SEQUENTIAL PRO-ONE - The Analog Rebel of the 80s

At the beginning of the 1980s, it was clear that music production was constantly evolving. Polyphonic synthesizers were becoming the standard in professional studios, but monophonic instruments still played a crucial role in creating aggressive bass lines, percussive sequences, and cutting lead sounds. Musicians sought instruments that would allow for deeper sound experiments while maintaining simplicity and instant usability.

Sequential Circuits had already achieved massive success with the Prophet-5, which became an icon of polyphonic synthesizers, but it was large, expensive, and targeted at high-end users. Dave Smith, founder of Sequential Circuits, wanted to create a more compact alternative that retained the pure analog character but offered the flexibility of a monophonic instrument. Thus, the Sequential Pro-One was born—not just a budget-friendly version of the Prophet-5 but a legendary synthesizer in its own right. Pro-One was manufactured between 1981 and 1984, with approximately 10,000 units produced.

Sound Architecture

The Sequential Pro-One is a monophonic synthesizer featuring two oscillators (VCOs) with a broad range of possibilities.

Key Features of the Sound Engine:

•  Two independent oscillators – A combination of saw, pulse, and triangle waveforms.
•  Curtis CEM3320 resonant low-pass filter – Delivers a rich and full tonal spectrum.
•  Oscillator synchronization (Hard Sync) – Creates metallic, aggressive sounds ideal for lead
    sequences.
•  Extensive frequency modulation options – The LFO and oscillators can modulate each other,
    producing organic and vibrating textures.
•  Independent ADSR envelopes for amplifier and filter – Precise control over sound dynamics.
•  Unison mode – Both oscillators can play simultaneously for a massive sound.

Thanks to this combination, the Sequential Pro-One is highly flexible, enabling intuitive and direct sound design.

Filter Types – Curtis CEM3320 and Its Characteristics

The Sequential Pro-One is equipped with a four-pole resonant low-pass filter Curtis CEM3320, known for its warm, full character.

Key Features of the Curtis CEM3320 Filter:

•  Resonance capability – The filter can self-oscillate, allowing for experimental sound creation.
•  Filter modulation – LFO, envelopes, and oscillators can modulate the filter’s frequency, adding
   complex sound textures.
•  Keyboard tracking – The filter can follow pitch, enabling harmonic changes across different notes.

Compared to other filters, such as the Moog Ladder or Oberheim SEM, the Curtis CEM3320 has a more aggressive character, making it ideal for cutting analog tones.

Manufacturing Process – How the Sequential Pro-One Was Made

Sequential Circuits manufactured the Pro-One in San Jose, California, where their main factory was located. Production took place in several stages:

•  Circuit assembly – The primary electronic components, including the Curtis CEM3320 filter,
   oscillators, and sequencer circuits, were built first.
•  Keyboard installation – Sequential used the J-wire keyboard, which was manually assembled
   and tested for contact precision.
•  Control panel assembly – Knobs, switches, and modulation wheels were mounted on the
   main panel and connected to internal circuits.
•  Oscillator and filter calibration – Each Pro-One underwent manual calibration to ensure stable
   tuning and proper filter response.
•  Final assembly – After assembling all components, the synthesizer was enclosed in plastic
   casing and prepared for testing.

Quality Control – Testing Before Shipment

Sequential Circuits had a rigorous quality control system to ensure every Pro-One functioned correctly.

•  Electronic diagnostics – All circuits, including oscillators, filter, envelopes, and sequencer,
   were tested.
•  Performance testing – Technicians played the keyboard to check key response and verify
   modulation wheel functionality.
•  Sound analysis – Each Pro-One was connected to an oscilloscope to analyze tuning stability
   and signal purity.
•  Stress tests – The synthesizers underwent extended use simulations to confirm reliability.
•  Final inspection – Once fully tested, each unit was assigned a serial number and prepared
   for distribution.

Technological Innovations During Development

Experimental Prototypes and Circuit Testing

Sequential Circuits experimented with various designs before finalizing the Pro-One model. Initially, three oscillators per voice were considered, which would have expanded the sound palette, but the concept was later discarded due to production costs. Eventually, the design settled on two oscillators, which provided broad tonal range, oscillator synchronization, and frequency modulation.

Refining the Filter Architecture

Sequential tested multiple types of analog filters before selecting the Curtis CEM3320, which provided clean and warm resonance, helping to shape deep bass, sharp lead sounds, and dynamic modulations.

Integrated Sequencer and Arpeggiator

While an advanced memory sequencer was initially considered, technical limitations led to the inclusion of a 40-step sequencer and arpeggiator, enabling quick phrase programming without external controllers.

Unique Features of the Sequential Pro-One

The Sequential Pro-One includes several rare functionalities, setting it apart from other monophonic synthesizers of its time.

Modulation Matrix

One of the Pro-One’s most unique features is its modulation matrix, which enables routing various modulation sources to different sound parameters. The LFO can modulate oscillator frequency, pulse width, and filter, opening doors for complex sound experiments.

External Audio Input

The Pro-One includes an external audio input, allowing users to process external signals through its filter. This means the Pro-One can act as an analog modulation unit for other instruments.

Sequencer with Transposition

The built-in 40-step sequencer allows real-time transposition, a rare feature for monophonic synthesizers of this era.

J-Wire Mechanism – A Keyboard with Character

The Sequential Pro-One uses a J-wire keyboard, a mechanical contact system commonly found in analog synthesizers of the 1980s. This system works by pressing a key, causing a thin metal wire (J-wire) to touch a conductive strip, triggering the tone.

Advantages of the J-wire Mechanism:

•  Immediate response – The mechanical contact provides instant reaction without latency.
•  Easy repair – Unlike membrane keyboards, the J-wire system can be easily serviced and
   adjusted.
•  Authentic feel – Many musicians appreciate its analog response, which differs from modern
   digital keyboards.

The downside is that J-wire contacts may lose elasticity over time, leading to unstable key response. A common issue is also dust accumulation, which can cause inconsistent note triggering.

Material and Construction – How Durable Is the Sequential Pro-One?

The Pro-One series has a plastic chassis with wooden sides, while the later Pro-One versions might have had all-plastic chassis. The plastic makes instrument lighter, but unfortunately also less durable than synthesizers with metal bodies.

The control panel is securely mounted, but knobs and switches can be prone to wear if handled carelessly. The keyboard uses a J-wire mechanism, offering quick response, but it can be sensitive to impacts. If the keyboard is damaged, it may cause unstable contacts, affecting tone triggering.

Live Performance – How Does the Pro-One Handle on Stage?

The Sequential Pro-One is excellent for live performance, but it requires careful setup.

Advantages for Stage Use:

•  Fast key response – Ideal for expressive solo playing.
•  Built-in sequencer – Allows phrase programming without external devices.
•  Powerful analog sound – Punchy basses and leads stand out in a mix.

Disadvantages for Stage Use:

•  Sensitivity to vibrations – Strong shocks may cause unstable keyboard contacts.
•  Potential tuning issues – Analog oscillators may require calibration before each show.

How to Ensure Reliable Performance on Stage?

To make the Sequential Pro-One stable for live use, it is recommended to:

•  Regularly calibrate oscillators – Maintains stable tuning during performances.
•  Use a sturdy stand – Minimizes vibrations and protects the keyboard from impacts.
•  Backup power supply – Analog synthesizers can be sensitive to voltage fluctuations.

Sequential Pro-One vs. Competitors in the 80s

Sequential Pro-One was released in 1981, a time when monophonic analog synthesizers were still highly popular. Although polyphonic synthesizers like the Prophet-5 or Roland Jupiter-8 started to dominate studios, monophonic synthesizers remained essential for bass lines, lead sounds, and experimental synthesis.

Several competing instruments offered similar features to the Pro-One. Each had its strengths and weaknesses, affecting their use in music production.

Moog Source (1981) – Digital Memory in an Analog Body

The Moog Source was one of the first monophonic synthesizers to feature digital memory for presets, a revolutionary feature at the time. Unlike the Pro-One, it had a simpler interface, but its classic Moog filter provided rich and warm sound.

Comparison with Pro-One:

•  Moog Source had digital memory, while the Pro-One required manual adjustments.
•  Pro-One featured a built-in sequencer, which Moog Source lacked.
•  The Moog filter was warmer, whereas the Curtis filter in the Pro-One was more aggressive.

Roland SH-101 (1982) – Portable and Simple

The Roland SH-101 was a cheaper alternative to the Pro-One, focused on simplicity and portability. It had a single oscillator, but included a sub-oscillator, adding depth to the sound.

Comparison with Pro-One:

•  SH-101 had a simpler architecture (one oscillator + sub-oscillator), while the Pro-One featured
   two full oscillators.
•  Pro-One had more complex modulation options, whereas SH-101 was more intuitive for
   quick settings.
•  SH-101 was lighter and more portable, while Pro-One had a sturdier build.

Korg Mono/Poly (1981) – Hybrid Polyphony

The Korg Mono/Poly was interesting because it featured four oscillators, which could be used polyphonically or monophonically in unison mode. This gave it a richer range of sound possibilities compared to the Pro-One.

Comparison with Pro-One:

•  Mono/Poly had four oscillators, while Pro-One had two.
•  Pro-One had a better sequencer, while Mono/Poly focused more on polyphonic modes.
•  Mono/Poly had a more advanced arpeggiator, whereas Pro-One’s arpeggiator was simpler.

ARP Odyssey (Rev. 3, 1980) – Aggressive Sound and Dual Oscillators

ARP Odyssey was one of Pro-One’s strongest competitors, featuring two oscillators, an aggressive filter, and flexible modulation options. Its dual envelopes allowed precise sound shaping, similar to the Pro-One.

Comparison with Pro-One:

•  ARP Odyssey had a different filter, which was more aggressive than the Curtis filter in Pro-One.
•  Pro-One had a built-in sequencer, which Odyssey lacked.
•  Odyssey featured a duophonic mode, while Pro-One was strictly monophonic.

Rarities – Unique Versions and Special Editions

The Sequential Pro-One had several rare variations, which are difficult to find today:

•  Prototypes with three oscillators – Some early versions had three oscillators per voice, but
   this design was never put into production.
•  Membrane keyboard versions – Some models featured a membrane keyboard, which was
   less reliable than the J-wire version.

Pro-One in the Hands of Legendary Synth Artists

Sequential Pro-One became a favorite among electronic producers and composers, who used it to create hypnotic sequences, massive bass lines, and expressive lead sounds. Some of the most famous musicians who used the Pro-One include:

•  Depeche Mode – Used it for deep synthetic basses and rhythmic sequences.
•  The Prodigy – Liam Howlett relied on it for aggressive analog frequencies.
•  New Order – Used the Pro-One to create minimalist electronic lines.
•  Vince Clarke (Erasure, Yazoo, Depeche Mode) – One of his primary monophonic synthesizers.
•  Soft Cell – The Pro-One became a cornerstone of their synthpop production.

Conclusion

The Sequential Pro-One is more than just a historical instrument. Its wide modulation possibilities, massive bass, perfect filtering, and unique sequencer secured its place among the most influential synthesizers of its time.

Thanks to its intuitive interface and ability to create expressive sounds, it remains an inspiration for today’s musicians.