[sdiy] Untangling the Oberheim OB-8 filter

[Magnus Danielson]

Hi Tom,

The resonator control cell either do the feed forward (2-pole) or 
feedback (4-pole). This is the less obvious one for the 2-pole scenario, 
while it is very simple and classic for the 4-pole scenario.
The output switches also helps.

If you redraw the CEM3320 as separate stages, it becomes much easier to 
unndestand what each contributes to.

Juergen Haible did his own modifications to his OB-8, and this also 
included a high-pass capability.

Cheers,
Magnus

On 06/09/2017 04:55 PM, Tom Wiltshire wrote:
> 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/
>
> 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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