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Microphone and camera using same power sounds like crap

Discussion in 'Audio' started by JohnWin, Feb 16, 2018.

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  1. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Or if it is lightly loaded, it might be running discontinuous at an audible hiccup frequency.

    ak
     
  2. JohnWin

    JohnWin

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    @dave9 @Audioguru and @AnalogKid after reading your responses I decided to make a small circuit with some stuff I have laying around. So I took an LM7805 and a ton of 82k resistors in a series to bring it down to 1.45V. And the put a 47uf capacitor between the output and the ground. So far so good.

    But when I turn on the mic the voltage drops down to nothing and it doesn't turn on at all. I'm such a novice at this stuff, I have no idea whats going on.. I might just have to except using a AA battery.
     
  3. dave9

    dave9

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    ^ You mean that you used an LM7805 and put a bunch of resistors in series AFTER it?

    If so, that's not the way to go. I'll tell you a secret many people never mention. You can use an LM7805 for variable or different (than reference 5.0V in this case) like you would an LM317, but when you do, consider the voltage calculation to include 5V reference instead of 1.2V when choosing resistors as seen in the LM317 datasheet example, or see this:

    http://www.rason.org/Projects/regulator/regulator.htm

    Edit: Come to think of it, I've never tried that below 5.0V, anyone know if there is an issue there?
     
    Last edited: Feb 17, 2018
  4. JohnWin

    JohnWin

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    Ok I'm trying this but I can't get it to go below 5v. It's going higher no problem but no matter what combination of resistors I use it won't go below 5v. So I looked for a schematic showing a circuit going from 5v to 1.5v, which you would think would be easy to find, but I'm not finding it anywhere.
     
  5. Audioguru

    Audioguru

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    You cannot make the output of a 7805 go less than 5V. Use an LM317 to make 1.5V.
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    That probably passed so little current that the poor little mic was starved :(.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The variable 3 terminal regulators are designed to have both a low and a relatively constant current going through their adj pin. This is not necessarily the case with non adjustable 3 terminal regulators.

    The result is that the performance of a non adjustable regulator, when used this way, is generally poorer than for an adjustable regulator.

    Just using a resistive divider, the output of one of these regulators cannot go below their reference voltage.
     
  8. JohnWin

    JohnWin

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    Nov 24, 2016
    I just ordered 10 of them. They should be here by thurs.
     
  9. JohnWin

    JohnWin

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    Nov 24, 2016
    Ah yes that makes sense
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    The electret mic that is powered from an AA battery cell is different from an ordinary 2-wires electret mic that needs a resistor from its output to a positive voltage so that the Jfet can draw about 0.5mA. the battery powered mic probably uses a 3-wires electret mic with its Jfet as a source-follower. Then DO NOT use a resistor to the positive supply, instead it might already have or needs an external resistor to 0V instead. The datasheet of the microphone powered with a battery might not even show you how to power it from a power supply.
     
  11. JohnWin

    JohnWin

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    Nov 24, 2016
    So after I get power to the mic with the LM317 set to 1.5v w/capacitor to -, if things still don't sound right or work I should connect a resistor between the audio output of the mic and a positive voltage? I apologize if I'm not understanding correctly.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,397
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    Jan 21, 2010
    Depending on how the power is connected to the mic, it may not be possible to power it the way you intend.

    Do you know, for instance, if the negative supply rail is common with the output ground?
     
  13. JohnWin

    JohnWin

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    Nov 24, 2016
    Well if that's the case so be it. It's not the end of the world using a AA. I'll try my luck with the LM317 and if not at least I've learned a little. Turns out they'll be coming in not this thurs but the thurs after that. I misread the ad.. Anyway I'll be sure to post the results good or bad once I get everything together. Thx!
     
  14. Audioguru

    Audioguru

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    Sep 24, 2016
    Ask the manufacturer of the mic how it can be powered with an LM317 that has its - common to the audio ground.
     
  15. JohnWin

    JohnWin

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    Nov 24, 2016
    I did contract the manufacture and they said I could use a LM317 to power it. I don't think they understood the whole common ground thing. I suspect English isn't their first language. Thank you for all you help on this!
     
  16. JohnWin

    JohnWin

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    Nov 24, 2016
    Sorry about the delay in get back with the results. I tried the LM317 and still get the pulsing sound. At this point I'm ready to except using a AA battery.

    Thank you all for the help!
     
  17. Sunnysky

    Sunnysky

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    Jul 15, 2016
    A Microphone load may be 1k~10k so to filter noise above 20Hz use ~100 in series to mic with 47uF to 100uF at output.
     
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