I'm digging up this thread in case anyone has run into the same issue.
I now have two different SDU4 units, and both exhibit the exact same behavior, which makes me think I may be missing something rather than dealing with a hardware fault.
My setup is:
SDU4 → Behringer UMC404HD (4 balanced XLR outputs)
Reaper with 4 mono tracks (one per input)
Left and Right outputs work normally. However, as soon as I connect the Center output, all four Reaper tracks start showing a periodic low-level signal/noise (see screenshot), even though only the Center cable was added.
I've already:
double-checked all Reaper routing,
tried both the Behringer ASIO driver and WASAPI,
tested two different SDU4 units,
verified that the issue follows the Center output.
Has anyone experienced this with an SDU4? Is there a known configuration setting, grounding issue, or something specific about the SDU4 outputs that I'm overlooking?