On 12 Aug 2012, at 10:29, Olivier Gillet wrote: > No, but to arrive at the "canonical" transfer function of the 4-pole > LP with resonance (1 / (4k + (1 + s) ^ 4)), you need to subtract the > output of the 4th stage from the input, not add it. I see. That's what I was missing. I assumed the resonance would be added. Thanks, Tom