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Silent microphone switching

Postby innerchord » Thu Feb 15, 2018 5:37 am

Hello everyone.

I am putting together a push-to-talk (PTT) switch for a balanced, dynamic microphone.

Does anyone have experience of the best approach. I know it can be done using various methods, so please weigh in with your favourite!

Thank you.
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Re: Silent microphone switching

Postby MarkPAman » Sat Feb 17, 2018 2:45 am

I may choose something more sophisticated, if it’s connecting to a big PA, or for recording, but a simple, normally closed, switch across pins 2&3 switches pretty much silently, and allows phantom to be used if needed.
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Re: Silent microphone switching

Postby ef37a » Sat Feb 17, 2018 8:12 am

innerchord wrote:Hello everyone.

I am putting together a push-to-talk (PTT) switch for a balanced, dynamic microphone.

Does anyone have experience of the best approach. I know it can be done using various methods, so please weigh in with your favourite!

Thank you.

I second Mark's idea of a shorting switch but, what is your specific application?
If an 'announcement' system (like supermarket* drivel) you might need a lever switch that has centre off and 'PTT' one way and a latched 'TT' the other. If peeps have to read more than a few words AND hold the button, they tend to forget and you get choppy Voice Over.
If a Talk Back system you really do not want a hold facility or if you do, arrange a flashing LED of 1k lumens! TB also usually needs 'dim' logic for monitors.

*My local Sainsburys is AWFUL! I know I am mutton but since the new place opened a few years ago I have asked customers AND staff many times what that last announcement said, often the response is "what announcement?" It is a classic example of a very badly treated (NOT!) space. Oh, and the installer has never heard of an HPF, think "19760s disco"!

Dave.
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Re: Silent microphone switching

Postby Hugh Robjohns » Sat Feb 17, 2018 10:51 am

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Re: Silent microphone switching

Postby Music Wolf » Sat Feb 17, 2018 12:55 pm


I have one. Not entirely silent switching, but almost. Well built.
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Re: Silent microphone switching

Postby innerchord » Tue Feb 20, 2018 6:43 am

Thanks for the replies.

I'm designing/building this myself as a little project. No commercial purchases allowed!

The application is studio to live room speaker link.
I have a choice of lighted switching available - just need to wire it up.
I did consider electronic switching, but was hoping to keep the audio switching passive since I'm sure it'll be fine in this application, and I'd want to keep the battery life as long as possible.

I'll experiment and let you know how I get on, shall I?
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Re: Silent microphone switching

Postby ef37a » Tue Feb 20, 2018 7:32 am

innerchord wrote:Thanks for the replies.

I'm designing/building this myself as a little project. No commercial purchases allowed!

The application is studio to live room speaker link.
I have a choice of lighted switching available - just need to wire it up.
I did consider electronic switching, but was hoping to keep the audio switching passive since I'm sure it'll be fine in this application, and I'd want to keep the battery life as long as possible.

I'll experiment and let you know how I get on, shall I?

Sorry, don't understand much of that. "Passive switching". Do you mean physical, mechanical switches? If so, where do the batteries come in? If you mean relays, you will never IMEx get rid of clicks at mic levels.

"Studio to live room speaker link"? Sounds like a Talk Back link? If so it is much easier to get a silent 'kill' at line or speaker (PA) levels.

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Re: Silent microphone switching

Postby Hugh Robjohns » Tue Feb 20, 2018 11:06 am

innerchord wrote:The application is studio to live room speaker link.

In that case, you would get much better results if you switch the line-level feed going into the studio after the mic preamp.

I'd configure a two-pole changeover switch (or relay) to open-circuit the balanced connections from the preamp line out while simultaneously short-circuiting to ground the connections into the amp.

passive mute switch.jpg


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Re: Silent microphone switching

Postby innerchord » Sat Feb 24, 2018 8:04 am

Yes, talkback if you like. I spelled it out just in case :smirk:

By passive I mean a switch box using no power, i.e. not active.
No phantom power.
The battery supply would be for switch state lighting only.

Also, the mic is a long way from the input, so switching the line-level signal is not practical.

Thus I was looking to build a simple box close to the mic if possible. If I'm not happy with the results I was then going to add a simple pre-amp and associated line-level switching.
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Re: Silent microphone switching

Postby ef37a » Sat Feb 24, 2018 8:46 am

Hmm, if you are having a battery for status LEDs you could use a j112 FET to short the mic line?

I think it is the j112 that has an 'on' resistance of about 50 Ohms but you can parallel FETS virtually ad.inf. to get enough attenuation. The advantage of FETs is you can 'remote' the gate feed and control the on/off speed and get very quiet switching.

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Re: Silent microphone switching

Postby innerchord » Sun Feb 25, 2018 12:56 am

Thanks Dave, that sounds like an interesting project, although I was hoping to keep things very simple.

Since your electronics knowledge is clearly much greater than mine, can you point me to some schematics I could use or modify?

When it comes to electronics, I'm a hobbyist builder, but not much of a designer.
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Re: Silent microphone switching

Postby ef37a » Sun Feb 25, 2018 9:12 am

innerchord wrote:Thanks Dave, that sounds like an interesting project, although I was hoping to keep things very simple.

Since your electronics knowledge is clearly much greater than mine, can you point me to some schematics I could use or modify?

When it comes to electronics, I'm a hobbyist builder, but not much of a designer.

Google for "FET audio switching" I shall soon to see what I find. Basically a 'junction' FET is 'on' unless you make the gate negative when they turn very high resistance. The j112 does indeed have an on resistance of 50R* and requires just -5V to turn it off, if you are using 9V battery supplies for the LEDs? Sorted.

*From the pages of D.Self's Small Signal Audio Design, a book I strongly advise you to get. The circuits in it go into details to reduce distortion in FET switches and improve the "offness" as he coins it. You need none of that sophistication. A FET or three, couple of Rs,diodes and 100nF caps should get you done.

http://sound.whsites.net/articles/muting.html#s2

^More complex than you need but shows the way and can be adapted I am sure.

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Re: Silent microphone switching

Postby Folderol » Sun Feb 25, 2018 5:55 pm

I can't help thinking this is arse about face.

Use a MOSFET in series, and with no bias it is effectively open circuit. Apply a voltage to the gate and it's about 5 ohms. A 3V coin cell should last almost indefinitely!
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Re: Silent microphone switching

Postby ef37a » Sun Feb 25, 2018 9:49 pm

Folderol wrote:I can't help thinking this is arse about face.

Use a MOSFET in series, and with no bias it is effectively open circuit. Apply a voltage to the gate and it's about 5 ohms. A 3V coin cell should last almost indefinitely!

Its a thought mate but leaving a mic circuit O/C? Could cause problems especially since the OP is looking at quite a long mic cable I believe?

I don't think battery life is much of an issue since he wants lights?

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Re: Silent microphone switching

Postby Folderol » Sun Feb 25, 2018 10:24 pm

Actually, re-reading the topic, and remembering this is for a talkback...
Sod all that. A 2 pole 2 way toggle switch.
One pole for the light.
The other pole wired so one way connects the common pin (amp input) to the mic, the other way connects it to ground.
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Re: Silent microphone switching

Postby ef37a » Mon Feb 26, 2018 1:15 am

Folderol wrote:Actually, re-reading the topic, and remembering this is for a talkback...
Sod all that. A 2 pole 2 way toggle switch.
One pole for the light.
The other pole wired so one way connects the common pin (amp input) to the mic, the other way connects it to ground.

Yes but for a balanced feed surely better to just short the mic? I agree though, for talkback a bit of a click is not likely to be a problem, you are not likely to talk AND record?

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Re: Silent microphone switching

Postby innerchord » Tue Mar 06, 2018 5:05 pm

I'm going to experiment with simple switching since small clicks are not an issue, and save the FET stuff for a rainy-day project.

Thanks for the book link, too. I have come across it before, but didn't purchase a copy.
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