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Old 30th October 2012, 01:20 PM   #8311
EarlK is offline EarlK  Canada
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Quote:
Originally Posted by krzys
<snip> A broad hump in the medium with a nasty drop at 3700 Hz. Does anybody sees any explanation? <snip>
- The explanation could be as simple as a poorly fitted diaphragm ( on your GPA 388 ) .
- Here's the distortion ( & FR ) plot of a poorly fitted ( new GPA ) diaphragm ( belonging to Pano ) put into one of his 291/288 driver bodies .

Click the image to open in full size.

More ( from Pano ) ;

Click the image to open in full size.

Notice the simularity to your ;

Click the image to open in full size.


- I recommend you remeasure the distortion content ( using STEPS, HolmImpluse or REW ) & then look in the 3.7Khz area.
- If H3 has climbed as high as H2 in the region , then your diaphragm is poorly fitted .
- If that's the case, simply refit/realign it .

H2 vs H3 should look something like this ( when the diaphragms are fitted properly ) ;

Click the image to open in full size.

or this

Click the image to open in full size.



Quote:
Originally Posted by Lynn Olson
<snip> Next, maybe the Altec is just plain defective. There were some years when Altec's quality control was pretty bad. You can usefully cross-check this by getting a pair of Radian 745 (I recommend the Neo) drivers - they're not that expensive from prosound vendors. <snip>
- He doesn't have an old Altec driver, it's a newer ( "pan-cake body" ) 388 made by GPA .

<> EarlK
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Old 30th October 2012, 01:24 PM   #8312
Pano is offline Pano  United States
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Earl, I see you also have that sharp dip circa 8300Hz, just like my 288-16K. Still haven't figured out what that is.
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Old 30th October 2012, 01:42 PM   #8313
EarlK is offline EarlK  Canada
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Pano,

- My plot (5th pic ) has a pretty small dip at 8K ( coinciding with a similar reduction in H2, fwiw ) / leading me to believe it is a mechanical phase cancellation ( of some sort ) .

- The (4th) pic with the 8300 hz suckout belongs to Coen , of Why-Do-My-288-8Ks-Sound-Like-Cr@p ? fame, ( sorry I wasn't a bit more clear on that ) .

<> EarlK
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Old 30th October 2012, 02:45 PM   #8314
Pano is offline Pano  United States
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Thanks Earl. I'll have to look back at my 288 measurements on a different horn, but I think the dip is always there. You see a little on the Emilar horn, but not nearly what I get. The wavelength is just about the same as the distance from the diaphragm to the exit of the driver throat.
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Old 30th October 2012, 03:08 PM   #8315
GM is offline GM  United States
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If it's like with the small format drivers, it, and the peaks/dips above it are all due to the discontinuity created by the driver/horn junction causing reflections back to the driver's internal horn throat. I smoothed out/extended the HF response of a pair of 802D/511 combos by replacing the cardboard spacer with a metal one sealed with wax paper gaskets, then align honed the assembly.

The downside of course is that this would have to be redone each time a different horn was used if you wanted to maintain this level of HF performance.

GM
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Old 30th October 2012, 03:16 PM   #8316
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Lynn ... can I get some recommendations for resistive slot ports on my new boxes. I see two choices for area, a % of the cone area or use a standard speaker program (Win ISD or UniBox) to get close. Length is the problem with slots as they are harder to adjust than typical round ports. There is also the Onken phenomena with the ports hugging a wall. I noticed the port area on my Jensen Ultraflexs is just about equal to the 12" Sd in the original design and are very short. I'm not sure if Cyr-Marc's Onken spreadsheet would be of much help.
Thanks, Zene
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Old 30th October 2012, 05:09 PM   #8317
krzys is online now krzys  Canada
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Hi Lynn, Earl K, Pano and others, thank you for your responses and help. Indeed, I bought two GPA 388 in April 2010. Both measure the same so I thought that it is normal with the aluminum membrane (part number 23763) but I got frustrated when I read that 288 measures flat on Azura horns. I have chosen GPA driver because I read some negative impressions about the SQ of Radian drivers.
My horns are Autotech JMLC 350 made from fibreglass with very smooth throat, 8 deg entrance angle, steel mounting flange and 388 have neoprene soft gasket. I think that I can conclude that neither the horn nor the gaskets are the problem.
Searching in the tread I found that we had the same discussion in December 2009, page 646 posts from 7th, post 6452.
I then must conclude that GPA drivers are not reliable and could not be recommended (or that the response isn’t flat at all). I cannot accept that a driver paid 340$ need some work and realignment. I will open them and try to align the membranes and will report the result.

At GPA site I found this:
http://www.greatplainsaudio.com/LgDrvrDiaRepl.pdf
The instruction recommends using a spider shim but I don’t have this shim!!!!!

Insert the spider shim that was shipped
with the new diaphragm/voice coil into
the voice coil gap, as shown in Figure 5.
Figure 5. Spider Shim Installed Into Voice
Coil Gap
10. Carefully place the new diaphragm/voice
coil assembly onto the dowel locating
pins. DO NOT install the diaphragm hold down
screws at this time. The shim sets
the voice coil into the precise position
necessary for proper sound reproduction,
while the free locating pins move to
accommodate the slight adjustment
needed to properly align the entire
assembly.

Christophe
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Old 30th October 2012, 06:17 PM   #8318
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Why not contact Bill and have this discussion with him. Perhaps he would offer other solutions, although you have not posted response to compare to. I have some 288K coming, so I guess we will see about them.
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Old 30th October 2012, 07:34 PM   #8319
krzys is online now krzys  Canada
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Quote:
Originally Posted by buzzforb View Post
Why not contact Bill and have this discussion with him. Perhaps he would offer other solutions, although you have not posted response to compare to. I have some 288K coming, so I guess we will see about them.
The FR response of my drivers is attached in post 8304 and EarlK posted a fair number of responses in his post. The problematic FR seems to be recurrent to Altec/GPA drivers and I would expect that the problem has been addressed by the manufacturer
Chris
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Old 30th October 2012, 07:34 PM   #8320
Pano is offline Pano  United States
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Chistophe, as far as I know, the spider shims come only with new diaphragms so that you can align them when you install them. My 288s were 291s, I added the new diaphragms to them. Alas, mine did not have the locating dowels, so alignment was very tedious, even with the spider shims.

Your 388 drivers should have come aligned.
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