An audio transformer is an electromagnetic device that is intended to isolate an input circuit from an output circuit and possibly also provide filtering to a signal that passes through it. It is an audio device because it is designed to operate on signals with a frequency spectrum in the audible band of frequencies (typically around 20Hz-20kHz).

 

A transformer isolates input and output because it AC couples (there being no DC signal path), the primary circuit by coupling its energy across to the secondary through induction by the alternating current creating a changing magnetic flux. This changing flux induces a varying electromotive force (voltage) in the secondary winding connected to the other circuit.

 

Isolation has the effect of removing the DC term on the input from the signal on the output. Input signals can also have different impedances on the inputs of the transformer (unbalanced) that can be transformed to a symmetrical output impedance (balance). A balanced circuit is less susceptible to noise if it is coupled onto both output conductor circuit paths equivalently (common mode noise). This transformation can contain a change of the ratio of the current to the voltage by designing the audio transformer to have different primary and secondary windings (step-up and step-down). As there is a fixed relationship between voltage, impedance and current (V = I . R) than this transformation has the effect of changing the effective impedance of the source signal as seen by the load. By making this source impedance the same as the output load impedance, the circuit is said to be matched.

 

Audio transformers operate in the audible signal range where there is considerable noise in the environment from sources like mains power and inverting power supplies. This noise can easily couple into a transformer and produce an effect that may lead to hum. As a result, audio transformers typically have magnetic shielding to protect against this noise coupling into its circuits.

 

Audio transformers can introduce noise and distortion due to their non-linear effects. Low-level signals can not be high enough levels to energize the magnetic core linearly, or be so high in level as to saturate the magnetic cores ability to contain the changing flux. These effects introduce harmonics and other products that distort the signal. High frequencies can also be attenuated by stray capacitive or leakage inductance effects and distort the signal.