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Amplifier for electret microphone

  1. Mar 1, 2012 #1
    So I posted a question regarding amplifying signals from electret microphone.

    I had a problem, in which I had to design such amplifier which uses unipolar power supply.

    I was suggested LM386, and I got somewhere. Its very neat little amplifier, but I still need more.

    I tried a lot of things. My goal is: low noises, and 4-5 V at the output. So yes very large amplitude.

    Currently I am using 9V battery for my power source, so I don't know how much power I can get out of it.

    I tried a lot of things.

    http://pokit.org/get/b446414024786bd4ba37c3ce805f9784.jpg [Broken]

    I tried this with a little bjt pre-amplifier. It was better but not as good as I wanted it.

    I tried cascading these
    http://pokit.org/get/1e2ed15bad7f97f328fd12f3fcdff81b.jpg [Broken]

    I get very weird sound at the output.


    Generally I need help with these amplifiers, because I am shooting things at random, because my knowledge of amplifiers is only theoretical, and as I see its a whole different story when you try to put it together on breadboard.


    I got 3 LM386, and few LM741 op amps, I got over a 70 BJTs both npn and pnp, help me design this amplifier, because I am currently nowhere.

    I may also add that I want this so high because I will be using capacitors at the output, to integrate this signal. So I need certain voltage, that can charge capacitors, ergo 4-5[max amplitude of the signal] V.
     
    Last edited by a moderator: May 5, 2017
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  3. Mar 1, 2012 #2

    AlephZero

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    You seem to be missing the fact that an "electret mic" contains not just the microphone but also the first stage of an amplifier, to convert the impedance to a reasonably low level.

    Your circut needs a resistor and capacitor to complete the "internal amplifier" circuit, to get a useful signal from the mic. See

    http://en.wikipedia.org/wiki/Electret_microphone
    http://www.zen22142.zen.co.uk/Prac/ecm.htm

    If you connect the output from the capacitor in those circuits to Vin of your amp circuit, things should work better.

    There's a complete circuit here. It's not quite the same as yours (which doesn't matter), but you should be able to see what's going on.
    http://www.zen22142.zen.co.uk/Circuits/Audio/lf071_mic.htm
     
  4. Mar 1, 2012 #3

    jim hardy

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    386 datasheet shows enough output swing for your application provided cap isn't just huge
    16 0hms it'll drive to about 7Vp-p wth 9 volt supply. See rightmost chart in first row page 5of11 in PDF, page 4 of National's datasheet

    http://www.national.com/mpf/LM/LM386.html#Overview [Broken]
    click datasheet PDF icon

    I hope Aleph's post helps you bias the microphone

    as best i recall the input to 386 is deceptively simple to bias - just dont feed ANY dc to either input, but provide a path for dc FROM each input to common.. it pumps out a small bias current from each input. See how last application circuit in datasheet, AM radio amp, uses a capacitor to block DC from detector. Your electret will need same treatment.

    If you need some easy gain look at LM324 single supply opamp. Not very fast but cheap and easy to use.

    old jim
     
    Last edited by a moderator: May 5, 2017
  5. Mar 2, 2012 #4
    I did power electret mic like that. I am using 10 nF as the output from the microphone, and I am powering it through 10k resistor.

    I heard that shunt resistors are needed or something, I really don't know what shunt resistors are for...
     
  6. Mar 2, 2012 #5
    I will try that.

    I don't know why I am getting this. It worked fine when I used +9 and -9 V supply with the LM741, two stage amplifiers. I don't know why I am getting so little gain...


    I should add that I am currently getting, with this setup, some 0.5 V max.
     
    Last edited by a moderator: May 5, 2017
  7. Mar 2, 2012 #6

    jim hardy

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    Wow i'm sorry you're having that trouble.

    How much gain are you getting?

    Looks like there's nothing magic about the electret
    here's a hobbyist preamp,, see schematic
    http://www.sparkfun.com/products/9964

    The TI datasheet they link is interesting see fig 7 page 11.


    hmm note schematic of LM386 internals.
    Input resistance is ~ 50K Ω

    10nf is 50K at 318 hz
    try a little bigger DC blocking cap?
     
  8. Mar 2, 2012 #7
    Yes about that, what determines the size of cap? What am I getting by using bigger or smaller capacitors?
     
  9. Mar 2, 2012 #8

    jim hardy

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    Is that unloaded?

    .5/200 = 2.5 mv in...
     
  10. Mar 2, 2012 #9
    My AC voltmeter, when I am blowing into microphone is giving me this, briefly.
     
  11. Mar 2, 2012 #10

    jim hardy

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    When cap impedance equals amplifier's input impedance , it drops the input voltage.

    So that makes your amp a high-pass at 1/ωC = Rin which is ~ 50kΩ at 318 hz
    Bigger cap lowers corner frequency
     
  12. Mar 2, 2012 #11
    I will try that tonight. Currently I am trying to clean the mess I made over last 2 weeks with all electronic projects I had. I found that underneath all this I had a desk :D
     
  13. Mar 2, 2012 #12

    jim hardy

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    I have a garage full of projects, am no stranger to clutter..

    I'm feeling a little embarassed - maybe 200 just isn't enough gain for your electret mike.
     
  14. Mar 2, 2012 #13
    When I used dual op amp, the gain that was ok was about 290.

    First one was 4,9 and the second was 59,8.

    But the thing is, I think in that setup I amplified all frequencies equally, because of dual supply.

    But with single supply I have less "window" of usable frequencies.

    Again, this was the setup that worked:

    http://pokit.org/get/641fcaca23c811ed20ebb0c16c0866c1.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
  15. Mar 2, 2012 #14

    jim hardy

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    Ahh i see..

    You want to capture the peaks?

    LM386 has output centered at mid supply, so in quiet would try to hold 4.5 volts at pin 5..

    I think you'll have to capacitively couple into your holding caps.

    draw this into that "gain of 200" LM386 circuit:

    1. Replace speaker with a diode, preferrably Schottky for low voltage drop, cathode up to 250uf's - side. Call it D1
    2. Add a second diode(also Schottky if you can) to junction of 250uf- and D1. Call it D2.
    D2's anode goes to junction of 250uf-/D1cathode, and D2's cathode hangs out to right. We'll connect it in a minute...
    3. Connect your holding capacitors to D2's cathode.

    Now is it a charge pump transferring peaks into your holding caps? or did i type before thinking again?.

    Something doesn't seem quite right yet, but an experiment would say if on right track.
    Probably adjust value on 250 uf for better setling time.

    This one is interesting !

    old jim
     
    Last edited: Mar 2, 2012
  16. Mar 3, 2012 #15
    I am aiming for this:

    When people are talking in background, I want that noise to charge up my capacitors, so that that voltage across those capacitor can trigger something, or in other words, I want to integrate that noise.

    So I don't want my trigger to go off on an amplitude, like a delta(somebody screams), but rather continuous noise.

    There is a Schmitt's trigger after this circuit, that is comparing this voltage across capacitors, with some reference voltage.


    I don't have Schottky diodes, but I do have small signal 1N4148 ones, I think they will have to do. I will try this and come back with results.

    Thank you mr. Hardy!
     
  17. Mar 3, 2012 #16

    jim hardy

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    Great !!

    1N4148's should work okay i'd think....

    Another thing i dont know about is effect of that big capacitive load on amplifier.
    But without a 'scope i'm not sure how to check for it.
    A quick experiment would be to place a small speaker in series with the 250uf, say between amp's output pin and + side of 250uf cap.. see if circuit behaves more like the 741's did. Also that little extra impedance would slow the charge time giving you an averaging effect that clips short duration peaks.
    If capacitive load turns out to be problematic, might you get away with 1uf integrating cap and 1 meg potentiometer, and 1 or 2uf coupling cap instead of 250??

    old jim
     
  18. Mar 3, 2012 #17
    Ok I am trying those things right now, will come back.
     
  19. Mar 3, 2012 #18
    Btw. what do I do with D1 anode?
     
  20. Mar 3, 2012 #19

    jim hardy

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    It goes to common - sorry..
     
  21. Mar 3, 2012 #20
    You are a wizard. That trick actually worked.

    I am detecting the level of sound by LED driver bar. I just need to work out the numbers with regarding resistors, to achieve desired trigger level!

    I will keep you posted.

    Can you explain what you did?
    :D :D
     
    Last edited: Mar 3, 2012
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