For two reasons, (I believe previously covered in other Clock threads);
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An external Clock is required in order to keep multiple units, like in the case of the Vivaldi stack, fully synchronised.
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But more importantly, a external master Clock with its own dedicated Power supply, isolation chassis, and control circuitry will be technically superior to one that’s built-in and has to share resources with the DAC.
In the case of MSB, since they don’t have an external master clock, their introduction of their Digital Director - effectively similar to the Vivaldi Upsampler - makes things very complicated They’re likely using some proprietary way to keep the units properly synchronised; not quite sure how exactly, probably exporting the DAC’s Clock over the Control Link Module. Greg @PaleRider, do you know?