LFO/ENV board of JH-4 finished

[Haible Juergen]

Subject already says it: The next board for the first voice module
of my Polysynth is finished. Tested and optimized it over the weekend.
It contains the following functions:

* Two ADS envelopes (ENV1 and ENV2). All parameters are voltage
controllable.
  Time constants are variable from 1ms to approx 1 minute.
  One switch that is shared by both ENV's swiches Release to approx.
20ms,
  similar as on the Minimoog. Otherwise, these are VC-ed Oberheim ADS
  circuits.
  Each ENV eats up 1 LM13700, one FET, and a handful npn's and pnp's.
* One Variable Waveform VC-LFO. It was designed to use as few as two
knobs
  for all functions. The first knob sets the initial frequency (from
approx. 1minute to
  approx. 200Hz). The second knob has two functions. If it is in center
position
  (12 o clock), the LFO produces a Sine Wave. Turning the knob ccw makes
the
  ENV1 to modulate the frequency of the LFO. Rate goes up to approx.
1kHz
  then, so you can produce the CS-80-like AD-enveloped Ringmodulator
effect. 
  Turning the knob in cw direction varies the waveform continuously from
Sine
  to Square to Sample&Hold to VCO2's SAW wave.
  There's obviously a big compromise in this: No FM on other than Sine
waves,
  and I have either crossmodulation by VCO2 *or* LFO modulation, but not
both
  at the same time. But I wanted to cramp as much as possible into a
design
  with fixed number of knobs, and LFO-FM on Sine is better than no FM at
all.
* This board also contains the VCAs for dynamic filter response
modulation.
  I described the scanning thru various filter modes in a previous
posting. Stefan
  came visiting, said if you bother to implement all this scanning you
*must*
  make it dynamical. Now I have a 3-position switch which selects the
mode
  in which the VCF-Mode Knob works: Either manual, or sweep from left to
  right (i.e. from 2pole LP -> ... other modes ...-> 4pole LP) by ENV1,
or sweep
  from right to left. In the dynamic modes, the starting values are
fixed to either
  extreme position (i.e. always starting with a LP), and the knob acts
as an
  ENV1 Amount control.
Unlike the VCF/VCA board, this one has OTAs (3080 and 13700) for all
control VCA functions. The interpolating scanner for LFO waveform
selection
is fully discrete, however.

I promise, you *will* see schematics for this (My main reason for not
using
SSM or CEM chips in this whole project was to make it available to
DIY-ers),
but please have a little patience. My fist scanning attempt (VCA) was
not that
great, and I'll work on a higher quality.

Hope you found this interesting - I know I have posted something similar
before,
but this is the final version.

JH.