| Tube Sound |
Article Index for Tube |
Website Links For Tube |
Information AboutTube Sound |
| CATEGORIES ABOUT TUBE SOUND | |
| valve amplifiers | |
| vacuum tubes | |
| high end audio | |
| audio amplifiers | |
| audio engineering | |
|
Tube sound (or ''' Valve sound''') is the characteristic sound traditionally associated with a Tube -based Audio Amplifier . All practical amplifiers distort to some degree; some Audiophile s prefer the sound that is produced by the distortion characteristics of tube-based amplifiers. The audible significance of measurable effects on signals is a continuing debate among audiophiles. HISTORY Before the commercial introduction of transistors in the 1950s, electronic Amplifier s used vacuum tubes (a.k.a. "valves"). By the 1960s, Solid State (transistorized) amplification became more common, due to its smaller size, lighter weight, lower heat production, and improved reliability. However, tube amplifiers have retained a loyal following amongst some Audiophile s, with some modern units commanding very high prices. Some musicians performing on electric guitar, electric bass, and keyboards in a range of popular and jazz genres continue to use tube Instrument Amplifier s or preamplifiers. EXPLANATION ''Tube sound'' is present in two distinct fields: Sound reproduction Some Audiophiles argue that the sound - classically associated with tube amplifiers - is richer or warmer than the sound from typical Transistor amplifiers, and for this reason more ''satisfying''. Superficially, in Sound Reproduction systems, accurate reproduction of the sound of the original recording is usually the goal, ie "High Fidelity" (HiFi), and thus gross distortion is nominally a bad thing designers do not deliberately seek to introduce. However at the upper end of audio systems ("high end" or "audiophile" systems) it is debated whether "accuracy" can best be described by simply having a wide frequency response and low measured distortion levels or whether highest quality reproduction is that which sounds subjectively "natural" or "musical". All real world designs distort to some degree, and designers seek a compromise in the nature of the distortion their design produces, that "sounds good", rather than that which simply measures well. In the high end world audiophiles will typically pay greater regard to ''how it sounds'' than to how it measures, to the point of just not caring about measured distortion levels at all. The reasons behind the "tube sound", and why some consider this to be better than the "transistor sound" are complex but discussed below. Musical instrument amplification Some Musicians also prefer the Distortion characteristics of tubes over Transistors for Electric Guitar , Bass , and other Instrument Amplifier s. In this case, generating deliberate (and sometimes considerable, in the case of electric guitars) audible distortion is usually a specific goal. See Also: Guitar effects Power chord SOLID-STATE EMULATION The term can also be used to describe the sound created by specially-designed transistor amplifiers or digital modelling devices that emulate the characteristics of the tube sound. PROBABLE CAUSES The "warmth" and "richness" typically associated with "tube sound" is due to significant levels of 2nd order distortion, typically coming from a single ended (classic tube class A amp design ) stage, often the output stage. However the monotonically reducing purely harmonic distortion spectrum of the simple SE gain stage is not of itself anything to do with using a tube, and a similar distortion spectrum and sound can be obtained using the same circuit topology with , for example, a MOSFET. Negative feedback Commodity transistor amplifiers, especially from the 1980s, typically used large amounts of negative feedback. This allowed a very low measure of total harmonic distortion. Typically tube amplifiers have little negative feedback applied, and in recent years there has been a tendency in transistor amplifiers to use less feedback and design more linear stages which has again narrowed the sonic differences between tube and transistor designs. AUDIBLE DIFFERENCES Some audiophiles prefer the sound of tubes over transistors. As above this is in reality partly a function of the circuit topologies typically used with tubes vs the circuit topologies typically used with transistors, as much as the gain devices themselves. But there are also real differences. Although there are also similarities between say the characteristics of a triode and mosfet, or a tetrode and a bipolar transistor Some sonic qualities are easy to explain objectively based on an analysis of the distortion characteristics of the gain device and/or the circuit topology. For example the triode SE gain stage produced a stereotypical monotonically decaying harmonic distortion spectrum that is dominated by significant second order harmonics that made the sound seem rich or even "fat". However, other audible differences in sound have proven difficult to define or measure, and it is difficult to explain these sound differences in words as the vocabulary available to describe sound is rather limited -even though the underlying sonic effects are real. Audiophiles often use words like 'warm', 'liquid', 'smooth' and 'midrange magic' to describe tube amplifiers' sound. Some claim that the midrange reproduction is more extended and smoother with tube amplifiers, but that high frequencies are somewhat rolled off. Historically this was often the case due to limitations in capacitor performance. Modern audiophile grade tube amplifiers however, using modern and/or premium quality (and cost) capacitors can have frequency response that are essentially flat to octaves beyond the audio range, -3dB above 65kHz would be normal, above 85kHz is quite common Similarly, some would characterise "tube sound" as bass response with less power and/or less definition (perhaps even "sloppy" bass or a bass boom with some speakers.) This again can be explained by many tube amplifiers having relatively high output impedance (Z out) compared to transistor designs, due to the combination of both higher device impedance itself and typically reduced feedback margins (more feedback results in a lower Z out). So for example a hypothetical design in two otherwise equal variants with just different amounts of feedback, might result in the higher feedback version having a "drier" midrange (due to reduced 2nd order harmonics due to greater reduction of distortion) but a "tighter" bass due lower output impedance. The speaker impedance divided by the Z out is sometimes referred to as the "damping factor" - the amplifier's ability to control the mechanical movement of the speaker. In general terms the sound from a tube amplifier will typically have a softer attack and the bass frequencies will be more prominent giving a warmer and "less harsh" sound. Instrumentation such as pianos and vocals sound softer and "fatter" than with transistor amplifiers. But as noted the reasons for these effects are not simply and unavoidably related to the gain device type, today a good designer using either technology has to make synergistic design compromise choices. And the sonic differences are less stereotyped than they used to be as a result. HARMONIC CONTENT AND DISTORTION Triodes (and Mosfets) produce a monotonically decaying harmonic distortion spectrum. Psychoacoustic effects include that the stronger, lower harmonic products tend to dominate and mask the sound of the weaker, higher harmonic products Even order harmonics sound as musical chords (notably octaves), which subjectively makes the sound "richer". Odd order harmonics sound less pleasant. aharmonic distortion is discordant and is often implicated in designs that sound "brash", "harsh", "brittle" etc Push Pull amplifiers use two nominally identical gain devices "back to back". One consequence of this is that all even order harmonic products cancel, leaving the - subjectively less musical, less "rich" - odd order products to dominate. The total (measured) harmonic distortion content is lowered, but subjectively the design may sound worse. A push-pull amplifier is said to have a symmetric ( Odd Symmetry ) Transfer Characteristic , and accordingly produces only odd harmonics. A single-ended amplifier has an Asymmetric transfer characteristic, and produces both even ''and'' odd harmonics. Ask the Doctors: Tube vs. Solid-State Harmonics — Universal Audio Webzine Volume cranked up in amp debate — Electronic Engineering Times |
|
|