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In Reply to: RE: Field coil dummy... posted by Alpha Al on December 05, 2016 at 12:43:47
In most vintage radios (consoles from the 1930s, for example) the field coils are indeed doing double duty as the magnet for the speaker and the choke for the power supply. The voltage may be as high as 300vdc so do beware. Sometimes the cone of the speaker gets damaged, in which case the suggested "keep the coil and use a permanent magnet speaker" approach works well. However, often the field coil itself fails due to aging and heat fatigue, or from failure modes in the radio/amplifier that caused high current draw. In that case you are kind of SOL. The coil in the field coil speaker cannot be easily repaired and I don't think anybody makes replacements.
Supposedly, field coils can create a stronger magnetic field than typical magnets, and therefore a better speaker. I have some doubt about this basic tenant though. A very powerful speaker magnet could be made with neodymium magnets, but I am not seeing such items in the marketplace.
Follow Ups:
You can only use so much magnetic strength before it becomes counter-productive. Qes values of less than 0.3 tend to choke off bass response, and even the original Alnico magnet D130 circa 1949 had a Qes of 0.27.
Neodymium magnet drivers have dominated the high end of pro-sound for over a decade. They aren't used to get lower Qes values than ceramic can provide, they're used to realize lighter weight and smaller magnet structures. They're also becoming popular in OEM auto sound drivers, for weight reduction. Most mainstream drivers still use ceramic, as it's cheaper than neo, mainly because the Chinese have a virtual monopoly and they're keeping the price up.
...check out the "Parts Express" website for starters. Lots of cone/dome type and compression drivers avail with Neo magnets. I'm using a pair of BMS Neo drivers on the 811s in my modified Altec Carmels (4552s). Physically small and very efficient although I think the ferrite version (4550) might sound better.
Steve
You may have seen it but you didn't,get over to Classic Audio Reproductions near Pontiac.John builds a set of Field coil speakers that are quite possibly the best dynamic speaker I've ever heard. They run about 70 grand a pair but they sounded like Quad ESL57s on steroids..They were that good.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
...driven by some Atmosphere OTL amps. Not my cuppa tea. YMMV
For me, they're the highlight of the Axpona show every year. And John is a great guy. If I had the $, I would have a pair.
He said last year may be his last Axpona; hope not.
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