In article ,
N. Thornton wrote:
The problem is a design one. Almost all suggestions have been right
off track. Theres nothing you can do about it unless you mod the
design on the audio side, or buy some decent kit.

You can buy plug in mains filters.

This is uk.d-i-y not sci.electronics.

It would be sad to exclude simple electronics.

However, it would help the the OP actually read the replies and answered
the points arising.

If the same thing happens when using headphones direct from the
computer, then I doubt external filtering on its supply will help. But
if it stops, then the fault lies with the external sound system.

--
*I went to school to become a wit, only got halfway through.

Dave Plowman London SW
To e-mail, change noise into sound.