Balanced and unbalanced audio are two ways of sending the same signal down a cable. Balanced connections reject most of the noise picked up along the way. Unbalanced ones do not. That is the whole reason balanced cables exist, and it explains why a microphone cable can run across a stage while a guitar cable should stay short.
Unbalanced: one signal, one shield
An unbalanced cable has two conductors. One carries the signal, and the other, the shield, acts as both the return path and the barrier against interference. Guitar cables, pedal patch cables and many consumer audio connections work this way.
The weakness is that anything the cable picks up from outside, such as hum from power wiring or buzz from a lighting dimmer, gets added to the signal. The receiving device cannot tell it apart from the music. Over short distances the noise is usually small enough to ignore. Over long ones it is not.
Balanced: two copies, one flipped
A balanced cable has three conductors: two for the signal and one for the shield. The sending device sends the signal on one conductor and an inverted copy, flipped upside down, on the other. The two conductors are twisted together, so any interference reaches both of them equally.
At the receiving end, the input flips one of the two signals back and combines them. The music, which was opposite on the two wires, adds up. The noise, which arrived the same on both, cancels out. This is called common-mode rejection, and it is why balanced connections stay quiet.
Why it matters more for some signals
Microphones produce a very small signal, so the receiving preamp has to amplify it a lot. Any noise picked up in the cable gets amplified along with it. That is why microphones use balanced XLR connections almost without exception.
Line-level signals, such as the output of a mixer or audio interface, are much stronger compared with the noise, so unbalanced connections work fine over short runs. For long runs through noisy environments, such as stages with lighting rigs, balanced is the safer choice.
Common myths
- Balanced does not mean better sound. It means less noise picked up on the way. The sound of the signal itself is not changed.
- A TRS plug does not guarantee a balanced connection. The same plug is used for unbalanced stereo, such as headphones. What matters is how the equipment is wired. See XLR vs TRS vs TS.
- A balanced cable on an unbalanced output does not make it balanced. Both ends must support it for you to get the noise rejection.
What to do in practice
- Use balanced cables (XLR or TRS) for microphones and for any long run between mixers, interfaces, monitors and PA gear.
- Keep unbalanced runs as short as practical. Guitar cables and pedal patch cables are the exception, because instrument signals have special needs, covered in how long a guitar cable can be.
- If a balanced connection still hums, the cause is probably a ground loop rather than picked-up interference. See how to fix ground loop hum.
- Where a source is unbalanced but the run is long, a DI box converts it to balanced near the source and back at the mixer.