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measuring THD audio amp

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surz90

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Hi all,
currently, i'm making a filter-free class D audio amp for my project.
i measure output THD from my amp using spectrum analyzer and get THD = 1.84%. quite high.. :-(
and i also measure THD from my function generator as an input in my amp. and i get THD = 0.32%.
my question is, is my THD amp = 1.84% - 0.32% = 1.52?
can i subtract it directly??

thank you..:)
 

Dear surz
Hi
A good THD ratio for a class D amp will be lower than 0.1 percent . which topology you are referring to ? Half bridge ? full bridge ? differential three level SPWM ? ..... ? because it seems pretty unusual . how much is the frequency of triangular wave ?
Best Wishes
Goldsmith
 

Hi goldsmith, thank you for reply
actually,this is the 1st time i make an amplifier, for my final project i choose class D amp because i think it is interesting.
i use three level modulation, and in power stage i use H-bridge with MOSFET.
and for modulator i use noise-shaping coding technique.
this is my journal reference
Yu, S. H. dan Tseng, M. H., “Modulation and Control of a Three-Level Class-D Audio Power Amplifier”, IEEE, 2005
frequency sampling that i used is 400kHz.
i use 3rd order filter.
from spectrum analyzer i see that noise-shaping work in my amp, but the noise floor is high (-40dB) in audio frequency and because of that i think the THD of my amp will be high.
until now i try to find what is the cause..:???:
 

Hi again
Can you show me your power stage circuit , please ? and value of your out put filter ?
BTW : why 3rd order filter ? you should use 4 order filter . first one is a notch filter and the other one is LPF .
BTW : did you see the spectrum of your SPWM ? perhaps it is not good ? do you know how it should be ?
Best Luck
Goldsmith
 

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