This simple source with zener regulation is ideal for experimental projects with operational amplifiers. Other voltages can be obtained by changing the zeners and the transformer.
In the figure, from a documentation from the 70s, we have a few-watt amplifier for general use....
This highly sensitive light sensor was found in a 1970 General Electric documentation. The circuit...
This simple audio oscillator produces signals reproduced on a loudspeaker, in a frequency range...