Mp3 faq - record by connecting stereo into soundcard

When I try to record by connecting my stereo into my soundcard I get a hum - why?

If you get a "loud humming sound" I'd be inclined to suspect some sort of ground loop between the equipment - which could be very dangerous (i.e. could trash your sound card, easily). Are both your stereo and your computer properly grounded (earthed)? If all you equipment is properly plugged into wall sockets using three-pin plugs, and the earth wires are properly connected to the equipment, and you still get a hum when you connect the two systems, I'd almost suspect that there is a problem with your house wiring. You might try separately grounding both systems by connecting the frames to a good ground (metal water pipes, for example). Or just plugging both pieces of equipment into the same wall circuit might eliminate the problem. It really is important that your equipment is earthed properly, and also that the polarity is correct (i.e. the line and return wires are actually connected up the right way round).

Another way to get around the problem to some degree might be to use something like a walkman as a source, which doesn't need to be grounded or to use mains power at all. If you still get a hum, especially if you get a hum only when you touch the walkman, then your computer is not grounded properly (the hum being the mains AC cycle frequency making its way through the sound card circuits...)

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