I am sorry for the late reply. The CS43131 has an Non Oversampling Filter, that one simply does skip the oversampling step.I had a $50 LEC plus a $8 code from Linsoul, so ended up getting FIIO KA15 for $37.
It's a great little toy! Very clean and detailed, leaning a bit towards warmth.
It has filters that I actually hear a difference in, I usually don't. And a 10 band PEQ, so it can have any sound you want.
It doesn't get warm.
Does anyone know what FIIO's non-oversampling filter does? Is it mimicing the sound of an R2R, or is it just skipping oversampling, making it a distortion fest?
Whatever's the case, I really like the non-os filter. To my ears it sounds "less digital" than the other filters.
And the best part: Yes, the wheels spin!![]()
When you disabling oversampling, everything should sound more digital, not less.
When you disable oversampling, you are turning this

Into this

So you make everything sound rougher and much more digital and the soft/smooth SINE wave turns into an stepped SINE wave and hence causes distortion.
This is the Stop Band with the Fast Roll-Off Filter (Excellent Performance, <=-110db)

This is the Stop Band with the NOS Filter (At best, <=-20db)

So when you use the NOS Filter, you turn the smooth analog signal into an very rough digital signal. That means everything should sound much more digital, not less^^