LJM MX50 kit amp

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#162
off1,

as far as I know all that have blue colored pcb are originals. LJM has a company that produces kits and then distributes them to various ebay sellers for export. There was several versions af MX50 amp, be sure to buy latest version MX50SE.
 
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#164
Looks very good Clive. I am using mine in a cheep n cheerful FAST setup for the garage. I have some Eagle L026B 6.5in full rangers in 15l sealed cabs that have an F3 of around 70Hz, so I have set the input and feedback caps to have a -3dB point of around 35Hz. And then for low frequencies I have a 3 x LM3886 on a variable 12dB crossover with an old 10in car sub in a 36l sealed enclosure.
 
#165
The quality of parts that LJM is supplying with his kits is VERY GOOD. For instance, all 0,1uF caps are highest quality polyester made by Pilkor, factory that was founded by Philips in Korea and later sold to Korean buyer. These parts are not cheap and not easy to find. D1047/B817 output transistors are "Y" grade for DC gain, meaning that they have DC gain in the range 100-200. That's really fantastic and very desirable for audio amplifiers.

Only Chinese company can offer this level of quality for such a fair price.
 
#167
So I got my kits in the mail yesterday and I built both channels right away. I also built a power supply using an old 45-0-45 transformer with fresh capacitors and diodes.

These are my results:

- AMP 1:
I'm not sure what I did wrong but the positive cable between the amp and the power supply started to smoke as soon as I turned it on. Nothing seems burned or bulged in the circuit. I tested the transistors with the multimeter and they seem OK. Not sure what I did wrong 🙁


- AMP 2:
This one worked fine... well, kind of. I plugged it to an 8-ohm cabinet on one end and to a Sony MP3 player on the other end. It sounded OK-ish but nothing to write home about.
So I plugged an old RC4559 preamp between the MP3 player and the power amp and it suddenly came to life. Loud and beautiful sound coming out of the speakers! 😀

Did any of you have to do that? (Adding a preamp)



Also, while on that subject. Would you recommend I add the volume potentiometer between the MP3 player and the preamp or between the preamp and the power amp?


Thank you!!!


- Ofir
 
#168
45-0-45 transformer is way too much for this amp. You have about 65V rails. It's voltage for some VERY high power amp. For this amp it it sure to fry it. You need 25-0-25 transformer for MX50SE, for guaranteed survival. I would not be sure that on the channel where there was some smoke something is not damaged. Always inspect assembled pcb with magnifying glass for solder bridges before first turn on or use high powersafety resisitors inline with positive and negative power rails.

For my taste hardly any power amp sounds really good without preamp. I usually use just what you did, some opamp based pre without tone controls. For that purpose ideal is LJM P7 preamp with onboard PSU, made to be mounted on the front panel using pot's hole. You just need some small transformer 9-0-9 V. LJM P7 is purpose made for LJM power amps. With it you do not need additional pots anywhere.

Or something like APEX OP pre that I built few weeks ago.🙂
 

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#169
ivanlukic said:
45-0-45 transformer is way too much for this amp. You have about 65V rails. It's voltage for some VERY high power amp. For this amp it it sure to fry it. You need 25-0-25 transformer for MX50SE, for guaranteed survival. I would not be sure that on the channel where there was some smoke something is not damaged. Always inspect assembled pcb with magnifying glass for solder bridges before first turn on or use high powersafety resisitors inline with positive and negative power rails.

For my taste hardly any power amp sounds really good without preamp. I usually use just what you did, some opamp based pre without tone controls. For that purpose ideal is LJM P7 preamp with onboard PSU, made to be mounted on the front panel using pot's hole. You just need some small transformer 9-0-9 V. LJM P7 is purpose made for LJM power amps. With it you do not need additional pots anywhere.

Or something like APEX OP pre that I built few weeks ago.🙂
Click to expand...


Thanks for your response!

I actually don't know the nominal specs on the transformer (got it from an old PA) but I'm getting 45V on the rails, positive and negative. Is that still too much?
 
#172
ivanlukic said:
With BC556 input transistors used in MX50SE absolute maximum rail voltage for safe operation would be +/-35V rails.
Click to expand...

I'm not being argumentative --I'm really grateful of your help and I'm honestly trying to learn more from those with more experience-- but where did you get that 35v max figure from? The data sheets for the BC556 show a 65v max safe voltage.




Sent from my LG-D851 using Tapatalk
 
#173
Safe rail votage is half of 65V, which is in fact below 35V. Sometimes, in the case of malfunction, these transistors can see combined voltage of both rails together. That's the reason. You need higher rated input transistors for higher rail voltages, and there are such transistors but not BC556.
 
#176
off1 said:
@LJM
There are many sellers on eBay selling your amp kit. Which one is you? I want to make sure I buy it from the designer and not from some shameless pirate.
Click to expand...

Can refer to pictures, my city in the mainland of China, it's not convenient to transport international,
So the merchants from some ports in sales.
Circuit board with an image, it seems that the only I use it.
In addition to the electrolytic capacitor, other components are basically will not change the appearance.
 

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#177
ivanlukic said:
Safe rail votage is half of 65V, which is in fact below 35V. Sometimes, in the case of malfunction, these transistors can see combined voltage of both rails together. That's the reason. You need higher rated input transistors for higher rail voltages, and there are such transistors but not BC556.
Click to expand...

Not recommended over voltage + －45V。
BC556 In the input side, CE voltage on both ends will not be too high.
I use 5551 5551 voltage amplification.
Within 40 v + - is safe, don't worry 😀
 
#180
I'm very new to building amps and power supplies but need to make a start somewhere so I purchased this amp because the designer is here and everyone else seems willing to help even if I ask a dumb question. Here is my first ones!
I would like to have most of the parts on hand when the amp boards arrive and I've found this transformer:
Avel Y236652 250VA 25V+25V Toroidal Transformer
Would this transformer be suited for this amp? If not what should I use to give the best performance without blowing up the amp?

Thanks in advance for the help.
 
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