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firedog

Isolating PC from sound system

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I listen in a home office which has my music server and stereo, and also a PC for work/pleasure. Typically I'm at the computer and listening to music from the audio system.

Recently seemed that I'd lost volume (3-6db) in the stereo. Was turning the volume up a couple of notches more than I'm used to.

Couldn't figure out the problem, and then yesterday had to do some serious maintenance on the PC (reformat and rebuild the main C drive contents).

Anyway, this obviously entailed several shutdowns and reboots of the PC. And guess what? Every time the power on the PC went off, the volume of the music playing went up and clarity of sound improved. Every time I pressed the power button to turn on the PC again, the Volume/SQ decreased, and stayed that way until the next time the PC was turned off.

Immediately had a small panic, and started thinking about all the possible expensive solutions: Power regenerator for the Audio system, noise filters on the power lines, installing a separate circuit just for the audio, etc.

Then I talked to 2 friends who are electricians. They suggested 2 likely possibilities:

a)Power suck caused by some problem with the PS of the PC - the PC on its own, they said, doesn't usually draw enough current on a continuous basis to have that kind of effect(again, it wasn't just on PC startup, but for as long as the PC was on).

b)Noise in the system introduced by the PC. Either general electrical/circuit noise, or some kind of interference transferred over to the sound system. Said it may not be a "power drop", but just lower apparent volume caused by noise in the system. Asked if the PC was connected to the stereo. I said no. They suggested, among other things, using an extension chord and plugging the PC in on a totally different circuit in another room to see if that made a difference.

But when I got home, I suddenly realized that the PC WAS directly hooked up to the stereo. I'd recorded an LP about 10 days before,and had listened to and edited the results on my PC, and to the editing played directly from the PC through to the stereo. I'd left in place the cables the went from the analogue out of my PC to the line in on my preamp.

Disconnected the pair of analogue cables...and the problem stopped.

I still don't understand exactly why the PC/pre connection would cause this, but relate the incident here b/c it may help others. Apparently the PC was sending out a signal over the analogue out, even when I wasn't "playing" anything, and even tough that set of "outs" wasn't chosen as the default output.

Computers are noise/interference generators, and you definitely want noise and electrical isolation between them and your sound system if you can manage it.
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  1. Jud's Avatar
    Have any compact fluorescents (or regular fluorescent lights) on while the stereo's playing? Try switching them off and see whether there's any difference in sound.



    Of course you could rapidly drive yourself bonkers straining to hear some imaginary differences (or not being certain if you've imagined a real difference), so this might be a good opportunity to put on that sleep mask and have a spouse/friend do the on/off switching for you.
  2. elcorso's Avatar
    [i]...and the problem stopped...[i]



    @firedog, I'm almost sure the noise source is from SPSU and chips in your PC. The 'common' ground injecting noise to your stereo system.



    That's why there is a lot of implementation regarding galvanic isolation between PC and stereo gear. In my case I have an low impedance ISO transformer on the AC cable that feeds the PC. The best galvanic ISO you can get is with fiber optic connection, but not the best SQ, plus sample rate limitation to 96kHZ.



    Jud comment is right, there is a lot of external noise sources, like fluorescent bulbs, fans, TV, microwave, freezer, AC, etc. Then it is also important to have a separate dedicated line & breaker for the stereo. There are a lot of "power isolators" in the market. Some of them improve the SQ, but others are worst than the noise they filter, since, for example, they cancel the low bass.



    Roch
  3. firedog's Avatar
    I've noticed that the AC or refrigerator compressor cycling on and off can make a difference.



    I'm thinking of getting something simple like the PS Audio Noise Harvesters to add in to several circuits. I've seen that they can make a difference in noise in the electrical system.
  4. 1audio's Avatar
    Is it possible you were connecting the output of the PC in parallel with the main audio source? I don't know enough about your system but the symptoms are those of two paralleled sources, where the output impedance of the PC audio circuit is loading the other audio circuit. When the PC is off the output amp for the audio becomes a higher impedance (but still very non-linear) that won't load the other source signal. Sometimes the internal connections on preamps are murky at best. Many times its possible to "backfeed" audio into a tape out and have the symptoms you described.
  5. ringenesherre's Avatar
    Hi Roch,



    I don't know where you got that idea from, but S/PDIF is not limited to any particular bitrate (and certainly not to 96KHz). The optical Toslink is limited to 125 MHz bandwidth leaving plenty of room (10x) for a 192 KHz signal (using less than 12.3 MHz of bandwidth). So even 1536 KHz should not be a problem.



    Cheers,

    Peter
  6. firedog's Avatar
    I think my preamp does have some leak between sources. It's actually an integrated amp and I'm using the pre-out to feed the power amp. Another reason to get a new preamp, or a DAC with a volume control.
  7. Jud's Avatar
    I think Roch referred to a 96kHz limit on at least some Mac optical outputs, not to any inherent Toslink limitation.
  8. Panelhead's Avatar
    Sometimes injecting noise makes the apparent volume louder. The drop in volume acts like an unbuffered "tape in" input. Usually the tape in and tape out circuits have active buffers to prevent these loading problems.

    If you used a regular input the pre is defective. Plugging in should make no difference with an unselected input. They make shorting jacks to terminate unused inputs. These would load down the preamp more than the computer in your case.



    George
  9. elcorso's Avatar
    Hi Peter,



    In the all the Macs I Know the limit is 96kHZ with the digital output (or line-out output).



    And only: 44.1, 48 & 96kHZ



    From the Mac Mini to the Mac Pro, but if you want more you can get an special audio card for the Mac Pro. Or, the USB output, that right now you can go to 5.6448 MHz. with DSD128, which is the most used interface on Mac addicts.



    I don't know on Windows PCs.



    Cheers,



    Roch
  10. Jud's Avatar
    From http://support.apple.com/kb/HT1596 regarding 15-inch MacBook Pro, dated June 2011:



    The headphone / line output jack accommodates optical digital audio output...up to 24-bit stereo and 44.1-192 kHz sampling rate



    I've been trying without success to determine what the limit is for my MacBook Pro 5,5 (13-inch 2.53 GHz mid-2009).
  11. elcorso's Avatar
    Hi Jud,



    The specs are confusing, at least to me, since they say "original" MacBook Pro, and if you see the ports, the firewire one is the 'old' one: 400?



    Then they talk about analogue capable form 44.1 to 192, but optical (digital), 44.1 to 96kHZ.



    I confess I never tried analogue, but digital. And is capable only to 96kHZ, 88.2 is dimmed in Audirvana, for example.





    Roch
  12. Jud's Avatar
    Agreed, confusing in relation to what era these MacBooks were from. Mine has FW800, so perhaps I'm restricted to 96kHz for optical output. I suppose it will be easy enough to tell either when I get my hands on my computer tonight, or certainly when my new 192kHz-capable DAC arrives in a week or two (I hope).



    Re 96kHZ, that's only mentioned regarding optical digital *in*put as far as I can tell. Digital *out*put from the MacBook Pro is specified as 192kHz.