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  1. #1

    Soft muting circuit?

    Does anyone have a design for a soft muting/unmuting circuit (either contact-closure controlled or switch-controlled, I will need both but can adapt either for my needs)?

    For the electrically inclined amongst you who don't know what a soft mute is, it's a method of bringing the output level to -inf. from the input level without a quick cut [which could cause a pop] and the unmuting is the same in reverse.
    Eric Adler (tonsofpcs) Chat at:
    Follow me on twitter: @videosupport @eric_adler

  2. #2
    What are you muting?

  3. #3
    Time to rip open an old mixer!

    To be honest it's never occurred to me how they make a soft mute. couldn't you slid a slider? you could use a potentiometer.

  4. #4
    karl: Audio (either line level or mic level).
    smifis: I don't have any mixers with built-in soft mutes. Yes, a pot will soft-mute, but I need this to be controlled digitally by a single pin/switch/closure.
    Eric Adler (tonsofpcs) Chat at:
    Follow me on twitter: @videosupport @eric_adler

  5. #5
    Well, yeah, I assumed it was audio or it wouldn't be in this forum.

    A quick and dirty mute for line level, if you don't need 100% mute, is to use a photocell and shunt resistor in an "L". I'm going to try to attach a diagram - I've never done an attachment in this forum so I don't know if this will work. Figure "A" is the photocell one. The 3.3K is totally non-critical. Sometimes the input impedance of the load will work.

    For the LED/photocell combination, I take a 1" piece of black heat shrink, insert LED and photocell in opposite ends, and shrink it down. While the tubing is still hot, take a needlenose and pinch the ends around the wires to make it light-tight. I make these up a half dozen at a time so I'll always have them on hand.

    The photocell approach doesn't work well with mic level because the hi-Z when off raises the noise floor. On the other hand, you can't use a photocell to short the line because few of them get below 50 ohms.

    For microphones, figure B is a mute using two power VMOS fets. They don't have to be very big; the reason to use power fets is to get a low on resistance for a good solid short. The drains are are connected to the audio lines and the sources are connected together. You have to use two, wired with opposite polarity, or otherwise the internal diode will clip one side of the signal when it gets above 1/2 volt. This circuit works best when there's no DC on the audio line. If there is, the DC+ for switching has to be at least 10V higher than the audio line DC voltage.

    If you don't really need a soft mute, but are just trying to avoid the pop you can get when shorting a mic line that has phantom power on it, try the circuit of figure C. This doesn't give you a perfect mute at low frequencies, because the capacitor is in the circuit, though the larger the cap, the better the mute. The 4.7K resistor bleeds off any DC imbalance across the pair. The cap doesn't have to be nonpolarized; an electrolytic cap can take about 1/2 volt reverse charge, and in this case we're talking millivolts.

    I wonder if there's a soft-mute box available on the market; I haven't seen one. Somebody should make it.
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  6. #6
    Thanks karl, exactly what I was looking for.

    Quote: karl eilers
    View Post
    I wonder if there's a soft-mute box available on the market; I haven't seen one. Somebody should make it.
    Well start selling them then!
    Eric Adler (tonsofpcs) Chat at:
    Follow me on twitter: @videosupport @eric_adler

  7. #7
    Oooh, I gonna be rich!

  8. #8
    Have fun marketing!


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