I had a mx50 fry the outputs on first power up. The KEC transistors supplied with thar kit looked like fakes. Really rough looking and copper showing through the uneven plating on the back.
The quality control is not the best it seems. Would gladly pay double if I could trust that the quality is consistent good.
The quality control is not the best it seems. Would gladly pay double if I could trust that the quality is consistent good.
I think it is kind of funny that I ordered two boards directly from LJM_LJM2 on Taobao through Superbuy,
Direct link to LJM_LJM2 web store to buy L20.5
and I was sent two different boards,
upper left cap, reads 10uf, measures 11uf
upper right cap, reads 106 (10uf), measures 8.4uf
bottom right cap, reads 106 (10uf), measures 10.4uf
bottom left cap, reads 106 (10uf), measures 8.4uf
at least mine are not broken.
Maybe it would be a good idea to buy bare boards and populate them with better/higher tolerance components.
Direct link to LJM_LJM2 web store to buy L20.5
and I was sent two different boards,
upper left cap, reads 10uf, measures 11uf
upper right cap, reads 106 (10uf), measures 8.4uf
bottom right cap, reads 106 (10uf), measures 10.4uf
bottom left cap, reads 106 (10uf), measures 8.4uf
at least mine are not broken.
Maybe it would be a good idea to buy bare boards and populate them with better/higher tolerance components.
I have gone that route more and more. When I get a kit with parts I usually toss the most expensive transistors and capacitors because I can not trust them. And all op-amps in kits with op-amps.
Now I have started to do my own PCB again after concluding that the PCBs on my last couple of DAC projects were beyond help and needed to be redone from scratch. At least the right LJM design seems to be ok with just a couple cap subs and new output. (And maybe a driver but that might be open to debate.)
It depends on where you get your +/- 47V DC from and what kind of power you want to know (peak or RMS). If you use a transformer unregulated supply with a say 33-0-33 V transformer and you mean RMS power, max RMS power is very roughly speaking the lesser of transformer VA rating and the equation 33V^2/4ohm = 275 w (less a bit in emitter resistors and output transistors and rectifier) - say 220W left for the load - that is asking a lot of the MX50X2! Note that your transformer VA ideally is a lot higher than the required Vrms^2/R RMS power rating (1.5-2 times more). Other readers have commented on the safe operating range (SOA) of the MX50X2 - +/- 47V may be too much for this amp. I would regulate the supply down - you loose only a little RMS power as long as Vdc > Vac, but you can keep the amp safe. Always add output RL circuit.
Thank you Grimrian and kozard for your answers.
The + -37vdc transformer is a 250va, therefore sufficient for the MX50X2. I have to say it doesn't lack power as I listen at a normal level and a few times louder. So for my needs it is quite powerful.
The other transformer is a 500va (+-47vdc), if I mention it it is already well installed in another box ready to use. I don't want to change the ouput transistors nor do I want to risk burning one.
I think it's better that I change the transformer in the other box for the 250va 37vdc.
With an idea of the power with 37vdc and 4ohms?
The + -37vdc transformer is a 250va, therefore sufficient for the MX50X2. I have to say it doesn't lack power as I listen at a normal level and a few times louder. So for my needs it is quite powerful.
The other transformer is a 500va (+-47vdc), if I mention it it is already well installed in another box ready to use. I don't want to change the ouput transistors nor do I want to risk burning one.
I think it's better that I change the transformer in the other box for the 250va 37vdc.
With an idea of the power with 37vdc and 4ohms?
Perhaps this chart will help: Rail Voltage chart for Amps
That chart shows that the output voltages needed at 4 Ohms are not that high. At 4 Ohms you can reach pretty high power levels at lower voltages. I think using 8 Ohm appropriate rail voltages with 4 Ohm loads/amplifiers is a common mistake that damages/blows outputs. I know I had some trouble with my "8 Ohm Compatible" Polk towers. I measured the impedance with REW and found that they are definitely 4 Ohms with dips close to 3 Ohms.
In other words 37 Vdc should take you to the limits of the MX50X2 with 4 Ohm speakers.
Also read the next few posts after the chart in that link.
That chart shows that the output voltages needed at 4 Ohms are not that high. At 4 Ohms you can reach pretty high power levels at lower voltages. I think using 8 Ohm appropriate rail voltages with 4 Ohm loads/amplifiers is a common mistake that damages/blows outputs. I know I had some trouble with my "8 Ohm Compatible" Polk towers. I measured the impedance with REW and found that they are definitely 4 Ohms with dips close to 3 Ohms.
In other words 37 Vdc should take you to the limits of the MX50X2 with 4 Ohm speakers.
Also read the next few posts after the chart in that link.
Last edited:
Do not forget that rail voltage for calculations must be measured under full load - for example w. pink noise - with DC voltmeter and ideally with scope on rail and output - idle rail voltage does not mean much. You will find that rail DC voltage under full and continuous load and with unregulated supply is close to transformer AC voltage.
Hi
I have a question, does anyone know this PCB from ljm?
1pair Original Clone British X50 Classic Musical Fidelity Power Amplifier Board | eBay
I have a question, does anyone know this PCB from ljm?
1pair Original Clone British X50 Classic Musical Fidelity Power Amplifier Board | eBay
The older MX50SE was blue and had a ferrite bead, for example. And the old LJM logo.
The new is green, does not have the ferrite and has the new panda LJM logo.
Others highly regard the MX50SE but I put mine aside early on after I did the REW impedance measurements that revealed that my advertised "8 Ohm Compatible" Polk towers are actually 4 Ohm with dips near 3 Ohm. So I didn't want to blow up the MX50SE so I put them aside and instead build the L20 and then L20.5. I had planned to try going back to the MX50SE later but with perhaps 2SC6145/2SA2223 outputs so that the 4 Ohms is not outside of the SOA. But then I bought another brand/implementation of the Blameless topology that has a two transistor VAS and came with Sanken outputs (or reasonable clones). That other design is similar to the MX50SE but has a few advantages over the MX50SE. (...then I got side tracked with the LJM QUAD405-2 which I upgraded with OPA228 and that is currently in my chassis.)
Perhaps there is someone else here who has compared the MX50SE to the MX50X2.
Since there are so many 4 Ohm speakers today I would suggest considering kits like the L20SE or L12-2 over the MX50SE. If you have truly 8 Ohm speakers and an appropriate transformer then I guess the MX50SE is fine.
And one other note: There are fake MX50SE kits (apparently) that use blue PCB.
The new is green, does not have the ferrite and has the new panda LJM logo.
Others highly regard the MX50SE but I put mine aside early on after I did the REW impedance measurements that revealed that my advertised "8 Ohm Compatible" Polk towers are actually 4 Ohm with dips near 3 Ohm. So I didn't want to blow up the MX50SE so I put them aside and instead build the L20 and then L20.5. I had planned to try going back to the MX50SE later but with perhaps 2SC6145/2SA2223 outputs so that the 4 Ohms is not outside of the SOA. But then I bought another brand/implementation of the Blameless topology that has a two transistor VAS and came with Sanken outputs (or reasonable clones). That other design is similar to the MX50SE but has a few advantages over the MX50SE. (...then I got side tracked with the LJM QUAD405-2 which I upgraded with OPA228 and that is currently in my chassis.)
Perhaps there is someone else here who has compared the MX50SE to the MX50X2.
Since there are so many 4 Ohm speakers today I would suggest considering kits like the L20SE or L12-2 over the MX50SE. If you have truly 8 Ohm speakers and an appropriate transformer then I guess the MX50SE is fine.
And one other note: There are fake MX50SE kits (apparently) that use blue PCB.
Last edited:
