Instruo Seashell

Instruo’s first foray into standalone synthesizers is both ambitious and original.

Instruo are an A‑list brand in the Eurorack community, known for their innovative, feature‑rich modules. The Seashell is the company’s first standalone synth, taking its inspiration from their Cš‑L complex analogue oscillator module. But it’s a more ambitious concept than a Eurorack module with its own case (although it will play with your Eurorack gear). It’s a versatile semi‑modular synth that runs standalone via MIDI or CV, or integrates into a computer‑based setup, with audio streaming and software control over USB.

The Seashell is beautifully built and surprisingly small, significantly smaller than the familiar 3U of Eurorack. Unlike some desktop synths from Eurorack shops (think Make Noise Strega or Bastl SoftPop II for example) you can’t pop the panel out and mount it in a rack: it’s an all‑in‑one metal enclosure, with connectivity on the rear as well as up front.

There’s a power supply but you can also power the unit from USB‑C, which, combined with its digital connectivity, makes the Seashell a terrific laptop companion. I’ve found that analogue synths powered by USB can suffer terribly from noisy interference, but happily this is not the case with the Seashell.

Standalone Basics

The Seashell’s compact panel is dominated by two large tuning knobs for the dual oscillators. It might not seem an obvious decision to devote so much of the limited real estate to oscillator frequency, but with quantisation, sync and cross‑modulation on board these are performable controls. In its initialised state, the Seashell’s two sources route into the filter, and the single envelope generator controls both the filter and the amplitude at the main output and headphone feeds on the rear panel. A gate/envelope trigger button lets you start exploring the synth with no connections other than some headphones.

As a synth the Seashell combines a ‘bottom‑up’ block that defaults to a sine wave passing into a wavefolder, with a harmonic oscillator (saw/pulse/triangle) that can run at audio or LFO rates. Similar to the Buchla Easel’s complex and modulation oscillator pairing, but with the addition of a filter like Majella’s Implexus.

At the back of the unit we find 3.5mm sockets for line/headphone output (with associated level knob), MIDI in and assignable CV in, along with a 5V power socket and USB‑C port.

Each oscillator in fact has multiple modes, or rather their various derived sources can be routed through the main voice path in different ways. When controlling the Seashell from the hardware these are selected by stepping through a series of combos via the Source button. The initial mode is folded sine plus pulse. Tapping Source moves you to dual (unfolded) sawtooths; a second tap and it’s folded sine plus saw, and so on. There’s no mixer for the oscillators, instead half the modes route just one source into the main filter path. Even here there are various ways the other oscillator can come into play. Osc 2’s three signal flavours are always available at dedicated patch outputs, and the folded osc 1 signal also has...