[sdiy] OBX-a Poly mod

[ulfur hansson]

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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