A balanced circuit is a method of wiring electrical signal conductors so that unwanted external noise is largely canceled before amplification. In a balanced line two signal conductors carry equal but opposite voltages relative to a reference, and a receiver amplifies the difference between them. Interference that couples equally into both conductors therefore appears as a common-mode signal and is rejected by the differential input, improving immunity to noise and hum. See interference and noise rejection for related concepts.
Typical construction and components
Balanced cabling normally places the two signal conductors close together inside a single jacket, often as a twisted pair, and usually includes a surrounding shield or drain wire tied to chassis ground. Important elements include:
- Two conductors: "hot" and "cold" (sometimes labeled + and −) that carry complementary signals.
- Shield/ground: a separate conductor or braid that protects from external fields and provides a reference for safety.
- Connector types: professional systems commonly use XLR connectors or balanced phone plugs such as TRS (phone connector). Differential inputs on mixers and preamps complete the circuit.
History and development
Balanced lines were adopted early in telephony to allow long cable runs between exchanges and subscribers without excessive noise or crosstalk. The practice spread to recording studios and public-address systems as professional audio technology matured. For an overview of the technology in its communications context, see historical material on telephone lines.
Uses and examples
Balanced circuits are widely used where low-noise transmission over distance is required. Typical applications include microphones and microphone preamplifiers, mixing desks, stage snake cables, studio patching and many instrumentation and measurement systems. Modern data networks such as Ethernet also employ balanced twisted pairs with differential signaling to carry digital data over copper.
Distinctions and practical notes
Balanced is distinct from unbalanced wiring (for example, typical RCA consumer connections) because the latter carries a single signal conductor referenced to ground and is more susceptible to pickup. Balanced systems depend on matched impedances and symmetric routing; if the pair is separated or one conductor is exposed to a different environment, common-mode rejection will degrade. Balanced cables can still encounter ground-loop problems if grounds are connected incorrectly, and some microphones use phantom power delivered over balanced XLR wiring—which requires compatible equipment to supply and receive the voltage safely.