[sdiy] Untangling the Oberheim OB-8 filter

Sat Jun 10 09:11:50 CEST 2017

Oh, ok, I get it now. What it appears they are doing (which Tom did
state in the first email but I just didn't twig that anyone would
bother making it this complicated) is switching the entire circuit
between being a 4 pole cascade and a 2 pole SVF - crazy complicated
stuff! I would have just grabbed either the 2nd or 4th output of the
cascade and called it quits right there.

Ok, so with that in mind, they lift the ground on the first cap to
bypass it completely, and then skip the last stages output, so now
you've got only two caps doing something. You then need to block the
feedback paths of those two caps to make them into integrators, and
then add external "in line back to back" diodes to increase the
damping at high signal levels to control the resonance.

If you check between the OB-Xa SVF
http://electricdruid.net/wp-content/uploads/2015/06/OBXa2PoleCEM3320Filter.gif
and the and the OB-Xa Cascade
http://electricdruid.net/wp-content/uploads/2015/06/OBXa4PoleCEM3320Filter.gif

you can see that both circuits are there and there is a whole bunch of
switching to have both of them together.

It took me a while, but now hopefully we're on the same page which bit
of the full circuit is causing trouble?

Cheers,

Andy

On 10 June 2017 at 11:03, Andrew Simper <andy at cytomic.com> wrote:
> Tom: another alternative to switching the input signal to the base of
> the cap to get the high pass response instead of low pass is to do
> this operation "manually", that is to take away input from the output
> of the low pass stage, and feed this differenced signal, which is the
> high pass, to the next stage. If the input to the low pass is already
> inverting then you need to sum the input with the output to generate
> the high pass response.
>
> Cheers,
>
> Andy
>
> On 10 June 2017 at 10:53, Andrew Simper <andy at cytomic.com> wrote:
>> A word of warning: Craig Anderton's multiple identity filter will not
>> give "correct" shapes for all responses, the resonance path doesn't
>> always work. You need to change the resonance feedback path to be
>> either at the input, or at the base of the first cap, and if I recall
>> correctly you also need to sometimes invert it depending on the
>> responses.
>>
>> Cheers,
>>
>> Andy
>>
>> On 10 June 2017 at 09:20, Doug Terrebonne <dougt55 at yahoo.com> wrote:
>>>
>>> Yes, Craig Anderton did this with the 3320 -
>>>
>>> http://www.synthdiy.com/files/2002/MIF.pdf
>>> Doug
>>> synthparts.com
>>> ________________________________
>>> From: Tom Wiltshire <tom at electricdruid.net>
>>> To: Sean Ellis <TensionType at hotmail.com>
>>> Cc: "synth-diy at synth-diy.org List" <synth-diy at synth-diy.org>
>>> Sent: Friday, June 9, 2017 6:09 PM
>>> Subject: Re: [sdiy] Untangling the Oberheim OB-8 filter
>>>
>>> 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
>>>
>>> ==================
>>>        Electric Druid
>>> Synth & Stompbox DIY
>>> ==================
>>>
>>>
>>>
>>>
>>>
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