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Thread: 60 cycle hum in audio equipment.

  1. #1
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    60 cycle hum in audio equipment.

    I have a student in my code refresher class that is looking for some help/advice for a customer. The customer is in a fairly new church building. The audio equipment for the church has a pronounced 60-cycle hum. There are two components that seem to be causing problems, a sound mixing board and an amplifier. This equipment is fed from the main service panel and the grounds and neutrals appear to be installed properly. They temporarily plugged the equipment in using 3-wire adapter plug which removed the equipment ground from the units. With this removed the hum is gone. Since this eleminated the hum, I suggested to my student that isolation transformers be installed for the equipment. They bought two isolation transformer and installed one for the amplifier and one for the mixing board and still had the hum. Any suggestions as to what they can try next?

  2. #2
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    Sounds like the equipment might be using the ground for a current path? If so, they have a ground loop?

    This is beyond my expertise, but if I was involved, I'd take the cover off of the equipment and check the power supplies.
    Lou (wannabe economist)

    If you relentlessly pursue perfection, you will eventually catch excellence.

  3. #3
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    And he needs to realize it's beyond his expertise too. Too many times churches rely on help from the congregation or from other unqualified people with deadly results. He may be an electrician but that doesn't make him a sound technician otherwise he wouldn't be asking what to do.

    I would strongly suggest that he not play with this any more and contact a sound contractor to safely handle the problem.

    -Hal

  4. #4
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    I often put all sound equipment on the same phase in a panel. This will help in some instances.

    I second the idea, that a qualified electrician with experience in sound look at the job. I dread church jobs more than any, because there are a bunch of church people with no knowledge and great plans, who I have to do battle with on each job.

    I was part of a design team of engineers and architects who got fired from a job, because we wouldn't let the preachers wife design the 50,000 square foot floor plan. She had no experience, no education in architecture, but her husband held the purse strings.

    Jim T

  5. #5
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    The most likely cause is a downstream neutral to ground connection, with proper troubling skills a good electrician should be able to locate the Neutral “SHORT”.

    As for placing the equipment on the same phase an often spec, requirement by some manufactures. And if it is spec’d one should comply, but IMO not necessary and comes from misunderstanding how their equipment works. After all most modern electronics operate on DC…

    I have been involved in several churches that have amazing TV and audio systems and after every electrician that attend services there had their shot at solving the issues, I was contacted.
    Last edited by brian john; 11-30-06 at 06:17 PM.
    Brian John
    Leesburg, VA

  6. #6
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    I run into this sort of thing all the time.

    First, put back any equipment grounds that were removed.
    Second, forget the power isolation transformers.
    Thrid, forget about running everything on one phase.

    The magic phrase here is They temporarily plugged the equipment in using 3-wire adapter plug which removed the equipment ground from the units. With this removed the hum is gone. That's the signature of a classic signal ground loop; nothing to do with the supply power.

    Do a web search for "ground loop" pages and study them, then get some quality signal isolation transformers and install them. (I'd even go so far as to suggest that the sound contractor may have "forgotten" to install them...)

    (Some places will have an isolated ground power feed to the mix position.That helps until an inadvertent ground, maybe from using an unisolated outlet for part of the system, messes things up.)

  7. #7
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    One thing I always like to do is run the power for the mix position (FOH) from the stage power (or where the amps are). In a smaller system, this could even be on the same circuit.

    Also, a simple isolation for the signal is to only ground 1 end of the offending audio cable.
    You can't scare me- I have teenagers

  8. #8

    Red face

    Maybe that's why most TV's and Radio's only have two prongs. Maybe some ding bat put the wrong cord on that equipment.

  9. #9
    Join Date
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    Quote Originally Posted by brian john
    The most likely cause is a downstream neutral to ground connection, with proper troubling skills a good electrician should be able to locate the Neutral “SHORT”.

    As for placing the equipment on the same phase an often spec, requirement by some manufactures. And if it is spec’d one should comply, but IMO not necessary and comes from misunderstanding how their equipment works. After all most modern electronics operate on DC…

    I have been involved in several churches that have amazing TV and audio systems and after every electrician that attend services there had their shot at solving the issues, I was contacted.
    Agreed. I have seen the EGC used as a neutral--on purpose too. And, believe it or not, this sort of effect occurs in analog ICs as well.
    Don't mess with B+!
    (Signal Corps. Motto)

  10. #10
    Try this:

    http://www.psaudio.com/articles/hum.asp

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