[sdiy] Serge Triple Waveshaper mods

[xamboldt]

On his version of this (the Wav1B), Tom G added a "Shape" pot - top of the
pot to +V, bottom to ground, wiper through a 3M3 resistor to the
non-inverting input of the 3900...

My wav1B is currently on my "to fix" pile as it left me a little
underwhelmed. I'll give your mods a try when I'm working on it. I think
there's something else screwy with it, as the closest thing to a triangle or
sine I can get out of it is what sounds like (no o'scope!) a little +DC bias
with a tiny triangular dip in it. Oh, well.

You said you're powering yours from +/-15V - did you mean +15V and ground?
I've only ever seen this running from +V (pin 14) and ground (pin 7).

-Chris

On 9/2/05 11:58 AM, "Michael Ruberto" <frankentron at hotmail.com> wrote:

> 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
> 
>