Sequential Fourm is a 4-voice polyphonic analog synthesizer that combines the classic Sequential sound with maximum expressive performance. The control panel, inspired by the historic Pro-One, puts modulations at your fingertips, eliminating annoying menus. The 37-key slim keyboard with polyphonic aftertouch opens up new expressive techniques. The sound synthesis architecture utilizes two oscillators and an LFO, allowing you to select all waveforms simultaneously. The classic 4-pole low-pass filter with resonance is supported by low-frequency compensation. There are two ADSR envelopes and the arpeggiator also offers a sequencer mode.

Sequential Fourm Review
by Giaga Robot

The Sequential sound in a compact format

The 37-note slim keyboard with polyphonic aftertouch opens up innovative control techniques: per-note bending, swell on individual voices, dynamic nuances, etc. This expressiveness makes the Sequential Fourm not only a studio instrument but also a stage companion capable of responding to the dynamics of touch with naturalness.

Sound generation section of the Fourm

The Fourm's synthesis engine is based on two voltage-controlled analog oscillators (VCOs) for each voice. Oscillator A generates sawtooth and square waveforms, while Oscillator B also offers the triangular option. All waves can be activated simultaneously, allowing for rich and complex timbral combinations. Pulse Width Modulation (PWM) is available on both oscillators, with extensive adjustability, even varying the phase of the waveform. Oscillator B also features two "LO" modes that transform it into a polyphonic LFO source, with or without key tracking, ideal for dynamic pitch and filter modulations. The hard sync function allows the subordinate oscillator to be synchronized to the cycle of the other, generating aggressive and harmonically complex timbres. In combination with variable intonation and fine detune, it will be possible to transition from classic Prophet timbres to modern and incisive sounds.

The Prophet-lineage filter

The timbral heart of the Fourm is its 4-pole resonant low-pass filter, a direct inheritance of the Prophet tradition. In addition to the classic warm and musical analog behavior, it integrates a bass compensation function, which maintains the presence of low frequencies even when the resonance is pushed to the maximum. This allows you to push the resonance to self-oscillation without sacrificing body and depth. The filter can be modulated by both internal envelopes and the LFO and Oscillator B when used in modulator mode, opening up possibilities ranging from progressive and dynamic sweeps to more extreme effects. The cutoff response is precise and immediate, ideal for incisive bass lines, airy pads, and percussive timbres sculpted with great precision.

Envelopes and dynamic control in the Fourm

The Sequential Fourm has two independent ADSR envelope generators, dedicated to the filter and amplifier respectively. The filter envelope shapes the cutoff over time, with attack, decay, sustain, and release that allow you to sculpt the brightness and body of the sound with precision. The velocity amount option also allows you to link the dynamics of keyboard touch to filter opening, making performance more expressive and natural. The amplifier envelope, on the other hand, shapes the volume of the sound, reproducing temporal curves ranging from soft attacks for the most ethereal pads to percussive hits with instantaneous attack. Both envelopes can be re-triggered with each key press, facilitating more complex articulations and accurate control in fast phrases. This section, in combination with the oscillators and the modulation matrix, provides the sound designer with extreme flexibility, capable of covering both classic timbres and advanced experimentation.