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LJM MX50 kit amp
LJM MX50 kit amp
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Old 19th February 2019, 06:54 PM   #311
t-minik is offline t-minik  France
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Join Date: May 2006
Location: Montpellier
alisson6101, perhaps you can try replacing input 3.3uf cap with a 10uf one.
You can find some decent quality "cbb 106j" on bay or Ali and tell us.
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Old 19th February 2019, 09:01 PM   #312
ekremy18 is offline ekremy18
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Join Date: Feb 2016
i think mx 50se does not need a separate preamp, i set the gain as 18kohm/300ohm=60 and i get a dead silent system, it use 29-0-29 450VAC toroid
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Old 25th February 2019, 10:34 PM   #313
bhartman1966 is offline bhartman1966  United States
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Default help with phase / grounding issue on LJM MX50SE

I'm wondering if anyone has any ideas relating to my MX50SE build. Both boards were pre-assembled so are presumably identical. I've checked my wiring repeatedly and have things grounded properly (star model). However, my outputs seem to be out of phase with one another. I get far better bass response when connecting one speaker with reverse polarity - the usual low-tech verification. (B&W speakers - definitely wired properly internally and verified by other DIY speakers) But, for what it's worth, I notice wider separation/imaging when the speakers are connected 'properly' (red-red, black-black). My guesses are that:
1) somehow the circuit boards were incorrectly factory-assembled... seems unlikely though
2) my adjustable power supply board, which accepts an AC or DC feed, outputs slightly different DC+ and DC– voltages against ground, even when both adjustment screws are 'wide open'
3) I need discrete power supply boards - one for L and R channels - rather than powering both amp boards in parallel
4) upcycling a 20v DC Apple laptop power source (the old yo-yo version) is not such a hot idea. Also, I'm grounding MAG straight to the AC receptacle, not the shielded strand from laptop PSU.

Any hypotheses or recommendations - for diagnosis or repair - would be greatly appreciated!

... the build otherwise turned out great. I used a spare, very compact polished stainless steel junction box, which also serves as an effective outer heat sink, with aluminum slabs internally between it and the output transistors. Photos to come.
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Old 26th February 2019, 11:33 AM   #314
Noise is offline Noise  Denmark
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Join Date: Dec 2005
Location: Denmark
Quote:
Originally Posted by bhartman1966 View Post
I'm wondering if anyone has any ideas relating to my MX50SE build. Both boards were pre-assembled so are presumably identical. I've checked my wiring repeatedly and have things grounded properly (star model). However, my outputs seem to be out of phase with one another. I get far better bass response when connecting one speaker with reverse polarity - the usual low-tech verification. (B&W speakers - definitely wired properly internally and verified by other DIY speakers) But, for what it's worth, I notice wider separation/imaging when the speakers are connected 'properly' (red-red, black-black). My guesses are that:
1) somehow the circuit boards were incorrectly factory-assembled... seems unlikely though
2) my adjustable power supply board, which accepts an AC or DC feed, outputs slightly different DC+ and DC– voltages against ground, even when both adjustment screws are 'wide open'
3) I need discrete power supply boards - one for L and R channels - rather than powering both amp boards in parallel
4) upcycling a 20v DC Apple laptop power source (the old yo-yo version) is not such a hot idea. Also, I'm grounding MAG straight to the AC receptacle, not the shielded strand from laptop PSU.

Any hypotheses or recommendations - for diagnosis or repair - would be greatly appreciated!

... the build otherwise turned out great. I used a spare, very compact polished stainless steel junction box, which also serves as an effective outer heat sink, with aluminum slabs internally between it and the output transistors. Photos to come.

Hi,

1) Unlikely, if they were build the wrong way, they most likely wouldn't work at all.

2) How much is "slightly different"? A few milivolts is no problem, but several volts can become a problem. Do you have any DC-offset on the output ? (more than +-50mV).

3) No, you don't need one power supply for each amp. They will work just fine in parallel, as long as the power supply has enough power.

4) Depending on how you did it, this one could be a problem. Do you use 2 mac chargers in series ? Do you use 1 charger with voltage multipliers, and then split it into a dual voltage ?

I strongly recommend not using any special "trickery", when building power supplies for power amps. The simplest way, is usually the best for DIY use. So find yourself a dual winding transformer with 2x25VAC (2x20VAC is fine too) and 160-300VA, and attach that to a rectifier bridge and a couple of caps (somewhere between 2x8200-2x15000uF minimum 50Volts rating).
Alternatively, if you insist on using switchmode, then you need to get one that is made for the purpose. You can get 2x30V 6 to 10 amps, if you search the web.

In theory, if you had 2 Mac chargers in series it could work, but I'm not sure if they have big enough peak current. Chargers are made for delievering a steady stream, and not sudden peaks. But I'm no expert on switch mode power supplies. If they are made properly, they work just fine. But from my experience, computer chargers are made as cheaply as possible, so don't expect any superior performance.
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