[sdiy] Gain staging in 4-pole VCFs

Rutger Vlek rutgervlek at gmail.com
Thu Nov 8 21:06:55 CET 2018


Hi list,

I just remembered something I read years ago on the Oakley sound website regarding gain staging inside filters (usually those with >2 poles). It was regarding the COTA filter (cascaded OTA), where Tony only briefly explains using integrators with varying amounts of gain throughout the filter. Hereby (I presume) generating a very smooth combined saturation curve, where each stage contributes its share (much like guitar distortion pedals with several stages of saturation involved). He refers to a famous filter that inspired him for doing this, and I'm curious which one (the design choice sounds like one Dave Rossum would make...). Also, I'm curious which other filters use this approach.

I'd be interested if anyone can contribute with knowledge about specific filters and their gain staging, And perhaps Tony, would you be willing to shed some more light on this?

Some years ago I design the filter that's now on the market as Dendrites, in which I tried to model (in analog) different resonance responses of filters I knew. Its gain staging (and therefore saturation characteristic) is fixed though, and at unity for each stage. I'd be interested to see if I could take this modeling approach a step further, perhaps with different saturation elements (diff-pair transistors, diodes) or with different gain staging.

Regards,

Rutger



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