What Causes Hum and RFI Noise?
Ground Loops
A system without ground-loops is surprisingly unsusceptible to hum and RFI pick-up. Unfortunately, most systems have them. Ground-loops are created when components have their grounds connected both through their AC cords and with single-ended interconnects. This creates one or more loops of varying diameters. Ground-loops provide a means for strong magnetic fields, differences in ground potential of the power system or strong RFI to interfere and cause audible noise. Lets look at each case individually:
Magnetic Fields
If a changing magnetic field (such as 60 Hz) intersects one of the loops, then a like current will be induced on the ground loop. If the resistance of the ground wires of the interconnect that is part of the loop is relatively high, a voltage will develop across this ground wire. The resistance of the interconnect ground wire is generally higher than the power cord ground wire. This voltage will appear on top of the signal that is being transferred across the interconnect.
Power System Potential Differences
Each separate circuit in house wiring has a separate ground wire that is usually connected to all other ground wires at the panel. Currents are not supposed to flow on these ground wires. However, in practice differences in AC potential can develop between grounds of different circuits. Therefore, when two or more of such circuits are used to power a single system, this ties together the grounds with different potentials through the power cords and interconnect ground wires. Currents between the different grounds can result, which can cause voltage drops across the ground wires in the power cords connecting between the different circuits. This is not in itself a problem, but the same voltages are developed across the ground wires of the interconnects between the components that are plugged into the two circuits. These voltages, usually 60 Hz hum, will appear on top of the signal that is being transferred across the interconnect.
RFI Susceptibility
Strong Electromagnetic fields can excite one or more of the loops. This happens when the size of the loop is fractionally related to the wavelength of the RF, usually 1/2 or 1/4 wavelength. The loops can act as loop antennas, actually tuning-in the RF signal by resonance of the loop-antenna. The RF signal can cause currents to flow and a voltage to build-up on the entire loop, including the ground wires in the power cords and the ground wires in the interconnects that make up the loop. The resistance and inductance of the interconnect ground wire will cause a voltage to develop that will appear on top of the signal that is being transferred across the interconnect.
The recurring theme in the above three cases is the resistance of the ground wire in the interconnect which causes a voltage to build-up. Therefore, one would conclude that making this wire very low in inductance and resistance would help to eliminate the noise problems. This is true, however the realization of this is more difficult than it appears at first blush. Making the interconnect ground wire very low inductance and resistance conflicts with the need for the same interconnect to be low capacitance, which is the most important factor for interconnect sound quality. Therefore, one can conclude that it is fairly straightforward to make either a low capacitance interconnect or a low-inductance interconnect, but not both. Strategies to accomplish this include using multiple small ground conductors (to minimize skin-effect). In effect, the only strategy that achieves a low inductance without significantly increasing interconnect capacitance is to rely entirely on the self-inductance of the ground wires, which equates to multiple separate ground wires that a intentionally decoupled magnetically from the signal wires.
Another strategy that can help eliminate at least the above RFI problem is to apply filtering to the power cord ground wire. This creates a low-pass filter in the loop comprising the power cord ground wires and the interconnect ground wires, but does not create a filter in the interconnects for purposes of signal transmission.
Other strategies include strategic system placement and selection of single-circuit outlets. Minimizing the physical diameter of each ground-loop by careful cable routing can minimize the susceptibility to magnetic fields and RFI. Using a single AC circuit for all components can minimize the chance of AC hum due to ground potential differences.