F4

About the F4

The First Watt F4 is a pure Class A impedance-converting power buffer amplifier designed by Nelson Pass, providing current gain with no voltage gain and no global feedback. It presents an input impedance of 47 kohm and an output impedance of about 0.2 ohm, making it suitable for driving high-sensitivity loudspeakers directly from a robust preamp or other line-level source, or for use in bi-amped systems where it is fed from the output of a conventional amplifier. In stereo operation with single-ended inputs/outputs, it delivers up to 25 W into 8 ohms and 50 W into 4 ohms, with a damping factor of about 40 into 8 ohms. Configured as a monoblock with parallel inputs and outputs, it can deliver about 100 W into 2 ohms, and in mono with balanced inputs and outputs it is rated at 100 W into 8 ohms at 1% THD. The F4 operates in pure Class A up to 25 W (50 W peak) with low-order 2nd and 3rd harmonic distortion that scales proportionally with output level. It is direct-coupled from input to output and specified for a frequency response of roughly 0.1 Hz to 200 kHz (-0.5 dB). Audiophiles value the F4 for its ability to pass the character of the preceding preamp or tube stage with minimal additional coloration, effectively acting as a high-current buffer that preserves signal purity. DIY versions and clones are popular and supported by community-supplied PCBs and chassis, but these are separate from the original factory-built First Watt F4 amplifier.