SIT-1 Power Amplifier

The 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.

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.

The owner's manual can be downloaded in pdf format here: SIT-1 Owner's Manual

You can read more about SITs here: SIT Introduction

The SIT-1 has only one active device in the circuit. All the other components are passive, 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-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:

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.

Because 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.

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:

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.