The term "ground loop" is used to describe the condition where multiple ground connections exist between two pieces of electronic equipment. Two different paths create a complete circuit or “loop.” Differences in voltage potential on the grounds cause an undesired current to flow on the ground connections, which can lead to an interference signal being added to the desired signal carried by interconnecting cables.
Impact on Audio
Any difference in the ground potential will result in a current flowing through the signal return of an unbalanced audio connection. In balanced connections, the undesired current can flow through the shield of the twisted-pair cable.
Virtually all audio circuitry which is not battery powered is reference to ground in some manner. In most audio equipment the audio ground is tied to the power supply ground, which is also tied to the "0 volts" of the circuitry’s DC power supply.
Sources of Ground Loop Currents
Even when plugged into the same AC outlet, leakage currents in the power supply can result in small currents flowing between different pieces of audio equipment over ground connections in the interconnecting cables.
If the interconnected audio equipment is plugged into two different AC outlets fed by the same phase of the AC power, differences in the ground connection of the AC outlets can lead to differences in the ground potential of the two devices.
One example of this condition is two AC outlets which are wired in series to the same circuit breaker. Any current on the ground of the outlet at the end of the wire exists only on the section of wire between the two outlets. At the outlet in the middle (between the end of the wire and the circuit breaker), the ground current from the end outlet combines with any ground current from the middle outlet. The result is a difference in the ground potential at the two outlets due to the finite resistance of the AC wiring and the combination of the currents at the middle outlet.
Using different AC power outlets which are fed from different AC power phases for interconnected audio equipment can increase the severity of the problem.
Ground loop interference can also be cause by interference signals inducing currents on the shield of interconnecting cables and AC power ground conductors. The worst case occurs when multiple connections between pieces of audio equipment take different physical paths, creating a “loop.” This results in the cables intersecting different electromagnetic fields, which causes different induced currents to flow on each section of the loop.
Balanced connections can be used to eliminate ground noise caused by ground loops, because balanced connections use shielded conductors for the signal and phase cancellation to reject induced noise common to both signal conductors. In an extreme case, the cable's shield can be disconnected on the receiving end to eliminate the ground loop. The two devices must have at least one common ground connection, such as 3-prong AC power cords plugged into a common AC power source, if the shield is to be disconnected in the audio cable.
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