The SIT-3 is the very latest example of single-ended / single-stage Class A amplifiers using the SIT (aka VFET) power transistor exclusive to First Watt. It is preceded by the successful SIT-1 and SIT-2 and very much follows their technical philosophy and construction.

There is a key difference in the SIT-3, which is that the Static Induction Transistor (SIT) that forms the heart of the circuit is operated in an entirely different mode, what is known as Common Drain Mode.

By way of explanation, any gain device with three pins can be used in one of three modes. For the Field Effect Transistor (FET), they are Common Source, Common Drain, and Common Gate.

The channels of the SIT-1 and SIT-2 consisted of a single SIT operated in Common Source Mode in which (conceptually) the signal comes into the Gate and appears amplified at the Drain pin, but phase inverted. The Source pin is grounded. The amplification with Common Source operation is both voltage and current, and the phase inversion is corrected by reversing the output terminals.

The SIT-3 goes in another direction, using Common Drain operation, where the signal goes into the Gate pin and comes out the Source pin and the Drain of the Fet is grounded (literally attached to ground). This mode only provides current gain – the voltage gain is provided by a high quality voltage step-up transformer which takes the input signal from a preamplifier (or other device) and boosts the voltage. It has no phase inversion.

In both approaches, the SIT does a good job of amplifying the signal without feedback, but Common Drain operation delivers the amplification with much lower distortion and noise and also a much better damping factor for the loudspeaker.

The trade-off is the addition of the input transformer, but I think you will find the compromise there is small with respect to the sound quality achieved.

Common Drain has the same simple spectral distortion character that graced the SIT-1 and SIT-2, and allows similar control of the amplitude and phase of the second harmonic content, but at a much lower distortion figure.

If like me you find 18 watts is enough power, then I think you will find that this is a really delightful amplifier.