Amplification of classical soloists in concert performances ?

I watched a YouTube video of John Williams performing a movement of Rodrigo's Concierto de Aranjuez with the LSO.

https://www.youtube.com/watch?v=ekznnxaGzNU

I noticed there was a B&W (I think) speaker near the soloist—probably to help the solo guitar match the volume of the orchestra. Is this a common technique for concerto performances?