60Hz Hum Problem

Discussion in 'Location Recording' started by dayn72283, Sep 2, 2009.

  1. dayn72283

    dayn72283 Active Member

    Hey all, please bear with me on this:

    I run sound in a small bar/venue, and everything was ghetto-rigged when I took over here. The other day I was in the ceiling running new power cables for the front of house, lights, and an RCA pair from our Yamaha MG20c's REC out to the house stereo so DJs and bands can be heard on the outdoor speakers.

    Due to a weird spacial setup and lack of power outlets, I decided to run the FOH speakers (Active JBL Eons) and the lights (2 par cans) to a single extension cord in the ceiling that drops down to a rocker panel and power conditioner in the rack. Everything is fine when I power up the speakers and lights (now all terminating at one extension cord), but as soon as I switched the house stereo to the AUX input coming from the board, I get a nasty 60 cycle hum from the house stereo, but not the FOH. I unplugged just the lights- no hum. Plugged in the lights, but didn't turn them on- hum. Ran the RCA out to the stereo with the FOH and lights unplugged, no hum. Did the same with them plugged in, but off, got the hum again. The RCA is about 75' and does not run across the power cables anywhere.

    Am I creating a weird feedback loop or something? Any ideas about what could be causing this and how to fix it? I'd prefer to have as few cables as possible coming from the ceiling, as we really have no way to hide them (historic building, can't do anything to the walls or floor). I'm fine for now without running to the outside speakers, but it is something I've wanted to do for a while now, so any help would be greatly appreciated!
     
  2. dvdhawk

    dvdhawk Well-Known Member

    What you've got is a ground loop. You're getting power from so many locations and interconnecting all the equipment from all ends of the building. Long runs of RCA usually don't fare very well anyway.

    There are interfaces you can use to lift the audio ground. And there are hum-eliminating gadgets you can buy too. But the most reliable solution is to try to power all the audio gear from the same circuit.

    Also, if you can, check the polarity of the electrical outlets. Mis-wired outlets can cause anything from hums to a lovely shower of sparks when you interconnect both ends of the building.

    Secondly, it's not a good idea plugging your speakers and your PAR cans into the same circuit. The PAR cans will seriously lower the amount of electrical current available for the Eons (and their subsequent output). And if any of the lighting is on dimmers, that's a whole new can of worms.

    Do some more testing and post your results.

    I hope that helps give you some ideas of things to look at.
     
  3. dayn72283

    dayn72283 Active Member

    I figured it was most likely due to having the pars and speakers on the same cord. The EONs rarely get pushed very hard (back volume pots stay at 1/2) because we are a very small venue (~100 cap). Currently they are all on the same circuit, so I dunno if that is a problem.

    The power is ALL coming from one location at the moment, so I'll look into the grounding situation. I'm not an electrician, and don't wanna die for a soundman gig, so is there anything I can do without frying myself?

    Mondays are the only days I can get in to do repairs, so I think I'll try rerunning the pars to another outlet or rocker switch and report back then!
     
  4. Davedog

    Davedog Distinguished Member

    I'll bet I must have made a zillion dollars opening up those ceiling tiles and fixing that kinda crap. Less
    before the fire....of course.
     
  5. sheet

    sheet Well-Known Member

    All of the audio should be one the same leg, same phase. Your audio should not share power with lights, kitchen equipment, anything with a motor that can back feed and contaminate the ground. since this sounds like funky dive, you will likely have to deal with it.

    You mentioned a long unbalanced RCA to RCA run. That is a no-no. That is unbalanced and unshielded cable. It acts like an antenna, picking up EMFs. So if you have a bunch of power cables, light fixtures, motors, etc around your cables, you can have hums.
     

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