As it has an “Upsample” switch, I assume that when it’s switched off it is not up/over sampling, IE. It’s a NOS DAC. Since digital audio was first proposed nearly a century ago, a fundamental requirement is to bandlimit the signal to half the sampling rate, IE. To have an anti-alias filter when converting to digital and an anti-image filter when converting back to analogue. NOS DACs made in the last 20 years or so for the audiophile market often break this fundamental requirement. This results in certain issues, including a gradual high frequency roll-off starting around 2kHz and extending to 22.05kHz (in the case of a 44.1k sample rate), plus relatively high levels of ultrasonic artefacts (“images”), which are likely to cause IMD downstream. Both of these are within the audible spectrum and therefore many NOS DACs can be differentiated (by human hearing). When some say “all DACs” they obviously don’t include broken DACs, and NOS DACs can be described as “broken”, although deliberately “broken by design” rather than because some component has failed.
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