Super Class-A 30W Amplifier

  
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. The schematic for my version of the Jean Hiraga's Super Class-A 30W Amplifier is shown..
Super Class-A 30W Amplifier - schematic

Super Class-A 30W Amplifier - img1

Super Class-A 30W Amplifier - img2

Super Class-A 30W Amplifier - img3

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. 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. 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. 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...



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