Oberheim Xpander

Not all analogue classics have keys. The Xpander was an early module that packed in more features than its moderate size would lead you to expect, and left a legacy still discernible in modern synth design.

On its launch, at the USA NAMM music fair in 1984, the Oberheim Xpander was touted as the last word in analogue synthesizers. Its design team included such newcomers as Marcus Ryle and Michel Doidic working on the software, and Tom Oberheim himself keeping an eye on specs and hardware. This mix of talent built a synth with the kind of features that every player has dreamed about at one time or other.

A 'Modular' Synth

One surprise for many people was the lack of keys. The Xpander, as you might guess just from the name, is a module, and certainly not a rackmounting one, measuring a fairly hefty 33 x 12.5 x 6 inches. But then I suppose even keyboards were bigger in those days!

Reviewers at the time quickly noted that the £2950 Xpander was a very complete instrument, with little obvious cost‑cutting or trimming of features. Oberheim aimed high. They were foresighted enough to put in a decent MIDI spec at a time when people were still trying to understand the implications of a computer interface, and also packed in features and a uniquely serious level of control. "A fantasy realised", as the advertising blurb said.

Being analogue in 1984 was not really a plus point for a synth. Everybody wanted digital, and Yamaha's DX7 was dominating the synth market like nothing else had before or since. But despite such competition, and a scary price tag of about the same as three DX7s, the Xpander flourished while other now‑revered classics sank without trace.

The Xpander Voice

At the time, much was made of the Xpander voice. Even today the module's complement is still undoubtedly special. Each of the six voices is made up of hardware for the audio path and software for controllers. There were two VCOs — real ones, too — for a richer sound, a 15‑mode filter and two audio VCAs, with software for the multitude of envelopes, LFOs, ramps, tracking generators and the like.

The VCOs have mixable Triangle, Sawtooth and variable pulse waves. Oscillator 2 has a sync setting and white noise, and you can also cross‑modulate the oscillators for an analogue FM fuzziness: although the effect here is not as strong as on a Memorymoog or Prophet 5, it still produces some good bell sounds. One nice touch is that each VCO has its own VCA, so you can adjust the volume of an oscillator with any control source, useful for creating complex layered sounds without eating polyphony.

Most synth users find the filter gives a synthesizer its character and uniqueness. On the Xpander, instead of the ubiquitous low‑pass design, there are 15 filter modes — low‑pass, high‑pass and band‑pass, notch (the reverse of band‑pass), and phase‑shifter — all in various rolloffs and combinations. And this was when high‑pass filtering was seen as exotic! The range of sound options instantly available is huge. Just grabbing the filter mode knob and changing from a simple low‑pass to a high‑pass with phase shifter will transform any sound. The filter also has a three‑pole low‑pass mode of the kind favoured by the TB303 and its clones, for instant dance appeal, with weird resonance too.

For the sake of cost, reliability, and probably portability too, the envelopes and LFOs are software generated. Some pundits claim software gives a different (read lesser) quality, when compared to real hardware. I've compared my Xpander to an older OBXa and there seems a common sound identity to both Oberheims, so I'm not sure I'd agree in this case. One drawback of using computer generation, however, is that everything must be quantised. Envelope times and LFO frequencies on the Xpander are quantised to 64 steps. (Some other parameters have 128 steps). At first this sounds like a shortcut to disaster, but I'm pleased to be able to tell you that things work out well. Chosen values are concentrated in the ranges you need most, so you do seem to get the setting you want. Interestingly, zipper noise seems to be almost undetectable too.

The Xpander's influence can be seen in most American designs, from the 80s to today

Let's All Modulate

Say "Oberheim" and old analogue anoraks think modulation. Like virtually every commercial synthesizer since the Minimoog, the Xpander has some standard modulation connections hardwired (12, in this case). But buyers of the Xpander had access for the first time to the now‑infamous Matrix Modulation system and, consequently, dynamic control of nearly every sound parameter — lots (and I mean lots) of sources, with loads of things to control. And where one LFO and a couple of envelopes were good enough before, the Xpander featured five of each. Per voice! These envelopes are no cut‑down versions, either — they're DADSR (Delay, Attack, Decay, Sustain, Release) types, including a delay parameter and various trigger and run modes. The LFOs have a claimed 64 different waveforms, including noise and a sample setting for creating staircase effects and the like. There's also a global LFO for normal vibrato duties. In essence, the Xpander delivered modular synth flexibility in a commercially available, programmable analogue package.

This is a synth with the kind of features that every player has dreamed about.

The Xpander's software matrix lets an intrepid sound hound connect any of 27 sources to one or more of 47 destinations, with up to 20 connections per voice. Some options are obvious, others less so. Oberheim's design thoroughness means that hard‑wired options are duplicated on the matrix, which is useful when further manipulation is needed. The result is not only limitless Joe 90 boffin appeal but also serious potential for sound creation, which can be used subtly or otherwise, depending on what you're after.

There are some unusual modules too — 5‑position tracking generators, three per voice, which the manual says change a linear input, such as a keyboard, into a non‑linear one. You may wonder why you'd want to do this. Well, perhaps you might want a low‑pass filter to track the keyboard, so that the sound is dull across the middle two octaves but bright at both ends, or maybe you'd like to vary the amount of PWM across the keyboard, to lessen the mushiness at one end.

And the features don't stop there. Each voice has four ramp generators, often used for fades and sweeps, and a lag generator, which slows down voltage changes. This is ideal for portamento, but it can process anything from velocity response to LFO shapes and pedal inputs. I'm sure you get the idea by now.

The Front Panel

With all this going on, you need a clear control panel, and Oberheim did a pretty good job of providing one. There was no way that an Xpander with one knob for each parameter could fit onto a keyboard stand, so there had to be a compromise. They chose lots of buttons, and six click‑stopped continuously rotatable knobs. The system works very well too — most of the parameters you'll want to twiddle can be adjusted quickly.

The panel is divided into easily digestible sections, which are called up in pages. A section's main parameters are then listed on one display for instant editing, with values listed on a second display below. In the case of an oscillator, for example, frequency, pulse width, volume and so on, are named, and each has its own dedicated knob. When you turn a knob clockwise it adds to the stored parameter value, while an anti‑clockwise turn subtracts from it. If you wanted to modulate something, you'd simply press the soft button next to the chosen parameter displayed and jump straight to a page where envelopes and LFOs can be assigned and their amounts adjusted.

To be fair, there is quite a bit of leaping from page to page, and this can be a bit of a pain. The alternative, however, could have been editing with a single slider and small display. Not nice!

Sounds

As well as having loads of features, the Xpander sounds good — very good, in fact. Compared to earlier Oberheims, or Moogs and ARPs, the Xpander has a smoother, more sophisticated timbre. Put it next to a Moog or ARP and it sounds mellower, with a more polished '80s feel. Although this was possibly due to different Curtis chips used in the later synths, the Xpander's sound seems to continue a trend by Oberheim towards a less raw tone than their earlier machines. Tom Oberheim has since said that he now prefers the sound of his earlier instruments over the OB8, Xpander and Matrix series, but didn't comment on whether this was what he thought at the time. Don't get me wrong — there's still clout and class here, easily enough to see off the virtual analogue brigade. But the heavy‑duty Van Halen patches are just not so convincing, and the Xpander's strengths lie in a slightly different area. What you get is a solid and much more expressive sound, helped, no doubt, by lots of controller possibilities, including touch response.

Multitimbrality & MIDI

The Xpander stores 100 single patches, and names for each of them. It's 6‑part multitimbral, which was virtually unique in 1984, and (beating the Korg M1 by a few years) it has a zoned multitimbral performance mode too. There are 100 of these 3‑zoned setups, which will seem very familiar to Korg and Kurzweil users! For each zone you can choose any MIDI channel and note range, for splits, layers or multi‑channel operation. Volume, transposition, voice assignments, and panning or separate output selection can be set for each voice. You can even edit voices within a multi on‑the‑fly.

Back Panel

Some people are going to love this. Thoughtfully labelled on the back panel are stereo audio outs and six individual voice outs, plus individual CV and gate inputs, MIDI In, Out and Thru. Yes, folks, it's a MIDI‑CV converter (except that CV has to be the controller). A Trigger In socket, three pedal inputs, including Program Advance, a cassette interface (though you are more likely to use SysEx today) and power complete the picture.

The Legacy

The Xpander is probably not going to be your only synth. It's analogue and only makes analogue synth noises. As I've already hinted, it lacks the raw punch of a Moog, and hard, percussive sounds are not its strongest suit. For modular analogue purists the fact that the signal path is fixed at VCO‑VCF‑VCA, and the knowledge that many parameters are quantised, could be a disappointment, although in reality it probably won't be. I could also reveal that setting up multis is a bit long‑winded. But hey, we're getting picky.

Make no mistake, this is a very fine, important and influential instrument. Its influence can be seen in most American designs, from the 1986‑vintage Ensoniq ESQ1 through to today's Alesis, Emu and Kurzweil machines, and matrix modulation is now widely available on instruments from a range of manufacturers, both American and Japanese.

For Oberheim, the Xpander was a very important instrument. They chose to stay with analogue. The Xpander and its offspring kept them in business when other purveyors of American analogue couldn't sell enough units to keep the doors open. People wanted new and interesting instruments, and the Casio CZ, Yamaha DX and sampling technology took the sales. Roland and Korg analogue‑based synths also took a serious knock from this trend, but these companies had the muscle to diversify into effects processors, pianos and amps. Oberheim continued, though it changed hands in later years, and even today, nearly 14 years on, you can buy a direct descendant of the Xpander in the shape of the Matrix-1000. High praise indeed.