Sequential Fourm is like a Prophet, minus one voice and several thousand dollars£799 / $1000 / €950, sequential.com
For most of us, it’s not realistic to splash thousands of dollars on instruments that, for all their history, rarity and desirability, have exceptionally limited capability when compared to modern software synths.

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Recently, though, we’ve seen a number of the ‘old-guard’ synth manufacturers releasing instruments that authentically embody the classic sound and character of their sought-after classics, but at prices that don’t demand an understanding bank manager and/or a significant lottery win — think Moog’s Messenger and Oberheim’s TEO-5.
Now, Sequential is getting in on the act with the release of its most affordable synth ever: Fourm.

It’s all in a name
Let‘s get one thing out of the way: Despite the synth’s name reading like something from a Swedish furniture maker’s showroom, I can confirm it’s pronounced “Form”. The unconventional spelling is simply an allusion to the synth’s four-voice architecture.
Given this architecture is based on the Prophet 5, and given the synth bears more than a passing resemblance to Sequential’s other example of a cut-down Prophet, 1981’s Pro One, I’m mildly surprised it isn’t called Pro Four. But OK, Fourm it is.
The synth is built into a mid-sized enclosure that’s surprisingly yet satisfyingly weighty for its size and, like the Pro One, features a three-octave keyboard. This so-called Tactive keybed, custom-designed by Sequential, features mid-sized keys. They’re not as small as the keys on something like Arturia’s MiniFreak, though not as large as standard keys, and the keyboard has a high quality feel thanks to a smooth, well-balanced action. The keybed is velocity sensitive and, more excitingly, features true polyphonic aftertouch; I love how this detailed expressiveness interacts with Fourm’s gorgeous-sounding synth engine.
Fourm sports a full complement of MIDI ports (in, out and thru), or can be connected to a computer via USB. There’s also a socket for connecting an expression pedal or footswitch, a headphone output, and a single mono line output.
Image: Press
Fourm’s Prophet-like tendencies
Fourm’s synthesis circuitry is 100% analogue, and closely follows the architecture (not to mention control layout) of the legendary Prophet series. So we have a pair of VCOs that can combine saw, pulse and, for Oscillator B, triangle waves, and deliver classic Sequential-style hard syncing of Oscillator A. Oscillator B can also serve as either a static or key-tracking LFO for those occasions when you need more than the single dedicated LFO.
The oscillators feed through a mixer, where noise and feedback can be added, and into a four-pole, resonant low-pass filter. This has been designed to embody the best qualities of the Prophet revs 1 and 2 filters, and delivers the characteristic smooth, rich filtering, powerful-yet-not-intense resonance, and ability to self-oscillate, just like the classic designs. Sequential has also added bass compensation to the VCF to counter a characteristic of many filters that causes low frequencies to become weaker as the resonance is increased. This deviation from the classic Prophet filter may not please purists, but it does make Fourm a more versatile bass machine.
Fourm’s pair of envelopes – one dedicated to controlling the amp, the other hard-wired to the filter and available as a modulation source – are of a classic ADSR flavour and have been designed to match the response characteristics of the Prophet. Sadly, I don’t have a Prophet 5 on-hand with which to make a direct comparison, but they certainly sound and feel like Prophet envelopes to me.
The synth’s modulation system is almost a direct facsimile of that found on the Pro One. The three sources – filter envelope, Oscillator B and LFO – can be mapped to destination parameters either constantly, under the control of the mod wheel, or a blend of the two. The keyboard’s polyphonic aftertouch has its own set of destinations.
Unison mode offers a choice over the number of unison voices, detuning amount, and an optional, rave-tastic auto-chord function. Detuning can also come from the Vintage parameter which introduces randomness to voice tuning, filter cutoff and such. Finally, there’s an arpeggiator / sequencer that, when operating as the latter, offers a choice between note or modulation sequences of up-to 64 steps in length.
Notable in absence are any on-board effects, but this is entirely in keeping with Fourm’s analogue credentials. After all, what’s the point of having all of that lovely rich-sounding analogue circuitry if you’re just going to push it through an AD/DA converter cycle just to add effects?
Image: Press
What is Sequential’s Fourm like to use?
Fourm provides dedicated controls for its most important parameters, consisting of conventional pots, backlit buttons, and a small number of continuous rotary encoders. It also has extensive digital control elements such as program storage and recall (four banks of 128 programs), automatic voice and filter calibration, and a fair few parameters that are only accessible via the synth’s display and menu system. The menu is navigated using the Select/Value continuous rotary controller located beneath the display – turn the rotary to select a parameter, press it to switch it to Value mode and turn again to set the desired value.
The ‘one-control-per-parameter’ paradigm is one of the big attractions of analogue synths, making the need to menu-dive annoying. Yet Sequential have done an excellent job of blunting the inadequacies that can stem from this cost versus functionality balancing act. Many of the buttons adorning Fourm’s fascia have a secondary label written in blue below the main label. When the button is pressed the secondary parameter’s page is called up on the display from where it can be adjusted using the Select/Value rotary. It’s a graceful solution that doesn’t break your flow.
The majority of knobs are conventional pots, and so have a natural and fluid feeling in use, but also raise the spectre of value-jumping when adjusting a parameter whose current value does not match its current pot position. Sequential has addressed this with three ‘Pot Mode’ options, Jump, Passthru and Relative, that determine how the synth reacts in this situation. In Jump mode, parameter values jump to the pot position, which is convenient during sound design. Passthru and Relative eliminate jumps and so are better when performing. The former holds a parameter’s value until the pot position passes through that value; the latter assumes a pot is positioned at the current value and increases/decreases the value from that point, although this means the full range of values isn’t available until the pot has been moved through its entire range.
Image: Press
Is Fourm a solid investment?
For an affordable and attainable price, Fourm boasts the classic Sequential looks while still appearing modern. It delivers an authentic Prophet-like sound without the headaches of working with a true vintage analogue synth. And it oozes genuine Sequential pedigree from every pore.
Fourm’s affordability may come with a menu-diving trade-off, but this has been so gracefully implemented that it rarely breaks your flow or interferes with a performance. And with modern benefits like USB connectivity, auto-calibration, and the VCF’s bass compensation, it’s a whole lot easier to live with than a vintage analogue beast.

Key features

2 voltage-controlled oscillators (VCO)
Noise generator
Mixer feedback
4-pole resonant low-pass filter with bass compensation
ADSR filter and amp envelopes
LFO with saw, triangle, square and sample-and-hold waveforms
Unison mode with optional auto-chord
Flexible modulation routing
Arpeggiator / sequencer with up-to 64 steps
3-octave, velocity sensitive keyboard with polyphonic aftertouch
2 user preset banks; 2 factory preset banks; 128 presets per bank
Mono line output and headphone output
MIDI in, out and thru sockets
Single pedal socket
USB port for connection to computer

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