Wall mounting on studs

Avoid opening wall? 

It is possible to surface mount a separate wood panel, behind your 'shelf system'.

wide enough to catch only 2 studs, and use many anchors to sheetrock or plaster for the extended end. Then you have limitless horizontal locations to mount anything to the wood panel. Choose wood thickness relative to weight.

Or, wider, catch 4 studs if heavy TT? full width? Depends on decor, ....

btw, brackets can go 'up' behind the shelf, rather than down, objects on shelf in front of them. and you can drill extra holes in the brackets with less final visibility.

galpi, some of the advice you're getting here is a bit over the top. Open your wall in the area needed, and install some horizontal blocking between the studs in the necessary location. Use 2x6 KD lumber, which is dry. Screw it in place with some strong screws (not sheetrock screws). If you're concerned about transmitting vibration, use some construction adhesive between the existing studs and your new blocking.
Additionally, before you open the wall, you might want to consider what's in it. Walk around to the other side, think a bit. This is just generic advice. Barring that, cut a small hole initially, look inside, make sure you're not running into a waste line, or the back of your electrical panel, etc.
Good luck.

While not construction related, have a look at member @slaw 's wall mounting system using springs.

galpi, some of the advice you're getting here is a bit over the top.

I strongly agree. For goodness sakes folks, we are talking about a 3 stud width space. There are a number of very efficient ways to solve any vibration concerns.

You might want to consider using a french cleat (or a pair of them) to attach your shelving.  Some people use this approach to hang kitchen cabinets. You'd avoid opening the wall this way. Plus, this would allow you to experiment more easily with positioning your shelving a little to the right or the left before finally fastening it down.