My old yamaha wont stop buzzing

Question

I have an old yamaha MX-800 amp and a yamaha CX-600 pre amp controler and when i plug the pre out of the CX-600 pre amp to the input of the MX-800 amp and turn them both on it makes a buzzing nouis, it doesnt stop untill i unplug the pre amps rca conection or tun off the amp. it still buzzes even if i turn off the pre amp. but when i bypass the pre amp and go straght from the CD player to the amp, it works fine and doesnt buzz. Whats wrong?