Why are ribbon microphones damaged by phantom power?

In theory, a microphone that doesn't need phantom power should just ignore it. So why are ribbon microphones supposedly so sensitive?

One of the beauties of phantom power is that microphones that need it use it; microphone that don't need it ignore it. But ribbon microphones are different. They can be severely damaged by phantom power. Why is this so?

The first thing to remember is that ribbon microphones are inherently delicate due to the way in which they are constructed. There are exceptions to this but it's best to regard a ribbon mic as needing special care unless reliably informed otherwise.

Any shock could damage the ribbon and its mounting. It is often considered good practice not to let air push against the diaphragm as you walk from the mic cabinet to the studio floor, and certainly don't run with it (although when do you ever see sound engineers run?)

And just as it is sensitive to physical shock, the ribbon mic is sensitive to electrical shock too.

In theory, phantom power should not affect any transformer-output mic that doesn't use it. This is because the coil of the diaphragm in a dynamic mic, and the ribbon in a ribbon mic, is connected across the primary coil of the transformer. In phantom power, +48 DC is connected to both ends of the secondary coil. Transformers to do not pass direct current, and in any case, both ends of the secondary coil are at the same voltage, so no current flows through the secondary coil due to phantom powering.

But that's the theory. How do things work out in practice?

The secondary coil of the microphone's transformer is connected to pins 2 and 3 of the XLR connector, which in turn are both connected to the +48 volts DC of the phantom power supply.

What could conceivably happen is that one pin makes contact a fraction of a second before the other. So for a moment, one pin has +48 volts connected, the other doesn't. Now in theory, until the other pin has connected, there is no circuit for any current to flow through, and when it does connect it will be at the same voltage so still no current will flow. Unless a microphone is unreasonably delicate therefore, connection to a phantom-powered input should pose no problems.

Patchbays are definitely a problem however. If you have a tip-ring-sleeve microphone patchbay (which some would argue against, but they can be very practical) with phantom power applied, then as you plug in the patch cable the microphone is exposed momentarily to the full 48 volts across pins 2 and 3 which, at the moment of connection, will transmit through the transformer to the ribbon. Needless to say, the ribbon will not like this and is likely to be stretched, at the very least. Continued plugging and unplugging on a daily basis will repeat the stress.

The scenario that is least likely to damage your ribbon mic is where phantom power is switched. Since switching occurs simultaneously on pins 2 and 3 of the XLR mic input, there is no possibility for any current to flow, other than a very small amount due to any slight mismatch of the resistors through which the phantom power is fed. This will only transmit through the transformer momentarily.

One final point - if you have a vintage ribbon microphone, then it might have a centre-tapped transformer and current from the phantom power supply will actually flow through the secondary coil. I don't have any personal experience of what damage this could cause, but my alarm bells would certainly be ringing.

Over to you. Have you ever damaged a ribbon mic? How did you do it?

P.S. Some ribbon microphones have internal preamplifiers and require phantom power. Clearly they are not going to be damaged by it.

Image: Marc Wathieu CC BY 2.0