This circuit, found in a 1958 magazine, is for reference only, since the components are not easy to find today. The transistors are germanium types and power supply is made by common cells.
Found in an American 1980s documentation, this circuit makes use of a sensor of the era uncommon...
This interesting configuration of optical triggering using an oscillator was found in a 90s...
This headphone amplifier has good gain and high input impedance. The circuit is from Electronics Now and...