F2J Power Amplifier Update

Unlike many companies (and I'm sure you can name some) First Watt is grateful to the early adopters who took a leap of faith and bought either an F2 or F1 amplifier. As First Watt moves into its twelfth year, this is how we thank those people: The F2J and the F1J update. This page is about the F2J (there is another about the F1J).

Most of what you might want to know about the F2J amplifier can be found in the documentation of the F2 power amplifier. As you may already know, the F2 is an active current source amplifier which offers unique characteristics, particularly to sensitive full/wide range loudspeaker drivers. I recommend downloading the owner manual for the F2 and the articles on current source amplification.

The F2J is an update to the F2 where the gain Mosfet transistor has been replaced by a new state-of-the-art power Jfet device. In addition, there have been changes to several resistor values and removal of the input zener protection diodes. As with the F2, the simplified circuit of the F2J looks like this:

The reason for bothering with this modification is very simple – better performance from faster, lower distortion parts. The new Silicon Carbide (SiC) Jfet transistors from SemiSouth are better in several ways. Here is a distortion vs power comparison between the original F2 and the F2J in the low to midband range where you see the newer parts have about one-third the distortion:

This improvement is consistent across the audio band, and there is essentially no alteration to the gain, power, or frequency response – just the much lower distortion.

There is another factor as well, which is that the F2J has a lower input impedance than the F2- approximately 25 Kohm, so the F2J is recommended for use with preamps with ordinary to low output resistance – tube preamps without cathode followers will work but will not reap as much benefit at high frequencies.

This caveat aside, the F2J delivers a dramatic improvement in current-source amplification. I have often ventured the opinion that improvements in measured performance in simple Class A circuits (particularly with no feedback) usually have the best correlation between what you measure and what you hear, and the F2J bears this out. I think it sounds about three times as good, and I feel certain that you will experience the same remarkable improvement.

The period for factories updates has elapsed, so at this time you are on your own. Here is the schematic (and some instructions) for the DIYers. Refer to the original documentation for original schematics and locations of components on the pc boards.

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