Introduction

In the world of DIY electronics, building an audio amplifier is a popular and rewarding project for enthusiasts looking to improve their skills while creating a practical device. In this article, we’ll show you how to design and build a high-power audio amplifier using the 2SB697K, a PNP power transistor. This transistor is well-suited for driving high-current audio signals, making it an ideal choice for building an amplifier that can drive speakers with substantial power.

The 2SB697K is capable of handling up to 80V and 15A of current, which gives it the power necessary to drive speakers with sufficient wattage for home audio systems, or even for small professional applications. We’ll walk through the steps of designing the amplifier, understanding the role of the 2SB697K, and then building and testing the circuit.

Understanding the 2SB697K Transistor

Before diving into the project, let's first take a look at the 2SB697K transistor itself. The 2SB697K is a PNP transistor designed primarily for high-power switching and amplification. It is often used in audio amplifier circuits, power supplies, and other applications where high current and voltage are required. Below are some of its key specifications:

● Type: PNP Power Transistor

● Collector-Emitter Voltage (Vce): 80V

● Collector Current (Ic): 15A (continuous)

● Power Dissipation (Ptot): 150W

● Gain (hFE): 30 to 150 (depending on the current)

● Package Type: TO-3, a large metal package that allows for better heat dissipation

The 2SB697K is commonly used in the output stage of audio amplifiers due to its high current capability. When used in conjunction with other transistors, it can form a push-pull output stage, providing efficient amplification for high-power audio systems.

How the 2SB697K Works in an Amplifier Circuit

In an audio amplifier, the 2SB697K typically works in a push-pull configuration, where two transistors (one PNP and one NPN) are used to amplify both the positive and negative halves of the input audio signal. The PNP transistor (2SB697K) conducts on the positive half of the waveform, while an NPN transistor will conduct on the negative half. Together, these transistors deliver a clean, full-range audio signal to the speakers.

The 2SB697K can handle high current, which is important for driving low-impedance speakers (typically 4Ω or 8Ω), especially in high-wattage applications.

Designing the Audio Amplifier

Now, let’s look at the basic design for a high-power audio amplifier using the 2SB697K. The design will consist of three main sections:

1. Pre-amplifier Stage: This stage boosts the input audio signal to a level that can be processed by the power amplifier.

2. Driver Stage: This stage controls the drive to the power transistors (including the 2SB697K) and ensures the proper voltage and current are provided for amplification.

3. Output Stage: This is where the 2SB697K and other power transistors amplify the signal enough to drive the speakers.

Key Components for the Project

● 2SB697K Transistors (2 units, one for each side of the push-pull configuration)

● 2N5401 or TIP31 (NPN Transistor for complementary pair)

● Resistors (various values for biasing and feedback)

● Capacitors (for coupling, filtering, and frequency response)

● Potentiometer (for volume control)

● Heat Sink (to manage power dissipation)

● Power Supply (for the amplifier, typically 30V-40V DC for high-power output)

● Speaker (typically 4Ω or 8Ω)

● PCB or Breadboard (for mounting the components)

Building the High-Power Audio Amplifier

Step 1: Design the Pre-Amplifier

The pre-amplifier stage is responsible for taking the low-level audio signal (from a source like a smartphone, computer, or audio player) and amplifying it to a level suitable for the power amplifier. For this, you can use a basic transistor amplifier or an op-amp circuit. The pre-amplifier should also include a volume control (using a potentiometer) to adjust the signal level before it is fed into the power amplifier.

The pre-amplifier should have the following features:

● High input impedance (to avoid loading the audio source)

● Low output impedance (to drive the next stage with minimal loss)

● Flat frequency response (to maintain the integrity of the audio signal)

You can design this stage with JFETs, bipolar junction transistors (BJTs), or an operational amplifier (op-amp) depending on the complexity you prefer.

Step 2: Driver Stage

The driver stage is where the 2SB697K transistor comes into play. The driver stage provides the necessary current and voltage to properly drive the power transistors.

For a push-pull configuration, you will typically use:

● A NPN transistor (such as the 2N5401 or TIP31) for the complementary pair.

● A biasing network (typically using resistors) to ensure that both transistors are properly turned on during their respective halves of the waveform.

● Driver capacitors to provide the necessary charge for switching the transistors quickly and efficiently.

You will also need to add diodes to protect the transistors from voltage spikes (especially during switching) and to stabilize the circuit under different load conditions.

Step 3: Output Stage (Using 2SB697K Transistors)

The output stage is where the real power amplification happens. In this stage, the 2SB697K PNP transistors are used in a push-pull configuration with complementary NPN transistors. This ensures that the full positive and negative cycles of the audio signal are amplified.

Biasing the 2SB697K Transistor

● Use resistors to create a biasing network that ensures proper quiescent current for the output transistors.

● The base resistors control the amount of current into the base of the 2SB697K and its complementary NPN transistor.

● Feedback resistors can be used to stabilize the circuit and ensure that the transistors work efficiently.

Heat Management

The 2SB697K can dissipate significant power under high loads, so it is important to use adequate heat sinks to prevent thermal damage. The TO-3 package is designed for good heat dissipation, but you will still need to attach a large heat sink to the back of the transistor to ensure it stays cool.

Step 4: Power Supply

The power supply for the amplifier should provide a regulated DC voltage that is high enough to support the desired output power. A 30V-40V DC supply will be appropriate for most medium- to high-power audio amplifiers.

You will need to use a high-current power supply capable of providing the necessary current (at least 5A-10A) for the amplifier to function properly under load. Make sure that the power supply has sufficient current headroom, as high-power amplifiers can draw significant current when driving speakers at high volumes.

Step 5: Assembly and Testing

Once the circuit is designed and all components are gathered, it’s time to assemble the amplifier on a breadboard or printed circuit board (PCB). Here are the steps to follow:

1. Place the pre-amplifier and driver stage components onto the board, ensuring that the input and output connections are correct.

2. Install the power transistors (2SB697K and NPN transistors) in the output stage, ensuring proper heat sinking.

3. Connect the power supply and ensure that the voltage is correct.

4. Install the speaker and connect it to the output terminals of the amplifier.

5. Test the amplifier with a low-level audio signal and gradually increase the volume to ensure proper operation.

Troubleshooting

● No output or low output: Check the biasing network and ensure that the power transistors are properly biased. Verify that the power supply is providing the correct voltage.

● Distortion or clipping: This could be due to improper biasing or insufficient power supply voltage. Ensure that the output transistors are operating within their limits.

● Excessive heating: Check the heat sinks and ensure that the transistors are properly cooled. If necessary, add larger heat sinks or use active cooling.

Conclusion

Building a high-power audio amplifier using the 2SB697K transistor is a great way to learn about power electronics and audio amplification. The 2SB697K PNP transistor is an ideal choice for this application due to its high current and voltage handling capabilities. By designing a push-pull amplifier circuit and carefully selecting supporting components, you can build a powerful amplifier that can drive speakers with clean, distortion-free audio.

With the knowledge gained from this project, you can explore more advanced audio amplification techniques, such as feedback networks, differential amplifiers, and even build multi-channel amplifiers for home theater systems.