FirstWatt SIT-2 Manual page 12

Third - like Triodes, SITs have a soft overload characteristic. When over-driven on peaks they
present compressed, rounded waveforms instead of sharp clipping, the result being that they
are more graceful under pressure.
The entire effort revolves around simplicity and minimalism in circuit design. Certainly you
can get good objective performance with multi-stage circuits and negative feedback. What
we want is the sound that can be had from a single gain stage giving both voltage and current
gain operated single-ended Class A without feedback or degeneration, and we want it with a
high input impedance and a low output impedance. And we want it with reasonably low
distortion with a simple low order character.
We can try this with Pentodes or Mosfets, but the results don't measure well, and they don't
sound as good, a clear-cut case where measurements and subjective performance agree.
It is worth noting that the original efforts by Sony and Yamaha were not minimalist – they
contained many parts in multiple gain stages and used a generous amount of feedback.
There are several reasons for the push toward minimalism. The first is simply aesthetic –
there is much to admire about an amplifier which performs well with only one transistor. And
of course there is an attractive challenge, which is "How good can you make such an
amplifier?".
There is another, more practical reason to explore simple circuits. It is generally agreed that if
you are going to have distortion, you will want it in a low order harmonic form, kept to only
second and third harmonic if possible. A single-ended Class A device is going to generally
give you the simplest version of this.
Lots of audiophiles dislike the sound of negative feedback, or at least think they do. It is true
that while negative feedback reduces the amount of distortion, it does tend to re-arrange it so
that the character is more complex.
Minimalism is the raison d'etre for SIT devices in audio. With them you can get good
objective and subjective performance from a single SIT in a very simple circuit: