[sdiy] ROSSUM'S LADDER FILTER with PNP Transistors

[Colin f]

You could try contacting Dave Rossum via his website.
I spoke to him at Superbooth and found him to be a really nice guy, happy to
answer my questions about the precise internal details of his SSM2044
design.
His Evolution filter is the ultimate incarnation of the Moog ladder
topology, IMO.

Cheers,
Colin f


> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of
> Juan Bermúdez Costa
> Sent: 25 September 2016 10:52
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] ROSSUM'S LADDER FILTER with PNP Transistors
> 
> This is my restoration of Emu Low Pass 2100 old schematic with v2 1100
> submodule...In my opinion it is not simply a classic moog ladder network
> implemented with PNP transistors. There are several variants of design not
> easy to understand. The first is the fourth filter cell, which uses a
capacitor 1n
> (the rest are 20n), disrupting certainly filter properties in resonant
mode. The
> second is the very low level audio (20mVpp) attacking the input filter,
when
> in moog filters it ranges from 40 to 100mVpp. The third is the DC levels
that
> sets the separation points of the filtering cells; while in use moog
filters is of
> the order of 1 to 2V, the Rossum is only 600mV. Zero offset buffers are
not
> identical; the right one includes a diode whose function is unknown. The
> differential output pair (QD102 NPN transistor) has a feedback resistor
(47K)
> that does not exist in moog NPN versions. I feel this circuit operation
hides
> some secrets that are not visible at first glance. It will be interesting
to find
> out why these variants ... Suggestions and comments are welcome.
> See schematic at :http://electro-music.com/forum/topic-67563.html
> 
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