Hiraga 20W class A


2000-12-31 9:31 am
I am an heretic and I am going to hell because I want to built a class A amp that is not a Pass project!
Having admitted to that, does anyone know anything about the Hiraga 20W class A amp? The somewhat incomplete schematic is posted in several sites so someone must have seen it and have some comments about it. It could be an interesting little project that distances itself from the usual crowd and that, most of all, does not involve buying mosfets, which I don't have.


2001-03-06 3:07 pm
well... I once built the 8W hiraga "Monster" refering to the 2F and battery operated powersupply....it was quite marvelous... i havent compared it to any passaps so i wouldent now how they compare...

But anyway i think the first thing to do is try to find the output transisors or suitable equivalents...(Good luck!)

I have also tried to find the layout of the 20W i know of one who has it but the "#Q#¤ wont send it to me....=//

Given the heat produced, I think anywhere a class A amplifier is operating could be rezoned as an annex to hell.
In my case--to have both class A solid state and tube circuits running in the same household (in the southeastern United States, mind you, and with summer coming on) would surely qualify as more than a mere annex. Free ticket to an insane asylum, most likely.
Given that the Mark Levinson ML-2 may rise from the grave, we'll at least have company. (Note that the ML-2 schematic showed bipolars, too.)
I once ran my tube circuit at class A for a few weeks, but it dropped the power down to something like 30 or 40W. The sound was glorious, though. Sometimes I think about going back to that circuit.
If you follow through and build it, report on the sound, please. I've seen schematics here and there, myself, but never quite managed to catch fire on the circuit.



2001-04-07 10:03 pm
Other Designs

Pass is a great designer, and wonderful to the DIY community.

Dosen't mean that other designs aren't just as good or better in ways, and that they shouldn't be discussed and built. Also dosen't mean this his trounce other designs too.. the moral? Discuss and build it all!


2000-12-31 9:31 am
that was the link I was looking for thank you very much! On the original schematic there were several mistakes and no output transistors. Nothing major really but somewhat bothersome.
What attracted me to this circuit is it's symmetry and the simplicity. And the fact that I got a bunch of large caps rated at 40V that I don't know what to do with!
I don't know if it will help but I am going to make the layout and post it here.
I've build several Hiraga's 20 Watt amplifiers (and Le Monstres too). The 20 Watter is one of the best transistor amplifiers in the world, the sound is very smooth and airy, almost tube-like. On the net the schematics can be found. The original transistors used by Hiraga are no longer available. Try 2SD718/2SB688 as power transistors and 2SA968/2SC2238 as drivers. The remaining little transistors are easily available.

Then for a joke: try build the amplifier with 3055/2955 and BD139/140 and a simple power supply, you can even use a lower voltage supply (around 16Volts DC, cheaper capacitors with high capactitance possible): it still sounds better than most of the stuff in the shops.



2000-12-31 9:31 am
I would like to design the board for this amp. Is anyone interested in sharing the setup costs of the Express PCB service? The way I see it: I design the PCB, send the interested party the layout so that cost can be personally verified by loading the free software on your computer and computing the board costs. I can accomodate requests for some specific type of connectors, or special components with different layout. Then, I can put the order thru on my credit card after I get the share of the payment.
I use express PCB before and the quality of the boards is absolutely incredible.
Also, if someone has a PCB to make, the layout can be added to my design and the board can be cut out after the board is made.
I make the PCB with the components at hand so there are no surprises at the end with components that don't fit. I don't mean to brag, but so far I have designed 100% error free boards.
Remember there exist a proven lay-out (though I guess the boards themselves are no longer available). Better check whether this lay-out is still usable for today's components (I anticipate it is). The lay-out has been published in the French magazine L'Audiophile and in a French book that contained reprints of a number of articles.

please also look at the forum tubes under 'Linsley Hood' amplifier (actually it is a transistoramp...). There now is a easy-to-build Linsley-Hood amplifier that is called the JLH evolution. It is based on the original 1969 schematics bus has a symmetrical power supply and consequently no blocking capacitor. The design is in many respects comparable (from a user's point of view) to the Hiraga:

* same power supply voltage (around + and - 21 Volts).
* both are class A (Linsley Hood has greater current through the output transistors)
* very simple and easy to build (Linsley Hood has only 4 transistors where the Hiraga has 8)
* apart from the power supply and cooling facilities very cheap to build
* Both have great sound (probably a matter of taste or of components used which of the two sounds best).
* The Linsley-Hood has a slight turn-on offset problem of around 1 Volt during 1-2 seconds. Not a serious problem for most people, but be warned.

Schematics can be found on home.wish.net/~tinholt


2000-12-31 9:31 am
are you saying that the Hiraga design has no DC-offset problem whatsoever? I already liked the design, if it is also free of transient DC offset I like it even more!
Speaking of DC, I am going to have to add an input decoupling cap. In fact, I am going to feed the amp with a signal form a crossover that spits out some DC (Ask Rod Elliot why that is). So the Hiraga philosophy of DC coupled amps is going out the window.
Haven't built the 20 watter but have built 3 pairs of 30 Watt Hiraga class A amps.

The First used a salvaged NAD 3020 transformer and about 16kuf of capacitance for both channels and was spooky quiet, the transformer got dam warm after about an hour but never made a noise.

The second had a 300VA toroid and about the same capacitance, the toroid hummed 75% of the time from switch on but never got that warm.

The third is sitting 3 feet from me waiting to have the PSU wired up.

TP9634 was originally 2SA634/2SC1096, you should be able to pick these up still at CB radio spare parts shop as they were used in the Audio amps in a few old CB radios.

The original outputs where 2SA627/2SD188 now discontinued.

For TP2788 I have used 2SB686/2SD716 with no problems

and Semelab SM3159S/SM3160S 200W/Tc=25C/15A/10-30hfe in a TO3 package, the bass seemed to be deeper and more controlled than the B686/D716.

I have 2 pair of Fujitsu 2SA1077/2SD2527 but haven't tried them yet, these have an ft of 60Mhz & Cob = 300pF

Check out:
not sure about the wirewound resistors, I use 5W Metal plate

Might change the simple Zener shunt reg on the input stage to a TL431 eventually.

From looking at French websites the MOSquito/BUZquito seems to be the successor to the 20/30 Watt Hiraga, check out
this site has lots of schematics/ideas v.interesting

also http://perso.club-internet.fr/ndaviden/mos/mosquito.html

I have a copy of the original PCB but no scanner at the moment, I also have the original Kaneda design that inspired the Hiragas', if anyone wants I can post an ascii schematic?

This 30Watt amp sound amazing on my speakers (94db/w). the sound stage fill the room not just left/right but top/bottom almost surround.
The Vangelis/Antarctica JPN CD leaves most people speechless.

Regards James
In answer to DC offset problems:

The Hiraga has no turn-on offset problems whatsoever.

The offset is not very stable in time, it shifts very slowly between - 100 and + 100 mV, not a problem at all. The monstre however (also a Hiraga design) stays put at whatever DC offset there is (preferably less than 10 mV).

Yes, the lack of an input cap can be a problem if your source has DC at the output, but that's the problem of your source (or cross-over) and has to be fixed at that point and not in the Hiraga.

power supply schematics

Well I'm a French guy interessted in High fidelity and of course DIY projects.
I'm going to build Hiraga's 20w Class A amplifier. I found many schematics on the net for the amplifier part but not so many schematics for the power supply.
Does anyone know where I would find it?
Maybe there is none because of its simplicity. The original Hiraga has a transformer of 2 x 15 Volts AC around 250 Watts for stereo operation (16 Volts works fine). It is no toroidal transformer, differences here between transformers clearly can be heard. Then there is a massive bridge rectificer, 25 Amperes. Then in total 6 capacitors are used, 68.000 mF 25 Volts, Sic Safco. Also here applies: good quality can be heard. The arrangement of the 6 is as follows: after the bridge 2 are used, one in the minus and one in the plus line. Then there are 2 R's in the supplylines of 0,47Ohm/25 Watts. The remaining 4 caps follow after the R's.

The caps are connected with big copper strips in between them.

2 small caps of brand ERO MKC 2,2mF 250 Volt are used to give the supply more 'speed' in the highs. The can be mounted on the circuit board.

Hiraga chooses to use a simple passive power supply with big amounts of mF. A active power supply is not recommended here (I believe that many people have tried this nonetheless).

Re: power supply schematics


You said that you have found many schematics on the net for the Hiraga's 20W-amplifier part.
So far I have only seen one or two.
Founding the transistors is another matter. Does any of these schematics you found use more modern devices?
Anyhow I'm interested in URLs you have gathered so far.