Many times we need to control the bass, treble and volume of the audio signal before passing through the amplifier stage to prevent sound distortion. The circuit that amplifies the audio signal before it enters the main speaker amplifier is called an audio preamplifier. Using an audio preamp 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 theater system. In addition, we can control the bass and treble of different songs and have extensive control over our audio system. In this article, we will build a stereo preamp circuit with bass and treble controls.

Components Required for Preamplifier Circuit

Our stereo preamp will have dual channels. Volume, bass and treble for each channel can be controlled independently using potentiometers; so it may look like a lot of components on a breadboard, but they are all simple components and should be readily available. 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 a two-channel preamp consists of two mono circuits combined into a stereo circuit, as shown in the figure below. As you can see the left and right channel audio is fed through two parts of the circuit, I used 3 mono 47k potentiometers to control volume, bass and treble. The audio source from the 3.5mm jack is provided as input through the 15k resistor of the (bass) potentiometer and is grounded through the 1k resistor on the other pin of the potentiometer for low frequencies. For treble (high frequencies), the sound signal goes through a 222 PF (polyester cap) to a 47k pot, and a 103pf and 10 uF cap for the volume pot to ground.

The main component of this circuit is the 2SC1815 transistor, which is a general-purpose NPN transistor that is often used in audio amplifiers to drive pre-amplifiers. 2SC1815 transistor as shown below

Silicon epitaxial NPN transistors are manufactured by Toshiba and are typically packaged in TO-92 as shown below. 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

Conversion frequency: 80MHz

Collector power consumption PC=400mW

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

Building a Preamplifier Circuit on a Breadboard

Since the preamp circuit does not involve large currents, we can build the circuit on a breadboard. My breadboard connections are shown below. I've also labeled these sections for ease of 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 with an audio source. Remember, this is an audio preamp circuit, not the amplifier itself. So you have to connect the output of the preamp to the audio amplifier and then to the speaker system. For the testing of this project, I used the LA4440 audio amplifier board we built in the previous tutorial.

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