In the world of DIY electronics, creating an audio preamp can be a rewarding project that yields immediate results. A preamplifier (preamp) serves as the first stage of amplification in an audio system, increasing the signal from low-level sources like microphones, electric guitars, or turntables, to a level suitable for further amplification. For audiophiles and electronics enthusiasts, designing and building a preamp circuit using high-performance components can lead to both educational insights and satisfying, professional-quality results.

In this project, we’ll explore how to build a simple yet effective audio preamp circuit using the OPA340UA/2K5, a low-power, low-noise operational amplifier (op-amp) with a rail-to-rail input and output. The OPA340UA is known for its excellent audio characteristics, making it ideal for sensitive audio applications.

We’ll break down the design, the selection of components, and step through the process of building and testing the preamp. This will provide a thorough, practical guide for those interested in learning about analog audio amplification without getting too bogged down in complex mathematics or design theory.

The OPA340UA/2K5 Op-Amp: Why Choose It?

Before diving into the circuit design, let’s briefly discuss the OPA340UA/2K5 op-amp’s features and why it’s a good choice for our audio preamp project:

1. Low Noise: The OPA340UA has a low noise density (typically 3.5 nV/√Hz at 1 kHz), which is crucial for preserving the integrity of the audio signal without adding unwanted distortion.

2. Rail-to-Rail Input and Output: This feature allows the op-amp to operate close to both supply rails, maximizing the dynamic range of the circuit, which is especially important when dealing with low-voltage audio signals.

3. Wide Supply Voltage Range: The OPA340UA operates on a single supply voltage from 2.7 V to 40 V, offering flexibility in power supply design.

4. Low Power Consumption: With a typical supply current of 0.3 mA, the op-amp is highly efficient, making it ideal for battery-powered applications or low-energy designs.

Project Overview: Building the Audio Preamp Circuit

The goal of this project is to design and build a simple but effective audio preamp circuit based on the OPA340UA/2K5. The circuit will amplify a low-level audio signal (e.g., from a guitar, microphone, or audio output from a media device) to a level that can be fed into a power amplifier or audio system.

Design Specifications

1. Single-Supply Operation: We will use a 9V battery or a 9V DC power adapter to power the circuit.

2. Input Sensitivity: The preamp will be designed to handle low-level audio signals ranging from a few millivolts (mV) to a few hundred millivolts (mV).

3. Output Level: The preamp will provide an output signal that can drive a power amplifier or active speaker, with a clean signal gain of around 10–20 times (20–26 dB).

4. Low Distortion: As we’re aiming for a high-fidelity design, we will prioritize low noise and distortion in the design of the circuit.

Circuit Design: A Simple Inverting Amplifier

For this project, we’ll design an inverting amplifier circuit, where the audio signal will enter through the input terminal, and the amplified output will be taken from the output terminal. An inverting amplifier is chosen because it provides simple gain control, and it’s easy to implement using the OPA340UA op-amp.​

Where:

1. RfR_fRf​ is the feedback resistor

2. RinR_{in}Rin​ is the input resistor

Component List

Here’s a list of the components you’ll need for this project:

1. OPA340UA/2K5 Operational Amplifier (1 piece)

2. Resistors:

1．1 x 10 kΩ (for RinR_{in}Rin​)

2．1 x 100 kΩ (for RfR_fRf​)

3．2 x 1 kΩ (for biasing and offset adjustment)

3. Capacitors:

1．2 x 10 µF electrolytic capacitors (for power supply decoupling and output coupling)

2．1 x 0.1 µF ceramic capacitor (for power supply decoupling)

4. Power Supply:

1．1 x 9V battery or 9V DC adapter

5. Miscellaneous:

1．Breadboard or PCB for assembly

2．Connecting wires

3．Audio input and output jacks (3.5mm or RCA, depending on your application)

4．Sockets for op-amp (optional)

Step-by-Step Construction of the Audio Preamp

Step 1: Circuit Schematic Design

Let’s start by designing the schematic of the inverting amplifier:

1. Power Supply Connections: Connect the positive terminal of the 9V battery (or adapter) to the V+ pin (pin 7) of the OPA340UA, and the negative terminal to the ground (GND).

2. Input Circuit:

1．Connect the input audio signal (from a source such as a microphone or guitar) to the non-grounded terminal of the input resistor RinR_{in}Rin​ (10 kΩ).

2．The other end of RinR_{in}Rin​ connects to the inverting input (pin 2) of the op-amp.

3. Feedback Loop:

1．Connect a feedback resistor RfR_fRf​ (100 kΩ) between the output (pin 6) and the inverting input (pin 2) of the op-amp.

4. Output Circuit:

1．Connect the output of the op-amp (pin 6) to the output terminal of the circuit through a coupling capacitor (10 µF) to block any DC offset.

2．The output capacitor ensures that only the amplified AC audio signal is sent to the next stage in your audio system.

5. Power Supply Decoupling:

1．Place a 10 µF electrolytic capacitor between V+ and GND near the op-amp to stabilize the supply voltage and reduce any power supply noise.

2．Add a 0.1 µF ceramic capacitor in parallel to the 10 µF capacitor for high-frequency noise filtering.

6. Biasing and Offset Adjustment:

1．To ensure proper operation of the op-amp, connect a 1 kΩ resistor from the non-inverting input (pin 3) to the ground. This will set the reference voltage for the input.

2．Optionally, you can adjust the biasing using a potentiometer or another resistor to tweak the input offset voltage for a more accurate signal.

Step 2: Breadboard Setup

1. Place the OPA340UA: Insert the op-amp into the breadboard, ensuring correct orientation of the pins.

2. Wiring: Connect the power supply to the op-amp, and wire up the components according to the schematic. Use jumper wires for the audio input and output connections.

3. Capacitors and Resistors: Place the capacitors and resistors in their respective positions, and ensure that all connections are secure.

Step 3: Testing the Circuit

Once the circuit is assembled, it’s time to test the preamp.

1. Connect the Input: Plug in an audio source, such as an electric guitar or a line-level audio device, into the input terminal.

2. Connect the Output: Connect the output of the preamp to a power amplifier or active speaker.

3. Power Up: Insert the 9V battery or connect the power supply. You should hear the amplified audio output from the speaker or power amplifier.

4. Adjust Gain: If necessary, tweak the feedback resistor RfR_fRf​ and input resistor RinR_{in}Rin​ to adjust the gain to your desired level.

Conclusion

Building an audio preamp with the OPA340UA is an excellent DIY electronics project for both beginners and experienced enthusiasts. The OPA340UA offers low-noise performance and high fidelity, making it ideal for audio applications. By following this project, you not only learn about op-amps and audio amplification but also gain valuable experience in designing and constructing analog circuits.

This simple inverting amplifier design provides a solid foundation for more complex audio circuits, such as tone controls, headphone amplifiers, or even full-fledged audio preamp systems with multiple stages. Whether you’re building the preamp for a specific audio application or just for the satisfaction of making something from scratch, this project is an excellent way to dive into the world of analog audio electronics.