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Electret microphone question

Discussion in 'Electronic Basics' started by MRW, Jan 15, 2007.

  1. MRW

    MRW Guest

    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!
     
  2. Tom Biasi

    Tom Biasi Guest

    How are you supplying power to the microphone?
     
  3. MRW

    MRW Guest

    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.
     
  4. Tom Biasi

    Tom Biasi Guest

    Did you actually measure the DC at the mic?
    Tom
     
  5. MRW

    MRW Guest


    Yep, I used a multimeter and an oscilloscope to measure the voltage
    levels.
     
  6. Phil Allison

    Phil Allison Guest

    ** Groper Alert !



    ** 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
     
  7. Guest


    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
     
  8. MRW

    MRW Guest

    Hi Phil

    Is it really wrong to be using Google Groups?

    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.
     
  9. MRW

    MRW Guest

    Thanks! Are there any other terms for 1+1 wire? It's just in case I
    don't find it in my search.
     
  10. Phil Allison

    Phil Allison Guest


    ** You are not a part of usenet at all.

    Just a fucking interloper.




    ........ Phil
     
  11. Phil Allison

    Phil Allison Guest


    ** LOL !!

    Grope, grope, grope......



    ......... Phil
     
  12. Guest

    Try using a larger resistor to bias your microphone. I would try
    something in the 100K-1Meg range.
     
  13. Ban

    Ban Guest


    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.
     
  14. MRW

    MRW Guest

    I used a 100k with the same results.
     
  15. MRW

    MRW Guest

    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?
     
  16. 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
     
  17. john jardine

    john jardine Guest

    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
     
  18. 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
     
  19. MRW

    MRW Guest


    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.
     
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