A friend of mine has this problem. His computer drives a usb soundcard,
the analogue output of which is connected to the amplifier.
I assume that the 'phone signal is not getting into the digital part of
the chain, and that the most likely entry point is the analogue cable.
Is that fair?
I'm not sure what cable he has, but I think that they're of the
audiophile tendency. I have a feeling that some of them use twin wires
with a screen that is only connected at one end. This leads me to think
that reversing his cables may be helpful. Is that likely?
Roger Thorpe