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Teisco SX400 [Retrozone]

Analogue Synthesizer By Gordon Reid
Published October 2001


Not every old analogue synth can be a classic. Gordon Reid reflects on one that is not desirable, probably won't appreciate and, in many analogue enthusiasts' opinions, does not deserve to be hunted down — except, perhaps, to be put out of its misery!

I must admit to being a bit surprised when I got a call from Sound On Sound suggesting the subject of this month's retrospective. Sure, it was on a list of possible instruments that I had submitted many moons previously, so the idea didn't come entirely out of the blue. Occasionally I begin a retro by suggesting that a given instrument is a classic, a near classic, or even 'one that got away', but I wouldn't be able to say these things about the Teisco SX400, which made me think: simply because an instrument is old, rare and analogue, does that mean that it is interesting? Then it dawned on me. There were not many old, rare analogue polysynths that were (commercially if not sonically) complete turkeys. Yet the subject of this month's Retrozone is precisely that — which may be the very thing that makes it interesting.

Teisco? Who?

I won't be surprised if you have never even heard of the original Teisco. This is because it was (so I am led to believe) a company acquired in the mid‑'60s by the hugely successful Japanese piano manufacturer, Kawai. Consequently, it seems strange that, when Kawai decided to turn its hand to synthesizers, effects, mixers and PA, it resurrected the name, producing five Teisco monosynths, three Teisco polysynths, and a Teisco string ensemble during the seven years between 1977 and 1984.

Keyboard aftertouch could be assigned to modify one of four parameters, including the mysterious 'Wow Wow'.Keyboard aftertouch could be assigned to modify one of four parameters, including the mysterious 'Wow Wow'.The first of these was the Synthesizer 100F, an intriguing but ultimately disappointing single‑oscillator monosynth. Released in 1977, but (as far as I can see) never advertised in the UK, this offered some interesting facilities. These included 64‑foot and 'Low' footages for its VCO; preset AD, ASR and ADS contours for each of its two contour generators; delayed vibrato; and three modes of portamento. It also featured sophisticated patching capabilities, including an external audio input and CV ins/outs. Unusually, for a low‑cost synth, there was also VCO pitch modulation from either of the EGs, and you could even modulate the VCF cutoff frequency using the external input and the VCO, as well as the more common LFO and envelope. Despite this, the S100F's sound was about as engaging as last week's lunch. (I know; I have one.)

Next came the S100P, a pressure‑sensitive preset synth in the same mould as the ARP Pro‑Soloist and Roland SH2000. Like the S100F, the S100P offered a surprising range of features, including an integrated spring reverb and a preset flanger. Although it was undoubtedly less flexible than its predecessor, I found the S100P to be slightly more useful, if only because its range of 32 thin, uninspiring sounds lent itself to unobtrusive accompaniment duties. (Not every sound has to be a screaming Minimoog with all three oscillators blaring, you know.)

As the '80s dawned, Kawai discontinued the 100F, opened a sales office in the UK, placed some adverts in the magazines of the era, and released two new monosynths to accompany the S100P. These were the S60F and S110F.

The S60F was in many ways a considerable improvement over the S100F, and with a street price of just £199 it was good value, too. It retained the single‑oscillator architecture and low‑frequency capabilities of the 100F, as well as the delayed vibrato and the high‑pass filter. Sadly, it lost one envelope generator, but added pulse‑width modulation, Sample & Hold, and waveshaping of the LFO — from zero percent pulse/sawtooth at one extreme, through square/triangle, to 100 percent pulse/ramp‑wave at the other extreme.

But it was the huge S110F that was the pick of the bunch. For one thing, it was duophonic — although only in the same, compromised fashion as the ARP Odyssey and the Octave Cat. For another, it offered an eight‑band fixed filter bank — much like the larger Moog modulars. It also has a pressure‑sensitive keyboard. Not bad for something that cost little more than £500. The S110F had numerous other tricks up its sleeve, too — for example, a ring modulator w hose output you could mix with the outputs from the individual oscillators. In addition, both of its oscillators offered the 'Low' range, allowing you to use either of them as additional LFOs, while the dedicated LFO shared the S60F's variable waveforms.

Next, in late 1981, Kawai released the instrument that signalled its leap into the dangerous waters of polyphonic analogue synthesis. Clearly designed as a competitor for the Roland Jupiter 4, it was this that has since become one of the world's least respected polyphonic synthesizers. Ladies and Gentlemen, I give you the Teisco SX400.

The Oscillators

On the face of it, the SX400 should be a very powerful synthesizer. For a start, it has four, independently‑tuneable VCOs that you can assign in four different ways to the keys that you play.

  • Quad mode is, as you might expect, the four‑note polyphonic mode, which makes the SX400 a conventional single‑VCO/VCF/VCA‑per‑voice polysynth. In this mode, the octave and fine‑tuning controls assigned to VCO1 (or 'Channel 1' as Teisco refer to it) tune all four oscillators simultaneously, as the global tuning control does on other synths.
  • Dual mode assigns the four oscillators to just two keys, making the SX400 duophonic. Hang on... let's be careful with our terminology here. The term 'duophonic' has been used and abused by many '70s monosynths, such as the aforementioned ARP Odyssey and Octave CAT. These divide their dual oscillators between two notes, but have just one VCF/VCA signal path. In contrast, the SX400 is truly duophonic, with two oscillators per note and independent signal paths for each note. This means that every note is articulated correctly — something that the Odyssey and CAT could only achieve if the notes were not shaped by their envelope generators.

Dual mode also allows you to tune the first and second oscillators independently, which creates some truly enormous sounds. Yes, I know the concept of 'fat' is a hideously over‑used one, but in this case it's justified. The Teisco's o scillators are as unpredictable as any old Moog's, and the sound can benefit (or suffer) accordingly.

The four white rectangular switches above the keyboard control the way in which the four VCOs affect notes played on the keyboard — see the explanation of VCO modes, right.The four white rectangular switches above the keyboard control the way in which the four VCOs affect notes played on the keyboard — see the explanation of VCO modes, right.

If there is one limitation to Dual mode, it's this: there's no oscillator mixer, so you can't determine the relative volumes of the oscillators: all are at 100 percent volume, all the time. Unfortunately, there's a second limitation, and it's another biggie. No matter which mode you're in, there's only one set of oscillator controls, so you can't select different waveforms for each. The whole instrument is set to either sawtooth, pulse, or triangle waves, but never to a mixture of them. This severely limits the range of 'Dual' tones that you can create.

  • Mono is the sonic monster of the set, because it turns the SX400 into a four‑oscillator monosynth. You can tune all four oscillators independently so, as you can imagine, the results can be monstrous, fully the equal — in terms of depth and loudness — of any other monosynth you might care to name. But, again, all four VCOs use the same waveform, and all four produce their sounds at 100 percent volume. Without these limitations, the SX400 would be a superb monosynth, and analogue aficionados would have rediscovered it many years ago.
  • Solo: Some commentators have questioned why Teisco should have included this mode. After all, it simply restricts the synth to acting as a single‑oscillator monosynth. But there are many times when you want a single‑oscillator sound — for example, any sort of solo brass, woodwind or string patch — and this is what Solo delivers.

And there's more. In particular, every SX400 oscillator incorporates a sub‑oscillator that offers four options: sub (a square wave one octave below the main pitch), noise, sub+noise, and 'off'. Given that the main oscillator can also be 'off' — thereby allowing you to use the sub‑oscillator and/or noise alone — the 16 available permutations increase the flexibility of the oscillator section by a considerable amount.

Then there are the remaining VCO controls: pitch modulation by LFO1, pitch modulation by EG1, manually variable pulse width for the pulse waveform, and pulse‑width modulation by either of LFO1 or EG1. When you compare these facilities to those offered by the other moderately‑priced polysynths of the era — the Roland Jupiter 4 and the Korg Polysix — the SX400 comes out looking pretty good.

VCFs, VCAs & Modulators

Looking at the control panel, the SX400's VCF (actually, four VCFs, one for each voice) seems very well specified for its day. Unfortunately, despite offering resonance, it doesn't come close to self‑oscillation. This, in itself, is quite a limitation. Furthermore, the fact that any amount of resonance decimates the bottom end, and does so without imparting any real character to the top, is perhaps the primary reason for the SX400's inability to impress.

Nevertheless, there are oodles of extra facilities to keep you interested, including keyboard tracking, modulation by either LFO1 or LFO2, and bi‑polar contouring courtesy of EG1 — a dedicated four‑stage (ADSR) envelope generator. Oh yes, and at the end of the signal chain there's a high‑pass filter. This sounds to me like a 6dB/octave or at best a 12dB/octave device, and it lacks resonance so, in truth, it's nothing much more than a glorified tone control. Still, you can say the same about the HPFs on other synths, so I shouldn't quibble.

Moving on, the VCA section offers another four‑stage contour generator (EG2), a level control, amplitude modulation courtesy of LFO1, and optional retriggering courtesy of LFO2. The LFOs themselves are very different from one another. Apart from the inevitable frequency control (0Hz to about 15Hz), LFO1 offers four waveforms (sawtooth, ramp, square and triangle), with delay of up to a second or so. As already noted in various places above, this modulates the VCO pitch, provides the waveform for PWM, can modulate the filter, and provides amplitude modulation. In contrast, LFO2 is a simple square wave routed only to filter modulation or VCA retriggering.

But to look at the top panel alone is to miss the third LFO, tucked away in the performance control panel to the left of the keyboard...

Performance Controls, Effects & Interfaces

If there was one thing that the SX400 offered over and above all of its contemporaries, it was this: aftertouch. I love aftertouch, and it has kept the ARP ProSoloist near the head of my 'most desirable synths' list for more than 25 years. The Teisco's aftertouch facilities may not be quite as highly developed as the little ARP's, but it still offers four parameters that you can modify by pressing down just that little bit harder. These are pitch‑bend (up or down), VCF cut‑off frequency, vibrato, and the beautifully misnamed 'Wow Wow', the last two of which are driven by a dedicated LFO.

A pitch‑bend/modulation lever complements the aftertouch facilities. The CV generated by this passes through the same 'sensitivity' control as the aftertouch itself, thus allowing you to control all the effects using the lever, if you like. Unfortunately, this means that you cannot use aftertouch and the lever independently, which proves to be a real pain in the posterior. Furthermore, the CVs generated by aftertouch and the lever are summed, so if you set up the correct pitch‑bend for the lever and then lean a little too hard on a key, you sharpen or flatten the pitch even further. It's just another example of how the SX400 got it nearly right.

While we're on the subject of effects, two more deserve mention. The first of these is (or maybe are?) portamento and glissando. These are truly polyphonic, so whenever you re‑use a voice in Quad or Dual modes the glide/gliss starts at that voice's previous pitch, and ends at its new one. This may be something that we take for granted today, but that was not always the case.

What's more, both the portamento and the glissando are 'constant‑rate' effects. This means that a single‑octave sweep takes a quarter of the time needed to sweep across the whole, four‑octave, keyboard. Many players prefer this to a 'constant time' implementation, in which the duration of the sweep is the same, no matter how far apart the notes may be.

Finally, there's the Ensemble effect. In contrast to so much else on the SX400, this is glorious. Indeed, the Teisco is one of the few synths that can produce an Ensemble convincing enough to rival dedicated instruments such as the Solina or Logan String Melody II. OK, like most analogue BBD (Bucket Brigade Device) effects, the Ensemble effect is noisy, and goes 'whoosh' even when you're not playing, but I'll forgive it that. If only the SX400 could play more than four notes at a time, it would be the thickest string synth of them all.

The SX400 is analogue, so it needs tuning... luckily, four ports allow oscillator tuning without the need to open the synth's case.The SX400 is analogue, so it needs tuning... luckily, four ports allow oscillator tuning without the need to open the synth's case.

Turning to the rear of the instrument, you'll find six sockets. Two of these are CV and Gate inputs routed to Channel 1. (The incoming pitch CV is summed with the keyboard CV generated on Channel 1, allowing you to transpose incoming sequences using the SX400's own keyboard.) The next is an EXP pedal input that controls the gain of the audio VCA. Then there's a headphone output and, finally, Left and Right signal outputs. "Why two outputs?" you may wonder. Simple: the Ensemble is a stereo effect, and it creates a massive sound field if you use both outputs. What's more, in a strange nod towards professional users, there's also an output‑level control that switches between +4dBm, ‑10dBm, and –20dBm. It's an odd facility to find on what is otherwise a budget synthesizer.

Before we move back to the front panel, I must mention the 'Note A' switch and VCO fine‑tuning controls that you'll also find around the back. With the switch on, the bottom four white notes on the keyboard (C, D, E, and F) act as A440 switches for each of the four oscillators. These then allow you to tune (or detune!) each of the oscillators using the recessed trimmers that lurk behind four cut‑outs in the rear casing. Given that the SX400 can drift considerably, it's a real boon to be able to retune it without opening the case.

Thanks For The Memories

The SX400 offers eight preset memories — trombone, trumpet, violin, human voice, electric guitar, harpsichord, electric piano, and sitar — plus eight user memories accessed using a 'Preset/Memory' switch in conjunction with the eight 'Preset' buttons. The first four presets are surprisingly pleasing, and all of them are far better than you might expect. Not that the electric guitar, harpsichord, electric piano, and sitar sound anything like the real instruments, of course, but they are exceptionally usable sounds.

The SX's eight preset voices, accessed by the eight white switches shown above, aren't what you might call realistic, but are more pleasing than you might imagine.The SX's eight preset voices, accessed by the eight white switches shown above, aren't what you might call realistic, but are more pleasing than you might imagine.

Unfortunately (harking back to the days of the Oberheim 4‑Voice and the Yamaha CS80 — 1975 or thereabouts), the SX400 is unable to store all its parameters in memory. None of the Channel 2, 3 or 4 oscillator pitches are stored, nor is the fine‑tuning position of Channel 1. Likewise, none of the performance parameters are stored, nor are the statuses of portamento and glissando, and the Ensemble effect. The last of these is particularly annoying.

Much better is the provision of an 'Edit' switch that allows you to modify the presets using one or more of the top‑panel controls. Should you require a small modification to a preset, this is much more convenient than trying to program the complete sound from scratch. If you want to edit the SX400 conventionally, simply press the 'Panel' switch, and the sound is then determined by the positions of all the controls as they appear on the panel.

In Use

Even in an era of bulky synthesizers, the SX400 was a lump: large, unwieldy, and very heavy. Despite this, it is not particularly sturdy, and I would have insisted on a flight case had I decided to use it for live work. Unfortunately, the total weight of the synth and case would then have been about 50kg, so I would have been very unhappy without some major contributions from roadies!

You might wonder why the SX400 is so cumbersome. Surely a four‑voice synth need not be excessively heavy? But when you open one up, you'll find that the case is stuffed full of electronics. Indeed, even by the standards of 1980, the boards are huge, as are the iron chassis that support them. This was far from an ideal instrument for the gigging player.

Even ignoring this inconvenience, I often find that I hesitate when I think about firing up my SX400. For some reason (and unlike any other keyboard I own) it makes me think about playing something else. Nevertheless, after a very short while I find that I'm enjoying myself, and then I realise that I'm coaxing some superb sounds from it.

I suspect that I know the reasons for my initial reaction. For instant synthesizer gratification you need powerful, resonant filters, snappy envelopes, and a wide range of mixable waveforms. In contrast, the SX400 offers you gutless filters, sluggish envelopes, and a limited range of oscillator options. On that basis, it's a turkey. On the other hand, perseverance with the four modes, experimenting with the detuning capabilities, aftertouch, and that glorious Ensemble can yield some startling results. I've already mentioned my enormous respect for the SX400's string sounds, but its poly‑brass can be equally good, and — like the vocal sounds on early Korg monosynths — its factory preset Human Voice is a classic of the genre.

What's more, if you consider the technical description above, you'll see that the SX400 offers a great deal more in the way of sound‑shaping facilities than many of its contemporaries. These allow it to generate polyphonic sounds and effects that lie beyond the dreams of a Roland Juno 6 or a Korg Polysix, and huge monophonic textures that outshine nearly every budget monosynth on the planet. Sure, the SX400 is limited to a maximum of four notes, and many of its capabilities — whether in monophonic, duophonic or polyphonic modes — are flawed, but perhaps it's this dichotomy that makes the SX400 such an enigma.


Despite a street price of around £899 — importer Chase did its usual trick of doubling the real price to £1690 and then offering a "never‑to‑be‑repeated, half‑price special offer" — the SX400 sold in tiny numbers. Consequently, it's unlikely that you'll find one today. And even if you do, it's almost equally unlikely that it's still fully functional. (See 'Chemical Weapon' box.)

Furthermore, Teisco's immediate follow‑ups to the SX400 were a motley bunch. The S60P was another single‑oscillator, pressure‑sensitive, preset monosynth that came and went without causing so much as a ripple in the collective consciousness of the synthesizer community. Then there was the EX300.

I've never even seen one of these, but a UK buyer's guide in May 1982 described it as simply a "four‑oct C‑C" string ensemble costing £649. However, a little extra research established that the EX300 was far more than a simple string synth, with four sections named Stringer, Brass, Human Voice and Bass. Each section offered at least one variable parameter, with the voice offering no fewer than four: Vowel Character ('umm' or 'aahhhh'), Ensemble, Brilliance and Attack Rate. You could even layer the strings, brass and voice, although the bass was rooted firmly in the bottom 20 keys of the instrument. This placed the EX300 in the 'multi‑keyboard' camp, alongside instruments such as the Korg Trident and Crumar Trilogy. But with no synthesizer section — monophonic or polyphonic — it's no surprise that it, too, disappeared without trace. And, given that in 1982 you could buy a Juno 6 for £649, I'm not surprised that I've never seen an EX300.

The following year saw the launch of the SX210, a five‑octave, 32‑memory synthesizer that was much more sophisticated than anything that had previously carried the Teisco badge. With a single DCO and sub‑oscillator for each of its eight voices, and a (then) fashionable 'spinwheel'‑controlled digital operating system, it looked good, and didn't sound bad, either. Much of the credit for this can be attributed to its eight SSM2044 filters, and another pleasing analogue chorus unit that, for some players, compared favourably with the classic choruses of the Roland Juno 6 and Juno 60. Indeed, in many ways the SX210 improved upon the Roland, with bonus features handed down from the SX400. These included dual and mono modes, dedicated filter‑ and amplitude‑ contour generators, a high‑pass filter, and glissando. There was even (remarkably for an instrument in this class) an LED display.

It's a shame that the SX210 never caught on, because it was a nice little synthesizer. But in the same year that the DX7 appeared, it never stood the slightest chance of commercial success. Nevertheless, Kawai had one final shake of the analogue synthesizer dice, launching the world's last Teisco synthesizer in 1984.

The SX240 was the only descendant of the SX210, adding a second DCO per voice, more memories, and a sequencer. Kawai also demonstrated remarkable foresight with this synth, because it embraced MIDI long before the majority of other manufacturers had realised the significance of the new standard. In retrospect, the SX240 was far more desirable than awful instruments such as the Siel DK80, or the disappointing Sequential Circuits SixTrak and Max. It even compared favourably against the Prophet 600. But it, too, was doomed to be a commercial flop that has since enjoyed nothing but obscurity. And that's a shame. I have a beautifully preserved SX240, and it's a superb little synth. Limited, yes, but what it does, it does very well.


By this time, Kawai had realised that the Teisco name was doing its products no favours, and most of them had already reappeared bearing the 'Kawai' badge. Indeed, it's unlikely that the SX210 or SX240 ever appeared in Europe or the USA bearing the Teisco brand name, this being confined to the home (Japanese) market. But it was too late for such minor Band‑Aids to make any difference, and Teisco, plus all the products that had borne its name, disappeared without trace. Fortunately for us, that was far from the end of Kawai's involvement with synthesizers because, in 1988, the company was to launch what would become one of the most successful digital instruments of all time... But that's a story for another day.

Teisco Pianos?

In 1982 there was a UK buyer's guide that referred to three Teisco electric pianos: the Pro5, the T77 (which was supposed to incorporate a small integrated amplifier and speaker) and the QP88. I have trawled through the literature of the late '70s and early '80s, but I can find no other mention of them. Did they ever exist in the UK, or were they the figment of some sales & marketing executive's fevered imagination? Get in touch via SOS if you can tell me.

Known Teisco Synths & Keyboards

The following list contains every synth known (to this author) to carry the Teisco name. Most of them later appeared in Kawai variants.
S100F 1977 £65 A basic and uninspiring single‑oscillator monosynth.
S100P 1979 £50 Of no conceivable value in 2001. Collectors only.
S60F 1980 £80 A huge improvement on the S100F it superseded.
S110F 1980 £250 The pick of the bunch, perhaps the only sought‑after Teisco.
SX400 1981 £180 It should be great, and it nearly is, but not quite.
S60P 1982 £50 Another basic pressure‑sensitive preset monosynth.
EX300 1982 £50 An ensemble keyboard released five years too late to be of interest.
SX210 1983 £150 A single‑DCO‑per‑voice polysynth that still sounds good.
SX240 1984 £250 Quirky and desirable, an early analogue polysynth with MIDI.
Note: The 'P' in Teisco's names referred to 'Preset' monosynths, while the 'F' denoted 'Free' (or programmable) monosynths.

The SX400 & ARP

On the back of my SX400 there is a small sticker bearing the words, 'Manufactured under license from ARP Instruments Inc., U.S.P. 3,965,78'. Since I've heard from a number of sources that the SX400 was based on an ARP patent, I thought that it would be fun to find out what this was.

In essence, US patent no. 3,965,789 describes a pressure‑sensitive keyboard mechanism that, in 1974, overcame a significant problem with previous designs. Early aftertouch‑sensitive keyboards would produce different sounds (for example, greater volume, or vibrato, or whatever, depending upon what the aftertouch was controlling) every time a key was pressed. The reason for this was, in retrospect, obvious: each time the player played a note, he or she did so with slightly differing pressure at the end of the key's travel, so the pressure‑sensitive effects were affected proportionally.

Alan R Pearlman (the founder of, and chief engineer at, ARP) overcame this by inventing a new pressure‑sensitive mechanism. This had two 'zones'. The first allowed the note to sound without the introduction of pressure‑sensitive effects, provided that you pressed the key with a certain minimum pressure. The second introduced the pressure‑sensitive effects once you exceeded a higher pressure threshold. This system permitted players to play normally within a range of pressures called the 'deadband', but allowed them to introduce the expressive effects by pressing much harder on the keys after the initial action.

An extension to the patent covered the use of multiple selectors and potentiometers for activating and controlling multiple aftertouch‑sensitive effects simultaneously. On the SX400, these were pitch‑bend, brightness, vibrato and Wow‑Wow, plus the sensitivity control in the performance panel to the left of the keyboard. ARP filed the patent on 1st February 1974, and it was granted on 29th June 1976.

Chemical Weapon

Like many other synths of its era, the SX400 used a rechargeable NiCd battery to maintain its memories. Unfortunately, these tend to leak when they get old, depositing nasty chemicals on the main board. These chemicals then eat away at the tracks, and the damage can lead to quite unpredictable side‑effects.

When the portamento and glissando on my SX400 started playing up (I couldn't switch them off, and something was interfering with the keyboard CVs), the last thing I thought to check was the memory back‑up. Fortunately, David Croft at the Synthesizer Service Centre soon found the problem. He then removed the offending component, cleaned away the spillage, re‑made the damaged tracks, modified the board, and installed a modern Lithium Ion battery. Although not rechargeable, this will last just as long as a replacement NiCd, and without the attendant risk of further damage.

If you have an old synth with NiCd backup (the Korg Polysix is another that can suffer from this problem) I recommend that you have it replaced. Immediately.

The SX400 & The Mono‑Poly

Given their common philosophy (four oscillators per voice in monophonic mode, or a single oscillator per voice in four‑note polyphonic mode), it's not surprising that people have compared the SX400 with the Korg Mono‑Poly. Indeed, I've read on a number of occasions that the SX400 borrowed heavily from the Mono‑Poly, and was to some extent inspired by it. What a load of old hogwash! Teisco released the SX400 in 1981. The Mono‑Poly followed a year later, in 1982. 'Nuff said.