We found this circuit in a Radio Electronics from October 1963. The circuit uses an output transformer with a center tap. The trimpot is adjusted for higher gain and lower quiescent current.
This circuit can also use other series operational amplifiers such as MC33172, MC33174, MC35171,...
Found in Popular Electronics from the 90s, this circuit mutes a scanner's speaker in the absence of a...
This circuit was found in General Electric documentation from 1965. The transistors are from that...