Buzz through speakers w/amp on & pre off

I'm experiencing a buzz through my speakers which makes it impossible for me to leave the amp on continuously. It only happens when just the amp is on. If I unplug the preamp the buzz gets slightly quieter, and if I unplug the cd player the buzz gets quieter, but barely enough to notice.

The second I turn on the preamp the buzz goes away completely. The second I turn off the preamp the buzz comes back. It doesn't interfear with listening, but I would like to be able to leave the amp on all the time.

Has anyone ever experienced this?

Here's what I'm using.
unmentionable cdp
Anthem Pre 1L
Classe ca-200
Thiel 3.5
It buzzez both with and without the Thiel EQ box connnected.

Any thoughts?
thanks,
kevin