If you've got four speakers and don't want to lash out on a new surround sound system (or 'er indoors refuses to have any more AV equipment in her living room!), try this Hafler circuit.
I first found this in Everyday Electronics in 1975 (thanks guys!), and have been using various systems (once in the car) ever since.
It's not a 7.1 system, but it will add hugely to your enjoyment of stereo - and especially to DVD film soundtracks. Plus it doesn't matter where you sit in the room - you always hear stereo.
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Step 1: Warning
I have heard that this system can overload your amp.
I have been using this circuit on a range of hifi equipment for thirty years, and I've never had a problem. But continue at your own risk.
Step 2: Wiring
The diagram shows what you have to do.
Stick two new speakers in the corners of the room OPPOSITE your existing ones.
Identify the + outputs from the back of your amp (your existing speakers will already be connected to them).
Now add an extra wire to each, without disconnecting the existing speakers, and connect those two wires to the + terminals of your two new speakers.
Now connect the two negative (-) terminals on your new speakers together.
Now wind up Guns n Roses - or whatever it is you like.
Hurrah! Instant surround sound!
Step 3: How It Works
The two extra speakers actually produce exactly the same sounds as each other. They play the difference between the original stereo speakers.
In other words, anything that is in the middle of the stereo image is not reproduced by the extra speakers: this gives the impression of surround sound.
Try a DVD: the swoops and whooshes of an action film do actually sound as if they are moving around the room.
Respect to the great David Hafler who invented this system.