Telephone balance unit

A balun /ˈbælʌn/ is an electrical device that converts between a balanced signal (two signals working against each other where ground is irrelevant) and an unbalanced signal (a single signal working against ground or pseudo-ground). A balun can take many forms and may include devices that also transform impedances but need not do so. Transformer baluns can also be used to connect lines of differing impedance. Sometimes, in the case of transformer baluns, they use magnetic coupling but need not do so. Common-mode chokes are also used as baluns and work by eliminating, rather than ignoring, common mode signals.

The origin of the word balun is “balanced to unbalanced”.

A variation of this device is the UNUN, which transfers signal from one unbalanced line to another.

In classical transformers, there are two electrically separate windings of wire coils around the transformer’s core. The advantage of transformer-type over other types is that the electrically separate windings for input and output allow these baluns to connect circuits whose ground-level voltages are subject to ground loops or otherwise electrically incompatible; for that reason they are often called isolation transformers.

This type is sometimes called a “voltage balun”. The ‘primary’ winding receives the input signal, and the ‘secondary’ winding puts out the converted signal. The core that they are wound on may either be empty (an “air core”) or equivalently a magnetically neutral material like a porcelain support, or it may be a material which is good magnetic conductor like ferrite in modern "high-frequency" (HF) baluns, or of ‘soft iron’ as in the early days of telegraphy.

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Wikipedia