Mac Computer Electrical Interference

[ThirdDegree]

I am trying to play a movie from my macbook pro with the video to a standard HD tv and the audio to a guitar amplifier.

I connected the guitar amplifier to my macbook pro through a 1/4" to headphone jack cable and the sound outputted to the amp was fine, just as I wanted it. As soon as I connect the HDMI cable to the mac's display port, an insistent, loud buzzing is heard through the amp, instantly going away when you unplug the display port cable. Can someone explain to me why this happening and if there is anything I can do about it?

[rcgldr]

What happens if you plug the HDMI cable into the TV first, then into the Mac? You could try connecting a ground cable between the computer and the amplifier case, and maybe the TV, although I'm not sure if there's a good grounding point for the TV.

[Proton Soup]

sounds like you've got a "ground loop" in your circuit. they're not that easy to deal with sometimes, but perhaps if you search the forum or google, someone with more experience on them can give you an idea.