AC-90: Speaker Stand Mount Diameter: 1-1/2" inches
Recommended Stand for AC-90:
ROLAND ST-A95: COMPANION SPEAKER STAND FOR KC AND BA SERIES
https://www.roland.com/global/products/st-a95/
Shaft Diameter: 38mm, 1-1/2 inches
Compatible with BA-330, KC-150/60, AC-90/60, SA-300 top unit.
NOTE: Typical Speaker Stands have Diameter: 1-3/8" inches