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In Reply to: RE: Field lines - the thumb rule posted by airtime on June 05, 2015 at 08:46:27
Some 25 years ago, a local audio shop had a pair of original B&W 801 Matrix speakers, and Jim the owner was experimenting with passive bi-amping. At one point, he was testing out the woofer section alone and heard this weird "pocka-pocka" sound coming from the unconnected midrange driver. He opened up the crossover and found that the main iron-core coil in the lowpass filter was mounted nose-to-butt with a cored coil in the bandpass filter, and the mutual inductance was sufficient for some of the signal going to the woofer to creep into the midrange circuit. B&W apparently got enough complaints from dealers and techs that they corrected the board layout in subsequent editions.
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I had a friend who produced a speaker with a passive crossover in a single box for both channels. The speaker had very large air core coils, one for each channel parallel to each other. Another friend took the coils out and hooked them up to long connectors and as the spacing between the coils increased we could hear the sound improve. Air core coils are the worst at this with their large external fields. Iron cores are not as bad.
Makes sense that air cores would emit a more diffuse magnetic field, while iron cores concentrate that field in the core. Another very good reason NOT to mount iron cores nose-to-butt the way B&W did.
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