Auto Fade

This automatic fader, drops the level of the background music when the narration comes up.  The control input goes through R10, a preset audio level control, to the input of an emitter-follower buffer stage (Q1). The buffer offers a high input impedance and makes sure that the source impedance is low enough drive the rectifier and smoothing circuit, which consist of D1, D2 and C5. The smoothed output drives a simple LED circuit. R8 and LDR1 form an input attenuator across which the output is fed via C6 and C7 to the output jack. The output at the emitter of Q1 couples to this socket through C4 and R5. R5 and R7 are a passive mixer.

When you’ve got 200 mV or less at the input, there isn’t sufficient voltage across C5 to make Q2 turn on. Over 200 mV, Q2 does turn on to a limit, and the LED gets power. That makes the LDR’s resistance fall, and signal loss through the attenuator increases. Increase the input to 350 mV RMS, and you get a signal reduction of better than 20 dB.