Unique Ground Loop problem...please help?

Discussion in 'Home Theater Projects' started by Trenton McNeil, Apr 9, 2003.

  1. Trenton McNeil

    Trenton McNeil Second Unit

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    I have the standard ground loop hum problem in my home theater since adding CABLE to my setup. With the cable box turned on, when I attach the coax, I get a 60 cycle hum in all my speakers, even with the volume turned all the way down. When I disconnect the coax, or simply turn off the cable box, the hum goes away.

    http://www.dplay.com/tutorial/cablehum.html#atten

    I performed the tweak with the 75-300ohm transformers as shown on that site, which completely removed the ground loop hum...>HOWEVER< this reduced my signal too much and made my HDTV channels unavailable, so this is not an effective solution, nor would a -6db attenuator, for the same reasons. I installed a -3db attenuator on the coax, and this reduced the amplitutude of the hum, but did not totally remove it from the signal path.

    So....HELP! I'm not sure what else I can do, but with speakers everywhere and a big subwoofer in the corner, the 60hz hum is quite distracting!
     
  2. Chu Gai

    Chu Gai Lead Actor

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    Well there are devices from Mondial and Jensen that likely wouldn't attenuate your signal, but before you go that route and drop some cash, is there a reason why your cable hasn't been properly grounded outside the home?
    As an aside, some people have reported that when they use a surge device with cable wire capabilities, that has eliminated their problem.
     
  3. Trenton McNeil

    Trenton McNeil Second Unit

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    I actually have a surge protector that works for rg6/59 type cable, but it did not isolate the problem.

    The cable co. ground IS intact....matter of fact, that is how the ground loops is generated. An electrical path is being connected between the outside ground, and my equipment's ground inside.

    A knowledgeable friend of mine (who happens to be an admin on this site) recommended that I use 'cheater' plugs on my amplifiers to remove the hum. I am a tad concerned about the safety of such a move, but at this point it seems like possibly the only viable solution, since inline transformers attenuate my signal too much.
     
  4. Lee Bailey

    Lee Bailey Second Unit

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    Trenton, if your cable is grounded outside at your water pipe, that is NOT where it should be grounded. It should be grounded back at your electrical panel ground. The other cheap solution, is to cut off the cable head at either the interior wall connection or at the cable box end. Prepare a piece of 16 gauge wire, and strip about 1/2" insulation off one end, and when you crimp on a new connector, have the wire inserted to be included in the crimp. Strip back another half inch or less on the other end, and attach it to the outlet cover screw for your AC that the rest of your system is plugged into. This should get rid of the ground loop.
     
  5. Trenton McNeil

    Trenton McNeil Second Unit

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    Lee I've heard of doing that, but I am horrible at crimping. [​IMG] Will try that tonight.
     
  6. Bob McElfresh

    Bob McElfresh Producer

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    Be careful. There are several sources of humm and the fix for one is not the fix for another.

    - Dual amplifiers (one receiver and one self-powered sub for example) can cause a ground loop. A cheater plug or a subwoofer cable with little arrows on it can break this.

    - CATV humm: The CATV coax outside your home are picking up 60 hz humm. This is being fed into the signal-ground on all your equipment. Guess what: all your amps look at the voltage difference between a "signal" and "ground". It does not care if the ground varies, it will try and amplify it.

    You need a "Humm Eliminator" that breaks the direct connection NOT ON THE CENTER WIRE, but on the outer shield. The power-strips with the "F" connectors tie the outer shield to the AC power ground, but...this sometimes does not work.

    I thought I saw Radio Shack carrying a inexpensive unit for this. The other $99 solution is the "Mondal Magic Box".

    Or try searching for "Humm Eliminator" on a search engine.
     
  7. Chu Gai

    Chu Gai Lead Actor

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    FWIW, you'll find the Jensen stuff here.
    VRD-1FF: $60 2MHz to 1300MHz CATV Isolator - F/F (Digital Cable compatible)

    I'd try the other suggestions first as I would not want you to incur expenses needlessly. Jensen though is a highly regarded company and can be reached at (818) 374-5857 where you might want to discuss your particular situation and what you've tried with a specialist. There's also a PDF for the product if you want to see some data. Mondial is a bit pricier.
     

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