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Old 14th August 2011, 08:46 AM   #11
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Just a thought, if the lavalier is phantom powered, (as most are) you need to get more canny, as just switching the signal lines will lead to pops and thuds.
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Old 15th August 2011, 02:07 AM   #12
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@nico:
I'm open to the idea of using a relay. That was, in fact, my first thought. But I was worried that a magnetic field so close to a mic level cable might introduce some issues. Secondly I worried that a relay would required more power than phantom power could provide.
If those are both non-issues, then I'm happy to use a relay.

My objective is simply a phantom powered noiseless muting circuit, with a remote switch. Any method that achieves this is fine with me.

@pinkmouse:
The muting cicuits I've seen generally use a capacitor and resistor to connect pins 2 and 3, with a normal unmuted condition. To mute, the resistor is bypassed leaving pis 2 and 3 connected by a capacitor. The tech sheets indicate this is a low pass? filter. The purpose is to ensure that the dc level is equal between pin 2 and pin 3...preventing pops.

Douglas Self shows a "series-shunt" transistor mute block that incorporate a few more items.

I'm sure I haven't learned enough to do this yet, but I hope I'm getting there.

Thanks

Last edited by TheFixer; 15th August 2011 at 02:15 AM.
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Old 15th August 2011, 03:21 AM   #13
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Default A single transistor works fine...

See page 25 of the Cirrus reference design for the CS42528:

http://www.cirrus.com/en/pubs/rdData...CRD42528-2.pdf

You can use FET's, but bipolars work just fine with standard CMOS logic levels.
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Old 15th August 2011, 10:45 AM   #14
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Hi Fixer,
I think that a relay mute/unmute is likely to give an audible click.

I further think that FET mute/unmute can be turned on/off sufficiently slowly to avoid that click.
It could probably be achieved in 100us to 1000us, rather than instantly.
A bit like a very fast resistor fade.
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Old 15th August 2011, 07:27 PM   #15
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@ neil: I may be reading that design wrong, but it looks like the transistor pulls the audio output to ground. I've seen many designs that do this, but I don't think that will work for me. My mute is based on connecting pin 2 and pin 3, not pulling them to ground. Which should, if it works properly, connect the + signal and the - signal, summing them, and producing a flat output. Meanwhile I have to keep those pins connected to the mic because they also provide power.

@andrew: thanks for the input. I had thought about a "slow mute" if for no other reason that it may sound better than having a sharp drop in system noise. We're talking in the millisecond range here, but it may be noticeable... I don't know. Of course, that adds yet another layer of complexity to this device. I'd have to bother with ramping up the power to a transistor in a controlled way.
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Old 15th August 2011, 07:39 PM   #16
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Click the image to open in full size.

Two mosfets in antiseries make an AC mosfet.
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Old 16th August 2011, 02:37 PM   #17
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@grenadier:
Thanks for the schematic. Could you explain it a bit?

It looks like T1 and T2 are loads, and when the switch is thrown positive the loads are allowed to go to ground through the transitors.
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Old 16th August 2011, 02:51 PM   #18
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Well current doesn't flow through the gate. The gate capacitance charges up and this turns the FET on.

When both fets are on, the electricity can travel through the body diode of one mosfet which for some reason is not shown there.

Click the image to open in full size.

Since the fets are in antiseries, and the diodes are in antiparallel with the fets, AC can either flow or get blocked depending on the state of the fets.
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Old 17th August 2011, 05:06 PM   #19
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Forgive my ignorance, but I'm not sure how this circuit would apply to my mute.

Would pin 2 = t1 and pin 3 =t2?

I didn't think my mute involved any ac at all.

I do understand the functioning of the fet transistor, but I'm not sure how your circuit is intended to work.
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Old 17th August 2011, 09:48 PM   #20
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Put it in series with the microphone. You could turn the mic on or off depending on your wishes.
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