This detector uses an unusual sensor today. The circuit is from an 80s documentation. Its sensitivity is 10 ppm. The transformer has two secondary voltages.
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...