The basis for the switch is the CD4053 analog multiplexer IC. This is a CMOS logic chip that contains three SPDT CMOS analog switches and the attending control logic to make them work. People who have tried these before have often complained that they pop or distort, and have then moved on and forgotten them.
There are some tricks to using this chip. First – the analog inputs and outputs need to be held somewhere near the middle of their power supply. The 4053 does have a “Vee” supply pin that lets it control signals near 0V, but that is often a fair amount of trouble to use. the simplest thing to do is to AC couple the in/out pins and bias them to the middle of the 4053’s power supply. In a +9V system, this only needs a two resistor divider and a cap, then a resistor and capacitor per in/out pin.
Yes, this is a pain, eats up board space, and is complicated. It works, though. When biased this way, the CD4053 has little or no distortion and no perceptible switching pops – exactly what we want.
The three SPDT switches are independent. As noted in the illustration, when “A” (pin 11) is low, “X” (pin 14) is connected to “X0” (pin 12). When “A” is high, “X” is connected to “X1” (pin 13). “B” and “C” do the same switching control for “Y” and “Z”. “Low” means below 1/3 of the Vdd voltage, or 3V in a 9V battery system. “High” is over 6V. Like all CMOS, it’s important to NEVER leave an input pin unterminated, so all control pins that are not being used must be tied to ground or some definite logic level.
When a switch is connected, it looks like a moderately nonlinear resistor between pins. The resistance varies with the signal voltage from pin to pin, which really means that for low distortion, you have to keep the current through the switch low. The switch looks like a 120 to 500 ohm resistor. So if it’s switching things that are themselves 10’s of K ohms or higher, the distortion will be low.
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