Yeah, well..
Hey, Mr. Olson, now that you got the woofers from GPA, you might convert them to THIS
Hey, Mr. Olson, now that you got the woofers from GPA, you might convert them to THIS
An externally hosted image should be here but it was not working when we last tested it.
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Your in Oklahoma...how far are you from Great Plains Audio? You could always potentially visit and listen to their setup(s).
Thanks Chris for the suggestion.
I live about 110 miles from Great plains Audio in OKC. But, I have not made it to their shop yet. I did hear the Iconic 704 at one of the Great Plains Audio Festivals that we had in Tulsa a few years back, and heard the GPA 604II implimented by Serious Stereo at RMAF 2007. I really like the midbass and lower midrange qualities of these coaxial drivers. But, I have yet to hear any of these from Altec, GPA, or Iconic that satifies my likes in the upper mids and treble region.
I do live about 25 miles from Pi Speakers in Tulsa. I know Wayne Parham, a great guy. And have had many listening experiences of his speakers.
I hope to one day make the trek to OKC and listen to the GPA system.
Not having the experience to DIY my own high efficiency speakers, or at least high quality. I have been looking forward to the outcome of this thread.
NW
I live about 110 miles from Great plains Audio in OKC. But, I have not made it to their shop yet. I did hear the Iconic 704 at one of the Great Plains Audio Festivals that we had in Tulsa a few years back, and heard the GPA 604II implimented by Serious Stereo at RMAF 2007. I really like the midbass and lower midrange qualities of these coaxial drivers. But, I have yet to hear any of these from Altec, GPA, or Iconic that satifies my likes in the upper mids and treble region.
I do live about 25 miles from Pi Speakers in Tulsa. I know Wayne Parham, a great guy. And have had many listening experiences of his speakers.
I hope to one day make the trek to OKC and listen to the GPA system.
Not having the experience to DIY my own high efficiency speakers, or at least high quality. I have been looking forward to the outcome of this thread.
NW
The problem you describe lies in the crossover. I attached an image of the unfiltered woofer response.
The following is a quote from Rick Craig that I lifted from AA, as he did my crossover after I found the stock one unlistenable (you can hear the promise in the driver through the stock crossover though):
I didn't really pay any attention to the stock crossover or specifications when creating the new design. The drivers were tested for frequency response and impedance and then everything was imported into our software optimization program.
This combination presents a difficult challenge in that there's very little overlap of the driver response curves due to the fairly small horn and large diameter woofer. The woofer has a significant breakup in the 1.8K-3K area The tweeter also has resonance issues in the 1-2K area. The shaping of the filters around the crossover point is critical and choosing the place to cross isn't easy.
I started first with a lower parts count crossover and experimented with different topologies. The objectives were to maintain good phase response in the crossover region, minimize the resonance and frequency response issues, maximize horizontal dispersion coverage, and provide plenty of headroom to keep the horn distortion low.
The crossover's acoustic slope target around the crossover point was 24db/octave. There are different ways to achieve this and a third order electrical filter was implemented for the tweeter. The woofer filter is a first order combined with a parallel resonance trap. Both are far from textbook values and designed to work in tandem with the driver responses to achieve the desired acoustical transfer function.
I also added a conjugate filter which creates a smooth system impedance. This is important for tube amps, in particular SET designs which typically have high output impedances. The less reactive load will provide better results and insures compatibility with virtually any tube amp.
Hey if you go visiting, maybe if you could get a chance, ask about the rumored "super 288 design" new back plate / modification that was in the works or at least in R&D before everything came to a close at Altec. I wonder if GPA or some one else would be interested in bringing these to light if they truly are an upgrade to the standard OEM part.... it has been very briefly mentioned over at the Altec hostboard in the past .... can any body add any additional info? Thanks
Re: chismercurio
It looks as if Rick Craig did a good job as he worked through the issues of the 604II when building your crossovers.
I would love to hear a pair of these speakers with a well implimented crossover under the right conditions.
To be fair to Dennis Fraker of Serious Stereo. I know he had several issues with his room at RMAF 2007, to include insufficient power to his system.
Knowing the quality of Dennis' 2A3 SET amplifier, and that he follows the KISS principle, leads me to believe that the GPA 604 can be a great speaker indeed.
Hopefully, I can hear them in the right situation when I get a chance to make it to GPA.
Re: JoMoCo,
I wil certainly inquire about the super 288 when I make it to GPA. I am interested to know more about this myself.
NW
It looks as if Rick Craig did a good job as he worked through the issues of the 604II when building your crossovers.
I would love to hear a pair of these speakers with a well implimented crossover under the right conditions.
To be fair to Dennis Fraker of Serious Stereo. I know he had several issues with his room at RMAF 2007, to include insufficient power to his system.
Knowing the quality of Dennis' 2A3 SET amplifier, and that he follows the KISS principle, leads me to believe that the GPA 604 can be a great speaker indeed.
Hopefully, I can hear them in the right situation when I get a chance to make it to GPA.
Re: JoMoCo,
I wil certainly inquire about the super 288 when I make it to GPA. I am interested to know more about this myself.
NW
Someone could call Steve and Bill at GPA and just ask. I don't currently need a driver like this, so I'm not going to. If you do, then just ask. They are cool guys.
Hi all, sorry for the lack of posting over the last couple of months. This has been a very busy year for Karna and myself, helping our assorted relatives move from where they were, staying at our place still they found a new place here in Colorado, and then getting them moved on. One group ended up in the nearby town of Lafayette (Colorado), and the other is going to boarding a plane from Christchurch, New Zealand to McMurdo Base in Antartica on Tuesday. He'll be coming back here again in March, staying at our place for a few weeks while looking for a permanent location here in Colorado.
Hifi is fun, but family takes priority. Just saw the thread re-appear, and here's the latest info. My friend over at Drivervault has offered to return the assortment of drivers he's measured, but I've been so busy we couldn't connect, much less spend time chatting about measurements. Based on previous e-mail communications, my understanding is that the before and after measurements of the two GPA 288 drivers are to the same scale - the drivers really are different. Once I get them back, I need to talk to GPA and find out what the problem is. There are plenty of good Altec vintage 288's out there, so I expect a teardown and comparison will answer the question.
The problem could be my fault - I requested GPA omit the mesh bug-screen, and that left a small indent in the throat where the bugscreen is usually installed. I'll be talking with GPA about whether this is the problem or not.
Hifi is fun, but family takes priority. Just saw the thread re-appear, and here's the latest info. My friend over at Drivervault has offered to return the assortment of drivers he's measured, but I've been so busy we couldn't connect, much less spend time chatting about measurements. Based on previous e-mail communications, my understanding is that the before and after measurements of the two GPA 288 drivers are to the same scale - the drivers really are different. Once I get them back, I need to talk to GPA and find out what the problem is. There are plenty of good Altec vintage 288's out there, so I expect a teardown and comparison will answer the question.
The problem could be my fault - I requested GPA omit the mesh bug-screen, and that left a small indent in the throat where the bugscreen is usually installed. I'll be talking with GPA about whether this is the problem or not.
Lynn, I posted plots in both 3db and 5db scale for the original 288 sample. For the second sample I only posted a 5db scale plot. In your post they were mixed causing some confusion. Anyway here are both samples with a 5db scale:
BTW sorry we couldn't hook up yet. Email me your address again, I may just have to ship your stuff back, as UPS is just down the street and I don't see having the time to get over to your place until well after the holidays, unfortunately.
BTW sorry we couldn't hook up yet. Email me your address again, I may just have to ship your stuff back, as UPS is just down the street and I don't see having the time to get over to your place until well after the holidays, unfortunately.
I've sent a private e-mail to Augerpro, advising we both wait until the deluge of friendly relatives and unfriendly weather comes to an end. The folks at GPA are very easy to work with, and know their stuff, so I'm optimistic that it can resolved in a straightforward way.
Truesound, are the diaphragms off-center or not firmly seated? Is that the problem?
Truesound, are the diaphragms off-center or not firmly seated? Is that the problem?
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The "original" Altec repair people would "bump" the new diaphragm while feeding it signal and watching for distortion on a scope. Evidently there is a little play even with the mounting pins and a sweet-spot for diaphragm alignment. I would have thought GPA would have fixed this problem. You should be able to do this yourself. Altec wasn't and isn't the only one with this problem. TAD, Beyma and B&C seems to have done it right from what I've worked with. The worst being Radian (Emilar)
It will also show up in the impedance plot as jagged vs smooth if you don't have a means to measure distortion..
It will also show up in the impedance plot as jagged vs smooth if you don't have a means to measure distortion..
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Hi,
I'm wondering, aren't there several pins/holes between the top plate and the diaphragm for alignment? Shouldn't they fit each other tightly?
If there's large enough tolerance for re-adjustment (to find the 'sweet spot'), how could/should it be done by bare eyes and hands? Or by some other tools?
Thanks
I'm wondering, aren't there several pins/holes between the top plate and the diaphragm for alignment? Shouldn't they fit each other tightly?
If there's large enough tolerance for re-adjustment (to find the 'sweet spot'), how could/should it be done by bare eyes and hands? Or by some other tools?
Thanks
Hi,
I'm wondering, aren't there several pins/holes between the top plate and the diaphragm for alignment? Shouldn't they fit each other tightly?
If there's large enough tolerance for re-adjustment (to find the 'sweet spot'), how could/should it be done by bare eyes and hands? Or by some other tools?
Thanks
The tolerances are **** poor on most of them. You can't expect to get the best sound unless you use the proper tools. I had a JBL 2440 once that went from 7 percent THD down to .25 % from proper tweaking. It is audible. Anytime they are shipped they should be gone over.