This is a functional AM modulator (and with the help of a rectifier, an amplifier) built entirely out of transformers and inductors: (It does not look like there is gain here, because of the different impedance’s, but this does actually produce a stronger output signal than the modulating input.) How is this possible without any transistors, vacuum tubes, or even diodes? Ferrite and iron cored inductors and transformers exhibit core saturation, where once a certain amount of magnetic field is flowing through the core, it becomes much harder to pass more through it.