Jean Hiraga's Super Class-A Amplifier
Dan W  
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Construction of a Class-A Amplifier
After building and listening to Jean Hiraga's "Le Monstre", a 8W Class-A Amplifier, I wanted to build "Le Monstre's" bigger brother "The 20W Class A" amplifier. Having read Jean Hiraga's article Construction of a 20W Class-A Amplifier and in copies of the old French magazine "L'Audiophile", I was interested to hear for myself how Le Monstre's bigger brother would sound. I decided to base my build on the latest version that I could find called the "Super Class-A 30W"
Figure 1: Jean Hiraga's Super Class-A 30W Amplifier Schematic and PCB
I used the original board layout shown in the magazine article (Figure 1) 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. The schematic for my version of the Jean Hiraga's Super Class-A 30W Amplifier is shown in Figure 2.
Figure 2: Super Class-A Amplifier Schematic with Toshiba 2SA1943 and 2SC5200
Photograph 1: DIY PCB for Jean Hiraga Super Class-A Amplifier
Above you can see the PCB with all components, note the bias resistors which are 1W carbon types I found they had better thermal stability in this application. The heat marks at the end of the legs of the resistors next to the Sprague Orange drop capacitor are from 0.6W metal film resistors which were operating at their thermal limit and were subsequently replaced with 2W metal film types.
My version of the Jean Hiraga Super Class-A Amplifier is running at a bias of about 1.65A @ 35V resulting in about 58W of continuous dissipation per transistor in the output stage (just over 1/3 their rating of 150W). As you can imagine, the heat sink runs quite hot, approximately 40 Celsius (100F) above room temperature.
Photograph 2: Super Class-A Amplifier Heatsink and PCB
The enclosure work is all done by hand using aluminum. Channel sides, 3 mm top plate, 3 mm base plate. The heat sink is comprised of two pieces cut to size and is approx 420 x 180 x 35 mm. The fasteners are mainly countersunk stainless M5 or M3 stainless. Heat sinking was increased. The capacitor bank was also increased to six 220000 uF capacitors (yes that is 1.2 Farads). A 500VA potted toroid transformer was used for the supply.
Photograph 3: Jean Hiraga Super Class-A Amplifier Power Supply
Above you can see the interior of the amplifier which has copper bus bars as per the original design. I have added a small toroid for a regulated supply running the DC protection circuit which I added. There is also a RF filter in the supply line.
Photograph 4: Jean Hiraga Super Class-A Amplifier - Front View
Photograph 5: Jean Hiraga Super Class-A Amplifier - Rear View
How does Jean Hiraga's Super Class-A Amplifier sound?
I am very happy with the sonic results of this amplifier. Jean Hiraga's deceptively simple and pure circuit topology once again shines through. The sound seems effortless and gives the impression of a much higher power amplifier yet still being able to produce all the low level details, the sound which class-A amplification is renowned for.
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Measurements: Jean Hiraga's Super Class-A Amplifier
Various measurements of the amplifier on an oscilloscope are shown in the photographs below. The amplifier is down -3dB at 208kHz and can deliver 58W at full power.
Photograph 6: 10Hz and 100Hz Sine Wave Response
Photograph 7: 1kHz and 10kHz Sine Wave Response
Photograph 8: 100kHz and 1MHz Sine Wave Response
Photograph 9: 10Hz and 100Hz Square Wave Response
Photograph 9: 1kHz and 10kHz Square Wave Response
Photograph 10: Full Power (58W) at 1kHz and Clipping Symmetry
Should you have any questions about this project or if you need some help putting your amplifier together, feel free to ask them in the Jean Hiraga's Super Class-A Amplifier questions thread on the forum.