"I solved this problem by audio transformers that are connected between the audio card and the external equipment. You need four transformers, two for (stereo) playback, two for recording. You must make sure that you do not connect the ground through, otherwise the ground loop persists, so either do not use a metal chassis or use connectors that are isolated from the chassis (that's what I did). The isolating connectors are the gold-plated ones with the higher price :-) Make sure they really isolate ground from the chassis... This is the circuit diagram used (you need this four times):"
transformer
-------X X-------
from Comp. X X to external equipment
-------X X-------
Be sure to read about disadvantages of using transformers, see AudioTransformerDisadvantages.
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