# FXBox: turning a Zynthian Kit V4 into a standalone multi-effect box (open source, GPL-3.0)

**URL:** <https://discourse.zynthian.org/t/fxbox-turning-a-zynthian-kit-v4-into-a-standalone-multi-effect-box-open-source-gpl-3-0/13820>\
**Category:** Software\
**Tags:** v4\
**Created:** [November 1, 2026, 6:03pm UTC](https://discourse.zynthian.org/t/fxbox-turning-a-zynthian-kit-v4-into-a-standalone-multi-effect-box-open-source-gpl-3-0/13820 "2026-11-01T18:03:14Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![mrz](https://discourse.zynthian.org/letter_avatar_proxy/v4/letter/m/3da27b/32.png) [@mrz](https://discourse.zynthian.org/u/mrz)\
**Post date:** [November 1, 2026, 6:03pm UTC](https://discourse.zynthian.org/t/fxbox-turning-a-zynthian-kit-v4-into-a-standalone-multi-effect-box-open-source-gpl-3-0/13820/1 "2026-11-01T18:03:15Z")

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Hi all,

I’d like to share a side project: **FXBox** , which turns a Zynthian Kit V4 (Raspberry Pi 4, ZynScreen, ZynADAC) into a dedicated, standalone effects unit for external instruments. Plug in, switch on, play.

To be clear up front: it does **not** run the Zynthian software. It’s a separate, single-purpose setup on Raspberry Pi OS Lite 64-bit (Bookworm), for people who want to use the V4 hardware purely as a stompbox-style effects processor. It does build heavily on Zynthian’s work: the hardware configuration (MCP23017 mapping, ZynADAC setup, MIDI UART) comes straight from zynthian-sys, zyncoder and webconf, and the ZynScreen overlays are included unmodified. Many thanks to the Zynthian team for documenting all of that so well.

**What it does**

- Fixed chain of 8 effects, each on/off: Filter (5 types, LFO, envelope) → Saturation → Distortion (5 types, 2× oversampled) → Chorus → Phaser → Tape Echo → Reverb → Compressor/limiter
- Controlled entirely with the 4 encoders and S1–S4, no touchscreen needed
- Analogue-style VU needles plus dBFS peak/clip metering
- MIDI clock sync via DIN or USB, tap tempo, dotted/triplet divisions
- 44 factory presets, 32 user slots, last state restored at boot
- Click-free bypass, effect switching and preset changes, with tails/spill-over

**How it’s built**  
C++ realtime engine on JACK with DSP from the Faust libraries (reverb is zita-rev1), and a Python + cairo UI drawing straight to the ZynScreen framebuffer. Three systemd services; the audio keeps running if the UI restarts.

**Status**  
Early. It installs and runs on my V4: hardware detection, screen and controls work, and a first measurement showed 7 % DSP load and 0 xruns with four effects on (128 × 2 @ 48 kHz). Latency, boot time, full-chain load and DIN clock from real gear still need testing. The test report in the repo lists exactly what has and hasn’t been verified.

**Repo:** [GitHub - Sreekmans/fxbox: Standalone multi-effect box for the Zynthian Kit V4: instrument input, effects (tape echo, reverb, chorus, phaser) out, MIDI clock sync and presets. · GitHub](https://github.com/Sreekmans/fxbox)  
Docs in English and Dutch. Note that the on-device UI is in Dutch for now.

Feedback, testers and ideas very welcome, especially from anyone with a V4 who’d like to try it, or with thoughts on effect choices.

Steven