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Oberheim

I have serviced Oberheims since they first entered the Country, often as 'grey' imports.
I was the official warranty repairer for the Importers, when these classics were built.

SEM SEM (Synthesiser Expander Module), a neat monophonic analogue subtractive synthesiser module. Very sought after.

Driven by an external CV (Control Voltage) & Gate. It is intended to provide, as its name suggests, an additional voice for a keyboard synthesiser such as the Minimoog. Inputs are provided for external signals to be processed by its two pole cell type VCF & OTA VCA.

Several external control options are provided for flexibility. The SEM forms the basis of the early polyphonic models, Two Voice, Four Voice & Eight Voice. A simplified main printed circuit provides voices for the OB-X, their first computer controlled model.

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4 Voice, one of their first series of polyphonic modular instruments. Uses four SEMs, shorn of their individual cases & power supplies. Mounted in a slope top rack style case with five octave keyboard, simple sequencer & programmer with sixteen preset memories. The common power supply has dual stage regulation, improving stability.

Some external control options are retained, giving flexibility. The programmer has an overall effect on all modules. Each can still be set to individual sounds & tuning. Alternatively, for true polyphonic results, they can all be set the same, which takes care & patience.

Four Voice Several keyboard modes are available, it took some time for most of the synthesiser world to catch up with this feature.

The Two Voice is a cut down version with duophonic capability. Some were fitted in 4 Voice cases, to allow for later expansion.

The Eight Voice is essentially two complete Four Voice units in separate cases, coupled by plug in cables. One case has the keyboard & control electronics. The other effectively functions as an expander, doubling polyphony. The master unit can be used on its own when four voices will do.

Working with this instrument one can see where the OB-X came from. Moderately reliable & mostly repairable. It is a rewarding addition to an enthusiast's portfolio. All that is needed is a love of early analogues, sufficient space (they are not small), patience for setting up & enough funds for skilled maintenance. Used on the road by brave souls but we would not recommend it. When they were current models, an all girl band from the South coast gigged with one, they were frequent customers.

Dot

OB-X, Oberheim's first computer based polyphonic. Programmable 8 voice, 16 VCOs, 32 patches. Voices based on the SEM sound module also used in the 4 & 8 Voice. Very unreliable, no maker's support, some parts unobtainable. Strongly built with legendary sound. Service access is good, with hinged panels, plug in boards & socketed ICs.

OB-x Like the modular Four Voice & Eight Voice two stage regulation is used. This however is not enough to make the tuning completely stable.

Auto-tune is employed, which 'sort of' works. I used to employ a design change of my own, which improved auto-tune capture accuracy. This should still work but all of this model have more serious problems (age).

It has a 'cross mod' feature, rather like polyphonic ring modulation. An interesting sound that puts the tuning even further out.
Too many cheap connectors & IC sockets make for constant intermittency. On one hand they make servicing practicable, on the other they make it more often necessary.

We serviced the first one in the country, brought in by its musician owner. It did not survive the trip over 'the Pond' in one piece. Discussion of multiple faults with an Oberheim engineer in California gave us a major reason for its troubles. I quote "Our accountants decided to save a cent on each of the connectors and IC sockets. So they give constant trouble". This was way back when they were new ! No precious metal used on contact surfaces, those connectors are now old and very corroded.

Non military grade ICs of course seldom ever had precious metal coated pins, unlike good IC sockets.

Dot

OB-Xa, an Oberheim masterpiece, programmable 8 voice polyphonic, 16 VCOs. Developed from its predecessor & has inherited some of the earlier unreliability. No maker's support, custom parts generally unobtainable. Well built & with legendary sound. Split & layered patches, later software expanded these from 32 to 128.

Auto-tune is a lot better than on the OB-x, but still tends to leave a 'warm' sound. A nicer way of saying 'not quite in tune'. We can improve these a lot, but don't expect them to be completely trouble free. They still have connectors, but would be un-servicable without them..

Dot

OB-8 OB-8, OB-Xa's successor, more reliable, better tuning. Split and layered sounds with more functions. Both models have spares problems, ensure everything works before buying. Again service access is good.

Start
Part way through production the keyboard changed. Out went steel shanked keys & gold contact wires. In came all plastic keys & polymer contacts. This may have improved short term reliability, at the expense of long term replacement possibility.

Dot

Xpander an Oberheim blockbuster, a 6 voice polyphonic analogue subtractive module with 12 VCOs. This one is almost frighteningly good. Oberheim were the one major American company that did not take part in the original MIDI negotiations, preferring to 'sit on the fence'.

They finally took the plunge with this model & redefined MIDI for ever. Key Zones, familiar to everyone now, came in with this machine along with full Matrix Modulation. Features were introduced that no-one has yet surpassed.

Fully multi-timbral, though with only six voices, what you can do with those voices is quite something. Each can be controlled by individual CV & Gate inputs. The results can then be sent by MIDI to other instruments etc. They can also each be set to respond to specific groups of notes &/or MIDI channels.

Common enough now, but no-one else had it then. Each voice has an individual output & can be programmed in the stereo mix.

Matrix Modulation has so many possibilities that mastering them all can take a lot of thought & practice. Any modulation or control source can be sent to any destination. The amount can be set individually for every connection & every voice. Few other machines can do this even now. The VCFs are multi-mode, several modes are distinctly different, clever.

As always Oberheim took Curtis chips & imbued them with their own sound.

Vacuum fluorescent alphanumeric displays (VFDs) show as much as possible of the large amount of information used in setting this beast up. Together with the usual LED indicators etc. Two eight bit CPUs are used, for the main board and the voice board. A single master clock drives both.

Auto-tune has been taken one stage further than everyone else. Each of the twelve VCOs has a chip temperature output. Read by the Voice CPU & used to correct the individual CVs for each voice. This results in an almost digital accuracy of pitch. Once Auto-tune has been pressed once it usually does not need to be used again, often for days.

A very definite & potentially serious problem exists. The Digital to Analogue Converter (DAC) is a multiplying fourteen bit type inherited from the OB-8. It was already obsolete when that model was in production, they had probably brought a large stock cheap & wanted to use them up.

No modern DAC that we have found will work, most are not multiplying & none are 14 bit. The multiplying feature is essential for the thermally tracked CVs.

A clever & dedicated engineer can possibly build a PCB to adapt a modern part but it will have to be good to maintain the tuning stability. Maybe someone has done it, all credit to them if they have. This instrument deserves to survive.

The usual problem with obtaining analogue custom ICs can plague this model. VFDs may also be a problem. Be very careful when working inside not to break them.

Dot

This example is seen during Corrective surgery at our Mercia workshops. After being 'worked on' elsewhere. An instrument of this calibre should not be given to just anyone for repair. Matrix-12

Matrix 12, developed from the Xpander with two voice boards, giving 12 note polyphony, & a 5 octave un-weighted keyboard.

Due to lack of space the CVs & Gates are omitted. A pity, more serious, individual outputs were originally not provided.

Late in production an ex-factory retrofit, for 12 outputs, became available, plus another giving mono pressure.

Certain later samples of both models were made in Japan, these are distinctly different. External finish is cruder & the power supply is a much cheaper line level switch mode type. The original type used a mains transformer with low Voltage switching regulation. Obviously the company was trying to cut costs later on, sound is still the same.

All the same parts availability problems as the Xpander. A worthwhile improvement to this model will be to replace the keyboard, if worn out or damaged, with a good weighted type. For such an expensive machine this should have been done originally. Both models are rewarding to play & can be frustrating to programme.

Start
Sound is typical of the marque, more versatile than previous models.

Dot

Matrix-6 Matrix-6, NOT a successor to the Xpander or Matrix 12. Six voice polyphonic with 5 octave keyboard. A pseudo analogue subtractive, using DCOs (programmable dividers) instead of VCOs. Design does not resemble the previous two.

Made in Japan, a different maker & probably a different designer.

Voices are implemented using a custom chip, integrating waveshaping, filtering & amplitude control. These were used in few other machines & are very hard to find. They frequently fail. Sound is acceptable but not what one expects from an Oberheim.

Matrix modulation is included but nowhere near as versatile. Changing the amount in one source link affects all other destinations from that source. In other words, in our opinion, it is not true matrix modulation. Clock sources for the DCOs are L/C oscillators.

Matrix-6R, a rack mount version, more practical, given the light keyboard of the M-6.

Dot

Matrix 1000, a smaller variant in a 1U case. Uses narrow versions of the same hard to get custom ICs. In our opinion the sound generating & modifying circuitry is not capable of 1000 noticeably different sounds. Probably not even a hundred. However it genuinely has a thousand patch locations. No editing controls, requiring a computer for everything except patch selection & volume.

Start

None of these models command a high price, DO NOT buy if they have any problems.

Safety rating "2".

Oberheim instruments comply with relevant safety requirements, there are no safety issues on any we have serviced. They are equipped with earth connections. Leave these intact for your own protection, especially on the Japanese built Matrix-12s. High marks.

Dot

All Brand & Model names are Trademarks and/or Copyright of their respective owners.

All opinions are those of the author, Ron Lebar.

Information given is generally brief & is based on our experience. If you spot any factual mistakes or 'typos' please feel free to let us know. We are not quite perfect & promise not to sulk over constructive criticism.

If you need more information on models listed or can suggest another instrument to add let us know & we will do our best. From time to time we may include comprehensive details of specific models.

For Technical queries, advice on operational problems etc. you are welcome to E-Mail us.

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Oberheim. Edited 5-9-2013. Ron Lebar, Author.