This power supply is based on the 723 integrated circuit, which is no longer very common today. The transistor must have a heatsink and the bridge can be formed by diodes 1N4002 or equivalent.
Found in a 1990s Popular Electronics, this circuit produces a tone on a small speaker. The keyer...
This second amplifier is based on the integrated LM108 and has a differential input. The resistor...
This 80's circuitry decodes stereo signals used in analog TV at the time. The circuit can be used...