Sequential Circuits Pro-One
| Spec | Detail |
|---|---|
| Manufacturer | Sequential Circuits |
| Year released | 1981 |
| Synthesis type | Subtractive |
| Architecture | Analog |
| Polyphony | Monophonic |
| Notable users | Vince Clarke, Depeche Mode, The Prodigy |
| Known for | The Prophet-5 sound in a compact mono with an onboard sequencer and arpeggiator |
Overview
The Sequential Circuits Pro-One is a monophonic analog synthesizer released in 1981. It is a compact, subtractive-synthesis instrument with a built-in sequencer and arpeggiator. Designed by Sequential Circuits, the company behind the Prophet-5, the Pro-One was intended to bring the core sound of that famous polyphonic synth into a more affordable, single-voice package. It has a 37-note keyboard, two oscillators, a resonant low-pass filter, and a modulation section. Because it is monophonic, it plays one note at a time, but it can produce thick, powerful tones thanks to its dual oscillators and sub-oscillator. The Pro-One became popular for bass lines, leads, and sequenced patterns. Its compact size and integrated sequencer made it a staple in early electronic and synth-pop music. Today, it is remembered as a classic mono synth that offers a direct, hands-on interface and a raw, aggressive sound. It remains a sought-after instrument among collectors and musicians interested in vintage analog gear.
History & design
Sequential Circuits introduced the Pro-One in 1981 as a lower-cost alternative to its flagship Prophet-5. The Prophet-5 was a revolutionary polyphonic synth, but its price put it out of reach for many musicians. The Pro-One took the same basic voice architecture—two oscillators, a four-pole low-pass filter, and two envelopes—and packaged it into a single-voice instrument with a built-in sequencer. The design was led by Dave Smith, the founder of Sequential Circuits. The Pro-One used a similar Curtis-chip-based filter and oscillator design, which contributed to its characteristic sound. It also included a 40-note sequencer and an arpeggiator, features that were uncommon at its price point. The instrument was housed in a metal chassis with a plastic front panel and wooden side panels. It had a 37-note keyboard and a compact footprint. Production ran until around 1984, when Sequential Circuits ceased operations. The Pro-One was not the company's best-selling product, but it earned a dedicated following for its sound and features.
How it works
The Pro-One is a subtractive analog synthesizer. It generates sound using two voltage-controlled oscillators (VCOs). Each oscillator can produce sawtooth, triangle, and pulse waveforms. The pulse width can be modulated. A sub-oscillator adds a square wave one octave below oscillator 1. The oscillators feed into a mixer, which also accepts an external audio input and a noise source. The mixed signal then passes through a voltage-controlled filter (VCF). This is a four-pole low-pass filter with resonance, meaning it removes frequencies above a cutoff point and can emphasize frequencies at the cutoff. The filter is followed by a voltage-controlled amplifier (VCA), which shapes the volume. Two envelopes—one for the filter and one for the amplifier—control how the sound evolves over time. These are ADSR (attack, decay, sustain, release) envelopes. A low-frequency oscillator (LFO) provides modulation for pitch, pulse width, and filter cutoff. The Pro-One also has a built-in sequencer and arpeggiator, which can trigger notes automatically. The sequencer stores up to 40 notes and can be clocked internally or externally. The arpeggiator plays notes held on the keyboard in various patterns. All parameters are controlled by knobs and switches, giving immediate, hands-on control.
Signature sound
The Pro-One is known for a bold, aggressive analog sound. Its two oscillators and sub-oscillator can create thick basses, piercing leads, and rich drones. The filter is a key part of its character: it can be smooth and warm, but also squelchy and resonant when pushed. The envelopes are snappy, making the Pro-One well-suited for percussive sounds and sequenced patterns. The onboard sequencer encourages rhythmic, repetitive lines, which became a hallmark of its use in electronic music. Because it is monophonic, it excels at single-note lines, but it can also produce complex textures through modulation and the sequencer. The Pro-One's sound is often described as raw and immediate, with a slightly gritty edge compared to later digital synths. It shares DNA with the Prophet-5, so it can approximate some of that synth's famous tones, though it is limited to one voice. Its external audio input allows processing of other instruments through its filter and envelopes, adding to its versatility. Overall, the Pro-One delivers a classic analog mono sound that sits well in a mix, especially for bass and lead roles.
Notable users & recordings
The Pro-One has been used by a range of influential artists. Vince Clarke, known for his work with Depeche Mode, Yazoo, and Erasure, is a notable user. Depeche Mode themselves used the Pro-One on early recordings. The Prodigy also incorporated the Pro-One into their sound. These artists helped popularize the instrument in synth-pop, electronic, and dance music. The Pro-One's sequencer and arpeggiator made it a natural fit for the rhythmic, sequenced styles of these genres. While specific recordings are not always documented, the Pro-One's distinctive tone can be heard on many tracks from the 1980s and beyond. Its use by these acts has contributed to its reputation as a classic mono synth. It is also used by contemporary musicians seeking vintage analog sounds. The instrument's association with these artists has kept it in demand among enthusiasts. Factual listings of users include Vince Clarke, Depeche Mode, and The Prodigy, as noted in synthesizer history references.
Legacy & influence
The Pro-One is remembered as a significant instrument in the history of analog synthesizers. It brought the sound and features of a high-end polyphonic synth into a more accessible monophonic package. Its built-in sequencer and arpeggiator were ahead of their time for an affordable synth, influencing later designs. The Pro-One helped establish the template for the modern analog mono synth with integrated sequencing. Its sound has been emulated in software and hardware, and it remains a sought-after vintage instrument. The Pro-One's design also influenced Sequential Circuits' later products, such as the Prophet-600 and Six-Trak. After Sequential Circuits closed, the Pro-One's legacy continued through its use in electronic music and its reputation among musicians. Today, it is considered a classic, valued for its raw sound and hands-on control. It represents an important step in the evolution of synthesizers, bridging the gap between the Prophet-5 and more affordable, feature-packed mono synths. Its influence can be heard in the work of many artists who followed.
Frequently asked questions
Q: Is the Pro-One polyphonic? A: No, it is monophonic, meaning it can play only one note at a time. However, it has two oscillators and a sub-oscillator, which can create thick, layered sounds. Q: Does the Pro-One have MIDI? A: The original Pro-One does not have MIDI. It uses control voltage (CV) and gate connections for interfacing with other gear. Some units have been modified for MIDI by third parties. Q: What makes the Pro-One's sound unique? A: Its analog signal path, based on the Prophet-5's design, gives it a warm, powerful character. The filter and envelopes are particularly praised for their musicality. The onboard sequencer and arpeggiator also contribute to its distinctive use in electronic music. These features make it a favorite for bass lines, leads, and sequenced patterns.