Audio Transformers are transformers designed to be used in the amplifiers and high-frequency audio and voice circuits, for coupling ad impendence matching applications. The transformers allow an AC input signal to produce a related AC output signal without the input and the output being physically connected together. This is accomplished by having two or more coils of insulated wire wound around a magnetic metal core. When the AC signal passes through the input winding, a related AC signal appears on the output winding via the inductive coupling.
Audio Transformers perform several functions:
They can step up (increase) or step down (decrease) a signal voltage
They can increase or decrease the impedance of circuit
Convert the circuit from unbalanced to balanced and from balanced to unbalanced
Block DC current in a circuit while allowing the AC current flow
They electrically isolate one audio device from another
There are two main types of transformers:
Step-up / Step-down transformers – The primary and the secondary coils have a different number of windings, that means they have different impedances. Different impedances cause the signal level changed as it goes through the transformer.
1:1 transformers – Each coil has exactly the same number of windings which makes the impedance identical. This transformer doesn't change the signal level but guarantees the physical electrical insulation between coils. These transformers are usually called Isolation Transformers.