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Old 17th January 2005, 08:58 PM   #1
kneadle is offline kneadle  United States
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Default What does thermal shutdown sound like?

Does it sound like this: "FZZZZTHUMP!"?

Or is that the sound of a transistor in my discrete preamp shutting down?

Oh, we're talking about the LM3875. I'm learning how to troubleshoot.

Thanks,
Dave

*Edit* It occurred to me that a transistor probably doesn't have a "shutdown" circuit, does it? And everything works properly after turning it off for a while...

heat? bad heatsinking?
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Old 18th January 2005, 06:42 AM   #2
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It don't sound like you describe.

It sounds more like like a old CD-player "chopping" if you understand. It don't sound nice at all, but both your amp and speakers will survive it.

(my ref is the LM3886, but I think it should be the same as your LM3875)

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Thomas
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Old 18th January 2005, 08:38 AM   #3
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Dave, it sounds as though the volume has been turned way down and doesn't increase when you turn up the volume control.

The FZZZZTHUMP that you describe is probably the PSU caps when you turn off. You didn't mention powering down but if you suspect it is a transistor switching off I guess that's what you mena.
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Old 18th January 2005, 11:40 AM   #4
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As the SPIKE protection sets in the distortion starts to increase very quickly -- you can see this if you have a scope with FFT, or a THD analyzer -- the distortion will go from 0.02% to 10% in a few seconds., then the device goes into a low output mode , a mandatory cooldown.

the problem with SPIKE is that there seems to be some hesitancy or latency period before it decides to set in --

if you want to see it enabled, drive the device with the maximum voltage on a smaller than recommended heat sink -- cool the heat sink with a computer fan -- the fan brings the effective C/W down to the recommended level -- when you turn off the fan you can see the waveform just about to clip as the heat rises -- just as the waveform starts to distort SPIKE kicks in and the power drops.
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Old 18th January 2005, 01:08 PM   #5
kneadle is offline kneadle  United States
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OK, thanks for helping me describe this.

This is what happened: I was screwing around with the wiring of my amp and preamp, just trying to understand the concept of "amplification," and I bumped the guts of the thing. I know for sure that one of the chips came off its heatsink a little.

Ever since then, that channel has been a little flaky, and I can't figure it out. However, since I reattached the chip to its heatsink, I haven't experienced a "shutdown" as I described.

But to re-describe: it sounded like a short burst of distortion, but with voltage (hence the ZZZZ sound), then the thump of the cap losing charge very suddenly.

Just before this happened, I noticed that the voltage was swinging wildly between 25mV to 200mV. After the shutdown, the voltage was a steady 25mV, but there was no amperage.

So, I think the voltage is still doing that.

Here's the other variable. When I test it, I use my nOrh 4.0s. When I listen, I use my Hohlflutes , which I believe are much more efficient. So when I test, the voltage stays constant. when I listen, the voltage swings, and I can hear some faint, occasional popping.

Man, I wish I hadn't messed with my amp and preamp setup! It was SO sweet! Live and learn...

Thanks for helping me learn,
Dave

*Edit* Nuuk, I wasn't turning the thing off; the shutdown happened while listening--actually, while I was testing, using the nOrh 4.0s.
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Old 18th January 2005, 01:11 PM   #6
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Check all the solder joints, you may have a dry or cracked one.
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Old 18th January 2005, 01:13 PM   #7
kneadle is offline kneadle  United States
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Heh--

ALL?



I was hoping the electron nation would send a delegate to inform me of the choke point.

*sigh*

Thanks,
Dave
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