First Watt J2 User Manual

First Watt model J2

Power JFET Amplifier

Design and Philosophy

Introduction

You may recall that nearly four years ago I introduced the model F3, an amplifier
using the Lovoltech JFET power transistors. The F3 remains one of my favorite
pieces, a 15 watt single stage / single-ended Class A power amplifier which
achieves very low distortion and exceptional subjective quality for such a simple
amplifier. Until now, it has been the only power JFET amplifier that you could buy,
excepting the efforts of Sony and Yamaha from more than 30 years ago.

The details of the First Watt F3 are found in the owner's manual (and the DIY
articles Zen Variations #8 and #9), and the charms of the amplifier's sound can be
found in the many reviews.

First Watt is nothing if not a moving target, so:

Meet the New FETs

Not the same as the old FETs. Advances in semiconductor technology have
resulted in new Silicon Carbide (SiC) JFET transistors with much greater voltage,
current, and power capabilities. How much more? How about 1200 volts, 30
amps, and 273 watts?

Offered by a new company in Missouri, SemiSouth, these parts are designed for
fast high power switching in solar power and electric car applications. They
achieve that last little bit more energy efficiency from conversion processes, and
that justifies a sampling price as high as $60 or so.

Tomorrow's transistors at tomorrow's prices!

Don't worry, I get a volume discount.

Given the manufacturer's intended use, there is a certain irony in making Class A
amplifiers with them. Obviously there are plenty of alternatives for power devices
if we simply want to make an regular amplifier.

So you have to ask, aside from the novelty such a new unique part, what makes it
special for audio? In two words:

Lower Distortion.

These are the lowest distortion big power transistors I have seen. To give you a
sense of the of the difference, Fig 1 shows the distortion between two different
transistors operated under identical conditions in a First Watt F2 amplifier:

Here we see the popular IRFP240 N channel Mosfet (top curve) versus a
SemiSouth SJEP120R125 N channel JFET, both operated in Common Source
mode, biased at 10 Volts DC and 2 amps by a constant current source and driving
an 8 ohm load without feedback.

Zowie! The JFET is about 20 dB better. At the higher power level in this particular
circuit, you will see that the difference declines, but that is simply the effect of the
triode-like character of the JFET at lower voltages across the device – you can
take care of that with a couple extra volts on the supply.

And with a 1200 volt rating you have some margin to burn, in fact you could stick

this in where a 300B used to go. Not that I would do such a thing....

Also, the distortion remains much lower at high frequencies as well. This part has
about half the input capacitance and about 2/3 the reverse capacitance of the
IRFP240 and the value is more linear, so in typical amplifiers we see about a
quarter of the distortion due to junction capacitances.

If we have a gain device that behaves a bit like a tube, then it is natural to try it out
in a popular tube amplifier topology. For many aficionados that would be single­ended Class A. 300B’s, 211’s and their cousins are often operated single-ended
and coupled to an output transformer and regarded by many as the low power
musical standard. Single-Ended Class A tube amplifiers are not very powerful,
and their measurements are nothing to write home to mom about, but there is no

denying that they have strong musical appeal to a sizable segment of the
audiophile population.

Here is the simplified schematic of the J2. The topology bears a resemblance to
the Aleph J, but is not quite the same.

The most important topological difference is that the output stage current source is
no longer an “Aleph” current source, but a version of the classic “Mu Follower”.
This approach was chosen because the JFETs can take better advantage of
distortion cancellation than Mosfets, but won't do so if the current source is tightly
regulated. This has a little less precision, but lower distortion.

The input stage uses parallel Toshiba 2SK170 and 2SJ109 JFETs for both the
current source and the differential input (the parallels are shown as single in the
drawing). The J2 is a nice blend of parts that are not yet available and parts that
are no longer available.

Being a single-ended Class A circuit, its characteristic is “second harmonic”, and
at 0.02% at 1 watt, it has about half the distortion seen in an Aleph J. Just as
important, it is achieved with a simpler circuit and half as much feedback.

An objectivist might say, “I've got a Carver that measures better than that.”