Nace, the other reply in this thread pretty much explains it. The
original U 87 was introduced in 1967 and the U 87Ai was introduced
as its successor in 1986. They use the same capsule type overall,
but in a slightly different arrangement: the backplates were kept
separate electrically in the U 87, but are connected in the U 87Ai.
Thus the newer one actually follows the same scheme as the old tube
models U 67 and M 269, and uses the same capsule head.

The U 87Ai is about 8 - 10 dB more sensitive depending on your
pattern choice, and 3 - 6 dB quieter in terms of "equivalent" noise
level. The one thing that was lost in the upgrade was about 5 dB of
headroom--the mike clips at about 117 dB SPL now, instead of the
ca. 122 dB SPL limit of the older model.

I wonder why they decided to make it SO much more sensitive than
before, when they could just as well have split the difference! The
U 87 is often used for close vocal miking, and there are singers who
can produce 120+ dB SPL. For that reason I prefer the older type of
U 87, but that's clearly at the cost of increased noise levels.

The one place where I disagree with J.P. Gerard's reply is that I'd
say the newer amplifier circuit has the same sonic qualities as the
older one (not "more open sounding")--it just has a different gain
structure and a DC/DC converter that boosts the 48 Volt phantom power
to 60 VDC to polarize the capsule. That circuit replaces the battery
compartment, so the newer model has to be powered externally. That's
no big loss for most people--it's getting harder and harder to find
those 22.5 V photoflash batteries. Until last year they were common
in Radio Shack stores, but not any more.

--best regards