Removing an audible “pop” from a CMOS switch - Ramping Power?

Discussion in 'The Projects Forum' started by danalog, May 14, 2012.

  1. danalog

    Thread Starter New Member

    May 8, 2012
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    Hi,

    I have a simple audio switching circuit that is giving me a problem. I am using a CD40106 Schmitt Trigger in conjunction with a CD4066 CMOS switch to route up to two audio signals between “wet” & “dry” circuitry.

    The circuit is DC and will run on 9 – 15 volts. It will always be under 100 mA so we’re not talking a lot of power.

    The circuit works well, except that with an amplifier turned way up I can hear a very small “pop” in the audio when the circuit gets engaged. I know, most of you think “Well turn the amp down dummy!” but I am trying to totally isolate all the sound so there is a seamless switch from one state to another.

    I know that the issue could be taken care of with pull down resistors or capacitors. However these seem to remove some volume and degrade the sound quality a bit, the goal is to have the volume level that enters the unit match the output volume so I am trying to avoid that route.

    I have read that this is due to a split second of Capacitance Coupling into the signal line. I believe that Ramping the control signals voltage up and down a few milliseconds with triangle waves to the CMOS can eliminate the issue but I cannot find an example of how to really implement the concept into my design.

    So what do think? Would an Astable Multivibrator creating triangle waves make the difference? And where would it go in the circuit?

    Thanks in advance for ALL of you out there helping.

    Danny
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U did quite well with that circuit.

    So ! U hearing popping sounds eh!

    To remove the pops u need to mute the amp when ever the circuit switches.
    A transistor mute circuit can be inserted between the out and amp input.
     
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  3. danalog

    Thread Starter New Member

    May 8, 2012
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    Thank you very much! Glad to know I am heading in the right direction so far.

    That sounds like a great idea and I am currently searching through the web to grok the idea.

    Would I need 4 transistor mute circuits in all, one for each of the outputs? (2 wet 2 dry)
    Could this be placed in front of the CD4066 to simplify the design and only need 2 of them?
     
    Last edited: May 14, 2012
  4. R!f@@

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    To simplify u need to find out from where it is being generated.

    Outputs are never muted. Inputs are
    The trick is to ground the input during switching period
     
  5. rogs

    Active Member

    Aug 28, 2009
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    You cannot eliminate all switching noise, using 4016 or 4066 CMOS analogue switches. You can get close ---making sure there is as little gain as possible after the switch --- making sure both switch poles are kept at the same DC potential (ideally half rail) at all times---- but you can't 'slow down' the gate transient, as you can with normal FET audio switching.

    The attached article, from an old National Semiconductor data book explains.
     
  6. danalog

    Thread Starter New Member

    May 8, 2012
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    I believe it is originating in the CD4066 as capacitance coupling due to the sudden surge of signal. I removed the CD40106 from the circuit to test my theory by using only the CD4066 being manually triggered. My results from that test were that it still have the pop when switching.

    I have read a few places that CMOS can leak a little control signal into the ins and outs which can result in an audible pop. That is what originally led me into thinking about ramping the control voltage in and out, or even an optoisolator.

    Can you send a circuit example link or pic for the transistor mute to help me understand a bit better? That would be very helpful.
     
  7. danalog

    Thread Starter New Member

    May 8, 2012
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    Thanks for that article, that does clear up some things. Do you by chance have the picture of figure 2.10.1 that it references? I'd love to take a peek at that.
     
  8. R!f@@

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    I need some time to draw up a circuit.

    In addition to the circuit which is just one SS transistor and some resistors, we have to make another circuit to activate the mute circuitry when u switch the device.

    Can u post some pictures of the device.
     
  9. danalog

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    May 8, 2012
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    I have the switch circuit on a breadboard if that is what you want a picture of.
     
  10. R!f@@

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    I wanted to know how it is switched.
     
  11. danalog

    Thread Starter New Member

    May 8, 2012
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    The momentary foot switch sends the signal to the CD40106 and causes the state change of the audio signal.
    The audio signal comes in the unit from a musical instrument on a 1/4" jack and enters the CD4066 on pin 2/3 or 9/10.
    Depending on the state of the CD40106, the audio signal is either routed into the actual effects circuit or on to the output section which again connects with a 1/4" jack. From there the signal will go on to other pedals or an amplifier.
     
  12. R!f@@

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    Apr 2, 2009
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    So, when ever the foot switch is pressed u are hearing POP. ?

    And tell me, U do have a Mixer connected to the power amps or is it a Mixer with an Amp.

    U have to connect the mute in between the Main amp and it's input. This is the best point to cut off pops
     
  13. danalog

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    May 8, 2012
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    Yes, I hear a quick small pop each time the foot switch is engaged.

    No mixer involved currently. In it's simplest form it would be guitar -> Router -> "Wet" or "Dry" -> Signals re-meet at the Output of the unit -> Guitar Amp.
     
  14. R!f@@

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    Need to know the connection type between pedal and amp.

    RCA or XLR ?
     
    Last edited: May 15, 2012
  15. danalog

    Thread Starter New Member

    May 8, 2012
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    Both quarter inch or XLR cables are used to connect to and from the pedal but for now lets think about it as quarter inch to simplify.

    tip sleeve style - tip = signal, sleeve = ground
     
  16. R!f@@

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    I just checked ur diagram.
    where did you got tht ?

    If you are dealing with audio, tht circuit is a poor design.
    No wonder u are getting pops. U need to use coupling caps to block DC.

    U don't need a muting circuit. Just add 2.2μf or 4.7μf rated at 50V Bipolar capacitors at the in and out put of the 4066. This will eliminate the pop noise.

    U will need a cap at each side of the cmos switches. One at the Input and one at the output.
     
  17. danalog

    Thread Starter New Member

    May 8, 2012
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    It's a design I came up with with much searching and reading.

    Is it really that bad? Will adding the 2.2μf or 4.7μf Bipolar caps make it good or just okay?

    Should I be going back to the drawing board now? What would be a better way to achieve the audio switching?
     
  18. R!f@@

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    What u did works.

    Bad means tht u avoided the coupling caps.

    Keep in mind that when dealing with audio amps and DC switches, u should use capacitors to couple the signal in where ever u can.

    Circuit is OK, if it serves ur purpose.

    2.2uf will have a lower bass response than the 4.7uf cap. so u need to try both and listen which gives the best dynamics. The lower the capacitance the higher the low range cutoff will be. Even 3.3uf will suffice in some case.

    All in all, 2.2 to 4.7 uf Caps are chosen for a typical coupling caps. It needs to be of higher voltage and should be bipolar electrolytic types. Good quality caps will have good frequency response.

    So get 'em caps and put them in between the inputs & outputs of every cmos switch u got
     
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  19. danalog

    Thread Starter New Member

    May 8, 2012
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    Great and thank you for all the info. I just happen to have some very nice audio grade Nichicon Muse Non Polar Electrolytics sitting right here. I'll throw them in tomorrow morning and begin testing the different sizes as soon as I can turn the guitar amp up loud again.

    Really, Thank you for your patience and for helping and especially teaching me.

    I will post results tomorrow as soon as I can to let you know.

    THANK YOU!
     
    Last edited: May 15, 2012
  20. rogs

    Active Member

    Aug 28, 2009
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    You can see how BOSS use FETs for switching from any of the schematics on this page :

    https://www.generalguitargadgets.com/tech-pages/45-schematics/55-boss-schematics

    You will not cure your switching 'pops' just by adding capacitors to your circuit.

    From what you show, the switching takes place before the amplifier. That means that any switch 'pops' will be amplified by the amplifier, making them worse.

    You also need to know that the analogue signal passing through a CMOS 4066 switch must be kept within the DC limits of the chip supply. Just using coupling capacitors will allow the signal to 'drift,' depending on the DC potential of the external input and output sources.
    At best this will allow 'pops' of different volume, every time you switch. At worst, it will introduce distortion into the signal path.

    Electronically audio switching a very low level guitar signal, without following some of the guidelines I suggested in my first post, will lead to some very erratic 'pops'.

    The Boss schematics show how some of the commercial devices go about it.
    Some others - like MXR - tended to use direct mechanical foot switches switching.

    Using CMOS 4066 is really only suitable if :

    1) you are switching with no significant gain after the switching
    2) You make sure the DC conditions each side of the switch are consistent, and within the supply rails.

    Otherwise you will get 'pops' of some sort.

    One other option you might like to consider. Have a read through this:

    http://www.silonex.com/audiohm/pdf/levelcontrol.pdf

    It's probably the simplest way of creating 'silent' audio switching, but not necessarily the cheapest!
     
    Last edited: May 15, 2012
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