basic synth logic...

[Gordonjcp]

It's worth looking at offloading some of the duties to a microcontroller, especially if you're using a repurposed "toy keyboard" keybed that is already matrixed.

[jamiewoody]

actually, nope, i'm not going to use "toy keyboards"....i have decided i hate sensor based things on pcb....i am going to use real reed switch full size keys harvested from a spinet organ! ;-)

[Bogus Noise]

Actually, it's fun to let the VCOs mod the filter cutoff frequency and pitch of other VCOs as well. Running these signals through an envelope (or LFO) to control the amount it affects them over the time of each keypress makes for a more dynamic sound. 

[Remork]

reed switches? don't mean to blow my own horn here, but maybe you should check out my blog.. i started modding a Hohner Bass3, which is basically a square wave oscillator (40106) controlled by reed switches. the square wave is then divided down into several suboctaves, and mixed together they form 1 'timbre'.
read more here: http://remork.blogspot.com/search/label/Hohner%20Bass3

[YashN]

Lots of useful suggestions in the thread. I would proceed in as gradual a way possible, with the simplest components at first just to learn things along the way.

I'd start with just a single VCO with a single waveform. Then, I'd do a simple lowpass VCF, not a multi-mode one. Then, I'd add the VCA. At each segment, I'd try to get a potentiometer for minimal control, e.g. just the frequency for the VCF.

This would give me the basic structure of a subtractive synth.

From there, you could explore each segment in more detail approximately like this:

1. VCO:
a. try to make it a multi-waveform one, i.e. it outputs Square or Sawtooth through a switch
b. next try to get both outputs simultaneously through a mixer
c. add more outputs

2. Add an LFO and make it control the VCO

3. Duplicate the VCO design in 1
a. Make VCO2 modulate VCO1 (Frequency)
b. Make VCO1 modulate VCO2 (x-mod)
c. Implement PWM
d. Implement OSC Sync

4. VCF (try to get 2 resonant 12dB filter chips, or two chips with 4 accessible poles, keep the second one for later)
a. Control the Resonance
b. Add Cutoff as LFO destination
c. Add Resonance as LFO destination as a switch

5. Think of a simple Envelope generator here
a. Add that to the Filter Cutoff
b. Add Faders for each Env segment
c. Duplicate the Env and add that to the VCA

From there, you can revisit every segment and improve each. Things that would be helpful:
6. Waveshapers
7. Ways to combine LFOs
8. Getting to Polyphony
9. Split keyboard operation (strive for 4 voice polyphony because then you can play a left-hand bass and a 3-note chord on the right, and that's a great basis for composing already)
10. Add Control Voltage input jacks for destinations and output jacks for sources, so that you can patch at will
11. Simple sequencer
12. Simple matrix modulation
13. Simple Flanger/Chorus/Delay effect
14. Cross-modulation/feedback possibilities with the Effects section and the main synth architecture
15. Implement multi-mode Filter segment
16. MIDI/CV operations
17. Micro-controllers
18. Digital Oscs inputs, e.g. Samples, Wave-Table

Some additional tips:
1. Think it terms of segments (i.e. basic synth architecture blocks). It will help you focus: say you have improved your VCO. Then, it's good to ponder on ways to improve the filter section, etc...
2. Try to read a lot about favourite existing synths and their architecture, find their block diagrams and then the schematics. Take note of particular features and the chips/circuits they use
3. When choosing mod sources/destinations on modifications to existing circuits, keep 'musical' mods, drop others.