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Microphone circuit assistance?

dbarak

Apr 25, 2010
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Hello everyone,

I have two computer microphone/headset units that I'd like to incorporate into a simple two-station intercom system, and I'm having a heck of a time figuring out how to do it. I'm hoping someone here can help. Each unit has two 3.5mm plugs - a TSR plug for the headphones and a TSR plug for the microphone. I guess the mics would be electret mics.

All of the electret amplifier kits I've found online have two pads for the microphone, so if these are usable with a TRS-style jack, I don't know how to work it out. I've found circuits online too, but I can't really decipher them all that well.

I'm open to using ready-made items, kits, or from-scratch circuits as long as I can build them on prototyping boards of some sort. Any ideas?

I wish I could offer electronics help here so I'm not only a "taker," but as you can tell I'm still at the stage of being a bit lost. If anyone has photo or video questions, maybe I can help in those areas .[ your email deleted for your privacy from spam]

Thanks everyone,

Dave
 
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davenn

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Sep 5, 2009
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hi Dave

so the mic have a 3 wire plug as well as the headphones?
a little unusual unless there are 2 mic's for stereo recording ?

show us a pic of your unit or a link to the same

look for responses in here
I deleted your email for your privacy which is respected by Electronics Point Forum

cheers
Dave
 

dbarak

Apr 25, 2010
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Hi Dave,

I'm attaching a picture of headphones with the same plug configuration. It's a stereo headphone with an attached boom mic. As I understand it, accessory microphones for computers are usually electret, which would explain the TRS configuration. Just for added information, I tried one of the headsets with a pocket-sized Radio Shack amplifier, but it's designed to take two-conductor plugs (tip and sleeve I guess). Each of the plugs is TRS, so for the headphones it's left, right, and common.

Also, thanks for fixing the email issue. : )

Dave

Handsfree-Headset-with-Microphone-for-VOIP-SKYPE-P13696613.jpg
 

KrisBlueNZ

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Check for continuity between the tip and the ring on the microphone plug. The microphone may be connected to both left (tip) and right (ring).

As you say, the microphone is mono, and AFAIK the microphone inputs on computers are mono as well.

Electret condenser mics need a power source.
 

dbarak

Apr 25, 2010
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Hi KrisBlueNZ,

Well, I checked two different model headphone/microphone sets, each with the two different plugs as shown in the photo. It turns out the tip and ring are tied together, so there are essentially two contacts - the tip/ring and the sleeve. However, I tried both in a mini amplifier that accepts two-conductor microphones and I got nothing. I checked to be sure the tip and sleeve were in contact with the appropriate contacts in the amplifier, but nothing came out. I'm baffled, but then again that's my normal state of being. ; )

Dave
 
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davenn

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I checked to be sure the tip and sleeve were in contact with the appropriate contacts in the amplifier, but nothing came out.

if the amp isn't supplying a voltage for the electret mic then no, you wont get anything
 

KrisBlueNZ

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Well, I just learned something interesting. On a standard TRS microphone input, the computer (or sound card) actually provides DC bias voltage for condenser microphones on the ring contact. (It takes the signal from the tip contact.)

https://www.google.com/search?q=sound+card+microphone+input+tip+ring&tbm=isch

So if your headset has tip and ring connected together, it will have an electret condenser microphone, and you will need to supply operating voltage to it, through a resistor. Several different resistor values are mentioned in the links found through that image search. I suggest 4.7 kΩ from a +5V supply.
 

dbarak

Apr 25, 2010
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Hi KrisBlueNZ and davenn,

Thanks for the follow-up! So, I've seen amplifier kits online that include electret microphone capsules that get soldered on to the board. I assume that since they're electrets they're getting voltage from the board in some way. Any thoughts about swapping out the kit microphone with a TRS jack? (The headphone half of the equation seems to be an easy thing to deal with.) The kits are cheap ($15 USD or less) so even if the opinion here turns out to be wrong, it's no big deal. ; )

Dave
 

davenn

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show us an example ... ie. give a link,
so we can see what you are referring to :)
 

dbarak

Apr 25, 2010
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I'm including a couple of links to amplifier kits below, but first, I found a decent write-up (for me at least) about using electret mics. It mentions positive and negative, so I'm making a guess that with my TRS plug for the mic, with the tip and ring being tied together, the T/R and the sleeve are positive and negative, or vice versa. First, the link to the write-up:

http://www.buildcircuit.com/how-to-use-electret-microphone/

And now a few kits...

http://www.canakit.com/electret-microphone-amplifier-kit-ck009-uk009.html

http://vakits.com/electret-microphone-amplifier-kit-1695

http://www.electronics123.com/kits-...-amplifiers/audio-amplifier-1W-LM386-kit.html

http://www.electronics123.com/kits-...r-amplifiers/TDA7052-Amplifier-Module-1W.html

To be honest, I'm not sure what the difference between an amplifier and a pre-amplifier is. Since my intention is to feed the output to headphones rather than regular speakers, would that need only a pre-amplifier? Basically, I'm trying to use a TRS-terminated electret microphone, and all the amps I see have two pads or conductors for a mic, even the ones that are advertised as being for electret mics.

Dave

show us an example ... ie. give a link,
so we can see what you are referring to :)
 

KrisBlueNZ

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Yes, they get their power from the board. Any amplifier board that has a built-in electret mic, or is designed to work with an external electret mic, should work with your headset.

Ideally you should separate the supply circuit (just a resistor coming from the positive supply) from the signal path (a series capacitor, feeding into an amplifier circuit) and bring the power source to the ring and connect the signal coupling capacitor to the tip, so that your board would also work safely with other types of microphones that don't need to be (and shouldn't be) powered from the preamp.

mic1.jpg
The diagram above shows how a condenser microphone is connected on your preamp/amplifier boards. There is a resistor, usually around 4.7 kΩ, from a positive supply rail to the electret, which provides power for the internal circuitry inside the microphone, and there is a coupling capacitor that passes the signal from the microphone to the rest of the preamp circuitry. Electret microphones are designed to use a single connection for positive supply input and signal output. The other connection, which is connected to the metal case of the microphone, connects to the 0V rail (aka ground).


mic2.jpg

The diagram above shows how an electret condenser microphone (for example one in a handsfree kit) connects to the microphone input of a PC or sound card. The external microphone is on the left, and the circuitry inside the computer or preamp is on the right. This circuitry now has the power feed (through the resistor) and the signal feed (through the capacitor) separated onto different terminals on the socket. Specifically the supply is provided on the ring, and the signal is taken from the tip.

If the external microphone is an electret condenser type, and needs a supply, then it is wired as shown, with the tip and the ring connected together. This completes the circuit between the resistor and the capacitor, so the result is like the first diagram.


mic3.jpg

This is how an external microphone works when it doesn't need (and doesn't want) power from the computer or preamp. The microphone is connected only to the tip of the plug; the ring terminal of the plug is not connected ("NC"). The positive supply through the resistor is not used in this configuration.


If you want to modify your preamp to use a jack socket instead of an onboard electret condenser microphone, I recommend that you wire it like the two examples above. Then you can use it with either type of microphone without potentially damaging a sensitive, high-quality microphone that isn't designed to have a supply voltage applied to it.


The difference between a preamplifier and an amplifier is the signal levels they use. A preamplifier is designed to amplify a low signal level, from a microphone, turntable pickup, etc, up to "line level" - something in the region of 0.5V RMS, although there are many different standards. A line level signal is not able to drive a loudspeaker; it may be able to drive a sensitive headphone but is not designed to do that either.

An amplifier takes a signal at line level and amplifies it up to drive a speaker. You can also get headphone amplifiers, which don't amplify the signal level much but provide a low impedance output suitable for delivering some power to the headphones. (Line-level signals are medium-impedance and cannot provide much power.)


I've just seen your recent post. Here are some comments on the kits you found.

http://www.canakit.com/electret-microphone-amplifier-kit-ck009-uk009.html
Combines a preamp and an amplifier, effectively. Can drive a small speaker, or headphones. I don't like the LM386; there are much better options available now, but this is usable. You should modify it as I described above to use a jack socket.

http://vakits.com/electret-microphone-amplifier-kit-1695
Ditto. This one has RCA phono sockets for input and output.

http://www.electronics123.com/kits-...-amplifiers/audio-amplifier-1W-LM386-kit.html
Ditto, except this one doesn't include the resistor to supply the electret condenser microphone. That might be an advantage, since it means no changes to the circuit when you add the jack socket; just add a resistor from the positive rail to the ring terminal on the socket.

http://www.electronics123.com/kits-...r-amplifiers/TDA7052-Amplifier-Module-1W.html
This one uses a type of amplifier that's designed for "bridge-tied load" (BTL) operation. These are not well-suited to driving headphones.
 

dbarak

Apr 25, 2010
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Wow, I can't thank you enough! This is great information, and it really helps me get my head wrapped around the concept! So it sounds like the third one is the best option.

A little about the project I'm working on... I'm a filmmaker here in San Diego, California, and I'm working on a space-based science fiction short film. I want the actors to be able to communicate with each other through actual headsets so they hear the kind of degraded radio transmissions we're used to. Not scratchy with static, but just... not live and standing next to you.

I've also actually expanded my electronics knowledge a bit in other ways with this short film. For the spacecraft interior set, I needed to have a bunch of LEDs flash in what appear to be slow random patterns, part of a caution light panel. I found out that 555 chips weren't going to work for what I needed - the durations were too short - but I found a circuit that uses a 4060 chip. I needed several of these circuits, so using Fritzing software I was able to design a simple board that would let me just tap in to whatever pins I needed for various timings. The miracle is that I didn't screw it up, and the boards I ordered worked perfectly. I'm actually learning now HOW the circuit works, not just that it does work. I'm sure there would have been a way for me to do this more simply, maybe with one board, but that would have made it more complex for me, and by having a simpler circuit repeated on multiple boards (with variations), I could mess up the first few and still have backups available.

So... maybe I can give you a "technical advisor" credit in the film? If so, let me know how you'd want it to read. It's still a ways off before shooting since it's pretty complex for a low budget short.

Dave

http://www.milestogo-movie.com
 
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KrisBlueNZ

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Wow, I can't thank you enough! This is great information, and it really helps me get my head wrapped around the concept! So it sounds like the third one is the best option.
You mean the third kit? Yes. You'll need to add a resistor, and mount the socket somehow.
A little about the project I'm working on... I'm a filmmaker here in San Diego, California, and I'm working on a space-based science fiction short film. I want the actors to be able to communicate with each other through actual headsets so they hear the kind of degraded radio transmissions we're used to. Not scratchy with static, but just... not live and standing next to you.
That sounds like a good idea.
I've also actually expanded my electronics knowledge a bit in other ways with this short film. For the spacecraft interior set, I needed to have a bunch of LEDs flash in what appear to be slow random patterns, part of a caution light panel. I found out that 555 chips weren't going to work for what I needed - the durations were too short - but I found a circuit that uses a 4060 chip. I needed several of these circuits, so using Fritzing software I was able to design a simple board that would let me just tap in to whatever pins I needed for various timings. The miracle is that I didn't screw it up, and the boards I ordered worked perfectly. I'm actually learning now HOW the circuit works, not just that it does work. I'm sure there would have been a way for me to do this more simply, maybe with one board, but that would have made it more complex for me, and by having a simpler circuit repeated on multiple boards (with variations), I could mess up the first few and still have backups available.
Cool!
So... maybe I can give you a "technical advisor" credit in the film? If so, let me know how you'd want it to read.
No thanks! But feel free to ask all your technical questions here on Electronics Point. Perhaps you should introduce yourself in the general chat forum and tell us about the project so we'll know the background when you ask questions.
It's still a ways off before shooting since it's pretty complex for a low budget short.
I had a quick look at the web site and I think it sounds cool! I like movie that are about real people in potentially real situations. My favourite author is John Wyndham, because he really understood human nature. Characters like Superman and James Bond don't do it for me because I can't relate to them. I wish you the best of luck!
 

dbarak

Apr 25, 2010
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Hi KrisBlueNZ,

I've got a couple of kits on order and they should be here in a few days. I have lots of spare parts (mostly from failed previous attempts at things), so I think I can get this going. Thanks again for the help! I'll post a bit on the general chat forum and once I have the electronic aspects of the film done I'll post them there.

Dave
 
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