Introduction

In the world of DIY audio projects, one of the most rewarding and practical designs is creating your own audio amplifier. Whether you’re looking to drive a high-fidelity (hi-fi) speaker, build a guitar amplifier, or create a headphone amplifier, building an amplifier circuit can be both educational and fulfilling. One of the most critical factors for audio amplification is ensuring low distortion and low noise, particularly in high-performance audio equipment where even the smallest imperfections can be noticeable.

The UPA2706, a high-performance operational amplifier (op-amp) manufactured by ON Semiconductor, is a perfect candidate for this kind of DIY audio amplifier project. This op-amp is designed specifically for audio applications and offers features that minimize noise, ensure low distortion, and provide high slew rate and gain bandwidth. It’s ideal for creating an audio preamplifier or a low-noise amplifier that can drive speakers or other audio equipment with superior performance.

In this article, we’ll guide you through the design and assembly of a low-noise audio amplifier using the UPA2706, exploring the key features of the component, the supporting components required, and the design process, all while considering optimal performance and sound quality.

What is the UPA2706?

The UPA2706 is a high-speed, low-noise op-amp designed for audio and signal processing applications. It belongs to the Precision Amplifier family and is often used in circuits that require high signal fidelity, such as audio preamps, active filters, and low-noise gain stages.

Key specifications of the UPA2706:

● Supply Voltage (Vcc): ±4V to ±18V (dual supply)

● Slew Rate: 10V/µs (minimum), which ensures fast signal response without distortion at high frequencies.

● Gain Bandwidth Product: 12 MHz (typical), ensuring stable performance even at high frequencies.

● Input Noise Density: 4nV/√Hz at 1kHz, which makes it one of the best choices for low-noise audio applications.

● Common-Mode Rejection Ratio (CMRR): 100dB (typical), which is essential for rejecting unwanted noise from the power supply and environment.

● Low Total Harmonic Distortion (THD): Less than 0.0003%, which ensures ultra-low distortion at high output levels.

The UPA2706 is housed in a 5-pin small outline package (SOP-5) and is well-suited for audio signal processing and preamplifier circuits where the focus is on minimizing distortion and noise while providing adequate gain.

Components Needed for the Project

To build a low-noise audio amplifier using the UPA2706, you will need several components to implement the amplification circuit. Below is a list of the essential components needed to create a basic preamplifier or low-power audio amplifier.

Core Components:

1. UPA2706 – The main low-noise op-amp that provides the amplification.

2. Resistors – For setting the gain, biasing, and stability of the circuit.

● Feedback resistors to control the overall gain of the amplifier.

● Input resistors to limit current and protect the op-amp.

3. Capacitors – For coupling, decoupling, and filtering.

● Input Capacitor (e.g., 10µF) – To block DC offset and allow only the audio signal to pass.

● Decoupling Capacitors (e.g., 100nF) – To filter out any noise from the power supply and stabilize the op-amp.

● Output Capacitor (e.g., 100µF) – To block DC from the output and prevent damage to speakers.

4. Power Supply – A dual power supply providing ±12V to ±18V to power the op-amp.

5. Volume Potentiometer – To control the input signal level.

6. Input Connector – To connect the audio signal source (e.g., RCA, 3.5mm jack).

7. Output Connector – To connect the amplifier to the speaker or other audio equipment.

8. PCB or Breadboard – To mount the components and make the necessary connections.

9. Thermal Management – For heat dissipation, although the UPA2706 has relatively low power consumption, appropriate capacitors and resistors should be used to ensure thermal stability.

Optional Components:

1. Bypass Capacitors – For further noise reduction and stability, you can use small-value capacitors (e.g., 0.1µF or 0.01µF) between the power supply pins and ground.

2. Current-Limiting Resistor – To prevent excessive current draw in case of a short circuit at the output.

3. LED Indicator – To show when the amplifier is powered on and working.

Circuit Design and Schematic

The primary design goal for this amplifier is to achieve a low-noise and high-gain audio amplifier suitable for driving speakers or headphones. We will design a non-inverting amplifier using the UPA2706, which will provide high input impedance and low output impedance, making it perfect for audio amplification.

1. Power Supply and Voltage Rails

The UPA2706 requires a dual power supply for proper operation. A typical voltage range is ±12V to ±18V, which is standard for audio op-amps. The op-amp is powered by connecting the positive supply (e.g., +15V) to the Vcc+ pin and the negative supply (e.g., -15V) to the Vcc- pin of the op-amp.

To ensure low noise and clean power for the op-amp, you can use a regulated power supply or low-noise DC-DC converter.

2. Input Stage

The input stage involves the audio signal entering the amplifier. The input capacitor (e.g., 10µF) blocks any DC offset from the signal source and allows only the AC audio signal to pass into the op-amp.

You can use an RCA jack or 3.5mm jack as the input connector. The input signal is then passed through a series resistor (e.g., 1kΩ) before reaching the non-inverting input of the UPA2706. The value of this resistor can be adjusted to control the input impedance.

3. Amplification and Feedback

For the amplification stage, we will configure the UPA2706 in a non-inverting amplifier configuration. This setup will provide a high input impedance (ideal for audio sources like instruments or microphones) and low distortion.

The gain of the amplifier is determined by the feedback network, which consists of a feedback resistor (Rf) between the output and the non-inverting input, and a ground resistor (Rg) connected between the non-inverting input and ground. The gain GGG of the amplifier is given by:

For example, to achieve a gain of 10, you could use a 1kΩ Rg and a 9kΩ Rf resistor.

4. Output Stage

The output of the UPA2706 is connected to an output capacitor (e.g., 100µF) to block any DC offset before driving the speaker. The value of the output capacitor is chosen based on the desired low-frequency response. You may also include a resistor in parallel with the output capacitor to ensure proper frequency response and load stability.

The output can be connected to a speaker or an external amplifier depending on the application.

5. Decoupling and Stability

To reduce noise and ensure stable operation, it’s important to use decoupling capacitors. Place 100nF ceramic capacitors between the Vcc+ and Vcc- pins of the op-amp and ground. These capacitors filter out any high-frequency noise from the power supply and prevent oscillations.

Additionally, place small-value capacitors (e.g., 0.1µF) near the op-amp’s power pins for local filtering.

6. Optional Volume Control

To adjust the volume of the amplifier, you can place a potentiometer (e.g., 10kΩ log potentiometer) in series with the input signal. This allows you to control the amplitude of the input signal before amplification.

Here’s a simplified schematic of the low-noise audio amplifier:

Assembly Instructions

Once you have gathered the components and the schematic, follow these steps to assemble the audio amplifier:

1. Prepare the PCB or Breadboard:

● Lay out the components on your breadboard or printed circuit board (PCB). Ensure that you leave enough space for the UPA2706, resistors, capacitors, and power supply connections.

● Make sure that the input and output connections are clearly labeled and easily accessible.

2. Place and Solder Components:

● Start with the smaller components like resistors and capacitors, followed by the UPA2706 op-amp.

● Carefully solder each component, ensuring that all connections are secure.

3. Connect the Power Supply:

● Attach the ±12V to ±18V power supply to the Vcc+ and Vcc- pins of the UPA2706.

● Ensure that the power connections are correct to avoid damaging the op-amp.

4. Attach Input and Output Connectors:

● Connect your audio input signal to the input connector and the output capacitor to the speaker or output device.

5. Testing and Troubleshooting:

● Power on the amplifier and apply a low-level audio signal.

● Measure the output signal with an oscilloscope to ensure that it is amplified cleanly without distortion.

● Adjust the feedback resistors if necessary to change the gain of the amplifier.

● Ensure that the output voltage is within the safe operating range for the connected speaker.

Conclusion

Building a low-noise audio amplifier using the UPA2706 is an excellent way to learn about audio signal processing and op-amp circuits. This op-amp’s high slew rate, gain bandwidth, and low noise characteristics make it ideal for high-fidelity audio applications, where clarity and distortion-free sound are paramount.

By following the steps outlined in this article, you’ll be able to create a highly effective audio preamp or low-power audio amplifier suitable for a variety of audio sources. The knowledge gained from this project can be applied to more advanced audio designs, such as power amplifiers or active filters, as you continue to explore the world of DIY audio electronics.