Valve audio amplifier
From Wikipedia, the free encyclopedia
A valve (UK) audio amplifier or vacuum tube (US) audio amplifier is a valve amplifier used for sound recording, reinforcement, or reproduction.
Until the invention of solid state devices such as the transistor, all electronic amplification was produced by valve (tube) amplifiers. Whilst solid-state devices prevail in most audio amplifiers today, valve audio amplifiers are still used where their audible characteristics are considered pleasing, for example:
- Music performance, especially guitar amplifiers. In the case of electric guitar amplifiers a degree of deliberate, often severe, distortion is intentionally added to the sound, and contributes directly to the "tone" of the guitar, being itself a major part of the instrument.
- Music reproduction in "high-end" audio.
[edit] 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 electrically driven 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.
[edit] The origins of the electric guitar amplification
The electric Guitar originates from Rickenbacker in the 1930s but its modern form was popularised by Fender and Gibson (notably the Les Paul) during the 1950s.
The earliest guitar amplifiers were probably "normal" audio amplifiers pressed into service, but the electric guitar and its amplification quickly developed a life of its own, supported by specialist manufacturers.
Guitar amplifiers are typically designed with excess gain, allowing the guitar, when played hard, to generate a signal in the pickup which as it propagates through the amplifier is sufficient to overdrive one or more tube stages and thus generate gross distortion deliberately.
The characteristics of the tube and the circuit design directly influence the tone that results. In addition guitarists may employ acoustic feedback, further modifying the resulting sound (noting that the feedback signal has a slight time lag relative to he original signal).
Guitar amplifiers are typically designed to withstand a lot of abuse both electrically and physically (since guitarists often travel to gigs etc.) In large systems the amplifier is separate from the speaker enclosure(s), but in smaller systems it is often integrated, forming a so-called "combo". Since the amplifier is usually at the top of the combo, the tubes often hand upside down facing into the body of the enclosure : They may (or may not) be held in with clips.
The circuit topology of most modern guitar amplifiers is a class AB1 push pull circuit using the mainstream tubes from the golden age, today usually 6L6 or EL34 but occasionally EL84 or KT88 / 6550 in ultralinear connection . This output stage is normally driven by at least two double triodes, invariably from the noval family (ECCnn or 12AX7).
See also Guitar amplifier
[edit] Amplifiers for sound reproduction
[edit] Early Development
The earliest mass usage of valve audio amplifiers was for telephony. Valve amplifiers were critical in development of long-distance telephone circuits and submarine telephone cables. Radio applications followed soon after, where valves were used for both the audio (AF) and radio (RF) circuitry. (RF is outside the scope of this article, see valve amplifier).
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.
Almost all amplifiers during this period were (by todays standards) of very low power, circuitry using being very simple, tyically the (Class A) "single ended triode" circuit topology, usually using directly heated tubes. Today this type of circuit retains a niche following at the very extreme of audiophile hifi, where it is often referred to by the acronyn DH-SET.
Prior to WWII, almost all amplifiers were Triodes used without feedback. The linearity of tubes makes it possible to get acceptable distortion performance figures 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 anode 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). Adding modest NFB to a circuit with reasonable open loop linearity may also yield further improvements.
[edit] The 1940s & 50's
During the post war period, widespread adoption of negative feedback in the push pull topology, notably following the publication in 1947 of the Williamson amplifier yielded greater power and linearity, the Williamson amplifier being a milestone that set the standard (and the dominat topology) for what was to follow.
Widespread adoption of push pull allowed smaller (and thus cheaper) transformers, combined with more power (typically ~ 10 to 15 watts) sufficient to drive higher quality domestic loudspeakers. The high fidelity industry was born.
Other developments included (among others):
- the introduction of the "Point One" series of amplifiers (in 1945) by LEAK in the UK, which first set a performance standard of 0.1% THD
- the Ultra-Linear output stage (a variation of push pull with tetrodes) was originated by Alan Blumlein in 1937 in the UK, but popularised following publication of a paper by by David Hafler and Keroes in the USA in 1951, and became the dominant topology during the post war recovery of consumer products
- Manufacturers bringing high quality domestic HiFi to a steadily widening audience, Eventually leading to Dynaco selling over 300,000 ST-70's ... to date the worlds most popular HiFi amplifier (of any type)
[edit] The "golden age" of tube Hifi, the 1960s
Valve amplification peaked as the mainstream technology during the 1960s and 70's, with device and circuits being highly developed, there have been only minor refinements since then.
The last generation of power tubes, typified by KT66, EL34 and KT88, in many ways represent the pinnacle of the technology, and also of production quality. Valve amplifiers produced since that time usually use one of these tubes, which have remained in continuous production (apart from KT66) ever since.
Small signal valves overwhelmingly changed from octal base tubes, notably the audio tube of choice, the 6SN6 family, to the smaller and cheaper noval base ECC81/82/83 (UK, in the US known as 12AX7/AT7 etc). The noval base EL84 power tube also became the dominant power tube in a class of ~ 10 watt Ultralinear power amplifiers
Commercial tube manufacturers also developed designs using their particular product - most notably, the Mullard 5-10 circuit, which as with the earlier Williamson were subsequently widely cloned either exactly or as derivatives (with and without due credit).
[edit] Valve preamplifiers
Due to the very poor technical performance of early gramophones, the lack of standardised equalisations, poor components and accessories (including loudspeakers), preamplifiers historically contained extensive and very flexible equalization, tone and filter circuits (different kinds of circuits designed to give a controlled non linear frequency response).
Valve preamplifiers are invariably triode circuits,in order to have low noise as well as good linearity.
Mains hum from the heater filaments is a major problem in valve amplifiers, but especially preamplifiers. During the early years batteries were often used so this was not a problem, but modern amplifiers invariably run from the mains. Today the heater supply is usually rectified and even regulated to avoid hum
A representative valve preamp from the 1950s is the Leak 'varislope' series of preamps, which included:
- switchable rumble filter
- switchable scratch filter with selectable slops and corner frequency, at 4.5, 6, 9 kHz
in addition to:
- continuously variable treble and bass tone controls
- a selection of 4 different gramophone equalisations (RIAA, ortho, RCA, 78)
The large number of complex filter circuits, combined with (by todays standards) poor capacitor quality and poor quality switching etc, resulted in severe sonic degradation.
[edit] Valve sound
Note that Amplifiers from and prior to this period often have a distinctive sound, that today is still widely referred to as the "valve sound", which might loosely (and certainly not rigourously) be described as a "warm" tone, perhaps with a relaxed top end and soft bass, but an excellent midrange.
NB this "valve sound" is not strictly due to valves being used rather than transistors per se, rather it is just a sound that was originally associated with amplifiers built using valves simply because that is what was available at the time : The origins of that particular "sound" are in fact in part due to
- the typical circuit designs of the time (class A or class AB1 with a heavy class A overlap), combined with
- capacitor quality (often very poor compared to modern types),
- under-dimensioned and unregulated power supplies,
- poor quality output transformers etc. in budget equipment,
- low damping factor (High Z out) output stages
rather than this being the "sound of a valve" per se (which typically have frequency responses running from DC to hundreds of MHz). For further discussion of this, see also the article valve sound
[edit] Notable historic designs
In addition to a huge range of indifferent commodity valve amplifiers made over the years, many very good amplifiers were made, some of which are still highly regarded today : the following is only a sampling of some of the most well known :
[edit] Leak TL/12
The first commercially produced amplifier with distortion of 0.1% was the LEAK Type 15 "Point One" of 1945, using KT66 vacuum tubes (valves) connected as triodes, with 26dB feedback over 4 stages including the output transformer. In 1948 LEAK produced the TL/12 which was also rated at 0.1% but featured improved performance with 26dB over 3 stages and the output transformer (giving better gain-margin and phase-margin). The TL/12 sold in large quantities to professional users, radio stations, laboratories, as well as to the emerging market for hi-fi equipment. It is highly-prized even today by audio enthusiasts.
See also a full article about Leak at LEAK.
[edit] Williamson
The Williamson amplifier was originally published in 1947 as an article in Wireles World magazine, and was a milestone which defined the mainstream topology for the majority of amplifiers thereafter. The design gave particular attention to a very high specification for the output transformer, in addition to being generally a consistently well worked-through design. It was not itself originally a commercial design, but many commercial versions and derivatives were subsequently made (with and without due credit).
[edit] Mullard 5-10
To promote their new 9-pin tubes EZ80, EF86 and EL84 (the still applied ECC83 wasn't a Mullard design), the Anglo-Dutch Mullard Company developed, in 1954, a famous and very popular mono amplifier circuit, the Mullard 5-10. The design featured 5 tubes with 10 watts output: EF86 in triode connection as preamplifier, ECC83 phase-splitting, two EL84 in push-pull konfiguration, EZ80 rectifier. The amplifier used the excellent Partridge output transformer and was well-known for its great sound reproduction.
[edit] Quad II
This iconic (but unusual) mono amplifier was popular and well regarded for many years in the UK / British empire. However its circuitry is not representative of the mainstream, very unusually having EF86's (pentodes) in the input stage and in having the output KT66 cathode as well as anode windings in the output transformer.
A detailed analysis of this amplifier is found in Morgan Jones (see references). See also a full article about QUAD at Quad electroacoustics
[edit] Dynaco
A major US manufacturer of completed and kit amplifiers of high quality, notably of the Stereo 70, claimed to be the most popular tube amp ever made with over 300,000 produced, and the Mk III.
[edit] MacIntosh MC 275
MacIntosh long held a reputation for producing very high quality (and very expensive and very heavy) equipment in the US.
The model MC 275 perhaps demonstrates most clearly the revival in tube amplifiers during the 1990s, since it was "re-released" at a suitably high (and presumably highly profitable)price.
[edit] Circuitry and performance
[edit] Characteristics of valves
Valves are very high input impedance (near infinite in most circuits) and high output impedance devices. They are also high voltage / low current devices.
While valves themselves are described under vacuum tube, these characteristics of valve as gain devices have direct implications for their use as audio amplifiers, notably that power amps need output transformers (OPTs) to translate a High output impedance high voltage low current signal into a lower voltage high current signal needed to drive modern low impedance loudspeakers (cf. Transistors and FETs (which are relatively low voltage devices but able to carry large currents directly).
Another consequence is that since the output of one stage is often at ~ 100V offset from the input of the next stage, direct coupling is normally not possible and each stage needs to couple via a capacitor (or exceptionally another transformer).
The capacitors and transformers have a secondary influence on the performance of the amplifier, but in particular the transformers add dramatically to the cost as well as size and weight.
[edit] Basic circuits
[edit] The single ended triode gain stage
Simplified diagram. Heater not shown.
The basic gain stage for a valve is the auto biased "single ended triode" (common cathode) stage, in which an anode resistor, the valve, and a cathode Resistor form a potential divider across the supply rails. The resistance of the valve varies as a function of the voltage on the grid, relative to the voltage on the cathode.
In the autobias configuration, the "operating point" is set by holding the input (loosely) about ground using a high value "grid leak" resistor, and selecting the cathode resistor such that the Ohmic voltage drop over the resistance sets the cathode the wanted number of volts above ground at the current wanted.
The anode resistor acts as the load for the circuit and is typically order of 3-4 times the anode resistance of the tube type in use. The Output from the circuit is the voltage at the junction between the anode and anode resistor. Simplistically, this will have voltage amplification by a factor, "mu", relative to the variation in the input voltage, depending on the tube type selected
Almost all audio preamplifier circuits are build using cascaded SET stages's. This stage is also often used as the input stage for power amps with unbalanced inputs, to buffer the input signal and provide some initial gain.
The signal is usually coupled from stage to stage via a coupling capacitor or a transformer,. although direct coupling is done in unusual cases
The Cathode resistor may or may not be bypassed with a capacitor. Feedback may also be applied to the cathode resistor
[edit] The single ended triode power amplifier
A simple SET power amplifier can be constructed by cascading two stages, using an output transformer as the load
The following circuit is a simplified conceptual circuit only, real world circuits also require a smoothed or regulated power supply, heater for the filaments (the details depending on if the selected tube types are directly or indirectly heated), and the cathode resistors are often bypassed, etc ..
[edit] Differential stages
Two triodes with the cathodes coupled together to form a differential pair. This stage has the ability to cancel common mode (equal on both inputs) signals, and if operated in class A also has the merit of having the ability to largely reject any supply variations (since the affect both sides of the differential stage equally), and conversely the total current drawn by the stage is almost constant (if one side draws more instantaneously the other draws less), resulting in minimal variation in the supply rail sag, and this possibly also inter stage distortion
Two power tubes (may be triodes or tetrodes) being differentially driven to form a push-pull output stage, driving a PP transformer load. This output stage makes much better use of the transformer core than the single ended output stage
[edit] The long tail pair
A "long tail" is a constant current (CC) load as the shared cathode feed to a differential pair. In theory the more constant the current, the more idealised the performance of the stage, ie a long tail linearises the differential stage.
The CC may be approximated by a resistor dropping a large voltage, or may be generated by an active circuit (either tube, transistor or FET based)
The long tail pair can also be used as a phase splitter.
[edit] The concertina phase splitter
As an alternate to the long tail pail, the concertina uses a single triode as a variable resistance within a potential divider formed by Ra and Rk either side of the tube. The result is that the voltage at the anode swings exactly and opposite to the voltage at the cathode, giving a perfectly balanced phase split. the disadvantage of this stage (cf the differential long tail pair) is that it does not give any gain. Using a double triode (typically octal or noval) to form a SET input buffer (giving gain) to then feed a concertina phase splitter is a classic PP front end, typically followed by a driver (triode) and (triode or pentode) output stage (in ultra linear in many cases) to form the classic PP amplifier circuit
[edit] The Push Pull power amplifier
The conceptual circuit shown is a simplified variation of the Williamson topology, which comprises four stages:
- a SET input stage to buffer the input and give some voltage gain
- a concertina phase splitter. This generates exactly equal but opposite drive signals for the following push pull circuitry, but gives no gain. Note that as shown, the Williamson topology concertina phase splitter is direct coupled (with a resistor) to the input stage. This requires careful design of the input stage since the nominal voltage of the Input valve anode will define the operating point of the concertina as well.
- a driver stage. This gives further voltage gain for each of the push / pull signals, and depending on the output stage tubes requirements may be a type selected for higher voltage or lower Z drive capability.
- The output stage, where the load is the transformer rather than an anode resistor. The original Williamson used KT66 pentodes "triode strapped" (ie operating as triodes, as shown in this conceptual schematic). The majority of later PP amplifiers used the ultralinear connection instead.
[edit] Cascode
[edit] Tetrode/pentode stages,
The tetrode has a screen grid (g2) which is between the anode and the first grid which can be used to provide higher gain at the expense of linearity and noise performance.
A pentode has an additional screen grid (g3) the tetrode kink.
Tetrodes have an extra grid, A pentode has yet another "screen" or grid although this is used for improved performance rather than extra gain and is usually not accessible externally, Such power tubes are sometimes knowns as "beam tetrodes".
It was realised (and many pentodes were specifically designed to permit) that by strapping the screens to the grid / anode a tetrode / pentode just became a triode again, as such making these late design tubes very flexible. "Triode strapped" tetrodes are often used in modern amplifier designs that are optimised for quality rather than power output.
[edit] Ultra-linear
In 1937, Blumein originated a configuration between a "triode stapped" tetrode and normal tetrode, that connects the extra grid of a tetrode to a tap from the OPT part way between the anode voltage and the supply voltage. As might be expected this gives a gain and linearity part way between the extremes as well - ie a compromise. Typically this tap is ranged to be ~ 43%. This configuration is often referred to as "ultra-linear" and after being republished by David Hafler & Keroes in 1941 becme popular and by the late 1950s became the dominant configuration for PP amplifiers
Due to the non linearities, Tetrode (and Pentode) power tubes are invariably restricted to push pull amplifiers that have significant NFB.
[edit] Single Ended Triode (SET)Power amplifiers
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.
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 filaments 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 |
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.
[edit] Push pull (PP) / differential power amplifiers
The use of differential output stages ("push-pull") cancels standing bias current drawn through the output transformer by each of the output tubes individually, greatly reducing the problem of core saturation and thus facilitating the construction of more powerful amplifiers at the same time as using smaller, wider bandwidth and cheaper transformers.
The cancellation of the differential output tubes also largely cancels the (dominant) even order harmonic distortion products of the output stage, resulting is a reduced THD, albeit dominated now by odd order harmonics and no longer monotonic.
NB however that due to tube variations the flux and even order distortion cancellations is not perfect, and PP OPTs will often stull have a (reduced relative to an SE OPT) gap to prevent saturation
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.
Push Pull output stages can use triodes for lowest Z out and best linearity, but unlike the SET output stage often use tetrodes/pentodes which give greater gain and power. 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 remaining significantly more linear than so-called "ultra-linear" connected tetrodes.
Class A Class A pure triode PP stages are sufficiently linear that they can be operated without any feedback whatsoever, although modest NFB to further linearise the design, reduce Zout, and control gain may be desirable.
Class A PP designs have no crossover distortion and distortion levels (excluding noise) reduce asymptotically to zero as signal amplitude is reduced. The effect of this is that class A amplifiers perform extremely well with music that has a large dynamic range, notably classical and acoustic music (among others) ... where the music is many dB below the peak for most of the time. This is one reason audiophiles favour class A.
Class AB1 (and class B)
Class B (and AB) amplifiers are more efficient and can deliver higher power output levels from a given power supply and set of tubes.
However, the price for this is that they 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.) For this reason class B amplifiers are arguably well suited to eg pop and rock music where high sound levels predominate
Class AB1 and B amplifiers must rely on NFB to attempt to reduce the poor open loop distortion this configuration produces. 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, aharmonic and typically non monotonic distortion spectrum.
[edit] 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 (typically pentodes in the so-called ultra-linear configuration), seeking some compromise between these extremes, although a small number of amplifiers run in pure class A to avoid crossover distortion entirely.
[edit] Circuit Topology
The typical topology for a PP amplifier 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.
[edit] Output transformers
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, which often requires shelving within the feedback circuit to keep within the Nyquist stability criteria at high frequencies and thus avoid oscillation
[edit] Negative Feedback (NFB)
Following its invention by Black, negative feedback (NFB) has been almost universally adopted in most amplifiers (of all types) and universally in amplifiers operating other than in class A, to provide substantially improved measured distortion performance, flatter frequency response, and more repeatable performance irrespective of component variations.
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 aharmonic components, which some allege audibly "sound worse", even though the design "measures better". It should be noted that in the extreme niches of valve amplifier design in particular there are often heated and religious opinions about the merits and demerits of NFB in the context of high end audio designs operating in class A that have been designed to deliver very good open loop linearity.
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). 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 amplifiers ability to control the movement of the loudspeaker diaphragm 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, that's aside from "boom" may result in excessive displacements that exceed Xmax and thus may damage the driver.
[edit] Tube noise and noise figure
Like any amplifying device, tubes add noise to the signal to be amplified. Even with a hypothetical perfect amplifier, however, noise is unavoidably present due to thermal fluctuations in the signal source (usually assumed to be at room temperature, T=295K). Such fluctuations cause an electrical noise power of k_B T B, where k_B is the Boltzmann constant and B the bandwidth. Correspondingly, the voltage noise of a resistance R into an open circuit is (4*k_B*T*B*R)^(1/2) and the current noise into a short circuit is (4*k_B*T*B/R)^(1/2).
The noise figure is defined as the ratio of the noise power at the output of the amplifier relative to the noise power that would be present at the output if the amplifier were noiseless (due to amplification of thermal noise of the signal source). An equivalent definition is: noise figure is the factor by which insertion of the amplifier degrades the signal to noise ratio. It is often expressed in decibels (dB). An amplifier with a 0dB noise figure would be perfect.
The noise properties of tubes at audio frequencies can be modelled well by a perfect noiseless tube having a source of voltage noise in series with the grid. For the EF86 tube, for example, this voltage noise is specified (see e.g., the Valvo, Telefunken or Philips data sheets) as 2 microvolts integrated over a frequency range of approximately 25Hz to 10kHz. (This refers to the integrated noise, see below for the frequency dependence of the noise spectral density.) This equals the voltage noise of a 25kOhm resistor. Thus, if the signal source has an impedance of 25kOhm or more, the noise of the tube is actually smaller than the noise of the source. For a source of 25kOhm, the noise generated by tube and source are the same, so the total noise power at the output of the amplifier is twice the noise power at the output of the perfect amplifier. The noise figure is then two, or 3dB. For higher impedances, such as 250kOhm, the EF86's voltage noise is 1/10^(1/2) lower than the sources's own noise, and the noise figure is ~1dB. For a low-impedance source of 250 Ohm, on the other hand, the noise contribution of the tube is 10 times larger than the signal source, and the noise figure is approximately ten, or 10dB.
To obtain low noise figure, the impedance of the source can be increased by a transformer. This is eventually limited by the input capacity of the tube, which sets a limit on how high the signal impedance can be made if a certain bandwidth is desired.
The noise voltage density of a given tube is a function of frequency. At frequencies above 10kHz or so, it is basically constant ("white noise"). White noise is often expressed by an eqivalent noise resistance, which is defined as the resistance which produces the same voltage noise as present at the tube input. For triodes, it is approximately (2-3)/g_m, where g_m is the transconductivity. For pentodes, it is higher, about (5-7)/g_m. Tubes with high g_m thus tend to have lower noise at high frequencies.
In the audio frequency range (below 1-100kHz), "1/f" noise becomes dominant, which rises like 1/f. Thus, tubes with low noise at high frequency do not necessarily have low noise in the audio frequency range. For special low noise audio tubes, the frequency at which 1/f noise takes over is reduced as far as possible, maybe to something like a kHz. It can be reduced by choosing very pure materials for the cathode nickel, and running the tube at an optimized (generally low) anode current.
[edit] Modern Audiophile HiFi amplification
For so-called "high end" audio, where cost is not the primary consideration, valve amplifiers have remained popular and indeed during the 1990s made a commercial resurgence.
Todays circuits in most cases remain similar to circuits from the golden age, however they benefit from dramatic advances in ancillary component quality (notably capacitors) as well as general progress across the electronics industry which gives todays designers a far more powerful insight into how the circuit behaves internally. Many push pull products also tend to be more powerful than products from the golden age, reflecting the needs of inefficient but high quality modern speakers, and the market segment they are targeting.
Taking advantage of modern components (sometimes including transistorised regulated power supplies etc) todays High end audio amplifiers deliver extremely low levels of noise, distortion and coloration needed to complement todays high quality source material, 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 typically 10-70,000Hz for power amps (>100,000 Hz for preamps due to there being no OPT) with extremely low distortion levels.
[edit] Modern valve pre amplifiers
The very high standards (notably improved frequency response accuracy) of modern sources, notably digital audio, has dramatically reduced the use of "tone control" and related filter circuits, and these are normally not provided in modern preamps, either valve or transistor based. It is common to drive valve power amps directly with line level signals from the source, using passive volume and input source switching integrated into the amplifier, or with a minimalist "line level" control amplifier, which again is little more than passive volume and switching, plus a buffer stage to drive the interconnects
However a small but intense demand remains for tube preamps and filter circuits for studio microphone amplifiers, audiophile phono stages in particular, and exceptionally for active crossovers.
[edit] Modern valve power amplifiers
[edit] Commercial 'Single Ended Triode' (SET) amplifiers
During the golden age, SETs more or less disappeared from western products except for low power designs (< 5 watts), push pull using "modern, standard" indirectly heated tube types such as EL84 becoming de rigeur.
However the far east never abandoned valves, and especially the DH-SET circuit, indeed the extreme interest in all things audiophile in Japan and other far eastern countries sustained great interest in this approach. At the time this was perhaps as much a reflection of eastern philosophy and attitudes as of technical considerations, although the different types of music in the very distinct cultures of the era may also have been significant.
One of the key connections between this far eastern understanding of the SET and the west was Jean Hiraga, long time editor of l'audiophile in France (and in French). Something about Jean can be found at http://www.nutshellhifi.com/library/europe3.html
A very extreme example of an almost "zen" or "peotic" approach to amplifier design in the far east - very different from the western engineering led approach - is the work of Sakuma San, which is described at http://www10.big.or.jp/~dh/, although it should be noted that Sakuma's own designs are far from mainstream, the defacto standard SET being a 300B driven by an even lower power valve
Since the 1990s a niche market has developed again in the west for low power commercial SET amplifies (~ 7 watt class or even lower) Single-ended triode, notably using the 300B in recent years which while undoubtedly an excellent tube has taken on a life (and price) of its own driven largely by what can only be called fashion. Even more rarely, higher powered SETs are produced commercially, usually using the 211 or 845, which are able to deliver ~ 20 watts in SE, but operate at ~ 1000V making them unacceptably dangerous for many domestic situations. Notable amplifiers in this class are those from Audio Note corporation (designed in Japan), including the "Ongaku", voted amplifier of the year during the late 1990s. This market is very small in commercial terms, yet a very small number of hand built products do sell at very high prices ($10,000 and up, and up). There is always an extreme, and very extreme SET is the Wavac 833 - arguably the worlds most expensive HiFi amplifier, delivering ~ 150 W
(Aside from ths Wavac and a very few other high power SETs,) SET amplifiers usually need to be carefully paired with unusually sensitive speakers, notably horns and full range drivers such as Lowthers and Fostex etc, which invariably have their own quirks, offsetting their advantages of very high efficiency and minimalism.
[edit] Commercial Push Pull (PP) amplifiers
Mainstream modern loudspeakers offer greatly improved performance compared to their historic predecessors, but typically are also much less efficient and thus require quite high power amplifiers to drive them, fuelled by the steadily increasing power output available from mainstream consumer amplifiers using transistors.
A side effect has been to drive the needed power levels beyond the capability of simple SE circuits, consequently the mainstream for commercial valve hi-fi power amplifiers has since the 1970s moved mainly to Class AB1 push pull (PP) circuits (usually using Tetrode/pentodes, and thus feedback (NFB) ).
A minority of commercial PP production continues to use pure class A, or can sometimes be switched between class A and AB; Another (partly overlapping) subset of commercial PP production uses (or can be switched into "triode mode".
Major manufacturers in the PP audiophile market today include:
- Audio Research
- Jadis
[edit] Hobbyist amplifier construction
The simplicity of valve amplifiers, especially Single Ended designs, makes them viable for hobbyists. To make an amplifier to the very highest quality standards of commercial production, possibly at a fraction the price, is, of course, also an attraction.
Indeed not only can hobbyist construct products that equal commercial production, they have a number of advantages, such as:
- Being able to use highly regarded tubes produced 30 years ago (or more) during the 'golden age', which are only available in ones and two's;
- The low parts count also makes it viable to tune the over all sound of that specific system, to balance the sound of other system components (speakers, the room acoustic etc,) especially in the case of amplifier circuits NOT using feedback) by the careful selection of each component individually, eg selecting one make of capacitor over another, or different makes (and vintages) of tube, of the same basic type. Auditioning many versions of a given tube type is sometimes known as "tube rolling".
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).
[edit] Construction
Point to point hand wiring rather than circuit boards tends to be used in low volume high end commercial constructions as well as by hobbyists. This construction style is satisfactory due to ease of construction, adapted to the number of physically large and chassis mounted components (tube sockets, large supply capacitors, transformers), the need to twist heater wiring to minimise hum, and the as a side effect benefiting from the fact that "flying" wiring minimises capacitive effects.
One picture below 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.
The wiring of a commercial amplifier is also shown for comparison
 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 |
[edit] 'Extreme' designs
[edit] SETs
Very occasionally, monster tubes (usually designed for use in radio transmitters) from decades ago are pressed into service to create one-off "statement" designs (usually at very high costs). 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.
Another problem is that the voltages for such such amplifiers often pass well beyond 1 kV , and such potentially lethal voltages form an effective disincentive to commercial products of this type.
Examples include the 212 and 833 tubes
[edit] Push pull amplifiers
Many commercial amplifiers (and some hobbyist constructions) place multiple pairs of output tubes in parallel to increase power while using the same readily obtainable tube types, operating from the same (reasonably) low voltage supplies. Another beneficial side effect of this is that the output impedance of the tubes is reduced as normal when paralleling resistances and thus the turns ratio needed from the transformer is reduced, making it easier to construct a wide bandwidth transformer
Some home constructed amplifiers use high power transmitting tubes (eg 813) to yield up to 100 watts (or beyond) per tube pair, but it is unusual to see commercial designs of this type, high power commercial products tending to use arrays of standard tubes (eg EL34 / KT88) in the parallel push pull (PPP) configuration instead (eg Jadis, Audio Research).
 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. |
[edit] Output transformerless amplifiers (OTL)
The output transformer (OPT) is a major component in all mainstream valve power amplifiers, and in addition to having a high cost (at least for a good one) they remain engineering compromises that deviate significantly from the idealised form.
One approach to avoid the problems of OPTs is to avoid the OPT entirely, and direct couple the gain device to the output (as is done with most transistor amplifiers for example). 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.
The problem with this is that that the characteristics of the valve itself thus directly limit the output performance of the speaker - high voltages (if anything goes wrong your speaker may be destroyed), low current capability and consequently very low efficiency when driving low impedance loads, very High Zout and consequently low damping factor.
These effects mean that OTLs are very selective about which speakers work well with them, many (normally considered to be excellent) speakers don't seem to work well with OTL's. Significantly, electrostatic speakers (often considered difficult to drive) often work especially well with OTL's.
The "discovery" in the west of the Russian 6C33C-B tube (designed for use as a series regulator pass element, and able to pass abnormally high currents at relatively low voltages) following the defection of a Mig pilot, and the (some years later) collapse of the Soviet Union was a key point in stimulating interest in the OTL, although some developments of this type (notably loftin-white) go back many decades. Concerns about 6C33C variability and reliability have in recent years made the 6AS7 the main tube of choice for OTLs (another regulator tube).
A serious problem is that in some OTL circuits, a failure (short) in one of the output tubes may result in the loudspeaker being connected directly across the power supply, and thus being destroyed (possibly quite spectacularly).
The "circlotron" topology for OTLs claims (among other benefits) to offer protection for speakers in the case of a tube failure, however as with parafeed this is accomplished by placing a very large capacitor in the signal path, which brings with it its own problems and may also affect sound quality. It must be stressed that OTLs remain a niche approach although one which is very interesting to enthusiasts willing to tolerate its implications.
See also [1]
[edit] Direct coupled amplifiers for electrostatics and headphones
In a sense this niche is a subset of OTL's however it merits treating separately because unlike an OTL for a loudspeaker, which has to push the extremes of a tube circuits ability to delver relatively high currents at low voltages into a low impedance load, some headphone types have impedances high enough for "normal" tube types to drive reasonably as OTL's, and in particular electrostatic loudspeakers and headphones which can be driven directly at hundreds of volts but minimal currents.
Once more there are some safety issues associated with direct drive fro electrostatic loudspeakers which in extremis may use transmitting tubes operating at over 1 kV. Such systems are potentially lethal.
[edit] See also
[edit] 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.
[edit] 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.
