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Oberheim OBMx

Analogue MIDI Synth Module By David Crombie
Published September 1994

It's not only the distinctive looks of the legendary Oberheim synths that the designers of the OBMx have borrowed; they've also incorporated its legendary sound quality into a '90s MIDI synth made in the true analogue mould.

When it comes to "Monsters of Rock", the name Oberheim is well up there — indeed, the Oberheim name is synonymous with a big sound. And in the early days it also meant a big instrument. Take a look at the photo of the original Oberheim 8‑voice (which appeared in 1977/8) — the top section consisted of eight voice modules and two mixing panels (left‑hand side); beneath this was a device laughingly called a programmer, and two keyboard assignments control panels. Oberheim synths were built like tanks, and were almost as heavy. This one also had in excess of 200 control knobs. Fifteen years on, the march of technology means that we now have the OBMx, which in essence is the same instrument (plus more) in a 6U rackmount.

Tom Oberheim, the founder of Oberheim Electronics Inc (the original company) is, sadly, no longer connected with Oberheim. He runs a company called Marion Systems which devotes itself to sampler technology, and has also recently produced a new synth module, the MSR2. However, it is nice to see that he does get credited in the OBMx manual as having pioneered many of the basic concepts of the instrument.

Oberheim is now a "Division of Gibson Guitars". The design team who built the OBMx was headed up by Don Buchla who, as many of you will know, was a synthesizer guru of the '60s and '70s, and responsible for the Buchla 100 Series modules (from 1966) and the 200 series (1970). Basically, these formed custom modular systems of the "telephone exchange" variety, but in 1975 he pioneered computer control of analogue and digital sound modules with the 300 Series.

The Box

The Oberheim OBMx is an analogue programmable synthesizer module. When I say analogue, I'm referring to the signal path; the OBMx's control circuitry is digital (as on all the early polyphonics, such as the Sequential Prophet 5), but all elements directly relating to sound construction are analogue — no modelling here. Consequently we have real voltage‑controlled oscillators, rather than analogue filters, as with the Waldorf Wave. You can tell that this is the case because when you first turn the instrument on the oscillators are all slightly out of tune — brilliant!

Racks aren't the most stimulating of physical entities, but the OBMx certainly does have that Oberheim feel to it and looks both elegant and functional. The panel is that Oberheim creamy‑white colour, and the knobs (and there are a lot of them — 32 — for a rack unit) are mostly small and black (though not the same as the original Oberheims). Old Oberheims also used to have "industrial" illuminating buttons, but these have been replaced by the widely‑used momentaries with integral LEDs, which have in fact been around since before the original Oberheims, and were widely used by Oberheim's original competitor, Sequential Circuits. And, of course, there's the famous Oberheim logo...

You can see the basic layout of the instrument from the picture. All the controls are centred around a 40‑character by 2‑line back‑lit liquid crystal display, underneath which is the main set of programming controls. All outputs (except headphones) are located on the back of the unit, but I'm pleased to find that the on/off switch is on the front panel — so many companies stick them on the back.

Voice Structure

The OBMx, at its most basic, is a two‑voice synth, which can be upgraded by plugging in additional two‑voice cards. A fully loaded OBMx is 12 voices strong.

Along the bottom of the front panel is a set of buttons marked VOICE STATUS; the LEDs in each of these buttons light up to indicate which voice is being played at a given time. As well as being quite useful, these offer the added benefit that anyone looking at the panel can at least see some action!

Each voice consists of:

  • Two Voltage Controlled Oscillators
  • Two Voltage controlled filters
  • A mixer module
  • A Voltage Controlled Amplifier
  • Four Envelope Generators
  • Three Low Frequency Oscillators.
The OBMx unashamedly harks back to the early days of synthesis, but utilises present day technology to deliver a truly superb piece of kit.

The structure is in the true analogue mould, with the only concessions to technology being the way in which parameters are selected and controlled. Each oscillator can produce triangle, sawtooth or pulse waves (the width of which can be modulated), and in addition there is the ability to sync VCO 2 to VCO 1, or vice versa. In case you're not familiar with this effect, I'll explain: if you sync VCO1 to VCO2, every time VCO2 starts a new cycle, it resets VCO1 back to the beginning of its cycle. If the two oscillators are tuned close to one another and Sync applied, they lock together to become one waveform with no beating. This isn't particularly useful. However, if you change the pitch of the VCO, the effect is quite startling as the harmonic structure of the waveform changes. As you raise the frequency of VCO1, its output sounds rather like the sweeping of a resonant band‑pass filter. So when using the Sync effect, you would tend to use it in conjunction with modulation to VCO1 — typically envelope modulation.

The VCO control section is located at the top left of the instrument, and is cleverly designed to give the user as much instant access as possible. Buttons labelled 1 and 2 select VCO1 and 2, at the same time calling up the relevant information in the display and assigning the control buttons and knobs to the respective oscillator.

The control knobs provided are for the usual pitch and modulation parameters, but in addition there is a knob marked 'Track'. This determines how the oscillator will respond to incoming MIDI note data. If set to TT (Tuning Table) it follows the selected tuning map (see below) — i.e, in normal operation it responds as you would expect: moving up a octave on the keyboard produces a pitch rise of exactly one octave. However, this can be attenuated such that an octave jump on the keyboard produces a semitone rise in pitch. In fact you can even reverse it so that an octave rise on the keyboard produces an octave fall in the oscillator. If you set VCO1 in this manner and VCO2 normally, you do get some pretty interesting effects — playing a simple minor scale makes you sound a bit of a jazz wiz.

There are 10 preset tuning tables (TT) including Classic Meantone (most often used), Repeated Major Thirds, and some weird ones! There are also nine memory locations for you to program your own. When you do so you can map each incoming MIDI key number to any pitch. All very nice, but I often wonder how many people actually utilise these tuning options?

Early analogue instruments always offered FM — it just wasn't controllable. With the OBMx, you can cross‑modulate VCO1 with VCO2 to produce harmonically rich timbres which are useful for effects and possibly certain dance applications. The other interesting aspect of the oscillators is the portamento control. There's nothing particularly interesting about the feature, which couldn't be more basic — a simple pitch‑sweep between notes — but you can have a different rate for each oscillator, which leads to rather wild effects (I'll leave the assignable modulation capabilities until a bit later on, as this is a pretty neat section).

The two oscillators are then fed, along with a noise signal, to the VCFs.

Voltage Controlled Filters

In 'olden' times, before we had all this digital technology, the filter was equivalent to the soundboard of a piano, or the body of a guitar. It was the filter that gave the synthesizer its characteristic timbre, not only removing unwanted frequencies but also introducing a degree of distortion — a kind of distortion that added to the tonal characteristic of the sound. With today's digital instruments, little attention is paid to the effects of this distortion, with the result that some present‑day instruments sound flat and lifeless when you take out the on‑board effects.

The OBMx actually has two filters — one designated 'OB' and one 'MM'. The 'OB' filter is based on the original Oberheim state variable design. This has a 12dB/octave roll‑off, and will simultaneously provide low‑pass, band‑pass and high‑pass outputs. This type of filter was responsible for giving early Oberheim synthesizers their incredibly full‑fat sound (no semi‑skimmed nonsense here).

Alongside the 'OB' filter is the 'MM' filter. This stands for Minimoog. Yes, Oberheim have loaded the OBMx with a Minimoog filter. Whereas the Oberheim sound was big and brash the Minimoog filter imparted a depth and a warmth to the sound. Moog's filter, essentially the same as that found in the original Moog modular synths, was a 'ladder'‑type 24dB/octave 4‑pole low‑pass filter.

As with the VCOs, there are two buttons, labelled 'MM' and 'OB', with a single set of associated control knobs. Selecting 'MM' brings up the Minimoog filter parameters in the LCD display and automatically assigns the knobs to the Minimoog filter. The parameters consist of:

  • Filter Cut‑off Frequency and Resonance
  • Two modulation amounts — one from an envelope generator, the other from an LFO
  • Three input level controls for selecting the amounts of incoming signal from VCO1, VCO2 and the Noise source.

If you select 'OB', all these parameters are re‑assigned to the other filter. Consequently each filter is fully independent.

Envelope Generator 1 and LFO2 are pre‑routed to modulate the 'MM' filter, whilst EG2 and LFO2 are assigned to the 'OB' filter. Keyboard tracking switches marked 1/3 and 2/3 are used to determine how the filter cut‑off frequency tracks the keyboard — when both are on, the filter follows note for note.

One other interesting, if not astounding, thing about the filters is that they can both be set to ring (self oscillate) by winding up the resonance. So what? That's nothing new. Well, the big deal is that they can be played over an 8‑octave span and they keep pretty well in tune over the entire range. If you've ever had an analogue synth, you'll know that this was unheard of — analogue filter design must have evolved a little, at least, over the past 15 years.

Final Mix & VCA

The last link in the voice chain is the Final Mix section. This takes the output from the 'MM' VCF and the high‑pass, low‑pass and band‑pass OB VCF, and feeds it into the VCA. An offset control can be used to bias the VCA so that it sounds continuously if desired. Otherwise, the VCA is modulated directly from Envelope Generator 4. Emerging from the VCA, the signal can then be panned across the stereo image. This is all shown simultaneously in the LCD display.

Envelope Generators

Four multi‑stage envelope generators are provided. These are fired by incoming MIDI note‑on commands or by pressing the trigger button on the front panel. Four buttons are used to select which envelope is assigned to the control buttons and knobs

Each envelope is a hybrid DADSR (Delay, Attack, Decay, Sustain Release) — the hybrid element being that there are Decay Delay, and Sustain Decay parameters available. Decay Delay is the time the envelope waits at the maximum value before beginning the Decay phase; Sustain Decay is the rate at which the sustain portion dies away even though the note is being held — releasing the note instantly switches the envelope into the Release phase. There is also a Velocity Sensitivity control for determining the effect that changes of key velocity have on the envelope.

Three of the envelopes are pre‑assigned — EG1 to the oscillators and the 'MM' VCF, EG2 to the 'OB' VCF, and EG4 to the VCA. But these, as well as EG3, can also be used for other modulation jobs, as we'll see when dealing with the Matrix Modulation section

A nice feature of the OBMx is the fact that all time parameters are expressed in actual milliseconds or seconds, and attenuation parameters are calibrated in dBs; even LFO waveforms are depicted in the display — all of which helps to make the instrument easier to use.

Low Frequency Oscillators

Three buttons select which of the three LFOs is 'live'. Each LFO offers ramp up, ramp down, triangle and random waveforms, with Delay and Offset variables. Again, nice touches such as having the display show both the frequency and the period (the time it takes to complete on cycle) are useful.

The Modulation Matrix

The modulation routing section is very neat and flexible. Select 'Matrix' in the main control section, and a Patch comes up in the display. This is the software equivalent of plugging in patch cords (derived, no doubt, from Mr Buchla's history in the field of modular synths), and each Patch equates to a cord. This is similar to the system used in earlier Oberheim Matrix Synthesizers. You simply select the source from:

  • Pressure
  • Velocity
  • Note Number
  • Mod Wheel
  • Pitch Wheel
  • Breath
  • Foot Pedal
  • MIDI A‑D
  • LFO1‑3
  • EG 1‑4
  • Random

Then decide how much of it you want (positive or negative), and what it's going to control, which can be virtually any parameter, from EG4 Attack time, to Pan, to Portamento time. Once that's done, you can set up a second 'cord', and so on. Simple. Effective.

So now you're familiar with each voice, what can you do with it? The OBMx has two modes — Single and Multiple. Single, as the name implies, is a polyphonic mode using one sound, whereas Multiple lets you use up to six different sounds at once. Single mode sounds pretty straightforward — but don't be fooled. As those of you who have used old Oberheims know, one of the interesting things about these instruments and, in fact, one of the reasons they sounded so good, was the way in which voices were assigned to keys. I should point out that the instrument reviewed was a 12‑voice unit, so this does help.

In 'olden' times, before we had all this digital technology, the filter was equivalent to the soundboard of a piano, or the body of a guitar.

A voice algorithm (Al) is used to determine which synthesizer voice is assigned to which note played. There are 12 different modes of assigning notes. These include:

  • Unison (monophonic): any note played is assigned all voices — so you can get 24 oscillators sounding at once.
  • Rotate: each new note played is assigned a new voice. So if you play the same note over and over, the OBMx steps through the voices. This is interesting, as each voice, being analogue, sounds slightly different.
  • Repeat: this is a bit complicated, but in essence each new note played is assigned a new voice, so playing the same note repeatedly re‑triggers the same voice. This, amongst other things, makes better use of the voices available.

Other modes stack up voices, so you may get three voices per note (Triple) or six (Hex). And then there are the Chorus modes, which slightly detune voices (not just oscillators) against one another to get a yet thicker sound. This kind of voice assignment really only works with analogue synthesizers where, as I said, each voice sounds slightly different and all the oscillators are not phase related to each other. Oberheim have always offered the most in this area, and although it can get a bit confusing, it does offer unique opportunities. The Voice Status lights are actually a great help in showing what is going on.

In Multiple mode, you have up to six Parts to play with. You'd probably tend to use Single mode to create individual sounds, while Multiple mode would be used for creating performance presets with splits and layers, and for addressing different sounds on different MIDI channels using a sequencer.


In addition to the headphone and main stereo outputs, the OBMx has separate outputs for each voice. These outputs are on stereo jacks, but using a mono jack you can take a feed from a particular voice and remove that voice from the Main outputs. If you use a stereo jack you can use the output as an insert point and having processed that voice (say), subsequently return it to the Main outputs. It's a shame that there is no external audio input that could be fed to the Oberheim's filters, as this would enhance considerably the facilities the OBMx provides.

As mentioned earlier, the OBMx is available as a two‑voice unit, or multiples thereof up to 12 voices. Installation of each set of two additional voices involves plugging in a new circuit card. If you take off the top panel, you will find a motherboard, into which is slotted a master CPU board that handles all the voice programming, assignment, and front‑panel control systems. Up to six further two‑voice boards can be simply inserted.

The internal design of the OBMx is very neat and tidy. My only worry is heat dissipation: there is no fan and the boards are densely packed. Ventilation is through holes in the side of the internal board rack and slats in the side of the external rack. The synth does run quite hot.


I came across few negative points during my time with the OBMx. It is necessary to leave the unit on for 10 minutes and then hit the Tune button when starting up, and it does tend to go out of tune very slightly, but occasional use of the Tune button sorts this out. I didn't get a chance to rack up the OBMx and use it in a really hot situation, but I was assured by Oberheim that, despite the apparent lack of sufficient ventilation, the tuning algorithm can handle the worst possible fluctuations in temperature.

I know that the revamping of old instruments, to some, is akin to re‑issuing Elvis Presley records on CD, but I'm of the feeling that synthesizers don't have to incorporate cutting edge technology to be worthwhile.

The MIDI implementation of the OBMx is still somewhat basic. You can perform SysEx dumps of sounds, but individual parameter control is, as far as I could tell from the operating system supplied, still strictly limited. Nevertheless, a simple MIDI spec does sometimes tend to mean a faster response and I was more than happy with the speed of the OBMx's reaction to Note On commands. Conceptually, analogue synthesis is pretty straightforward, so programming the OBMx is virtually effortless. If I had to voice more gripes, I could say that the LCD display is a bit of a weak link, unexciting and a bit cramped. I would also have preferred all the knobs to be stepped (not continuous), so that each click would advance the parameter value by one unit; as it stands, the Data Entry knob is the only control that is stepped. In addition, since many parameters go from negative to positive with a central zero position, it is quite fiddly to null a value — so even if stepped controls weren't incorporated, a centre‑detent or some form of zeroing facility would help considerable. Another little grouse is that jumping between programs with keys held down isn't smooth and does sometimes cause a glitch.

But these are all minor points to justify my position as reviewer. Pricewise, the OBMx is on the expensive side, but do consider that the original Oberheim 8‑voice cost some $6,000 dollars 18 years ago!

I know that the revamping of old instruments, to some, is akin to re‑issuing Elvis Presley records on CD, but I'm of the feeling that synthesizers don't have to incorporate cutting edge technology to be worthwhile. The basic design of the modern piano is some 150 years old. It doesn't have to incorporate the latest carbon fibre technology or whatever to sound good. Synthesizers are established and the utilisation of established principles to produce pleasing and desirable sounds can't be wrong. I just think it's a pity that Oberheim didn't come out with the OBMx a couple of years ago.

Don Buchla is one of the few 'originals' still involved in making synths. The OBMx unashamedly harks back to the early days of synthesis, but utilises present day technology to deliver a truly superb piece of kit. Buchla's input has no doubt helped to make this a 'must‑have' device. Indeed, the OBMx is, to my mind, the best new synthesizer to come out for ages. (Perhaps 'new' isn't the most appropriate term, but you know what I mean.) The sound quality of this instrument just blows you away — paralleling a Minimoog filter with an Oberheim filter was a masterstroke.

Being a bit of an old synthesizer fart, I just love analogue synthesizers, so I wasn't too sure if this one would live up to my expectations. It did.


The OBMx comes with 128 Single Sounds and 128 Multiple Sounds stored in ROM. These are duplicated in RAM, so you have 128 user‑programmable memories for both Single and Multiple Sounds.

With 128 analogue presets to program at the factory, there are obviously going to be some 'fillers', but perhaps there were one or too more fillers than I would have expected. Don't be put off, though, if you step through the Single Sounds expecting to have your mind blown away. Some of the Single Sounds do appear a bit naff, but when you listen to the Multiples you realise that these sounds make up just part of a bigger sound, and this composite is awesome. To be fair, distributors MCM are aware that the presets don't really do the machine justice, and there is a possibility that some will be replaced before the machine is generally on sale.

A lot of the multiple sounds utilise a split, with a bass in the left hand. Both Oberheim and Moog synths were particularly good at bass patches, and presets such as 'Seq Bass & Lead', 'Monster of Ceremonies', 'Doomed!', and 'Triple Booger Bass' illustrate this. The Oberheim sound was also typified by big brass stabs — shown here, not surprisingly, by a preset called 'Brass Stab'.

There are hardly any imitative presets — no straight pianos, strings, etc — most presets being abstracts. I think, though, that you really only need presets on analogue synths to (a) give you an idea of what is possible, and (b) to act as a springboard from which to create your own sounds. So although there probably aren't too many sounds you can say "Yes I'll use that...." of, there is a good collection of "can‑do's".

It's easy to create you own sounds from scratch — and there is a patch called 'Initial Conditions' in the Single Sounds which is a basic starting point. A corresponding Multi preset is called 'Default Program'.

Having said all this, I will point out that there are one or two presets on the OBMx that you should try out if you get the chance:

  • 'HOLD MIDDLE C' is the synth explosion to end all synth explosions.
  • 'THEREMIN' is a simple monophonic patch, but is nicely voiced to give that ethereal sound.
  • 'ICY SOLO KEYS' is a great lead sound.
  • 'EPHEMERAL' is an unusual, atmospheric pad that would work well in a lot of applications.
  • 'SYNCSPLIT' utilises the oscillator sync facility to give an harmonically rich solo voicing that really cuts through.

In terms of sound quality, the OBMx cannot be faulted.


  • Dimensions 6U rack, 480mm (W) x 220mm (H) x 200mm (D)
  • Display: 2‑line x 40‑character back‑lit LCD
  • Single Sounds: 256 memory locations (128 preset)
  • Multiple Sounds: 256 memory locations (128 preset)
  • Multitimbrality: 6 part
  • Modulation Patches: 12 (max)


  • Audio: Main Out (x2); T/R/S inserts (x12)
  • MIDI: In, Thru, Out
  • Headphones
  • Mains Power In

Feature Summary

  • Fully Programmable, Multitimbral Analogue Synthesizer Module.
  • Oberheim and Minimoog‑type filters (VCFs).
  • Hardware Expandable (up to 12 voices)
  • Voice Complement: 2x VCOs, 2x VCFs 24dB/oct. LP and 12dB/oct HP/BP/LP, VCA, 4x EG, 3x LFO, Mixer.

OS version reviewed 1.04.


  • Superb sound quality.
  • Minimoog and Oberheim filters.
  • Modulation section.
  • All Parameters expressed as true values — for example, Decay Time .001s to 30s.
  • Excellent manual.


  • Relatively high price.
  • Locating knob null positions difficult.
  • No external audio inputs.


In short, a superb sounding module. An analogue dream.