Search for information about mic preamps (microphone preamplifiers) on the interwebs, and you’ll likely be overwhelmed with results. Many folks will go on and on about the out-of-this-world sound of their vintage tube-fueled preamp and how it makes everything sound like a million bucks. Others will brag about their colorless, super-transparent, transformerless, solid-state preamps that sound like “a straight wire with gain.” Some describe preamps in more practical terms, noting how they can use their gain-hungry dynamic and ribbon mics on super-quiet sources without excessive noise. So, what is a mic preamp really? A magic bullet for better sound? A practical device? Both? In this article, we’ll explore all facets of these oft-used yet oft-misunderstood tools and explain how you can best utilize them in your studio.
What’s a Preamp?
Preamps are an essential piece of studio gear. Although preamps are frequently used for creative coloration in the studio, their primary function is to boost a weak mic- or instrument-level signal up to a robust line-level signal. Ideally, they accomplish this task with minimal added noise and distortion. Why do you need a line-level signal? The answer is simple. It’s because your outboard studio gear, your audio interface, and pretty much everything else in your studio is designed to work with professional line-level signals (commonly referred to as +4 [plus 4]).
Most audio interfaces include at least one built-in mic preamp. There are also many standalone mic preamps available at varying price points. (A search for “preamps” on Sweetwater’s site reveals over 200 products!) Regardless of their form factor and feature set, all these devices perform this same essential function — boosting gain.
What Exactly Is Gain?
Gain refers to the change in voltage level in the audio signal path when an amplifier is inserted. It’s specifically how much an electronic circuit amplifies a signal expressed as a decibel value (in other words, 70dB of gain).
Gain occurs at the input stage — it’s applied before any other processing. On a microphone preamp, you’d adjust the gain depending on the sound level of the source you’re trying to capture. For example, if you’re recording a quiet vocalist, then you’ll turn up the gain to achieve an optimal signal level. Also, because gain regulates the level of the signal that passes into your other processors, it has a profound effect on the quality of your sound. Turning up the gain too far will distort and possibly overload your system; turning it down too low will result in a noisy recording with a poor signal-to-noise ratio. Proper gain staging will ensure a distortion-free result with minimal unwanted noise.
To learn more about gain staging, check out our article “Gain Staging Like a Pro.”
What Does “Line Level” Mean?
Professional line-level signals, such as those output by an analog console or pro studio gear, are measured at +4dBu, which translates to 0 on a VU meter. Consumer line-level signals, on the other hand, such as those output by CD players or other semipro audio gear, are measured at -10dBV.
When connecting gear, these levels must match; you interface +4dBu gear (typical signal level for professional gear) with +4dBu gear and -10dBV gear with -10dBV gear. Otherwise, you’ll experience issues due to the level discrepancy. For example, plugging a device with a +4dBu output into a -10dBV input will likely sound distorted because of the level mismatch — the output is overdriving the input. Conversely, plugging a unit with a -10dBV output into a +4dBu input will be quiet and will require turning the volume way up.
To learn more about different signal levels and how to make them work together in harmony, check out Lynn Fuston‘s article, “Understanding Signal Levels in Audio Gear.”
How Do Preamps Work?
Okay, what kind of sorcery do preamps use to convert mic- and instrument-level signals to professional line-level signals? Actually, there’s no alchemy involved. It all comes down to science. Professional line-level devices operate at an average output voltage of 1.23V. A preamp amplifies a mic-level signal (in the range of 0.001V to 0.1V) into a usable 1.23V line-level signal by replicating it and sending it through one or more amplification circuits. The resulting line-level signal is then output to the rest of your signal chain and ultimately to your audio interface, DAW, or recording device.
How Much Gain Do I Need?
We’ve established that preamps add gain to mic-level signals. Different preamps offer differing amounts of gain — some as little as 40dB while others provide 80dB or more. So, how much gain do you need? The answer depends on the type of microphone you’re using and what instrument is in front of it.
Condenser microphones typically have hotter outputs and thus only require 10dB to 30dB of gain. Condenser mics have active electronics built right into them that increase their signal output. Because of their hotter output, condensers generally produce acceptable results with almost any preamp — even the built-in preamps found on value-priced audio interfaces.
Dynamic and ribbon mics are another story. These lower-output microphones often need as much as 50dB to 70dB of gain, depending on the loudness of the instrument you’re trying to record and how distant it is. Simply put, a mic placed in front of a quiet source will require more amplification than a loud source. On top of that, the further a source is from the mic, the more amplification it will need.
Do All Preamps Sound the Same?
The sound of a preamp is determined by the sum of all the components in the signal path — discrete transistors or integrated circuits, vacuum tubes, transformers, wiring, capacitors, or any combination thereof. While the primary purpose of a preamp is to amplify your mic-level signal, the coloration imparted by different preamp circuits is often used as a creative tool.
Some microphone preamps deliver clean, transparent sound, amplifying mic-level signals without imparting any sonic influence or harmonic coloration. Both the Grace Design m101 and the Millennia HV-3C are examples of this type of preamp. Many modern music makers prefer this style of preamp as a clean, uncolored signal makes it easier to add additional post-processing to your tracks using software plug-ins.
Other microphone preamps contain transformers, such as the Neve 1073 and its countless derivatives. Transformers are coils of wire wound on a core of magnetically permeable material. When you put a signal through a transformer, it alters the sound. Depending on the input level to the transformer, the result may be saturated, imparting ear-grabbing harmonic content. This gives you the delightfully punchy sound that’s graced countless classic recordings.
Another type of character preamp is a tube preamp, which employs vacuum tubes for amplification. Vacuum tubes are renowned for their thick, pleasant sound that is chock-full of even-order harmonics. Most tube preamps can deliver everything from a warm-sounding character to a full-on crunch. There’s a reason why this antiquated technology is still around — nothing else sounds quite like it!
What Features Are Found on Preamps?
Different models of preamps contain different features. Here are some standard features you may find on preamps and an explanation of what they do. Most standalone preamps will offer one or more of these options.
- Phantom power is DC voltage, typically +48V, used to power the active electronics in condenser microphones.
- Variable input impedance enables you to tailor the impedance of the preamp (the load that the microphone output sees) to your mic and offers additional sonic flavors. Changing the input impedance can have a radical effect on a microphone’s tone. Some mics, most notably passive ribbons, will benefit from this feature so the mic sounds as intended. Some mic preamps even offer a “ribbon mic input” or a “ribbon mic” setting optimized for ribbon mics.
- Input pad allows you to lower, or “pad down,” the preamp’s input stage to avoid distortion. This is important for recording snare drums, cranked guitar amps, and other extra-loud sources.
- Output trim allows you to vary the output level so you can drive the preamp’s input stage harder (increase gain) for added saturation and then reduce the actual output that is delivered to the next device.
- Polarity reverse (sometimes erroneously referred to as “phase reverse”) does exactly what its name suggests — it reverses the polarity of your signal. This is a must-have feature when using more than one microphone simultaneously.
What Preamp Is Right for Me?
So, what preamp is right for you? There’s no easy, one-size-fits-all answer for that. Your budget, microphone(s), and application(s) will all play into that, as will your personal preferences regarding sound. Luckily, we’ve created “best of” articles to get you started.
- Best Preamps for Snare Drum
- Best Mics and Preamps for Acoustic Guitar
- Best Preamps for Recording Vocals
Want to hear how big a difference a vocal preamp can make in your recordings? Head over to our vocal mic preamp comparison and take a listen!
Further Reading
Microphone Preamp Buying Guide
Why Do Boutique Preamps Cost So Much?
We hope you’ve learned everything you need to know about preamps. As always, if you have any questions, then call your Sweetwater Sales Engineer at (800) 222-4700.