The Neumann U77 is a vintage electret microphone that is made for use in the recording studio.  Never bring this outside the recording as it is a vintage and delicate microphone that needs to be treated with care at all times.  The mic comes with a power supply and I believe it can also be powered with a battery if needed.  This is a major difference when compared to the other Neumann mics that look like this one like the U87, but as long as you have the original power supply, you shouldn't have any problems with powering this mic properly.

OVERALL OPINION

I've first used the Neumann U77 a couple of months ago and I was instantly impressed with the crystal clear sound quality and clarity that I heard while using the mic for recording vocals.  It has a really rich tone and an extreme amount of detail, making it perfect for recording all sorts of vocals that you would want a lot of clarity with.  This mic is pretty rare and is hard to find if you want to buy one.  This doesn't mean that you can't find one, they are just hard to come by since they weren't made for all that long of a time.  These are somewhat similar to the other mics in the 'U' line like the U87, but there is definitely a different tone that you get with each, even if the difference is subtle.  For some things I would prefer to use the U77 as it tends to have a little rounder of a sound and is probably better suited for male vocals because of this, while with a U87 this might not always be the case.  If you are looking to buy a Neumann U77 you will be able to find one at a reasonable price even if it is steep by a lot of home studio owner's standards.  This is a great vintage mic that doesn't get enough credit probably because of its scarcity.