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RSF Kobol Expander

Anyone who read the article on Depeche Mode in the May 1982 issue of E&MM will no doubt be familiar with the item under review this month. The RSF Kobol is a synthesiser expander and is meant to accompany another synth that has a keyboard. Alternatively it could be used with a pitch-to-voltage converter for use with, say, a guitar, or it could be used with a sequencer or computer controller. It has the usual complement of oscillators and filters found on commonly available synthesisers, but the feature that puts this particular item into a class of its own is the fact that practically every control on the front panel can be regulated by an external control voltage.

There are two rows of standard jack sockets at the top and bottom of the unit; these are inputs and outputs to and from the various control functions. These were originally intended so that sounds could be stored by a programmer made by RSF to accompany the Expander. This programmer unit stores preset control voltages in its computer memory and these can be recalled by selection of a particular memory, in much the same way as sounds are stored in a Prophet 5 or Jupiter 8. There is a space on the rear of the unit presumably to house a multipin socket to interface these functions to the programmer. These jack sockets also serve a double purpose in that they transform the Expander into a very versatile mini modular synthesiser.

Voltage Controlled Oscillators

The RSF has two VCO's which range from 10Hz to 10kHz (expandable by voltage control). There are control voltage inputs for external control of pitch and VCO level, and also separate outputs from each VCO. What makes the VCO's unusual is the waveform selection control; instead of the usual switched waveform selector found on most synths the controls on the RSF are totally variable between the waveforms. The waveforms provided are, rotating the control clockwise, triangle, rising sawtooth, square and pulse. By setting the control midway between any two waveforms shown on the panel interesting mixtures can be obtained and many unusual sounds are available, this control is also voltage controllable which means that you can automatically sweep through the waveforms by applying the output from one of the envelope generators or the low frequency oscillator to the waveform control input.

The two VCO's are identical and provide a wide range of sounds even without the aid of filtering. The volume of the individual oscillators is controlled by a voltage controlled amplifier (VCA) for separate voltage control of the level of each VCO and there is also a switched hard sync facility so, as you can see, there are endless possibilities for the creation of unique waveforms — far more than any other analogue synthesiser currently available.

Voltage Controlled Filter

This is a standard 24dB/octave voltage controlled lowpass filter. There are four controls associated with the filter which are Cut-off Frequency, Resonance, ADS 2 Amount and Keyboard Amount. There are also inputs for external voltage control of each of these controls and also an audio input for processing an external signal. A versatile but fairly straightforward filter.

Envelope Generators

There are two envelope generators on the RSF, one for the filter and one for the final VCA. They require a positive going V-trigger to gate them on and off. This makes them compatible with ARP, Roland, Sequential Circuits and other synths but you will have trouble interfacing the RSF with Moog, Korg and Yamaha because they use different triggering systems. The envelope generators operate in much the same way as Moog contour generators in that the final release time is linked to the initial decay control and is switched in or out. Those of you who have a Minimoog or Prodigy will be familiar with this mode of operation. The range of control with the attack and decay/release times is very wide, from 1ms to over 20 seconds. Once again, there are control inputs which allow you to vary the attack and decay/release times and sustain level with an external CV. By applying the keyboard CV to, say, the decay input, low notes will have a shorter decay than high notes. By routing the keyboard CV via the inverter in the voltage processor, low notes will have a long decay which will become progressively shorter the higher up the keyboard you play thereby simulating acoustic instruments more closely. Of course, it is possible to control all these parameters for an even wider range of articulations.

Low Frequency Oscillator etc.

These modules are fairly straightforward. The LFO has triangle and square wave outputs for vibrato effects, trills, sweeps, and so on, but this can be slightly limiting, and it would be better to have made a wider range of control waveforms available, preferably via a variable control like on the VCO's. The rate of modulation is very wide indeed, ranging from 0.01Hz to 100Hz and this rate is also voltage controllable. The noise generator outputs white or pink noise which is selectable by an internal switch — again, a variable control would be better. The output is very high and can also be used as a control source.

Finally, we come to the voltage processor. This is a really useful device which is capable of boosting voltages by up to x4, and there are normal and inverted outputs.


The RSF Kobol is a pleasure to use, requiring only two interface leads for basic operation; one for CV which is internally routed to both VCO's and the VCF, and one for the envelope gating. It is ideally suited for use, for example, with an ARP 2600 and it also works well with Roland equipment. The module measures 19" x 7" x 4" and so could be mounted in a 19" rack mount along with other units if so required. At a cost of only £350 plus VAT it will open up the exciting world of modular synthesis to musicians without a vast capital outlay. For the more adventurous, the possibilities of using the RSF (or a few of them) with a computer controller such as the Roland MC4 are very exciting as you could assign the computer's multiple control voltage outputs to the various inputs on the module(s) and program some fascinating patterns wherein the structure of the sound could vary with the music. As previously mentioned, RSF also make a programmer, and also another expander module all of which could be used together to create a very versatile system indeed.

It is a great pleasure to use a high quality product that doesn't originate in the Orient but just across the water in France. The RSF is capable of producing some unique and powerful sounds and is sure to appeal to anyone interested in the more creative aspects of synthesis.

The RSF Kobol Expander Module is available from Syco Systems, (Contact Details).

Previous Article in this issue

Ibanez Pedals

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Francis Monkman

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


Electronics & Music Maker - Mar 1983


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Gear in this article:

Synthesiser Module > RSF > Kobol Expander

Review by Steve Howell

Previous article in this issue:

> Ibanez Pedals

Next article in this issue:

> Francis Monkman

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