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Old 30th October 2012, 07:38 PM   #8321
Pano is offline Pano  United States
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Quote:
Originally Posted by GM View Post
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.
I like this idea! At the moment I'm using 2 or 3 layers of aluminum foil for the gasket. Maybe the wax paper and different thicknesses could help.

Do you remember any of what you did to "hone" the FR response? Got any tips to get me started?
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Old 30th October 2012, 07:50 PM   #8322
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Quote:
Originally Posted by krzys View Post
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
Once again, just call him. Until you have spoken with him about the issue, you cannot say what has or can be done. I have seen the graphs and even viewed attached threads, and from what i have seen, some people seem to have solved the issue while others have not.
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Old 30th October 2012, 07:57 PM   #8323
EarlK is offline EarlK  Canada
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Quote:
Originally Posted by krzys
<snip> I will open them and try to align the membranes and will report the result.
- First ( before doing that ) check with GPA to see if you might void an existing warranty by opening them up .

- Second, look at your older captures of these drivers on your horns . You should ( if you kept them ) be able to simply turn on the distortion display ( & then compare what you have to the healthy driver specs I posted ) .
- If you need to, then make some new data captures ( of your drivers & horns ) .

- Third , if the distortion specs are out of whack ( & only if GPA says its OK ) then try aligning the diaphragms ( for better frequency response & lower distortion ) .

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Old 30th October 2012, 09:58 PM   #8324
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krzys - it looks a bit similar to the problem I had with one of my GPA 288H drivers, which was traced to faulty material on the pole-piece. I would have hoped this sort of problem was solved now, as I lost a considerable amount of money in repeat customs charges after sending one of them back from the UK to Oklahoma .....
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Old 30th October 2012, 10:25 PM   #8325
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Reported issues with the GPA drivers have me hesitant to start the project. It would be nice to know that the issues are resolved.

A round trip Australia to OKC for repair under warranty is almost as costly as the driver.

Last edited by jeff mai; 30th October 2012 at 10:30 PM.
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Old 30th October 2012, 10:32 PM   #8326
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Default Problems with GPA Drivers

I recently bought a pair of GPA 902 drivers. They work great!
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Old 31st October 2012, 02:42 AM   #8327
krzys is offline krzys  Canada
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Hi Earl K and others. I measured both GPA 388 drivers. I found that they are quite different in FR and distortion. One of them displays the distortion pattern of a poor fit example EarlK gave. The other one does not although both present similar but not identical FR. I don't know what to think about.

This once again raises the question of quality and consistency of those drivers. I am not very glad of that. I’m able to EQ them flat with 2/3 parametric filters but it isn’t the ideal situation. I must admit that maybe apart some occasional harshness on loud music I can not hear any major problem.

I contacted GPA and asked if they can send me a spider shim. We will see. I would like to be able to at least check the phragms alignment. This is doable although a very delicate operation. I once damaged a diaphragm on an 806 driver. Somebody knows where I can buy a stainless steel or aluminum screwdriver?
Christophe

Measurements where taken at the horn mouth. SPL is not calibrated. In order: SPL and distortion of both drivers, SPL overlay and group delay overlay
Attached Images
File Type: jpg gpa 388 no 1 distortion.jpg (309.5 KB, 897 views)
File Type: jpg gpa 388 no 2 distortion.jpg (292.0 KB, 653 views)
File Type: jpg gpa 388 spl.jpg (222.5 KB, 878 views)
File Type: jpg gpa 388 group delay.jpg (226.7 KB, 636 views)
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Old 31st October 2012, 06:52 AM   #8328
GM is offline GM  United States
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Quote:
Originally Posted by Pano View Post
Do you remember any of what you did to "hone" the FR response? Got any tips to get me started?
I didn't 'hone' the response per se, I align honed the driver, steel shim and horn entrance with a three stone brake master cylinder hone attached to an electric drill to make the transitions as seamless/smooth as practical. Then made a tapered wood alignment tool, added indexing marks on everything, so they could be taken apart, reassembled, i.e. a typical motor blueprinting type of mod.

I had easy access to a comprehensive metal fabricating shop at work, so finding a suitable steel shim might be a problem if you canít make one out of a fender washer or maybe alum. sheet.

There were other tweaks I did that helped in other BWs, grinding down/smoothing welds, rounding over corners, cutting out vane welds, glass beading to a high smoothness, mucho stiffening, damping, etc., but I was trying to make a 'silk purse out of a sow's ear' [Altec 511], just to see if I could and was very pleased with the results, though they were still too 'colored' for SOTA HIFI even back then, so moved on to scratch building horns, ultimately winding up with a huge conical WG that was about the same overall size as a 1803 multi-cell to get good loading down to 500 Hz.

The downside to this was a way too large an acoustical ctc spacing to the vertically oriented dual 15Ē bass horns, so at that point went back to the tweaked 511s plus added another pair of ~stock 511s [with inductors] stacked on top to get the big horn systemís Ďfullí mids.

GM
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Old 31st October 2012, 01:09 PM   #8329
EarlK is offline EarlK  Canada
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krzys,

Click the image to open in full size.

- For a "Pro" driver those variations are not out of line ( & sadly, still expected from the established names ) .

- I'm in Pro audio and don't consider $ 340.00 ( per compression driver ) as a ton of money / so my sympathies are pretty limited here .

- If you had wanted "matched" drivers ( for audio-phile purposes ), you should have offered GPA a premium price (per driver ) to go through their inventory to find two the same.

- I know GPA can match drivers ( when asked ), here are two of their 902s , as obtained by our dearly departed "Zilch" . IMO, these traces are wonderfully matched !

Click the image to open in full size.


Click the image to open in full size.

- I would work on getting the distortion content down for this driver ( fyi, from 500hz to 10K, the third harmonic should average @ 15db below H2 on the "full-body" 288 / the pancake-body 388 is anyones guess ) .

Click the image to open in full size.
Click the image to open in full size.

- I published these 2 blow-up pics ( of Brandons "overly small" FR studies ) almost 3 years ago to better highlight the variations in GPAs blind-pulls.
- This was before you bought your GPA 388s .

- So I'm now wondering where you got the idea that these "blind-pulls" from the GPA inventory would give you matched responses ? .

- I don't think it would have been from this post ! Please note my opening line .

- A close scrutiny of AugerPros measurements implied a diaphragm fit up problem for one driver ( indicated within his distortion findings ) from more than 3 years ago .
- Here are Brandon's original words of caution ( regarding distortion ) as found in the first measured GPA 288H .
- The data ( on his site ) for the first driver was removed & replaced by a second set of measurements ( I suppose after it was determined that the first driver with the 2K-4K "problem" was not a healthy specimen ).

<> EarlK
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Old 31st October 2012, 02:58 PM   #8330
GM is offline GM  United States
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Quote:
Originally Posted by EarlK View Post
- I'm in Pro audio and don't consider $ 340.00 ( per compression driver ) as a ton of money / so my sympathies are pretty limited here .
More than me, I have zero sympathy since inflation adjusted, their pricing is for the most part somewhat lower than what I paid way back when after a discount and by the mid '70s, Altec's QA had dropped to the point where I quit buying/recommending them except for replacements where cost effective.

WRT distortion testing, I've no experience or even considered it since I don't recall seeing anything published by Altec in either cut sheets or in tech bulletins beyond recommending that in any type of high power app the speaker's components be limited to a decade, implying that in any consumer app short of a frat party it was a non-issue, so wondering what the distortion looks like when the driver is BW limited 2nd order to 500-5k, etc..

GM
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