Function of Microphone Transformer

The audio transformer can be used for matching microphone to input of amplifier.
The main function of the microphone transformer is to matching impedance between microphones, connecting cables and amplifier input.
It should be noted that there is no signal reduction during impedance mismatching.

Microphone Cable

Figure shows long microphone cable consists of two conductors twisted together which are surrounded by conducting metal foil or braid.
As two conductors are twisted, magnetic fields generated by each conductor tends to cancel out.
The earthed conducting foil protects conductor from external magnetic field.

A microphone is connected to cable through single primary winding and center taped secondary winding transformer.
As cable is fed from a center taped transformer, signals on two conductors are in antiphase.
The difference between two input signals applies at the amplifier input, the output signal is doubled in amplitude.
Any noise at the input signal cancel out automatically because two conductors are twisted each other in the opposite direction.
The combination of signal at the amplifier can be carried out either by differential amplifier or balun.
A balun is a balanced input to unbalanced output device.

Figure shows microphone transformer that plugs directly into high impedance amplifier input.
The XLR socket at the other side allows low impedance microphone to connected via a long lead.
A metal screening case in the transformer acts as a balun and impedance matching device.