[cabinetman]

If it doesn't sit on the floor, you are limited to what the studs can provide. No way to determine that from here.

.

 

[cabinetman]

Wow, thanks guys -- that's super helpful.

The lower shelves, below the TV, are the deepest. 16". The biggest weight down there is the LPs, at the bottom. They are 12" deep and will be centered on the shelf. (There are really two shelf units, upper & lower with two sets of brackets. The TV sits in the middle hanging on a wall mount.)

The brackets are designed for #6 screws and apparently can't be modified for bigger ones. I wanted to use bigger screws, but the company that makes them has reviewed my drawings and thinks they are okay. (They also pointed to the top screw as the key issue.) I'm planning to use 2.5" screws. Brackets are 1/2", plaster is 3/4", so they'll be 1.25" into the wood. Could go to 3" if you think it would help.

Picture of brackets is linked below. They're aluminum.
http://rakks.com/portfolio/PopUp/tstyle.html

Thanks for all the help --
Steve

I really don't like the idea of #6 screws that long. I would ream out the holes and countersink accordingly, and use #8. For support for the bottom that would be relatively not that obvious, you could make a clear Plexiglas box or display that would help support the bottom shelf.

.