|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
68.183.113.63
In Reply to: Re: Double loaders... posted by Paul Eizik on February 6, 2007 at 21:12:50:
Hi Paul,RCA did build a compound horn system for small theatre use in the late 1930s which closely resembled Figures 1,2,3. I've never seen one of these rarities in person, and don't know if the front horn actually contained the resonating chamber(s). Seems like a hip idea though to keep the front horn from honking below its cutoff, since there was no electrical crossover to limit low frequencies being fed to the driver.
The partial obstructions of the rear horn shown as #29 in Figure 9 are described in Olson's text as being embodied by 1) the reduction in cross section from chamber 27 to the beginning of the horn sections 19a and 19b, and 2) the reduction of cross section at 21 where the horn path travels through panel 11.
RCA built the design shown in Figures 5,6,7 as the 64A Monitor (late 1930s) and 64B Monitor (mid 1940s). The two models differed in external styling but seem to be the same internally. I have a pair of the 64Bs, and (not having studied the patent carefully) have been puzzled by the restrictions to the rear horn path. As built, the first obstruction is actually much more severe than indicated in the patent. A crosswise panel at the rear of chamber 27 leaves only a vertical slit barely wide enough to admit fingers at the point where the path splits in two. Perhaps this was an example of Olson's cleverness as you indicated, describing the function well enough for the patent but not tipping the competition as to the actual embodiment. The second obstruction is similar to the patent drawing, perhaps just a bit more severe than shown.
Follow Ups:
Hi SteveThe rareness of the theater version of the double loader speaks to the power of the Altec monopoly which you described so well in your legendary presentation at MAF '03. I recall you mentioning the monitors before. It's interesting how they seem to depart from the patent in the size of the connecting holes. Olson never mentioned anything about customizing the cabinets to a particular driver, something which is almost second nature to us today. Maybe this is the reason for the variations in the hole sizes in your monitors. I did a sketch of the backloader version scaled up to accomodate a 15'er for John K.. I also did a second sketch with a larger bass horn mouth and a somewhat different top manifold section. In the second sketch I made the holes larger (almost as a reflex), with the idea that they should be adjustable, but I did'nt give it any more thought untill these latest threads. I was fixated on area 27, the back/front chamber, We now usually evaluate this chamber solely in how it effects the highs, but Olson concieved it as having both acoustic capacitance and inductance, which becomes more important in a double loader. Olson also investigated multiple chambers connected in series in his Dynamical Analogies book which has recently become available as a download on the net (thanks for the link Freddiy!). It's interesting trying to probe the mind of the master.
Thanks for your input.
Hi Paul,I just reread your post and caught the part about the "...legendary presentation...." LOL! I guess the 10 or 12 people in the audience were also legendary attendees. The MAF 2003 was a mighty fine audio show, and we have the irrepressible Mike Baker to thank for that.
Hi Paul,Olson was indeed a genius; just ask RCA-fan (himself a bass horn master) who knew him! In many of Olson's writings I also detect the uncredited insights of Frank Massa, who collaborated with Olson at RCA all through the 1930s and refined the science of voice coil and transducer design to an amazing extent.
The 64 Monitor design was no doubt tailored to Olson's dual voice coil driver. These amazing 8" (6" cone) relics still define what is possible in a single wide range driver. At the last Oswald's Mill gathering, Jonathan Weiss had scored a 64A Monitor cabinet with altered exterior cosmetics for something like $35 on ebay, and had procured the correct RCA dual voice coil driver to install. The assembled audio crazies listened to that system in stunned silence except for the occasional "Wow." Though a bit limited in bandwidth in both extremes, these systems possess incredible resolving power and see-through clarity.
Olson's dual voice coil driver design can be studied in detail in his U.S. Patent #2,007,748. The dual voice coil was incorporated in the driver (can't remember the model #) used in the 64A, which otherwise was the same as the MI-1425A 8" cone driver used in most of the early 1930s RCA horn systems. By the 1940s there were modernized versions of the driver, MI-4410 (permanent magnet) and MI-4411 (field coil) which were fitted to the 64B monitor; either available at equal cost. Then the trail went cold, and to my knowledge Olson's dual voice coil driver has not been heard from since.
The 64B contained at least one acoustical improvement over the A, a sheet metal vaned high frequency diffusor. It is decribed in Olson's patent #2,102,212.
Until carefully reading the patent we discussed earlier, I assumed that the purpose of the two necked-down sections was to limit the output of the bass horn. The output of the horn does seem a bit timid in comparison to the driver's front output. Perhaps Olson did seek to limit the bass output a bit, as these monitors were listened to in the nearfield in small control rooms.
SteveAdd my "wow" to the 64 Monitor, and I have'nt even heard it! Some of the patents from the late 30's I ran across had Massa's name on them. The double loader design of 1936 pictured in Acoustical Engineering has both Olson and Massa's name, but the 1937 patent application lists just Olson. Bruce Edgar told us at a CHC meet once, that Massa resembled Larry Fine of the Three Stooges, and was prone to play practical jokes on the rather stiff Olson. The RCA R&D lab must have been a great place to work in the 30's. BTW, I have some vague memories of some German field coil designs from the late 30's with dual, or maybe even tripple, voice coils (Telefunken?). Thorsten Loesch posted some pics on one of these forums several years back, with similar comments that a pinacle of driver design had been reached, and then forgotten. I hope you guys do get to incorporate the dual VC in your FC designs.
Hi Paul,I seem to recall Thorsten posting pics and discussing the System Eckmiller 015 coaxial speakers of the 1930s. There is quite a bit of information on these drivers at the site below. Click on pictures and scroll down to category 2.4. Drivers. There are plenty of other fabulous cinema products documented on this site as well.
Steve,How was the signal fed to the Olson dual voice coil? Was it used like the Watkins woofer, where the second voice coil would kick in at the point where response started to fall off, thus assisting the first voice coil and extending the low end? Or were the two voice coils used to create a sort of servo for tighter control of driver, etc.?
I'm asking because Mr. Sano's Exact drivers -- at least the later ones -- all used dual voice coils (actually it's a bifilar winding so at first glance it appears to to be a single coil, until you notice that there are four leads) and he used them in conjunction with a passive circuit that not only extended the bass but enhanced the treble as well. Feastrex dropped that approach and went back to a conventional single voice coil, because they felt the gains achieved by Mr. Sano's approach came at the cost of imparting the "sonic signature" (or "veiling effects") of the passive components, somewhat akin to what one tends to get with passive crossover components in a multiway loudspeaker (versus using an electronic crossover or just a single driver).
Feastrex is getting good results with their conventional approach, but I often wonder if maybe it wouldn't be worth going back and taking another look at dual voice coils in a fullrange driver. Not necessarily the way Sano used them, but perhaps the way Olson used them. I'm quite curious to find out exactly what Olson had them doing.
When Feastrex parted ways with Mr. Sano, it was definitely the right decision for them to drop the dual voice coils. They were going through the "reinventing the wheel" process that every transducer-building acolyte has to go through, and eliminating that variable probably made their life a lot simpler. But now that they have thoroughly debugged their initial product line, as soon as their balance sheet stabilizes I hope they'll go back and take a second look at that. As you know well Steve, there are too many gems that have been consigned to technology's trash heap without good reason!
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.
This post is made possible by the generous support of people like you and our sponsors: