audio circuit brainstorming

[Chris Weiss]

I have a hacked IP phone with a headphone jack in place of the speaker, then some powered PC speakers plugged into the jack.  it's perfect when audio is playing, but has random noise when the phone is outputting nothing.

basically it seems like a ground loop issue, which I'm not sure how to fix in this case since the phone is powered via POE from a switch on a UPS, and the speakers are on a common DC brick on a properly wired circuit.  

would a resistor from tip to common on the headphone jack as a "pull down" help?  would I use a small or large value?  would it cause any harm to the phone's simple 2W amp or the speaker's amp?

[EschewObfuscation]

First experiment I'd suggest is you run the speakers off a suitably sized stack of batteries, see if the behavior remains. That will tell you whether to look in the direction of the power supply or the signal. You might also try that with the phone. Those two data should narrow the search considerably.

[nfolken]

Maybe a ferrite ring around the speaker wire would help? I know they work great for getting rid of the EMI caused by early cell phones