Do tube preamps really sound better than solid-state preamps? As with many, discussions in which technology, artistry and aural perception are combined, this issue has no clear answer. But we can look at some basic details objectively, and hopefully help you make a more informed buying decision.
Sol•id State adj.
Electronic components and circuits where signals pass through solid semiconductor material such as transistors and diodes as opposed to vacuum tubes where signals pass through a vacuum, or relays, which are electromechanical devices. "Solid State" was a big buzzword in the early 1970's because solid state equipment was not only less expensive, but often thought to be more reliable in the field than the tube based predecessors.
Vac•uum Tube n. Abbr. VT
An electron tube where virtually all the air has been removed (creating a vacuum), thus permitting electrons to move freely, with low interaction with any remaining air molecules. Early tubes had two electrodes (cathode and anode, or plate) and were known as diodes. Later (1907) a grid (or control grid) was added, creating what is known as a triode. The grid provided real-time control of output levels, where a small voltage applied could regulate the current passing from the cathode to the plate, creating the first practical amplifier.
First of all, understand that everything in the signal path can have a noticeable effect on the sound of your track - the microphone, the cable, the power supply (for condenser mics), the room noise, the impedance of the preamp, even the humidity! So before you make a long-term decision on a preamp, be sure to look into all the elements of your
Tube circuits are sometimes elevated into mythological status, primarily because they were all we had for about 50 years before solid state came along. But in real world terms a good solid-state preamp can sound much better than a poorly designed tube preamp. Conversely, a good tube preamp usually sounds better than a poorly designed solid-state preamp. That's a pretty simple quality issue.
It's safe to say that both tube and solid-state preamps - when properly designed - exhibit low distortion throughout their normal amplitude range. The difference in sound becomes evident when the circuits run out of headroom. Solid-state devices tend to abruptly transition from low distortion to extreme distortion (clipping). This is actually a good thing because, when operated right up to their maximum level (generally a higher level than a tube circuit), solid-state preamps can maintain excellent performance.
It's the nature of the distortion that makes the two types sound different. Solid-state circuits run out of headroom when the output voltage exceeds the power supply voltage. The result is gross distortion - the output becomes a square wave. Square waves are not sounds that we normally usually consider musical, so our subjective response to them is negative.
When a tube circuit distorts, the primary distortion product is even order harmonics, with the second harmonic dominant. It so happens that musical instruments also produce primarily even harmonics. By definition, that's what makes them "musical." So you could say that tube circuits can add a musical component to recorded sound. Fortunately, you can take your choice - keep the level reasonable and obtain good clean audio, or run the circuit into distortion and generate some harmonics that weren't there to begin with.
In circumstances when the preamps aren't being driven into distortion, it's reasonable to say that tube circuits always generate a certain degree of harmonic distortion, simply as a result of the way they work. Some tubes, specifically triodes, also exhibit a form of low pass filtering attributed to the "Miller Effect" - the charging and discharging of the plate-to-grid capacitance as the input signal changes. These factors are a large part of what is often described as "tube warmth," and we - meaning most musicians, recording engineers and the listening public - have basically agreed that this is a pleasant sound. Solid-state circuits, with relatively low distortion and no inherent low pass filtering, might actually provide a more accurate capture of the sound, but this is often perceived as "thin" or "sterile" in contrast to tubes.
So we're really saying that we like tube distortion to a certain degree, even in vocals and other critical recording situations. And many solid-state preamps actually incorporate circuitry that attempts to recreate tube-style distortion. And the message for you, the recordist in search of a preamp, is to use your ears and choose what sounds best to you.