[sdiy] simple 2164 linear to expo CV?

[Neil Johnson]

Hi Neil,

(yay, another Neil on synth-diy :) )

> can a single 2164 cell be used to convert a linear voltage to roughly an
> exponential voltage?

Yes what you're after is easily done with a 2164 and a couple of op-amps.

> basically i have some already finished modules (mfos vcas and vcfs) that
> only have linear CV inputs. i have a little cv output interface with a
> bunch of linear faders in them that i use to control these and would
> like to make a really simple 'linear to expo' CV module to use between
> these so the controls feel more natural.

Ok, sounds like you have a linear CV in the range 0V to, say, 5V, and
you want to convert that to an exponential voltage in the range 0V to
+5V?

For this you'll need a 2164 gain cell, a -5V reference voltage (at a
push you could use a 5V1 zener), an op-amp to convert the 2164's
output current into the output voltage, and an op-amp to invert and
shift the incoming CV to match the range of the control pin of the
2164.  You want to convert the 0-to-5V range to 3.3-downto-0V.  The
3.3V end sets the range of the exponential CV, so you may find you
need to reduce this down to, say, 2V.  You can use the same -5V
reference as the offset input, suitably scaled.

Cheers,
Neil