QSC RMX-850 eww the major noise! Can it be filtered?

Discussion in 'AV Receivers' started by scott>sau, Sep 13, 2003.

  1. scott>sau

    scott>sau Stunt Coordinator

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    I finally isolated the noise in my system to my QSC sub amp. The noise doesn't go away when the power is turned off on the amp, it goes away when I unplug it. "Bob, from QSC where art thou?" Can I unplug the internal fan? Is there a capacitor from Parts Express that will eliminate the very loud 60Hz hum? TIA for any remedy. It's a good amp, but man is it loud!
     
  2. RichardHOS

    RichardHOS Second Unit

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    Sounds like you have a ground loop problem. As often as I've had them, and as successful as I've been in getting rid of them, I'm not sure there is a straightforward way to tell you how to fix it without much more detail about your system.
     
  3. scott>sau

    scott>sau Stunt Coordinator

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    Thanks for the reply. A ground loop seems to happen if there are more than one earth grounds in the system. I may be wrong. I use three Hafler power amps with a Lexicon prepro and DVD player. I tested each peice and no noise save the QSC.
     
  4. Gregg Loewen

    Gregg Loewen Video Standards Instructor, THX Ltd.
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    You need to have all of your gear grounded to the same spot / location. It has nothing to do with how good your other gear is.

    Regards

    Gregg
     
  5. scott>sau

    scott>sau Stunt Coordinator

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    Gregg, thanks. It is strange but only my QSC (sub amp) has a three-prong power cord. Do I run wire to each of the components chasis? Then the other ends to a grounding rod into the earth?
     
  6. RichardHOS

    RichardHOS Second Unit

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    Using a "cheater plug" to convert the 3-pronged QSC plug into a 2-pronged one would be the quick fix, but isn't recommended from a safety viewpoint.

    Fixing it "correctly" may require some serious investigation. Since the QSC has TRS inputs for a balanced connection, you may be introducing an improper ground through incorrect coversion of unbalanced to balanced signals. Or, it could be that the QSC being the only amplifier with a chassis ground is the only one that suffers from the ground loop. Or, it could be a combination of the two, or something else entirely.

    Man, this is a hard problem to diagnose remotely. [​IMG]
     
  7. scott>sau

    scott>sau Stunt Coordinator

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    Yes, it is. And thanks Richard for the insight. I will try the cheater plug adapter.
     
  8. Michael R Price

    Michael R Price Screenwriter

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    Isn't it not a good idea to defeat the ground on a high power amplifier? Where is the speaker output current going to go? Through the signal cables and to the ground of the other equipment? (It's possible that the QSC amp is not designed to conduct output current back to earth ground, and that the cheater plug will work fine. But be careful and try other things first.)
     
  9. RichardHOS

    RichardHOS Second Unit

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  10. scott>sau

    scott>sau Stunt Coordinator

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    [​IMG]
     
  11. Kenneth Harden

    Kenneth Harden Screenwriter

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    I would hope QSC would design the amp to run in a less-than-ideal situation - as in, it might not be 100% OSHA, but it should be fine - I hope... [​IMG]
     
  12. Greg_R

    Greg_R Screenwriter

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    If an internal power wire shorts against the case then you will become the ground (if you touch the case). The ground wire prevents this (i.e. it provides a better path to ground). Pro amps are more likely to implement this robust scheme (vs. ignoring the ground problem) since they are abused more.

    99% of ground loop problems can be traced back to Cable TV or Satellite TV cables. Yes, I know your sub is humming but the problem is originating somewhere else. (the 60hz hum is merely a result of the problem) Unplug the CATV and Sat cables and see if that fixes the problem (powering off the equipment is not a test).
     
  13. scott>sau

    scott>sau Stunt Coordinator

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    Greg, that was it! When I unplug the coaxial f-fitting (from the cable provider feed) that connects to the TV (direct-view) the noise is silenced. Dead quiet. Let me tell you the amplifed hum was heard in all nine of my speakers (2 15" subs and the sats). What can be done now short of getting a line conditioner and run the coax thru its filters? Would removing the ground on the slipper rectify anything?
     
  14. Michael R Price

    Michael R Price Screenwriter

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    Oops. This is why I am not so hot in English classes. It isn't a good idea to defeat the ground, indeed.

    You can try a ground loop isolator/signal transformer on the cable line, but I know someone else may have a quicker solution.

    I'm glad to hear the problem was not the big amplifier. I have a little ground loop hum (thankfully not enough to hear, really) between my two DIY monoblock amplifier channels, and there's no way in hell I am disconnecting the ground. (I may soon try disconnecting the ground on one of the signal cables, though. I know some audio cables have the ground connected at only one end.) They are unbalanced amplifiers and the speaker negative lead is connected directly to earth. I have a feeling some other amplifiers work this way and in the event of a fault it could be dangerous if the third AC prong was not connected.
     
  15. scott>sau

    scott>sau Stunt Coordinator

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    I wanted to thank everyone who helped me with this issue. The QSC third prong is grounded. I am running the amp with 1/4" phono plug-to-un-balanced RCA sum adapters. I will try experimenting with the ground going to the cable splitter this weekend. Funny that intense hum was caused from the cable line.
     
  16. Greg_R

    Greg_R Screenwriter

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  17. scott>sau

    scott>sau Stunt Coordinator

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    Greg thanks for all the insight.
     

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