The Source Audio C4 Synth is a digital synth pedal for guitar or bass that, in the maker's words, "unites all the sound creation tools of a classic Eurorack modular synthesizer" and "packages them in a compact and easy-to-use effects pedal for guitar or bass"1. Released in 2019 by Source Audio, it tracks your instrument's signal and feeds it to up to four parallel oscillators whose wave shapes, pitch shifts and harmonies you can mix, edit and store as presets2. Where most synth pedals give you a handful of fixed voices, the C4 is built like a modular system: the sound design happens in the free Neuro app, and the pedal itself keeps a streamlined set of controls for live use.
A modular synth in a pedal
The C4's engine is built around four independent voices that you can mix together, each assignable as an oscillator with a selectable wave shape, a monophonic or polyphonic pitch shifter, an intelligent harmonizer and more1. The pedal tracks a monophonic signal and feeds it to up to four parallel oscillators2. Source Audio describes the result as the sound creation tools of a classic Eurorack modular synthesizer in pedal form1, a lineage Pedal of the Day echoed when it called the pedal "forged from the depths of Eurorack modular synthesizers"5.
On the surface the pedal is deliberately simple. A single footswitch handles effect engagement and bypass, with a status LED showing whether the effect is active6.
| Control | What it does |
|---|---|
| Input/Sensitivity knob | Sets input level and tracking sensitivity2 |
| Mix/Volume knob | Sets the effect mix |
| Two Assignable Control Knobs | Functions set per preset in the Neuro app2 |
| Three-Position Toggle Switch | Selects between presets 1, 2 and 36 |
| Toggle Switch in ALT Mode | Accesses presets 4, 5 and 66 |
| Mix Knob in ALT Mode | Serves as a Volume control for overall output6 |
| Control Input button | Engages ALT Mode6 |
ALT Mode, engaged by pressing the Control Input button, temporarily reassigns the knobs to alternative functions and exits automatically after five seconds of inactivity6. There is also a Control Input jack for the Source Audio Dual Expression Pedal, and the pedal supports external expression and tap tempo control47.
The Neuro app and presets
Deep editing happens in Source Audio's Neuro app rather than on the pedal. The app provides a preset library of over 10,000 user-generated and Source Audio presets6, and the Neuro 3 Editor's panning controls let you route each voice to any location in a full stereo spread1. The pedal holds 128 presets: six are reachable from the toggle switch, and all 128 can be recalled with MIDI program change messages sent over the mini USB port1. Guitar World's Trevor Curwen noted that with 128 onboard preset slots, these digital stompboxes are hugely powerful4.
The C4's sound design lives in the Neuro app. On the pedal itself you get six presets and a handful of knobs, so players who want to build their own voices from scratch will spend their editing time in the app.
MIDI and connections
Disaster Area Designs lists the C4 Synth among its supported MIDI devices, alongside the Spectrum Filter and UltraWave pedals8. Connections are standard inputs 1 and 2, standard outputs 1 and 2, USB and a control input4, giving you stereo output with per-voice panning when you run both outputs1.
History and the hand-painted limited edition
Source Audio, based in Boston, released the C4 Synth in 2019, joining what Delicious Audio called the small number of manufacturers offering a true synth pedal design2. The pedal was unveiled around Summer NAMM 2019 with a $239 street price37, and Source Audio said it would be the first in a new line of C4 Synth pedals9.
Before the official release, Source Audio sold fifty limited-edition, hand-painted C4 Synths to the public, and with nearly no promotion the fifty units sold out in minutes3. The limited edition's logo was designed by Jamie Bulmer3.
The standard pedal comes in a brushed anodized aluminum enclosure1, made in the USA4, and carries a 2 year warranty1.
What reviewers said
Premier Guitar's 2021 review summed up the pedal's scope: "The C4's sound-generation capabilities often seem to verge on the infinite," and the reviewer found it can open up a near-limitless world of exploration for newcomers to guitar and bass synth9. The same review praised the "streamlined control set" and "easy preset switching"9, but flagged two cons: "Classic synth sounds can be hard to generate. Little control beyond Neuro app."9 On the question of traditional synth tones, the reviewer was direct:
"An oscillator-driven keyboard synth, the C4 is not."
Premier Guitar found that classic sounds like a simple flute tone are hard to conjure compared with a Korg MS-10 or Minimoog9. But the review also found value in the pedal's unpredictability: the randomness that comes from a guitar signal meeting synthesis is half the fun, and sounds can range from gentle bubbling background atmosphere to musical exclamation points in a band arrangement9.
MusicRadar's reviewers found that "careful technique and judicious edits via the Neuro app yield great synth sounds for electric guitar, some classic, some weird, and some very weird indeed"10. Bass Musician Magazine's Cláudio Ribeiro took a deep dive for bass players in 2025, covering the controls, the synth engine, Neuro App customization and tracking performance6.
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Questions
Is the Source Audio C4 Synth true bypass?
It offers what Source Audio calls Universal Bypass: you can select between high-quality signal relays for true bypass or transparent buffers for analog bypass. Either mode can be chosen, so the pedal works in true bypass or buffered bypass depending on your preference and the rest of your board.
Does the C4 Synth work on bass?
Yes. Source Audio builds the C4 Synth as an effects pedal for guitar or bass, and Bass Musician Magazine published a deep-dive review of it in February 2025 covering its controls, synth engine and tracking performance. The Input/Sensitivity knob lets you match the pedal to your instrument's output level.
Do you need the Neuro app to use the C4 Synth?
No, the pedal works on its own with six presets reachable from the toggle switch, but the Neuro app unlocks the deep editing. It provides a preset library of over 10,000 user-generated and Source Audio presets, and Premier Guitar noted there is little control beyond the app for classic synth sounds.
Is the Source Audio C4 Synth stereo?
Yes. It has standard outputs 1 and 2, and the Neuro 3 Editor's panning controls let you route each of the four voices to any location in a full stereo spread, so each voice can be panned independently in a stereo rig.
How many presets does the C4 Synth hold?
It has 128 preset slots. Six presets are accessible via the pedal's toggle switch (presets 1 to 3 normally, presets 4 to 6 in ALT mode), and all 128 are available via MIDI program change messages sent over the mini USB port.
Can the C4 Synth be controlled by MIDI?
Yes. MIDI program change messages can recall up to 128 effect presets via the pedal's mini USB port, and Disaster Area Designs lists the C4 Synth among its supported MIDI devices. External expression and tap tempo control are also supported.
Sources (10)
- C4 Synth, Source Audio
- Source Audio C4 Synth Pedal, Delicious Audio, 2025-06-04
- Introducing the C4 Synth, A Eurorack Modular Synth in a Pedal for Guitar and Bass from Source Audio, MusicPlayers.com, 2019-07-16
- Source Audio C4 Synth review, Guitar World, 2021-11-05
- Source Audio C4 Synth, Pedal of the Day, 2020-09-15
- Review: Source Audio C4 Pedal Deep Dive, Bass Musician Magazine, 2025-02-03
- Summer NAMM 2019: Source Audio Unveils New C4 Synth Pedal, Guitar World, 2019-07-18
- Supported Devices, Disaster Area Designs, 2021-10-11
- Source Audio C4 Synth Review, Premier Guitar, 2019-09-21
- Electro-Harmonix String9 String Ensemble review, MusicRadar, 2023-01-03
