[sdiy] A few Memorymoog questions

[Don Tillman]

   > Date: Sun, 15 Apr 2007 16:59:23 -0700
   > From: "Tim Parkhurst" <tim.parkhurst at gmail.com>
   > 
   > Ah, now I've noted that the "summing" amps (CEM3360 VCAs) for
   > VCO1 and VCO2 are non-inverting. Now this is a new one for me: It
   > looks like the summing node for the 3360 is a non-inverting pin,
   > and no feedback is used (at least, no external feedback).

Um, that's not actually what's happening; there's no "summing node" in
the sense of a classic opamp inverting summer circuit.  Here the VCO
waveforms are simply switched on and off with the 4016's, and those
that are on are averaged together by the resistors, and that signal is
sent through the 3360 VCA.  The 3360 only does non-inverting
operation, and if you did apply negative feedback around it you would
lose most of the VCA function.

   > Could the same configuration be used to acheive averaging of
   > several signals with a regular op-amp (without CV control of
   > gain, of course)?

Sure.

   > Again, I'm told that the Memorymoog gets a little more distortion
   > and thickness than the Mini when all three VCOs are used, and the
   > internal mixer was set up this way on purpose.

I've never compared a MiniMoog to a MemoryMoog side by side, but I can
say for sure that the MemoryMoog is designed to have the VCO's
overdrive the VCF, and a bunch of factory patches make use of it.

And hey, it is kind of cool where you expect overdriving a stage to
add harmonics, but overdriving a smooth-clipping circuit with a
triangle wave will actually remove harmonics.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com