\tikzstyle{main}=[circle, minimum size = 10mm, thick, draw =black!80, node distance = 16mm]
\tikzstyle{connect}=[-latex, thick]
\tikzstyle{box}=[rectangle, draw=black!100]
  \node[main, fill = white!100] (S1) [label=above:S\textsubscript{1}] { };
  \node[main] (S2) [right=of S1, label=above:S\textsubscript{2}] { };
  \node[main] (S3) [right=of S2, label=above:S\textsubscript{3}] { };
  \node[main] (M) [below=of S1,below=of S2,below=of S3, label=below:mixer] { };
  \node[main] (D) [right=of M,label=below:D] {};
  \path (S1) edge [connect] (M)
        (S2) edge [connect] (M)
        (S3) edge [connect] (M)
        (M) edge [connect] (D);