Transformers are electrical devices that allow an AC input signal (like audio) to produce a related AC output signal without the input and output being physically connected. This phenomenon is called inductive coupling. Inductive coupling occurs by having two or more coils of insulated wire wound around a magnetic metal core. These wire coils are called windings. When an AC input passes through the input winding (the primary), a related AC out signal appears on the output winding (the secondary). This wireless magnetic field communication is preferred over expensive and inefficient downtimes of physical connectors that wear down quickly due to mating and de-mating. In this article, we will focus on transformers designed for audio.
There are two basic types of audio transformers, step-up / step-down transformers and Unity 1:1 transformers. In a step-up / step-down transformer, the primary and secondary have a different number of windings, thus they can create different impedances. These impedances cause a signal level change as it passes through the transformer. When the secondary has more windings than the primary (a higher impedance), the secondary signal level will be a higher voltage than the primary. When a transformer has multiple taps access to multiple impedances and different signal gains or losses can be achieved. For example, a microphone may utilize a step up transformer that ultimately steps up the signal level and impedance of input currents, resulting in an amplified audio output.
Unity 1:1 transformers, often called isolation transformers, use the same number of windings on each coil. Identical impedance of the primary and secondary, ensures that the signal level does not change. These transformers transfer an unmodified audio signal from the primary to the secondary and also block DC voltage and radio frequency interference (RFI).
Limitations for audio transformers include frequency response, a maximum input level, and the inability to step up a signal more than 25 dB (when typical audio circuits are used). Audio transformers will reduce or block signals that are below or above the audio range of 20 - 20,000 Hz. When the max allowance is surpassed, distortion occurs and the transformer is considered “saturated” and cannot hold anymore signal