Posted 10 November 2005 - 06:51 PM
I'm fairly sure your problem is caused by an "Earth loop" which is an audio problem caused by different components having different zero volts (or Earth) connections. The USB connector is going to the zero volts of the PC power supply, the audio jack is connected to the zero volts of the amp. Because the two have different paths, and there is mains power involved, you can get an annoying 50Hz buzz that can be loud.
I don't know how you can fix that, it is really caused by the amp but there is no guarantee that another amp would be any different.
A PC sound card, capable of driving your headphones, would fix the problem because then you would have both USB and the audio jack connecting to the same earth.
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