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Old 28th June 2006, 07:11 AM   #1
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Default My Lm3886, Dead or not? DC at OUTPUT!

Hi,
Been working on a chip amp project based on lm3886.
I built it all got it working, but thought that 50watts was not enough.
So I thought about adding a second chip and bridge them.
Once they were bridged, all I was getting was DC at the output, around 60volts.

I took it appart and am now trying to get it working again with a single chip.
And no matter what I do I get around 30volts dc at the output.

What could be causing dc at the output?
Are my chips dead?
Thanx for your help!
Mathieu
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Old 28th June 2006, 08:02 AM   #2
Wynand is offline Wynand  South Africa
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What schematics did you use and can you post some pics? Maybe somebody can spot a mistake.

If you used the same board for the single chip the second time round, I'd think something is wrong.
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Old 28th June 2006, 08:48 AM   #3
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I have used the same board both times.
I have removed everything now, and reconnected it through aligator clips so I can move things around easier.
I don't think the chips are dead, the heatsink still gets hot if I let it open for a while.

note that I could not find any 1% resistors for the output....though it should work fine with a single chip....unless I don't get something....
thanx for the help!
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Old 28th June 2006, 02:24 PM   #4
ampmade is offline ampmade  Belgium
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What type of power supply do you use?
Symmetrical or assymetrical?(+/- 30V or +60V and GND)
If you use an assymetrical one it's why you have the 30V at the output .The op amp always work at the half of the power supply if you use symmetrical the midlle point is zero and for assymetrical the middle point is 30 volts.
The two can work but with assymetrical version you must use capacitors to block the dc.
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Old 28th June 2006, 02:37 PM   #5
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ampmade wrore :
Quote:
The two can work but with assymetrical version you must use capacitors to block the dc.
For the bridge version therz no need for the DC blocking capacitor with assmetrical (dual psu).
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Old 28th June 2006, 04:45 PM   #6
sangram is offline sangram  India
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The posted schema shows Parallel, not bridge, which did you use?

For the other chips, what resistor values did you use, were they the same as the first chip (just checking).

It's possible your chip is busted. I would likely think some sort of short caused the chip to blow, or some misconnection.

In parallel mode the offset is slightly higher due to mismatch between chips.

But it should not exceed 150 mV no matter what.

ampmade is right, if your supply is a single supply you have to use an output blocking cap .sivan is also right, if you're bridging you do not need a DC blocking cap on output even with a single supply.

But I suspect you're using a dual supply and getting the +V rail on the output. That is most likely a blown chip, if you're measuring it correctly.

Be also sure that Rin (the first 47K resistor in your schema) is correctly connected. Its disconnection can create huge DC-offsets.
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Old 28th June 2006, 05:05 PM   #7
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the ground pin may need resoldering.
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Old 28th June 2006, 07:41 PM   #8
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I am using a dual supply.
I use 2 transfos
24vac 10amp each.
2 bridge rectifiers and 4 6800uf filter caps.
The center tap of each transfos goes to the center of each capacitors in series.
My ground point is in the complete middle.
I have +35 and -35.

Though I did change the power supply completly.
I use to have a dual supply
with 28vac 4amp *2
making it a +44 -44 supply.
SO, yeah Maby I killed the chips with to much power, but its only at first with that supply that I got everything to work!
Then I tried parralleling 2 chips adding a full second circuit.
Wich did not work (I actually got some sound, but It would stop every 2-3 seconds and would have a huge sort of stuttering hum coming from the transfos) I thought it was my power supply that did not have enough current, so I but in the bigger psu, and never could get any sound after that....

If there is a short in the chips, would the chip heat up?
I am to beleive that all my connections are good, but if the chips are dead, well, yeah this sucks...

thanx so much for your help!
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Old 29th June 2006, 09:19 PM   #9
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alright.
Seems my chips are dead.
I got 2 more pluged one in and I get sound.
And a low 1.5mv dc output!

Thanx for your help!
This amp sounds pretty good through a good speaker!
I will bridge 2 lm3886 if I need more power. As of yet Its not really necessary.

I have 97db 1w/1m woofers and 108db 1w/1m tweeters so a lot of power would be maby a little overkill for my application, at least....for now.

thanx again!
you guys are great!
so much great info around!
Mathieu
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