Audio-frequency transformers

Transformers for use at audio frequency (AF) are similar to those employed for 60-Hz electricity. The differences are that the frequency is somewhat higher (up to 20 kHz), and that audio signals exist in a band of frequencies (20 Hz to 20 kHz) rather than at just one frequency.
Most audio transformers look like, and are constructed like miniature utility transformers.
They have laminated E cores with primary and secondary windings wound around the cross bars.
Audio transformers can be either the step-up or step-down type. However, rather than being made to produce a specific voltage, audio transformers are designed to match impedances.
Audio circuits, and in fact all electronic circuits that handle sine-wave or complex- wave signals, exhibit impedance at the input and output. The load has a certain impedance; a source has another impedance. Good audio design strives to minimize the reactances in the circuitry, so that the absolute-value impedance, Z, is close to the resistance R in the complex vector R + jX. This means that X must be zero or nearly zero.
In the following discussion of impedance-matching transformers, both at audio and at radio frequencies, assume that the reactance is zero, so that the impedance is purely resistive, that is, Z= R.