Simple Condenser Mic Preamplifier Circuit using 2N3904

Today let’s try to make a microphone. By using a condenser microphone preamplifier circuit.

Why would we need said circuit?

Because of its high sound sensitivity, easy to use, small, and cheap. For example, It can be connected directly to your power amplifier through the AUX channel. Then, you could hear your voice clearly from the speakers.

I hope you will apply this circuit to a wide variety of applications.

What is Electret Condenser Microphone

It is a type of electrostatic capacitor-based microphone. whose audio pickup section has a structure of a condenser consisting of a diaphragm and a back-plate opposite thereto, is called a condenser microphone.

The motion of the diaphragm by sound is picked up as a variation of capacitance between the diaphragm and the back plate.

In this case, usually, a voltage of tens or hundreds of volts should be applied externally as a condenser polarizing voltage.

However, electric charge can be maintained in a polymer film by the electret effect, thereby eliminating the polarizing direct-current high voltage. Such is an electret condenser microphone.

Almost all circuits involved in receiving audio this kind of microphone is often used for example mobile phones, intercoms, voice control switches, etc. because it is very sensitive, small size, easy to use and cheap, good sound quality. Suitable for medium and high-frequency sound.

Look at a structural schematic of the Electret Condenser Microphone. There are diaphragm and back-plate to receive external audio signal and FET inside to amplify the signal and have a very high sensitivity. therefore responding to sound frequencies almost throughout the sound frequency range.

Look at a typical basic electret microphone preamp circuit. The two-terminal electret capsule contains FET a common source configuration. It must be externally powered by supply voltage V+.

The R1-resistor sets the gain and output impedance. The audio signal appears at the output, after a DC-blocking C1-capacitor.

Therefore, If we connect it to the wrong polarity and no power supply voltage. It does not work at all.

Electret microphone on Wikipedia

How it works

Look at the circuit diagram below. It has very few components.

First of all, we connect a 9V battery to the circuit. Secondly, a resistor R1 passes an electrical current to bias the Microphone (MIC1).

Also, R1 is a current limiting resistor for MIC1. It is ready to work. When we make a sound to MIC1, it causes the electrical signals to change.

Then, the sound signal flows through a coupling capacitor-C1. It will block DC current not passing. But the signal is very weak. Therefore, it needs help with a transistor Q1 to increase the signal.

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

After that, the coupling capacitor C2 passes the AC signal to the output, a tone control preamplifier.

Also, you can connect the output with a headset to convert an electrical signal into sound.

The function of many resistors

• The R1 is a current limiting resistor for MIC1.
• Resistor R2 is a biased feedback signal from output to input by connecting between  B and  C of the transistor Q1. To provide stability for the good work.
• Both R3 and R4  act as to keep a bias voltage level to be suitable.

Learn: How to use LM386 audio amplifier circuit

Component list

Q1: 2N3904, 40V 0.2A NPN transistor
Electrolytic Capacitors
C1, C2: 10µF 25V
0.25W Resistors, tolerance: 5%
R1: 27K
R2: 100K
R3: 10K
R4: 1K
B1: 9-volt batteries, with Snap connector Or 9V power supply circuit
MIC1: Electret condenser microphone
Perforated PC board

Learn: Designing small signal amplifier circuit using transistor simply

How to apply it

We try to assemble the above circuit on the breadboard.

If you are a beginner, here is Connecting a Condenser Microphone Preamp with an amplifier system. Look at below, You should have a tone control preamplifier. Because the output of this circuit has a weak signal.

Noise reduction

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

• 9-volts battery Best results! I didn’t hear any noise.
• 9V DC Regulated power supply. Little noise! If you don’t notice, you can hardly hear.
• 5V mobile charger/ USB port. It is a kind of switching mode power supply. As it operates, it generates a very high frequency pulse and having too many harmonics. which is one of the noises, definitely enter the preamplifier.

Assembling Condenser Preamplifier

This circuit has a few components therefore we can assemble them on the universal PC Board or perforated board. Look at below, the components layout.

Step-by-step to build the circuit

Here is the specific method for building this circuit:

• Cut the small piece of perfboard out of a sheet. This PCB can be easily divided with the press forces of your thumbnail.
• Put components and carefully insert them through holes in the board, counting the holes to make sure everything is in the right place.
• Flip the board over and bend the wires from the components to anchor them to the board and create connections as shown.
• Make the joints with soldering iron.

• Trim the wires approximately with your wire cutters.
• Check each joint using a magnifying glass. If there is not enough solder, reheat it and add more. If the solder connection should not be there, use a knife to make two points away.

Note: This circuit only consumes about 0.75mA, so if we use a 9V 200mAh Ni-MH battery, be able for long around 266 hours (about one year).

This circuit uses transistors that are easy to build and cheap. If you want better performance, go for a high-quality Op-amp IC instead.

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Apichet Garaipoom

I love electronics. I have been learning about them through creating simple electronic circuits or small projects. And now I am also having my children do the same. Nevertheless, I hope you found the experiences we shared on this site useful and fulfilling.