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Behringer MonoPoly

Analogue Synthesizer By Gordon Reid
Published July 2022

Behringer MonoPoly

We test Behringer’s take on an underrated Korg classic.

Korg launched the Mono/Poly in 1982 but, from the outset, it was seen as a bit of an evolutionary mistake and was always overshadowed by its six‑voice sibling, the Polysix. Perhaps this was because people saw it as an under‑endowed polysynth rather than as an interesting monosynth. Its complement of a single VCF and a single VCA placed it firmly in the paraphonic rather than the truly polyphonic camp: you could play four separate notes at once, but they wouldn’t each have their own filter or envelope. The Polysix — and, to an even greater extent the Roland Juno‑6 — thus seemed to render it obsolete even as it was announced. With the Roland SH‑101 and the Sequential Pro‑One gobbling up the monosynth market and Roland’s new Juno‑60 redefining what was possible for a low‑cost polysynth, it had nowhere to go and was discontinued just two years later.

But some time after the turn of the century, people started to get rather excited about the Mono/Poly. At first, I thought that this was just a knee‑jerk reaction to the ballyhoo regarding all things analogue, but its reputation continued to gain ground until it assumed the mantle of one of the more desirable of the early‑’80s analogue synthesizers. And there is some logic to this: it offered some powerful features when used as a monosynth, not the least its uncommon complement of four oscillators, as well as oscillator sync and cross‑mod, good interfacing and a pitch CV input that used the V/oct standard rather than the less common Hz/V system found on previous Korgs.

Fast forward to 2021 and Behringer have released an unofficial recreation, to meet a demand that can (perhaps) no longer be satisfied by the second‑hand market. So, how does the MonoPoly (the Behringer, without a slash) compare with a factory‑original Korg MP4 Mono/Poly (with a slash)? Is it a clone, an homage, or paintwork engineering?

Face To Face

My first impressions of the MonoPoly were two‑fold. Firstly, it looks and feels very smart. Secondly, it’s much smaller than I had expected, to the extent that I had to place a plank across my X‑stand and sit the synth on this so that it didn’t fall between the supports. Much of the saving in size has been achieved by losing seven notes from the bottom end of the keyboard. Fortunately, the keys are full size, although the action is extremely light, and those who are expecting a bit more resistance may not like it.

Having placed it on its plank I lifted the control panel to angle it toward me, using the metal prop with the company name printed on it to hold it in position. Hang on... this felt familiar, which wasn’t surprising with an Arturia MatrixBrute sitting just a few feet away.

With my original Mono/Poly on the adjacent stand and the two connected via their CV+gate I/O, I hooked them both up to my sound system and to a signal analyser, the latter of which proved to be invaluable as the test progressed. Next, I upgraded the MonoPoly’s firmware to the latest version using the Synthtribe software (see box). There were some significant bugs in the version installed when it was delivered, but many of these had been corrected in v1.0.6.

I was now ready to compare the two, so I started by testing the oscillators to see how closely the Behringer’s four waveforms echoed the Korg’s. Since there’s no way to tap the oscillators’ outputs directly, I opened the filter fully on each synth, set the resonance on each to zero, defeated all forms of modulation, set the VCA contour to an organ shape, and played. I was very surprised to find that, with the exception of the triangle wave (which has weak high harmonics), all of the MonoPoly’s waveforms sounded dull when compared with the original’s. Nothing in the oscillator section seemed to explain this, so I turned to a more likely culprit — the filter.

It was the work of just minutes to identify the problem. With all of the other controls set to zero, the maximum cutoff frequency of the filter in my Korg is (in round figures) 19.6kHz. With the filter contour or key tracking pushing it even higher, the maximum reaches 21.1kHz. But the same settings on the Behringer revealed maximum cutoff frequencies of 6.5kHz and 7.6kHz respectively. This meant that the amplitudes of harmonics at 15kHz (around the upper limit of many adults’ hearing) were about 24dB down when compared with the original. Even half‑deaf musicians with a history of standing stupidly close to big speakers will be aware of that much attenuation. However, all hope was not lost, so please read on because we’ll be returning to this issue in a moment.

My next job was to test the contours. With both contours set to 0,0,0,0, the original produces a squelchy click. The same settings on the MonoPoly resulted in a slightly longer transient, although the difference is not great enough to concern me. Further tests showed that the maximum contour times were also different, with the Korg’s maximum attack and decay each lasting 13 seconds compared with the Behringer’s nine seconds. This means that you can create longer sweeps on the original. Unfortunately, the Behringer’s contours share two issues with the original’s. If you set the attacks of the VCA and VCF contours to their maxima and press a key, you’ll obtain a low‑amplitude click. Then, when you release it, you’ll obtain another. In addition, if you reduce the attack of the VCA to zero you’ll hear a significant bump. These are both attributes of the original, although perhaps a little more obvious on the Behringer.

I then tested the two modulation generators. Here, the results were rather closer, with the slowest and fastest LFO cycles being much the same on both, with similar amplitudes for each. So I moved on to the Effects section, which contains the controls for oscillator sync and cross‑modulation (FM). Sync can be a raw and abrasive sound on the original, but I wasn’t able to obtain the same level of aggression on the MonoPoly and, when sweeping the pitch of the slave oscillator, the huge ‘tear’ in the sound was muted in comparison. Likewise, the cross‑modulation was less engaging on the Behringer.

Getting In Trim

I now want you to imagine me standing with my chin resting between the thumb and index finger of my left hand, my brow furrowed, and an audible “??” escaping my lips. It’s an accurate image because I couldn’t imagine that anyone attempting to clone the MonoPoly would get so much right, but impair its sound so considerably. So I turned to the trimmers on the rear panel (see box) and identified the one controlling the filter cutoff frequency. A quick twiddle showed that this raised and lowered the frequency without obviously affecting any other aspects of the sound, so I set the filter to self‑oscillation, watched the frequency analyser and turned, and turned... and turned some more. Many rotations later, I reached the point beyond which it would go no further, and found that the maximum cutoff frequency was now 22kHz. What’s more, although the lowest cutoff frequency was slightly raised I could still get it down to 20Hz, which is low enough for any sensible needs. I also checked whether the filter would still track correctly, and it did. Having done so, I retested the oscillators. What a difference this had made! Everything on the Behringer sounded brighter and more alive, the four waveforms sounded indistinguishable between the two synths, and even the differences in the sounds generated by oscillator sync and FM were almost eliminated.

We contacted MusicTribe UK to ask about this and they told us, “During the years after the release of the Korg Mono/Poly, many have had their sonic characters changed, whether in the form of modifications or non‑original components being fitted. The engineers at Behringer made all these trim pots accessible so that users could tune the MonoPoly’s characteristics without the need for opening it up. The factory shipped Behringer MonoPoly is close to how an original non‑modified Korg Mono/Poly should sound.” I’m not sure that I can agree with this because, while I wholeheartedly approve of the provision of these trimmers, my Mono/Poly is ‘factory original’ and I don’t recall any other Mono/Polys that I’ve played sounding less than bright and lively. Perhaps my memory is playing tricks on me, perhaps my Mono/Poly is an aberration, perhaps Behringer’s engineers had access to poorly maintained originals when they were voicing theirs, or perhaps Korg’s documentation misled them. Whatever the reason, the review unit was dull when it arrived but now, having spent two minutes becoming intimate with a small screwdriver, it sounds greatly improved and within a gnat’s wotsit of my original.

Despite being smaller than the original, the MonoPoly retains full‑size keys, albeit fewer of them.Despite being smaller than the original, the MonoPoly retains full‑size keys, albeit fewer of them.

In Use

Other than a quick start guide, there’s no documentation for the Behringer MonoPoly. The manual for the original is freely available online, and you might think that would be all you need to learn to program and use the MonoPoly, because using a copy of a vintage synth is going to be much like using the original. As it happens, however, there are several reasons why that’s not true in this case.

Despite covering just 60 percent of the area of the original and lacking the wider spacing that made the Korg Mono/Poly such a pleasant and accessible synth to program, the Behringer MonoPoly’s flip‑up panel still looks and feels very nice. The same is less true of its keyboard. The reduction from 44 keys to 37 may not appear significant, but it is. Three octaves is pretty much par for monosynths, but one of the things that made the original stand out from its immediate competition was its wider keyboard. The loss of those seven keys is even more inconvenient should you wish to play the MonoPoly paraphonically.

Next, there’s a design error. Neither the clone nor the original have dedicated pitch‑bend and modulation wheels. Instead, each of the two wheels can control the filter cutoff frequency, or the overall pitch of the synth, or the pitch of VCO1 or, when the Effects section is activated, the frequencies of the slaved oscillators. The difference between the two wheels is that the one marked MG1 determines how much Modulation Generator 1 affects the chosen destination, while the one marked Bend sends a control voltage directly to the destination. On the original synth, neither wheel is sprung, which means that you can use Bend to determine and modify sync’ed and cross‑modulated tones. On the Behringer, the Bend wheel is sprung and returns to zero the moment you release it. This means that you have to hold it in position to maintain a modified tone — which is clearly nonsensical. Were I to use a MonoPoly, the first thing that I would do would be to open it up and disconnecting that bloody spring. Furthermore, tests showed that, when using Bend to sweep sync’ed and cross‑modulated sounds, the Behringer’s response wasn’t quite as smooth as that of the original.

With v1.0.6 installed, the arpeggiator now works correctly (including the original’s method of ignoring notes that lie above the maximum permissible) when clocked internally and via MIDI, but not always when clocked using the Arpeggio Sync input. It’s fine when either of 1pps, 2ppqn or 24ppqn are selected as the sync rate but, when 48ppqn is selected, it sometimes triggers just the lowest note played, and at other times it produces the expected notes with the expected durations when the rate is above a certain threshold, but not when it’s below it. Furthermore, the MonoPoly doesn’t seem to like irregular clock sources. The original doesn’t care about these — indeed, they can be used to generate patterns — but the Behringer sometimes drops notes. Hopefully these faults will have been fixed in the firmware by the time you read this.

I should also note that there’s a trigger latency of a few milliseconds when using the Arpeggiator Sync input. Some people have suggested that this is a problem, but I must admit that it doesn’t bother me. What’s more, it’s little different from the original’s performance so I think that it would be a bit disingenuous to criticise it.

Now let’s turn to MIDI, which is provided via 5‑pin DIN and USB sockets. When receiving MIDI messages, the MonoPoly recognises just note number, pitch bend and modulation. When sending, you have the advantages of full keyboard polyphony and (even though the synth itself doesn’t recognise it) velocity, but no CCs are generated by any of the knobs or switches. So far so good... until I turned up the volume of my sound system when it was connected via USB. Although the MonoPoly is, in isolation, significantly quieter than my original with respect to background hum, I found that it was now generating background burble and repeated noises: clicking when connecting to my MacBook Pro via a passive hub, and more extended noises when the two were connected directly. A bit of gentle hiss is to be expected from an analogue synth, but these noises were neither hiss nor gentle. Unfortunately, this problem is far from unique to the MonoPoly, but it meant that I disconnected it from USB and used 5‑pin MIDI (if needed) when I played it.

But let’s finish this section with an improvement. On the original Mono/Poly, turning on the Effects section always selected Mono/Unison mode and left it there when you switched the Effects off again. The Behringer MonoPoly remains in the selected mode throughout, which makes it much easier to skip from (say) paraphonic playing or multi‑pitched solos to single‑pitched sync and cross‑mod sounds, and then back again. It may sound like a small point but, for live performance, it’s more significant than you might think.

It’s therefore a fine monosynth for leads, bass, some great off‑the‑wall effects, orchestral imitations and more.

The Sound & The Money

Despite the MonoPoly’s shortcomings, there’s a lot to like here. The four oscillators can offer the same, rich swirl as before, and the filter retains its ability to resonate without sacrificing too much of the bottom end. Sure, I found minor sonic differences between the original and the copy when I went hunting for them, but these differences were too small to be of interest in anything other than a forensic comparison. Indeed, many sounds were all but identical, even when I pushed both synths into weird and wacky territory. So, if you crave the sound of an original Mono/Poly but can’t find one on the second‑hand market (or can’t find one at a sensible price), the MonoPoly might well do the job. You may not like the narrower keyboard or its action but, if these are not deal‑breakers, I don’t think that there’s anything to stop you checking it out. At £449$769 or thereabouts it’s not in the super‑cheap bracket that Behringer have occupied with many of their recent releases, but neither is it expensive.

Having copied many of the more popular monosynths from the ’70s and ’80s — the Minimoog, the Odyssey, the Pro‑One, the CAT, the SH‑101 and the Wasp to name but six — the MonoPoly may seem a strange choice for their next release, but Behringer obviously feel that there’s a market for it. So let’s return to my original question: is it a clone, an homage, or paintwork engineering? It’s clearly not the last of these, but neither is it quite close enough to be called a clone — the smaller format, the narrower keyboard and the sprung pitch wheel ensure that it can’t be viewed as such. Nonetheless, the sound is close enough to earn that accolade, and it’s therefore a fine monosynth for leads, bass, some great off‑the‑wall effects, orchestral imitations and more. But if you’re going to try one in a shop or at an exhibition, I would advise that you take a small screwdriver with you. If I had tested it without adjusting the filter cutoff frequency trimmer, I would have reached a very different conclusion.  


You’ll find no fewer that 22 trimmers exposed on the rear of the MonoPoly’s flip‑up panel. Twelve of these adjust the oscillators’ tuning and scaling, three adjust the filter, three adjust the amplifier, one adjusts the noise level, and three adjust the voltages of the ±15V and +5V power rails. None of this is described in the quick start guide so, with no manual to guide you, I would advise that you approach these with caution because any uninformed twiddling could seriously harm the performance of the synth.


The Synthtribe app.The Synthtribe app.The Synthtribe application manages several of the company’s products. When run on a computer connected to the MonoPoly it reveals the screen shown here and... umm, well, that’s not quite true. The window it opens is too small to display everything, so you have to scroll up and down to see all of the options. I created this image so that you could see everything at a glance.

I first used it to update the synth to the operating system that was current at the time of the review, and then to experiment with all of the settings that you see here. With the exception of the issues noted in the body of the review, everything appeared to work as it should.

The I/O Panel

Behringer MonoPoly's I/O panel.Behringer MonoPoly's I/O panel.

The analogue inputs and outputs to the right of the MonoPoly’s I/O panel echo those of the original. After the audio and headphones outputs you’ll find 1V/octave CV inputs and outputs as well as the misnamed Trigger In and Out. A switch allows you to select whether the gate pulse conforms to the S‑Trig or V‑Trig standards. Next come ±5V control inputs for the pitch and filter cutoff frequency, followed by a socket for a portamento on/off switch and an input to clock the arpeggiator externally. Happily, Behringer avoided the temptation to make it ‘Eurorack compatible’, and all of these connections are nice, friendly quarter‑inch sockets.

To the left you’ll find 5‑pin DIN sockets for MIDI In, Out and Thru, as well as a USB input for computer communication and MIDI. The final socket is for a 1A 12V DC external power supply. It won’t surprise you to read that I view the external PSU and its flimsy cable as inappropriate. The MonoPoly isn’t a small desktop box for which such things can sometimes be justified; it’s designed to be a real, performing instrument and it deserves an integrated PSU and a robust IEC cable.


  • It can sound almost identical to the original Mono/Poly.
  • (That means that it can sound very nice).
  • It’s smart and attractive, and the flip‑up panel is a nice touch.
  • It’s much cheaper than a second‑hand Mono/Poly in good condition.


  • The review unit needed adjustment before it sounded correct.
  • The keyboard is seven notes narrower than the original and the panel is 40 percent smaller.
  • The Bend wheel is sprung, which is neither original nor desirable.
  • Connecting it via USB generates noise in the audio output.
  • There’s no manual.
  • It has a lightweight external power supply.


Once adjusted, Behringer’s MonoPoly sounds almost indistinguishable from the original. There are still issues to be addressed but, if you want the sound and facilities of the Mono/Poly and like the design and smaller format of the copy, you should try it.


£449 including VAT.