Q: “What, exactly, is ‘tube warmth?’ What causes it, and why would I want it in my mic preamp?”
A: Two of these questions can be answered in highly subjective terms. First, “tube warmth” is a widely-overused phrase that attempts to describe the tonal differences between preamps – and audio gear of all types – that use vacuum tubes in their amplification stages, and solid state equipment, which replaces tubes with transistors. No two people have the same definition of this reported warmth, but it can be roughly identified as having a sound like “vintage” preamps and amplifiers, which often produced a slight to moderate emphasis on midrange frequencies while attenuating high frequencies somewhat, and typically had higher levels of distortion and noise compared to high end equipment available today. This statement alone could generate emails to your InSync editor. For example, it would be a mistake to read the above statement and conclude that tube gear has a darker sound with less high end. In fact, many tube advocates swear by the openness and airy quality of their most prized tube gear. We are talking about what are in many cases subtle variations in sonic character that are often signal dependant. Broad generalizations must be used with caution.
The “cause” of this tonal characteristic is easier to define. Part of “tube sound” is the result of harmonic distortion that is inherent in tube circuitry. A second element, particularly in circuits that use triode tubes such as the ubiquitous 12AX7, is a behavior called the Miller Effect. This is a cycle of resistance and capacitance occurring between the tube’s plate and grid that creates a primitive low pass filter. As input stage gain increases, the greater this effect becomes. Guitar players like this; it means they can drive their amp’s tubes to the point of distortion while the Miller Effect rolls off high frequencies, thus preventing broken glassware and shattered windows. However, in microphone preamps, manufacturers work to suppress this effect by reducing the resistance that drives the triode tube, for example, or by reducing the tube’s gain. Mic preamps have almost always been designed with accurate signal reproduction in mind. But a good portion of tube warmth can be attributed to this low pass filter, or high frequency rolloff, however you choose to think of it.
Why would you want tube warmth in your mic preamp? Another highly subjective issue! Many vocalists (and engineers) feel that a tube preamp delivers a “smooth” sounding vocal performance with well-rounded mids and high frequencies that aren’t harsh. This can be an appealing choice for a singer whose natural (or at least recorded) timbre is rather thin, with weak fundamental pitches and overly strident upper harmonics.
The final answer is that a preamp is as much a creative tool as it is a utility. Besides amplifying a mic signal to acceptable levels it adds its own tonal color, and the color of a tube preamp is simply one more creative option you have when recording.