[sdiy] Untangling the Oberheim OB-8 filter

Sat Jun 10 06:24:20 CEST 2017

Yes, it does, but not in the same way, exactly.  We take all four output and
the feedback, and with those five signals, we can get a whole slew of other
modes.  Resonance works perfectly on all of them.

  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Jason
Proctor
Sent: Friday, June 09, 2017 8:18 PM
To: Andrew Simper
Cc: synth-diy at synth-diy.org List
Subject: Re: [sdiy] Untangling the Oberheim OB-8 filter

the Intellijel Polaris also does something similar, if i'm not mistaken... 

https://intellijel.com/eurorack-modules/polaris/

On Fri, Jun 9, 2017 at 8:03 PM, 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/
<http://www.synthdiy.com/files/2002/MIF.pdf> 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.
<https://mutable-instruments.net/archive/shruthi/build/4pm/>
net/archive/shruthi/build/4pm/
>>
>> I believe it's a similar topology.
>>
>> ________________________________
>> From: Synth-diy <synth-diy-bounces at synth-diy.
<mailto:synth-diy-bounces at synth-diy.org> 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/
<http://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/>
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-
<http://electricdruid.net/wp-content/uploads/2017/06/ob8-sm-24.jpg>
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/
<http://electricdruid.net/cem3320-filter-designs/> 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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