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Monitor hum

Discussion in 'Sound, Music, and Intercom' started by supperclubsound, Jan 30, 2009.

  1. supperclubsound

    supperclubsound Member

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    Hey everyone.

    We have several tv's around our club that get cable signals. Problem is that when some of them are plugged in it causes and awful hum in the monitor rig. It sounds like a ground issue.

    Any idea as of why or how to fix it?
     
  2. avkid

    avkid Not a New User Fight Leukemia

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    Is your cable grounded?
     
  3. supperclubsound

    supperclubsound Member

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    it's coax so I guess it would be. Don't know a lot about television stuff.
     
  4. avkid

    avkid Not a New User Fight Leukemia

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    Do you know where it comes into the building?
     
  5. lakota651

    lakota651 Member

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    If I am understanding you correctly; just some, not all, of your TV's create a hum in your stage monitors when turned on. Correct?

    Is the problem in your mains as well?
    Is this a recent development or an existing problem?
    How are your return lines ran? Do they go anywhere near those TV's or their outlets?
    Do you have a diagram of how your system is setup?
     
    Last edited: Jan 31, 2009
  6. supperclubsound

    supperclubsound Member

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    Yes, two of the tv's cause the hum. The snake doesn't return anywhere near the tv's or any outlets.

    You know, I've never taken the time to diagram the setup, sorry.

    oh, the problem is also in one of my front top boxes. It's very strange because all of the amps are on they're own breaker box.

    Thanks for taking the time.

    -wade
     
  7. museav

    museav CBMod CB Mods Departed Member

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    Is the problem when the TVs are 'plugged in' to the cable or the power? Or only with both connected? Does the cable system at all tie into the audio system?

    You might try taking an extension cord and seeing if powering the problem TVs off another outlet, such as the same ones serving TVs that don't have a problem, gets rid of the noise. Not a long term solution but might provide some useful clues.
     
  8. mnfreelancer

    mnfreelancer Active Member

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    What is the nature of the tv's? Are they CRT, LCD or plasma?
     
  9. FMEng

    FMEng Well-Known Member Fight Leukemia

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    This is a classic ground loop problem. At least one of the TV's must be connected to the sound system in some way. Does TV audio feed the console? Break that connection and see what happens.

    If the problem goes away, make the audio connection via an isolation transformer. Jensen Transformers, and ART make them, among others.
     
  10. sounddune

    sounddune Member

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    Howdie,
    Chances are you got yourself a ground loop being completed between the cable and the AC plugs on the problem TVs...they probably share a conduit ground with some circuit(s) that feed your sound system. I 'spect your tv's just have a two wire A/C plug which means that there is a good chance that your cable shield and your tv chasis' are all beign sent to ground through an A/C neutral that is tied to ground or conduit. Like has been already suggested...start by disconneting one cable or one tv A/C at a time and see what happens.
    Ain't audio fun?!!
     
  11. Hughesie

    Hughesie Well-Known Member

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    You can try minimizing the issue by turning the source volume up on the desk and the monitor volume down. Its not perfect but it should give you a bit of relief.
     
  12. DarthFader

    DarthFader Member

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    Here is an easy fix if it is a ground problem.

    Cable drops should be grounded to the utility ground where they enter the building, but my experience is that it usually doesn't happen. The closest ground may be outside the building or even several poles away from the building.

    Very inexpensive matching transformers are available ...used to connect 75 ohm cable TV to a set that has flat wire (300 ohm) inputs. Take two of them, hook the 300 ohm sides together. TV cable input goes on one end, and the TV on the other end. I've had the problem dozens of times in home theater systems and the this fix has never failed me. For convenience I usually use a standard matching transformer on one side, and a push on matching transformer on the other. The push on is the same electronics internally, but made to go the other way. (connecting a coaxial input TV to a 300 ohm antenna) Hook em together, use the push on on the TV input, and screw the cable into the other end and the problem goes away.

    The TV can no longer see the cable ground through the transformers and signal loss through the pair of transformers isn't significant in any well designed TV distribution system. I'm in the TV and home theater business, so buying these pieces in bulk is under two bucks for me, but you could probably buy them at Radio Shack or any other retailer for less than five. It a whole lot cheaper than audio isolation transformers!

    JC
     
  13. museav

    museav CBMod CB Mods Departed Member

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    The cable feed is supposed to be grounded where it enters the building, this is a safety issue and in many areas is a code requirement, so wouldn't it be more professional to make sure the grounding is in place and working properly?

    I think we need some responses on issues such as whether it is the power or cable connections that cause the problem, whether the problem tracks with particular displays or outlets and whether the audio system in any way ties into the cable system before getting into what solutions are appropriate.
     
  14. FMEng

    FMEng Well-Known Member Fight Leukemia

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    What JC says is true, but there is a catch. Not all matching baluns are transformers. Some 75 to 300 ohm baluns do not isolate. The Jensen VRD-1FF does isolate for sure. Jensen also makes some fine audio isolation devices.
    JENSEN TRANSFORMERS, INC. - ISO-MAX® Audio Isolator Products
     
  15. supperclubsound

    supperclubsound Member

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    Thank you for all the advice. When the cable and the s-video lines are both connected it cause the ground loop. very strange.
     
  16. mnfreelancer

    mnfreelancer Active Member

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    What's the s-video source and where is it located relative to the rest of your gear, specifically the amps driving the monitors? Also what type of displays are these?
     

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