lm386 and electret microphone problem

Discussion in 'The Projects Forum' started by Stratos15, Aug 23, 2015.

  1. Stratos15

    Thread Starter New Member

    Aug 23, 2015
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    Hello,
    I have completed an amplifier circuit using the lm386 and an electret microphone as shown in the following link: *How to Build a Microphone Amplifier Circuit

    When I blow onto the electret microphone, I can hear the sound of it. However if I start talking onto the microphone, no sound is coming out of the speaker.

    I have tested the electret microphone on my pc and works fine as I can record myself talking and then hear it back.

    What am I doing wrong?
    Thank you very much for your time.

    Regards,
    Stratos
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What did you do about R1?
     
  3. Stratos15

    Thread Starter New Member

    Aug 23, 2015
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    R1 is 2.2 KOhm
    This is the normal value for R1, along with the 0.1 microFarad for C1 when dealing with electret microphones
     
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  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Stratos15
    Add some resistance in series with your capacitor across pins 1 and 8. Right now, with a 10uF capacitor, your gain is 200 and you may be saturating your output. Alternatively, you can attenuate the input with your potentiometer - turn down to a low value until you can start hearing something.

    Blowing into the mic is not an efficient frequenc for the mic and may be attenuated enough to play through without saturating the amp. Look at the datasheet of the 386 - try a 20x amplification instead of your 200. circuit examples are at the middle to end of the datasheet.
     
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  5. ian field

    Distinguished Member

    Oct 27, 2012
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    That's a reasonable choice for R1 - but 5V might be cutting it fine for an LM386, the TDA7052 BTL amplifier performs better at that low voltage, the A suffix version has DC volume control. Most electret capsules require about 2V, so 5V supply will cramp the output swing a bit.
     
  6. Marcus2012

    Member

    Feb 22, 2015
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    I would suggest removing C2 to set the gain to 20 just to see if it was saturating the output (this happened to me recently) but in addition to this it may be worth changing R1 to a 10K trimpot in order to adjust the input impedance to the Lm386, it may be a little low atm.

    EDIT: Quick Q: What LM386 variant are you using and what power speaker? I only ask as this circuit seems to be more set up for a line out configuration than a speaker amplifier. (i.e. LM386N-1 only has an output power of 325mW @ 8ohms).
     
    Last edited: Aug 23, 2015
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Good call on the power supply voltage, Ian.

    @Stratos15
    You should look for 9 v power supply or better. At 5 volts, you will get (without clipping), less than a half watt. Not much to hear if you are trying to hear more than your own (un-amplified) voice.
     
  8. Marcus2012

    Member

    Feb 22, 2015
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    It should work on 5 volts no problem unless its the N-4 but if its a N-1 it will only have 325W at 8ohms anyway. Ofc as you say Gopher increasing the supply will increase the output power but I can't help but think this circuit wasn't intended for this.
     
  9. Stratos15

    Thread Starter New Member

    Aug 23, 2015
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    It's the lm386 power amplifier and I am testing it on either 8 Ohm 0.5 Watt speaker or 4 Ohm 6 Watt speaker
     
  10. Marcus2012

    Member

    Feb 22, 2015
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    I doubt you'd get anywhere close to 6 watts out of it at 5V, The graphs below show different load impedances with respect to device heat dissipation and output power with each curve being a different source voltage. As gopher said if your intention is to have the output directly to speakers you might want to use a 9 or 12V supply
    .
    Untitled[1].png
     
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  11. Stratos15

    Thread Starter New Member

    Aug 23, 2015
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    I am using a 9V battery.
    I removed the C2 to set the gain to 20 but I still hear noise instead of my voice.
    I also changed R1 to a 10K but I still hear voice, although this noise now follows my voice's ups and downs as I am talking.
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I run my LM386 with a 9 volt battery, but I notice that when the voltage in the battery begins to sag, there is more noise in the amp output.

    It's been a while since I tried to use a mic with an LM386, but if I recall, I had to build a JFET preamp in order to make it work.
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I've seen this before, and there are lotsa low cost "preamp" modules on ebay that use the 386, but it never works well. The LM386 was not designed to bring a microphone level signal all the way up to speaker level. That's a gain of 1000 or more, 12 dB or more above what the chip can provide. So it is not a surprise that the output volume is low. A little gain-of-10 or -20 preamp in front of it will make a big difference.

    ak
     
  14. tracecom

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    Here is the preamp I think I used. It's shown in stereo, but you can use one channel for mono. MFP102 Preamp Schematic.PNG
     
  15. jjw

    Member

    Dec 24, 2013
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    2.2k parallel with 10k + 0.1uF form ~ 900 Hz high pass filter.
    You could try C1 = 1uF.

    Edit: I made a mistake. -3dB response is at ~ 200Hz. Still C1 could be bigger, for example 0.47u
     
    Last edited: Aug 24, 2015
  16. Stratos15

    Thread Starter New Member

    Aug 23, 2015
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    Thank you all for your replies.
    As far as I understood I need a preamp for the electret mic to work with lm386 as expected.
    Thus I would like to make my own preamp. I have seen that a NPN transistor is needed for this.
    Can I use any NPN transistor, because I have got BC547 (50V 100mA)?
    In such a case what values for transistors and capacitors will I need?
    Cheers
     
    Last edited: Aug 25, 2015
  17. Marcus2012

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    Feb 22, 2015
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