This crystal or galena radio was found in documentation from the 1990s. The earpiece must be a high impedance crystal or magnetic earpiece and D1 can be any germanium diode.
This filter, from an audio publication from 1977, passes the frequency range corresponding to the...
In the figure we have three voltage doubler configurations that can be used in high and low...
This circuit was found in Motorola documentation from 1966. The lamps are according to power and...