Harmonic distortion inquiry

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

Ralph Karsten (Atmasphere), would be the person to either chime in or PM.

Bob

Whatever the answer is, your setup is at a level NOT to be concerned. 

Since THD is calculated as root-sum-square of rms voltages of individual harmonics as a percentage of fundamental I wouldn't just add THDs of two devices.  Most likely it would be again root-sum-square of two (or three) THDs.

Thank each of you for the helpful responses! 

Well it didn't help me much. My brain just exploded all over the floor and I have to mop up!🤣

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%? 

@gs5556 has this one correct.

THD doesn't tell the whole story though. The human ear is fairly unresponsive to the 2nd harmonic but responds a lot (since it uses them to sense sound pressure) to higher ordered harmonics, the latter of which it translates as brightness and harshness.

So the ratios of these harmonics will play a role in what you hear, more so than the actual THD value.