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DIY Class-A 2SK1058 MOSFET Amplifier

 Mark Houston  Australian Flag   To email Mark, type out the email address.

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 1.

Single-Ended Class-A MOSFET Amplifier Schematic

Figure 1: 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 2.

Hitachi 2SK1058 N-Channel MOSFET Pin Diagram

Figure 2: 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

Single-Ended Class-A 2SK1058 MOSFET Amplifier

Photograph 1: Single-Ended Class-A 2SK1058 MOSFET Amplifier


Power Supply: Class-A 2SK1058 MOSFET Amplifier

The power supply consists of a 18VAC, 160VA EI transformer into a 25 amp bridge rectifier which provides about 24 volts DC. The power is filtered using 10000µF capacitors and a 5 amp Hammond 10mH choke in a pi configuration, Cap - Choke - Cap. Bias is provided by a 100K pot and a 1M resistor. Simply adjust the pot until half of the DC voltage is across the MOSFET and the load resistors.

Class-A MOSFET Amplifier Power Supply Schematic

Figure 3: Power Supply Schematic: Class-A 2SK1058 MOSFET Amplifier

Power Supply for Class-A 2SK1058 MOSFET Amplifier

Photograph 2: Power Supply for Class-A 2SK1058 MOSFET Amplifier

Inside View of Class-A 2SK1058 MOSFET Amplifier

Photograph 3: Inside View of Class-A 2SK1058 MOSFET Amplifier

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Sound: Class-A 2SK1058 MOSFET Amplifier

I am using this Class-A Amplifier with a 12AU7 based valve preamp. It produces the most purest sound. I have no idea of distortion levels etc. but it has a very fine grain and delicately textured quality. With only a few watts of output (the power is less than 5W RMS but I have seen clean 15W pulses on the CRO), sensitive speakers must be used. Bass is better than the design predicts and the amp drives my 12" 63L based 3-way speakers with ease.

Not quite Zero Components but close to it. A single 2SK1058 MOSFET works it's butt of to give you a great listening experience provided you don't ask it to deliver more than is expected. It plays all acoustic instruments with an incomparable earthy natural quality. Simple jazz trios, classical quartets or delicate male / female voice show this amp for what it really is - beautiful!

Rear View - Class-A 2SK1058 MOSFET Amplifier

Photograph 4: Rear View of Class-A 2SK1058 MOSFET Amplifier

Front View - Class-A 2SK1058 MOSFET Amplifier

Photograph 5: Front View of Class-A 2SK1058 MOSFET Amplifier


Updates: DIY Class-A 2SK1058 MOSFET Amplifier Project

July 2007 - I was extremely delighted to find out that Jochem from the Netherlands has also built the ZCA project following the same circuit but using a 2SK2221 N-Channel MOSFET. For more information and photographs, see Jochem's build DIY Class-A 2SK2221 MOSFET Amplifier Project.

December 2007 - Mike also from Australia sent me an email to let me know he also built this amplifier. I pleased to read his positive comments. Photographs of Mike's build are available in the Photo Gallery: DIY Class-A MOSFET Amplifier - Inside View.

January 2008 - John sent me an email and some pictures of his build of this amplifier. Photographs of John's amplifier are available in the Photo Gallery: DIY SE Class-A Mosfet Amplifier.

August 2009 - I came across the best looking builds of this project to date which have been put together by Alex Kap. Check out his website for photos and his experiences with the ZCA amplifier - 2SK1058 Single-Ended Class-A Amplifier and the follow-up in a Retro Style Design.

July 2010 - Dinesh has also completed the ZCA project. See the ZCA project completed thread on the forum for photographs and listening impressions.

August 2011 - Taoybb in Thailand has also put together the amplifier using premium capacitors. Here are some photos and the power supply used. For more information see the Thai ZCA Design thread on the forum.