[sdiy] Untangling the Oberheim OB-8 filter

[Tom Wiltshire]

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…)

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================


On 10 Jun 2017, at 01:13, Sean Ellis <TensionType at hotmail.com> wrote:

> The 4-pole Mission filter for the Shruthi is at least a bit clearer to read:
> https://mutable-instruments.net/archive/shruthi/build/4pm/
> 
> I believe it's a similar topology.
> 
> From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Tom Wiltshire <tom at electricdruid.net>
> Sent: Saturday, 10 June 2017 12:55 AM
> To: synth-diy at synth-diy.org List
> Subject: [sdiy] Untangling the Oberheim OB-8 filter
>  
> Hi All,
> 
> 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):
> 
> http://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/
> 
> 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:
> 
> http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg
> 
> 
> 
> 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:
> 
> http://electricdruid.net/cem3320-filter-designs/
> CEM3320 Filter designs | Electric Druid
> electricdruid.net
> 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 ...
> 
> 
> 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.
> 
> 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.
> 
> Thanks,
> Tom
> 
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>        Electric Druid
> Synth & Stompbox DIY
> ==================
> 
> 
> 
> 
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