After setting up sympatic boards, LM4780s on a heat sink and temporarilly connected antec transformers to the boards, I confirmed that both channels are making good music, but with some hum, so I have connected the shield leads of transformer to the heat sink and to the main earth. Made sure there was discontinuaty between chips and heat sinks and heat sink to the ground ( I think it was actually like 0.7 Mega ohm though), Back to input shorted to ground, measured and confirmed AC dc between both speaker outputs to ground to be less than 30mV, connected speakers and there was no hum any more... Shut off the power and then I connected my usb sound card ( single-ended ), but UNFORTUNATELY this time, I forgort to unshort the + input from ground and turned the power back on...
Immediately one of the LM4780 was on fire! So shut off the power right away, but both chip were eminating bad burning smell. And found the computer used for the source was triggered to shut off. Needless to say I put away the usb sound card, never try to do anything after that. Only silver lining was that the computer was fine.

What did I do? I know both LM4780 burned. Would it be worth to get new LM4780s and try to test these boards? I found that Mouser has what appears to be LM4780. Is this right part?

http://www.mouser.com/Se...alkey926-LM4780TA%2fNOPB

If so, could any one let me know how to check the boards without further demage after new LM4780 installed? If I do try again, I will definitely use source like old radio that I will not miss.... Sympatico seems currently is out of stock as well.

Thanks for the help!

Edited by user Saturday, November 19, 2011 7:50:31 AM(UTC)  | Reason: Not specified

Yes. I had the thermal pad between them and tried to measure the conitunity between the legs exposed upside of board, and speaker out and heat sink. I don't remember the values between legs and heasink, they were I think megaohms and between speaker out and heat sinks, DVM was showing infinity.

Thanks for the reply Russ. I took off the boards from the heat sink and I can't find any obvious solder short. For both chips the spark blew away V+ and V- pins. pin number 17 18 and 9 10. The flame I saw seems to be that the spark burning a thermal pad.

Do these tell you something? I don't see any burn marks on the boards other than LM4780, but wonder what other parts could have been demaged and how I could be test them?

Thanks again!

Edited by user Monday, November 21, 2011 2:30:49 AM(UTC)  | Reason: Not specified