Transistor switch audio mute

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After puzzling over your diagram I have to admit defeat. I don't understand how that is supposed to work. The mic is phantom powered, so disconnecting the signal lines will power down the mic just the way pulling the plug would. This leads to pops and whatnot. It's a balanced mic, so the signal travels down two lines equally...down pin 2 as the + (positive phase) signal, and down pin 3 as the - negative phase signal. And in all of this, I've seen no ac current at all.

The idea of all the mutes I've seen to date to to effectively short the 2 signal lines together, combining the out of phase signals and summing to a flatline signal output.


In other questions, I've been looking into latching switches. Could anyone steer me in the right direction to figure out how to allow a monetary switch to work as a latching switch?

That is, a selectable momentary or latching switch as the occasion dictates. Something to do with "flip-flop" chips? I haven't found a good page describing these circuits.

thanks
 
I'm about ready to order the parts to test a prototype.... does anyone have any more input on anything that may make this more successful?

At this point I'm planning on basically connecting a single mosfet across pins 2 and 3 to short them together when voltage is applied to the mosfet gate.

the voltage used will come off the phantom power.
 
Hi Guys

As long as the voltage difference between pins 2 and 3 is less than 600mV, then you could use a single mosfet to short the pins together for muting. If voltage can be higher than this then a single mosfet will clip half the wave. In general this is flaw in most mosfet switching circuits overlooked by the designers of such things.

BJTs can be made to switch on/off as slowly as jfets in this application.

If you had access to the inside of the mixer, the mute options would be a lot easier and could be made silent as you would like.

The least distortion for the mute at the mic input is provided by relays. use subminiature types but not the kind where the armature is magnetic - Omron makes many like that (G6 series). Apart from reading data sheets, one indication that is fairly reliable regarding whether the armature might be magnetic is if the coil polarity is critical. If it is pick a different relay. The Axicom IM-series has a few that are very low current coils, like the IM26TS but this is 12V.

The muting circuits shown by Self are ones operating at higher signal levels and that can be powered by the mixer internal supply. The current-mode switching requires a virtual-earth stage to function and to keep the jfets in their linear voltage range. The method presents just millivolts of signal.

For your app you need the lowest "short" resistance possible. A switch or relay is the winner here. A very low-Rds-on mosfet would be next close, as would a BJT. Effective BJT on-resistance as a muting switch varies with the device but is typically tens of ohms. In your app this would allow significant signal to get to the mic.

You should try just using a switch to experiment with how much noise that causes during transitions. You should try this also with a non-phantom mic. Upsetting the DC power to the mic will cause a thump even if shorting pins 2-3 is otherwise quiet on a non-phantom-powered mic.

An alternative to shorting the signal lines together is to interrupt the phantom feed. This transition should be slowed slightly with a cap. During power up/down the mic may produce a distorted signal, so that is something you should test.

Have fun
 
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