Valve sound

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 sound either from a valve amplifier or a specially designed transistor amplifier. It is described by some audiophiles to be a richer, warmer sound than that from transistor counterparts. Some musicians also prefer the distortion characteristics of tubes over transistors for instrument (usually guitar) amplification.

The term "Valve sound" is used in two main areas that are sometimes confused:

In sound reproduction systems (stereos) where accuracy to the sound of the original recording is usually the goal.
In guitar amplifiers, where audible distortion is usually the goal.

Valves are claimed by some to sound better in both situations, though the criteria are vastly different.

History

Before the introduction of transistors in the late 1940s, most electronic amplifiers used tubes. Since then, solid state (transistorized) amplification has become commonplace due to its size, price, and portability. However, tube amplifiers, especially single-ended triode (SET) models, have retained a loyal following amongst some audiophiles, with modern units from companies commanding very high prices.

Audible differences

Many audiophiles prefer the sound of tubes over transistors but the actual audible differences in sound have proven difficult to define. What most valve enthusiasts will agree on is that valves "sound better" than transistors, and audiophiles claim that there is indeed an audible difference. However, it appears that no results from scientifically conducted listening tests are available to confirm or deny the audiophile claims.

'Texture'

The problem is explaining these sound differences in words, as few words exist to describe the characteristics of sounds. Audiophiles use words like 'creamy' and 'crunchy' to describe tube harmonic distortion, which are words to describe texture. They state that tube amps sound much 'warmer' and 'creamier' than solid-states.

Improved bass

Some claim that the bass response is more extended and smoother with valve amplifiers.

Harmonic content and distortion

A widely-used argument claims that valves produce only even-numbered harmonics, while solid state amplifiers produce only odd-numbered harmonics. It has been claimed that even-numbered harmonics are "more musical", since they all correspond to named notes in the western musical scale, while only some of the odd harmonics do. However this argument is highly suspect, as any even harmonic can be viewed as merely an odd harmonic transposed up by one or more octaves, so the odd and even sets of harmonics contain the same proportion of named notes. (See Mathematics of musical scales)

In fact, the harmonics produced by a non-linear device depend on the topology and symmetry of the amplifier; not the type of device used. 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.Ask the Doctors: Tube vs. Solid-State Harmonics — Universal Audio WebzineVolume cranked up in amp debate — Electronic Engineering Times

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 obviously not desirable in audio reproduction systems, 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 should be irrelevant, since proper amplifier design can reduce all harmonics to inaudibility, and they should never see overload conditions. It is, of course, possible that the greater amount of distortion in class A valve amplifiers is the actual reason for the perceptually "improved" sound, even if it is degradation from an engineering standpoint.

Measurement differences

Attempts to measure the difference between them produce confusing results. Testing for distortion, frequency response, and noise at normal signal levels and assuming linear operation of the test amplifier, no truly significant differences exist.

Under severe overload by signal transients (30% THD) however, tube and transistor amplifiers measure differently.

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 say 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.

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.

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.

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 indeed no, negative feedback, and thus have very simple distortion spectra.

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.

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 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.

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 powers 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.

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 say that measured sound performance is a very bad indicator of real world sound performance. In the 70s, 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 in the opinion of some listeners sounded cold, dull and clinical. In the years since this fashion has reverted to giving much greater attention to making an amplifier which has modest gain but good open loop linearity, and then deploying this with only minimal levels of NFB.

Despite their linearity, SETs do distort. SETs have a unique distortion pattern: a simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic distortion, which 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 say that this "distortion" can actually enhance the music, making it sound somewhat richer, as if a quiet chord is being played in the background. It is a form of distortion. Whether is considered bad distortion depends on personal taste.

SETs usually only produce about 5 to 10 watts or less; the most expensive amp in existence, the $350,000.00 Wavac SH-883 monoblock SETs only produce 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.

Intentional creation of distortion

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 from 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. Needless to say, this lacks the nostalgia of using tube gear.

Transistor sound from valve amplifiers

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".

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.

Valve sound enthusiasts

Several 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, etc. 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.

References

External links

Vacuum tubes | High end audio | Audio amplifiers | Audio engineering

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