Class D output filter Design and component selection

[Amini]

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

 

[airwavesystem]

Hi,maybe you can refer to app note~

 

[nigelwright7557]

if the cutoff frequency is: 4500Hz
Vielen Dank

Around 30,000Hz would be a better breakpoint.

I use 27uH and 470nf output filter.

 

[Amini]

Thank you nigelwright,but I want only human voice passes the filter...
Vielen Dank

 

[DF96]

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.

 

[Amini]

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 ?