[AudioCats]

starting a thread to document the Le Monstre build process....

Le Monstre = 8w class-A speak amp.
Maybe good for driving large ortho-dynamic phones as well.

I bought two Jim's audio le monstre kits quite a while back. These are red board kits, marking on the board is Y50087. The reason for choosing these particular kits was that they are more loyal to the original design, most semiconductors are original models (except the output transistor pair changed to "more modern devices").

I then lost focus and never got the motivation to actually look into building it. Until now.

The red PCB is 2mm thick and looks well-made, but it only shows reference-designators in the silk screen, no parts value. Fortunately the le monstre circuitry is rather simple and not difficult to trace out. Picture below shows JimsAudio (red board, Y50087) reference-designators and part values on the original Hiraga schematic.

 

[AudioCats]

hFE values of transistors in kit #1
2sc1775: 563
2sa782: 432
2sb716: 312
2sd756: 368
w0281g: 92
w0302g: 118

JFET's:
2sk170: grade marking "BL"; idss measured 6.6mA
2sj74: grade marking "BL"; idss measured 6.5mA

The assembly was quick. Connected to +/-12v test supply --> voltage across R11//R12 about 0.6v, so the bias current for the final stage is only about 1/2A.

connected dummy load (6 ohms) and turned trim pot to adjust output offset to 0mV;

connect to music source and hook up to a speaker --> clean music. Not very loud but seems to be very clear.

after about 1hr of warm up --> output offset drifted to 3mv, stable enough

Net steps:
--build the other board,
--come up with a better +/- 12v supply. The the test supply can do very high current up to 15A, but has some hissing noise in the back ground. I know the xbox-360 power brick is very quiet, maybe I can find another one and do +/-12v that way.
--come up with a pair of higher sensitivity speakers. testing was done using magnepan mmgw, which is very inefficient.

 

[AudioCats]

Added some low value bypass caps to the input side of test power supply, between the DC/DC converter's case pin and Vin- ==> no more hissing. Now dead quiet.

Listening test: done using a Fostex T40rpmk2, paralled to the 6 ohms dummy load. ==> Sound is clean and clear but a bit bright, has some sibilance. The reservoir caps on the board are nichicon UKZ 470uf/25v. UKZ is not known to cause bright sound.

I will let it run for a few days and listen again. Maybe the sound will soften up a little.

3-27-2021
tried the Le Monstre with a Lundahl transformer box, to drive a pair of stax 007. Not enough power to push the transformers hard, noticeable distortion when played loud. Sound is good as long as level is below 10% (1k/10k). Sound is not bright. Smooth vocals and enjoyable full bass even though the volume is low.
Not a bad combo. Not being able to turn the volume up is probably good for my hearings, long term.




Changing R2//R4 and R3//R5 values:
R2//R4=486 ohms: Voltage across R2//R4 location is 1.28v, across 486 ohms =2.6mA thru the JFET's; output stage current: 0.6v across 1.1 ohm = 0.54A
486+330=816 ohms: input stage 2.56mA, output stage 1.4v/1.1 ohm=1.27A
486+39+47=562 ohms: output stage 0.8v/1.1 ohm = 0.72A
750 ohms: 1.2v/1.1 ohm = 1.1A, close enough

 

[AudioCats]

signals at R3//R5 and R2//R4 locations. Yellow trace is the input signal.
not sure if this means the gain is only 2x for the input stage? resistor value is 750 ohms.

 

[rubahganteng]

Pls help advices,

what R 1K replace value 390R ?… and pot dc offside 100R Vr keeped or replace with 200R Vr how to get run 1A or close to original 0.6-0.8a with my existing transistor

K170/J74-BL input
BC550/BC560-B hFe 200/180
Driver BD140/BD139 hFe 40.0/40.0
Output Tip41/42-C hFe 15.0/15.0