I've been thinking about this issue with filters & linear power supplies. The way I see it is even if the 0s & 1s data comes across perfect, there still might be an opamp inside the DAC that controls the 2V signal out of the DAC.
So if your 5V power into the DAC is a little under, or the 5V regulators in the DAC are a liitle inaccurate, 'MAYBE' the DAC 'MIGHT' produce some distortion or crackle, which some people might (quite reasonable) believe is interference.
Recently I was told that you can get 'dirty power' where your AC is polluted with DC from other items in your room/system, so I decided to invest in a AG500 AC ReGenerator 500W, which takes your fluctuating AC from your mains power socket any provides a stable with no spikes AC output for all your sensitive equipment.
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AC ReGenerator is a true online double conversion system providing the highest degrees of power protection, with a stable unwavering high quality power waveform for sensitive and critical loads."
I think that cleaning up/regulating your power supplies is better than filtering out noise causing by dirty power, or other low/non stable power issues. So prevent the problem, rather than try to remove the symptoms.
All of these power & filter remedies will totally be dependent on the rest of the system, which is why some people will swear that they can hear a difference & others can't.
I hope this information, based on my own personal experience, might help someone else with a similar problem, and maybe help others from spending too much fixing the symptoms rather than finding the root of the problem.
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I spent the week looking at 'decrapifiers' / USB purifiers, because of what I thought might be interference, but I followed the advice from my knowledgable friend & invested in an AC Regenerator; and I have not had any problems at all.
Now that my system has the cleanest power possible, I am happy that I have reached the limits in terms of quality of my current Audioengine D1 DAC, and want to buy something like the SMSL M8.
If the power supply for the M8 is a SMPS (switched mode power supply), I may be tempted/'talk myslef into thinking I need it', and buy a SMSL P1 Linear power supply, but I am gonna wait and see, and just used the supplied wal wart to start with. If I don't have any problems I'll stick with the supplied wal wart. I think it is worth noting that the quality of SMPS can vary, it is quite easy to get a dud or model that hasn't been tested to within a particular tolerance (even from the same manufacturer). The SMSL P1 looks to on the list of items to test in the future.
But if I do have problems with the SMSL M8 (and have money to improve my system) I'll look 1st to a linear power supply (P1), 2nd to a decent shielded USB cable (no need to go to extremes, Wireworld Ultraviolet 7 0.5m is the best I would get (If I had some money to burn)).
My current mode of thinking (and the advice of my friend) is that USB filters WILL change the sound of your DAC (for better or worse), so 3rd I won't be looking at USB filters.
Reclockers are a totally different conversation I think, and repeaters in USB filter products might help some people 'FIX' their system with long USB cable runs (but it would be better to use a screened/shorter cable in the first place).
So just a final note to say that I can totally believe that some people can hear a difference & others can't, because it is dependent on the other items in your system (including your mains power supply on 'THAT' day).
If you do a search on youtube for 'Audiophile USB Cables - Are they worth it?', you'll find someone who has proof of a difference, but remember that is proof dependent on his setup, and the guy also states clearly that although he can prove that their is a 'difference', it is only his 'opinion' as to which is 'better'.