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  • EQ generates a power hum

    I have a DBX 1231 EQ and it is generating a power hum. I have disconnected everything between the EQ and the JBL 612's except the output. When the output cable is unplugged the hum stops. It is in both channels. There are no extreme settings on the EQ, everything is around 0db with just some minor adjustments. There is no ground lift on the EQ or the 612's. Any ideas?

  • #2
    Are they both powered from the same power source?

    There are 4 possible (likely) causes...

    1. Noise being electro-magnetically coupled into the eq circuitry via a large power transformer located nearby (like above or below in the rack)

    2. Ground loop caused by a simple difference in ground potential between the two pieces (eq and powered speaker). Lifting pin 1 at the output of the eq should solve this.

    3. A more complicated ground loop that is causing larger common mode voltage to be present on the balanced pair. For this, a 1:1 line level audio isolation transformer with pin 1 of the input and output connected to their respective isolated electrostatic shields within the transformer. I have these available on the shelf if you need some. They are not too expensive either. PM me.

    4. There is a failure within the eq. Not too likely as they are quite reliable.
    -------------------------------------------------------------------------------------------------------------------------------------------------------------------
    Former product development engineer: Genz Benz, a KMC Music/FMIC/JAM Industries Company, continuing factory level product support and service for Genz Benz

    Currently product development engineer: Mesa Boogie

    Comment


    • #3
      One thing I forgot to mention is that when the EQ bypass button is pushed it gets real quiet. Push the button to get it into EQ mode and there it is. It happens in both channels. I have switched cables numerous times, I removed the connection strips from the barrier strips (ground lift), I switched power cables as well as used a different circuit in the shop, it still does it. I think it may be time to call DBX and possibly send the unit in for a checkup.

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      • #4
        Also in the rack is a Pendulum SPS 1 preamp and a Furman rack mount power distro ( I have disconnected that and went to a J box too). The yammy MG124 sits on top of the rack box. When I have tested the EQ I have had everything disconnected (power and audio) and it doesn't change anything.

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        • #5
          Ok, Ok, OK!! I got it.

          Promise not to laugh (I deserve it)
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          I had the mic/line button in the wrong position on the PRX ......Geeeeez I feel dumb.

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          • #6
            Well at least that's an easy fix
            -------------------------------------------------------------------------------------------------------------------------------------------------------------------
            Former product development engineer: Genz Benz, a KMC Music/FMIC/JAM Industries Company, continuing factory level product support and service for Genz Benz

            Currently product development engineer: Mesa Boogie

            Comment


            • Telecruiser
              Telecruiser commented
              Editing a comment
              Yep, but I still feel dumb.












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