[sdiy] OBX-a Poly mod

[Terje Winther]

Thanks for the positive feedback on this.

The PolySix VCF FM modulation:
At first I tried to duplicate Jürgen´s schematics for VCF FM-modulation directly, but it failed for me. Working on version two (which worked better, but not good enough), I understood the general idea better and came up with the solution for the final version, which included a lot more parts than anticipated. As several has mentioned here; you need a complete and separate path for all signals. So for the Polysix I needed 6. In addition I choose to have one CV-control common for all voices, just like on the Prophet-5.

The general path is like this:
- Each VCO output goes to an op-amp, and is then fed to each individual VCF, using the CV-input to the filter
- The amount of modulation is controlled by a pot, which generate a CV that controls each individual path (op-amp). One common control for all six voices. The pot drives an op-amp for this.
- I also added an on-off switch for each VCO-FM-VCF path, which is especially handsome when doing arpeggios to get changing rhythms, or when in unison mode and some voices are modulated and others not
- The Polysix analog board actually has a small diode (D10) on the PCB to indicate if the voice is on (taken directly from the gate signal). I removed these LEDs, and…
- The on-off switches for each voice FM-modulation have a LED on the top of the switches, and the LED-signals from the analog board was routed to these front panel switches, to easily see which voice is being modulated

I had to carefully choose values for the various parts to make it work. There is a limited CV-window where this works from zero to full FM-modulation.
There are also various support circuitry, like CV-smoothing, decoupling, LED-drivers, power etc.
Finding the various input/output points was fairly easy, especially since I first did all the other modifications mentioned.
All the circuitry was put on a veroboard, and fitted on top of the analog voice card inside, close to the edge of the keyboard frame inside. There is enough room between the analog PCB and front panel for this.


Terje Winther
WintherStormer



> 5. okt. 2025 kl. 15:06 skrev Todd Sines <sines_list at scale.la>:
> 
> This demo is otherworldly— can you share more details on how to do this for the PolySix?
> 
> 
> Todd
> 
> 
>> On Oct 5, 2025, at 7:34 AM, Terje Winther <terje.winther at wintherstormer.no> wrote:
>> 
>> I just want to add….
>> 
>> Jürgen Haible explained what he did with his Oberheim OB-8 here:
>> http://jhaible.com/legacy/hj_pmob8.html <http://jhaible.com/legacy/hj_pmob8.html>
>> 
>> I used this as a starting point for me doing the same on an Korg Polysix. Sound examples and a snippet of a photo here:
>> https://soundcloud.com/terjewinther/polysix-test <https://soundcloud.com/terjewinther/polysix-test>
>> 
>> 
>> 
>> Terje Winther
>> terje.winther at wintherstormer.no <mailto:terje.winther at wintherstormer.no>
>> http://wintherstormer.no/
>> 
>> 
>> 
>> ________________________________________________________
>> 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

Terje Winther
terje.winther at wintherstormer.no
http://wintherstormer.no/



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20251005/81aac93a/attachment.htm>