How to interface a microphone to a speaker?

Discussion in 'The Projects Forum' started by yaantey, Dec 28, 2011.

  1. yaantey

    Thread Starter Member

    Oct 7, 2011
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    Hey guys,

    Can anyone point me to a simple circuit that can be used to wire a MIC to a speaker so that the speaker can output the inputs to MIC. Also then I can connect the output from mic to micro controller to see the maximum and minimum voltage output of the MIC.

    Im a newbie, so please help me out. Thanks.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    What you need is an amplifier, but it is possible for something usually not wanted to happen if a microphone feeds a speaker so that the speaker can return the input to the microphone.

    Have you heard what happens when a microphone is placed near the loudspeaker that it is feeding?
     
  4. bertus

    Administrator

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  5. yaantey

    Thread Starter Member

    Oct 7, 2011
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    Hello, people. Regarding the attached circuit, can someone answer to some questions about it?
    • I would like to know why a non-inverting input op-amp is used and why not an inverting op-amp?
    • Why do we need a feedback loop?
    • Also why is that the gain is determined by the ratio of the two resistors forming the divider bridge at the output of the OpAmp?
    • Also what are the specific uses of the capacitors and resistors used?

    The link for the circuit (http://didier.longueville.free.fr/arduinoos/?p=1057) and please be kind, I am a newbie on electronics

    [​IMG]
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You have pressed the "new thread" button and created a new thread.
    I have merged the two threads now.
    If you want to reply to a thread use the "post reply" button.

    Bertus
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    I suspect there is no reason, the originator of that schematic wanted it that way. When you are designing there is no right or wrong way, there are only choices.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    • The non-inverting configuration has a conveniently high input impedance. Edit: As Bill Marsden points out, an inverting configuration would be possible, but this setup for a single-supply amplifier is quite commonly used. Perhaps trying to work out an inverting configuration would be a useful exercise, when you are ready for it.
    • Without a feedback loop the gain of an op-amp would have a very large but not closely specified value. The bandwidth would be low, and the linearity may be poor. The DC condition will be unstable, the output possibly limiting at one or other extreme value.
    • When negative feedback is applied to a high-gain differential amplifier, the circuit can often be assumed to have a negligible voltage difference between its inputs. The gain of a circuit such as this can be predicted from this assumption. My rambling version of this analysis might hinder rather than help you, so see these links: http://www.allaboutcircuits.com/vol_3/chpt_8/index.html http://en.wikipedia.org/wiki/Operational_amplifier
    • It is possible to describe the function of each of the components:
    C1: main supply bypass (or decoupling or filter) capacitor. C5: microphone supply bypass/decoupling/filter capacitor.
    R5: microphone supply dropper/filter resistor. R6: microphone DC load (or feed) resistor.
    C2: amplifier input coupling capacitor. R3, R4: bias resistors, setting half-supply DC voltage at amplifier non-inverting input.
    C3: feedback coupling capacitor - allows feedback to define AC gain without upsetting the DC output at half supply condition.
    R1, R2: Feedback network - sets the AC gain. This is approximately given by Av = (1+R1/R2), which here comes to 11 times.

    Perhaps such descriptions may not help much unless some underlying facts are also understood. Working "backwards" from a circuit may be confusing unless basic ideas such as bias, AC coupling, decoupling , feedback etc. are mastered first.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    The circuit is a poor choice:
    1) The LM358 has too much hiss and crossover distortion to be a mic preamp, use a low noise opamp like a TL071 instead (its minimum supply is 7V). The LM358 has two opamps but the TL071 is a single opamp (a TL072 is a dual and a TL074 is a quad).
    2) Resistors R3 and R4 have values of only 4.7k which is much too low and they short most of the level from the mic. Use 47k for each of them instead.
    3) The gain of the circuit is only 11 so you must scream loudly into the mic. Change R1 to 100k or 200k for a gain of 101 or 201.
     
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