It was back to yusynth.net for the voltage controlled amplifier (VCA). This is the circuit that turns the output of the VCO's and filter on and off so you can have notes with a finite duration instead of just running on non-stop. Pretty straight forward but in order for it to be useful you also need an envelope generator that takes the key press and release and translates it into voltages that control the opening and closing of the VCA.
You usually don't want the notes to start and stop instantly upon
pressing and releasing a key, it's nice to have some sort of gradual
change over time. This is what the envelope generator does, takes the
key "on" and "off", and creates an adjustable time varying envelope to
smooth out the attack and decay of the notes.
This was where I ran into my first big problem with a yusynth
circuit. While I'm sure the yusynth envolope generator works quite
well, it's not a voltage controlled envelope generator. Instead of pots
set up as voltage dividers to send control voltages to the circuit, the
pots were incorporated directly into the timing circuits of the
envelope generator. This was a problem for my ultimate goal of 8
voices, since every voice needs it own envelope generator but I need all
8 of the envelope generators to be controlled by the same set of
knobs. I had to pass on this circuit and look elsewhere.
So the search was on for a voltage controlled envelope generator. I
eventually came to a circuit by Aaron Cram called "Envylope V3", the
core of which was based on a design by Jürgen Haible. This appeared to
be a fairly well designed circuit more or less suitable to my needs. I
may tweak it a bit to adjust control voltage input levels but other than
that it's looks very nice. However it was just a schematic, no layout
I had already acquainted myself somewhat with Eagle PCB for adapting
the surface mount components I had used in previous circuits, but this
was another level. I had to carefully enter the schematic into Eagle's
schematic entry while searching out library components with a footprint
that matched what I would be buying from Mouser. That was quite a
lesson in studying data sheets. Then the layout place and routing.
Very tedious and a challenge unlike any I've ever tackled before, trying
to find a compact and organized arrangement that allowed all the
signals to go where they needed to, all while maintaining design rules
like trace width and spacing that were workable for producing a circuit
board by hand.
After many evenings of shuffling things around in the layout editor, I
finally had something I was happy with and built the board. This board
took quite a bit of trouble shooting. I learned that things can look
very big and easy to work with when zoomed in on the computer screen,
but end up very small on the actual board leading to a lot of broken
traces and solder bridges. Noted for future reference.
Eventually it was working, and along with some more programming in my
Arduino midi controller to add gate and trigger signals that went to the
envelope generator, I had a more-or-less "proper" working monophonic,
dual oscillator, 24dB/OCT LPF analog synthesizer. I built one more copy
of the envelope generator to add some movement to the filter cutoff
frequency and that was it - the basis of a single voice prototype of my
dream synth that I could play from my K2600 through midi, minus an LFO
and some other features to come later.
Time to start integrating.