You probably know more or less what preamplifiers do. Commonly known as preamps or just pres, these devices are used to amplify low level signals and prepare them for further processing. But there's a lot more to it than that, and if you want to get the most out of your preamps, there are a few important things you should consider.

In the studio, when we talk about preamps we usually mean microphone preamps. As we discussed in our Knowledge Base feature on microphones back in FM212, mics produce a very low voltage signal that must be boosted before it can be recorded, mixed or processed. Whether you're recording or just mixing a couple of m;cs for a live PA, preamps are specifically designed to amplify the low level output, bringing it up to line level. At their most basic, the settings are simple: a gain control to adjust the level of amplification and a peak level indicator to warn about the onset of clipping.

Most mic preamps also include switchable 48V phantom power for condenser mics. As things start to get more complex, preamps may feature additional options such as phase inversion settings, pads (fixed, switchable attenuators on the input to reduce the level of a hot signal before it hits the gain stage), filters and line level gain stages to boost the signal further after the preamp stage. Although not strictly part of the mic pre itself, more advanced units may also include features such as EQ, compression and noise gates in order to offer a complete channel strip.

Walk in to any studio these days and you'll almost certainly find a selection of rack-mount outboard preamps. This wasn't always the case. Back in the '60s and '70s, most studios would simply plug microphones directly into the mixing console's inputs, using the preamps built into the channel strips. The lack of options was made up for in convenience.

Over the last couple of decades, the balance has swung in favor of high-quality external preamps which provide a number of different options from clean and neutral to dirty and colored. In general, consoles with neutral, transparent preamps and summing stages now tend to be favored, providing a blank canvas which can be colored using a variety of standalone character preamps. At the risk of making a sweeping generalization, most producers would now prefer to have various different preamps on hand rather than a full console of identical units. The availability of classic preamp designs as standalone units means that producers can pick and choose the best sounding option for each recording task.

Although we're going to concentrate on microphone preamps, very similar units are used for a variety of other purposes in the studio and in live setups. Instrument amplifiers, for example, typically consist of a preamp hooked up to a power amp. The preamp colors the signal and brings it up to a level high enough to drive the power amp stage. Line level amplifiers and internal gain stages operate along broadly similar electronic principles but are designed to amplify higher level signals and so offer lower maximum gain.

So far it all seems pretty straightforward, right? The preamp makes the signal louder so we can feed rt to the next piece of equipment or the next link in the signal chain. Unfortunately, that's only half the story. Practically every piece of studio equipment colors the sound to some extent and preamps are no exception. So what's actually happening to affect the signal? Let's consider what the ultimate clean, neutral preamp would offer: essentially, it would increase the level but leave everything else untouched Such a perfectly neutral preamp doesn't exist, so there will always be some level of coloration, from mild distortion to noise, inaccurate reproduction of transients, uneven frequency response or waveshaping and mild compression from clipping. The nature of this inaccuracy determines the characteristics of the preamp, from slightly colored to downright dirty.

Is there such a thing as a transparent preamp? Yes and no. 100% accuracy and neutrality are impossible to achieve, but certain preamps are known for being relatively neutral. The other important thing to consider is that preamps are adjustable. It may seem like a banal statement, but the sound of any preamp can change dramatically over the range of its settings. It might often be tempting to push the gain close to the clipping point in order to maximize the signal to noise ratio, but m some cases the sound may not necessarily benefit. Some preamps break up pleasantly as they approach clipping, whereas others start to sound bad when pushed hard. Some preamps will sound agreeable at high gain but others may reward you with harshness and noise. Resorting back yet again to the well-worn cliche, it's important to trust your ears and find a setting which offers the best compromise between neutrality, color and signal to noise ratio.

If different preamps can sound so different, what exactly affects their sound? Electronically speaking, there are dozens of variables which we need to take into account. The most obvious defining factor is whether the circuit is based on solid state components (like transistors and microchips) or valves. Generally speaking, all the usual rules of thumb apply: valve preamps tend to be warmer, less accurate and more colored than their solid state counterparts.

Many preamps also include transformer-coupled inputs and/or outputs, which also color the sound. Transformers are typically used to unbalance or balance signals, block DC signals, balance impedances or adjust voltages but they also tend to impart a distinct sound to the signal, saturating at higher levels and adding harmonics. All other things being equal, transformerless designs tend to be more neutral than transformer-coupled equivalents. As a result, some modern preamps include transformers but allow them to be bypassed in order to make the sound cleaner. It's easy to get bogged down trying to describe preamps in subjective audiophile terms but ultimately the best sense of each unit's sonic characteristics comes from experience.

Way back in FM209 we discussed the importance of 'matching' source (output) and input impedances in order to maximize voltage transfer and avoid signal degradation. Matching is something of a misnomer here, since the most common solution is actually to bridge the impedances by ensuring the input impedance is at least five times higher than the source's output impedance. Like any other connection, the impedance match between a microphone and its preamp can affect the sound significantly. However, most modern low-impedance mics will function quite happily with most modern pres. Even so, it's still

worth considering the impedance of your mic pre input. A large number of modern preamps offer switchable input impedances, allowing users to experiment with loading combinations in order to get the best sound. Ribbon mics in particular need special consideration. Many older models were designed with 300 ohm output impedance and intended to be matched to 300 ohm inputs. Newer models, on the other hand, often benefit from much higher input impedances. Specialist ribbon mic preamps may offer input impedances up to 20 kohm and beyond.

Despite the name, mic preamps aren't just used to amplify microphone signals. Electromagnetic pickups found in guitars, basses and electric pianos offer a low level output which requires amplification to line level and microphone preamps a perfectly capable of boosting the signal of an instrument in just the same way that they take a low voltage mic signal and bring it up to line level. However, the high output impedance of these instruments means that connecting directly to a preamp input is likely to result in a highly undesirable uneven frequency response.

A DI (direct injection/input/interface) box offers an easy solution, combining a high impedance input (usually greater than 500 kohm, often 1 Mohm or more) with a preamp-friendly low output impedance. DIs also convert unbalanced signals to balanced, meaning that the signal can be transmitted much further without signal loss or noise. There are two types of DI box to choose from: passive and active. Passive Dis typically consist of a simple transformer, requiring no power and offering very little in the way of adjustment. Some may include basic additional options such as a ground lift switch or signal splitter.

Active Dis, on the other hand, require power and operate (as the name suggests) on active circuitry. As a result, they may offer more advanced features such as pads, high-pass filters, phase inversion and even adjustable gain stages. Neither design is inherently better than the other, although active Dis typically offer marginally more accurate frequency response. Generally speaking, more expensive units will offer more accurate frequency response and less colored sound, with the obvious exception of units specifically designed to color the sound. Many preamps and interfaces feature high-impedance (hi-Z inputs as standard to allow instruments to be plugged in directly. These essentially route the signal through a built-in DI unit, but the quality of the results can vary significantly so it's a good idea to try a dedicated DI box even if it seems like your interface can do the job.

It's no exaggeration to say that preamps are one of the most important elements of any studio. From a simple acoustic recording to a synth masterpiece, careful selection and use of preamps can make the difference between a clean, neutral sound and a colored, character-filled tone.