Audio synthesis via vacuum tubes/Tube VCA: Difference between revisions

The simplicity of the '''tube VCA''' circuit makes feasible the use of a chassis-mounted tube socket and a low-cost terminal strip.

 

== Background ==

It is ironic that the modern concept of a voltage-controlled amplifier has become rigidly fixed into a single topology, that of the differential amplifier with controlled current source used for gain control. This circuit came about due to the need for a two-quadrant multiplier for use in analog computers, an application which has been rendered meaningless by modern digital computers. Although the diff-amp VCA can be implemented with tubes in the usual manner, it is universally in the form of a monolithic IC today. Amusingly, this is due to mental inertia and conservatism among instrument designers, and not because there are no alternate schemes available. (Indeed, some experts have chosen to 'gild the lily' by pursuing ever-more-complex diff amp designs, in order to utterly exterminate small technical issues such as control-voltage feedthrough. Often, such pursuits become far more important than the original intention, the making of music!)<ref name="tvsm">[https://web.archive.org/web/20110912043448fw_/http://www.cgs.synth.net/tube/vca.html The VCA: simplest module] (archived) by Eric Barbour, 1997, with permission of the author</ref>

In a tetrode, there are only two grids; control (the usual signal input point) and the screen. The latter was added to increase the gain of the tube, and to greatly decrease the Miller-effect capacitance between plate and grid, thus making high-frequency operation of the tube easier. The new grid 'screens' out the effect of the plate, by absorbing low-energy electrons. And this grid changes the electrical behaviour of the tube if its applied voltage is varied.<ref name="tvsm"/>

 

 

== How it works ==