Dear Anklebiters, I have a mixer with a global on/off switch for phantom power. If I am using just one or two mics that need power, will it hurt anything else to leave it on?
Charles Cox Moapa, Nevada
Paul: Condenser microphones require a polarizing voltage and power for their built-in amplifiers. Sometimes provision is made to supply this voltage directly through the microphone cable. The procedure is called phantom powering, and the most common phantom supply voltage available in mixing consoles is 48 VDC, although 24V supplies are widely used. Most phantom powered mics can operate on a wide range of supply voltages, from as little as 1.5 volts or 9 volts up to 50 volts. In a phantom power system, the polarizing supply voltage is placed on both of the signal lines in a balanced connection, with the same polarity on each line. Dynamic microphones connected in a balanced system with a phantom power input are then protected from damage, theoretically, since the system results in a net zero DC potential across the coil. A dynamic mic connected unbalanced to a phantom power input may be destroyed, however!
It is therefore very important to be aware of whether a mixing console input is wired for phantom power. Most inputs provide a switch to disable the phantom power when it is not needed. "Always be sure that this switch is off when dynamics, or electret condensers with internal batteries, are connected to the input." That quote is from the Yamaha Sound Reinforcement Handbook, Second Edition.
I realize that many of the smaller consoles/ mixers have a global phantom power switch that puts the voltage across all of the microphone inputs. I haven't had any problems with the Mackie 1402 mixer in this regard. Just be careful, and don't use it when it is not needed. Brian, what are your thoughts on this subject?
Brian: As long as you are using a balanced connection between your microphone and your phantom power source, and you are using modern microphones, you are unlikely to have a problem with running global phantom power. I have found that some older dynamics don't behave well when connected to an input that is energized with phantom power. I have one particular vintage microphone, an AKG D12e, which doesn't produce a clean audio signal, and even emits an audible hum or buzz from the microphone itself. I had taken it to a small club one evening to use on the kick drum and got stuck with a global-phantom console. Since my vocal microphones were all condensers, I had to come up with a solution to work around the problem. I wound up inserting a ground lift adapter in the microphone end of the XLR line. This eliminated the 0V reference from the phantom circuit, and the microphone no longer "saw" the 48 volts of phantom power. And since the ground was only lifted at the microphone end of the cable, my signal was shielded from the adapter all the way back to the mixer's input. I opened myself up to some RFI interference, and probably increased the signal-to-noise ratio of the microphone, but with the loud source material of a kick drum, a small increase in noise was not a noticeable problem in a club PA.
The other possibility, and really the more technically correct one, is to use an outboard phantom power supply for your condenser microphones and leave the global phantom power turned off at the console. These outboard supplies are made as accessories by many microphone manufacturers, such as Audio-Technica, Crown and Shure. To use one, it is simply connected between the microphone and the console input. Most models have standard XLR connectors to make interfacing easy.
Paul: The question that comes to mind when using a monitor console and a FOH console is: Which console provides the phantom power? I have tried variations of this problem and found some interesting things. I tried phantom from monitor world but used a snake that had transformer isolation, and I couldn't get power to the mics. I then had to use the FOH phantom power. I have tried it with ground lifts and had some mixed results, depending on what was lifted. I recently spoke with Howard Page of Clair Bros./Showco, and he told me to have phantom power turned on the channels on both FOH and monitor world. This method balances out the load on the consoles and gives the full 48 VDC to all of the items needing the voltage. He specifically mentioned that Countryman DIs like the full 48 VDC or they won't sound as good. The snake must not be transformer isolated, and the ground (pin 1) may be lifted. Brian, what do you think of that statement?
Brian: I'd definitely agree that the venerable Type 85 likes its healthy dose of current. What most people don't know is that there is also a battery compartment inside, and if you have a battery installed and it goes dead, that DI doesn't want to work with or without the phantom power.
As for the idea of running phantom from both consoles, it certainly seems like a valid idea to me, assuming that you are using a non-isolated snake with all of the grounds landed. The current flow from the two consoles will sum, providing more total power to the microphones. And when you are sending power down a 24 gauge wire, you probably need every bit of help you can get. This is especially true when even the monitor console is likely to have 100 feet or more of cable between it and the microphone. That may seem a bit much, but if you have a 50-foot monitor leg, a 25-foot sub snake out onto the stage and a 25-foot XLR cable plugged into that, it all adds up quickly.
Before you had mentioned the concept of running phantom from both consoles, I always thought it was best to run it from the closest source. However, some guest engineers would find it strange when I was running my phantom from monitor world. They would protest that that wasn't the "standard." Where do people learn about these phantom standards?
Paul: I think that habits start and then they become "the standard." I haven't yet tried this, but on my next gig I am going to do it and read the voltage and see what happens. I am glad that people, who are much smarter than me, figure these things out and teach us a better way. I guess that we need a "myth buster" in the sound business. If you, the reader, will try this out and let us know what your experience is, we would love to hear from you. Also, tell us any stories you have regarding phantom power and its effects on your mics.
Got a question? You can reach the Anklebiters at [email protected].
