As explained on another board (https://www.gearslutz.com/board/10509494-post13.html):
There are two reasons why phantom power mic inputs should never be connected directly together with a "Y" cable or adapter. (Such adapters or "Y" cables are fine for use with unpowered dynamic mic inputs, but not for phantom powered inputs. That includes inputs on the same device like (2) side by side channels on a mixer.)
- Potential mic damage:
Modern phantom powered microphones use the phantom power to power the impedance converter and the output line driver stages, and to supply the capsule bias (on non-electret mics). By placing two powered inputs in parallel the source impedance of the phantom power source is halved. On a P-48 circuit, instead of have a single 6810 ohm resistor on each input line feeding the mic you now have an effective power source impedance of 3405 ohms on each line. The P-48 circuit can now supply twice the current. The lowered P-48 resistance changes the operating voltage at the mic. That can change the operating point of the output stage drivers and the intended impedance converter operating point, and possibly the capsule bias.
Some mics (not the majority), use a voltage regulator (usually a single Zener diode) to keep the mic active-electronics operating at a constant voltage. On those mics, using paralleled powered inputs will greatly increase the power dissipation in the Zener regulator. The Zener current can more than double.
A modern phantom powered mic is designed to work with a known source impedance for the phantom power current. Halving that impedance is risky. Additionally paralleling the inputs drops the load impedance the mic "sees" to half of a single input. That's not a mic safety issue like lower the P-48 current source impedance, but may affect the sound of some mics.
Incidentally, cutting the pin-1 return for one of the inputs may not change the current supplied from the input. There will be a good chance that there will be another ground path like a power line common connection or another audio cable between the two pieces of gear which will still provide a return path for the phantom power current.
- Input circuit damage (not relevant to your case, but potentially interesting nonetheless):
If phantom powered inputs on different pieces of gear have their inputs paralleled using a simple direct "Y" cable it's possible to damage the mic input stage of one when it's operating power (not phantom Power) is off and the phantom power on the other input is switched on or off. Most modern op amp input can be damaged if either of the differential input terminals is allowed to reach a voltage that exceeds the power supply rail voltage. When one input amp is powered down, it's rails are at or very near "0" voltage. Even the application of a few volts coupled in from an external power source (the PP on the "other" mic input) may produce a input voltage transient that exceeds the (now "0" ) power rail voltage. Some input's have voltage "clamps" to prevent that but many do not. Remember, the designers did not design the inputs to withstand [externally-applied] phantom power when the device was switched "off".
That is not an issue if you are running (2) mixer inputs in parallel (same power supplies), but can be if you are using separately powered devices.
Bottom line: If you care about protecting your powered mics and your mic pre inputs, don't ever use a directly coupled parallel "Y" cable on a phantom powered input no matter what anyone tells you. An isolated transformer splitter, or capacitively-coupled "splitter" is the only wise solution.
Or, in your case, use the internal battery power and split the mic signals as if it were a self-powered dynamic mic.
