[sdiy] 2164 4P LPF perfected

[cheater cheater]

On 24/10/2010, David G. Dixon <dixon at interchange.ubc.ca> 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

You mean octature? (since quadrature implies four equally-important
items, as in four quarters of a whole)

This intrigured me, so after about 20 minutes research I came up with
this word genealogy:
quadro -> quadrare -> quadrat -> quadrature
Similarly, you could do:
octo -> octare -> octat -> octature

:-)

> 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

What's the basic difference here? Does this mean just gain of the loop
around the whole filter, vs gain of smaller loops from the last stage
to the input, middle stage to the input, etc?

> 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.

Sounds really great!

Now I'm also waiting for it to have resonance output in triangle
shape. Especially on lower resonance settings 8-)

BTW, what happened to your plan of making it possible to dial in the
amount of resonance? Did you ever realize this? I thought it was one
of the more interesting features.

Cheers,
D.