Is my LM4780 burnt ? - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th March 2015, 10:59 AM   #1
Kgs is offline Kgs
diyAudio Member
 
Join Date: Nov 2013
Default Is my LM4780 burnt ?

Hello everyone,

I built a LM4780 2x60W amp based on Veteran (Unisonus) PCB.

I had a problem when puting it first in the metal case with the heatsink connected to V- which shorted to GND and made it burn. I checked all other components (seemed fine) and replaced IC, plus isolating every GND from chassis and V- from heatsink.

Since then it worked quite fine but sometimes one channel had a problem : loud hum and no source anymore. In that case I used to simply shut down the amp and all would be fine when restarting it. Never had the time to understand from where it came from.

It didn't seemed related to temperature as sometimes it started directly when powering on amplifier.

Now my problem is that this channel is definitly broken. I measure between OutB and GND -20VDC with input shorted. When I remove mute resistor I get the same result.

I checked every resistor/capacitor and everything is symmetric between channels. I guess the IC is dead.

I ordered new one but was wondering why did it happen ?
Anything I should check ?
Anyone experienced the same problem ?

Thank you,
Julien.
  Reply With Quote
Old 9th March 2015, 12:26 PM   #2
diyAudio Member
 
Join Date: Sep 2004
Location: Minneapolis, Minnesota, USA
Can you post the schematic? Can only guess without more info.

Mike
  Reply With Quote
Old 9th March 2015, 12:43 PM   #3
Kgs is offline Kgs
diyAudio Member
 
Join Date: Nov 2013
Sadly no the website went offline.

https://web.archive.org/web/20120209...ka-drukowana/7

I think it was very close from the layout in the datasheet Click the image to open in full size.
  Reply With Quote
Old 9th March 2015, 05:49 PM   #4
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
If you short VEE to GND through the case, the chip draws significant current. This usually fries the chip. In some cases, I've seen PCB traces burn as well. You've already replaced the chip, so my money if on a vaporized trace. Inspect the PCB closely to make sure all connections are solid.

~Tom
__________________
Modulus-86: Composite amp achieving 0.00014 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com - Engineering : : Done : : Right
  Reply With Quote
Old 9th March 2015, 06:58 PM   #5
diyAudio Member
 
Join Date: Sep 2004
Location: Minneapolis, Minnesota, USA
The output spectrum shown in the link you posted is hard to read, but it looks like there's a lot of HF garbage there. I'm guessing the power supply bypassing is probably inadequate and the layout might not be the best, which can cause HF oscillation. Is there any HF filtering at the input? What are you using for the power supply, and what is it's output voltage ?

Mike
  Reply With Quote
Old 9th March 2015, 08:51 PM   #6
Kgs is offline Kgs
diyAudio Member
 
Join Date: Nov 2013
Quote:
Originally Posted by tomchr View Post
If you short VEE to GND through the case, the chip draws significant current. This usually fries the chip. In some cases, I've seen PCB traces burn as well. You've already replaced the chip, so my money if on a vaporized trace. Inspect the PCB closely to make sure all connections are solid.

~Tom
Wow amazing ! You saved my amp :-) I didn't thought it could happen.
I discovered that pin 12 was not anymore linked to GND. I soldered a strap and voilą.

Now only problem is little noise (noticeable when source is silent).
I took the GND from one leg of a capacitor, do you think if I take it from my star ground connection point it will improve this ?

Anyway thank you very much for the advice.
  Reply With Quote
Old 9th March 2015, 08:55 PM   #7
Kgs is offline Kgs
diyAudio Member
 
Join Date: Nov 2013
Quote:
Originally Posted by Michael Bean View Post
The output spectrum shown in the link you posted is hard to read, but it looks like there's a lot of HF garbage there. I'm guessing the power supply bypassing is probably inadequate and the layout might not be the best, which can cause HF oscillation. Is there any HF filtering at the input? What are you using for the power supply, and what is it's output voltage ?

Mike
The power supply is made of a toroidal transformer 300VA 2x25V (when I measure it is more about 33V) which is then rectified by diode bridge.
There is only 2 small capacitor (100nF) near power supply.
  Reply With Quote
Old 9th March 2015, 09:19 PM   #8
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
Quote:
Originally Posted by Kgs View Post
Now only problem is little noise (noticeable when source is silent).
I took the GND from one leg of a capacitor, do you think if I take it from my star ground connection point it will improve this ?

I don't think a star ground will help. Look for burnt traces to VEE. You probably have a bad connection somewhere.

Glad I could help.

~Tom
__________________
Modulus-86: Composite amp achieving 0.00014 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com - Engineering : : Done : : Right
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
4 x LM4780 mono, OR 2 x LM4780 stereo: Will I notice the difference? mondogenerator Chip Amps 23 1st November 2013 05:40 AM
IRFP064 gets burnt xyz9915 Power Supplies 30 5th February 2009 02:57 AM
Need Help - Burnt Sanken p.heimpold Solid State 16 30th May 2007 05:05 PM
burnt transformer A'af Digital Source 1 20th May 2005 11:23 PM


New To Site? Need Help?

All times are GMT. The time now is 02:50 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright ©1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2