First Watt SIT 5 Class A

"Think, Build, Measure, Listen, repeat."

The SIT-5 is the most recent amplifier produced by First Watt, and is the fifth to use a Static Induction Transistor (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 also 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.

The success of the monoblock SIT-1 led to the SIT-2, which offered similar performance at higher efficiency, and two channels in the same chassis. 2018 saw production of the SIT-3 which operated the SIT transistor in Common-Drain (follower) mode, again without feedback which was available until 2023.

Some years prior, SemiSouth suffered bankruptcy and we found ourselves unable to acquire more of this special part. However there was a company in Japan – Tokin, known for making industrial Silicon SIT parts suitable for audio use. Their production facility was destroyed in the Fukushima earthquake and tsunami, but we were able to acquire these parts from existing inventories of these devices over a period of several years. These large Tokin SITs enjoy that same Triode character, but at much higher voltage, current and power ratings than our original SITs.

A special variety of Jfet invented in Japan in the 1950’s, in the 1970’s SITs enjoyed popularity in the “Vfet” power amplifiers from Sony and Yamaha that are still highly regarded in high end audio. Ultimately the difficulty and expense of SIT manufacture made them less competitive compared to later bipolar transistors, and it is only more recently that their superior qualities have been recognized for high end audio.

SIT devices have a unique characteristic which is of particular value for audio amplifiers. Quoting inventor Nishizawa’s patent abstract, “(The) Drain-current to Drain-voltage characteristic simulates the Anode-current to Anode-voltage characteristic of the Triode vacuum tube very closely.”

As with Triodes, the characteristic curves of the SIT allow operation on Class A “load lines” that can determine the relative values of second and third order harmonics and have little in the way of higher order distortion. It is a now a common observation that the most appealing sound tends to come from a dominant second order harmonic character followed by a lesser values of higher order harmonics.

Historically SITs have been used in microwave, radar and other exotic applications, but found use in audio amplifiers produced by Sony and Yamaha in the 1970’s and 80’s. After that they pretty much existed as industrial parts from Tokin. Recently there has been renewed interest in these high power tube-like devices, partly because two audio companies stepped up and spent the money required to fabricate new devices suitable for audio power amplifiers.

The first of these was Digital Do Main in Japan, which produced at least two audio amplifiers based on newer Tokin versions of original Yamaha parts. The other was First Watt, with the custom SemiSouth Silicon Carbide SIT for the SIT-1, 2 and 3. The SIT-4 and SIT-5 amplifiers use the Tokin THF51s rated at 600 volts, 30 amps and 400 watts in a 10 watt/ch amplifier."

Personal Comments from Nelson Pass…

The comparison between the SIT-4 and the SIT-5 is not simply about power, although there is significant difference there. The SIT-4 has the SIT operated in Common-Source mode, where it supplies both voltage and current gain. The SIT-5 operates the SIT in Common-Drain mode where it only has to follow a voltage, giving it an advantage in lower measured distortion, bandwidth and output impedance. You should not expect them to sound the same.

My experience is that people who like 5 or 10 watt Triode tube amps driving efficient/high impedance loudspeakers will have an appreciation of the SIT-4 that is different than that of the SIT-5, designed with higher power around less efficient/low impedance loudspeakers.

It’s not merely about watts, but human taste and the relationship the amplifier has with the loudspeaker. In any case, it’s worth experiencing both sides, and you can actually do that with the same single gain device…

I always keep in mind that the graphics and numbers only hint at the sonic performance.

It’s 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 amplifier tests don’t necessarily tell the whole story (but are at least reliably repeatable) but they main thing is that they are in service to the listener’s experience.

Human perception is perhaps not the most reliable, but humans are the customers, and I rely on whatever information I can get, having developed some sense of the relationship between design, measurements and listening over the last 57 years.

I design mostly for myself. Only a few make it to market, but I am happy with all of them. It’s a lengthy task, but the process is simple: Think, Build, Measure, Listen, repeat.

For the SIT-5 this process began in 2020, and the design is now finished as we enter 2024.

These particular amplifiers are aimed at those audiophiles who appreciate detail, warmth, depth and imaging. They aren’t for everyone, although they seem to have considerable acceptance in the audiophile community.

Anyway, I hope you enjoy them as I do.

– Nelson Pass 10/29/24