Audio-frequency transformers:

Transformers for the use at audio frequency (AF) are similar to those employed for 60 Hz electricity. The differences are that frequency is higher (up to 20 kHz), and that audio signals exist in the band of frequencies (20 Hz to 20 kHz) instead of at just one frequency. Most audio transformers look like, and are constructed like miniature of the utility transformers. They have laminated E cores with along the primary and secondary windings wound around cross bars, as shown in the Figure given below.

Audio transformers can be the step up or step down type. However, instead of being made to produce the specific voltage, audio transformers can be designed to match impedances. Audio circuits, and in fact all the electronic circuits which handle sine wave or complex wave signals, exhibit impedance at input and output. The load has some impedance; a source has other impedance. The good audio design strives to minimize reactances in circuitry, such that the absolute value impedance, Z, is close to resistance R in complex vector R +jX. This means that X should be zero or nearly zero.

In the following discussion of the impedance matching transformers, both at the audio and at the radio frequencies, imagine that the reactance is zero, such that the impedance is purely resistive, which means, Z =R.