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I have two computers connected through this device:

http://www.audioholics.com/news/press-releases/impact-acoustics-audio-crossplay

to two sets of commercial audio speakers, designed for connection to the computers. (3.5 mm plugs typical to consumer computer sound)

Obviously the idea is to be able to switch the speakers to one source or the other for certain applications but be able to switch them both to one source for other applications.

One of the computers connected through the Crossplay is a MacBook Pro. The other is a largeish "SuperMicro" PC. The signal from the server contains a high-pitched noise in the upper audio frequencies. I haven't put the signal to a counter or a scope but it sounds like it's in the upper audio bands, between 18kHz and 20 kHz if my ear is at all good.

The "Crossplay" itself is powered by what I imagine is the cheapest possible USB-level power supply. Connecting the power supply to different sources, predictably, didn't change the noise on the line. Neither did using different unshielded cables, though the same cables which "had noise" didn't have the noise when connected directly to the MacBook Pro.

Therefore, the noise appears after testing to come from the sound card on the computer. However, since the noise isn't apparent when the speakers are connected directly, they must be getting amplified through the Crossplay.

My EE days are long past. The questions are:

What is the electrical source of the noise? My guess is that since the PC's inner spaces are electrically noisy, the source is the ground plane of the PC. Further, that the very inexpensive and ungrounded circuitry in the Crossplay is part of what amplifies the noisy signal. (My other guess is that the Mac's circuitry is simply better designed, hence, no noise from that source; the Crossplay is still amplifying it but 0x0=0...)

And, how would you filter it? Find a way to ground the Crossplay? Something else?

I'd prefer to just find a way to shield a cable or clamp on some kind of high-pass filter, but I'm handy with a soldering iron and Radio-Shack parts if that's what it takes.

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I predict that someone is going to start throwing around the term "ground loop" soon. –  user3624 Jun 8 '12 at 18:55
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I also seriously doubt that you can accurately detect 18-20 KHz tones. The hearing of most 40-ish year olds top out at 14 KHz, with only a few going to 17 KHz. If you can indeed hear beyond 18 KHz then you are a unique individual. –  user3624 Jun 8 '12 at 18:57
    
I checked it with some reference WAV's, which I guess makes me "a unique individual". As a child I could reliably hear the whine from CRT flyback transformers. –  Rob Perkins Jun 9 '12 at 3:08
    
It's rare, but not unheard of. Old CRT Flybacks were around 15 KHz. I used to hear them, but not anymore. I'm you're age. Also, most people overestimate the freq. of a tone. It's good you checked with some WAV files. –  user3624 Jun 9 '12 at 4:24
    
Do you get the sound from the computer when the computer is connected but turned off? Leave everything connected and on, but power down only the computer. –  user3624 Jun 9 '12 at 4:29
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1 Answer

My guess is that it may be the cheap Crossplay supply, which is coupling (capacitive from the mains side) it's switching frequency onto the signal/PC/mains ground.
I would try using another (preferably better quality) supply of the same voltage rating (with same or better current rating) to see if things improve. Or you could try a ferrite bead clamped on the cable of the current supply if you have one handy.
If you have any means of isolating the Crossplay from the PC (e.g. little 1:1 audio transformer or similar between Crossplay/PC) then that should also do the trick if it is as suspected.

EDIT - guess 2 :-)

If it's not the Crossplay supply, then it's almost certainly the PC supply. In this case I think the easiest way to sort it is probably to buy the little isolator you link to, rather than trying to filter. These things are very handy to have lying around if you are into audio in any way.
In the meantime you could maybe try things like plugging the PC supply into the same socket as the Crossplay if it isn't already, or using shorter/thicker cables.
Audio noise problems can be quite difficult to diagnose, but IME (I have a small recording studio) an isolator will usually fix things, and often the easiest option.
One more thing - if your Crossplay happens to have a differential input (unlikely I guess) then use that.

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It occurs to me that if the PS is coupling then the noise ought to come from all the sources. But its shape changes when there's a lot of CPU and disk activity on the PC and it's present ONLY when either speaker set is switched to the PC input, and ONLY on the speaker set switched to the PC input. I just tried feeding the power from the MacBook's USB through a USB cable with some ferrite beads built-in, with no change. Seems to rule out the PS. –  Rob Perkins Jun 9 '12 at 3:23
    
Ah okay - sounds like you have a scope handy which is good. I updated the answer - I'd go for the isolator. In the meantime you could have a poke around with your scope and try changing your setup as described above. –  Oli Glaser Jun 9 '12 at 4:52
    
I don't have a real scope handy; I have scope software that came with my son's 300-in-one kit. It would be good enough for these tests, but I haven't used it, since the noise is audible I just used my ears. I'll buy the isolator tonight. :-) –  Rob Perkins Jun 9 '12 at 6:03
    
Ah I see. I just read it can be powered via USB cable - just out of interest try powering it from the PC. –  Oli Glaser Jun 9 '12 at 6:39
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