Found in a 1924 book by Gernsback, this circuit uses a double variable capacitor, a component not very common at the time. Listening was done on a loudspeaker at the time.
This circuit is used to test an inductor (coil) allowing the observation of its opration in an R-L...
We found this circuit in a Radio Electronics magazine from June 1992. The circuit has a power...
This different audio amplifier was found in a 90's Electronic Design. The circuit uses components...