This circuit was found in a Mallory documentation from 1968. The integrated circuit is of the time, unusual in our day. Input sensitivity is 20 mV for 2 W output.
In the figure we have the way to activate a red LED that needs 1.7 V or even others with higher...
This circuit was obtained in a 1979 publication, but it can be assembled with the BD135 instead of the...
Found in Radio Electronics from the 90's, this circuit employs a 3140 operational amplifier, still...