Gray, Hurst, Lewis and Meyer has an excellent and rigorous discussion of fully-differential amplifiers, with a good, non-hand-wavy presentation of common-mode feedback.

A *discrete* op amp?  Really?

I don't think you're going to find much to help you there.  Generally discrete designs have a big enough power supply that they can do fine single ended, especially audio frequency circuits.

If it must be differential, an easy way is to make a pseudo-differential amplifier using two op-amps, one inverting and one non-inverting.  There is a phase difference associated with this, of course, but if you design fast circuits it shouldn't be a problem at audio frequencies.  

This is an old technique, and predates fully-differential op amps (this was done in the late 70s for example).  A good reference for this is Gregorian and Temes "Analog MOS Integrated Circuits for Signal-Processing".  This is a great book to read in general.

Good luck!

Carl

yeah it has to be diff because the mic output, having a long cable has a high level of hum, buzz and RF which is CM signal at the amp end. The mic capsule on the other hand, it's floating.

Out-board amp for mics are routinely used for mic signal in the audio circle. Wireless Mic are of lower quality. Out board amp can and do have better SNR.
Discrete transistors at the front end have up to 50% less noise then manufactured opamp/inamp.