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 from an old Argentine publication shows how it is possible to use a tunnel diode in a...
To invert the rotation of a motor by changing the levels of an Arduino output or another...
This ultra-low distortion driver was found in 80's Analog Devices documentation. The circuit has a...