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I use my laptop with an external/Pro sound card. The biggest problem I have is that of ground loop/hum/noise when the laptop is plugged in. I don't want to 'lift the earth' fix as this is pretty dangerous. What I would like to know is what laptop makers out there DON'T have this problem - or if anyone has known models that don't..

Right now my experience is the following:

1) Every Dell I have tried has the problem 2) Every Apple Mac does NOT have the problem

I would like to use a non-Mac laptop based on specific software I use that only runs on Windows..

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My experience is that it's more linked to your electrical installation than to the laptop, but I could be wrong. –  Gnoupi Jul 28 '10 at 13:18
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+1 the above - I have seen given machines have issues in one location or not another. It's usually the actual electrical installation and/or adapter (although it's certainly possible that there are some manufacturers who just make lousy adapters or internal converters etc.) –  Shinrai Jul 28 '10 at 14:46
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3 Answers

Sorry for bringing up this old question, but I think this question deserves an in-depth analysis as I see similar question answered multiple times...

Noise, hum and ground loop are three related but different concepts.

First, noise refers to anything other than the signal in the sound system. Harmonic distortions ( http://en.wikipedia.org/wiki/Total_harmonic_distortion ) are noise, Other distortions includes intermodulation distortion, clipping (power supply), slew-rate related distortion (slow transistors), flutter & wow (due to medium), quantization noise (caused by encoding). For different kinds of audio distortion, see ( http://en.wikipedia.org/wiki/Distortion )

And then hum sound, yes, are usually caused by ground loops, and ground loop is usually treated by either treating the source or the recipient. The easiest way to solve amplifier-source ground loop problem is to connect them to the same ground (e.g. same power strip/same power outlet)

But for most notebook computer, the problem is often caused by the inadequacy of (1) transformer to filter noise (bad switching power supply comes to mind) - try to see the voltage from the notebook transformer with a CRO if available , (2) soundcard to handle noise in the power supply correctly.

Curiously, what sound card/pro interface are you using? Most pro external sound card had pretty good power supply rejection characteristics, i.e. filter caps in and out, external power supply for the sound card instead of using bus power, etc. And most important of all, remember to use the same power strip for both the sound card & the computer (and the amplifier/active monitor).

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At the Video Projection Computer in our Church's sound room we had a fairly bad hum on the audio from that computer to the main sound mixer. Also that connection would put hum bars on projection Screen. Not Good!

Several fixes that only helped a little were tried. Including Audio transformers!

Found a device for automobile audio and it FIXED the problem - no Hum in the audio and NO Hum Bars on the screen! Hallelujah.

PAC SNI-1 is the device. Just search on dogpile or google for "pac sni-1"

The Projection computer has a very low noise floor in it's sound board. Check to be sure your computer has quiet sound board. But it should be better than -80 dB

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I've had similar problems with more than one laptop, it doesn't seem to be possible to determine it on a brand by brand basis. I know of some HP laptops that are totally fine and others that that have the noise issues. I also have had hum free experiences on some Lenovos, but wouldn't want to state that all the laptops in either range are hum free.

The usual way of solving such issues is to break the link in the ground between the affected PC and the sound desk. The can be done using a soundcard and desk with optical interfaces, or by balancing the audio and then lifting the ground, or by the approach explained by BJ Russel, which uses a transformer to achieve the isolation required.

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