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Old 12th January 2007, 05:11 PM   #1
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Unhappy Problem with LM4780

Hello,

I have a problem,
I found circuit in this forum:
www.e-ideja.com/dielkius/schematic.gif

And I made PCB for that:
www.e-ideja.com/dielkius/pcb.jpg
www.e-ideja.com/dielkius/pcb2.jpg

And there is a part schematic:
www.e-ideja.com/dielkius/parts.jpg

I have 2 transformers, one 33-0-33 and one 15-0-15 (there is values after diode bridges). I connected +33 G -33 then I conected +15 -15 and that transformer ground connected with PCB ground.

And a problem: When I turn it on nothing happens, LM's get warm, but speaker silent, no buzzz or etc. Whats wrong? Maybe you can help me..
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Old 12th January 2007, 05:40 PM   #2
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probably impossible to tell you what went wrong.
It needs some troubleshooting with scope, signal generator and so on
at least there is one design error on the schematic...
a bias resistor at U4 is missing, you could try 47k from pin 3 to ground.
without a path for the bias current the operational amplifier could latch up. Maybe thats just your problem.
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Old 12th January 2007, 06:09 PM   #3
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btw. are the inputs of U5/U6 shorted?
Quote:
I have 2 transformers, one 33-0-33 and one 15-0-15 (there is values after diode bridges). I connected +33 G -33 then I conected +15 -15 and that transformer ground connected with PCB ground.
why a second transformer?
Regards
Juergen
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Old 12th January 2007, 08:32 PM   #4
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Second transformer is for bufer and driver (OPA134 and DRV134) I don't use that part of schematic where is LM317 and LM337, I use another transformer to power up that two micro's.. Now i'm going to try put resistro 47k...

Thank you juergenk,
And sorry for my bad english
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Old 12th January 2007, 08:32 PM   #5
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Nop, 47k resistor not help...
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Old 12th January 2007, 10:24 PM   #6
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Default LM4780

Check your mute pins if the mute is alright bypass the ins on the 4780 inject the signal there without the buffer and DRV134
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Old 13th January 2007, 09:14 AM   #7
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Hi halo0925,

Thank you for reply, i tryed to put signal without bufer and driver, first to one LM and it worked, and then to another LM, and it worked too..

So problem is driver or bufer..yes?
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Old 13th January 2007, 12:47 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
I thought chipamps needed decoupling (electro + film/ceramic) right at the supply pins. Neither of these parallel pairs have any local decoupling.
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regards Andrew T.
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Old 13th January 2007, 01:06 PM   #9
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I take another schematic without buffer, just driver (DRV134) and everything works OK..need another buffer, and new PCB
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