Microphone Miscellanea Part 3
By Phil O'Keefe |
Microphone Miscellanea - Part 3
Phase and polarity, and powering your microphones
by Phil O'Keefe
In Part 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.
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.