[sdiy] A few Memorymoog questions

[Don Tillman]

   > Date: Sat, 14 Apr 2007 14:49:14 -0700
   > From: "Tim Parkhurst" <tim.parkhurst at gmail.com>
   > 
   > A few questions about the Memorymoog voice card:

Hey Tim,

   > 1) I noticed that the card uses 4066 switches for CV routing, but
   > 4016s for the audio path (VCO 1 and 2 waveform select). Any
   > particular reason for this? My guess is that 4066s perform a
   > little better, and their lower Ron is a benefit for CVs, while
   > the 4016 is less expensive and worked just fine for the audio
   > path. Right? Wrong?

Probably.

   > 2) The selected waves from VCOs 1 and 2 go into the inverting pin
   > of the audio summing amp (U12A), while VCO 3's signal gets
   > inverted (this signal is used as audio and as a CV source for
   > "voice mod") and then sent to the NON-inverting input of the
   > audio summing amp. This configuration is an interesting one, in
   > that it saves one op amp, and lets the inverted and non-inverted
   > signals to be mixed (without canceling). 

That's an interesting subtlety; 'good eye...

(I'll write this so that folks without a MemoryMoog schematic in front
of them will understand...)

Each Memorymoog voice has three VCO's, and each VCO has three
waveforms, and those waveforms are selected with three pushbutton
switches.  For VCO1 and VCO2, when you press more than one waveform
button the waveforms average, but for VCO3, when your press more than
one waveform button the waveforms arithmetically add.  This is because
VCO1 and VCO2 are dedicated to being audio sources and you want the
waveform buttons to change the timbre of the waveform without altering
the level.  VCO3 can be used either as audio or an LFO, so the having
the waveforms add makes more sense when VCO3 is a modulation source.

Also note that VCO1 and VCO2 are AC coupled while VCO3 is not.

The summing of the VCO3 waveforms adds an extra opamp inversion into
that path, so when the VCO's are summed together after their
individual VCA stages, that extra inversion is accounted for in that
audio summing stage that you noticed.

   > Is there a noticeable (audible) difference between a standard
   > inverting summer and a difference summer?

Yes, there are several differences.  One is that a standard inverting
summer isolates the inputs from each other, while with the difference
summer the noninverting input interact with each other and with the
inverting inputs.

   > And finally: I'm watching "The Mummy" and I'm noticing that
   > Rachel Weisz is a very beautiful woman... sorry, got distracted
   > there. Guess I drifted OT.

That depends, could there be a MemoryMoog in the soundtrack?

  -- Don

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