[sdiy] 2164 4P LPF perfected

[David G. Dixon]

> > 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

Well, you might be right, but I notice that people use the word "quadrature"
even when only two waves are being generated.  In fact, "quadrature" refers
to waves which are 90 degrees (i.e., pi/4) out of phase.  Since the waves on
the 2164 4P LPF are 45 degrees out of phase, you are correct.  It is
"octature".

> > 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?

No.  In my filter, there are four separate VCAs adding gain to the signal:
one between each successive filter stage, and one feeding the inverted
output of the fourth stage back to the first stage.  At zero resonance, the
gains of these four VCAs are 1, 1, 1, and 0, respectively.  At
self-oscillation, the gains are 1.414, 1.414, 1.414, and 1.414,
respectively.  They are all driven by the same control voltage.  The
different gains are achieved by the clever selection of input and feedback
resistors.  These gain ratios ensure that the signal gain is compensated
with increasing resonance, and that the octature sine waves are all of the
same amplitude.

> Sounds really great!

Thanks!

> 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.

I have adjustable 5.6V zeners to alter the octature sine amplitudes from
+/-5V to +/-10V.  However, this has only a barely discernible effect on the
output when the filter is not oscillating.  The resonance is very robust
regardless.  However, this same basic circuit does have a dramatic effect on
the amount of resonance vs the amount of signal on my 2164 Korgasmatron dual
MS-20 clone.  This is because the resonance is generated in completely
different ways in the two circuits.