lm4780 Zobal resistor burns immediately on Power ON - 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 20th July 2008, 10:30 AM   #1
diyAudio Member
 
kuldeepsingh's Avatar
 
Join Date: Aug 2006
Location: Chennai, India
Send a message via MSN to kuldeepsingh
Unhappy lm4780 Zobal resistor burns immediately on Power ON

I wired an LM4780 amplifier as per the special amplifier circuit mentioned in NS LM4780 datasheet. picturs are enclosed.

After making the PCB i connected it to an audio source and speakers and appplied +-25 vDC and it worked fine.

Now today when i assempled it in the body of amplifier and just applied DC voltage (+-25 V). The output zobal resisters "Burnt Off" in the first second. immediately i switched off the supply and checked the ICs, visually OK. But the Zobal resistors were completly burnt.

Now i Did not connected any source or speaker to the amplifier. -> IS that may be the reason that resistor "Rsn" burned off??

please comment, all the four modules of LM4780 are down..
Attached Images
File Type: jpg sch.jpg (16.0 KB, 491 views)
  Reply With Quote
Old 20th July 2008, 10:32 AM   #2
diyAudio Member
 
kuldeepsingh's Avatar
 
Join Date: Aug 2006
Location: Chennai, India
Send a message via MSN to kuldeepsingh
The output resitor and inductor R, L are on down side of the PCB.
Attached Images
File Type: jpg pic.jpg (81.0 KB, 494 views)
  Reply With Quote
Old 20th July 2008, 10:42 AM   #3
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
The Zobel resistor would burn if you had RF oscillations.
Short the inputs to ground and test again .
Never power up all channels at the same time on a new build.
If something was wrong they would all blow up , like here !

Reconnect one channel ( Disconnect all other channels.)
with a good Zobel resistor and short input to ground and power up. If it still burns up , you need to check everything again.
If it's OK , you will need to make it stable with an open input socket .
After checking if it's stable with an open input socket , add another channel and re-test. Add other channels one by one , just in case some wiring loop on any channel is causing instability.
Cheers.
__________________
AM
  Reply With Quote
Old 20th July 2008, 11:00 AM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
fig 5 is a good circuit to build and it usually avoids the worst of the problems that the fewer component circuits have.

BUT,
the three grounds on the left hand side are all INPUT grounds.
The five grounds on the right are power grounds. Be very careful that your layout does not mix these up.

It is very confusing when most use the same word (ground) and the same symbol, but they intend different things. The builder has to interpret what he/she sees and assemble accordingly. Amp building is a thinkers hobby, don't expect to jump in and achieve consistent success unless you make the effort to understand what it is you are trying to build.

Did you install both the electrolytic and the nonpolar parts of Cs?
How close to the supply pins are the nonpolar caps?
__________________
regards Andrew T.
  Reply With Quote
Old 20th July 2008, 12:25 PM   #5
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
the 2.7 ohm resistor should be 2W. I don't see the 0.1uF cap in series.

That particular design should be oscillation proof.

Looking at your pix I think that you have to consider the lead dressing which is a nice way of saying that you should get some tie wraps and neaten it up a bit. If you can't find tie-wraps you can use short lengths of radio shack or CAT5 wire to hold the wires together. The power leads can pick up Radio Frequency Interference and kick the amp chip over into oscillation. You are running a gain of 21 so the chip-amp will do a very good job of picking up RFI.

Make sure that the chipamp is isolated from the heat sink with a mica or Berqquist (insulated) washer -- and the heat sinks should be tied together and connected to ground. Elsewise they are just another surface which picks up electromagnetic radiation.

Put 100nF/50V ceramic caps at each power supply pin to ground.

I don't see where you have put in the 200pF cap between the inverting and non-inverting inputs...
  Reply With Quote
Old 20th July 2008, 02:31 PM   #6
diyAudio Member
 
kuldeepsingh's Avatar
 
Join Date: Aug 2006
Location: Chennai, India
Send a message via MSN to kuldeepsingh
Well, i guess the high freq oscillations damaged the zobal resistors, coz the same thing ws working when input and speakers were connected.
i am putting new resistors and try again tomorrow. here's the pcb layout and the signal ground and power ground are joined by 10 ohm,1W resistance
Attached Images
File Type: jpg 4780pcb.jpg (32.0 KB, 416 views)
  Reply With Quote
Old 20th July 2008, 04:22 PM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
2caps and a resistor are missing between the PCB layout and the schematic.
the assembled PCB has even more missing.

When you replace the burnt resistors, do not connect the speaker.
Use a bulb tester to connect the transformer to the mains and short the input to signal ground.
switch on and measure the output offset.
__________________
regards Andrew T.
  Reply With Quote
Old 20th July 2008, 04:41 PM   #8
diyAudio Member
 
kuldeepsingh's Avatar
 
Join Date: Aug 2006
Location: Chennai, India
Send a message via MSN to kuldeepsingh
Quote:
Originally posted by AndrewT
2caps and a resistor are missing between the PCB layout and the schematic.
the assembled PCB has even more missing.
just let me clear. Cr & Rf2 are Not there in PCB (and these are optional component - won't hinder the basic functioning). Rin &Cc are located on lower side of PCB (though i have not provided holes for putting Cc i n PCB).
  Reply With Quote
Old 20th July 2008, 04:52 PM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
where's L & R.
is there anything else missing?
Rf2 and Cf reduce the high frequency gain by about half (+20.8dB from +26.4dB below 160kHz).
__________________
regards Andrew T.
  Reply With Quote
Old 20th July 2008, 05:00 PM   #10
diyAudio Member
 
kuldeepsingh's Avatar
 
Join Date: Aug 2006
Location: Chennai, India
Send a message via MSN to kuldeepsingh
Quote:
Originally posted by AndrewT
where's L & R.
is there anything else missing?


sorry not to mention them.. They are also on the other side of PCB, and the resistor is inside the inductor coil.
Attached Images
File Type: jpg picture 021-1.jpg (64.7 KB, 393 views)
  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
Precision Power PC6600.2 Amp turns on then immediately turns off. Need help! autolink Car Audio 4 15th March 2009 02:12 AM
output resistor matching for LM4780 paralleled Audiophilenoob Chip Amps 8 7th August 2005 10:03 AM
Philips cd880 player channel dead, resistor burns out canuck Digital Source 1 2nd September 2004 10:38 PM
Kenwood DP3300D tray closes after opening immediately Bernhard Digital Source 1 28th June 2004 12:40 AM


New To Site? Need Help?

All times are GMT. The time now is 12:44 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2