Help with Hum in Power Supply Circuit

Discussion in 'The Projects Forum' started by acoles1619, Mar 20, 2011.

  1. acoles1619

    Thread Starter New Member

    Mar 20, 2011
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    Hello Everyone,

    So I am working on a little circuit to supply power to 2 devices at the same time. One requires 12V and the other 5V. I am using a 12V battery for the power, and a 7805 to regulate to 5 volts. These are both audio devices, and they will power up fine, but upon connecting them together with an xlr cable, I get a hum on the headphone outputs. What I find a bit odd is that on both devices the ground for the power input and the ground for the xlr are tied together. Could this be causing some kind of loop? I attached a rough wiring diagram. Any help is much appreciated.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    Hey Welcome

    U have to tell us a bit more about ur project.
    But A battery should not generate any hum. It could be the you are wiring the differential XLR signal that is generating it
     
  3. acoles1619

    Thread Starter New Member

    Mar 20, 2011
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    What else would you like to know? I am using a standard XLR cable, not one I made. So do I need a special cable or something? Thanks.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    I like to see the whole setup.
    I like to know if there is any connection to the mains.
    Can you post pics
     
  5. acoles1619

    Thread Starter New Member

    Mar 20, 2011
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    It's currently all torn down because I was troubleshooting, and I was doing this at work so it is not near me. It Is connected only to a battery for power. Assuming mains means 120VAC. It's a pretty straightforward setup, not sure why I am having problems with it.
     
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    Hi acoles1619,

    The 7805 needs capacitor decoupling at in and out pins. Download the specification for your brand of part and see what is needed. This may be the cause of your "hum" if the 120VAC is not used by your circuit.

    What is the frequency of the hum?

    Regards,
    Ifixit
     
  7. acoles1619

    Thread Starter New Member

    Mar 20, 2011
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    0
    I will try using an input and output capacitor on the regulator to see if that solves the problem. How would I tell what the frequency is? I don't have test equipment other than a multimeter. Thanks for the replies!!
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    The caps are pretty much required, because the regulators will oscillate without them. Figure a 0.1µF on both outputs of the 3 terminal regulators and a 0.1µF and a 100µF capacitor on the inputs.

    You've heard 60Hz hum I'm sure, is it the same?
     
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