Audio Signal Switch

all right today's video is going to be about how to switch audio signals let's say you got a couple of different sources maybe an FM radio and a tape deck and you want to feed them into some kind of device which you can select with buttons which one's going to be played but it's stereo output you have a stereo if a left and right of each so you want to be able to select both channels to be to go to the output and that output is also stereo obviously and one way you can think of this is with a a multiplexer or MUX and in the digital world this is used everywhere for to bus digital signals but this paradigm can be applied to analog electronics as well the most basic switch to switch audio or what you know one of the most the basic practical switches I suppose would be a J FET simply turn on its side and if you think about it a J FET is basically like a variable resistor controlled by its gate so if you just enable the gate by providing the appropriate voltage this will act as as a conductor then you'll get you can pass your signal or not pass your signal depending on what your control voltage is now the problem with that is this on resistance varies with the signal and the resistance changes and that causes distortion and that are on resistance is kind of high it's like hundreds of ohms like low hundreds made hundreds maybe a better way slightly better way is to use CMOS or complementary MOSFETs complementary meaning if a p-channel and an in channel together being driven by the same control voltage and by the same source so it's like they still have the same resistance curves but the opposite so in parallel together you get this much less extreme change in resistance as your signal varies to get less distortion of your signal because there's less on resistance and less change one way I implemented this was with a forty sixty six CMOS chip this is exactly what what this is it's basically a complimentary CMOS switch but there's four of them in the chip I'm only showing two here but there's four of them in the chips you can switch to stereo devices with this in this example I'll only be switching to on the left and a right channel coming out of a tape deck and it's feeding this thing now I have to have this 47 K resistor divider here on both and I figured this out after much experimentation and carefully reading the datasheet that did max input voltage is from negative point 5 volts to the drain voltage plus 0.5 volts which means if you put in a line voltage signal which could be up to 2 volts peak-to-peak anything below negative 0.5 anything in that negative half the sine wave is going to actually get through it'll pass through so what I did is I went ahead and put a voltage divider to put that input at half the voltage so it sits between the two extremes and so you don't get a this weird distortion crosstalk sound which is pretty terrible but that does mean you have to have capacitors for that to interface with your equipment because then now you have DC sitting on on your input output and so what we can do then is take these two control voltages and tie them together and we could just modulate that one we can switch both of these on and off and we'll do that with a transistor latch if you haven't seen one of these before it's basically two transistors hooked up such that there's positive feedback basically if you press one of these two buttons it grounds out one of the bases of these transistors these bjts and that circuit will change and then latch in that position or if it hasn't changed I'll just stay in that position you can connect these two collector hold or one of these two collector voltages to these two here so you press switch one it will like enable it if you press wish to it'll be enable or disable what the heck it'll disable both those controls at once thus turning on and off your your equipment now if you press switch one that will enable the control one like I just said but also it'll do that by turning off t1 because if this if this goes away if this turns off that's going to look a lot like the rail voltage 12 volts and but it's going to turn on q2 and it's going to pull control to low and it'll stay like that because of the positive feedback now here's an example of such a circuit right here we have the transistor latch transistor latch right here our left and right switch this turns it on this turns it off and we have a forty sixty six chip with our input and output capacitors it's also on a twelve volt rail right over there twelve volt rail so I've got this thing also hooked up to my tape deck right here that tape deck is running out into this thing and back into this amplifier and you can hear in a moment got my tape here and I'm going to play that the amps powered up okay fit play [Music] coming through the meters now I press this button gone nothing [Music] not tea bag a

Channel: The Tube Roaster

Views: 43,501

Keywords: MOSFET switching, analog signal processing, push buttons to control audio, audio signal circuit, how to switch an audio signal

Id: -5JOKztteVU

Length: 7min 27sec (447 seconds)

Published: Tue May 30 2017