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The F1J is an update to the F1 where the two gain Mosfet transistors have been replaced by new state-of-the-art power Jfet devices.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.

Key Improvements

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:

• Less than half the distortion in low to midband range
• Dramatic reduction in distortion at 20 KHz
• Approximately 1.5 dB more gain due to higher transconductance
• Lower input impedance (approximately 30 Kohm balanced and 15 Kohm single-ended)
• More extended bandwidth

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.

Recommendations

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.

The manufacturer believes the improvements in measured performance correlate well with perceived sound quality, stating that it sounds "about twice as good" compared to the original F1 model.

The period for factory updates has elapsed, so at this time, DIY updates are possible with the provided schematic and instructions.