Hiraga 20w Class-A Amplifier (re-working)

This amplifier was original built into an old car amplifier chassis using the original 12v to 24v SMPS. After deciding that the heat sink was not adequate enough for a Class-A amplifier I decided to re-mod the whole thing.

The modification ended up taking 4 days instead of the 4 hours I original expected .. just goes to show how the ideas in our head can be completely off the mark! the one thing I have learn about the matrix we live in is often things are not as simple as they seem.

This amplifier was built using recycled materials and anything I could find lying around the house!


A crazy mess on the floor as I have no workshop!


Reworking power supply and testing it.



hmmm where shall I put things!  thermal grease applied under the transistors.



Working out how ill add more space for more circuit boards and electronics.





Me Looking happy


How and where shall we put the capacitors, at this point I was agonizing over placements and overthinking everything being a perfectionist!


It’s coming together!


Testing and working out what the hell is going on!



Looking nice now!



By this time the place is getting very untidy and the girlfriend is going insane!



Back of heatsinks, pitty they are also a bit of a mess but I got given these for free so won’t complain, perhaps I could spray them ?




Looks nice to me anyway! interesting to note that the driver transistors with mini heatsinks are getting hotter perhaps because the rail voltage is now around 26v, you can also see in this picture the power supply to convert from 240v to 12v, its a 600w SMPS so hopefully enough power there. I have taken the fan out of it however as it was very noisy!


The main Hiraga amplifier section, not sure what the capacitors are for.


Ive attached the thermal sensor to this brass piece which is used to transfer heat from one heatsink to the other, well that was the idea. The thermal sensor switches off the amp if it gets too hot


Capacitors are held tight via the solid core wire but I will place something between them so that they cannot ever touch…. otherwise …. BANG!



Well there we go, one finished amplifier or perhaps not until I find a suitable mesh to place over the front and a nice internal lighting system to make it look nice! 🙂



Now with 26v rails and a large heat sink attached for extra cooling, Interesting the bias takes longer to rise ? and it does not reach past 0.38v so Ill re-adjust the bias. The conversion from the original amp to this new one went well but it did take me 4 days instead of the 4 hours I thought it might!

For some reason I have one transistors on one side getting a lot hotter than the other and I cant think why. I’m touching the aluminum metal bar holding down the transistors shown in the pics any thoughts ? the bias is actually a little lower on the hotter side. Ive tried replacing the thermal grease and pads on that channel but no change. also I realized that the bar holding the transistors down has pressure in the middle so the pressure is not uniform over the transistors, perhaps this may be the issue perhaps not as its only one channel that’s getting too hot.

There is however on the hotter side a small out dent on one of the transistors in other words the transistor is not perfectly flat it has a small defect on the bottom which may be stopping it stay flush and uniform on the heatsink.

Oh by the way, the amplifier sounds fantastic much better with a higher rail voltage and higher bias so happy days all round 🙂 I would say its sound is almost as good as the Pass F5 with perhaps a little more mellow sound yet still authoritative and revealing, the sound-stage is lovely and instruments really have their place compared to many other amplifiers that sound muddy this one is fairly transparent but I would like to do more modifications if anyone has any ideas… comments are below.


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