You’ve heard about famous preamps such as the Neve 1073, API 312, or the REDD.47. You’ve heard they make your signal sound warm, thick, fuzzy, crispy, airy… You own an ok mic, but what you really need is an awesome preamp, right?  Wrong.
Many popular audio interfaces contain at least one mic preamp, and many companies claim that these built-in preamps are all you need for perfect audio. In this article, you’ll learn why an external preamp could be a good investment for your particular situation. Additionally, you’ll learn how to choose the ideal preamp, what a preamp does, the limitations of the preamps on your audio interface, and the pros and cons of plugin emulations of preamps.

What’s a preamp and what does it do?

A preamp can refer to two things: 

  • a preamplifier circuit in any device, like the preamp part of an interface
  • a stand-alone device or module that performs as a preamp—a dedicated mic pre.

The purpose of a preamp is to amplify low-level signals to what is referred to as “line level.” Line level is the professional level standard and represents an RMS voltage of about 1.23 volts.

Generally, microphone signal levels are very quiet, so they need a lot of gain to be brought up to line level. Usually, you’re looking at anywhere between 30-60dB of gain increase; some microphone types, such as ribbon mics, require even more gain. See sidebar for some technical info about preamp gain.

Passive instruments, such as guitars and basses require much less gain, but still around 20-30dB, while synths and drum machines may also need to go through a preamp, though their output level is even higher than mics or passive instruments. Passive instruments, with unbalanced outputs, may require a dedicated direct box, or your interface may provide an input suitable for them.

Preamp Gain Facts:
A microphone typically produces a signal that is between .005 and .05 volts. A microphone preamp boosts that signal up to line level, which is about 1.2 volts. That increase requires 30 to 50dB of gain. Think about that—60dB of gain increases the signal level by a factor of 1000! 
So we see that microphone signals are quiet, delicate signals and mic preamps do a lot of work to increase their level up to line level. That is why mic preamps can impart so much color to the sound, and also why the noise floor is such an important consideration with preamp design.

Note: Voltage Gain in dB = 20log(output voltage/input voltage)

Benefits of external preamps

  • More gain: The built-in preamps inside your interface usually offer less than 60dB of gain. Ribbon mics and other low output microphones sometimes require even more than 70dB, especially when recording very quiet sources, like acoustic guitar.  
  • Better sound quality: The higher you turn that gain knob, the more apparent the limitations of your preamp become. Interface preamps sound okay if you boost only by 35-50dB, but lose quality at extreme settings. External preamps continue to sound great, or even add more interesting color as you increase their gain.
  • Specific sound or character (creamy, airy, round, etc): These colorations are most likely why people buy external preamps. The built-in preamps in most audio interfaces generally sound clean and transparent, especially at reasonable gain settings. Each external preamps let you spice up your signal with their distinct flavors, such as a 60’s tube sound, 70’s transistor sound, etc.  Some external preamps simply provide pure, uncolored sound at any gain level, useful for purist recordings.
  • Lower noise: Many modern interfaces have low noise preamps. However, if you record very quiet sources (grass growing, whispers, paint drying, etc), or you’re using ribbon mics, you might still benefit from an external preamp doing this hard work.
    Beware: Specifications can be deceiving. Manufacturers typically claim a preamp noise floor of around -128dBu. This is excellent, but only if that noise floor remains constant at all gain settings. Dedicated preamps typically have better specifications at their extreme settings than USB-powered preamps in inexpensive interfaces.
  • Design Quality: Since preamps do so much heavy lifting, each component, from the specific electronic components to the power supply design and shielding affect the audio quality. Inexpensive interfaces with preamps necessitate inexpensive components, while external preamps can provide higher quality parts and designs, usually with a considerable increase in price.
  • Compression: Not the least important, but often overlooked feature of an external preamp, is the ability to run the preamp into an analog compressor (and maybe an EQ) before hitting your A-D converter. This signal path adds color, saturation, and dynamic control in the analog domain before you capture it as a digital signal. Interface preamps rarely provide an analog insert point for patching in a compressor during recording.
Cost Considerations
A typical external mic preamp contains high-quality components, a well-built power supply, a durable enclosure, and quality switches, and connectors. Typical parts cost for a decent single-channel preamp might range from $100 to $300, so their retail price would be $400 to $1200 per channel.
Inexpensive interfaces include preamps as part of a total package, designed to meet an inexpensive retail price point of perhaps $200 to $600. That price includes preamps, instrument inputs, a headphone amp, monitor control, A-D and D-A conversion, and packaging. That means the preamp itself probably uses only a few dollars in parts. 
Preamps in interfaces, therefore, are designed to provide relatively clean and quiet gain for not-so-difficult recording applications. While those preamps function pretty well, it should be easy to see the quality improvements that an external preamp will provide. 

Features of Preamps

Most preamps share similar basic features, regardless of whether they’re part of an audio interface or external ones.

  • A preamp has to have inputs. Usually, they provide both XLR connectors for microphones and TRS for line inputs and instruments such as guitars and basses. It’s important to bear in mind that the input dedicated to guitars/basses is usually labeled Hi-Z or Inst. If you use a line input or mic input for a passive instrument, the instrument may sound distorted, have a strange frequency response, or simply be too quiet.
  • Line inputs are used to connect line-level gear that do not need additional amplification, such as high output synths or guitar processors. If your interface has line inputs, check the manual to see if these inputs bypass the mic preamp. If they do, hooray! You can use external preamps with your interface. You don’t want to plug an external preamp into a built-in preamp, plug an external preamp into a line input only! 
  • The most important function of a preamp is the gain. As previously mentioned, the main point of a preamp is to bring a very quiet audio signal up to the full operating level. The quality of the mic preamp is most obvious at high gain settings. Once you go above 40-50dB of gain, cheap preamps start sounding more and more lifeless, and the noise level may also increase.
  • Phantom power (P48) is required by condenser mics. USB powered interfaces need to generate 48-volt phantom power from 5 volts of USB bus power. External preamps use 120-240 volt wall power to easily create phantom power.
  • The phase reverse switch is used to reverse the polarity of the signal. If a source is recorded with more than 1 microphone or DI, you’ll want to check whether both microphones are in phase with each other.
    An example is recording a snare drum with 2 mics, one from the top and one from the bottom. The mics are basically facing each other, so when the snare drum is hit, the bottom microphone sees the drum head moving towards it, whilst the top microphone sees it moving away from it. When these two opposite signals combine, they’ll cancel each other, resulting in a weak sound which no amount of EQ boosting will fix. If you flip the phase switch on the bottom mic, the signal from both microphones will combine correctly, giving you a full, natural sound.
  • Some preamps provide a high pass (low cut) filter. This circuit removes low frequencies while allowing everything above them to pass roughly unharmed. Cutting these low frequencies is the most common way to remove rumble and unnecessary sub-bass frequencies. Removing thumps and plosives at this stage allows better processing and headroom down the line and compressors won’t react to low frequencies that we don’t need in our signal.
  • The pad switch is used to lower a very loud microphone signal before it has a chance to overload the mic preamp circuit. Pads typically bring the lowest gain setting of a mic preamp down to 0dB of gain. Beware that pads may slightly affect the sound of some microphones.

Types of Preamp Circuits

An important difference between various preamps is their tone. Do you want a preamp designed to color the sound or so you simply need clean, transparent gain? A colored preamp can add body, weight, sparkle to a thin-sounding voice, whereas a transparent preamp’s whole point is to reproduce the source and microphone as accurately as possible.

Engineers tend to have different preamps in order to choose the best color during the recording stage. Some interfaces, like the UAD Apollo, provide software-based preamp emulations. These processors affect the sound after the conversion to digital, while external preamps react to the analog signal and color the sound before it becomes a digital signal. Further, preamp emulation plugins can also be applied during mixing. Each coloration has its own benefits.

Preamps can be based on different circuit types and each circuit provides its own characteristic coloration. These colorations are based on vacuum tubes (valves), transformers, transistors, and various combinations and implementations of each. Preamps in interfaces almost always use integrated circuit chips without tubes, transformers, or discrete transistors, to keep costs down. These built-in preamps provide clean, uncolored gain with few options for tone control or artistic coloration.
Some external preamps let you switch between different tube setups or types of transformers, essentially giving you 2 preamps for the price of 1. Examples of these preamps are the Warm Audio TB12 Tone Beast Microphone Preamp, the Audient ASP800, the Slate Fox Quadtone preamp, and the Retro 500Pre, among others. Some preamps, like the Chandler Little Devil and the Coil Audio preamps, let you further affect the tone by changing input impedance, transformer settings, and even amplifier bias settings.

Tube preamps

These use vacuum tubes to boost the input signal. The tubes are known to add deep bass, airy highs, and warm presence in the midrange frequencies. Tube coloration can range all the way from super clean to larger-than-life to creamy to gritty and distorted.

As the level of the signal increases, the tube starts producing mild harmonic distortion. These even harmonics stack up on top of the fundamentals, adding more and more depth, body, and weight. “Fat” and “beefy” are two words used by engineers to describe this sound.

Another way a tube preamp can color your sound is through the compression characteristics of the tube circuit. If you look at waveforms of sounds recorded through tube preamps, you’ll see that the curve at the top of the wave becomes subtly compressed, flattened as more gain is applied. Tubes can also smooth out transients, taking the edge off of harsh high-frequency content.

Tube Preamps can be very colored like the Universal 610 preamp, or extremely transparent like the Avalon 737 preamp. Some tube preamps gain even more tone from their input and output transformers, like the Chandler Limited REDD.47 preamp. Because of their tubes, these preamps may need routine servicing every few years to maintain their original tone.

Solid-state Preamps

These preamps use transistors or opamps to amplify a signal. The specific sound of a solid-state preamp comes from its components and design. Different opamps and transistors provide different amounts of distortion and some designs also include various styles of transformers. Neve preamps sound warm, while API preamps sound more mid-focused, but both use transformers and opamps.
This type of preamp provides a more consistent, clean, and natural sound than tubes do. Some special solid-state designs, like those of the GML 8300, the Millennia HV-3C, or the Avalon AD2022 utilize sophisticated circuit designs that provide rich and lifelike audio, free of many types of distortions common to standard preamp designs. 

Other solid-state preamps like the Neve 1073 or API 512 provide their own unique flavors through a combination of transformers and proprietary amplifier circuits. Other solid-state preamps, like the Grace Designs M101, provide what some consider the most neutral sound possible.

As you can see, solid-state preamps run the range for colorful to ultra-pure and will typically remain trouble-free for many years. 

Hybrid preamps

…combine the best of both worlds, usually employing solid-state components at the input stage, and tube components are the output stage. This gives you clean gain and the ability to add as much color to your sound as you want. Manley’s TNT provides one channel of tube and one channel of solid-state preamp. Other examples of hybrid mic preamps are the Universal 710 preamp and the Millennia TD-1 and STT-1 channel strips that allow users to mix and match tube and transistor components in their signal path.

Which type of preamp should I use for my recordings?

There are so many choices of preamps available, how can we choose the best starting point? Now that we know the basics of each style of preamp and its colorations, we can formulate our choice. Keep in mind that the colorful character of a preamp becomes more intense as the gain is increased, so many preamps offer a wide range of coloration.

  • Classical music recording engineers tend to favor solid-state preamps for a pure, transparent, clean sound. Some tube preamps also fit this criterion.
  • A thin source, be it a singer or a sax, can be fattened up using a tube preamp or a colorful solid-state preamp.
  • Acoustic instruments, such as the acoustic guitar, might benefit from using a tube preamp since it can add ‘warmth’ to it. However, many engineers prefer to capture a clean sound, so they go for solid-state preamps.
  • If you want your drums to sound fatter, go for a tube mic preamp, since the harmonic distortion will help in that direction. Its slight compression artifacts also add to that effect.
  • If you want punchier, more beefy drums, go for a solid-state preamp to capture as much of the transient as possible.

Limitations of built-in preamps on interfaces

The good news is that, although we almost always record through a preamp, we don’t necessarily need an external preamp. Some interfaces include preamps that are really good. In most cases, more expensive interfaces come with way better preamps than beginner interfaces do. 

I have seen many top artists record famous records using the built-in preamp on their interface. To be fair, they have a lot of experience recording, use a very good microphone, record in well-treated rooms, and use the preamp for moderate-level vocal and DI guitar recording only.

Most preamps in interfaces don’t provide a pad, a low-cut filter, or a phase switch. As previously mentioned, those are all useful options to have on your preamp, especially the pad. Additionally, you’re stuck with the tone, or lack of color, of the built-in preamps your interface offers you.
If you are mainly concerned with the ability to record quiet sources or low-output microphones (like the Shure SM7 or ribbon mics) a quick, cheap fix to this issue is buying an in-line microphone booster such as the Triton Audio FetHead or the Cloudlifter CL-Z. These devices provide clean gain so the preamp doesn’t have to work so hard. These devices cost $100-$250 per channel, so factor that in if you’re thinking of purchasing a new interface with challenged mic preamps.

Some interfaces, though, do provide extra features, like reamp outputs, remote gain control (via their software control panel) and some do provide switchable circuit options. The affordable Focusrite RedNet X2P interface, for example, provides 2 mic preamps with high-pass filters, pad, phase reverse, and even switchable transformer-type inputs. They also provide excellent instrument inputs and great-sounding headphone amps. All these features in one interface for under $1000.

How to Best Use a Preamp

Gain

I’m going to assume you’re recording straight to your computer and not to analog tape. Our goal is to record in the analog electronic’s sweet spot, where the level is near optimum, but not overloading. This usually means our average level should be around 0VU for vocals, but maybe 5 to 10dB less for percussive instruments.
DAW meters look deceiving since the meter usually shows only the top 60dB (24-bit recording allows up to 144dB) and the scale is logarithmic, so the meter has more resolution near the top. Therefore, the middle of the meter represents about -15dBFS, which is a very hot signal to analog equipment, about 8dB above 0VU. See the diagram below to help you understand how loud to record in a DAW.

Pad

When recording loud sources, microphones may produce too much level for the preamp to handle. That’s when a pad can become a session saver. Try switching in the pad if you hear distortion and your levels look safe, then turn up the preamp gain a bit. If you still hear distortion, you may be overloading your microphone or something else in your signal chain.

If you hear undesirable distortion taking place, do this in order:

  1. If you are using an external preamp, make sure to set your interface to receive a line input, which may be the same or a different physical input than the interface’s mic input.
  2. If your preamp is clipping your interface bring down the gain on the preamp.
  3. If you still hear distortion switch on the preamp’s pad to attenuate the signal before it gets to the preamp’s input.
  4. If you’re still hearing unwanted distortion, use the pad on your microphone, if it has one. This will bring down the signal right at the microphone capsule before it gets to the mic’s electronics. Unfortunately, this pad increases the noise floor of the microphone and can affect the quality of your recording. Using the microphone’s pad should be your last attempt at combating unwanted distortion. 
  5. If all of the above fails, try to move the microphone further away from the source.
  6. Lastly, consider changing the microphone to one that can handle higher SPL.

Phase reverse

Using this feature of your preamp won’t change the sound quality of a single microphone. As previously mentioned, phase reverse is very useful when recording a sound source using multiple microphones or a mic and a DI at the same time.

However, the phase reverse switch is useful when recording vocals with one microphone. The singer hears their voice through their bones and also through their headphones. If the two are out-of-phase with each other, they won’t hear themselves properly. When you’re tweaking the cue mix with the singer in front of the microphone, flip the phase and check with the singer which position sounds better to them. This won’t affect the sound of your recording and you could also flip the phase in the monitor path instead of the mic preamp. 

Understanding how your preamp level relates to your DAW meter.

Preamps are analog devices and they sound best when they create a signal around 0VU, or +4dBu. This is 0 on a VU meter and the same level as -18dBFS on your DAW meter.

This level only looks like halfway up the meter in Logic or Pro Tools. If you try to push your mic preamp louder, be it an interface’s preamp or an external preamp, you will start to overload the analog electronics. 

This analog overload may happen before your digital meters overloads since the digital meters have 18dB of headroom above analog 0VU. This distortion may not be noticeable on one track, but it adds up quickly!

A good way to keep this organized is by watching the peak meter in your DAW and making sure it never goes much above -10dBFS. 

Plugin Emulations of Preamps: Are They Worth It?

Yes, they are. If you’d be buying an external preamp mainly to add some color to your recording path, emulation plugins can save you some money, while offering you enough coloration options to last a lifetime.

During Tracking

What these plugins, like the UAD Unison preamps, do is digitally emulate the characteristics of the analog gear they model. Used during recording, this type of preamp modeling can help you commit to a specific type of coloration to enhance the recording.

One thing veteran engineers know is that each microphone sounds different through every different preamp. Each preamp interacts and loads a microphone in a way that changes its frequency response, distortions, and overall tone. It is common studio practice to pair specific mics with specific preamps and modeling preamps may or may not accurately emulate this type of interaction. Analog preamp circuits interact with mics in a non-linear way, reacting to level, frequency content, and a particular mic’s own electronics in ways that interface preamps and plugins can only represent to a limited extent. That is to say that analog gear has complex interactions and modeling preamps provide less complex, so less interesting, interactions. That is not to say modeling is not a good way to go, but it is different. 

During Mixing

During mixing, plugins like Brainworx’s bx_console line of plugins, emulate every single component in a particular circuit. All the tiny inconsistencies in harmonic distortion, frequency response, and noise, all add up to give an analog circuit its sound. Use these emulations on some or all the tracks in your mix and you’ll probably find that the tracks gel together a bit better and that the whole mix has a bit more depth.

Note: Pay attention to how low end-heavy tracks such as kicks, 808s, basses, subs, synths, etc, behave when going through preamp/console emulations. Some of the plugins are faithful emulations of vintage gear, that were never meant to handle modern low end.
Airwindows’ Console6 is one of the best ways to get a more analog-sounding mix without breaking the bank…that’s because Airwindows’ plugins are donationware. You get them for free and, if you want to, you can support the plugin creator on Patreon.

These plugins virtually replace your DAWs summing (mix bus), doing it all inside the Console system, and simulating the sound of an analog mixer.

Slate Digital produces preamp, console, and tube emulation plugins, including the Virtual Tube Collection, which are excellent tools for enhancing clean recordings. There are several distinct preamp/console flavors, modeled after classic preamps. 

  • “London” is inspired by vintage tube circuits originating from Europe. It’s smooth and gentle. It usually works well on non-aggressive vocals.
  • “New York” is taking after some vintage tube circuits with a dash of NYC solid-state circuits to make the sound tighter and give it more impact. Use it on bass to make it roar.
  • “Hollywood” is modeled after some tube and solid-state circuits from California. It works great on drums.
  • “FG-73” is modeled after a Neve 1073 preamp. It’s bold, present, and warm. 
  • “FG-76” is modeled after a Telefunken V76 preamp. It’s thick, warm, and colorful. 

Next on our list is Waves NLS, which models 3 classic analog consoles: an SSL 4000G (called Spike), an EMI TG12345 (Mike), and a Neve 5116 (Nevo).

Plugin Alliance has quite a few console preamp emulations. At the time of writing, there are 5 console emulations, with more on their way.

  • bx_console N models a Neve VXS console.
  • bx_console SSL 4000 E, G and the 9000 J model their namesakes.
  • bx_console Focusrite SC is modeled after one of the best and rarest recording consoles ever made. 10 were made, and six exist today. I strongly suggest you watch the documentary about the legendary Focusrite Studio Console.

Note: With all the plugin emulations, it’s very important to gain stage properly. Since these plugins are modeling actual gear, they’re calibrated to give you the best results when they’re dealing with sound averaging at -18dB.

Use a trim/gain plugin right before the preamp emulation and adjust the level until it averages out at -18dB. Alternatively, use HoRNet’s VU Meter to automatically have the level set appropriately. 

What should I buy?

If you’re in the beginning stages of your walk in the audio world, forget about buying an external preamp. Focus on buying a high-end interface with good built-in preamps and features. External preamps can drastically improve the sound of dynamic mics such as ribbons but have less of an impact on condenser mics. 

I strongly suggest you go with one in the Audient ID line, such as the ID22. These feature the same preamps as their large format consoles, as well as really really good converters. They’re cheap and they’re awesome and they easily outperform interfaces that are more expensive than they are. UAD Apollo interfaces and the Focusrite X2P have similar features in their preamps. Once you get above that price range, many interfaces have feature-rich preamps.

Consider buying an external preamp if you feel the audio benefits are worth the extra cost and complexity. Remember an external preamp does allow for an external compressor! Recording a source to sound like it should for the final product will help you during the mixing process. You’ll need less mix processing to achieve the sound you’re looking for.

Conclusion

Your interface preamps are likely fine to record with. If it is missing some features that you desire or if you have outgrown its sonic pallette, then consider upgrading to an external mic pre. For some, using DSP-based preamp modeling, like the UAD Unison system, is a worthwhile step up. For others, simply applying analog preamp emulations in the mix stage will suffice.

For those of you who wish to get the most out of analog preamps, remember that each preamp will lend its own flavor to your recording, so choose wisely. You may decide to go with a versatile hybrid preamp or one that compliments your particular style of music.

Do your research and read reviews in respectable magazines and websites and research your favorite artists to see what preamps they use for songs you are familiar with. Rent some time in a big studio with lots of preamp choices and try out a handful. The money you spend on the studio time will reward you with invaluable knowledge!