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Headphone output on MacBook Pro has electronic noise

Posted on 2008-11-02
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I have a MBP. It has a mini-plug stereo line-out/headphone jack. Headphones sound fine when plugged in. However, when I plug in a mini-plug to 1/4" Y cable in, and send that cable to my desktops external sound card (firebox), electrical noise becomes very prominent (eg. if I move the mouse, noise changes, etc). Is there a way to fix this?

sample of affected audio:
http://www.box.net/shared/yxa4cnjlx1
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Question by:rose1013
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Frosty555 earned 125 total points
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I'm assuming the firebox has an AC power source? If so, the problem is most likely caused by ground loop issues. Incorrect electrical wiring causes noise in audio equipment.

http://www.gbaudio.co.uk/data/ground.htm

Either find the problem (usually it is another device connected to the same fuse causing issues. so start unplugging things until you hear the noise stop).

Or, get a ground loop isolator. E.g. http://www.amazon.com/Xitel-GLI1-X1-Ground-Loop-Isolator/dp/B00023XDYA
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by:Frosty555
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>>Incorrect electrical wiring causes noise in audio equipment

Okay, that's a vague, and not entirely correct statement. But more details are explained in the link below it.
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by:rose1013
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I have heard grounding issues before, and they usually don't react to changes in the computer circuitry firings, do they? I'm going to see if my college has any isolators that I could borrow to see if that does fix the problem, since 1/4" can be fairly expensive. I'll get back to you on that. Thanks.
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by:Frosty555
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Ground loops wouldn't affect your computer. Your computer is sending the exact signal that you would expect out the sound card. But somewhere inbetween the sound card and your speakers, there is an alternative paths to ground that is causing small amount of current to flow through those wires, and that makes electrical noise on the line which you hear as an audible hum. That's what a ground loop isolator tries to prevent.

The other possibility is that the noise is coming from a device inside your computer itself. My laptop for example makes a faint "ticking" sound when I use the onboard headphone jack, because it is close to the CPU and some kind of cross-talk is occuring. But I don't think this is the case for you, because you said it works fine when you use headphones.
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by:rose1013
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At the same time, cross-talk seems more likely to me: there is a distinct change in the tone of the noise when I move the mouse, or the comp starts thinking, as I open a program, etc. The ogg i provided was this::

Idle sound
Moved mouse in the first 3rd.
Opened a program in the second third.

You can make out distinct changes in the recording. If it is crosstalk, how would I avoid that?
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If it is some kind of cross-talk or interference coming from within your computer, you could try moving around some of the PCI cards, changing position of memory, etc. But normally there is not much you can do about it. It would be a design flaw in the motherboard or in a peripheral attached to it.

That's why I'm leaning towards the ground-loop-isolator thing and hoping it solves your problem ;)
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by:rose1013
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Here's to hoping! I'll check with my tech breakout room to see about borrowing.
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