[lug] audio interference question

[karl horlen]

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.