Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Comp Sci Major Gives Up

  1. Apr 17, 2010 #1
    Okay I talked to all my comp sci friends they can't get this one either. I must go to my poor friends in elec engineering. To be honest I kinda just started using bluetooth headphones to get around this problem and the people living below me told me i cant use my sound system anyways (they get pissed easily) but I still want to get to the bottom of this.

    I refuse to say I “need help” because I am a very competent computer user. But this is a problem that I can’t get to the bottom of and goes beyond the scope of most the idiots I have talked to. That is why I am posing this question as a challenge since I know I will most likely just have to ignore it. I’m going to post all the details and maybe someone out there will humble me who knows what to do.
    The Situation: I have a Sony laptop (VGN-SR540G), an Acer P244W monitor, a Sony Bravia home theater speaker system. Naturally, there is nothing more I would like to do than use all three at once.
    However, when my laptop is connected to both the monitor and the sound system there is audio noise.
    Here are the details:
    The noise is a low frequency hum. (Sounds a LITTLE like this maybe not as low frequency and not as creepy sounding ) The video is borrowed from something else I didn’t bother to look at
    This noise occurs when my laptop is plugged in AND when my laptop is unplugged.
    The audio system has a two prong (ungrounded) plug.
    The monitor has a three prong plug.
    The laptop has a two prong plug.
    The audio system noise is there when the monitor is plugged in through the HDMI output AND when it is plugged in through the VGA output.
    Audio output goes through my headphone jack.
    Audio noise DOES NOT occur if I am plugging headphones into my laptop HOWEVER if I plug in my audio system to the laptop and then plug in the headphones to the audio system THEN you can hear interference through the headphones.
    Audio NOISE is MINIMIZED when LAPTOP SOUND is MAXIMIZED. In other words audio noise does not depend on laptop volume only amplifier volume.
    The noise occurs even if the laptop is muted
    I have tried a few “moving around the plugs” to deal with ground loop but it really has done nothing at all.
    The noise comes from all speakers not just the bass.
    If the monitor is not connected to the laptop then there is no noise.
    If the monitor is connected to the laptop there is noise.
    If the monitor is OFF but connected to the laptop there is noise.
    If the monitor is unplugged but connected to the laptop there is not noise.

    Okay kids have at it and tell me if you need more details.
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Apr 17, 2010 #2
    Go to mcmastercarr.com. Order 70245K71 for $75.

    Plug the speakers into that and your hum should go away.
  4. Apr 17, 2010 #3
    i hate expensive solutions -.-
  5. Apr 17, 2010 #4
    It's the safe way. You can put together an isolation transformer for a lot less but I prefer to dispense safe advice.
  6. Apr 18, 2010 #5
    So I guess you understand that your main problem is a ground loop. Your monitor is grounded, and your amplifier is. They form a loop through the laptop which catches a magnetic 50/60 Hz field of some sort, which gets amplified by the amplifier. Amplifiers usually have some grounding tab. You could try to ground that to different things to see if you problem goes away. I suggest the outer shield of the VGA plug.
  7. Apr 18, 2010 #6
    His amp is ungrounded. The problem is that he's got current leaking through the galvanic isolation of the amp's power supply.

    I suppose he could try an adapter that lifts the groung off the monitor. It's very cheap and therefore worth trying first but I don't think it will solve the problem.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook