This crystal or galena radio is one of the many versions available on this website. This is from a documentation from the 70s. The coil is formed by 50 + 50 turns of wire 28 in a ferrite rod.
Using a 741 op-amp this circuit high-pass to 10 kHz was found in the February 1995 issue of...
Found in 1965 General Electric documentation this circuit applies to a LASCR used as an optical...
Using transistors and lamps, this simple circuit was found in Popular Electronics. The circuit can...