Electret mic non-sensitivity

Discussion in 'General Electronics Chat' started by tracecom, Oct 11, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    With a sound card purchase long forgotten, I got a free mic; it was one of those electret elements on a long skinny plastic stick with a sort-of adjustable plastic base. I never used it much because it seemed that I had to "eat it" in order to be heard; pretty soon, I threw it in the junkbox.

    Today, I got to thinking about the JFET preamp I have been working on, and decided to try it with a mic. Being the tightwad that I am, I dug out the old mic, took the electret element out of it, rewired it with a 10k power resistor and a .1μF DC blocking cap, and hooked it to my preamp.

    Well, the performance is exactly the same. I still have to get right up against the element to produce any output, and then it seems that the output is excessive.

    Because I don't know anything about electret mic's, I am wondering if this element is just bad or is that characteristic of the electret?

    Thanks.
     
  2. radiohead

    Active Member

    May 28, 2009
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    Measure the voltage across the 10K resistor. It should be about 2V or so. If you are getting less than that, there may not be enough voltage to bias the mic. Try different values for the mic's bias resistor. You might have to use a value as low as 1K or 2.2K in order for it to work. Is it a two or three terminal electret mic? If it is a 3-terminal mic, one wire is the negative connection, one wire is the bias voltage and the other is the audio output (connected to the .1uF cap). Hope this helps.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Most electret mic cartridges are omni-directional and pickup sounds from all around it. Your mic is on a "gooseneck". Some gooseneck mics have a mic cartridge with "rear ports" on the sides that cancel background sounds that are distant to the mic. Then you must "eat the mic" to be heard. Maybe you can block the rear ports with tape?

    Oh yeah, the Jfet preamp you were working on was to provide a very high input impedance to a guitar pickup. Its voltage gain is nothing or is about 3dB which is slightly more than nothing.
    But an electret mic needs a preamp with a voltage gain of 100 to 400.
     
  4. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    It's a two wire; I will try your suggestions. Thanks.

    There are no ports on the side, but there are these tiny holes in the back. Are they the rear ports?

    And with regard to the gain, you are correct. The gain in my preamp is only 3.4 with a 12 V supply. Are there component changes that I can make to increase the gain dramatically, or do I need another stage?

    Thanks.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The holes in the back of your mic looks like the rear ports I talked about that cancel distant sounds.

    You need a simple and cheap omni mic cartridge.
    You also need a preamp circuit for an electret mic, not for a geetar. An electret mic already has a Jfet impedance reducing circuit inside.
     
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  6. bertus

    Administrator

    Apr 5, 2008
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