Originally Posted By: Lawrie
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.


Lawrie, you're absolutely right about the GPU. Minor brain fart there.

Yes, the AudioBox (PAB) is pretty much the heart of the system. Hadn't thought of it that way until you pointed it out.

I'm pretty good at making and repairing cables. I just bought some heavy-duty molded Hosa brand cables to go from the PAB to the sub and thence to the satellites (FL and FR). I'd really rather not hack into those.

Question is, how do I tell whether the mixer is double insulated? It's a Behringer Eurorack MX 2004A. The mains cord has two prongs, but goes thru an external transformer to a cord with a multipin connector into the mixer. I have no problem mucking about with the short (<1 m) audio cables with what look like Switchcraft 1/4" plugs running from the mixer to the PAB.

Final question for this session is, how risky is it to leave the PC plugged into the 3/2 adapter? I'm sure enjoying the noise-free thing here.

R.

"My primary musical instrument is the personal computer."