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Audio signal source selector

2014-11-26 06:19  
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A very simple circuit that can be used to switch between two signal sources can be constructed using this diagram . This circuit helps you to choose one ( from two ) audio signal sources connected the audio amplifier stage .

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For this audio signal source selector you don’t need to use any mechanical contact or push buttons , because you can choose between two stereo sources by simple touch of your hand. This circuit is very compact and can be fixed within the audio power amplifier cabinet and can use the same power supply source.

The circuit uses just two CMOS ICs and a few other components. The ICs used are MC14551/CD4551 (quad 2- channel analogue multiplexer) and CD4011 (quad 2-input NAND gate). When touch-plate S1 is touched, gate N1 output (IC1, pin 3) goes high while the output of gate N2 at pin 4 goes low. This causes selection of CD outputs being connected to the power amplifier input, which is indicated by lighting of LED 1.

When touch-plate S2 is touched, the outputs of gates N1 and N2 toggle. That is, IC2 pin 3 is pulled 'low' while its pin 4 goes 'high'. This results in selection of tape recorder outputs being connected to the input of power amplifier. This is indicated by lighting of LED 2.
Pin 9 is the control pin of IC2. In the circuit, the state of multiplexer switches is shown with pin 9 'high' (CD source selected). When pin 9 is pulled 'low', all the switches within the multiplexer change over to the alternate position to select tape player as source.
Pin 7 (VEE) of IC2 can be connected to ground, but for operation with preamplifier signals going above and below ground level, one must connect it to a negative voltage (say, -1V to -1.5V) to avoid distortion.
This audio signal selector needs to be powered by a 5 volts DC power supply .
This circuit is very good if you don’t want to hear pop noise in speakers when you change between two audio signal sources .
If want to try another signal switch circuit that supports many inputs , you can try this circuit diagram based on TDA1029 IC.
audio signal source selector circuit