This fm stereo noise suppressor circuit is to be used when the reception of far FM stations is bad but improves when you switch the radio receiver to mono. The cause of this effect is that in stereo reception the biggest part of the noise in the right channel is in opposite phase with the noise in the left channel.
Once the 2 channels are switched togheter (mono), the noise signals cancel each other and the result will be a clean and relatively noise free mono signal.
In order to use this effect in improving the signal but keeping the stereo character, the two channels should be switched together only in higher audio frequencies.
The stereo noise suppressor circuit is built in 2 identical channels each composed of 2 emitter-follower circuits. The switch S1 (single throw/3 poles switch) can connect the 2 channels together at 3 points. The circuit functions as a bridge between the left and right channel for opposing signal components above 8 kHz. These signals get “short-circuited” cancelling the noise. Otherwise, all other frequencies get through to the output via their respective channels.
If you want to lower the “short-circuit” freq. to 4 kHz, just double the values of C2, C3 and C4.
T1 = T2 = 2SC734
Sent by Jorge Valamos, ES.