wdf-amp-2203

Real-time circuit model of the Marshall JCM800 2203 (100 W master-volume head) in C++20, using Wave Digital Filters (chowdsp_wdf). Every block is verified against an ngspice reference model of the full amp with a Python harness, and the engine is wrapped as a JUCE plugin (AU / VST3 / Standalone).

Status: milestone M0 (build skeleton and verification harness) is in progress; the engine is still a pass-through. Milestones M0–M5 are defined in docs/SPEC.md; current state and decisions are in docs/PROGRESS.md and docs/DECISIONS.md.

How it works

• Reference: reference/spice/jcm800_2203.cir is a full-amp ngspice netlist (preamp, tone stack, long-tail-pair phase inverter with negative feedback and presence, 4× EL34, output transformer, HT supply, bias) with Koren tube models. It is the ground truth.
• Engine: the circuit is split into blocks, each one WDF tree with at most one nonlinear (tube) root, solved with a bounded multiport Newton iteration. Blocks are joined by Thevenin junctions whose error is measured in SPICE before they are used. The engine is JUCE-free; only plugin/ uses JUCE.
• Verification: the harness generates block testbenches from the netlist, renders SPICE references at 768 kHz, runs the engine on the same signals through the ampwav CLI, and writes reports with ESR, harmonics, frequency response and DC tables against fixed tolerances (docs/VERIFICATION.md).

Repository layout

docs/          SPEC, CIRCUIT, WDF_DESIGN, VERIFICATION, DECISIONS, PROGRESS
reference/     SPICE netlist and tube models, reference plots, DI recordings folder (not included)
engine/        real-time engine (C++20, JUCE-free)
plugin/        JUCE plugin wrapper
tools/         ampwav (WAV in/out CLI for the harness) and a benchmark
tests/         Catch2 unit tests; tests/data/ holds SPICE device DC sweeps
harness/       Python verification harness, generated testbenches, committed reports
.github/       CI: build and unit tests on macOS (plugin off)
CLAUDE.md      working rules for Claude Code, which develops this project with the owner

Quick start (macOS)

brew install ngspice cmake ninja
python3 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt

cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release   # add -DAMP2203_BUILD_PLUGIN=OFF to skip JUCE
cmake --build build
ctest --test-dir build --output-on-failure

python3 reference/spice/spice_tools.py                     # reference DC operating point
python -m unittest discover -s harness/tests -t .          # harness tests (need ngspice)
python -m harness.compare --block b12 --preset DEFAULT --milestone M0

All commands are listed in CLAUDE.md ("Commands").

Circuit source

The netlist was transcribed from Marshall's "2203 STD Preamp" (issue 4, 19-5-88) and "2203 STD Output Stage & PSU" (issue 6) drawings. The drawings are not included in this repository. Values that are not on the drawings are either taken from other sources or assumed: the output transformer uses the datasheet of Hammond's drop-in replacement part (1750U), with its primary inductance fitted to the published response curves, and the choke uses the datasheet of Hammond's replacement choke (194F); the HT source and bias are assumptions. All are listed in docs/CIRCUIT.md §3 and §6 and in reference/README.md.

Licence

Copyright (C) 2026 CatastrophicCoder. This project is licensed under the GNU Affero General Public License v3.0 (LICENSE).

Third-party dependencies are downloaded at build time (pinned in cmake/Dependencies.cmake, see docs/DECISIONS.md D-011) and keep their own licences: chowdsp_wdf (BSD-3-Clause), Catch2 (BSL-1.0), dr_libs (public domain / MIT-0) and JUCE 8 (AGPLv3 or a JUCE commercial licence; distributed plugin builds must meet the terms of one of them).

Disclaimer

This is an independent modelling project. It is not affiliated with, endorsed by or sponsored by Marshall Amplification plc. "Marshall" and "JCM800" are trademarks of their owner and are used here only to identify the circuit being modelled.