Laptop Computer Ground Loop Induced Noise

[Craig Maier]

Laptop computers can create high levels of noise when either playing or recording audio via external audio equipment. This primarily occurs when the laptop is operating on its external power supply, rather than on its internal batteries. The noise is induced via a ground loop formed between the audio system and the laptop power supply. It sounds like random whistles, clicks, pops, sizzling, and other strange sounds. It can happen during recording or playback. Here is what happens. If the sound system is grounded (like the soundboard in a venue), it is grounded for safety reasons. If you are recording through one of the aux outputs of the soundboard and into the laptop via a coaxial cable, and the computer is also grounded via its in-line power supply (sometimes called a lumpy cable), the safety ground of the sound system and computer are effectively tied together. The coaxial cable shield will try to equalize the difference in ground loop potential Voltage and the I x Z Voltage drop created will create common mode to normal mode noise conversion which will appear as an unwanted signal into the computer (during recording) or into the sound system (during playback).

The simplest "fix" for this situation is to operate the laptop on it's batteries when used as a recorder off of a sound board or when it is used as a sound source into a sound board. If the laptop is a permanent installation and battery operation on a continuous basis is not practical, then the laptop power supply can be isolated via the use of a suitable isolation transformer, which will break the ground loop. Alternatively, the computer audio line input and/or line output can be isolated from the soundboard (or audio system in general) via the use of audio isolation transformers (one needed for each channel).

The use of balanced inputs and output will also help, but that would require an external USB based soundcard.

Craig

[Craig Maier]

We have been told by one customer who has had this problem that there is another solution. The customer told us to use a "ground cheater" plug inserted between the computer power power supply module and the power line. The claim is that this eliminates most of the noise in the system. We do not know if this works - - - we have not tested it. Furthermore, we do not recommend it as it may preset an electrical safety hazard.

Craig