Dave Smith Instruments Mopho x4 Analog Synth

March 14, 2013

When it comes to modern analog synths with a pedigree, the trinity of Dave Smith, Moog, and Tom Oberheim have led the way for the better part of a decade now. Each has their sound and strengths, so saying that one is “better” than the others is like saying that chocolate is better than bacon—I can’t survive without either. In addition to my collection of vintage gear, I’ve had a Prophet ’08 since its introduction, and the Mopho-based products have made appearances in my studio as well, so I’m intimate with the sound. Naturally, when the Mopho x4 was announced, I was all over it, so here are my thoughts.




Unboxing the Mopho x4, I was struck by how compact it is. Part of this comes from the fact that the pitch and mod wheels are on the front panel as opposed to the end of the keyboard, but that presented me with no problems soloing. Despite its size, the construction of the x4 inspires confidence. Its charcoal metal casing feels like a tank, and the retro wood end panels seem quite hardy. The velocity- and aftertouch-sensitive keyboard feels identical to the Prophet.

The front panel layout is the same as the original Mopho, which means many of the knobs have multiple functions. For example, the two oscillators share identical functions, so there’s one set of knobs for both and users toggle between oscillators via a dedicated button. There’s a similar approach with three of the four envelopes. While I prefer a one-function-per-knob approach, the overall design of the x4 panel is sensible and quick to master.

Flipping the x4 around, the back panel is surprisingly uncluttered. In addition to both USB and five-pin MIDI, there are stereo outs, sustain and expression pedal inputs, a headphone out (with lots of headroom, I might add) and Dave Smith’s Poly Chain MIDI connector, which lets you add other Dave Smith synths, including the Tetra, original Mopho, Mopho Keyboard, and even the Prophet ’08, for up to 16 voices of polyphony. (With identical sound programs loaded into each one, the master keyboard sees the whole kit and caboodle as one giant synth. Note that the Prophet lacks the Mopho line’s sub-oscillators and feedback, so you’ll want to avoid patches that require those if poly-chaining.) I’d like to see an input for processing external audio like on the Mopho desktop module, but implementing that on a polyphonic synth can be a little tricky compared to a monosynth.


Since they’re based on the Mopho, the x4’s oscillators are actually more flexible than those found on the Prophet ’08. In addition to sawtooth, triangle, saw/triangle hybrid, and fully adjustable pulse waves, the x4’s oscillators also include sub-oscillators for each. These generate additional square waves—either one or two octaves below oscillators 1 and 2. As a result, the x4 can deliver massive textures, especially when the main oscillators are set to octaves or fifths at higher registers. Great stuff.

The oscillators can also be hard synced, which is great for those vintage squawky leads made popular by the Cars, and that my colleague Jerry Kovarsky has been studying in the past three issues’ “Art of Synth Soloing” columns. Hard sync is also super-useful for morphing and evolving riffs, especially when paired with the X4’s extensive modulation.

A noise generator, located in the mixer section, works wonders for pumping up techno and electro leads. Also in the mixer section is a feedback knob, which does that nifty old-school trick of routing the synth output back into the filter. At low settings, it’s a great way to add grunge and dirt to your sounds. At high settings, with a touch of detuning in the oscillators? Full-on audio mayhem.


The filters on the x4 are based on the legendary Curtis chip and are identical to those on the Prophet and Mopho, so they sound freakin’ awesome. The roll-off slope can be switched between a bright and lively two-pole (12dB-per-octave) and a subjectively warmer four-pole (24dB-per-octave) mode, the later of which can self-oscillate. If you’re going for that classic Oberheim brass sound, two-pole is the way to go. For darker ambient pads and screaming resonance, four-pole is the ticket.

What’s more, there’s an audio mod option that lets you use oscillator 1 as an FM modulator for the cutoff frequency. Boost the resonance into self-oscillation and this knob expands the x4’s palette into metallic and bell-like sounds as well as sideband-heavy insanity.

The rest of the filter implementation is straightforward stuff: cutoff, resonance, keyboard tracking, and bipolar envelope amount. Can’t argue with classic, right?


Four LFOs, four step sequencers, and slew of matrix modulation options deliver an embarrassment of riches in this department. The x4’s two-line LCD keeps you informed of what goes where if you start getting lost in the knobs and switches.

The four LFOs are identical to those on the Prophet ’08: five waveforms (including random), tempo sync via MIDI, key sync (for restarting the LFO every time you hit a key), and the ability to route the LFO to virtually any parameter in the x4, which is saying a lot when you start doing magic tricks with hard sync, resonance, and feedback amount.

One of the hallmarks of Dave Smith’s modern designs is the inclusion of four step sequencers in almost every product. These aren’t simple note-only affairs. Like the LFOs, you can assign them to an astonishing range of synth parameters. Quite a few factory presets make great use of these tools, notably “MetalNoizLoop” and “SteppingStone.” Sequences can be up to 16 steps and support odd lengths, so you’re not trapped in an eight- or 16-step world. 

As if that weren’t enough, the x4’s can route the LFOs and sequencers to additional destinations, as well as serving as the nerve center for MIDI controllers like the mod wheel, velocity, and aftertouch.

With so many modulation possibilities, it’s easy to get lost in the details as you tweak your sounds. Fortunately, Soundtower.com has an x4 software editor that delivers a lot of visual clarity, like their other editors for the Prophet, Mopho module, and Tetra. If you’re not going too deep with your own sound design, and instead tweaking with an eye to live performance, you can easily handle it all from the front panel.

In Use

To put the Mopho x4 through its paces, I made it sit in for my Prophet ’08 for two weeks. During that time, I discovered that the Mopho sounded considerably more aggressive than the Prophet, thanks to the dual sub-oscillators and feedback knob.

On the other hand, I never really felt the same intuitive connection that I have with the Prophet ’08, thanks to its plethora of single-function knobs and switches. On a Prophet, I can whip up an entirely new sound with sequences and complex LFO routings within a matter of minutes. With the x4, things go a tad slower because of all the toggling, which for most players won’t be a deal-breaker, because essentials like cutoff, resonance, and envelopes are right there.

In a track, the x4 slides right into place and does a fantastic job at everything from silky pads to nasty, in-your-face leads. One of my Facebook fans commented that it’s more of a “band” keyboard than a dance music tool, and while I’d gladly use it as my main lead synth in a band context, I also had zero trouble coming up with EDM sounds of all kinds and strongly recommend the x4 to any electronic producer looking for a true polyphonic analog synth.

That said, I was disappointed that Dave Smith didn’t include the Tetra’s multitimbral mode as a performance option, as it would have been a great way to add even more value to a feature-packed synth. Maybe in a software update?


You could say that the Mopho x4 is like half a Prophet ’08 for half the price, but that would miss the point. The Mopho architecture has a different sound. It’s way more aggressive in some ways and maybe a little leaner in others, but still light years better than any software emulation you’ll find in today’s market—and unlike much of the competition in the hardware realm, it’s polyphonic. The simple fact is this is the most cost-effective and sonically flexible analog polysynth on the market today. Here’s to another slam-dunk from a living legend.


Fully analog signal path. Curtis-based lowpass filters. Dual-sub oscillators, filter FM, feedback and hard sync deliver aggressive sonics and a boatload of flexibility. Keyboard senses aftertouch.


Programming the step sequencers is a tad fiddly from the front panel alone. Not multitimbral like Dave’s four-voice Tetra.

Bottom Line

Want real analog? Need polyphony? The Mopho x4 is the biggest bang for buck of anything out there.

$1,429 list | $1,299 street


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