J2 Power Amplifier

The J2 is a stereo power amplifier rated at 25 watts per channel. It has a two-stage circuit that operates in pure single-ended Class A mode, with signal JFET devices forming the input stage and power JFET devices for the output stage.

JFET transistors have long been recognized as having the highest audio quality of any transistor; the input devices of the J2 are known through the industry as the standard for low noise and linearity, and are found on the inputs of the finest phono stages, line level preamplifiers, and power amplifiers.

But for over 30 years robust power JFETs have not been widely available since the short-lived efforts of Sony and Yamaha. In the last few months, advances in Silicon Carbide (SiC) technology have resulted in new power JFET transistors with high voltage, current, and power capabilities – as high as 1200 volts, 30 amps, and 273 watts.

Created by SemiSouth Corporation of Mississippi, these robust new JFETs are designed for very fast high power switching in solar power and electric car applications. However, they also have a very low distortion characteristic that makes them superb for use in linear amplifiers. In apples-to-apples comparisons with comparable MOSFET type power transistors, they can achieve 10 to 20 dB improvements in distortion performance.

For over 10 years First Watt has been pushing the design envelope with simple high performance Class A amplifier circuits, and when a better transistor like this comes along, it can mean getting the same distortion performance with a lot less feedback, or lower distortion with the same amount of feedback. The J2 does amplifier does both,and that helps make it a better sounding amplifier than the best of its predecessors.

With a device that behaves a little like a tube, it is natural to consider popular tube amplifier design topologies. Single-Ended Class A tube amplifiers are not very powerful, and their measurement numbers are not exceptional, but there is no arguing that they have strong musical appeal to much of the audiophile population. The J2 amplifier uses a classic JFET differential input stage followed by a single power JFET transistor. This power JFET is biased by another JFET in what is known in tube circles as a “mu follower”.

Here is a slightly simplified schematic of the J2 circuit. It could be a classic tube amp, except that the P channel input JFETs would have to be fabricated from anti-matter. The single-ended Class A output stage is “second harmonic” in character, and it uses about half the feedback of a comparable MOSFET circuit but with half the distortion and twice the bandwidth.

The J2 power amplifier is simple, clean, and measures astonishingly well. It achieves a sound which is warm and relaxed, combining precision and detail without sterility. With a pair of sensibly efficient loudspeakers, it will give you a toe-tapping experience that other solid state amplifiers do not. The design is extremely reliable and will never need adjustment.

Download: J2 Manual

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