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    This mic preamp uses the low noise IC uA739. The circuit is an example of how a good preamplifier can be designed for dynamic microphones. The IC houses two identical integrated preamp circuits. The second preamp is used in identical manner for the second channel of the stereo microphone.

    Diagram bellow shows the pin numbers (in brackers) for the second identical channel. All external parts are identical to those shown in the schematic.
    ua739-ic

    The non-inverting input is biased at about 50% of the power supply. This bias voltage is set by the voltage divider circuit R1 and R4. The point between R1 and R4 is used commonly for both channels.
    The unwanted HF signals coming from the microphone are filtered out by the RC-circuit composed of R3/C4. Frequency compensation is done by the R7/C6 circuit. The values of R7 and C6 were designed to avoid oscillation at the amplification level of 100.

    The input impedance is about 47K. This means that a normal dynamic microphone gets connected to a high impedance preamp which in turn produces good results. The output impedance is about several hundred ohms.

    THe maximum peak-peak output voltage is about several volts lower than the supplied power. The frequency range is from 20Hz to 20KHz (-3dB). The upper cutoff frequency is 80KHz when the low-pass filter is removed from the circuit. The IC shown can be replaced with TBA231 or SN76131 without changind the external circuit.

    Dynamic microphone preamp circuit diagram

    dynamic microphone preamplifier schematic

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    9 Responses to "Dynamic Mic Preamplifier Circuit"

    1. I have designed and constructed a microphone amplifier circuit using LM301 with a line driver SSM2142 and a line receiver SSM2141.The circuit is supposed to pick up any voice/audio by the mic and send it via 100m Twisted Pair Cable to a PC for recording and analysis.I am using an omnidirectional Yodata Laptop Microphone,but i get nothing on the output.The circuit(amplification etc.) on its own is perfectly working but the audio/voice part is not,cracking my head.I have tried a ready-made Phillips microphone on my circuit and i works perfectly but the mic described for the job turns me down..Please help

    2. betontalpfa2 says: on March 9, 2013 at 3:32 pm

      Hi!

      You wrote:
      “The unwanted HF signals coming from the microphone are filtered out by the RC-circuit composed of R3/C4.”

      But the low-pass filter is R3/C3. Isn’t it?

      Betontalpa2

    3. betontalpfa2 says: on March 9, 2013 at 7:09 pm

      What is the unit of C3 and C5 condensator’s capacity?

      • C3 & C5 are in uF

        The LM739 is really an ancient op amp–haven’t seen that one in years–there are many better ones available that do not require compensation (R5 & C5).

    4. Is there a good way to put a dc component in the microphone line so I can detect an open line?

    5. I’d rather not guess, what is the unit for C1, uF?

    6. So I’ve wired it up with a voltage divider between the supply and ground with its output tied to the junction between the mic and C1. And the other mic lead passes through a similar resistor to go to ground. It will apply a 1V bias to that side of C1 while the other side of C1 maintains around 6.25V (9V supply). My oscilloscope picks up this bias when in DC mode. It looks to be a success.

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