Sequential Circuits Pro One

The sequential circuits manufactured Pro One was designed to be a monophonic version of the prophet five [which was five note polyphonic, there’s always a reason for a name!], it was designed and built after the prophet five which was a more unusual route than the more normal building if a monophonic synthesiser and them producing a polyphonic version of the keyboard. The Pro One came out in 1981 as opposed to the 1978 release of the polyphonic prophet 5 and as a result ended up using different parts to the original prophet 5, the solid state music devices circuits of the original prophet five were redesigned to use the CEM Curtis Electromusic range as a result in my opinion the Pro One didn’t sound like a monophonic prophet five, instead it sounded like itself if you know what i mean. I wanted to get a synthesiser after my experiments with building them myself from kits and magazines, i guess to see if they sounded better than the ones i had built. I set my target on the moog prodigy and began earning money prestigiously to secure the synthesiser as i got closer to the target amount of money that i needed [this took ages due to the amount of money involved]

I had plenty of time to read every review and comment that was ever published about the Moog Prodigy and just about every synthesiser that was available at the time, i began to prefer the Pro One instead, although it was more money i decided to pursue this instead. The modulation routing, the sequencer and Vince Clarke using it with Yazoo swung me!

Upon purchasing the Pro One and getting it home i eagerly plugged it into the mains and my hi fi amplifier, because of the amount of time i had to save the required money and hence the time reading about it i practically knew how to operate it already! The Pro One had a three octave keyboard with a modulation wheel and a pitch bend wheel to the left of it. it was a versatile two oscillator synthesiser, the versatility was basically down to the modulation matrix but well get to that later, the oscillators were called oscillator a and b. Oscillator a had a variable frequency control and a four octave switch adjacent to it. It then had two slide switches to output the waveform. The first was for sawtooth and the second was a square wave, being slide switches meant that you could have them both on simultaneously, one thing you couldn’t do with synthesisers that used a rotary switch to select the waveforms. The next control was the width control to alter the pulse width of the afore mentioned square wave. After this came the sync switch which rather unsurprisingly synchronised the pitch of the two oscillators together giving you the option of producing much more harmonically striking waveforms!

Oscillator b had similar controls except it had an extra waveform switch for its ability to produce a triangle waveform and it had no synchronisation switch. Instead it had two slide switches, the first switched the oscillator from normal frequency to low frequency to basically use it as a low frequency oscillator and to aid that use the next slide switch along disconnects the pitch of the oscillator from the keyboard as normally the frequency of the LFO is not normally related to the keyboard pitch [although with the aid of this switch it now can be!]

Apart from the two oscillators there is also a noise generator [there is also a fourth sound generator mentioned later] this comes into play in the mixer section, three controls enable you to mix the volumes of the three sound sources, oscillator a oscillator b and the noise level, the noise level control also doubles to introduce an external sound source, if you plug one into the external input on the rear panel this over rides the noise generator and mixes in the external input.

The output of the mixer section is fed into the filter section; this was a 24 dB’s per octave low pass filter. It had the usual cut-off and resonance controls with an envelope amount modulation control and a keyboard level control, this alters the amount that the filter tracks the keyboard, i found at about the setting 7 on the control knob this tracked to make it harmonically track so that the sound filtered similarly over the keyboard, the best explanation of this if you haven’t heard the effect is that you can turn it up to max to make the higher notes much more harmonically brighter and subsequently turning the effect down produces sounds the become harmonically “ duller” the higher you play up the keyboard. Underneath these four control knobs are the traditional four envelope controls for the filter of ADSR. By turning the resonance control up to maximum you could get the filter to oscillate and hence then become the fourth sound generator. This produced quite a powerful sine wave and the envelope generator when turned up to maximum modulation could sweep it over its entire audio range and with minimum attack and a very short delay you could produce the classic Kraftwerk drum sounds.

The amplifier section is next again with the same four ADSR controls for the traditional volume contour

The LFO also doubles as a clock for the sequence and arpeggiators of the synthesiser, the first control is the frequency with the next three slide switches providing ramp, triangle and square wave in similar fashion to oscillator b and also similar in that they can all be selected together.

There is also a glide control which when set to zero has no effect. Turning the control up introduces the effect in that it controls the rate of the glide effect. That is if the next slide switch next to it is set to normal. If it is set to its other option, auto it has no effect unless you keep hold of the original note, when the then press the next one the itch glides to it at the set rate, this can produce some quite clever effects as you play and then suddenly sweep pitches!

Ok now to the aforementioned modulation matrix [I’ve been putting the explanation off wondering if i can put it adequately into words to explain it properly!] here goes! There are three modulation sources of the modulation matrix. They are the filter envelope, oscillator b and the low frequency oscillator. Each of these has an amount control and a slide switch to select between direct and wheel. Let’s assume that they are all set to “direct” for now.

There are then five modulation “paths”; these are oscillator a frequency, oscillator a pulse width, oscillator b frequency, oscillator b pulse width and the filter. Each of these has a three way slide switch that selects between wheel, direct and off.

Right then let’s try to explain how this all works together, we have all the three modulation sources set to direct, turn all the amount controls down to zero and  switch all the modulation paths to their central off position except oscillator a frequency. Now if you turn up the amount control from the LFO it will modulate the frequency of oscillator a. If you turn up the amount control from filter envelope it will be mixed in with the LFOs modulation of oscillators a frequency! You could also turn up the third amount control from oscillator bus output and mix in that as well so you could have all three sources modulating the frequency of oscillator a.

If you now turn everything back to how it was with all the amounts set to zero but switch the LFO switch to wheel instead of direct and turn the LFO amount control up nothing will happen, now move the modulation wheel and you will hear the effect. So you can route through the modulation wheel to let that change the depth as well.

By using the sources and routing with the ability to mix and split gave you a whole host of possibilities when it came to patching and it was almost as versatile as a small modular set up. Some of my favourite tricks were to use the modulation envelope that you would usually use to alter the cut-off of the filter to modulate the pulse widths of one or both of the oscillators. This would alter it dynamically throughout the length of the note and by adjusting the starting points using the manual controls which both oscillators possessed you could produce an effect that i particularly liked and found useful  and was more subtle than the more normal using the LFOs to modulate the pulse width.

By setting oscillator a to synchronise its pitch to oscillator b [unsurprisingly using the sync slide switch] you could then route the envelope modulation to oscillator a frequency and this would automatically sweep the harmonics through the as you held and released the notes, because of the modulation matrix you could also alter the amount and feed it to the pulse width as in the above trick at the same time, you can see why I compare this to an almost modular way of connecting the synthesiser together.


The Pro One also had an arpeggiators which was pretty standard at the time, notes held down were played, up, down or up and down the keyboard, at a speed set by the LFO or an external trigger would override the LFO if connected. To be fair this was something that I didn’t find much of a use for because of the more interesting sequencer feature which for me seemed to do all that and more! You had two sequences to program. A and b each could contain 40 note and could be switched between and by sliding the record switch to play and back to record a rest could be programmed. You could do a fair amount with forty notes and not only did it also work like the arpeggiators in that you could trigger it from an external trigger [which meant for excellent use with the trigger out from a drum machine] you could also transpose the sequences in real time by holding down the notes on the keyboard, which could lead to some excellent ideas!

Incidentally the album “upstairs at Eric’s” by Yazoo uses the Pro One for most of the album and when I say uses and Vince Clarke is involved the word takes on almost different concept as the guy manages to extract the maximum of brilliance to about everything he did at that time.

Using oscillator b to modulate while it was set to audio frequencies and linked to keyboard control was also something that i found to produce more unusual and still useful effects especially when modulating the pitch of oscillator a. Metallic and strange harmonic effects could be produced and you could also mix in some envelope modulation and send some to the PWM again.