In today’s article we’re going to talks about preamps.
We’ll talk about what a preamp is, the types of preamps, what they do, and why you need a preamp in your rig. We’ll also talk about how ot choose the right preamp for your purposes.
It’s hard to overstate the importance of preamps.
We know that secret of a good recording is a good mic, good placement, a good preamp, and a good room.
Let’s dive right in.
What is a Preamp?
A preamp is a kind of amplifier that’s designed specifically to take really weak audio signals and make them “louder”. It is designed to give audio signals the amount of gain necessary, without inducing a lot of distortion or inteference into the signal, so that devices further down the chain can use that signal.
You might wondering, why feed a weak signal in the first place?
Think of how a mic works. It responds to really tiny vibrations of molecules in the air that we perceive as sound and tranduces that into an eletrical device such as a mixer. When that energy gets transduced, it’s a really, really, tiny signal.
The signal is so tiny its only a couple of millivolts, but our devices like mixers or audio interfaces, or monitors, can’t really use such a tiny signal. It’s too low. That is why mics have preamps.
The preamps job is to take an audio signal, typically from an instrument or microphone, and give that signal it’s sonic shape before it’s amplified.
Preamps add a lot of character, warmth, and dimension to your recordings.
They are a kind of specialty amplifier because they have to take that weak signal and make it louder without inducing a lot of noise.
Any electrical device, just by it’s very nature, is going to add a little bit of noise to your signal. Preamps have to be designed in a specific way so they can minimize that noise while giving you the sound you really want. They add clean gain.
Preamps do this in different ways, there are different circuits and components that are used in different types of preamps to make this happen. Some use tubes, others transistors, others transformer.
We’ll discuss all of these shortly.
You’ll see this things listed in the technical specifications of preamps when you’re picking them up.
You’ll see distortion listed as THD (Total Harmonic Distortion), noise floor (idle noise when no signal is passing through).
These are important specs to keep in mind.
Do You Need a Preamp?
Let’s talk about different scenarios you might need a preamp.
#1 — You’re Performing Live
If you’re performing live with your band, and you’re singing into a microphone, chances are that signal is going into a mixer or a console of some sort.
The mic input itself is a kind of preamp.
It works by taking the signal coming from a mic and adding gain to it so that the rest of the mixer can use that signal.
#2 — You’re Recording
The same logic goes if you are using preamps in a recording environment.
Whether that’s your home studio where you’re recording vocals or an instrument, chances are you’re routing the signal through an audio interface.
The mic input on that interface will also be a preamp.
In any case, in any of these situations (and more not mentioned here), you’re still using a preamp which is helping to take your mic signal and add gain to it so that devices down the chain can use that signal.
Audio Transformers: Color vs Transparency in Preamps
One of the main factors that determines color vs transparency in preamps is as a result of the use of transformers.
A transformer is a passive electrical device that transfers electrical energy from one electrical circuit ot another.
The audio world is split between the use of transformer based circuitry to provide color and transformerless circuitry that delivers the purest, most transparent, signal possible.
When included in a signal path, a transformer will always add some color to the signal. Reducing the number of active colorful components in the signal path is, arguably, the way to keep more audio transparency.
Most often, you’ll find preamps without transformers of the solid-state type (we’ll look at types of transformers shortly).
Transformerless tube preamps do exist but are much harder to come across and tend to be really expensive due to the difficulty in building them. The better the build of the transformer, the flatter and broader the frequency response. They are also shielded better.
A hotter input signal can be put through an expensive transformer without saturating it.
Shielding produces pickup of hum and interference from outside sources such as power supplies. Not only does shielding keep unwanted signals out of the transformer but it also keeps the desired signal within the transformer.
Many budget transformers have little to no shielding.
Limitations of Audio Transformers
#1 — By design, audio transformers only pass audio signals. Therefore, an audio transformer will reduce or block signals that are below or above the audio range of 20-20k Hz.
This can be a limitation or a benefit depending on your sonic goals.
#2 — Audio tranformers have a maximum input level that cannot be exceeded without causing distortion, When the maximum level is exceeded the transformer is said to be saturated.
#2 — Audio transformers cannot step up any signal by more than 25 bB when used in typical audio circuits.
Because of this limitation, an audio transformers is not a substitute of a microphone preamp. If more than 25 dB of gain is required, an active preamplifier must be used instead of a transformer.
Types of Preamps
It’s important to know the different types of preamps.
Tube preamps use valves or vacuum tubes (thermionic tubes) to create gain.
Tube preamps are known for adding deep bass, opening air highs, and a warm presence in the mid range. Tube preamps are associated with a warm, fat, sound.
As the signal level increases, the produces distortion which in turn produces a deep, warm, sound. This distortion is produced gradually, making it really sound good to our ears.
Most tube amps will color your sound a lot. The additional of even ordered distortion comes off as color. This effect is often referred to as harmonic distortion.
Tube preamps also provide color from the natural compression characteristics provided by the tube circuit’s design.
In gain staging, we saw that the signal becomes more compressed as gain is applied gradually or when the tube is overdriven in our case. Have a look at that if you haven’t already. The human ear hears this natural compression as pleasing.
Natural tube compression might be subtle effect, but is is considered the audio glue that binds everything together.
Tube also act as low pass filters.
This filtering effect smooths outs distortion at the high end of the spectrum reducing the high frequency content of the audio signal. This is either desirable or undesirable depending on your sonic goals.
For instance, it is desirable for warmer vocals but undesirable for drum and percussions where the attack information is essential.
Solid State Preamps
The development of transistors to achieve gain has been the basis for the creation of solid state preamps.
Transistors perform optimally, and with minimal distortion, at high levels of gain.
They create gain more efficiently, and with less heat, than tubes. They also operate more consistently as gain increases maintaining very low distortion up to the maximum levels beyond which they produce extreme distortion or clipping.
The transparency of solid state preamps is largely due their capacity of accepting significantly higher gain levels without distorting.
Digital preamps convert an analog signal to a digital signal (increasing it’s sonic signature) before it hits a DAW.
The idea is to allow the user to process a signal with a dedicated preamp, bypassing the builtin ADC supplied by audio interfaces, which are inferior converters by comparison.
Some digital preamps offer a digital output sound card that can be installed in the users computer. Others have the processing and conversion built into the preamp itself.
If the ADC convertion is built in these preamps can be considered digital interfaces as they efficiently convert an analog signal t a digital signal for the DAW even though the units are designed as preamps.
The ADC is just a convenient addon of the primary function.
Which Preamp is Right for You?
Choosing the right preamp largely depends with what you want the preamp to do for you.
If you are mainly looking to add warmth or body to an audio signal, your first pick should be a tube preamp.
That will fatten your sound.
If you are looking for an intense clarity sound, and high dynamic range, that’s more solid-state territory.
If you’re seriously looking into getting state-of-the-art vocals, you should also look into investing in a channel strip since it will include a lot of the tools you need in a convenient package that will save you time and money in the long run, even though they are really expensive.
There is always the channel count factor, always start with, at least, a 2 channel solid state preamp. That way you’re not limited to only a single channel while recording.
As you start getting more experienced, you can add more preamps to your arsenal for different purposes and tone shaping.
Another important thing to consider when choosing a preamp is the type of microphone you’re using with it.
Different types of microphones output different signals streams. This is something to keep in mind when choosing a preamp.
For instance, the condensor microphone we looked at in dynamic vs condensor vs ribbon microphones typically gets phantom power meaning it’s going to output a hotter signal, it’s more sensitive and it might not need as much gain as a dynamic microphone.
Dynamic microphones are low-gain microphones because the need more gain from the preamp. The signal coming from that type of microphone is lower.
It need a little bit more gain before it can go to further down the signal path.
Ribbon microphones, by their very nature because of how their transducing mechanism works, also output a much lower signal. In fact, there are preamps that are designed specifically for ribbon microphones.
The idea is still the same, to help you get gain in a really clean way, to preserve the character of the ribbon microphone while also giving you the amount of signal you need for the rest of your path.
As you can see, there are a lot of factors to consider, when selecting a preamp and we’re just starting to scratch the surface.
Regardless of the kind of preamp, at the end of the day, all it’s doing is taking your signal and essentially adding gain in a clean (or colored) way so that devices further down the chain can use that signal.
Some preamps might color our sound in a way that is pleasant to our ears, others are designed to be very clean or transparent meaning they don’t add color or extra harmonic saturation to your signal but allow you to get clean gain.
Whether we are using a clean preamp or a colored or character preamp you should now know the different types that are available. They are all ultimately different tools that we can use in our musical setups.
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That’s it for this article.