Simple MIC Preamplifier Circuit using an Op-Amp

This article explain how to build a microphone amplifier circuit using a chip called the IC LM358.

Regular microphones have very weak signals and this amplifier makes them much stronger.

This circuit is better than simpler designs because it uses a special chip called an operational amplifier op amp.

This makes the signal stronger without adding any scratches or background noise.

To use with various microphones or applications the circuit includes a knob to change the level of signal booster.

What is a MIC Preamplifier Circuit:

A microphone MIC preamplifier circuit is an electronic circuit designed to improve the weak electrical signals which are generated by a microphone.

The low level signals produced by a microphones are enhanced to control and communicate with other audio equipment.

In an audio system the preamplifier helps raise the signal to the noise level and confirms that the audio signal is loud enough for the next steps.

Circuit Working:

Parts List:

The heart of this simple electret condenser MIC preamplifier is IC LM358 which operates as an inverting amplifier to amplify the input signal from the condenser microphone.

In input source the condenser microphone produces low level signals that require amplification for further processing.

Resistor 820 ohm and capacitor 1µF is connected in series with the condenser microphone determining input impedance.

82k resistor limits the current.

100k resistor provides feedback to control the gain.

C1 is the coupling capacitor that allows the AC signal from the condenser microphone to pass through while blocking DC.

Pin 1 provides an interface for connecting the pre amplified audio signal to external audio devices.

The circuit is powered within the specified range between 6V to 12V DC.

Formulas:

Below are the formulas for Simple MIC Preamplifier Circuit using an Op-Amp:

The output gain (Av) of the circuit is calculated using the formula:

Gain_(Av) = (V_out / V_in) = -(R_f / R_in)

where,

• Gain(Av) represents the voltage gain of the amplifier.
• Vout / Vin is the ratio of the output voltage Vout to the input voltage Vin.
• Rf represents to 100k between pin 2 and 3
• Rin represents to 820 ohm.

How to Build:

To build a Simple MIC Preamplifier Circuit using an Op-Amp follow the below steps for connections:

• Connect the DC power supply to the input ensuring correct polarity.
• If required LED connected in series with 82k serves as a power indicator.
• Connect the output of the condenser microphone to the inverting pin of the LM358 through C1 and 820 ohm.
• The LM358 is configured as an inverting amplifier.
• The input signal is applied to the inverting negative terminal and feedback from the output is fed back to the same terminal through 100k.
• If a variable resistor is used connect it in place of 820 ohm to control the input signal to the operational amplifier.
• The pre amplified audio signal from pin 1 of the LM358 is connected to the audio output

Testing:

• Apply power to the circuit and observe the LED indicator if used.
• Verify the output using an audio recording device by adjusting the variable resistor if included for best performance.

MIC Preamplifier Circuit using a Single Transistor Working:

Parts List:

A common emitter and collector to base bias setup is used for transistor BC547 in the circuit diagram above.

Capacitors C1 and C2 block the DC bias and permit only the audio signal to pass through.

As a result the preamplifier circuit produces audio output that is free from noise and distortion.

This single transistor MIC preamplifier circuit is powered within the range of 1.5V to 6V.

Conclusion:

The LM358 Condenser Microphone Preamplifier circuit provides a flexible way to boost low level audio signals allowing for modification and efficiency in a range of audio applications.

References

Analysis and Design of a Microphone Preamplifier

Microphone preamplifier