This two-microphone amplifier was found in a April 1961 Radio Electronics Magazine. The circuit is powered by battery and has a 600-ohm line output.
This radio version with a transistor for a crystal phone was found in a Japanese magazine from...
In this circuit of Texas Instruments an incandescent lamp is used as a light source, and lenses are...
This source whose voltage depends only on the zener diode used, can supply currents up to about 3 A....