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Old 10th December 2009, 08:52 PM   #1
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Default LM3886 PA100 Problem

hi! so this is my first diy amp, and i decided to use lm3886 because of its great audio quality....
I made 2 differets pcbs with one Lm3886 and i tried each one and every LM3886 work! suond is good... I really like that...
now, the two pcbs work and I want to do Parallel configuration 100W on 4 Ohms....
but there is a problem... when i connect two LM3886 in parallel on 4Ohms load they go in protection mode and they overheath...
I 've checked wirings and they are rights, the heath sink is too big for 2xlm3886 so , I haven't got power dissipation problems....

what is the problem??? i don't understad why each pcb work in single mode, and don't work in Parallel mode... help me!!!
thanks and sorry for my bad english...
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Old 11th December 2009, 06:47 AM   #2
ratza is offline ratza  Romania
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Can you post the schematic you used?
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Old 11th December 2009, 10:44 AM   #3
sangram is offline sangram  India
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Do you have current sharing resistors on the outputs? Are you using 0.1% resistors in the feedback loop?

Your problems sound like one chipamp is driving the other. 0.1% resistors are not strictly necessary, I use 1% resistors and a larger output resistor (0.25 ohm) to balance the chipamp out. But output resistors are critical, the amp will not function without them.
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Old 11th December 2009, 10:55 AM   #4
AndrewT is offline AndrewT  Scotland
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the gain of both amps must be matched. that is why you need the 0.1% resistors shown for PA100.
The input offset current of the two amps may interact with each other if you simply parallel them. Do you have DC blocking caps on the inputs of both chipamps?
The 0rX output resistors are a must.
Although the AN1192 does not show the output Zobel, I believe it is necessary and further that every chipamp needs the Zobel on it's output.
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Old 11th December 2009, 10:14 PM   #5
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yes, i've used 0.1% resistors , like blue resistors.... so, i follow my schematic...
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Old 11th December 2009, 10:20 PM   #6
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Quote:
Originally Posted by AndrewT View Post
the gain of both amps must be matched. that is why you need the 0.1% resistors shown for PA100.
The input offset current of the two amps may interact with each other if you simply parallel them. Do you have DC blocking caps on the inputs of both chipamps?
The 0rX output resistors are a must.
Although the AN1192 does not show the output Zobel, I believe it is necessary and further that every chipamp needs the Zobel on it's output.
yes, i've used the zobel on output, and DC blocking caps, but the problem is: if i turn on one LM3886 in "single mode" (not parallel) it work!!!! and if i do a parallel configuration there is the problem...
i post the pcb image
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Old 11th December 2009, 10:43 PM   #7
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When you make a schematic you NEED to make the little dots where you want the wires to connect. Now it's pretty much guesswork as you use the dots somewhere and other obvious places not.

Eagle will warn you about it if you do a Electrical Rule Check(Tools->Erc), I find its a good thing to run that now and again, especially if one change something.
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Old 13th December 2009, 04:56 PM   #8
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Originally Posted by coolbeer View Post
When you make a schematic you NEED to make the little dots where you want the wires to connect. Now it's pretty much guesswork as you use the dots somewhere and other obvious places not.

Eagle will warn you about it if you do a Electrical Rule Check(Tools->Erc), I find its a good thing to run that now and again, especially if one change something.
yes, but only parallel configuration don't work, single config, with one lm3886 on 4 Ohm works perfectly!!
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Old 14th December 2009, 05:49 PM   #9
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Take out C2 and C5 and see, if the parallel configuration becomes stable without them.
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Old 14th December 2009, 09:22 PM   #10
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Originally Posted by pacificblue View Post
Take out C2 and C5 and see, if the parallel configuration becomes stable without them.
so, i've just tried to take out C2 and C5, but i have the same problem, lm3886s goin protection mode and they overheat! what is the problem???
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