The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This circuit was found in the June 1982 issue of Radio Electronics magazine. It provides reference...
With this source, it is possible to obtain a higher output voltage than that obtained by the...
This circuit was found in a 70s American documentation. The operational amplifier supports a modern...