[sdiy] "Summing" Point for CV's on VCO

[David Ingebretsen]

I am continuing my very slow progress on a Synthacon re-do (public thanks to
Paul Schreiber for all his off line help).

I put a partial grab of one of the triple VCO schematics here (since I don't
know who owns the copyright, I don't want to publish all of it):

http://www.xmission.com/~dingebre/page9.html

Even I understand the gain for an ideal inverting op-amp circuit :), but I
am a bit baffled as to the resistor selections.

First, I think I understand that for most of the inputs, the 47K in the
feedback and 47K in the input make for a unity gain input buffer, but why
use a 110K on the keyboard input? In the Synthacon, the scale tuning was
done on the keyboard for VCO1 and then VCO2 and VCO3 were trimmed to follow,
does that make a difference? I do not intend to build a Steiner keyboard (I
already built one when I was 17...) and so will use a standard 1V/Oct
keyboard CV. Should I use a 47K on the keyboard CV input, too? If I add an
external jack or two, do I put a 47K resistor on those inputs?

For VCO2 and VCO3, the input from VCO1 uses a 10K making a gain of almost 5?
I'm not seeing it.

To that end, I will replace the 47K in the feedback with a pot and resistor.
But, do I leave the 110K resistor in the keyboard input or do I change that
to 47K as well?

I thought I'd just leave the frequency and fine frequency as they are.

One other question, the power supply for this is a +12 / -10 VDC. Will I run
into problems connecting to CV's/triggers/gates/audio from newer systems?

Is it worth the trouble to redesign circuits to work on a +/- 12VDC or even
a +/- 15VDC supply?

I may end up abandoning the idea of a Synthacon and rather make it more of a
patchable Synthasystem. At the least, adding some jacks to make the
inputs/outputs available for patching as well as the toggle switches Nyle
used.

David