<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">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><br></div><div>Tom<br><div><br><div apple-content-edited="true">
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<br><div><div>On 10 Jun 2017, at 01:13, Sean Ellis <<a href="mailto:TensionType@hotmail.com">TensionType@hotmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><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-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div id="divtagdefaultwrapper" dir="ltr" style="font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; "><div style="margin-top: 0px; margin-bottom: 0px; ">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; "><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></div><div style="margin-top: 0px; margin-bottom: 0px; "><br></div><div style="margin-top: 0px; margin-bottom: 0px; ">I believe it's a similar topology.</div><div style="margin-top: 0px; margin-bottom: 0px; "><br></div><div style=""><div><hr tabindex="-1" style="display: inline-block; width: 760px; "><div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size: 11pt; "><b>From:</b><span class="Apple-converted-space"> </span>Synth-diy <<a href="mailto:synth-diy-bounces@synth-diy.org">synth-diy-bounces@synth-diy.org</a>> on behalf of Tom Wiltshire <<a href="mailto:tom@electricdruid.net">tom@electricdruid.net</a>><br><b>Sent:</b><span class="Apple-converted-space"> </span>Saturday, 10 June 2017 12:55 AM<br><b>To:</b><span class="Apple-converted-space"> </span><a href="mailto:synth-diy@synth-diy.org">synth-diy@synth-diy.org</a><span class="Apple-converted-space"> </span>List<br><b>Subject:</b><span class="Apple-converted-space"> </span>[sdiy] Untangling the Oberheim OB-8 filter</font><div> </div></div></div><font size="2"><span style="font-size: 10pt; "><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><div style="margin-top: 0px; margin-bottom: 0px; "><img title="" src="http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg" style="max-width: 99.9000015258789%; "></div><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><div id="LPBorder_GT_14970531652260.26908333971034815" style="margin-bottom: 20px; overflow: auto; width: 776px; text-indent: 0px; "><table id="LPContainer_14970531652200.6739743926864977" role="presentation" cellspacing="0" 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); "><tbody><tr valign="top" style="border-spacing: 0px; "><td id="TextCell_14970531652220.31412000655914185" colspan="2" style="vertical-align: top; position: relative; padding: 0px; display: table-cell; "><div id="LPRemovePreviewContainer_14970531652220.7776566927147515"></div><div id="LPTitle_14970531652220.05649051262562932" style="top: 0px; color: rgb(0, 120, 215); font-weight: 400; font-size: 21px; font-family: wf_segoe-ui_light, 'Segoe UI Light', 'Segoe WP Light', 'Segoe UI', 'Segoe WP', Tahoma, Arial, sans-serif; line-height: 21px; "><a id="LPUrlAnchor_14970531652230.12078022455548065" href="http://electricdruid.net/cem3320-filter-designs/" target="_blank" style="text-decoration: none; ">CEM3320 Filter designs | Electric Druid</a></div><div id="LPMetadata_14970531652240.7929267362589624" style="margin: 10px 0px 16px; color: rgb(102, 102, 102); font-weight: 400; font-family: wf_segoe-ui_normal, 'Segoe UI', 'Segoe WP', Tahoma, Arial, sans-serif; font-size: 14px; line-height: 14px; "><a href="http://electricdruid.net/">electricdruid.net</a></div><div id="LPDescription_14970531652250.6249472220350711" style="display: block; color: rgb(102, 102, 102); font-weight: 400; font-family: wf_segoe-ui_normal, '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><br>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><br>==================<br> Electric Druid<br>Synth & Stompbox DIY<br>==================<br><br><br><br><br><br>_______________________________________________<br>Synth-diy mailing list<br><a href="mailto:Synth-diy@synth-diy.org">Synth-diy@synth-diy.org</a><br><a href="http://synth-diy.org/mailman/listinfo/synth-diy" id="LPlnk533177" previewremoved="true">http://synth-diy.org/mailman/listinfo/synth-diy</a><br></div></span></font></div></div></div></blockquote></div><br></div></div></body></html>