Analog Synth Modules | Big Sounds in Small Packages

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Clockwise from top: Tom Oberheim SEM, Patch Panel edition; Dave Smith Instruments Tetra; Doepfer Dark Energy; and Dave Smith Mopho

Photo: Bill Schwob

As interest in analog synthesizers continues to increase, more and more manufacturers are getting into the game. The world of modular systems, in particular, is exploding with new products, especially in the Eurorack format.

But whether it''s a keyboard or a rack of modules, an analog synth can take up a lot of room. Sometimes you just need a simple synth voice—an oscillator or two, an LFO, a filter, an envelope generator, and a VCA—in a portable package to put that fat bass line, searing lead, or space-age burbling where you want it. That''s where a standalone synth module comes in handy.

These modules offer subtractive synthesis in a small footprint, providing the perfect complement to a DJ setup, a laptop rig, digital keyboard workstation, or the digital plug-ins in your DAW. Although all-in-one modules date back to the ''70s, technological advances over the years have allowed manufacturers to make them smaller and more powerful and increase their stability, without sacrificing the vintage tone.

In this article, I examine three single-voice, analog-synth modules released in the past few months, as well as a related 4-voice module, all of which are priced less than $1,000 (some considerably so). I admit that comparing the features of these instruments is very much an apples-to-oranges-to-bananas affair, but this isn''t a shootout to see which synth ranks highest. Rather, I want to show the differences in design philosophy because, on top of the sound quality of an instrument, the voice architecture and feature set inspires each musician differently.

Let''s begin with the instrument that has the longest pedigree.

Tom Oberheim SEM
Designed in the early ''70s to inexpensively augment monosynths by ARP and Moog, as well as support the company''s digital sequencer, Oberheim''s Synthesizer Expander Module (SEM) offered a basic feature set with a distinctive sound that has remained popular over the decades. The instrument''s creator, Tom Oberheim, has finally given in to the pressure of friends and fans by reissuing the SEM, following the original specifications as much as current parts availability would allow. Yet, like any restless developer, he took the design a little further by adding a few welcome features.

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FIG. 1: The Tom Oberheim SEM, Patch Panel edition, adds 33 patch points for increased sonic control.

The Tom Oberheim SEM is available in three configurations: the Patch Panel edition ($899; see Fig. 1), the MIDI edition ($899), and the Panel Only edition ($599). I received the Patch Panel edition for this roundup because the MIDI edition wasn''t shipping yet. But that suited me just fine as I prefer to work with patch cords. Because many readers will want to know to what degree the new version resembles the original, I will compare features between the two.

The classic SEM had a straightforward synth-voice architecture, with two VCOs; two 3-stage envelope generators (EGs); a 2-pole (12dB per octave) multimode, resonant filter; a sine-like LFO that goes into the audio range; and a VCA. The audio and CV I/O were on 3.5mm jacks. The new SEM is exactly the same, with a nearly identical front panel in layout and size. The main physical difference is that the new SEM isn''t as tall as the original, but the left panel adds an extra 2.25 inches to the module''s width. Both have rear panel power switches, and the new version has a rear panel ¼-inch output, which means you don''t need a 3.5mm-to-¼-inch adapter to use the module with a mixer or amp.

The SEM''s front panel has a slightly unusual layout. Each VCO can produce a sawtooth or pulse wave, but the waveform is selected in the filter section. The knob is actually a level control: Fully counterclockwise gives you the sawtooth at full volume; at the 12 o''clock setting, you get no signal; and the pulse wave is at full volume when the control is fully clockwise. At full level, the VCOs overdrive the filter in a very musical way. Each pulse wave can have its own duty cycle (10 percent to 90 percent), as well as individual pulse-width modulation or frequency modulation using the LFO or EG.

Another interesting design feature is that the knob that selects the filter type is continuously variable, from lowpass through notch to highpass. In the original version, the control would click into bandpass mode in its full counterclockwise position. The update has a separate bandpass switch.

The filter''s cutoff frequency can be modulated (positive or negative) by the LFO, Envelope 2, or an external source. Thankfully, the new SEM has separate controls for fine- and coarse-tuning of the VCOs. The vintage model had dual-concentric pots that stacked the tuning controls. (Fine-tune was on top of the coarse-tune.) Everyone hated them, including Oberheim, because it was easy to accidentally bump an oscillator out of tune.

Original SEM owners would often customize the synth by making internal features accessible for modulation by adding jacks to the panel, and that''s pretty much what the Patch Panel version offers. Sticking to the 1-volt-per-octave standard for CVs, the panel has 33 passive patch points that allow you to control parameters from external synth sources, process audio through its filter, send its audio and CVs to other modules, and, of course, interconnect the various features to broaden the unit''s sound palette. You can make this synth scream with only two patch cords.

So how close does the SEM redux sound to the original? Keep in mind that analog circuitry changes over time, and any 35-year-old synth is going to sound different than how it did when it was built. Yet the new version has that unmistakable SEM sound when compared to my vintage piece (serial number 100). The biggest difference I noticed was the LFO behavior: My vintage module has a slightly different waveform with a longer frequency time at the bottom end. But that might be due to its age.

The filter is largely responsible for the SEM''s signature timbres, and the update has the same growly and aggressive character as the original, offering attractively gritty harmonics when the cutoff frequency is swept with the LFO. Use it to create fat, buzzy basses or driving lead lines—it nails the classic sounds (see Web Clips 1a, 1b, 1c, and 1d).

My main beef is that on the passive Patch Panel version, the audio inputs do not have a preamp—a clear decision on Oberheim''s part to keep it as much like the original instrument as possible. Consequently, there''s not enough gain to help an external line-level input compete with the internal VCO output levels. Not only do you have to keep the VCOs at a low level if you want to combine signals, but you can''t overdrive the filter with an external source.

The MIDI version addresses this issue by adding preamps to the two rear panel ¼-inch audio inputs. According to Oberheim, one input goes through a high-gain preamp (with a level control), while the other input''s gain is merely doubled. However, there are four destinations for the inputs: VCO 1 and 2, the filter, and the VCA.

Of course, the MIDI version also features a MIDI-to-CV converter via a rear panel MIDI port to give you internally patched pitch CV and gate signals. You get a second CV that is assignable to three sources and three destinations, selectable note priorities (high, low, and last), LFO reset, keyboard tracking, a portamento control, and octave transposition.

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FIG. 2: With its FM capabilities, Doepfer Dark Energy packs a lot of punch into a small synth.

Overall, the SEM sounds as good as I had hoped. Although they don''t come cheap, I''d happily purchase a new one before dropping more cash on a vintage model. The multiple patch points are great for interfacing with other synth modules, the controls work well, and the ¼-inch output is handy. But it looks like my wish list includes both models: Although I like the flexibility of the Patch Panel edition, I also want the MIDI features and external signal boost of the MIDI edition.

Doepfer Dark Energy
German synth designer Dieter Doepfer takes the single-voice module concept a step further by increasing the internal modulation capabilities while retaining patchability. Dark Energy ($625) is a standalone version of the A-111-5 module designed for the Eurorack A-100 system. However, the standalone module augments the front panel I/O nicely by adding a USB/MIDI interface with associated gate and CV outputs, allowing the unit to cover a wide range of performance needs (see Fig. 2). Dark Energy is powered by an external 15V supply and housed in a rugged steel case with stained wooden sides. Its small footprint allowed me to set it conveniently on the top corner of my keyboard controller.

Although Dark Energy crams a lot of features into a small area, it''s fairly easy to use because, like the SEM, all of the parameters are at your fingertips. The top panel has 16 knobs, 12 3-position switches, and nine 3.5mm patch points (four CVs, a gate, and external audio as inputs; LFO, envelope, and audio outputs). Even with all of these controls filling out the top, you still have the option of adding one more knob (more on this in a moment).

The rear panel has the USB and MIDI In ports, a Learn button, and the four CV outputs and gate output on 3.5mm jacks. As you''d expect, Dark Energy is compatible with most modular synths. The instrument ships with a power supply, a pair of 3.5mm patch cables, a 3.5mm-to-¼-inch cable, and a USB cable.

Dark Energy has a fairly standard feature set: a single VCO offering a square wave and a triangle/ramp wave, a resonant 4-pole lowpass VCF, a 4-stage EG, a VCA, and a pair of LFOs. (LFO 1 routes to the VCO and VCA, and LFO 2 goes to the VCO and VCF.) The VCO tops out around 12kHz, and the LFOs go into the audio range. Right off the bat, Dark Energy can create very rich sounds when you mix the pulse wave, the triangle/ramp wave, and an external audio signal.

The MIDI-to-CV converter internally routes CV 1 to the VCO and a gate signal to the EG. While MIDI controls the VCO''s pitch, you can simultaneously patch a CV into the VCO''s frequency input to transpose sequences or add modulation—that''s handy. The CV 2, 3, and 4 outputs derive their signals from MIDI Pitch Bend, Velocity, and Modulation, respectively (although CV 4 can be assigned any Continuous Controller using the Learn button). You can patch the CV outputs back into Dark Energy''s inputs or use them with other analog gear—pretty much bread-and-butter stuff. But that''s just the beginning.

The real power of this little brute is in its modulation capabilities (see Web Clips 2a, 2b,2c, 2d, and 2e). To begin with, you can add exponential frequency modulation to the VCO from LFO 1 or the EG. However, you can also add exponential and linear frequency modulation, simultaneously, to the filter, resulting in an unexpected variety of crystalline and metallic sounds. The filter''s FM sources are LFO 1 or the EG for exponential and the VCO''s triangle/ramp wave for linear. The two types of FM can also be used on the sine-like signal of the resonating filter. Overall, the filter sounds remarkably lively, yet it''s fairly easy to get a nasty tone from it when you need some bite.

The manual offers a thorough explanation of the synth''s features, including patching examples. For instance, you can patch a back-panel CV to the front panel to control the filter''s cutoff frequency via MIDI. You''ll want to use long patch cables when patching the MIDI CVs to the front so they don''t obscure the top-panel controls. The unit also includes a simple 6-note arpeggiator—notes play in the order you hit them—that synchs to MIDI Clock or an internal clock (which you control with your keyboard''s modulation wheel).

You can link multiple Dark Energy modules into a polyphonic instrument using internal link connectors; the required cable is included. There is also room for customization within Dark Energy using internal jumpers. For example, you can change the rear-panel CV 4 jack to a second audio output if you don''t want a cable coming from the top panel. You can also add a pot to the front panel to access portamento. (The review unit included this option, and I found it to be a welcome addition when playing Dark Energy with a keyboard.)

The main complaint I have is with the MIDI converter: The resolution of the pitch bend and mod wheel data is not very high, resulting in noticeable steps. Connecting my Kenton Pro Solo MkII MIDI-to-CV converter gave me the smooth bends that I wanted, but Dark Energy''s portamento feature no longer worked. Hopefully, the data resolution can be increased with a firmware update. Other than that, the synth''s MIDI functionality is fairly straightforward, and during this roundup, I was happy to let Dark Energy handle my routine MIDI-to-CV needs so I could leave my standalone converter at home.

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FIG. 3: Don''t let its size fool you: Mopho takes the fat sound of a single Prophet ''08 voice one step further.

Overall, Dark Energy is a fun and great-sounding module to work with. Its compact size makes it an excellent patchable synth where portability is a concern, and it is user-friendly enough for someone just starting out in modular synthesis.

In January, Doepfer announced Dark Time, a 16-step sequencer featuring CV/gate and USB/MIDI I/O. Housed in the same form factor as Dark Energy, it''ll be a nice complement to the synth module when it ships later this year.

Dave Smith Instruments Mopho
The Dave Smith Instruments (DSI) Mopho ($399) is also a monophonic analog module, but it follows a distinctly modern paradigm, with its LED screen, patch-storage capabilities (128 patches in each of its three Banks), and full MIDI support (see Fig. 3). Unlike the other two synths in the article, Mopho doesn''t send or receive control voltages. But its clever design addresses many of the issues that took the fun out of synth programming since the ''80s (menus, menus, menus), while offering the ballsy sound of an analog signal path. It''s easily the most powerful standalone analog synth of its size and price range, yet it gives you real-time access over most of its parameters. Think of it as a MIDI-controlled expander module that fits in your carry-on bag.

Starting with the voice architecture of Prophet ''08—two digitally controlled analog oscillators, a noise generator, a filter, and a VCA—Mopho adds a sub-octave generator to each of the oscillators and an audio input. (Visit to read a review of Prophet ''08.) You can choose between ADSR-controlled 2-pole and (resonant) 4-pole lowpass filters—CEM-based filters that harken back to Prophet 5. You can also internally feed the left audio-output channel back into the filter to build extreme textures.

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FIG. 4: Tetra is a massive-sounding, 4-voice analog synth module that can fit in your backpack.

Like Prophet ''08, Mopho includes an arpeggiator and a gated (16x4) step sequencer. A free software editor (Mac/Win) is available online if you want to program the synth from your computer.

This sturdy metal module is about the size of a paperback book and features 12 knobs and six buttons, including the red button labeled Push It!, which can be used as a trigger. (The note it plays is user-assignable.) The four lowest knobs—assigned to filter cutoff, filter resonance, attack, and decay/release—give you immediate control over a patch in an intuitive way. Turn one, and the LCD shows you the parameter and the levels as they''re changed.

The four knobs in the middle of the unit are user-assignable—just hit the Assign Parameters button to their right. (They are pre-assigned with useful functions for each patch.) These allowed me to turn the arpeggiator on and off in real time, as well as change the filter type I was using for a patch. And it''s easy to reassign the knobs on the fly.

The other controls are straightforward and handy: increment/decrement buttons for program and bank; a knob to change the program; a button that takes you immediately into Program Mode/Global Mode; and a Write button for when you want to store your patch. The remaining knobs are for pitch (semitones), output level, and input gain. The rear panel is spartan, offering a pair of ¼-inch outputs, a ¼-inch headphone jack, a ¼-inch audio input, MIDI I/O, and a connector for the power supply.

Like other DSI synths, the factory patches in Mopho are both sexy and useful: sexy in that they show off the wealth of programmability in the unit, and useful in that they are musical while providing a great jumping-off point for experimentation. Although it''s helpful to know something about MIDI and subtractive synthesis to get the most out the instrument, it''s designed so that you can just go for it and tweak parameters. And it''s easy to get back to the original sound if you paint yourself into a corner.

Mopho is capable of some very big and expressive sounds (see Web Clips 3a and 3b). And with the internal arpeggiator and sequencer, it provides groove-oriented projects a flexible palette with which to work. But why stop at one voice?

Quadruple Your Fun!
Dave Smith thought it would be an interesting challenge to create a polysynth version of Mopho in roughly the same form factor. Tetra ($799) is exactly that: a 4-voice version of the little yellow monosynth in a case that''s only a half-inch larger and a few ounces heavier than the original (see Fig. 4). Yet it''s only twice the price.

Tetra''s front panel has a similar layout to that of Mopho, but with a few additions that prove handy with a polyphonic synth. The Program/Global button adds Combo to the list, which provides 128 patches that simultaneously use all four voices, whether as multi-timbral sequences, polyphonic patches, or monophonic stacks. The Edit B/Combo button lets you create and edit Combos, and the factory patches are a great place to start. (Download the software editor if you want to make it easier to edit.) As it turns out, Tetra''s Program 1 and 2 banks are taken from Prophet ''08, and the keyboard''s controls map directly to Tetra''s via MIDI.

Four LEDs ring Tetra''s Push It! button, and each one is assigned to a voice. They light as each voice is triggered in a multitimbral sequence, which I found to be very useful when editing or playing a patch.

Tetra''s rear panel also adds some features, such as four ¼-inch outputs that can be set up as Mono, Stereo, or one of two Quad settings so you can separate the voices by output. Another welcome addition is a USB 2 port that acts as a bidirectional MIDI interface so you control Tetra directly from your computer. And like other DSI synths, Tetra features a PolyChain port that lets you connect up to four Tetras, connect a Mopho (using its MIDI In port), or use Tetra as an expander for Prophet ''08 (see Web Clips 4a and 4b).

I was not at all surprised to find that Tetra''s factory patches sounded massive, were punchy, and covered a wide variety of musical styles. I particularly enjoyed exploring the sequenced Combo patches, many of which were immediate song-starters for me—they''re very inspiring.

If I had to decide between Mopho and Tetra, it would probably come down to how I wanted to use them more than the cost. Although Tetra isn''t difficult to use, there is more to think about when you''re programming. If I only needed one voice (for basses and lead lines) and was looking for a simple, great-sounding analog module that could store patches, I''d go for Mopho. (By the time you read this, DSI will have released a keyboard version of Mopho.) But if there was even a remote chance that I would play polyphonically or that I wanted to build 4-voice grooves, Tetra is a no-brainer. It''s about as powerful and user-friendly as any analog synth module will ever be.

Besides writing his blog, “The Robair Report,” Gino Robair is editorial director for Gearwire .com and a former editor of EM.