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Summary of Contents for FirstWatt SIT-2
- Page 1 First Watt SIT-2 Power Amplifier...
- Page 2 Red output as ground, and the Black as active. Warmup - The SIT-2 takes a while to fully warm up. During this time you can listen to it, but you will find that the initial operating point will drift a bit and will require some readjustment.
- Page 3 After a few months I had a small batch of SITs with my name on them, and I started playing. It took about a year and a half to arrive at the designs of the SIT-1 and SIT-2 amplifiers, and it has been a revelatory experience.
- Page 4 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.
- Page 5 The distortion performance of the SIT-2 into both 8 and 4 ohm loads is shown here: In both curves we see a second harmonic character culminating in a very soft overload, which gives the amplifier more apparent power than its distortion ratings suggest.
- Page 6 To address this, There is the option of a simple P channel Jfet follower on the input which raises the input impedance to 100,000 ohms and about 10 pF. Anything will drive this without problems. The distortion of the P channel Jfet is incredibly low, and doesn't alter the sound in any significant way, but it assures no impedance interation and a bandwidth to 500 Khz.
- Page 7 This consistency of performance over the audio band is an important element to the sonic character of the SIT-1 and SIT-2. The frequency and phase response is flat, distortion harmonics are consistent in amplitude and phase relationship, and the damping factor...
- Page 8 A Short Introduction to Static Induction Transistors In the mid 1970's a special variety of Jfet invented in Japan called a Static Induction Transistor (SIT) found its way into the “Vfet” power amplifiers produced by Yamaha and Sony. These amplifiers were produced for several years and then discontinued, but are still highly regarded in the high end audio community.
- Page 9 The behavior of an ordinary Jfet or Mosfet is very similar: As with the Pentode, the current flowing from Drain to Source in a Fet will flatten out as the voltage between those pins increases. By contrast, Here is a set of Triode (300B) curves:...
- Page 10 In a Triode the current through the device depends not only on the control pins but also the voltage across the device. In this sense the gain device acts in a manner similar to a resistor whose resistance value is controlled by the grid voltage. Similarly with the SIT device seen below: As with the comparisons between Pentodes and Fets, the curves have a similar...
- Page 11 characteristic, but the scales have been altered by a factor of about 10, so that the voltages are much lower and the current is much higher. Audiophiles often go to great expense to achieve as little as 5 watts of power using Triodes because of their specific sonic character.
- 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.
- Page 13 Here is a picture of First Watt's SIT device: Of course the audience for such an amplifier will only want to know about the sound. How does it compare with SETs from a subjective point of view? Having made a variety of these amplifiers at this point I can say that first off, they can be made to sound exquisitely in many of the systems that would enjoy the performance of a SET amplifier.
- Page 14 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.
- Page 15 First Watt within the U.S. Amplifiers under warranty outside of the U.S. not purchased from an authorized distributor can be serviced in the U.S. by First Watt at the cost of freight and customs charges. Copyright 2012 General Amplifier www.Firstwatt.com nelson@passlabs.com...
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