This balance indicator for stereo amplifiers was found in an 80's Elektor. The components used are common. Zeners are 1.8 V or close to that value.
In the figure we have a circuit suggested by National Semiconductor, using an operational...
For experiments with ionization chambers and Geiger valves this circuit generates high voltage from...
We found this circuit in a 1979 documentation, but it can be changed to use more modern...