Dave Plowman (News) wrote:
In article ,
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.

Yes.

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

However, surely the PS would be a switch mode type, so producing no
audible hum?

It's an interesting problem...

Update: Apparently it's an HP laptop brought over from the US. Would there
be a difference in the earthing arrangements for a US-spec machine I wonder?