Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier. The concept of tube sound did not exist prior to the invention of transistors, as all electronic amplification of audio signals used vacuum tubes. After the introduction of solid-state amplifiers, tube sound became the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. Modern solid-state amplifiers are made with a neutral sound compared to tube amplifiers; thus, tube sound typically refers to euphonic distortion now associated with tube amplification. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts. (also found in )
Even though tube amplifiers are generally more expensive than solid-state devices, tube instrument amplifiers are still preferred by many musicians for electric guitar, electric bass, and keyboards. Additionally, some audiophiles prefer tube amplification for listening to prerecorded music.
History Before the commercial introduction of transistors in the 1950s, electronic amplifiers used vacuum tubes (known in the United Kingdom as valves). By the 1960s, solid state (transistorized) amplification had become more common because of its smaller size, lighter weight, lower heat production, and improved reliability. Tube amplifiers have retained a loyal following amongst some audiophiles and musicians. Some tube designs command very high prices, and tube amplifiers have been going through a revival since Chinese and Russian markets have opened to global trade—tube production never went out of vogue in these countries. Many transistor-based audio power amplifiers use MOSFET (metal–oxide–semiconductor field-effect transistor) devices in their power sections, because their distortion curve is more tube-like.
Musical instrument amplification
Some musiciansFor example, Robert Walser Running with the Devil: power, gender, and madness in heavy metal music, Wesleyan University Press, 1993 pages 43-44 discusses the "tube sound" sought by Eddie Van Halen prefer the distortion characteristics of tubes over transistors for electric guitar, bass, and other instrument amplifiers. In this case, generating deliberate (and in the case of electric guitars often considerable) audible distortion or is usually the goal. The term can also be used to describe the sound created by specially designed transistor amplifiers or digital modeling devices that try to closely emulate the characteristics of the tube sound.
The tube sound is often subjectively described as having a warmth and richness, but the source of this is by no means agreed on. Possible explanations mention non-linear clipping, or the higher levels of second-order harmonic distortion in single-ended designs, resulting from the tube interacting with the inductance of the output transformer.
Harmonic content and distortion Triodes (and MOSFETs) produce a monotonically decaying harmonic distortion spectrum. Even-order harmonics and odd-order harmonics are both natural number multiples of the input frequency.
Psychoacoustics research suggests that high-order harmonics are more offensive than low-order. For this reason, distortion measurements should weight audible high-order harmonics more than low. The importance of high-order harmonics suggests that distortion should be regarded in terms of the complete series or of the composite waveform that this series represents. It has been shown that weighting the harmonics by the square of the order correlates well with subjective listening tests. Weighting the distortion waveform proportionally to the square of the frequency gives a measure of the reciprocal of the radius of curvature of the waveform, and is therefore related to the sharpness of any corners on it. Based on said discovery, highly sophisticated methods of weighting of distortion harmonics have been developed. Since they concentrate in the origins of the distortion, they are mostly useful for the engineers who develop and design audio amplifiers, but on the other hand they may be difficult to use for the reviewers who only measure the output.
Some subjective aspects of audio perception is difficult to address with purely objective audio measurements. This is the case when designing or reviewing instrument amplifiers, as they tend to have design goals of very different than that of HiFi audio amplifiers. HiFi design largely concentrates on improving performance of objectively measurable variables. Instrument amplifier design largely concentrates on subjective issues, such as pleasantness of a certain type of tone. Fine examples are cases of distortion or frequency response: HiFi design tries to minimize distortion and focuses on eliminating offensive harmonics. It also aims for an ideally flat response. Musical instrument amplifier design deliberately introduces distortion and great non-linearities in frequency response. Former offensiveness of certain types of harmonics becomes a highly subjective topic, along with preferences towards certain types of frequency responses (whether flat or un-flat).
Push–pull amplifiers use two nominally identical gain devices in tandem. One consequence of this is that all even-order harmonic products cancel, allowing only odd-order distortion.A First Course in Electronics, pg 414-416. Anwar A. Khan and Kanchan K. Dey This is because a push–pull amplifier has a symmetric (odd symmetry) transfer characteristic. Power amplifiers are of the push–pull type to avoid the inefficiency of Class A amplifiers.
A single-ended amplifier will generally produce even as well as odd harmonics.Ask the Doctors: Tube vs. Solid-State Harmonics—Universal Audio WebzineVolume cranked up in amp debate—Electronic Engineering Times A particularly famous research about tube sound compared a selection of single-ended tube microphone preamplifiers to a selection of push–pull transistorized microphone preamplifiers. The difference in harmonic patterns of these two topologies has henceforth been often incorrectly attributed to the difference between tube and solid-state devices (or even the amplifier class). Push–pull tube amplifiers can be run in class A (rarely), AB, or B. Also, a class-B amplifier may have crossover distortion that will be typically high order and thus sonically very undesirable indeed.
The distortion content of class-A circuits (SE or PP) typically monotonically reduces as the signal level is reduced, asymptotic to zero during quiet passages of music. For this reason, class-A amplifiers are especially desired for classical and acoustic music since the distortion relative to signal decreases as the music gets quieter. Class-A amplifiers measure best at low power. Class-AB and B amplifiers measure best just below max rated power.
Loudspeakers present a reactive load to an amplifier (capacitance, inductance and resistance). This impedance may vary in value with signal frequency and amplitude. This variable loading affects the amplifier's performance both because the amplifier has nonzero output impedance (it cannot keep its output voltage perfectly constant when the speaker load varies) and because the phase of the speaker load can change the stability margin of the amplifier. The influence of the speaker impedance is different between tube amplifiers and transistor amplifiers. The reason is that tube amplifiers normally use output transformers and cannot use much negative feedback due to phase problems in transformer circuits. Notable exceptions are various OTL (output-transformerless) tube amplifiers, pioneered by Julius Futterman in the 1950s, or somewhat rarer tube amplifiers that replace the impedance matching transformer with additional (often, though not necessarily, transistorized) circuitry in order to eliminate parasitics and musically unrelated magnetic distortions.Tubes vs Transformers: An esoteric exploration of tubes, transformers, tone and transcendence In addition to that, many solid-state amplifiers, designed specifically to amplify electric instruments such as guitars or bass guitars, employ current feedback circuitry. This circuitry increases the amplifier's output impedance, resulting in a response similar to that of tube amplifiers.
The design of speaker crossover networks and other electro-mechanical properties may result in a speaker with a very uneven impedance curve, for a nominal 8 Ω speaker, being as low as 6 Ω at some places and as high as 30–50 Ω elsewhere in the curve. An amplifier with little or no negative feedback will always perform poorly when faced with a speaker where little attention was paid to the impedance curve.
Design comparison There has been considerable debate over the characteristics of tubes versus bipolar junction transistors. Triodes and MOSFETs have certain similarities in their transfer characteristics. Later forms of the tube, the tetrode and pentode, have quite different characteristics that are in some ways similar to the bipolar transistor. Yet MOSFET amplifier circuits typically do not reproduce tube sound any more than typical bipolar designs. The reason is circuit differences between a typical tube design and a typical MOSFET design.
Input impedance A characteristic feature of most tube amplifier designs is the high input impedance (typically 100 kΩ or more) in modern designs and as much as 1 MΩ in classic designs. The input impedance of the amplifier is a load for the source device. Even for some modern music reproduction devices, the recommended load impedance is over 50 kΩ.Sony Corporation 1999. Sony compact disc player CDP-XB930 Operating Instructions. (1). Specifications, p.20. This implies that the input of an average tube amplifier is a problem-free load for music signal sources. By contrast, some transistor amplifiers for home use have lower input impedances, as low as 15 kΩ.Rotel stereo integrated amplifier RA-935BX owners manual. MN10002975-A. p.4 Since it is possible to use high output impedance devices due to the high input impedance, other factors may need to be accounted for, such as cable capacitance and microphonics.
Output impedance Loudspeakers usually load audio amplifiers. In audio history, nearly all loudspeakers have been electrodynamic loudspeakers. There also exists a minority of electrostatic loudspeakers and some other more exotic loudspeakers. Electrodynamic loudspeakers transform electric current into force and force into acceleration of the diaphragm, which causes sound pressure. Due to the principle of an electrodynamic speaker, most loudspeaker drivers ought to be driven by an electric current signal. The current signal drives the electrodynamic speaker more accurately, causing less distortion than a voltage signal.
In an ideal current or transconductance amplifier, the output impedance approaches infinity. Practically all commercial audio amplifiers are voltage amplifiers. Conversely, their output impedances have been intentionally developed to approach zero. Due to the nature of vacuum tubes and audio transformers, the output impedance of an average tube amplifier is usually considerably higher than that of modern audio amplifiers produced completely without vacuum tubes or audio transformers. Most tube amplifiers, with their higher output impedance, are less ideal voltage amplifiers than the solid-state voltage amplifiers with their smaller output impedance.