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Clusterchord, thanks for your update. I stand corrected. Thanks for expanding my knowledge on these machines. I checked with my brother and it was not an OB-Xa that we saw in the workshop but an OB8, as he reminded me that it had MIDI and the exposed pan pots for the voices on the side panel. I have no doubt that you are correct, and your photos have helped resolve this mystery.
I have not seen the insides of an OB-X. The only ones I have seen used were by Richard Barbieri of Japan and Brian May of Queen on the Flash Gordon sound track. I have not heard of anyone having to tweak OB-SX voice cards to make them fit an OB-X, but was going off what my service engineer said, although he now deals mainly with digital stuff.
I've re-read the OB-SX manual with your corrections in mind and note that in one section Oberheim say that it "... uses much of the same circuitry as the OB-X", then on the back cover says "... utilizing the same circuitry as the celebrated OB-X". This seems to be a contradiction, but both statements could in fact be true. It all depends on what we read into "much of the same circuitry" and "same circuitry". This was a user's manual afterall, so is not going to go into great detail about the small circuitry differences between the OB-X and OB-SX. Maybe Oberheim were trying to say that most of the circuit board concepts were derived from the OB-X, but leaving room for "improvements" or "updates" by switching to CEM based sound cards, yet still in essence giving an OB-X sound.
One point made earlier is that the OB series is an ever evolving project. Therefore, it could be that the OB-SX is in fact a hybrid or transition model between the OB-X and OB-Xa. Maybe the basic circuitry connecting the VCO's, VCF, VCA's and Envs is the same as per OB-X, but they substituted CEM chips for the discreet functions of the OB-X.
As I originally stated, a CEM3020 can be modified to operate in 12dB/Oct mode, even though it is mainly used in 24dB/Oct mode, so it might be that Oberheim customised the circuitry surrounding the CEM3020 chip to get as close as possible to the discreet circuitry of the OB-X and hence "the OB-X" sound and 2 pole (12dB/Oct) filter they refer to.
The Voice architecture is, TTBOMK, as the OB-X, but using the CEM chips instead of discreet circuits, as a forerunner to the OB-Xa. There are some sounds I get on my OB-SX that clearly have cross modulation between Osc 1 and Osc 2, to get Honner Pianet type piano sound, or metalic sounds. From what I remember using the OB-Xa, it replaced Cross Modulation with a Filter Envelope, so you could use the Filter Env to modulate the pitch for those falling pitch Prophet V-ish type of sounds.
Your comment that the OB-SX sounds closer to an OB-Xa is as valid as mine that I have heard OB-X sounds in mine, as I've recognised ABC8 as close to a sound Brian May used with his OB-X on Queen's soundtrack to the Flash Gordon movie. Then again some sounds are as you say OB-Xa identical.
The more I think about it the more I'm convinced that the OB-SX is a Hybrid beast forming the stepping stone from the OB-X with it's discreet S.E.M style circuitry, to the CEM based OB-Xa. Oberheim might have used the OB-SX as a proof of concept in the CEM based production techniques as a forerunner to the OB-Xa.
I can only envy you for having an OB-Xa and an OB-SX side by side.
I've enjoyed learning more about the X family from you all, so thanks for taking the time to add your comments and pics.
Pete
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