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What is a preamp, what it does, and why do you need one?

If you speak to anyone about recording, one of the most discussed pieces of equipment will be their preamp. As a newbie, you might be thinking, what is a preamp? Don’t worry; we’ve all been there.

A preamp is a unit that amplifies a low-level signal to line level; that’s the primary function.

It comes after your microphone/instrument level signal and before your interface or outboard gear. Preamps can be more than just a signal booster, though, so let’s take a closer look.

What is a preamp?

As we mentioned above, a preamp boosts a low-level signal, making it line level. A preamp can be a circuit inside a device like an audio interface or a dedicated external unit.

Line level is the audio signal that is standard for transmitting analog sound between equipment/components. The specified strength of line level varies slightly between consumer audio equipment and professional recording gear slightly.

For consumer audio and entry-level recording gear, line level was -10 dBV as standard, although, these days, almost all entry-level gear is designed for +4 dBu. Pro audio gear has a more definitive line level of +4 dBu.

So far, a preamp might not sound like the most exciting bit of gear you can buy. But, to give you an idea of their importance, microphones require anywhere around a 30-60 dB gain to reach line level.

For other instruments, like guitars or basses, it’s maybe around a 15-30 dB gain. Even electronic instruments require some gain boost of around 10 dB, give or take a little.

Preamps do get more exciting, trust me, but you can see how important they are even in their most basic form.

What does a preamp do?

Well, we have covered the primary job of a preamp, so let’s take a look at the common features and their functions. Not every preamp offers the same features and functions.

Inputs

As far as inputs go, a preamp will have a minimum of one microphone input (XLR). Preamps can have multiple inputs for stereo recording and can also have line inputs (TRS). That applies to be audio interface preamps and external preamp units.

Generally, a preamp will have at least one instrument input (usually unbalanced), although not always.

An instrument input is also TRS, making it look like a line input, but they are very different.

Instrument inputs have a very high input impedance, usually around 1 MegaOhm, and no less than 400 kOhms. Line inputs don’t have the same high input impedance; they are generally around 10 kOhms.

You might be wondering why that matters, well, that 1 MegaOhm impedance matches the input impedance of the average guitar amplifier.

In simple terms, plugging a guitar or bass straight into a line input doesn’t deliver the same result as an instrument input. You’ll find that you lose a lot of the personality and life and get a duller tone altogether.

Phantom power

Condenser microphones require an external power source. Phantom power (48 V) is common on preamps, interfaces, and mixers and is used to power condenser microphones.

It means you can power the microphone by activating phantom power, without the need for any additional power cables.

If you own a unit where phantom power is always engaged, it won’t affect dynamic microphones. The name phantom power comes from the fact that it’s virtually invisible to dynamic microphones. Microphone giants Neumann are credited with inventing phantom power.

Gain control

Gain control is the most critical aspect of any preamp, after all. Boosting the signal to the full operating level is its main purpose.

On average, a preamp will offer around 60 dB of gain (some provide more), and that’s generally enough for most uses.

Condenser microphones have a much higher output level than dynamic microphones; therefore, almost any preamp will do. However, if you are using low output dynamic microphones, then you may require more gain.

Gain control isn’t just important as a primary function; it also gives you a good idea of the preamp’s quality.

Let’s imagine you have a low output ribbon microphone and a cheap preamp. That could be a $5000 microphone, but when you crank the gain control to 80 dB, you will suck the life out of your high-end microphone.

Instead of the lush, creamy texture that you’d want from a high-end ribbon microphone, you are left with a flat, uninspiring response.

Now, if you put that same microphone through a high-end preamp, the quality won’t degrade even at the highest gain settings.

As well as downgrading the performance, you can also run into some unwanted noise issues with lesser preamps. It’s something that tends to happen again when you push the gain further.

That means you are less likely to run into these issues if you are using a condenser mic, but if you regularly use dynamic microphones, then you should consider your choice of preamp more carefully.

We should say that it’s not just about the price; some excellent budget preamps perform very well. But, it’s worth being wary that some cheaper units do have these problems.

Low-cut filter

On many preamps, it will be called a low-cut filter or high-pass filter, but of course, it’s the same thing.

A high-pass or low-cut filter allows the higher frequencies to pass through untouched while attenuating the bass frequencies.

It’s both a technical and creative tool. It can be used to remove low-end noise or boominess, and it can be used more creatively to shape your sound. These filters on a preamp can be particularly handy when recording vocals or guitar.

Applying a high-pass filter is generally something that can be done on your microphone, on the preamp, or in your DAW, and it won’t make too much difference.

One of the main reasons to apply a high-pass filter at the recording stage is if the low-end rumble/boom makes it hard for musicians to monitor themselves.

As a general guide, if you are recording instruments, cutting everything below 30-40 Hz is a good way to eliminate unwanted noise. Kick drums and bass instruments can contain sub frequencies, and you might need to adjust your cut a little.

For male vocalists, cutting 60-80 Hz is a good place to start, and 100-120 Hz for female vocalists. Guitars can be filtered very similarly to male vocals. Obviously, you will need to tweak depending on the style of playing/singing and the genre.

Reverse polarity

The reverse polarity or phase reverse function of a preamp simply reverses the phase polarity.

It’s a function that you will put to good use when recording an instrument with more than one microphone.

For example, let’s say you are recording a snare or tom drum, with one microphone above and one underneath. The signals from each microphone will be out of phase, which means they will largely cancel each other out, and you’ll be left with a very thin sound.

To remedy this, you can reverse the polarity of one, which means they are now in phase and will capture a much fuller sound.

Can a preamp improve my sound?

As we just covered, a good preamp can reduce or remove noise, so in a practical sense, that does improve your sound. On top of that, a better preamp will help you get the most out of low impedance microphones.

But, when musicians ask if something will improve their sound, they really want to know what it will do for their tone.

For the most part, preamps tend to be reasonably transparent, meaning they don’t color your sound in any particular way.

Whether a preamp colors your sound or not depends largely on its components and circuit design. The quality of components plays a big part in determining is the sound is transparent, creamy, smooth, harsh, etc.

When it comes to a tonal preamps value, it comes down to individual taste. There is no definitive way to say one is better than another; it’s about what you prefer.

For the most part, preamps that you find in entry-level to mid-price interfaces are very similar in sound. In fact, they are sometimes exactly the same, using the same preamp chip. With a few exceptions, you won’t find too much coloration with these preamps.

If you do want something that colors the sound more, you might want to look towards a vintage preamp. In particular, many people crave the tube sound of vintage 50s/60s preamps.

Older tube technology relied on input/output transformers to interact with low impedance equipment because tubes are high impedance.

When you want that analog warmth and ooze, there’s nothing quite like vintage tube gear. But, the transformers are a massive part of that sound, too.

The price of some vintage gear is enough to make it a complete non-starter, but there are manufacturers who make recreations of classic equipment at a much cheaper price.

Purists will tell you that you can never truly match the real thing, and they would be right. That doesn’t mean that the cheaper recreations don’t have a lot to offer. We are in a pretty good time at the moment for affordable “clones” of iconic studio gear.

If you do your research, you can find units that get you closer to the real thing than most plugins, let you work with an intuitive hands-on flow, and don’t cost a fortune.

If you want something even cheaper, you can find retro tributes to classic preamps that have a tube stage to add a little distortion, and that natural analog break up, while the gain stage is a transformerless solid-state circuit.

The downside of these preamps is that they are more artificial, and as a result, they can often overshoot the mark, creating an exaggerated sound.

Ironically, in the 50s and 60s, when tube preamps were standard, many producers dreamed of a completely transparent preamp. Now, most of us want that coloration.

Despite vintage tube preamps being inherently more colorful, you can create a sort of analog crunch with a transistor preamp, to some extent.

To do this, you can turn the gain control up beyond what is actually needed to make the input stage overdrive the output stage. To avoid digital clipping in your audio interface, simply reduce the output level.

In a nutshell, if you want completely transparent, you have a spend some cash. If you want authentic coloration, you have to spend some cash.

The great thing is that there are so many options in the middle that offer unbelievable value for money.

Is an external preamp better?

The easy answer is most likely yes. A dedicated preamp is likely to be of higher quality and deliver a more authentic version of the sound you want.

Again, this is especially true when working with low impedance dynamic microphones, like ribbon microphones.

One of the most significant issues with a built-in preamp is that they tend to suffer beyond around 50 dB, whereas dedicated preamps maintain their transparency at high gain. They tend to offer more gain, too, going beyond the 60 dB that most interface preamps offer.

If the vintage sound and coloration that we discussed are what you are after, you won’t find it with the average interface preamp.

So, it’s difficult to say that an external dedicated preamp isn’t better, but the expression walk before you run comes to mind.

What we mean by that is that if you are fairly new to recording or buying equipment, you probably don’t need an expensive external preamp.

Chances are, if you are a newbie, you’ll be working with a condenser microphone for vocals, and they won’t benefit as much from an expensive preamp.

In that scenario, you’d be better taking a little of the cash you’d have spent on a preamp and use it to buy a better audio interface.

Anyone with a passion for recording equipment will tell you that it’s an expensive interest to have. One of the easiest mistakes to make is to buy things that you don’t need yet. Start from the ground up, learn your craft, and put your money to good use; walk before you run.


Preamps are something that will become more and more important to you as you progress in your passion for recording. Your preamp is right at the start of your recording chain, and you don’t want to start on the wrong foot.

As a newbie, it’s vital that you understand how important they are and that you know it’s not the time to overspend white yet.

Over time, as you develop your sound and your ears, suddenly the smallest sonic difference will matter to you, which is both a curse and a blessing.

Next time you find yourself in a recording equipment debate, you won’t be thinking, what is a preamp?

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