The baseband-audio input comes from the pole of switch Sl in the stereo decoder, and is coupled to IC1 (a CA3089) via a 78.6 kHz bandpass filter that consists of capacitors Cl and C2, and inductor Ll. IC1 is a combination i-f amplifier and quadrature detector normally used for FM radio systems operating within an i-f of 10.7 MHz. The device works equally well at 78.6 kHz. Capacitors C6 and C7, and inductor L2 tune the detector section to 78.6 kHz, while C5 provides the necessary 90-degree phase shift for proper quadrature detector operation.
Second-audio-program-adapter - schematic

The output voltage at pin 13 of IC1 is proportional to the level of the incoming signal. When the voltage at the wiper of potentiometer R3 reaches a predetermined threshold level, Ql conducts, grounding pin 5 ofIC1, enabling IC1"s mute function. Detected audio output from pin 6 of IC1 goes to IC2a, which is configured as a 12-kHz, -12 dB per octave, low-pass filter. The output of IC2a appears across potentiometer RlO, which provides a means of adjusting the drive level into IC3b, the 2:1 compander. Audio from the wiper of RlO is split into two paths: a high-pass filter (C14 and R8) provides a path to the rectifier input of the compander, and a bandpass filter (R9, C16, and C15) that feeds the audio input of the compander. A fixed 390-~ts de-emphasis network is formed by C18 and Rll in conjunction with IC3b. Corrected audio appears at pin 10 of IC3b and is coupled to IC2b, and output buffer amplifier. Audio from pin 6 of IC1 is also coupled to an audio high-pass filter, R5 and ClO, and fed to an audio rectifier, Dl, D2, and Cll. When a SAP signal is detected by IC1, it is rectified by D1 and D2; the resultant de charges Cll. An increasing positive voltage at the base of Q2 causes its current flow to decrease, so the voltage at Q2"s collector also decreases. That in tum causes the base voltage of Q3 to drop, which causes Q3 to conduct, thereby lighting the LED.

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