This circuit was found in an English magazine from 1977. Signal inputs and outputs must be shielded, and the circuit can be powered by battery, since its consumption is low.
Found in Precision Monolithics documentation from the 90's, this circuit equalizes octave signals...
This nice analog indicator circuit was found in the January 1974 issue of Radio Electronics...
This small amplifier, with output less than 1 W, was found in a 1960 transistor manual from...