Not from me but from Nelson:
F4 Power Buffer
Like everything else from First Watt the F4 is a unique amplifier. It has no feedback and no voltage gain, only current gain. This design is sometimes called an “impedance converting amplifier”, but more commonly referred to as a buffer – in this case, a power buffer.
The F4 is Class A impedance converting amplifier, having no voltage gain or feedback. Its input impedance is 47,000 ohms, and its output impedance is about 0.2 ohms. It is suitable for driving a high sensitivity loudspeaker with the output voltage of a preamp or other line-level audio source. It is also useful with a less sensitive loudspeaker in a bi-amped configuration where it takes input from the output of a conventional amplifier.
As a stereo amplifier with single-ended inputs and outputs, it will deliver up to 25 watts into 8 ohms with a damping factor of 40. It will do 50 watts into 4 ohms, and as a mono-block amplifier with parallel inputs and outputs, it will do about 100 watts into 2 ohms. As a mono-block amplifier with balanced inputs and outputs the power output rating is 100 watts into 8 ohms at 1%.
The amplifier operates Class A to 25 watts (50 peak), and the distortion is 2nd and 3rd harmonic in character, rising or declining in proportion to the output power. The amplifier has a direct coupled input and output, with a -0.5 dB rolloff around .1 Hz and 200 KHz. It does a clean square wave at 100 Khz.
The simplified circuit of the F4:
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.
Yeah, I was also thinking about this. Basically if I understand correctly the F4 is a pass-through window for the pre-amp so any gain would have to come from there.
--> If the voltage swing theory is correct then it might not be a good idea for the HE-6.
Sounds like my F3 might not be the optimal pairing then...hmmm...will sleep on it. Guess I could still pull the plug on the deal without too much costs. Hard to judge what the impact of the voltage limitation is in real terms, especially because I don't listen to heavy bass slamming music. But the description about ultimate clarity and detail is alluring, as is its harmonics profile which gives a teeny bit of sweetness according to Nelson Pass.
If anyone has any additional insights, do share 'm.