Just pointing out the obvious, but you need to have the amulet equipped when you consume Source, so the Source will go into the amulet instead of to you.

There's a Source tank in Arx which you can use to replenish Source indefinitely. It's a bit hard to get to. SPOILERS:

You have to deal with Linder Kemm and his vault.

"We make our choices and take what comes and the rest is void."