For a short time after Update 10.6, this mod could have been acquired via Transmutation. This was due to a bug where all mods, including unreleased ones such as this one, became obtainable through transmutation. This has since been fixed, but players who obtained the mod this way were allowed to keep them. There is currently no way to obtain this mod.
It later became re-obtainable as an 'Endurance' reward from Hallowed Nightmares Tactical Alert. However, this too was an error, and was promptly omitted from the drop tables the same day.
In Hotfix 17.9.2, Affinity Amp was removed from player inventories entirely. Legendary Cores were distributed to affected players as compensation.