PLAN B analog blog

A comment came up about the functionality of the Model 33 Tri/VCA. 
Why I kept the two of them at log only and the bottom unit the only
one with a linear option.

The M33 uses the Analog Devices SSM2167 quad precision VCA chip as
it's engine. This is one of the last surviving products from AD's
acquisition of SSM and thank God the pro mixing industry embraced the
part because it would be gone as well had they not been used in many
high-end mixing desks over the last decade. The decision to go with
the 2164 is a wise one as it assures better than .02% harmonic distorion and a 120 dB dynamic range.   However, the basic nature of these devices is non-linear.  To change that I incorporated Mike Irwin's EDN published
2164 linearization circuit and Phil Gallo's later addition to this, also
widely recognized  that incorporates the circuitry for offset-free (read
seamless) lin/log switching.  Both of these however require two
segments of the 2167 to do their job - two of the four.  Knowing that
half of the chip is taken by this single VCA, it leaves two others
with Log response.

Never being the type to compromise performance without finding a
solution, please remember the Model 10's contour switch.  With
that you can adjust for the non-linearity of the two top VCAs quite
easily.  You can achieve the same results as if they were linear
themselves.

Hope this helps,

- P