Microphone help

Discussion in 'The Projects Forum' started by SilvrEclipse, Feb 16, 2009.

  1. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I am building something and have a small amp from an old set of computer speakers and I'm trying to hook a small electret microphone to the input of the amp and get it to play through the speaker. I didn't think it would work by just connecting it to the input wires but tried it anyways and of course I got nothing. Is there anything I can do to get this to work? Thanks guys
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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  3. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Thanks man
     
  4. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I had some free time today and put together this circuit.
    http://www.reconnsworld.com/audio_simplepreamp.html

    Im a little confused on how to connect this preamp to the speaker amp. The preamp only has 1 output which I would assume would go to the positive on the amp input but what about the negative? Would I just ground that to the battery ground? I went ahead and tried some variations of the wiring and I cant get this microphone to work. All Im getting is some radio station in the back ground. Any advice?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    The grounds will have to be tied together. That would be the battery negative and the amp negative.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Maybe your microphone is not an electret type. Look in Google Images for Electret Microphone.
    It must have the preamp compactly built (not on a breadboard) and use shielded audio wire between the mic and the preamp, and from the output of the preamp to the audio amp input.

    If the mic hears the speaker then you will have acoustical feedback howling as the sound goes around and around.
     
  7. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    I still cant get this thing to work correctly. The Mic just is not working. I drew a pic to make sure everything is connected correctly. The only other thing I can think of is I pulled the mic from an old cell phone headset. I know it worked but I kinda guessed that it was an electret mic. Do you guys see anything wrong with this setup??

    http://i189.photobucket.com/albums/z173/SilvrEclipse/Circuit-1.jpg?t=1234922231
     
  8. flat5

    Active Member

    Nov 13, 2008
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    Use 9v and change the caps to 10uf.
    Plus side to the mic and plus side to the transistor collector.
    But I expect you will need even more power to the amp.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Your schematic should work fine if the input impedance of the amp is 10k ohms or more.
     
  10. flat5

    Active Member

    Nov 13, 2008
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    I'm more concerned with the output cap. Many PC amplifiers expect to see a speaker output - not a line out. Or they did in the old days...say <1993.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    You forgot to post the schematic or part number of your audio amp. Maybe its input must be connected to 0V with a resistor or to a voltage divider at half the supply voltage.
     
  12. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    It dont have a schematic or anything for this amp. I just took it out of a set of desktop speakers. The amp works, when I hooked up the microphone I got nothing from it but when I touched the negative wire for the amp input I was pickin up radio stations.
     
  13. flat5

    Active Member

    Nov 13, 2008
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    Can you provide a good photograph of what you have? The preamp and amp.
     
  14. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Yea I will get one when I get home.
     
  15. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Ok here are some pics I just snapped. The on the preamp, the top of the board is the +. The bottom is the - and the middle wire is the signal. Im running 12v to the amp and ran another resistor inline with the preamp to knock the voltage down to 8v or so. The mic is on the left of the preamp. I ran the signal from the preamp to the right and left channel of the amp and all the negatives tie together. Power source is not connected right now but everything else is wired up.

    http://i189.photobucket.com/albums/z173/SilvrEclipse/DSCF2716.jpg?t=1235753324
    http://i189.photobucket.com/albums/z173/SilvrEclipse/DSCF2717.jpg?t=1235753383
    http://i189.photobucket.com/albums/z173/SilvrEclipse/DSCF2718.jpg?t=1235753411
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Then you knocked down its gain from 180 to almost nothing.
    You should use a voltage regulator, not a resistor.

    Get rid of the extra resistor and add a 9V battery with a 100uF capacitor to power the mic before you make a voltage regulator.
     
  17. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Ok I will give that a try. Hopfully it will fix it.
     
  18. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    Thanks AudioGuru, the mic works now. I didn't add the 100uF cap yet, what exactly is it going to do?
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    As the battery runs down, its internal resistance rises which causes its voltage to swing up and down with the audio that causes positive feedback.
    Then the output will make a motor-boat sound: "putt, putt, putt etc".
    The 100uF capacitor smooths the voltage from the battery so it does not jump up and down.
     
  20. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    The mic is working great now. I went ahead and put the 100uF cap on it earlier. It seemed to take some of the interference out of the circuit. I am getting a much cleaner signal now. I went ahead and added a 5v regulator to the preamp also. The whole thing will run off a 12v power source now.

    Only issue is the volume is a little low. Is there anyway to make it louder? It may be fine for what Im using it for but I have to keep the volume on the amp all the way up.

    Thanks again guys
     
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