DIY Class-A 2SK1058 MOSFET Amplifier
Mark Houston 
Page 1: Introduction, DIY Class-A 2SK1058 MOSFET Amplifier
Page 2: Power Supply, Sound
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Introduction: Class-A 2SK1058 MOSFET Amplifier
I wanted to construct an amp that had zero negative feed back (0NFB), was pure Class-A and definitely single-ended. Nelson Pass has done a lot of work in this area with his Zen builds, but I wanted to go beyond Zen. I wanted to build a Zero Component Amplifier (ZCA). Was I trying to find the holy grail of amps - the straight piece of silver wire that has pure distortion-free gain?
DIY Class-A 2SK1058 MOSFET Amplifier
Well of course there has to be some active components in an amp in order to get the gain to be called an "Amplifier". I was always impressed how simple single ended tube amplifiers could be. A single valve, a resistor or two and an output transformer. So I fashioned a FET amp around the same simplicity of design. One audio grade MOSFET, a couple of resistors and capacitors and a heavy duty well filtered power supply. The schematic for the amplifier section is shown in Figure 01.
Figure 01: Single-Ended Class-A MOSFET Amplifier Schematic
The amplifier is built around the Hitachi 2SK1058 N-Channel MOSFET. A schematic and pin diagram for the 2SK1058 is shown in Figure 02.
Figure 02: Hitachi 2SK1058 N-Channel MOSFET Pin Diagram
I used Sprague capacitors for the input coupling and a large electrolytic on the output with a 10µF polyester bypass capacitor. The four 10W non-inductive wire wound resistors act as the load. The four resistors are 15ohm and 10W each which I wired two in series for 30 ohms and then the two sets of 30 ohms are wired in parallel to give a total resistance of 15 ohms. These get extremely hot and burn about 30W at idle. Yes, Class-A amplification is very inefficient. I burn over 20 Watts to get only about 4.8 audio Watts per channel! The heatsink I used is rated at 0.784 °C/W
Photograph 01: Single-Ended Class-A 2SK1058 MOSFET Amplifier
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Power Supply, Sound |