Valve audio amplifier

A valve (US: tube) audio amplifier is equipment intended for the amplification of audio signals, typically music or speech from radios, phonographs, CD players and other audio playback devices.

History

Initially, all audio amplifiers were based around the thermionic valve but after the introduction of the of the transistor in the late 1960s, valve amplifiers started to lose popularity due to their fragility, weight and heat dissipation disadvantages. Today valves are obsolete in mass-market applications.

Today, Audio amplification is the dominant application for valves a broadband low frequency amplifiers. Voice Telephony is very undemanding, nominally requiring only 300-3,000 Hz bandwidth, and low demands on fidelity.

At the other extreme, as the name "HiFi" (Originally "High Fidelity") suggests, Audio amplifiers for hi-fi audio amplification demand extremely low levels of noise, distortion and coloration. Frequency response is expected to be essentially flat across the audio band (20-20,000 Hz) to within a fraction of a dB, requiring -3dB points often > 4 orders of magnitude apart (5 - 100,000 Hz for some preamps) with extremely low distortion levels.

For so-called "high end" audio, where cost is not the primary consideration, valves amplifiers have remained popular and during the 1990s have made a resurgence.

Early Development

The earliest mass usage of valve audio amplifiers was for telephony. Radio applications followed soon after.

Among the first applications of sound recording and electronic replay around the 1920s was its use in many cinemas equipping for exhibiting the new 'talkies'. Cinema sound systems of this period were predominantly supplied by "Westrex", related to the Western Electric company, a telecoms supplier, who were also the makers of the 300B DHT tube that today is central to current production DH-SET audiophile amplification.

1940s

Technical developments in the late 1940s, particularly by D T N Williamson in Britain and by David Hafler in the US, made further improvements in output power and fidelity.

During this period, widespread adoption of negative feedback in the push pull topology following the Williamson amplifier yielded greater power and measured linearity, despite using smaller (and thus cheaper) transformers, bringing more power (typically 12-20 watts) to the masses. This was sufficient to drive better loudspeakers than the simple elliptical full range driver used in many cheap record players, radio and TVs and the high fidelity industry was born.

Valve amplification peaked as the mainstream technology during the 1960s and 70's. The last generation of power tubes, typified by EL84, KT66, EL34 and KT88, in many ways represent the pinnacle of the technology, and also of production quality. Commercial tube manufacturers also developed designs using their particular product - most notably, the Mullard 5-10 circuit.

Valve amplifiers produced since that time usually use one of these tubes, which have remained in continuous production (apart from KT66) ever since. See also Williamson amplifier

Circuitry and performance

Characteristics of valves

Valves are very high input and output impedance devices. Valve amplifier designs typically couple stages either capacitively or using transformers and these devices can limit the bandwidth at both high and low frequencies.

Output transformer limitations

Because of their inability to drive low impedance loads directly, valve audio amplifiers must employ output transformers to step down the impedance to match the loudspeakers.

Output transformers are not perfect devices and will always introduce some odd harmonic distortion and amplitude variation with frequency to the output signal. In addition, the excessive phase response of transformers can be problematic when applying overall negative feedback to valve amplifiers.

This is one reason for only small amounts of NFB being used on valve amplifiers. Another reason is that valves are relatively low gain devices so there is less gain available for feedback (compared with typical transistor alternatives).

Some designs without output transformers (OTLs) were produced by Julius Futterman in the 1960s and '70s, and more recently by others, in an attempt to escape the problems of transformers. There was great interest in the Soviet 6C33C tube in the years shortly after the fall of the Soviet union, this tube has very low Z and can drive high currents. However reliability is frequently questioned, and the consequence of a failure in some OTL circuits can be that the loudspeakers are destroyed, so OTL's remain a niche technique. [1]

Distortion, feedback, post war development

Prior to WWII, almost all amplifiers were Triodes used without feedback. The linearity of tubes makes it possible to get acceptable distortion performance figues without any form of compensation or error correction.

Amplitude distortion in a class A triode stage can be small if care is taken to prevent the andoe current from too closely approaching zero,and by ensuring that there is no grid current allowed to flow. In this case, distortion will be largely second harmonic in nature with the percentage of the second harmonic being closely proportional to the output amplitude.(Terman p338)

Triodes used in a Single Ended (SE) circuit without feedback produce a characteristic distortion spectrum (monotonically decreasing harmonic distortion only) that is said to sound especially musical due to the dominance of second order harmonics in the distortion spectrum.

Following its invention by Black, negative feedback (NFB) has been almost universally adopted in more powerful push pull power amplifiers to provide substantially improved measured distortion performance, flatter frequency response, and more repeatable performance irrespective of component variations. A side effect of NFB is that the output impedance (Z out) of the amplifier is effectively reduced as a function of the level of NFB applied (which may vary as a function of frequency in some circuits). However a more controversial side effect is that while the measured peak distortion is dramatically reduced, the distortion spectrum becomes more complex and often contains significant inharmonic components, which audibly "sound worse", even though the design "measures better".

Output impedance

Non-NFB valve designs typically have output impedances of several hundred ohms. NFB designs (typically class B push pull) have lower output impedance (typically fractions of an ohm).

This has two important consequences :

Loudspeakers that have impedance versus frequency functions that deviate substantially from flat will develop substantially non-flat frequency responses when used with High Z out amplifiers.

High Z out equates to a low "damping factor" - the ampliifiers ability to control the movement of the louspeaker diaphram based on its inertia. The most serious consequence of poor damping factor is often boomy/"sloppy" bass, possibly with serious resonances at the drivers mechanical resonance frequency, thats aside from "boom" may result in excessive displacements that exceed Xmax and thus may damage the driver.

Circuit configurations

The basic "single ended" triode (common cathode) gain stage. Typically capacitively coupled, with a resistive/inductive plate load and a cathode resistor. The cathode resistor is often used to generate a positive voltage relative to the negative supply at a level proportional to the current passing through the valve, thereby automatically generating a bias voltage on the grid. The grid is held at about the negative supply voltage using one more (large impedance) resistor, known as a "grid leak".

The Cathode resistor may or may not be bypassed with a capacitor. The plate load may be a resistor or the primary of a transformer (typically in an output stage)

Two triodes with the cathodes coupled together to form a differential pair. A "long tail" is typically added to linearier this fundamental differential circuit, either a high value resistor or possibly an active constant current source. For convenience, cost (and historically also to minimise patent royalties), small signal triodes very often are packaged two per glass envelope. Notable examples are the ECC81/82/83 series (6AX7.6AT7 etc in the US)

Two power tubes (may be triodes or tetrodes) being differentially driven to form a push-pull output stage, usually driving a transformer load. This output stage makes much better use of the supply voltage and the transformer core than the single ended output stage

A phase splitter
A constant current source
Cascode

Instrument and vocal amplification

Valve amplifiers for guitars (especially, vocals and other applications to a lesser degree) are usually designed to very different specifications from those of hi-fi amplifiers. There is usually a greatly reduced demand for fidelity and the valves are pushed much harder to maximize output power at the expense of increased distortion levels. Output stages are usually Push Pull class AB1 (often using the El34 output tube.)

Small signal circuits are often deliberately designed to have very high gain, driving the signal far outside the linear range of the tube circuit, to deliberately generate large amounts of harmonic distortion. the distortion and overdrive characteristics of valves are quite different from transistors (not least the amount of voltage headroom available in a typical circuit) and this results in a distinctive sound. Amplifiers for such performance applications typically retain tone and filter circuits that have largely disappeared from modern HiFi products. Amplifiers for Guitars in particular may also include a number of "effects" functions

HiFi amplification

Although valves were largely displaced by the transistor from the 1970's and consumer products are today almost exclusively transistor based, the extreme upper end of the market saw a dramatic resurgence in demand for tube based amplifiers in the late 1990's, and today tube products hold a significant market share in the niche of amplifiers costing over $1000.

Preamplifiers are invariably triode circuits. Preamplifiers historically contained equalization, tone and filter circuits (different kinds of circuits designed to give a controlled non linear frequency response). However the decline of the vinyl record, the inclusion of equalization inside modern tape and cassette decks, and especially the introduction of digital audio devices that all have a "flat" frequency response at line level, needing neither amplification (prior to the power amp) correction or equalization circumvented the need for preamps as a mainstream product.

The majority of hi-fi power amplifiers is Class AB1 push pull circuits (usually using Tetrode/pentodes) but a small niche of low power (~ 5 watt class) Single-ended triode amplifiers continue to be popular in a segment of this market and very small numbers of hand built triode power amplifiers continue to be marketed at high prices (> $10,000)

Notable audio hi fi amplifiers

There are many but the following are some of the most well known, and in most cases remain highly regarded even today.

Leak TL/1

This first major commerically produced amplifier to reach only 0.1% THD. In part due to the output transformers on these amplifiers being abnormally large.

Williamson.

The Williamson design was a milestone which defined the mainstream topology for the majority of amplifiers thereafter. Particular attention was given to specifying the (complex) construction of the output transformer. It was not itself originally a commercial design. It was published as an article in "Wireless World"

Quad II

This amplifier was popular and well regarded. However its circuitry is not representative of the mainstream, very unusually having EF86's (pentodes) in the input stage and in having cathode as well as anode windings in the output transformer. A detailed analysis of tis amplifier is found in Morgan Jones.

Home amplifier construction

The simplicity of valve amplifiers, especially Single Ended designs, combined with the ability to select components that are not in production anymore and are not available in commercial volumes, makes valve amplifiers easy and very attractive for hobbyists, often extremists, to construct: There are many hobbyist constructed amplifiers, many of them unique, albeit based closely on mainstream designs.

Modern valve audio amplifiers

In comparison with modern transistor amplifiers, valve power amplifiers tend to have rather low power, depending on the power tube used, and low efficiency. Economic factors mean that today tubes are only used by hobbyists or for high priced commercial products at the upper end of the HiFi field. These amplifiers are often known as "high end" or "Audiophile", where percieved concern about sonic excellence is paramount to the purcahser and over-rides cost.

Some specialist valve preamplifiers (e.g. microphone amplifiers)are widely and generally considered to be excellent, and are still used in recording studios.

Most historic pre-amplifiers have extensive tone and filter controls that severely degrade modern high quality source material, (due primarily to the capacitors of the time having poor performanc, but also due to the phase shifts introduced by the filter circuit itself even in idealised form).These products are today of historic interest only. Modern Audiophile replay systems rarely have filter or tone controls.

Single Ended Triode (SET)Power ampifiers

Some valve amplifiers uses the Single Ended Triode(DH-SET) topology that uses the gain device in class A.SETs are extremely simple and have low parts count. Such amplifiers are expensive because of the output transformers required, and also they run at lethal voltages.

This type of design results in an extremely simple distortion spectrum comprising a monotonically decaying series of harmonics. Some consider this distortion characteristic is a factor in the attractiveness of the sound such designs produce. Compared with of modern alternative designs, SETs adopt a minimalist approach, and often have just two stages: each a simple triode gain stage. However, variations using some form of active current source or load have become commonplace. Such an active load is not normally considered a gain stage.

Power output problems

One reason for SET's being (usually) limited to low power is the extreme difficulty (and consequent expense) of fabricating an output transformer that can handle the standing bias current in addition to the music signal, without saturating, while avoiding excessively large capacitive parasitics.

The low power output available from the DH-SET inevitably requires (by modern standards) extremely efficient loudspeakers to be used.

Valves

The typical valve using this topology in (rare) current commercial production is the 300B, which yields about 5 watts in SE mode. Rare amplifiers of this type use tubes such as the 211 or 845, capable of about 18 watts. These tubes are of a type known as "transmitting tubes", and have thoriated filiaments which glow like lightbulbs when powered up.

The pictures below are of a commercial DH-SET amplifier, and also a prototype of a hobbyist amplifier.

A commercial SE amplifier
A prototype hobbyist constructed DH-SET amplifier

Push pull (PP) / differential power amplifiers

Since the 1950s the vast majority of high quality valve amplifiers, and almost all higher power valve amplifiers have been of the push -pull type.

The use of 'push pull' output stages removes the problem of a standing bias current through the output transformer, facilitating the construction of more powerful amplifiers at the same time as using smaller and cheaper transformers.

In contrast to the single ended variety, a push pull topology almost perfectly cancels the (dominant) even order harmonic distortion products of the output stag

Push pull designs

Push Pull output stages can use triodes for lowest Z out and best linearity, or tetrodes/pentodes which give greater gain. Many of the classic output tubes from the golden age (KT88, EL34, EL84) were specifically designed to be operated in either triode or tetrode modes at the designers choice and some of these PP amplifiers can be switched between these modes (when powered off). Post Williamson, the majority of commercial amplifiers have used tetrodes in the so-called "ultra-linear" configuration. This term is a misnomer because ultra-linear is a compromise between triode and tetrode configuration and gives performance between these two extremes. The pure triode mode is significantly more linear than so-called "ultra-linear" tetrodes.

Class A Class A pure triode PP stages are sufficiently linear that they can be operated without any feedback whatsoever. They have no crossover distortion and distortion levels (excluding noise) reduce asymptotically to zero as signal amplitude is reduced. This is one reason audiophiles favour class A.

Class AB1 (and class B)

In contrast Class AB1 (and class B) circuits suffer from crossover distortion, the amplitude of which remains more or less constant regardless of the amplitude of the signal. This means that class AB1 and B amplifiers produce their lowest distortion figures at near maximum amplitude, and return extremely poor distortion performance at low levels, regardless of how good the quoted figures (measured at high power) may be.

Class AB1 and B amplifiers rely on NFB to attempt to reduce the open loop distortion performance. Measured distortion spectra from such amplifiers show that peak distortion amplitude is dramatically reduced by NFB, but at the expense of the residual distortion having a much more complex, inharmonic and typically non monotonic distortion spectrum.

Biasing

The biasing of a Push Pull output stage can be easily adjusted (at the design stage, usually not in a finished amplifier) between class A (giving best open loop linearity) and class B (giving greater power and efficiency from a given power supply, pair of output tubes and output transformer). Most commercial amplifiers operate in Class AB1, seeking some compromise between these extremes, although a small number of amplifiers run in pure class A to avoid crossover distortion entirely.

Circuit Topology

The typical topology has an input stage, a phase splitter, a driver and the output stage, although there are many variations of the input stage / phase splitter, and sometimes two of the listed functions are combined in one tube stage. The dominant phase splitter topologies today are the concertina, floating paraphase, and some variation of the long tail pair.

The gallery shows a modern, hand wired, fully differential, no negative feedback, pure class A Audiophile amplifier of ~ 15W, based on 6SN7 and KT88 tubes. This example is a home constructed hobbyist amplifier.

Construction

Point to point hand wiring rather than circuit boards tends to be used. This is for ease of construction by and the fact that "flying" wiring minimises capacitive effects. A hand wired commercial product is also shown for comparison.

One picture shows circuit constructed using "standard" modern industrial parts (630V MKP capacitors / metal film resistors). One advantage a hobbyist has over a commercial producer is the ability to use higher quality parts that are not reliably available in production volumes (or at a commercially viable cost price). For example the "silver top getter" sylvania brown base 6SN7's in use in the external picture date from the 1960s.

Another picture shows the exact same circuit constructor using Russian Military production Teflon capacitors and non inductive planar film resistors, of the same nominal values.

External view
Schematic
Internals using normal industrial quality parts.
Internals using using "up-spec" Teflon caps and planar resistors.
Internal construction of a commercial PP amplifier

Hobbyist valve amplifiers

One of the virtues of (in particular) DH-SETs is their extreme simplicity and low parts count. This simplicity lends itself to hobbyist construction, and this has become a field with many enthusiastic participants.

Component selection

The low parts count also makes it viable to tune the sound (especially of circuits NOT using feedback) by the careful selection of each component indiviually, eg selecting one make of capacitor over another, or different makes (and vintages) of tube, of the same basic type.

Selecting a preferred vintage tube version is a luxury denied to commercial producers who have to restrict themselves to tube types and brands that are still in volume production, such that they have a secure supply (and low costs).

This is one reason why some consider that hobbyists actually have the advantage over commercial producers in some respects, and conversely the dividing line is blurred, with the very highest end of "commercial production" being essential hand made one offs (or extremely limited runs of just a handful)

Auditioning many versions of a given tube type is sometimes known as "tube rolling".

'Extreme' designs

Acoustic and classical music is mostly much quieter than the musical peaks typical of a finale and so designers seek to optimize for performance at low levels. This favors class A.

Arguing that minimalist designs and such early production components have sonic benefits some hobbyists adopt a "less is more" approach is common. Many build amplifiers in the 2 watt class using tube types that became obsolete pre war and that can only be sourced in ones and twos.

Some home constructed amplifiers use high power tubes to yield up to 25 watts (or beyond) in class A, although there are engineering difficulties in constructing suitably massive coupling transformers that maintain acceptably wide bandwidth. The main problem is constructing output transformers able to sustain the bias current and resultant flux density without core saturation, at the same time as maintaining the desired wide bandwidth. This problem becomes more difficult and expensive as power levels are increased.

Hobbyists have also constructed class AB1 Push Pull amplifiers and OTL's.

Hobbyist constructed single channel 100W, Class A, parallel push pull amplifier based on 813/QB2-250 power tubes. Weight is 48 Kg, Dissipation is 1 kW.

References

Valve Amplifiers, Morgan Jones, Third Edition 2003 ISBN 0 7506 5694 8 - about the design and construction of valve audio amplifiers
Tube Amplifiers, Allegro Verlag, Vienna. ISBN 3-901462-00-7 - Contains a short introduction, the rest of the book is lots of photographs of some tube amplifiers.
Glass Audio. A long running journal devoted to tube amp construction, published by the Audio Amateur (TAA) Corp
Radio Designers Handbook, Classic Edition, F Langford-Smith et al. First published 1934, revised until 1967ISBN 0 7506 3635 1 - Compendium of articles of historic interest to people in this field
http://www.stereophile.com/reference/70/ - discussion of the limitations of NFB in audiophile systems
[2] - Theory paper on OTL designs.

External links

The Audio Circuit - An almost complete list of manufacturers, DIY kits, materials and parts and 'how they work' sections on valve amplifiers.
[3]A detailed discussion of the limitations of NFB as a panacea.