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Behringer TD-3-MO

Synthesizer By Robin Vincent
Published September 2021

Behringer TD-3-MO

Behringer’s TD‑3‑MO isn’t just a TD‑3 with extra knobs: it’s a completely different instrument.

A ‘modded out’ Roland TB‑303 is a particular and peculiar synthesizer. You start with something sweet, bouncy and juicy, take it apart and poke it with a stick until it gets angrier than a bag of wasps. And with the TD‑3‑MO, Behringer have applied this daring and, some would say, abusive procedure to their own TD‑3, revealing broader sonic territories, wilder tones and an angrier disposition. Even the smiley face on the front panel looks annoyed with the process.

The TD‑3

Behringer’s initial clone, homage or reimagining of the iconic Roland TB‑303 Bassline, the TD‑3 captured everything that made the 303 uniquely fun to play with. Offering bags of nostalgia for the original generation and something quirky for the uninitiated, it nailed the layout, the box and the workflow, and was close enough in the sound department to fool everyone other than the really nit‑picky. But then 303s were never that close enough to each other for that level of analysis. The important thing for most people is that you can turn one on, finger‑fudge your way to a sequence, and have it sound exactly as you imagine it would; and it does.

The TD‑3 added useful things like MIDI and USB, as well as a decent distortion circuit modelled on the Boss DS‑1 and a built‑in random pattern generator for some instant inspiration. The TB‑303 has a famously annoying sequencer that splits the pitch and time aspects into separate linear processes; this can make for laborious programming and editing, though I’ve no doubt that many of the best acid bass lines came out of fudging that sequencer and going with it rather than going back and trying to edit it. Behringer faithfully reproduced this, but improved the experience by adding the ability to tap in the timing to a metronome. They also added an alternative way to sequence the TD‑3 using the Synth Tool app, which means that if you’re not looking for an authentically frustrating sequencing experience you don’t have to find one.

Round the back of the MO we find a quarter‑inch audio output, full‑size MIDI I/O sockets, a USB port and an input for the external 9V power supply.Round the back of the MO we find a quarter‑inch audio output, full‑size MIDI I/O sockets, a USB port and an input for the external 9V power supply.

Devil Fish

The TD‑3‑MO is a ‘modded out’ version of the TD‑3, which incorporates many modifications that were hacked into the TB‑303 over the years by enthusiastic people with soldering irons, who felt there were secrets yet to be discovered inside the very pliable box and inner circuitry. Specifically, it is an unofficial reproduction of the famously mischievous and unruly Devil Fish TB‑303 modifications designed by Robin Whittle of Real World Interfaces. Adding new features like extended bass response, overdrive, expanded synth controls and CV connectivity, the first Devil Fish was introduced in 1993 and version 2, which is still offered today, came along in 1996.

The idea behind the modifications is to expand the range of the filter, pull out more envelopes, mess about with resonance, boost the bass, flirt with FM and overdrive the signal. The result is an altogether angrier and grittier tone that loves to squeal, throb and slap you about. Behringer sent me a standard TD‑3 for comparison and it’s safe to say that even with the distortion circuit, it’s a cuddly synth in comparison; the TD‑3‑MO is an entirely different animal.

The only supplied manual is a Quick Start Guide, which is lacking in detail: it tells you what everything is, but if you want to know what the controls are for, or how they vary from the original, you would do better to download the Devil Fish manual.

If you were thinking that this is just a 303 clone with a few extra functions, then you’d be dead wrong, because these modifications change the nature of the tone at some sort of fundamental level.

Fishy Fury

So, what’s different? Well, for one thing, filter cutoff now closes completely and goes all the way up to raucous. The Decay knob now controls the volume envelope rather than the filter envelope, which is taken care of elsewhere. It can be wound down to nothing, and then up through ticks and blips and bleeps to drone indefinitely, provided you hold the gate open. The Soft Attack knob softens the front end of non‑accented notes into some tiny semblance of attack. The filter envelope is controlled by some of the new knobs and switches on the right‑hand side. Normal Decay and Accent Decay shapes the movement of the cutoff for normal and accented notes independently. An Accent Sweep switch dictates how much resonance is available in the envelope decay from off to lots. These all then work with the original Envelope, Cutoff and Resonance knobs to wangle the filter into something approaching predictable behaviour.

With the Accent Sweep set to high resonance, the filter easily trips into self‑oscillation. This works in tandem with the Overdrive which, when wound down to nearly zero, will completely close out the VCA, leaving the self‑oscillations to stand on their own. The new Filter Tracking knob pulls these tones in line with pitch while working inversely with the Envelope knob. The Overdrive reintroduces the main signal at around the 2 mark and then pours on the warmth and saturation. It’s completely different from the Distortion found in the TD‑3, which I was surprised to find was missing from the MO.

One thing I’ve always found with the TB‑303/TD‑3 is that I just can’t leave it alone. I’m always fiddling with those knobs, tweaking and adjusting and enjoying every change within its limited range of tone. It’s intuitive, like dancing to your favourite tune: you know what’s going to happen and it’s always groovy. The TD‑3‑MO wants you to do the same, but this time you’re dancing with twice as many knobs and that sense of certainty gives way to a more dangerous, but no less fun, unpredictability.

The Filter FM knob and its companion, the Filter FM input, allow the entire signal chain to be routed back into the filter. This has the most effect with loud signals so ideally when accented and overdriven. I found that not a lot happened from zero to three o’clock and then it tripped into a crunchy bit of chaos. Putting audio into the Filter FM input results in similar noises, which are complex and interesting if you like that sort of thing, but nothing that resembles FM synthesis as found in classic DX‑series synths.

As well as all the new knobs and two exciting red buttons, the MO also gains a generous number of extra CV points.As well as all the new knobs and two exciting red buttons, the MO also gains a generous number of extra CV points.

The Sweep Speed controls the Accent Sweep Circuit, which pulses the filter frequency to add to the volume on accented notes. With a high resonance, you can hear the filter sweep up at the start of the note, which is what gives the 303 that familiar ‘wap‑wap’ sound. Sweep Speed can be set to Fast, Normal or Slow: to my ears, taking it from a spicy ‘wup’ to ‘wap’ to a throaty ‘whoa’. But the best thing about them is that they are cumulative. So, as you pound the keys or program in a series of fast accented notes, the wap and whoa get mangled into a satisfyingly sadistic scream. Not so much the wup.

As a final stage process, we have something called the Muffler, which dictates how an overdriven accented note is handled. If you whack up the overdrive and hold the Accent button then settings 1 and 2 do seem to take the edge off, but you really have to be pushing it to appreciate it. The Accent button is also a mod. It’s a momentary switch that engages the Accent level and envelope while pressed, providing a neat way of dropping in a boost or an alternative filter sweep.

The TD‑3‑MO includes one mod that didn’t originate with Devil Fish: the three‑octave sub‑oscillator. It switches in an unbelievably thick tone, which you can set to low, mid or high. It undoubtedly packs a punch and extends the reach into darkness, but there’s no level control and it’s a bit too dominating for my taste, as it seems to erase all remaining traces of that 303 sound. I really enjoyed the contrast, though, and the way the machine bounces back towards the 303 when you disengage it.

Conversely, there are a couple of optional Devil Fish mods that didn’t make it onto the TD‑3‑MO. One option was a number of switches for Dynamic Bank Switching, which enabled you to switch instantly to another pattern mid sequence, rather than waiting for a pattern to finish. Another one was a switch for the audio input to the filter. Both of these could be switched in or out or held momentarily, which is a really interesting way of bringing in a few new notes or an external source and then springing back to where you were.

CV Connections

The TD‑3‑MO offers a nice selection of new patch points to play with, including control of the filter, activating Accent and Slide and running the synth via CV and Gate inputs. Of course, you can already play the TD‑3 via MIDI, but it’s great to keep it in the realms of voltage control, and the Accent circuit displays a whole new dimension once you start throwing in some modulated gates. The throw of the filter from a ±5V LFO is a little underwhelming, but on the other hand it sits in there really nicely, keeping things moving and interesting without troubling the extremes. Playing it from a keyboard I was only able to get about two octaves over CV and four octaves over MIDI, which I guess is enough for a bass synth.

Playing the TD‑3‑MO from a keyboard has quite a remarkable effect because it encourages you to lengthen notes and listen more closely to the tone. All sorts of subtlety that’s so often lost in frenetic bass lines can be rediscovered. The Filter FM knob starts to show its quality, the differences between the Filter Tracking and Envelope settings becomes apparent, and the Slides and Accent add a lot of instant flavour and fun to your noodlings. I didn’t know a 303 could sound like this.


Behringer have shown themselves to be adept at replicating synthesizer circuits, and while I don’t have a Devil Fish modded TB‑303 for direct comparison I have few doubts that they would sound similar. I can tell you that all the features and functions on the TD‑3‑MO work as described in the Devil Fish documentation and if the vibe you’re after is one of a little synth being pushed to its limits, you’ll find that here.

Considered as an instrument in its own right, the TD‑3‑MO is quite different to the TD‑3. If you were thinking that this is just a 303 clone with a few extra functions, then you’d be dead wrong, because these modifications change the nature of the tone at some sort of fundamental level. There are shadows of that 303 sound in here, but these quickly get lost as soon as you start fiddling. And fiddle you will. So, if you are after the cuddly acid of the 303, don’t buy this: get the TD‑3.

The Devil Fish modifications are really a way of hacking a little synthesizer to make it behave in ways it wasn’t intended to. The results are quite distinctive, and I wonder how many people are looking for a 303 with this rare level of adjustment. You could argue that Behringer didn’t have to work within the limitations of the 303 design in the same way: perhaps they could have developed the TD‑3 into something more expansive and fresh. And why leave out the distortion that made the TD‑3 that bit more interesting? But then the Devil Fish mods are legendary and brilliant, and would usually set you back around $1500. The TD‑3‑MO is £159$199.

The TD‑3‑MO is different from what I expected. In many ways, it’s more fun and more engrossing than the TD‑3, at the cost of leaving the familiar 303 sound behind. While some mods feel like a stretch, the main combination of highly resonant filter, sweeping envelopes and fat overdrive makes for a very happy time. Get the TD‑3‑MO only if you want to explore something different, something more unruly, something that screams and groans and spits at you like it’s been infected and become a hazard to itself and others. Get it if you enjoy the subtleties of tones feeding back into the filter, if you have an unhealthy interest in decay, if you want to push in slides and accents in unexpected places and then fatten the whole thing out with a really deep sub‑bass. Get it if you like the 303 sequencer, get it if you don’t. It shouldn’t be your first 303‑style machine, but it might well be your last.


  • Peculiar and different in a good way.
  • Faithfully reproduces the Devil Fish mods.
  • Loads more to play with.
  • Three‑octave sub‑bass.
  • Synth Tool app and other functions for easier sequencing.
  • The price.


  • Poor documentation.
  • Mods are a bit specific.
  • That 303 sound gets lost.
  • Small octave range over CV.
  • Some things do less than expected.


The Devil Fish modifications move the TD‑3‑MO away from the classic TB‑303 sound and into interesting new territory.


£159 including VAT.