[sdiy] Untangling the Oberheim OB-8 filter

[Tom Wiltshire]

I found the link to Juergen's filter mod:

http://jhaible.com/legacy/hjob8mod.html

The other things he did was add Polymod, and improve bass response:

http://jhaible.com/legacy/hj_pmob8.html
http://machines.hyperreal.org/manufacturers/Oberheim/OB-8/mods/OB-8.mod.txt


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




On 11 Jun 2017, at 12:15, Tom Wiltshire <tom at electricdruid.net> wrote:

> Thanks Magnus.
> 
> Redrawing the diagram is exactly what I'm currently engaged in, so as soon as I've got something worth showing, I'll post it. I totally agree that it seems like the best way to make it clearer what's going on.
> 
> Is any of Juergen's OB-8 stuff online still? That sounds interesting. Do you have a link? 
> 
> Regards,
> Tom
> 
> ==================
>       Electric Druid
> Synth & Stompbox DIY
> ==================
> 
> 
> 
> 
> On 11 Jun 2017, at 10:57, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
>> 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
>>> 
>>> ==================
>>>      Electric Druid
>>> Synth & Stompbox DIY
>>> ==================
>>> 
>>> 
>>> 
>>> 
>>> 
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