Information article and example circuits for both input and output.
http://www.dself.dsl.pipex.com/ampins/balanced/balanced.htm
http://home.btconnect.com/woodgreen/tech_tips/techtip/art11.htm
http://sound.westhost.com/project51.htm
transformer balanced input and output with amplifier in between.
http://www.opamplabs.com/425brln.htm
for Maximum Common-Mode Rejection in Real World Environments.
http://www.jensen-transformers.com/an/ingenaes.pdf
http://www.jensen-transformers.com/as/as052.pdf
http://www.jensen-transformers.com/as/as078.pdf
for Silicon Graphics Workstations - balancing using transformers.
http://www.jensen-transformers.com/as/as075.pdf