First Watt Sit 2

SIT-2
Power Amplifier

The
SIT-2 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.

This
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.

You
can download the owner's manual here: SIT-2
Owner's Manual

You
can read more about SITs here: SIT
Introduction

The
gain of the SIT-2 is the result of the single gain device in the
circuit, the Pass SIT transistor. The other semiconductors form a
nearly constant current source, as illustrated in the following
diagram:

In
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 1.

There
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.

The
SIT-2 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:

The
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 character of the
SIT device is a function of the voltage/current values
(“load-line”) it experiences with a load and the SIT-2
amplifier has been adjusted for most optimal sound in the region
around 8 ohms.

It's
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 soft.

A
typical critical comment on the sound:

The
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.

You
can also read some commentary describing the prototype SIT-2 in
6moons.

Regular
production starts in December 2011 and is expected to be delivered
in late January 2012.

The
owner's manual will be posted on-line as soon as it is completed.

Local pick-up welcome. Stop by our store and take a look at our other

fine product lines we have to offer, such as KEF, Lumin, Antipodes,

Parasound, Hegel, Audio Physics, Acoustic Signature, Acoustic Solid, and

more. We carry a full line of Solid state & Tube Amps, Preamps,
Loudspeakers, Headphones, DAC's, Streamers, Transports, CD players, as well as Turntables & Cartridges.