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

[Altec Best]

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"THE ALTEC USER BOARD" IS "PRO GPA" ! FOR A REASON ! THEY PRODUCE ORIGINAL ALTEC PARTS !
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If I can count on the quality of GPA....

Coen

And you CAN ! I've been dealing with Bill/GPA for a longtime and have yet to have a problem.Oh wait a minute. I had one issue Bill forgot to add a few washers in a hardware kit I bought for some 288's,(Studs,nuts,washers,gaskets) so he sent me double for the mistake. GPA will stand behind any of their products.You won't find better for Altec parts or diaphragms anywhere else IMHO...

You seem to be doing a great job investigating the issue yourself !! Listen to your own opinion, it won't steer you in the wrong direction.

The best advice I can give you here is go with your instincts... Do your own investigation... It will be the best advice for your application !! Graphs will only get you so far (For Building Crossovers & Enclosures).In the end it is how it sounds to you that is important.

[Coendert2011]

@Vuki,

Thanks for the instruction. I guess GPA is driving the driver into serious distortion anyhow with 200Hz @2V unloaded. I am curious to the experience of moving the diaphragm around and hearing the distortion vary.

@Pano:

You are referring to the 'adjustable Dowel Locating Pins' wich can move a little to center the phragm and be clamped afterwards by a screw. Won't GPA be kind enough to deliver a pair of those for you to get the job done right for once and for all?

So I take it you have a set of permanently misaligned 288s. I saw no distortionlines in your measurementgraph with the 1005 horn (post 48). Does thie misalignment result in bad sound or in "could be better" sound.


@Altec Best

My experience so far has been excellent too. Your confidence is reassuring!

Coen

[Panomaniac]

If you ask GPA for the Dowel locating pins, they may be able to send some. They did not come with my GPA frams as they are supposed to be with the driver. Apparently, they tend to stick in the fram surround and get thrown away at change out time. That most have happened to mine before I got the drivers.

Although it's supposed to be easy to align by ear, I had a devil of a time with it. Sure wish I had the dowels. Maybe you have more finesse than I and will have no trouble.

[Earl K]

Below are Pano(s) distortion sweeps from a couple of years back for one of his ill-fitted 288 diaphragms from GPA ( one can see from the first pic [which is more recent] that he has achieved a somewhat better fitup compared to a couple of years back ) .



Above was made with STEPS , while below was made with HolmImpulse .
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vs his 291 diaphragm .





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[Panomaniac]

Thanks Earl, I could not find that 291 measurement file. At least you've found the chart! I see that the symbiotic also has the 8.3K hole.

[Coendert2011]

Earl,

For some rason I only see the top graph in Chrome and none of the graphs in Explorer.
Does anyone know what setting to change?

Coen

[Earl K]

For some rason I only see the top graph in Chrome and none of the graphs in Explorer.

Does anyone know what setting to change?

The bottom 4 pics are hosted over at diyAudio . I'm now thinking that one needs to be registered there to see them .

3 are .png files while the other is a .jpg / so I doubt it's a browser setting .



PS : The simplest solution ( I guess ) is to simply register at diyAudio ( unless Pano wants to retrieve the pics & then host them somewhere that will retain their original size ) .

[Coendert2011]

@ Earl, I will logon DIYaudio to get the cookie.

Just a little note from the centering activity of tonight.

The 288-Ks don't have the dowel pins. There are the two little bolts that have an oversized washer around them that is part of the diaphragm. I think the adea is to get audibly rid of the nasty resonance caused by the scraping of the voicecoil against the sides of the gap and then fix um.

I had a little rattle on one unit (the measured one) and the other is 'totally' clean. 200Hz at 2V works fine, you have a lot of movement in the diaphragm and any misalignment is easily observed (heard).
It was a hell of a job to get rid of the rattle, there is only one position that the rattle disappears (almost). I have the impression that the gap isn't evenly distributed, iow the phaseplug isn't totally in the centre. I have to measure to be shure.

Sorry no measurements yet of the recentered unit with the horn in place, next week more results in this theatre.

At last, I checked the other 802 and it exibited a higher distortion. After a lot of searching it turned out that the metal strips on the diaphragm (where the Voice Coil is soldered to) stuck out a little. I carfeully scraped the excess away with a watchmakers tool. Putting it back it didn't work out completely satisfactory. A definite improvement yet I discovered that I had to play with the pressure on the fixing screws to make distortion disappear. Thia was also the case with the better diaphragm of the other unit when inserted this one, so this is an imperfection of the unit itself (I should check the gap here too!).

A very instructive night!

Coen