<html><head></head><body><div style="color:#000; background-color:#fff; font-family:Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:16px"><div id="yui_3_16_0_1_1496437498173_575481">Yes, Craig Anderton did this with the 3320 - <br></div><div id="yui_3_16_0_1_1496437498173_575534"><br></div><div dir="ltr" id="yui_3_16_0_1_1496437498173_575482"><a href="http://www.synthdiy.com/files/2002/MIF.pdf" id="yui_3_16_0_1_1496437498173_575533">http://www.synthdiy.com/files/2002/MIF.pdf</a><br></div><div id="yui_3_16_0_1_1496437498173_575483"><span></span></div><div class="qtdSeparateBR" id="yui_3_16_0_1_1496437498173_575479"><div>Doug <br></div><div id="yui_3_16_0_1_1496437498173_575572" dir="ltr">synthparts.com<br></div></div><div class="yahoo_quoted" id="yui_3_16_0_1_1496437498173_575467" style="display: block;"> <div style="font-family: Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;" id="yui_3_16_0_1_1496437498173_575466"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;" id="yui_3_16_0_1_1496437498173_575465"> <div dir="ltr" id="yui_3_16_0_1_1496437498173_575478"> <font id="yui_3_16_0_1_1496437498173_575531" size="2" face="Arial"> <hr id="yui_3_16_0_1_1496437498173_575532" size="1"> <b id="yui_3_16_0_1_1496437498173_575566"><span style="font-weight:bold;" id="yui_3_16_0_1_1496437498173_575565">From:</span></b> Tom Wiltshire <tom@electricdruid.net><br> <b><span style="font-weight: bold;">To:</span></b> Sean Ellis <TensionType@hotmail.com> <br><b><span style="font-weight: bold;">Cc:</span></b> "synth-diy@synth-diy.org List" <synth-diy@synth-diy.org><br> <b id="yui_3_16_0_1_1496437498173_575574"><span style="font-weight: bold;" id="yui_3_16_0_1_1496437498173_575573">Sent:</span></b> Friday, June 9, 2017 6:09 PM<br> <b><span style="font-weight: bold;">Subject:</span></b> Re: [sdiy] Untangling the Oberheim OB-8 filter<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_1_1496437498173_575464"><br><div id="yiv5193598260"><div id="yui_3_16_0_1_1496437498173_575463">Thanks Sean, but they're not very similar. 4PM is a 2164 VCA-based pole-mixing filter, similar to the Oberheim Xpander and not very much like the OB-Xa/OB-8 CEM3320 filters. OB-Xa/OB-8 use a standard 4-pole cascade as per the CEM3320 datasheet for the 4-pole filter and a 2-pole state variable filter (completely *unlike * the datasheet) for the 2-pole filter. The 4-pole version was then developed into the pole-mixing filter that appeared in the Xpander/Matrix-12, but by that point, it was using the CEM3372 rather than the 3320 (although I don't see why you couldn't do the same trick with the CEM3320 - now, there's a thought…)<div id="yui_3_16_0_1_1496437498173_575477"><br clear="none"></div><div id="yui_3_16_0_1_1496437498173_575462">Tom<br clear="none"><div id="yui_3_16_0_1_1496437498173_575461"><br clear="none"><div id="yui_3_16_0_1_1496437498173_575841">
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<br clear="none"><div class="yiv5193598260yqt9681886687" id="yiv5193598260yqt41419"><div id="yui_3_16_0_1_1496437498173_575460"><div>On 10 Jun 2017, at 01:13, Sean Ellis <<a rel="nofollow" shape="rect" ymailto="mailto:TensionType@hotmail.com" target="_blank" href="mailto:TensionType@hotmail.com">TensionType@hotmail.com</a>> wrote:</div><br class="yiv5193598260Apple-interchange-newline" clear="none"><blockquote type="cite" id="yui_3_16_0_1_1496437498173_575459"><div dir="ltr" style="font-family:Helvetica;font-size:medium;font-style:normal;font-variant:normal;font-weight:normal;letter-spacing:normal;line-height:normal;orphans:2;text-indent:0px;text-transform:none;white-space:normal;widows:2;word-spacing:0px;" id="yui_3_16_0_1_1496437498173_575458"><div dir="ltr" id="yiv5193598260divtagdefaultwrapper" style="font-size:12pt;font-family:Calibri, Arial, Helvetica, sans-serif;"><div style="margin-top:0px;margin-bottom:0px;" id="yui_3_16_0_1_1496437498173_575474">The 4-pole Mission filter for the Shruthi is at least a bit clearer to read:</div><div style="margin-top:0px;margin-bottom:0px;" id="yui_3_16_0_1_1496437498173_575473"><a rel="nofollow" shape="rect" class="yiv5193598260OWAAutoLink" id="yiv5193598260LPlnk839774" target="_blank" href="https://mutable-instruments.net/archive/shruthi/build/4pm/">https://mutable-instruments.net/archive/shruthi/build/4pm/</a></div><div style="margin-top:0px;margin-bottom:0px;" id="yui_3_16_0_1_1496437498173_575472"><br clear="none"></div><div style="margin-top:0px;margin-bottom:0px;" id="yui_3_16_0_1_1496437498173_575457">I believe it's a similar topology.</div><div style="margin-top:0px;margin-bottom:0px;"><br clear="none"></div><div style="" id="yui_3_16_0_1_1496437498173_575845"><div id="yui_3_16_0_1_1496437498173_575844"><hr tabindex="-1" style="display:inline-block;width:760px;"><div dir="ltr" id="yiv5193598260x_divRplyFwdMsg"><font style="font-size:11pt;" id="yui_3_16_0_1_1496437498173_575843" face="Calibri, sans-serif"><b>From:</b><span class="yiv5193598260Apple-converted-space"> </span>Synth-diy <<a rel="nofollow" shape="rect" ymailto="mailto:synth-diy-bounces@synth-diy.org" target="_blank" href="mailto:synth-diy-bounces@synth-diy.org">synth-diy-bounces@synth-diy.org</a>> on behalf of Tom Wiltshire <<a rel="nofollow" shape="rect" ymailto="mailto:tom@electricdruid.net" target="_blank" href="mailto:tom@electricdruid.net">tom@electricdruid.net</a>><br clear="none"><b>Sent:</b><span class="yiv5193598260Apple-converted-space"> </span>Saturday, 10 June 2017 12:55 AM<br clear="none"><b>To:</b><span class="yiv5193598260Apple-converted-space"> </span><a rel="nofollow" shape="rect" ymailto="mailto:synth-diy@synth-diy.org" target="_blank" href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a><span class="yiv5193598260Apple-converted-space"> </span>List<br clear="none"><b id="yui_3_16_0_1_1496437498173_575842">Subject:</b><span class="yiv5193598260Apple-converted-space"> </span>[sdiy] Untangling the Oberheim OB-8 filter</font><div> </div></div></div><font size="2"><span style="font-size:10pt;"></span></font><div class="yiv5193598260PlainText" id="yui_3_16_0_1_1496437498173_575846">Hi All,<br clear="none"><br clear="none">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 clear="none"><br clear="none"><a rel="nofollow" shape="rect" id="yiv5193598260LPlnk737967" target="_blank" href="http://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/">http://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/</a><br clear="none"><br clear="none">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 clear="none"><br clear="none"><a rel="nofollow" shape="rect" id="yiv5193598260LPlnk37201" target="_blank" href="http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg">http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg</a><div style="margin-top:0px;margin-bottom:0px;" id="yui_3_16_0_1_1496437498173_575848"><img title="" src="http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg" style="max-width:99.9000015258789%;" data-id="bf89c5b5-0bc7-c50a-53ed-7cc5ae94cff4" id="yui_3_16_0_1_1496437498173_575847" class=""></div><br clear="none"><br clear="none">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 clear="none"><br clear="none"><a rel="nofollow" shape="rect" id="yiv5193598260LPlnk964064" target="_blank" href="http://electricdruid.net/cem3320-filter-designs/">http://electricdruid.net/cem3320-filter-designs/</a><div id="yiv5193598260LPBorder_GT_14970531652260.26908333971034815" style="margin-bottom:20px;overflow:auto;width:776px;text-indent:0px;"><table id="yiv5193598260LPContainer_14970531652200.6739743926864977" style="width:698px;background-color:rgb(255, 255, 255);position:relative;overflow:auto;padding-top:20px;padding-bottom:20px;margin-top:20px;border-top-width:1px;border-top-style:dotted;border-top-color:rgb(200, 200, 200);border-bottom-width:1px;border-bottom-style:dotted;border-bottom-color:rgb(200, 200, 200);" class="yahoo-compose-table-card" cellspacing="0"><tbody><tr style="border-spacing:0px;" valign="top"><td colspan="2" rowspan="1" id="yiv5193598260TextCell_14970531652220.31412000655914185" style="vertical-align:top;position:relative;padding:0px;display:table-cell;"><div id="yiv5193598260LPRemovePreviewContainer_14970531652220.7776566927147515"></div><div id="yiv5193598260LPTitle_14970531652220.05649051262562932" style="color:rgb(0, 120, 215);font-weight:400;font-size:21px;font-family:'Segoe UI Light', 'Segoe WP Light', 'Segoe UI', 'Segoe WP', Tahoma, Arial, sans-serif;line-height:21px;"><a rel="nofollow" shape="rect" id="yiv5193598260LPUrlAnchor_14970531652230.12078022455548065" target="_blank" href="http://electricdruid.net/cem3320-filter-designs/" style="text-decoration:none;">CEM3320 Filter designs | Electric Druid</a></div><div id="yiv5193598260LPMetadata_14970531652240.7929267362589624" style="margin:10px 0px 16px;color:rgb(102, 102, 102);font-weight:400;font-family:'Segoe UI', 'Segoe WP', Tahoma, Arial, sans-serif;font-size:14px;line-height:14px;"><a rel="nofollow" shape="rect" target="_blank" href="http://electricdruid.net/">electricdruid.net</a></div><div id="yiv5193598260LPDescription_14970531652250.6249472220350711" style="display:block;color:rgb(102, 102, 102);font-weight:400;font-family:'Segoe UI', 'Segoe WP', Tahoma, Arial, sans-serif;font-size:14px;line-height:20px;max-height:100px;overflow:hidden;">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></td></tr></tbody></table></div><br clear="none"><br clear="none">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 clear="none"><br clear="none">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 clear="none"><br clear="none">Thanks,<br clear="none">Tom<br clear="none"><br clear="none">==================<br clear="none"> Electric Druid<br clear="none">Synth & Stompbox DIY<br clear="none">==================<br clear="none"><br clear="none"><br clear="none"><br clear="none"><br clear="none"><br clear="none">_______________________________________________<br clear="none">Synth-diy mailing list<br clear="none"><a rel="nofollow" shape="rect" ymailto="mailto:Synth-diy@synth-diy.org" target="_blank" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br clear="none"><a rel="nofollow" shape="rect" id="yiv5193598260LPlnk533177" target="_blank" href="http://synth-diy.org/mailman/listinfo/synth-diy">http://synth-diy.org/mailman/listinfo/synth-diy</a><br clear="none"></div></div></div></div></blockquote></div></div><br clear="none"></div></div></div></div><div class="yqt9681886687" id="yqt21126">_______________________________________________<br clear="none">Synth-diy mailing list<br clear="none"><a shape="rect" ymailto="mailto:Synth-diy@synth-diy.org" href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br clear="none"><a shape="rect" href="http://synth-diy.org/mailman/listinfo/synth-diy" target="_blank">http://synth-diy.org/mailman/listinfo/synth-diy</a><br clear="none"></div><br><br></div> </div> </div> </div></div></body></html>