Simple Mic Pre-Amp based LM358

Discussion in 'The Projects Forum' started by mark63534, Oct 23, 2012.

  1. mark63534

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    Nov 24, 2011
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  2. bertus

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  3. mark63534

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    Nov 24, 2011
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    I'm confused. Single supply as in each op amp will have its own power supply/battery ? In that case, I'm not trying to do that...
     
  4. bountyhunter

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    Sep 7, 2009
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    Is that a crystal mike?

    What is the input signal level to the amp?

    What signal level is needed for the following stages?

    The 358 has such high noise and poor frequency bandwidth it is not suited for audio use for anything but very poor signal quality.
     
  5. mark63534

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    Nov 24, 2011
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    I'm not sure about where the crystal idea came from...

    The input signal to the amp would be coming from a typical 3.5mm audio out jack, from an electret microphone, as shown in the circuit.

    As for the signal level, I do not know what it could be in the next stage, but I was hoping that at the end of the circuit I would be able to interface with a ordinary "pro audio" mixer.

    Since the lm358 is so bad, do you think the lm386 would be a better choice ?
     
  6. Audioguru

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    Dec 20, 2007
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    It is a STUPID mic preamp circuit:
    1) The lousy old LM358 has much too much noise (hiss) for a mic preamp.
    2) The lousy old LM358 has problems above only 2kHz so it should never be used for audio.
    3) The lousy old LM358 has crossover distortion so it should never be used for audio.
    4) The opamp is inverting so its input resistor SHORTS the fairly high resistance from the electret mic. The opamp should be non-inverting then it can have an input resistance high enough.
    5) The circuit is missing a very important supply bypass capacitor.

    The video shows the LM386 amplifier oscillatiing at a high frequency, maybe because a breadboard is used instead of a pcb. Also its output capacitor value is much too low causing the severe phase shift shown and the slanted clipping.
    The LM386 has a capacitor between pin 1 and pin 8 which makes its gain too high when a preamp is used.

    This electret mic preamp works well:
    Sorry, the Manage Attachment button doesn't work now.

    The LM386 is a little power amplifier for a speaker, not a low noise mic preamp. use an audio opamp like a TL071 to make a mic preamp.
     
  7. mark63534

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    Nov 24, 2011
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    Unfortunately, I did not get to see that preamp you supposedly added.

    In that case of the TL071, what circuit do you think is suitable?

    This is a lot of guidance, thank you, all of you.
     
  8. Audioguru

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    Recently this website has problems that cause a few of us to not be able to post schematics with our replies. My browser is IE8.
    I will try again now:
    Nope, I cannot attach a schematic.
     
  9. mark63534

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    Nov 24, 2011
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    any chance you can upload it to a image hoster like

    Code ( (Unknown Language)):
    1. imageshack.us
     
  10. Audioguru

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  11. mark63534

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    Thank you for that circuit. I'll build it as soon as possible. However, I still have a question:

    Can I still power or use one 9v battery on multiple builds of this circuit?

    (i.e. I build 4 circuits that are all identical, but I use one 9V battery as a power source for all those circuits.)
     
  12. Audioguru

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    If you look at the datasheet for the TL071 opamp then you will see that it typically draws only 1.4mA. Then if you look at the datasheet of an Energizer 9V battery you will see that it can power 20 preamps for 6 hours when the voltage will have dropped to 7V.
    One 9V alkaline battery will power 4 preamps for 30 hours.
     
  13. Audioguru

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    I am testing to see whether I can attach my preamp schematic today:
    Yes, today it works!
     
  14. bountyhunter

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    Sep 7, 2009
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    AG's circuit is good assuming the microphone is OK looking into a 50k input impedance. I'm not sure if that's going to load the mike too much (?) I believe the two 100k input resistors could be increased in value quite a bit. It's a J-FET input op amp so the bias current is microscopic.



    ADD: reference I saw says typical electret output impedance is 2k - 5k Ohm. Not sure what looking into 50k is going to do. Should be only a small signal amplitude loss, not sure about added distortion.
     
    Last edited: Oct 24, 2012
  15. Audioguru

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    An electret mic works perfectly when loaded with 50k ohms.
    One person (Collin55, where is he lately?) powers the Jfet in an electret mic with 47k from 9V which is too high a value.
     
  16. mark63534

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    Nov 24, 2011
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    wow. nice. Any Idea where I would add a LED to this circuit? I've been thinking I could insert it before C4, where i also plan to add a SPST switch.
     
  17. Audioguru

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    You can add an on-off switch between the positive terminal of the battery and the +9V terminal of the circuit.
    An LED draws much more current than the opamp so the battery will run down much quicker than without it. The anode of the LED connects to the +9V terminal of the circuit and its cathode is in series with a current-limiting resistor that connects to ground.
     
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  18. bountyhunter

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    I have some super bright red LEDs (with clear lenses) that glow very brightly with only about 0.5 mA. I use them in portable junk I need indicators in. But it is a waste of power.
     
  19. Audioguru

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    Super bright LEDs (at only 0.5mA?) are simply ordinary dim old LEDs in a package that focuses the light into a very narrow beam. The focussing makes them bright only if they shine directly at you. If they don't shine directly at you then you can barely see them.
     
  20. bountyhunter

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    I fix that by "sanding" a flat (dull spot) at the end of the LED for the light to hit which makes them pretty non directional. They show up pretty well.
     
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