alternative to the_content filter

In functions.php:

function myExcerpt($text) {
    $text = str_replace(']]>', ']]>', $text);
    $text = strip_tags($text);
    $excerpt_length = 50; // max number of words
    $words = explode(' ', $text, $excerpt_length + 1);

    array_pop($words);
    array_push($words, '-N');
    $text = implode(' ', $words);
    return $text;
}

In your loops:

echo myExcerpt ( get_the_content() );