Since magnetic field intensity is proportional to current in the winding, very low frequencies produce very strong magnetizing forces on the core. A given core material will concentrate the magnetic field only up to a certain level. Above this point, even though the winding may produce an ever increasing magnetizing force, the magnetic field in the core will not change. Another distortion mechanism involves the core material's behavior at low magnetic field levels. While controlling this distortion is not a consideration in power transformers, for audio transformers it can makes the difference between simply useable and excellent.
Distortion is almost entirely due to the core material.
Power transformers use silicon steel alloys because it is cheap and has good magnetic saturation properties, but its hysteresis distortion properties are horrible. Only a few very specialized audio applications exist for this material, although it is frequently seen in very cheap audio transformers turned out by manufacturers who know nothing about audio. High performance, low distortion transformers are made with cores of a special nickel, iron, and molybdenum alloy that has been processed to enhance the uniformity of its crystalline structure. Noisy and sometimes quite large voltages exist between them. In theory, the solution is simple.