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Old 13th October 2004, 04:22 PM   #1
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Default Why did my amp start on fire?

I had built a subamp using 2 OPA541 in non inverting mode, a DRV134 inverting one of them and an LM833 as the crossover.

I had it all working, the sound was amazingly loud and distortion free (except I did notice a couple high frequencies getting into the amp that were really distorted (probably just something with the crossover).

I turned it off, let it cool (I wasn't using a large heatsink to test, didnt let it get too hot though).

I then turned it on again, and cranked the volume (not to max but to close. WOW! the speaker case was shaking and no distortion.

But then, I heard a few pops, hisses and then the transformer started buzzing. Then, a ton of smoke came out of both chips, and they both started on fire. It wasn't hot at all, in fact very cool. This started after about 2 seconds of it cranked.

edit: my trafo is a torrid pulled from a set of logitech z680s. I don't know the amperage, but the secondary voltage is 26.2-0-26.2, way less than the max of the 541. And nothing happened to to the 7812, 7912, DRV134 and LM833.


Why is this happening? And how do I fix it? I only have two more chips so I can't waste any more.

Thanks!
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Old 13th October 2004, 04:42 PM   #2
Netlist is offline Netlist  Belgium
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My guess:
You cranked it too high without a decent heatsink.
The amp oscillates.
Bad connections.

Next time, try to burn in at low volume and keep one eye on the temperature and the other on the scope. The third on the DMM.

/Hugo
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Old 13th October 2004, 04:46 PM   #3
lazyfly is offline lazyfly  Australia
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You say they weren't to hot. Perhaps they weren't mounted properly to the hs. Just a thought.
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Old 13th October 2004, 04:57 PM   #4
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Well, I did have a heasink, it wasn't completely touching the chips, but I only had about 5 seconds before I cranked it.

It is on a PCB so it shouldn't be bad connections.

Even if it isn't hot with a bad heatsink could this happen?

Oh, and by the way, everything was connected with alligator clips as I was just testing it, including the power supply to amp board, input and speaker output.

Should I just replace the chips, and try connecting it properly, burning it in, then cranking it later? How long should I burn it in for?
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Old 13th October 2004, 05:12 PM   #5
Netlist is offline Netlist  Belgium
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Sorry to say but bad solderings also occur on a PCB.
The alligator clips are certainly not the best connections.
There's quite a lot of current running when you "crank" it and a loose connection can't take that. They are really only good to do some test setup, not to make a device work at high power, not even for five seconds.
By "burn in" I meant testing.

/Hugo
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Old 13th October 2004, 05:58 PM   #6
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Location: Wherever I hang my hat...
Check out this page:

http://sound.westhost.com/heatsinks.htm

It gives some great advice about heatsinks, cooling etc etc...
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"Everything should be made as simple as possible, but not simpler." - Einstein
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Old 13th October 2004, 06:10 PM   #7
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I think I figured it out.

Before the chips started on fire, one had a shorted Power supply, so one blew. I guess it took the other with it. Because now that I have replaced both, it sound amazing again (testing with just a +/-20VDC 1A power supply).

I'll let you know if anything else blows up
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Old 13th October 2004, 07:29 PM   #8
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Quote:
Then, a ton of smoke came out of both chips,
It's amazing how much smoke modern technology can pack into a small chip. Very impressive.
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Old 13th October 2004, 08:16 PM   #9
johnnyx is offline johnnyx  United Kingdom
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Quote:
Originally posted by analog_sa


It's amazing how much smoke modern technology can pack into a small chip.
Yes, I've noticed this trend.
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Old 13th October 2004, 08:57 PM   #10
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Its all about the magic smoke!!!!!

the trick is not to let it out!

Now if i could just figure out how to get the magic smoke back in! i would be rich!!!




Zero
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