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Odd grounding problem

Discussion in 'Cables & Switches' started by meshman, Sep 29, 2005.

  1. meshman

    meshman
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    I have this odd grounding problem I can't make sense of. The setup is a bit elaborate.

    I have a home recording studio. Keyboards, mixer, rack-o-effects, etc. All works great. I also have a network of 5 Windows machines. All works well. Every sound source in the room connects to a 32ch mixing console, including all of the computers audio outputs. All this works and sounds fine.

    I upgraded one of the machines to include an ATI All in Wonder TV/Capture card. Agaiin, it works fine at first. As soon as I plug the cable TV into the video card, a hum comes through the mixer. I can't isolate where the problem is. Some notes:

    - I'm in Canada :)
    - I've checked every electrical socket in the room. All sockets and power bars are showing proper wiring and grounds.
    - The hum occurs even if all audio sources on the mixer are muted and the mains turned down.
    - The hum is worse if I un-mute the machine with the TV card in it.
    - The cable comes out of the wall from the pole outside the house right beside this machine. If this machine is the only thing plugged directly into this cable, I still have the hum.

    My first guess is that there's an imbalance between the ground the cable TV has and the ground the house's electrical system has. So I figure I'll balance it by connecting the cable and the house grounds together at the source. It does reduce the hum considerably but it's still there. If I disconnect the cable TV from the computer and the formentioned ground bridge, the hum disappears.

    Here's where it gets weird. If I mute the computer and turn the mains on the mixing board up to half, the hum almost completely disappears. If I turn it all the way uip or down the hum increases.

    Any ideas? I'm somewhat stumped. Sorry if this is in the wrong place, it's an odd topic.

    Thanks!
     
  2. demostenes

    demostenes
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    Hi meshman,

    From reading your post i believe u are right in saying that there's a imbalance ground between the cable and the house.

    The coaxial cable that runs from the pole might not be grounded (the external aluminium braid as to be connected to the ground somewhere far along the lines). If u have a "switch" such as the 1 in the picture, try connecting the aluminium case of the switch to the house ground and see if u still have that hum (be carefull and unplug all the sensitive equipment 1st).

    Or u could connect to the pc casing (if its connected to the ground)


    If it doesnt work please say so :)
     

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  3. Troon

    Troon
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    If you can't find an cheap and easy solution, there is a more expensive one: connect your PC to a cheap AV receiver via optical SPDIF, and take the analogue audio output from that to your mixer.
     
  4. meshman

    meshman
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    "If u have a "switch" such as the 1 in the picture"

    Would a splitter be the same thing? I have a 4 way splitter at the moment where it enters the house. (Like I said above, I've tried it without) I grabbed a blank eletrical plug and connected a single wire to the third pole (green) only. I plugged that into the same sockets/curcuit that the studio stuff runs from. The other end of that is connected to the splitter. It has a handy ground screw. Same problem. At the moment the cable is connected to the PC and the ground bridged. It's less than any other configuration but it's there.

    Is a switch something different?

    Thanks for the replies!
     
  5. vex

    vex
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    It may not be the cabel feed at fault.

    We has a similar effect here in the UK and it ended up being the fact that the tv amp in the loft was being powered by a different circuit than the av equipement in other rooms. THe potential earth difference was causing the hum.

    This actually got worse because the amp was on the same circuit as a dimmed light, which of course chops a power source to neutral to decrease its power. Causing a increase in hum as the lights were dimmed.

    Hope that gives you soemthing else to try.

    Chris
     
  6. meshman

    meshman
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    Thanks Vex. I did some web searching and it appears I have a ground loop between the house and the cable. I disconnected the machine in question completely from the audio system. I connected external speakers so I could see if I plugged the cable into the machine, do the speakers hum? As soon as I plug the cable into the PC, I get hum... through the audio system it's completely disconnected from, not the PC speakers. The mixer now indicates a different machine in the room is the source of the hum.

    To solve it, I disconnect the PC from the cable. So a ground isolating transformer is on my shopping list. :) I'll post the results.

    Thanks!
     

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