FYI: JBL/Edgar Wooden Tractrix going on eBay tomorrow

[SOUNDJUNKIE]

Thanks Tom. I have some further questions for you if you wouldn't mind taking the time to straighten me out on a few things. If you recall, I was going to run the Altec 515's as direct radiators in a set of Jensen Imperials, but I kinda liked the Altec 828 cab idea instead(as the cabs were designed for them)until I stumbled upon this: (http://www.users.qwest.net/~ccfrye/index.htm). What do you think of this endevour? It's a bit slimmer(WAF you know) than the 828, and it looks like, based upon this calculator(http://www.melhuish.org/audio/exphypcalc.htm), that it would be a 40-45Hz radial horn? I was going to further modify the split by toeing the sidewalls outward slightly to prevent standing waves and extensively brace/damp them without the wooden crossmember. Do you think this is a better implementation of a short horn than the 110Hz loaded 828 with reflex to 40Hz or even the direct approach with even more reflex as in the Imperial?

This message has been edited by SOUNDJUNKIE on 05-13-2002 at 11:01 PM

[TBrennan]

SOUNDJUNKIE----Ah, the 210 Split. I like the idea. The horn will certainly go lower than the 828's, in the 70s a I recall (I've a book around somewhere that says how low those horns go but it's not handy). I've no idea of how the low bass below the horn cutoff would sound but I'm not to keen on the low bass from the 828-515B anyway. It certainly looks cool and oughta sound real good in it's range. I think it would probably benefit from a sub though as my 825-515s do though it probably would sound pretty good without a sub, I mean my Altecs sound good without the subs, they just sound better with them. As I recall the Imperial has no reflex action but is rear horn; the front of the driver direct-radiating and the rear into a folded horn. JBL rear horns got pretty solid down to about 50, dunno with the Imperial. So as usual no clear answers. :-)

[djk]

"your low frequency limit is set by the compression drivers resonance. " Only partly true.There is no point in having a horn cut-off lower than 1/2 the resonance times the Qts, fLC=(Fs*.5)*Qts .In fact the JBL 375/2440 have a fLC=160hz base on the taper rate of the throat from the diaphragm through the phase plug to the driver exit. Newer designs based on shallow ceramic and neo magnet strucures have higher cut-off frequencies.

[SOUNDJUNKIE]

Thanks Tom. My bad, Imperial = no reflex. I meant to say reflex off the back of the 210, as in the article. I really wasn't looking for any concrete answers, I try to save those for the formulas. I figured you had seen or heard something about the split somewhere along the line so I was just curious if your impressions of it were good. Can you hear the wood ripping from there?

[JLH]

Edster00,

The amps I have used in the past have run the full gambit. Ive done the 300B thing and the 2A3, 45, and 10. They all had their positives, but also had a common problem, noise from the directly heated filament. To my ears AC heaters sounded much better than any form of DC heating the filaments, but I could not tolerate the hum with my horn speakers. Then I began to experiment with the TV triode sweep tubes like the 6AH4, 6BL7, 6CK4, and 12B4. Once again, just according to my ears, I felt like I was able to make fewer compromises and get better sound with the IDHT sweep tubes. Right now I run a small SE 6AH4 direct reactance drive amp on the mid range horns, and a PP 6AH4 direct reactance drive amp on the bass horn. These are my favorite amps up to this time. Until I get the itch to just build something, I think Im done with my amps.

John Hasquin (JLH)

[JLH]

Djk,

Yep, I neither can imagine anyone ever wanting to run a compression driver down to half its resonance frequency. If you run a compression driver below Fs, you will pretty much just get distortion below that point. However, there is more to what I was trying to say. I was really just referring to tractrix mid range horns in particular. The lowest a tractrix horn will start to provide any efficiency is when the horn is half a wavelength. The actual flare only has a minimal affect on its low frequency reproduction capabilities. This can be seen in McBeans (sp?) horn program. Model a free space, or full size tractrix horn with a depth of quarter a wavelength and see how close it comes to the horns Fc. Now start reducing the throat area to lengthen the horn to half a wavelength and you will see it is pretty much making it to the horns Fc. I have several examples and experiments in which have proven this over and over again. One example is the TAD TD-2001 on a full size 300Hz tractrix. Even through the TD-2001s throat is on the order of a 100Hz exponential horn, the 300Hz tractrix horn only provides loading and efficiency to 400Hz. The depth of a full size 300Hz tractrix horn with a 1 throat is 17 ¼ deep. It so happens that 17 ¼ is half a wavelength at 391Hz. The next example is Bruce Edgars 350Hz tractrix horn he uses with the 375/2440 and TAD TD-4001 compression drivers. Note that even through Bruce calls this horn a 350Hz horn; in reality it is a 340Hz tractrix. He calls it a 350Hz horn because it is a rounder number and is more pleasing to refer to it as 350Hz than 340Hz. Anyway, even through the throat of the 375/2440 and TD-4001 is on the order of a 160Hz exponential horn, it will not go any lower than about 500Hz on the 340Hz full size tractrix horn. The reason why is as follows: The depth of a full size 340Hz tractrix horn with a 2 throat is 14 7/8 deep. This depth corresponds to half a wavelength at 454Hz respectively. I saw Bruce's LEAP print outs of this compression driver and horn combination at the Midwest Audio show in Lima. Just as I stated above, efficiency was nonexistent below 400Hz, but as soon as it reached 450Hz the horn started to turn on and by 500Hz it was up to the zero dB reference line on the graph. The last example is the Radian 475 compression driver mounted on a full size 380Hz tractrix horn. The depth of a full size 380Hz tractrix horn with a 1 throat is 11 13/16 deep. This is equal to half a wavelength at 562Hz. However, this combination is good to 535Hz because the Radian 475s resonance is centered at 530Hz and helps the horn just enough to squeeze out another 35Hz or so on the bottom end. This is no different than what we do with bass horns when we tune the rear chamber to resonate the driver at or just below the Fc of the horn to gain a little more bandwidth. I used the Radian 475s resonance to annul the throat reactance to get just a little bit below the half wavelength barrier of the tractrix horn. I have done this on numerous tractrix horns over the last three to four years and have been able to reproduce these exact types of results in every case. This is what I mean about tuning all my mid range horns to their specific driver. It is fine if you dont need the extra bandwidth for your particular design, you can just go with a smaller horn. However, the broader bandwidth we can exploit out of a single driver, the closer it brings us to the magic of point source sound reproduction.

John Hasquin (JLH)

[HDBRbuilder]

I have been considering glueing up some Honduran mahoghany, and making some copies of the LaScala midrange horn lens...I think they would be classy additions to a nice pair of LaScalas. If I pre-miter of the four pieces it will take to make each horn lens, and get a satisfactory fit prior to shaping the interior of the lens panels on a disc sander, I should be "good to go", as I see it. My question is this...does the thickness of the body of the horn make any difference?...In other words, can I leave them thick towards the throat (2" or so for each of the 4 panels)and have them about one inch thick at the mouth?

I spent over 5 years as a pattern-maker in a foundry...got plenty of experience in this sort of thing...can make the body thickness any thickness, but prefer more thickness for stability of the wood over time. I COULD just turn some bowls, but dont need that much vertical dispersion...and it would take alot more of that EXPENSIVE mahoghany to do it!!

Any thoughts here?

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This message has been edited by HDBRbuilder on 05-16-2002 at 11:15 AM