Condenser mics are nothing new. In fact, the first such transducer was invented by Western Electric research engineer Edward Charles “E.C.” Wente in 1916 — 12 years before the first moving coil dynamic mic was conceived.
Condenser microphone designs have a few advantages over dynamic mics including excellent high-frequency response, faster transient response and multi-pattern capability. Unlike dynamic microphones, condenser mics use two metal plates placed thousandths of an inch apart with an electrical charge (“bias”) between them. One of these plates (the backplate) is stationary. The other (the diaphragm) moves in response to sound waves. As the diaphragm moves back and forth, the voltage between the plates varies. This very small change in voltage is our audio signal. A condenser mic capsule is essentially a capacitor, and that’s why condenser mics are sometimes referred to as “capacitor microphones.”
The bias voltage can be supplied in one of two ways: (1) permanently charge the diaphragm (this is known as an electret condenser) or (2) use an external power source. Regardless, the output of the capsule will be incredibly low, and if you send it down a cable it will be overcome with noise by the time it gets to the preamp in your mixing console. That’s one of the reasons that just about all condenser microphones feature built-in amplification. A power supply is needed for this amplifier, and also to produce the voltage required to bias the capsule.
What a Trip
Tube-based condenser microphones employ dedicated power supplies that you can see and trip over on the floor of a recording studio (check out old pics of The Beatles’ recording sessions). Not only are these big, bulky power supplies designed for a particular microphone, they’re often tweaked to one specific microphone (e.g. “Power supply #1 to be used only with Neumann U47 serial number XX”). Needless to say there’s little compatibility.
Wouldn’t it be great if we could use one common power supply for all of our condenser microphones? And if we could send that power down the same audio cable that connects the microphone to the mixer? We could make those external power supply boxes disappear!
Pioneered by Neumann and Schoeps in the 1960s, phantom power addresses those concerns. A standard was created (Ha!) and that’s how phantom power was popularized. It didn’t hurt that the first 48-volt phantom-powered microphone was the Neumann KM84, which is an absolutely wonderful mic.
Standard?
Phantom power requires a balanced microphone cable and requires +48 volts DC to be applied equally between pin 1 and pin 2, as well as between pin 1 and pin 3 of a three-pin XLR connector, simultaneously. When executed correctly, there is no measurable DC voltage between pins 2 and 3, so they can happily carry a balanced audio signal with no noise or damage to the microphone. This enables the power supply to be hidden inside the mixing console (or mic preamp), thus the term “phantom.” A dynamic mic simply ignores the phantom power — unless it’s one of those rare dynamic mics that has built-in amplification.
Before we go any further, let’s dispel the myth that phantom power will damage a ribbon microphone. However, be aware that phantom power can damage a ribbon microphone if the cable is mis-wired or damaged. Let’s suppose, for example, that the wire connecting pin 2 of an XLR cable is broken and you switch on phantom. The ribbon might be exposed to the +48 volts DC across pins 1 and 3, which will likely fry it. This does not apply to active ribbon microphones with onboard electronics that require phantom power for operation. Also, if the ribbon mic has a transformer-isolated output, there’s a good chance that the transformer will block the DC. The moral of the story: be careful with phantom power when using ribbon microphones (FYI: You can also damage a ribbon mic by patching it through a TRS or TT patch bay where there is phantom power, but that’s a story for another time).
Back to the Standard
When is a standard not a standard? When a manufacturer decides that their way of doing things is better. Let’s take the positive side of that equation first. There are microphones that can operate across a wide range of phantom power voltages. For example, the Shure SM81 shown in Fig. 1 — a popular, roadworthy multipattern condenser that debuted in 1978 and is still in production — can operate on phantom voltages ranging from 12 to 48 VDC. Why? Because Shure recognized the fact that someday you might need to plug an SM81 into a battery-powered field recorder that simply isn’t capable of delivering +48 volt phantom, but can generate +18 VDC phantom. No problem for an SM81.
The tradeoff is that lower power supply voltages can increase the noise floor, reduce headroom or increase distortion.
Field recorders notwithstanding, here’s where you run into problems: there are some mixers out there aimed at the professional market that do not provide +48 VDC phantom. They might provide +20 or +24 VDC. That’s not a problem for many microphones, but at some point or another, you may run into a situation where a mixer doesn’t deliver the voltage required by a particular condenser mic.
Not long ago, this happened to me, where I was using a mixer that (unbeknownst to me) provided +18 VDC phantom power. I plugged a Neumann U87 into it and it didn’t work, because the U87 requires +48 VDC (±4 V). I nearly had a heart attack thinking that the mic was broken. Why would anyone want to build a mixer or preamp with a phantom power supply of less than +48 volts? You guessed it — it’s easier and cheaper to design a power supply with a lower voltage.
Current Events
All phantom powered microphones require a small amount of current to operate, usually just a few milliamps (mA). The phantom power supply in your mixing console will have a spec with the total number of milliamps it can provide. It’s conceivable that you could exceed this if you had condenser microphones plugged into every channel, powered up simultaneously. This could show up as distortion, noise or some of the mics not working at all. If you have any concerns, simply add up the current requirements for all of your microphones (it’s a commonly published spec), and make sure it doesn’t exceed the amount of mA the console can provide.
In such cases, my solution is to use an external phantom power supply (Fig. 2) that provides +48 VDC, patching it in between the microphone output and the input to the mixer. Regardless of the source for phantom power, it’s a smart idea to mute the channel on your mixer because I can almost guarantee that turning phantom power on or off will create a nasty pop that could damage a speaker. For the same reason, it’s wise not to plug in a microphone when phantom is already turned on.
You may also encounter some active DIs with similar phantom power requirements, and while you may not have a snappy solution, being aware of such ghosts can at least give you hope that your gear hasn’t gone out to lunch. Or maybe you’ll get lucky and the thing can run on batteries.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
