moog sonicV diode ladder

[Terry Michaels]

Hi list members:

The recent discussions on Moog transistor ladder filters, and diode ladder
filters, brings up a question:  Is there a consensus that these filter
types sound significantly different?  If so, one reason might be the
coupling between filter sections.

The 4 stages of a transistor based ladder filter have "reverse isolation" a
term used in RF circuitry.  Each section is isolated from the others, each
section adds a single pole, all 4 poles should fall onto the same point
(same cutoff frequency).  The diode ladder filter has no "reverse
isolation", each section is coupled to the one next to it. The poles will
be in different places, because each filter section has a different amount
of loading.  The loading depends on where it is in the circuit, and if it
is connected to the input buffer, or output buffer, or another filter
section.

Another way to look at is: the transistor ladder is equivilant to 4 RC
lowpass sections in tandem with a buffer between each one, and the diode
ladder is equivilant to 4 RC lowpass sections connected together in tandem,
 with no buffers between them. 

If the filter topology is extended to the rf range, a multiple section
filter with high isolation between sections will have a higher Q, and a
sharper cutoff, than a filter with low isolation between sections.

I do not have a modeling circuit to graphically analyze this.  Has anyone
on the list run these filters on Spice or anything else?  I would be
curious to see what the lowpass functions looks like.

Any comments?

Terry Michaels