[sdiy] OBX-a Poly mod

[Florian Anwander]

I found that interview from 1987

https://www.muzines.co.uk/articles/inside-views-j-l-cooper/2516

"I stayed at Oberheim for five years as Chief Engineer. During that time 
we produced the OBX and the OBXa polysynths and we had just started 
working on the design of the DSX and the DMX drum machine when Tom and I 
had a parting of ways. At the time, I was doing customising work on the 
side, strictly on Oberheim equipment (one of the big things I was adding 
were four-pole filters to OBX synths). Tom basically didn't think that I 
should have been doing that, so I left in 1981. For the next two years I 
worked out of my garage doing consulting work and scraping by."

So it looks, like he even started modifying those synths when he was at 
Oberheim.


Florian

Am 03.10.25 um 11:13 schrieb ulfur hansson via Synth-diy:
> here are pictures of the JL-Cooper poly mod kit mentioned in my post, 
>  installed in an OBX-a;
>
> https://www.matrixsynth.com/2015/12/oberheim-ob-xa-with-mods.html
>
> the post says the unit comes with a Kenton Midi kit installed. I 
> wonder if one of the additional midi controlled CV can control the 
> Poly-Mod depth - if not it looks like the depth is not CV-controlled 
> at all, but rather adjusted manually with the two pots on the panel.
>
> it is ugly, but I'm intrigued. maybe one of the expression jacks on 
> the back panel could be repurposed for depth ctrl?
>
> fös., 3. okt. 2025 kl. 07:51 skrifaði brianw <brianw at audiobanshee.com>:
>
>     That sounds promising.
>
>     Before you get too deep into the schematic, you might want to make
>     sure that the DW-8000 CPU can update those unused CV outputs. Some
>     of these old 8-bit CPUs can only manage to update a limited number
>     of CV outputs, even when the hardware is capable of accessing more.
>
>     But I've certainly seen hardware that was capable of more than it
>     shipped with. Sometimes, the firmware even has subroutines to
>     support the unrealized features, so part of the work was already done.
>
>     As a suggestion, selecting the filter mode could be as simple as
>     another bit on a GPIO port, or some other way of expanding digital
>     outputs without consuming an analog CV output for a binary value.
>     Hopefully, there's a way to add a GPIO bit to control an analog
>     switch.
>
>     Brian
>
>
>     On Oct 3, 2025, at 12:04 AM, Mattias Rickardsson wrote:
>     > On the topic of adding poly-mod to old polysynths:
>     > A colleague of mine is reverse-engineering the Korg DW-8000
>     (hardware and firmware) and just found both an unused 12dB/oct
>     filtermode and a couple of unused CVs on the board. One of these
>     CVs could select filtermode and the other control poly-mod amount
>     to filters, globally. I hope to sketch him a solution adding 2 x
>     SSI2164 VCAs and analog switches on a small daughterboard, and he
>     could include these parameters in a new firmware.
>     >
>     > But perhaps all synths don't have these hidden possibilities :-)
>     >
>     > /mr
>     >
>     > Den fre 3 okt. 2025 04:18brian skrev:
>     >> Agreed.
>     >>
>     >> If there is a schematic for any poly-mod, each OBX-a voice
>     would require a VCA for mod depth, and a couple of analog switches
>     to choose between filter envelope and OSC2 sources, plus some more
>     analog switches to choose the destinations. Not only would the VCA
>     need a dedicated CV in the patch, but those switches would need a
>     few bits of storage to select the modulation path in each voice.
>     >>
>     >> Brian
>     >>
>     >>
>     >> On Oct 2, 2025, at 6:36 PM, Michael E Caloroso wrote:
>     >> > I have never heard of an OB-Xa mod that adds P5-style
>     polymod.  Frankly it would be useless unless a new programmable CV
>     for mod depth can be implemented.
>     >> >
>     >> > MC
>     >> >
>     >> > On Thu, Oct 2, 2025 at 3:14 PM ulfur hansson wrote:
>     >> >> i came across this matrix-synth post that features images of
>     an obx-a fitted with a mysterious JL-Cooper mod, allowing for
>     prophet-5 style poly-mod.
>     >> >>
>     >> >> does anyone happen to have any information on the kits design?
>     >> >>
>     >> >> ...or if i should end up feeling brave and design a board to
>     achieve something similar - any thoughts?
>     >> >>
>     >> >> thanks y'all!
>     >> >> sincerely,
>     >> >> -úlfur
>     >
>
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> -- 
> •
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