A variable-gain (VGA) or voltage-controlled amplifier (VCA) is an
electronic amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...
that varies its
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
depending on a control voltage (often abbreviated CV).
VCAs have many applications, including
audio level compression
Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or ''compressing'' an audio signal's dynamic range. Compression is ...
,
synthesizers
A synthesizer (also spelled synthesiser) is an electronic musical instrument that generates audio signals. Synthesizers typically create sounds by generating waveforms through methods including subtractive synthesis, additive synthesis and ...
and
amplitude modulation
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the amplitude (signal strength) of the wave is varied in proportion to ...
.
A crude example is a
typical inverting op-amp
An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
configuration with a
light-dependent resistor
A photoresistor (also known as a photocell, or light-dependent resistor, LDR, or photo-conductive cell) is a passive component that decreases resistance with respect to receiving luminosity (light) on the component's sensitive surface. The resi ...
(LDR) in the feedback loop. The gain of the amplifier then depends on the light falling on the LDR, which can be provided by an
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
(an
optocoupler
An opto-isolator (also called an optocoupler, photocoupler, or optical isolator) is an electronic component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the ...
). The gain of the amplifier is then controllable by the current through the LED. This is similar to the circuits used in optical
audio compressor
Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or ''compressing'' an audio signal's dynamic range. Compression is ...
s.
A voltage-controlled amplifier can be realised by first creating a
voltage-controlled resistor
A voltage-controlled resistor (VCR) is a three-terminal active device with one input port and two output ports. The input-port voltage controls the value of the resistor between the output ports. VCRs are most often built with field-effect transist ...
(VCR), which is used to set the amplifier gain. The VCR is one of the numerous interesting circuit elements that can be produced by using a
JFET
The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifiers. ...
(junction field-effect transistor) with simple biasing. VCRs manufactured in this way can be obtained as discrete devices, e.g. VCR2N.
Another type of circuit uses
operational transconductance amplifier
The operational transconductance amplifier (OTA) is an amplifier whose differential input voltage produces an output current. Thus, it is a voltage controlled current source (VCCS). There is usually an additional input for a current to control t ...
s.
In audio applications
logarithm
In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...
ic gain control is used to emulate how the ear hears
loudness
In acoustics, loudness is the subjectivity, subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The rel ...
.
David E. Blackmer's
dbx 202 VCA, based on the
Blackmer gain cell The Blackmer gain cell is an audio frequency voltage-controlled amplifier (VCA) circuit with an exponential control law. It was invented and patented by David E. Blackmer between 1970 and 1973. The four-transistor core of the original Blackmer cel ...
, was among the first successful implementations of a logarithmic VCA.
Analog multiplier
In electronics, an analog multiplier is a device that takes two analog signals and produces an output which is their product. Such circuits can be used to implement related functions such as ''squares'' (apply same signal to both inputs), and ''s ...
s are a type of VCA designed to have accurate linear characteristics, the two inputs are identical and often work in all four voltage quadrants, unlike most other VCAs.
In sound mixing consoles
Some
mixing console
A mixing console or mixing desk is an electronic device for mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals from electric or electronic inst ...
s come equipped with VCAs in each channel for
console automation
In music recording, mix automation allows the mixing console to remember the audio engineer's adjustment of faders during the post-production editing process. A timecode is necessary for the synchronization of automation. Modern mixing console ...
. The
fader, which traditionally controls the audio signal directly, becomes a DC control voltage for the VCA. The maximum voltage available to a fader can be controlled by one or more master faders called VCA groups. The VCA master fader then controls the overall level of all of the channels assigned to it. Typically VCA groups are used to control various parts of the mix;
vocals
Singing is the act of creating musical sounds with the voice. A person who sings is called a singer, artist or vocalist (in jazz and/or popular music). Singers perform music (arias, recitatives, songs, etc.) that can be sung with or without ...
,
guitar
The guitar is a fretted musical instrument that typically has six strings. It is usually held flat against the player's body and played by strumming or plucking the strings with the dominant hand, while simultaneously pressing selected stri ...
s,
drum
The drum is a member of the percussion group of musical instruments. In the Hornbostel-Sachs classification system, it is a membranophone. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a she ...
s or
percussion
A percussion instrument is a musical instrument that is sounded by being struck or scraped by a beater including attached or enclosed beaters or rattles struck, scraped or rubbed by hand or struck against another similar instrument. Exc ...
. The VCA master fader allows a portion of a mix to be raised or lowered without affecting the blend of the instruments in that part of the mix.
A benefit of VCA sub-group is that since it is directly affecting the gain level of each channel, changes to a VCA sub-group level affect not only the channel level but also all of the levels sent to any post-fader mixes. With traditional audio sub-groups, the sub-group master fader only affects the level going into the main mix and does not affect the level going into the post-fader mixes. Consider the case of an
instrument feeding a sub-group and a post-fader mix. If you completely lower the sub-group master fader, you would no longer hear the instrument itself, but you would still hear it as part of the post-fader mix, perhaps a
reverb
Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abso ...
or
chorus effect
Chorus (or chorusing, choruser or chorused effect) is an audio effect that occurs when individual sounds with approximately the same time, and very similar pitches, converge. While similar sounds coming from multiple sources can occur naturally, ...
.
VCA mixers are known to last longer than non- VCA mixers. Because the VCA controls the audio level instead of the physical fader, decay of the fader mechanism over time does not cause a degradation in audio quality.
VCAs were invented by
David E. Blackmer, the founder of
dbx, who used them to make
dynamic range compressors. The first console using VCAs was the Allison Research computer-automated recording system designed by
Paul C. Buff
Paul may refer to:
*Paul (given name), a given name (includes a list of people with that name)
*Paul (surname), a list of people
People
Christianity
* Paul the Apostle (AD c.5–c.64/65), also known as Saul of Tarsus or Saint Paul, early Chri ...
in 1973. Another early VCA capability on a sound mixer was the series of
MCI JH500 studio recording desks introduced in 1975.
The first VCA mixer for live sound was the PM3000 introduced by
Yamaha Yamaha may refer to:
* Yamaha Corporation, a Japanese company with a wide range of products and services, established in 1887. The company is the largest shareholder of Yamaha Motor Company (below).
** Yamaha Music Foundation, an organization estab ...
in 1985.
Digital variable-gain amplifier
A digitally controlled amplifier (DCA) is a variable-gain amplifier that is digitally controlled.
The digitally controlled amplifier uses a stepped approach giving the circuit graduated increments of gain selection. This can be done in several fashions, but certain elements remain in any design.
At its most basic form, a toggle switch strapped across the feedback resistor can provide two discrete gain settings. While this is not a computer-controlled function, it describes the core function. With eight switches and eight resistors in the feedback loop, each switch can enable a particular resistor to control the amplifier's feedback. If each switch was converted to a relay, a microcontroller could be used to activate the relays to attain the desired amount of gain.
Relays can be replaced with Field Effect Transistors of an appropriate type to reduce the mechanical nature of the design. Other devices such as the CD4053 bi-directional CMOS analog multiplexer integrated circuit and digital potentiometers (combined resistor string and MUXes) can serve well as the switching function.
To minimize the number of switches and resistors, combinations of resistance values can be utilized by activating multiple switches.
See also
*
Automixer
*
Console automation
In music recording, mix automation allows the mixing console to remember the audio engineer's adjustment of faders during the post-production editing process. A timecode is necessary for the synchronization of automation. Modern mixing console ...
References
External links
Examples of non-optical VCAsSome schematics for VCAs
University of Toronto undergraduate lecture explaining how to implement a Voltage Controlled Amplifier using an operational amplifier and a photocellAllen & Heath's Guide to VCA Sound Desk Mixing
{{DEFAULTSORT:Variable-Gain Amplifier
Frequency mixers
Electronic amplifiers
Dynamics processing
Synthesiser modules