We are asked lots of technical audio questions during our travels on the HOW-To church sound workshop tours. One of our most frequently asked questions is “what is phantom power?”
Phantom power in its basic form is a powering system that permits condenser (capacitor) microphones to operate. Condenser microphones have a charged plate as their diaphragm (part of a capacitor), and therefore need a particular DC voltage to function. This DC voltage (power) comes from the mixing console, and usually is marked on your console with the words “48V” or “phantom.” In short, we need phantom power because condenser microphones need it to function.
A Little History…
Phantom power was invented by Neumann (www.neumann.com) in 1966 as a way to eliminate batteries in their condenser microphones; it is now a common feature in most modern mixing consoles. Phantom power is typically activated by a switch or group of switches either near the top of your channel strip or next to each XLR input. Many times there is a single switch near the power cable, which sends phantom power to all XLR inputs at once. Alternatively, some consoles have a sub-surface switch that needs the point of a pen or tiny screwdriver to turn it on.
As we alluded earlier, back in the 1960s a special type of microphone was invented that used a charged capacitive plate rather than a voice coil to generate an output signal. These mics sounded great; however, the battery needed to operate them was heavy, expensive and tended to die during a broadcast. Neumann invented a way to power these microphones directly from the mixing board without any additional wires or batteries.
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| Figure 1: Phantom power schematic diagram |
Tech Talk
Take a look at the diagram in Figure 1. Note in the bottom part of the diagram there is a 48-volt power supply connected to the XLR inputs on the channel strip through a pair of bias resistors. The two capacitors block this voltage from the red balancing amplifier of the next input stage. This effectively sends 48 volts backwards up the mic lead to the microphone. While not explicitly shown in the diagram, the shield acts as the neutral conductor for phantom power.
Now look at the top of the diagram that shows a condenser microphone. As part of its design there is a small transistor amplifier that requires DC power to operate. Also, the diaphragm itself needs an electrical charge applied to it in order to generate a voltage when sound moves it in and out. Both of these elements need between 9 to 48 volts to power them. In the case of the microphone shown, this is accomplished with a resistor coming from the center-tap transformer that picks off this voltage and supplies it to both places. We should note that some modern condenser microphones use electro-stati- cally-charged elements that don’t actually require bias voltage from the phantom supply, but which, nonetheless, need phantom power for their output amplifiers.
This is a simple and elegant solution that works without any problems most of the time. If a dynamic microphone, which does not need any phantom power, is plugged in, it is unaffected by the phantom power. Hence, the name “phantom” – the 48 volts is used only when needed and otherwise is not utilized. However, there are some issues to consider, which if ignored, may spoil the party.
Handling Phantom Power
First, not all mixing consoles produce true 48 volts from the phantom power supply. While most modern consoles do in fact generate 48 volts, some older consoles provide voltages as low as 9 volts and as high as 52 volts. Most modern microphones will accept this wide variation in voltage, but many earlier condenser microphones need a true 48 volts to operate. If, for example, all of your modern condenser mics work on a particular console but your older, “vintage” microphones do not, then it is possible you have low phantom voltage. You can purchase individual phantom supply adapters or upgrade to a new console that does provide 48 volts when budget allows.
Next, while some higher-end consoles have individual switches for phantom power on each channel strip, many less expensive consoles can switch it on only globally or in banks of four or eight channels at a time. Since we now know that dynamic mics are not affected if they are accidentally supplied phantom power, this is not an issue most of the time. But you will certainly hear a loud “pop” or “boom” anytime a microphone is plugged into a phantom powered input — which can be alarming if your worship service is already in progress. This noise will be routed to all outputs for that channel (such as the primary loudspeakers), creating great distraction. It’s best to switch off phantom power on all channels unless needed for a particular condenser microphone or other sound sources such as active direct boxes.
In addition, you cannot use a simple phone-plug-to-XLR adapter if you want to plug any audio device having a ¼-inch phone plug output into an XLR input. Doing this will certainly short out the phantom power on that input channel resulting in a loud pop, and sometimes will short out the phantom power supply in the console resulting in every phantom microphone going dead at the same time. If you do need to plug in a ¼-inch phone plug signal source, you must use a DI ( “direct inject”) box to isolate the DC voltage from the phone plug. As a side note, many active DI boxes can use either battery power or, you guessed it, phantom power as well. This is a great convenience since batteries have a way of failing at the worst possible time.
Lifting the ground on a balanced audio cable will also make the condenser microphone or active DI not function. As the ground shield for the cable is also the neutral conductor for phantom power, lifting the ground will effectively disconnect the phantom power supply for that channel.
Another potential “gotcha” with condenser microphones and phantom power is related to the electrical current available from the phantom power supply. Some lower-priced mixing consoles provide a limited amount of current. If the console is “loaded up” with condenser microphones and active DI’s there may be problems due to excessive current draw on the power supply.
Lastly, moisture in an XLR cable or connector can induce a leakage current on a phantom-powered input. This creates a sound like a steady crackle, and it occurs even with a dynamic microphone. If you find yourself with this problem, a quick fix is to use a hair dryer to dry any XLR connectors. If the cables have been sitting in a rain puddle, you may also need to fl ush them with denatured alcohol. Cheap microphone cables can wick moisture up inside and remain damp for weeks, so if you have a suspect cable, mark it as such and give it a sunny vacation out on the back deck for a few days to heat up and dry out.
Mike Sokol and Hector La Torre, from Fits & Starts Productions, LLC, conduct the HOW-To Church Sound Workshops across the United States. For information on the workshop tours see www.howtosound.com.
