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Old 8th July 2012, 07:37 AM   #1
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Default ClassD output filter equation.

While searching for design information on classD amp, I came across this information hosted on national.com.

Audio Fundamentals Course 5 - All About Class D Audio Amplifiers

In the link they have given equation for solve the values for L & C for the given RL.
I have put them on mathcad sheet. There if found discrepancy in results.

The equation they have mentioned for finding L & C don't get the same frequency when verified using the mentioned equation.

wondering whats going wrong.

Attached the pdf of mathcad results.
Attached Files
File Type: pdf Mathcad - LCFIlter for ClassD.pdf (32.4 KB, 76 views)
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Old 8th July 2012, 03:12 PM   #2
JinGeo is offline JinGeo  Slovenia
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Try with

L=RL/(2*pi*Fc)
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Old 8th July 2012, 07:23 PM   #3
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thanks JinGeo,

I tried as you said, I could equations matches. now the Q changes to 1. where as it was 0.8 prior. which generates hump. ( i simulated it using the values.)

on the national site they say Q should be 0.707.
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Old 8th July 2012, 07:35 PM   #4
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I am still seeing lot of confusion.

got another site ,http://www.eetimes.com/design/audio-design/4015847/Understanding-output-filters-for-Class-D-amplifiers

THe equation seems ok here. even with Q calcualtions.

But the simulation of graph do not agree with this. it shows small bump. so the Q must be more than 0.707
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Last edited by Aucosticraft; 8th July 2012 at 07:39 PM.
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Old 9th July 2012, 01:57 PM   #5
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Let me ask you a question.


Most driver impedance curves rise pretty much in the higher audible frequency areas.

Why is this not considred in those formulas???
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Old 9th July 2012, 09:08 PM   #6
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Sound check, I agree with you.

Each speaker must have zobal network connected to each driver. and so calculated. Speaker Zobel / Impedance Equalization Network Circuit Calculator
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Old 10th July 2012, 08:26 AM   #7
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Most of the Class-D output stages come with Zobels on the output.
These obviously won't be "One Size Fits All" configurations.

I'm wondering how you'd match that Zobel with a complex impedance ( multi ways + crossover )


How exact does it has to be?
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Old 10th July 2012, 09:36 AM   #8
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...probably it requires a Zobel in front of each driver, similar as it's done with Tube amps
...best would be to run active setups with perfect amp/driver alignment
and no crossover in between

...then. What's actually in for us if we'd put that much effort into such a project?? Do we really gain that much?
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