Everyone knows the balanced output circuit that acts like a transformer. If you ground one output pin, the level in the other rises by 6dB so that the total output level remains the same. I saw the circuit first time in HP Journal a long time ago. I don't know if it is invented by HP engineers, probably not.
HP Journal, August 1980 (pages 12-13)
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1980-08.pdf
In addition to the original HP circuit, I have noticed that a version without resistor R4 and R9 is quite popular too. You can also drive it differentially (feed inverted signal to the points marked with ground symbol).
Studer has their own, relative complex version (page 11 upper left corner).
ftp://ftp.studer.ch/Public/Products/Mixing_Digital/Analog_Mixer_Assemblies/Technical_Info/1911220/1911220.pdf
I have used this circuit only once. It turned out to be unstable despite carefull layout. I have also seen several commercial audio equipment (using this circuit) which start to oscillate when driving long lines. I did some Spice simulations today and even the simulations oscillated at high frequency.
The circuit is so widely used that there must some trick to make it work properly...
HP Journal, August 1980 (pages 12-13)
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1980-08.pdf
In addition to the original HP circuit, I have noticed that a version without resistor R4 and R9 is quite popular too. You can also drive it differentially (feed inverted signal to the points marked with ground symbol).
Studer has their own, relative complex version (page 11 upper left corner).
ftp://ftp.studer.ch/Public/Products/Mixing_Digital/Analog_Mixer_Assemblies/Technical_Info/1911220/1911220.pdf
I have used this circuit only once. It turned out to be unstable despite carefull layout. I have also seen several commercial audio equipment (using this circuit) which start to oscillate when driving long lines. I did some Spice simulations today and even the simulations oscillated at high frequency.
The circuit is so widely used that there must some trick to make it work properly...