Now that I had a nice base tone to work with from my 2 VCO's, and could sort of "play" them by controlling the frequency with a keyboard, it was time to get to the heart of the synth by adding a low pass filter. I headed back to the yusynth.net website and decided to go with his version of the classic Minimoog filter.
This circuit uses a 3046 transistor array, and the only version I
could find on Mouser was a surface mount package while the yusynth
layout called for a through-hole package. So I had to get a little
creative but was able to make it work by etching a small board to mount the 3046 and break out the pins which I then connected with wires
to the main filter board. Looks funny but works. I'll need to redo the
filter board layout for the final build (at this point I'm just
prototyping for proof of concept and plan to redo everything anyway).
With the filter I was now able to get some nice warm synth tones
instead of the bright pure unfiltered tones of the VCO's. However I was
getting quite tired of the constant droning (VCO's are free running, generating a constant output as long as it's turned on), it was time to add a VCA
and envelope generator so I could start making some actual notes instead
of non-stop wailing.