MQA DACs have had two different issues related to the addition of MQA capability. Each issue comes from the need to deal with the same core issue: how to handle playing a sequence of tracks that are a mix of MQA and non-MQA (e.g. streaming or shuffle/random play of a library).
Some DACs exhibit a momentary click or similar noise in the transition between MQA and non-MQA tracks (or non-MQA and MQA tracks). I don't know for sure what causes this, but my understanding is that it comes from switching the digital reconstruction filter while the music stream is playing. MQA "final render" decoding with a DAC assigns a supposedly DAC-specific reconstruction filter, and that filter might be different from either the DAC's normal default filter, or from the user-selected filter (if the DAC allows the user to do that). So I believe the click comes from the automated on-the-fly change of the filter.
Other DACs, when they added MQA capability, simply switched the MQA filter on and left it on, presumably to avoid the problem noted above. This avoids clicks or any other obviously noticeable operational nuisances. But it does force all music to be played with an MQA-dictated reconstruction filter. As
@ClicketEKlack notes above, this might very well be a non-issue sonically. But from a technical point of view, MQA filters do not have the characteristics that one would desire if one reads up on and is persuaded by the technical literature on filters.
@amirm ,
@mansr , and many others here can elaborate if need be, but basically MQA filters are "leaky," with poor suppression of ultrasonic frequencies.
So owners of the D90 or any other MQA-capable DAC should very easily be able to tell you whether or not the unit exhibits a click between tracks when switching between MQA and non-MQA sources. But if the DAC permanently engages a MQA reconstruction filter, they can't necessarily tell you that. You'd have to search around and see if the manufacturer or anyone else has posted info about that online.
