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Balanced connections

The IEEE dictionary defines a "balanced line" as:

"A transmission line consisting of two single or two interconnected groups of conductors capable of being operated in such a way that when the voltage of the two groups of conductors at any transverse plane are equal in magnitude and opposite in polarity with respect to ground, the total currents along the two groups of conductors are equal in magnitude and opposite in direction."

Balanced interconnects in audio systems are used to reduce or prevent "hum" and other power related noice sources. Single ended or "unbalanced" lines often exhibit "hum" due to differences in ground planes between devices. Balanced lines avoid the problem by providing two equal current sources to ground in opposite phase. This can be accomplished either with active circuits or with transformers. Both have advantages and disadvantages.