Sequential’s latest is a miniaturised, analogue, four‑voice polysynth inspired by the Pro‑One and Prophet lines. But does it still feel like a Sequential?
Sequential entered a new era in 2021 when Focusrite bought the company out. The synthesizer industry is relatively small, so it can be a fraught time when someone takes over a beloved, well‑established brand. Even though this new synthesizer is built on the legacy of the mighty Sequential Circuits Pro‑One, it feels different to anything Sequential have done before or after the Focusrite buyout. Sequential has always been a premium brand, so is a smaller desktop synth with mini keys really a good idea?
Top Fourm
The Fourm is a four‑voice analogue synthesizer with two VCOs, a four‑pole resonant low‑pass filter, two ADSRs, one LFO, an arpeggiator and a sequencer. The front panel should be recognisable to any synth fan. The Pro‑One obviously inspires it. Where it deviates from its 40‑year‑old ancestor, aside from the addition of polyphony, is the miniature keys, which are a new design Sequential are calling ‘Tactive’ (a fancy term for polyphonic aftertouch). Plus, as we’ll find out, a lot more.
If the idea of polyphonic aftertouch on a mini keybed worries you, then dismiss those negative thoughts. The aftertouch response is fantastic, better than on many full‑sized keyboards.
If the idea of polyphonic aftertouch on a mini keybed worries you, then dismiss those negative thoughts. The aftertouch response is fantastic, better than on many full‑sized keyboards. In fact, the build quality, finish and hands‑on experience of the Fourm are superb throughout. It feels like a premium product, just in a smaller form, which is a smart move from Sequential, as it maintains their reputation as a high‑end brand. In essence, the Fourm is a low(er)‑cost, well‑built, classic analogue synth that feels like a full‑sized product, and should replicate many familiar mono and polyphonic sounds.
At 563 x 259 x 82mm, the Fourm will sit easily on a desk. It is lightweight but features an all‑steel chassis, which means it will travel well. The one‑knob‑per‑function (almost) interface feels authentic, and there is a small screen for settings, patch naming and so on.
Synth Engine
The architecture is straightforward but well‑balanced, and sticks reasonably closely to that of the Pro‑One. Firstly, there are two voltage‑controlled oscillators. Both oscillators can generate sawtooth and pulse waves, with the second oscillator also adding triangle waveform. You can combine these waveforms, as each one has a simple on/off switch. Oscillator 1 has a classic Prophet‑style sync mode, and oscillator 2 has two low‑frequency modes — one with keyboard tracking, and one without.
A simple mixer offers volume controls for the two oscillators, plus noise, which can be switched between white, pink and violet modes for different noise profiles. There’s also a feedback circuit that will take the filter output and feed it back into the input for beefier sounds, or outright chaos, if that’s your thing.
You may notice that the Noise and Feedback amount controls are missing on the front panel. Sequential have clearly done their best to include a knob for every function, but there are limits. Instead, the Noise and Feedback functions have a button that serves two purposes: one is to turn the function on or off, and the other, activated with a long press combined with turning the data encoder under the LCD screen, is to change the amount (with appropriate feedback on screen). Any button with a blue label on the UI indicates a secondary function that involves the data encoder. Luckily, many of these are functions you won’t need to reach for often, although I personally would have loved a dedicated noise amount knob.
The mixed signal heads to the filter next. It’s a simple four‑pole resonant filter derived from the Prophet series. There are controls for cutoff, resonance and envelope amount, and then there are four levels of key tracking: off, quarter, half and full. There isn’t much more to say about the filter, except that it’s one of my favourite‑sounding filters in recent years. It loses no bass end when the resonance is increased, and it sounds well balanced at any frequency. There is no harshness and no unexpected volume bumps. It’s just a lovely filter.
As with all Sequential synths released in the last decade or more, there’s a Vintage parameter that adds detuning and instability to oscillators and the filters to emulate old gear. It’s been nice to witness these parameters evolve over the years, because what started as oscillator ‘slop’ (I was never keen on that name!) has developed into a very musical and, I suspect, far more complex parameter that emulates the ageing of electronic circuits and affects the oscillators, filter, envelopes and individual voices. The result can range from subtle note‑to‑note variations to “Is this broken?”. I love it.
Modulation
With such a simple synthesizer section, a lot rests on modulation and playability. The Fourm replicates the Pro‑One‑style modulation section almost exactly. That means no modulation matrix. The front panel handles everything. There are four potential modulation sources: filter envelope, oscillator 2 (for some lovely cross‑mod sounds), LFO and aftertouch. Sources can be assigned to one of two ‘buses’: the Modulation Wheel bus and the Direct Modulation bus. The first is multiplied by the position of the modulation wheel, allowing you to assign that modulation to the wheel, and the second is a direct connection. Any source can be assigned to either bus, and any destination can be affected by either bus, or by both (except aftertouch, which is always direct). This is achieved using buttons of various colours: blue for the Modulation Wheel bus, red for the Direct bus, and purple (for destinations) to receive from both buses. The Amount knobs allow for positive and negative modulation and control the amount of modulation sent to both buses.
These buses can be routed to oscillator pitch (either), oscillator pulse width (either), filter cutoff, amplitude, LFO frequency and LFO amount. Aftertouch can be assigned to all those, except pulse width. I particularly love the cross‑modulation possibilities of using the second oscillator as a modulation source going to filter, oscillator 1 pitch, or amplitude. The second oscillator also makes an excellent second LFO when in low‑frequency mode.
Enjoying an almost one‑knob‑per‑function front panel, the Fourm does have some hidden depths, but not too hidden.
The front‑panel modulation system takes a bit of getting used to, and is perhaps the only part of the Fourm which failed my “Can I work it out without reading the manual?” test. The original Pro‑One design was slightly more friendly, but only slightly. Once you figure it out, it’s a flexible system that offers plenty of options without resorting to menu diving.
The main LFO, assignable through the modulation panel, offers sawtooth (up and down), triangle, square, sample & hold, random, pink noise, white noise, violet noise and DC waveforms. It is sync’able to MIDI Clock, or the internal tempo of the sequencer. The DC waveform can be useful for sending a constant voltage through, for example, the modulation wheel to allow you to control a parameter from the wheel directly. Alternatively, you could use it through the main modulation section to have the Amount knob change several modulation destinations at the same time. Along with the other bonus waveforms, it’s a clever and thoughtful addition to the vanilla waveforms offered by the original Pro‑One LFO.
The two envelopes, which are dedicated to filter and amplitude, are simple ADSR types, with adjustable velocity scaling. The filter envelope can be used through the modulation section. And there isn’t much more to say about that.
Sequencer, Arpeggiator & Unison
A sequencer and arpeggiator based on those of the Pro‑One are present but, as we’re starting to see everywhere, much enhanced. The arpeggiator offers six playback modes, and up to a three‑octave range, as well as number of repeats per note. There is also a hold mode and a re‑latch function that prevents notes from being added to the arp, instead starting a new arpeggio every time notes are changed. You set the tempo with tap tempo, bpm, or sync to MIDI Clock with clock divisions.
The sequencer mode, which is another function of the arpeggiator, can record up to 64 notes, with ties, glides and rests, and the ability to edit a sequence step‑by‑step. To accommodate the Fourm’s four voices, each step can record chords. Once you’ve recorded a bunch of notes into the sequencer, you can play back the sequence in a variety of ways. These include retriggering or continuing the sequence on each new note, a one‑shot mode that plays through the entire sequence (great for trills and chiptune‑style note flourishes), and a step mode where each new note triggers the next step in the sequence.
The sequencer can also function as a modulation source where the note pitches represent a value, with higher pitches meaning higher values. In the parameter menu (the on‑screen list of all the parameters which don’t have dedicated front‑panel controls), you can choose a single destination for the modulation, which includes oscillator frequencies, filter cutoff, modulation amounts, LFO frequency, pulse width for both oscillators, and feedback. Unfortunately, you can’t have both notes and modulation at the same time. It’s one or the other.
Four‑voice polyphony means an opportunity for massive unison sounds, and the Fourm doesn’t disappoint. You can choose the number of voices to use, between two and four, and the amount of detune. There is also a chord memory mode, which allows you to memorise any chord up to four notes and transpose it on the keyboard. I have to admit, I’m a sucker for rave and techno chord stabs. The combination of the old‑school Pro‑One sound, four‑voice polyphony, and chord memory (maybe with a bit of glide and legato) is absolutely perfect for these sounds. Chef’s kiss!
The Other Bits
We’ve covered everything on the front panel, and what’s left is various settings and patch‑management stuff, all of which happens on the screen using a few buttons and a single data encoder. The Fourm offers four banks of 128 patches for a total of 512. Two banks are factory patches, and two are user patches. Unfortunately, the factory patches cannot be overwritten, which leaves 256 slots for custom sounds. Loading and saving patches is intuitive enough, and you can even categorise presets, but the lack of any compare function, and the inexplicable lack of any display showing you the name of the patch you are about to overwrite, makes it impossible to know what you’re about to lose when saving a patch. As someone who likes to make a lot of patches, I found this really frustrating, and I hope that Sequential might add this function in a future update.
For the purists among you, there is a live panel mode that disables the loading of presets and uses the front panel to generate the current sound. I love this mode, and it’s perfectly implemented. I particularly like that you can still save presets without leaving live panel mode, in case you stumble upon a sound you want to keep. It feels like the perfect marriage of old‑school simplicity and modern flexibility.
Another killer feature is the alternative tunings function, which supports 64 additional microtonal tunings. The included tunings are fantastic, and there is an excellent section in the manual that explains a lot of them, their history and some of their potential uses. Alternative tunings can be global or per preset, as you wish. It’s a great implementation and makes it really fun and easy to experiment. The factory tunings can even be replaced via System Exclusive if there is a tuning you want to use that isn’t included.
The only menu items to mention are various dull but necessary settings such as pot mode (relative, pass‑thru, jump), footswitch assignments (sustain, arp hold, seq start and stop, various note trigger options), expression pedal assignments (filter controls, LFO and osc modulation amounts, and volume), aftertouch type (mono, poly or off), the usual MIDI settings, preset category filtering (can be turned off), and MIDI dumps for backing up presets.
Great sounds tumble out with minimal effort, and it’s a testament to the original Pro‑One design that this is still relevant and kicking ass in 2025.
Fourm & Function
The ideal synthesizer delivers on the following: sound, hands‑on experience, playability, flexibility, build quality and cost. On the first item, sound, I am happy to report that the Fourm sounds amazing. It is, of course, a traditional analogue synthesizer, so it’s not going to give you groundbreaking, never‑heard‑before sounds. Instead, it’s one giant sweet spot of classic analogue goodness. Great sounds tumble out with minimal effort, and it’s a testament to the original Pro‑One design that this is still relevant and kicking ass in 2025.
Whether it sounds like an original Pro‑One or not is, in this reviewer’s humble opinion, irrelevant. I’m sure there will be YouTubers doing one‑to‑one comparisons, and that’s fine. But there are so many improvements, large and small, to the original format (it’s polyphonic for goodness’ sake!) that we should only be judging this synth on its own merits. The Fourm will easily spit out solid dance basses, silky leads, brass polys, analogue strings, hard‑sync rippers, heat‑warped wobbly pads, rave stabs, electro blips, and techno sequences galore.
Incidentally, I’m quite glad that Sequential resisted the urge to add effects. I’m sure some will disagree, but in general it’s fair to say that onboard effects are not the best. The decision to keep this synth ‘pure’ actually makes it sound better. Put it through your favourite reverb or delay, and the results are far more likely to impress.
Sequential have done a stellar job of keeping the ‘big synth’ feel whilst reducing the size and cost.
So it sounds great. What about the other stuff? Well, Sequential have done a stellar job of keeping the ‘big synth’ feel whilst reducing the size and cost. For the most part, you will be doing all your sound design using knobs and buttons — the way it should be. The build quality is not compromised either. Everything feels solid, like it will last a lifetime. The new Tactive keybed is quite a revelation, too. I have never played a mini keybed that felt so good, and the polyphonic aftertouch response is perfectly balanced, consistent across the keyboard, and feels musical to play. Bravo.
In terms of flexibility, there are all the modern considerations: USB MIDI, enough patch storage, alternative tuning support, plus grown‑up features like pedal input, poly aftertouch, chord memory, unison, a fantastic sequencer, and even the anti‑theft K‑slot on the rear.
Which leaves the only metric left to measure: cost. At £799$999, this represents excellent value for money. You get a polyphonic analogue synthesizer that feels and sounds like something that costs three times its price tag. It boasts an incredible pedigree, a vintage Prophet/Pro‑One sound, grown‑up features, and is both fun to play and eminently musical. It would make an ideal first synth, but equally will appeal to experienced synth‑heads thanks to its well‑thought‑out design and awesome vintage sound. Sequential have managed to miniaturise without compromise. I love it.
Round The Back
The Fourm’s miniaturisation hasn’t resulted in cheap connectors on the rear, though 12V power is from an external power supply rather than a built‑in PSU. Otherwise, the USB‑C connector is for USB MIDI. There’s a quarter‑inch input that can be configured for a footswitch or expression pedal. MIDI in, out and thru are on 5‑pin DIN connectors. Audio and headphone outputs are on quarter‑inch mono and stereo connectors, respectively. There’s even a K‑slot security cable lock to keep your precious synth away from jealous types.
Pros
- All the fun of the Pro‑One, and much more.
- Four‑voice polyphony on a monosynth structure makes a lot of sense here.
- Polyphonic aftertouch on a mini keybed (which feels great to play!).
- The sequencer is great fun.
- It sounds phenomenal.
Cons
- Only the preset saving system.
Summary
Sequential’s latest synth aims to lift a Pro‑One‑style design into the modern age by upgrading it, and making it smaller and more affordable at the same time. They haven’t just had their cake and eaten it — they’ve pulled it off, with icing on top. The result is a four‑voice analogue synthesizer with a very playable mini keybed that supports polyphonic aftertouch. It has all the familiar weighty sound of a Pro‑One with polyphony and modern features. What’s not to like?
Information
£799 including VAT.
$999
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