In the receiver, stereo radio station. Usually have more noise than listening in mono. Because, the stereo system is splits signals-to-noise ratio. (S/N) There level lower, cause interference, in mono with less. becausePhase noise of the 2-back phase. When combined into a mono signal was decreased by default.
As principle the noise is high frequency, thus if both high frequency be mixed together. The noise will go out while the sound still normally stereo system.
Figure 1 Noise Filter for Stereo system
Figure 1 include the emitter common follower output circuit in series (cascade) in each channel. The high frequency noise in 2 channel will be refuted,by through high pass filer include R3 to R7 and C3 to C5. This frequency is in 8 Khz range. When switch S1 close circuit and S1 open circuit the operation will stop but there are resistors R9 to R11 drop across S1 to maintain DC voltage level for capacitor C3 to C5. Thus when turn on switch S1 no sound “tup” out to the speaker.
The frequency will cut of without changing with add or reduce value of C3 to C5, which if add capacitance cause frequency be low down.