When building audio circuits, whether for a home stereo system or a DIY speaker project, one of the most essential components is the audio amplifier. The STK022, a power amplifier IC from Sanyo (now ON Semiconductor), offers an excellent solution for such projects. Known for its high output power, efficiency, and ease of use, the STK022 is an ideal choice for DIY audio enthusiasts looking to build a high-quality audio amplifier circuit.

In this article, we will walk through the design and construction of a DIY audio amplifier using the STK022. We will discuss its features, how to properly use it in a circuit, and tips for building a robust amplifier system.

Understanding the STK022

Before diving into the specifics of the project, let’s first explore what makes the STK022 a good fit for audio amplification.

The STK022 is a stereo power amplifier designed primarily for audio applications. It features a dual power amplifier configuration (one for each channel) and can handle a wide range of speaker impedances and output power levels. It operates with a supply voltage of ±18V to ±40V, making it suitable for both high and low-power applications. Its typical output power is around 40W per channel at ±30V, though this can vary depending on the exact operating conditions.

Some key features of the STK022:

1) High Output Power: Typically 40W per channel, which is sufficient for most home audio systems.

2) Low Total Harmonic Distortion (THD): This ensures the output audio remains clear and clean, which is crucial for high-fidelity sound.

3) Short Circuit and Thermal Protection: It includes built-in protections, which makes it more reliable and robust.

4) Easy to Use: The IC has built-in circuit design elements, so building an amplifier around it is relatively straightforward.

The combination of these features makes the STK022 a popular choice for DIY audio amplifier projects.

Circuit Design Overview

The audio amplifier circuit using the STK022 can be broken down into a few key sections:

1) Power Supply: Provides the necessary DC voltage to power the amplifier IC.

2) Input Stage: This handles the incoming audio signal and conditions it for amplification.

3) Amplification Stage: Here, the STK022 amplifies the audio signal.

4) Output Stage: This drives the speakers, ensuring that the amplified audio signal is strong enough to produce sound.

5) Protection Circuitry: To prevent damage to the amplifier and connected speakers, short circuit and thermal protection are incorporated.

Let’s now take a deeper dive into the details of each section.

Power Supply Design

The power supply is the heart of any audio amplifier. It provides the necessary voltage for the amplifier to function correctly. For the STK022, the recommended supply voltage is ±30V DC (or 60V total), which is within its operating range. A dual-rail supply (+V and -V) is required for the STK022 to work.

Components Needed for the Power Supply:

1) Transformer: A 30V-0V-30V center-tapped transformer is ideal for the dual-rail power supply.

2) Bridge Rectifier: To convert AC voltage from the transformer to DC voltage.

3) Filter Capacitors: Large electrolytic capacitors (typically 4700µF or higher) are needed to smooth the DC output from the rectifier.

4) Voltage Regulator (optional): If more stable voltage is required, a voltage regulator can be used, though this is often unnecessary for a basic amplifier.

The transformer steps down the voltage from AC mains to the required levels. The rectifier and capacitors then convert this AC into smooth DC voltage, ready to power the amplifier.

Input Stage

The input stage serves as the point where you connect your audio source (e.g., a phone, CD player, or computer). The audio signal from your source is usually weak and needs to be conditioned before it can be amplified effectively.

For the input stage, you’ll need the following components:

1) Capacitor: A coupling capacitor (typically 1µF to 10µF) blocks any DC offset from the input signal and ensures that only the AC audio signal is passed into the amplifier.

2) Resistor: A resistor (typically 10kΩ) is used for biasing the signal and setting the input impedance.

3) Potentiometer: A volume control potentiometer can be used to adjust the level of the input signal.

Once conditioned, the audio signal is fed into the amplifier’s input pin (Pin 1 of the STK022). The signal will be amplified in the next stage.

Amplification Stage (Using the STK022)

The STK022 is a dual power amplifier IC, meaning it amplifies two channels of audio simultaneously. It features both the input and output stages integrated into one chip, making it a relatively simple solution for building a stereo amplifier.

Pinout and Connections:

The STK022 has several pins, but here are the most important ones for the amplifier:

1) Pin 1 (Input): The audio signal is input here.

2) Pin 2 (Ground): The ground connection for the IC.

3) Pin 4 and Pin 5 (Power Supply): These are the Vcc and Vee pins, which connect to the positive and negative rails of the power supply.

4) Pin 6 (Output): The amplified audio signal is output from here to the speaker.

5) Pin 8 (Speaker Protection): If your design includes speaker protection circuitry, this pin can be used for this purpose.

In the amplification stage, the STK022 will take the conditioned input signal and amplify it. This is where the IC really shines, as it offers significant power amplification with minimal distortion.

Output Stage and Speaker Connection

The final stage of the amplifier is the output stage, which involves connecting the output of the STK022 to the speakers. The amplifier will drive the speakers with the amplified audio signal, producing sound.

Components for the Output Stage:

1) Speaker: The output power of the amplifier will drive the speaker. Typically, you’ll use 8-ohm speakers for this application, though the circuit can also support 4-ohm speakers if needed.

2) Capacitor: A coupling capacitor (typically 100µF or higher) is often used between the output pin of the STK022 and the speaker to block any DC component from reaching the speaker.

You’ll need to ensure that the power supply can provide enough current to drive the speakers at full output power. For a typical 40W per channel amplifier, your power supply should be able to provide around 3A of current per channel, or 6A in total.

Protection Circuitry

While the STK022 has built-in short circuit and thermal protection, adding external protection circuitry can further safeguard your components, especially the speakers.

Key Protection Components:

1) Fuses: A fuse on each rail of the power supply can protect the circuit from short circuits or excessive current.

2) Thermal Shutdown: The STK022 already includes thermal protection, but external heat sinks may still be required if you plan to run the amplifier at high output levels.

A heat sink attached to the STK022 will help dissipate heat generated during operation, preventing thermal shutdown.

Putting It All Together

Once all the sections are designed and components gathered, the next step is to assemble the circuit. Here are the steps for building your DIY audio amplifier:

1. Assemble the Power Supply: Begin by building the power supply with the transformer, bridge rectifier, and filter capacitors.

2. Connect the Input Stage: Add the capacitor, resistor, and potentiometer to the input stage, and then connect it to the input pin of the STK022.

3. Wiring the Amplifier: Wire the STK022's pins to the power supply, input stage, and output stage according to the datasheet and the pinout.

4. Speaker Connection: Connect the output of the amplifier to the speakers through a coupling capacitor.

5. Testing and Adjustment: Power on the circuit and test with a known audio source. Adjust the volume using the potentiometer and check for distortion or overheating.

Conclusion

Building a DIY audio amplifier with the STK022 is a rewarding project that can yield excellent results. The STK022’s high power output, low distortion, and integrated protection features make it an ideal choice for audio enthusiasts. With the right components, you can create a reliable and high-quality amplifier that will power your audio systems with impressive clarity and power. Whether you're creating a new stereo amplifier or refurbishing an old one, this project will deepen your understanding of audio electronics while giving you a functional and impressive piece of technology.