This two-stage audio amplifier takes a line-level audio signal from a mobile phone and drives an 8 Ω speaker. The design was taken from initial hand calculations through Proteus SPICE simulation, breadboard prototyping on both dual and single supply and a final custom PCB.
Documentation • FAQ • Gallery • Full Report • Journal • Contributing • Citation
The amplifier is two cascaded stages:
- Stage 1 (TL071 active band-pass filter): provides frequency selectivity across the human hearing range (5 Hz to 28.54 kHz) and voltage gain, bringing the 0.87 Vpp phone output up to 3 Vpp.
- Stage 2 (OPA551 unity-gain buffer): replicates the Stage 1 output voltage at high current, driving the 8 Ω speaker without adding further gain.
| Parameter | Value |
|---|---|
| Input source | iPhone 14 Pro Max |
| Input voltage | 0.872 Vpp at 440 Hz |
| Output voltage | 3 Vpp at 440 Hz |
| Speaker load | 8 Ω |
| Lower cutoff frequency | 5 Hz |
| Upper cutoff frequency | 28.54 kHz |
| Stage 1 IC | TL071CP |
| Stage 2 IC | OPA551PA |
| Feedback resistor | 82 kΩ |
| Supply configuration | Single supply |
PCB measurements at 440 Hz:
| Stage | Input | Output | Notes |
|---|---|---|---|
| Stage 1 (TL071 active filter) | 0.868 Vpp | 3.000 Vpp | Meets 3 Vpp target |
| Stage 2 (OPA551 buffer) | 0.872 Vpp | 2.980 Vpp | Unity gain confirmed |
Frequency response was verified across the full audio band on both breadboard and PCB. Full results with simulation comparison are in DOCUMENTATION.md.
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| Proteus | Git | GitHub | VS Code | Excel | Word |
| Tool | Model | Purpose |
|---|---|---|
| Oscilloscope | Tektronix TBS1052C | Waveform capture and peak-to-peak voltage measurement |
| Function generator | Hameg HM8030 | Generating sine wave test signals for frequency response measurements |
| Audio source | iPhone 14 Pro Max | 440 Hz sine wave input for amplifier characterisation and testing |
| DC power supply | Dual-output bench PSU | Providing split supply (±15 V) for breadboard op-amp testing |
| Breadboard | 830-point | Dual supply and single supply circuit prototyping |
| Multimeter | - | Verifying virtual ground bias voltage before fitting ICs |
| Soldering iron | - | PCB component assembly and rework |
two-stage-audio-amplifier/
├── design/
│ └── proteus/
│ └── exports/ Schematic, PCB layout and simulation exports (PNG)
├── media/
│ ├── images/ Circuit figures, PCB photographs and oscilloscope traces
│ ├── block-diagrams/ System block diagrams and design flowcharts
│ ├── assets/ Icons used in this README
│ └── GALLERY.md Curated image gallery with descriptions
├── report/
│ ├── REPORT.md Full technical report in markdown
│ └── JOURNAL.md Project retrospective and build journal
├── DOCUMENTATION.md Complete technical reference
├── FAQ.md Frequently asked questions
├── CONTRIBUTING.md Commit and workflow standards
├── NOTICE.md Third-party media attribution
└── LICENSE CC BY-NC-ND 4.0
Note
This repository is registered with Zenodo and has a permanent, citable DOI: 10.5281/zenodo.21903757. See CITATION.cff for the full citation. The "Cite this repository" option on GitHub can also be used.
Tip
This project is maintained by Isaac "Zac" Adjei. Questions about it can be directed to any of the following:
- GitHub: zaccesss
- Website: isaacadjei.me
- Email: eng@isaacadjei.me
Project Status: Completed | Last Updated: August 2026