[sdiy] Inverting an envelope?

[David G. Dixon]

Having negative envelopes is nice, but it's actually much more useful to
have bipolar attenuators on the inputs of the modules you wish to control
with envelopes (primarily filters).  In fact, my ADSRs have both positive
and negative outputs, but all my filters have one bipolar attenuator for
envelopes, one monopolar attenuator for LFO modulation, and one unattenuated
1V/oct input for keyboard tracking.

Here's how I build bipolar attenuators:

Wire up the CV input to an opamp inverter with input and feedback resistors
of R/2, where R is your pot resistance (i.e., if you're using a 100k pot,
use 49.9k resistors).  Wire the top end of the pot to the CV input, the
bottom end to ground, and the wiper to the non-inverting input of the opamp.
This circuit is essentially a differential amp with matched impedances of
R/2 when the pot is in the centre of its rotation (i.e., when no CV is going
through the opamp).  When the pot is fully CCW, it is a unity-gain inverter
(the + input is grounded).  When the pot is fully CW, it is a non-inverting
amplifier with a gain of two superimposed with a unity-gain inverter, such
that one unit of gain cancels out, leaving one unit of non-inverted gain.

With two filters in series, one highpass and the other lowpass, manipulating
the amplitude and direction of the envelopes to both filters results in a
very broad range of formant effects with only minor tweaks to the bipolar
pots.  It's a good thing!