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2. Signal goes from back panel USB port—which MAY be USB 3, I don't know how to tell—to PreSonus AudioBox (PAB) USB. Signal also comes from Behringer 16 x 2 mixer to PAB preamps. Signal comes out of the PAB to (a) Behry 4-ch. headphone amp, and (b) JBL PSW-D110 sub ins, and out of Xover to (c) Behry TRUTH B2031A active two-way monitors.

Note: It ain't the speaks. There is abso no noise when the PAB us D/Ced from USB. The mixer has also been positively ruled out as a source.

OK, so the PAB is like the centre point of a star.

Grounded PC to USB I/O port on PAB, Possible double insulated mixer to audio I/P on PAB, OUTPUT of PAB to headphone, OUTPUT of PAB to SUB and thence Speaker system via X'over...

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3. I have no idea how to lift ground in the signal chain. Please say more.

Hmm, if I am correct and the mixer is double insulated then I would open the ground wire in the audio cable between the PAB and the Sub.

If the mixer is NOT double insulated, AND the previous doesn't do the trick, then I would open the ground in the AUDIO cable(s) between the mixer and the PAB.

To open the ground:
Balanced cables - cut the shield wire at ONE END ONLY
UnBalanced cables - cut the -ve (normally shield wire, though the sheld and the -ve may be bonded together - possibly only at one end, in which case cut both) at ONE END ONLY.

As I am skilled with making cables I would just "do it" but if you're not then I would test with cables that are (old) on their way out, or which I have spares for, so as to not be too worried about the damaged/lost cables if it didn't work.

Remember, you are still testing at this stage.

--=-- My credo: If it's worth doing, it's worth overdoing - just ask my missus, she'll tell ya laugh --=--
You're only paranoid if you're wrong!