Full wave rectifier as Saw-to-Tri converter

[Roy Tate]

I recently tried out a few of the saw-to-tri converter schematics
I've collected, and in most cases, I wasn't happy with the results.
The switching point would either have a spike to +15 or -15 V or
the inverted section would not meet the non-inverted section of the 
wave.

After looking at Walter Jung's OpAmp Cookbook, page 249
(figure 5-17 - see attachment), and noticing that Rick Jansen's
wave shaper uses this circuit, I tried it out.  Given a +/-5V saw
input, it generates a positive 5V triangle with a 1 volt offset
between the positive and negative halves of the peak.  The 
resulting triangle is ...

      |\        |\    
     /  \      /  \   
    /    \    /    \  
\  /      \  /      \  /
 \/        \/        \/

This sounded okay, but I would rather have a "perfect" triangle,
as it will make sine wave shaping easier, and will not sound as
bright as a dirty triangle.  So, I experimented with the circuit
and finally came across a tweak that makes the output triangle
nearly perfect.  I simply replaced the 10K resistor (R2) with a 20K
trimmer and dialed it up to 11.3K Ohms (R2 goes between the 
inverting input and output of the second op-amp.)  Any resistor 
between 11.2K and 11.5K would probably give satisfactory results,
but a trimmmer allowed me to line up both peaks.

Obviously, to use this in a synth, I would add an output buffer 
with a gain of 2 and a negative 5V offset.  So 3 op-amp sections 
are required to make a usable synth module.  By the way, I used
the TL072A (the low offset version).

Regards,

Roy Tate
roytate at ionet.net
http://www.ionet.net/~roytate
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