Balanced vs. unbalanced

Every cable is an antenna. Running past lighting ballasts, power cables and network gear, it picks up interference — and for a low-level audio signal, even a little interference is audible as hum or buzz. Balanced lines are the professional answer, and they are why AV systems can carry microphone signals across a building.

Unbalanced: one signal, one reference

An unbalanced connection has a single signal conductor and a ground, usually the shield around it. The receiving device measures the voltage between the two. Any interference induced in the signal conductor is measured too — the receiver has no way to tell it apart from the audio.

That is fine over short distances at line level: RCA leads behind a TV, a 3.5 mm cable from a laptop. Over longer runs, or at mic level where the signal is tiny, the noise becomes audible.

Balanced: let the noise cancel itself

A balanced connection uses two signal conductors — hot and cold — plus a shield. The two conductors are twisted together, so interference along the run hits both almost equally. The receiver is a differential input: it measures only the difference between hot and cold.

• The wanted signal appears differently on the two legs, so it survives the subtraction.
• Interference appears the same on both legs — it is common-mode — so it cancels.

How well that works is the input's common-mode rejection ratio (CMRR), in dB. It depends on how equal the two legs are — in impedance as well as pickup. Many balanced outputs don't drive an inverted copy at all; they simply present equal impedances on both legs, and the cancellation still works.

Recognising connectors

Phantom power rides on balanced lines

Condenser microphones need power for their electronics. Phantom power (usually +48 V, labelled P48) is applied equally to both hot and cold, relative to the shield. Because it is identical on both legs, the differential input ignores it — the same trick that rejects noise lets power and audio share the cable.

Ground loops and hum

When two devices are earthed at different points, a small current can flow along the cable shield between them. The result is a steady hum at the mains frequency — 50 or 60 Hz — and its harmonics. Unbalanced connections are especially vulnerable because the shield is also the signal's reference.

Getting between the two worlds

Real systems mix consumer and pro gear. A short unbalanced lead into a balanced input usually works but loses the noise rejection. For long runs or noisy environments, put a line-level converter (an audio balun) close to the unbalanced source. It converts to balanced and usually raises −10 dBV to +4 dBu at the same time. In live sound you'll also meet the DI box, which converts an unbalanced source to a balanced mic-level signal for a mic input.