In this article, we will walk through a DIY electronics project where we build a high-power audio amplifier using the 2SB1390 transistor. The 2SB1390 is a high-power NPN transistor designed for audio applications, and it is known for its excellent power handling capabilities, making it an ideal component for building an audio amplifier. This project will guide you through the process of designing and constructing a simple, high-quality amplifier circuit that can drive high-impedance speakers and deliver clear sound at high volumes.

The goal of this project is to create a straightforward audio amplifier that can be used in home audio systems, DIY speaker projects, or any other application that requires moderate to high power amplification of an audio signal. While we will be focusing on the 2SB1390 as the primary transistor in this build, we will also incorporate a few other essential components to ensure the amplifier’s stability and performance.

Components Needed

1．2SB1390 Transistors (2 pieces)

2．Resistors (1kΩ, 100Ω, 10Ω, 100kΩ)

3．Capacitors (100nF, 1µF, 220µF)

4．Diodes (1N4007 for protection)

5．Heat Sinks (for the 2SB1390 transistors)

6．Speaker (8Ω, typically)

7．Power Supply (12V or 24V depending on desired output)

8．PCB or Breadboard for mounting the components

9．Jumper wires and connectors

10．Transistor Socket (optional, for easier replacement of transistors)

Project Overview

The 2SB1390 is a high-power transistor typically used in audio applications due to its excellent linearity and high current handling capacity. The goal of this project is to build a class AB audio amplifier, which uses the 2SB1390 as part of the output stage to drive a speaker with clear sound reproduction. This type of amplifier operates in a push-pull configuration, where two transistors (in this case, the 2SB1390 and its complementary PNP counterpart) alternate conducting, ensuring efficient power usage and reduced distortion compared to a class A design.

Design Considerations

Amplifier Class (Class AB)

We will use a Class AB amplifier design, which combines the efficiency of a Class B amplifier with the reduced distortion of a Class A amplifier. In Class AB amplifiers, both transistors conduct for more than half of the input signal cycle, leading to lower distortion than a pure Class B design while still being more power-efficient than a Class A design.

Biasing

Proper biasing of the output transistors is crucial to prevent crossover distortion (a common issue in Class AB amplifiers). To avoid this, we’ll use a diode-based biasing network that helps set the operating point of the transistors correctly. This ensures the transistors will operate in the correct region for linear amplification of the audio signal.

Feedback Loop

A feedback loop is used to stabilize the amplifier’s gain and reduce distortion. The feedback network will monitor the output and adjust the input signal slightly to maintain a consistent, high-quality output.

Speaker Protection

Since the amplifier is capable of delivering significant power, we’ll include a capacitor and diode for speaker protection, which will help avoid damage from sudden voltage spikes, especially during power-on or power-off events.

Circuit Design

Transistor Selection and Push-Pull Configuration

The 2SB1390 will be used as part of a push-pull output stage, with a complementary NPN transistor (such as the 2SD529) to drive the load (the speaker). The transistors are placed in a configuration where one transistor handles the positive half of the signal, and the other handles the negative half. This reduces the overall distortion and improves efficiency.

Input Stage and Pre-Amplification

The input stage consists of a simple voltage divider network to set the input signal levels correctly. If your input signal comes from a source like a smartphone or a microcontroller, you may also need a capacitor in series to block DC components of the signal and only pass the AC (audio) signal through to the amplifier. This ensures that the DC bias of the amplifier is not disturbed.

Biasing Network

The biasing network will involve a pair of diodes (such as 1N4007) in the emitter of the transistors to provide thermal stability. The diodes will help adjust the voltage across the emitter resistors, ensuring that the transistors are always slightly conducting, even when no input signal is present, thus avoiding crossover distortion.

Feedback Loop

A feedback loop will be used to stabilize the gain of the amplifier. This consists of a resistor from the output to the non-inverting input of the transistor stage. The feedback reduces the overall gain slightly but ensures a more linear and stable output.

Output Stage

The output stage consists of the 2SB1390 transistors, which will provide the necessary current to drive a speaker. The power supply voltage can range from 12V to 24V, depending on the desired output power. For higher voltage supply, you’ll get more power and louder sound, but this will also require better heat dissipation, as the transistors will be generating more heat at higher voltages.

Speaker Protection

To protect the speaker from power surges, a 100nF capacitor is placed in series with the output to filter out any DC components. Additionally, a 1N4007 diode is placed in parallel with the speaker to absorb any back EMF generated by the inductive load (the speaker).

Assembly Instructions

Step 1: Prepare the Components

Gather all the components listed above. If you're using a breadboard, arrange the components according to the schematic for easier testing. For a more permanent setup, you can design a PCB for better component layout and soldering.

Step 2: Set Up the Input Stage

1. Begin by connecting the input signal to the base of the first transistor (NPN). You’ll need to use a capacitor (e.g., 1µF) to couple the input signal to the base of the transistor.

2. Attach a resistor between the base and the ground to provide a bias current to the transistor.

Step 3: Bias the Output Transistors

1. Connect the 2SB1390 NPN transistors in a push-pull configuration with their complementary PNP transistor (e.g., 2SD529).

2. Use a diode-based biasing network to set the emitter current and avoid crossover distortion. This will ensure that the output transistors are slightly conducting when no signal is present.

3. Place a resistor between the base and the emitter of each transistor to stabilize the operation.

Step 4: Connect the Output Stage

1. Connect the collector of the NPN transistor to the positive rail, while the emitter will be connected to the speaker.

2. Connect the other terminal of the speaker to the negative rail (ground).

3. Use a 100nF capacitor in series with the speaker to prevent DC from reaching the speaker and to ensure the audio signal is passed through cleanly.

Step 5: Add the Feedback Loop

1. Add a feedback resistor from the output (after the capacitor) to the input of the amplifier, ensuring stable gain.

2. You can also place a 100Ω resistor in series with the feedback path to adjust the feedback loop gain.

Step 6: Power the Circuit and Test

1. Connect the power supply (12V or 24V) to the appropriate rails of the circuit.

2. Power on the amplifier and feed an audio signal to the input.

3. Adjust the volume and observe the output on the connected speaker. Ensure the amplifier is driving the speaker with clean sound without distortion or excessive heat generation.

Step 7: Heat Management

The 2SB1390 transistors will likely get hot during operation, especially when driving higher-power loads. Attach heat sinks to the transistors to dissipate heat effectively and prevent thermal damage. Ensure there is proper ventilation around the amplifier for heat dissipation.

Troubleshooting Tips

1．Distortion or crackling sounds: This could be due to improper biasing of the output transistors. Double-check the biasing network and make sure the diodes are correctly installed.

2．No output or very low volume: Check the transistor connections, especially the base-emitter voltage and biasing. Ensure that the power supply is providing adequate voltage to drive the transistors.

3．Excessive heating of transistors: Ensure that the heat sinks are properly installed and that the transistors are not operating too close to their maximum power rating. If necessary, reduce the supply voltage or use higher-current-rated transistors.

Conclusion

This DIY audio amplifier project using the 2SB1390 transistor provides a simple yet effective way to build a high-power audio amplifier suitable for driving speakers in home audio systems. By leveraging the high current capabilities of the 2SB1390 and using a Class AB configuration, we can achieve high efficiency and clear, distortion-free sound. With the correct biasing, feedback, and heat management, this amplifier can deliver excellent performance for your audio projects.

The 2SB1390 is an excellent choice for audio amplification due to its power handling capabilities, and this project demonstrates how to use it in a practical and efficient manner. Whether you're building a personal speaker system or designing an amplifier for a larger audio setup, this design provides a solid foundation for high-quality audio amplification.