HeadphoneAmp

🎸 Guitar Headphone Amplifier

A compact, single-supply headphone amplifier for electric guitar, built around a paralleled NE5532 output stage

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📌 Overview

The Guitar Headphone Amplifier is a custom PCB designed to drive low-impedance headphones directly from an electric guitar signal — no computer audio interface required. The board operates from a single 9V supply using a virtual ground at VCC/2, and features a paralleled op-amp output stage capable of cleanly driving 32Ω loads without a discrete transistor buffer stage.

Hardware design is complete. The board was designed, simulated, and laid out in full at Cornell University in 2022.

⚡ Specifications

🏗️ Signal Chain

┌──────────────┐     ┌──────────────────┐     ┌──────────────────────────┐     ┌──────────────┐     ┌──────────────┐
│              │     │                  │     │                          │     │              │     │              │
│  Guitar In   │────▶│  Input Buffer    │────▶│  ×4 Parallel Output      │────▶│  Volume Pot  │────▶│  Headphone   │
│  (J3)        │     │  U1A — NE5532    │     │  Stage  U2–U3 — NE5532   │     │  RV1 100kΩ   │     │  Out (J1)    │
│              │     ├──────────────────┤     │                          │     │              │     │              │
│              │     │  Active Filter   │     │                          │     │              │     │              │
│              │     │  U1B — NE5532    │     │                          │     │              │     │              │
└──────────────┘     └──────────────────┘     └──────────────────────────┘     └──────────────┘     └──────────────┘

A virtual ground at VCC/2 enables single-supply operation. Coupling capacitors block DC from the headphone output.

🔧 Design Details

Op-Amp Selection — NE5532 over LM358

The initial design used an LM358. Transient simulation in LTSpice exposed a critical flaw: crossover distortion appearing at frequencies as low as 5–10 kHz. This notching artifact near the zero-crossing is clearly audible and disqualifies the LM358 from audio signal paths.

The pin-compatible NE5532 was substituted and resolved the issue entirely:

• Industry-standard audio op-amp with input voltage noise of 5 nV/√Hz
• Significantly higher slew rate than the LM358
• Simulation confirmed clean, undistorted output across the full 20 Hz – 20 kHz audio band

Paralleled Output Stage

Driving 32Ω headphones requires more output current than a single op-amp section can supply without distortion. Rather than adding a discrete BJT output stage, four NE5532 sections are wired in parallel:

• Multiplies available output current by 4×
• Averages individual op-amp noise contributions
• Eliminates bias complexity associated with class-AB transistor stages

PCB Layout

The 2-layer board was laid out in KiCAD with the following considerations:

• Copper pour ground plane on the bottom layer for noise rejection
• Decoupling capacitors placed directly at each op-amp supply pin
• Ferrite bead (FB1) and protection diode (D2) at the audio output
• All panel connectors (barrel jack, potentiometer, audio jacks) placed at board edges for clean enclosure mounting

🗺️ Roadmap

• Schematic capture and component selection
• LTSpice simulation — LM358 vs NE5532 comparison
• PCB layout and design rule check
• Bench measurement of THD+N and SNR; comparison against simulation
• Baxandall tone control in place of simple volume potentiometer
• Laser-cut or 3D-printed enclosure

🛠️ Tech Stack

👤 Author

Luis Martinez

Built with KiCAD · LTSpice · NE5532