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First Watt SIT-4
SIT-4 Amplifier
The SIT-4 is the most recent amplifier produced by First Watt, and is the fourth to use Static Induction Transistors (SIT) as the power amplifying device. In 2011 we introduced our first SIT amplifier using a custom Silicon Carbide (SiC) transistor part made by SemiSouth. The amplifier used a single power device, operating without feedback in single-ended Class A "Common-Source" mode to deliver 10 watts of power emulating the characteristic of a Triode, but operating at voltage and currents directly needed by loudspeakers, eliminating the output transformer.
Do simple SIT amplifiers sound the same as single-ended Triodes (SETs)? Maybe not – it would be a matter of opinion. Without an output transformer with its significant limitations, the SIT has a bandwidth and distortion edge. I suppose if it glowed as well, it might be perfect ;)
Circuit Design
Above is the simplified diagram of the circuit of the SIT-4. This is a single-ended Class A circuit using a SIT (Q2) in common-source mode without feedback. It is biased by a conventional Mosfet "mu follower" current source Q3 and is input buffered by Q1, the NOS Toshiba 2SK170, the finest of Jfets. It has an easily driven high input impedance.
It has plenty of gain, and handles 4 ohm loads – my test speakers are SR-1's at 3.8 ohms with 86 dB/watt sensitivity, and the SIT-4 drives them surprisingly loud, although it was designed around Altecs, Klipsch, JBL, Lowthers, and other high efficiency speakers.
Performance
Here are some performance graphics. Keep in mind that they are achieved in a single-stage without negative feedback. Here is the frequency response curve, showing bandwidth to -3 dB at 80 Khz.
The square wave at 10 Khz at 1 watt, 8 ohms
The distortion/power curve shows the 2nd harmonic dominance up to 10 watts:
Distortion waveform (blue trace) @1Watt, showing the negative phase 2nd harmonic.
I always keep in mind that the graphics and numbers only hint at the sonic performance.
Development Philosophy
It's always interesting to compare the test data to the listening experience. Sometimes the two line up well for good or bad, but sometimes an amplifier that tests well still leaves critical listeners disappointed. Sometimes the reverse is true.
Objective tests don't necessarily tell the whole story, but they are at least reliably repeatable.
Human perception is perhaps not the most reliable, but humans are the customer.
When developing product I rely on whatever information I can get, having developed some sense of the relationship between measurements and listening over the last 50 years.
Still, developing audio product is a lengthy task if you do it right.
The process is simple: Think, Build, Measure, Listen. Then repeat.
For the SIT-4 this process began in 2018, and the design is now finished as we enter 2024.
Target Audience
This amplifier is generally aimed at those audiophiles who appreciate the detail, warmth, depth and imaging. It is not necessarily for everyone, although I believe it will be generally appreciated for its special subjective qualities.
I hope you like it. And Happy New Year.
– Nelson Pass 1/1/24
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Professional Reviews 93/100
Deliciously Crispy
Sound Quality
95/100
Build Quality
88/100
Value for Money
89/100
The low-power class A SIT-4 delivers a richly textured, immersive sound that feels organic, punchy, and almost holographic with the right speakers. 6 Moons praises its timbre complexity, vivid presence, and startling imaging, while also noting it’s a specialist amp that really rewards efficient widebanders and careful system matching.
What Reviewers Are Saying
Pros
Exceptional timbre richness, with organic, vivid, and fleshed-out tonal color.
Creates a highly immersive, three-dimensional soundstage with striking image focus.
Delivers punchy, grounded bass and strong presence for a low-power design when matched well.
Works especially well with efficient wideband speakers like Zu and Cube Audio models.
&
Cons
Very low power output means it is a specialist amplifier, not a universal match for demanding speakers.
Runs hot and draws significant power, so heat management is a real consideration.
Bass control can be slightly underdamped compared with very low-output-impedance amplifiers.
SOUND QUALITY PERFORMANCE 95/100
Reviewers describe the SIT-4 as exceptionally organic, vivid and saturated, with striking timbre complexity, fleshiness and resolution. It is also praised for strong grip and a presentation that feels immersive and physically present, though the reviews focus most heavily on its distinctive tone and imaging rather than broad all-rounder neutrality.
Transparency Resolution
95/100
The review calls the amp highly resolved, with 'density, saturation and resolution' singled out as strengths. The presentation is described as present and material rather than veiled or softened, though the evidence comes from a single detailed review.
Tonal Balance Musicality
96/100
This is where the SIT-4 is most lavishly praised: 'timbre complexity, moisture, fleshiness and vividness' are all highlighted, along with an 'organic, elastic' character. The language points to a deeply natural, emotionally engaging tonal balance that seems especially rewarding with the right speakers.
Dynamics
93/100
The amplifier is described as 'more grounded, punchy and juicy' and as adding 'a grander tauter profile of additional ease' in the bass. It sounds forceful and assured for a low-power design, although the reviews also make clear that speaker matching matters and do not claim unlimited drive.
Soundstage Imaging
96/100
Imaging and spatial immersion are major strengths here: the SIT-4 is called 'exceptionally three-dimensional, enveloping, present and material,' and its effect on the Vox loudspeakers' imaging is described as spectacular. That said, this is still one reviewer’s report, so the praise is extremely strong but not broadly cross-confirmed.
Assessments of Build Quality, Usability, and Overall Propositions.
Build Quality
88/100
The reviews note the amp’s substantial 200-watt class-A heat output, its 32-lb chassis, and the fact that the output devices are described as generously underused and likely to last. There is little detailed discussion of fit, finish, or long-term construction beyond those engineering and thermal comments.
Value for Money
89/100
At roughly $5,000, the SIT-4 is clearly a premium component, but the review is strongly positive about the proposition for the right system: 'If you have the right speakers for it, this is the one not to consider but get.' That makes the value case compelling for compatible widebanders and high-efficiency speakers, even if it is not a universal bargain.
Reviews
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Authorized dealers for First Watt SIT-4
- 8th Dimension Audio — United Kingdom (website)
- Atelier 13 Audio — in-store — Forest Hills, Tennessee, United States (website)
- Dream Audio — in-store — Innere Stadt, Wien, Slovakia (website)
- ljudshopen — in-store — Norrböle, Västerbottens län, Norway (website)
- Mains Cable R US — in-store — Elland, England, United Kingdom (website)
- Moon Audio — online and in-store — Cary, North Carolina, United States (website)
- Perrotta Consulting — in-store — Bethel, Connecticut, United States (website)
- Suncoast Audio — in-store — Florida, United States (website)
- Wide Screen Audio — in-store — Gent, Vlaanderen, Belgium (website)