microphone amplifier

Discussion in 'The Projects Forum' started by tryinghard, Mar 14, 2008.

  1. tryinghard

    Thread Starter New Member

    Mar 14, 2008
    4
    0
    hello all,
    my first post so be gentle!

    I have produced a circuit diagram - see attachment - of what I believe is a microphone amplifier. I would like the opinion of people who have a better knowledge than me before I build it as to whether it will work.
    any advice appreciated.
     
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  2. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    First off, you are creating a midpoint of ~4.5V with the two 10k resistors and the 10µF cap, but not using it, so why did you do that?
    Did you mean to use it for the "ground" of the mic (which is the only way it makes sense)?
    Why the resistor of 47 Ohm on the output?
    If you remove the cap between the 1k resistor and ground, the op-amp output will have proper DC-reference.
    For a mic-amp, consider if you need a higher gain than 11x.

    If you ask to learn, consider why you placed any and all of the components in the circuit as you did - why, what would it do with/without it, what is the output of the mic and how are you planning to feed it bias (hopefully not a direct feed).

    Lastly, why do you wanna re-invent the wheel when the net is floating with mic-amps?
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Greetings tryinghard,

    Have you selected an opamp for your project?

    What mike are you using and what are you planning to use your amplifier to drive?

    Sorry for all the questions but the more information you can provide the better we can assist you with your project.

    I gather you have decided to take this on as a fun project. If not, as Soren has remarked, there are many fairly inexpensive off-the-shelf amplifiers available on the web.

    Look forward to hearing more details about your project plans.

    hgmjr
     
  4. tryinghard

    Thread Starter New Member

    Mar 14, 2008
    4
    0
    Thanks for the replies.
    The 4.5V divider is for a seperate part of the circuit and should not have been included in the sketch ( I needed to reduce the 9V to a level taht could be used to supply a voltage regulator, sorry about that. The 10uF capacitor and 1k resistor are working as a filter for the amp output cutting off frequencies below about 15Hz.
    This is for fun just to make an outside recorder for birdsong and I wnted to cut out wind noise. the op-amp is an ne5532. The mic is a small electret condenser microphone
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I think you should consider using the 4.5 volt bias to set the positive input of your opamp at the middle of your power supply. You will then need to use a dc blocking capacitor between the output of the mic and the input to your amp.

    hgmjr
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I guess you should use a dedicated 4.5V bias rather than one you already have provided for other parts of the circuit. The input bias network would not have cap to ground. You will also need to use larger valued resistors to form the bias network.

    hgmjr
     
  7. tryinghard

    Thread Starter New Member

    Mar 14, 2008
    4
    0
    Thanks for the reply hgmjr, do I need to put a DC bias on the op amp or will it work as is?
    What would be the benefit of the bias to the op-amp?
    What would the capacitor do on the outputof the mic? wouldn't this disrupt he output while charges and discharges?
    Sorry for the questions but I want to improve my understanding of electronics
     
  8. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    You'll get much better wind noise reduction from a cover of (acoustically open) foam.
    Allways cut noise at the earliest stage possible!
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Hi Soren,
    If the capacitor to ground that connects to the 1k resistor is removed then the opamp's DC gain of 11 will saturate its output, especially if the input of the opamp is biased at +4.5V and it had more gain.

    Many audio opamps are high-speed like the TL071. They oscillate at a very high frequency if they directly drive the capacitance to ground of a shielded cable. The 47 ohms in series with the output prevents this oscillation.
     
  10. tryinghard

    Thread Starter New Member

    Mar 14, 2008
    4
    0
    thanks for the advice.
    I will also be using a foam windshield but though that a low frequency filter would also improve the signal, I read somewhere that wind noise is worst at low frequencies.
    I am still unsure though if I need to include a DC offset to the op-amp (NE5532) or not. Would this be added to the + input or -? The negative gives me a feedback from the output so I would assume that it would need to be connected to the + input, is this correct?
    If so is it really necessary and what beneifit would it give me?
    Thanks again
    Tryinghard
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The (+) input of the opamp should be biased at half the supply voltage for it to have the max possible output voltage swing. The microphone's output voltage might be too low for the opamp's input since its minimum allowed common-mode input voltage on the datasheet is 3V higher than its negative supply voltage. The microphone's DC output voltage might be 1.0V which is too low. Your circuit has ground as its negative supply voltage.

    Then a coupling capacitor is needed between the microphone and tha opamp's input to pass the signal but block the DC.
     
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