Pass Labs First Watt 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 which helps make it a better sounding amplifier than the rest 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”.
Measured at 120 V AC and an 8 ohm load:
Distortion @ 1 watt .025% @ 1 KHz
Input Impedance 100 Kohm
Balanced input CMRR -60 dB @ 1 Khz
Gain 20 dB
Input Sensitivity 280 mV = 1 watt, 1.4 V = rated power
Damping Factor 20
Output power 8 ohms 30 watts @ 1% THD, 1KHz
Output Power 4 ohms 15 watts @ 1% THD, 1 KHz
Frequency response - 1 dB @ 4 Hz, 100 KHz
Noise 100 uV unweighted, 20-20 KHz
Power consumption 200 watts
Fuse 3AG slow blow type - 2.5 Amp for 120VAC, 1.25 Amp for 240 VAC