Electret mic circuit

Discussion in 'The Projects Forum' started by Mr.Whatever, Jan 25, 2007.

  1. Mr.Whatever

    Thread Starter New Member

    Jan 25, 2007
    3
    0
    So I duplicated the following circuit on the breadboard:

    [​IMG]

    It works great. But my problem is that the omni-directional electret microphone that I'm using cannot pick up my voice from 5 inches away. I have the output of this circuit connected to a PC speaker with built-in volume control (i.e. has an amplifier built-in). I even turned up the speaker volume all the way up.


    How come I can't speak 5 inches away from the microphone and hear my voice?

    :confused:
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What power supply scheme are you using? The circuit you have posted is an instrumentation amplifier with the final amplifier stage's positive input referenced to ground. That would normally be the case if it is used with a dual power supply. For example, +5V and -5V would be one possible choice.

    Is it possible that you are trying to power the circuit with a single supply?

    hgmjr
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Some electret mikes have three leads - the + voltage and audio are separate.
     
  4. Mr.Whatever

    Thread Starter New Member

    Jan 25, 2007
    3
    0

    Hmm.. I do have it in single supply configuration. I'll try out dual. How would I convert this into single supply?
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Once you get it working with the dual supply we can then look at conversion to a single supply.

    The basic requirement is that a voltage reference created that would be equal to your positive supply voltage divided by 2. That voltage reference would then become the point to which you would connect the 10K ohm, the two 10 nFd caps, and the two 1.6K ohm resistors.

    hgmjr
     
  6. Mr.Whatever

    Thread Starter New Member

    Jan 25, 2007
    3
    0
    Thanks! I think I see how to convert it to single supply. I can simply send a VCC/2 output to an opamp voltage follower and just supply the nodes you mentioned above.

    For the input stage of the mic, should I make the gain at 1000? I was doing some calculations and noticed that without any amplification the microphone would output around 7 uV at 50 dB SPL re 20 uPa. What do you think?
     
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