In this article, we will guide you through a DIY audio preamp project using the UPC1358H, a low-noise operational amplifier designed for high-fidelity audio applications. If you’re interested in audio electronics, building a preamp is a great way to learn how to amplify weak audio signals and prepare them for further processing, such as driving speakers or feeding into power amplifiers.

The UPC1358H is an ideal choice for this project because of its high-quality performance and low distortion, which are essential characteristics for achieving high-fidelity sound in audio systems. Whether you’re building a preamp for guitar, a microphone, or any other audio source, the UPC1358H can help you design a clean and precise signal path.

The goal of this project is to create a high-quality, low-noise audio preamp that can take an unamplified audio signal from a source, such as a guitar, microphone, or audio interface, and boost it to a level suitable for further amplification or processing. We'll focus on designing a mono preamp, but the design can be adapted for stereo applications if needed.

Components Needed

1．UPC1358H Operational Amplifier (op-amp)

2．Resistors: Various values (10kΩ, 100kΩ, 1MΩ)

3．Capacitors: 100nF, 10µF (electrolytic)

4．Potentiometer: 10kΩ (for volume control)

5．Power Supply: ±12V DC (or a regulated +12V and ground)

6．Input Capacitor: 1µF (for signal coupling)

7．Output Capacitor: 10µF (for signal coupling)

8．PCB or Breadboard for assembly

9．Enclosure (optional for housing the preamp)

10．Input jack (e.g., 3.5mm or RCA)

11．Output jack (e.g., 3.5mm or RCA)

12．Thermal paste (if required for heatsink)

13．Power connector (for connecting the power supply)

Why Use the UPC1358H?

The UPC1358H is a low-noise, high-performance operational amplifier designed for audio applications, specifically high-fidelity (Hi-Fi) audio systems. It features a low total harmonic distortion (THD) and high slew rate, making it ideal for amplifying audio signals with minimal coloration or interference.

Here are the main reasons to use the UPC1358H for this preamp project:

1. Low Noise: The UPC1358H is designed for low noise, making it perfect for audio applications where clarity and detail are important.

2. High Slew Rate: The op-amp has a high slew rate, meaning it can accurately track fast-changing signals, such as those found in audio recordings.

3. Wide Bandwidth: The UPC1358H can handle a wide range of frequencies, which is crucial for audio applications where you want to preserve the full range of sound from low bass to high treble.

4. Compact and Easy to Use: The UPC1358H comes in a convenient package (usually an 8-pin DIP), making it easy to incorporate into DIY projects.

Step 1: Designing the Circuit

The basic function of an audio preamp is to amplify weak audio signals without introducing too much noise, distortion, or coloration. We will start by designing the core of the circuit, which consists of the UPC1358H op-amp in a non-inverting configuration.

1.1 Non-Inverting Amplifier Configuration

The most straightforward configuration for an audio preamp is the non-inverting amplifier configuration. This means that the input signal will be amplified in-phase with the original signal (i.e., the signal will not be flipped upside down).

In this configuration:

1．The input signal is fed into the non-inverting input of the op-amp (pin 3).

2．A feedback resistor connects the output of the op-amp to the inverting input (pin 2).

3．The input capacitor (1µF) is placed in series with the input signal to block any DC offset and pass only the AC audio signal.

The feedback resistor, in conjunction with a ground resistor, determines the overall gain of the amplifier. In this case, we will choose resistor values that allow us to achieve a reasonable gain of around 10 to 20 times (20dB), which is typical for a clean, general-purpose audio preamp.

1.2 Signal Coupling Capacitors

To ensure that only the AC audio signal passes through the amplifier, we will use capacitors for signal coupling. The input capacitor (1µF) blocks any DC offset that may exist at the input (e.g., from the microphone or instrument). Similarly, the output capacitor (10µF) blocks any DC components from reaching the output stage, preventing DC voltage from being sent to the next stage (e.g., power amplifier or speakers).

1.3 Power Supply Requirements

The UPC1358H op-amp requires a dual power supply (positive and negative voltages) for optimal performance. For this project, we will use ±12V DC. You can use a regulated power supply that provides these voltages or create a split supply from a single 12V DC source using a voltage divider and appropriate filtering.

Step 2: Assembling the Circuit on a Breadboard

2.1 Preparing the Components

Gather the necessary components:

1．UPC1358H op-amp (8-pin DIP)

2．Resistors (10kΩ, 100kΩ, 1MΩ)

3．Capacitors (100nF, 10µF)

4．Potentiometer (10kΩ)

5．Input and output jacks

6．Power supply (+12V, -12V)

2.2 Building the Amplifier

Op-Amp Connections:

1. Connect pin 4 to ground (GND).

2. Connect pin 7 to +12V (positive supply voltage).

3. Pin 2 (inverting input) is connected to the feedback resistor (e.g., 100kΩ).

4. Pin 3 (non-inverting input) is where you’ll connect the input signal via the input capacitor (1µF).

Feedback Resistor:

1. Place a 100kΩ resistor between pin 2 (inverting input) and pin 6 (output) to set the gain of the amplifier.

Gain Control:

1. To adjust the volume, place a 10kΩ potentiometer in the feedback loop between pin 2 and pin 6, allowing you to fine-tune the gain as needed.

Output Capacitor:

1. Place a 10µF electrolytic capacitor between the output pin (pin 6) and the output jack to ensure only the AC signal is passed to the next stage.

Input Coupling Capacitor:

1. Place a 1µF capacitor between the input signal (e.g., from a microphone or guitar) and pin 3 of the op-amp. This will block any DC components from the input signal.

Power Supply Connections:

1. Connect the positive supply (+12V) to pin 7 of the op-amp.

2. Connect the negative supply (-12V) to pin 4 of the op-amp.

3. Ensure that all ground connections are properly tied to the power supply ground.

2.3 Testing the Preamp

Once the circuit is assembled, use a multimeter to check for proper connections and ensure that there are no shorts or incorrect wiring. After double-checking the connections, connect the input signal (such as a guitar or microphone) to the input jack and the output to a speaker or further amplifier.

Step 3: Fine-Tuning and Enclosure

Once you have verified that the preamp is functioning correctly, you can make adjustments to optimize its performance:

Adjusting the Gain:

1. The potentiometer allows you to adjust the gain, which controls the volume of the output signal. Start with the potentiometer at a low setting and gradually increase the gain to avoid distortion.

Adding a Power Supply:

1. If you are using a separate power supply, ensure that it provides a stable and regulated ±12V output.

Enclosing the Preamp:

1. For safety and durability, it’s a good idea to place the circuit in an enclosure. This protects the circuit from damage and provides a professional look. Make sure the enclosure has sufficient ventilation to prevent overheating.

Step 4: Calibration and Testing

Finally, test the preamp under various conditions, using different audio sources such as a guitar, microphone, or even a line-level audio source. Use a volume knob or potentiometer to adjust the gain and listen for any distortion or clipping. The circuit should produce a clean, amplified signal with minimal noise or distortion.

Conclusion

By following this guide, you’ve built a high-fidelity audio preamp using the UPC1358H op-amp. This preamp can be used in various audio applications, including musical instruments (e.g., guitar or bass), microphones, or as part of a larger audio system. The low-noise characteristics of the UPC1358H ensure that the signal remains clean and accurate, making it suitable for high-fidelity sound reproduction.

This project not only provides you with an understanding of how to amplify audio signals, but it also introduces you to the principles of op-amp circuits, signal coupling, and low-noise design. With this knowledge, you can explore more complex audio systems and experiment with other op-amps and circuit configurations to enhance your DIY audio projects further.