steve marnier:
Hi Tony,
Thanks for your reply. The laptop is an Acer Extensa 5620, and it is linked to the hi fi system via it's headphone jack/screened cables/aux amp input. The harmonics are not a mains hum as such, rather a mixture of white noise, harmonic ringing and staccato morse code ! And totally intrusive. So the PSU (presumably switched mode) is almost certainly the culprit, as it is the same if I plug it into the mains on an extension lead from next door !
Do they make suitable low noise PSUs ? It's output is 19v, 3.42A
Regards
This does sound like a faulty PSU - however, I would have thought that if it's generating that much noise, you would be getting some kind of interference elsewhere.
If you plug headphones (only - leaving your amp disconnected) directly into the laptop, does the noise persist?
Also, does the laptop have a low power mode, as used when watching DVD's etc.? You could try switching it to that mode and see if the noise reduces.
The noise you describe doesn't actually sound PSU related ( I do realise you stated it's fine on battery though! ).
What I suspect is happening, is that in battery only mode, the laptop is "stepping down a gear" ( i.e. the clock / bus speeds are all slower ) and hence no interference - but when you run off mains, it switches to full power ( CPU speed increases / bus speed increases etc. ) resulting in the noise you describe.
Can you try the above experiment with the headphones and give some feedback on the result?
Tony.
P.S. I'm not aware of any special low noise PSU's but I'm sure they must be available.