Simple MIC Preamplifier Circuit using an Op-Amp

This article shows you 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.

The circuit also has a knob to adjust how much the signal is amplified so you can use it with different microphones or applications.

What is a MIC Preamplifier Circuit:

A Microphone MIC Preamplifier Circuit is an electronic circuit designed to amplify the weak electrical signals produced by a microphone.

Microphones generally produce low level signals, and a preamplifier is used to boost these signals to a level that can be further processed or transmitted by other audio equipment.

The preamplifier helps improve the signal to noise ratio and ensures that the audio signal is strong enough for subsequent stages in an audio system.

Circuit Description:

Parts List:

Below mentioned are the circuit descriptions:

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

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

820 ohm connected in series with the condenser microphone determining input impedance.

82k limits current for the LED indicator.

100k at pin 1 provides feedback for controlled gain.

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

Indicates the presence of DC power supply, offering a visual confirmation of operational status.

Allows adjustment of the input signal to the operational amplifier offering customization of the circuits sensitivity.

Provides an interface for connecting the pre amplified audio signal to external audio devices.

Powers the circuit within the specified range of 6V to 12V DC.

Formulas:

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

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

where,

• Gain(Av): This represents the voltage gain of the amplifier.
• Vout / Vin: This is the ratio of the output voltage Vout to the input voltage Vin.
• Rf: corresponds to 100k between pin 1 and 2
• Rin: corresponds to 820 ohm.

Construction Details:

• Connect the DC power supply to the input, ensuring correct polarity.
• The 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 connector J1.

Testing:

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

Simple MIC Preamplifier Circuit using a Single Transistor

Parts List:

In the above circuit transistor BC547 is configured in a common emitter collector to base bias arrangement.

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 can be powered within the range of 1.5V to 6V.

Conclusion:

The LM358 Condenser Microphone Preamplifier circuit offers a versatile solution for amplifying low level audio signals, providing a platform for customization and optimization in various audio applications.

References

Analysis and Design of a Microphone Preamplifier

Microphone preamplifier