VCO Designs .. Stability and co.

[tomg]

On 23-Jun-98, Tadej Bozicko wrote:
>Howdy!

>I am up to modular synth and I've decided to build one myself. I've checked
>several designs, but am still hesitating which one to choose. Since I've
never
>built and VCO before, my question would be:
>How stable is really a CA3046/86 based oscillator (well, exp. converter)? How
>much improvement is done by heating the CA3046 up (the design Rick Jansen
>use)? What about LM394 for matched transistor pair? Anyone used this before?

Stability depends on one hell of a lot of things heat is just one of them.
Layout, transistor matching, even cap ,resistors and trimmer choices have an
effect. The first thing to realize is. "If it is a analog oscillator it's gona
drift." I said it and you can quote me to anybody you want. I have lots of
boxes full of "high stability vcos" and they all need the tuning buttons that
come on them. There is a reason.

The best thing to do is pick a design you like and try to make it work. You
might not be successful but you could be.. it could happen. If it doesn't DO
NOT! give up. Try something else but go back when you have a little more
knowledge. Chances are more than a couple are in your future unless you take
the CEM route. I really like the 3374 and synthtech.com has them in stock. BTW
they drift too, but what a wonderful chip... 

The Maxx and Mini4cv oscillators they are fairly easy to build and a lot more
stable than some. I let them warm for about 5 minutes before adjusting them.
Once adjusted I let them both sit for about 4 hours checking the scale and
tuning about every 30 minutes. The Mini4cv won the stability contest still
being almost dead on after 4 hours but the Maxx came in a very close second at
less than a quarter step drift. Flat? For some reason? This is not to say if
you build one of them it will perform the same way...but it could. 

Oh the heating thing..well the designs I have tried have not really made any
real difference...except in power consumption. Of course I don't gig my "home
manufactured" darlings. If they ever left the house I might have a problem. 

Luck and Fortune.

-Tom