MVRANDAZZO asked a good question. What is driving the amplifier?(C.D. player, mixing console, microphone or house system?) How is it connected? (XLR, RCA or 1/4" jacks?). A "stand alone" speaker/amplifier that is supplied by one of the receptacles that you have described and is being driven by mixing console (or etc) that is pluged into the other receptacle that you have described, and are interconneccted by RCA or 1/4" or improperly terminated XLR will produce the noise you have described. The solution to the problem is "most likely""fix the defective circiut and this will eliminate the ground loop".
