[sdiy] OBX-a Poly mod

[brianw]

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
>