PASS LABS FIRST WATT SIT-4

The SIT-4 is the First Watt power amplifier using the industrial grade Static Induction Transistors. It is preceded by the ground breaking SIT-1, 2, and 3 and follows their technical philosophy and construction. An important difference in the SIT-4 is the SIT device itself. The previous three designs used Silicon Carbide to make a custom vertical power Jfet with a low impedance Drain whose character nicely emulated a power Triode tube, but directly operating at the current and voltage that would drive loudspeakers without the requirement for an output transformer. SITs lend themselves to high quality audio performance with very simple Class A circuits with little or no dependence on other active circuit elements or negative feedback. The Common Source mode SIT-1 could do the job surrounded by only passive elements, and the SIT-2 required only bias from a current source. The SIT-3 upped the game with Common Drain design biased by a novel Pass circuit, MUFF (Mu Follower/Follower). The SIT-4 follows in the Common Source footsteps but ups the game with an industrial SIT. The SIT-1/2/3 gain transistors were rated at a few amps and about 50 watts dissipation. This amplifier uses a Tokin THF51s rated at 600 volts, 30 amps and 400 watts. As a practical matter, the limitations of the SIT-4 circuit are the size of the heat sinks. And this big Jfet manages a bandwidth of 50 megaHertz… The circuit of the SIT-4 is simple. Q1 is a new-old-stock Toshiba 2SK170 which drives the Gate of the Q2 SIT, whose output is biased by a mu-follower current source Q3. The actual circuit is only slightly more complicated.


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