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Harmonic distortion inquiry

As much as I’ve searched and read on audio component distortion ratings, I’m unsuccessful finding one elusive answer. Does the THD of each individual component accumulate, or is the effect limited to the highest (noisiest) one component? Simple example question; if each device has .03% of THD for a phone pre-amp, pre-amp, and power amp, is the system distortion .09%? 
 

sdguyer87

Showing 1 response by gs5556

gs5556
1,213 posts
 

THD is a ratio of voltages (harmonics to fundamental) so it can be expressed as dB. For 0.03% THD the distortion attenuation is:

dB = 20 Log [THD/100] or 20 Log [0.03/100] = -70.5

Now all you have to do is add the three devices (-70.5 * 3) and that comes out to -65.7 db. (You can easily google adding dB’s). To convert that to THD:

10^(-65.7/20)(100) = 0.05% THD

 

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