[attachment=15558]
Circuit of an automatic switch for audio power amplifier stage is presented here. The circuit uses stereo
preamplifier output to detect the presence of audio to switch the audio power amplifier on only when
audio is present. The circuit thus helps curtail power wastage. IC1 is used as an inverting adder. The
input signals from left and right channels are combined to form a common signal for IC2, which is used
as an open loop comparator. IC3 (NE556) is a dual timer. Its second section, i.e., IC3(b), is configured
as monostable multivibrator. Output of IC3(b) is used to switch the power amplifier on or off through a
Darlington pair formed by transistors T1 and T2. IC3(a) is used to trigger the monostable multivibrator
whenever an input signal is sensed. Under ‘no signal’ condition, pin 3 of IC2 is negative with respect to
its pin 2. Hence the output of IC2 is low and as a result output of IC3(a) is high. Since there is no
trigger at pin 8 of IC3(b), the output of IC3(b) will be low and the amplifier will be off. When an input
singal is applied to IC1, IC2 converts the inverted sum of the input signals into a rectangular waveform
by comparing it with a constant voltage which can be controlled by varying potentiometer VR1. When
the output of IC2 is high, output pin 5 of IC3 goes low, thus triggering the monostable multivibrator. As
soon as the audio input to IC1 stops, pin 5 of IC3 goes high and pin 1 of IC3 discharges through
capacitor C3, thus resetting the monostable multivibrator. Hence, as long as input signals are applied,
the amplifier remains ‘on.’ When the input signals are removed, i.e., when signal level is zero, the
amplifier switches off after the mono flip-flop delay period determined by the values of resistor R8 and
capacitor C3. If no input signals are sensed within this time, the amplifier turns off—else it remains on.
Power supply for the circuit can be obtained from the power supply of the amplifier. Hence, the circuit
can be permanently fitted in the amplifier box itself. The main switch of the amplifier should be always
kept on. Resistors R1 and R2 are used to divide single voltage supply into two equal parts. Capacitors
C1 and C2 are used as regulators and also as an AC bypass for input signals. Diode D1 is used so that
loading fluctuations in power amplifier do not affect circuit regulation. Transisitor T2 acts as a high
voltage switch which may be replaced by any other high voltage switching transistor satisfying
amplifier current requirements. Value of resistor R10 should be modified for large current requirement.
The LED glows when the amplifier is on. The circuit is very useful and relieves one from putting the
amplifier on and off every time one plays a cassette or radio etc