Loud humming, help!

Discussion in 'Archived Threads 2001-2004' started by Peter Kasting, Jun 17, 2002.

  1. Peter Kasting

    Peter Kasting Auditioning

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    My current frankenstein-like setup has the TV using the VCR as its a/v source, and my (analog) cable TV line and Super Nintendo plugged into the VCR. Separately, I have a pair of speakers driven by a 2-channel power amp, to which my cd player and tape deck are connected. I'd like to hook the VCR's audio outs to the amp, so that I can hear movies (and other things I'm watching on TV) on my speaker system, not just the TV speakers. However, when I do this, I get a loud hum over the speakers. If I disconnect the cable TV connection from the vcr, the hum stops (but then I can't watch TV, only movies). I have all of these components plugged into a single power strip, but I'm not convinced that's a problem since things sound fine with the cable TV cable unplugged. I don't have a cable box, the cable just comes in from the wall. How do I stop this humming? Do I need to get a power conditioner, or can I buy/build a simple circuit that will take care of things, or what? Is there some obvious thing I'm missing?
     
  2. Mike Matheson

    Mike Matheson Second Unit

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    Peter,

    What you're describing sounds like a ground potential problem between the cable and the rest of your system. This isn't at all uncommon (and it is solvable). There have been a number of threads here and over at AVS that go into great detail regarding the cause of the problem, various solutions, and some devices that might help.

    Maybe try searching using keywords "hum cable" or similar.
     
  3. Saurav

    Saurav Cinematographer

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    I believe Radio Shack sells some cheap device(s) that can be used to fix this easily. I think you can do this by connecting a 300-75ohm adaptor and a 75-300ohm adaptor back-to-back, but don't quote me on that [​IMG]
     
  4. Chu Gai

    Chu Gai Lead Actor

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    I believe in the vast majority of cases, Saurav's solution will work. You may also want to consider contacting your cable company to describe your problem. These types of problems are not unknown to them and they often provide a device which they will install free of charge as well as provide several checks to ensure all is well. Of course, someone needs to be home [​IMG]
     
  5. Peter Kasting

    Peter Kasting Auditioning

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    Well, no surprise... my cable company service rep (Cox) had never heard of this problem and had never heard of an "inline attenuator". Looks like it's the Radio Shack route for me.
     
  6. Saurav

    Saurav Cinematographer

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    I don't think the RS devices are inline attenuators. They end up breaking the ground path somehow, maybe because they use transformers for the impedance matching.
     
  7. Mike Matheson

    Mike Matheson Second Unit

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    Peter,

    Seriously, do a search. The RS devices don't seem to work in many cases (mine for instance) and if you correctly described the problem you're experiencing (vs. mentioning an inline attenuator) then they should definitely have heard of the problem before--it's common.
     
  8. Chu Gai

    Chu Gai Lead Actor

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    What did you find worked for you Mike since the Radio Shack approach didn't?
    Peter: with all due respect to your rep, the person sounds like a putz. Just have them send a person over to check your cable. I'm sure that person has a better grasp on things. I believe that what's happening is your cable is on a different ground than the rest of your system, hence the ground loop. There is also the possibility that your cable itself is defective. If the person continues to blow you off, ask for their name and then have them put you through to their supervisor.
     
  9. Peter Kasting

    Peter Kasting Auditioning

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    The RS devices are definitely not inline attenuators.
    I'm reluctant to have them send someone out to check my cable because I'm pretty sure they'll charge me $10 for a service call [​IMG] (haven't confirmed that though).
    The guy and his supervisor both had never heard of this problem. I could have tried to make them pass me around to other people, but I had to go to work.
    I'll try the RS route in the next couple days and see how it goes.
     
  10. Wayne A. Pflughaupt

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    Peter,

    The CATV signal is inducing a ground loop. Make sure the cable feed is properly grounded to the same stake as the electrical service. The service guy can check this for you.

    Regards,
    Wayne A. Pflughaupt
     
  11. Mike Matheson

    Mike Matheson Second Unit

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    Chu Gai,

    As usual I had a hum in my system when the cable was connected to anything. I tried using the RS devices but found most channels to then be unusable (I've not actually spoken with anyone for whom they did work, though I've seen postings to that effect). I researched a bit and found the Mondial M.A.G.I.C. devices and similar ground isolation devices. Hated to pay the $100 and more some of them cost though (later I saw ground isolation devices listed for less).

    Finally managed to talk my cable company into sending some techs out. They didn't believe me that it was a grounding problem at first I did convince them to at least check (and if nothing else, to shut me up). In fact they found there was no ground attached between the incoming cable and my electrical cable. We quickly rigged something and Viola! The hum was gone.
     
  12. Saurav

    Saurav Cinematographer

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    OK, you have a good point. I don't have cable, so my suggestion was based on hearsay, not first-hand experience.
     
  13. Chu Gai

    Chu Gai Lead Actor

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    Mike: you know, you think about a hum and the first thing that should've popped up was ground problems. its really a sad commentary when the consumer has to direct the service personnel.
    Saurav: regardless, a good idea, for when all else fails.
     
  14. Reece

    Reece Stunt Coordinator

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    Peter try the xantech 63400 ground breaker. Ever since I tried these...no more hum.
     
  15. brucek

    brucek Second Unit

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    Actually Saurav is quite correct in his solution. The devices he suggests are 300 ohm to 75 ohm impedance matching transformers and will work fine if connected back to back. There are a couple problems though.

    A lot of the transformers of this type have DC continuity on the shield side of the 300 ohms through to one lead of the 75 ohm wire to preserve the ground path. They are designed that way and as such won't solve the problem. In the case where the ground potential of your cable is different than that of the safety ground at your equipment, you'll experience the "humming" ground loop problem. If the DC continuity of the transformer is preserved, then the problem will still exist. This is why this trick doesn't work for most people. A simple check with an ohmeter will reveal if the transformer you want to use has this feature. You want to use the kind that have no DC short through the device - then they'll work.

    The other small problem is one of insertion loss. These transformers will introduce a small insertion loss, so two together will marginally reduce the signal - no big deal.

    These "fancy" premade cable hum reducers are simply a 300 ohm to 300 ohm transformer with no DC continuity, sold for ridiculous prices......

    Of course, as already mentioned, you have to first ensure that the cable coming into your home is grounded, or you'll have trouble for sure. But many people still experience ground loop hum with a completely and properly grounded system. The culprit is usually through the cable shield potential.

    brucek
     
  16. Chu Gai

    Chu Gai Lead Actor

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    BruceK: great reply. Other than the 300 ohm approach, how could an individual treat this problem say on the outside of the house? Could a ground wire be run to the same point as one's home is grounded? Also are you aware of any inexpensive sources of 'hum eliminators'? Perhaps by a different name?
     
  17. brucek

    brucek Second Unit

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    Chu gai,
     
  18. Mike Matheson

    Mike Matheson Second Unit

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  19. Dennis Oblow

    Dennis Oblow Stunt Coordinator

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    I had good results using the xantec isolator and it is inexpensive with no negative impact on my cable picture
     

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