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.
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.
Circuit Mosfet Amplifier 2SK2221
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.