You are correct. But all DACs, except for NOS DACs, employ a low-pass filter, which will handily eliminate any RF-noise from upstream. NOS DACs don’t. They, themselves, produce copious amounts of high frequency noise (in addition to whatever is passed on from upstream).
And yet some audiophiles prefer them.
Which is yet another demonstration that all this blather about RF noise from upstream ethernet switches and whatnot is just blather.
Here are measurements of one “well-regarded” NOS DAC. Read the whole thing, but pay particular attention to figure 2.
There are countless other measurements of NOS DACs available with a simple Google search. They all suffer from similar ultrasonic spuriae. Many, but not all, suffer from outrageous levels of harmonic distortion, too (see, e.g., fig 9).
Thanks for the link. I can see that the NOS DAC reviewed by John lacked an output filter and he notes that is consistent with other NOS DACs he has looked at.
There may be a some/all distinction here.
Some NOS DACs do not have an analog output filter. I don’t understand that is because NOS necessarily means absence of such a filter, it just happens to be a further design choice that is made, possibly even commonly or ubiquitously made, by modern NOS DACs. Historically, before oversampling became routinely used, I understand that DACs in CD players had analog low pass filters.
Some DACs which have a NOS option have an analog output filter.
I use a Holo Audio Spring, which is a discrete resistor R2R DAC with a NOS setting. It has 4 boards of resistor ladders, two for PCM and two for DSD. The second boards are used to maintain linearity.
In my case I use HQ Player to upsample to DSD 256 and feed that signal to the Spring in NOS mode. In HQ Player I use the ext2 filter. Archimago has tested HQ Player and found that:
“HQPlayer provides a nice collection of resampling digital filters to choose from. They work without overloading and suppress ultrasonic artifacts very well …”
“These filters would be a great complement if you’re running NOS DACs that can accept high sample rate material”.
I understand from the above that external filtering in HQ Player avoids ultrasonic artefacts that might otherwise result from a NOS DAC lacking an analog output filter.
I haven’t been able to find whether NOS operation of the Spring bypasses the analog low pass filter.
If the input sample is 44.1/48k, then you will get a lot aliasing effect due to nearby image get reflected back to audio range. Aliasing effect is known to cause intermodulation distortion. Some audiophiles find this kind distortion ’pleasing’. On the hand, NOS DAC excels in having no steep digital oversampling filter; this greatly improve the timing response resulted in improved sound. For input sample >88.2/96k and above will show great reduction in noise and aliasing as the sample image is moved further away from the audio range.
Also note that the analog output from NOS DAC is also subjected to further noise filtering (LPF) to pre and power amp to speakers.