This power supply has a resolution of 10 pA in the current range from -100 to +100 nA. The circuit is from a 90s Electronic Design operating with voltages from -4 to +3.5 V.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This traditional crystal radio circuit, but which has a modern version with diode, was found in an...
This headphone amplifier using a pickup coil can be used as an extension for devices such as TVs,...