[sdiy] FatMan Hacking ( and I dont mean hitting me with an Axe!)

[Theo]

I tried a 24dB SVF, gave interesting results, highly resonant sound.
However, the BP and HP functions became depended on the resonance level
and the best feedback level for HP response, was not the best level for the
BP,
was not the best for the LP.
Therefore both feedback paths (one for the resonance, one for HP&BP)
needed to have variable gain.

Silly me thought I was trying something new,
but then a Electronotes 24dB SVF popped up.
While I hoped for some smart solution to the feedback level problem,
it turned out to be the same circuit, only OTA based while mine used
diode bridges.

A 18dB SVF should also work.
HP and LP response should be no problem.
The other 2 tap points will probably give a sort of BP function.
Kinda expect a 6dB/12dB slope on the first tap and
a 12dB/6dB on the second, this is more a guess though.
Won't be surprised to see the same kind of feedback level problems as
with the 24 dB version.

The idea to build a 303 filter and add this to the Fatman is probably
a better idea than a 18dB SVF.
Make a switch to toggle the first cap in the ladder from 2xC to 1xC,
C being the value of the other caps in the ladder.
And voila, you have a 18dB 303 to 24dB Moog switch :)

Theo




From: harry <harrybissell at prodigy.net>


> I have not seen an 18db State Variable Filter...
>
> I do not have sufficient "guru" points to say that it cannot be done...
>
> but the SVF is basically a summing amp and two inverting integrators...
> If you add one more the feedback is going to get VERY funny... at least.
>
> Why not just build a clone of the TB-303 filter and ADD it to the
fatman...
> that would comemuch closer to the sound u want.
>
> H^) hary
>
> matti wrote:
>
> > On Sat, 27 Oct 2001, adam wrote:
> >
> > > Hi
> > >
> > >     One thing that you could try is use the unused half of the 13600
/13700
> > > used for the VCA as a third stage for the VCF to get the 18 db
resposnse, it
> >
> > can this be done? I don't know how OTAs work, but what I'd gathered is
> > that you need two stages for any sort of filtering to work.
> >
> > Oh ye sages, intervene before we render out fatmans inoperable please.
> >
> > -
> > the bills were all due yesterday
> > you've failed
> > you're through
> > you've failed
> > you're through
>
> --
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>
>