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Sci.Electronics.Basics -> Electret microphone question
There are 26 messages in this thread.
You are currently looking at messages 20 to 26.
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Author: Michael BlackDate: 00:02 17-01-07
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"MRW" (mr.whatever@gmail.com) writes:
> Michael Black wrote:
>> 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.
>
>
> 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.
>
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
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Author: BanDate: 02:12 17-01-07
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Michael Black wrote:
> "jfeng@my-deja.com" (jfeng@my-deja.com) writes:
>> 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
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.
--
ciao Ban
Apricale, Italy
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Author: MRWDate: 09:39 17-01-07
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Michael Black wrote:
> 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
> 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.
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!
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Author: MRWDate: 18:57 17-01-07
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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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Author: THERES RAJ, BLRDate: 06:32 31-01-07
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On Jan 16, 12:42 am, "Tom Biasi" <tombiasi@********optonline.net>
wrote:
> "MRW" <mr.whate...@gmail.com> wrote in message
>
> news:1168885085.063760.40050@s34g2000cwa.googlegroups.com...
>
> > 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=423-104...
>
> > 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!
>
> 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
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Author: THERES RAJ, BLRDate: 06:36 31-01-07
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On Jan 16, 1:12 am, "MRW" <mr.whate...@gmail.com> wrote:
> Tom Biasi wrote:
> > 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.
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