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Old 18th November 2012, 06:42 AM   #1
Amini is offline Amini  Iran
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Question Class D output filter Design and component selection

Hi Everybody,
A BIG .... question: is anybody sure about the output filter calculation?
and really has anyone built the filter?
Actually I have calculated according to ClassDAmplifierFAQ.pdf .
if the cutoff frequency is: 4500Hz
then L1,2(Iron Core) will be 100uH, Cs1,2(Electrolyte Nonpolar) =0.56uF and
Cdif(Electrolyte Nonpolar) = 5.6uF
but the quality of output sound is terrible.
is something wrong with calculation or maybe the type of components?
the attached is the schematic of Amplifier and output filter.
Vielen Dank
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File Type: pdf Schematic Prints.pdf (13.9 KB, 114 views)
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Old 18th November 2012, 09:30 AM   #2
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Hi,maybe you can refer to app note~
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File Type: pdf ti - Class-D LC Filter Design.pdf (525.3 KB, 97 views)
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Old 19th November 2012, 11:04 PM   #3
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Quote:
Originally Posted by Amini View Post

if the cutoff frequency is: 4500Hz
Vielen Dank
Around 30,000Hz would be a better breakpoint.

I use 27uH and 470nf output filter.
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Old 20th November 2012, 04:05 AM   #4
Amini is offline Amini  Iran
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Thank you nigelwright,but I want only human voice passes the filter...
Vielen Dank
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Old 20th November 2012, 11:34 AM   #5
DF96 is online now DF96  England
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Put a filter on the input too. A Class D amplifier is meant to see an approximately resistive load at signal frequency. You have given it a highly reactive load at upper audio frequencies, so you need to ensure that these frequencies don't appear at the output.
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Old 21st November 2012, 03:53 AM   #6
Amini is offline Amini  Iran
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Hi DF96
voice is acquired from PC microphone and transmitted through UDP socket to PIC18F97j60 then Micro sets the PWM duty cycle, and finally the amplifier stage.
it was the project general description.
Now input filter... at which stage ?
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