PASS LABS FIRST WATT SIT-5 MONO BLOCK
Nelson Pass bespoke First Watt SIT 5 mono block amplifiers
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.
First Watt by Nelson Pass
My experience is that people who enjoy 5 or 10 watt Triode tube amps driving efficient/high impedance loudspeakers will have an appreciation of the SIT-4, which offers a different experience compared to the SIT-5. The SIT-5 is designed with higher power to suit 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 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 in 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.
Summary of the nominal specifications:
Distortion @ 1 watt: 0.07%
Input Impedance: 100 Kohm
Gain: 19 dB
Damping Factor: 25
Output power: 35 watts @ 1% THD, 8 ohms
Frequency response: 5 Hz to 200 kHz (-3 dB)
Noise: 30uV unweighted, 20-20 kHz
Power consumption: 200 watts
Fuse: 3AG slow blow type, 1.25 Amp for 240 VAC
Weight: 32 lbs
Dimensions: 17” W x 15” D x 5” H
More information at https://www.firstwatt.com