On 5 Jan 2005 19:51:44 GMT, Al wrote:

He tells me that everything works fine when the
laptop is on battery power but as soon as he plugs in the power
supply, there's a loud humming audible through the stereo speakers.

It's an earth loop hum.

Most unlikely. All the laptop PSs I've seen have been double
insulated.

Much more likely to be poor decoupling in the PS / computer and
appearing directly on the audio output.

Bill