lm4780 Zobal resistor burns immediately on Power ON

[kuldeepsingh]

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..

[kuldeepsingh]

The output resitor and inductor R, L are on down side of the PCB.

[ashok]

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.

[AndrewT]

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?

[jackinnj]

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...

[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

[AndrewT]

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.

[kuldeepsingh]

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).

[AndrewT]

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).

[kuldeepsingh]

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.