Tau Systems 1010 Filter

[WeAreAs1 at aol.com]

Hello Jim and DIY folk,

Thank you, Jim, for drawing and sharing that filter schematic with the rest of
us.  It's interesting that Tau Systems chose to make their Moog filter clone
with PNP transistors rather than NPN devices.  I haven't ever seen this
variation.  Did they do this in order to achieve their multimode filter
possibilities?  Since we don't know exactly what's going on inside the mystery
part at the output, it's hard to figure out how they are achieving the
Highpass response.  Well, hard for me, at least.  Maybe someone else on the
list has an explanation for it.  

Jim, I can see why you think the mystery part at the output is an OTA and an
opamp.  It would appear that the signals from the FET's enter the differential
inputs of an OTA (probably a 3080), with the OTA control current connecting at
Pin X (strapped "on" by the 39K resistor to ground).  The signal then goes on
to an opamp, but is it entering the opamp through its inverting or non-
inverting input?  It's likely R29 and R30 are the feedback loop of the opamp,
controlling its gain, connecting to pin S at the opamp's inverting input.
It's also possible that pin R (the Highpass input) connects to its non-
inverting input.  Maybe I'm dense, but I get lost trying to figure out signal
phase going through all those transistors...  There may be some internal
resistors that we can't see, between the OTA and the opamp.  I'm assuming also
that there are some power connections on the part that you didn't include on
your drawing, right?

Does this filter really get Highpass response simply by inputting audio at the
Highpass input, or must some signal also be input at one of the other inputs?
(at the inverting Lowpass input?)  One would think that Bandpass response
might also be available by putting equal amplitude signals into both the
Highpass and Lowpass inputs.  Does this make sense?

I am curious about all of this because I would very much like to know how to
modify a standard Moog ladder to make it act like a Highpass filter.  Does
anyone know how to do this? (and hopefully without rebuilding the filter with
PNP transistors!)  As some of you may already know, the Moog modular system
offered a Highpass module, but it does not appear to have differential ladder
filter topology.  Rather, it looks more like four simple 6dB highpass filters
in series.  Also, unlike their lowpass module, it did not have a resonance
loop. (why not??)

The classic Minimoog lowpass ladder filter is a sound we all know and love, so
it stands to reason that a highpass version of this filter would be equally
elegant and musical.  It's curious that there aren't any examples of this
already out there, at least to my knowledge.  Is there a technical reason why
it can't be done?  I wonder if there is a way to somehow implement Tau's
Highpass input arrangement into an existing Moog ladder.  Any ideas?

Michael Bacich