MBP Buzz when Plugged In

Discussion in 'MacBook Pro' started by airkarol, Mar 2, 2008.

  1. airkarol macrumors 6502

    Joined:
    Nov 12, 2005
    #1
    Hey,

    I was doing a live recording last week, and whenever the MBP was plugged in with the power adaptor, there was a huge buzz/hum coming through the house PA and in the recording. This didn't occur when on battery. The setup was the MBP, with an external M-Audio Firewire 1814 (bus powered), with two microphones plugged into it going through a snake (for those who don't know, it's basically multi cable). Those two were not connected to the house PA at all. There were two others, going through the same snake, that were plugged into the mixer, and coming through the house PA. When I unplugged the two microphones from the firewire interface, the buzz went away.

    Any ideas?

    Thanks.
     
  2. gr8tfly macrumors 603

    gr8tfly

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    #2
    Are you using the grounded (3-prong) extension that came with the MBP power adapter?
     
  3. airkarol thread starter macrumors 6502

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    Nov 12, 2005
    #3
    I've tried it with both the 2 and 3 prong adaptors.
     
  4. MarkW19 macrumors 65816

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  5. rezonat0r macrumors 6502

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  6. MarkW19 macrumors 65816

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    #6
    It's not a ground loop, that would be a constant 50Hz hum...wouldn't it?

    The sound I'm getting when plugged in is a lot more random, like sort of interference. Running on the battery reduces it, and curiously unplugging my USB (music) keyboard 100% cures it! So I guess it's related to the USB for me.

    Still, I need my USB keyboard for live use - so, any advice to get rid of the interference?
     
  7. gr8tfly macrumors 603

    gr8tfly

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    #7
    If there isn't a choke on the keyboard's USB cable, try adding one (or replace it with a cable that has one). The choke should trap the noise coming down the USB cable. (The choke is usually the cylindrical thing around the cable ends).
     
  8. MarkW19 macrumors 65816

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    #8
    Good advice, thanks. I don't think any of my USB cables have chokes - what exactly would it look like?
     
  9. rezonat0r macrumors 6502

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    Nov 2, 2007
    #9
    For the OP it certainly sounds like a ground loop ("hum" or "buzz").

    Your issue is being exacerbated by a ground loop - the fact that the noise decreases on battery would seem to prove that. A lot of isolation transformers cut down on the noise floor in general. Is your keyboard purely bus powered? Also, if you are using any unbalanced cables or connectors, that will probably be the main problem.
     
  10. Killyp macrumors 68040

    Killyp

    Joined:
    Jun 14, 2006
    #10
    I'm guessing the snake you were using uses a single ground return? Many of them do...

    If so, this may be the issue...
     
  11. MarkW19 macrumors 65816

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    #11
    No, my keyboard is mains powered. It just uses the USB for MIDI.

    My cables are unbalanced yes, as the outputs on my controller/piano are unbalanced. But, it does just seem like a USB problem though - playing the piano directly into the PA (unbalanced) is perfectly clean, then when the USB is plugged back in, the noise comes back...
     
  12. airkarol thread starter macrumors 6502

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    Nov 12, 2005
    #12
    It sounded like more than one frequency. When the magsafe was plugged in, there would be one sound while the light was green, and once it changed to amber, there would be another, much worse.

    Another thing-- the sound went away when the 2 XLRs were removed from the interface. Would the source of the problem be the computer, or the snake?
     
  13. rezonat0r macrumors 6502

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    Nov 2, 2007
    #13
    Still sounds like there's an imbalance in the ground between your notebook and keyboard, and the power over the USB cable is completing the loop. Electricity is nasty stuff : (

    Both of you should try to pickup a little conditioner/isolator (from Guitar Center or something), it would nip that in the bud. If everything is running XLR that probably isn't the issue, but it only takes one unbalanced RCA or something to cause problems.
     
  14. airkarol thread starter macrumors 6502

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    #14

    Everything in my setup was running XLR, although there were snakes involved, I don't know if they're all sharing a ground inside.
     
  15. rezonat0r macrumors 6502

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    #15
    Nah, those XLRs in the snake will all be balanced. They are transporting your unfortunate noise with pristine quality :D
     
  16. Heston macrumors regular

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    Feb 13, 2007
    #16
    Yeah, it's definitely the ground loop. I used to live over seas and I would get a more pronounced hum because the incoming frequency wasn't as doctored up as it is in the US. I think, too, if your ground is better, you should hear the sound less. Don't most houses just ground to the pipes? Obviously, when you don't have it plugged in, you're not completing the circuit.

    I'm not sure if it's been mentioned, but I do think they have some devices to clean out the sound.
     
  17. airkarol thread starter macrumors 6502

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    #17
    I'll pick up one of those when I have a chance, thanks for the help.
     
  18. MarkW19 macrumors 65816

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    #18
    Yeah, thanks guys...I'll go to Maplins or something (UK).
     

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