Sequential Fourm review - Legendary sound, excellent value

Originally called Sequential Circuits, the synth maker Sequential was founded 50 years ago by Dave Smith. After filing for bankruptcy in 1987, Smith regained use of the brand in 2015, courtesy of Yamaha, which had bought out the assets. Before his death in 2022, Smith sold the company to the Focusrite group. His last contribution, released after the sale, was Take 5, a compact polyphonic analog synth full of good ideas.

The Fourm, an ultra-compact polyphonic analog synth, promises the sound of a Prophet-5 at a price under $1,000. At that price, Focusrite is targeting the entry-level segment of the hardware synth market. Let’s take a look at the features of this new instrument and see if Sequential has produced a winner.

A very compact three-octave keyboard with slim keys and polyphonic pressure

The compact Fourm was designed in California and produced in China, like the vast majority of the Focusrite group’s products. Construction is exemplary: a rigid metal shell, well-anchored knobs (though a little hard to turn), straightforward switches, OLED screen, screwed connections.

The synth measures 56 × 25 × 7 cm (22 × 9.8 × 2.8 inches) and weighs 4 kg (8.8 lbs.). Its keyboard consists of 37 slim, elongated mini-keys, coupled with two transposition keys (+/- 2 octaves). In-house designed, it is sensitive to velocity and polyphonic pressure, a first for the brand since the luxurious Prophet-T8 of the '80s. Although the keys and mechanics are molded, the response is pleasant; it doesn’t snap and allows nuanced polyphonic modulations (bend, vibrato, filter, swell…), assignable via a specific modulation matrix, which we’ll talk about later. Excellent news!

The other good news is that Fourm is easy to get up to speed on, thanks to numerous direct commands and clear organization of the modules, access to additional functions via the Param button, access to secondary functions by holding down the blue screen-printed buttons, a push-button encoder for selecting/editing parameters, and multicolored LEDs for visualizing source/destination assignment in the small modulation matrix.

The Fourm is almost self-explanatory! A Basic Program function (initialization) is accessible via the Global menu or by key combination (Transpose Down + Program). We couldn’t find the Compare function, which is a shame. We also regret that the display only shows the names of programs and global parameters, but not the value of direct front-panel controls when editing.

Minimalist connectivity is located on the rear panel: a mono audio output, a headphone output, a switch/continuous assignable pedal socket (all in 6.35 (1/4”) jack format), MIDI In, Out and Through DIN jacks, a USB-C socket (MIDI, CC/NRPN, programs, firmware update) and a cheap 12VDC/1.2A external power supply terminal. We’re really in the entry-level range here.

Comfortable program memory totaling 256 rewritable slots and 256 factory presets

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The Fourm is capable of storing four banks of 128 programs, two banks of factory presets, and two banks of user programs. Our test model (OS 1.0.0) included only one bank of presets. Presets can be selected by bank number + program, or from one of the 16 predefined categories. This is done with the Select push-button encoder located below the display, which is not the most practical way to reach extreme numbers. The synth has a high output level, and there’s no background noise or noise gate — everything is very clean. It’s advisable to calibrate the machine for the first time — the operation takes a few minutes — but we didn’t notice any particular instability. 

Right away, we found the great standards of polyphonic analog synthesis: progressive layers, PWM-based strings, thick filtered brass, sharp organs, varied basses (peachy, fat, resonant, aggressive, thick in unison), ripping synchros, FM bells, thumping percussion, and hyper-modulated special effects. We appreciated the envelope fishing and the filter compensation, which is also very raw if you abuse the feedback, allowing you to keep a powerful bass even if you push the resonance. We took pleasure in the polyphonic pressure, which allowed us to add a little vibrato or filter opening on the keys of our choice, without affecting the others. What’s missing are more elaborate filtering modes, which confine the Fourm to a fairly classic register, even if potentially more teasing than the Prophet-5.

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Fourm_1audio 01 FB Bass

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• Fourm_1audio 01 FB Bass00:23
• Fourm_1audio 02 Sync Bass00:53
• Fourm_1audio 03 ATP Brass00:34
• Fourm_1audio 04 ATP FC00:51
• Fourm_1audio 05 EDM Stack00:24
• Fourm_1audio 06 Fear Pad00:28
• Fourm_1audio 07 ATP Pluck00:30
• Fourm_1audio 08 ATP Gratte00:38
• Fourm_1audio 09 Big Rez00:36
• Fourm_1audio 10 Dirty Sync00:28

 ALL_WAV

A fully analog signal with VCO-VCF-VCA

The Fourm is a four-voice monotimbral polyphonic synth whose sound architecture is similar to the Prophet-5. The SMD components are close to the Take 5. The sound is produced by two VCOs (component SSI2131, a simplified version of the SSI2130 found on the Take-5), capable of generating cumulative waves (sawtooth and variable pulse for oscillator A, triangle, sawtooth and variable pulse for oscillator B). They can be tuned over four octaves, with low-frequency positions for oscillator B (with or without keyboard tracking), and over more or less seven semitones continuously.

They can also follow one of the 65 internal temperaments (rewritable via Sysex). Oscillator B can synchronize the cycle of oscillator A. There’s also a noise generator (white, pink, and violet). All this is adjustable in the Mixer before attacking the VCF (component SSI2140 used in Take 5).
There’s a resonance-compensated 4-pole low-pass filter (retaining bass if the resonance is pushed), capable of self-oscillation. Coupled with the adjustable feedback circuit, resonance can do some damage. The response of the cutoff frequency potentiometer is quite smooth. This is modulated by keyboard tracking (via the menu), a dedicated envelope (bipolar modulation + velocity control), B oscillator, LFO and pressure (more on this later).

We appreciate the strong character of the classic (but compensated) filter, but its compensation can’t be deactivated (never happy, on Audiofanzine!). Finally, the final amp can be modulated by a second dedicated envelope (also velocity-controlled), B-oscillator, LFO and pressure. The overall volume can be memorized per program, thank you. Voices can be played in unison (one to four voices), with fine detune or chord memory. A rather subtle (not cartoonish) Vintage parameter simulates inconsistencies in behavior or calibration between voices (VCO, VCF, and VCA, envelopes), just like on a vintage synth. It can be accessed via the menu.

A small Pro-One-style modulation matrix directly accessible from the front panel that facilitates source/destination assignment

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There’s a Glide function with constant speed or slope, detached or linked mode, but it’s only continuous (no chromatic mode, too bad). Next comes a global (monodic) LFO, like the Prophet-5. It can produce sawtooth, ramp, triangle, square, S&H, random, different noise colors, and DC waves, all of which can be cumulated. Its frequency can be clock-synchronized in different time divisions and oscillate up to audio. It can be assigned to eight destinations (details later).

Regrettably, there is no polyphonic mode for this LFO, but for this, you can fall back on oscillator B via the modulation matrix (see below). The two ADSR envelopes are routed to the VCF and VCA, with the VCF envelope also routed to the modulation section (see below). Their curves are modeled on the Prophet-5 envelopes, allowing for everything from slamming attacks to very long beats. Velocity can modulate the amount of envelope action, thanks to a dedicated key for each, which triggers and doses modulation (positive action only).

Their cycle can be retriggered each time a note is pressed.
The polyphonic pressure keyboard, for its part, allows each of the Fourm’s four voices to be modulated independently. For this purpose, there are three buses with two destinations, each of which can be routed separately or simultaneously. The amount of modulation, which is bipolar, is set globally via a dedicated knob.

Destinations are oscillator A frequency, oscillator B frequency, filter cutoff frequency, amplitude, LFO frequency and LFO quantity. For the LFO, the modulation is monodic (based on the largest captured value), as the synth contains only one LFO, which is a shame, as the LFO is digital! Finally, there’s a small Pro-One-style modulation matrix with three sources (filter envelope, oscillator B and LFO) assignable to eight destinations (oscillator A frequency, oscillator B frequency, oscillator A pulse width, oscillator B pulse width, filter cutoff frequency, amplitude, LFO frequency, and LFO quantity).

Modulation quantities are controlled by a potentiometer (one for each source, with bipolar adjustment) and by the modulation wheel; thus, there are  two simultaneous control buses, with each destination controlled via one, the other, or both. Route buttons are used to assign sources to destinations (those you want, without exception). They are equipped with multicolored LEDs to distinguish the control buses used, which is a well-thought-out feature in this tiny space.

A small arpeggiator with sequencer mode memorizable in each program

The Fourm is equipped with an arpeggiator/sequencer, capable of operating in play up, play down, pendulum, alternate, random, according to order played, note sequence, or automation sequence. The arpeggio can be transposed over 1–2–3 octaves, while the sequence is transposed on the keyboard. Arpeggiated notes can be repeated 0–1–2–3 times, while the sequence can be played back from the beginning with each new note, continuously, once, or in step increments.

The sequencer is 4-voice polyphonic and has up to 64 steps. In note mode, recording is by step increments. A step can contain note(s), silence, link or glide. In automation mode, movements of a single parameter to be defined are recorded step by step, somewhat blindly. Settings are saved in programs. Arpeggiated/sequenced notes can be transmitted via MIDI. In short, very basic stuff here, we’d have liked a bit of swing, randomness, and ratchets.

Specifications

• Polyphony: 4 voices, monotimbral; unison modes (1 to 4 voices), fine detune, chord memory; vintage parameter for voice variations. 

• Keyboard: 37 slim keys, velocity, polyphonic aftertouch (poly pressure), 2 transposition keys (±2 oct.). 

• Oscillators: 2 VCOs (SSI2131) with cumulative waves (A: sawtooth + variable pulse; B: triangle + sawtooth + variable pulse), 4 oct. tuning, B→A sync, noise generator (white/pink/violet), temperaments (65). 

• Filter: 4-pole low-pass (SSI2140) with compensated resonance, self-oscillation, adjustable feedback circuit. 

• Envelopes: 2 ADSR (VCF/VCA), inspired Prophet-5 response, routable velocity on envelope amount. 

• LFO: global, multiple wave cumulants (sawtooth/ramp/triangle/square/S&H/random/noise/DC), clock sync, going all the way to audio. 

• Modulation matrix: Env-VCF/Osc B/LFO sources to 8 destinations (freq A, freq B, PWM A, PWM B, cutoff, amplitude, freq LFO, amount LFO) + wheel; poly aftertouch with 3 buses/2 destinations each. 

• Arpeggiator/Sequencer: arp modes up/down/pendular/alternated/random/order; note repeats 0–3; poly 4 voice sequence up to 64 steps; step-by-step recording (notes, silence, link, glide) or parameter automation; keyboard transposition; MIDI send. 

• Memory: 4 banks × 128 programs (2 factory + 2 user); selection by bank/number or 16 categories. 

• Controls & interface: OLED display, numerous direct controls, Param key, screen-printed secondary functions, Select push-button encoder; Basic Program function; no front-panel display of values during editing. 

• Connections: mono audio output, headphones, assignable pedal (switch/continuous) in 6.35 mm, trio MIDI DIN, USB-C (MIDI, CC/NRPN, programs, update), external power supply 12 VDC/1.2 A. 

• Construction/format : screwed metal shell; 56×25×7 cm (22 × 9.8 × 2.8 inches) ; 4 kg. (8.8 lbs.)

FAQ

Does the Fourm offer a polyphonic aftertouch that can be used for different modulations per note?

Yes. The slim keyboard supports polyphonic aftertouch with three buses and two destinations per bus, assignable independently on each voice. 

What type of filter is it, and can resonance compensation be disabled?

It’s a resonance-compensated 4-pole low-pass (SSI2140); compensation cannot be disabled. 

How much program memory is available in total?

512 locations: 256 factory presets and 256 user programs (4 banks × 128). 

Is the LFO polyphonic?

No, it’s global (monodic), but it has many shapes that can be accumulated, synced to the clock, and goes right up into the audio. Constant low-frequency oscillator B can be used in the modulation matrix to create a polyphonic LFO.

Are the arpeggiator and sequencer recordable in steps?

Yes, up to 64 steps in note mode (notes, rests, slurs, glide) and one automation mode per step for a defined parameter. 

What MIDI and USB connections are planned?

Trio MIDI DIN and USB-C for MIDI, CC/NRPN, programs and firmware updates.