[sdiy] Best AVAILABLE OTA for building new designs?

[David G Dixon]

 >>  Each ADSR requires one 2164

 >  Wouldn't one VCA per envelope generator be enough? Depends on the
design, of course.
 
No, because all of the controls must be voltage controlled.  I tried
switching the A, D and R voltages to a single VCA, but it was not practical.
What I ended up doing was using separate VCAs for A, D and R.  The control
voltages from the front panel are locally buffered and sent through three
channels of a 4066 analog switch.  When each switch is off, the voltage at
that VCA's control pin is pulled up to about +5V by a 10k resistor to the
15V rail.  This effectively turns that VCA off.  All three VCAs are in the
feedback loop of the EG integrator (remember, 2164 VCAs must terminate to
virtual ground).  Believe me, I tried many different ways to do this, and
this was the only way that worked perfectly.  The fourth VCA of the 2164 is
a linearized VCA (the log amp is on the front panel) which controls the
amount of the envelope being sent to its destination.
 
>> Will there be a problem mixing 48 key outputs together?  Will clipping be
a problem?  Obviously, all 48 keys will never be played at the same time,
but still.

 >  Gainstaging a synth with lots of voices is a nontrivial problem. But at
least it's just one gain factor that needs to be decided upon..
 
I may have to come back here for some advice when I get to the point of
adding all of the voices together.  Right now, I'm just thinking of summing
them all at unity gain at a single opamp (i.e., 48 10k resistors into the
inverting input, and a 10k feedback resistor).  Maybe I'll have to make sure
that the voicecard outputs are suitably attenuated so that I can add many of
them together without fear of clipping.  This is something about which I
haven't given much thought yet.  How does an electronic organ manage this
problem?
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