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Korg Mono/poly

Analogue Synthesizer (Retro) By Derek Johnson
Published June 2000

Korg Mono/poly

Derek Johnson collects £200 as he passes 'Go' on his tour of one of Korg's more obscure vintage 'boards.

It was 1987. The Conservatives had just been re‑elected, making Mrs Thatcher the first PM of the 20th century to achieve a third term. In Italy — and in stark contrast! — porn star La Cicciolina was making her mark on that nation's parliament, while Oliver North made his on the USA's Iran Contra hearings. In the pop charts M/A/R/R/S Pumped Up The Volume, and at the cinema audiences flocked to see Fatal Attraction.

On the hi‑tech music front, magazines focused on early digital synthesis, reviewing Yamaha's TX81Z FM module, Casio's AZ1 Phase Distortion keyboard, Kawai's K5 additive synth, and Roland's breakthrough S&S D50. Sampling was becoming cheaper, with the launch of machines such as Casio's FZ1, and SOS offered in‑depth coverage of the first affordable digital mixer, Yamaha's DMP7. In the free ads, analogue synths' fall from grace was painfully apparent. Korg MS20s, for example (these days about as trendy as analogue gets and selling for around £500), were fairly plentiful as digit‑struck owners dumped them at less than £100. Oh, for a crystal ball.

For the impoverished would‑be synthesist, and one who had coveted the instruments now being sold off as dinosaurs, analogue was still the way to go. After scrutinising spec sheets and the essential Keyfax Volume One reference book, I decided the 1982‑vintage Korg Mono/Poly, which had been priced at £689 on its launch, was just the thing to go with my dinky Roland MC202 Microcomposer synth/sequencer. At a second‑hand price of under £200, a Mono/Poly wouldn't break the bank, and there were several available in the music free ads. Spotting a likely candidate miles away in Bury St Edmunds, I (then car‑less) asked my parents if they would mind going to collect it, showing a touching faith in the essential goodness of human nature. I wouldn't be present, and they didn't know the first thing about analogue synthesizers. If it didn't work when it got back to my place, tough.

Happily, it did. In fact, it was perfect, and even came with a semi‑famous association, from the studio of a member of Sniff 'n' The Tears, whose one massive '70s hit, 'Driver's Seat', made it onto the soundtrack of 1997 film Boogie Nights.

Reasons To Be Cheerful

The Mono/Poly is handicapped in polyphonic mode by the fact that its four oscillators share a single filter and the same amplitude envelope.The Mono/Poly is handicapped in polyphonic mode by the fact that its four oscillators share a single filter and the same amplitude envelope.

Why had I chosen this particular synth? Back then, before I had even heard it, I was swayed by very practical considerations. Its second‑hand price seemed low for the facilities on offer; the 44‑note keyboard was larger than that of some of the competition; it had four oscillators; it had separate filter and volume EGs; the filter EG intensity control offered a negative value, something I'd always loved from a friend's Yamaha CS40M; and its gate and CV standards were compatible with my MC202. Like the 202, the Mono/Poly uses 1 Volt/octave CVs and positive triggering, though it has an S‑trig (negative) option too — all a bit of a departure for Korg. Though a second‑hand MS20 would have been cheaper for me at the time, it wouldn't have interfaced easily with the MC202, since the MS20 used Korg's more usual standard of Hz/V CV conversion and S‑triggering only. (For more on analogue control systems see the article in SOS March 1995 on interfacing MIDI with analogue synths.)

Another feature that I'm sure played a part in my decision to go for the Mono/Poly was its option to play with four‑voice polyphony as well as monophonically, hence the name. That was a big draw in those financially challenged days, in spite of the fact that most contemporary reviews found the polyphonic mode a bit poor, especially when compared to the power of the synth with all four oscillators stacked for a monophonic sound. Of course, so far I hadn't even heard what the Mono/Poly could do. Thinking back, I don't recall even being worried that it might be a lemon! Once I had it at home and started playing with it, though, it became clear that youthful optimism had not been misplaced. Even now that I've accumulated a few more years of sonic experience (and cynicism), the Mono/Poly remains a favourite.

Not that it's the same Mono/Poly. In the intervening years, I sold my original machine, and regretted it almost immediately. It was essential to buy another, but the price hike brought about by the analogue revival meant that I spent several years waiting around for a cheap example to turn up. It never did, so recently I bought one at almost the going rate, £375.

Returning to the younger me, I recall that the Mono/Poly was a little daunting at first. I didn't have a manual, either, which didn't help. Still, a knobby analogue synth without a manual is an entirely different proposition than a DX7 or D50 without a manual. Nothing is hidden, so you just tweak away until it sounds good. In this sense, the Mono/Poly is an easy synth to come to grips with, and it's rare to accidentally encounter a combination of control settings that result in no sound at all, as is possible with some analogue synths. Newcomers to the Mono/Poly who lack manuals these days can, predictably, benefit from the Internet: www.synthesized‑ has a manual to download as a series of JPEGs. Helpful tips are available at the Zen Mono/Poly site (, which is run by a knowledgeable enthusiast. Magazine articles discussing analogue subtractive synthesis in general can also help, since the Mono/Poly has a fairly traditional signal path.

Mono Tone

For a non‑MIDI synth, the possibilities for remote control are surprisingly comprehensive.For a non‑MIDI synth, the possibilities for remote control are surprisingly comprehensive.

Though the Mono/Poly has two modes, essentially monophonic and polyphonic, I imagine the majority of users would mostly use it as a monosynth. However, even then the polyphonic side of the instrument often comes in handy. For example, a 'Chord' mode lets you play four notes as a chord, freeze it, and then play (or arpeggiate) that chord monophonically. And all MP owners discover sooner or later that an arpeggio played in Polyphonic mode is a unique experience, making up for some of the disappointment of not having a sample and hold option on either Modulation Generator (Korg‑speak for LFO).

Each of the MP's four oscillators, which are stacked in mono mode, has a ±50 cent tuning control, waveform selector (triangle, sawtooth, pulse, and pulse with modulated width), and octave range control to transpose the oscillator's fundamental pitch. The synth can be further transposed, as a whole, one octave up or down.The pulse — or square — wave oscillator option is rather flexible, since you can define the pulse width manually or have it modulated by one of the Modulation Generators or the VCF's EG. One slight disappointment is that there's only one set of pulse‑width and pulse‑width modulation controls, which affect all four oscillators simultaneously.

The output of the oscillators (and the separate white‑noise generator) passes through a chunky 24dB/octave low‑pass filter, which will resonate to self‑oscillation. The filter is a gem, and can sweep from inaudible, house‑shaking bass to a rather piercing whistle. A tracking control is provided, to vary filter response according to what note is being played on the keyboard, and the dedicated ADSR filter envelope generator can be applied in negative or positive amounts. This has the effect of closing, rather than opening, the filter during the envelope cycle, with an effect that's harder to describe than it is to produce!

Not‑So‑Pretty Poly

The Mono/Poly's Effects section covers not reverb and chorus, but those analogue‑synth staples, cross‑modulation and oscillator sync, while its arpeggiator is basic but effective.The Mono/Poly's Effects section covers not reverb and chorus, but those analogue‑synth staples, cross‑modulation and oscillator sync, while its arpeggiator is basic but effective.

Next in line after the filter is the Voltage Controlled Amplifier, with its own ADSR envelope generator to set the volume (amplitude) response of a sound over time. The observant amongst you will now have spotted the reason why the Mono/Poly is relatively weak when used polyphonically: just one VCF and one VCA for all four voices, rather than one for each, as would be provided on a true polyphonic instrument. The result of this compromise is that the filter and amplitude EGs won't trigger for each oscillator, retriggering instead whenever a new note is played and cutting off the envelope of any notes already playing. A number of triggering options can help, however. Firstly, you can choose Multiple or Single triggering: Multiple causes every new note to retrigger the EGs, regardless of whether another key is already being held down (the held note will retrigger too); Single allows the EGs to be retriggered by a new note only if all other keys have been released. In the latter case, notes played over the top of a held note or chord will sound, but will not retrigger the EGs. Secondly, there is an Auto Damp switch, which automatically silences each note of a chord as soon as its individual key is lifted. Normally, if you play two notes as a chord, releasing one key will not stop either note sounding, and this can blur rapid legato scales played in polyphonic mode.

The Mono/Poly's sound is, in truth, rather thin when the synth is played polyphonically, though it does have its own appeal, with a sharp purity that for me provides welcome contrast to the all‑encompassing fatness of even budget Moog synths. There are two modes, which allocate oscillators in different ways for polyphonic playing: in Poly mode each note played sounds one oscillator, while in Unison/Share mode, oscillators are apportioned according to how many notes are being played. Play one note and all four oscillators sound; play two notes and each note gets two oscillators; play three, and each note gets one oscillator, with one remaining silent. If four notes are held down, each is allocated an oscillator.

In both modes some interesting results can be achieved, since unless you're extremely precise in your playing (and sometimes not even then) the notes that sound in a series of chords won't always be played by the expected oscillator. Giving the oscillators different octave settings can produce a nifty 'hocketing' effect, where the notes played will be correct in pitch but differing in timbre as they jump around the oscillators and different ranges. (Hocketing, by the way, is a medieval music term describing the practice of dividing a melodic line between different instruments or voices.)

They're Effects, Jim...

The inclusion of an 'Effects' section on the Mono/Poly might appear to have been a startlingly forward‑looking move on Korg's part. They're not effects as we now know them, however: the Effects switch introduces the delights of X‑Mod (cross modulation, something like ring modulation) and oscillator Sync, available individually or together. X‑Mod, variable with an Intensity knob, causes oscillators 2, 3 and 4 to be modulated by the frequency of oscillator 1, or (in the so‑called Double mode) organises the oscillators into pairs, where oscillator 2 is modulated by 1, and oscillator 4 by 3. When a 2‑note chord is played in Double mode, each note is thus modulated differently, for extra sonic variety. Oscillator Sync locks the pitch of oscillators 2‑4 to oscillator 1, creating a commanding, focused effect for lead and bass sounds. Like X‑Mod, Sync also has a Double mode which locks pairs of oscillators. In addition, a Frequency Modulation knob in the Effects section allows the slave VCOs (those being modulated or sync'd) to be further modulated by the VCF EG or MG1, which produces a very 'digital' sound.

Subtle and rich Effects 'treatments' can be created, but overall the Effects section tends to an aggressive, metallic spikiness. Back in the days when very few synths had patch memories, Korg saw the bright‑yellow Effects button as a way of changing instantly from one type of sound to another, as it alternates between a given patch and its 'effected' self — an idea something like the two 'Scene' buttons on Yamaha's recent 'CS' control synths. It works, too: with a little planning, it's possible to create a drastically changed version of the current patch at the touch of a button. Great in performance.

Fiddler's Elbow

In the years immediately preceding MIDI, when sequencing was nowhere near as widespread or sophisticated as it is now, an arpeggiator was a valuable facility for a synth to offer. Compared to modern arpeggiators, which often provide auto‑accompaniment‑like patterns, sophisticated groove and swing parameters, and the ability to trigger chords, the Mono/Poly's arpeggiator seems incredibly simple, yet it produces brilliant results. Just three switches control it: one selects arpeggiation mode (up, down or up/down), another is a range control (one octave, two octaves and full, which transposes arpeggiated notes up to a five‑octave limit), and the final one turns the arpeggiator on and off. This last switch also has a 'Latch' option, which continues arpeggiating the last chord played even when you remove your hands from the keyboard. The MG2 control provides the arpeggiator's tempo, which varies from roughly one step every 10 seconds to 30 steps per second. A 'Trigger In' connection allows an external trigger to advance the arpeggiator, but it requires a negative pulse. Most modern devices that provide interfacing between MIDI and pre‑MIDI gear (such as my Kenton Pro 4 interface) should be able to compensate for this slight anomaly, but I found that the need for a negative pulse meant I couldn't reliably trigger the arpeggiator from a pattern played by a suitable drum sound. Nor could I use the Pro 4's single trigger output to trigger the arpeggiators on two synths requiring different polarities.

As mentioned earlier, using the Mono/Poly's arpeggiator with the synth's Polyphonic mode is rather interesting. In what might be an unsuspecting precursor of the wave‑sequencing technology that appeared on Korg's Wavestation family nearly a decade later, each step of arpeggiation in Poly mode plays a different oscillator. The potential here is huge: since the four oscillators may have wildly differing settings — waveforms, octave ranges, tunings — each note can have a completely distinct character from the others. This can be as effective with a fast 16th‑note feel as with a slow speed that produces more of a textural effect. The results are very much a trademark sound of the Mono/Poly, though they remind me somewhat of the random effects achieved on other synths using sample and hold. Depending on what chords you arpeggiate, and their range, a pseudo‑algorithmic feel can be produced. More variety can be introduced by turning down the level of an oscillator or two during the performance, which has the effect of inserting a rest into the arpeggio pattern. Playing chords made up of odd numbers of notes enhances the random effect, by ensuring each oscillator plays a different note on each pass of the arpeggiator.

Connect For...

Interfacing is an area where the Mono/Poly scores rather highly. In addition to audio and headphone outputs, it has CV In and Out, gate (labelled as Trig) In and Out, a VCF cutoff frequency input, a VCO pitch input, a portamento footswitch input for switching the portamento facility on and off, and the arpeggio trigger input mentioned earlier. The VCF cutoff input accepts either a volume pedal or a control voltage to alter cutoff frequency; the same goes for the VCO input, to change VCO pitch.

What this connectivity means is that a reasonably sophisticated MIDI‑CV converter can provide all the control voltages and triggers necessary to access all the Mono/Poly's real‑time control potential via MIDI. The downside of its interfacing is that there's only one set of CV and gate I/Os, so it's not possible to play the Mono/Poly polyphonically from a MIDI sequencer. Nor can you arpeggiate notes played on a MIDI keyboard connected to the Mono/Poly (via a MIDI‑to‑CV interface) with the Mono/Poly's arpeggiator. Another MIDI limitation is that in normal circumstances the Chord option explained in the 'Mono Tone' section, earlier, is unavailable over MIDI. However, moving the CV lead from the CV In to the VCO input allows you to set a chord with the Mono/Poly's keyboard, and play that monophonically via MIDI. (Thanks to the Zen Mono/Poly web site for that tip.)

Warm & Cool

Though the Mono/Poly is easily capable of being as warm, fat and fuzzy as other analogue classics, it offers several additional interesting avenues for exploration. When tweaked correctly, it has something of the thin, metallic edge that many in the mid‑'80s thought unattractive in the MS20 (retro'd in SOS November 1996). And if one does the aural equivalent of squinting, it's not a million miles from the '70s‑vintage, John Carpenter‑esque, ARP 2600's electronic precision. The effect becomes more pronounced when you use fewer oscillators and set up simple patches with most controls left at minimum. Adding a little FM and oscillator Sync from the Effects section can enhance the haunting effect; the sound becomes even more purely electronic. It's good for generating random weirdness, courtesy of the polyphonic mode, especially in combination with the arpeggiator. In monophonic mode, the four oscillators combine to produce a range of harmonically rich, complex timbres — distinctive lead sounds that stick out a mile in a mix; rumbly, bass‑heavy textures; and solid bass‑line sounds, often with a digital FM‑style edge. Subtle, acoustic‑like timbres are also possible, with plenty of expressive possibilities courtesy of the foot‑pedal and footswitch inputs, and the flexibly configured pitch‑bend and mod wheels. (These can both be routed to alter filter cutoff frequency, overall pitch, or the pitch of oscillator 1. Such assignability was pretty unusual for the time.) One clue to inventive sound creation is to not necessarily use all the oscillators, nor fix them all to full on.

Pre‑MIDI Magic

Despite the fact that the Mono/Poly appeared at around the same time as the DX FM revolution and the start of mass adoption of MIDI, thus becoming old‑fashioned almost as soon as it was released, Korg apparently sold quite a few worldwide. It might not feel that way when you try to find one, however. It seems that folks who own a Mono/Poly these days tend to keep it. It's not a rare synth as such, but it's relatively difficult to find one second‑hand, which affects its price. A perfect example could cost £400 or more from a specialist dealer. In my opinion, though, that would be £400 well spent.

We Will Mend It...

Neither of the two Mono/Polys I've owned has been especially problematic in terms of reliability and maintenance. My current machine shows signs of not having been pampered in its lifetime, having a couple of cracked keys and a few dents, yet it has functioned perfectly so far. It's worth knowing, however, that the contact strip beneath the keyboard can cause problems. On my first Mono/Poly, this strip eventually became thoroughly gunged up, causing misfiring keys and keys that appeared to be completely dead. The cleaning process (which I undertook myself) is fiddly. Fortunately, accessing the insides of a Mono/Poly is as simple as removing a few screws and hinging the top panel upwards.

A quick straw poll of synth service specialists suggested that there are no insurmountable problems with servicing the Mono/Poly. Panic Music's Mike Swain did note, however, that the keyboard contacts shouldn't be cleaned with isopropyl alcohol or any solvent, for fear of corroding the contacts or perishing the rubber backing. A dry tissue or cotton bud can be effective. Interestingly, Mike also reports that Panic have a straightforward technique for recoating worn or corroded keyboard contacts, which is quicker than waiting for a replacement contact strip to appear.

I've always found the Mono/Poly's tuning stability particularly good: it occasionally (but not always, oddly) needs a warm‑up, and once tuned it barely needs touching for ages — certainly not during an average session. Fine‑tuning is relatively easy, since the main oscillator tuning pots are easily accessible, but Korg UK report that of the few Mono/Polys that come in for service, many need attention due to inexpert tampering with the tuning preset pots. So Mono/Poly owners should be careful, and if they're not sure of what they're doing they should call a professional!