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The Microphone Preamplifier

There are many elements that go into making a professional recording studio a great place to record music. Each is as important as the other, as a chain is only as strong as its weakest link. Your A\D converters, the cleanliness of your signal path, the acoustic properties of your recording room, your choice of microphone and choice of monitors (so you can hear the fine differences) are all critical to quality.(1) I would like to address one of these elements – the microphone preamplifier (in short “preamp”, or “mic pre”) – why this is such an important element to your sound, and some of the differences between a high quality preamplifier, costing well over a thousand dollars for one channel, and a mic pre in a unit that costs less than that for several channels. In a recording studio you find a large number of microphone preamplifiers, usually of various makes and models. Each one of these high quality preamps has a different characteristic. Choosing which mic pre to use for each instrument is one of the ways in which an engineer affects the sonic characteristic of a recording.

The task of a microphone preamplifier is to amplify the very low voltage that comes from a microphone (“mic level“) to a “usable” voltage, which can be manipulated by studio equipment and sent to a recording device (“line level“). To understand why this process is such a critical one, consider that the voltage coming from a microphone is around 2 mV (milliVolt), or 0.002 V. Line level signals are typically around 1.23 V. The mic pre has to amplify a signal by roughly 615 times. With such a drastic change in level, every little detail will have great effect on the amplified signal.

One very important and basic characteristic that sets a high quality preamp apart is its noise level. A lower quality preamp can add relatively large amounts of noise into the signal it amplifies. The noise – which may not initially be heard by an untrained ear – degrades the clarity of the tone. When you consider the large amounts of tracks that make up a recorded song (especially these days with virtually endless amount of tracks at our disposal, track counts tend to get larger and larger) the seemingly small amount of noise in each track can really add up. Another very important component of a preamp is its power supply. The voltage amounts mentioned earlier are average levels, measured in RMS, which is a mathematical way of averaging a sine wave. In reality sounds produced by most instruments have peaks that are high above this average level. Think of plucking a guitar string, or hitting a cymbal. The initial attack of the sound is much louder than the average level the instrument produces, and the peak happens for a fraction of a second. These peaks are called transients. A good microphone will pick up these transients, and it is up to the mic pre to faithfully amplify that information. In order to do that, the preamp will need to draw significant amounts of current very fast. If you ever get to lift a high quality mic pre you will notice it is fairly heavy. Most of this weight comes from the power supply needed for this demanding task. Now let’s consider a basic audio interface (or sound card) which may be connected to your computer at home. Usually these are entirely powered from the USB or Firewire port. This one small source of electricity powers the entire device. You have invested in a decent microphone, which is connected to the built-in preamp. Do you think the preamps faithfully amplify the transients that make up the detailed sound, which the microphone has picked up? Or is a lot of that information distorted and lost during the amplification process?

Some other aspects that make up a preamp’s characteristic are the components used to amplify the signal (e.g. tube or solid-state transistors) and its circuit design. Some preamps are considered to be “transparent” – producing a signal that is as faithful as possible to the source; while others are known for adding coloration to the tone. The personal preference of the engineer goes into play here, as he decides which preamp to use on each instrument to achieve the desired tone. Some preamps look simple; all they have on the front panel is two knobs, or maybe even one. The idea behind these designs is to maintain a very clean signal path. Other preamps may have an equalizer, a high-pass filter and other switches such as phase reverse and an impedance selector switch. Some preamps even offer two different signal paths: one with tube amplification and one with solid-state – all in one device! These options give the engineer more ways to shape the tone. You have to remember, though, that in order to maintain good quality each one of the components the signal passes through must be of high quality. Again, the chain is only as strong as its weakest link. Cheap components and switches can add noise and distortion and degrade the signal.

There is much hype around tube preamps, which are marketed as “better” preamps. I would like to stress here that among high quality preamps there is a lot of choice and neither type is considered to be better. There are high quality solid-state preamps that perform much better than some tube pres. I like to think of the different options as colors. It is not better or worse, rather different. In choosing the desired preamp for the task, an engineer also considers which microphone to use. Of course, the wide choice of high quality microphones is another feature of a good recording studio, and an article can be written on this topic alone. The interaction between the microphone and the preamp gives the engineer many options and a large palette of tone, and making these choices is a true art. If you ever get a chance to experiment with several preamps, try to use the same microphone with the same sound source, and connect it to the different preamps. You will be surprised at how different one can sound from the other.

Each input on a recording console has a mic pre. This is an important factor that makes a high quality console. In a good recording studio you will also typically find a selection of “outboard” mic pres. These are external units that are brought into the studio and patched into the signal path. This way, the engineer has many different options to choose from for a variety of tone when recording. Choosing the right microphone for each instrument, placing it well and pairing the microphone with a good preamp will produce a great sounding recording with detailed and rich tone.

Sources:
1 – http://www.tweakheadz.com/microphone_preamps.htm

References and Links:
“Mic Level” from Sweetwater Glossary – http://www.sweetwater.com/expert-center/glossary/t–MicLevel
“Line Level” from Wikipedia – http://en.wikipedia.org/wiki/Line_level
“RMS” from Sweetwater Glossary – http://www.sweetwater.com/expert-center/glossary/t–RMS
Short summary from wikirecording.org – http://www.wikirecording.org/Choosing_a_Microphone_Pre_Amp