[sdiy] OBX-a Poly mod

[brianw]

Nope.

What you're missing, Michael, is that the Poly-Mod feature of the Prophet 5 is already repeating the components per voice. So, the equivalent of Sequential's Poly-Mod feature for the OB-Xa would already need 8 duplicates of the components, assuming an 8-voice OB-Xa. When in split/layer mode, that would be 4 voices in each layer, but they would already have per-voice components.

You would be correct if this were enhancing the mono-mod LFO, which does require duplication for the Upper and Lower to avoid having different patches restricted by sharing an LFO. But Sequential also separated their regular modulation circuits from the Poly-Mod circuits, because the latter need to be duplicated for every voice (just 5 times in the Prophet 5; 10 times in the Prophet 10).

Granted, the firmware would have to be enhanced to treat poly-mod settings as part of a voice patch, such that calling up different patches for Upper and Lower would result in different settings for the Upper voices than the Lower voices. In this situation, the hardware doesn't change for split/layer mode, but the firmware control of the CV and other settings does have to handle the differences.

I didn't realize that the OB-Xa was available in 8-voice, 6-voice, and 4-voice variations - at least according to the 3rd edition of the owners manual - but this highlights that the modifications will have to span several sections of the circuits.

Brian


On Oct 3, 2025, at 8:38 AM, Michael E Caloroso wrote:
> > 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.
> 
> Double the components, CV, and logic controls for the split/layer feature that the OB-Xa has.
> 
> MC
> 
> On Thu, Oct 2, 2025 at 10:19 PM brianw <brianw at audiobanshee.com> wrote:
>> 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
>