Jensen A12 OB + Altec 802-8D Horn Project - diyAudio
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Old 29th January 2008, 08:34 PM   #1
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Default Jensen A12 OB + Altec 802-8D Horn Project

Hi All --

There's been a recent plague of open baffle + horn/waveguide threads, and here's what it's gotten me: a 2.5 way with the Jensen A12 12" field coil pair doing sub-1500hz and the Altec 802-8D/ AH!1000 covering from there up. Up until now, I've been listening to Fertin 20EX on the JELabs open baffle and been pretty content. But, as one sage put it, you can't ask a 8" 'chinless' widerange driver to do justice to large-scale music.

Click the image to open in full size.

I measured the theile/small #s for the Jensen A12 using WT2, and here is that data, matching up pretty well with what my expectations were going into the project. They roll off around 100Hz on my baffles, but I always assumed I'd need some form of sub/woofer so the trade-off of extension for sensitivity was expected/ hoped for. I don't yet have a measuring set-up, so I got Meniscus Audio to do a first pass on the freq. response and you can get that jpeg here, for what it's worth.

The Altecs came from the classifieds at AudioCircle (I got Bill at GPA to check them out), and the 1000hz conical horns are courtesy of Bill Woods at Acoustic Horn Co. I'm using the Altecs because, er, well since the Jensens are from circa. 1947 shouldn't the tweeters be old-school as well? I heard the conical horns at VTV in northern NJ a couple of years ago, and really liked them so I stuck with what I knew.

Right now, the crossover is (electrically) 2nd-order highpass + CD compensation and 3rd-order lowpass, around 1500Hz. The .5 woofer is a first-order at ~250Hz. Next up is to get some gear + ARTA and revisit some of the design decisions to-date. I only have one channel up and running as I'm still building a DC supply stout enough to handle all four drivers -- listening in mono when evaluating a new speaker is just brutal. It's still early, but I like the set-up very much. There's clearly some 'anomalies' to be worked out, but even as-is it's tough going backing to the now-seemingly 'undynamic' Fertins.

Regards,
John
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Old 30th January 2008, 12:53 AM   #2
CLS is offline CLS  Taiwan
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Beautiful! Congratulations!

I didn't see any bass (baffle) compensations or EQ, did I? Anything done on this?

Looking at this elegant baffle, I think of somewhere in this forum that fixing drivers by their magnets (instead of frames) and claimed good results. On this baffle, it seems not very difficult to convert to that style....

About the 8 inchers, I beleive they CAN, at least, deliver decent (if not thundery) bass to some extent. And you got 2 per side.

I got very good overall proportion by a single 8" fullranger per side in a small room, to some level. They give proper, satisfied sound on even big scale orchestra music (like the <Rhapsody in Blue> ), by some helps from bass EQ. In that small room, it's already quite loud. (Loud enough to bother neighbors nearby). But of course if you got a big room, that's another story.

Again, congratulations! It's beautiful!
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Old 30th January 2008, 12:57 AM   #3
limono is offline limono  United States
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Hi
Do you feel that Jensen has better midrange than fertin 20EX ?Also did you consider d'Appolito configuration with horn in the middle ? Regards L
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Old 30th January 2008, 02:49 AM   #4
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Quote:
Originally posted by CLS
Beautiful! Congratulations!

I didn't see any bass (baffle) compensations or EQ, did I? Anything done on this?

Looking at this elegant baffle, I think of somewhere in this forum that fixing drivers by their magnets (instead of frames) and claimed good results. On this baffle, it seems not very difficult to convert to that style....

About the 8 inchers, I beleive they CAN, at least, deliver decent (if not thundery) bass to some extent. And you got 2 per side.

I got very good overall proportion by a single 8" fullranger per side in a small room, to some level. They give proper, satisfied sound on even big scale orchestra music (like the <Rhapsody in Blue> ), by some helps from bass EQ. In that small room, it's already quite loud. (Loud enough to bother neighbors nearby). But of course if you got a big room, that's another story.

Again, congratulations! It's beautiful!
Thanks for the compliment; very much appreciated.

The second woofer helps compensate for the baffle roll-off. Without it, I'd expect the speakers would start rolling off at ~200Hz.

I might look into baffle decoupling, but more likely once I've got the form factor to something I like I'll just go heavy and relatively less resonant, building with something like 5/4 maple.

These actually don't reach any lower than the Fertin 20EX -- only to 80-100Hz. I wanted to try the large diameter midbass/ compression driver + horn for reasons of distortion, not bass extension. For example, Sonny Clark sounds just great on the Fertin OBs, but my favorite local band Marah not so much so. The difference in the ability to handle complex music between the two setups is readily apparent -- it simply can not be that one 8" audiophile driver is as distortion-free as two 12" prosound drivers.

That said, they are any number of people, whose opinions I hold in high regard, that are unwilling to forgo the sheer coherency of a widerange driver for the sake of wider dynamic range. And likewise there is an equally vocal crowd that will not sacrifice realistic dynamics, no matter the complexity and sonic penalty for mixing/ matching disparate drivers. Each must make their choice...

Quote:
Originally posted by limono
Hi
Do you feel that Jensen has better midrange than fertin 20EX ?Also did you consider d'Appolito configuration with horn in the middle ? Regards L
I'd say its a push. The Jensens/Altec are so dynamic and realistic, but the point source advantage of the Fertin is pretty strong. I will say that certainly the compression driver + horn makes for a waaaaaaay better treble response - even if the Altecs are rolling off at 10kHz (and they do).

The second Jensen is there just to compensate for baffle roll-off, so I didn't think d'Appolito would be the better way to go. I did think about it, but this is my first multi-way project, and I don't need to make it any harder on myself than I already have.

Regards,
John
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Old 30th January 2008, 04:22 AM   #5
limono is offline limono  United States
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'I did think about it, but this is my first multi-way project, and I don't need to make it any harder on myself than I already have.'

Perfecly understood and yes , good horns are wonderfull .Your baffles look tasty !
I will be cutting wood tomorrow for Visaton NoBox baffle with one Jensen FC (midbass) and Lowther DX4 .Thanks for speakers data in the table -seems like 98 dB Lowther should match Jensen nicely.
Are you going to add tweeter to your altecs ?? HE ribbon-perhaps?
Regards, L
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Old 30th January 2008, 12:04 PM   #6
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Hey L --

I'll probably do something about the highs, but I don't know what. Options are: adding a supertweeter, goosing up the high-end with EQ, or trying some modern compression drivers -- I'm toying with ordering 1 ea. of BMS 4550, Beyma CP380M, B&C DE250, and Radian 475PB once I get some way of measuring them. I'm curious to see how the current production compression drivers differ from vintage as much as anything else, and I think it would be nice to have that comparison 'out there'...

Best of luck with your project. I haven't heard any Lowthers on OB, but they do have a good reputation as long as you relieve them of <200Hz. Will you be using a subwoofer to cover where the Jensens leave off? I think that no matter the baffle size/ eq applied, Fs is still only ~60-70Hz and the lowest octave and a half will be MIA regardless.

Regards,
John
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Old 30th January 2008, 12:32 PM   #7
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What's nice about field coils (among other things like liquid sound) is you can adjust the QTS on the fly with current changes at the coil.
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Old 30th January 2008, 02:10 PM   #8
limono is offline limono  United States
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John
I believe we've met at AA discusing Jensen in OB. I will use my Klipshorns in Bass duty . Below 100 Hz they probably have ~95-98dB so I might get away with passive crossover without too much padding or EQ just 6dB slopes. I don't have much experience with modern CD's but Jesens own RP-103 horn tweeters are sweet sounding and not very expensive (they probably die ~15Khz too) I like EV T350 a lot. Also I heard good things about Fountek 5iPro 102dB true ribbon ($600;(
Do you think 2A3 for Jensens's midbass would be enough ?
Regards, L
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Old 30th January 2008, 03:02 PM   #9
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Quote:
Originally posted by Magnetar
What's nice about field coils (among other things like liquid sound) is you can adjust the QTS on the fly with current changes at the coil.
Absolutely. I have my DC supply hooked up to a variac at the moment, but once the beefier one is up and running I'll be sticking with ~280Vdc. I think that works out to be about 16W dissipation for the field coils.

Quote:
Originally posted by limono
John
I believe we've met at AA discusing Jensen in OB. I will use my Klipshorns in Bass duty . Below 100 Hz they probably have ~95-98dB so I might get away with passive crossover without too much padding or EQ just 6dB slopes. I don't have much experience with modern CD's but Jesens own RP-103 horn tweeters are sweet sounding and not very expensive (they probably die ~15Khz too) I like EV T350 a lot. Also I heard good things about Fountek 5iPro 102dB true ribbon ($600;(
Do you think 2A3 for Jensens's midbass would be enough ?
Regards, L
For me, 2A3 would be plenty. I have a small room and I listen @ 75-80dB on my RS meter. Sounds like a very interesting project. If you think of it, shoot me an email once you get it up and running; I'd very much like to hear your impressions.

Regards,
John
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