Simple Condenser Microphone Preamplifier Circuit Based on 2N3904

Today, let's try to make a microphone by using a condenser microphone preamplifier circuit.

Due to its high sound sensitivity, convenient use, small size and cheap price. For example, it can be directly connected to your power amplifier through the AUX channel. Then you can hear your voice clearly from the speaker.

What is an electret condenser microphone?

It is a microphone based on electrostatic capacitor. Its audio pickup part has a capacitor structure, and the capacitor consists of a diaphragm and a back plate opposite to it, which is called a condenser microphone. The movement of sound on the diaphragm is picked up as a change in capacitance between the diaphragm and the back plate.

In this case, a voltage of tens of volts or hundreds of volts should generally be applied externally as the capacitor polarization voltage.

However, the electret effect can hold the charge in the polymer film, thus eliminating the polarized DC high voltage. This is the electret condenser microphone.

Almost all circuits involving receiving audio often use this type of microphone, such as mobile phones, intercoms, voice control switches, etc., because it is very sensitive, small, easy to use and cheap, with good sound quality. Suitable for medium and high frequency sounds.

Check out the structural schematic diagram of the electret condenser microphone. The diaphragm and back plate are used to receive external audio signals, and there is a FET inside to amplify the signal, which has very high sensitivity. Therefore, it can respond to sound frequencies in almost the entire audio frequency range.

Let's look at a typical basic electret microphone preamplifier circuit. The two-terminal electret capsule contains FETs, which are a common source configuration. It must
be powered externally by the supply voltage V+. The R1 resistor is used to set the gain and output impedance. The audio signal appears at the output, after the DC blocking C1 capacitor.

So if we connect it with wrong polarity and there is no supply voltage. It will not work at all.

How it works

Please see the circuit diagram below. It has very few components.

First, we connect the 9V battery to the circuit. Secondly, the resistor R1 biases the microphone (MIC1) by current. Also, R1 is the current limiting resistor of MIC1. It is ready to work. When we
send sound to MIC1, it causes the electrical signal to change.

Then, the sound signal flows through the coupling capacitor - C1. It will block the DC current from passing. But the signal is very weak. Therefore, it needs the help of transistor Q1 to increase the signal.

Next, the signal goes to B of Q1 to allow a larger signal to pass from C to the output.

Afterwards, coupling capacitor C2 passes the AC signal to the output, which is the tone control preamplifier.

Additionally, you can connect the output with headphones to convert the electrical signal into sound.

Many resistors have the following functions:

R1 is the current limiting resistor of MIC1.

Resistor R2 is the bias feedback signal from the output to the input through B and C connecting transistor Q1. It provides stability for good operation.

R3 and R4 both serve to maintain the bias voltage level appropriately.

Learn: How to use the LM386 audio amplifier circuit

Component List

Q1: 2N3904, 40V 0.2A NPN transistor

Electrolytic capacitor C1,

C2: 10μF 25V 0.25W resistor, tolerance: 5%

R1：27K R2：100K R3：10K R4：1K

B1: 9 volt battery with snap connector or 9V

Power circuit MIC1: Electret condenser microphone

Perforated printed circuit

plate

How to Apply It

Let's try assembling the above circuit on a breadboard.

If you are a beginner, here is the connection of condenser microphone preamp with amplifier system. Look below, you should have a tone control preamp. Because the output signal of this circuit is very weak.

noise

Nobody likes noise. I experimented with 3 different power supplies with this circuit.

9 volt battery works best! I didn't hear any noise.

9V DC regulated power supply. Little noise! ​​You can hardly hear it if you don't notice.

5V mobile phone charger/USB port. It is a switch mode power supply. When it works, it generates very high frequency pulses with excessive harmonics. This is one of the noises and is sure to get into the preamplifier.

Assembling the Capacitor Preamplifier

This circuit has several components so we can assemble them on a general purpose PCB or perfboard. See below for the component layout.

Build the circuit step by step

Here's how to build this circuit:

Cut a small piece of perfboard from a piece of paper. This PCB can be easily split with the pressure of a thumbnail.

Place the components and carefully insert them into the holes on the board, counting the holes to make sure everything is in the right place.

Flip the board over, bend the wires of the components to secure them to the board and create the connections as shown.

Use a soldering iron to connect the joints.

Trim the wires with wire cutters.

Use a magnifying glass to inspect each joint. If there isn't enough solder, reheat and add more. If the solder connection isn't there, use a knife to separate the two points.

NOTE: This circuit only consumes about 0.75mA, so if we use a 9V 200mAh NiMH battery, it will last for about 266 hours (about a year).

This circuit uses transistors which are easy to build and cheap. If you want better performance, choose a high quality op-amp IC.