This audio power is the famous design of Jean Hiraga. The original 20 W amplifier is described in Jean Hiraga Class-A Amplifiers. The transistors are discontinued, so the transistors here are all replacements. The original 30 W amplifier is similar to the 20 W amplifier, with a few modifications to increase the power output. With my layout presented here, you may adapt the design to what output power you would like. I have used the same schematic as Hiraga, but have pointed out the options you have if you want to lower the output impedance and distortion.
The amplifier schematic is shown in the figure below. The
components are all placed on the same PCB. J1-J6 are the connectors on the
values shown for the components in the schematic are suitable for a 20
W amplifier with a power supply of about +22/-22 V. Please observe that
R20 and R21 set the bias point. If these resistors are reduced, the
current in the output transistors are increased. It is advisable to use
a variable resistor to set the bias point and solder the resistors on
the PCBs afterwards.
With the exception of the power resistors, 0.6 W metal film resistors with 1 % tolerance can be used. The driver transistors Q24/Q25 are fast devices with a very low Cob. However, the difference to the traditional pair BD139/BD140 is probably insignificant. The output transistors used for Q34/Q35, are the couple KSC5200/KSA1943 from OnSemi. They come in a plastic housing, and can be mounted directly on the heatsink. For a 20 W amplifier, the power dissipation for these transistors is about 25 W per transistor, so do not underestimate the cooling demand.R1 680 ohm