Copyright (c) 1999-2002 by Rick Chinn. All rights reserved.
Revision date: 1/06/2003
Condenser (capacitor) microphones differ from dynamic and ribbon microphones because they are not self-generating. That is, they cannot generate electricity in response to an impinging sound wave. A condenser microphone modifies an external source of electricity to reflect the effects of a sound wave striking its diaphragm. Dynamic and ribbon microphones use magnetism to generate electricity in response to a sound wave: they are self-generating. Furthermore, both of these types of microphones are inherently low impedance devices. It is possible to connect a dynamic microphone element directly to a balanced, low-impedance mixer input. Many commercially made dynamic microphones do just that.
On the other hand, a condenser micorphone is an inherently high-impedance device. How high? Verrrrrrry high. On the order of a billion ohms (1 Gigaohm). This is high enough that the inherent capacitance of a foot of shielded cable would audibly reduce the output of the microphone. All condenser microphones have an impedance converter, in the form of a vacuum tube or field-effect transistor (FET), built into the microphone, and located extremely close to the microphone element. The impedance converter and the microphone element itself require an external power source. <To be strictly correct, electret condenser microphones are a bit different as the microhone element does not require a power source for operation (it is more or less permanently self polarized). Regardless, the impedance converter still requires an external source of power.>
The obvious external power source for any modern microphone is a battery. About the only electronic advantage that a battery has is that its output is pure DC. The only other advantage (to the battery company) is that you have to keep on buying them.
Tube microphones require several different voltages for operation. This invariably means a multi-conductor cable and non-standard (not XLR) connectors. A tube microphone will always have an associated external power supply.
In the late 1960’s, Neumann (you know, the folks that brought you the U47 and U87 microphones) converted its microphones to solid-state, adopting a system of remote powering that they called, and trademarked, Phantom Powering. Because of the trademark, some manufacturers use terms like Simplex Powering, etc. Over the years, the trademark has become genericized and now refers to any device that is powered according to DIN standard 45 596.
So, where is the Phantom of Phantom Powering? Because
(like the Phantom in the old comic strip) it’s there when you need it,
and invisible when you don’t. This technology is not new, it actually predates
rocket science. Like so many other things in audio, it was brought to you
by the telephone company, who used it to get an extra circuit from a pair
of wires. In effect, so does your phantom powered microphone.
What is important is: phantom powering is a compatible system. Your dynamic/ribbon microphones as well as your condenser microphones work side-by-side, from the same microphone inputs, without further thought on your part.
Technically speaking, phantom powering refers to a system where the audio signal is applied to the balanced line in differential-mode <across pins 2 and 3>, and the DC power is applied common-mode <pins 2 and 3 plus pin 1>. The audio travels via pins 2 and 3 differentially, and the power travels between pins 2 and 3 simultaneously and pin 1. Microphones that do not require power simply ignore the DC present between pin 2, pin 3, and pin 1. If you measure with a voltmeter between pin 2 and pin 3, you will read 0 Volts DC. This is what your dynamic microphone sees. Measuring between pin 2 and pin 1, or between pin 3 and pin 1, you will read the phantom power voltage, usually 48V, without a microphone connected. The dynamic microphone, as well as your balanced mixer input, ignores this voltage.
Lately, the term phantom power has been perverted to refer to any remote powering system. In the strict sense of the DIN standard, this is not true. Furthermore, microphones or transducers that claim to use this system are not compatible with the DIN standard and will almost certainly be damanged if connected into such a system. Fortunately, these systems use tip-ring-sleeve phone plugs or miniature XLR connectors and they are usually associated with instrument pickup applications.
Phantom powering is defined in DIN standard 45 596 or IEC standard 268-15A..
If you are fortunate enough to own any tube condenser microphones, such as the AKG C12, Neumann U47 or U67, these microphones may be connected in a phantom powered system and will operate without regard to the presence or absence of phantom power. They will always require their external power supply (which must be plugged in and turned on).
Older condenser microphones (Sony C22, EV 1777, PL76, etc.) that were either made before the adoption of phantom power, had the phantom power feature omitted as a cost savings measure, or omitted it for product differentiation reasons (stupid, really stupid). These microphones will operate, but they will always require their own batteries. Consider them a battery-powered dynamic microphone. There is one tube condenser microphone that can operate using phantom powering. It is the Microtech Gefell UM900. For further information, consult their website
The list is short:
The problem is that their balanced output is transformerless, and (worse) can’t tolerate the 48V power present. Usually, the output coupling capacitors are 25V parts, and most often connected so that their + terminal goes to the opamp that drives the output connector. When connected into a phantom powered system, the 48V reverse biases the capacitor and causes trouble. This trouble can be temporary and sometimes it can be permanent.
If you’re a musician and this scares you, it should. What can you do? Call the manufacturer and ask. Have a service tech who understands the problem check the schematic. A really bright service tech can study the circuitry without a schematic by looking at the unit and figuring out what the manufacturer did circuit-wise.
I have seen at least one brand of amplifier (G-K) with outputs that are supposedly phantom power proof that aren’t. In this case, I spoke to their chief engineer, and we both agreed that what he was doing should work, but in practice it doesn’t. If you are connecting to a G-K/Gallien-Kruger amplifier, you should use a direct box.
Electronic keyboards are also suspect. I’ve had other musicians tell horror stories of blown instruments caused by assuming that the balanced outputs on the instrument were phantom power proof. When in doubt (and that’s my usual state), use a direct box.
I’m assuming that the circuitry of the balanced output is driven by a pair of opamps, with their outputs arranged to be out-of-phase with each other. It doesn’t matter if the output is cross-coupled or not.
Find the output coupling capacitors. They need to be at least 25V units, polarized so that the plus side connects to the XLR connector.
There should be a resistor on the XLR side of the coupling capacitor going to ground. There should be one for pin 2 and one for pin 3. If not, add them. The value needs to be 10k or lower (equal value).
The resistors put enough of a load on the phantom power coming from the mic preamp to reduce the phantom voltage to something that the capacitor can withstand.
| Do | Don’t |
|---|---|
| If you are plugging in a condenser microphone, do verify that your microphone can be phantom powered. | Worry about your other microphones as long as their outputs are balanced and floating. |
| Ensure that the microphone’s output is low impedance, balanced and floating. This is especially important for vintage ribbon microphones like the RCA 44BX and 77DX. | Connect microphones or devices that do not conform to the DIN 45 596 standard. |
|
Mute the sound system when turning
the phantom power on or off, or when connecting or disconnecting microphones.
Lest you forget, the resulting loud, nasty POP may be your last. |
Don’t connect A-B or T-system microphones (another remote powering system) without suitable adaptors. |
| Turn the phantom power off when connecting ribbon microphones (cheap insurance). It's OK to turn it back on after making the connection. Be sure that there is some way to bleed the charge off of the power supply if you chose to do this. See the discussion on patch bays, below. | Worry about your tube condenser microphones. They’re compatible; they just can’t be phantom powered. |
| Do read the discussion about phantom powering causing magnetized transformer cores. | Put your microphone lines thru a TT or TRS patch bay. See further discussion, below. |
If you're building a studio, this is a bad idea. Avoid it.
Use XLR connectors instead. Yes, they're not as high density as a TT bay. Don't hot patch microphones when the phantom powering is turned on. Turn the phantom power off first. Even this may not be sufficient.
The problem is that the tip of the plug can contact ground as the plug is being inserted. With a transformerless input, this can discharge/charge the input blocking capacitors thru the microphone's output transformer. This could magnetize the core of the transformer, zap a ribbon microphone, or (remote possibility) damage a dynamic microphone.
With a transformer coupled input, you risk magnetizing the core of the console's input transformer, ruining its distortion characteristics.
If the microphone's output is transformer coupled, the microphone's output transformer could also have its core magnetized, ruining its distortion characteristics.
This is not good.
There are several ways to disable or turn off the phantom power in a mixer.
Disconnect the input to the rectifier/filter system in the phantom power supply. Mackie mixers with a global phantom power switch use this method. There should be a bleeder resistor somewhere within the supply to discharge the filter capacitors.
Break the feed to the 6k81 phantom feed resistors, either individually or in groups. In transformerless consoles, there should be a discharge path for the input coupling capacitors.
Using a voltmeter, or the console schematics, determine which method your console uses and how long it takes for the voltage at the microphone inputs to fall to zero volts after the phantom power has been switched off. If you can't do this yourself, get someone who can and have them explain it to you until you understand it. Have them read this article.
When patching microphones through a patchbay, either turn off the phantom powering first, and/or unplug the sensitive microphones (ribbons) before repatching any of these lines.
This issue at hand is the magnetization of the transformer core caused by unbalanced current pulses through one of its windings. Once this occurs, it is quite difficult (although not impossible) to undo. Once the core has been magnetized, the transformer's low distortion characteristics go by the wayside.
First, let's look at the different ways that this can happen:
I guess there IS a GOOD reason to have phantom power switches on every input!
There are several ways to split a microphone's output. They range from cheap (wye-cord or paralleled inputs) to expensive (BSS active mic splitter). Somewhere in the middle there is the venerable old transformer coupled splitter.
For sure, the active splitter is the best solution. The microphone only sees one load, which is the splitter's input impedance. Each of the outputs is buffered from the rest. The only rub is that the splitter's sonics are imposed on everyone that takes one of the split outputs.
In this case, the splitter supplies phantom power to the microphone and could care less what the mixers are doing.
Transformers are the next-best solution. Unfortunately the usual 1:1:1 transformer still parallels the input impedances of the two consoles. From a microphone loading perspective, this is no better than simply paralleling the mixer inputs. If you run into a nasty grounding situation, the transformer coupled splitter will save your bacon. For phantom power purposes, what usually happens is that the FOH mixer gets a direct feed from the microphone and the monitor mixer gets a transformer-isolated split (in reality, a parallel feed) from the mic. The FOH mixer supplies phantom and everyone is happy. In a really nasty scene, both mixers might get transformer splits, and the splitter itself has phantom feed resistors and a 48V DC source.
In many instances, simply connecting the mixers in parallel works just fine. You can get into trouble if you aren't really careful about how things get grounded, especially with regard to the FOH console's ground system and the monitor console's ground system. If either of the boards has pin-1 trouble, this may be exacerbated. I've used this method myself for over 30 years and only occasionally had problems. In each instance, I can trace the source of trouble back to not following the rules. The big rub with parallel connected mixers is the old question: "Who supplies phantom?"
Some consoles don't like having phantom power applied via their microphone inputs when their internal supply is not switched on.
This is especially true of mixers that don't have individual phantom on/off switches or at least switches for groups of inputs. What happens here is that the manufacturer took the easy way out to turn the phantom power supply on and off: they switched the AC feed to the phantom supply rectifier. The voltage regulator that supplies 48V to all of the phantom feed resistors is always connected. When the mixer that isn't supplying phantom has it's inputs 'backwashed' with phantom power, the regulator doesn't quite know how to deal with this voltage coming at it from what it thought was its output.
The cure here is a simple series diode connected at the output of the regulator, and a large electrolytic capacitor to ensure that the output impedance of the regulator stays low in spite of the series diode.
The Mackie SR series mixers that I had a hand in have this series diode. Other mixers may, and it pays to check the schematic for this feature.
Now, having said that, let's explore the situation when both mixers supply phantom.
Quite simply, we have two 48V supplies driving the mic through 6800 ohm resistors on each leg. As far as the microphone is concerned, it's no different than if we halved the value of the feed resistors to 3400 ohms each. So, again from the microphone's perspective, the mic is seeing 48V thru 3400 ohms in parallel with 3400 ohms. This means that twice the current is available to every microphone. Looking inside the microphone, we typically have a zener diode with or without some extra current limiting resistance. So, the effect of the extra current is that the zener diode works a bit harder.
I have used sound systems where both the monitor and FOH boards supplied phantom powering to the microphones. I've done this with just about everything from cheap electrets to Neumanns and AKGs. It works, and the microphones have all said, "so what." The Neumanns are especially immune to this. The 9-52V AKGs (old C451's, C414's) have to work their zeners a bit harder, but it's no big deal. They're a worst case example. My C451's and C414's still work just fine, thank you.
