English: Circuit diagram of an early radio receiver using a carbon microphone as an amplifier from 1919. Before the triode vacuum tube a few radio receivers used a combination of an earphone and a carbon microphone to amplify the signal. This drawing is from an advertisement for microphones by the G. Boissonnault Co. showing how an earphone and carbon microphone taped together can function in a radio circuit as an audio amplifier. The carbon microphone, consisting of loose carbon granules held in a "cell" between two electrodes, worked by the varying pressure of sound waves changing the resistance of the carbon.
The radio waves picked up by the antenna pass from the primary to the secondary of the tuning coil (LC). The variable capacitor (VC) forms a resonant circuit with the secondary of the transformer (S) which selects the frequency of the radio station desired. The detector rectifies the radio signal to pulsing DC, extracting the modulation (audio signal) from the radio carrier wave. The fixed capacitor (FC) smoothes the rectified signal, removing the carrier pulses, leaving the audio signal. This audio signal is applied to an earphone (sensitive 1000 ohm radio telephone receiver) taped to a carbon microphone (super sensitive microphone transmitter). In the earphone driver it passes through the windings of an electromagnet creating a varying magnetic field which vibrates the earphone's diaphragm, creating sound waves. The sound waves strike the diaphragm in the carbon microphone, vibrating it. This puts varying pressure on the carbon granules, varying their resistance. A current from the battery passes through the carbon. The varying resistance causes a varying current to flow, creating an audio signal that is a duplicate of the signal applied to the microphone. Since the current from the battery is much stronger than the current from the detector, the output audio signal is much stronger than the input; it has been amplified. The current from the microphone passes through the loudspeaker ("loud-talking telephone") producing sound. The volume can be controlled by the rheostat.