A Haffler circut is a simple way to extract difference/out of phase signals and send them to a rear speaker or two. Run a wire from the negative terminals of the left and right channel speaker outputs to one rear channel speaker (two if set up in parallel) If you have gain or volume controls on the rear channel power amp, use them to set the front/rear balance, by increasing or decreasing the rear channel level. With proper speaker positioning, the sound can be spooky real and with no processing.
Some vintage receviers and preamps have a mono channel out (a throwback to mono recordings- for those who had not yet bought into stereo at the time), in addition to left and right stereo. These three channels, when combined with a Haffler circut to set up the rears create a very credible five channel theater, with out any processing and entirely in the analog domain. Not a bad way to crash together a two channel rig with home theater, assuming proper speaker placement.
