I believe it depends whether the DAC is powered via the connected device or the DAC's internal power supply.
For example, I demoed the new Creek 50CD earlier this year and had intended running a 5 mtr long USB cable from a laptop into it. As the USB input in the 50CD was powered through the cable it did not recognise that a device was connected as it was too long. Creek informed me that a 3 mtr cablle was the maximum length it could support, but actually when I tried a 3 mtr cable that wouldn't work either!
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