Balanced circuit

"See also Balanced line"A balanced circuit is a common method of transmitting many types of communication signals between two points on two wires. In a balanced circuit the individual signals on each wire are inverses of each other as referenced from an arbitrary midpoint between the two signals.

As the signals travel the wires, noise is induced into both by the surrounding environment. Due to the proximity of the wires, this induced noise is very much the same in each wire. At the end nodes, the two out-of-phase signals along with the common noise that each wire had induced into it over the transmission path, are captured. One of the two signals, "signal" or "signal inverted", is fed into an inverter. The resulting wave is combined with the unaltered signal. The noise on each signal is the inverse of the other, and the signal wave is imposed on itself. The result is twice the signal, and essentially no noise.

This explains the lack of significant noise in properly balanced audio circuits. Two common examples of this type of connection include RJ-11 and XLR connectors. Not all XLR connectors are balanced, but there are good odds to it. Also 1/4" connectors may also be used to create a balanced circuit. When done so they are known as TRS connectors or tip ring sleeve.

A common and simple method of designing a balanced circuit is to drive both ends of the loop circuit with matched transformer output (and input) coils. As stated above, the two wires forming the basis of the circuit traverse the same environment and, if the are a twisted pair, can reduce interference and stray radiation to a minimum. Coaxial lines by design are unbalanced in that the center conductor and the shield are not referenced to each other but to other conductive external systems. A triaxial cable or a twin-axial with two center conductors can be made into a balanced circuit by proper driving devices (usually transformer windings, which should be reasonably matched.)

For ordinary purposes it is not necessary to attempt to 'balance' the two sides of the circuit to the nth degree. Most practical applications can be successfully designed using similar transformers at each end. Stray unbalances can usually be filtered out for a very small price.