Guitar Headphone Amplifier Complete

Design Process

From Simulation to Board

This project started as a way to practice playing guitar late at night without bothering anyone. The requirement was simple: low noise, flat frequency response across the audible band, and enough drive current to push a pair of headphones to a comfortable volume.

Op Amp Selection — LM358 vs NE5532

The initial design used the common LM358 dual op amp. Running LTSpice transient and AC simulations quickly revealed a serious problem: the LM358 introduces noticeable crossover distortion at frequencies as low as 5–10 kHz — well within the audible range and far too early for an audio application. The simulated output waveform showed characteristic "notching" near the zero crossing at mid-to-high audio frequencies.

Paralleling Output Stages

Headphones present a relatively low impedance load (~32Ω typical). Rather than use a discrete output buffer transistor stage, four NE5532 op amp sections are wired in parallel at the output. This multiplies the available output current by four while averaging out individual op amp noise — a clean way to get more drive without adding complexity or BJT-related design headaches.

PCB Layout

The 2-layer board was laid out in KiCAD. Power and ground planes on the back copper help with noise rejection. Decoupling caps are placed directly at each op amp supply pin. A ferrite bead (FB1) and protection diode (D2) guard the audio output. The barrel jack, potentiometer, and both audio jacks are positioned at the board edges for easy panel mounting.

Future Work & Notes

What's Next

The project is functionally finished, but there are still things I would like to measure and evaluate or improve on such as:

• Measure THD+N and SNR on bench and compare to simulation
• Use baxandall volume control instead of just potentiometer
• Consider adding mcu for the option to do digital processing on the guitar signal
• Design a laser-cut enclosure or 3D-printed case