Pass Labs First Watt F6 Power Amplifier
F6 Design and Philosophy By Nelson Pass
The F6 is a very interesting amplifier, first designed in 2012 and presented in October at the Burning Amp Festival in San Francisco. The transcript of my presentation there is available on the First Watt website under articles - F6 Talk at Burning Amp Festival 2012, and contains numerous comments and details about the original design.
The amplifier was not initially slated for commercial production, and I continued to refine the design over the course of the next year and a half, working with different parts and adjustment values until both I and my listeners were satisfied that I had gotten the most out of this topology.
The F6 is a very simple Class A push-pull design which uses N channel power Fets driven by a high quality line level transformer which splits and isolates the drive signal to the output stage while also serving as the feedback element for the amplifier.
Measured at 120 V AC and an 8 ohm load:
Distortion @ 1 watt 0.05%
Input Impedance 100 Kohm
Gain 14 dB (5x)
Input Sensitivity (1 watt) 0.57 V
Input Sensitivity (max output) 2.83 V
Damping Factor 16
Output power 8 ohms 25 watts @ 1% THD, 1KHz
Output power 4 ohms 50 watts @ 1% THD, 1KHz
Frequency response - 3 dB @ 1 Hz, 50 KHz
Noise 200 uV unweighted, 20-20 KHz
Power consumption 180 watts
Fuse 3AG slow blow type - 2.5 Amp for 120VAC, 1.25 Amp for 240 VAC