[sdiy] Help understanding Serge 73 Oscillator

Guy McCusker guy.mccusker at gmail.com
Tue Feb 20 22:27:01 CET 2018


Hello list!

I'm having a hard time figuring out how the exponential response is
generated in the Serge 73 oscillator design. The schematic is here:
http://electro-music.com/wiki/pmwiki.php?n=Schematics.OldSergeVCOUsingLM3900

Like a lot of Serge designs there is heavy use of LM3900s which I do
not understand properly. It seems to me that the first stage (U1a) is
a more or less regular summing stage, the second (U1b) is an
integrator, and the third (U1c) is a comparator to reset the ramp via
the transistor across the integrating cap. So far so straightforward.

But how (if at all) is an exponential CV response generated? The notes
on the schematic hint that somehow the transistors within the LM3900s
are being pressed into service as an expo converter but I can't wrap
my tiny brain around it.

Can anyone help please?

Cheers,
Guy.


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