Differential Stereo

I first heard about setting up a differential speaker from the liner notes of "On Land" by Brian Eno. I have included information and an explanation below, but beware, as by doing this you may damage your equipment, electrocute yourself or cause cows to jump in nearby fields. It is also frowned on by most audiophiles and HiFi purists, as the effect is unnatural. This is based on the 'Hafler Matrix' published by David Hafler in the mid 1960s. There is more information about more complicated Hafler matrices here.

A speaker of between 8-16 ohms is wired between the positive terminals of the left and right speaker outputs of the amplifier. The speaker is placed away from the main speakers, usually forming a triangle around the listener with the new differential speaker behind the listener. Basically, any sound which is placed in the centre of the stereo image will produce a voltage in both the left and the right speaker outputs. There will be no difference and therefore no sound through the differential speaker.

Any sound which is located in the left of the sound stage will produce a voltage in only the left speaker. Because the differential speaker is wired to the two positive speaker outputs of the amplifier, any differences in voltage to either speaker will pass through that circuit, and sound will come from the differential speaker (the differential speaker reacts to the differences between the left and right channel). The same is true for any sound that is only (or mainly) in the right channel. There will be some contraction of the width of the stereo stage from the front speakers, but this will not be noticeable generally, as long as the speaker used for the differential is not too low an impedance (most speakers are 8 ohms, which should be fine).

Most recordings place the vocal in the centre of the stereo image, so little or no vocal should come through the differential Channel. This system gives similar results to passive Dolby Surround, and can be used for stereo TV or Video as well. Some people find the effect inaccurate, but it is worth a try. Brian Eno records generally sound great using this effect, as do a lot or orchestral classical records (as long as you like to feel like you are in the centre of the orchestra!)

The front speakers should be placed equidistant from the listener at around a 45 - 70° angle. Place the differential speaker directly behind the listening position, roughly the same distance behind the listener as the front speakers are to the front.

I should also point out that setting up your speakers like this is not what most amplifiers are designed to cope with. Many will probably be fine, but it may damage your amplifier and will not be within the uses expected by your warranty. If in doubt, do not attempt.