It is not the Ground Loops that are the problem and is a myth. The problem is caused by leakage currents or stray currents entering the ground reference circuit. For example a prime offended is the DC power supply, and AC line filters. In a DC power supply you have large filter capacitors tied directly across the rectifier output to filter out the AC component. Well if you know anything about capacitors is they conduct AC signals, and block DC. The impedance is completely dependent on the size of the capacitor measured in Farads, and the frequency of operation. In a DC filter cap is designed to be a very low impedance at 60 Hz. Therefor it is dumping AC current right to ground causing the ground circuit to carry load currents. Currents flowing through any circuit will cause voltage differences along the length of the conductor due to the resistance and reactance. So when you pick up the ground reference at different points along the length of that so called common ground, there is minute voltage differences. In the case of audio equipment it is an AC 60 to 120 Hz signal that is picked up and amplified by the high gain amps and you hear it as HUM. It is referred to as Common Mode Noise Secondly our electrical architectural infrastructure in the USA is very prone to Common mode current because we use an unbalanced electrical system of 120 Vac where one circuit conductor is grounded called the Neutral by laymen terms or grounded circuit conductor by the pros. So when we take a 120 volt circuit from the breaker panel we have 3-wire circuits, Line, Neutral, and Ground. What do we have with wiring between wires? Answer = capacitance. So both Line and Neutral leak AC current into the ground circuit. That is Common Mode Noise. There are ways to deal with it but it takes some equipment. In modern recording studios they use what is Called Balanced Power Systems. It is still 120 Vac so all the equipment works with it, but what it does is take the unbalanced 120 Vac, into a transformer, and on the secondary is center tapped with the center tap grounded. So what you have is L1-L2 of 120 Vac to power equipment, there is no Neutral. That leave the Ground circuit sterile and free of noise and currents because the L1-L2 is symmetrical 180 degree out of phase and at zero sequence with respect to ground so all noise is cancelled out. But in fact all circuits get noisy because of unbalanced signal circuits because ground is used as a signal path or referenced to ground. What you should take away from this is use balanced signal transmission. That is why signal modes like RS-232 are not used much anymore as they are very prone to noise over any kind of distance, or where they are connected to two different power sources. One fix is to use ground loops using a MESH GROUND Topology.