Hello and welcome back to the Sounds From The Apartment blog! This week we are getting back to the “What is an Audio Interface” series discussing about the Preamps!

Many people seem to not know a lot about (or even not understand at all) Preamps! As we briefly mentioned on the Input Section article of these series:

The main purpose for a Preamp to be at the beginning of our signal chain, is to amplify any Microphone Level signal up to a Line Level Signal without distorting its sonic qualities.

But let’s dive into more specific details about the Preamps! I am pretty sure you will be amazed!

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Preamps as the first point of entry!

Have you ever heard the phrase “There’s no smoke without fire“? Well… There’s no microphone without a preamp! They are literally everywhere, behind every microphone, in different forms.

The reasons why Preamps are so important is physics. Remember what we discussed on the Sensitivity article on the Microphone Characteristics series?

In simple terms, the output of a microphone is not meant to be directly handled by professional audio equipment. After all, it’s just some voltage which has been transduced to that form from the acoustic energy that moved the microphone’s diaphragm. Preamps are specially designed to handle and amplify such weak signals.

They are there to help us out and they can be found everywhere. Your phone and laptop have preamps (small integrated ones that you cannot see, but they are there), and then you can find them on every mixing desk, no matter how small or big, on audio interfaces and on stand-alone versions as external hardware units that do exactly just that: Amplify the microphone signal, without affecting its quality, so it can be used by the rest of the audio chain!

Preamp Characteristics & Types

As you can imagine, there are lots of kinds of preamps out there but there are a few characteristics that are common and then some that depend on the type of the preamp.

Noise Floor

Like any electronic device (remember Microphone Characteristics: Self Noise) Preamps have self-noise too. And they can introduce it to the signal they are outputting.

Fortunately, since Preamps were built for this specific use, the do not add a massive amount of noise, and most of the times will be bearably noticeable. It’s worth noting that if you crank up the Preamp gain, you might introduce more noise above 50 or 60 db, depending on the quality of your Preamp.

Choosing a preamp with a low noise floor will keep our signal noise-free and clear!

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Solid State Preamps

Solid state preamps are pretty much transistor chip preamps. Common on most audio interfaces that are out there and are getting the job done!

Typically you will find them have the below characteristics:

  • Low Noise Exhibit
  • Transparent and clean
  • Not easy to drive as the introduce distortion

You will find a lot of other preamps sold in the market as stand-alone hardware units using a transistor so you may ask yourself what is the difference between that one and the ones you have got in your interface. With the low budget ones, the difference might be insignificant, but usually, more expensive Solid State preamps might have a better design and components that can lead to a lower noise floor and better performance in high dB gain values!

Tube Preamps

The Tube Preamps used to be modern back in the 50s till 70s, maybe till the 70s, when the Solid State preamps began taking over. These preamps have actual tubes in them which come in between input and output transformers.

Generally speaking Tube preamps have the below characteristics:

  • “Vintage” sound
  • Colourisation of the sound
  • Introduce Total Harmonic Distortion (THD) when the gain is overdriven

That “Vintage sound they offer is usually defined as smooth and creamy bass, with warm mid and airy or pristine high frequencies but the truth is that Tube preamps can sound differently between them in a vast degree.

Some might have richer bass and less pristine highs or the opposite might happen. Or you might find out that some have really warm mids but lack to reproduce the bass frequencies well enough. Or some, as a whole, might sound more direct and not as smooth as others. This all comes down to the design of the Preamp itself, the tubes, the input and output transformers, and the rest of the components used.

All these are features of previous, vintage, models that had they own sound, or “colour”, and now modern manufacturers are trying to reproduce these Preamp models so these signature sounding preamps live forever.

And although the choice of a Tube preamp might be difficult, once you choose your “colour” you need to think of the down sides.

Tubes don’t last forever. You will find out that you will have to replace a tube after a certain amount of time as its life will come to an end. And the closer the time comes to that moment, their performance will be degrading over time. Therefore that means, they need attention and maintenance.

To add to that, the warmer they get, their performance is changing and that also affects the sound. Generally speaking, the warmer the tube gets, the richer the sound it will output, but for a tube to be properly heated, it might even take hours.

This is the same reason why tubes perform better when pushed more, as more current flows through them, therefore they are heated more. But this does not make them ideal for situations when they are not being used to their limit and, of course, is contradictive to the solid state preamps, who can perform brilliantly when not pushed too much.

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Total Harmonic Distortion (THD)

THD is a benefit of the Tube Preamps, but first, let’s see what its definition is as defined on Wikipedia:

The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion factor, a closely related term, is sometimes used as a synonym.

Cool huh? But what does that mean?

Practically it tells you that all sounds have a Fundamental frequency which then has some Harmonics amongst other frequencies. The harmonics are strictly, any multiples of the Fundamental frequency.

If these Harmonics are boosted properly they introduce a kind of distortion (also known as Saturation) that enchances the sound in a way that colourises the sound instead of distorting it.

This is why boosting the gain on a Tube Preamp will result in a smoother, more compressed rather than a harsh, almost clipping sound when doing the same thing on a Solid State Preamp.

Extended Preamp functions

Finally, any type of Preamp can have extra controls on it that can vary.

A lot of them offer what an Input Section of a channel strip would offer and some others offer their own, unique functions. For example, some of the Tube preamps might have tone controls that affect the colour of the sound even further than just having a tube in the way.

The posibillities are endless, and when it comes to a Preamp purchase, it comes down to the type of sound you want to introduce to your mixes!

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What do you think about preamps? Do you understand their use and differences now? Let us know in the comments!

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