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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.
Performance
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.
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.
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.
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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First Watt SIT-4 — static‑induction Class A intimacy
First Watt's SIT-4 resurrects the rare Tokin static‑induction transistor in a single‑ended Class A design to deliver unusually organic, dense and vividly textured sound. According to Srajan Ebaen of 6 Moons, the amp excels in timbral complexity and presents an enveloping, three‑dimensional soundstage that brings instruments forward with natural weight. Its low‑power performance is surprisingly punchy and grounded, translating into taut bass and lively dynamics when paired with high‑sensitivity wideband speakers. The chassis is compact and built to last, using heavily over‑specified output devices that promise longevity. Ideal for listeners with sensitive monitors or small rooms, the SIT-4 is a niche, highly musical choice that prioritizes tonal saturation and imaging over brute wattage. Immersive soundstage and timbral richness are its standout traits.
Pros
- Exceptionally rich timbral complexity and presence that reveal instrument texture and color.
- Immersive, three‑dimensional imaging and staging that enhances separation and musical immediacy.
- Surprisingly punchy dynamics and taut low‑end for a low‑power Class A amp when matched to sensitive/wideband speakers.
- Robust construction and long‑lived, over‑specified Tokin SIT output devices aimed at decades of reliable service.
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Cons
- Very low output (≈10 Wpc into 8 Ω, 5 Wpc into 4 Ω) and relatively high output impedance—requires high‑sensitivity or specially matched speakers.
- Class A heat load and ~200 W idle consumption—runs hot and needs ventilation/consideration for placement.
- Compared with low‑output‑impedance amplifiers, some listeners may notice slightly less damping in the deepest bass with certain speaker/load combinations.
Sound Quality
91/100
Dynamics
91/100
Class-A, low-power design (≈10 Wpc into 8 Ω; 5 Wpc into 4 Ω) but reviewers repeatedly praise its punch, grounding and muscular presentation. 6moons notes it sounded 'more grounded, punchy and juicy' than a 3-stage class AB comparison and delivered a tauter low end and greater ease, indicating strong headroom and transient authority for its power class.
Soundstage Imaging
92/100
The SIT-4 is credited with creating a notably three-dimensional, enveloping and present soundstage that enhanced the Voxativ speaker's imaging; the reviewer says it elevated the speaker's imaging skills and created a strong sense of immersion. That spatial performance held up even as the amp emphasized density and presence, suggesting stable channel separation and convincing depth for a low-power Class A amp.
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Build Quality
88/100
Solid physical construction (32 lbs, conservative chassis size) and durable output devices that 'should coast for decades' were praised, indicating good component selection and longevity. Thermals are a real trade-off for the always-on Class A topology (≈200 W consumption at idle), so while construction is robust, the hot-running nature and required ventilation temper the score slightly.
Value for Money
90/100
Retail cited at US$5,000 with output roughly 10 Wpc into 8 Ω (5 Wpc into 4 Ω); the reviewer explicitly recommends the amp for listeners with suitable, sensitive wideband speakers (AER, Camerton, Cube, Lindemann, MonAcoustic, Rethm, Voxativ & Zu) and states that 'if you have the right speakers for it, this is the one... It deserves the red.' Given the niche low-power Class A market, reviewers present the price as a justified premium for its sonic strengths when paired with compatible speakers.
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