Now that I had a reasonably well working single voice prototype, it was time to start figuring out how to hook up multiple copies of this thing. Each of the modules had control voltages for adjusting their parameters, these control voltages just need to be buffered before distributing to each voice, not a big deal. But the routing was all hard-wired. I needed to add switches for things like selecting one of the 4 waveforms out of each VCO and a voltage controlled mixer to be able to control the volumes of the two VCO output signals.
With just one voice it's simple, I can simply run the signals through mechanical switches and pots. However, with 8 voices I can't simply run the 8 signals into the same components or they'll all just combine together in one jumbled mess. Each voice has to have its own set of switches and pots in order to keep the signals from each voice distinct, but of course that would mean that to make each voice sound the same I would have to adjust all 8 sets exactly the same. Not very practical. Instead what I needed was for each voice to have electronically controlled switches and amplifiers for the signal routing and mixing.
For the routing I decided to go with the dg41x series of analog switch IC's as they seemed the most compatible with the voltage range I was working with and easy to implement. These are basically MOSFET switches that take a 5V logic input that turns the switch on or off. This allows me to have one mechanical switch on the front panel whose signal can be distributed to a number of the electronic switches in each of the voices.
For example, to select on of the 4 waveforms, I have a 4 position rotary switch with common connected to ground. Then each of the four contacts is connected to the control input of an active low electronic switch in each voice. The inputs have pull-up resistors to deactivate (open) the switch unless that connection is grounded through the rotary switch which then allows the signal from the corresponding VCO waveform output through the switch to the next stage of the voice.
For the mixing, I have two voltage controlled amplifiers (VCA's), one for each VCO, implemented using LM13700's (dual OTA). A dual ganged pot sends control voltages (arranged so they move opposite to each other when the knob is turned) to the VCA's, allowing the mix between the two to be controlled.
There are more switches and VCA's for routing and controlling amplitude of other signals like LFO, envelopes, cross modulation etc., but that's the idea.
So lets start building this beast.