Single supply differential balanced preamp
Every Electret mic that I've seen is inherently unbalanced. The output signal is developed between the metal case of the mic and center electrode. 

Electret mics are often used in Professional Audio and are connected so as to present a balanced output. This is done by putting equal-value resistors in series with the (+) and (-) output terminals of the mic capsule, then capacitively-coupling to the balanced output connector. Additional resistors take care of the bias supply. 

But even these mics aren't truly balanced - there is more capacitance between the mic capsule case and Ground than there is between the mic capsule (+) and Ground. You will sometimes see an extra capacitor added to help compensate for that. 

Bottom line is: If you are dealing with a bare Electret mic, treat it as unbalanced.