What makes an amp hum/ground loop?

I just hooked up a pair of monoblocks in place of a Coda stereo amp. Nothing changed. I am and was, running directly out of a cd player with an analog o/p. When I hooked up the m-blocks, I got a hum in my speakers. It only happens when BOTH monos are hooked up to the cd player. But that was no problem going into the Coda. Suggestions anyone? Thanks!
It usually is a ground loop mismatch between components. Are your A/C outlets grounded? Did you try cheater plugs (2 prong no ground)? Are both mblocks plugged into the same circuit, if so, is it feasible to plug one into a seperate circuit in the house/apt? Some cables being so sensitive will cause this as well? Did you change cables? So many qestions! Good Luck...Kent
Some amp makers suggest the use of (1) cheater plug on just one of the monoamps. A good way to tie the two monos together is with a heavy duty 1 to 3 a/c outlet adapter cord. Home Depot has a nice 12ga pigtail type. Using one of those you can plug in both amps on one wall outlet. Sometimes it takes alot of experimenting with different powercord positioning. Get an extension cord and plug the preamp into another outlet on a different circuit and listen for change. It can be other things also but moving the preamp to another circuit works alot of the time. Good Luck.
Hey Cspra, Thanks for your comments. However, I mentioned that I was connected directly to the cd player. It has analog o/p's, so there is no preamp. Thanks again for your thoughts. John
Johnwowen, I have had an almost identical situation. One dedicated outlet for source components plugged into a powerwedge 114 and one other dedicated outlet for stereo amps. Always was very quiet with a wide variety of stereo amps. However, I purchased monoblocks and got a very loud hum when they were either plugged into the same dedicated outlet or into separate outlets. One cheater plug did not work, I had to use two. Also, I have tried out another set of monoamps from a different manufacturer and they did the exact same thing! Quite frustrating! This doesn't happen at my friends house. Yes, all circuits are properly grounded, though not with separate "dedicated grounding". I will be trying balanced interconnects (really my only guess at what might cause this) in a week or so to see if this helps. Otherwise, I'm sticking with the cheater plugs.