Adding waveshapers to the ASM-1

[gstopp at fibermux.com]

     Hi DIY List,
     
     I saw a post yesterday that mentioned adding triangle and sine wave 
     outputs to the ASM-1 VCO. In yet another case of totally off-the-wall 
     coincidence I had been recently toying around with that very same 
     idea, and that made me poke around in my papers till I found the 
     one-transistor saw-to-triangle trick.
     
     So I tried it - it works! One 2N3904, two resistors. Sawtooth in, zero 
     to +5, triangle out, +2 to +4.5.
     
     Looks like this:
     
                          V+
                         ---
                          |
                          >
                          >  Rc
                          >
                          |
                         o------------o Triangle Out
                       |/
        Saw In  o------|  NPN
                       |\
                         V
                         |
                         >
                         >  Re
                         >
                         |
                        ---
                         V-
     
     
     In my test Rc = 10K and Re = 50K trimpot which gets adjusted till the 
     triangle looks right.
     
     After this stage a gain/level shift op-amp can be used to get it to a 
     +/- 5v triangle centered around ground, and then the normal sine 
     shaper can be added after this. I used the overdriven 3080 trick for 
     this.
     
     The triangle symmetry (up vs. down slopes) looks just ever-so-slightly 
     off, so maybe some experiments with resistor values or transistor 
     types may be in order. Careful - if you're using +/- 15v power 
     supplies like I do there's 30 volts across the rectifier, so don't go 
     too low in your resistor values!
     
     - Gene
     gstopp at fibermux.com