Found in a 1970 documentation, this circuit makes use of 741 operational amplifiers that are common today. Note that the power supply must be symmetrical.
Found in a 1968 US publication, it is a font filter. The transistor is according to the desired...
This circuit has an interface for a multimeter or electronic voltmeter. We found it in a 90s...
Found in an American publication from the 90s, this circuit allows the supply of 3, 6 and 9 V...