Description
The SIT-4 is the First Watt power amplifier using the industrial grade Static Induction Transistors. It is preceded by the ground breaking SIT-1, 2, and 3 and follows their technical philosophy and construction. An important difference in the SIT-4 is the SIT device itself.
The previous three designs used Silicon Carbide to make a custom vertical power Jfet with a low impedance Drain whose character nicely emulated a power Triode tube, but directly operating at the current and voltage that would drive loudspeakers without the requirement for an output transformer.
SITs lend themselves to high quality audio performance with very simple Class A circuits with little or no dependence on other active circuit elements or negative feedback. The Common Source mode SIT-1 could do the job surrounded by only passive elements, and the SIT-2 required only bias from a current source. The SIT-3 upped the game with Common Drain design biased by a novel Pass circuit, MUFF (Mu Follower/Follower).
The SIT-4 follows in the Common Source footsteps but ups the game with an industrial SIT. The SIT-1/2/3 gain transistors were rated at a few amps and about 50 watts dissipation. This amplifier uses a Tokin THF51s rated at 600 volts, 30 amps and 400 watts.
As a practical matter, the limitations of the SIT-4 circuit are the size of the heat sinks. And this big Jfet manages a bandwidth of 50 megaHertz… The circuit of the SIT-4 is simple. Q1 is a new-old-stock Toshiba 2SK170 which drives the Gate of the Q2 SIT, whose output is biased by a mu-follower current source Q3.
The actual circuit is only slightly more complicated.
It is this single-ended Class A simplicity which delivers the much desired sound quality, that nearly pure negative phase 2nd harmonic signature identified so well in the original SIT-1:
The first prototypes of the SIT-4 were operated without feedback, and had lots of this 2nd harmonic quality, but extensive listening tests concluded with a design which used 6 db of feedback to lower this a bit and improve the damping factor. The result is this distortion vs power curve:
You can see where the 2nd harmonic character gives a straight line for distortion vs output power all the way to 10 watts, and then some 3rd harmonic starts bending the line with some compression.
Realistically, you can listen to 40 watt peaks before your ears will get start to hurt. Me, I hardly ever need more than a watt, but I know that many, if not most audiophiles want more power and dynamics. Has it really been 12 years of SIT amps? Seems like yesterday…
View the owners manual here.
"...in a single 300B's power range, the SIT4 is the best amp I've yet heard. Hard stop."
""FirstWatt amps aren’t for everyone. It takes a particular listener intelligence and awareness to appreciate their kind. Enthusiasts with fitting loads and brand familiarity already know the drill and address. The SIT4’s casual exterior, price to sell, technological pedigree and sublime performance are hardly any news to this audience. Meanwhile, my role is to spread the word about this superbly voiced marvel here. Many amps are special in one way or another. Nelson Pass’ latest creation went significantly beyond just that in my book.
I don’t know how closely it mimics regular DHT designs and I couldn’t care less. At the end of the day this is by far the most accomplished and exquisite low-power amp I have ever listened to. In my room it proved excellent on every sonic front I can think of. If you have just the right speakers for it, this is the one not to consider, but to get."
Steve Guttenberg Audiophiliac:
The Absolute Sound introduces the First Watt SIT 4:
Q: Why would I want one of these?
A: Maybe you don’t. These amplifiers are all out of the mainstream, reflecting quality in simplicity and intrinsic linearity, often with little regard for the usual performance specifications. I presume a subset of audiophiles will appreciate them. Q: What about all the other amplifiers that measure better?
A: The ear is not a microphone, the brain is not a tape recorder, and measurements are limited in describing subjective quality. I like to have low distortion and so on, but these things take a back seat to what I experience when I listen. There are plenty of products which have great specs – I will not be offended if you buy those. Q: Why such simple circuits?
A: It is my experience that simple linear circuits tend to sound better. I think they are more interesting and subtle than complicated circuits, and I find that measurements and subjective sound quality correlate better with simple gain paths. Exploring their potential for high performance is what I like to do. Q: Why do they run hot?
A: They are Class A amplifiers. They are very linear, but they run hot. Can I make them sound good without Class A? No. Because they run hot, it is important that they get good ventilation. Do not place them in a closed cabinet or on top of another hot amplifier. Q: Isn't Class A bad for the environment?
A: Everything is bad for the environment, in case you haven’t noticed. When I look at the cost of enjoying my 25 watt/ch class A amplifier as compared to watching a big screen television or running a porch light all night, I see that I get as much or more value for the same sort of consumption. If I drive down the hill to shop at Fry’s, I consume about as much power as running one of my amplifiers for 2 hours a day for a year. So I go to Fry’s less often, and I turn the amp off when I’m done. Q: What's so bad about feedback?
A: Feedback isn’t so bad. Sometimes I use it, sometimes I don’t. It all depends on what makes a particular design sound better. In the ideal case, we wouldn’t need feedback to improve the performance, but our parts are not ideal.
Watt's the deal?
Dick Olsher famously remarked that “The first watt is the most important watt.” This sentiment has also been expressed by others as “Who cares what an amplifier sounds like at 500 watts if it sounds like crap at one watt?” With this in mind, I created First Watt in 1998 as a “kitchen-table” effort, exploring unusual low power amplifiers with an emphasis on sound quality.
Small amplifiers have a number of advantages over “big iron” in that very high quality can be achieved with simple Class A circuits using little or no feedback.
There is no such thing as a perfect amplifier. All audiophiles and their associated equipment have specific needs, but in each case there is such a thing as a best amplifier – the one that makes you happy.
First Watt exists because I wanted to explore a variety of amplifier designs in what I think of as neglected areas – amplifiers that might not fit into the mainstream and are probably not appropriate to my more commercial enterprise, Pass Labs.
With oddball characteristics and output power ratings of 25 watts or less, First Watt is not for most people. If you have efficient loudspeakers, listen at reasonable levels and are obsessed about subjective performance, then you probably have come to the right place.
If you want reliable audio product, then you really have come to the right place. For twelve years First Watt has had a near-zero failure rate.
First Watt amplifiers are routinely compared with tube amplifiers, although I make a point that they are not designed to mimic tubes as such. These amplifiers share some of the characteristics of the better tube products in that they have simple circuits with minimal or no feedback and emphasize performance of individual gain devices. In some ways they are better than tubes, in other ways perhaps not.
With the new releases of the SIT-1, SIT-2, B2 and B4, the total is now up to sixteen designs released, all simple Class A circuits, each offering a unique approach to sonic quality. The F1 and F2 are both current source (as opposed to voltage source) amplifiers with no feedback. The F3 offers single-stage single-ended operation with new Jfet power transistors. The F4 is a push-pull current buffer with no voltage gain or feedback, and the F5 is a uniquely high performance push-pull amplifier with feedback. The J2 and M2 continue to push the envelope in unique ways.
And there is more to come.
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