Logiktester (mit 3 LEDs (hi, low, undef.))

This very simple circuit, giving an indication about the pres- ence of stereo information in a music signal, is connected to the loudspeaker outputs of a stereo amplifier. The input level is adjusted with stereo poten- tiometer PI. Both the R- and the L-signal are half- wave rectified via the base-emitter junctions of T 1 and T2, respectively , producing `pulsating` collector currents. If the L- and R- signals are equal (mono information), the collector voltages of TI and T2 are equal, too, so neither of the LEDs will light up. If the signals are unequal (stereo) one or both of the LEDs will light.