FYI:
Robert Harley -- Mon, 06/09/2008 - 13:17
From Anthony H. Cordesman:
The Hafler "quad" circuit is actually a derived rear channel that drives the rear speakers with out of phase data. There are some old Stereo Review and Audio articles on how to make your own unit, but they are hard to find. The Wikepedia suggests making a connection from the right-positive post on the power-amp to the positive post on the right rear speaker. Then running a single wire from the negative post of that speaker to the negative post on the left rear speaker. And finally, running a single wire from the left rear's postive post back to the left-positive post on the power-amp.
The resulting sound creates an out of phase signal in the rear, without a clear sense of direction wi. The circuit does not somehow extract ambience, alter the timbre, or have any delay effect. All it does is add a sense of "space" with some recordings -- although the effect can sometimes be weird. It is a good idea to listen to a given recording with the front channels off to know exactly what is coming out in any given case. Performance is also very much a matter of set up, and room effects.
The user should be aware that this is a passive circuit, and effectively adds a new form of load to the amplifier and stereo speakers. This interaction rarely has a critical impact on sound, but is scarcely what high end gear is designed for, and can sometimes present problems.
It is not a good idea to use an old Halfer box designed for much lower wattages and far less sophisticated speakers. Anyone who experiments with this should research the various options, and then make their own box as a do it yourself project with connectors and wire gauges suited to a modern stereo system. The user should also know that speaker efficiency match will be a problem and it may be necessary to add a L-pad(s) into the circuit to match speaker levels to get the best effect.
Quite frankly, the various ambience circuits in a good AVR modern preamp or receiver will sound far better, although any synthetic rear channel effect will always be a "midfi" cludge. The Lexicon AV preamps contain the best options for creating such rear channel data that I have yet heard, but even these are not my thing. It takes a surround recording to get anything approaching high end surround sound. Barring that, I'd stick with stereo.
Anthony H. Cordesman