[sdiy] saw to tri/sine options

[David G. Dixon]

Scott,

You might also have a look at the discrete diff-amp version in Thomas
Henry's VCO-1 (on schematic page 2):

http://mypeoplepc.com/members/scottnoanh/birthofasynth/id33.html

It's a very simple and inexpensive design.

Personally, I've used the Ray Wilson LM13700-based one in my VCO-1 builds,
mostly to use up the spare OTA on the dual chip.  Diodes are the
"old-fashioned" way to do it, and typically three diodes are used for lower
harmonic distortion (at least according to my copy of Thomas Wells' textbook
on electronic music techniques).  I've never seen the diode circuit in
action personally, but the scope-shots in Wells' book look a fair bit
crappier than the sine waves from my Ray Wilson shaper.

I believe that each of these methods gives about the same quality of sine
wave -- typically, nothing less than about 2% THD is to be expected with any
of these methods (which are ultimately all the same method, really).  None
of them completely eliminates the little pointy peak from the triangle.
Also, I've started with a triangle core from the VCO-1, which I believe
gives cleaner sine waves than a saw core (saw-derived triangles typically
have a bit of garbage at the apex of the wave which bleeds into the sine,
but which can be mostly filtered out with one or two well placed little caps
and some care in how the reset FET is wired up).  Personally, I really like
the VCO-1.  I'm very pleased with the quality of all of the waveforms.

If you really need perfect sine waves, use a quadrature oscillator.
Otherwise, any of the three will give sines which are good enough for
government work.  If you have a spare OTA in your circuit, I'd recommend
Ray's circuit.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Scott
> Sent: Monday, June 01, 2009 1:37 PM
> To: Synth DIY
> Subject: [sdiy] saw to tri/sine options
> 
> I'm trying to pick out a saw to tri/sine converter for a VCO I'm building
> (j.h.living).  Is there much difference between the various types?
> Looking through my bookmarks I see a few..
> 
> Bergfotron uses transistors
> http://hem.bredband.net/bersyn/VCO/Saw%20to%20tri-sin%20W.gif
> 
> ASM uses diodes
> http://www.elby-designs.com/asm-2/vco/vco1-shaper-asm2-cct.pdf
> 
> rwilson uses a 13700 (nothing direct, but it's on the VCO schematic)
> http://www.musicfromouterspace.com/analogsynth/VCO200604REV01/vcoschematic
> page2.gif
> 
> Thanks!
> 
> 
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy