The speaker transformer, audio transformers or AF transformers are working at low frequencies that lie between 20Hz to 20kHz and are basically used in circuits audio amplifiers. These transformers were instrumental in the days of vacuum tubes to match the high impedance amplifier outputs with low impedance speakers.
audio transformer is a transformer of "broadband", in essence it is a transformer two or more windings attached by a common magnetic field. That magnetic field which provides the means for passing voltages and currents of the primary winding to the secondary winding.
The Audio Adapter
Essentially, the main purpose of an audio transformer is to isolate the DC with minimal loss and engage complex impedances to the AC signal. But currently the transistor amplifiers do not have the need for audio output transformers and their isolation is not necessary due to low working voltage.
Basic specification Audio Transformer
Impedance: 1kohm / 8ohm
Frequency response: 300Hz to 3 kHz ± 2,5 dB
Primary DC Resistance: 100 ° ± 10%
Secondary Resistance DC: 0.7The ± 10%
Normally the audio transformer has a primary impedance 1 k Ohms and secondary 8 Ohms, but there are a multitude of impedances, and even audio transformers 1:1 which are used only for insulation.