[sdiy] ROSSUM'S LADDER FILTER with PNP Transistors

Michael E Caloroso mec.forumreader at gmail.com
Sun Sep 25 19:33:27 CEST 2016

The schematic may not be complete or correct.  Designers back then have
been known to do this to deter copycats (I confirmed errors in Oberheim
schematics).  The first omission I spotted in the 1100 submodule schematic
is the lack of AC coupling caps at the differential recovery circuit.  Does
the actual module match this schematic?

The 20mvpp input signal may be required because of the lower 600mv DC
bias.  Bipolar transistors have a limited linear range in transconductance
mode that varies with DC bias.

The output coupling stage is different.  Moog uses a pair of bipolar
transistors off the top of the last filter stage; the 1100 uses JFETs right
off the filter cap nodes.  The high input impedance at the gate minimizes
loading of the RC component.

Part of the answer with the 1nf cap may be the 3x diode biasing at that
stage, which is not 600mv but 1.8v wrt to the previous stage.  The
filtering action is based on the emitter resistance which is set by DC bias.

I talked to Dave at superbooth and he was happy to answer technical
questions about the OTA-based SVF used in Oberheim SEMs, since his name
appears on the patent 3,969,682.  I'm sure he would be open to exchanges
about the 1100.

A very good analysis of the Moog filter was done by Timothy Stinchcombe,
which can be found on google.  Tim may very well be on SynthDIY.


On Sun, Sep 25, 2016 at 6:56 AM, Colin f <colin at colinfraser.com> wrote:

> 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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