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Old 7th November 2012, 04:20 PM   #1
dvisves is offline dvisves  United States
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Default Testing Class D amplifiers

I have ruined two Class d modules by testing them in the conventional way.I attached the signal generator to the input and loaded the module with a wirewound 50 watt resistance of 8 ohms.I used a compuper based oscilloscope to monitor the out put waveform.As i increased the input signal the heatsink got very very hot and the module stopped working.

I need to explain the reason why i started testing the class d module.There was inconsistency in the advertisements for the amplifier module.The ads said 100watts rms at 4 ohms.Assuming that the the Class D has an efficiency of 85% which is typical,the heatsink would have to dissipate 15watts per channel and total 30 watts for stereo.The heatsink attached to the module was tiny and could not dissipate 30watts by no stretch of imagination.Hence my experiment to determine the true rating of the amplifier.
Can any body point me in the right direction.
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Old 7th November 2012, 04:30 PM   #2
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Perhaps if the wire wound resistor is too inductive, it can load incorrectly the amplifier, and may did it to oscillate, and then kaput.
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Old 7th November 2012, 08:56 PM   #3
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I have run my class d designs quite loud without heatsinks with no problems.
Maybe the amp has a tight deadtime and the mosfets cross conduct?
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Old 7th November 2012, 09:56 PM   #4
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Often what people do with low cost Class D systems is design thermally for music, which has a high peak to average power; even B&O modules are NOT designed thermally to run for full power sine wave testing for a significant time, especially not at high frequencies. They work Ok for music, though.

Also, many CLASS D modules aren't particularly "happy" if you clip them continuously, so always be sure to monitor the output waveform as well as RMS voltage, and take some care about going beyond the rated power, and also be sure to monitor the power supply rail voltages, to be sure they aren't drooping.
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Old 7th November 2012, 10:36 PM   #5
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Schematics? Part numbers? Pictures? Power supply details?
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Old 7th November 2012, 10:49 PM   #6
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Some class D amps have floating outputs. Meaning that none of the outputs are connected to ground both the red and black are active and driven. by connecting the output to your PC based O-scope to the speaker outputs, you grounded one side of the amp. if this amp has floating outputs, then you just shorted half the amp to ground!

With the amp off you should be able to measure from each speaker terminal to ground. IF one of the terminals is ground referenced then you should measure a low resistance. either zero ohms or maybe 1-10 ohms to ground depending on how the amp is built. if you have a much higher resistance to ground and the resistances seem to be even between red to ground and black to ground then the output is most likely floating.

Zc
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Old 8th November 2012, 01:41 AM   #7
dvisves is offline dvisves  United States
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Thanks guys for the feedback.The class d amp in question is a TDA8920 amplifier module driven by Dual votahges of +29v -29v.
I have been reading some aspects of testing class d on the audio precision site where they us a prefilter designed to AES 17 standard to get rid of the high frequency and only get the audio envelope.I was wondering whether because i didnot use this filter the module went kaput!
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Old 8th November 2012, 01:46 AM   #8
dvisves is offline dvisves  United States
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the configuration is a stereo single ended one according to the datasheet
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