Fig 1. How a balanced line cancels out noise.
Fig 2. The shield of the cable (1) connects to ground and the audio signals flow in two conductors (2) not connected to ground.
A balanced line is an audio cable that has two current-carrying signal wires surrounded by a ground (shielding) wire. The signal wires are of equal impedance relative to the ground and are twisted along their length so that any noise is picked up equally by both conductors, no matter where in the cable it occurs. One of the signal wires carries the original sound signal and the other a polarity-inverted copy. The inversion is done using either transformer-based or electronic output stages.
At the destination, a transformer or electronic differential amplifier flips the inverted signal, returning it to its correct polarity. At this point, the wanted signals on each conductor are both the same polarity, whereas any noise that has been induced is of opposite polarity. Summing the signals from the two conductors together reinforces the wanted signal and cancels anything unwanted, leaving a recovered signal that is very close to the original (Fig 1). This process is known as common mode rejection, which means that any signal that's the same on both conductors is eliminated. Balanced lines are thus less susceptible to hum and can carry audio signals over longer distances. They typically use 3-pin XLR or 1/4-inch tip-ring-sleeve (TRS) phone connectors (Fig 2) and are used for carrying microphone and line level signals.
Even balanced lines aren't perfect: slight differences in impedance, twisting, or input gain will allow some noise to slip through. However, this isn't usually a problem unless the line is very long or is being used where there's a lot of interference.
Balanced output from unbalanced input
Unbalanced lines consist of just two wires – one providing the signal and the other a reference (i.e. ground) – and are the type used with guitars, most domestic audio equipment, and some keyboards. Often, 1/4-inch jack connectors or phono/RCA connectors are used to connect unbalanced equipment.
A straightforward way to create balanced output from unbalanced equipment is using a DI box (see direct injection). A DI box uses a transformer or electronic components to isolate the input from the output while still passing audio. It can take an unbalanced input (usually of instrument, line, or speaker level, depending on the pads available) and give a balanced output, to feed the balanced line.