Here is one piece of additional information to my ground loop postings.
Ground loops can be introduced when different components in audio systems are connected with standard audio cables, and these loops can cause annoying interference. Grounded audi components generally carry significant currents in their ground returns.
Texas Instruments Application Report SLOA143 – October 2009: Ground Loop Break Circuits and Their Operation tells that in many cases, the interference can be reduced significantly with a “ground loop break” (GLB) circuit including a low-value resistor and differential amplifiers.
Ground Loop Break Circuits and Their Operation paper explains this approach and how to design a GLB circuit. The following schematic illustrates a GLB line input. The circuit uses a low value GLB resistor, Rgbk, typically 5 to 20 ohms, plus two differential amplifiers.
The resistance inserted in series with the audio cable shield is called a “ground loop breaking resistor, Rgbk.
More properly it’s a “ground noise reduction” resistor. The resistor can be inserted at either the source or the receiver.