[sdiy] question about a Mankato filter

[Andre Majorel]

On 2010-06-04 00:29 -0700, David G. Dixon wrote:
> On 2010-06-03 21:25 -0700, David G. Dixon wrote:
> > On 2010-06-03 20:32 -0700, David Brown wrote:
> >
> > > I used a 1 KHz square wave and adjusted the output (input to
> > > Mankato) to 10.0 V pk-pk.
> > >
> > >  Input: 10.0V pk-pk
> > >   6 dB:  5.0V pk-pk
> > >  12 dB:  6.9V pk-pk
> > >  18 dB:  9.9V pk-pk
> > >  24 dB: 13.7V pk-pk
> >·
> > The Mankato seems to be optimized for use as a quadrature
> > oscillator, but the filter outputs shouldn't be giving these
> > disparate levels.  This is the result of using dividers and
> > inverters at various gain levels to equalize the sine waves
> > during self-oscillation.  Distributing the resonance gain
> > across the four stages overcomes this problem, but does
> > require another quad VCA, and hence, increases the cost of the
> > filter.
>
> In any case, having filter outputs at increasing levels isn't
> really that big of a deal, since output levels tend to
> decrease across a cascaded-stage filter anyway, when the
> filter is actually filtering.  In this way, having a bit of
> gain across the filter might even be a good thing.

Given the choice between optimising levels for use as an
oscillator (i.e. all equal to 10 Vpp) and optimising them for
use as a filter (i.e. equal loudness or whatever), I'd do the
former.

Equal loudness is ill-defined and depends on the input. And
filter outputs are certainly going into a mixer anyway.

If all oscillator outputs have the same amplitude, you can switch
between them without having to tweak the modulation levels every
time. That's worth another 3.50 USD to me.

Or one could use three op amps for make-up gain and save a couple
dollars. Inelegant ? Maybe, but in the big picture, an elegant
interface is worth a lot more than an elegant circuit.

-- 
André Majorel http://www.teaser.fr/~amajorel/