Stereo Indicator

On most FM tuners, the stereo indicator lights upon detection of the 19 kHz pilot tone. How- ever, this need not mean that the programme is actually stereophonic, since the pilot tone is often transmitted with mono programmes also. A similar situation exists on stereo amplifiers, where the stereo LED is simply controlled from the mono-stereo switch. The LED-based stereo indicator described here lights only when a true stereo signal is fed to the inputs. Differential ampli- fier Al raises the difference between the L and R input signals. When these are equal, the output of Al remains at the same potential as the output of A2, which forms a virtual ground rail at half the supply voltage. When Al detects a dif- A1...A4 = IC1 = LM 324 D1...D6 =1N4148 8...30V 0 07 36 SUPPLEMENT C1 202/16VRi C2 202/16V R3 T1 BC 547 R13 87420 ference between the L and R input signals, it supplies a positive or negative voltage with respect to the virtual ground rail, and so causes C3 to be charged via ...