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Ask Jonah: Addressing A Long-Standing Phantom Power Myth

Searching for the truth about ribbon microphones and the potential damage from DC voltage.

By Jonah Altrove March 30, 2018

Got a question? Send it to Jonah via email at [email protected]

Dear Jonah:

Does phantom power burn up ribbon microphones? – Emily G., Nashville

The short answer is no, even though we’ve all heard otherwise. I knew this myth was old, but I hadn’t realized just how old until researching it – in fact, it seems to predate the modern live sound reinforcement industry entirely. (It dates back to at least 1938.)

Let’s start with a quick review of phantom power since the rest of the answer won’t make sense otherwise. Phantom power is a means of supplying a DC voltage via a mic cable for active devices such as condenser mics and active direct boxes (DIs).

When phantom power is engaged, the mic preamp applies +48 volts DC to both XLR signal conductors (pins 2 and 3). Balanced signals are differential – derived from the difference between the two conductors – so having the same DC voltage on both doesn’t interfere with the AC signal from the mic.

A dynamic mic doesn’t need phantom power, but it won’t be harmed by it either since the mic’s coil is wired between XLR pins 2 and 3. Phantom power would apply the same DC voltage to both ends of the coil, so no voltage is dropped across it, and no current flows. (Some dynamic mics have output transformers, in which case the DC wouldn’t even make it to the coil.)

Functionally, ribbon mics are similar to dynamics: a thin, delicate metallic diaphragm – the ribbon – is suspended between two magnets. The vibrating ribbon cuts through magnetic field lines, inducing a small signal in the ribbon. [And I do mean small – ribbon mics require a very generous amount of gain at the preamp.]

Ribbon mics have output transformers, which in theory should assuage any concerns because transformers don’t pass DC, so there should be no way phantom power could imperil the ribbon. Why, then, are we so worried?

In his book “The Audio Expert,” audio mythbuster Ethan Winer traces the origin of this myth back to the RCA Model 44 ribbon mic, which had a center-tapped output transformer. Apparently, this caused some issues with phantom power, until the center tap was disconnected.

But center tap or no, I couldn’t understand how DC could get through to the ribbon, so I called Ethan for more background. This myth actually predates most living audio professionals – Ethan showed me a Model 44-B schematic from 1938 – so finding someone who knew its origins was tricky. Ethan called fellow guru Mike Rivers, who tracked down Wes Dooley, one of the few living experts on vintage ribbons, at the recent NAMM show.

After a napkin-scratching session, what emerged is a “worst case scenario” that could plausibly lead to phantom power damaging a ribbon: when an XLR cable is plugged in, it’s possible that one of the DC-bearing pins (2 or 3) could make contact before the other, especially with an aged, worn XLR connector. Although this would only last an instant, current would flow across the coil.

The DC can’t get through the transformer to the ribbon, but the transient can – think of the loud “pop” that occurs when engaging phantom power on an open mic input. This transient surge would slam the ribbon around pretty violently, and damage would be a possibility.

Ethan also points out that even the tiny current flow generated by this jolt could be damaging because the ribbon is basically acting as a fuse in this situation – a flimsy conductor that would be the first thing to melt. Damage would therefore be either mechanical (over-excursion) or thermal (melting), which, not coincidentally, are the two main ways to blow up a loudspeaker.

For the same reason, 1/4-inch TRS patchbays are a bad idea. TRS jacks can momentarily short hot and cold to ground when the plug is inserted. This would briefly impress the full +48-volts of DC across the mic’s coil or transformer, and the resulting transient will bump the diaphragm around. It’s probably uncomfortable for dynamic mics, but is potentially damaging to more delicate ribbons.

The bottom line: turn off phantom power before patching or unpatching, which is good practice anyway, and there’s nothing to worry about.

Will the accidental application of phantom power melt your prized vintage ribbon into a molten puddle? No. Rest easy.

Got a question? Send it to Jonah via email at [email protected].

 


About Jonah

Jonah Altrove
Jonah Altrove

Veteran Live Audio Professional
Jonah Altrove is a veteran live audio professional on a constant quest to discover more about the craft. Send him your "Ask Jonah" questions at [email protected]

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