So to use the filters, the ares II has to be in OS mode ?
Not quite. Based on Golden Sound video (this has been discussed in another thread but I could not find the link), it appear Denafrips is implementing a
First order hold DAC.
Standard R2R and Weighted Resistor DACs are usually
Zero order hold DACs. A Zoh DAC generates a sample voltage which is held for the entire duration of the sample period; 22.676 μs for a standard CD. This gives that classic "staircase on a sine wave" look to the explanatory drawings that must be included in all articles about DACs. As a result Zoh DACs have a drooping frequency response. This droop is usually corrected in the post conversion anti-alias filter. The droop starts around 17 kHz, is -3 dB at 20 kHz and -3.9 dB at 22.05 kHz.
Foh DACs interpolate the missing values between 2 adjacent samples by integration. The staircase is replaced with a series of sloping lines. A test signal looks more like sine wave but it's still a bit wonky. Foh DACs do not suffer from droop. Maybe it's less droop; I did not find a definitive answer. Denafrips method of integrating the samples is to over sample so the large step between 2 samples is replaced with a series of smaller steps. Zoh and Foh are valid techniques but both still need an anti-alias filter.
Why does the subjectivist camp which seems to include Golden Sound, (he does do the odd measurement) claim NOS R2R DACs with no anti-alias filter sound superior? Could it be the frequency droop that is the standard behaviour of Zoh DACs.
Finally just in case people have the wrong impression, I am no expert on signal processing. Most of what I said is a rehash of Professor Google. I remember reading all about CDs in the early 80s. I had this vague recollection that many articles mentioned that the anti-alias filter needs to apply a 2-3 dB hf boost prior to rolling off so it was off to the interwebs. The observation about the subjectivist camp; that's mine.
EDIT: Golden Sound not goldenears. Explains why I could not find the link!