What's wrong with my ljm_ljm L25D board ?
 

Dear folks,

I am asking this way because yubing or steinar from eastern_audio (ebay seller) cannot help me obviously and my questions in the ljm-ljm thread regulary are not given attention although I eMailed ljm directly. No response. Thanks to member Alkasar who cleared some issues for me via PM but I have no face meanwhile to spam his PM box. So I hope to get here some useful feedback.

In short: my L25D monoblocks are working fine when only powered without speakers connected - at least the blue LEDs went on. But connecting the speaker wires one monoboard seems to fail. No LED on and no noise out of this speaker. After disconnecting speaker wires both amps work. What could be reason ? Shortcut test made. All so far so good O.K. Showing you also a pic of the building and thanks for your attention :)

Can you measure the DC voltage? both rails with speaker connected and disconnected?
the rails need to be at least +/- 60VDC, should be around +/-70 for margin against the low volt cutout. If you have 60VDC, no spekaers connected, your problem is low voltage. Be safe, it only takes 0.002 amps to make your heart skip a beat.

Did you buy a kit to solder yourself? or ready made?

Are you powering DC rails down between connection to speakers, then powering up?

FYI - sitting the DC recitifier cap bank on the styrofoam is probably not a good idea as it is probably electrically conductive. Try using something a bit more electrically insulating. You could start a fire pretty easy, or worse get a little tingle. Most foam packaged with electronics is electrically conductive to protect them from static.

Thanks mate for help. You are writing reasonable things. I will check up and tell results here. Have to go to work now.

Have you tried only connecting one amp at a time to verify they both function properly on their own?

I think self oscillating amplifiers on the same supply can cause interference with each other. If they do both work fine independently of each other, try cleaning up the wiring and put more distance between the two boards. Braid or twist the DC power wires and twist the speaker leads.

Good idea. Firstly I will remove the styrofoam and power up again. But I suspect the minimum voltage issue. Without load and of course both boards working I could remember about 60,2 volts for both monoboards. With load this should be more less. I remember also that I measured DC offset 4,5 and - 4,6 V. Maybe the minus value is also a reason because it is more minimum supply demand sensitive and one channel won't work.

looks like under voltage protection to me , swap the UV protection zeners with one value lower and see what happens .

Cheers ,

Rens

I have 5 L15D's in one enclosure pretty stacked up together on one 1 KW SMPS , no interference ( or any other ) problems :hohoho:

Cheers ,

Rens

So folks, I made a test independent from each other. Powered only one board. The working board shows me with load 59,7 V and the not working one the same value. If there should be a under voltage protection why one works and the other not ? And by the way: The not working board gives light scratching noise every few seconds out from the speaker. Of course when powered but the blue LED is off.

Could it really be that the material parameters from the not working board are closed to the UV ? But if yes, then I would do this zener replacement in last step. I have really suspicous that anything wrong with one board. Better to send them back although shipping costs to china/hongkong makes less sense.

Do you have an SMPS you could test them with. Chances are it's not your PSU, but you never know. As has been said... double check your supply voltages.

Sending things back to China is generally out of the question for me. It takes long enough just to get here! If money, is not too much of an issue, I'd just buy 2 more L25Ds. They are only ~$42USD for the pair. That's pretty cheap for a 175+wpc stereo amp! :) I'm using 2 (L25D) in a nice little rig I put together and quite like the sound. They are however using an SMPS @ roughly +-72vdc. Dead quiet and all fitted into a rather small enclosure. Pics can be found here.

Glad for you that you have less problems with this boards. Double check is done. I used same power connections of the working board for the problematic one.

I cannot understand how sellers could send boards without testing and now these gentlemen (one of them an all time busy chinese) care less in support. I am going meanwhile angry. This is my 2. "defect" board of them:mad:

Hey LJM - Mr. Developer !!! I was also referred to you by the sellers. Please clean your PM box that I can send a message to you.