The latest iteration of the OCD has quite a few differences from the earlier models. There's an added transistor stage at the output after the volume control, and a JFET buffer that can be toggled for buffered or true bypass. I've laid it out so it will still fit in a 1590B with board mounted pots.
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The off-board wiring's not the most intuitive, so here's a diagram for wiring up the buffer switch and 3PDT bypass footswitch.
