Are The THD Of The Amplifier/player And The THD Of The Speaker/headphone The Same Thing?

- Jul 13, 2020-

Are the THD of the amplifier/player and the THD of the speaker/headphone the same thing?


Or in other words, do amplifiers and speakers with the same THD sound the same thing? Do amps and headphones under the same THD sound the same thing?


No.


First of all, this is a pseudo-proposition. The THD of the amplifier and the THD of the speaker can only be the same on the product parameter label, or the same when the measurement is very simple.


For example, the power amplifier marked its own THD is 0.5%, the speaker also marked its own THD is 0.5%. But I have said many times that neither audio nor acoustic parameters can be replaced by a single value. At least one curve. The THD of the power amplifier is 0.5%, which may be measured only at 1kHz half power, and the THD of the speaker is 0.5%, which is most likely measured at 1kHz 1m 1w. (THD value is just an example)


But in fact, the THD of the power amplifier is different in different frequency bands, and it is even different under different gains. Speakers have a greater difference in THD in different frequency bands. Generally, the low frequency THD of each unit will be larger, but the tweeter unit will have a crossover filter, so in general the low frequency THD of a speaker is higher. And, generally speaking, the THD of the speaker will increase as the SPL increases.


Okay, then we assume a more ideal premise. It is assumed that the THD of the power amplifiers in the entire frequency band are the same, 0.5%, and the THD of the speaker after 200Hz is 0.5%. We only look at the two after 200Hz, does the sound sound exactly the same?


Of Course Not!


First of all, we still have to understand, assuming that the speaker of the dynamic coil unit, usually, the THD of the speaker/speaker will increase with the increase of the SPL (if it is small to a certain extent, there may not be significant changes). This is because in the case of small SPL, the movement displacement of the unit voice coil is small and is always in the linear region, so the nonlinear distortion is small, but when the SPL of the speaker is large, the closer the movement displacement of the voice coil is to xmax, the nonlinear Distortion will increase greatly.


As for the power amplifier, I have tested some level sweep THD myself, but for various reasons, I still found a few test results on the Internet.

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Of course, this is just an example. In many cases, the power amplifier (especially the amp) will not change significantly with the voltage change within a certain level. Even in many cases, THD will increase as the voltage decreases in the level range that is more suitable for listening to songs. The specific reasons for this situation are many and varied, and here is just the following phenomenon.


So back to the problem of the test signal itself, the music is not a sine wave with constant Vrms used in Freq sweep THD, and the music Vpp is constantly changing. At the volume of our daily music appreciation, a few signals in the music will reach a higher level, most of the main energy such as vocals, such as accompaniment pianos will not have too high SPL. If the THD curve is measured under a certain Vrms, for the speaker, usually only the music signal with a large Vpp, such as drumming, is more in line with or exceeds the THD curve under this Vrms. The components of most music are actually It will be played back with lower distortion than this test value. In contrast, most of the main components of music such as vocals and most musical instruments, because the actual Vpp value is smaller than the test signal, it is easy to appear that when the music signal of these parts is played back, the actual THD is higher than the test value. . And the THD caused by this power amplifier will affect every component in the music, and for the speaker, only a small amount of energy in the music will be affected by the nominal value of the speaker test.


So generally speaking, under the same THD, the THD of the amplifier and the THD of the speaker are different. If you want to achieve a more ideal playback system, the THD of the power amplifier and sound source is usually less than the THD of the speaker. This is actually not a question of principle, but a question of whether the test method can reflect the actual effect.


Headphones and amps have the same conclusion. (But for headphones, a significant change in THD will only occur if it exceeds a certain SPL, but usually, it is difficult to achieve such a high SPL at normal listening volume)