Hi Cathryn and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, bear with us; re-confirm and post back with the complete model number of the "...Sternberg mixer..."
Appreciate the additional information.
What external mic would be used please?
Have you considered/tried the XLR connector at the front for MIC?
Sorry but the Rode NT1-A mic would not directly work with the iMac as it needs phantom voltage to operate (not unless there is an add-on power source 24/48 V).
If connected by XLR to the UR12, does the NT1-A work?
Have you checked/turned on the "+48V" switch at the back of the UR12?
Let's review the basics + the above:
I understand though I am inclined to suspect the mixer.
Still, good luck with the UR12 and the NT1-A.