Hiraga 30W A class Amplifier
This is one of the most popular A class amplifiers. I used the original board layout shown in the magazine article (as well as the original driver transistors. However, due to the unavailability of the original power transistors, I opted for a more robust output stage using Toshiba 2SA1943 and 2SC5200 complimentary transistor pairs. This transistor change also allowed me to increase the rail voltage which allows the amplifier to deliver more power. However this build is for expirienced users because setup of this amp is not easy . I suggest this amp to be used together with very good power supply, Emi filter and if it is possible with speaker protection. The trimmer on board is for dc offset correction and it need to be slightly negative, however if you need to play with bias the best solution is to change 1W / 47k resistor to lower values eg. 33k or 27k .