[sdiy] OBX-a Poly mod

[Michael E Caloroso]

Hard to know the specifics from the pictures.

Thanks for the info.

MC

On Fri, Oct 3, 2025 at 5:17 AM ulfur hansson via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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
>> >
>>
>> ________________________________________________________
>> This is the Synth-diy mailing list
>> Submit email to: Synth-diy at synth-diy.org
>> View archive at: https://synth-diy.org/pipermail/synth-diy/
>> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
>> Selling or trading? Use marketplace at synth-diy.org
>>
>
>
> --
> •
> ulfurhansson.com
> instagram.com/ulfur_
> .•°~ Úlfur - Arborescence on Spotify
> <https://open.spotify.com/album/118CEJliIg47Jcr70y7Kww>
> ________________________________________________________
> This is the Synth-diy mailing list
> Submit email to: Synth-diy at synth-diy.org
> View archive at: https://synth-diy.org/pipermail/synth-diy/
> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20251003/582da9c6/attachment.htm>