These circuits were found in the February 1995 issue of Electronics Now magazine. The circuits have normal response in (a), boost in (b), cut in (c), and plate response in (d).
This amplifier with gain control was obtained in a documentation from Signetics of the 80's....
Found in 1960's Radio Electronics, this circuit provides an audio tone that depends on the logic...
This circuit can be used as a signal when powered by a car battery, as it also works with 12 V, or...