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Subwoofer hum!

Discussion in 'DIRECTV HD DVR/Receiver Discussion' started by jbauer, Mar 7, 2011.

  1. Mar 7, 2011 #1 of 20
    jbauer

    jbauer Mentor

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    Hi All,

    So I had an HR20 for the last several years, but just got a 3D TV, so I had to upgrade to a later version...

    DTV sent me an HR21, but it was seriously dented and made my subwoofer hum loudly. Decided not to hook that one up, so they send me another box. This time, I got an HR22, which also caused my subwoofer to hum. DTV technician came out yesterday, and we hooked up an HR24-500, and we are STILL getting that hum!

    The weird thing is that it only hums with ONE of the coax cables, not the other one. He is coming back today with a Yoda guru technician that is supposed to be a master on this kind of issue. Hope they can solve it, since that hum is a show stopper for me...

    Anyone else have a similar issue? If so, what solved it?

    - Thanx
    - Jon
     
  2. Mar 7, 2011 #2 of 20
    Chris Blount

    Chris Blount Creator of DBSTalk Staff Member Administrator DBSTalk Gold Club

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    Without looking at the system my first impression would be a ground loop. Something is not grounded correctly which a good DirecTV technician should be able to find.
     
  3. Mar 7, 2011 #3 of 20
    Stuart Sweet

    Stuart Sweet The Shadow Knows!

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    Most hum is caused by noise across the line. Perhaps if you described how the whole thing is set up... such as how the HR gets audio to your receiver and whether it's possible that the line from the HR to the subwoofer is running parallel to the line out to the subwoofer.

    I would suggest swapping cables... DIRECTV's supplied ones are just fine in many cases but the shielding isn't as good as it could be. I would also experiment with Dolby Digital On/Off on the DVR just to rule that out.
     
  4. Mar 7, 2011 #4 of 20
    jbauer

    jbauer Mentor

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    Thanks for the note. The technician said that he didn't think the dish on the roof needed to be grounded... Where might a ground loop manifest itself? I mean, all of my components in my stereo are off the shelf...

    - Jon
     
  5. Mar 7, 2011 #5 of 20
    veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    More than likely this is a ground loop, where the receiver SAT cable is acting as a better ground than the subwoofer has.
    If the hum goes away when the SAT cables are disconnected, then the SAT cables are a better ground.
     
  6. Mar 7, 2011 #6 of 20
    jbauer

    jbauer Mentor

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    The HR24-500 is hooked up to my receiver (Marantz AV7005) via HDMI cable. Any time the HR24 and that coax is connected to any part of my system in a closed loop, I get the hum... I tried hooking up the HR24 HDMI cable directly to the TV, and if I had another HDMI cable going back to my pre-amp, I get the hum...

    The RCA cable for the subwoofer IS right next to the other cables, but when I move stuff around, I'd expect that the hum would change a bit if it were some kind of cable shielding issue. That's not the case.

    Another interesting thing is that the technician hooked up a DTV diagnostic device to the coax, and it read a .1 voltage on that line. Is that normal? He didn't seem to know if it was or not...

    - Jon
     
  7. Mar 7, 2011 #7 of 20
    Stuart Sweet

    Stuart Sweet The Shadow Knows!

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    Oh boy... the tech is wrong. There does need to be a ground on the dish. If there is not a green wire running from the dish to something else on your house like a metal conduit... there's no ground. I would call and have another tech dispatched.
     
  8. Mar 7, 2011 #8 of 20
    jbauer

    jbauer Mentor

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    Hm. Ok, so in that scenario, how do you fix it?

    - Jon
     
  9. Mar 7, 2011 #9 of 20
    jbauer

    jbauer Mentor

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    Hm. He said that the dish was far enough away, that grounding the dish was not necessary... That's not true?

    - Jon
     
  10. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    This is very hard to tell through the internet.
    It's even hard in person, but you'll need to "break the loop" to find it.
    Try removing cables between equipment until to goes away and then connect them one at a time until it comes back. This really is a trial and error process.
     
  11. Stuart Sweet

    Stuart Sweet The Shadow Knows!

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    Jon, I am not looking at your installation, so maybe he's right. But personally I make sure every piece of metal on my roof is grounded. And... now you're experiencing a possible ground hum, so it sure sounds like he was wrong.
     
  12. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    [​IMG]

    This is kind of what you have and what you are hearing is the voltage going <--> between the grounds because one of them is more resistive than the other.
     
  13. Mrmiami

    Mrmiami Legend

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    Far enough away from what and why would the distance away matter at all? Is this dish not mounted on your house or outdoor mast and wired to cables and splitters inside the house? If the dish is hit by a lightning strike it will follow the path of least resistance to get to the ground and without a ground in this case it would follow your cable from the dish into your house right to the splitter and branch off to each of your connected receivers before hopefully reaching the household electrical boxes and reaching it's ground through them. I may be wrong about this and hope that if any part of it is I get corrected but that is how I believe it would go. Unfortunately, without a proper (separate Copper grounding cable) running from the dish directly to the house ground or say as in my case the main cold water feed pipe, you run this risk of frying all your receivers,TV's, etc...and worse yet starting a house fire.

    As for your Sub Hum...is this connected by RCA cables? have you made a single connection to the sub or a stereo (right and left)? how does that connect to your receiver single, stereo (R & L) because if you plug in two rca's on the sub you should use two on the receiver (even if you only have the one plug input you can use a RCA Combiner/splitter) or only use One Cable to the Left input on Sub and the Sub Out on receiver. You may have a bad cable also use a Sub specific cable for they have more interference rejection.

    Hope this helps you out some.
     
  14. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    You've got the right idea, but depending on the strike, even all this may not be enough to keep this from happening.
     
  15. mobandit

    mobandit Hall Of Fame

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    The ground for the dish is not to protect from lightning strikes. It is to allow a safe dissipation of built up static electricity and to ensure that a failure in the attached equipment doesn't "electrify" any metal components on the dish itself.
     
  16. TomCat

    TomCat Broadcast Engineer

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    It sounds like even if you did not have a sub there would still be hum, its just that the sub makes it easy to detect.

    1) make sure both/all components are powered from the same AC circuit. That keeps the possibility of a ground loop minimized between circuits.

    2) a powered sub, fed by RCA cables, is a non-balanced circuit. Hum can be induced. Make sure no AC cords run parallel with the audio cables.

    3) a passive sub, powered by speaker cable, will likely not have the potential to produce hum. Usually the hum is induced earlier in the chain.

    4) try a ground lifter on the AC plugs of various equipment

    5) in the broadcast world we rarely if ever have hum issues, yet we have hundreds of cables running in parallel through computer flooring for long distances. Of course most are balanced circuits, and more and more are digital. But we have found that if you float the shield at the destination, that can cure ground loops for analog unbalanced. You can do this by using a ground lifter for RCA audio, or you can simply cut the shield away from the RCA connector at the destination end (connect just the center conductor). If this results in no audio, you likely do not have a ground loop in that circuit. If it works, that is a dead giveaway that there is a second ground, as the signal finds its signal ground through the chassis ground and completes the circuit (and eliminates the hum).
     
  17. jbauer

    jbauer Mentor

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    Hi All,

    Looks like DTV found the problem. My dish is feeding 3 units in my building. When they disconnected the other units from my multiswitch, the hum STOPPED.

    So they are going to install another DTV dish that is dedicated to me tomorrow. Phew!

    - Jon
     
  18. Stuart Sweet

    Stuart Sweet The Shadow Knows!

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    All's well that ends well. I wonder, was that a legal installation?
     
  19. jbauer

    jbauer Mentor

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    Update: After the decision was made to set up a new dish, the installer found that 2 coax cables that were hooked up to the multiswitch were causing the hum, and that they were actually not connected to any equipment! He disconnected them, and now all is well.

    Yes, I believe that this was a legal installation.

    - Jon
     
  20. TomCat

    TomCat Broadcast Engineer

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    I love a happy ending. I am glad you did not have to resort to any of the workarounds or fixes we proposed.

    If the cables were unconnected, it probably was not a ground loop issue. A 75-ohm shielded coax cable is only really shielded when connected to a 75-ohm termination, such as the connector on a STB or TV, so if the cables ran parallel to any power lines for any distance, that could have induced the hum.
     

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