Smoke at LM1876's output resistor having inductor - diyAudio
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Old 27th February 2012, 02:22 PM   #1
pra3718 is offline pra3718  India
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Default Smoke at LM1876's output resistor having inductor

Now I am busy with Bi-amp with LM1876. I build LM1876 as per datasheet's page # 2 Typical Application circuit. Changes are.

1) Added Output resistor with inductor i.e. 10R resistor wound with 0.5mm enamelled copper wire.

2) Ci cap for Tweeter Amp used 1uf.

3) Ci cap for Woofer Amp used 100uf.

4) Gain resister used 4.7k/100r

Observations :

1) Output resistor with inductor running very hot. And after 2 min. it smoked. (copper wire burned)

2) I skipped output resistor then I found Diode at power supply running hot.

3) I replace 1uf with 100uf at tweeter side amp then Amp was stable. But I damaged cheap tweeter.
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Old 27th February 2012, 03:02 PM   #2
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chk the resistance of inductor.
0.5mm dia looks quite thin. it should be atleast 1mm thick.
and whats the wattage of output resistor, it should be atleast 1 Watt.

whats the condition of zobal resistor..
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Old 28th February 2012, 09:24 AM   #3
pra3718 is offline pra3718  India
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Quote:
0.5mm dia looks quite thin. it should be atleast 1mm thick.
yes, i will try.

Quote:
whats the wattage of output resistor,
1 Watt.

Quote:
whats the condition of zobal resistor..
No zobel.
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Old 28th February 2012, 09:26 AM   #4
pra3718 is offline pra3718  India
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My doubt is Ci cap, is there any harm to use 1uf ?
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Old 28th February 2012, 02:15 PM   #5
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Ci cap is no problem.
First put a zobal network of 2.2 ohm, 2W and 0.1uf cap. If it also get burn, this means there are high freq stray vibrations across the feedback loop. put 200nf cap between +- signal inputs of IC and also try to use the special application amplifier circuit as proposed by NS in LM4780 datasheet.
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Old 28th February 2012, 03:05 PM   #6
pra3718 is offline pra3718  India
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Quote:
put 200nf cap between +- signal inputs of IC
Is this 200nf or 200pf
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Old 28th February 2012, 09:30 PM   #7
tuhkam is offline tuhkam  Estonia
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I just finished the Auxiliary Amplifier Application
Circuit on page 6 - it seemed a more complete setup than the one on 2nd page.
Changed:
- Cin from 1uF to 3.3uF
- Ci from 10uF to 22uF
- Rf from 20k to 47k

The amp runs well, no signs of overheating components, it has been running at almost full blast for 3-4 hours several times. Supplies are +/-20V.

Output inductor is 10 turns of some 0,8-1mm copper wire around 1W resistor.

I wonder what could be wrong with your application. Do you use an input capacitor? Perhaps some DC has made it's way in.
Maybe you should not use such low values for gain resistors. My amp works with 47k/1k, I have not tried 4.7k/100R.

Hopefully you get it working, it's a nice chip!
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Old 29th February 2012, 09:48 AM   #8
pra3718 is offline pra3718  India
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Quote:
Supplies are +/-20V
Is that DC ? or AC

Quote:
Do you use an input capacitor?
No, No input cap used. but I will use it no problem.

Quote:
Maybe you should not use such low values for gain resistors.
Yes, I should immediately replace to 47k/1k



tuhkam : I still fighting for MUTE/STAND-BY. How do you manage it ?

Thank you for your reply
Best Regards.
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Old 29th February 2012, 10:56 AM   #9
tuhkam is offline tuhkam  Estonia
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+/-20V is DC, after it has passed the rectifier and filter.

I'm still having problems with the mute (I left the standby pins floating). I connected both mutes together on the PCB and routed them to a pad where I can now supply voltage from outside the PCB.
I used 2V, but it didn't work. I will try it with 5V next. The datasheet mentions a "logic high", but a graph there suggests it should mute with a couple of V already.

A pic of the thing! The yellow/brown wire is where the mutes come together.
The empty holes on the lower edge are summed mono line output for sub.
Click the image to open in full size.
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Old 29th February 2012, 11:41 AM   #10
pra3718 is offline pra3718  India
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Thank you for posting picture which is very clear and informative. About MUTE/STAND-BY same story with me. I have also connected MUTE/STAND-BY together for both amps and left floating.

@tuhkam : I am looking for the solution, you are requested, if you got it. please post.

Thank you tuhkam
Best Regards.
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