Phantom Powering for Condenser Microphones

Post date: Sep 26, 2009 9:10:44 PM


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. (the cable capacitance and the capsule capacitance form a capacitive voltage divider) 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. ››

What is it, exactly?

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 almost 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. ‹‹ Neumann says that they gave the use of the trademark to public domain. ››

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 (phantom circuit).  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 <From pin 2 AND pin 3 to pin 1>. The audio travels via pins 2 and 3 differentially (across the pins), and the power travels across pins 2 and 3 simultaneously and pin 1.

Phantom Powering — What Really Happens

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 any 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 nearly always use tip-ring-sleeve phone plugs or miniature XLR connectors and they are usually associated with instrument pickup applications or microphones for camcorders.

Some of the terms used to describe these "foreign" powering systems are: PIP, Electret Bias, Inline power, and (grrrr)Phantom Power. This is not only misleading, it's just plain wrong.

Regardless of what any marketing propaganda says, these units are NOT phantom powered and NOT compatible with the system defined in the DIN standard. Applying phantom power will damage these units.

Phantom powering is defined in DIN standard 45 596 or IEC standard 268-15A.

What works?

To be compatible in a phantom powered system, a device (microphone, preamp with a microphone-style output, or direct box) must have a balanced and floating, low-impedance output. This includes all microphones commonly used for sound reinforcement and recording such as the Shure SM-57, SM58, Electro-Voice RE-15, RE-16, RE-20, ND series, Beyer M88, M160, M500, AKG D224, D12, D112, Sennheiser 421, 441, and MANY, MANY others.

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). Audio Technica and Microtech Gefell have developed tube condenser microphones that are phantom powered and do not require an external power supply. For further information, consult the Microtech Gefell  website

Older condenser microphones (Sony C22, C55, 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.

The older EV microphones (EV 1777, PL76) can be converted to phantom powering. It's a simple conversion, but it does require some electronic work (you need to know how to build things with parts). Click Here  for the conversion instructions.

What doesn’t work?

The list is short:

Many pieces of electronic equipment aimed at the musician marketplace have direct outputs intended to make connection into a professional sound system easier. In theory at least, you eliminate needing a direct box. The problem is that many of these manufacturers apparently haven’t heard of phantom power.

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. If you're not sure, use a direct box.

I have seen at least one brand of amplifier (G-K) with outputs that are supposedly phantom power proof that aren’t (meaning that the presence of phantom power didn't harm the amplifier, but it did cause noise that was intolerable). 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.

A Simple Modification

This modification makes a balanced transformerless output phantom power proof. I came up with this idea when I worked at Mackie Designs. It works. It has been incorporated into all of their mixers that have XLR connectors for their outputs. I’ll describe the mod in words. This should be crystal clear to a competent service tech.

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. Reverse the connections to the capacitors if necessary. If you have to desolder them, I'd replace them with 63V parts, correctly polarized, of course.

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.

What about T-Powering?

T-powering is another remote powering system. It is sometimes known as A-B powering or as "Modulation Lead Powering." It uses pins 2 and 3 to carry both power and audio. It is NOT compatible with dynamic, ribbon, or phantom powered microphones. For further information, please refer to my article   about T-powering and to Richard Hess'  excellent page.

Do/Don’t Chart