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High Efficiency Speaker Asylum Need speakers that can rock with just one watt? You found da place. |
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In Reply to: field coils who agrees with this? posted by setsound on July 2, 2011 at 07:25:29:
The following is an outline of my experiences with field coil speakers.
I firct encountered FC speakers in the 50's as a teenager trying to gain experience with radios and audio. I was able to get several old late 40's console radios from neighbor's garages for free. They all used FC speakers, usually 12" or 15" and came with a SW band and a 6 Watt push pull amp. The FC was used as a filter choke in the power supply with a hum bucking coil but always had some hum that would annouce that they were working. As soon as I figured out how this setup worked, I cut the FC from the basket (the cone or VC was shot) and connected the output transformer to a small alnico cone speaker. The sound always improved.
When FC speakers started to gain some recognition several years ago, I wondered why anyone would take up that old technology again, based upon my experiences in the 50's. Of course, people said that to me when I started to experiment with horns in the 80's. But then I heard Steve Schell's experiments with FC RCA compression drivers and changed my opinion. Then my driver guy, Sam, thought that he would try his hand in designing FC drivers. Sam went through many trials and errors but eventually worked up a complete set of modified JBL drivers for my Titan horn system. I have these drivers in my present Titan setup at home. John at Classic Audio is now manufacturing these drivers except for the 18".
The first experience was that FC drivers out trump many horn problems. In my Subwoofer horn, the bass sometimes was lacking any response below 35, depending whether you had a wood or concrete floor or lacked the proper boundary setup. My living room wood floor was a real problem for me compared to the concrete floor in my sound rooms at my industrial site. But when the 18" FC driver was installed instead of the JBL 2241 ferrite driver that I formally used, the bass low end went all the way to 20-25 Hz instead of wimping out at 35-40Hz with the ferrite driver. The low end of my midrange horns was also improved with FC compression divers.
The explanation for the differences in low end response characteristics is that horn low end response is limited by horn mouth size, flare frequency, horn flare type (i.e. hyperbolic, exponential, conical or tractrix) and reflections. Given that your design is designed well, reflections are the big restriction. Mouth or floor reflections can come back to the cone and try to push the cone back in spite of the input audio trying to push the cone forward, resulting in VC interactions and cancelations. The FC speaker has better control of the VC in the gap because of the stiff power supply and thus can resist the push back of refected waves.
Steve Schell gave me the genesis of the following explanation. A ferrite or alnico speaker magnet can be thought of a storage battery of magnetic energy. When a battery reaches the limit of its current capacity, it just gives up. In the case of a FC, a stiff DC power supply will look like an infinite capacity storage battery, i.e. a flashlight cell compared to a large marine storage battery. In the case of FC magnets the magnetic field in the gap is always in control of the VC and thus reflections can not cancel out the low end response.
You could improve the performance of regular loudspeakers with additional magnets, but manufacturers generally won't do it because magnets cost money and add to material costs. But I have observed exceptions. I have tried the early JBL Neo compression driver (I don't remember the model number) which used a stack of Ne0 slugs in its design. I didn't notice any difference in performance between it and my favorite 2441 driver. Several horn customers wanted to use their new JBL small "hockey puck) Neo compression drivers. I was able compare them to my standard 2441 driver in listening tests. The 2441 was clearly better. Part of the difference may be due to the minimal Neo magnet (which actually measured better than the 2441 in response tests).
The other example is the Fostex T-500 tweeter which people have been replacing the old Fane tweeter with in my Titan systems. It definitely sounds better than any horn tweeter on the market. But the horn is not any flare that I would design for a tweeter. The magnesium diaphragm may have some influence, but the real reason for its good sound is the double stacked alnico magnet.
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Follow Ups
- RE: field coils who agrees with this? - Bruce Edgar 13:09:52 07/04/11 (0)
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