August 27, 2013
After settling on a layout tool I finally got down to some serious layout. But after getting it nearly complete, I decided the board was just too big (about 12"x6"). So I decided to take a hint from previous synth designs and make some of the sub-circuits into modules that plugged into a host board. This pretty much makes moot all the effort I put into finding a layout tool that would let me do sub-layouts, but it gets the voice board size down to about 8"x4" which I think will be much more manageable. The layouts for the main voice board and modules are mostly complete, probably just a little fine tuning so I've started doing the first prototypes. I'm etching these by hand and being double-sided with small features to keep the total size down it's a little messy, but should work well enough to give me confidence when ordering boards. Modules will be VCOx2, VCF/Ax1 and ENVx2. I combined the VCF and VCA into one module since they each use one OTA so they can share a single LM13700 dual OTA. The main board will contain the circuitry for controlling/routing control voltages and selecting/mixing VCO waveform outputs, as well as +/- 15V regulators so I can connected an unregulated power supply to it. Regulators will be bypassable with jumpers so I can also connect a regulated supply.
VCF/A module prototype in progress: