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Need some feedback on a PA issue

Discussion in 'Live Sound [BG]' started by buldog5151bass, May 11, 2019.

  1. buldog5151bass

    buldog5151bass Kibble, milkbones, and P Basses. And redheads.

    Oct 22, 2003
    Last week, I brought my backup head to a gig (Fender Rumble 500). I plugged it into the PA through the DI, and got some background hum through the PA. When I turned the amp on, the sound went away.


  2. DI cable might act as a big antenna like a guitar cable with no instrument?
  3. eJake


    May 22, 2011
    New Orleans
    Sounds like a ground issue maybe?
  4. s0c9

    s0c9 Supporting Member

    Jan 9, 2014
    1964 Audio artist, Fractal Audio Beta Tester
    If it went away when you turned on your amp, why is there an issue?
    Zbysek likes this.
  5. Wasnex


    Dec 25, 2011
    There was another thread on this a few weeks ago where these symptoms were causing a problem because the amp was used in a worship service. The amp was not always on and there was not always an audio tech there to mute the bass channel in the PA.

    What you describe is not unusual. I have a Sony Walkman hooked up to my mixing board and it hums when it turns off as well. But the noise goes away as soon as the unit is powered up. I even have the unit connected to the mixer through Ebtech isolation transformers. I have also encountered this with various amps while working as an audio tech.

    Unless there is some weird circumstance where this is actually causing a problem, I recommend you don't worry about it. Either power up the amp or mute the channel on the mixer, problem solved.

    Some possible solutions: 1.use isolation transformer, 2. lift the ground on one end of the XLR , 3. make sure the mixer and bass amp are plugged into the same AC circuit, 4. check the ground on all extension cords and power strips.

    For the isolation transformer consider the Ebtech HE-2 XLR. EBTECHAUDIO.COM

    Regarding lifting the ground on the XLR. Usually DIs have a switch to lift the ground on the amp, but this is not always the case. Try lifting the amp side and also the mixer side, but not both, using an inline XLR ground lift adapter. LIFTER - Catalog - Whirlwind

    When you plug into different AC circuits the outlets may be on different ground buss bars, which can inject noise into audio systems. I believe technically this is not supposed to be within code, but it happens sometime in old venues when you plug the mixer into the back of the hall and plug the amps into back stage receptacles. I've seen venues where separate breaker boxes existed for various plugs.

    An inexpensive outlet checker can be used to check extension chords and power strips for proper wring.
  6. LowFactor


    Jul 6, 2018
    Totally normal. Line outs will cause noise if they are not powered on. Not gonna try the technical explanation of why cause I will more than likely mess it up. But perhaps one of the smarter members of this forum can chime in with the exact reason this happens.
    s0c9 and Wasnex like this.
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    Primary TB Assistant

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