[sdiy] Untangling the Oberheim OB-8 filter

[Jason Proctor]

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/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
> >> ==================
> >>
> >>
> >>
> >>
> >>
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >>
> >>
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >>
> >>
> >>
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at synth-diy.org
> >> http://synth-diy.org/mailman/listinfo/synth-diy
> >>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20170609/39ba8564/attachment.htm>