If your condenser microphone is plugged in but producing no signal, the answer is almost always “phantom power is off.” Here’s what phantom power is, which mics need it, and which mics it can damage.
Phantom power is the +48 volts of DC current that an audio interface or mixing console sends UP an XLR cable to power a condenser microphone.
It’s called “phantom” because the audio signal coming back DOWN the same cable is unaffected by it — the voltage is “invisible” to the audio. The condenser’s electronics use the +48V to charge the capsule and run the internal preamp circuit.
Every audio interface and mixer with XLR inputs has a phantom power switch — sometimes labeled +48V, sometimes P48, sometimes just PHANTOM. On a Focusrite Scarlett, it’s a button on the front. On a MOTU M2, it’s on the back. On a Behringer mixer, it’s usually a master switch for all the channels at once.
If your condenser mic is plugged in and producing no signal, the first thing to check is whether phantom power is on.
Phantom power was standardized at 48 volts in the 1960s by Neumann (the German microphone maker). They needed a way to power their condenser mics without running a separate power cable. The +48V standard let condensers and dynamics share the same XLR jacks and cables, with the dynamic mics simply ignoring the voltage.
Some older mics use 12V or 24V phantom (mostly Schoeps and Sennheiser condensers from the 1970s). Modern interfaces all output +48V.