After reading Jean Hiraga’s article Le Monstre I was interested to hear for myself how this simple 8W class A amplifier would sound.

I used the original board layout, transistors and JFETs, and made some modifications. Heat sinking was increased to approximately triple the amount recommended. Instead of using the standard bridge rectifier, capacitor bank and battery setup, I opted for a fully regulated supply with a total of 127,0000 uF capacitance per channel and a 500 VA toroid transformer (rather than a 160 VA EI or C core transformer as per the article).

Figure 1: Jean Hiraga Le Monstre 8W Class-A Amplifier Schematic

Figure 2: Jean Hiraga Le Monstre Printed Circuit Board (PCB) Layout

Photograph 1: DIY PCB for Jean Hiraga’s Monstre / Monster

My “regulated monster” is currently running 0.5-0.6 Amps bias per Hiraga’s recommendations in the article. I tested the power supply which you may recognize as the PCB for the John Linsley-Hood simple Class A amplifier 1996 version from Williams Hart Electronics. I have upgraded the supply with LM338K TO-3 case regulators in lieu of the original LM338T TO-220 regulators (see LM338 Voltage Regulator Calculator for more information). On the bench the supply measures very well in this configuration with below 160-200uV of ripple peak-to-peak under a 2A series load. Despite this low level of noise, I would like to try the battery supply for its huge current capability and as close to zero noise as possible.

Figure 3: John Linsley-Hood LM338 Regulated Power Supply Schematic

I did the case work by hand, it is sand blasted aluminum. 3 mm rear panel, 4 mm base panel and lid with a 10 mm front panel. The Heat sinks are approx 350 x 180 x 35 mm. The fasteners are mainly countersunk stainless M5 and the front panel has hidden fixings with a single 3 mm blue led to indicate power is on.

Photograph 2: Jean Hiraga Monstre / Monster 8W Class A Amplifier

Photograph 3: Jean Hiraga Monstre / Monster 8W Class A Amplifier

Sound – Jean Hiraga’s Le Monstre Class-A 8W Amplifier

I am very pleased with the sonic results of this amplifier. It really does not disappoint. Even using fairly standard 3 way speakers in a large room, surprisingly there is ample power. What strikes me the most is the ability of this amplifier to differentiate between instruments and noises in the sound stage. This clarity is what I like most and I think this is achieved by Jean Hiraga’s deceptively simple and pure circuit topology.

Overall I would recommend Hiraga’s Le Monstre amplifier design to any one who appreciates listening to music.

Update June 2008: Added some oscilloscope shots of the amplifier under load.

UpdateFebruary 2009: Dan has constucted another Jean Hiraga Class-A amplifier. This time following the “Super Class-A 30W” design. See the project page – Jean Hiraga’s Super Class-A Amplifier for full details.

Photograph 4: 1kHz and 10kHz Sine Waves at Full Output into 8ohms

Photograph 5: 10kHz Square Wave and 300kHz Sine Wave

Photograph 6: Stability at Clipping – 1kHz into 8ohms