[sdiy] Serge Triple Waveshaper mods

[Michael Ruberto]

Hi all,

I have been playing around with the fascinating triple waveshaper by Serge. 
I am using Ken Stone's schematic:
http://www.cgs.synth.net/synth/serge/pic/schem_tri_wave.gif

It's an interesting circuit and truly does convert saw waves to sine. I did 
find, however, that it does so with a big glitch, might this be because I am 
powering it from +/- 15V?
I eliminated the glitch completely by replacing the 100pF cap with a 2200pF.

While I found the circuit to operate properly I wasn't too excited by it's 
sound. Rather boring for my tastes. So of course I had to experiment...
I managed to get a much more interesting sound by removing the diode in 
parrallel with the 2200pF cap (formerly the 100pF). Now the circuit will not 
only transform the saw to a sine but will continue on to a trapezoid and 
then square.
This makes for a wider range of timbres over the CV sweep and really reduces 
the yawn factor.

Also, reversing the polarity of the diode between the 2 inputs yields some 
rather different looking waves.
I guess since I'm building 3 of them I'll have a normal one and 2 
differently modded ones.

The LM3900 is suprisingly still available and easy to find. I bought mine at 
a local marine electronics store of all places but many places on the web 
are carrying it. Allied has it for 77 cents and their part # is 735-5442.

If there are any more interesting mods for this circuit I would love to see 
them.

M. A. Ruberto