What is a Mic Preamp?
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Getting into the world of recording vocals and audio production means learning and understanding the tech surrounding your microphone. One such piece that you’ll need to familiarize yourself with is the microphone preamplifier.
A mic preamp is a very important device to get to know. It can often be found in professional recording settings, but can also be incredibly useful in the home setting.
If you’ve been asking the question, “what is a mic preamp?” and wondering how it helps your microphone and overall sound quality, keep reading to find out everything you need to know about mic preamps and whether it’s worth investing in a dedicated outboard mic preamp.
What Does a Microphone Preamp Do?
A microphone preamplifier is a device that takes the output of a microphone and amplifies it to line-level output. When recording sound with a microphone, the microphone signal produced is too weak to be used by any of the other equipment in your studio. The mic preamp works by taking the mic signal and boosting it to a level that can then be used by the rest of your studio equipment.
There are a few different kinds of electrical signals, and a mic preamp turns this level signal into a line-level one. A mic level signal typically ranges from -60 dBV to -20 dBV, while level signals operate at a nominal level of +4dBu (1.78 DVB), which is quite a bit higher.
As line level is the professional standard used by all recording and mixing gear, we need a device to turn up the output of the microphone so we can use it both in professional recording studios and the home studio.
Mic Preamp Types
Microphone preamps can be found as standalone units or as a single component in audio interfaces and larger devices. Here are some examples of where are mic preamps found.
Standalone Mic Preamps
Standalone mic preamps are a type of preamp that is separate from other audio interfaces and recording interfaces. This refers to a microphone preamp that is not built into mixing desks or recording devices.
Standalone preamps work to boost microphone signals to line level with gain. Some are more complex and have greater functionality than others, while some have more mic input and output than others. However, each of these mic preamps is its own audio unit.
Dedicated mic preamps can be a very expensive investment, and you will have seen an external preamp in professional recording studios, commonly mounted inside rack units. Engineers typically have their favorite preamps that they use over and over again, and most of them have familiar microphone/mic preamp combinations that give them the specific tonal quality that they’re after for a particular recording.
When an engineer chooses to use a dedicated outboard mic preamp, it bypasses the preamp found in audio interfaces and mixing desks.
Mixing Desks
Large format mixing consoles have multiple inputs for microphones, instruments, and line-level mic signals. Because mixing desks utilize line level signals within their circuitry, the mic inputs on the mixing desk have microphone preamplifiers.
You’ll also find a microphone preamp at the XLR input connection of each mic input channel on both analog and digital mixing desks. Audio interfaces also have mic preamps built into them for each XLR input.
Audio Interfaces
With the popularity of digital recording, you can find many built-in mic preamps in audio interfaces. So, if you own an audio device with a microphone input, it will contain a mic preamp, and if you own an audio interface with numerous audio inputs, it will include a mic preamp for each channel.
This audio interface has analog-to-digital converters and digital-to-analog converters to effectively allow communication between analog audio equipment–like microphones, instruments, headphones, speakers, and digital devices, such as computers and their digital audio workstation software.
It’s also worth noting that more expensive audio interfaces will still have a better quality preamp, and if you go one step further, dedicated outboard preamps offer the most significant sonic potential.
Internal Microphone Amplifiers
Besides a standalone preamp, many microphones have built-in mic preamps as well. Though true signal amplifiers are active and require power to function, there are other passive means of effectively boosting a mic signal.
Types of internal microphone preamplifiers include FET impedance converters, vacuum tubes, step-up output transformers (commonly found in passive dynamic and ribbon microphones as well as in a condenser microphone), and amplifier circuits.
Are Microphone Preamps Necessary?
If you’re already working with a mixing desk or audio interface that has a mic preamp built-in, then you might be wondering if there’s really any point in investing in external outboard preamps.
Outboard mic preamps in particular offers improved sound quality than a microphone preamp that comes as standard with a cheaper audio interface or as a plug-in. This becomes more apparent at higher gain settings, especially for low output microphones. In addition to this, the most common reason why someone would buy dedicated outboard preamps is because of the unique sound characteristics of the equipment.
So to answer the question of “what is a mic preamp, and why does a mic need one?” A mic preamp is necessary to amplify mic output signals to a level where the signals can be effectively used in other audio equipment, making it very important to the process of audio recording.
Do I Need a Preamp for My Mic?
In the process of figuring out whether you need a mic preamp and what best mic preamps suit your needs best, let’s take a look at the key features found in such devices.
Tube vs. Solid State
The technology that is employed inside the box to amplify the mic signal is either a vacuum tube or a solid-state transistor, with both offering different tonal qualities. Vacuum tube transistors offer a vintage, overly saturated sound, while solid-state preamps have a tendency to be cleaner and clearer.
One of the key issues with any piece of studio equipment is if it’s overloaded, and this includes a mic preamp. When something is overloaded, it means the signal entering the device is too high and the electronic circuitry cannot deal with it.
One of the biggest differences between tube and solid-state is how each one reacts and behaves when it is overloaded. When a tube is slightly overloaded, the effects can be quite pleasant. Small amounts of overload can lead to a pleasant and warm distortion.
Gain Control
Gain is the measurement of a mic preamp’s ability to increase a signal’s amplitude from the amp input to the amp output. An amp supplies gain to an input signal to make it stronger at the output of the amplifier. Gain works by adding energy to the signal. This energy is converted from an external power source, whether that’s an AC wall plug, phantom power, or batteries.
Mic preamp gain increases the amplitude of a microphone signal level. Gain boosts signal strength from mic level to line level, so the signal path is compatible with professional audio equipment.
Channel Strip
One way of getting around the high cost of a mixing desk with an inbuilt mic preamp is to look into a channel strip. Some mic preamps incorporate features such as equalization or compression, which are described as channel strips.
The name channel strip comes from mixing desks, which have a number of channel strips that are generally comprised of mic preamps, equalization, and sometimes compression, or inserts for other analog gear. You can use a channel strip to manipulate audio on the way in while you’re recording or connect them to your audio interface.
48V Phantom Power
Microphone preamps are active devices and require power to function, typically plugging directly into the wall unless they’re battery powered. This power allows the active components of a mic preamp unit to work, including the differential amplifier and the phantom power source. A mic preamp is used to amplify the microphone signal, while the role of phantom power is to supply power to the microphone for it to work.
A condenser microphone must receive 48v phantom power in order to function. Phantom power is often supplied straight out of the microphone preamplifier, so once a microphone is working through the phantom power, the mic preamp is then employed to boost the signal to increase it to line level.
It should be noted that all good mic preamps come with phantom power capabilities, whether audio interfaces with built-in preamps or external microphone preamps. This is important information to know as phantom power is key for all condenser microphones to work.
High Pass Filter
A high pass filter is a common feature found on a mic preamp and is the ability to filter out any low frequencies. This is helpful when recording instruments such as voice or electric guitar and acoustic guitar, where there isn’t a lot going on in that region. The exact frequency where the bass roll-off begins will differ from unit to unit, but it is usually somewhere between 80Hz and 160Hz.
Phase Reverse
The phase reverse feature flips the polarity of your mic signal. It is helpful when recording with two or more microphones simultaneously so you can ensure all of the signals are in phase.
Input Impedance
Note that some microphones have a rated load impedance specification. This refers to the minimum input impedance a mic preamp must-have for the microphones to function properly.
Input impedance is another concern with ribbon mics since it is frequency-dependent. Therefore, a preamp will actually alter the sound of a ribbon microphone.
Ribbon microphones, which notoriously output very low-level mic signals, sometimes require their own specialty preamplifiers. This preamp is able to provide a great amount of clean gain so that the ribbon microphones can be used effectively.
Some microphone preamps also double as an instrument preamp as well. Instruments such as electric guitar output signals with high impedance and benefit from an input/preamp with high impedance.
Final Note
Now that we’ve answered your question of “what is a mic preamp?” you can take a look at some of the industry’s favorite preamps to get the best sound quality you can afford.
However, before getting yourself a new mic preamp, it’s also good to know that it’s perhaps not worth purchasing a dedicated preamp when you’re at the beginning stages of setting up your home studio. You can still get good results from inbuilt preamps found in a mixing board or a recording interface.
But, if you have the money to spend and you’d like to further improve the quality of your recordings, then looking into the possibilities that a microphone preamp can bring will definitely help. A good preamp can do so much more for the quality of your recordings than you might realize.
