If you're using condenser microphones in your set up, you're going to need phantom power to them. Condenser mics will not work without some kind of power source. Period.
Condenser mics aren't the only thing that can take advantage of phantom power. Some active DI boxes can use it as their power source, as can a few preamps and effects pedals (the Sansamp Tech 21 series being a good example). So lets start the discussion today with exactly what phantom power is.
Special Delivery
Phantom power is a way to deliver DC power down a balanced audio connection without causing problems for the audio coming back the other way. The original specification was written to allow 1mA (.001 amps) per connection, but as with everything else electronic, power demands have increased over the years, and now that power budget is 10mA.
Thinking back last week to our discussion of balanced and unbalanced connections, keep in mind that phantom power can only be done through a balanced system. The positive voltage is applied to pins 2 and 3, and the shield carries the ground return current. At the mixer (or voltage source end), the DC is blocked using capacitors as a high pass filter (DC is essentially a 0Hz signal, so we can filter that out long before we get to audio frequencies).
Electrically, here's what it looks like:
At the microphone end, the transformer that's built in keeps the DC from messing with the output of the preamp (DC can't cross a transformer, only AC / audio can). At the mixer input side, you see the blocking capacitors I talked about. Capacitors don't pass DC after they've charged (which only takes a few microseconds in a circuit like this).
So now you can see "how" we get the phantom power down to the mic and get the audio signal back. I suppose you could think of it like a double-decker highway. The audio is riding down the upper deck, and the DC the lower deck.
Did You Want Medium, Large, or Extra Large?
I was brought up thinking that phantom power was always supposed to be 48 volts DC. When you're using phantom for condenser mics, that's where they operate best.
However, it's a well know fact that most consumer grade electronics don't put out that much...it's typically 15 volts or so. Why? Easy...it saves them the cost of putting another power supply in their product (as most audio electronics run off of +/- 15 volts or less). And in all fairness, you're not going to hear the difference with a mid-grade mic in a live situation.
It's interesting to note that the first mic to use phantom power (the Schoeps CMT-20) was built in 1964, and was designed to use 9-12 volts. It was built for French radio, and that value was picked because that's what was easily available in the studio.
It was in 1966 that Neumann Audio of Berlin built a mic with a transistorized preamp for the Norwegian Broadcasting Network. That mic was designed for the now accepted 48 volts. Again, why? Because the emergency lighting in their studios ran off of 48 volts. Again..it's what they had available. And you thought there was a bunch of math and egg heads involved in picking these values.
There was a 24 volt standard proposed in the 1970's, but it never really caught on.
But I Don't Want Any...
So, what happens when you hook a plain old dynamic mic (that doesn't require phantom) up to a cable that is providing it? Nothing. Since the same voltage is being applied to both sides of the mic's transformer, there's a net zero potential across it. In other words, it doesn't need it so it ignores it.
Big Bang Theory
Now, it's never a good idea to connect or disconnect an audio cable unless the channel is muted. But with phantom power on a cable, it's worse. Even if you're working with something that doesn't use phantom power, connecting or disconnecting the cable with the channel unmuted is going to sound like a .22 caliber weapon going off. You won't damage the device you're hooking up to, or the mixer. But, there's a good chance you'll blow a high frequency horn out. This is why (when the equipment allows it) I only turn phantom power on the channels that need it. Most digital and higher end consoles allow this. But for less expensive gear, it's typically all or nothing.
You've been warned.
No, You Can't Do That
I mentioned earlier that some active DI boxes and preamps can use phantom power instead of their internal batteries. This is cool (as anytime you can take a battery out of the equation is a good thing in my book). But, there's one major drawback. That is, when using phantom power you can not use the ground lift if you need to. Phantom counts on the ground being there as the return path for the current. If you have to use the ground lift, you'll have to run the device on batteries.
We've been at this for almost a year now, and I've wondered how much of this stuff I've been writing about has stuck with the readers. So next week...we're having a quiz! Yeah, you read that right. It will be multiple choice, and all of the questions come from what we've covered here.
And just to give you a little more motivation, the first person to email me with all of the correct answers....heck, I'll send you an iTunes gift card. In the event no one aces the test, the highest score with the earliest time stamp wins.
Study up and get some rest.
Ken
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