LJM MX50 kit amp

[ljm_ljm]

The image. And the test data.

Test data: testing equipment: Audio Precision 2

Frequency response = 20 HZ - 25 k HZ - 0.2 DB


THD+N=0.002% 20hz-20KHZ 10W RMS 8 OHM

 

[gudriukas]

more pics

Hello,
What is the module with four transistors?
Thanks!

 

[Drdorkmeister]

I threw a pair in an old DBX sub amp case. I used the existing heatsink and transformer. Sounds OK. very passable

 

[ivanlukic]

I'm surprised the bias current is only a small fraction what it probably should be.

Bias setting using fixed resistors results in huge errors compared with the calculated values of what it should be. Therefore, if it is necessary to have fixed resistors for bias setting one must first solder precision 25 turn trimmer in place of the bias setting resistor, set the bias, desolder it and than measure the value on the trimmer, buy the appropriate value of the resistor and than solder it. Very complicated, time consuming process. Simply soldering calculated value resistor will never give anything remotely close to the calculated value.

 

[ivanlukic]

The described procedure should be performed independently for each channel! Measured value on the trimmer for one channel will certainly be very much different than the value needed for the other channel.

 

[tonza75]

The described procedure should be performed independently for each channel! Measured value on the trimmer for one channel will certainly be very much different than the value needed for the other channel.

330R + 1k pot. Insted of 1K metal resistor.
I recommeded reading everyday practical electronics the was a artickle of this amplifier and fine tuning, a few mods for stabilatity. Wet elek. Caps removel. Good artickle for those intrested.

 

[ivanlukic]

I think that LJM opted for the fixed resistor bias circuit in order to avoid setting procedure and make assembly simpler for the builder. To be honest, even with huge bias current errors this amp will sound very good. But if the builder want to have exact and precise bias level, trim pot is the way to go.

 

[madel88]

Hello! I've built the amplifier with modifications to the choice of spare parts. I used better transistor capacitors and resistors. I placed bias trimer. I pumped the amplifier with + - 40 volts. The result is fantastic!

 

[ivanlukic]

Hi madel88,

Can you give details about 4R dummy load resistors? Power rating, price, seller, etc...

 

[augustoriodosul]

Congratulations madel88 on the upgrade: what was good should have improved.

 

[madel88]

Hi madel88,

Can you give details about 4R dummy load resistors? Power rating, price, seller, etc...

The resistance is this:HS100 4R F ARCOL / Ohmite | Mouser Deutschland

 

[thimios]

Hello! I've built the amplifier with modifications to the choice of spare parts. I used better transistor capacitors and resistors. I placed bias trimer. I pumped the amplifier with + - 40 volts. The result is fantastic!

R30,C14 can't be correct connected

 

[madel88]

R30,C14 can't be correct connected

The schematic is wrong, the board is correct. I had seen it and checked it.

 

[thimios]

The schematic is wrong, the board is correct. I had seen it and checked it.

Please post the corrected

 

[madel88]

Hello! The first schematic is according to pcb. I think I did not make a mistake. The second schematic is my amplifier, according to the changes I made.

 

[madel88]

A small correction, R10 does not exist at all.

 

[tonza75]

Do you have hum with one shared power supply for the two power amps.
No 10R ground lift resistor.

 

[AndrewT]

Do you have hum with one shared power supply for the two power amps.
No 10R ground lift resistor.

I see R24.

 

[ivanlukic]

madel,

The schematic you found on the net is not MX50SE but an earlier version of MX50. Here is proper schematic of MX50SE reverse engineered from the pcb.

 

[dagfaune]

Sorry, maybe the wrong thread
Can anyone tell what is the audible difference between the MX50, MX50SE and MX50X2