Source Audio C4 Synth Review

A streamlined synth pedal and its accompanying app open up a treasure trove of guitar-synth possibilities.



Neuro app is fun and intuitive. Streamlined control set. Easy preset switching.

Classic synth sounds can be hard to generate. Little control beyond Neuro app.


Source Audio C4 Synth


Ease of Use:



My musical predilections and processes typically favor crude, old-world technologies. When recording and manipulating music digitally, I typically sabotage anything too precious with analog pollution. Given that, I’m always pleasantly surprised at how much I enjoy working with Source Audio’s digital effects editing tools.

Source Audio’s primary digital editing tool is called the Neuro app. It’s available for desktop, iOS, and Android applications. You don’t have to use it to make pedals like the company’s True Spring reverb sound great. But it’s easy to use when you want to customize a Source Audio pedal with a tweak that makes it your own. In the case of the new C4 Synth, that’s a good thing, because you’ll definitely want to dive deep into the customization possibilities the Neuro app can unleash. You can also use the app to easily share and download presets from other users.

Don’t Fear the Neuro
The C4 is, to a significant extent, the work of Source Audio chief sound engineer Bob Chidlaw, who has a deep affection for synthesis and a sprawling array of Eurorack modular synthesizers in particular. Modular synthesis is not for chickens. In complex synthesizer arrays you either have to approach the process with a neophyte’s recklessness—happy for whatever randomness your synthesis solution yields—or attack it with the rigor of a lab-coated brainiac. The beauty of the C4 and the Neuro App is that it, too, permits both approaches: enabling disciplined sound sculpting or random tweaking at the app level, as well as more limited tailoring options on the pedal itself.

Perhaps more than any other Source Audio pedal, the C4 relies on the Neuro app. Source Audio opted for a streamlined pedal interface with just four knobs: an input level control, a wet/dry mix, a toggle that moves between three top-level presets, and two “control” knobs that change function depending on how you assign them. The minimal control set means the lion’s share of twiddling is done at the app level.

You should be prepared to create sounds akin to a space gamelan or alien sitar.

The Neuro app interface for the C4 is complex. But it’s also fun and inviting. The ease with which you can navigate controls for the four voices, filters, envelopes, LFOs, sequencers, and more is a testament to the interface designers at Source Audio. And while it helps to have a background in synthesis to move around the interface and understand signal flow, it readily rewards casual, intuitive navigation.

Seek and You Shall Search
Conjuring exactly the sound you need on the C4 isn’t always easy. And with four voice generators that can be set for nearly four octaves in either direction, and powerful processor, envelope, LFO, and sequencing sections, you can get bogged down in options fast. But even when I kept things simple and used a single voice, a sine wave, and very basic filtering and LFO settings, it wasn’t easy to get a simple flute tone the way I would with my old Korg MS-10 or a Minimoog. Much of this limitation is down to the way a guitar signal, with its tangle of harmonics and peaks, interacts with synthesis. Such randomness is half the fun of the C4. But you should be prepared to create sounds akin to a space gamelan or alien sitar, with odd attack and decay qualities, rather than luscious beds of future-medieval oboe with predictable decay properties. An oscillator-driven keyboard synth, the C4 is not.

To the extent the C4 generates fatter sounds (often from big, compound four-voice settings), they tend to align with the sonorities of EDM. If you’re on the prowl for more classic sounds, simpler is better, and starting with a single voice and a fairly regular waveform is a good start point for approaching the C4 and Neuro app’s capabilities anyway. But if it’s vintage Pink Floyd and Tangerine Dream textures you hope to extract from the C4, the search can be tricky. Thankfully, Source Audio makes it simple to share presets, effectively creating a community of artists, some of which may be more adroit at fashioning old-school sounds.

The Verdict
The C4’s sound-generation capabilities often seem to verge on the infinite. And when you factor in the inevitable randomness created when guitar signal meets synthesis, that’s probably true. For all its expansiveness, the C4’s voices still tend toward the modern digital-synthesis palette. Framed within a more complex musical piece or a band arrangement, these sounds can be everything from gentle bubbling background atmosphere to smack-you-silly musical exclamation points. But if you’re new to guitar and bass synth or the world of sound design through synthesis, the C4 and the Neuro app can open up a near-limitless world of exploration.

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