If you thought that synth expander modules were a post-MIDI phenomenon think again - the Oberheim Synthesiser Expander Module was a box to be reckoned with over a decade ago.

SOME TIME AGO I saw an advert in an old Melody Maker offering an Oberheim expander module for sale for £90. I phoned instantly on the offchance it hadn't already been sold. "It's still here", came the reply, "but it's not what you're expecting. It's not the recent Xpander but an old analogue box I've had for years." Well, the secondhand market may not favour the seller of musical equipment at the moment, but I hadn't really expected to pick up three grand's worth of synth module for 90 quid. I had other hopes about what the mystery expander might be. My luck was in - the man was selling an old Oberheim Synthesiser Expander Module - or SEM. We met, we talked, I bought...

When I got my new toy home I found that it wasn't working too well. No problem - this was a machine that came from a time when circuit diagrams were part of the handbook. Although I'm no whizz-kid, 10 minutes with a soldering iron cured a simple earth problem. Eagerly I set it up and played. I still haven't entirely recovered.

The innocuous little white box you see in the photograph is actually a complete synthesiser. It contains two VCOs, a multimode VCF, a VCA, two EGs and an LFO. Along the top panel are facilities for all the inputs and outputs you're ever likely to need - apart from MIDI, of course. Nineteen seventy-four is as closely as I can date this particular example of the SEM, at which time it was being sold as an expander for layering sounds with the Moog Minimoog and ARP Odyssey. Later it appeared as the basis of the Oberheim OB1 monophonic and duophonic synths and the four and eight-voice multitimbral synths.

So what makes the SEM kick? The short answer is I don't know - you try one and tell me. There are plenty of synths around that have similar specs but not that sound. Perhaps it's the oscillators: rich sawtooth and square/pulse waveforms that can overdrive the filter for smooth, controlled distortion. Or perhaps it's the characteristic Oberheim filter, or the punchy envelope generators. Most likely, it's a magical combination of all these things, but there's no denying that this diminutive parcel of analogue electronics has a unique sound quality, parallelled only by the Minimoog and Odyssey in the same era.

ON CLOSER EXAMINATION, you'll find that the oscillators have coarse and fine frequency controls. At the control inputs you can derive signals from the EGs, the LFO or an external signal. These control signals can be independently inverted for further modulation flexibility. Oscillator pulse width is variable in ratio from 10/90% through 50/50% to 90/10% and can be modulated by the same control sources as the pitch. Sadly, only one waveform is available from each oscillator at any time but this doesn't seem to detract from the power of the SEM. The oscillators also feature a switchable hard sync facility capable of producing a dirty sound that should satisfy even the most fanatical analogue devotee.

The only simple way to describe the filter is "unique". It is totally voltage controlled with the same modulation facilities as the oscillators. It also offers no less than four filter modes: low pass, band pass, high pass and notch reject. Interestingly, the selection between low pass, notch and high pass is totally variable - you can set up a combination of low pass and notch reject or a combination of notch reject and high pass in completely variable amounts. These filtering options give rise to a wealth of sounds that would take the sophistication of something like the PPG system to reproduce today.

"Final pages of the manual detail options for user-modification, and the board itself has pins that can be used to solder sockets to, according to your own requirements."

The two envelope generators are similar to those found on the Minimoog in that the initial decay and final release times are controlled by one knob. In some ways this is a limitation but it doesn't seem to have handicapped the Minimoog too greatly, does it? The transient times of the SEM are comprehensive - from the fastest attack time on a synth (1msec), to a leisurely 15 second plus release time.

The LFO generates only a triangle wave for vibrato and sweep effects - if you want anything more whacky you'll have to "import" a more unusual control signal through the unit's external inputs. One of the more unusual aspects of the SEM is the VCA - the only evidence of its existence on the front panel is a switch that allows you to set up a drone. Again, anything more adventurous necessitates the use of an external control signal.

THE MANUAL THAT accompanies the old Oberheim should be made required reading for all writers of today's equipment manuals. It gives a comprehensive run-down on the SEM's controls and gives clear and detailed information on interfacing the SEM with various other synths. The best is kept 'til the end, however, as the final pages are devoted to offering various options for user-modification. There's a diagram of the circuit board showing the inputs and outputs of the various modules, and the board itself has a number of pins that can be used to solder sockets (and so on) to, according to your own requirements. To complete your modification Oberheim have thoughtfully provided a row of 3.5mm jacks - though there is room to drill holes in the case to fit more. Briefly, the SEM allows you to fit: individual waveform outputs, sync inputs and outputs for each oscillator, control inputs for the VCOs, separate control and audio inputs to the filter(s) and independent filter outputs for simultaneous access to all four filter types (although they will all share the same modulation), final VCA output and input, LFO output and trigger input, separate EG voltage outputs and various triggering options and a variety of external audio signal inputs. I was impressed. But the story doesn't end there as Oberheim provide nice little stickers for you to give your mods a professional look. Of course, you can take things a stage further than Oberheim intended by adding facilities like external control of pulse width and EG sustain level. The world, as they say, is your oyster.

But, as I mentioned earlier, even without any of these modifications the SEM is not a synth to be quickly dismissed. Bass sounds that will kill at 1000 paces, lead sounds that would cut through a Motorhead gig and, using the filter options, a fantastic range of almost digital textures are all part of the expander's character. Fast filter attack transients also make it well-suited to the generation of percussive sounds and, if that weren't enough, I found it more than passable as an imitative synth - and all for 90 quid (to me at least).

If all this has whetted your appetite for the SEM, you could do worse for a demo than to listen to some of Jan Hammer's early albums - I'm reliably informed that his "lead guitar" patch is a combination of SEM and Minimoog. Larry Fast is another SEM user and Weather Report's Joe Zawinul made great use of the SEM's close relative, the Oberheim four-voice.

Although I managed to pick up my SEM for under a ton, you might not be as lucky as I was. Unfortunately, these wonderful boxes weren't as popular in the UK as they were in the States, consequently they're a little thin on the ground. You might find it easier to pick up a four-voice, which is basically four of these beasts chained together in one box, or an OB1 which incorporates two SEMs and an analogue sequencer. In today's FM-obsessed world the SEM sounded just great to these ears.