[sdiy] ROSSUM'S LADDER FILTER with PNP Transistors

[Juan Bermúdez Costa]

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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