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<p>The 4-pole Mission filter for the Shruthi is at least a bit clearer to read:</p>
<p><a href="https://mutable-instruments.net/archive/shruthi/build/4pm/" class="OWAAutoLink" id="LPlnk839774" previewremoved="true">https://mutable-instruments.net/archive/shruthi/build/4pm/</a></p>
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<p>I believe it's a similar topology.</p>
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<div id="x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" color="#000000" face="Calibri, sans-serif"><b>From:</b> Synth-diy <synth-diy-bounces@synth-diy.org> on behalf of Tom Wiltshire <tom@electricdruid.net><br>
<b>Sent:</b> Saturday, 10 June 2017 12:55 AM<br>
<b>To:</b> synth-diy@synth-diy.org List<br>
<b>Subject:</b> [sdiy] Untangling the Oberheim OB-8 filter</font>
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<div class="PlainText">Hi All,<br>
<br>
I've been doing some research on filter designs that use analog switches to reconfigure the filter (as part of a series of blog posts on multimode filters that I'm working on):<br>
<br>
<a href="http://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/" id="LPlnk737967" previewremoved="true">http://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/</a><br>
<br>
Among these is the Oberheim OB-8, which uses a 4016 and a 4053 to mimic the effect of the two independent filter circuits in the OB-Xa (one four-pole lowpass, one 2-pole state-variable, used as lowpass only). The circuit looks like this:<br>
<br>
<a href="http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg" id="LPlnk37201" previewremoved="true">http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg</a>
<p><img style="max-width: 99.9%;" title="" src="http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg"></p>
<br>
<br>
I'm struggling to untangle this. It's safe to assume that the two separated circuits are close to the OB-Xa designs (they are - that much is clear). I've already documented those here:<br>
<br>
<a href="http://electricdruid.net/cem3320-filter-designs/" id="LPlnk964064" previewremoved="true">http://electricdruid.net/cem3320-filter-designs/</a>
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<a id="LPUrlAnchor_14970531652230.12078022455548065" style="text-decoration: none;" href="http://electricdruid.net/cem3320-filter-designs/" target="_blank">CEM3320 Filter designs | Electric Druid</a></div>
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electricdruid.net</div>
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Like my SSM2044 page, this page is a look at how various synths implemented the CEM3320 filter, with the datasheet design as a reference standard. Whereas the SSM2044 ...</div>
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Despite having a good idea what I'm looking for, I haven't yet been able to get the two different OB-8 designs untangled. I've got the schematic open in a image editor with multiple layers, so I can delete irrelevant parts of the schematic from the "4 pole"
and "2-pole" layers and so on. This is getting me closer all the time, but I'm not there yet. I think it's also the case that in 2-pole mode there are a number of components hanging off things that are connected but basically irrelevant, since they're there
for the other circuit, and similarly for 4-pole mode. That makes life more of a mess.<br>
<br>
Any advice on how to untangle this? I'm feeling a bit like I might have to redraw the schematic into a clearer layout first, and then deal with separating the two versions, but that seems like adding another step.<br>
<br>
Thanks,<br>
Tom<br>
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