Hi, For every book or journal I have looked at, the method for amplifying the difference between two low-level voltages is op-amps, mainly instrumentation amplifiers.

Can anyone suggest other ways of doing this?

 

You can built the equivalent of an op-amp by using one transistor at a time or one vacuum tube at a time. You can apply the two voltages, one to the base of a transistor and one to the emitter. That would make a difference amplifier.

Just a couple of ideas.