Digital switch control of analog audio signal

Discussion in 'General Electronics Chat' started by dfro, Mar 20, 2009.

  1. dfro

    Thread Starter Active Member

    Feb 6, 2006
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    I would like to digitally control the switching of an audio signal from a pickup like a guitar pickup. My guess is that the signal can swing +1V and -1V.

    I was looking at the 74HC4016 "Quad analog switch" as a possible choice, but I am having trouble understanding the data sheets.

    Can I have Vcc at +5V and Vss at 0V and have it carry an analog signal that is swinging from 1V to -1V? Or do I have to do a setup where I set up Vcc at +5V and Vss at -5V?

    Any help would be appreciated.

    dfro
     
  2. dfro

    Thread Starter Active Member

    Feb 6, 2006
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    0
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If the signal is 2V p-p, you could bias it at 2V to feed to the switch, then remove bias with a decoupling capacitor at the output.

    I'll wait for other's opinions on this, as I'm not sure of the impact with Guitar level impedance signals.
     
  4. Darren Holdstock

    Active Member

    Feb 10, 2009
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    I recall one guy published such a circuit in a popular electronics magazine, many years ago, but he ran it off a single 9V battery and forgot to bias the signal chain. The next month the letters pages were full of people ripping him to shreds for this omission. The odd thing was, if one actually built his circuit, it worked, and didn't clip the negative half of the waveform as one might expect. One day when I'm not so busy I shall work out exactly why this should happen. Still wrong though.

    The safe bet is to AC couple the inputs and outputs (don't forget to add a pulldown resistor to 0V on the inputs and outputs or your signal may float and your cables will become crude microphones and crackle), and then bias the signal to half the supply voltage.

    Some things to look out for with CMOS switches:

    - The switching voltage is referred to Vss, so bear this in mind when biasing.
    - The switch on resistance is non-linear wrt voltage, so make the CMOS Rdson much smaller than the load resistance.
    - There is always feedthrough from the gate signal to the output. A sneaky way to minimise this is to use two CMOS switches in a potdown configuration, and then switch them in antiphase. In this way, the glitches tend to cancel out; it helps if the switches are in the same IC package as the gate characteristics will match closely. The dual-switch technique also helps attenuation, as you may find that a switch and a resistor won't give you the attenuation you need, and you'll still hear the switched-out signal very faintly.

    Alternatively, use one of the very nice SS switches from THATS corporation, as 4016s/4066s are a bit nasty for audio, even guitar.
     
  5. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Clarification: When I said "one guy" I didn't mean "thatoneguy" who posted before. The magazine article is probably about 20 years old now, and I don't recall the designer's name.I'll put that down to crosstalk in my brain.

    Also, after some checking it seems that THAT Corp don't currently do a CMOS switch, but the SSM2012 from Analog Devices is very nice - it's optimised for audio, has a 36V maximum dynamic range, and can be switched without pops and clicks.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Just doing my part to improve grammar worldwide. :D
     
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