MIC Pres & Phantom Power

You’ve bought a cool, new digital audio workstation. You’ve saved up for a really great vocal microphone. The only thing left to do is to get the signal from the mic into the DAW. For this you need a microphone preamplifier, or “mic pre.” Populating the marketplace is a dizzying array of mic pres from which to choose. Some of the differences are in function or features, while others are purely aural or aesthetic. The one (or ones) you choose depends on your needs, your taste, and of course, your budget.

Not long ago, an engineer would use whatever preamps were built into the console. The concept of a rack full of outboard mic pres in a control room was unknown a couple of decades ago. A mic pre was part of a console, not a product unto itself. With Neve and API consoles in common use, this was certainly not a disadvantage. As consoles grew in size and complexity, the expensive, power hungry features like class A amplification and discrete electronics gave way to integrated circuits. Engineers began to miss the sound of the classic consoles, and as the aging fleet was decommissioned, the mic pres began to find their way into racks that lived alongside the new consoles. The market for outboard preamps was born.

A dynamic microphone creates a small current when its diaphragm, excited by sound pressure, moves a coil within a magnetic field. In a condensor microphone, a charged capsule consists of a diaphragm that moves in relation to a fixed back plate, thereby varying the capacitance and creating a current. The current created by microphones is too small to be of use to an audio mixer or recorder. It needs to be brought up to a standard line level to interface with other equipment. This is the job of the mic pre.

Among the features one normally finds in a mic pre, phantom power is one of the most common. This is a voltage, usually +48V, that is sent up the XLR cable to power condensor microphones. The current produced by the variable capacitance in the capsule is so diminutive that there is a small preamp contained in the mic. Phantom power is used both to charge the capsule and power this small preamp. Some mic pres, especially those taken from early consoles, lack phantom power. Early tube mics had their own separate power supplies, and some consoles had switchable phantom power supplies apart from the actual mic pre circuit. When rack mounting these pres, your tech will need to add phantom power to enable use of most condensor mics. There are also outboard phantom power supplies available for just such a circumstance.

When shopping for mic pres, you will notice that many have additional features, such as EQ, filters, dynamics processing, and metering. Products with all of these features are often called “channel strips” because it is like having a full processing channel from a high-end console. Some also have an input impedance selector. This is very helpful if you have a large collection of vintage and ribbon mics. The proper setting for each mic will minimize output noise and delay the onset of distortion.

Whether you prefer a minimalist pre with a transparent sound, or a tube design packed with features and oozing colorful charm, there exists a pre to fit your needs and budget. As your collection grows, so does your palette of available colors. -Jay Frigoletto

Question for Jay? Visit him online at www.promastering.com or www.myspace.com/sslmixer.