This circuit was found in the June 1990 issue of Radio Electronics magazine. The maximum current is 1 A and the integrated circuits must be mounted on heat sinks.
In the figure we have the way to implement a 5 to 45 V zener diode with 50 mA of maximum current...
This circuit is from a Radio Electronics magazine from December 1958. The indicator instrument is analog...
These integrated ones consist of class B amplifiers with gain fixed 24 internally for better...