Microphone mute switch

[boatman]

I have been asked to make a microphone muting relay box with local and remote operation. The system design isn't a problem, but I'm not sure of the best way to mute the mic feed. Is a simple short between pins 2 & 3 on the mic OK or do I need buffer resistors or even a balanced pad? It will be used with dynamic mics.

[bruce]

A simple switch should be OK for a dynamic mic.

 

There's some discussion on http://www.crownaudio.com/popless.htm about how phantom power could cause a "pop" if used with a condenser mic, and a circuit to overcome this.

 

Also see the circuit on http://www.procosound.com/download/datasheets/Old%20Version%20of%20Cough%20Drop.pdf

[revbobuk]

I've made a popless switch using the circuit referenced above, and it works perfectly. It also helps with dynamic mics when phantom power is switched on globally, as it often is with cheaper desks.

[fincaman]

Hello There, a short between pins 2 and 3 will create an inductive loop and pickup allsorts of noise especially dimmers. Some years ago I was called out to a working men's club in Sheffield funnily enough the one where the Full Monty was made. There were shorting switches between 2 and 3 on the mics and it all buzzed like mad, I just cut the shorting links and it was fine. The best way in my opinion is just to arrange for pins 2 and 3 to be disconnected perhaps with a relay, you could have a local switch and a remote one. With a balanced mic input using dynamic mics it should be fine but if there is phantom power look at the previous post to combat this.

[boatman]

Thanks guys, that's just what I needed.

 

The design will have a local box for muting with two momentary push buttons and two LEDs and it will have an internal PP3 battery. There will be capability for an external remote control which will supply external power to extend the battery life.

 

I'll design the PCB so that it can be used for both local and remote boxes by fitting different components. For the quantities I envisage building, this will be more economic than having two PCB designs.

 

Here's a quick spec:

 

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1. Control will be an 8-pin PIC12F615. This will detect buttons (2 pins), drive relay & LEDs (2 pins) and monitor the supply voltage (1 pin) leaving 1 spare pin for later use. There are versions of this chip which have an internal shunt regulator, but an external low dropout regulator is easier.

 

2. Muting will be done by one contact on a 5V DPCO signal relay as per this circuit - http://www.crownaudio.com/popless.htm. The other relay contact will be available on screw terminals to drive an external mains switching relay (preferably solid state for low drive requirements). It could be designed so that the mains switching relay isn't available when running on battery power.

 

3. Microphone connection will be male and female 3-pin XLR connectors mounted directly on the PCB to avoid the possibility of loose contacts.

 

4. When there is no power the mic will be un-muted.

 

5. There will be two high intensity LEDs one red and one green. Local and remote LEDs will be driven by constant current sources so supply voltage and remote control cable length won't affect the brightness.

 

6. Remote control will require two wires in for remote buttons, two wires out for remote indication and two wires for supply volts.

 

7. The box has an internal Alkaline PP3 battery and external 12v input (on remote control connector) with automatic supply switching using diodes. There will be a slide switch to turn off the local battery.

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[Chappie]

Shorting pins 2 and 3 together is simple and works fine. All our voice-of-god boxes are wired this way and they have never caused us any issues.

[jamesperrett]

Hello There, a short between pins 2 and 3 will create an inductive loop and pickup allsorts of noise especially dimmers.

 

This seems very odd to me. A properly made shorted microphone cable with a screened twisted pair fed to a balanced input shouldn't pick up any interference. I suspect that there were other problems with the system in question.

[alistermorton]

I'm glad I'm not the only one who though that. Not being a noise boy, I thought I'd missed something.

[mackerr]

Hello There, a short between pins 2 and 3 will create an inductive loop and pickup allsorts of noise especially dimmers.

 

This is very unlikely. The loop is a twisted pair that has very high rejection of induced magnetic interference, and is very low impedance, so appears as a short relative to the input impedance of the mixer. Leaving the line unconnected and therefore very high impedance creates an antenna which leaves the input open to RF interference.

 

Mac

[fincaman]

Hello There, a short between pins 2 and 3 will create an inductive loop and pickup allsorts of noise especially dimmers.

 

This seems very odd to me. A properly made shorted microphone cable with a screened twisted pair fed to a balanced input shouldn't pick up any interference. I suspect that there were other problems with the system in question.

 

I can only say that in 50 years experience I have had this problem many times, the instance I quote was at Shiregreen Working Mens club Sheffield, there was a mic socket in another room for the bingo checker, it had been arranged for the twisted pair to short together when the mic was unplugged, the system had a dreadful buzz I removed the short and it was silent.

[timsabre]

I can only say that in 50 years experience I have had this problem many times, the instance I quote was at Shiregreen Working Mens club Sheffield, there was a mic socket in another room for the bingo checker, it had been arranged for the twisted pair to short together when the mic was unplugged, the system had a dreadful buzz I removed the short and it was silent.

 

A short to ground/pin1 would do this...