Hiraga 20-30 W - A Class A Power Amplifier


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.

Amplifier schematic

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 PCB. 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.

For a 30 W amplifier it should not be necessary to change other component values than R20 and R21. For an amplifier with an output power of less than 20 W, the zener diodes should be replaced by lower value ones. Then R7 and R8 should be reduced in value, too. To reduce distortion and output impedance, one option is to halve the value of R23.

PCB Layout

The layout of the amplifier board is shown below.It measures 99x52 mm. For all replacements, be sure that the size of the components and pinning is correct, especially for other transistor types, when mounting the components on the circuit boards.


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
R2 33 kohm
R7-R8 27 kohm for D5-D6 20 V zener
R9-R10 220 ohm
R13-R14 220 ohm for D5-D6 20 V zener
R15-R16 1.5 kohm
R20-R21 Select on test (typically 680 ohm for 20 W)
R22 10 ohm
R23 220 (200) ohm min 2 W 1 % for 20x gain (Reduce for lower gain)
R26-27 2.2 kohm
R28-29 0.47 (0.5) ohm min 3 W 1 %

RV19 500 ohm Potentiometer Bourns 3296W

C3-C4 22 nF Axial Film L 12.0mm D 6.5mm P 15.0 mm (Value not critical)
C34-35 68 pF NP0/C0G (or radial film) P 5.0 mm (Value affects stability)
C32-C33 10 µF Radial Film L 18.0mm W 9.0mm P 15.0mm (Value not critical)

D5-D6 20 V 500mW Zener DO-35

Q11, Q18 KSA992 TO-92
Q12, Q17 KSC1845 TO-92

Q24 KSA1381
Q25 KSC3503
Q30 KSC5200
Q31 KSA1943

J1 Screw Terminal 01x02 P 5.0 mm
J2-J5 PCB Tab P 5.0 mm


Please notice:
This project description is for non-commercial use, only. Using this document on a site and charging a fee for download is vialation of non-commercial use and prone to demand for payment. So, for commercial use, contact me for agreement of terms. This page, however, can be downloaded for own use, and linked to, not violating term of non-commercial use.

Knut Harald Nygaard