[sdiy] 2164 4P LPF perfected

[Tom Wiltshire]

Excellent work! This is fascinating reading.

Do you have a schematic we (SDIYers) can see?
I'd be interested to compare with your earlier circuits in documents like your filter analysis.

Thanks!
Tom

On 24 Oct 2010, at 20:38, David G. Dixon wrote:

> Hey Team,
> 
> I am a happy camper today, and since the list has been so quiet lately, I
> thought I'd send a wee message and toot my own horn a little.
> 
> As you probably know, I have designed a cascaded-stage 4-pole lowpass filter
> with 8 quadrature outputs based on the SSM2164 quad VCA.  There are two
> innovations in this design which distinguish it from other COTA-type
> filters:
> 
> a) distributed resonance gain to equalize the quadrature output amplitudes
> b) reverse exponential gain-VCA response to smooth out resonance control
> 
> You probably also know that I've been developing a multimode matrix filter
> add-on for this filter.  However, as part of my simulations, I realized that
> the distributed gain aspect of my filter would give the wrong responses in
> the multimode interface when the resonance was turned up.  Hence, I altered
> my circuit by adding a switch so that one could select between distributed
> resonance gain and conventional resonance feedback.  This switch does two
> things in "conventional feedback" mode:
> 
> a) it grounds the VC pins of the first three resonance gain VCAs to give
> unity gain through the first three stages, and
> 
> b) it adds input resistors in series with the mixer input and the fourth
> stage output on the fourth stage's gain summer to increase the current
> through that summer by a factor of 2.82 (twice root 2), thereby increasing
> the overall gain on that summer at oscillation from 1.41 to 4.
> 
> The switch is 3PDT, with one pole grounding the first three gain VCAs and
> the other two adding the summer input resistors in parallel.
> 
> The good news is that this works beautifully, but I've discovered something
> very interesting that I didn't know before about this circuit.  When the
> resonance is turned all the way down, the filter gives identical results
> regardless of the position of the switch (of course).  However, when
> conventional feedback is employed, the input signal is sacrificed as the
> resonance is increased, such that the signal nearly disappears when the
> filter is near the point of self-oscillation.  However, when distributed
> resonance gain is employed, the signal remains at unity gain across the
> entire spectrum of resonance.  That means that resonance can be changed
> manually or controlled with CV and there will be no decrease in audibility
> of the filtered signal.
> 
> The very first filter I ever built had this distributed gain feature (it was
> the 13700 4P LPF, based on OTAs rather than VCAs), so I've never actually
> used a filter that suffered from this loss-of-signal problem (I've never
> bought a module or even a PCB).  Indeed, I never realized just how lame
> these filters can be with increasing resonance until I installed this switch
> on my filter.
> 
> The only downside of distributed gain is that self-oscillation is just a
> little bit less robust at low frequencies than in the conventional feedback
> case, and thus requires the resonance trimmer to be set a bit higher if one
> wants to use the filter as an LFO.  Part of this has to do with the reverse
> exponential response as well, which allows for less overdrive at the top end
> of resonance.  It may be worthwhile to replace these trimmers with small
> panel pots.  In fact, it would be nice to put the tuning trimmer on the
> panel as well.
> 
> In any case, I think this circuit is now just about perfect.  My goal was to
> have a COTA-type filter which would be a robust 4P filter, an excellent
> quadrature oscillator, and a suitable core for a matrix filter interface,
> all without compromise, and I think I've achieved all three.  Please email
> privately if you would like more information.
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy