This very low current circuit was found in the April 1995 Electronics Now magazine. The frequency should be chosen for maximum efficiency according to the small transformer used.
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...