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Electronics Forum / Basics / January 2007Electret microphone question
Good morning everyone: I have the following crude circuit on the breadboard: http://img116.imageshack.us/my.php?image=micpreampbl4.jpg The opamps are part of an LM6134 IC. Here is a link to its page: http://www.national.com/pf/LM/LM6134.html For my microphone, I am using this one that I got from Digikey: http://catalog.digikey.com/scripts/partsearch.dll?Detail?name=423-1043-ND Has anyone have any experience with microphones? The mic is supposed to be omnidirectional, so I was thinking that I should be able to hear its surroundings well. However, even by varying the 10k pot to amplify the signal I really don't hear the surroundings amplified that much. The microphone is a few inches away from an oscilloscope with a loud fan. I was expecting to hear the fan from the speakers. The only noticeable sound that I can hear if the tapping sound of my finger near the breadboard. Are there any tips to improve the sound pickup of the mic? Is my crude circuit the cause of this? Or do I need to enclose the mic in a custom enclosure? I really don't know much about mic setups. All I know is that when my friends call up and I'm using my bluetooth headset, they complain about hearing the surroundings. I'd like to be able to duplicate that to some extent. Thanks! > Good morning everyone: > [quoted text clipped - 27 lines] > > Thanks! How are you supplying power to the microphone? > How are you supplying power to the microphone? Hi Tom, The + terminal of the mic is connected to VCC via a series resistor. That's the configuration they showed on the mic datasheet. >> How are you supplying power to the microphone? > > Hi Tom, > > The + terminal of the mic is connected to VCC via a series resistor. > That's the configuration they showed on the mic datasheet. Did you actually measure the DC at the mic? Tom > Did you actually measure the DC at the mic? Yep, I used a multimeter and an oscilloscope to measure the voltage levels. > > How are you supplying power to the microphone? > > Hi Tom, > > The + terminal of the mic is connected to VCC via a series resistor. > That's the configuration they showed on the mic data sheet. on end of the terminal is grounded with microphone body, which will be negative, other terminal will be connected to input of preamp where voice data is converted to analog speaker data, power supply should be in milli amp lessthan 3v. exceeding the power supply will damage the microphone. On Jan 16, 12:42 am, "Tom Biasi" <tombiasi@********optonline.net> wrote: > > Good morning everyone: > [quoted text clipped - 29 lines] > > How are you supplying power to the microphone? power supply to the micro phone is in milli-amp no external power supply required, microphone pre-amp supply will acitve the micro phone (lessthan 500mv ** Groper Alert ! > I have the following crude circuit on the breadboard: > http://img116.imageshack.us/my.php?image=micpreampbl4.jpg [quoted text clipped - 14 lines] > speakers. The only noticeable sound that I can hear if the tapping > sound of my finger near the breadboard. ** If you want to hear amplified room background noises - you MUST listen to the mic via headphones, OR place the speakers in another room acoustically isolated form the first. There is NO WAY the crude PA system you have created will audibly amplify any such sounds without breaking into a feedback howl. ....... Phil Hi Phil > ** Groper Alert ! Is it really wrong to be using Google Groups? > ** If you want to hear amplified room background noises - you MUST listen > to the mic via headphones, OR place the speakers in another room > acoustically isolated form the first. > > There is NO WAY the crude PA system you have created will audibly amplify > any such sounds without breaking into a feedback howl. I actually wanted to hear feedback, but I never got any. At least the feedback would tell me that it is picking up the sound from the speakers. >> ** Groper Alert ! > > Is it really wrong to be using Google Groups? ** You are not a part of usenet at all. Just a f.cking interloper. ....... Phil > Hi Phil > [quoted text clipped - 12 lines] > feedback would tell me that it is picking up the sound from the > speakers. So you take a step back, and take out the microphone. Inject an audio signal where the microphone would be (make sure to use a coupling capacitor). If you don't hear the tone, then you know something else is wrong. If the point where the microphone is supposed to go is high enough impedance, you should be able to touch the point with a finger, and hear some hum. If the injection doesn't work, then go to the output of the first stage, and inject a signal there. If you hear something, then you know the problem is ahead of that point. If you don't hear something, you know the problem lies later in the circuit. And so on. Break things down into small sections, and make sure they work by themselves. It's easier to figure out a problem the fewer components you have to deal with. Michael > So you take a step back, and take out the microphone. Inject an audio > signal where the microphone would be (make sure to use a coupling [quoted text clipped - 13 lines] > themselves. It's easier to figure out a problem the fewer components > you have to deal with. Hello Michael: Yep, I did this, too. Even before I put the mic, I fed a 1kHz signal to the input and got a nice 1kHz output waveform. I then added the microphone and was able to hear the output if I tap or blow on the mic. However, when I took a headphone connected to a walkman and held it above the microphone, I was only able to hear a faint sound from speaker even when I turned the speaker volume all the way up. I was expecting to hear a louder output from the speaker since I can hear the headphone output with my ears. I also had the gain of the amplifier high. But then if I turn it all the way up, I get some feedback from the speakers. >> So you take a step back, and take out the microphone. Inject an audio >> signal where the microphone would be (make sure to use a coupling [quoted text clipped - 26 lines] > high. But then if I turn it all the way up, I get some feedback from > the speakers. That's a start, but it still leaves multiple possibilities. You do have an amplified speaker connected to the second op-amp? Because the schematic shows a speaker, but it won't really drive a speaker. The first op-amp should give a gain of a hundred, and the second stage gain up to 10, which compounded is quite a bit of gain. ONe of the stages could be wired up wrong, or isn't being coupled properly, and the gain could make up for it, even though it results in a weak signal. You do have the non-inverting input of the second op-amp connected to something that supplies half the Vcc? The schematic doesn't show that outright, but if that 1K resistor isn't connected to 1/2 the Vcc (normally you'd have a something like two 1K resistors in series from Vcc to ground, and that 1K resistor to the non-inverting input of the second op-amp would go where the two 1K resistors are joined), then that second stage won't work properly. It will be way out of the range where it can do any amplification. Or if that 1uF capacitor from the microphone to the first op-amp is missing, or maybe even it's in there but in reverse polarity, the power to the microphone will get to the first op-amp and mess up its operation. Assuming you are using an amplified speaker, put its input at the output of the first op-amp, and work on that stage until things get workin. I think you should get enough gain from that stage to hear sounds from the microphone if things are working right. If it works, then you can move on to the second op-amp. Michael > That's a start, but it still leaves multiple possibilities. > > You do have an amplified speaker connected to the second op-amp? Because > the schematic shows a speaker, but it won't really drive a speaker. Actually, the speaker is a standard plug-in power computer speaker with built-in volume control. I'm assuming since it has volume control that the input is some kind of opamp stage. > The first op-amp should give a gain of a hundred, and the second stage > gain up to 10, which compounded is quite a bit of gain. ONe of the stages > could be wired up wrong, or isn't being coupled properly, and the gain > could make up for it, even though it results in a weak signal. Would it be better if I get the gain of the input stage a bit lower so I don't increase the overall system noise? From the texts that I've been reading, low noise design always specify a lower gain to reduce the noise figure. > You do have the non-inverting input of the second op-amp connected to > something that supplies half the Vcc? The schematic doesn't show that [quoted text clipped - 4 lines] > work properly. It will be way out of the range where it can do any > amplification. Yeah, the second stage is in single supply configuration, so am biasing the non-inverting input with half of VCC. > Or if that 1uF capacitor from the microphone to the first op-amp is missing, > or maybe even it's in there but in reverse polarity, the power to the > microphone will get to the first op-amp and mess up its operation. I've also checked this capacitor aspect before hand. > Assuming you are using an amplified speaker, put its input at the output > of the first op-amp, and work on that stage until things get workin. I think > you should get enough gain from that stage to hear sounds from the microphone > if things are working right. If it works, then you can move on to the second > op-amp. This I haven't done, yet. I'll try connecting the output to the first stage. Thanks again, Michael! > Good morning everyone: > [quoted text clipped - 27 lines] > > Thanks! The opamps used by u is LM6134 IC. try replacing the with LM 324 with is a universal purpose opamps. Mike should be properly shielded wire, buy 1+1 wire with is available in market ( one is normal and other one will be shielded ) now u will able to hear any sound > Mike should be properly shielded wire, buy 1+1 wire with is available > in market ( one is normal and other one will be shielded ) now u will > able to hear any sound. Thanks! Are there any other terms for 1+1 wire? It's just in case I don't find it in my search. >> Mike should be properly shielded wire, buy 1+1 wire with is available >> in market ( one is normal and other one will be shielded ) now u will >> able to hear any sound. > > Thanks! Are there any other terms for 1+1 wire? It's just in case I > don't find it in my search. ** LOL !! Grope, grope, grope...... ........ Phil Thanks, Phil! http://members.iinet.com.au/~rutlidge/alanindex.html Try using a larger resistor to bias your microphone. I would try something in the 100K-1Meg range. > Try using a larger resistor to bias your microphone. I would try > something in the 100K-1Meg range. I used a 100k with the same results. > Try using a larger resistor to bias your microphone. I would try > something in the 100K-1Meg range. You do realize there's nothing in there that's either going to be damage, or requires such a high value resistor? There's the actual electret element, and then there's an FET to buffer the signal. The electret is really high output impedance, and the FET doesn't load it down, but provides a lower output impedance. Take one apart, and you'll likely find an actual garden variety FET. That's what I found when I took one apart, I was actually surprised that it was some recognizeable device and not some blob of epoxy. You aren't biasing the microphone with that resistor. An electret microphone is charged when manufactured and requires no external charge or bias. The resistor is there to power the FET buffer. HOw often do you see a circuit that runs off low voltage and yet the voltage is run through such a large value resistor? You don't see it much. You're likely to starve the circuit with such a high value resistor and the low voltage. And if you run up that resistor, you are back to a relatively high impedance output from the microphone, even if the internal FET buffer works okay with that large value resistor. No, most resistors to power electret microphones are far lower, no more than 10K and likely lower. As for the other poster suggesting the microphone is blown, there is virtually no reason that that could happen. There is no reason to put in an FET that is so finicky that it won't run at reasonable voltages. ANd of course, long before one should be wondering if the microphone is bad (and it can easily be checked by powering it up and feeding it's output to an existing amplifier through a coupling capacitor), the circuit itself could be faulty, or something been's wired wrong. Michael >> Try using a larger resistor to bias your microphone. I would try >> something in the 100K-1Meg range. [quoted text clipped - 37 lines] > > Michael 2SK3857 for example. Has a gate resistor and -protection diode integrated. If you hook up the mike with reversed polarity, it will blow the internal fet, even with 5V. I have already damaged some Knowles mikes with static discharge from handling only.  Signatureciao Ban Apricale, Italy > Good morning everyone: > [quoted text clipped - 27 lines] > > Thanks! You probably have blown your mike with overvoltage. It is rated for 2V operation with that 2.2k, absolute max is 10V. You are operating it with higher voltage I suppose, and blew the internal FET. I would also filter a bit on that line, since any noise is amplified together with the signal. Signatureciao Ban Apricale, Italy > You probably have blown your mike with overvoltage. It is rated for 2V > operation with that 2.2k, absolute max is 10V. You are operating it with [quoted text clipped - 4 lines] > ciao Ban > Apricale, Italy Hi Ban The highest voltage setting that I've set my power supply is only 5V. So I don't think that it's blown. What kind of filtering techniques can I use? I already have some bypass caps close to the VCC pin of the IC. I read about a guarding technique this website: http://sound.westhost.com/dwopa.htm, but I don't know much about it, yet. Also, after looking at the microphone datasheet again and doing some calculation, I got the following numbers: Referring to 1kHz, at 30 dB re 20 uPa, the mic ideally outputs 4.74 uV. At 80 dB re 20 uPa, 1.58 mV. These numbers are lower than my original assumption. How would I handle signal levels like this properly besides using low noise op amps? > Good morning everyone: > [quoted text clipped - 27 lines] > > Thanks! I'm a bit wary of that "Vcc/2" voltage fighting against the static DC output of the first opamp. Those 1M resistors are very high and the opamp bias current will cause some shift away from 2.5V. If the static output of the 2nd opamp is about 2.5V (a volt either way say), then no problem. Otherwise, I'd stick a 10uF cap in series with the 1k resistor between the opamps and just for luck I'd stick a 10uF cap on the output of the second opamp. john SignaturePosted via a free Usenet account from http://www.teranews.com I was trying out this circuit: http://tinypic.com/view/?pic=2a8oppl (classic instrumentation amplifier) The only value that I replaced was the 1.6kohm resistor to 2.2kohm. The audio input definitely sounds cleaner if I speak directly on the mic. However, if I place a headphone with some audio playing directly above the mic, I don't hear anything out of the speakers even if I turn the speaker volume all the way up. I also tried to speak directly in front of the mic with my distance about 10 inches away from it (the same distance when speaking into my PC mic). I didn't hear anything out of the speaker. Is this possibly a mic issue? I'd like to be able to duplicate the mic pickup from my speaker phone or cell phone headset. With my headset, my friends sometimes complain if I have my car radio slightly up. The headset can apparently pick up the background noise better compared to just using the cellphone microphone. Thanks! |
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