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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: Steve, how was the signal fed to the Olson dual voice coil? posted by Christopher Witmer on February 8, 2007 at 01:11:52:
Hi Christopher,"...too many gems that have been consigned to technology's trash heap..."
You sure got that right. When it comes to the technology trash heap, I'm a dumpster diver!
Olson's dual voice coil driver was fed a normal audio signal, but contained an onboard crossover of sorts, partly electrical and partly mechanical. A careful reading of his patent # 2,007,748 will explain it pretty well, but I can add a few things.
These drivers had 8" baskets and a 6" thin, seamed, concentrically corrugated paper cone that was sprayed on both sides with a dull black compound resembling auto primer. RCA's literature states that the compound was applied to help the paper resist weather changes; I believe it has a damping effect as well, like the Aquaplas used by JBL. The outer suspension was leather or cloth, and the spider was in the center of the 1.5" voice coil assembly, located with a screw (like some of the finer compression drivers :) ).
The voice coil assembly is the heart of the driver. Two half shells of thick copper foil comprise the bobbin, like brake shoes, and do not quite touch each other. The bobbin halves contain a half roll compliance midway across their width- this can be seen in the patent drawing. Two voice coils are wound on these half shells; an aluminum coil close to the cone junction and a copper coil in the rear, on the other side of the compliance. The half roll acts as a mechanical crossover, isolating the two coils above 2.5kHz. The coils are wired in series with a center tap lead brought out along with the other two. The patent drawing shows four lead outs, but there are only three on the actual drivers. A pair of paralleled oil caps (2 mfd. total I think) is fastened to the rear of the driver, and is connected in parallel with the copper coil to keep the high frequencies directed to the aluminum coil.
So what we have is Olson's stellar design where both voice coils drive the cone at low frequencies and the aluminum coil drives the apex of the cone at high frequencies. The corrugated cone likely decouples to progressively smaller areas at higher frequencies, eliminating excess mass and preserving dispersion in one fell swoop. The dispersion of the highs is augmented in the 64B Monitor by the vaned diffusor, described in the second patent I mentioned above.
The response curve shown in the patent is about right. About three years ago Rich and I evaluated three different drivers on a small TL; a 4" aluminum cone Jordan, a 6" Fostex, and the Olson permanent magnet dual coil MI-4410. First we listened, then we measured. Subjectively we thought the RCA had the sweetest, most extended highs and best, most lively sound overall, followed by the Fostex then the relatively inefficient, lifeless, but still not bad sounding Jordan. When we made LMS measurements the RCA driver tanked above 8kHz., the Fostex was strong to about 12k and the little Jordan was almost ruler flat to 16 or 18kHz. Despite the measurements, I would much prefer the clean, lively RCA driver for a desert island system. I am still not sure exactly what is at work here, but I think the RCA obtains its response honestly, with no whizzer nasties or other breakup BS to gain h.f. level.
A back burner project is to tool up and produce this voice coil design again someday, perhaps fitting it to a 20 kilogauss field coil motor. If someone like Fostex (or Feastrex? Exact?) were to do it they would have a unique, world beating driver; a real Lowther killer IMO. If anyone does get around to building the Olson driver before Rich and I do I'll be the first in line to buy a pair.
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Follow Ups
- Re: Steve, how was the signal fed to the Olson dual voice coil? - Steve Schell 23:40:13 02/08/07 (0)
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