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Valve sound

From Academic Kids

Valve amplifier (British English) means the same as tube amplifier (American English).

The valve sound or tube sound is the sound often associated with music amplified by a valve amplifier, although this association is not strict. The valve sound is often described as being "warm", "rich", "relaxed" etc., in contrast to a so-called transistor sound that is sometimes said to be "faster" or even "harsh" and "brittle". Both audio enthusiasts and musicians with amplified instruments have a strong interest in these properties.

One important aspect of tube sound is the soft ceiling in the dynamic range of tube amplifiers. A tube radio or tube amplifier will increase in volume to a point, and then as the volume is further increased beyond the linear range, it gently reduces in gain. Unlike transistor amplifiers, tube amplifiers don't tend to produce the harsh clipping of saturation and cut-off that transistor amplifiers do. Thus turning up the volume control, the sound will get louder and then quieter again in a very smooth and gentle sort of way. This soft ceiling behaves very much like a dynamic range compressor/limiter. In the same way that photographic film has a filmic look especially owing to a soft compression of highlights (i.e. even when massively overexposed, the film still responds, to some degree, to increases in light), tube amplifiers provide a nice compressive effect. For this reason, tubes are often still used to impart a pleasant response characteristic to solid state amplifiers (e.g. through the use of one 12AX7 tube within an otherwise transistorized amplifier). Alternatively, one may use a light bulb in the feedback loop of an infinite gain multiple feedback (IGMF) circuit. The sluggish response of the light bulb can thus be used to moderate the sound and attain a valve-like "soft ceiling" in dynamic range. Microprocessor-based or microcontroller-based solid state signal processing devices can also be used for this purpose, and there are a wide variety of products that can shape the dynamic range of an audio signal. These typically operate in four regimes: expansion at low levels; no change at mid levels; compression at high levels; limiting at extremely high levels.

Some other aspects of valve sound have more to do with the circuit topology and circuit design of the amplifier, than with the use of valves rather than transistors as the active gain devices. There is also a degree of influence from the passive components used.

Another aspect of the valve sound is that early valve amplifiers often had only limited bandwidth, in part due to passive component technology available at the time, notably capacitors. It must be remembered that the source material available at this time (usually vinyl records) lacked the definition of today's CDs. There was no electronic music with extreme bass, and high frequencies mostly contained noise and distortion, so it was advantageous to not emphasise these.

Early amplifiers were by definition valve amplifiers since the transistor did not become common in consumer amplifiers until the late 1960s. The very earliest amplifiers usually had single-ended topologies with the most basic type of vacuum tube, known as a triode. An audio amplifier using this topology will always be in class A. Class A single-ended triode amplifiers (known as SET's) have a characteristic asymmetrical distortion spectrum, a simple and monotonically decaying series of harmonics, dominated by modest levels of second harmonic distortion and followed by both even- and odd-numbered harmonics. Second harmonic distortion (multiplication of the original frequencies by 2) is musically equivalent to adding the same tone one octave higher, to form. In this case the added tone is at a lower level (typically 5% or less at full power) but the effect of the distortion is said to "fatten" the sound.

Transistor amplifiers are almost always class AB push-pull, and this symmetrical topology tends to cancel even-order harmonic distortion products. The resulting distortion is therefore dominated by odd-order harmonics (multiplication of the original frequencies by 3, 5, 7, ...) which some find "harsh", etc. Transistor amplifiers made during the 1980s typically also had extremely high open-loop gain, but poor open-loop linearity, and relied on large amounts of negative feedback (NFB), to linearise the devices. Some consider that NFB does not sound "natural" or "musical", due to errors in the way it reacts to transients. These errors also reveal themselves in very complex distortion spectra, which humans find "discordant".

In contrast audio valves typically have only modest gain, and are extremely linear in the area of interest. This makes it possible to design very simple valve circuits that rely on this inherent open-loop linearity and have little, or indeed no, NFB, and thus have very simple distortion spectra.

The subject is further complicated by the way triodes and MOSFETS have certain similarities in their transfer characteristics, whereas later forms of the valve, the tetrode and pentode have quite different characteristics, in some ways reminiscent of the transistor.

All these factors contribute to the valve sound for audio applications.

In more modern times, transistor amplifiers have become dominant, mainly because they are cheaper to produce, and operate on lower voltages (including batteries, in applications such as car radios.) However, valve amplifiers have retained a loyal following among audio fanatics, especially for SET's, in Japan, and in recent years there has also been a resurgence in the west.

Modern amplifiers are generally designed to have very low distortion (<0.1% THD) and reproduce the original sound as closely as possible, leaving tone and compression to the recording studio. Valve amplifiers however are still widely used for electric guitars, in this case for the sake of desired distortion: the way they distort when overdriven is quite different than transistor amplifiers (both BJT and FET), again leading to a very distinctive sound, which many find superior.

Using modern passive components, and modern digital sources and wide band speakers, it is possible to build valve amplifiers that have the characteristic wide bandwidth and "fast" sound of modern transistor amplifiers, including using push pull circuits, class AB and feedback, and some enthusiasts have built amplifiers using transistors and MOSFETS that operate in class A, including single-ended, which often have the valve sound but lack the valve dynamics.

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