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Beware The Phantom

March 28, 2005
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From whence came the phantom?

Phantom power, or simplex power as it was called, originated back in the early days of the telegraph industry. According to Chris Woolf of Rycote Microphone Windshields (www.microphone-data.com), “Telegraph engineers realized that they could treat a balanced pair as a single conductor and, using a separate ground, create an additional circuit without any extra wires.” Pretty neat trick, eh?

According to Greg Gualtieri of Pendulum Audio (www.pendulumaudio.com), Bell then adapted this system to power the carbon mics and ringers in original telephones. The beauty of the system was that it worked with only two wires coming into a home, no separate ground was needed, and the voltage was low enough not to pose a threat to customers.

Fast-forward to 1966. Ribbon, dynamic, and powered tube mics are mainstays of the recording industry. But a new type of mic, the solid-state mic, appeared on the scene. In order to power these mics without using a separate supply, Neumann GmbH considered two alternatives: T-power (or AB or parallel power) and phantom power. It should be pretty evident which system won. (A-B Power, which uses 12V, is still common in the television and motion picture industries.) While experimenting with powering systems, Neumann considered different voltage standards as well. Oliver Archut, of AMI Tab-Funkenwerk (www.tab-funkenwerk.com) says that Neumann considered many different voltages, all the way up to 103V. The goal was to implement the new standard without having to change the existing cables, connectors, and so on. The design engineers realized that higher voltages would be better, but an order from the Norwegian Broadcasting Network cinched the standard. The Norwegians were changing all their mics to solid-state and wanted to power them with their existing 48V system. Neumann filled that order and we’ve had 48V phantom power ever since.

Convenience or Fidelity?

To many preamp and microphone designers, the decision to use 48V phantom was a “compromise of convenience,” not sonic fidelity. According to Archut, the German Institute of Broadcast Technology (IRT or Institut für Rundfunk GmbH, the audio gurus in Germany from 1940 to 1975), rejected Neumann’s +48V phantom system because it was a sonic compromise. Ever heard of a Neumann U 77 or KM 74? They were non-phantom powered versions of the ubiquitous U 87 and KM 84 that used standalone power supplies.

There are several problems with the phantom powering system. First 48V is too low. The original preference from Neumann was a system with 70-80V. 48 volts, in the opinion of many mic manufacturers, is inadequate for the polarization voltage of (non-electret) condenser mics. While 48V is considered adequate for the head amp (the amp inside the mic that interfaces the mic element with the outside world), it doesn’t have enough juice for the capsule.

Standard? Which one?

Another problem is that some manufacturers don’t adhere to the 48V standard for phantom power. Phantom voltages of 12, 15, 18, or 24V aren’t uncommon. One of the most prolific console manufacturers has a phantom system that only puts out 41V. Why is that a big deal? If manufacturers are making consoles with 12V phantom, then mic manufacturers are compelled to make sure their mics work at 12V in order to not lose sales, so they aim for the lowest common denominator. That in turn means compromises in the design stage.

Why would a console manufacturer have a 41V phantom supply instead of 48V? Instead of spending another $10 for a transformer with a 48V tap, they use an inexpensive voltage-doubler circuit on the AC voltage used for the +15V supply so that it can also be used for phantom, which results in 41V.

Greg Gualitieri comments, “I don’t know any recording gear manufacturer that would allow a third party to design the power supply for their gear. It’s simply too important to the sound. Why should condenser microphones be any different?”

So if the power supply is running at a lower voltage, what impact does that have? First, headroom of the mic will be compromised. Distortion will likely increase. Noise figures may increase. The overall capabilities of the mic will be compromised.

Strong Reservations

Then there’s the power supply design: Does it have adequate reserves to handle as many condensers as you might plug in? When phantom power was first standardized, an upper limit of 2mA (milliAmps) of current was established as the guideline, but some mics today can draw as much as 10 mA. What if the designer spec’d the system to provide current for 20 condensers at 2mA each (40 mA), but you plug in eight mics that each draw 10 mA? In a situation like a big explosive drum hit with kick, snare, and cymbals, will that supply be able to provide all the current the mics need to keep up with the dynamics of the drummer? Maybe, maybe not.

The channels of a mixer can also interact. If one channel is shorted, say, by a bad cable or mic, what happens to the current available to the other mics? If the power supply isn’t hefty enough to keep up, then every mic’s performance will be compromised. Even if you aren’t using that mic or that channel. It’s just sitting there draining the power supply reserve.

Blasting Zone

Did you know every time a condenser mic is plugged into a phantom-powered input with a transformer that there’s a short microblast of voltage that’s destroying the insulation of the transformer’s secondary? According to Archut, the transformer dies over time because of this action. He should know. He rewinds transformers for vintage preamps, like the Telefunken V72 and V76. “People buy these vintage modules and strap a phantom supply on them (since they were designed without phantom) and then wonder why they have to have the transformers replaced or rewound. I see it all the time.”

What’s a mic to do?

The good news is that there are options. Probably the biggest proponent of increasing power voltage for mics is DPA Microphones (www.dpamicrophones.com) with their 130V system. All of their studio mics use a 200V backplate voltage for the capsules, and some use a 130V supply for the head amp. By increasing the voltage available to the mic, DPA is able to take the already impressive specs for their 48V 4006 mic and make them even better in their 130V 4003 (for example, headroom up from 14 to 25dB, and maximum SPL handling up from 143 to 154dB SPL). According to DPA’s Bruce Myers, DPA is the only manufacturer in the world making 130V mics. Many engineers who hear the difference become high-voltage converts and they’ve convinced several preamp manufacturers (Millennia, Grace, Avalon, along with DPA) to make preamps capable of 130V power for these mics. DPA uses a modified 3-pin XLR so you can’t plug in a standard XLR cable and destroy 48V mics. DPA also uses a 6-pin system for their 4041 series (130V preamp/200V backplate), which allows the audio to travel on dedicated lines while keeping the supply voltages on separate lines.

But are there other options, short of replacing your mics? Greg Gualtieri rebuilds his mics so that they use 5-pin XLRs, so the power supply voltage doesn’t have to piggyback on the audio lines. He also makes dedicated power supplies for his condensers. He believes using dedicated supplies, just as with tube mics, can make a big sonic difference. Gualtieri has also been known to replace the head amp when he feels the original design was compromised to accommodate lower voltages. He believes that if mic manufacturers would quit compromising their designs to accommodate lower voltages than 48V, then consoles and preamps with lower phantom voltages wouldn’t be able to power some mics, and manufacturers would have to put in solid 48V supplies.

Oliver Archut’s solution for transformer corruption, which he has adopted in all his preamps, uses a 48V power supply that ramps up slowly on power up and helps extend transformer life. “$25 worth of parts can save you a $300 rewinding fee.” One other benefit is that you don’t hear that huge pop in the speakers when you turn on phantom. He just wishes everyone else would follow his lead.

What about budget solutions?

So, you’re convinced that the current phantom system is insufficient but you don’t have the money to replace or rebuild your mics, or buy or rebuild your preamp collection. What can you do?

There are three solutions: The first is using standalone preamps with sufficient phantom powering. A second option is to have a technician build a solid, high-current capacity +48V power supply that can be added to your existing console. It will require some modifying of the internal wiring and another plug in the back of the console for the external phantom to come in. I’ve done this before; it’s a good option and not terribly time consuming or expensive. Finding someone qualified to do it shouldn’t be that difficult.

The third choice is to buy an external phantom supply for your mics. Standalone phantom supplies are available from companies such as Audio-Technica, AKG, Shure, Neumann, Rolls, ART, and Crown for as little as $35, although the cheaper ones might not be an improvement over existing powering. (Some use 9V batteries for power.) This is probably the easiest and cheapest solution.

In the end, only your ears can tell you if phantom power problems are compromising your recordings. Try it for yourself: make two identical (as close as possible, anyway) recordings; one with poor phantom, the other with a known good phantom supply. Can you hear the difference?

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