Voltage Controlled Amplifier, a component used in higher-end analog mixing consoles.
See this post, from this thread: http://www.controlbooth.com/forums/stage-management/11574-td-helping-out.html#post125506
See this post, from this thread: http://www.controlbooth.com/forums/stage-management/11574-td-helping-out.html#post125506
