There are two types of the audio transformer, which are the single level compatibility audio transformer and the impedance compatibility or matching audio transformer. The step up audio transformer's job is to increase voltage and the step down transformer is to decrease voltage. The output has more windings than the input. This difference in the number of windings of the audio transformer helps the signal to change as it moves through the transformer.
The single level compatibility audio transformer, unlike the step up and down transformer, has the same number of windings in both the input and the output. When the impedance of both the input and the output is the same, the signal does not change. Since the audio signal passes without modification, this kind of audio transformer can block DC and radio frequency waves. The benefit is that the signal can pass through many secondary windings of the audio transformer and the sound in each can be isolated.
The impedance compatibility audio transformers are of three types. One type of transformer is used for DC audio current blocking. The second type is radio frequency interference blocking (RFI) and the third type audio transformer is a ground lift device used to isolate audio devices.
The above is just the brief introduction about the main features of the two types of theaudio transformer.