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Thursday, March 22, 2018

Q. Is it safe to apply phantom power to dynamic mics?

By Hugh Robjohns

I did a recording session recently using a mixture of dynamic and condenser mics, and realised my desk does not have switchable phantom power for each individual channel — they're either all on or all off. Luckily, I had a second mixer and some external channel strips which I ran the condensers through, but is it safe to apply phantom power to dynamic mics?

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Technical Editor Hugh Robjohns replies: People get very hung up about phantom power. As long as your mic cables are all wired properly (balanced, with the correct pin connections) and well made, and you are using decent XLRs everywhere — and all your microphones are modern — there is no problem at all.

In BBC radio and TV studios, for example, phantom power is provided permanently on all wall box connections. It cannot be turned off. And engineers are plugging dynamic, condenser and even ribbon mics in and out all day without any problems whatsoever.

Clearly, it is vital that dynamic and ribbon mics are properly balanced internally and well maintained, but this should be a given with any modern mic. The female connectors on good-quality XLR cables should have the contact of the earth pin socket (pin 1) slightly forward of the other two so that the earth contact mates first, and are designed so that the other two pins mate simultaneously. There is therefore little chance of subjecting the mic to significantly unbalanced phantom voltages.

There will be a loud 'splat' over the monitors when connecting a condenser mic as the circuitry powers up, but it is good practice to always keep the channel fader down when plugging in mics anyway. I don't disagree that plugging mics in with phantom off is a safe way of working, but I have never really bothered about it, and have never destroyed a mic yet — not even a ribbon, and I've used a lot of those over the years.

It's perfectly safe to apply phantom power to modern ribbon mics, like the Oktava ML52, and dynamic mics, like the Sennheiser e903, provided you use good quality XLR-XLR cables. 
It's perfectly safe to apply phantom power to modern ribbon mics, like the Oktava ML52, and dynamic mics, like the Sennheiser e903, provided you use good quality XLR-XLR cables.

The important issue about ribbon mics is that it is safe to plug in ribbon mics on circuits carrying phantom power, provided the ribbon mics in question are compatible with phantom power. Some vintage ribbon mics employ an output transformer which is centre-tapped, and that centre tap is earthed. This arrangement essentially short-circuits the phantom power supply and can cause damaging currents to flow through the transformer, potentially magnetising it or even burning it out (although that is extremely unlikely). So it is sheer lunacy to be using vintage ribbon mics with centre-tap grounded transformers in an environment where phantom power is also used. Sooner or later, a ribbon will get plugged into a phantom supply by accident and will be permanently damaged. If you want to use vintage ribbons with centre-tap transformers in the same room as phantom-powered condensers, get the ribbons modified before it's too late.

The bottom line is that all modern mics with balanced outputs terminated with XLRs, whether they be dynamics (moving-coils and ribbons) and electrostatics (condenser and electrets), are designed to accommodate phantom power, and can be plugged in quite happily with phantom power switched on, provided you are connecting XLRs, not jack plugs/sockets. Some vintage ribbon mics, and any mic wired for unbalanced (sometimes also referred to as high-impedance) operation will be damaged by phantom power unless suitably modified.

Published January 2004

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