Oberheim OB-Mx Preservation Page

Oberheim OB-Mx Preservation Page

Oberheim OB-Mx Preservation Page

Oberheim OB-Mx Preservation Page

This page is devoted to information about a simultaneously fascinating
and infuriating synth known as the Oberheim OB-Mx. I’m trying to collect
all the data I can about this curious and quirky beast.


The schematics were provided by
Mark Glinsky of
Manual Manor.

  • Voice Board: CPU Section,
    CV Demultiplexing,
    Voice A VCO,
    Voice B VCO,
    Voice A Processing,
    Voice B Processing
  • Master Board:
    Digital Section,
    Analog Section
  • Front Panel

    ROM Images

    I’m not sure what the exact differences are between the various software
    They’re straight binary
    files. Many thanks to Tom Collins of GTRI for making the images for me; few
    people around the department still had equipment that could read them! Tom
    included some text files comparing the different versions; I’m not sure how
    useful the comparison files are, but here they are anyway.

  • Master board software:
    Version 1.04,
    Version 2.00;

    Comparison of V1.04 and V2.00
  • Calibration ROM and
    (courtesy Tom Virostek) – The calibration procedure is pretty complex and
    requires a scope (and preferably the voice board extender card that lets
    you get to the board while the unit is on).
    The OB-Mx uses some digital pots with built-in nonvolatile memory. I had
    always assumed that these were recalibrated upon booting up the OB-Mx, but
    it turns out they are set in this calibration procedure. I suspect that a
    lot of errant voice boards that are out there just need recalibrated.

    Source code

    Lynx Crowe
    has kindly provided his original source code for the OB-Mx,
    provided for noncommercial personal and educational use. This code is provided as-is with no guarantees of completeness or correctness; it’s a compilation of various bits of relevant code that Lynx found going through his archives. If you want to delve into this, you are “on your own.” Knowledgeable people with time on their hands could potentially do all sorts of neat things with this. Please
    give Lynx proper credit on anything you do with this.

    Here is the source
    code as a 7-zip file
    . (Some quick googling
    will find something that will decompress 7-zip files on your particular
    system.) Recompiling this will be quite a challenge. Lynx suggested that
    the macroassembler AS
    might be worth trying out.

    See the following notes Lynx included in an e-mail:

    You have my permission, as the copyright holder, to publish any of the
    source code or documentation, so long as it’s not for profit. If you
    or anyone else wants to make a profit on it, contact me first and we’ll
    come to some sort of mutually acceptable arrangement.

    I never got a chance to properly complete the OB-Mx code, since they
    released it while I was in the hospital recovering from a heart attack,
    so I can almost guarantee that it’s not up to my usual standards for a
    completed project, but it will give anyone studying the OB-Mx a good
    look at what the code looked like in an “almost finished” state. There
    is some debug code still active that slows down processing, particularly
    the LFO, and it should be removed, or at least disabled, if you should
    happen to want to actually run the code.

    In any case, I hope you find this stuff useful.

    [I’d also note that, although Lynx won the rights to the source code he
    created, as far as the whole OB-Mx goes, other parties, such as Gibson
    and Don Buchla, probably own rights to various bits of IP, such as the
    front panel designs, so I’d recommend treading carefully.]


    Anyone have patches they want to share?

    Last modified 9/23/09
    Maintained by Aaron