[sdiy] [AH] Re: [AH] KORG 700S CV Hz/V / gate retrofit

[David G. Dixon]

> But isn't this weighted out by the wider temperature range you can use it
> at?

I tend to use my synthesizers at room temperature...

But seriously, with proper tempco, temperature should never be an issue.  Of
course, one can go a bit overboard with this, since only one part of the
circuit (the trannies) is ever tempco'd, but everything in an electronic
circuit has a temperature effect (such as the opamps, the resistors, the
pots, etc, etc).  At some point, one just has to live with residual
temperature effects as a side-effect of using analog circuitry.  Generally
it isn't that big of a deal, and may even add to the charm of analog.

The main attraction of the 2164 expo for me is the fact that it is easily
built from parts which are easy to obtain, plus I never liked the idea of
having to glue parts together, and having to make special provision in a
layout to place the summer feedback resistor close to the transistor pair
can be a bit of a drag.  From that standpoint, the 2164 expo is certainly
more elegant.

However, I think the key to a good expo converter is how well the resulting
VCO tracks.  For good tracking, one needs excellent log conformance,
generally over at least five or six octaves, and ideally over eight or nine
octaves.  2164s will generally give four or five good octaves, but they must
be hand-picked for excellent tracking over seven or eight octaves.  There
are transistor pairs which are specially formulated for excellent log
conformance (such as the Linear Systems LS358 Log Conformance Monolithic
Dual PNP Transistor, which will probably be my first choice when I return
inevitably to VCO design -- it's almost as if it was created with synth DIY
in mind (perhaps it was) -- and it comes in metal cans and PDIPs as well as
SOIC).  I believe one of our fellow listees was going to try them out for an
expo converter.  I don't remember hearing how that all turned out, but I'd
like to...