[sdiy] expo converter /vco questions

[Aaron Lanterman]

> Dan Snazelle wrote:
>
>> 3. has anyone had good luck with the SSM2164? and once again, is it  
>> to replace an NPN or PNP pair?
>> at 6 bucks it seems like a good soloution (As you could get an expo  
>> AND a vca out of one chip right?)

Well, it's not really a replacement for an NPN or PNP pair. Although  
it does have expo conversion built in, but the expo conversion aspect  
is built into the structure of the VCA itself. There has been some  
discussion of using SSM2164 to drive VCOs. I had a student a while  
back look into building a triangle core with one - didn't get very  
far, but it was an ambitious project compared to many of the others.

As an aside, the SSM2164 is a great chip. One of my students built a  
four-pole lowpass with variable slope - there was an external voltage  
you could put in that would move each of the poles by a different  
amount. He based it off one of Osamu Hoshuyama's awesome circuits.

http://users.ece.gatech.edu/~lanterma/studentprojects

See Siddharth Bhandari's project.

>> also...can anyone recommend a good NPN pair Triangle VCO?
>
> No immediate recommendations myself. The Buchla oscillator core  
> might be a good one to play with. It has an expo converter and a  
> current mirror, so you have both NPN and PNP pairs.

I had a student build the Bergfotron VCO, which is a Buchla triangle  
core with a more modern expo converter grafted on. Good stuff. Google  
will find it for you.

- Aaron