Sharing strikingly similar aesthetics, it’s no wonder I get this question 3 times a week. What is the difference between the 3? They look the same, therefor they must sound the same, right?

No. They’re all different. Worse, different revisions might even sound different.

To make matter’s even worse, there’s a mound of conflicting info out there on the internet to make it even more confusing… often from those who not only don’t own an OB, but may have never even played the models they’re in question!

In this blog post, we’re going to go through what we’ve seen in these machines after restoring over 100 OB’s. We’ve seen what seems to be every possible configuration at this point, and thusly write the following with some authority. If you believe we’re incorrect, please do email us to discuss. We’re happy to correct where needed.

I’m going to try and avoid subjective phrases like “warmer” “wider” etc as much as possible.

The OB-X

This is the original “Oh-Bee”. Sporting a greyscale aesthetic complete with arcade cabinet carpentry, the OB-X was Tom Oberheim’s answer to the Prophet 5 as well as his streamlining of the SEM module.

The OB-X was available in 4, 6, and 8 voices from the factory. Unlike the Xa and 8, people actually chose less voices at the time to save money. This means 4 voice OB-X are actually in the wild!

Note: The OB-X computer is very primitive, utilizing 2708 ROM’s, 6508 RAM, and AMD Static RAM which fail often and are not available new (however plentiful used). For this reason, we consider the Encore Electronics kit to be an important part of the battery of servicing done to an OB-X whether you mount the midi jacks or not. It brings the computer to exactly where it needs to be to be reliable. There are other updates to do, however this is not a repair guide.

Unique features compared to other OB’s:
Cross Mod (X-Mod)
Discreet voice architecture (sans envelope)
“Edit” Button
No dedicated LFO
AutoTune does -not- defeat failed or dead voices


Rev 1: Earliest rev we’ve seen had the OB-X in alternative logo on right side of keyboard above the top octave. Overall texture seemed different on both units, but could have been storage conditions affecting that. The end cheeks were different with lighter, cheaper looking veneer and smaller finishing washers for the screws.

We refer to the top later board with the Z80 as the CPU board. We refer to the lower board as the lower control board.
Electronically, this revision has a different CPU board than later revisions. The overall topology and component designators are different. The 2 early OBX’s we’ve seen had all 3 ROM sockets populated. Installing Encore in these revisions is different in regards to the reset signal.
The CPU and Lower control boards in this revision did not have any solder mask and sported a tan “bare PCB” color.
Voice boards in this revision have no gate LED. The overall topology is identical to later boards however seems to have a few less resistors in the oscillator area. Otherwise they are identical. We have no found any real sonic differences between voice board revisions, and have seen OBX’s with both revisions mixed in with no adverse affect.
This revision has different switched for test modes and voice defeating. I will update this post in the future with their functions.

Rev 2: Physically, this revision looks much different than rev 1. This is the revision that is what people know as the OB-X, aesthetically at least. The “chord mode” button may read “Reset”. The end cheeks are glossy dark veneer and sport the same 8/32 screws and #8 black zinc finishing washers found on all future OB’s.

This revision uses the rev 2 voice cards which have the gate LED, and an updated computer board that is simply “rationalized”. Instead of the odd switch scenario in Rev 1, this one follows the topology in the service manual, with a DIP switch for voice defeat, and test mode toggle with the big slide switches.

Rev 3: Physically, this revision reads “Chord” instead of “Reset”. The Bender cage has grey blocks silk screened below the paddles, and may have a plastic trim piece inserted as well (this is not found on all OBX!).

Inside, the computer board is extended for the computer interface to connect a DSX sequencer. All other properties are identical to Rev 2.

Is it a programable SEM Poly? Kind of? They do sound quite similar assuming the SEM is only using the LPF.
Both utilize the same basic architecture for their oscillators, using a CA3046 transistor array and tempco resistors. For the low pass filter, they each use the same architecture of CA3080 OTA’s. The envelopes in the SEM are discreet and use 3 stages (ADR). The OB-X uses a CEM3310 envelope IC giving it ADSR. I like the CEM3310 a lot as an envelope and I’m glad they went with those. The VCA is similar in architecture if not identical as well.

Unstable- Yes, these drift! But with choice modifications to the auto circuit and voice card, they lock right in every time. Other unreliability are related to your technicians skill level and experience with the OB-X architecture.
Full of CEM chips- Nope. But does use CEM3310 EG’s, which are now available through Alfa.
Parts impossible to find- Definitely not the case. Everything is still available perhaps except the diode arrays on the voice mother boards which are easily replaced with standard 1n4148 diodes.

The OB-Xa

This OB went through a lot of changes through it’s history. Oberheim was keen to make an updated poly with new features to be the successor to the OB-X. However, they only share an aesthetic (I am not 100% sure, but I believe the front panel is literally the same except the graphics…)

The Xa, like the X, was supposedly available in 4, 6, and 8 voice configurations. With the Jupiter 8 on the market, perhaps simply no one ever went for less than 8 voices, because after restoring dozens of these, I to this day have never seen an OB-Xa with less than 8 voices.

The Xa was the first Oberheim synthesizer to utilize programmable split & layer functions. However, this came at a major price for field service then and restoration level service now, as the Xa operates more like two 4-voice synths rather than 1 8 voice synthesizer. This means discrepancies can occur between top and bottom voice trays (each tray holds 4 voice cards) that are noticeable by the user.
It’s my opinion that these discrepancies are what have given the Xa an unreliable reputation. It’s not uncommon to find an Xa having 1 set of trays play sharper than the other and other similar issues. These usually come down to poorly matched resistors, rather than IC failure, but can really be a pain to track down and reliably repair.

Unique features (see OB-8 for other features both 8 and Xa share compared to X):
– First OB use of CEM chips, including CEM3340 oscillators, CEM3320 Filters, CEM3310 EG’s, and CEM3360 VCA’s (2, 1 for each 4 voice motherboard).
– Fully analog 2nd LFO
– Selectable defeat of voices if they don’t pass auto tune

Rev 1: Physically these units have a sticker for the Oberheim logo and may not say Oberheim above the top octave of keys.

These Xa’s have 40 programs and the lower control board is more like the OB-X using an array of TL082’s and CA3080’s for the portamento circuitry. These Xa’s don’t follow the service manual too well and may have trimmers floating in various places that trim the same function you may be confused about in the service manual. These Xa’s end up needed the most work, and often an entire suite of precision resistor changes. The PSU on these Xa’s may also seem to have flying capacitors compared to later ones.

Rev 2: These Xa’s have the common graphics with the Oberheim logo silk screened without use of stickers. The 120 program badge will likely be affixed to the panel. This is not rare nor a selling point. 99% of Xa’s have 120 programs!

Electronically, these Xa’s have a well rationalized set of control boards. The PSU looks better designed. The boards have a set of trimmers that can be trimmed per the service manual instructions. These units also can suffer from voice tray discrepancies, but not as bad as earlier units.

Rev 3: This is the best Rev of the Xa but can also be the hardest to fix. Physically it’s identical to Rev 2.e

Electronically, rev 3 strays away from the OB-X portamento scheme and utilized 2 TL084’s instead. This rationalizes the circuit considerably. However, since the manual makes no mention of this, it makes calibrating the DAC more difficult. Pro-Tip: take the DAC measurement from the voice card itself on the KEYCV pin while in unison mode.
Unlike previous revisions, many chips that fail are no longer in sockets. This makes service more difficult, as the top control board often has computer issues in this revision. Worse, often these issues are affected by chip speeds. For this reason, we consider Encore (with or without midi mounted) part of a comprehensive restoration for Xa’s of this rev and earlier as well, as it eliminates the failing part of the computer (as well as the need for a battery).

All in all, the Xa is an instrument worthy of restoration. While the work needed may be vast, the result is always spectacular if your tech knows what they’re doing with it.

The OB-8

Loved by those looking for the Minneapolis sound and Armenian’s looking for a popular lead sound, the OB-8 was the true rationalization of the OB line.

The OB-8 brings many new features to the basic Xa architecture by adding a suite of digitally controlled functions. Utilizing the same Z-80 as the it’s older brothers, the OB-8 sports a fully digital LFO that can be inverted, quantized, tracking, and modulated. It also digitized a lot of the calibrations including oscillator tuning, envelope offsets, DAC offsets, and more. And thats not even mentioning the addition of Page 2, which at the time brought never before seen modulation and control over an otherwise standard poly synth.

The OB-8 of course went through a few revisions as well. However, it’s feature set is quite similar to the Xa, so for this one, we will list their similarities and following that, what’s unique to the OB-8

New features in the Xa and 8 (compared to X)
Dedicated LFO for modulation paddle with 2 waveforms
Split/Later function
No edit button
Osc Mod by Filter EG
Auto tune defeats non-tuning voices (if enabled)
2 filter modes, 2 and 4 pole
Noise on or off, no “half” mode

Unique to OB-8
Page 2 (which adds so much)
Volume modulation by LFO
Auto Tune checks many octaves, thusly more precise than earlier OB’s
Split/Layer can be independently transposed and tuned
Split/Later can be saved in RAM (like JP8)
Triangle wave available
Panasonic (masushita) keyboard (on most models, earliest had Pratt-Reed like Xa and X)
Factory midi (on later models)
Pan pots accessible through the right end cheek

Overall, the OB8 greatly rationalizes (how many times have I typed that word?) the OB-Xa with a new single computer board (2 if midi is present) with a full computer, DAC, and PSU on one PCB. Instead of the failure-prone voice card motherboards found in the Xa and X, the OB-8 uses larger circuit boards with 4 voices each with their own digital end on the boards themselves to handle CV duties for the split and layers.

Rev 1: The earliest OB-8’s we’ve seen have no page 2 graphics (but have page 2 functions!) and Pratt-Reed keyboards. These also use the older style buttons that have a satisfying *click*

The computer board in this Rev often has kynar wire in many places and flying components, likely last minute changes. The panel boards use the same Molex connectors as the Xa and X. The voice cards have no IC designators, often have tons of kynar wire flying everywhere, and also have a CEM3360 per voice that does make it sound different than later OB-8’s. These earlier OB’s, while having that added chip, sound more like the Xa than later 8’s (this is ignoring the Jurgen Haible modification that is popular, do your own research). These are a little harder to service and calibrate and making sure they’re on B-5 firmware is critical.

Rev 2: Panel still has no page 2 graphics and it has a Panasonic keyboard. These also use the older style buttons that have a satisfying *click*

Inside, the voice board loses the CEM3360 per voice. The voice architecture is now essentially the same as an Xa. Usually midi is found (not usually from factory) on these and is mounted to the left side end cheek. Component designators are found on the voice boards and they’re less kludged. These boards usually are populated with failure prone RCA CMOS but we recommend replacement regardless.

Rev 3: Page 2 graphics now appear on the panel. Midi is from factory and mounted on the left end cheek with a custom plate. These also use the older style buttons that have a satisfying *click*

Electronically, these are usually like Rev 2. There are no notable differences that we’ve noticed thus far, except that the pot board may appear difference and use solid core wire between panel boards.

Rev 4: The final realization of the OB-8 and OB line before the Matrix-12 came into the fray. These have Page 2 graphics and an updated back panel with mounting for midi. The buttons change to ones that have no satisfying click.

Inside, the unit is usually free from kludges unless a previous tech wrecked something. The panel boards come with solid core wires between the panel boards and may appear different than the older ones in general. These were shipped with B-5 firmware and midi.

Some think Rev 2-4 sound worse than the Xa or the early rev OB-8. They surely do sound different, however consider the following:
– They do in fact have different keyboards. I would be lying if I didn’t think my OB-8 sounded better when I put a Pratt-Reed inside of it. I know scientifically it makes no difference, but the Panasonic keyboard does make it feel cheap
– There is a design flaw in the output stage, and the output stage is inherently different than the Xa. The Jurgen Haible mod seems to be the popular solution, however we find that not all OB-8’s need it, inexplicably.
– The Auto Tune routing in the 8 is much more precise than the Xa and X. The Xa and X rely on the V/Oct trimmer being solid and accurate across temperature ranges, and thusly there is room for error, as auto will only adjust the initial tuning. The OB-8 samples different octaves and tunes more accurately. Page 2 gives you a scaling detune that helps bring back some of the out of tune life you might be missing.

Because the OB-8 was so ahead of it’s time with digital control, when the battery dies they can often behave erratically. It’s important to reset all programmable functions, including split functions, when an OB-8 is restored to factory settings. Look at the user manual to find these functions.

Which One Do I Want?

Well, that’s totally up to you. With the above info, hopefully you can now understand the differences between models and their revisions to make an informed choice. It’s important to remember that each revision can be brought to the same spec with different levels of work, and none should be avoided over others as a hard rule. Each of the synths sound a bit different with the only sonic similarities being between the OB-Xa and OB-8. The X sounds objectively different as it’s voice architecture is objectively different.