Most consoles that supply phantom power follow the P48 Standard (also called 48-volt phantom). This entails the use of a 48-volt DC power supply with the negative side connected to the console ground, and for each microphone input, a pair of 6.8 kohm (kilo ohm) resistors.
One resistor connects from +48 volts to pin 2 of the XLR while the second resistor connects from +48 volts to pin 3 of the XLR. The pair of resistors must be matched within 1 percent, and matching within 0.1 percent or better is desirable. The power rating of the resistors must be 0.5 watts or higher.
If a microphone line is shorted to ground, or has an unbalanced source connected to it with phantom power turned on, the phantom power feed resistor will dissipate around 1/3 of a watt. If resistors with 1/4- or 1/10-watt power ratings were used (all too common), they will overheat and go off value. Typically this results in a permanent change of resistor value, and that input of the console is damaged.
Many phantom powered microphones require exactly matched phantom power feed resistors in order to perform correctly. This can result in a console having certain inputs that don’t sound right when certain mics are used on that input.
To test the console, start with the phantom power turned off and measure each mic input from pin 3 to pin 1, and from pin 2 to pin 1. No DC voltage should be found on the inputs. Turn the phantom power on and measure each mic input from pin 3 to pin 1, and then from pin 2 to pin 1.
The voltage measured should be 48 volts DC and must be between 44 and 52 volts DC, while the voltage on each input pins 2 and 3 must be exactly the same. Record the voltage measured.
Prepare dummy plugs for testing from male 3 pin XLR connector inserts and a pair of 6.8 kohm resistors. One resistor connects from pin 1 to pin 2 of the XLR, while the second resistor connects from pin 1 to pin 3 of the XLR. The pair of resistors must be matched within 1 percent, and matching within 0.1 percent or better is desirable. The power rating of the resistors must be 0.25 watts or higher.
Plug a dummy into only the first mic input to be tested, and measure the voltage on the dummy from pin 1 to pin 2 and from pin 1 to pin 3. Record the voltages measured.
The voltages should be exactly half the voltage measured without the dummy plug, and the voltages on pin 2 and pin 3 should match each other. If the voltage did not drop exactly in half from the voltage measured without the dummy plug, this means the phantom power feed resistor(s) are not the correct 6.8 kohm value.
Move the dummy plug to the next input and repeat the test above. Repeat the test for every mic input. Note that only one dummy plug is used for this series of tests and is moved to the input being tested.
Prepare multiple dummy plugs, plug one into every mic input to be tested, and measure the voltage on each dummy from pin 1 to pin 2 and from pin 1 to pin 3. Record the voltages measured.
The voltages should be exactly the same as measured with only one dummy plug. If they’re not, the 48-volt power supply in the console is not properly regulated to a constant 48-volt output and/or can’t supply the needed current.
These tests are extremely useful in detecting some of the most common phantom power problems with consoles.