[sdiy] Ladder filter pole (Moog Taurus filter)

[Gene Stopp]

I would think that you could use the buffer 3080 as the resonance control,
but if you have a compensating 3080 for the final VCA, why not just parallel
it to the resonance VCA and control it normally? Perhaps there would be
loading effects on the ladder. Soundwise, the Taurus filter is extremely
fat. At least it is with the rest of the Taurus circuitry. :)

I'm trying to imagine how the tube stuff would work in a ladder setup. Now
I'm not big on the math side of filters - I've done the math, years ago, but
I forgot it all. I can't speak fluently about poles and the S-plane and so
on. These days I'm more of a circuit-copier and seat-of-the-pants designer.
So correct me if I'm wrong, but the Moog ladder lowpass functions by
modulating the standing current in the transistor ladder with the audio
signal, and the imbalance between the ladder sides are picked off at the top
of the ladder as the output voltage. The standing current in the ladder is
variable, modified by the CV controlling the transistor at the base of the
ladder. Higher standing current = higher cutoff.

I'm under the impression that "tubes are voltage devices" and "transistors
are current devices", to oversimplify. Would a ladder of cathode-to-plate
valves with caps as the "rungs" between the sides of the ladder do the same
thing as a transistor ladder? Would the variable cutoff range be wide
enough?

Another popular filter type is the cascaded integrator, typically incarnated
as four OTA's in a row, each set up as a simple R/C filter with the R being
the OTA. Feedback from end back to beginning allows for resonance. Can a
tube (voltage device, like a FET) work as a variable integrator here? Or
would it be better to use the tubes as Hi-Z followers, and use LDR's as the
R and gang them together for frequency control?

Then what about a state-variable filter? This is (simplistically again) a
two-integrator quadrature oscillator with feedback from the middle to
stabilise things, and then an audio signal added to the input of the first
integrator.

Perhaps it would be easier for me to visualize this if we started with VC
filters that use FETs as the controlling element, and then pull them out and
drop tube circuits in. Anybody have any workable FET designs? I've seen
FET-based allpass filters in phase shifter schematics, but I don't remember
seeing any VCF's.

Darn it, these 1/2 watt resistors don't fit into the plug-type protoboards
very well :(

Best Regards,

- Gene

-----Original Message-----
From: mikko.a.helin at nokia.com [mailto:mikko.a.helin at nokia.com]
Sent: Tuesday, April 23, 2002 7:02 AM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Ladder filter pole (Moog Taurus filter)


Found the Taurus schematics here:
http://www.sonicstate.com/synthfool/schematics/taur1.gif
(also http://home.debitel.net/user/jhaible/jh_minotaurus_2.pdf looks much
similar)

CA3080 is used only as differential amp (there's another for VCA), the
output is buffered with a transistor to 4016 switch for resonance control.
Would it be possible by varying the 3080 gain to control also the resonance
by a CV? Obviously the output gain would also change at the same time but
couldn't that be compensated in the VCA by subtracting part of the resonance
CV from the VCA CV? That way you could get a really low part count VCF+VCA
for polyphonic synth. How does that filter sound like (vs. mini/memorymoog
filter)?
-Mikko