This simple general-purpose transistor mixer was taken from an article in Electronics Now magazine in March 1994. You can choose a low-noise transistor such as the BC549.
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...