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Old 9th October 2006, 05:29 PM   #61
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Hi GM,

Most stuff easily available here is the modern pro stuff. JBL, Beyma, B&C, etc. Not much vintage stuff. I have been looking for a pair of jbl 2440's to play with.

Altec stuff hardly ever comes up. I am planning to import from JamminJersey when I'm ready to get the drivers. I bought my LE5 drivers from US ebay as they don't come up often here. Will probably get the 2440's at the same time.

Cheers,

Rob.
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Old 12th October 2006, 06:23 AM   #62
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Greets!

Try some used B&C DE250 (or new if they're cheap enough) and use the Altec 20472-2 or other's 1" i.d. x 0.71" thick high density cardboard gasket if available. Otherwise you'll have to make your own. Most folks either don't realize how critical a smooth driver/horn transition joint is required for best performance or don't care, figuring they can EQ it.

FWIW, the ones that I was forced to sell had close tolerance DIY metal shims, then the assemblies were align honed. Final assembly included waxed paper gaskets. There's a reason the original compression horn designers used no initial expansion in the driver and a flange collar joint to the horn.

GM
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Old 12th October 2006, 05:12 PM   #63
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Hi GM,

I should have mentioned I have a pair of Beyma 380 1" cd's. They are recomended to go down to 800Hz. I thought that modern drivers wouldn't get down to the 500Hz xo point that the old Altecs do?

Luckily my job is making press tools for cutting gaskets etc so that won't be a problem.

Cheers,

Rob.
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Old 12th October 2006, 09:20 PM   #64
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Greets!

Well, like most things, it depends. The 1" WE 755 that the original 1" Altec and JBL drivers are derived from went to 50 Hz, but it took a ~8 ft^2 mouth/15 ft pathlength and the same basic Altecs of yesteryear were only rated to 500 Hz, then increased to 800 Hz, then to 1 kHz in more recent times due to one thing, the same thing that keeps todays 1" from going lower, and that's the rear loading cap's Vb kept shrinking to allow the driver to handle increasingly more power as it became readily available with the trade-off being a higher Fc.

Either replace it with a larger Vb rear cover or gap the exisitng one as required and the typical 'humped' response flattens out/LF lifts up for a flatter response over a wider BW at the expense of LF power handling. No 'free lunch' and obviously not acceptable for PA since it drops to typically 30-35 W peak/500 Hz, but fine for all but the most extreme HIFI apps.

No personal experience with the Beyma, but these are all basically the same design concept, so I don't see why it wouldn't work unless you're either half deaf and/or one serious Punk Rock 'headbanger'. Anyway, since you have the parts and some DIY skills it'll give you another project to keep you busy during the Winter.

GM
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Old 12th October 2006, 09:36 PM   #65
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Hehe! Yep I'll give the beymas a go over Christmas. One more for the list. Any idea why that list keeps growing ?.


Rob.
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Old 15th October 2006, 05:14 AM   #66
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Recently I've discovered the value of back to back driver mounting which is used in the Lab horn to reduce box vibration. I discussed this in another thread:

dual driver vibration cancellation really does work!

I have a new design which uses both back to back mounting and push pull, like Wayne Parhams 12pi horn.

I think this new design is more interesting in terms of looks, not quite "form follows function," but more of a "form expresses function" design.

Click the image to open in full size.

The bottom part is an audio rack, with cutouts for source componets. On the right is the sealed chamber for the drivers with a port hole to show the driver. This is for two reasons:

1. I want to see the driver excursion for safety (probably not really necessary but I like to know)

2. I have a hard time giving up seeing these drivers - they look great

On the left hand side is another concealed rack, probably with pull out drawers. Audio stuff is kept from prying fingers, and its all quite neat.

The screen pulls down and you can see most of the horn. I'd probably build it in a few parts which can be assembled - perhaps three. I think this new design could be made more solid.

Now for some more detail. This is a section showing the push pull part:


Click the image to open in full size.

The driver section is based on an accurately measured model, although I had to guess the pole piece (and it's not likely I got it right).

Unlike most bass horns, I'd like to actually curve as shown for 2 reasons:

1. I like the look
2. Stiffness

I realise it could be faceted with no acoustic penality.
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Old 21st October 2006, 04:09 PM   #67
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Just out of curiosity, I decided to work out how many sealed subs using the same driver would be needed to match the output at 20 Hz. I was a little surprised. This horn with two 12" drivers (AV12) with an xmax of 23mm has a maximum SPL of 140 db @ 20 Hz. A rumble filter is required, otherwise the max SPL is only 135 db. 140 db only requires 800 watts.

To achieve the same output with the drivers in a sealed box, how many do I need and how much power? I need around 64 drivers! That is 147L of displacement!

The power required is a killer at 50 kw!

So the sealed sub requires around 60x the power and 64x the displacement. The size of the subs combined would be 2240L (approx 35L each which is quite small). In other words, the small sealed subs are around the same size but need more power and cost a lot more.

Let's look at it in terms of cost.

Drivers cost AU $320 each.
Behringer EP2500 ~$600
Material cost about the same so won't be included.

The horn version costs $1240 + cost of the box >>> $1,600

Sealed version

64 Drivers: $20,480
20 Amps: $12,000

>>> $33,000

In other words, the sealed version costs 20x as much!

Of course, the sealed version has one nice feature - a wall of subwoofers could look great!

Now I wonder why more don't think seriously about doing a bass horn.
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Old 21st October 2006, 05:24 PM   #68
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Thumbs up You're fun, but silly

I have four 18" woofers in two 15 cu.ft. enclosures in my great-room. The room measures 25 feet by 25 feet with 13 foot ceilings. I can literally shake the table cloth on a nearby table. During deep bass passages, the air around me feels as if I turned on a small fan! The first train scene in Polar Express brought about a great deal of excitement in my household!

I run a Behringer Europower 2500 with the Ultra-Curve EQ. There's about 600 watts available per channel with a four ohm load.

The woofers cost me $90.00 each from Mach 5 Audio. The amp cost me $300.00. The UltraCurve was around $140.00 on EBay. The plywood was free scraps from a job-site. Now that's a lot of entertainment for $800.00!!!

Your horn is a wonderful experiment, but unless you're loading a stadium, you're simply delighting your mind with an exciting, but self indulgent hobby. I'm all for exciting and self-indulgent hobbies!

John:
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Old 21st October 2006, 05:40 PM   #69
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Quote:
Originally posted by paulspencer

The bottom part is an audio rack, with cutouts for source componets. On the right is the sealed chamber for the drivers with a port hole to show the driver. This is for two reasons:

1. I want to see the driver excursion for safety (probably not really necessary but I like to know)

2. I have a hard time giving up seeing these drivers - they look great

Make sure you double check what pressure that glass is going to see. A horn decreases the swept volume required by increasing the pressure the cone must exert, right? It seems to me that if you see 140 dB at the horn mouth, you would probably get significantly higher (160+) inside that sealed chamber.

I'm sure you can figure out on your own how to keep the glass from breaking, but I just thought I'd chime in and say that I think it's an issue.
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Old 22nd October 2006, 03:07 AM   #70
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John there is something I like about extreme sub designs! I have seen those drivers you are talking about - are they musical?

I have ultracurve and EP2500 (great minds think alike). For this bass horn with a rumble filter running I can get 140 db @ 1m with just 400w from each channel, so in reality the fanless A500 amp would be more than enough. But I like the EP2500 so much I don't like the idea of getting rid of it and may even find another use, or put it in another room to get around fan noise.

In my current similar room (to the one I have in mind for this design), listening position SPL is about 9db down from the 1m SPL, so it follows the -6db/double distance rule perfectly, confirmed by measurements. I listen at 3m.

So to get 120 db I need 130 db @ 1m. That gives me 10db headroom, which is a nice number to have, and I doubt I'm crazy enough to go past that. My mains would need serious upgrading to keep up. I have found that I'm quite happy to have the subs 10db louder, so the mains would need to be 110 db @ 3m and 119 db @ 1m. They can squeeze out about 95 db @ 3m but at that they have lost their finesse so I'll need to get at least 15 db more output and lower distortion to pull it off.

One thing that is interesting about the newer version of hornresp is that it shows GD,and the plots are unlike any I've seen for a sealed or vented box. It goes all over the place - on average lower than a vented box but will NEGATIVE GD in places with big peaks and dips.
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