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Microphone Miscellanea - Part 3
Phase and polarity, and powering your microphones

 

by Phil O'Keefe

 

 

IPart 1 of this 3-part series we went over impedance levels and SPL, while in Part 2 we covered the basics about microphone sensitivity, microphone preamplifiers, and connecting your microphones. In this third and final installment we're going to talk about powering your microphones, as well as phase and polarity. As with the two earlier installments, we'll try to keep the math to a minimum and still give you the basic info you need. Ready? Let's get some power to your mic.


POWERING YOUR MICROPHONES

So you've connected your new condenser microphone to a suitable microphone preamp and turned it up, but you're not hearing anything - why not? Chances are you haven't provided any power to the microphone. All condenser microphones require some sort of power source. This usually charges the condenser capsule itself (except for "electret" condenser models, which have permanently charged capsule backplates) and also powers their onboard impedance conversion electronics and built-in preamps. These shouldn't be confused with the outboard microphone preamplifiers you connect microphones to in order to bring their signal up to line level. Condenser microphone capsules are very high impedance, low output devices that need to have their output signal converted to low impedance and raised up to mic level in order to be further boosted to line level by the outboard mic preamp. For that reason, nearly all condenser mics have electronics built inside of them to accomplish these tasks.


Some condenser microphone models power their internal electronics with a battery or two and will have an onboard battery compartment and usually a built-in switch for turning the battery on and off. This powering solution is great for field use (when recording remotely where no AC power is available) and even isn't too bad for studio use as long as the mic uses standard battery types, and especially if the battery life is measured in thousands of hours. But that's not always the case, so it's always very important to evaluate the battery type and life expectancy before you buy.


Since dealing with batteries can be a pain in the posterior and the active electronics inside condenser microphones still need a power source of some kind, audio engineers developed a powering method called phantom power that sends power to the microphone over the XLR cable. This doesn't interfere in any way with the microphone's output signal. How it works is less important than knowing that unless the condenser mic has onboard batteries or some other alternative method of powering it, your condenser microphone won't work without phantom power engaged. So if you connect a condenser mic and it doesn't seem to be working, first make sure the phantom power is turned on.


Some microphones require less voltage than others to operate properly. Most (but by no means all) phantom power supplies provide 48-52V. Be aware that if your mic requires 48V phantom power but your device supplies a lower voltage, the microphone may not operate at all, or may suffer from reduced audio performance.


Some ribbon microphones (such as the Cloud 44-A and Royer R-122 MkII shown in Figure 1) also have built-in preamplifier circuits, because passive ribbon microphones usually have relatively low output levels that require more clean gain from the mic preamp than many other microphone types. So, it makes sense to increase their levels at the source instead of relying solely on the external microphone preamp. This means you don't need an expensive, high-end external mic preamp with lots (60-70 dB) of clean gain, because the signal can be amplified adequately with more modest amounts of mic preamp gain.



Figure 1: The Royer R-122 MkII is one of the few ribbon mics with an onboard preamplifier, and it requires phantom power to operate properly


Occasionally you'll hear dire warnings about how phantom power can kill a passive (unpowered) ribbon mic and destroy the sensitive (and expensive) ribbon element, and while this is true for some older models, it's not very likely to happen with most modern ribbon mics. Still it's a good idea to get into the habit of disabling the phantom power on any mixing board or audio interface channels (or on your external mic preamp) before plugging in a ribbon mic. If your device has only a global phantom power switch, it's recommended that you turn it off whenever connecting and disconnecting your ribbon microphones because this is when they're at the greatest risk of being damaged by phantom power. Once they're connected, it's usually safe to turn the phantom power back on until you're ready to disconnect the ribbon mic, at which time you should turn it off again before you unplug.


Tube microphones require far more power to run than solid state condenser microphones with FET electronics, and phantom power supplies usually don't provide enough current for them to operate. Instead, condenser microphones with tube electronics usually have their own external, AC-powered "brick." This provides power to the tube mic itself through a special multi-pin cable that connects to the mic, and also returns the microphone's output signal back to the external power supply unit, where a standard XLR output jack provides the mic's output. You can then route this signal to your external microphone preamp or computer audio interface with built-in mic preamplifiers.



PHASE AND POLARITY  

The sound that we hear travels through the air in waves - a cyclic pattern of compressed and rarefied air molecules that are emitted by the vibrating sound source and occur at a frequency, or number of waves per second, that corresponds with the vibrations of the sound source. In the world of audio the frequency of those cycles is known as Hertz (Hz) or, as it used to be called, Cycles Per Second (CPS). Human hearing extends from about 20 Hz to 20,000 Hz. Just like waves in water, each sound wave traveling through the air has a positive point on its cycle (the wave "peak," where the air molecules are most heavily compressed) and a negative point - the wave's "trough" where the air molecules are more rarefied. And just as with waves at the beach or on a lake, when the peak of one sound wave meets the trough of another wave their energies cancel each other out to some degree. The more similar the two waves are in size and energy and the more precisely the peak and trough align, the more completely they will cancel each other out.


The same thing that happens with waves in the water happens with sound waves as they travel through the air. Placing a microphone a foot in front of something like a guitar amp might result in positive pressure on the microphone diaphragm (the part that, like your eardrum, vibrates when the sound waves from the amp's speaker hits it), and the microphone will usually output a positive AC voltage. Moving the mic a little closer or further away from the amp might result in negative pressure on the diaphragm and negative AC output voltage from the microphone. If you take two mics and put one mic in each of those two locations and listened to them individually you won't notice any problems, but if you try to listen to both of them at the same time over a mono speaker the sound will be weak and hollow compared to listening to either microphone individually. The reason is that the two microphones are out of phase with each other; combining the signals from them causes some of the sound to be cancelled out, just as when a wave trough meets the peak of another wave in a pond, resulting in a weak sound that usually lacks fullness.


Most mixing boards and outboard mic preamps have "phase" or "polarity" switches (sometimes indicated by a "slashed zero" symbol - Ø ) that allow flipping the polarity. If you're using just one microphone you don't need to worry as much about polarity, but whenever you use multiple microphones in close proximity to each other, it's always a good idea to listen carefully in mono while flipping the polarity button on one of the two channels to determine if it improves the sound quality. "Out of phase" signals will usually sound weaker, quieter and thinner, with noticeably reduced bass. -HC-

 

Do you have questions or comments about this article? Then be sure to join the discussion in this thread in the Studio Trenches forum right here on Harmony Central!




__________________________________________________

 




Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.  

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