On (Aune M1p) DAC filter
@DaveStarWalker @Scuba Devils
So, for context, I've researched DAC filters since a while now — purely as a hobbyist.
Researching the M1p, I stumbled upon
DaveStarWalker's post re: filters which got me back into it.
In a nutshell, lots of DAC have filters and it seems that it's up to the integrator to decide whether to make them visible for the end user or not. This applies equally to desktop, portable as well as dongles.
On page 57 of the M1p's
DAC datasheet (which is an ES9038Q2M), you'll see the different PCM filter's impulse response. This is where things get tricky for me — I struggle to find a layman's interpretation of the filters' effects.
The best I have found so far is
here (you'll find similar graphs from AKM as well as other reproductions online) as well as a fairly comprehensible description of their effects
here which I lightly adapted below. Note that they apply to ESS DACs but shouldn't be a far cry from other manufacturers.
Fast Roll-Off, Linear Phase Filter
This filter ensures a high-frequency response, but there is a lot of ringing that does not exist in the original sound source. The phase is in the correct position on the time axis, but pre-ringing occurs. However, the sound is powerful.
Slow Roll-Off, Linear Phase Filter
Starting from around 18kHz, it attenuates gently, operating similarly to human hearing characteristics. The ringing is small. The sound may be a bit dry, but it reproduces the sound present in the original source. It may sound sharp.
Adodizing, Fast Roll-Off, Linear Phase Filter (as per the ES9038Q2M's datasheet, it should be the DAC's default filter)
Similar to Fast Roll-Off, Linear Phase Filter, but the ringing occurs for a shorter time, and the high-frequency band attenuates more naturally. It creates a powerful yet slightly smooth/natural sound.
Fast Roll-Off, Minimum Phase Filter
It ensures a high-frequency response and suppresses pre-ringing. Rich post-ringing creates a good sense of reverberation and a full sound.
Slow Roll-Off, Minimum Phase Filter
Starting from around 18kHz, it attenuates gently, operating similarly to human hearing characteristics, and perfectly removes high-frequency sounds. It suppresses pre-ringing and produces less post-ringing. It delivers a sound close to the original, but compared to Slow Roll-Off, Linear Phase Filter, it has a relatively more reverberation, creating a smoother sound.
Hybrid Fast Roll-Off, Minimum Phase Filter
Similar to Fast Roll-Off, Minimum Phase Filter, it doesn’t completely suppress pre-ringing. Instead, the phase position is slightly closer to its original position on the time axis. In most cases, it’s not easy to distinguish between Fast Roll-Off, Minimum Phase Filter and this one, but there are golden ear listeners who notice the awkwardness. However, there are also golden ear listeners who think it sounds more natural.
Brickwall Filter
Similar to Fast Roll-Off, Linear Phase Filter, but with a higher cut-off frequency and more ringing. It is very difficult to distinguish the sound from Fast Roll-Off, Linear Phase Filter.

I hope this was interesting — and for those in the know, please let me know if anything needs to be corrected.