Introduction

When it comes to high-fidelity audio systems, the quality of the amplifier plays a crucial role in ensuring clean, distortion-free sound reproduction. For audio enthusiasts and hobbyists, building your own amplifier not only offers a chance to achieve high performance but also provides a rewarding experience in understanding the fundamentals of audio electronics.

In this DIY project, we will design and build a high-quality audio amplifier using the 2SK2655 MOSFET. The 2SK2655 is a N-channel MOSFET designed for high-speed switching and amplification applications. With its high transconductance and low distortion characteristics, it’s an ideal component for audio amplification. This project is focused on creating a Class AB audio amplifier, which is well-suited for driving speakers in home audio setups, providing a balance between efficiency and sound quality.

We will walk through the step-by-step process of designing the circuit, assembling the components, and testing the amplifier with real-world audio signals.

Materials Needed

Here is the list of components required for the project:

1. 2SK2655 N-channel MOSFET (2 units)

● These MOSFETs will be used as the main amplifying elements in the amplifier circuit.

2. IRFP240 N-channel MOSFET (2 units)

● These are complementary MOSFETs used for the output stage of the amplifier to provide the necessary power and drive capability.

3. Resistors (various values)

● 1kΩ, 100Ω, 10kΩ, and 330Ω resistors for biasing and feedback.

4. Capacitors (various values)

● 100μF, 10μF, 1μF, and 0.1μF electrolytic and ceramic capacitors for coupling, bypassing, and smoothing.

5. Thermal Compound and Heatsinks

1. MOSFETs need to be kept cool, especially under load. Heatsinks are necessary for thermal dissipation.

6. Power Supply: ±30V DC

● A dual power supply (±30V) will be used to drive the amplifier’s output stage.

7. Speakers (8Ω, 50W or similar)

● The speakers are the load that the amplifier will drive.

8. Miscellaneous: Potentiometer (10kΩ for volume control), PCB, Sockets, Wires, and Soldering Iron

● Basic tools for assembling and testing the circuit.

Circuit Design and Assembly

Step 1: Understanding the 2SK2655 MOSFET

Before we start building the circuit, it's important to understand the role of the 2SK2655 MOSFET. The 2SK2655 is a high-power, N-channel MOSFET, designed for use in audio applications due to its high transconductance and low distortion. It features a high-current handling capability and can easily switch large amounts of power with low distortion, making it an ideal choice for driving speakers in audio amplifiers.

The MOSFET has the following important characteristics:

● Vds (Drain-to-Source Voltage): 150V

● Id (Drain Current): 18A

● Rds(on) (Drain-Source On-Resistance): 0.15Ω (low resistance for efficient switching)

In the audio amplifier, the 2SK2655 will primarily serve as a part of the output stage, amplifying the signal to a level capable of driving the speakers.

Step 2: Design Overview of the Class AB Audio Amplifier

We will build a Class AB amplifier because it combines the efficiency of Class B with the low distortion characteristics of Class A. In Class AB, the output transistors (in this case, the 2SK2655) are biased to conduct for more than half the signal cycle, but less than the entire cycle, reducing distortion and improving power efficiency compared to pure Class A.

The core components of the circuit are:

● Input stage: This includes capacitors for signal coupling and resistors for biasing.

● Differential amplifier: This ensures that the audio signal is properly amplified and matched for the output stage.

● Output stage: This is where the 2SK2655 MOSFETs come into play, driving the load (speakers) efficiently.

● Feedback loop: A feedback network is used to reduce distortion and stabilize the overall gain.

The overall design is split into two major parts: the driver stage and the output stage.

Step 3: Building the Input and Driver Stage

1. Signal Coupling: The audio input signal, typically from a preamplifier or audio source, is first fed into a capacitor (100nF) to block any DC offset. This ensures that only the AC audio signal reaches the amplifier circuit.

2. Differential Amplifier: The input signal is then passed through a differential amplifier stage. This stage is made up of resistors and transistors that amplify the input signal and prepare it for the output stage. The feedback loop around the differential amplifier helps maintain the gain and stability of the amplifier.

3. Biasing Network: The MOSFETs require a biasing network to set the correct operating point for the Class AB operation. This is achieved by using biasing resistors to set the gate-source voltage of the 2SK2655 MOSFETs. In this case, the 10kΩ resistor is used to bias the gate of each MOSFET, ensuring that they remain in their active region when the signal is applied.

4. Capacitor Filtering: Capacitors are placed at various points in the circuit to smooth out any noise or power fluctuations. A 10μF electrolytic capacitor is used for bypassing in the driver stage.

Step 4: Output Stage with 2SK2655 MOSFETs

1. Connecting the MOSFETs: The 2SK2655 MOSFETs will form the core of the output stage. These MOSFETs are responsible for amplifying the signal to a level that can drive the speakers. Each MOSFET’s drain will connect to one terminal of the speaker, and the source will connect to ground.

2. Complementary Pairing: In a typical Class AB amplifier design, complementary MOSFETs are used in the output stage. In this case, the IRFP240 MOSFETs will serve as the complementary pair to the 2SK2655. The IRFP240 is also an N-channel MOSFET, but the two types work together to provide push-pull amplification, where one MOSFET conducts during the positive half of the audio waveform, and the other conducts during the negative half.

3. Thermal Management: Since the 2SK2655 and the IRFP240 are power devices, they will generate heat during operation. To ensure the MOSFETs remain within safe operating temperatures, attach heatsinks to each MOSFET. This helps dissipate heat and prevent thermal runaway.

4. Flyback Diode Protection (Optional): If the load is inductive (such as a speaker with a large coil), a flyback diode can be added across the load to prevent any voltage spikes when the MOSFETs turn off.

Step 5: Feedback and Stability

The feedback loop is a critical part of the amplifier design. A small fraction of the output is fed back to the input via a network of resistors and capacitors. This feedback helps control the gain and reduce distortion, resulting in a cleaner, more accurate output signal.

A typical feedback network will consist of a 1kΩ resistor and a 1μF capacitor, which ensures that the frequency response is flat and that there is no unwanted oscillation or instability in the amplifier.

Step 6: Power Supply

For a high-performance audio amplifier, a dual power supply is typically used. A ±30V DC power supply will provide the necessary voltage to drive the MOSFETs into their active region. The power supply should be able to provide at least 2-3A of current, depending on the power rating of the load.

Step 7: Assembling the Circuit

1. PCB Layout: Once the circuit design is finalized, create a PCB layout for the amplifier. Make sure to route power traces thick enough to handle the high current, and provide ample space for the heatsinks.

2. Component Placement: Carefully place all components on the PCB according to the circuit diagram. Pay special attention to the MOSFETs, capacitors, and resistors in the input and driver stages. Ensure that the power components are securely mounted and connected.

3. Soldering: After placing the components on the PCB, solder each connection carefully. Double-check for any cold joints or shorts before proceeding.

Step 8: Testing the Amplifier

1. Initial Power-Up: Power up the amplifier with no load connected and measure the voltage at various points, including the output terminal. Ensure that there are no excessive voltages and that the amplifier is operating as expected.

2. Load Test: Once the initial power-up is successful, connect a speaker to the output and play a low-volume audio signal. Gradually increase the volume and listen for distortion. Adjust the biasing network if necessary to ensure clean output.

3. Thermal Monitoring: During the test, monitor the temperature of the MOSFETs to ensure they do not overheat. If they do, consider adding additional cooling or improving the thermal management.

Conclusion

Building a high-quality audio amplifier using the 2SK2655 MOSFET is an excellent project for audio enthusiasts and hobbyists looking to improve their knowledge of audio electronics. The combination of the 2SK2655 and the IRFP240 MOSFETs in a Class AB configuration allows for efficient and low-distortion amplification of audio signals. By following the steps outlined in this project, you’ll end up with an amplifier capable of delivering clear, high-fidelity sound to your speakers.

The 2SK2655’s characteristics make it a perfect choice for high-quality audio applications, and by carefully assembling the circuit, managing heat, and tuning the biasing network, you’ll be able to achieve impressive results in your DIY audio projects.