Op-Amp based guitar direct box
The main amplifier is IC1A.  R1+R13 and R3 set the gain, C1 and C2 are DC blocking caps, and most of the remaining mess of resistors and capacitors is for providing a low-noise DC bias at (presumably) 1/2 the supply voltage.  The circuit around IC1B takes the output of the main amp and inverts it.  The output of IC1B is then just the output of IC1A flipped around the DC bias point, so together they make a differential signal.  C5 and C6 are DC blocking caps, this time for the output.

DC blocking the signal on the way in and then on the way out again decouples the DC operating point of the circuit from the input and output levels, which are both centered around ground.  Since you don't care about frequencies below 20 Hz, it's perfectly fine to pick a arbitrary DC (0 Hz) level internally.

My only quibble with this circuit is R2 and C4.  Although you didn't show the values, I'm expecting C4 to be much less than C7.  R2 and C4 probably filter high frequencies beyond what the amp is intended to handle (20 kHz for "HiFi"), but that could have been accomplished by tying the bottom end of C4 to ground instead of the internal bias supply.