OT: how can I measure THD of my amplifier if my sound card is 0,02% THD ?

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Sadly, no.

Assuming you obtained .022% distortion figure using this sound card, you can only say that it is in same order or less distortion than the sound card. You need a sound card with wide bandwidth, like 24 bit 192 kHz sample rate and distortion <,= 0.001% for useful THD estimates.

If you only need to measure at 1 kHz, then a good quality 16 bit card specified to 20 kHz will do. Even older genuine Creative cards like Live or Vibra 16 were low noise and fair on THD. Generic and clone cards usually do not give original performance on noise or THD. Second hand or swap of these from old computers is sometimes fine if you have the genuine part with specs. that suit this task.

I hope this is helpful to you. Happy hunting.
 
I suggest that you consider a USB interface with a laptop on battery. Otherwise you may have issues with mains/line noise/harmonics/ground loops.

There are useful reviews/tests online which can help you select a good interface with suitable noise and THD.

One example: Motu M4 Audio Interface Review | Audio Science Review (ASR) Forum

Besides USB there are reasons to look for other helpful features such as balanced inputs/outputs if you want to measure low THD and noise. Near the bottom of the following linked post (first post in that thread) there is a diagram including a useful balanced attenuator, for example:

https://www.diyaudio.com/forums/sof...-distortion-measurements-rew.html#post5806493


Key features and specifications for a suitable audio interface include USB interface, class compliance, low noise (preamps/inputs), low THD and balanced inputs/outputs.


Interfaces that require additional power adapters can also cause problems with mains/line noise/harmonics/ground loops.
 
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Erm...this thread was opened 11 years ago! Soundcards, and audio testing software have changed and improved by at least an order of performance and flexibility since then. Packaged test systems like Quantasylum's QA400,401 products have also appeared but in any case, I think the OP would have saved up and bought a decent card or given up on the idea by now.