Virtual Valve Amplifier

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A Virtual Valve Amplifier (VVA) is software algorithm designed and sold by Diamond Cut Productions, Inc. for simulating the sound of various valve amplifier designs. It can be found within their DC8 and Forensics8 software programs.

Contents

A VVA can be used to color the sound of a digital recording by adding "tube-warmth" or "fat-bass" in addition to adding subtle harmonics to enhance very old or muffled recordings. The algorithms behind a VVA are based on real vacuum tube circuits and non-linearities, mathematically simulating the large-signal transfer functions of various vacuum tubes and output transformers found in amplifier designs. A majority of this data was originally derived from extensive bench measurements on real vacuum tube amplifier circuits under varying operating conditions by engineers Craig Maier and Rick Carlson in the early 1990s. A VVA is a direct mathematical reconstruction of the same signal passing through a physical electron tube amplifier. The VVA algorithm can be found in the Diamond Cut DC8 and DC Forensics8 software packages. It is also sold as a VST plug-in.

Basic operating parameters

VVA designs generally include a number of parameters that may be configured to change the sound and operating characteristics of the amplifier design:

Operating point

Historically referred to as the "Q" or bias point by engineers, the operating point of a vacuum tube is a condition generally fixed by the amplifier manufacturer. In general, the operating point determines the device's bias value at zero signal input and determines the distribution of harmonics introduced into the output of the amplifier. Tubes that operate with a higher operating point close to cutoff give more "headroom", enabling greater volume gains to be applied before signal degradation in the form of "breakup" or saturation results. By contrast, a lower operating point introduces more harmonic distortion into the final output as a result of the different non-linearity distribution near cut-off as compared to operation in the nominally linear portion of the characteristic. Some guitar amplifiers are designed to produce this type of distortion as its sound effect is considered desirable. [1] [2]

Drive

This describes how loudly the "physical" equivalent of the virtual valve amplifier is set. However, the output level of a VVA generally remains constant independent of drive due to internal gain compensation algorithms. Instead, the drive determines the amount of distortion that can be introduced into the output signal. As such, the Drive of a VVA describes the degree of modulation applied to a given vacuum tube amplifier circuit centered about the set operating point. The higher the drive level setting, the greater will be the production of predominantly even order harmonics due to the circuit's asymmetrical non-linearity. As a result, the VVA "effect" increases with increasing drive. [1] [2]

Common vacuum tube types

Triode 12AX7/ECC83

This is a high-mu dual triode that is generally incorporated into an RC coupled class A audio pre-amplifier configuration and its design is optimized to minimize harmonic distortion. This vacuum tube is still the industry standard pre-amplifier valve. It has a relatively flat linear operating region in the middle of its dynamic operating range, producing relatively lower levels of distortion compared to some of the other devices listed here. But, by moving the Operating Point to either the saturation or cutoff extreme, more "tube-warmth" effect can be produced by this device. Some VST effects emulate this tube. [3]

Triode 12AT7/ECC81

The 12AT7 high-mu dual triode was designed primarily for RF mixing applications where it was incorporated into the oscillator/mixer stage and used to heterodyne incoming RF signals with the local oscillator to create an intermediate frequency in TV and FM sets. Thus, it is intentionally designed to be extremely non-linear. Thus, circuits based around the 12AT7 exhibit a larger degree of non-linearity throughout the entire dynamic operating range, including the middle. As a result, it produces greater even-order harmonic distortion (less objectionable than odd-order distortion).

Triode 12AU7/ECC82

The 12AU7 is a medium-mu dual triode often found in the driver / phase inverter stage of a push-pull power amplifier and also results in significant non-linearity in the middle of its dynamic operating curve.

Pentode 6EJ7

The 6EJ7 pentode and its equivalents are often found in high-gain vacuum tube microphone amplifiers which require the sharp cutoff of a pentode. It generally produces a very pleasant "tube-warmth" effect when the operating point is properly set. This device is the same as the European type EF183.

Pentode 6267/EF86

The 6267 / EF86 pentode was a vacuum tube well suited for use in low-level pre-amplifiers where low noise and minimal microphonics were important. Its high-gain characteristics and family of operating curves make for useful harmonic distortion and signal compression properties[ clarification needed ].

Common vacuum tube amplifier types

Circuits may be "single-ended", using a single output device (or several connected in parallel), or "push-pull", using paired devices configured to cancel out even-order distortion products and reduce output transformer magnetisation. Bias of a push-pull amplifier may be set to make both sides conduct at all times (amplifier class A), to make only one side conduct at a time (class B), or intermediate (class AB). Class A uses more power for the same output (i.e., is less efficient), can produce less output power from the same devices, and produces lower distortion, than classes AB and B.

2-Stage Class A

These devices generally consisted of a 12AU7 medium-mu triode driving a single 6L6GC beam power pentode audio output valve or similar. Its effects are distinctive due to convolution of the non-linearity of the triode interacting with those of the pentode, with both devices operating in class-A mode. The 6L6GC is similar in performance to the industrial type 5881, and also the European type KT66.

2-Stage Class AB

Often consisting of a 12AU7 phase inverter / driver, pushing a pair of 6L6GC beam power pentodes. The symmetrical push-pull circuit cancels out and reduces even-order distortion products compared with a single-ended circuit. The operating point is hard-wired and cannot be adjusted.

2A3 push-pull

Also known as a "retro–triode" amplifier, it was invented in the 1930s and incorporated a directly heated cathode resulting in a high power output. It was often used in theatrical applications and public address systems. This vacuum amplifier typically exhibits a more linear output transfer characteristic than its pentode push-pull counterpart and as a result produces a characteristic clean sound. The particular devices used to create the 2A3 VVA models were of the "dual–plate" variety taken from unused stock manufactured for the military by RCA Victor in 1953. This configuration is favoured by many jazz musicians including Les Paul who reportedly used this amplifier configuration to cut all the records made from his home studio. [4]

2A3 single-ended

This is a single ended class A vacuum tube power amplifier implemented using the 2A3 power triode. It exhibits reasonably good linearity with dominant even distortion products.

Vacuum tube exciter

Because of the non-linear properties and distortion products of vacuum tubes and their associated amplification circuits, they are useful in the simulation of a vacuum tube rectifier (6X4) to produce harmonics. Asymmetry between the positive- and negative-going transfer function establishes the relationship between the degree of even and odd harmonics produced. [5]

See also

Related Research Articles

Amplifier electronic device/component that increases the strength of a signal

An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.

Triode

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the pleasantly (warm) distorted sound of tube-based electronics.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Audio power amplifier Audio amplifier with power output sufficient to drive a loudspeaker

An audio power amplifier is an electronic amplifier that amplifies low-power electronic audio signals such as the signal from radio receiver or electric guitar pickup to a level that is high enough for driving loudspeakers or headphones. Audio power amplifiers are found in all manner of sound systems including sound reinforcement, public address and home audio systems and musical instrument amplifiers like guitar amplifiers. It is the final electronic stage in a typical audio playback chain before the signal is sent to the loudspeakers.

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.

Valve amplifier Type of electronic amplifier

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications and very high power radio and UHF television transmitters.

Control grid

The control grid is an electrode used in amplifying thermionic valves such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve to create the first amplifying vacuum tube, the Audion (triode).

Push–pull output

A push–pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load. This kind of amplifier can enhance both the load capacity and switching speed.

Beam tetrode

A beam tetrode, sometimes called a beam power tube, is a type of vacuum tube or thermionic valve that has two grids and forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode when the anode potential is less than that of the screen grid. Beam tetrodes are usually used for power amplification, from audio frequency to radio frequency. The beam tetrode produces greater output power than a triode or pentode with the same anode supply voltage. The first beam tetrode marketed was the Marconi N40, introduced in 1935. Beam tetrodes manufactured and used in the 21st century include the 4CX250B, KT66 and variants of the 6L6.

Pentode

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube, which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode consists of an evacuated glass envelope containing five electrodes in this order: a filament for indirectly heating a cathode, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.

Grid-leak detector

A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.

Single-ended triode Vacuum tube electronic amplifier that uses a single triode to produce an output

A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted. Single-ended amplifiers normally operate in Class A; push-pull amplifiers can also operate in Classes AB or B without excessive net distortion, due to cancellation.

A valve audio amplifier (UK) or vacuum tube audio amplifier (US) is a valve amplifier used for sound reinforcement, sound recording and reproduction.

Distortion (music) Electronic manipulation of audio signals to increase their gain

Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may also be used with other electric instruments such as electric bass, electric piano, and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. While overdriven tube amps are still used to obtain overdrive, especially in genres like blues and rockabilly, a number of other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of a distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, and heavy metal music, while the use of distorted bass has been essential in a genre of hip hop music and alternative hip hop known as "SoundCloud rap".

Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube to a load.

Valve RF amplifier Device for electrically amplifying the power of an electrical radio frequency signal

A valve RF amplifier or tube amplifier (U.S.) is a device for electrically amplifying the power of an electrical radio frequency signal.

Technical specifications and detailed information on the valve audio amplifier, including its development history.

Tube sound Characteristic quality of sounds from vacuum tube amplifiers

Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. However, solid state amplifiers have been developed to be flawless and the sound is later regarded neutral compared to tube amplifiers. Thus the tube sound now means 'euphonic distortion.' The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.

In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period. A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse-width modulation output is obtained from the stage.

References

  1. 1 2 "Analog In". TubeDepot.com.
  2. 1 2 "Vacuum Tube Technology: A Perspective Circa 1950". www.ampbooks.com.
  3. "Archived copy". www.xeniumaudio.com. Archived from the original on 6 November 2011. Retrieved 14 January 2022.CS1 maint: archived copy as title (link)
  4. DC 7.5 & DC 7 User's Manual
  5. Diamond Cut DC8 Users Guide