November 24, 2022
 by Paul McGowan

Ever wonder why harmonic distortion figures include the word total? Surely we don't benefit from knowing partial harmonic distortion.

Or do we?

Harmonic distortion happens when higher frequencies that are not part of the music are added to the signal when passing through our equipment chain.

If we take a single tone, say 1kHz, then inevitably when we run it through our equipment, some extra harmonics are generated. These harmonics occur at odd and even multiples of the fundamental frequency.

So, 1kHz is the fundamental frequency in this example. Harmonics are then generated at even frequencies (2kHz, 4kHz, etc.) and odd frequencies (3kHz and 5kHz).

If we look at just one of these added harmonics that’s simply called distortion. If we look at all the generated frequencies in total (2kHz, 3kHz, 4kHz, 5kHz, etc.) that’s called THD or Total harmonic distortion.

THD is all the added frequencies together that are not supposed to be there.


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15 comments on “THD”

  1. Occasionally composers instruct the first or second harmonic to be played on the violin, to great effect.

    THD seems to be an issue that for all practical audible purposes has been overcome many years ago, which is why THD measurements are somewhat redundant.general background noise from electronics seems more of an issue, and that had improved (or disappeared) to a large extent since I started streaming 10+ years ago.

    1. Very much so. THD measurements are close to useless for assessing whether some piece of kit will allow a system to deliver subjectively satisfying SQ; very little effort has been put into understanding why some rigs perform well as a means of "enjoying the music", whereas other don't. In my audio journey it became clear, very early on, that overall integrity of the playback chain is absolutely critical, noise and interference factors are what does the damage - but very few take this seriously enough.

  2. Curiously, mention of THD caused me reluctantly to visit the site that worships good THD. The latest review was of a Schiit $130 DAC, and it concludes: "Great to see Schiit continue the (new) tradition of optimizing objective performance as they cater to their traditional audience." What a load of tosh. I hope they are better at electronics than they are at English. A "new tradition"? A great oxymoron.

    One of my favourite products was the Cambridge Audio CXA81. That range, CXA 60, 80, 80 and 81, have been hugely popular for being feature-rich, value and subjective performance. The THD is acceptable and there is no audible distortion, but it got slated on objective measurement. I don't know about Schiit's "traditional audience" but apparently the company was formed in 2010, 12 years ago. Cambridge Audio was formed in 1968, 54 years ago, and was top end, making the first CD player with external DAC in 1985. So rather more tradition. I doubt that there is a single manufacturer that does not consider the levels of distortion, but it seems just one of the things that goes into the mix and, if there is a new thing, it I to use THD as a marketing tool.

    Devialet is a brand that launched in 2010 on the back of extremely low THD levels, it was all over their marketing. The figures are really quite meaningless these days. What is relevant is when you listen to a digital device and it has a noise floor that is so low as effectively to be non-existent.

  3. THD numbers are often given at 1KHz 1W into an 8ohm

    When you find measurements for the whole ‘accepted audio band’ they are often 10x higher but at the same 1W 8ohm level.

    Why aren’t systems spec’d across at both the 1W and the max rated output power level across the whole audio band? (Answer: because that’s the way it’s always been done?! 😀 ✌️)

    Not only that, when looking at THD numbers there is no way of knowing what the breakdown of the harmonics within the total is. Are they mostly even or are they mostly odd? Of the even / odd, Are they mostly 2nd or 3rd order or something higher? Do the harmonics sum and difference frequency mix with the original tone and themselves?

    So in theory the lower the THD number the less of that type of distortion as stated & measured.
    Numbers look good in print, but are they ‘real world’?

  4. What are the aspects of tubes that typically result in higher distortion vs. their solid state counterparts? Is it inherent with the tubes themselves, or circuit design related?

  5. I have been told by people who make amps that tubes produce more even order harmonics and transistors produce more odd order harmonics. I have never investigated this myself. I have no idea what kind of tubes they were talking about or what kind of transistors they were talking about. I have read in reviews that FET's are better sounding than bi-polar transistors.

    1. That’s probably why Nelson Pass bought out tens of thousands of FETs when the company that produced them went out of business. He knew he could use these FET’s in all of his future designs.

  6. Paul was discussing THD.
    Strangely, he emphasized 'total'.

    Me, I'd have emphasized 'harmonic'

    Because intermodulation distortion is more pernicious to the ears than (most) harmonic.

    And, incidentally, to address an issue raised in prior comments, intermodulation distortion is a useful measure of the circuit's response to 'music' - better than trying to somehow sum THD across the spectrum.

    And good circuit design can improve the linearity of tubes and transistors of all sorts. What tube power amps suffer from is output transformers, which are horribly non-linear (by comparison to good circuitry) and have a time-based behaviour: what they do magnetically *now* depends on what they were doing in the past - hysteresis effects in the core. Plus, the core can saturate.

    Happy Turkey Day to all (but we're having lamb)


    1. That’s the latest I’ve been hearing and reading Pete. I’ve known that for quite few years that’s why the Toroidal Transformer came into being.

      There are some manufacturers that are making their own output transformers, which I believe solve many of their inherent problems

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