This circuit was found in a Delco 1965 publication. Transistors may be current equivalents according to the desired output voltage and current.
This 1965 Philips circuit uses transistors from the era. Modern equivalent components can be used....
This circuit reproduces the signal from a nearby transmitter in a phone. The earphone must be...
This more sophisticated version compared to the previous one in this section was also found in 90s...