MC1490

The amplifier drives the base of a PNP transistor operating commonemitter with a voltage gain of approximately 20.The control R1 varies the quiescent Q point of MC1490P so that varying amounts of signal exceed the level Vr. Diode D1 rectifies the positive peaks of Q1's output only when these peaks are greater than Vr
7.0 V. The resulting output is  filtered by Cx, Rx.Rx controls the charging time constant or attack time. MC1490P is involved in both charge and discharge. R2 (the 150 kW and input resistance of the emitterfollower Q2) controls the decay time. Making the decay long and attack short is accomplished by making Rx small and R2 large. (A Darlington emitter follower may be needed if extremely slow decay times are required.)The emitterfollower Q2 drives the AGC Pin 5 of the MC1490P and reduces the gain. R3 controls the slope of signal compression.