THD correlation between DAC - AMP - Headphone for consumer?

[nhatlam96]

Hello guys,
I'm a newbie and a simple consumer and want to understand the practical meaning of the THD value for us.
I did some research and found THD values across: DAC - AMP - Headphone and I wonder how to link these.

For example:
DAC: Pro-Ject DAC Box S: THD 0,005% https://www.pro-ject-shop.de/inhalt/de/pdf/dacboxsusb.pdf
AMP: SMSL SP200: THD 0,00006% http://www.smsl-audio.com/productshow.asp?id=158
Headphone1: DT 1990: THD 0,05% https://www.beyerdynamic.de/amfile/file/download/file_id/4535/product_id/3971/
Headphone2: Focal Elex: THD 0,3% https://drop.com/buy/massdrop-x-focal-elex-headphones/details#details

How I see those values:
A) AMP THD is extremly low and "easily classifies as reference quality headphone amplifier", so the AMP isn't a problem(?)
B) The DAC THD far exceeds Headphone THD, does that mean that the DAC isn't an audible concern? DAC blind test

 

[Fluffy]

THD as percentage is kind of misleading. It's basically an expression of the difference between the signal and the sum of all other disturbances. It's easier to perceive that difference in the form of db, meaning – how much weaker are the unwanted signals (noise and distortion products) relative to the wanted signal (the music).

THD of 0.005% is equivalent to -86 db of attenuation.

THD of 0.00006% is like -124 db.

THD of 0.05% is like -66 db.

THD of 0.3% is like -50 db.

You can get a sense of how hard it is to actually notice these levels of distortion in the test discussed in this thread: https://www.audiosciencereview.com/forum/index.php?threads/distortion-listening-test.8152/

In short, hearing distortion products at even -30 db is not trivial.

Above that, the listed THD values for audio products are a best case scenario, and the manufacturer have every incentive to write the lowest value they can without being completely incorrect. The THD value can differ across the frequency spectrum and listening level. This is what numerous measurements had shown. For example, the highest probability for distortion with the gear you mentioned will be in the low bass area in the headphones, when driven to a high enough volume (which will probably be higher than the mentioned value, which is a sort of an average).

You also need to remember that the value is usually for THD+N, meaning – distortion and noise. The distortion can be low while the noise is high, and vice versa. While distortion is hard to hear as shown above, a high noise floor can easily reveal itself if the device is turned on but playing no music. if there is an audible hiss, that means the noise is too high.

So, that means the AMP will likely not affect the sound audibly regarding THD+N. And your last point is correct – the DAC is almost never the issue.

 

[RayDunzl]

I'm a newbie and a simple consumer and want to understand the practical meaning of the THD value for us.

It can be difficult to eyeball decibels and percents, so I made a silly tool that helps (maybe)

https://www.audiosciencereview.com/forum/index.php?threads/the-shoutometer.2555/