F1J 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 F1 or F2 amplifier. As First Watt moves into its twelfth year, this is how we thank those people: The F1J and the F2J update. This page is about the F1J (there is another about the F2J).

Most of what you might want to know about the F1J amplifier can be found in the documentation of the F1 power amplifier. As you may already know, the F1 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 F1 and the articles on current source amplification.

The F1J is an update to the F1 where the two gain Mosfet transistors have been replaced by new state-of-the-art power Jfet devices. In addition, there have been changes to several resistor values and removal of the input zener protection diodes. As with the F1, the simplified circuit of the F1J 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 F1 and the F1J in the low to midband range where you see the newer parts have less than half the distortion:



The difference is even more dramatic when we compare the distortion at 20 KHz:



We can also see this difference at a fixed wattage vs frequency:



The F1 has always favored balanced input operation over single-ended input, but the characteristics of the Jfets help to narrow the gap between the two. As a result the improvement is even more dramatic between the F1 and F1J when operated single-ended:



Noting the improved performance for distortion at high frequencies, it is not surprising that the F1J also has more extended bandwidth as seen below:



You may notice that the F1J also has approximately 1.5 dB more gain. This is because the Jfets have a higher transconductance figure that the original Mosfets.

There is another factor as well, which is that the F1J has a lower input impedance than the F1- approximately 30 Kohm balanced and 15 Kohm single-ended. The modifications that help achieve this bandwidth take advantage of lower impedances, so the F1J 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 F1J 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 F1J bears this out. I think it sounds about twice as good, and I feel certain that you will experience the same remarkable improvements.

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