A proof of concept prototype for a high yield, high altitude, 30 block radius, spherical mob spawner floating just below max build height.
The inner surface is constructed at the ideal mob spawning distance from the player who stands in the middle. All the mobs can move, none can despawn, and they can't see the player. The lower half is cyclically flushed by a ring of dispensers wired to a piston clock. The mobs are collected and killed, and the items are delivered to the player via an automatic item elevator--even the xp orbs if you want to do some archery practice.
The inner surface is constructed at the ideal mob spawning distance from the player who stands in the middle. All the mobs can move, none can despawn, and they can't see the player. The lower half is cyclically flushed by a ring of dispensers wired to a piston clock. The mobs are collected and killed, and the items are delivered to the player via an automatic item elevator--even the xp orbs if you want to do some archery practice.
Download: http://www.mediafire.com/?j14mprk997diq9j
I might improve it with shelves of spawning pads in the upper hemisphere, also built in the mob sweet spots.
I'll probably build this next in survival on my server. Most likely I'll get up to it via a nether portal.