In the world of DIY audio electronics, building a high-quality, low-cost audio amplifier is a common and rewarding project. For this particular build, we will create a simple yet efficient audio amplifier circuit using the UPA68H and UPA68HA transistors. These are complementary NPN-PNP power transistors, designed for audio applications, particularly in the amplification of low-level audio signals.
This project will guide you through the step-by-step process of designing and building a basic audio amplifier circuit that can be used in various audio systems, from small home audio systems to personal portable speakers.
Project Overview
In this project, we will build a Class AB audio amplifier using the UPA68H (NPN) and UPA68HA (PNP) transistors, which are perfect for audio amplification. The amplifier will have enough power to drive small speakers while maintaining audio clarity and minimizing distortion. This simple design will feature basic components like capacitors, resistors, and transistors, which are essential for controlling gain, frequency response, and overall circuit stability.
The audio amplifier will be capable of driving 8-ohm speakers with a moderate output power, making it suitable for use in small audio devices. This project is designed to be simple and does not require complex circuitry or advanced design skills, making it an excellent entry-level project for DIY enthusiasts interested in audio electronics.
Components Required
Before we start assembling the amplifier, let’s list the components you will need for the circuit:
UPA68H (NPN Transistor): This transistor will serve as the main amplification component.
UPA68HA (PNP Transistor): This complementary transistor works alongside the UPA68H to provide the necessary power for the speaker.
Resistors: These will be used to set the biasing and gain levels of the transistors.
Capacitors: Used for frequency response control, noise filtering, and coupling the signal.
Potentiometer: A variable resistor for adjusting the volume level.
Diodes: For protecting the transistors from voltage spikes and ensuring the stability of the circuit.
Speakers: 8-ohm speakers are ideal for this amplifier.
Power Supply: A 12V DC supply to power the amplifier.
PCB (Printed Circuit Board) or Breadboard: For assembling the circuit components.
Heat Sink: Necessary for cooling the power transistors during operation.
Soldering Tools: Soldering iron, solder, wire cutters, etc., for assembling the circuit.
Step-by-Step Assembly of the Audio Amplifier
1. Preparing the Power Supply and Initial Connections
First, start by connecting the power supply. We will be using a 12V DC power adapter, which is ideal for driving this amplifier circuit. The UPA68H and UPA68HA transistors require a stable 12V input for optimal performance.
● Power Supply Wiring: Connect the positive terminal of the power supply to the collector pin of the UPA68H transistor. The negative terminal of the power supply will connect to the ground of the circuit.
● Transistor Placement: Mount the UPA68H (NPN) and UPA68HA (PNP) transistors on the PCB or breadboard. Ensure that you follow the pinout configurations of these transistors:
— UPA68H: Collector (C), Base (B), and Emitter (E).
— UPA68HA: Collector (C), Base (B), and Emitter (E).
Ensure proper placement of each component to avoid short circuits or incorrect connections.
2. Biasing and Gain Control
To ensure that the amplifier works efficiently, we need to bias the transistors and control the gain. Biasing involves setting the operating point of the transistors to ensure that they work in the desired linear region, where they amplify the audio signal without distortion.
● Base Resistor (R1 and R2): Place resistors between the base of the transistors and the power supply rails. These resistors set the bias voltage for the transistors and control the overall gain of the amplifier. You will typically use values in the range of 10kΩ to 100kΩ for these resistors.
● Emitter Resistors (R3 and R4): Emitter resistors are used to stabilize the operation of the transistors by providing negative feedback. These resistors also control the gain of the circuit and improve thermal stability. Use resistors with values ranging from 100Ω to 1kΩ, depending on your desired gain level.
● Coupling Capacitors (C1 and C2): Capacitors are used to block DC components and only allow AC signals (audio signals) to pass through. These capacitors are placed between the input signal and the base of the UPA68H transistor. Use capacitors with values between 10µF and 100µF for effective signal coupling.
3. Frequency Response Control
The frequency response of an amplifier determines how well it can handle different frequencies, especially in the audio range. To control the frequency response and reduce unwanted noise, we will use a combination of capacitors and resistors.
● Decoupling Capacitors: Place capacitors between the power supply rails and ground to filter high-frequency noise. These capacitors smooth out any fluctuations in the power supply, ensuring that the transistors receive a clean, stable voltage. A 100nF ceramic capacitor is typically used for this purpose.
● Bass and Treble Control: You can add a potentiometer between the input signal and the base of the UPA68H transistor to allow users to adjust the volume or gain. You can also experiment with adding additional components to shape the frequency response and adjust the bass and treble output of the amplifier.
4. Output Stage and Speaker Connection
Once the signal is amplified, it needs to be sent to the speaker for output. The output stage of the amplifier circuit connects the emitter of the UPA68H transistor to the speaker.
● Speaker Connection: Connect the output of the amplifier (from the emitter of the UPA68H) to the positive terminal of the speaker. The negative terminal of the speaker will be connected to the ground of the circuit.
● Protection Diodes: To protect the amplifier from voltage spikes that might occur when switching on or off, place a diode across the speaker terminals. The diode will protect the circuit from reverse voltages and prevent damage to the transistors.
● Heat Sink: Power transistors can get quite hot during operation, especially when driving speakers. Attach a heat sink to the UPA68H and UPA68HA transistors to dissipate heat and prevent thermal damage.
5. Testing and Fine-Tuning
Now that the circuit is assembled, it’s time to test it.
● Power On: Apply power to the amplifier by connecting the 12V DC power supply. Check for any obvious issues such as overheating components or short circuits.
● Input Signal: Connect an audio input signal (e.g., from a smartphone or audio source) to the input terminals of the amplifier. Adjust the volume potentiometer and ensure that the audio signal is being amplified and outputted to the connected speaker.
● Sound Quality Check: Listen for any distortion or noise in the output. If you notice any, adjust the biasing resistors or fine-tune the coupling capacitors to improve the frequency response. Ensure that the output is clear and free from unwanted noise or distortion.
6. Final Assembly
Once you are satisfied with the performance of the amplifier, finalize the assembly:
● Enclosure: Place the entire amplifier circuit into a protective enclosure. This will not only protect the circuit from damage but also provide a clean and professional appearance. Make sure the speaker terminals are accessible from the outside, and the potentiometer is easy to adjust.
● Mounting: Secure the components inside the enclosure. Ensure the transistors have proper ventilation and that the heat sink is attached securely.
Conclusion
By following these steps, you will have successfully built a simple, efficient audio amplifier using the UPA68H and UPA68HA power transistors. This amplifier can be used for small audio systems, portable speaker setups, or as an educational tool to learn more about audio amplification.
While the design is relatively simple, there are several opportunities to enhance the circuit, such as adding tone control, improving power output, or refining the frequency response. Regardless of your experience level, this project provides an excellent foundation for exploring audio electronics and learning the principles behind transistor-based amplification.
Through this project, you’ve not only built a functional audio amplifier but also gained valuable insights into audio circuits, component selection, and the importance of proper component placement for optimal performance.