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Oberheim OB-X

The Oberheim OB-X was Oberheim's answer to the Sequential Circuits Prophet-5 released in 1978 which was the first fully programmable polyphonic analog synth. Its appearance put the concurrency into severe difficulties and also the sales of Oberheim went downhill. So they had to react and released the OB-X in 1979.

It was partly superior to Prophet-5 because it could have up to 8 voices (it could be ordered in 4, 6 and 8-voice configuration) and was very easily serviceable since everything was immediatly accessible simply after opening the lid for which you only had to unscrew 4 screws.

Also - other than the Prophet-5 which relied on itegrated chips for oscillators and filters - the Oberheim OB-X still used the discrete circuits (so no inegrated chips, only transistors, resistors and op-amps) of the former SEM (Synthesizer Expander Module) with the exception that it didn't offer the multimode functionality (only the 12bB lowpass part) and for the envelopes used two Custis chips (CEM 3310) to save space which still were fully discrete in the old SEM.

That's the reason why the OB-X is said to sound quite a bit different than the later models OB-Xa and OB-8 which both used Curtis chips also for the oscillators and filters. It somehow sounds more organic, more lively and wilder, and the filter clearly sounds depper and nobler than the a bit "pinched" Curtis chip sound. And it's very bright because of the temco-regulated discreete oscillators. Also it has more potential for wild sounds since the OB-X offered X-Mod (cross modulation, which is sort of a simple FM) which OB-Xa and OB-8 didn't offer anymore.

This fact (so the discreete ciruits) and that the OB-X only was bult for about one year before it was replaced by its follower OB-Xa makes it very rare and thought after on the used marked. It's been reported that only about 800 units were produced in total.

Regarding sounds the OB-X has benefits and losses compated to its followers OB-Xa and OB-8. It's unbeaten for lush, vivid and organic pads and sweeps. On the other hand it's not as good for snappy brass because of the missing 24dB mode and the way the envelopes act. So both the OB-X and the followers OB-Xa and OB-8 have their benefits and losses. Because of it's rareness the OB-X however has a much higher value on the used marked and so is the better asset.

In this video we are going to open the lid of an OB-X (Rev. 2) and have a look at the voice boards and how to tune them, and compare this to our plugin synth OP-X PRO-II:

If you have a look at the voice boards and search for a picture of the circuit board of the old SEM you will clearly notice that the voice boards of the OB-X look very similar. On the top right of the SEM circuit board you have the temco-regulated oscillators, on the left the filter with the CA3080 and LM741 opamps (the small chips), and at the bottom left and right the still fully discreete envelopes for filter and amplifier which in the OB-X voice cards are the two Curis chips (CEM3310) at the bottom left which of course consume much less space than the discreete circuits.

The OB-X had three revisions, Revision 1 (rev.1), Revision 2 (rev.2) and Revision 3 (rev.3). The OB-X featured in both videos above is a revision 2 (rev. 2) one with 6 voice boards mounted (2 slots still are free) on which the old tripots (prone to failure, the same as the tantalum condensors) were replaced with newer more pecise precision cermet trimpots which make tuning a lot smoother and easier and don't suffer from contact issues. Also in this device the hole for the pitch and and modulation levers was custom-framed with a custom-made black plastic frame since the hole without any frame looks somehow basic and crude.

The specs of the 3 revisions in short:

Revision 1 (OB-X Rev. 1):

The OB-X logo was on the right side above the keyboard and was only small. Different CPU-board. No gate-LEDs and the voice boards and no dip-switches to activate and deactivate single voicees, which both makes it harder to tune the device.

Revision 2 (OB-X Rev. 2):

The OB-X logo now was at the top left of the case and much bigger. Changed CPU board, gate-LEDs on the voice boards and dip-switches to activate and deactivate single voices, which makes it easy to tune the device. The Rev. 2 model is featured in the videos above. The only thigs changed in the featured device is the added plastic frame around the levers as well as the replaced trimpots on the voice boards.

Revision 3 (OB-X Rev. 3):

The three switches below the pitch- and modulation-levers now have grey square-areas around them, the lever hole often has a narrow black plastic frame (which in our Rev. 2 model we added by ourselves, but much broader), the on/off switch is on top of the case (used to be on the backside), the text label below the former "Reset" button on the left now reads "Chord" to clearly label this function which was hidden in the previous models, and on the CPU board there's now a computer interface socket to attach the DSX sequencer

Thanks for reading!