Hi,
I actually have two 900 series desks. I like them a lot. The inputs are transformer balanced (the mix pre is) line input and return input is balanced as well. the groups out are also transformer balanced. On each channel you can find two inserts (pre fader and pre filter) and also a direct out. Chances are you'll have to wire them yourself although that is notdifficult. But..... they are unbalannced. The prefader insert can be balanced depending on the fader you have. Take a look at the mono fader PCB.... if it's not fully stuffed then the prefader insert is not balanced.... You can populate that board yourself .... it's not difficult and studer is known to only use of the shelf components in most cases.... NE5532......
Most of the time you have boosterinserts on these channels.... in this case the inserts get balanced by the use of balancing eurocards you can find in the bay below. The inserts on the groupfaders are most of time balanced as well using this method.
If you end up buying this desk... you'll have a good deal as it sounds really good. It's very neutral so don't expect this to be a rock board as are those tridents and MCI desks. But this is a very very good desk.... I have some schematics for a mod, switching the opamps in the channelelectronics.... it must give it a more coloured sound but haven't tried it out myself. I like the desk the way it is. Depending on the size of the desk..... if it's a 902 dont pay more then 4000 euro, don't get a 901.... they sell for 4000 euro as well as they are relatively small and easy to handle.
If I can be of further help...... please let me know.... Look in the schematic meta for the schematics on the studer server all your questions will be addressed by reading through those files..
Geert