Tube sound
Encyclopedia
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
associated with a vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
-based audio amplifier
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.
Many electric guitar
Electric guitar
An electric guitar is a guitar that uses the principle of direct electromagnetic induction to convert vibrations of its metal strings into electric audio signals. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker...
, electric bass
Electric Bass
Electric bass can mean:*Electric upright bass, the electric version of a double bass*Electric bass guitar*Bass synthesizer*Big Mouth Billy Bass, a battery-powered singing fish...
, and keyboard
Keyboard instrument
A keyboard instrument is a musical instrument which is played using a musical keyboard. The most common of these is the piano. Other widely used keyboard instruments include organs of various types as well as other mechanical, electromechanical and electronic instruments...
players in a range of popular, rock, funk, blues, reggae and jazz
Jazz
Jazz is a musical style that originated at the beginning of the 20th century in African American communities in the Southern United States. It was born out of a mix of African and European music traditions. From its early development until the present, jazz has incorporated music from 19th and 20th...
genres also prefer the sound of tube instrument amplifier
Instrument amplifier
An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...
s or preamplifiers.
History
Before the commercial introduction of transistorTransistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s in the 1950s, electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s used vacuum tubes (known in Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
as "valves"). By the 1960s, solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
(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 musician
Musician
A musician is an artist who plays a musical instrument. It may or may not be the person's profession. Musicians can be classified by their roles in performing music and writing music.Also....* A person who makes music a profession....
s. Some tube designs command very high prices, and tube amplifiers have been going through a revival since Chinese
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
and Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
n markets have opened to global trade—tube production never went out of vogue in these countries.
Sound reproduction
Audiophiles disagree on the relative merits of tube and solid state amplification. Some prefer the sound produced with tube amplifiers on the grounds that it is more natural and satisfying than the sound from typical transistor amplifiers. Those who subscribe to measurement and scientifically-based approaches to high fidelity note that in general, solid state designs can be manufactured without output transformers, and are therefore immune to speaker-dependent impedance mismatches and other transformer effects which alter the system spectral response. Typically, in sound reproduction systems, accurate reproduction of the sound of the original recording is the goal; distortion and uneven spectral response within the audible frequency band is something designers deliberately seek not to introduce.Musical instrument amplification
Some musicians also prefer the distortionDistortion (guitar)
Distortion effects create "warm", "dirty" and "fuzzy" sounds by compressing the peaks of a musical instrument's sound wave and adding overtones. The three principal types of distortion effects are overdrive, distortion and fuzz. Distortion effects are sometimes called “gain” effects, as distorted...
characteristics of tubes over transistors for electric guitar
Electric guitar
An electric guitar is a guitar that uses the principle of direct electromagnetic induction to convert vibrations of its metal strings into electric audio signals. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker...
, bass
Bass guitar
The bass guitar is a stringed instrument played primarily with the fingers or thumb , or by using a pick....
, and other instrument amplifier
Instrument amplifier
An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...
s. In this case, generating deliberate (and sometimes considerable, in the case of electric guitars) audible distortion or overdrive is usually the goal. The term can also be used to describe the sound created by specially-designed transistor amplifiers or digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
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. It may be due to the non-linear clipping that occurs with tube amps, or due to the higher levels of second-order harmonic distortion, common in single-ended designs resulting from the characteristics of the tube interacting with the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
of the output transformer.
Audible differences
The sound of a tube amplifier is partly a function of the circuit topologies typically used with tubes versus the topologies typically used with transistors, as much as the gain devices themselves. Beyond circuit design, there are other differences such as the electronic characteristics of a triode and MOSFETMOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
, or a tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
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 produces a stereotypical monotonically decaying harmonic distortion spectrum that is dominated by significant second-order harmonics making the sound seem "rich" or even "fat", while each higher order harmonic is smaller than the nearest lower order harmonic. Some audio professionals regard the effects of the use of tubes as distortion - which can be used creatively in certain scenarios.
The low frequency roll-off can be explained by many tube amplifiers having high output impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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 output impedance).
A hypothetical amplifier design in two otherwise equal variants with just different amounts of feedback, might result in the higher feedback version having a "drier" mid-range (due to reduced second-order harmonics due to greater reduction of distortion) but a "tighter" bass due to lower output impedance might result. The speaker impedance divided by the Z out is sometimes referred to as the "damping factor
Damping factor
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
"—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. Instruments such as pianos and vocals sound softer and fatter when compared against transistor amplifiers. The reasons for these effects are not simply related to the gain device type; today an amplifier designer using either technology may make synergistic design compromise choices. 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. Even-order harmonics and odd-order harmonics are both natural numberNatural number
In mathematics, the natural numbers are the ordinary whole numbers used for counting and ordering . These purposes are related to the linguistic notions of cardinal and ordinal numbers, respectively...
multiples of the input frequency.
Psychoacoustic phenomena include the effect that high-order harmonics are more offensive than low. Thus, in distortion measurements this should be taken into consideration to 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 wave-form 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 wave-form proportionally to the square of the frequency gives a measure of the reciprocal of the radius of curvature of the wave-form, 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.
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 odd order products to dominate. A push-pull amplifier is said to have a symmetric (odd symmetry
Even and odd functions
In mathematics, even functions and odd functions are functions which satisfy particular symmetry relations, with respect to taking additive inverses. They are important in many areas of mathematical analysis, especially the theory of power series and Fourier series...
) transfer characteristic, and accordingly produces only odd harmonics.
A single-ended amplifier has an asymmetric
Asymmetric
Something which is asymmetric displays asymmetry. Specific uses of the term may include:*Asymmetric relation for information on such relations in mathematics and set theory*Asymmetric warfare for information and theories of modern war...
transfer characteristic, and produces both even and odd harmonics. As tubes are often run single-ended, and semiconductor amplifiers are often push-pull, the types of distortion are incorrectly attributed to the devices (or even the amplifier class) instead of the topology. Push-pull tube amplifiers can be run in class A, AB, or B. Also, a class AB amplifier may have crossover distortion that will be typically inharmonic and thus sonically very undesirable indeed. However, guitar effects unit designer Zachary Vex has specifically designed a device that can generate crossover distortion in a controlled and potentially pleasing manner.
Another factor is that 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 etc. cf. class B and AB amplifiers, for which the amplitude of the crossover distortion is more or less constant, and thus the distortion relative to signal in fact increases 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
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
, inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
and resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
). This impedance may vary in value with signal frequency and amplitude. This variable loading affects the amplifier's performance both because the amplifier has finite 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, principally because tube amplifiers normally use output transformers, and cannot use as much negative feedback due to phase problems in transformer circuits.
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 transistorBipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
s. Triodes and MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
s have certain similarities in their transfer characteristics, whereas later forms of the tube, the tetrode
Tetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
and pentode
Pentode
A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...
, have quite different characteristics that are in some ways similar to the bipolar transistor. Despite this, e.g. MOSFET amplifier circuits typically do not reproduce tube sound any more than typical bipolar designs, due to the circuit topology differences between a typical tube design and a typical MOSFET design. But there are always exceptions: for example some very interesting designs such as the Zen series by Nelson Pass
Nelson Pass
Nelson Pass is a noted and widely respected designer of audio amplifiers.Unlike some audio engineers, Pass remains vocal that listening tests remain valuable and that electrical measurements alone do not fully characterize the sound of an amplifier.Mr. Pass holds at least seven U.S...
which can be found on the web.
Input impedance
A characteristic feature of most tube amplifier designs is the high input impedanceElectrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
(typically 100 kΩ
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
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. 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.
Soft clipping
Soft clipping is a very important aspect of tube sound especially for guitar amplifierGuitar amplifier
A guitar amplifier is an electronic amplifier designed to make the signal of an electric or acoustic guitar louder so that it will produce sound through a loudspeaker...
s, although a Hi-fi
High fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
amplifier should not normally ever be driven into clipping. A tube amplifier will reproduce a wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
relatively linearly to a point, and as the signal moves beyond the linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
range of the tube (into overload
Overload
-Bands:* Overload * Overload * Overload -Albums:*Overload *Overload *Overload -Songs:*Overload , a dance song by Voodoo and Serano...
), it distorts the signal with a smooth curve instead of a sudden, sharp-edged cutoff (or even ringing and/or lockup) as occurs with transistors. The harmonics added to the signal are of lower energy with soft clipping than hard clipping. However, soft clipping is not exclusive to tubes, it can be simulated in transistor circuits (below the point that real hard clipping would occur). (See "Intentional distortion" section).
Note also that tube circuits often have huge headroom (overload) margins due to the high voltages they run from, so hard clipping is in reality very rare in a tube stage itself. However core saturation in the output transformer may be "designed in" to some guitar amplifiers when driven hard, and/or the tube biasing may be designed so that the tube passes from class AB1 to class AB2 and starts to draw grid current etc. (these effects are perhaps beyond the scope of this article)
Circuit design may also play an important role in the tube sound; tube circuits are often less complex and laid out differently. It is argued that simplicity is usually best, as the length and complexity can change the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
and capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
of a circuit. A more complex circuit will have a more complex sonic distortion characteristic. Minimalist DH-SEs for example typically have a dominant very simple harmonic distortion spectrum. Complex modern transistor designs often have low level but extremely complex harmonic distortion spectra.
In recording industry and especially with microphone amplifiers it has been shown that amplifiers are often overloaded by signal transients. There is a major difference in the harmonic distortion components of the amplified signal, with tubes, transistors, and operational amplifiers separating into distinct groups.
Bandwidth
Early tube amplifiers often had limited response bandwidth, in part due to the characteristics of the inexpensive passive componentsPassivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...
then available. In power amplifiers most limitations come from the output transformer; low frequencies are limited by transformer core saturation and high frequencies by winding inductance and capacitance. Another limitation is in the combination of high output impedance, decoupling capacitor and grid resistor, which acts as a high-pass filter
High-pass filter
A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
. If interconnections are made from long cable (for example guitar to amp), tube input impedance with cable capacitance acts as a low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
. However, modern premium components make it easy to produce amplifiers that are essentially flat over the audio band, with less than 3 dB attenuation at 6 Hz and 70 kHz, well outside the audible range.
Negative feedback
Tube amplifiers could not use as much negative feedbackNegative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
(NFB) as transistor amplifiers due to the large phase shifts caused by the output transformers and their lower stage gains. While the absence of NFB greatly increases harmonic distortion, it avoids instability, as well as slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
and bandwidth limitations imposed by dominant-pole compensation in transistor amplifiers. Since transient intermodulation distortion was mainly caused by negative feedback, tube sound never suffered much of that kind of distortion.
Power supplies
Early tube amplifiers usually used unregulated power supplies. This was due to the high cost associated with high-quality high-voltage power supplies. The typical anodeAnode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
supply was simply a rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
, an inductor and a filter capacitor. When the tube amplifier was operated at high volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
, the power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
would dip as the amplifier draws more current, reducing power output and causing signal modulation. This dipping effect is known as "sag", which may be preferable to some electric guitarists when compared with hard clipping. Electric guitar amplifiers are almost invariably using a class AB1 amplifier, whereas in a class A stage the average current drawn from the supply is constant with signal level, and consequently does not cause supply line sag, until the clipping point is reached.
In contrast, modern amplifiers often use high-quality, well-regulated power supplies. In theory, the output voltage remains constant, but in reality it never does—not least due to resistive losses in the cabling from the power supply to the gain stage. This problem is proportionately much worse in transistor amplifiers because they operate at low voltage and high current, whereas tube voltage amplification stages operate at low currents and high voltages. Ohmic losses are a function of current through resistance.
Tube versus solid state rectification
Some high end tube amplifier designs also include vacuum tube rectifier circuits instead of modern silicon diode or bridge rectifier circuits. A cheap solid state rectifier does introduce audible noise into the circuit. Audibility of the effects is disputed by many. In unregulated power supplies the switching noise from silicon diodes can affect the amplifier's performance by introducing noise into the high voltage circuit. In guitar amplifiers, tube rectification is used in order to intentionally cause the high voltage supply to sag in order to add distortion and compress the output signal.The practical advantage to tube rectification is that rectifier tubes require some time to warm up before they begin to conduct. This provides some time for the heaters in the output tubes to warm up as well and therefore extend their lifespan. If the high voltage supply is brought up too quickly, the cathodes might be damaged. Some high end manufacturers, such as Welborne Labs in their premium kits, feature ultra-fast soft-recovery silicon diodes bridged by snubber networks on the basis that the cost and power required to operate a vacuum tube rectifier does not yield any measurable improvement in the sound.
Class A
The special feature of all Class A amplifiers is the absolute absence of crossover distortionCrossover distortion
Crossover distortion is a type of distortion which is caused by switching between devices driving a load, most often when the devices are matched...
(which is substituted with non-linear higher harmonic distortion, q.v.). The crossover distortion was found especially annoying after the first silicon-transistor Class B and Class AB transistor amplifiers arrived on the consumer market; the earlier germanium-based designs with the much lower turn-on voltage of this technology and the non-linear response curves of the devices had not shown large amounts of cross-over distortion. Although crossover distortion is very fatiguing to the ear and perceptible in listening tests, it is also almost invisible (until looked for) in the traditional Total harmonic distortion
Total harmonic distortion
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
(THD) measurements of that epoch.
Push-pull amplifiers
A Class A push-pullPush-pull
Push–pull may refer to:In electronic technology:*Push–pull output, type of electronic circuit*Push–pull converter, in electronics, is a type of DC to DC converter that uses a transformer*Push–pull connector, an electronic cable connector...
amplifier produces low distortion for any given level of applied feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
, and also cancels the flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
in the transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
cores, so this topology is often seen by HIFI-audio enthusiasts and do-it-yourself builders as the ultimate engineering approach to the tube Hi-fi
High fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
amplifier for use with normal speakers
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
. Output power of as high as 15 watts can be achieved even with classic tubes such as the 2A3 or 18 watts from the type 45. Classic pentodes such as the EL34 and KT88 can output as much as 60 and 100 watts respectively. Special types such as the V1505 can be used in designs rated at up to 1100 watts. See "An Approach to Audio Frequency Amplifier Design", a collection of reference designs originally published by G.E.C.
Single-Ended Triode (SET) amplifiers
SET amplifiers typically show poor measurements for distortion with a resistive load, have low output power, are inefficient, have poor damping factorDamping factor
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
s and high measured harmonic distortion. But they perform somewhat better in dynamic and impulse response. Also, SET-amplifiers allegedly (there is little agreement on this) sound good at low power levels without the perceived need for higher sound levels like transistor amplifiers allegedly do.
The triode, despite being the oldest signal amplification device, also can (depending on the device in question) have a more linear no-feedback transfer characteristic than more advanced devices such as beam tetrodes and pentodes. However no amplification device has a sufficiently linear characteristic for acceptable distortion performance unless it is linearised by the use of negative feedback. The low gain of a single triode makes it impossible to use more than a minimal level of negative feedback and so THD levels of a few percent are the rule in this type of amplifier. NFB is used in most post 1950s amplifiers and although it usually reduces the measured distortion level, if poorly implemented it results in an unpleasant combination of harmonics to some ears; this is not the case with a competent implementation.
Audiophiles who prefer SET-amplifiers state that measured sound performance is a poor indicator of real world sound performance and distortion level is not the only criterion for good sound reproduction. There are measurements not using resistive load but actual loudspeakers to back this up. In the 1970s, designers started producing transistor amps with higher open loop
Open loop
An open loop is a rhetorical device to instill curiosity by creating anticipation for what will come next. The device is sometimes also called a tension loop for the tension and anticipation it creates.- Short Example :...
gain to support a greater value of negative feedback. In the following years, amplifiers were built with modest gain but good open loop linearity, deployed with only minimal levels of NFB.
All amplifiers do distort, so do SETs. This for the most part harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
distortion is a distortion with a unique pattern of simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
. The result is like adding the same tone one octave
Octave
In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems"...
higher. The added harmonic tone is lower, at about 1–5% or less in a no feedback amp at full power and rapidly decreasing at lower levels. It has been also claimed that a single-ended power amplifier's second harmonic distortion could reduce similar harmonic distortion in a single driver loudspeaker, if their harmonic distortions were equal and amplifier was connected to the speaker so that the distortions would neutralize each other.
SETs usually only produce about 2 watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
(W) for a 2A3 tube amp to 8 W for a 300B up to the practical maximum of 40 W for a 805 tube amp. The resulting sound pressure level depends on the sensitivity of the loudspeaker and the size and acoustics of the room as well as amplifier power output. Their low power also makes them ideal for use as preamps. SET amps have a power consumption of a minimum of 8 times the stated stereo power. For example a 10 W stereo SET uses a minimum of 80 W, and typically 100 W.
Single-ended pentode and tetrode amplifiers
The special feature among tetrodes and pentodes is the possibility to obtain ultra-linearUltra-Linear
Ultra-linear is a term used to describe a type of electronic circuit that is used to couple a tetrode or pentode vacuum-tube to a load ....
or distributed load operation with an appropriate output transformer. Ultra-linear connection is a negative feedback method, enabling less harmonic distortion.
Class AB
The majority of modern commercial Hi-fiHigh fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
amplifier designs have until recently used Class AB topology (with more or less pure low-level Class A capability depending on the standing bias current used), in order to deliver greater power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
and efficiency, typically 12–25 watts and higher. Modern designs normally include at least some negative feedback, although in the old times of High fidelity
High fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
use of feedback was totally out of question. It should however be noted that Class D topology (which is vastly more efficient than Class B, and has garnered some respect from audiophiles) is more and more frequently applied where traditional design would use Class AB.
Class AB push-pull topology is nearly universally used in tube amps for electric guitar applications that produce power of more than about 10 watts. Whereas audiophile amps are primarily concerned with avoiding distortion, a guitar amp embraces it. When driven to their respective limits, tubes and transistors distort quite differently. Tubes clip more softly than transistors, allowing higher levels of distortion (which is sometimes desired by the guitarist) whilst still being able to distinguish the harmonies of a chord. This is because the soft profile of the tube amplifier's distortion means that the intermodulation products of the distortion are generally more closely related to the harmonies of the chord. All sides of the question are inclined to agree about valve guitar amplifiers offering a very useful sound, though there are also some well-respected solid-state designs.
Tube sound from transistor amplifiers
Some individual characteristics of the tube sound, such as the waveshapingDistortion synthesis
Distortion synthesis is a group of sound synthesis techniques which modify existing sounds to produce more complex sounds , usually by using non-linear circuits or mathematics....
on overdrive, are straightforward to produce in a transistor circuit or digital filter
Digital filter
In electronics, computer science and mathematics, a digital filter is a system that performs mathematical operations on a sampled, discrete-time signal to reduce or enhance certain aspects of that signal. This is in contrast to the other major type of electronic filter, the analog filter, which is...
. For more complete simulations, engineers have been successful in developing transistor amplifiers that produce a sound quality very similar to the tube sound. Usually this involves using a circuit topology similar to that used in tube amplifiers.
In 1982, Tom Scholz
Tom Scholz
Donald Thomas "Tom" Scholz is an American rock musician, songwriter, guitarist, pianist, inventor, and mechanical engineer, best known as the founder of the hard rock band Boston. He is also the inventor of the Rockman guitar amplifier...
, a graduate of MIT and a member of Boston
Boston (band)
Boston is an American rock band from Boston, Massachusetts that achieved its most notable successes during the 1970s and 1980s. Centered on guitarist, keyboardist, songwriter, and producer Tom Scholz, the band is a staple of classic rock radio playlists...
, introduced the Rockman, which used bipolar transistors, but achieved a distorted sound adopted by many well known musicians. Advanced digital signal processing
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
offers the possibility to simulate tube sound. Computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
algorithms are currently available that transform digital sound from a CD or other digital source into a distorted digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
sound signal.
Using modern passive components, and modern sources, whether digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
or analogue, and wide band loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s, it is possible to have tube amplifiers with the characteristic wide bandwidth and "fast" sound of modern transistor amplifiers, including using push-pull circuits, class AB, and feedback. Some enthusiasts have built amplifiers using transistors and MOSFETs that operate in class A, including single ended, and these often have the "tube sound".
Hybrid amplifiers
Tubes are often used to impart characteristics that many people find audibly pleasant to solid state amplifiers, such as Musical FidelityMusical Fidelity
Musical Fidelity is a low volume producer of high-end audio equipment. Founded in the United Kingdom in 1982, they are known for the unusual industrial design applied to their products....
's use of Nuvistor
Nuvistor
The nuvistor is a type of vacuum tube announced by RCA in 1959. Most nuvistors are basically thimble-shaped, but somewhat smaller than a thimble. Triodes and tetrodes were made, although tetrode nuvistors are rare. The tube is made entirely of metal and ceramic. Making nuvistors requires special...
s, tiny triode tubes, to control large bi-polar transistors in their NuVista 300 power amp. In America, Moscode and Studio Electric use this method, but use MOSFET transistors for power, rather than bi-polar. Pathos, an Italian company, has developed an entire line of hybrid amplifiers.
To demonstrate one aspect of this effect, one may use a light bulb in the feedback loop of an infinite gain multiple feedback (IGMF) circuit. The slow response of the light bulb's resistance
Conductance
Conductance may refer to:* Electrical conductance, the ability for electricity to flow a certain path* Fluid conductance, the ability for fluid to transmit through materials* Thermal conductivity, the ability for temperatures to transmit through materials...
(which varies according to temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
) can thus be used to moderate the sound and attain a tube-like "soft limiting" of the output, though other aspects of the "tube sound" would not be duplicated in this exercise.
Tube sound enthusiasts
Different uses of tube amplifiers can be found due to the different personal preferences of the enthusiasts. From those who opt to restrict their use as active devices to those who opt to include them in the audio circuit, accepting the use of semiconductor gain devices in the power supply or as constant current sources. Others, still, will use tubes for the main amplification circuit but add semiconductors (such as solid-state diodes) for clipping purposes, particularly in the preamp section, which is often debated in advertised vintage instrument amplifiers such as the Marshall JCM900 or the Vintage Modern as to their integrity due to their utilization of solid-state devices in the tone-generation circuit. Other schisms concern the use of triodes vs. tetrodes and pentodes, and the use of directly heated tubes vs. indirectly heated tubes.Many of the explanations relate to the circuit topologies pioneered using tubes, and traditionally associated with them ever since, regardless of whether they are built using tubes today, notably the directly heated single-ended triode amplifier circuit, which operates in class A and often has no external negative feedback; this topology is a classic source of the tube sound.
Feedback paths coupled through the secondary of the output transformer reduce distortion because they compensate for the transformer's distortion to some extent. However only limited NFB can be used around the transformer, as there is phase lag caused by the transformer, and this causes instability if NFB is incorrectly (without any phase / frequency correction) used.
See also
- Audio system measurementsAudio system measurementsAudio system measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are...
- British Valve AssociationBritish Valve AssociationThe British Valve Association was a cartel of valve manufacturers in the United Kingdom of Great Britain and Northern Ireland that was designed to protect their interests from foreign competition...
- European triode festivalEuropean triode festivalSince 2000 each year the "European Triode Festival" has been held in various venues in Europe Since 2000 each year the "European Triode Festival" (ETF) has been held in various venues in Europe Since 2000 each year the "European Triode Festival" (ETF) has been held in various venues in Europe (the...
- Virtual Valve AmplifierVirtual Valve AmplifierA Virtual Valve Amplifier is a modern software development for simulating the sound of various valve amplifier designs that would normally consist of valves/electron-tubes....