# Help drawing an Electronic System with tikz

I have this Code:

% Title: Block diagram of Third order noise shaper in Compact Disc Players
% Author: Ramón Jaramillo
\documentclass[border=10pt]{standalone}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}

\begin{document}
%Blocks
\tikzset{%
block/.style    = {draw, thick, rectangle, minimum height = 3em,
minimum width = 3em},
circle/.style      = {draw, circle, node distance = 2cm},
}

%Symbols
\renewcommand{\sum}{\Large$+$}
\newcommand{\cross}{\Large$\times$}

\begin{tikzpicture}[auto, node distance=2cm, >=triangle 45]
%nodes
\node [] (start) at (0,0) {};
\node [circle, right of=start] (plus) {\cross};
\node at (7,0)[] (end) {};
\node at (6.8,0)[] (send) {};
\node [block, below of=send] (T) {T};
%##### I don't know how to place the last node - it should be in the midlle between plus and send and 2x the distance from send to T.
%arrows
\draw[->] (start) -- (plus);
\draw[->] (plus) -- (end);
\draw[--] (send) -- (T);
\end{tikzpicture}
\end{document}


And it should look like this at the end:

But at the left side of the node (start) there should be the letter a, and a letter b at the right side of the node (end) and there should be a short horizontal arrow showing on the cross (which I couldn't draw).

Problems It didn't compile. And I don't know how to make the last note. And I don't know how to put the labels on. And I don't know how to make the arrows change direction.

Thank you very much in advance! Kind regards!

Something like that?

The main problem with your code is the redefinition of circle which calls circle. I replaced it by my circle. For the elements, I used absolute positioning which is not so elegant, but allows all degrees of freedom.

\documentclass[border=10pt]{standalone}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}

\begin{document}
%Blocks
\tikzset{%
block/.style    = {draw, thick, rectangle, minimum height = 3em,
minimum width = 3em},
my circle/.style   = {draw, circle, node distance = 2cm},
}

%Symbols
\renewcommand{\sum}{\Large$+$}
\newcommand{\cross}{\Large$\times$}

\begin{tikzpicture}[auto, node distance=2cm, >=triangle 45]
\node (start) at (0,0) {a};
\node[my circle] (plus) at (2,0) {\sum};
\node[my circle] (times) at (4,-2) {\cross};
\node[block] at (6.5,-1) (T) {T};
\node (end) at (8,0) {b};
\coordinate (send) at (6.5,0);
\draw[->] (start) -- (plus);
\draw[->] (plus) -- (end);
\draw (send) -- (T);
\draw[->] (T) |- (times);
\draw[->] (times) -| (plus);
\end{tikzpicture}

\end{document}