iMac 3.5mm Line Input Pre-Amp?

Discussion in 'iMac' started by elithrar, Sep 20, 2013.

  1. elithrar macrumors 6502


    May 31, 2007
    I'm looking for a 3.5mm microphone pre-amp so I can plug a 3.5mm mic into my 2011 iMac's line input.

    USB microphones aren't a solution for me unfortunately as my iMac suffers ground loop issues and short of betting > $200 (typically more) on a power conditioner I can't safely resolve the issue. Old house, HAM radio operator down the street on a low freq, etc.

    Note that I've tried a powered USB hub to no avail. I assume an external USB sound card would suffer the same problem.

    (so 3.5mm it is!)
  2. BrianBaughn macrumors 603


    Feb 13, 2011
    Baltimore, Maryland
    Sounds like your getting interference on anything you plug into the wall. I'm wondering why a pre-amp would be any different, unless it's battery powered.

    My experience with a ground loop was that I got a hum when audio was connected from a device plugged into a different circuit. Plugged into the same circuit as the computer eliminated the hum.

    I also had some noise once on my powered audio monitors that was eliminated by putting "ground floats" on the plugs.

    What is the brand/model of your mic?
  3. elithrar, Sep 20, 2013
    Last edited: Sep 20, 2013

    elithrar thread starter macrumors 6502


    May 31, 2007
    The kicker is that the headset is USB powered. No external supply whatsoever. So I can't use cheater plugs on the power supply (because I won't use on one on the iMac) and the HumX is only available for 110VAC/60Hz and I need 240VAC/50Hz.

    A cheap-ish ($100) pair of Skullcandy SLYR's, thanks to this Head-fi guide. They are just for gaming/voice comms: I have a pair of Ety IEM's for serious listening.

    I have a pair of Steelseries 'phones that have 2x 3.5mm jacks, however without an appropriate mic pre-amp the microphone won't work: the iMac's line in is unpowered, and most pre-amps give +48VDC, not +5VDC. This won't be friendly to the headphones.

    I've been recommended the GreenKey USB isolator, but I'm concerned it might limit the current draw too much to work with a headset.
  4. elithrar thread starter macrumors 6502


    May 31, 2007
    I've solved this with a Creative X-Fi Surround 5.1 Pro + Soundflower ( to allow me to control the volume with the standard OS X controls/keyboard.

    The X-Fi doesn't exhibit the "hum" I suffered from over a set of USB headphones (which had their own integrated soundcard) and it obviously provides the correct "PC-style" +5V microphone input.

    Cost me $86, which is cheaper than the USB isolator—Firestone indicated it wouldn't be a good solution as the current draw limitation would likely stop most headsets from working.

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