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DIY Class-A 2SK2221 Mosfet Amplifier Project

 Jochem Jubels   Netherlands Flag     To email Jochem, type out the email address.

Introduction - DIY Class-A 2SK2221 Mosfet Amplifier

Prior to graduation, I was in my final 6 month work term with an electronics company as a trainee. Within this company there are a few audiophiles who are tube lovers and spend astronomical amounts of money on their audio equipment. My training supervisor had found Mark's Class A MOSFET Amplifier Project and suggested that I design a PCB and build this project as part of my training.


Class-A 2SK2221 Mosfet Amplifier

I followed the Class A MOSFET Amplifier Schematic with the following exceptions. Instead of the 2SK1058, I used the 2SK2221 mosfet which I had on hand. I compared the datasheets of the 2SK1058 and the 2SK2221 and the differences between the two were very small. The other change was to use a 6800 uF capacitor on the output instead of a 4700 uF capacitor.

Amplifier Section - Single Ended Class A Mosfet

Photograph 1: Amplifier Section - Single Ended Class A Mosfet

I was very fortunate as this company is planning to move to a smaller location, and was trying to give away some old inventory and parts. The building is an old Philips factory with an enormous cellar. The cellar holds many interesting parts. Every part used in this amplifier project was available in the cellar, with the exception of the 15 ohm load resistors that had to be purchased.

RCA Inputs, Speaker Outputs and Heat Sink

Photograph 2: RCA Inputs, Speaker Outputs and Heat Sink


Power Supply - Class-A 2SK2221 Mosfet Amplifier

For the power supply, I made some changes. For the rectifier bridge, I used four BYW 29/100 diodes. Around the diodes I placed 100 nF capacitors to filter out any switching noise. Wima 1 uF capacitors were placed in parallel with the 10000 uF capacitors used for filtering. Initially, I used the secondary coil from a transformer which measured about 1H. With this configuration, I was experiencing hum in the output. After an email exchange with Mark and Gio, I was informed that a transformer cannot be used as a choke. A transformer does not have an air gap and will therefore "saturate" quickly. This means it will have no inductive effect and is therefore no better than a resistor. I settled on two toroid chokes which measured about 1H each. With proper chokes, the hum was essentially inaudible. A schematic of the modified power supply is shown below.

Power Supply Schematic for Class A Mosfet Amplifier

Figure 1: Power Supply Schematic for Class A Mosfet Amplifier

Power Supply Section - Single Ended Class A Mosfet

Photograph 03: Power Supply Section - Single Ended Class A Mosfet

Finished Class A Mosfet Amplifier

Photograph 04: Finished Class A Mosfet Amplifier

Underside View of Class A Mosfet Amplifier

Photograph 05: Underside View of Class A Mosfet Amplifier

Rear View of Class A Mosfet Amplifier

Photograph 06: Rear View of Class A Mosfet Amplifier

Listening to Class A Mosfet Amplifier

Photograph 07: Listening to Class A Mosfet Amplifier


Sound - Class-A 2SK2221 Mosfet Amplifier

Once everything was built, I tested the amplifier everything sounded great. Above expectation! The music I tried ranged from Dire Straits to Enigma, Dj Tiesto and Metallica. It is important to note that this amplifier will require good filtering and a choked power supply to reduce hum. Overall, this is a very simple and rewarding project.

I want to thank Mark for sharing his Class A MOSFET Amplifier Project over the internet. It really got me going and wanting to do more with audio.