There's a new f-word just around the corner. It's F4 as in four times f, why didn't anyone think of this sooner. The F4 is Nelson Pass' newest entry into the special application stable of his First Watt kitchen-table venture from whence he issues, at a rate of 100 units per installment, amplifiers that break with convention.

The F1 was a transconductance or current-source amp primarily intended for crossover-less single-driver speakers. The F2 was another current-source amp but single-ended. The F3 uses a special JFET as output device but only makes 10wpc. The F4 looks at the über-gain scenario today where hi-output sources plus high-gain preamps mean that many amps, into many speakers, run well below unity gain most of the time. The F4 asks a good question. What'll happen if such redundant amplifier gain was essentially eliminated?

This amp can be operated in single-ended stereo or balanced mono mode, making 25 or 100 per channel respectively. The amp will deliver +/- 20 volts in single-ended mode, double that in mono yet gain is below 1dB. Frequency response is 0.1Hz to 200kHz -0.5dB and power consumption 160 watts. The topology is a "Class A impedance converting amplifier, having no voltage gain or feedback. Its input impedance is 48,000 ohms and its output impedance is about 0.2 ohms... you'll need adequate voltage and/or loudspeakers sufficiently sensitive to be able to work with the voltage and gain you already have without an amp. You would think offhand that this might be a real problem but it is my experience that this is often not the case.

"First, you will find that the more sensitive drivers deliver good volume levels at quite low wattage. My Lowther DX55s (93dB/watt) mounted in Alerion enclosures adequately fill my 11,000 cubic foot listening room with about 5 volts (the equivalent of a 4-watt amplifier). I can achieve good levels with headroom to spare with my phono stage or DAC feeding an X1 preamp, which has a maximum of 14dB gain. If I lived in an apartment, I could get complaints from my neighbors.... As you drop below 87dB speaker sensitivity, you will want to consider 100-watt balanced mono-block use, with a preamp capable of swinging 14 volts per balanced output and having a gain of 20dB+. As your loudspeaker increases in sensitivity, you need less gain and voltage swing."

The master is presently still tweaking the circuit before unleashing it on the world at large. At large then could mean just 100 units as has been Nelson's custom thus far for the First Watt models which he handcrafts after hours in his own digs. There's a number of interesting applications for the F4, including headphone drive, as power buffer for micro-power SETs and for bi-amp scenarios where the unity gain of the amp won't require a dedicated pot for
the bass circuit. At First Watt, creativity is the norm to insure that the designer at the helm -- good for far more circuit variations then there's commercial room to exploit -- doesn't get bored. Sez Jon Ver Halen of Lowther America who's already heard an F4: "Very, very nice amp. It has the JFET sweetness like the F3 but also has push/pull control and is direct coupled. An awesome little amp for those who can live with its limitations (low gain and low power). If your system does not work with these limitations, consider this amp to be a great excuse to build a better preamp and get into high efficiency speakers. Yes, it is that good."
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