This circuit scan all the preamplifier entries to identify where is a audio signal and that entry remains selected. After a sign of signal absence, scanning resumes. Scanning action is supported by rectangular signal generator IC2c. Scanner’s entries are connected to the audio entries from volume control board of the preamplifier.
The audio signal is amplified by 40 times with IC1c and IC1d, then summator amplifier IC1a combine left and right signals. As soon as a signal of music or vocal appears on entry lines, C1 partialy discharges, very quickly. As long as it receives the input signal, a part of the capacitor C1 load is discharched through IC1a.
If the signal is missing for a period of time (adjustable with P2 between 3s and 25s), the capacitor will charge completely through P2, R8 and D1. Once voltage at the terminals with C1 increase over the comparator threshold, oscillator is activated again and repeats the action of scanning.
A full scan of all entries ends in 3 s (determined by R11-C2).
Input sensitivity can be adjusted between 10 mV and 4V with P1. Scanner absorbed current does not exceed 10 mA.