I was just curious how the analog inputs of my soundcard (MOTU Traveler Mk I) would deal with very low frequencies. So I sent a cycle~ to one analog out and took it back from an analog input and compared output and input signals.
First, I have a small softening even for audio range frequencies despite of a full balanced output to input connection.
However, there is no significant difference between 100 Hz and 5 Hz. Below that point it’s not so good. There’s not only an expected softening but also some distorsion. Please have a look at the pictures below. The last one shows a 5 Hz to 0 Hz continuous sweep over 8 seconds.
The test was run at both 44.1 kHz and 192 kHz. The results are identical.
Of course one cannot know whether the output or the output is responsible for the results but I’d say both.