Makes you realise how small synthesisers really are when you get all the noise making bits of a Korg Poly 800 in a box not much bigger than a telephone directory.

What the EX800 doesn't have to worry about, of course, are keys. Keys we have none. This is purely the sound making, memory and sequencer sections of the aforementioned synth, destined to be connected to your main keyboard via the infamous MIDI technology – four Din plugs two leads and a buzzword.

Expanders are the rage at the moment. We've already reviewed those from Siel and Oberheim and Roland's miniature JX3P is on its way. I suppose there are two primary reasons for synth manufacturers to go for this modular approach – (a) to enlarge upon the sound of your existing MIDI-ed keyboard synth and, (b) provide a sound source for sequencers and composers based around MIDI software on computers... noise making for those who wish to tap the typewriter rather than the keys.

It's housed in a slim, oblong black box with the control panel face upwards. Electronically the EX800 is virtually identical to the Poly 800 – certainly the two digital oscillators, two digital envelope generators, mod, noise and VCF sections are familiar. So that means like the original synth, you're only gifted with one filter for all eight notes. There's the usual choice of single or multiple triggering, much like early string and brass machines. The difference is this – in single triggering, if you've selected a 'wangy' sound, then while you're holding down a chord, you can't make any new notes start with that wang. They come on at the level and tone of the notes you're holding down... no independence, y'see. Alternatively, on multiple triggering, the new notes will 'wang', but so will all the others you're already pressing down. They'll be retriggered.

A drawback, it's true, but on the other side of the coin, the Korg has two digital envelope generators which allow you to specify decay and sustain points more precisely than a normal ADSR, and let you set up both oscillators so they behave differently – separate attack times, varied release rates etc. Few analogue synths can do that.

Otherwise the specs read out the same – 64 memories, a 256 note, step time sequencer, and all the memory selection and editing called up on eight push buttons.

Definite improvement here. The EX800's buttons are half an inch square, individually marked and boast a positive, click action – smarter than the Poly 800's easily confused, sliver sized switches. In program mode, they'll dial in the memory number; in parameter mode they'll call up one of the 51 synth sections. Each parameter has its own identifying number and graphic explanation in a list inscribed on the top half of the EX800's panel. For example, filter cut-off is 41 and has a range of 0 to 99. Trigger selection is 46 with a 1 for single and 2 for multiple. Once you've selected your parameter, its value is displayed in the third of the EX's red LED readouts, and you can edit that figure using up and down incrementing buttons in the bottom right hand corner.

So, there are your sounds stored and editable. The EX800 comes factory fitted with the Poly 800's voices, but is accompanied by a cassette bearing two more batches of 64. At this point it's worth mentioning the EX's MIDI capabilities since they have been upgraded from the Poly 800's. You can either select the EX's memories from its own bank of buttons, or keep it chained to the Poly 800 so dialling in, say, 61 on the keyboard will drag a 61 out of the expander as well. Both the father and son Korg can send and listen to their info on any one of 16 MIDI channels, but in fact the expander puts more material down the line.

For example, you cannot edit the parameters of the EX800 with the controls of the Poly 800. The Poly doesn't include that information in its MIDI transmissions. The EX800 does, so it could receive parameter instructions from, say, a MIDI interfaced computer, or another EX.

As a further bonus, it's possible to give the sounds in the EX a different bend rate from the Poly! Normally, when you jiggle the joystick on the keyboard, both synthesisers will fall or climb in pitch by the same amount. Reprogramming the EX800 will reduce the joystick effect so the synths can be made to bend apart to weird intervals. Distinctive.

And so we finally make it to this layering business... EX plus Poly. Certainly there are the options such as two string sounds back to back, piano and harpsichord, brass and bass line, etc. But bearing in mind the Poly 800 already has two oscillator banks and a chorus unit, the simple act of adding a near identical sound from the expander to thicken the overall result isn't so effective, nor, for that matter, essential.

The best effects are frequently from entirely different memory settings... new voices fading up under your original chords, fresh sounds beginning once the old ones have finished. Korg's digital envelope generators are particularly adept at this, only starting up some voices once you lift your fingers off the keys. This extra dynamic endurance adds more than just 'thickness', it adds interest, and expands the variety of sounds available, as well as enforcing them.

If you're already a Poly 800 owner, the tram lines of the front panel won't worry you. If this is the first time you've come across Korg's graphic layout of its functions you'll be completely bamboozled. It looks eye wrenchingly confusing but there is a lot to explain and I suppose familiarity breeds content... er, is that right?

So far the best bargain among expanders – not too difficult when the Siel is £699 and the superior Oberheim is £3,500. You could even argue that the 800 is better with "EX" in front of it than "Poly".

KORG EX800 synth expander: £350