Editing Variable-gain amplifier

{{Short description|Electronic amplifier that varies its gain depending on a control voltage}}

A '''variable-gain''' ('''VGA''') or '''voltage-controlled amplifier''' ('''VCA''') is an [[electronic amplifier]] that varies its [[Gain (electronics)|gain]] depending on a control voltage (often abbreviated CV).

VCAs have many applications, including [[audio level compression]], [[synthesizers]] and [[amplitude modulation]].

A crude example is a [[Op-amp#Inverting amplifier|typical inverting]] [[op-amp]] configuration with a [[light-dependent resistor]] (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]] (an [[optocoupler]]).  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]]s.

A voltage-controlled amplifier can be realised by first creating a [[voltage-controlled resistor]] (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]] (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]]s.

In audio applications [[logarithm]]ic gain control is used to emulate how the ear hears [[Fletcher–Munson curves|loudness]]. [[David E. Blackmer]]'s [[Dbx, Inc.|dbx]] 202 VCA, based on the [[Blackmer gain cell]], was among the first successful implementations of a logarithmic VCA.<ref>{{cite web |url=http://www.thatcorp.com/History_of_VCAs.shtml |archive-url=https://web.archive.org/web/20200703115259/http://www.thatcorp.com/History_of_VCAs.shtml |archive-date=2020-07-03 |title=A Brief History of VCA Development}}</ref>

[[Analog multiplier]]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]]s come equipped with VCAs in each channel for [[console automation]]. The [[Fade (audio engineering)|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.<ref>{{cite web |title=Allen & Heath ML3000 Block Diagram |url=http://www.allen-heath.com/media/ML3000-Block-Diagram.pdf|archive-url=https://web.archive.org/web/20221014092711/http://www.allen-heath.com/media/ML3000-Block-Diagram.pdf |archive-date=2022-10-14 }}</ref>  Typically VCA groups are used to control various parts of the mix; [[Vocal music|vocals]], [[guitar]]s, [[drum]]s or [[Percussion instrument|percussion]].  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 [[Musical instrument|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 [[Reverberation|reverb]] or [[chorus effect]].<ref>{{cite web |url=http://www.allen-heath.com/media/ML3000-USER-GUIDE.pdf |title=Allen & Heath ML3000 User Guide}}</ref>

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 (company)|dbx]], who used them to make [[Dynamic range compression|dynamic range compressor]]s. The first console using VCAs was the Allison Research computer-automated recording system designed by [[Paul C. Buff]] in 1973.<ref>{{cite book |author=Richard James Burgess |title=The History of Music Production |page=134 |url=https://books.google.com/books?id=8J6zAwAAQBAJ&pg=PT134 |date=2014 |publisher=Oxford University Press |isbn=9780199385010}}</ref> Another early VCA capability on a sound mixer was the series of [[Music Center Incorporated|MCI]] JH500 studio recording desks introduced in 1975.<ref>{{cite book |last=Self |first=Douglas |title=Audio Engineering Explained |date=2012 |page=249 |publisher=Taylor & Francis |isbn=9781136121258 }}</ref> The first VCA mixer for live sound was the PM3000 introduced by [[Yamaha Pro Audio|Yamaha]] 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]]
*[[Mix automation]]

== References ==
{{reflist}}

== External links ==
* [http://www.schmitzbits.de/vca.html Examples of non-optical VCAs]
* [http://www5b.biglobe.ne.jp/~houshu/synth/ Some schematics for VCAs]
* {{cite web |url=http://www.cgs.synth.net/tube/vca.html |title=Vacuum tube VCAs |archive-url=https://web.archive.org/web/20080513201131/http://www.cgs.synth.net/tube/vca.html |archive-date=2008-05-13}}
* {{web archive |url=https://archive.today/20130221013802/http://glogger.mobi/v/31470/ |title=University of Toronto undergraduate lecture explaining how to implement a Voltage Controlled Amplifier using an operational amplifier and a photocell}}
* {{web archive |url=https://web.archive.org/web/20081203003921/http://www.allen-heath.com/DL/vca_book.pdf |title=Allen & Heath's Guide to VCA Sound Desk Mixing}}

{{DEFAULTSORT:Variable-Gain Amplifier}}
[[Category:Frequency mixers]]
[[Category:Electronic amplifiers]]
[[Category:Dynamics processing]]
[[Category:Synthesiser modules]]