Audio Level Threshold Control
Circuit diagram:Audio Level Threshold Control Circuit Diagram
The signal at "C" is inverted by transistor Q1 to produce "D". These two signals are used to control a pair of CMOS switches (S1 & S2), which either pass the audio signal to the output or short it to ground. The signal from the CMOS switches is buffered by IC3, which in conjunction with the 10kΩ resistor and 10nF capacitor filters out the switching artefacts. In practice, the offset voltage is adjusted until there is little or no breakthrough of the noise background at the output. Thereafter, only audio signals exceeding the threshold are passed. Inevitably, this produces some crossover distortion but this is of little consequence compared with the benefit of the quiet background.
Author: Graham Jackman - Copyright: Silicon Chip Electronics