Most readers are clear on what a mic preamp is and what it's for, but once terms like solid state, tubes, Class A and impedance start coming up, things can get complicated.
This component is a very important link in the signal chain, and there are often many myths and beliefs about it that aren't necessarily true. On the other hand, the widespread adoption of computers as tools for recording, mixing and producing music means that many users find themselves using all kinds of plugins every day.
Because each plugin takes a certain amount of time to process the signal, each one introduces a delay in the signal passing through it. Some DAWs have systems to compensate for this automatically (known as delay compensation systems), but many others don't.
Knowing how to combat this delay and what effects it can have on our work is essential day to day. Below are some of the most common questions and answers on these two topics.
Mic Preamps
Without going into detailed definitions or analysis, and to keep less experienced readers up to speed, you could say that a mic preamp is a device that matches the level of the signal coming from a microphone (or other sources) to the operating level of other gear, such as a mixing console, an A/D converter, an equalizer...
The way this process is carried out can have a big effect on the character of the original signal, which is why so much importance is given to choosing this piece of gear. The preamp's internal circuitry interacts with the signal coming from the mic, and factors such as impedance, tubes, transistors and so on can give the unit one “color” or another.
Solid State vs. Tube
There's often a belief that tube preamps are “better” than solid-state ones, or that solid-state units are “cleaner” than tube ones.
In reality, if they're designed correctly, both types of preamp can deliver low noise figures, and both will work properly within their linear range. The difference begins when the preamps are pushed beyond that linear zone. Tubes move into nonlinearity gradually, and the effect is that the signal gets rounded off, which predominantly generates even-order harmonics.
The ear interprets these even-order harmonics as a “pleasant” distortion, which is why preamps are often deliberately overdriven to get this tube effect.
Some types of tubes even create a certain amount of harmonic distortion within their linear range too, and the way others work involves a slight low-pass filtering of the signal. The sum of these factors is often what is defined as tube “warmth.”
By contrast, solid-state components move into nonlinearity abruptly, so the signal gets clipped. In this case, mostly odd-order harmonics are created, which are generally perceived as “less pleasant.”
That doesn't mean solid state is worse, only that it “shouldn't” be overdriven. Then again, there are guitar distortion pedals that generate odd-order harmonics, so some guitarists will disagree that these harmonics are undesirable...
Which preamp is better? Perhaps a more appropriate question would be: what are you looking for?
The effect produced by tubes can be just right on some occasions, where a good tube preamp driven into saturation is the ideal ingredient to give a passage or instrument a certain character.
At other times, however, that may be precisely what you want to avoid, and a good solid-state preamp is the best choice for a faithful capture of the original sound (hence the saying that these preamps are cleaner).
In any case, the key to it all probably lies in the word “good” preceding the word “preamp” in the previous two paragraphs.
Class A?
This term also tends to create some confusion. In general, a Class A preamp seems to signal good quality, when in fact the term only tells you about the topology of the preamplifier's output circuit.
In other words, there are different types of amplifier design, also known as “classes.” Class A is one of them, but there is also Class B, Class AB, Class C, Class D... each one carries out amplification in a different way and therefore has certain characteristics of its own.
In theory, Class A is the least efficient in terms of power consumption, but it's also the most accurate at its job, and that's why it tends to be preferred for audio applications.
That doesn't mean other classes can't be used for audio, though. For example, Grace Designs uses hybrid AB topologies in its preamps, which are widely recognized for their quality.
On the other hand, a Class A design is no guarantee of excellence. As with any other piece of gear, the quality of the components used to implement the design (capacitors, resistors, wiring...) has a lot to say about its final quality.
Thinking otherwise would be like assuming all cars are the same because they all have the same parts... Obviously, some engines are better than others, some brake discs are better than others, and so on.
Impedance
The relationship between the mic's output impedance and the preamp's input impedance has an effect on the resulting sound.
It's sometimes said to be a kind of EQ, because if you vary either of these impedances (for example, by using a preamp with adjustable impedance), the audible result is similar to what you'd get by equalizing the signal.
However, this effect is not usually so drastic that a mic can't be used with a given preamp. In fact, the truth is that (speaking of the pro and semi-pro market) most mics can be used with most preamps without any problem.
What is advisable is to run tests with the gear you have, to find out how each mic sounds with each preamp and get to know intimately the possibilities available to you when recording.
As for exact impedance matching (in reference to the Maximum Power Transfer Theorem), it should be noted that, in audio, such matching isn't always desirable, since it can mean a reduction in signal-to-noise ratio. With dynamic or condenser mics, the ratio of the mic's impedance to the preamp's is usually between one-fifth and one-tenth.
