If a critter can drop a level 50 common recipe, they can drop a purple.
Typically thats 47 or higher
The boss has a higher chance of dropping a Pool A recipe (and therefore a chance at a purple) than a minion, though typically you can defeat several minions in the time it takes to defeat a boss.
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Generally, 'only drop from 50 mobs' is pretty much effective shorthand because generally once you're the level to be fighting those you'll be 50 pretty soon anyway.
I personally have gotten a purple drop from a lvl 49 mob. I was running at -1 difficulty and all the mobs in the group I was targetting were 49, so it had to come from one of them. Of course, it was from the confuse set, which is one of the lower value sets, and I only made 5 million off of it, but a purple is a purple.
The Purple Sets follow their own rules:
1.The enhancements are level 50 only. Their recipes drop from Pool A (defeat of foes) very rarely whenever you would normally have a chance at level 50 drops (when fighting critters around 47+).
2.You must be level 50 to slot them.
3.The Set Bonuses of Purple Sets never shut off due to exemplaring more than three levels lower than the level of the IOs (which are level 50).
4.The Special IOs of the Purple Sets which are 'Procs' (see below) have a higher chance of firing (33%) and do more damage that the Procs of the other Sets.
5.You cannot buy them from Merit Vendors. They only come from Pool A drops. The recipes and crafted purple IOs can be put up on the Markets.
This is from Paragon Wiki. . . notice it says 47+ mobs drop purples (Sighted at end of #1)
I have asked plenty of people in game and they all say, you can only get a purple from level 50 mobs. Is this true or false? Is paragon wiki wrong? Next question, does it matter what mob you fight for drop rate, i.e. does a boss drop a purple the same amount of time a minion does? Thanks for answering these newbie questions, just confused from feed back of ppl in game and PW.