Major ground loop that I can't seem to isolate.

Discussion in 'Mac Basics and Help' started by Kingsly, Apr 14, 2007.

  1. Kingsly macrumors 68040

    Kingsly

    #1
    I just recently made a change to my office's sound system. My old setup: Minijack-to-RCA out to a 8 channel mixer, from there RCA out to some M-Audio studio monitors and minijack out (via the mixer's headphone port) to some other monitors with a sub.

    I also ran my iPod into the mixer, via 1/4" TRS. The mixer, Sub and M-audio are all powered (no ground).

    I got some big Bose speakers so that I can get the full range (high from the small monitors, mids from the M-audio, and lows from the Bose) and needed a way to power them. So, I dug up a aiwa receiver (simple, six input stereo, also not grounded) and ran the RCA out from my mixer into the receiver, looped the M-audio speakers through the receiver's OUT, and powered the Bose with aiwa's amp. The top-of-the-range speakers remained connected via the mixer's headphone out.

    Everything seemed to work okay and I was pleased with the range I was getting.

    Today I plugged in my external monitor, a cheap :Hansol 15" LCD. It's grounded. Buzzzzzzzzzzzzz!

    I cant, for the life of me, figure out why the monitor is causing feedback, when my old system worked perfectly fine. The only thing I can think of is since all the audio cables and the VGA and power for the monitor run together (they say not to do that...) that that would be causing a problem, but it was like that before and was okay.

    Help? :(
     
  2. bleachthru macrumors member

    bleachthru

    Joined:
    Nov 13, 2003
    #2
    Try ground lifting the monitor, or plugging it into it's own breaker. This should resolve your issue.
     
  3. Kingsly thread starter macrumors 68040

    Kingsly

    #3
    That's the odd part. The monitor is plugged into a different place on the wall.

    EDIT: reread your post. Yeah, they're all on the same circuit. I guess I'll have to lift it. Is that safe and all? :confused:
     
  4. faintember macrumors 65816

    faintember

    Joined:
    Jun 6, 2005
    Location:
    the ruins of the Cherokee nation
    #4
    You should have the monitor on a different circuit period, as the pre-monitor setup was more than enough for one circuit (in most household instances) IMO.

    If that is not a option you can try something like this instead of trying to lift the ground. I have had some success with this type of product in the past. And yea, lifting a ground loop can fry your gear.:(
     
  5. bleachthru macrumors member

    bleachthru

    Joined:
    Nov 13, 2003
    #5
    If you cannot easily move the plug to it's own breaker, groundlifting can be done safley, here is a link that explains more:
    http://www.audioholics.com/tweaks/c...ground-loops-eliminating-system-hum-and-buzz/
    Also, if this is something that you run into often, or you need to do this as a permanent thing, you might want to look into the cost of an isolation transformer. As Ground lifting should only be done safely and as a temporary solution. Good Luck!
     
  6. stroud macrumors newbie

    Joined:
    Apr 25, 2007
    #6
    Ground Loop

    I too had this ground loop problem when using a LCD monitor (Acer 21" Wide) with my KRK monitors. The hum would only surface if I powered on the monitor.

    I tried everything in my studio to try and find the problem with no success until I started playing with the mains cable attached to the monitor. It was the earth wire in the mains cable! So I disconnected it and the hum went away instantly. I had previously tried powering it from ALL the different circuits in my house but with no success so I have removed the earth! This may seem dangerous to some, but the power supply is very similar to the one I use for my laptop wich only has a two prong lead so (as it is only 12v), I found this the best solution.

    It does worry me a bit that it is not earthed but I unplug it at night, and ocassionally connect it back to the earth to discharge any built up electricity, if there is any.

    I have searched the web also for the dangers of operating a 12v power adapter without an earth connection but have found very little.

    I will post agin if my monitor explodes but it has been two months now and anything is better than that annoying buzzing hum!!! :)
     
  7. ElectricSheep macrumors 6502

    ElectricSheep

    Joined:
    Feb 18, 2004
    Location:
    Wilmington, DE
    #7
    I found This to be a very informative read.

    In summary:

    Best and correct way to get rid of th hum: Use balanced lines and tie the shields to the earth-grounded metal chassis at the point the signal enters the chassis, and at both ends of the line. Do not use the signal ground.

    Second best way: Use isolation transformers on all unbalanced signals.

    Least best way: Use special cable assemblies that let you lift one end of the shield.
     

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