[LAU] balanced <--> unbalanced

[Chris Caudle]

On Tue, January 10, 2017 7:02 am, Ralf Mardorf wrote:
> in this German document [1] Sengpil [2] added a note for
>   balanced out ---> unbalanced in
> for electronically balanced out without galvanic isolation to ground. He
> mentioned to connect the + signal _and_ ground (not _with_ ground,
> just to prevent against a misunderstanding), but the - signal should be
> unconnected.

There are multiple circuits commonly used for symmetrical outputs, some
need the "-" signal left unconnected, some need the "-" signal connected
to the circuit ground, but the optimal location could be at the receive
end, or could be directly at the output connector.

The Sengpil approach would be required for circuits which just drive both
legs by similar op-amp output circuits with signals of opposing phase.

Some circuits attempt to mimic the behavior of a transformer by
cross-coupled feedback paths, such that you can terminate one leg to
circuit ground and the other leg should increase amplitude to keep the
total amplitude the same.
This paper presented at an Audio Engineering Society conference about 16
or 17 years ago describes some of those circuits and various problems:
http://www.thatcorp.com/datashts/AES5152_Improved_Balanced_Output_Driver.pdf

-- 
Chris Caudle