Not necessarily singletrack, but a nice loop:
B4401 through Llandrillo
At Rhydyglafes, R & climb to Wayfarers Memorial. Cont. alonsigside Nant Rhywilym, to L D Ceiriog.
Folllow tracks of choice south of B4500 line, to meet B4579, and head NW to Glyn Ceiriog (lunch?)
Head up through Ceiriog forest, & drop down to Glyndyfrdwy
Cross R. Dee, R to Rhewl, & track up, over & along flank of Llantysillio. Drop back to Carrog.
Cont. B5437/A5104/A494 to Ty'n-y-cefn, SO TL & follow dead-end road.
Wade Afon Alwen
& cont. ~1Km minor rds N. of Dee.
Cut up onto Mynydd Mynyllod, drop down to LLanderfel.
Back to Bala & bathtime...