The Cool Sound of Tubes
One of the last remaining tube domains is in music application, but there the devices flourish and even innovate.
IEEE Spectrum August 1998 Volume 35 Number 8
By Eric Barbour
Although solid-state technology overwhelmingly dominates today's world of electronics, vacuum tubes are holding out in two small but vibrant areas. They do so for entirely different reasons. Microwave technology relies on tubes for their power-handling capability at high frequencies ["Tubes: still vital after all these years," Robert S. Symons, IEEE Spectrum, April, 1998]. The other area--the creation and reproduction of music--is a more complicated and controversial story.
The complications and controversy stem from the fact that music is played to be heard by human beings, whose nonlinear ear-brain hearing systems are far from fully understood. Since no one knows exactly how to model the human auditory system, no one knows exactly what engineering measurements are appropriate to evaluating the performance of audio equipment... the only way to judge audio equipment is by listening to it. Hence the controversy: subjective human perception--especially when flanked by questions of artistic merit--is made to order for arguments and disputation.
...Why tubes--subjective reasons
The three areas of tube audio tend to be mutually exclusive and appear not to influence each other, even though all three directly involve the production or reproduction of music. It is common to see the same tube types, such as the popular EL34 power pentode, in electric-guitar amplifiers and in high-end stereo amplifiers. Often, too, these disparate products employ similar circuit topologies.
Electric-guitar amplifiers, it is estimated, consume as many as three out of four of the world's production of audio tubes. This is hardly surprising, since the tubed guitar amp seems unshakably enthroned at the top of the rock 'n' roll world. In this case, the use of tubed amplifiers in the early rock of the 1950s and '60s caused their distinctive distortions to become the standard tonal effect for the electric guitarist. A cultural bias formed during those years among U.S. and British musicians in favor of the particular nonlinearities of those amps, which typically were quite simple and had little or no negative feedback to improve their linearity.
...Discussion of an amp's merit frequently hinges on the clipping effect, which is often described as yielding a sound like a brass wind instrument. The saturation distortion of the output transformer, which couples the power tubes to the speaker, also plays a key role in determining an amplifier's sound. Another amplifier parameter--its touch sensitivity--is affected by circuit nonlinearities and loose regulation of the plate-power supply.
Tubes have also been cited, albeit not without controversy, as facilitating a controlled so-called infinite sustain effect because of the way their signal compression interacts with acoustic feedback from speaker to guitar string. These effects are well-known among musicians, yet seem difficult to reproduce accurately with solid-state equipment. The many designers who have tried to build tube simulators over the past 30 years have achieved varying levels of musical and financial success. Such equipment has its supporters, but most amateur and professional guitarists remain faithful to tubed amplifiers.
...Clipping distortion is not the only issue. In semiconductors, the shift of characteristics with temperature along with their relatively low maximum operating temperature has led to extensive use of Class-B amplifiers to keep power dissipation down. In many designs, the result has been audible crossover distortion, which often does not show up in published specifications. Those specs are typically based on measurements made at full power, where crossover distortion is at a minimum. Consumer tube amplifiers use Class-A or Class-AB designs, which have vanishingly low distortion even at low signal levels.
...Speaking about solid-state designs, Whitlock asserted that they "...depend on huge amounts of negative feedback to 'fix everything', including crossover distortion. Op amps commonly have open-loop THD in the 20 percent to 70 percent range. Stabilization generally requires open-loop gain to fall at 6 dB per octave. This means that, for ultrasonic input signals, the op amp has little gain margin to fix its own distortion. The ultrasonic signals, along with distortion products, are fed to the next stage for further distortion and intermodulation. This intermodulation creates audible, but non-harmonically related, artifacts which contaminate the noise floor and mask many subtle features of the music."
...In addition to the active devices themselves, the passive components customarily used with them apparently also favor tube designs. According to Atwood, the low impedances of bipolar transistors necessitate the use of large (high-value) coupling capacitors, which for the most part means electrolytics. High dielectric absorption, imperfect high-frequency characteristics, and aging of electrolytics put them at a sonic disadvantage compared with the good-quality film capacitors nearly universally used for coupling the stages of tubed amps. "While it is possible to build solid-state amps with no electrolytics, it is rarely done," he said.
Whitlock said he felt that the main problem with electrolytics is actually at low frequencies: "The major problem with electrolytics is dielectric hysteresis, which produces high THD at low frequencies, where there are ever-increasing voltage swings across the capacitor." He also stated that direct coupling is the most desirable scheme, whether semiconductors or tubes are used.
Parallel views are held by guitar-amp designers. As Rick Perrotta, a longtime recording engineer and cofounder of guitar-amp manufacturer Matchless Amplifiers, Santa Fe Springs, Calif., explained, when a transformer saturates, it has a compressing effect. "When someone says that a particular guitar amplifier has a big fat sound, that is due, in part, to the output transformer saturating along with the output tubes. Solid-state amps can't perform that trick."
...Professional audio gear
...The advantage of tubes in condenser microphones is their very high input impedance, which does not load down the capsule significantly. Many recording professionals agree that the soft clipping exhibited by the tube--along with its high-voltage operation--can also be important, since some singers have very powerful voices, capable of producing peaks far beyond the dynamic range of typical solid-state electronics.
Almost as much the rage as tubed microphones are special preamps and so-called direct input boxes, which in studio signal chains serve to provide voltage gain, impedance conversion, and (in the case of direct input boxes) conversion from unbalanced to balanced connections. In spite of their lesser usage in studios, compared with tubed condenser microphones, these preamp devices are also riding high on a resurgence of popularity. Among the best-liked designs of this category are units that are solid-state but for a single 12AX7 tube operated at a very low plate voltage--as little as 12 V. This so-called starved-plate operation delivers high distortion, which some equipment designers consider the only useful characteristic of vacuum tubes in audio.
...A key use of tubes in the recording chain is in signal compressors, also called limiters or leveling amplifiers. In general terms, a high-quality audio compressor consists of a preamp, a so-called sidechain with audio-signal rectifier and peak detector, and a voltage-controlled attenuator or amplifier driven by the peak-detected voltage. The control is arranged so that signal gain in the preamp rises as the signal level on the input falls, giving a narrowed dynamic range. Compressors of this type are considered mandatory in recording and production studios, for a variety of sonic effects. Rock music is often heavily compressed to give the illusion of greater loudness.
...About the author
Eric Barbour has been an applications engineer with Svetlana Electron Devices Inc., Portola Valley, Calif., since July 1996. His work involves testing and characterizing new vacuum-tube types and constructing and testing amplifier circuitry for Svetlana audio and RF tubes. Earlier, he was a senior technician and then an engineer with the U.S. Department of Energy, Intelligent Electronics Co., and Dionex Corp.
A staff editor of Vacuum Tube Valley magazine, Sunnyvale, Calif., since its founding in 1995, he also contributes to Glass Audio magazine, Peterborough, N.H.
Copyright 1998, The Institute of Electrical and Electronics Engineers, Inc.
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