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Old 14th January 2004, 06:16 PM   #1
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Default LM3886 problem

hi guys,
I am new in this: hope u guys can help. recently Ive built an amp using the LM3886 IC. I followed the exact schematics from the application datasheet. When the amp is first turned on, it sounds really sweet, untill something like 5 minutes. then it started to distorts heavilly, and the volume decreases. After shutting it down and turning it on again, it sounds ok, and the whole distortion process starts again....-(
I suspected thermal problems, but Ive changed the heatsink to a super duper large one, Ive even installed a fan to cool it. still the problem stays. can anyone help me with this? thanks
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Old 14th January 2004, 09:15 PM   #2
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What speaker are you running? and... how big of a heatsink are you using? When it starts distorting badly, feel the heatsinks you are using...see if they are scorching hot...i suspect they are...which means they are too small or you are running a speaker that this amp can't handle

-Matthew K. Olson
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Old 15th January 2004, 12:05 AM   #3
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You may want to post this in the chip amps forum if you haven't already.

I agree you should check your heatsink temp and the top of chip (metal part, t model) for burning heat and if there is a noticable diff in the temp's between the 2 you may need to look at your chip to sink junction.

Unless the chip is faulty or your power supply/load are incorrect this is all I can think of.

Could you post a pic of your setup in question?
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Old 15th January 2004, 04:42 AM   #4
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I seem to recall something similar, where it turned out the centre tap of the transformer wasn't connected to ground.

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Old 15th January 2004, 06:29 AM   #5
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Default yep

that was the problem with my second set of gainclones.

I had connected every ground cable that was there, except I had overlooked one thing, to test the IEC receptacle

You see I recycle them from PC enclosures, and after i remove them i trim then, it is to say I desolder the pieces of wire that hang on the rear end. Well I do that with a solder GUN 100 watts.
I must have burned something while doing so because after I checked out the ground wiring( i did that after havind sought for 2 days... to test oout everything from the schematic to my building outlay, I noticed it was obstructed because of the faulty receptacle.

and the description you give is exactly the same !
same thing!!! first few minutes nice sound, then hum and some music at very low volume! Must be the groundloop somewhere obstructed from going to the earth......

J-P.
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Old 15th January 2004, 07:46 AM   #6
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skyraider, can you take a photo? Do you use the TF package and a small heatsink?
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Old 15th January 2004, 06:47 PM   #7
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thanks guys, i am going to check on my earth line again. Matt, youve mentioned about speakers, that the amp could not handle. Currently I am using a pair of cheap speakers salvaged from a panasonic hi-fi, rated 8 ohms. Would this effect the amp?

another thing, Im pretty sure my heatsink is big enough, could it be the thermalpaste that i am using is not conductive? I will try to post the photo after i can rob a camera off someone-)
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Old 15th January 2004, 08:16 PM   #8
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I had the same problem with one of my gainclones when I first started biamping with passive XO.

The problem was crossed wires (0 ohm impedance) in the crossover.

It would play nice for a few seconds or at low volume, but then started to distort big time.
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Old 15th January 2004, 08:58 PM   #9
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Skyrader, if you have a multimeter, stick it in the terminals of the speakers and give us a reading of the resistance in ohms. ...do this to both speakers, you may have a short

-Matthew K. Olson
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Old 17th January 2004, 04:20 PM   #10
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matthew,
it is 6.xx ohms. I am using a cheap sunwa multimeter
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