Ajax & Class Output

I'm trying to give data through an output class

PHP Code:

$this->output
  ->set_content_type('application/json', 'UTF-8')
  ->set_output(json_encode($my_array)); 


As a result I get problems with Ajax

Code:

VM1166:1 Uncaught SyntaxError: Unexpected token o in JSON at position 1
   at JSON.parse (<anonymous>)
   at Object.<anonymous> (main.js:260)
   at i (jquery.min.js:2)
   at Object.fireWith [as resolveWith] (jquery.min.js:2)
   at A (jquery.min.js:4)
   at XMLHttpRequest.<anonymous> (jquery.min.js:4)

If I do through the echo...

I do not understand where the pre