The noise reduction circuit is based on the observation that noise is always disturbing in intervals when music stops. It will attenuate the signal output by about 45 dB, when the musical signal input is null or very weak.
If the musical signal begins to increase, the attenuation will decrease proportionally reaching the value of 0 dB during normal or strong passages.
Noise reduction circuit sensitivity can be regulated with potentiometer P1: if it is higher, the more rapidly decreases the noise suppression. In this way, the sensitivity of the circuit can be adapted to different sources of music.
The maximum audio signal which circuit can work is 210 mV. Distortion is less than 0.01%. Time delay of the noise reduction circuit is determined by the time constant R7C4. For the schematic values, it is 1 second, but obviously can be changed at will. The electronic circuit can be supplied with a voltage between 12V and 30 V, and current consumption will be 2 to 3mA. Here are some 12Vdc power supply circuits.