Buyer's Guide: Microphone Preamp

A crucial link in the audio chain is the microphone preamplifier, aka the mic preamp or mic pre. The output level of a microphone is very low and requires amplification to bring it up to line level. That’s what a mic pre does. But a mic preamp can’t drive a pair of speakers; that’s the job of the main amplifier.A key part of the job for a mic pre is to keep input noise to a minimum and to amplify without distortion.

Being the first amplification stage of the microphone signal, the mic preamp is essential to getting a good sound. Choose a poor one and you’ll be working harder to overcome its limitations. Choose a good one and processing your sound will get a lot easier. But what are the differences between average preamps and better ones? Let’s look at some of them.

Types of Microphone Preamps

There are three main varieties of mic preamps, based on how they amplify the signal: tubes, transistors, and integrated circuits, or ICs (also known as chips).

Vacuum Tubes

The granddaddy of preamplifier design is based around vacuum tubes, or valves. Until the advent of transistors, tubes were the only game in town. After falling out of favor due to size, weight, and cost, vacuum tubes have made a resurgence. There are many models to choose from nowadays.

Vacuum tube mic pres, like all tube gear, are known for their warmth, which is a result of the even-order harmonic distortion they create. Tube mic pres are a good choice if you want to soften the signal a bit, especially before going into a digital audio workstation (DAW).

Transistors

With the advent of transistors in the 1960s, just about every part of the audio signal chain was affected. Transistors do the work of tubes, but with much less size, heat, and cost. The earliest designs were "discrete," meaning all components were placed individually on a circuit board or hard-wired together (often by hand-soldering). Many of the classic mic preamp designs were discrete transistor types. Known for their punchy sound and accurate reproduction, discrete transistor designs remain a favorite today.

Integrated Circuits

The integrated circuit, or IC, is exactly what it sounds like: a circuit where all the components are put together on a single board. Modern ICs, like in your computer, can have billions of transistors – try hand-wiring those! An obvious evolution from discrete components, the IC, or chip, made the circuitry components even smaller and more powerful. But many feel the benefits are outweighed by the loss of "color," as ICs tend to be accurate to the point of being sterile. There is certainly a place for them in modern designs, but at the crucial preamp stage, most still prefer a discrete design.

These three types of preamps have their basic amplification function determined by the type of component. But this is far from the whole story when it comes to preamps. The next major contributor to the sound is the transformer, or lack thereof. Transformers are also called X-formers, or trafos.

Transformers

Mic pres can be transformer-coupled on the input and/or output, or they can be transformer-less. Which is better? There is no right answer. What is beyond question is that transformers alter the sound, and impart a certain color (a pleasant word for distortion). The design of transformers is almost as much an art as a science. Generally, no two manufacturers’ transformers will sound alike. Rupert Neve’s classic designs all incorporated transformers, and he has long been a vocal proponent of their use.

Trafos are used in a preamp design to isolate the signal from RF interference, provide for lower input noise, and allow for a higher maximum signal before overload. But since they will impart some color, however slight, many designs do away with them altogether. Such a transformer-less preamp will be more accurate, due to the absence of distortion, but will also sound more clinical. If you’re looking for full transparency and want to add color later, a transformer-less preamp is the way to go. Many like them on acoustic instruments where they’re looking for lots of clarity, such as pianos or acoustic guitars, or when they want to preserve the transients as much as possible, like on drum overheads.

Different Types of Amplifiers

The next thing to consider when searching for the right mic pre is: what type of amplifier is it? Whether run by tubes or transistors, with X-formers or not, there are two main types of mic preamplifiers: Class A and Class A/B (Class B is not generally used for mic preamps). Since the heat associated with power generation is the main drawback of a Class A amp, and since preamps generate very little power, most preamps will be Class A.

Class A Amplifier

The term Class A amp refers only to the design – not the components or quality – not whether it is hand-wired or done by automated machinery. It simply means the amplifier is always ‘on,’ in the sense that current is constantly available to it. The result of such designs has a major benefit and a major drawback.

The good news is that because the amplifier deals with the entire 360 degrees of the waveform, it produces a very accurate analog of the incoming signal. The bad news is that being constantly "on" means that lots of power is wasted while the amp is merely sitting idle, waiting for a signal. As such, the amp is very inefficient in terms of power in vs. power out.

This is especially troublesome in tube preamps where lots of power is converted into heat, and the tubes are always working to some extent, diminishing their longevity. Since a Class A amp can use a single device for its amplification, these types are called “single-ended” output stages, as opposed to the “push-pull” output stages of the two devices in a Class B amp.

Class A/B Amplifier

A Class B amplifier splits the waveform into two halves, with a pair of amps working on each half. This makes it more efficient than Class A, but also provides for distortion where the two halves cross the zero point of the waveform. Since this is undesirable, some designs compromise and take the best aspects of each type to create a Class A/B amplifier.

In this scenario, the amplifier’s two devices work in an on/off cycle to reproduce more than half the waveform, say 181 to 200 degrees, depending on design. This greatly reduces (or eliminates) the point where both amplifiers are ‘off,’ thus reducing the possible distortion. As a bonus, it will also operate more efficiently than a Class A amp.

Impedance Matching

Another consideration with microphone preamplifiers is impedance matching, a subject that has proponents on all sides of the issue. All microphones have an operating impedance, a measurement of their resistance to current. Many mic preamps will have a high fixed impedance, to allow for a wide range of mic impedance on their input. But several designs are available that allow for a choice of a few different impedance settings, or even a continuously variable impedance within limits.

It’s important to remember that exact impedance matching is not a necessity in modern amplifier design. An exact match is only necessary when trying to achieve optimum power transfer. But in this scenario, what’s more important is audio fidelity via voltage transfer, and not power transfer. So lower impedance mics can certainly be output to higher impedance mic pre inputs without a problem.

The point of providing various impedance choices is all about tonal color. The sound of a given microphone, as well as the frequency response, will change based on the preamp input impedance, in ways ranging from subtle to obvious. The outcome will also depend on whether the microphone itself has an output transformer or not, and whether it’s a dynamic, ribbon, or condenser.

Older ribbon mics (and some modern transformerless types), often operate with 50 ohms impedance, and can benefit from a mic pre that can go below the typical 1500 or 2K Ohm resistance. This typical range for mic pres is based on the rule of thumb that the input impedance of the preamp should be about 10x the microphone’s source impedance (aka the ‘bridging load’), to increase the signal-to-noise ratio.

Since most mics have an output impedance of 150-200 Ohms, the typical mic pre is set to this range. But it’s fun to break the rules too! By giving the user options, multi-impedance mic pres allow creative choices to be made in regards to the timbre of the incoming signal. Thus, the choice of impedance becomes another tool in the box of the audio engineer.

Different Form Factors Of Microphone Preamps

Microphone preamps come in all shapes, sizes and formats to help you achieve your goals in the studio. Continue below to discover the three predominant form factors of microphone preamps and some suggested pieces of gear for each format.

Rackmount Mic Pres

The traditional 19-inch rackmount mic preamp has been the go-to for replicating the classic sounds of console mic pres for decades.

500 Series Mic Pres

The 500 Series format is an easy way to add more signal processing to your workflow in the tightest of quarters or when you're on the go. Mic pre modules for 500 Series racks can offer up the same vintage-inspired warmth or pristine sounds of their larger 19-inch rackmount counterparts.

Channel Strips

Some mic preamps are combined with an equalizer or compressor, or both, to create a "channel strip." Channel strips allow you to get all of the basic signal processors all in one 19-inch rackmount unit.

Desktop Preamps

These are portable mic preamps that, as their name implies, reside on your desktop rather than in a rack.

If you’re interested in learning more about microphone preamplifiers or ordering some for your studio, contact a Vintage King Audio Consultant via email or by phone at 866.644.0160.

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