Exponential current source blues

[Christopher_List at Sonymusic.Com]

Hi List -

My $0.02.....

> The difficult and touchy parts of the standard discrete VCO's and
> VCF's is the exponential current source.

Things like the average state variable VCF do not self
oscillate reliably, so I have never used any fancy transistors or temp.
compensation in a filter's exp converter.
Just doesn't seem worth it.


> 1. Why do VCO's (and VCF's), that at heart are made up of simple
> op amp oscillators (like LFO's), need the exponential current
> source as given in most common circuits?  Is it mainly for scaling
> of CV's? (1V/oct to 18mV/oct).
> Can another structure do this
> besides tempcos and transitors (either matched OR arrays)?

The scaling to 18mV/Oct happens at the CV summing op-amp. The exp converter
simply creates a current source that is exponential with this voltage.

Maybe this could be done with some sort of multiplier and a resistor, but
the matched trannys and tempco
resistors aren't THAT hard to find and they work perfectly.

> The FH's seem the cheapest.
> The 5.15 price was from the First Stop division of Pioneer
> Standard (1-216-349-9310, I talked to Mike Boyer).

I've always used FH's and they work just fine.
The others are mil-spec and higher tolerence models - the FH is very high
tolerence to begin with.
That's a great price, BTW, I've been paying $9 at Newark Electronics....

Lastly, the exp. converter will work just fine with a normal resisitor.
I had mine set up this way for a long time before I found a tempco
supplier, and hardly noticed
a difference when I put in the tempcos (then again, I do mostly dance music
-
which tends to be less demanding on melodies). I just put the tempcos in
for "completeness".

I got my tempcos from:
KRL/Bantry
(603) 668-3210
Spoke to Maureen.
Get the "C-2AQ" model.
They may have a minimum qty, I've always ordered at least 25....

- CList