[sdiy] VCO minimum frequency and V/oct tuning

[David Ingebretsen]

I put a schematic on my web site temporarily:

http://www.xmission.com/~dingebre/page11.html

The VCO core is page 1 with the VC in on the left. The input summing section
is on page 3.

I have a couple of questions, but first a bit of background.

In the Steiner VCO, the tuning was accomplished in the keyboard section. The
idea was to input the keyboard CV into a VCO, then to use the "VC VAR"
control to tune the other VCOs to the primary VCO. The interval tuning was
accomplished by a tuning control on the keyboard.

To make these VCOs friendly, I wanted to put a V/Oct trim pot on the VCO,
leave the VC VAR control for fun. Let the user tune weird tuning scales,
without messing up the primary scale of the VCO.

It was suggested to me to replace the feedback resistor on the input summing
amplifier with a trimmer and slightly lower resistor. When I did this, I
could not get a 1 V/Oct tuning. Playing around,  I found if I change the
feedback resistor (R44 on page 3) to about 15k, I can get good tracking over
the my 5 octave keyboard. There is no high frequency compensation, so it
does go a little flat, but I figure my goal is not to design a new VCO, but
make this one work the way they used to. If they went flat at the high end,
so be it.

First question, why do I have to so radically change the feedback/gain of
this guy to get the V/Oct tuning to work?

But, when I change the feedback loop, I now find I can't get lower than
about 36 Hz with a 0.0 volt CV input. I don't want to re-design this circuit
to death. I want it as faithful as possible to the original. That said, 36
Hz for a low end is not low enough. 

Second question, how do I get a lower low end? Do I just add a negative DC
offset? Do I connect the Frequency pot between positive and minus supplies
instead of positive and ground? Do I change the Frequency knob summing
resistor?

Honestly, I don't understand how the exponential convertor is working well
enough to see what is going on.

Many thanks in advance.

David