The TBU (Telephone Balance Unit) provide high degree of separation between send and receive signals, enabling 4-wires communications system to interface with the telephone network.
In telephone IFB applications, the high drive capacity at the 4-wires output, enables a presenter`s earpiece to be connected directly to the unit without an external amplifier.

This unit is powered from the telephone line and provides an interface to a 4-wires circuit with separate level control of send and receive signals.
Optimum rejection of the input signal on the 4-wires output is achieved by adjusting SIDETONE NULL trimmer.
This can be used to compensate for local line variations or to adapt to the telephone systems of other countries, where line characteristics may differ.
To enable communication between the 4-wires circuit and the telephone network, the 4-wires an line connections are made to the unit and the "hook" switch is pressed to power the unit from the line.
This will be indicated by the ON LED, and can either be done after an outgoing call has been dialed on a telephone set connected to the unit, or to answer an incoming call after RING LED is seen flash (a telephone set is not required for incoming calls unless an audible ring is required).
If the sidetone level at the 4-wires output is found to be excessive, the outgoing signal level should be reduced or the balance controls adjusted to minimize it.
To ensure compatibility with all systems, a switch is provided to select either the inner or outer pair of conductors on the RJ11 connector.