This simple metal detector was found in a 1960 Sylvania transistor manual. The phone must be high impedance or crystal.
This circuit was found in ancient American documentation. It provides a 70 Vpp signal at...
This circuit appears in various parts of this site and this section, with diverse origins. This is from...
This stage of IF to 455 kHz operates with voltages from 1.5 V. We find it in a 90s Popular...