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Phantom Power and Microphone Interconnect Basics

From the "ground" up!

Eddie Ciletti for the July issue of EQMagazine

If you are new to the audio scene, let's start with a few basic electronic concepts then move on to Microphones and see how all of the connections are made, audio and phantom power.


What's that you say? You don't know your AC from your DC' Audio is considered an Alternating Current, a.k.a. "AC." (So is 120-volt "wall" power.) But electronic circuits need Direct Current (DC) to turn them on, from batteries or power supplies. Like a speaker in reverse, a dynamic mic consists of a coil of wire suspended in a magnetic field. When vibrations move the cone or "diaphragm," the energy stored in the magnet is transferred to the wires. (A Dynamic mic is passive and needs no power.)

The preamplifier inside Vacuum Tube microphones requires both plate and filament voltages.  Power and audio are delivered via special, multi-conductor cables and non-standard connectors from a dedicated power supply. Only then does the mic-level signal appear at a standard three-pin XLR connector. Transistorized microphones require much less power and can operate from a battery, hence the idea for phantom power, a system of distributing a DC voltage through a standard mic cable.  All condenser mics (except electrets) requires a fairly large, but low current DC polarizing voltage that is applied to a diaphragm ' similar to a drum head, but thinner and plated  with a molecularly thin conductive layer that is typically gold. The signal is not strong enough to venture into the outside world without an internal buffer / preamp (active electronics) that also requires power. 


Compared to both consumer (-10dBV) and professional (+4dBu) Line levels, Microphones produce a signal that can be considerably lower in level, hence the need for an external preamplifier. Every precaution is taken to minimize noise. By design, this begins with using two wires for the signal ' referred to as "balanced" ' plus a shield. Contrast this with a passive electric guitar ' that is, one with no active internal electronics (i.e., a battery is required). A guitar cable uses a single conductor plus a shield, an unbalanced signal.

PHANTOM POWER: First you see it, then you don't

The rear of a Female XLR is shown in Figure One with a Red wire on Pin-2 and a Black wire on Pin-3. Pin-1 is called "ground" and the reference to terra firma implies that the metal body of the mic will ultimately connect to the "earth" and is therefore safe to touch even if you are barefoot in a pool of water (the Green wire). A good ground connection also improves noise immunity.


In most cases, a "balanced" signal appears on both pin-2 (hot) and pin-3 (cold.) "Hot and cold" refer to Polarity, plus and minus (+ and '), respectively. As you can see on the left side of Figure One, the same signal appears on pin-3 with reverse-polarity, that is, 180 degrees "out-of-phase." Even though the AC signal is constantly changing, it is important to establish a reference, in the same way that a loud speaker has "polarized" terminals. A kick drum creates air pressure that moves the mic diaphragm, generating a positive going voltage on pin-2. After trips to and from all of the processing and recording gear you can imagine, a woofer should recreate that same in-your-face gust of air. (Only better, of course!)


With properly designed balanced gear, output polarity equals input polarity, so it doesn't matter whether Pin-2 or Pin-3 is designated as "hot," so long as the input and output wiring is consistent ' with regard to polarity ' throughout your system. You will occasionally have to interconnect unbalanced gear, in which case the tip (of a quarter-inch plug) or the center-pin (of an RCA connector) are always "hot." However, be very careful when interfacing unbalanced gear that uses XLR connectors. If one is designated pin-2 hot and the other as pin-3 hot, all you will get is NOT!


A microphone's source impedance is 200 ohms (W ). Speakers are lower at 4W , 8W or 16W , but a Professional "lo-Z" mic is optimized to drive a long cable without signal degradation. This is in direct contrast to a passive electric guitar, which is an unbalanced, hi-Z device that is susceptible to every snap, crackle and pop technology can generate.

Please Note:

The built-in preamps in modern condenser microphones do not necessarily produce signal on pin-3, but the impedance of pin-3 must match that of pin-2 for noise immunity.


Two (identical but) out-of-phase signals do not combine on a mixer, they cancel. A Mixer, by nature, "sums" all of the channels together. But subtracting a balanced signal ' by using a "differential" input amplifier (active) or a transformer (passive) ' generates two results. First, the audio signals "add up" instead of cancel. Second, any noises common to pin-2 and pin-3 (like the red "spike" in Figure One) are left behind. "Common Mode Rejection" is the term used to describe how well a preamp can ignore the fact that you ran a mic cable right next to a wall wart. Try that with a guitar cable!


A vintage preamp is not likely to have phantom power because either dynamic mics were used or the condenser mics of the day had their own external supplies. In addition, you will occasionally run into a piece of gear with a quarter-inch mic input. To connect a balanced dynamic mic, you will need a transformer/adapter, Radio Shack part number 274-016. It may not be audiophile-grade, but it will get you started.


Though miniaturization started before "solid state" electronics, transistorized circuitry ushered in a new era of devices whose power requirements were considerably lower than their vacuum tube counterparts. Older versions of the Neumann U-87 included a "window" in the case to expose a meter that indicated the presence of either the internal Battery or the "external" Phantom power.

Battery operation implies that very little current was required. That a system was devised to send power down a standard mic cable was even more clever. Quite simply, +48 volts DC is piggybacked on top of both AC signals ' on pin-2 and pin-3 ' via two resistors, without disturbance. (This is the "Phantom" signal.) The negative or "return path" to the DC supply is connected to pin-1.


To create a phantom power adapter, your project box should include two connectors: a Female XLR as mic input and a Male XLR to feed the mic signal to the preamp. Then ' with a suitable 48-volt DC power supply ' make the positive and negative connections to the Yellow and Green wires, respectively, as shown in Figure One. (Most parts are available from such "catalog" suppliers as Radio Shack, MCM Electronics, Digi-Key and Mouser Electronics.)


The typical value of the two resistors is 6.81k-ohm. More than just the audio and the source impedance (of pin-2 and pin-3) must be balanced. The "phantom" resistors must also match to a tolerance of at least 1% ' .1% tolerance would be even better.

The resistor color code is interpreted as follows. The first three stripes from left to right are the significant figures. For example: 1st stripe (blue = 6), 2nd stripe (gray = 8), 3rd stripe (brown = 1). The 4th stripe is the decimal multiplier, in this case brown = one "zero" instead of the number "one." The fifth stripe is the tolerance: brown = 1%. So, from that you can conclude that the value is 6.81kW . Low-noise metal-film resistors were used in Figure One along with a "local" capacitor to filter any noise.

Good luck!


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