Buchla Eurorack Conversion Module

Evergreen's Buchla 200 I concieved this module as a link between The Evergreen State College’s sizeable Buchla 100/200/200e systems and their new mixed make Eurorack synthesizer. The unit features 4 channels of control voltage scaling- 2 allocated for each direction. The gain factor is intended to be x1.2 for Buchla to Eurorack and x.833 for the reverse. Each channel also features variable offset for accomadating a mixture of singular polarity and bipolar modules. In addition this device contains a bipolar bargraph for visualizing applied control voltages and offsets. Another feature I included for fun is a 2 channel fixed amplifier for converting Buchla audio signals to modulation levels. I found myself wanting this after some satisfying experiments with driving model 258 dual oscillators into sub audio frequencies in order to provide some more conventional LFO’s than are typical in an Electric Music Box.

Evergreen's Hybrid EurorackI chose to use 1% or higher precision passive parts and very low offset op-amps due to the critical nature of control voltage scaling. Some parts of the design could be implemented more elegantly, but at the expense of simplicity I chose certain technologies because I wanted to become comfortable with them. For example, 4000 series CMOS is used for momentary switch debouncing, and a special MAX313 analog switch is used to route signals from each of the channels to the bipolar bargraph section. Furthur CMOS is used to create an exclusive flip flop action (“radio button” in software speak) so only one channel is routed to the destination at a time. I probably could have done away with all of these chips and used a small uC, but I really wanted to match the look and feel of existing Electric Music Box modules without adding any new power requirements.
Initially I built the module on 2 layers of veroboard, which got annoying. I’m having a PCB etched for the final revision, which allowed me to shrink it all down to one board. I’m giving batchpcb a shot, they make single board runs reasonably affordable by outsourcing orders to China on big sheets which they pare down with a CAD script. Neat.