If you want it to be usable with any preamp without needing to know details of the pre-amp input circuit you should have DC blocking capacitors so that 48V is not applied to the input of the preamp, and you should have protection diodes. Typically protection diodes would dump fault currents into the power supply rails of the preamp, but in an external phantom power supply the only thing you could do is have some kind of voltage clamp, like a transient suppression diode (essentially a high current zener style device).