Project: PCB AUX Bass Booster

Objective

To gain practical experience in custom PCB design by creating a hardware-based bass equalizer. The objective was to design, simulate, and fabricate a board that could intake a standard AUX audio signal, apply an adjustable bass boost, and output the modified signal, providing a tangible improvement to my bookshelf speakers.

Technical Approach & My Implementation

I executed this project from initial concept to a finished, functional product, handling the circuit design, simulation, PCB layout, and assembly.

Circuit Design: Active Filter with Op-Amps

The core of the project is an active high-pass filter circuit, but used in a non-intuitive way to boost bass.

Theory of Operation: A capacitor's impedance ($Z_C = \frac{1}{2\pi f C}$) decreases as frequency increases. I designed a circuit where this frequency-dependent impedance is placed in the feedback loop of an operational amplifier (op-amp). At high frequencies, the capacitor's low impedance reduces the op-amp's gain. At low frequencies (bass), the capacitor's higher impedance results in a higher gain, thus amplifying those frequencies selectively.

Adjustability: To make the bass level tunable, I replaced a fixed resistor in the feedback network with a variable resistor (potentiometer) in parallel with the capacitor. Adjusting the potentiometer changes the effective impedance at low frequencies, allowing for precise control over the amount of bass boost.

Simulation and Validation with LTSpice

Before committing to a PCB, I rigorously tested the circuit design.

I created a detailed schematic in LTSpice to model the circuit's behavior.

I performed an AC analysis simulation to plot the frequency response, confirming that the circuit indeed provided a significant gain boost at frequencies below ~200 Hz while leaving higher frequencies relatively unchanged. This simulation step was critical for selecting appropriate values for the capacitors and resistors to achieve the desired bass range.

PCB Design and Layout 

Translating the theoretical circuit into a manufacturable PCB was the primary focus of the project.

Schematic Capture: I began by recreating the validated circuit in KiCad, selecting specific components (e.g., TL072 dual op-amp for its low noise and common availability). 

PCB Layout: This involved the strategic placement of components and routing of traces to minimize noise and crosstalk, which is critical for audio fidelity.

Iteration: The initial design used terminal blocks for the AUX cables. However, I revised the layout in a second iteration to include solder pads for direct wire attachment, creating a more robust and compact final product.

Assembly and Testing

After receiving the custom-fabricated PCBs, I assembled and tested the unit.

I hand-soldered all components, including the potentiometers and the headers for the op-amp ICs.

For integration, I salvaged AUX cables, soldering the ground, left, and right channels directly to the designated pads on the PCB for a permanent and reliable connection.

Results 

The project successfully met all its objectives, resulting in a functional and useful piece of audio equipment.

• Functional Success: The final PCB operated exactly as intended. The two potentiometers provided independent adjustment of the bass boost for the left and right channels. The device was fully compatible with any standard AUX input/output.

• Subjective Outcome: After tuning the bass levels to my preference, the modified speakers produced a richer, more powerful low-end, significantly enhancing their perceived audio quality.

Practical Lessons Learned:

• Design for Assembly: The shift from terminal blocks to direct soldering was an important lesson in Design for Manufacturability/Assembly (DFM/DFA), teaching me that the easiest design to model isn't always the most reliable in practice.

• Simulation is Crucial: Using LTSpice prevented costly errors and gave me high confidence in the design before fabrication.

• Audio Layout Matters: I gained firsthand experience in the importance of PCB layout for analog signals, understanding how proper grounding and routing are essential for achieving clean audio output.

Conclusion: This project provided invaluable, hands-on experience in the complete electronics design workflow. It solidified my understanding of analog filter design, demonstrated my ability to use industry-standard tools (LTSpice, EasyEDA), and proved I can take a concept from a theoretical sketch to a polished, functional PCB.