First Watt SIT-4

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.

It is this single-ended Class A simplicity which delivers the much desired sound quality, that nearly pure negative phase 2nd harmonic signature identified so well in the original SIT-1:

The first prototypes of the SIT-4 were operated without feedback, and had lots of this 2nd harmonic quality, but extensive listening tests concluded with a design which used 6 db of feedback to lower this a bit and improve the damping factor. The result is this distortion vs power curve:

harmonic character gives a straight line for distortion vs output power all the way to 10 watts, and then some 3rd harmonic starts bending the line with some compression. Realistically, you can listen to 40 watt peaks before your ears will get start to hurt. Me, I hardly ever need more than a watt, but I know that many, if not most audiophiles want more power and dynamics. Has it really been 12 years of SIT amps? Seems like yesterday…