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Cheetah MS6

Analogue Synth Module (Retro)
Published April 2001

Cheetah MS6

British computer peripherals manufacturer Cheetah ventured into the hi‑tech music jungle in the 1980s and went down in budget gear history. David Harman goes on a synth safari with their MS6 analogue rack synth.

To many British keyboard players and home studio owners, the arrival of Cheetah on the music scene in the late 1980s seemed too good to be true. "Affordable Excellence", proclaimed their logo. Well, "affordable", certainly. UK manufacturing and generally sensible component choices meant that British users got some great kit for low prices. As for "Excellence", this is always in the eye of the beholder — but I'd certainly let them have "Innovation".

Child of The '80s

The author's MS6 has been accused of resembling a "rackmount alarm clock". Looks like someone's late for work then...The author's MS6 has been accused of resembling a "rackmount alarm clock". Looks like someone's late for work then...

The late 1980s was a time of great change in the synthesizer market, as digital synths established their dominance. However, the launch of Oberheim's Matrix 1000 analogue module in 1988 showed that at least one major manufacturer still felt there was a demand for good old analogue, but in an easily portable rackmount form. The thought must have been in the air, because it had also occurred to Cheetah, and their MS6 was released at almost exactly the same time.

Physically very similar to the Matrix 1000, the MS6 is housed in a 1U rackmount chassis, the front panel sporting a power switch, 12 editing switches, a four‑character, seven‑segment LED display, and some decidedly British‑looking early '80s‑style artwork. Appearance was never really a strong point of the MS6. The one in my rack has been variously accused of being a cheap synchroniser, a MIDI router, and a rackmount alarm clock. That last one seemed a bit harsh, but I have to admit that with the display showing 1 47 it does have that look about it...

The Engine

Cheetah gear wasn't known for its stylish looks — check out the MD16 drum machine and its rackmount version pictured here — but the price and facilities made up for any cosmetic shortcomings.Cheetah gear wasn't known for its stylish looks — check out the MD16 drum machine and its rackmount version pictured here — but the price and facilities made up for any cosmetic shortcomings.

It's what's inside that counts, though — especially when it's this interesting. The MS6's sound engine has a classic two‑oscillator structure, each oscillator (DCO, or Digitally Controlled Oscillator, in MS6‑speak) offering square, triangle and sawtooth waveshapes, and the second oscillator also featuring a noise option. The DCOs both have coarse‑tuning controls, and DCO B can be fine‑tuned, too. DCO A may also be synced to DCO B for creating sharp sync basses and leads.

Pulse width is independently adjustable for each DCO, and can be modulated via a dedicated 'DCO PWM Rate' parameter. This means that you don't have to tie up the main LFO for pulse‑width modulation, leaving that free for pitch or filter duties. Another nice touch is that the synth's envelopes can be used to control the mix between DCO A and B.

In all, I'd say the MS6's source section is versatile and varied. But its 24dB/octave low‑pass filters, offering cutoff‑frequency and resonance parameters, with LFO, envelopes, key follow and aftertouch selectable as modulation sources, are really something special, as befits an instrument with Curtis chips at its heart.

If the name Curtis rings a bell, it's because Curtis chips are also used in Oberheim synths, and Sequential instruments including the post‑1980 Prophet 5s. The Curtis package used here is the CEM3396, essentially a synth‑on‑a‑chip, as found in the Oberheim Matrix 6/6R and Matrix 1000. This replaces the independent Curtis VCO, VCF and EG chips, employed in more expensive polysynths, with a single chip, resulting in a much more cost‑effective arrangement that still carries that trademark Curtis sound.

Bringing up the rear of the synthesis parameters is a pair of ADSR (Attack, Decay, Sustain, Release) envelopes. I find it refreshing to go back to envelopes that have these four basic parameters, after enduring the mental torture of the Manhattan skyline‑shaped envelopes available on many of today's synthesizers. The MS6's envelope shapes may be simple, but I think they're all the better for it.

All the sound‑creating and sound‑shaping elements mentioned above go together to make a Tone — a single sound played up to six‑note polyphonically. The MS6 comes with 320 ROM Tones, arranged in five banks of 64 each and covering all the sound classification areas you'd expect.

Not A Jack Of Every Trade

Cheetah MS6

Sonically, the MS6 can turn its hand to just about anything the analogue enthusiast might want to hear: warm, lush strings; detuned PWM pads; hollow voices; wet, acid squelching; and rock‑solid basses. Though the synth's modulation options may lack the complexity of Oberheim–style matrix cross‑modulation or the Prophet's Poly‑Mod capabilities, with judicious noodling the MS6 can be made to produce some fairly complex timbres. For those who are interested, yes, the filter will self‑oscillate at extreme settings, but one annoyance is that there is apparently no way of turning both the oscillators off, so producing lunatic howling is not as easy as it should be.

You may have noticed my use of the term "just about anything" when I described the breadth of the MS6's talents a few lines ago. Although it's very Oberheim‑sounding (particularly reminiscent, perhaps unsurprisingly, of the Matrix 6) and capable of generating timbres very close to the more modest Prophet patches, there are some limitations to its sound. I find that it lacks top‑end aggression, and doesn't cut through a mix too well. This tends to preclude it from searing lead duties and trouser‑flapping basses (although these can be achieved by layering sounds internally, as described later). MS6 Tones almost always mix and layer well with those of other synths, particularly analogue monosynths, but where the Cheetah really shines is in laying down a warm backing pad that seems to fit immediately into a mix with little or no external processing.

Taking Control

The MK5 and its relatives, such as the MK5V pictured below, made buying a controller keyboard a realistic proposition even for musicians on a budget.The MK5 and its relatives, such as the MK5V pictured below, made buying a controller keyboard a realistic proposition even for musicians on a budget.

So far, the synth I've described is pretty much on a par with the Oberheim Matrix 1000 — a 1U analogue rack boasting some great sounds and capable of producing many more.

But here come the differences. First, it's possible to edit all the MS6's synthesis parameters using just the front panel — something you can't do with a Matrix 1000. No computer software or external programmer required here (although SysEx is supported, for use with generic librarians/editors).

Parameters are accessed by an editing system that may at first seem arcane, but very quickly becomes second nature. Pressing the Edit switch puts the MS6 into Tone Edit mode (or Performance Edit mode if a Performance is selected — more in a moment). Two numbers are now shown in the four‑character display. The one on the left is the two‑digit reference number of the current parameter, while the right‑hand one is the current value of that parameter. A new parameter number can be selected with the eight front‑panel numeric switches, while the parameter value can be edited with the increment/decrement buttons.

The tricky part is remembering the parameter numbers and their associated ranges. Helpfully, Cheetah decided to print the entire parameter list on the top panel of the unit. This initially seems a tremendous idea, particularly if you're picking one up, second‑hand, that has long since parted company with its manual. But when you slide your new beastie into the already crammed studio rack, you notice a minor flaw in the plan: you can't see the top of a rackmount unit when it's in a rack. Still, at least all the parameters are there, and I should think a good number of second‑hand MS6s have been slapped feet‑upwards onto the glass of a photocopier. (See 'Second‑Hand News' box for information on getting hold of a complete manual.)

Where the MS6 seriously pulls away from the Matrix 1000, though, can be summed up in one word: multitimbrality. As well as its 96 Tone memory locations, called Banks 6 and 7, it has a further 64 Performance memory locations. Into each of these may be saved a six‑Tone setup, each Tone (or Instrument) with its own MIDI channel and Top and Bottom note limit for zoning.

Although it's great for basic sequencing, this mode also suggests other creative applications; for example, assigning all six Instruments to use just one voice and the same MIDI channel results in a stacked effect, which certainly goes some way to improving the MS6's ability to cut through a mix. Unfortunately, it's not possible to detune individual Instruments, so the resulting sound is a bit 'thick'. The manual suggests the following method to get around this problem: save six instances of the same sound in different memory locations, each with slightly different tuning, then layer these six for a nice fat sound. The method does work, of course, and makes it possible to achieve some fairly seismic results, but it's not quick to do. I certainly miss the quick A/B comparison that a proper dedicated Stack or Unison function would provide.

Too Good To Be True?

Before too many champagne corks are popped, and everyone flips to the back pages to inundate the one poor soul who is advertising an MS6 this month, the synth does have a few problems that are worth pointing out.

First and foremost, the rear panel of the MS6, along with its MIDI In/Out/Thru sockets, features just a single mono audio output. I imagine cost was the reason for this stinginess, but it does rather take the shine off fully exploiting the MS6's sonic potential. Admittedly, most polysynths only have a single output, but it's a problem on the MS6 because it limits the usability of its multitimbral modes.

In practice, I tend to use my MS6 mostly for fast, arpeggiated synth lines that intertwine, and I find careful placement in the mix and a send to an appropriately timed stereo delay, with a little feedback, produces a very 'wide' sound from just its mono output. Otherwise it's used for general 'fill' pads and doubling bass lines, usually in monotimbral mode. A note for arpeggiator and bassline nuts here: the factory MS6 bottoms out at C0, but it looks as though this will be addressed in a future third‑party firmware update (see 'On The Up' box).

Another curiosity is that sounds from Bank 7 (the second user bank) cannot be included in Performances at all, which really makes that bank only useful as a scratchpad Tone‑editing memory vault.

The final problem with multitimbral mode is the assignment of voices to the six available Instruments. This is fixed at the Performance level and is not, therefore, dynamic. So if you have a string sound, a bass and a lead, you can have one voice each for the bass and lead and four for the strings, but even if the strings aren't playing at a particular point, the lead part can never play additional notes.

On which note, the MS6 responds to Program Changes 0‑127, but does not support Banks. So you can recall the 128 patches in either banks 1‑2, 3‑4, 5‑6 or 7‑8, but in order to recall Tones from a specific bank the MS6 must first be set to a memory location within the appropriate Bank pair. It is also not possible to send Program changes on specific MIDI channels to change Instruments within a Performance — the MS6 will simply select another Performance. Some of these problems may be eliminated when third‑party firmware upgrades become available (see 'Taking It Further' box.)


The MS6 makes an excellent addition to any synth arsenal, whether it's mainly analogue or mainly digital. Current and future third‑party upgrade possibilities are encouraging, and the support that this synth receives so far into its life is a testament to its charm and capabilities.

The MS6 makes an excellent starting point for anyone interested in experimenting with polyphonic subtractive synthesis for the first time, at minimal cost. Although the parameter‑access concept is less intuitive than dedicated controls, the nomenclature used throughout (oscillators, filters, ADSR) is very standard, and the results yielded by even a small amount of tweaking are instant and rewarding.

Used among other analogue gear, the MS6 dutifully handles day‑to‑day analogue synth requirements, freeing up other, more aggressive synths to do what they do best. Used with digital gear, it will nicely fill those gaps in the mix that the strings and choirs from most modern digital synths are just too dominating to fill, or cheerfully layer with any older digital synths that you may have lying around (Yamaha's higher numbered DXs, for example) to thicken up sounds. It's always been fun setting up my MS6 in other people's totally digital synth rooms and suffering their scorn — until the first bit of MIDI hits the MS6...

Second‑Hand News

An MS6 today should sell for £150‑200, depending on condition. It is essential to hear a second‑hand unit in action before handing over any cash, as reliability was not one of the MS6's strongest points. Watch out for mains hum, a problem that affected earlier units. A remedial modification is explained at the web site shown below, although it's not an easy or advisable mod if you're not electronically experienced.

Those turning up to audition an MS6 hopefully clutching a pair of headphones will be disappointed, as there is no headphone jack on the MS6. The seller will have to be prepared to put it through some amplification, or you could take your own MIDI keyboard and powered monitor if you want to be sure of being able to properly check out your prospective purchase.

Don't be put off by the lack of a manual, or if repeated movement into and out of a rack has scraped away the useful legending on the top of the unit, as this information is available both in text file and Acrobat PDF form at (However, if the legending has been scraped away, that could at least give you some idea of the type of use the synth has had.)

On The Up: MS6 Upgrades & Support

Some enterprising souls have taken it upon themselves not only to support users of the MS6 via the web, but are now also offering their own firmware upgrades. Kristopher Maad, for example (, has a couple of projects under way that include some very saucy additions to the MS6, such as Program Change selection of Tones within Performance, MIDI Bank Select, Continuous Controllers assigned to each editing parameter (hooray!), and SysEx dumping and receiving of individual tones and patches rather than entire user banks.

All these features, apart from the SysEx modification, are available now in a firmware upgrade — see the web site for details. Further ideas on the horizon include accessing the hitherto‑unused capabilities of the CEM3396 chip to give more waveforms than just basic saw and triangle; dynamic voice allocation; matrix‑style modulation; and slow, random voice detuning to simulate VCOs. Kristofer has also written his own patch editor, MS6ed, for Windows 95, which is available for download from his site.

These mods and others in the pipeline show that the MS6 is not only still around, it's also still improving.

More More More...

The MS6's MIDI overflow mode made it possible to use several of these six‑note polyphonic units in tandem. With this mode active, when a seventh note is called for the first MS6 will pass the note information to the MIDI Out port, which is expected to feed a second MS6 (into which the sound data of the first would presumably already have been loaded). The MS6 manual also suggests that a third unit can be added, for 18‑note polyphony.

Some Cheetah History

Known previously for computer peripherals, Cheetah turned their hand to the music instrument business in 1987 with the arrival of the MK5 controller keyboard. I remember jokes about their first keyboards having a joystick with a trigger, but since a certain well‑known Japanese manufacturer also majors in furniture and fitted kitchens, I never thought Cheetah's background presented any problem.

The MK5 and its successors (MK5II/V and MK7VA) offered home studio users and the burgeoning computer music market a range of very cost‑effective controller keyboards and filled a niche in the market. Master keyboards had been around for years, but the vast majority had been £1000+ 'Stage Controllers' for live musicians trying to shepherd a flock of feisty modules. For the studio musician on a limited budget, £1000 is an awful lot of money to spend on something that can't make a sound. Enter Cheetah, with their £100‑400 range of controller keyboards — basic, but very welcome, with more than enough capability for the majority of their target users.

The next Cheetah product was the MD8 drum machine. Launched in the same year as the MS6, the MD8 was a pattern‑based programmable beatbox supplied from the factory with just eight onboard sounds. And there was no danger of using them all up at once — the MD8 was only four‑note polyphonic. Despite its limitations, the MD8 was to establish Cheetah in many people's hearts as the champions of the budget‑conscious studio owner. Its limited polyphony and meagre sample count ruled out any Vinnie Colaiuta moments, but for under £150 the MD8 offered a useful feature set for a very reasonable price.

The follow‑up to the MD8, the far superior MD16 and rackmount MD16R, followed in the early 1990s, as did Cheetah's wavetable synth (MS800), sampler (SX16), and slightly more up‑market controller keyboards (7P/770), but the music instrument side of Cheetah ceased to be in 1993, and went to the great rack in the sky.