Two side effects are introduced when adding a transformer in the audio amplifier system. First, the transformer has a frequency response limit. If the frequency is too high, the power losses in the iron core of the transformer increase and lead to lower efficiency in the transformer. If the audio frequency is too low (especially less than 50 Hz), the winding in transformer acts as a DC wire with a very low resistance, which also leads to higher power loss and potential system failure. The audio system with transformers has less bass response than normal. Also, the transformer can introduce extra noise, which leads to higher THD+N for in the audio output.

Second, the transformer in the up-conversion and down introduces additional power loss. For a typical transformer, there are two kinds of power loss: copper loss and iron (core) loss. Copper losses are dominant at low frequencies, and iron losses are dominant at high frequencies. For copper loss, the main parameter is DC resistance (DCR), and a higher DCR leads to higher copper loss.

For core loss, when the frequency is higher, it induces eddy currents flowing in the core according to Faraday’s law, which causes more power loss in the core (typically made of iron, so also called iron loss).