[sdiy] Gain staging in 4-pole VCFs

David G Dixon dixon at mail.ubc.ca
Fri Nov 9 01:15:09 CET 2018


I do gain staging in the Dr Octature 4-pole filter.  I'll explain it in
detail later when I'm not in a hurry to leave work.  It was one of my
cleverer moments.


> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Rutger Vlek
> Sent: Thursday, November 08, 2018 12:07 PM
> To: Synth DIY
> Subject: [sdiy] Gain staging in 4-pole VCFs
> 
> 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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