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The OB-SX is a preset analog polysynth with limited programmability. I always wondered if it was closer to the OB-X or OB-Xa. According to the owner's manual notes below it has the same circuitry as the OB-X.
Update via the comments: "The OB-SX manual says its the same as the "Xa" not "X"! Looking at it right now..." Note: check out the update towards the end of this post as well.
"From the Owner's Manual:
'The Oberhiem OB-SX brings polyphonic synthesis to the discerning keyboardist at an affordable price. It's as simple to use as the popular "string machines" with the versatility of sounds achievable only with a variable synthesizer. The OB-SX is a real synthesizer utilizing the same circuitry as the celebrated OB-X. User programmability is unnecessary since the 'SX' is programmable with 24 sounds including those sounds most used and requested, such as: strings, brass, clavinet, organ, electric piano, bass, flute, and synthesizer solo. Other great sounds, many unique to the 'X' family, are also included. Each program can be modified to ones' own personal taste by adjusting the front panel controls (since they are constantly in edit mode). The programs (sounds) are permanently stored on a plug-in computer memory chip. Should 24 programs not be enough, an additional 24 are available for a nominal charge*. A switch on the back panel is provided to switch from each group. Custom chips can be used by sending an OB-X cassette tape to Oberheim (first 24 programs can be used). The key ingredient to the OB-SX is: although it's as simple to operate as your typical string machine, you can update your sounds by changing program chips.'"
Update:
"Strangely enough, the original sales flyer for the OB-SX (printed around the summer of 1980 I presume) claims it utilizes the same circuitry as the OB-X.
Read for yourselves here (the white text on black background): link
Maybe the SX was originally meant to utilize the same circuitry as the OB-X but that in the end Oberheim chose to base it on Curtis chips seeing as the OB-Xa was soon to replace the OB-X at the start of 1981.
The first OB-SX's (mk I) started to ship in the late summer/fall of 1980 so it did predate the OB-Xa. I'm not sure when the first mk II's shipped though...possibly in the spring/summer of 1981!?
Btw, does anyone here have the 2nd (or 3rd) edition of the service manual for the OB-SX (mk II)? It should be mentioned in there when the mk II first appeared.
Micke"
Update via PeteM in the comments:
"I'm a little late here, but also have a 56 Preset, Blue striped, genuine 5 Voice OB-SX, despite all references stating that the OB-SX only came in 4 or 6 voice configurations.
IMHO it is probably best to think of the OB-SX as a hybrid between the OB-X and OB-Xa, as it shares features with both. I did wonder if the OB-SX was a proof of concept for changing the production techniques from the discreet circuitry of the OB-X, to the more reliable Curtis Chip based voice card circuitry of the OB-Xa. Personally, I consider the OB-Xa as an advanced continuation of the OB-X model, rather than its replacement.
My Owner's Manual First Edition August 1980 states:
"The 0B-SX has been developed using much of the same circuitry as the Oberheim OB-X in order to get true polyphonic synthesizer sounds. This gives you the sound of an 0B-X along with the ease of operation of a pre-set synthesizer."
Owners with later User Manuals can also verify that their manuals refer to the "OB-Xa" instead of the "OB-X", so both camps are correct. This would suggest that the OB-Xa was the next evolutionary step of OB-X project.
As others have already stated, the OB-SX has the Voice Architecture of the OB-X, but using a manufacturing process relying on Curtis chip based voice cards like the OB-Xa. Some of the sounds in the first 24 presets bear testimony to the Cross Mod of the OB-X, yet it generally sounds more like an OB-Xa in 12dB/Oct mode. The sonic similarities between the OB-SX and OB-Xa are largely due to the CEM3340 Oscillators, CEM3320 Filter and CEM3310 Envelope Generators that were common to both machines.
The main differences in voice architecture between the OB-SX and OB-Xa are that the OB-SX uses Oscillator Cross Modulation like the OB-X, whereas the OB-Xa uses Filter Envelope to modulate the Oscillators instead. The OB-SX voice card only has a single CEM3320 Filter chip in 12dB/Oct mode, whereas the OB-Xa has both a 12dB/Oct and 24dB/Oct filter chip on each voice card.
Kenton can partially MIDI retrofit an OB-SX to recieve only Note On/Off data using an OB-X MIDI kit rather than an OB-Xa kit, which probably creates further debate about which OB family member the OB-SX takes after."
