Altec 291-16L diapragms (21531 and 25885)

[Todd W. White]

You should be using 290-series drivers for something like this.

BUT, since you already have the 291's - let's see what we can do....

What are you doing to limit the frequency range of the amplifier? I know you are sending a 1500 Hz blast through them, but is the sound signal band limited? You should use a passive filter prior to the signal being sent to the amplifier to band limit the upper and lower harmonics of the amplifier.

Sounds to me like the amplifier is doing its job, but the harmonics are eating you alive.

P.S.: Oh, and you WON'T need beryllium diaphragms for this appication....

<font color="#FFFFFF" size="1">[ February 01, 2003 10:38 AM: Message edited by: Todd W. White ]</font>

[Tom Bellinger]

The signal is band limited prior to going to the amp. Is it possible that the damage I get is beacuse I have reversed the input leads or installed the diaphragm 180 degrees rotated from where it shoule be? That is... I may have hooked the wires inside the driver cover to the wrong post.I don't remember how it was orginally. I'm constructing it with the limited pictures I have. Would a small gap in the driver cover gasket cause this? Need input on remag as well.

[Jim D]

What is your transmission to the air, a horn, plane wave tube,---? I wonder if you have a proper load to the driver. Were these drivers used in this situation before? Let me know, this is a fascinating application. Jim

[Murray Keidge]

Hello again,
I have looked at your set up & most significantly the way in which the 25885 failed. I have a couple of points to consider.

Firstly, Todd is correct in advising that a 290 would be the best driver for your application. I have had many of them in service for over 20 years and only ever had one fail, they are extremly robust, reliable and perfectly suited to your set-up.
However, the 290 diaphragm does not fit the 291 driver, so you cannot change them without replacing the complete driver.

Secondly, in my experience, the 21531 diaphragm is much more robust than the 25885. The sonic quaility is not as good, but in your unique application a proven performer.

The damage to the 25885 described as "flakes" is very common in my experience. I have had this problem on 25885 and 25884 diaphragms on many occassions and it appears to be a fatigue problem in the forming of the dome.

The problem becomes obvious when the diaphragm is working at it's limit. Bill at GPA and I have spoken about this and he has modifed the manufacture process. Since then I have noticed a significant improvement in their reliability.

However, I would suggest that you continue to use the 21531 diaphragms and I think you will find your problem is solved. You have proven that the 21531 has worked well for many years so it would be best to continue with the diaphragm that works.

If you cannot find any 21531 diaphragms, please call as I do have some surplus to my needs.
Thanks, Murray.

[Tom Bellinger]

Thanks for all your wonderful replies. Much for me to think about.
Here is my dilemma. The 21531 diaphram has been a reliable performer for my application but it is longer available. It seems to me I have several opptions:
1) I suppose I could try to hoard as many of the 21531's that I can find and pay a few bucks for them. I'm not sure that there are oddles of them out there.
2) Murray indicated that Bill at GPA has changed the manufacturing process of the 25885. Getting a hold of a few of these newer 25885's may be an option. I bought my small stock of the 25885's back in 2000. Has the process changed since then?
3) A couple of you have noted that the 290 driver is really best suited for my application. I agree. I would love to get into the phenolic diaphragms. If I go down the 290 driver road, are there 290 drivers available for sale somewhere? Which one 290-G,H,J,or L? Do they make decent replacement diaphragms for the 290's?
4) What if I just went out and got the newest avaliable driver (ie. P Audio, Radian, RCF, etc.) What would you get for this application?

Any help here?
Thanks in advance.

[Todd W. White]

If Bill @ GPA doesn't have any NOS Altec 290's left, I think I know where you can get some.

How many do you need?

<font color="#FFFFFF" size="1">[ February 03, 2003 10:54 AM: Message edited by: Todd W. White ]</font>

[Tom Bellinger]

I would need three and possibly a backup. what do you think these would cost?

[Steve Burger]

We may soon be coming into some 290's. If so you might want to think about them. We could probably work a trade.

[marcuswilson]

When the 299 driver came out I got 2 of the new pascalite diaphragms. They lasted a very short time before the diaphragm disintegrated, voice coils still working. I went back to the symbiotic types, with no repeat of the disintegrating diaphragm, just breaking lead in wires.

[Murray Keidge]

Hello again,
With regard the 21531 diaphragms, I do not think that Bill is looking at making them in the short term, but you should check with him to confirm that.
Most people now use the 299 diaphragm to replace the 291 & the 288, however in your application I would still suggest the 291 over the 299.

I do have some 291 diaphragms available if you need them. If you have only lost one in 12 years then the possible shortage of supply may not be such a problem.

I know that Bill has some 290 diaphragms in stock, and is intending to make more. If you get stuck I also have a number of them available in the short term.

I do not think that in the long term you will have a problem finding 290 drivers and supply of diaphragms. I am certain that with some looking around you will find some in the US as there have been lots installed, and sadly removed, over the years.

With regard the 25885's, it was after 2000 that Bill changed the process so I would say yours are the "old" type.
However, even knowing that Bill has made significant improvements, I would suggest that because of your application, the 21531 will still outlast the 25885 due to the different construction of the diaphragms.

Murray.