Here's a circuit diagram of automatic muting for audio systems using IC LB1403. The signal from pre-amplifier such as LA3160, LA3161 or HA1032 is connected to the base of the amplifier transistor BC548 (T1). Variable resistor VR1 is used to control the gain of input signal.
Working of the automatic muting Circuit
Comparator 2 output at pin 2 of LB1403 is used for generation of muting signal at the emitter (point A) of transistor T2, which can be directly connected to muting pin 4 of amplifier employing IC LA4440. As long as the audio input to the circuit of Fig. 1 is below a certain level (say, 150 mV peak to peak), the output at point A will be high. Once the input crosses this threshold level, the output will be around 0V. Capacitor C4 determines the ‘on’/‘off’ muting delay. Higher the value of this capacitor, greater will be the muting delay period.
Slight circuit modification will be needed if this circuit is used with STK series amplifiers, such as STK 4141, 4142, 4152, and 4191, because they need negative polarity voltage for muting. The additional circuit to be connected at point A in that case is shown in Fig. 2.