Balanced circuit

Balanced lines are a common method of transmitting many types of electrical signals between two points on two wires.

By measuring the voltage difference between the two wires at the receiving end, the original signal is recovered while the noise is rejected.

A balanced circuit will normally show a symmetry of its components about a horizontal line midway between the two conductors (example in figure 3).

The advantages of symmetry are that the same impedance is presented at both ports and that the circuit has the same effect on signals travelling in both directions on the line.

A balanced signal is one where the voltages on each wire are symmetrical with respect to ground (or some other reference).

In all methods, for the continued benefit of good noise immunity, it is essential that the driving and receiving circuit maintain the impedance balance of the line.

The conceptually simplest way to connect to a balanced line is through transformers at each end shown in figure 5.

Transformers were the original method of making such connections in telephony, and before the advent of active circuitry were the only way.

When discussing a two-wire circuit primary and secondary cease to have any meaning at all, since signals are flowing in both directions at once.

In fact, one leg of the equipment side can be earthed without effecting the balance on the line as shown in figure 5.

Each of the two wires is driven by an op amp circuit which are identical except that one is inverting and one is non-inverting.

To achieve a floating output additional feedback paths are required between the two op-amps resulting in a more complex circuit than figure 6, but still avoiding the expense of a transformer.

1/4" tip/ring/sleeve (TRS) phone connectors were once widely used on manual switchboards and other telephone infrastructure.

Such connectors are now more commonly seen in miniature sizes (2.5 and 3.5 mm) being used for unbalanced stereo audio; however, professional audio equipment such as mixing consoles still commonly use balanced and unbalanced "line-level" connections with 1/4" phone jacks.