There has been considerable debate over the characteristics of tubes versus
bipolar junction transistors. Triodes and
MOSFETs have certain similarities in their transfer characteristics. Later forms of the tube, the
tetrode and
pentode, have quite different characteristics that are in some ways similar to the bipolar transistor. Yet MOSFET amplifier circuits typically do not reproduce tube sound any more than typical bipolar designs. The reason is
circuit differences between a typical tube design and a typical MOSFET design.
Input impedance A characteristic feature of most tube amplifier designs is the high input
impedance (typically 100
kΩ or more) in modern designs and as much as 1 MΩ in classic designs. The input impedance of the amplifier is a load for the source device. Even for some modern music reproduction devices, the recommended load impedance is over 50 kΩ. This implies that the input of an average tube amplifier is a problem-free load for music signal sources. By contrast, some transistor amplifiers for home use have lower input impedances, as low as 15 kΩ. Since it is possible to use high output impedance devices due to the high input impedance, other factors may need to be accounted for, such as cable capacitance and
microphonics.
Output impedance Loudspeakers usually load audio amplifiers. In audio history, nearly all loudspeakers have been electrodynamic loudspeakers. There also exists a minority of electrostatic loudspeakers and some other more exotic loudspeakers. Electrodynamic loudspeakers transform electric current into force and force into acceleration of the diaphragm, which causes sound pressure. Due to the principle of an electrodynamic speaker, most loudspeaker drivers ought to be driven by an electric current signal. The current signal drives the electrodynamic speaker more accurately, causing less distortion than a voltage signal. In an ideal
current or transconductance amplifier, the output impedance approaches infinity. Practically all commercial audio amplifiers are voltage amplifiers. Their output impedances have been intentionally developed to approach zero. Due to the nature of vacuum tubes and audio transformers, the output impedance of an average tube amplifier is usually considerably higher than that of modern audio amplifiers produced completely without vacuum tubes or audio transformers. Most tube amplifiers, with their higher output impedance, are less ideal voltage amplifiers than the solid-state voltage amplifiers with their smaller output impedance.
Soft clipping Soft clipping is a very important aspect of tube sound, especially for
guitar amplifiers. A
hi-fi amplifier should not normally ever be driven into clipping. The
harmonics added to the signal are of lower energy with soft clipping than hard clipping. However, soft clipping is not exclusive to tubes. It can be simulated in transistor circuits (below the point at which real hard clipping would occur). (See
"Intentional distortion" section.) Large amounts of global negative feedback are not available in tube circuits, due to phase shift in the output transformer, and lack of sufficient gain without large numbers of tubes. With lower feedback, distortion is higher and predominantly of low order. The onset of clipping is also gradual. Large amounts of feedback, allowed by transformerless circuits with many active devices, lead to numerically lower distortion but with more high harmonics, and a more abrupt transition to clipping. As input increases, the feedback uses the extra gain to ensure that the output follows it accurately until the amplifier has no more gain to give and the output saturates. However, phase shift is largely an issue only with global feedback loops. Design architectures with local feedback can be used to compensate the lack of global negative feedback magnitude. Design
selectivism is again a trend to observe: designers of sound-producing devices may find the lack of feedback and resulting higher distortion beneficial, designers of sound reproducing devices with low distortion have often employed local feedback loops. Soft clipping is also not a product of lack of feedback alone: Tubes have different characteristic curves. Factors such as bias affect the load line and clipping characteristics. Fixed and cathode-biased amplifiers behave and clip differently under overdrive. The type of phase inverter circuitry can also greatly affect the softness (or lack of it) of clipping; the long-tailed pair circuit, for example, has a softer transition to clipping than a cathodyne. The coupling of the phase inverter and power tubes is also important, since certain types of coupling arrangements (e.g., transformer coupling) can drive power tubes to class AB2, while some other types can't. In the recording industry, and especially with microphone amplifiers, it has been shown that amplifiers are often overloaded by signal transients. Russell O. Hamm, an engineer working for
Walter Sear at
Sear Sound Studios, wrote in 1973 that there is a major difference between the harmonic distortion components of a signal with greater than 10% distortion that had been amplified with three methods: tubes, transistors, or operational amplifiers. Mastering engineer R. Steven Mintz wrote a rebuttal to Hamm's paper, saying that the
circuit design was of paramount importance, more than tubes vs. solid state components. Hamm's paper was also countered by Dwight O. Monteith Jr and Richard R. Flowers in their article "Transistors Sound Better Than Tubes", which presented transistor mic preamplifier design that actually reacted to transient overloading similarly as the limited selection of tube preamplifiers tested by Hamm. Monteith and Flowers said: "In conclusion, the high voltage transistor preamplifier presented here supports the viewpoint of Mintz: 'In the field analysis, the characteristics of a typical system using transistors depends on the design, as is the case in tube circuits. A particular 'sound' may be incurred or avoided at the designer's pleasure, no matter what active devices he uses.'" Electric guitar amplifiers often use a class-AB1 amplifier. In a class-A stage, the average current drawn from the supply is constant with signal level; consequently, it does not cause supply line sag until the clipping point is reached. Other audible effects due to using a tube
rectifier with this amplifier class are unlikely. Unlike their solid-state equivalents, tube rectifiers require time to warm up before they can supply B+/HT voltages. This delay can protect rectifier-supplied vacuum tubes from cathode damage due to application of B+/HT voltages before the tubes have reached their correct operating temperature by the tube's built-in heater.
Class A The benefit of all
class-A amplifiers is the absence of
crossover distortion. This crossover distortion was found especially annoying after the first silicon-transistor
class-B and class-AB transistor amplifiers arrived on the consumer market. Earlier germanium-based designs with the much lower turn-on voltage of this technology and the non-linear response curves of the devices had not shown large amounts of crossover distortion. Although crossover distortion is very fatiguing to the ear and perceptible in listening tests, it is also almost invisible (until looked for) in the traditional
Total harmonic distortion (THD) measurements of that epoch. It should be pointed out that this reference is somewhat ironic given its publication date of 1952. As such, it most certainly refers to
ear fatigue distortion commonly found in existing tube-type designs; the world's first prototype transistorized hi-fi amplifier did not appear until 1955.
Push–pull amplifiers A class-A
push–pull amplifier produces low distortion for any given level of applied
feedback, and also cancels the
flux in the
transformer cores, so this topology is often seen by HIFI-audio enthusiasts and do-it-yourself builders as the ultimate engineering approach to the tube hi-fi amplifier for use with normal
speakers. Output power of as high as 15 watts can be achieved even with classic tubes such as the 2A3 or 18 watts from the type 45. Classic pentodes such as the EL34 and KT88 can output as much as 60 and 100 watts, respectively. Special types, such as the V1505, can be used in designs rated at up to 1100 watts. See "An Approach to Audio Frequency Amplifier Design", a collection of reference designs originally published by G.E.C.
Single-ended triode (SET) amplifiers SET amplifiers show poor measurements for distortion with a resistive load, have low output power, are inefficient, have poor
damping factors and high measured harmonic distortion. But they perform somewhat better in dynamic and impulse response. The triode, despite being the oldest signal amplification device, can also (depending on the device in question) have a more linear no-feedback transfer characteristic than more advanced devices such as beam tetrodes and pentodes. All amplifiers, regardless of class, components, or topology, have some measure of distortion. This mainly
harmonic distortion is a unique pattern of a simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic. The result is like adding the same tone one
octave higher in the case of second-order harmonics, and one octave plus one fifth higher for third-order harmonics. The added harmonic tone is lower in amplitude, at about 1–5% or less in a no-feedback amp at full power and rapidly decreasing at lower output levels. Hypothetically, a single-ended power amplifier's second harmonic distortion might reduce similar harmonic distortion in a single driver loudspeaker if their harmonic distortions were equal and the amplifier was connected to the speaker so that the distortions would neutralize each other. SETs usually only produce about 2
watt (W) for a 2A3 tube amp to 8 W for a 300B up to the practical maximum of 40 W for an 805 tube amp. The resulting
sound pressure level depends on the sensitivity of the loudspeaker and the size and acoustics of the room, as well as amplifier power output. Their low power also makes them ideal for use as
preamps. SET amps have a power consumption of a minimum of 8 times the stated stereo power. For example, a 10 W stereo SET uses a minimum of 80 W, and typically 100 W.
Single-ended pentode and tetrode amplifiers The special feature among tetrodes and pentodes is the possibility to obtain
ultra-linear or distributed load operation with an appropriate output transformer. In practice, in addition to loading the plate terminal, distributed loading (of which ultra-linear circuit is a specific form) also distributes the load to the cathode and screen terminals of the tube. An Ultra-linear connection and distributed loading are both, in essence, negative feedback methods, which enable less harmonic distortion along with other characteristics associated with negative feedback. Ultra-linear topology has mostly been associated with amplifier circuits based on research by D. Hafler and H. Keroes of Dynaco fame. Distributed loading (in general and in various forms) has been employed by the likes of McIntosh and Audio Research.
Class AB The majority of modern commercial hi-fi amplifier designs have until recently used
class-AB topology (with more or less pure low-level class-A capability depending on the standing bias current used), in order to deliver greater
power and
efficiency, typically 12–25 watts and higher. Contemporary designs normally include at least some
negative feedback. However, class-D topology (which is vastly more efficient than class B) is more and more frequently applied where traditional design would use class AB because of its advantages in both weight and efficiency. Class-AB push–pull topology is nearly universally used in tube amps for electric guitar applications that produce power of more than about 10 watts. ==Intentional distortion==