Expo converter trouble - need some help

metal_head_82

Hey everybody!

I tried toget my single opamp SAW VCO to 1V/Oct (thread here: http://electro-music.com/forum/topic-49901.html ).

I swapped the opamp with an inverting schmitt-trigger from a 40106 IC. Works just like the opamp and is already a single supply part (basically it's the same circuit - the opamp is used as schmitt trigger).

The idea was to keep this design as simple as possible. So I tried the expo converter from René Schmitz's 4069 VCO. It can be found here:
http://www.uni-bonn.de/~uzs159/vco4069.htm
I just put both transistors to ground (0V) because I don't have a negative rail in my circuit. I can get the converter to work over 3 octaves. After that the response of the VCO seems to flatten. So if I try to adjust the 1V/Oct trimmer further I can get that 4th octave working but this will mess up the setting for the other octaves. Perhaps the expo converter needs some "high frequency compensation".

Anyone any ideas? I'm stuck here...
Thanks in advance!

umschmitt · Posted: Fri Jan 06, 2012 7:11 pm Post subject:

Wow cool ! Three octaves are already nice for such a small number of components ! (Now you understand I'm not going to be of any help)
Actually I have a couple of questions : did you try the converter also with a regular op-amp ? And what did you do with that 22k resistor (initial CV ?) to negative rail in R. Schmitz design ?