The hafler surround method uses inverse phase to provide the surround
channel.  The speaker is connected out of phase between the two hot
leads of the left and right channels.  Make sure your amp does not have
bridged outputs as it does not work with this type of amp.  The circuits
costs one nothing but a power resistor and came out during the 4 channel
days.
--- PCBoard (R) v15.3/M 2
---------------