[sdiy] Later MS-20 filter with 2164

[Richard Atkinson]

I'd have thought the presence of those feedback limiting diodes was a big 
part of the MS-20 filter sound. Maybe not as much on the cascaded OTA 
filter you're building, but certainly on the Sallen & Key Korg35 filter.


On Jul 1 2010, David G. Dixon wrote:

>Further to my previous postings...
>
> I've just simulated my 2164-based MS-20 filter clone, and I am shocked at 
> how beautiful the simulations are! The filtered waveforms look fantastic. 
> And, with no limiting diodes or anything, this filter oscillates just 
> beautifully! (This is very different from the state variable filter, 
> which hits the rails hard at oscillation, creating trapezoids.) At a 
> feedback gain of exactly 2, the oscillations are stable and perfect. The 
> sine waves are +/-6.7V from each filter separately (when one is 
> oscillating and the other is at zero resonance). They will be very easy 
> to trim. If the first filter is oscillating, the second filter will give 
> identical sine waves 90 degrees out of phase at a feedback gain of 
> exactly 1. THDs are in the neighbourhood of 0.03% to 0.04%. If the gains 
> are cranked beyond these limits, the sine waves distort, looking more 
> like sharks' teeth, with larger amplitudes, but nowhere near hitting the 
> rails (!).
>
> I wouldn't want to put any limiting diodes in the feedback loops of these 
> resonance gain cells, or if I did, I'd put them on a selector switch, or 
> use back-to-back 6.8V zeners (which would clip at about +/-7.5V) so as 
> not to lose those perfect sines.
>
> I think this might be the nicest filter simulation I've seen. I'm really 
> in shock. Design is not supposed to be this easy! I'm definitely building 
> this filter.
>
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