<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p>I found that interview from 1987</p>
<p><a class="moz-txt-link-freetext" href="https://www.muzines.co.uk/articles/inside-views-j-l-cooper/2516">https://www.muzines.co.uk/articles/inside-views-j-l-cooper/2516</a></p>
<p>"I stayed at Oberheim for five years as Chief Engineer. During
that time we produced the OBX and the OBXa polysynths and we had
just started working on the design of the DSX and the DMX drum
machine when Tom and I had a parting of ways. At the time, I was
doing customising work on the side, strictly on Oberheim equipment
(one of the big things I was adding were four-pole filters to OBX
synths). Tom basically didn't think that I should have been doing
that, so I left in 1981. For the next two years I worked out of my
garage doing consulting work and scraping by."</p>
<p>So it looks, like he even started modifying those synths when he
was at Oberheim.</p>
<p><br>
</p>
<p>Florian <br>
<br>
</p>
<div class="moz-cite-prefix">Am 03.10.25 um 11:13 schrieb ulfur
hansson via Synth-diy:<br>
</div>
<blockquote type="cite"
cite="mid:CAG9O7x3bY=07ZfUsegeR1ippKUQOg3j12_7m5oKzebF+46phLw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">here are pictures of the JL-Cooper poly mod kit
mentioned in my post, installed in an OBX-a;
<div><br>
</div>
<div><a
href="https://www.matrixsynth.com/2015/12/oberheim-ob-xa-with-mods.html"
moz-do-not-send="true" class="moz-txt-link-freetext">https://www.matrixsynth.com/2015/12/oberheim-ob-xa-with-mods.html</a><br>
</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>it is ugly, but I'm intrigued. maybe one of the expression
jacks on the back panel could be repurposed for depth ctrl?</div>
</div>
<br>
<div class="gmail_quote gmail_quote_container">
<div dir="ltr" class="gmail_attr">fös., 3. okt. 2025 kl. 07:51
skrifaði brianw <<a href="mailto:brianw@audiobanshee.com"
moz-do-not-send="true" class="moz-txt-link-freetext">brianw@audiobanshee.com</a>>:<br>
</div>
<blockquote class="gmail_quote"
style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">That
sounds promising.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Brian<br>
<br>
<br>
On Oct 3, 2025, at 12:04 AM, Mattias Rickardsson wrote:<br>
> On the topic of adding poly-mod to old polysynths:<br>
> 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.<br>
> <br>
> But perhaps all synths don't have these hidden
possibilities :-) <br>
> <br>
> /mr<br>
> <br>
> Den fre 3 okt. 2025 04:18brian skrev:<br>
>> Agreed.<br>
>> <br>
>> 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.<br>
>> <br>
>> Brian<br>
>> <br>
>> <br>
>> On Oct 2, 2025, at 6:36 PM, Michael E Caloroso wrote:<br>
>> > 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.<br>
>> > <br>
>> > MC<br>
>> > <br>
>> > On Thu, Oct 2, 2025 at 3:14 PM ulfur hansson
wrote:<br>
>> >> 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.<br>
>> >> <br>
>> >> does anyone happen to have any information
on the kits design?<br>
>> >> <br>
>> >> ...or if i should end up feeling brave and
design a board to achieve something similar - any thoughts?<br>
>> >> <br>
>> >> thanks y'all!<br>
>> >> sincerely,<br>
>> >> -úlfur<br>
> <br>
<br>
________________________________________________________<br>
This is the Synth-diy mailing list<br>
Submit email to: <a href="mailto:Synth-diy@synth-diy.org"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">Synth-diy@synth-diy.org</a><br>
View archive at: <a
href="https://synth-diy.org/pipermail/synth-diy/"
rel="noreferrer" target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">https://synth-diy.org/pipermail/synth-diy/</a><br>
Check your settings at: <a
href="https://synth-diy.org/mailman/listinfo/synth-diy"
rel="noreferrer" target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">https://synth-diy.org/mailman/listinfo/synth-diy</a><br>
Selling or trading? Use <a
href="mailto:marketplace@synth-diy.org" target="_blank"
moz-do-not-send="true" class="moz-txt-link-freetext">marketplace@synth-diy.org</a><br>
</blockquote>
</div>
<div><br clear="all">
</div>
<div><br>
</div>
<span class="gmail_signature_prefix">-- </span><br>
<div dir="ltr" class="gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div>•</div>
<div><a href="http://ulfurhansson.com/"
style="color:rgb(17,85,204)" target="_blank"
moz-do-not-send="true">ulfurhansson.com</a></div>
<div><a href="http://instagram.com/ulfur_"
style="color:rgb(17,85,204)" target="_blank"
moz-do-not-send="true">instagram.com/ulfur_</a></div>
<div><a
href="https://open.spotify.com/album/118CEJliIg47Jcr70y7Kww"
target="_blank" moz-do-not-send="true">.•°~ Úlfur -
Arborescence on Spotify</a><br>
</div>
</div>
</div>
</div>
</div>
<br>
<fieldset class="moz-mime-attachment-header"></fieldset>
<pre wrap="" class="moz-quote-pre">________________________________________________________
This is the Synth-diy mailing list
Submit email to: <a class="moz-txt-link-abbreviated" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a>
View archive at: <a class="moz-txt-link-freetext" href="https://synth-diy.org/pipermail/synth-diy/">https://synth-diy.org/pipermail/synth-diy/</a>
Check your settings at: <a class="moz-txt-link-freetext" href="https://synth-diy.org/mailman/listinfo/synth-diy">https://synth-diy.org/mailman/listinfo/synth-diy</a>
Selling or trading? Use <a class="moz-txt-link-abbreviated" href="mailto:marketplace@synth-diy.org">marketplace@synth-diy.org</a>
</pre>
</blockquote>
</body>
</html>