This circuit is from a Radio Electronics from August 1992. The transistor used allows the control of motors up to about 5 A. It must be equipped with a heat sink.
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...