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Old 22nd August 2010, 10:51 PM   #1
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Default Daughter blew up my favourite amp.

Posted this in commercial but had no reply;

While I was in europe my daughter decided to test the limits of my LM3875 Peter Daniels amp, and turned the source right up and now it makes an occilating, drumming noise through the speakers.
Has anyone got an idea of what needs replacing?
Ive been very happy with the amp it is my lounge room mainstay.
Ive had to fall back on an airboarded LM3886 that for some reason does not sound quite as good.
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Old 22nd August 2010, 10:58 PM   #2
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...and now you're just left questioning it, "man was all this worth it?"
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Old 23rd August 2010, 02:01 AM   #3
star882 is offline star882  United States
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Open it and look for burned parts. And the problem would likely be due to poor design - a good amp will have protection against overload.
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Old 23rd August 2010, 02:20 AM   #4
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Dats what I thought.
Apparently not enough protection.
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Old 23rd August 2010, 06:18 AM   #5
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Could it be that the source input was too high and damaged the input?
What type of source are you using?

Worse case scenario is that you need to replace the two lm3875 chips.
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Old 23rd August 2010, 07:00 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by tangmonster View Post
Worse case scenario is that you need to replace the two lm3875 chips.
and the speakers
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Old 23rd August 2010, 08:23 AM   #7
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The source was my daughters ipod shuffle. The speakers are fine they are working perfectly with the LM3886 amp.
I just wondered if anyone recognised the problem and could say, "this same thing happened to me and its the......."
Ill try pulling the chips and replacing them.
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Old 23rd August 2010, 08:31 AM   #8
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yeah but he said "Ive had to fall back on an airboarded LM3886" So i figured the speakers are fine.

I have only had 3 lm3886 broke in the last 4 years of building and using them

1) was my stupidity and I had the +35V rail touch the output pin while connected to my test speaker. Test speaker was gone within a few seconds.

2) I was using a lm3886T with the metal body and i touched the heatsink ground to the metal body instantly making it burn up. Luckily only dummy load connected.

3) I had 3 in parallel and my feedback resistors was not closely matched and 2 chips basically fed current back into the one output pin which destroyed it. This happened only after 'n month of heavy(subwoofer) use on a test system. luckily didn't damage the speaker but if i didn't jump and switch it off it would have happened fairly quickly.
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Old 23rd August 2010, 08:37 AM   #9
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Is there a dc blocker cap on the amplifier?

After my run-in with the 3 chips in parallel. I am going to fit speaker protect to all my current amps.

Currently investigating on building a box that is a speaker switcher and speaker protect system in one.

Still trying to find the correct circuit I can use. Not sure how to design a speaker protect that will detect dc and will work with bridge or non bridge amplifier running on a separate supply from amplifiers? Any ideas would be appreciated.
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Old 23rd August 2010, 09:12 AM   #10
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No no blocker cap. Im very gentle with my speakers and didnt expect to need protection.
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