SIT-1 is the first of a new generation of power amplifiers
using Static Induction Transistors (SIT) in single-stage,
single-ended Class A circuits delivering superlative sound
without feedback or degeneration. This is made possible by the
unique characteristic of First Watt's custom SIT devices, which
combines a square-law input character with a low impedance
output to form the only solid state gain device which behaves
like a Triode tube.
character has long been sought as a potential replacement for
tubes, but the realized devices have several advantages. The
Pass SIT device has higher gain and lower distortion and also
conveniently operates at the voltages and currents required by
loudspeakers. This means that an output transformer, with all
its bandwidth and distortion limitations is eliminated.
owner's manual can be downloaded in pdf format here: SIT-1
can read more about SITs here: SIT
SIT-1 has only one active device in the circuit. All the other
components are passive, as illustrated in the following
this circuit, the signal is presented at the Gate of the SIT
and appears at the output with a voltage gain of about 18 dB.
The input impedance is about 47000 ohms, and the output
impedance is about 4 ohms, or a current gain of about 10,000 to
are other amplifiers which operate without negative feedback
loops for the purpose of improving performance, but it is a
poorly understood fact that the bulk of them use degeneration
on the gain devices to improve the performance, which is widely
regarded as just another form of feedback.
SIT-1 uses neither loop negative feedback nor degeneration. As
a result, the sound of the amplifier entirely reflects the
character and chosen operating point of this single device. It
is no exaggeration to say that the amplifier speaks with a
single voice. Nevertheless the measured performance is quite
good compared to the single-ended tube circuits (SETs) operated
with multiple stages and feedback:
distortion is second harmonic in character, and this is
constant across the audio band. The frequency response is flat
from 1.5 Hz to 200 Khz. The amplifier operates at 200 watts,
and is designed to operate with loads from 4 ohms to 16 ohms.
the unique character of the SIT device is a function of the
voltage/current values (“load-line”) it experiences
with a load, the SIT-1 amplifier has been equipped with an
adjustment to help optimize the performance with different
loudspeakers. A knob on the front panel is used to tweak the
operating point of the gain device as indicated by a meter
display. It is this feature which further distinguishes the
monoblock SIT-1 from the stereo SIT-2, as it allows the last
bit of performance refinement.
interesting to try to describe the effect of this amplifier on
the performance of high sensitivity loudspeakers. The sound is
fast but very smooth, with a very organic, musical character.
The spatial presentation is very three dimensional, with
exceptional localization. You will find it easy to “drill
down” so as to focus on individual instruments in an
ensemble. Also, the amplifier appears to have more power than
it's rating suggests, as the overdriven characteristic is very
typical critical comment:
amp sounds "big". There is a large, open sound stage
that fills the room, that is the first thing that you notice
when the amp is plugged in. The effect is a little
intoxicating. ...the SIT seems to bring a lot more control &
detail to the speaker. The tube sweetness is present, yet the
presentation is very clean. This is like a very nice low
distortion SET. The SIT presents details, but with proper
dynamic contrast. Details do not poke you in the eye, they just
naturally unfold as part of the music.
can also read some commentary describing the prototype SIT-2 in