Guitar preamp to unplugged MacBook has hum

Discussion in 'General Electronics Chat' started by blah2222, Aug 10, 2013.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    33
    Hi all,

    I wanted to get started with basic guitar recording in GarageBand on my MacBook Pro, so I picked up a 1/4" to 1/8" adapter to access the line-in port.

    Reading Apple's spec, input resistance was stated at minimum 20K so I figured I would need to build a quick little JFET preamp to give my signal a bit of a boost and lower its output impedance.

    After wiring everything up, the sound quality and volume level is great but unfortunately there is 120Hz hum. I am using a breadboard so a bunch of stray inductance and capacitance is around but I am certain it is coming from the wall-wart (no batteries for testing).

    The part that is confusing me is how it is so prevalent in my signal as it appears with no change when I plug-in and unplug my laptop from the wall. The wall-wart is the only component grounded (two-pronged) so I can't see how there would be a ground-loop problem.

    I've been scratching my head on this one, and wondering if anyone can spot something that I am not picking up on.

    Attached is a rough sketch of my schematic.

    Thanks again,
    JP
     
    Last edited: Aug 12, 2013
  2. MrChips

    Moderator

    Oct 2, 2009
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    3,361
    Replace the wall wart with a 9V battery.
     
    blah2222 likes this.
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    You should be aware that the jfet is a high enough impedance to pick up power line hum. If using a battery doesn't fix this, put it in a metal box to shield it.
     
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  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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    You are using shielded cables for the audio connections, correct ?
     
  5. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
    554
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    I know that would be the easiest solution, I just don't want to waste batteries at this point.

    Shielded patch cord from guitar to breadboard and breadboard to laptop adapter.

    I think it's a noisy wall-wart tbh but I guess the breadboard doesn't help.
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,058
    3,820
    1) Make double sure that your circuit ground is going to the ring on the connector.

    2) Do not try to cover left and right channels by connecting left and right channels in series and leaving the ground floating.

    3) if you want left and right channels, just connect both of the left and right channel rings together but make sure the circuit ground goes to connector ground on the plug (usually the ring closest to the cord (but check - apple loves to set up unique pin-outs just because they can).

    If ground is reversed (even with battery powered circuits), you will get power line hum.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    +1

    No way you'll avoid wall wart hum without doing this.

    Well, using a voltage regulator circuit would help a lot too, if you don't want to waste batteries. Look up 7805 regulator IC. Or power with USB from the Macbook.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,442
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    There is a 9V battery in my Fender Jazz Bass. I replace it about once a year.
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You can automatically switch the battery on/off by using a RTS (stereo) jack
    as one of the jacks on the preamp. Connect the circuit grounds to the gnd on the jack, but connect the battery gnd to the ring connection.
    Using a regular mono plug will connect the battery.
     
    Last edited: Aug 11, 2013
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