CA3046 exponential converter

[Paul Schreiber]

I'm restoring a Moog 15 modular, and the 921 VCO uses 3046 arrays as the
expo gen.
It also uses 1K tempcos UNDER the chip (as opposed to on the top). The chip
is
in a socket, which is just tall enough to allow the 3046 to sit on top. I
did this in the MG-1
as well.

In fact, the MG-1 VCO is pretty much the same as the Moog modular. The main
difference
is the modular uses a CA3080 as the comparator (!!?!) to switch the PNP
across the integration
cap. Else, it's full of hand-selected 741s (BARF!!). No wonder they drift
like mad!! I suggest looking
at the OP-07 "family" of op amps for input summers (OP-07, OP-27, etc).
Input bias current is
not the issue, it's offset voltage and current drift. The Linear Tech LT1013
is also a good part.

As an aside, the circuitry in the Moog modular is sort of like looking under
the hood of a Model T.
Most of the pc boards don't even have solder masks or silkscreens. This
particular one was built
in 1977, and most of the electrolytic caps are starting to split apart. The
measured signal-to-noise
ratio (like on the filter and VCA) is around 58dB, which is like FM radio
(not to brag, but the SNR of
the MOTM VCA is 82 dB).

Paul Schreiber
Synthesis Technology
www.synthtech.com