This circuit is from a 2004 documentation serving for signals from 1 MHz to 160 MHz. The antenna is of the telescopic type and the analog instrument is of the type found in old sound equipment.
This polarization step for electret microphones was found in an Electronics Now from the 90s. The...
This circuit was found in a 1998 documentation but uses common components that can still be found...
This circuit is from a 1974 publication, but it can still be easily assembled today with BC548...