Is it possible to subtract one voltage from another without the use of an op-amp?
LVDT, a differential transformer, has AC output.

It's easy to subtract AC signals, if they're produced by one 60Hz source, and coming from two separate (electrically floating) transformer windings.  (Just connect the two outputs in seires.)  

Also, a Wheatstone bridge is the device for performing the subtraction.  Just use AC drive and AC meters, rather than DC.