Most of today’s audio amplifiers use a Class-D architecture. This architecture benefits from offering high power efficiency compared to traditional audio amplifiers. However, at their output, these amplifiers require a low-pass filter which is designed to remove the unwanted switching components which are generated by the switching stage. This low-pass filter is typically formed by a coil and a capacitor. Some more advanced low-pass filters operate with two coils and two capacitors.
However, the coil material has an impact on amplifier performance. In particular, the material will impact the distortion as well as the losses of the amplifier. Most notably, one parameter affects performance. It is called permeability. This parameter describes how many windings are required in order to achieve a certain inductance given a certain type of coil. The larger this parameter, the lower the number of windings per inductance.
Having a low number of windings has the advantage of having low losses due to resistance within the wire. However, oftentimes there will be hidden losses caused by eddy currents. Therefore, inductors which have a low number of windings usually have a larger loss when connected to a switching voltage.
Another important parameter when evaluating inductors is how much distortion is being caused by the inductors. Usually, an air-core inductors the ideal inductor. This type of inductor has no distortion whatsoever. Unfortunately, the disadvantage of using an air core is that the magnetic field will actually spread out quite a bit and not stay focused within a small area. Also, these types of inductors require large number of windings.
Typically, iron powder materials are well-suited for inductors that are used for audio amplifiers. These inductors offer a low amount of distortion and still allow inductors to be fairly compact.