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.
Somebody posted the question on www.diyaudio.com about the relative sound of the various amplifiers, and got the following answer from Jon Ver Halen at Lowther America:
Q: I am curious, since you are the only two who have listened to all the First Watt amps with Lowthers, as to which amp you personally like best?”
A: Aha! Down to the meat of it all. There is no right answer, and it is part dependent on your taste, part on your music, and your source.
The F1 is super clean and dynamic. If your source is lean, your music will be lean. If your source is rich, your music will be rich. It is the real truth machine, with nothing taken out and nothing added. Ruthlessly revealing.
The F2 adds some 2nd harmonic to this. On lean source material this sounds better than the F1. It is putting a little color in your music, and on dry material it is a pleasant addition. Some like that additional color, some do not. Your choice.
The F3 sounds has a very light touch of sweetness. Sounds more like a great tube amp than any solid state amp I have heard, except that it has the same basic tonal characteristics from highs to lows – something most tube amps dream about.
The F4 sounds like a cross between the F1 and the F3. Cleaner than the F2, a touch of sweetness, but the truth is still front and center. I have not heard this amp in my system (yet), so my thoughts about this may change. It is a very exciting product.