speaker buzzing

I have bought a new power amp and am now getting a buzzing sound (NOT a hum) from my speakers even when everything except the amp is turned off. If I disconnect the cable TV line it clears up. If I connect the shield part of the cable but not the inner wire, it is still fine. Will a ground loop isolator fix this and where can I get one?
I think a Mondial magic box will help. I Solved a similar problem with one. //www.klipsch.com/mondial/index2.asp?path=/products/mondial/index.asp?frame=y&id=&line=&1
Viewsonics makes a simple, in-line ground lifter that is around $25.
You are getting a ground loop from your crapy cable tv company. You will have a hard time convincing them that is there fault. If you want a cheep solution, you can get some 75om to 300om converts and reverse them. In other words, go to radio shack, get 2 75-300ohm converters. One that has the 75-300 with leads and the other that has the 75-300 that just has the screws on the 300 ohm side. Take the leads on the 300ohm side and connect them to the 300 ohm side of the one with the screws. This will leave you with a 75-75 converter. Since these converters are just transformers there will be no grounding problems and you will not see any reception loss. Good Luck
The Radio Shack matching transformers back to back (parts # 15-1253C and 15-1140B) fixed the problem.
Thanks for all the help