First Watt SIT-4 and SIT-5 – Class A Amplifiers Based on SIT Transistors

Nelson Pass’s First Watt offers two power amplifiers built around Static Induction Transistors (SIT): the stereo SIT-4 and the monaural SIT-5. Both models operate in pure Class A without global negative feedback and use industrial Tokin THF51 transistors rated for up to 600 V, 30 A, and 400 W of power dissipation. However, their circuit designs differ significantly, which makes them suitable for different types of loudspeakers.

The First Watt SIT-4 is based on an extremely simple single-ended topology. Its SIT transistor operates in a Common-Source configuration, providing both voltage and current gain, while the input stage uses NOS Toshiba 2SK170 JFETs. This architecture eliminates the need for an output transformer while retaining some of the operating characteristics associated with single-ended triode amplifiers. The SIT-4 delivers 10 W per channel into 8 ohms, has an input impedance of 100 kOhm, a gain of 20 dB, and a frequency response from 4 Hz to 80 kHz. Its relatively high 4-ohm output impedance makes it best suited to sensitive loudspeakers with fairly high impedance.

In the SIT-5 monoblock, the same SIT transistor operates in a Common-Drain configuration, effectively functioning as a source follower. Voltage gain is provided by a preceding complementary push-pull stage, while the output section remains predominantly single-ended and also operates without negative feedback. This arrangement reduces output impedance and distortion while increasing power output and extending the operating bandwidth.

The SIT-5 delivers 35 W into 8 ohms at 1% THD and up to 60 W into 4 ohms. At 1 W into an 8-ohm load, total harmonic distortion is approximately 0.07%, gain is 19 dB, and the damping factor is 25. The -3 dB frequency response is specified as 5 Hz to 200 kHz. Both amplifiers consume around 200 W, which reflects the continuous operation of their output stages in Class A.

The First Watt SIT-4 and SIT-5 therefore represent two different implementations of the same basic concept. With its 10 W per channel and highly minimalist topology, the SIT-4 is aimed primarily at owners of high-sensitivity loudspeakers and is conceptually close to low-power single-ended tube amplifiers. The SIT-5 is significantly more powerful, has lower output impedance, and is better suited to less sensitive and more demanding loads. Nelson Pass himself emphasizes that the choice between the two depends not only on measured specifications, but also on the characteristics of the loudspeakers and the listener’s personal preferences.