Film-Tech Forum ARCHIVE: Info for Altec Lansing Horn 1505B

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Posted by Lindsay Morris (Member # 1430) on 10-21-2007, 04:12 AM:

I am looking at a couple of Altec Lansing multicellur horn units type 1505 B equipped with drivers which look the same but one is branded Altec 291-18A and then another white paper label alongside says 291 8A. The other driver just has a white paper label 288 8A. Anyone have any ideas or info if these are the same type with similar response and what crossover freq do they work at?
I have cruised the Altec site but nothing there bobs up and bites me which is not surprising as these must be at least 20years old.
Steve Guttag or anyone else shed some light please.
Lindsay

Posted by Louis Bornwasser (Member # 3063) on 10-21-2007, 09:35 AM:

The older Altec stuff is 500 hz crossover, the 288 is 8 ohm. The other one is too old for me, but, if it looks similar, it probably is similar.

BTW: voice coils are still easy to get here in the US. I have a drawer full myself. Louis

Posted by Dave Macaulay (Member # 813) on 10-22-2007, 12:04 AM:

The 291 drivers had several diaphragms available with different impedances, and different materials appeared as Altec developed them. The magnet structure stayed pretty much the same, I think the phase plug changed to a tangerine type sometime late in production but I'm not sure of it. It's common to replace a higher impedance diaphragm with an 8 ohm one when bi-amping the system, so the label is not trutworthy, measure the DC resistance to see what's inside the case.
You should inpect the diaphragms, the older metal ones like to shatter when driven hard and sound pretty nasty even though the coil is fine. Check the little connector wires between the coil and the mounting ring: look for a smooth arch shape. They develop sharp bends just before breaking.
Be careful when removing the rear cap, if a screw gets loose when you lift it off it will probably rocket towards the magnet gap... destroying the diaphragm.
If you have to remove or replace a diaphragm don't use a multi-tip screwdriver... the magnetic field is ridiculous at the mounting screws and will pull the tip out and rrrrip! Even with a normal screwdriver, hold the shaft firmly with one hand while working so close to the gap.

Posted by Louis Bornwasser (Member # 3063) on 10-22-2007, 08:40 AM:

Dave: good advice!

Also: impedance is not too critical on the h.f. driver. The most noticeable effect is to shift the crossover frequency slightly because of slight gain changes. I have successfully used anything from 8 to 24 ohms in a pinch, along with retuning.

You may know that in some applications, the passive crossover (n500) does not have quite enough loss to balance at crossover. Using a higher impedance would reduce slightly the h.f. level and allow balance without resorting to external resistors like Cinerama did. Louis

Posted by Steve Guttag (Member # 268) on 10-22-2007, 04:10 PM:

The 288 came in several forms thoughout its life...ending with the 288L. The 288 was offered in 8 - 32 Ohms (288D was 32-Ohms). The most popular theatre drives were the 288C (and 288B which also had a Simplex version) at 24-Ohms. All of the 288s used aluminum diaphragms and as Louis said, the various impedances could all fit in those drivers though one needs a 288C or later to use current diaphragms (longer voice coil formers).

The 291 is essentially the same driver but they made a number change to denote it is supposed to have a Symbiotik diaphragm (aluminum with a kapton support)...it will have less HF response but more power handling. That was the reason to denote it as a different driver...however it can take a 288 (or 299) diaphragm and have the better HF response.

The 1505 part of the system is the horn which reads that it has 15-cells (5 across, 3 tall) and a 500 Hz crossover. The last digit denotes lowest usable crossover point in a multicell. On a sectorial, the first digit is the lowest crossover point.

Steve

Posted by Lindsay Morris (Member # 1430) on 10-24-2007, 04:49 AM:

Thanks guys,

Info much appreciated... [thumbsup]

both of these are 8 ohm but as Steve said the 291 has a different looking dome/voice coil assembly.

Both sound very similar but then I am only feeding them with a modest power level in my test setup.
Any ideas on what power handling the things can take... they came out of a bi amped setup and another driver I opened up had a totally shattered dome and VC... I left that one there.
Had a fairly beefy amp driving it.

Lindsay