Think about the analog RCA pair that connects, say, the analog outputs of a CD player to an AVR. If you start at the CD player's left output ground, you can trace a wire (the RCA cable's shield) to the AVR's left channel input; you can then trace to the AVR's right channel input; you can then trace back to the CD player's right output on the RCA cable's right channel shield; and this is connected together inside the CD player.
That's about as complete and obvious a loop as you can have. And it's all ground connections and wire, so that's a ground loop. The thing is, it isn't a PROBLEM ground loop.
That's why you can't just say "ground loop" and let it go at that.
We ignore ground loops all the time when we don't have hum or other problems. We usually don't run into circumstances where adding more cables -- more ground interconnections and actually more ground loops -- solves a hum problem.
So look for other things first.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
