Valve sound
From Wikipedia, the free encyclopedia
- This article uses the term valve amplifier, which in English speaking countries outside North America means the same as tube amplifier in North America.
Valve sound is the characteristic sound produced from a valve-based audio amplifier. 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 valve sound.
Valve sound is present in two rather different fields:
- Some audiophiles argue that the sound from valve amplifiers to be "richer" or "warmer" than the sound from transistor amplifiers. In sound reproduction systems, accurate reproduction of the sound of the original recording is usually the goal.
- Some musicians also prefer the distortion characteristics of tubes over transistors for electric guitar, bass, and other instrument amplifiers. In this case, audible distortion is usually the goal.
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[edit] History
Before the introduction of transistors in the late 1940s, electronic amplifiers used vacuum tubes. 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, including but not limited to single-ended triode (SET) models, have retained a loyal following amongst some audiophiles with some modern units commanding very high prices. As well, performers of electric guitar, electric bass, and keyboards in a range of popular and jazz genres continue to use valve instrument amplifiers or preamplifiers.
[edit] Audible differences
Some audiophiles prefer the sound of tubes over transistors. However, the audible differences in sound have proven difficult to define or measure. Valve enthusiasts claim that valve amplifiers "sound better" than transistors.
It is difficult to explain these sound differences in words as the vocabulary available to describe sound is rather limited. Audiophiles use words like 'warm', 'liquid', 'smooth' and 'midrange magic' to describe valve amplifiers' sound. Some valve enthusiasts claim that the midrange reproduction is more extended and smoother with valve amplifiers. Many note that high frequencies are somewhat rolled off, while bass response has less power and definition.
[edit] Harmonic content and distortion
Some valve enthusiasts claim that valves produce only even-numbered harmonics, while solid state amplifiers produce only odd-numbered harmonics, and that even-numbered harmonics are "more musical". However, these claims are questionable, because the harmonics produced by a non-linear device depend on the topology and symmetry of the amplifier; not the type of device used. (For more information on scales, see Mathematics of musical scales.)
An amplifier with a symmetric (odd symmetry) transfer characteristic, like a solid state push-pull op-amp, produces only odd harmonics. An amplifier with an asymmetric transfer characteristic, like a class A valve amplifier, produces both even and odd harmonics.[1][2] [3] As valves are often run in class A, and semiconductor amplifiers are often push-pull, the types of distortion are incorrectly associated with the devices instead of the topology.
In order to produce only even harmonics, the device needs a transfer characteristic with even symmetry. A simple example is a solid state full-wave rectifier. Note that the fundamental, which is an odd-numbered harmonic, would not be reproduced at all. (The lowest frequency produced by a full-wave rectifier is double the original; or the second harmonic.) The production of only even harmonics is not desirable in audio reproduction systems, even though it is used in guitar distortion.
Additionally, this simplification fails to note that harmonic distortion only occurs when a single sine wave is input. For more complex signals (any other form of audio), the frequency components produced by non-linear distortion are not harmonics, but more complex intermodulation products.
In audio reproduction systems, the types of harmonics produced are irrelevant, since proper amplifier design can reduce all harmonics to inaudibility, and they should never be in overload conditions. While it is commonly believed that the greater amount of distortion in class A valve amplifiers may be the cause of the perceptually "improved" sound from the listeners' point of view, there are many other plausible explanations that are often ignored.
Because an audio signal is not composed of a single frequency, severe distortion can occur due to slew rate limiting, asymmetrical slewing, thermal effects, and other characteristics. These problems are far more common in solid-state circuits than in tube circuits, and cannot be detected with a simple THD measurement.[citation needed] Furthermore, it is possible for a solid-state amplifier to be underdamped or unstable when connected to certain speakers. This can cause severe distortion artifacts, which cannot be detected on a simple resistive load typically used to measure performance.
[edit] Measurement differences
It is difficult to measure the difference between valve and transistor amplifiers. When valve and transistor amplifiers are tested for distortion, frequency response, and noise at normal signal levels (and assuming linear operation of the test amplifier), no significant differences exist. Under severe overload by signal transients (30% THD) however, tube and transistor amplifiers do produce different measurement results. It is important to note that the standard 1 kHz total harmonic distortion (THD) test is of little use when discussing high-performance amplifiers, since it does not measure distortion at high frequencies (which is always higher than distortion at 1 kHz due to dominant-pole compensation) or parameters such as slew rate, intermodulation distortion, or spectral content.
[edit] Valve and transistor amplifier designs compared
There has been considerable debate over the characteristics of valves versus bipolar junction transistors. Some audiophiles have argued that the quadratic transconductance of tubes compared with the exponential transconductance of transistors is an important factor. This has not been proven.
Some audiophiles argue that devices are not as important as circuit topology, since MOSFETs exhibit a transfer characteristic similar to tubes but fail to reproduce valve sound in modern amplifiers. Triodes and MOSFETs have certain similarities in their transfer characteristics, whereas later forms of the valve, the tetrode and pentode, have quite different characteristics that are in some ways similar to the transistor.
[edit] Soft clipping
An important aspect of tube sound is the soft clipping characteristic of tubes. A tube amplifier will reproduce a wave relatively linearly to a point, and as the signal moves beyond the linear range of the tube (into overload), it distorts the signal with a smooth curve instead of a sudden, sharp-edged cutoff. The harmonics added to the signal are of lower energy with soft clipping than hard clipping, though the type of harmonics will be the same for both (dependent on symmetry). However, soft clipping is not exclusive to valves; see section "Intentional creation of distortion" below.
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 and capacitance of a circuit. Of course a more complex circuit can cancel out these effects with other components.
[edit] Bandwidth
Early valve amplifiers often had only limited bandwidth, in part due to passive component technology available at the time, notably resistor - capacitor-coupled stages and output transformers. Tube stages were usually capacitively coupled, reducing low frequency response. Tubes could not directly drive speakers, so output transformers were used which further reduced both high and low frequency response.
[edit] Gain
Audio valves typically have only modest gain. This makes it possible to design very simple valve circuits that rely on this inherent open-loop linearity and have little, or no negative feedback, and thus have very simple distortion spectra.
[edit] Negative feedback
Tube amplifiers could not, and did not need to, use as much negative feedback (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 slightly increases harmonic distortion, it avoids instability, as well as slew rate and bandwidth limitations imposed by dominant-pole compensation in transistor amplifiers.
[edit] 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 anode supply was simply a rectifier and a filter capacitor. When the tube amplifier was operated at high volume, the power supply voltage would dip, reducing power output and causing signal modulation. This dipping effect is known as "sag", which may be preferable to some electric guitarists.
In contrast, modern amplifiers often use high-quality, well-regulated power supplies. The output voltage remains constant, even at the peak of the amplifier rating. For this reason, the power supply is near ideal and does not affect the sound.
[edit] Push-pull amplifiers
A Class A push-pull amplifier produces exceptionally low distortion for any given level of applied feedback, and also cancels the flux in the transformer cores, so this topology is seen by some as the ultimate "engineering" approach to the tube hi-fi amplifier for use with normal speakers. Output power of 10W is possible using standard tubes, and up to 25W using "reasonable" extreme tubes.
The majority of commercial HiFi amplifier designs are Class AB, in order to deliver greater power and efficiency, typically 12 - 25 watts upwards. Such designs will invariably use at least some NFB.
Class AB push-pull topology is nearly universally used in tube amps for electric guitar applications. 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.
[edit] Single-Ended Triode (SET) amplifiers
SET amplifiers typically measure very badly - they have low output power, are inefficient, have poor damping factors and high measured distortions.
The triode, despite being the oldest signal amplification device, also has the most linear transfer characteristic, and thus requires little or no negative feedback for acceptable distortion performance. NFB is used in most post 1950s amplifiers and although it usually reduces the measured distortion level, it results in an unpleasant combination of harmonics to some ears.
Some audiophiles argue that measured sound performance is a poor indicator of real world sound performance. In the 1970s, designers started producing transistor amps with higher open loop gain to support a greater value of negative feedback. These amps produced near perfect measured results but some listeners believed that these amplifiers sounded "cold" or "dull". In the following years, amplifiers were built with modest gain but good open loop linearity, deployed with only minimal levels of NFB.
Despite their linearity, SETs do distort, with a unique distortion pattern of simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic distortion. The result is like adding the same tone one octave higher. The added tone is usually lower, at about 5% or less in a no feedback amp. Some argue that this "distortion" can actually enhance the music, making it sound somewhat richer.
SETs usually only produce about 5 to 10 watts or less; the most expensive amp in existence, the Wavac SH-833 monoblock SETs (which cost about US$350,000) produces about 150 watts. Large amounts of power are not necessary in amplifiers, as only a few watts are required to drive most audiophile speakers to a SPL of nearly 100 dB at 1 m. Their low power also makes them ideal for use as preamps.
[edit] Intentional creation of distortion
[edit] Valve sound from transistor amplifiers
Some individual characteristics of the tube sound, such as the waveshaping on overdrive, are straightforward to produce in a transistor circuit or digital filter. 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, a graduate of MIT and a member of Boston, introduced the Rockman, which used bipolar transistors, but achieved a distorted sound adopted by many well known musicians. Advanced digital signal processing offers the possibility to simulate valve sound. Computer algorithms are currently available that transform digital sound from a CD or other digital source into a distorted digital sound signal.
Using modern passive components, and modern sources, whether digital or analogue, and wide band loudspeakers, it is possible to have valve 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 "valve sound" [citation needed].
[edit] Tube/transistor hybrid amplifiers
Tubes are often still used to impart a distortion characteristic that most people find audibly pleasant to solid state amplifiers, such as Musical Fidelity's use of Nuvistors, tiny triode tubes, to control large bi-polar transistors in their NuVista 300 power amp.
Alternatively, one may use a light bulb in the feedback loop of an infinite gain multiple feedback (IGMF) circuit. The sluggish response of the light bulb's resistance (which varies according to temperature) can thus be used to moderate the sound and attain a valve-like "soft limiting" of the output.
[edit] Valve sound enthusiasts
Some enthusiasts consider that "pure" valve amplifiers should not use anything except valves as active devices. Others, in contrast, will use valves for the audio circuit, but will accept the use of semiconductor gain devices in the power supply or as constant current sources. Other schisms concern the use of triodes vs. tetrodes and pentodes, and the use of directly heated valves vs. indirectly heated valves.
Many of the explanations relate to the circuit topologies pioneered using valves, and traditionally associated with them ever since, regardless of whether they are built using valves today, notably the single ended directly heated triode amplifier circuit, which operates in class A and often has no negative feedback; this topology is a classic source of the valve sound.
[edit] See also
[edit] General Audio Topics
- Physics of music
- Audiophile
- Audio system measurements
- British Valve Association
- European triode festival
[edit] Instrument Amplification Topics
- Distortion (guitar)
- Electric guitar
- Guitar effects
- Guitar amplifier
- Instrument amplifier
- Power chord
[edit] References
- ^ Ask the Doctors: Tube vs. Solid-State Harmonics — Universal Audio Webzine
- ^ Volume cranked up in amp debate — Electronic Engineering Times
- ^ W. Bussey and R. Haigler (1981). "Tubes versus transistors in electric guitar amplifiers". IEEE International Conference on Acoustics, Speech, and Signal Processing, Volume 6 p. 800–803.
- Russell O. Hamm (September 14, 1972). "Tubes vs. Transistors: Is There An Audible Difference?". Presented at the 43rd convention of the Audio Engineering Society, New York
