Sequential Pro-One – best vintage monophonic?

Sequential’s Pro One is a miracle. It was released at a time when monophonic synths had to fight a battle against polyphonics, which became more and more popular.

Quite at the end of the great analog aera Sequential Circuits presented their first and only monophonic in 1981. Interesting, considering this company itself somehow introduced the polyphonic aera way back in 1978 with the all-time-classic Prophet-5.

Sequential Pro-One Synthesizer

While the original five-voice Prophet was called Model 1000, its smaller single-voice brother was released as Model 100

The majority of Pro-One owners doesn’t know that at all, simply because many instruments don’t have the serial-plaquette these days. Wheather they were never sticked on Pro-Ones at all or if they all fell off after some time … who knows!

While the cute Pro-One Model 100 might be only one tenth of the great Prophet-5 Model 1000 considering hardware, it has its own musical territory, its own power that never will be beaten. It must rate as one of the most complete of the commonly available (vintage) monosynths ever.

Sequential Pro-One Synthesizer

A wolf in sheep’s clothing

The Pro-One is NOT a simple synthesizer with just nice VCOs and a great filter. And it is NOT one single voice of a Prophet-5. That rumour seems to be very, very insistent but when listening to each instrument’s power you’ll recognize it’s not true, not at all.

Further, the Pro-One won’t build up another battle line between ARP-enthusiasts on the one side and MOOG-admirers on the other side. It is not a dominating instrument and seems to be camouflaged with some really weak hardware components.

That might be an unexperienced musician’s downfall! When playing the instrument for the first time, when recognizing first small damages on the speakers, most people might be worried about that strong audio signal leaving the mixing console. Sure, it must be the mixing console, it’s simple not gained correctly, isn’t it? What the hell is happening …?

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Might it be that small unimposing synthesizer named Pro-One? Well, it is.

Some features of this monophonic are truely dangerous:

  • a high-class concept of the signal path with strong VCOs and a powerful VCF
  • a unique modulation matrix system
  • an equally unique (and simple!) sequencer/arpeggiator unit

Two facts seem to be less excellent:

  • the minor-quality hardware
  • a clunky and clattering keyboard (on most Pro-Ones)
Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

But times are changing. Upgrades available today for the Pro-One essentially eliminate the above-mentioned negative points.

Facelifting – the Pro-One of the 21 Century

What’s remarkable about those wooden cases is that the Pro-One now (for the first time) looks like a smaller version of the legendary Prophet-5. The new wooden frame replaces that less appealing plastic chassis. New knobs and a unit of illuminated wheels are included.

Information at

The conversion kit requires minimal technical knowledge – you need a drill (4 holes), a screw driver and a soldering iron. But more important, you need a lot of time and much patience. It might take a whole weekend (and a few bottles of beer) to completely restore a Pro-One (including cleaning all the keys).

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Now, a Pro-One Deluxe of this sort is object of our test report. It looks very classy and is a pleasure to operate. Two modifications were carried out in addition to the above mentioned: the crumbly rubber pads under the keys were replaced (keyboard upgrade from Virtual Music) and all pots were stabilized with shims (available in any DIY store) – so the knob action now feels more reliable.

Pro-One: A hard worker

The Pro-One features:

  • 2 VCOs: all waveforms are available simultaneously. VCO 1 has a sync switch. VCO 2 can be set to LFO mode (with or without keyboard voltage control)
  • VCF: a powerful 24 dB low pass filter with brilliant resonance
  • 2 envelopes: one ADSR for VCF and VCA each
  • LFO: as with the VCOs – all waveforms are available simultaneously. LFO-clock serves as masterclock for SEQ and ARP
  • a powerful modulation matrix
  • noise, glide, various trigger modes, 2 wheels
  • sequenzer and arpeggiator (with EXT clock input)

Well, the Pro-One is simply the best! No other vintage mono-synth matches its excellent sound. At the most, it could be surpassed by the much more expensive ARP-2600.

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

However, a Pro-One is a hard worker, no doubt! In some way it is truely unique flexible, maybe it is the most versatile allround-monophonic on earth (I believe so). Sure, a Korg MS-20 is outstanding as well, but its special sound character will always put it in its own corner in any musical context. No Minimoog will be used for generating extreme effect-sounds, whilst it is the unchallenged king of lead- and basslines. That’s why an ARP Odyssey is a very effective addition with its vast modulation routings and a rather neutral sound character (whatever “neutral” means, I’m not sure up to now).

So, Sequential’s Pro One is part of all! Not character-wise (sure!) but regarding its versatility. Musical-wise there are not limits at all! It offers scarifying bass-sounds (punchy dry analog beats as well as warm, softsounding ones). Further, it’s one of the best FX-machines ever made (just have a look at the modulation matrix and you know what’s going on…). Last but not least high-quality leadsounds and especially outstanding sync-leads are just a simple job for the all-time-classic Pro-One as well.

Envelopes are very short and percussive sounding and the filter is strong (with that great resonance!). All waveforms of the VCOs may be used at the same time (just like on old granny ARP-2600). If the single LFO is not enough for your musical inspiration, use VCO2 as great high-end LFO that lets you slide from Low Frequency to the oudio range (whilst modulating the filter – it is great!).

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Modulation matrix and vast filter-cv routings

If you’re one of those who love to create new, crazy sounds, the modulation matrix is your thing!

Following three modulation sources are offered:

  • LFO

Modulation destinations are:

  • VCO 1 PWM
  • VCO 2 PWM

Two possibilities of ROUTING: either via WHEEL or DIRECTly

This small modulation matrix offers incredible sonical possibilities. Just imagine you synchronize both VCOs, while the second oscillator is in LOW FREQ position and via the matrix system you let it (and VCO1) modulate by itself. This is not at an end: route any modulation source to FILTER FREQ and use the FILTER-IN port for extended (external) filter modulation… Maybe you got an idea how flexible the Pro-One is. It can produce sounds that an ARP-2600 or even more elaborate modular system might offer.

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Comprehensive control of the filter is of great importance for good sound effects. So it’s good to hear the Pro-One has a FILTER IN jack. That means the filter can not only be modulated by its envelope, by keyboard tracking and by any of those weird routings in the modulation matrix, but also (simultaneously) by any external CV-source (e.g. analog sequencer, an additional LFO, another envelope).


The Pro-One is – almost – perfectly fitted with:


“Perfect” would include the addition of two more connections: VCA IN (like on the early Oberheim OB-1) and VCO1 IN (for cv-control of oscillator synchronization, as on the Crumar Spirit). Such modifications might be possible by skilled technicians, I’m sure they’re worth the money.

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Trigger Modes and Glide

There are several Trigger Modes (like repititon via LFO clock and external triggering) as well as DRONE (= hold)… The only LED found on the Pro-One shows the trigger signal – like on the OSCar.

GLIDE is not extraordinary, but very important. First, there are two modes (NORMAL > each note glides, AUTO > legato played notes glide). Second, when triggering another synthesizer via the Pro-One sequencer (that’s possible, great!), glide also effects on the external synth … sounds very good!

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Arpeggiator and Sequencer

As mentioned above, sequencer and arpeggiator are simple but brilliant. Two arp-modes are offered: UP and UP/DOWN. What a shame Random – like with those great Roland Jupiter arpeggiators – is not implemented.

But the Pro-One arpeggiator features an interesting extra. Peter Forrest describes it as follows:

“[…] the arpeggiator looks basic at first, with just Up and Up/Down options, but by a bit of dual-purpose switching, SCI have made it very flexible and usable: you can latch notes by holding them down and sliding the sequencer switch to RECORD. (It might mean using your nose, if you want to hold down a lot of notes, but it’s almost worth it.) […] You can then add bits to the arpeggio in real time, which will disappear again when you take your hands off, leaving the original arpeggio still going strong.”

(Peter Forrest, “The A-Z of Analogue Synthesizers Part Two: N-Z”, revised, page 163/164)

The sequencer comprehends two rows (useable separately, not together – it’s a monophonic machine) with a maximum capacity of 40 steps… not too much, I agree… but for simple but effective patterns it’s a perfect tool! Rests may be programmed as well to give you at least some rhythmical possibilities. (Just trigger the Pro-One sequencer by a rhythmical external trigger signal, and you’re the king!).

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Finally – and that’s tremendously important – sequences can be transposed in realtime (not over the whole keyboard length but over two octaves, that’s fine!). Best of all, any other monophonic synth (1V/Oct, sure) can be controlled via the Pro-One sequencer as well. Making music really gets easy as soon as the groovy Pro-One pumpes out some weird FX-noises, while the ARP Odyssey or other synth plays in parallel a nice bass line.


One simple reason the Pro-One is worth every Cent: its strong and versatile musical sound. Great VCOs and a powerful filter (with that clear and effective resonance), fast & percussive envelopes, vast modulation routings, good connection panel – what else do you wish for?

Peter Forrest perfectly describes the instrument:

“[The Pro-One is] simply one of the most powerful and enjoyable monosynths ever made. It’s capable if the full range of the monophonic repertoire, from bizarre twiddlings to fat bass: soundwise it seems to get as much out of the standard (3340/3320/3310) Curtis chips it uses as any non-modular design has ever done. You could play it as your only instrument for years, and still be coming up with new sounds.” (Peter Forrest, “The A-Z of Analogue Synthesizers Part Two: N-Z”, revised, page 163)

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

Pro-One today

It becomes immediately obvious that the Pro-One is the most versatile of all vintage monophonic (non-modular) synthesizers. Not necessarily the best (if there can be such an assessment for any instrument) and not even the most powerful synth. But its flexibility – due to modulation matrix and sequencer / arpeggiator – has not been achieved by any other mono-synth.

For many years, the Pro-One was easily available on the second hand market (just like the Korg MS-20). This situation has changed. It is rarely seen and is becoming increasingly expensive. In 2020 you’d have to calculate 1800 – 2500 Euros/Dollar for a Pro-One in fair condition. However, the price does remain reasonable considering the instrument’s incredible sonic strengths.

Finding vintage synthesizers in reasonable condition at reasonable prices, is getting more and more difficult, I have to admit.

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

On the other hand, many new high-quality analog synthesizers are available on the market today.

For the above-mentioned 1800 – 2500 Euros you could get (more or less):

Or a few DSI Mophos:

“The Mopho is an excellent choice for anyone looking for large, aggressive analog sound combined with extreme flexibility at a very low price. It’s tempting to make the Pro-One comparison, but it’s much more accurate to think of the Mopho as a descendant rather than a reproduction.” (

Finally, the sounds of new analog synthesizers and vintage synths are of pretty much equal quality. And the modern instruments often offer significant advantages, such as additional MIDI or USB.

Sequential Pro-One Synthesizer

Sequential Pro-One Synthesizer

So, there are cheaper alternatives to the Sequential Pro-One, but you may prefer to own a classic – a true legend. 30 minutes of Pro-One sound samples are waiting for you below. Enjoy listening …

Sequential Pro-One

Monophonic Analog Synthesizer

Pro-One TURBO CPU Upgrade (Music Technologies Group)
Kenton MIDI retrofit for Pro-One
DSI Pro 3 / Pro 3 SE

Pro-One – modifications and upgrades (part 1)
Pro-One – modifications and upgrades (part 2)
Pro-One – modifications and upgrades (part 3)

Filed under 2012, Reviews

“Es genügt, einen Ton schön zu spielen” sagte der Komponist Arvo Pärt im Jahre 2005. Diese Aussage ist ebenso einfach wie ich auch exzellent: Es braucht kein Meer an Tönen, denn entscheidend ist der Klang. Dass so mancher Vintage-Synthesizer der 70er und 80er Jahre teils unerreicht hochwertige Klänge liefert, steht außer Frage. Doch tatsächlich leben wir “heute” in einer nahezu perfekten Zeit. Einerseits hat man – mehr oder weniger – noch Zugriff auf die Vintage Analogen, andererseits wird auch bei Neugeräten die wichtige Komponente des hochwertigen Klanges wieder zunehmend berücksichtigt. Doepfer, Cwejman,, MacBeth, Moog, GRP, Studio Electronics, COTK, John Bowen und andere Hersteller bauen hervorragende Synthesizer, die den “Klassikern” in nichts nachstehen. All diesen (alten wie neuen) “großartigen” Instrumenten ist Great Synthesizers gewidmet. _________________________________________________________ In 2005 composer Arvo Pärt said: “Playing one tone really well is enough”. In other words, it is sufficient to play one tone 'beautifully'. I agree with that. All musical efforts are focused on the sound itself. Although I studied classical music (piano and drums), it’s the electronic sound that inspires me. Synthesizers are the epitome of new sounds and exciting tonal spheres. Today, many companies produce high-quality - excellent! - synthesizers: Doepfer, Cwejman, MacBeth, Moog, GRP,, COTK, Studio Electronics, John Bowen and others. It's their products I'm really interested in ... apart from Vintage Synthesizers, which I have been collecting for 20 years. Subsequent to our former websites Bluesynths and Blogasys, Peter Mahr and I have now created GreatSynthesizers. We hope you like it.


  1. There are no real alternatives to the Pro-One.
    I used to have one in the early 2000s – which I exchanged with 2 other synths for my modular system.
    Gosh! I loved the Pro-One and I missed it – but I’ve never regret the exchange.

    If you don’t like double assignments and you prefer one controller for every single parameter – go and get this masterpiece of a synth!

    2 weeks ago I finally got a Pro-One again.
    With this one I’ll be buried…

    Best regards,

  2. Dean Dukes

    How can I gate this wooden cabinet shown here for my Pro One? It looks absolutely awesome!
    I have both Prophet 5 and Pro One and would love to my Pro One look like the one shown here…any info you can provide to me will certainly be appreciated.

    • Theo Bloderer

      Hello Dean!

      Synthwood offers these cabinets. The company is located in Seattle (if I’m right), and its products are just beautiful. I made 4 of those Pro-One “deLuxe” – it was a lot of work (two days for each of them), but it was worth every Cent (and every minute I spent on it). The cabinets come in pieces and need to be put together on your own. They are available in Pacific Northwest Walnut (similar to the Prophet-5) or Figured Maple. Recommendable!



  3. Great article. I have the pro one and want to sync it cheaply. You mentioned a rhythmical external trigger signal. What did you use to make your sound samples? Would a click track sound from a DAW work? Thanks

    • Richard Faith

      You could always sync it via MIDI if you put something like an Arturia Minibrute in between the Pro One and the sequencer. The Minibrute has a CV/Gate output and I believe it will perform the MIDI-to-CV function for you as well as providing another potent analogue monosynth.

      • Richard Faith

        …granted, that isn’t “cheaply” but then you are getting yourself another great little synth too!

        • Theo Bloderer

          … or take the MicroBrute :o) – it’s even cheaper and has that cute patch panel so you can use the Arturia LFO for weird VCF mod on the Pro-One. MIDI sync etc is all the same as with the MiniBrute …

          Anyway, using one of those potent new analog synths for MIDI sync is a brilliant idea!

          • Ersatz Rat

            I recently used a korg volca keys sync out to the gate in of my pro one. Magic!

  4. Do you think that the DSI Mono Evolver Keyboard is the 21st Pro One now that it has been discontinued?

    • Theo Bloderer

      No. I’ve got the DSI Mono Evolver Keyboard. Interesting concept, some decent sounds. But: less-than-average hardware (many of the plastic knobs have problems – why? This instrument is new!), no in-depth analog sounds, complicated to use, and you need sunglasses (the blue lights are very irritating). I don’t believe the DSI MEK will turn out to be a “classic” synthesizer …

  5. Great article!!! Thank you for this goldmine of info about the Pro One! I am the privileged owner of one of this legend and love the sound samples you’ve added here, any chance you’d share some of your patches at all? david

  6. Wow!! I was just emptying my closet to throw things out and I found my Sequential Pro-One in the corner. I always loved it but forgot all about it when life got in the way. It appears to be in excellent shape — just as I left it. Think I’ll be cleaning it up in the next few days!! Your article was awesome!! Thanks for re-inspiring me!! :)

  7. El Cangrejo

    Great read. I’m refurbishing one now and loving every step of it. Solid, large and clean. Everything you want from a mono. I enjoyed your passionate review.

  8. Angelo Gaetano Spicola III

    Can the Pro-One be triggered externally from another synth/sampler via gate/hold/trigger jack w/o Midi (having typical J-wire keybed problems) what’s the Audio IN for?

    • Theo Bloderer

      … sure, the Pro-One can be triggered by any Trigger or Gate-Signal – (analog) drumcomputer (CR-78, TR-808, etc – trig out), trig-out of a step-sequencer, any envelope generator (trig/gate out), possibly even by an external LFO … also by the TRIG Signal of a MIDI-CV/Gate interface, of course.

      The Audio-In is exactly what it says: Audio signal input for sound processing – the audio then being processed by the Pro-One Filter / AMP section …

      Regards …

  9. did anyone listen to the author’s demo?
    it’s like a mini Boards of canada ep
    Great work man
    i have played with a pro one in the past and now lust after one more thank ever
    i already own an array of mono synths but dammit of i need this one….more!
    thanks for this amazing music and the article
    well done M8

  10. Funnily enough Pioneer Dj company have released what people are calling a modern day Pro One. The Pioneer Toraiz AS-1 has all the internal workings of a single voice of the Prophet 6, as confirmed by DSI, yet it has the subtle yet important difference in terms of it being a monophonic synth. It’s actually more rich and powerful than a single Prophet 6 note, and for the price and compact nature it’s an amazing machine.

    It’s got a somewhat more limited set of controls, but it’s actually very straightforward, and like the Pro One it has modulations available in conjunction with a slider that can effect multiple modulations. It has a touch keyboard which allows for unusual performance styles, I find it very fun for doing quick soloing up and down the touchkeys, which i could not do in the same way on a keyboard. It’s very well built and there’s an excellent comparison by InHalt on youtube where he compares it in detail with the Pro One. I listened and preferred the AS-1 hence why I bought it, used for £300. I think the spirit of the Pro One is in the Toraiz AS-1, but it’s a shame that people haven’t caught on, as they tried to market the device to DJ’s whereas i don’t think most DJ’s want such a powerful monosynth, I can’t imagine many of them crying out for that kind of setup. Anyway, I think it’s a fantastic device and it would be worth you reviewing as it’s very unrepresented/dismissed due to it’s labelling.

  11. I bought my first Pro-One in the late eighties. I used it for my first cd. It was not reliable in terms of tuning and broke down after a couple of years. Later I bought a second one with – no working – midi but I sold as I found it no match for my Alesis Andromeda at the time. I still love the sound but in fear of it breaking down I resorted to the Behringer clone, which, in my opinion sounds the same, thought it could have benefitted from a better calibration of vco 1’s frequency pot.

    • Theo Bloderer

      … thanks for the note. Interesting – I had like 5 or 6 Pro-Ones during the last decades (still have 3 of them), and none ever broke down. The VCO octave switches possibly need to be replaced (they are still the original ones), and the keyboard needs some service once in a while. Other than that there aren’t any problems. Possibly it’s the history of each unit that decides whether that specific synth is in good or bad working order. Seems as if you weren’t lucky, in this respect.

      Good to hear you like the Behringer clone. A lot cheaper, brand-new, with a good sound (as you tell us). Great!

  12. Milan Zivadinovic

    Hello Theo,
    I was just wondering how does the Pro One compare to the 1981 Octave Cat SRM2 and his mixable waveforms, double subs, vco1 and vco2 cross mod, loopable ADSR, S+H with vco1 or noise input and also CEM3340 vcos ?
    What would be your own preference and why ;-)
    All the best and greetings from France.

    • Theo Bloderer

      Hello Milan

      Oh oh oh oh … Octave Cat SRM2 is … gorgeous, amazing, from-outer-space, fantastic.

      Not sure if it’s possible to compare The Cat and the Pro-One. The latter is a powerful, universal allrounder, a synth “for-your-entire-lifetime”, a performance machine with mod matrix and step sequencer. Reliable, fun, instant-performance with first-rate, high-quality analog sounds. This is the Pro-One.

      The Cat SRM2 is magic, but in a different way. It misses sequencer/arpeggiator (sadly), no wheels, just the simple pitch-slider … the instrument is nowhere as good for instant performance as the Pro-One. Aside from that, it has a tremendously attractive sound, it is one of the rare machines that produces some sort of transistor-like, extra-warm analog sound (despite the CEM chips its based on), a refreshing sound you don’t come across too often.

      As a near-to-perfect allround-synth, the Pro-One is probably the one to go for. But if it was for lead-lines and basses with that extra “character” (that only few synthesizers have), I’d go for The Cat SRM2.

      Best if you have both (o:

  13. Milan Zivadinovic

    Hello Theo,
    Now that’s an awsome answer, thank you so much !
    I guess the Ody would have been a more valid comparison for the Cat.
    I enjoy reading your detailed reviews and listening to the very musical demos from the beginning, such an invaluable ressource for classic synth lovers.
    I’m more into fx sounds, audio rate mods, ring mod, s+h, full range pitch osc, so more Odyssey than Mini. But not so much into patching cables, I prefer integrated synths.
    Even then, so many choices, both old and new.
    The sky is not the limit, but space, time and money most definitly are ;-)
    Keep up with this fabulous site !
    All the best.

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