Why do my 288-8Ks sound like cr@p?

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Originally Posted by Earl K

Sheesh AB,

The newer series 288s include a 2-piece dowel system ( which includes a hollow over-sized sleeve & a small bolt that holds that sleeve in place ) .

It is not, a solid metal pin that can be simply replaced by a small piece of metal rod ( cut to length and stuck in the threaded hole as you & GM suggest ) .

BTW, Pano is missing the 2 over-sized sleeves that are snugged-up inside the 2 locator holes within the diaphragm & held there by the set-screw . ( Look at the picture I posted of Panos open-backed driver to see the set-screws ) .

The center "hole" in the sleeve is larger than the diameter of set-screw, essentially allowing the sleeve to "float" or slop around a bit ( when the set-screw is loose ). This "play" when used in conjunction with the spider shim is what allows for an accurate fitup in the new system .

http://www.hostboard.com/forums/hbmc.../2012/06/1.png


Coen's description of what he has seen in his 288s suggests he might have a fully functional ( adjustable dowel ) system ( compare to the pic above ) .

He simply needs to read the instructions on how to use it ( along with the included spider shims coming from GPA ) .

DownLoad & Read ;Altec Lg-Format Driver Diaphragm Replacement Instructions

:)

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I'm a bit shocked that Earl had to post this as i thought it was fairly common knowledge here.

I've never seen it, Never knew there was such a thing as adjustable locating pins. It does make great sense to do it that way.

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Originally Posted by Panomaniac

Well they were hesitant to send me some, so suggested I try the 200Hz tone. If that didn't work, I think they'll see some. I'd be happy to buy a set and pay all postage, so I'll let GPA know that. I really don't know if they have a bunch of them hanging around. I think they were afraid of charging me too much postage for a few little pins. :)

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I don't know why they would be hesitant to sell them to you.It makes it much harder to align them without them.Agree with the last part probable don't want to charge you too much.Just ask for a dozen make it worthwhile.

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Originally Posted by bowtie427ss

I'm a bit shocked that Earl had to post this as i thought it was fairly common knowledge here.

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And it is !!! When you don't have them in front of you it is easy to make a mistake..

I'm glad you guys zero'd in on the alignmentof the VC in the gap.
When I swapped the OEM frams out of my 904 duplex (symbiotic) to new GPA frams, the speaks just sounded bad. So I set up my spectrum analyzer with tracking gen and a mike and looked at the response. Huge suckouts. I put the original frams back in and they were gone. So just because this later model uses the od of the fram assembly to center, I tried loosenig the screws and guess what, suckouts gone! These speakers actually had a thin gasket between the fram and the hf driver body. Anyway, I ended up moving the fram around a bit and found the sweet spot and simply tightened the screws just enough to keep it that way.
It about drives you nuts listening to the thing sweep, but the results were worthwhile.
I am fortunate to have the test setup. a Tek 7L5 Sa withtracking Generator option. My mike is simply a Radio shack SLM with an output that feeds the SA. Not accurate for sure, but for this it worked perfectly.

Ron

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Originally Posted by RonSSS

I'm glad you guys zero'd in on the alignmentof the VC in the gap.
When I swapped the OEM frams out of my 904 duplex (symbiotic) to new GPA frams, the speaks just sounded bad. So I set up my spectrum analyzer with tracking gen and a mike and looked at the response. Huge suckouts. I put the original frams back in and they were gone. So just because this later model uses the od of the fram assembly to center, I tried loosenig the screws and guess what, suckouts gone! These speakers actually had a thin gasket between the fram and the hf driver body. Anyway, I ended up moving the fram around a bit and found the sweet spot and simply tightened the screws just enough to keep it that way.
It about drives you nuts listening to the thing sweep, but the results were worthwhile.
I am fortunate to have the test setup. a Tek 7L5 Sa withtracking Generator option. My mike is simply a Radio shack SLM with an output that feeds the SA. Not accurate for sure, but for this it worked perfectly.

Ron

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Hi Ronn,

This matches exactly my findings. For the small formats and 604's don't tighten the new GPAs!

Coen

[EDIT]

The first unit had the most problems with getting centered. As you may observe the other unit does not have the remnant H2 distortion arount 1KHz, but has a higher H3 distortion:

Right unit:
http://www.hostboard.com/forums/hbmc.../2012/06/2.png

Left Unit (better):
http://www.hostboard.com/forums/hbmc.../2012/06/3.png

Peaks and valleys are also due to the location of the microphone and reflections within the horn. I can see some move and disappear when I move the mic.

Hi All,

Here are some fresh results!

I've done a FR STEPS measurement on both 'aligned' 288-8K drivers. Unit1 is the one that has been the mother of all previous 288 graphs.

Unit1:
http://www.hostboard.com/forums/hbmc.../2012/06/4.png

Unit2:
http://www.hostboard.com/forums/hbmc.../2012/06/5.png

Looks much better now, with some rising distortion after 3kHz (mass loading?!). No Gremlins in here anymore.

Though the units match quite good on FR, distortion is similar with odd peaks on unit1:

Unit1:
http://www.hostboard.com/forums/hbmc.../2012/06/6.png

Unit2:
http://www.hostboard.com/forums/hbmc.../2012/06/7.png


Not much goin on here. Unfortunately :Sad no:.

I dug a little deeper. See the next post:

Coen

Ok,

Here's some more.

I have another hunch that the harshness may have to do with the 12k peak. For this purpose I made seperate FFTs of a 12.5kHz sine and ones with both a 12.5kHz and a 2kHz tone.

Unit1 @12.5kHz:
http://www.hostboard.com/forums/hbmc.../2012/06/8.png

The above gives rise to some elbrows. There is actually a new peak at 1/2 the fundamental, that is also echoed at the other side of the 12.5kHz. This one is not there in unit2, so is not related to the sound of -both- units. This must be some side effect of my imperfected centering capabilities.

Unit2:
http://www.hostboard.com/forums/hbmc.../2012/06/9.png

In the dual tone test this doesn't show up, I think because both tones are much lower in amplitude individually.

Unit1:
http://www.hostboard.com/forums/hbmc...2012/06/10.png

Unit2:
http://www.hostboard.com/forums/hbmc...2012/06/11.png

No real surprises here I'd say. With a lot of midrange in the music I can get a little crowded around the 12.5Khz though...

Well, I'm still waiting for the new GPA diaphragms. In the meanwhile I will do an impedance measurement. I might be that the highish output impedance of my favourite tube amps doesn't match well with the impedance peak likely to be associated with the 12.5KHz peak (amps increase output with rising impedance). Favourable for LF response, yet with an allready firm 12kHz bump, it can be too much on the HF.

Thanks all for the support. The story continues...

Coen

Coen, the 12kHz bump is what you get with the 288's - that's normal. The bump goes away off-axis, so the power response is usually quite smooth. I think that the response of your realigned 288's is very acceptable. I suspect that GPA diaphragms won't bring anything much better.

288-8k on Lecleach 450Hz horn
Attachment 1368

same with hf correction
Attachment 1369

at 30 deg off-axis
Attachment 1370

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Originally Posted by vuki

Coen, the 12kHz bump is what you get with the 288's - that's normal. The bump goes away off-axis, so the power response is usually quite smooth. I think that the response of your realigned 288's is very acceptable. I suspect that GPA diaphragms won't bring anything much better.

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Hi Vuki, maybe I was a little overconfident that the new diaphragms would cure the harshness and a little too fast in ordering for shure. Well they are paid for and on the road so let's wait and see for the final verdict.
Anyhow, even if these do not make the desired difference I really would like to proceed to get to the bottom of this and make them sing in my setup. Your off axis graph for instance makes a lot if sense to me.

I noticed the 12k bump in the other 288-k graphs before. Save for what the diaphragms may bring this is suspect nr 1 next on the list. Furthermore I'm interested in your opinion on the 288-8k, do you like them all theway up, or do you consider a seperate (super) tweeter like gm suggested?

Coen

I'm using them without supertweeter. They sound (to my ears) better than 902s on 511 or even tractrix. BTW those were GPA dias in the 288k's that were measured.
I suggest you to measure 902 on 511 with the crossover than the 288, keeping the exactly same measuring conditions (distance, mic placement, amp gain etc). Using ARTA's overlay function you'll probably find out more about sound difference between those drivers.