When phantom power was first invented, it was designed so that any mic that didn't need it wouldn't be affected by it. So why do we now worry about switching it off?
Here's a quote from the user guide for the Focusrite Forte audio interface...
If you are using a capacitor (condenser) microphone, click the 48V button to supply phantom power to the mic. Don't turn this on if you're using any other type of mic.
I suspect the reason for saying 'don't turn this on...' is that a user might have an overly-delicate microphone that doesn't require phantom power. Because of the microphone's inadequate design it is damaged, then the user wrongly blames Focusrite! So if Focusrite tells users to switch the phantom power off, then their complaints department can sleep soundly at night. (Although I can't imagine Focusrite gets enough complaints to warrant having a whole department for them!)
Go back a few years when microphones were built for professional studios, broadcast and live sound. They found their way into home studios too. But mics had to be professional in sound quality and also design and build quality, otherwise they simply wouldn't do their job.
It would be standard practice therefore to leave phantom power switched on all the time. Capacitor mics would use it. Other mics would ignore it. Mixing consoles would be designed such that phantom power was all-on or all-off. And everything worked fine!
Ribbon microphones are delicate flowers
Where we come into problems with phantom power is with ribbon microphones. Ribbon microphones come from an era of audio even before phantom power was invented. But they have had a resurgence in popularity of late because they have an interesting sound quality. They are however quite delicate.
A well-designed ribbon microphone shouldn't care about phantom power any more than a normal dynamic mic. However, any condition that puts 48 volts across the ribbon will blow the mic, or potentially severely damage it.
This is very unlikely to happen when mics are connected with XLR cables, unless there is a damaged or incorrectly-wired cable. It is much more of a risk where a mic is connected through a patchbay, but this is less commonly done in home recording studios than in professional studios.
There is a useful guide to this problem at Royer Labs' website. It says clearly that, "Royer ribbon microphones are not usually affected by the presence of phantom power." However they go on to give good reasons why you should take care.
In practice, if microphones are properly designed then there should be no difficulty with phantom power. In fact I would go so far as to say that, in this day and age, if a microphone had a problem with phantom power, then it shouldn't be considered adequate for professional use.
But to a certain extent it is inevitable that we have to accept things as they are, so I propose three alternative courses of action...
1. Only use microphones that are OK with phantom power, and simply leave it on all of the time, like they do in broadcast studios.
2. Make it your habit to switch phantom power on for capacitor microphones that need it, and switch it off immediately after use.
3. Avoid ribbon mics. But that would mean denying yourself their special tone colour. As an alternative, you might consider buying a dedicated ribbon mic preamp.
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