Background-Noise Suppressor

06 BACKGROUND-NOISE SUPPRESSOR Hiss, crackling, and other discordant sounds are disconcerting and frequent sources of annoyance to most music lovers. Unfortunately, the sources of this background noise are not easy to eliminate, but the circuit proposed here will be of help. It should be ap- preciated, however, that the suppres- sion of noise is always a last resort: the best way of getting rid of it at source. The circuit is based on the fact that background noise is always at its most annoying during quiet music passages. It attenuates the output signal by some 45 dB when there is no or very low music signal input. When the input rises, the attenuation decreases propor- tionally, becoming 0 dB with normal to loud passages. The input signal is taken direct to the output terminals via RI: and R12 respect- ively. At the same time, they are summed via R: and R2 and applied to non-inverting amplifier IC: via poten- tiometer Pi. The cross-over point in the gain characteristic of IC: i...