FirstWatt SIT-2 Manual page 4

Both amplifiers deliver about 10 watts output per channel. The SIT-1 is a monoblock drawing
about 200 watts, and the SIT-2 is a stereo amplifier also drawing about 200 watts.
So what's the difference?
First off, the SIT-1 gain device is surrounded by passive components, giving it a lower
efficiency of around 5%. It is biased through 800 watts of power resistors mounted on the
heat sink. The SIT-2 is biased by a constant current source created by another FET device,
and it has an efficiency of about 10%. While these two approaches give nearly the same
sound at the default setting, I have always been intrigued by the sound available from the SIT
without any other active devices involved. I suppose you could think of it as slightly more
purist, if twice as impractical.
The SIT-1 has a knob and meter on the front panel which allows the user to adjust the load
line setting of the SIT device to tweak it into the most preferred setting for different
loudspeakers and listening taste. It also helps compensate for small variations in AC line
voltage.
The SIT-2 operates a fixed operating load line as determined by listener's preferences in
various systems. This setting corresponds to the middle position setting on the SIT-1. Being
biased by a constant current, it does not need compensation for AC line variations.
Apart from these, they are very similar. I live with both happily.
The Harmonic Structure
The SIT device allows the possibility of arbitrarily deciding on the specific harmonic structure
of the distortion of the amplifier. Like Triodes, the characteristic curves of the SIT device allow
operation on load lines which can set the relative values of second and third order harmonics.
My own experience echoes that of Jean Hiraga, which is that the best sound tends to come
from a dominant second order harmonics followed by a lesser value of third harmonic and
essentially no harmonics above those. It looks a lot like this: