A premium DIY build with help from Nelson Pass and the diyaudio.com/passlabs community. Built with outboard power suppies using independent 400v transformers for both the + & - rails (1600v total), perfectly matched Harris IRFP9240 & Vishay IRFP240 MOSFETS, Vishay naked z-foil resistors throughout the signal path, VH Audio 28ga .99999 solid core silver input wire, Tempo Electric 14ga .9999 solid core silver output wire, 384,200uF of total power supply capacitance with inductor filters and 50uF film bypass capacitors, and pure silver WBT RCA connectors.
From the F4 manual: "The combination of a simple Class A circuit operated without feedback and the good objective performance gives us a superb sounding amplifier. The low distortion, bandwidth extension, and high damping results in midrange clarity, treble detail, and control on the bottom end. While these are available from most good solid state amplifiers, the F4 also brings depth, imaging, midrange warmth and top-end sweetness. Overall, it is one of the best sounding amplifiers, and if you can live with unity voltage gain in your amplifier, it is possibly your best choice."