[sdiy] Dual Log/Linear Irwin-Gallo VCA

[David G. Dixon]

I'm very happy to report that my latest module, a dual log/linear VCA based
on a modified Irwin-Gallo circuit using linearized 2164s, is up and running
perfectly.  Scope images look exactly like my simulations.

I used the Coolaudio clone V2164 from Small Bear Electronics ($3 each for 10
or more; check it out!).  The module involves two identical, independent
VCAs, each with attenuated signal and CV inputs, 0-to-5V CV bias controls,
and + and - outputs.  The one-sided board is 2.1" x 4.4" and includes one
V2164 and three TL074s.  The panel is pseudo-MOTM format, 2U x 5U.

As usual, any interested parties can email me offline for the schematic, the
FPD front panel file, my funky Excel-based pcb layout file, and a Multisim
simulation file.

I'm totally sold on the 2164.  The Irwin linearization basically gives you
the equivalent of two "perfect" OTAs -- no fussy biasing or trimming, and no
need to attenuate the input signal to just a few millivolts.  I will
definitely be using it in place of LM13700 in subsequent designs, and may
even go back and redesign some OTA-based circuits around it.