[lug] audio interference question

[Stephen Queen]

On Thu, Jan 29, 2009 at 8:35 PM, karl horlen <horlenkarl at yahoo.com> wrote:
> i plugged the stereo out of my laptop into a really nice power amp / speaker combo i normally use for recording.
>
> when the laptop is NOT plugged in (running purely on battery), the sound is pristine... exactly what i'm looking for.  fwiw, i made sure i've muted the microphone to eliminate any extraneous sound.  if i don't do that, you can hear whitenoise when no music is playing.. probably picking up internal drive and fan noise..
>
> here's the catch.. the minute i PLUG the laptop in to POWER IT (non battery), i get a an annoying but constant static / hum / interference sound.  it does NOT go away, even if i totally mute the sound on the laptop.  so it's electrical in nature.
>
> i've tried plugging the laptop in all of the outlets that are within reach of the power amp.  buzz / hum / static is still there.. it sounds like a motor or something and or possibly even acting as an antenna...
>
> the odd thing is i've got another pc and 3 other audio gadgets that are hooked up to this same power amp and none of them have this this same issue.. pristine sound.  so i can't think it's a grounding problem in the house or circuit.  only the laptop plugged into the wall outlet generates the hum.
>
> i tried unplugging all other devices on the power strip, nothing helps.
>
> - is there anything i can do to make this work with power?
> - any test to eliminate the problem (though my tests are already telling me fairly clearly that the laptop (psu or brick?) itself is the source of the problem)?
> - are laptops or their psu / bricks known to not be grounded or shielded very well?
>
> thanks to anyone that can unlock the mystery.  i'd like to make the laptop i have rarely ever used become my full time mp3 / media player.
>
It sounds to me like you may have a ground loop problem. A ground loop
occurs when there are multiple grounds and the current through the
ground causes a voltage to develop between the different ground
points. The fact that the problem disappears when you run your laptop
on its battery is a big clue.

The fix is to allow only one path to ground. Somehow you must isolate
your laptop power ground from the power amp power ground. A more
distant power outlet would probably agravate the problem. You could
try one of those 3 prong to 2 prong adapters they use to sell at
hardware stores for your laptop. The more expensive approach would be
to buy an isolation transformer.

See
http://en.wikipedia.org/wiki/Ground_loop_(electricity)
http://en.wikipedia.org/wiki/Isolation_transformer

Steve