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.”
Maybe, but here's the deal:
There's a certain amount of “zero-sum game” involved. You can have lower
distortion, but you have to trade off for it. Maybe you need a more complicated
circuit, maybe you need more negative feedback, or both.
A subjectivist viewpoint might be that distortion numbers don't matter anyway.
Probably this is true, but only up to a point. Somewhere above 1%, distortion gets
pretty tiresome. That amplifier may find itself in the closet next to the one with .
0000001% – it doesn't sound that good over the long haul.
Complexity and feedback come with their own baggage, and it's a fact that many
audiophiles will spend more money on amplifiers with higher distortion. Why is
that? My own opinion is that our ears seem to be able to discern something about
the process of amplification.
Perhaps amplifier that works hard to remove errors sounds different than one that
doesn't have to. The more simple and linear amplifier sounds more relaxed, and it
makes me more relaxed when I listen to it. Maybe I'll even fall asleep....
Where was I? Oh, I rememberMaybe you just need gain devices that distort less in the first place.
These new parts give us more leverage when we are making the trade-offs
between low and high distortion, simplicity and complexity, feedback and no
feedback. With the proper choices, this makes for better amplifiers.
Better sounding amplifiers.
Read the brochure if you don't believe me:
“The J2 has 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.”
Now everybody trying to sell you an amplifier will say something like that.
Fortunately, if you are reading this owner's manual, you've already bought the
product.
So Congratulations. You made a great choice.
Now I'm going to show you some stuff that is perhaps not already apparent:
Here's the distortion into 8 ohms at 1 Khz from 0.1 watts to 30 watts
Here's the distortion into 4 ohms at 1 Khz
Here's the distortion into 8 ohms at 1 watt from 20 to 20 Khz
Here's what that second-harmonic distortion looks like
Here's the frequency response from 10 to 100 Khz
Here's the square wave at 20 KHz
Setup
The initial setup of the amplifier is very straight-forward. Place the amplifier in a
well-ventilated location, as it draws nearly 200 watts during operation and requires
as much opportunity to cool itself as possible. You should be able to put your
hands on the heat sink during operation. If you can't do this for 5 seconds or so,
they need more ventilation.
On the front panel there are two blue LED lights, one for each channel, indicating
regulated power to the channel. If the light is on, the power supply for that
channel is delivering voltage. On the rear panel you will find pairs of RCA inputs,
XLR balanced inputs, speaker outputs, a fuse holder, an AC power receptacle,
and on/off switch and a label.
If you are using the single-ended RCA inputs, make sure that the jumper is
present between pins 1 and 3 of the balanced XLR connector, or you won’t get
any gain. If you lose the jumper, contact First Watt and I will send you a pair.
The XLR balanced input has pin 1 = ground, pin 2 = (+), and pin 3 = (-).
The label will indicate a serial number and also what AC line voltage the amplifier
is set for. If the voltage is 120 VAC, then the fuse value will be a 3AG slow blow
fuse rated at 2.5 amps. If the voltage is 240 VAC, then the fuse will be rated at
1.25 amps. Do not substitute a larger value fuse. Contact First Watt if you have
any questions.
I'm assuming that you know how to attach the speaker cables to the 5 way output
connectors provided. I recommend that you make all the connections with the
amplifier power switch in the OFF position, but you will not damage the amplifier
circuit by accidentally shorting the output or overdriving the input - my concern is
more for the safety of any fragile loudspeaker driver you might be using.
With everything connected up and the source equipment powered up first, you can
proceed to turn on the power switch to the amplifier. Turn-on and turn-off thumps
and noise are small in this amplifier, and should not present any hazard to delicate
drivers. It takes about 1 minute for the regulators to come up to full voltage, so
don’t expect full output power for that time.
At this point you should be able to listen to music.
The power supply of the amplifier is isolated from the chassis and AC earth
ground by a thermistor which connects the circuit ground to the chassis and earth
ground. This helps to prevent ground loops, but the thermistor stands by to
conduct AC line voltage to ground until the fuse blows in case of transformer or
other such failure.
The input impedance is 100 Kohms, and the input capacitance is very low, so you
will find it easy to drive it with single-ended tube equipment if you like.
The J2 has enough damping factor at 20 to work well with loudspeakers that mate
well with tube amplifiers in general, and it delivers good performance into 4 ohms
also – see the distortion curves at the back of this manual. It is designed around
relatively high efficiency speakers and it particularly shines with those that have 90
dB/watt sensitivity or greater, but you can hook it up to anything you like, as long
as you adjust your expectations as to how loud it will play.
The amplifier requires about 1 hour of operation to reach normal operating
temperature, and this warm-up time is appropriate for the most critical listening,
but is not otherwise an issue. The amplifier’s final adjustments were been made
after an hour, but the performance difference between 1 minute and 60 minutes is
not very great.
I do not personally see a reason to run the amplifier all the time, but you can do
that if you want to. The power supply capacitors are likely to last about 15 years
or so, and while they will slowly dry out just sitting there, they will have a shorter
life span with the amplifier running constantly. Also, at 200 watts it makes
economic sense to shut the amplifier off if you aren’t planning on using it for the
rest of the day.
Again, the heat sinks on this amplifier run fairly hot, and you want to make sure
that they get adequate ventilation. They will run at around 25 degrees C. above
the ambient temperature, which puts them around 50 degrees in the average
listening room. At this temperature you should be able to put your hand on them
for about 5 to 10 seconds or so.
If you have any questions, contact First Watt. I answer all questions, even if the
answer is no.
www.Firstwatt.com
nelson@passlabs.com
Now the following is for your protection –
Do not defeat the AC line Earth ground connection on the
amplifier power cord. It provides an extra barrier to prevent
potential shock hazard.
Do not replace the fuse with a type other than specified.
Do not operate the amplifier outside in the weather, or in and
around water or anything resembling water. If you spill a drink
in the amplifier or if your dog/cat/child urinates on it, turn it off
immediately, unplug it, and do not operate it until cleaned by a
qualified technician.
If something gets loose or rattles around inside or smells funny,
or if you can’t touch the heat sinks for 5 seconds or so, then turn
it off, unplug it from the wall, and contact First Watt.
There are no user serviceable parts inside. Do not open the
amplifier, and if you do anyway, don’t operate it with the cover
off. There are hazardous voltages inside. If you need to
change the operating AC voltage, contact First Watt.
Once Again:
If you have a problem, contact First Watt. We are much happier
helping you solve problems so that we can be certain that it’s
done properly. If you are far away and don’t want to ship the
product for repair, we will assist your technician with information
and parts.
www.Firstwatt.com
nelson@passlabs.com
Summary of the nominal specifications:
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
Warranty: Parts and labor for 3 years, not covering shipping costs or
consequential damages.
Copyright 2009 General Amplifier
General Amplifier Inc.
PO BOX 7607
RENO NV 89510-7607
www.Firstwatt.com
nelson@passlabs.com
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