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SCI Pro-FX 500

The Pro-FX 500 from Sequential Circuits is a fully programmable, microprocessor-based, modular signal processing system. It consists of a mainframe 19" x 3U high, rack-mounting chassis containing the System Controller and space for up to six effects modules. It is capable of storing the control settings for every knob and switch as a 'program' in memory. Up to 64 such programs (eight banks of eight programs each) can be retained in non-volatile memory, as well as 64 user-created sequences.

Up to 30 effects modules can be controlled by one System Controller alone which only remembers and reprograms each module according to their slot location within the rack. All interconnections of modules are done using mono jack leads, and since modules only have one output you require 'Y' cords to patch one module to two destinations. Modules can be removed as long as their cage (chassis) slot remains vacant.

The System Controller front panel contains eight Program Select black pushbuttons below which is a two digit LED display. This shows the program number selected eg. 34, which describes bank 3, program 4 in memory.

The Pro-FX has three modes of operation: (1) Manual (2) Preset and (3) Edit; these shall now be described.


On power up the Controller defaults into Preset mode and the effects are programmed for the default (11) setting. Pressing the 'Preset' button to the left of the LED display once, disengages this mode returning all control settings to their current front panel status. In Preset mode the knobs do not indicate their value, so switching to Manual can be hazardous if you have a level control turned up full, for example.


The Pro-FX 500 stores settings of every front panel control as one 'program'. You can create Presets by setting your patch up in the Manual mode and recording them into the memory; or by editing an existing Preset, then recording your new version into the same location or a different one.

64 programs are available as eight banks of eight programs. To select one of them you press a combination of the Program Select buttons. The programs run from 11 to 18, 21 to 28 etc.

To record a program you first need to set the back panel slider to Enable, then press the red Record button — it should light. Select the location you wish to record in eg. 74, and it's recorded. You verify your program is safe by hitting 'Preset' and select 74. Switching back the slider to Disable prevents erasure of your data — it's ideal for live use where you don't want people fiddling with your control settings.


Any program can be selectively edited in Preset mode by adjusting any front panel controls, which remain 'active' all the time. If you get it wrong, pressing the Preset program number again restores the original values, as they weren't recorded.

If you move a control in Edit mode nothing happens. Edit is only activated by turning the control until it reaches the programmed knob setting, when the Preset button LED will blink and Edit LED lights up. That control only, is now in Edit mode. The idea is that slowly turning a control until the LED lights will then show you at what position the previous programmed control was left. Simply press 'Preset' to return to start if you turn the knob too far. To store that edited program, press Record plus the program number!


Up to 64 sequences of program changes can be recorded then played back, advancing them one at a time via a footswitch or sync pulse from tape — superb for mixing, on stage or even in the studio. These sequences can be programmed in any order and then looped automatically to cycle through the changes. All sequences are saved and loaded along with the programs through the tape interface.

To record a sequence you need to connect the footswitch to the socket marked FTSW 2 on the back panel, then press and hold it down. Enter the required sequence number (the SEQ display will light), press the Record button and select the numbers of the programs, one after the other, and release the footswitch.

To playback that sequence you press and hold the footswitch, select the sequence number (display lights up) and release the switch. Now every time the footswitch is depressed the program will advance. A clever feature, enables you to manually select another program at anytime from the front panel without destroying the program running order.

It is, however, impossible to erase sequences; you have to overwrite a long sequence with a single program sequence in order to free extra program areas. If the sequence memory is full to capacity then the display will read 'FF' and no more sequences can be constructed.

The Pro-FX System is an extremely easy unit to get on with. There's even a self-diagnostic memory test procedure available if the System Controller refuses to obey commands, which makes troubleshooting so much more simple a task.

Rear Panel

This features a CV In socket. Connecting an external control voltage to this lets you programme those module functions having 'External' front panel switches. The Data jack socket is used to send and receive data between several 'slaved' System Controllers, whilst the 24 pin connector 'daisy-chains' the expansion cages of modules, so that they can all be controlled by the one overall System Controller. Lastly, there are the Record Enable/Disable switch and the XLR (FTSW 1) socket. The former allows you to write to the Controller memory locations or not, whilst the latter socket is for future connection to an 'intelligent' footswitch that will incorporate an LCD display and give access to programs.

Tape Interface

On the rear of the back panel are the To Tape and From Tape jack sockets which are used to store program/sequence data onto cassette or tape, thus allowing a library of effects patches to be maintained. These sockets also read sync pulses in/out from tape.

It's advisable to get into the habit of dumping all important programs and sequences to tape as soon as is convenient, thus safeguarding against computer failure. Tape sync pulses are designed to be recorded onto one track of a multitrack machine using the To Tape socket fed to the Mic or Line input of the recorder, and recorder output fed into the From Tape socket. Then, every time a program is advanced (either manually or automatically via the footswitch) a synctone is output to the tape recorder. However, this is not the case when reading pulses off tape; if it were, you'd end up with another (unwanted) pulse being recorded back onto the multitrack.

Right hand panel modules.


Now we shall take a look at the range of effects modules available for the Pro-FX.

Digital Delay

This unit offers a delay range of 0 to 1 second at 15 kHz bandwidth, or 666 milliseconds to 2 seconds at 7 kHz. It occupies three times the width of the usual module and has a 4 digit LED display for Delay Time. To the left of this are In and Out, 5 segment horizontal LED bargraphs giving indications of the system headroom.

Both input and output Levels are provided with a dry/delay Mix control for setting the balance of effect to original signal.

The Delay section has controls for Fine and Coarse tuning of the delay time, the latter selecting the delay in a 15 step range, whilst Fine is used to set precise times. Fine is adjustable over a 3:1 range set by the Coarse value. These control knobs were fairly jerky to use and prevented really accurate setting of a delay. Pressing in the External switch below the Fine knob effectively halved the overall delay time.

Regeneration Level sets the amount of signal being fed back into memory, resulting in multiple echoes at long delay and hard reverberation on faster delays. A novel feature is the inclusion of 24dB/octave high and low pass filters on the regeneration circuit which can be used to filter the echo/reverberation effects, creating the feel of various size rooms.

The modulation section is used to produce flanging, vibrato and chorus-type effects and has Rate, Depth and Shape controls. Depth is variable from 0 to 4:1 sweep of the delay time, Rate from 0.03 to 18Hz (a nice range), whilst Shape sets the type of modulation in use. Sine, square and envelope follower are all possible. Sine produces a very good chorus, and some weird vocal treatments, not too unlike a vocoder, can be produced on envelope follower with Depth and Rate near full.

The rear panel features a Repeat footswitch jack, which is extremely useful; an external modulation input, and Regeneration Input/Output sockets to allow external signal processors to be inserted in the Regeneration loop.

The sound quality from the unit is high; as expected from a 14-bit device. When you consider that every control is programmable and delay 'patches' can be stored and saved on cassette, you begin to realise how powerful this unit is and how useful it is. RRP £895 inc. VAT.

Parametric Equaliser

One of the best modules, having a comprehensive selection of dual controls that caters for almost all equalisation requirements. There are two overlapping frequency bands each with EQ cut and boost of 16dB, a bandwidth control adjustable from one-sixth to 2 octaves range and centre frequency can be adjusted from 20Hz-2kHz (Low Frequency) and 200Hz-20kHz (High Frequency). An overall Level cuts the input signal to prevent distortion, which is easy to produce when cutting or boosting levels by excessive amounts, and the peak LED indicator should prove helpful on this count.

LED switches change the high frequency band shape to a shelving response which gives a frequency cut-off on one side of the band only, whilst the HF/Filt switch converts the HF band into a tunable low-pass filter for removing top end hiss. In this mode, the bandwidth control functions as a resonance adjustment.

Using the CV input facility on this module lets you control the LF band externally from a synth to produce swept bandpass 'wah-wah' effects which were particularly appealing on a direct-injected bass guitar signal, for example.

Setting the exact amount of equalisation is always quite difficult, but one way of simplifying matters is to continually switch the EQ from effect to bypass after each adjustment in order to hear the difference. All in all a very versatile module that would provide a powerful EQ system for live work or even the studio. RRP £383 inc. VAT.


This module provides synthesiser effects and is designed for guitars/basses. It will only accept a monophonic input signal, but by using the controls it can produce outputs one octave above and either 1 or 2 octaves below the original. The switch labelled 'Lower Fuzz' is used to boost the extremely low frequencies through distortion, which introduces an upper harmonic content boost to add a bit more definition to the bottom end.

A dual dry/effect control lets you mix the relative balance between the various signals. The upper voice can be selected to track the original at any interval between a -1 octave/+2 octave range when voltage controlled via the rear panel CV input, or made to sweep up and down in frequency. This was excellent on lead guitar, and utilising the upper sync LED switch, the upper voice could be 'hard synced' to the input producing timbral modulation effects that totally transformed vocals and virtually turned a rather thin Casio MT-30 into a Prophet 5! If this was polyphonic it would be tremendous. RRP £395 inc. VAT.


Four rotary controls provide adjustments to Tone, amount of Distortion, input Level and Sustain. The latter controls a built-in compressor which can be used to obtain long, sustained guitar lines without the need to resort to distortion if required.

The Distortion is variable in depth and produces a smooth overdrive sound when set below halfway, and a warm raspy effect on full. External voltage control is possible of both Level and Distortion, but the CV input signal is the same for both parameters, so you need to alter the panel functions to vary the relative depths of each. Considering the price of this module it is the most disappointing with a limited range of sounds on its own. RRP £228 inc. VAT.


The unit utilises a 6-spring delay line, built into a 1U high, black 19" rackmounting case, with active limiting to help overcome the usual springline side-effects on transient material. The actual module houses controls for dry/effect Mix and there's a three band EQ which can be used to tune the reverb signal bandwidth to create different 'acoustical environments'. This has controls for Low Pass (200 Hz - 20 kHz) and High Pass filters (20 Hz - 2 kHz) and a Mid tone control whose frequency could be varied as well as the amount of signal cut and boost. Whatever you do don't mount the reverb on top of the System Controller as this tends to result in excessive 'hum'. A fairly versatile module. RRP £315 inc. VAT.

4x2 Mixer

Each of the four programmable inputs have both Pan and Level controls with a peak LED indicator to help set mixing levels. The input is panned between outputs A and B which are available at the rear panel ¼" jack sockets. Also on the rear are two A and B input sockets which permit several 4x2 Mixers to be cascaded to increase the channel inputs. This module has no in/out switch, since it is the one unit that you'll always have need to use and so it's left permanently in circuit once patched.

An interesting use of the unit is to provide a programmable level control for an external delay, for example, by patching the delay into a mixer channel and varying the mixer's Level control (and effectively altering that of the delay!). A basic module this one. RRP £290 inc. VAT.

Phase Shifter

Six 180° phase-shift networks are employed to produce three frequency notches which, when combined with the original input, produce phasing. A triangle wave LFO is used to sweep the notches up and down the frequency band set by the Range control. Regeneration is used to vary the intensity of the phasing, and controls are available for Speed and Depth of modulation which are indicated by a pulsing LED. Selecting Vibrato defeats the 'dry' signal completely to give a pleasant effect that resembles a Leslie speaker on faster Speed settings.

A built-in noise gate helps clean up the background modulation noise leaving a clean signal. Voltage control of both Speed and Range is possible on this unit which can lead to some interesting treatments. RRP £290 inc. VAT.


A stereo module offering delay times ranging from 0.5mS to 38mS, Regeneration and four sources of modulation: LFO triangle waveshape, LFO sine, Envelope Follower and an External control voltage. The modulation sweep ranges from 0.05 Hz to 10 Hz and LED indicators display the signal peaks, depth and rate of modulation.

Individual output levels for dry and effect help set up a wide range of flanging effects, and the Effect Invert pushbutton produces a 'hollow' colouration. The 12 kHz bandwidth ensures a bright, clear sound and a very rich chorus. RRP £448 inc. VAT.


The System Controller is a well designed, easy to operate device. Programmability is without doubt the plus factor of the whole Pro-FX system, and tied in with the modular format, makes this package highly desirable. The major hurdle is that of price. At £771 inc. VAT the System Controller isn't exactly budget-priced, and neither are the majority of the effects modules. As stand alone units (if that was possible) they lack in sound quality slightly in comparison to similarly priced devices. All said and done though, you are paying for the exclusivity of having the programmable function. Is it worth it? Well, that's for you to decide.

The Pro-FX 500 package is distributed in the UK by HHB Hire & Sales Ltd., (Contact Details).

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Sep 1983

Gear in this article:

Studio FX > Sequential Circuits > Pro-FX

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Review by Ian Gilby

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