Усилители мощности моно — First Watt SIT-5

The SIT-5 is a mono power amplifier from First Watt, the small-production brand of the American designer Nelson Pass, and it is sold in pairs at about USD 11,000. It is the fifth First Watt amplifier built around a Static Induction Transistor, with design work running from 2020 to 2024 and the manual dated 29 October 2024. The series began with the mono SIT-1 of 2011, which used a custom silicon carbide SIT from SemiSouth. After SemiSouth went bankrupt, First Watt turned to industrial Tokin SITs from stock left after Tokin's plant was destroyed in the Fukushima disaster.

Key specs that matter

Output is 35 W into 8 ohm at 1% THD and 60 W into 4 ohm. Distortion is 0.07% at 1 W, input impedance is 100 kOhm and gain is 19 dB. Output impedance is 0.2 ohm, giving a damping factor of 25. Frequency response is 5 Hz to 200 kHz at -3 dB, and noise is 30 µV unweighted from 20 Hz to 20 kHz. Consumption is 200 W, and the fuse is a 2.5 A slow-blow type at 120 V or 1.25 A at 240 V. The only input is RCA. Each amplifier weighs 14.5 kg and measures about 43 x 38 x 14 cm.

Engineering and use context

A push-pull voltage gain stage feeds an output stage that works mostly in single-ended Class A. The output device is one Tokin THF51 SIT, rated at 600 V, 30 A and 400 W with response to 50 MHz, used in common-drain, or follower, mode without feedback. It is current-biased from a -60 V rail through a P-channel MOSFET rated at 500 V, 40 A and 890 W, which supplies about 20% of the output. Ratios of power resistors and capacitors trim the load line so that each device gives the same negative-phase second-harmonic distortion character, as individual SITs vary. Compared with the common-source SIT-4, the follower mode lowers distortion and output impedance and widens bandwidth, which makes the SIT-5 suitable for less efficient and lower-impedance speakers. Unlike the SIT-4, its terminals have normal polarity, with red as positive. The aluminium chassis doubles as the heat sink and shares its dimensions and transformer with the other First Watt amplifiers. About 30 minutes of warm-up are recommended, the amplifier is built for either mains voltage and First Watt covers it for three years.

Pairing

Speakers of moderate sensitivity with either nominal impedance suit the SIT-5, and the high input impedance lets valve preamplifiers drive it through RCA.

Alternatives

The First Watt SIT-1 is the earlier mono SIT amplifier with 10 W into 8 ohm and a single-stage common-source circuit.