[sdiy] Filter slopes

[Tom Wiltshire]

> On 11 Jun 2018, at 09:32, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> 
> For a practical implementation hunt out the Oberheim multimode filter circuit:
> http://www.synfo.nl/servicemanuals/Oberheim/OBERHEIM_MATRIX-12_SERVICE_MANUAL.pdf
> 
> See page 4.

While the Oberheim is definitely the original source of this idea, Olivier’s or David’s 2164 implementation of this idea is more practical, in my view. The benefit of the inverting filter stages and the well buffered output from each integrator makes it as simple as just adding a bunch of resistors for each output you want. This can either be switched, as in Olivier’s original Shruthi "4-pole mission” filter, or you can implement a separate mixer for each required output and have them available simultaneously.

Another simple possibility is to do the same thing using the AS3320/CEM3320. Again, this includes the buffers, making life much simpler, and in the case of this chip, you get VC-resonance for free too, unlike the 2164.

These pole-mixing filters are ideal for creating asymmetrical-slope bandpass responses though, that’s for sure.

Tom