Two boards are required for stereo. I have four pairs available to anyone willing to be a beta-tester. Cost is $20 shipped (this is at-cost pricing, I'm just interested in having a few more people try this out.) PM if interested.
The Sapphire amplifier is a fairly standard op amp + diamond buffer. Zener diodes are used for voltage references. The boards have a jumper to switch from class A open loop to class AB closed loop operation, and the circuit values can be optimized use either as a headphone amplifier or a preamp. My own interest is class A open loop for 16 ohm headphones.
schematic (parts values shown are nominal placeholders, the final values depend on the application)
I will have an excel worksheet/BOM available by the time the boards ship. I'm also working on a webpage for the project.