Усилители мощности стерео — First Watt SIT-4

The SIT-4 is a two-channel power amplifier from First Watt, Nelson Pass's own line of low-volume designs, and it is the fourth First Watt amplifier to use a Static Induction Transistor. Its design work ran from 2018 to 2024, the manual is dated 1 January 2024 and the price is about USD 5,000. It is designed around high-efficiency loudspeakers of the kind made by Altec, Klipsch, JBL and Lowther.

Key specs that matter

Rated output is 10 W into 8 ohm at 2% THD and 5 W into 4 ohm. Distortion is 0.6% at 1 W, input impedance is 100 kOhm and gain is 20 dB. Output impedance is 4 ohm, which gives a damping factor of about 2. Frequency response runs from 4 Hz to 80 kHz at -3 dB, and noise is 75 µV unweighted. The amplifier draws 200 W, fused at 2.5 A slow-blow for 120 V or 1.25 A for 240 V. Input is RCA only. It weighs 14.5 kg in a case of 43 x 38 x 13 cm.

Engineering and use context

Each channel is a single-stage, single-ended Class A circuit with one Tokin THF51 SIT, rated at 600 V, 30 A and 400 W, working in common-source mode. In this mode the SIT supplies both voltage and current gain, and its curves resemble those of a triode. A MOSFET mu-follower current source sets the bias, and an NOS Toshiba 2SK170 JFET buffers the input, which gives the high input impedance. The load line is chosen so that negative-phase second harmonic dominates up to full rated power, and the amplifier runs into a soft, rounded overload, so distortion rises gradually rather than abruptly near the limit. There is no output transformer, and the First Watt product chart lists the circuit without feedback. As in any Class A design, the power consumption stays constant whatever the volume, since the bias current flows all the time. The high output impedance means the amplifier behaves partly as a current source, so the response follows the impedance curve of the speaker, which is why First Watt pairs it with efficient designs. Nelson Pass also tested it on SR-1 speakers of 3.8 ohm and 86 dB. The output polarity is inverted, with the red terminal as ground and the black as live, which matters when an active subwoofer is connected. About 30 minutes of warm-up are needed, the amplifier is made for either mains voltage and the warranty is three years.

Pairing

Horn-loaded and other high-efficiency speakers are the natural match, as a few watts drive them to full level. Its 100 kOhm input suits valve preamplifiers and passive attenuators.

Alternatives

The First Watt SIT-3 is the stereo SIT amplifier with a common-drain output stage, 18 W into 8 ohm and 11.5 dB of gain.