[sdiy] ROSSUM'S LADDER FILTER with PNP Transistors

[Juan Bermúdez Costa]

Hi Michael. 
As far as I know the scheme is complete and correct.


The DC coupling was a deliberate design decision. According Rossum in the EVOLUTION's brochure  "I Envisioned to DC-coupled design with a cutoff frequency range well beyond 10 octaves, as well as variations of the height Eliminating of the resonant peak oscillation amplitude or frequency with voltage control".

The nominal level of 20mVpp pursues a much lower distortion than usual in moog ladder filters. Again Rossum says "The 1100 used a Moog ladder as ITS core element, but I wanted to isolate the audio innate characteristics of the ladder filter from colorations resulting those from the input level-shifters and output amplifier used in the Moog 904A".


Regarding the 1100 output stage, Dave says "I then a designed a completely new output stage for the ladder; esta circuit has never (to my knowledge) Been used outside my designs The entire signal path was DC coupled and the resonant feedback. path phase compensated. The 1100 was my favorite sounding filter (I liked it more than the SSM2040 I later invented, and kept the 1100 as E-mu's modular lowpass in preference to a cheaper 2040 design). The operational range of the cutoff frequency was from 0.1Hz to 25kHz acerca, with Q's stable throughout. "

You say "The filtering action is based on the emitter resistance which is set by DC bias" Partially agree. The filtering action takes place by varying the current through the PN junction, not the voltage on it.

Pure technical curiosity, but I find fascinating some aspects of this circuit...

Cheers, 
Juan Bermúdez Costa
Barcelona

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