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FET Audio Mute

Discussion in 'Electronic Basics' started by Jon Slaughter, Aug 13, 2007.

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  1. I'm trying to design a simple audio mute using a fet but I can't seem to get
    it to function properly. (Actually this is essentially equivalent to what
    I'm trying to do... So I need help on this specific method and not some
    other method which might be easier(such as just a switch)). The audio signal
    is is < 2V p-p.

    Essentially audio signal passes through a enhanced n-mosfet connect to
    source. The drain goes into a load which is what ever load the would have
    been connected to it in the first place.

    The base of the fet is connected to -5V. The gate is connected to a STDP
    switch that selects -5V or +5V for turning on and off the fet.

    The circuit does work but the signal is extremely attenuated. Its about 1/10
    of what it should be. If I double the supply voltage then I can half the
    attenuation. I just want the fet to pass the signal completely(with almost
    no attenuation) or completely turn it off(effectively).

    Is there something I'm missing? I thought that a fet could essentially act
    as a voltage controlled switch? So whats wrong with my circuit? I've posted
    a schematic in binaries.

  2. Eeyore

    Eeyore Guest

    Use a jfet.

  3. tempus fugit

    tempus fugit Guest

    The audio signal

    I always watch your posts with interest because it looks like you and I are
    attempting to design similar things. Is your mute circuit for a guitar by

    Have you tried connecting both the input and the output to the source? Then
    when you switch the MOSFET on, it will conduct the signal to ground.

  4. lol. Well, its not really for a guitar mute but it is something for a
    guitar. For a guitar mute I just use a switch.
    I'm not sure I follow.

    Ultimately I'm going to use an audio analog switching ic but I want to
    understand the basic idea first but can't get it to work. Well, its working
    but its attenuating the signal way to much. (so its not acting like a switch
    but more like a resistor)

  5. Ross Herbert

    Ross Herbert Guest

    Maybe you should use a circuit designed by a broadcast engineer - he
    should know what he is talking about...
  6. Ross Herbert

    Ross Herbert Guest

  7. tempus fugit

    tempus fugit Guest

    Thanks for that Ross. I'm still in the process of trying to implement JFETs
    to bypass a string of effects and this could be a big help, although if
    anyone has a way to configure the JFET switch to act as a SPDT or DPDT
    relay, that'd be even better.
  8. tempus fugit

    tempus fugit Guest

    What are you trying to do with this circuit?

    I've successfully done what you're trying to do here with a CMOS switch IC.
    Instead of running your signal through the MOSFET, just connect both your
    signal input and output to node 1 in your diagram. When you apply voltage to
    close the MOSFET switch, the signal will pass through the MOSFET to ground,
    thus muting the output. When the switch is opened again, the signal will be
    audible again since there is no longer a path to ground to mute it.
  9. Oh, I think I see what your saying. You mean that instead of having the
    signal pass through the fet you have the fet ground the signal at that
  10. tempus fugit

    tempus fugit Guest

    Oh, I think I see what your saying. You mean that instead of having the
    Exactly. You also don't have to worry about altering your signal since it
    doesn't actually pass through any extra silicon (if that's a concern).
  11. Yeah. Only thing I'm worried about is that for a guitar the ground is sorta
    "floating"? Else it seems like a good idea... although for my specific
    application I actually need the signal to go through the fet because I need
    to disconnect the audio completely from the circuit(grounding at a point
    might cause some problems although maybe can be fixed other ways). The main
    issue I think is that I want to try to use some ic's and not go discrete
    because the application requires a ton of "switches".

    I ordered some analog switches though and maybe these will work for now.

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