Yet another ground hum . . .

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  • Brandon B
    Super Senior Member
    • Jun 2001
    • 2193

    Yet another ground hum . . .

    OK here's one I can't figure.

    Got a rack ful omc stuff, and the only place I get any noise is from my sub (ground hum). To make it worse, it is only when my Symmetrix 551E eq iss in engaged mode.

    In other words, no hum at all when the system is used with the eq in the loop power and signal wise but bypass button is pushed, but activate it and hum occurs. Tried adding a cheater plug, tried plugging into a power conditioner. It is receiving an unbalanced output from my preamp, and going balanced from there to my amp. They are on different circuits (amp is on a dedicated 30 amp by itself), but all my circuits are on the same 120 leg of my box.

    Am I looking in the wrong place, and this is coming from interaction with the eq and something else? My sub amp has a ground lift switch, but it also has no effect.

    Thoughts?

    BB
  • David Meek
    Moderator Emeritus
    • Aug 2000
    • 8938

    #2
    Brandon,

    This doesn't sound like a ground loop issue. That'd be pretty constant. It does sound like some time of interaction or short when the switch is engaged. Have you tried running balanced all the way (if possible), or un-balanced all the way, just as a test?




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    • JonMarsh
      Mad Max Moderator
      • Aug 2000
      • 15274

      #3
      Where is the symmetrix located in relation to other equipment in the rack? Is there a possibility of noise pickup in the active circuits from a nearby power transformer? Either that, or in the bypass mode, there's no active circuitry engaged, and in the active mode, the active circuitry is picking up noise from it's own defective power supply.

      Regards,

      Jon




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      • brucek
        HTG Expert
        • Aug 2000
        • 303

        #4
        Brandon,

        I have to agree with David and Jon, that the fault likely lies with the 551, and not a ground loop.

        The 551 in bypass has a relay that completely bypasses the electronics of the unit and hard-wire connects the input to output jacks. The unbalanced to balanced convertion by shorting the connector sleeve(ground) to ring(-signal) is then passed to your subwoofers input differential receivers as opposed to the 551's input receivers when bypass is dis-engaged.
        In this bypassed condition the subwoofer is silent, while still using the 551 only as a connector mounting point. The 551 is still passing the ground and still provides safety ground. Only its electronics is removed. Not a ground loop problem....

        When you dis-engage bypass, you enter the 551's electronics into the mix. There are now many possibilities of faults, but they all point to the 551 electronics, including the power supply may have a faulty filter capacitor (my guess), and any op amp may be faulty...
        In that regard, most of this can be quickly eliminated by disconnecting the processor to 551 cable and only have the 551 feeding the sub amp. Is the hum still there? If so, the 551 likely has an electronics fault.

        I also wonder if you may have a small signal hum eminating from your processor all the time that you also have a filter boosting, and as such, "sounds" much louder when bypass is dis-engaged..

        Anyway, those are a few ideas. Heck it could still be some weird ground loop, although the "cheater" would tend to refute that. But, there are more paths for a ground loop than I've had hot dinners.....

        brucek

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        • ThomasW
          Moderator Emeritus
          • Aug 2000
          • 10934

          #5
          Brandon

          There is a switch inside the 551/552 that will eliminate the hum..

          I had the same problem you describe. Sent the unit back for service. The tech (very nice guy) I talked to called and apologized for having me send it in for service. He said he should have told me over the phone about it.

          Now unforutnately that was 2yrs ago so I don't remember any of the specifics. Pop open the top and look for a DIP switch. Change it's position and see if the hum goes away.

          If you can't find an appropriate switch call their tech support line and they'll walk you through it over the phone.




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          Comment

          • Brandon B
            Super Senior Member
            • Jun 2001
            • 2193

            #6
            Great! Thanks to all of you for the answers.

            The unit is right under my 3600 watt crown sub amp, so it very likely could be picking something up from there. I will move it first, and if that fails, will pop it open and check for the switch.

            I had considered maybe I was boosting an already present hum, but I am actually cutting pretty heavily in the 60 Hz band due to a hump. Also, can't run balanced all the way, as my HT preamp (outlaw) gots no balanced outs.

            Thanks. I'll tackle it this weekend and let you know what the culprit was, location or switch.

            BB

            Comment

            • Brandon B
              Super Senior Member
              • Jun 2001
              • 2193

              #7
              Originally posted by JonMarsh
              Where is the symmetrix located in relation to other equipment in the rack? Is there a possibility of noise pickup in the active circuits from a nearby power transformer? . . .
              Jon
              We have a winner. It was the 2 crown amps bleeding into it. I foolishly assumed pro audio equipment was intended to all be jammed inn a rack together and would be shielded.

              It's now VERY quiet. Spiffy.

              Thanks again guys.

              BB

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