Building an Audio Preamplifier Circuit

Many times, we need to control the bass, treble, and volume of an audio signal before it passes through the amplification stage to prevent distortion of the sound. The circuit that amplifies the audio signal before it goes to the main speaker amplifier is called an audio preamplifier. Using an audio preamplifier ensures good audio quality and provides the option to modify our sound system by using it as the main audio circuit/device and then feeding the audio signal to the amplifier/subwoofer/home theatre system. Moreover, we can control the bass and treble for different songs and have extensive control over our audio system. In this article, we will build a stereo preamplifier circuit with bass and treble control.

Components Required for Preamplifier Circuit

Our stereo preamplifier will have dual channels. The volume, bass, and treble of each channel can be independently controlled using potentiometers; therefore, it may look like a lot of components on a breadboard, but they are all simple components that should be easy to obtain. The list of materials required for the audio preamplifier circuit is as follows.

Transistor-based dual-channel stereo BT circuit diagram

The complete circuit diagram of the dual channel preamplifier consists of two mono circuits combined into a stereo circuit as shown in the figure below. As you can see, the left channel audio and right channel audio are fed through two parts of the circuit, and I have used 3 mono 47k potentiometers to control the volume, bass and treble. The audio source from the 3.5mm jack is given as input through the 15k resistor of the (bass) potentiometer and is connected to ground through a 1k resistor on the other pin of the potentiometer for low frequencies. For treble (high frequencies), the sound signal passes through a 222 PF (polyester capacitor) to the 47k potentiometer and is connected to ground for the volume potentiometer through a 103pf and 10 uF capacitor.

The main component of this circuit is the 2SC1815 transistor, which is a general-purpose NPN transistor commonly used in audio amplification and for audio driving preamplifiers. The 2SC1815 transistor is shown in the figure below.

Silicon epitaxial NPN transistors are manufactured by Toshiba and are typically packaged in a TO-92 package as shown below. The important technical specifications of the 2SC1815 NPN transistor are as follows.

Vceo = 50v

Collector current IC = 150mA

Absolute maximum ratings at Ta = 25°C,

Collector base voltage Vcbo 60V

Collector-emitter voltage Vceo 50 V

Emitter base voltage Vebo 5v

General purpose NPN transistor

DC current gain (hFE) 70 to 700

Continuous collector current (IC) is 0.15A

Switching frequency: 80MHz

Collector power consumption PC = 400mW

We use 2 transistors for each part of the circuit as a dual stage amplifier configuration, a 560k resistor from VCC and a 47k resistor from ground are used to make a voltage divider circuit, providing the collector of the first power/gain transistor and the audio signal through the 10uF capacitor of the volume potentiometer. The emitter is connected to a 2k variable resistor, a capacitor 47uF and a 1k resistor for frequency selection and audio clarification, and the base of the first transistor is connected to the collector of the second transistor for later amplification. Finally, the output comes from the emitter of the second transistor, through a 47uF capacitor and a 2.7k and 1k resistor to GND for noise filtering.

Building the Preamplifier Circuit on a Breadboard

Since the preamplifier circuit does not involve high currents, we can build the circuit on a breadboard. My breadboard connections are shown below. I have also labeled the parts for easier understanding.

You can simply follow the circuit diagram above to build your own circuit. The most important component in our circuit is the C1815 NPN transistor. The pinout of the transistor is shown below

Once the circuit is built, you can test it directly using an audio source. Remember that this is an audio preamplifier circuit, not the amplifier itself. Therefore, you must connect the output of the preamplifier to an audio amplifier and then to a speaker system. For the testing of this project, I am using the LA4440 Audio Amplifier Board that we built in a previous tutorial.