Great Plains Audio diaphragms

[Edacnz]

I purchased a pair of replacement diaphragms from GPA for the 902 drivers in my Altec 19's

After installing them carefully, I am finding them quite 'icy' sounding. There is definitely more high frequency information making it through, but I seem to have lost the smooth, melodic mids which have been drawing me in to the 19's, with a Prima Luna El34 amp at the helm.

Also, one seems to have a 'gap' in its frequency response. It's hard to define and I don't have the means to measure it - its just like a hole in the sound. Particularly noticeable when audience is clapping at the end of a song, this channel is almost empty of clapping sounds. I have isolated the problem to the one diaphragm after swapping everything else around (I mean everything!) I've checked and double checked correct placement of the phragm inside the magnet etc.

Do these things have a break-in period?

I took the old units out because one had a slight rip, which I glued up. It still buzzed a bit. If the GPA units aren't going to get any better, I might swap them back out.

Any ideas or suggestions? And yes I will get in touch with Bill, but always appreciate the communal wisdom. Thanks.

[Earl K]

If you don't have test gear to acoustically test what you are aligning, then my recommendation is to simply buy new GPA 902-8a drivers ( with new 34647 diaphragms pre-aligned within them ).



Altec 902 drivers/diaphragms are very hard to set up ( at the best of times IME ) even with proper test gear .

Since your GPA purchase is recent why not contact Bill and see if he'll take back the diaphragms if you agree to buy new GPA drivers from him ( he can load the diaphragms that you send back into GPA magnets ).

Zilch ( Evan ) demonstrated 8 years back that the GPA version of the 902 is more linear than the original Altec.

Good Luck!

[Edacnz]

If you don't have test gear to acoustically test what you are aligning, then my recommendation is to simply buy new GPA 902-8a drivers ( with new 34647 diaphragms pre-aligned within them ).



Altec 902 drivers/diaphragms are very hard to set up ( at the best of times IME ) even with proper test gear .

Since your GPA purchase is recent why not contact Bill and see if he'll take back the diaphragms if you agree to buy new GPA drivers from him ( he can load the diaphragms that you send back into GPA magnets ).

Zilch ( Evan ) demonstrated 8 years back that the GPA version of the 902 is more linear than the original Altec.

Good Luck!

Thanks Earl

Its not a simple proposition for me unfortunately. Getting the new diaphragms here was challenging and costly in itself.

I need to work with what I have.

I must admit, your post makes me realize perhaps I went in to this unprepared. I didn't know that there could be alignment issues given that the diaphragm ring fits the rebate in the magnet assembly perfectly and the positioning screwholes prevent in other travel within..

Oh well. Might engage a pro.

Any other comments about the sonics of these vs altec phragms?

[Chas]

I recently replaced the 806A with GPA 902 in a pair of Altec Valencia’s (846B). The GPA drivers are nothing short of amazing. They have extended highs that are clearly audible and measurable. Well worth the money IMHO.

[Earl K]

Drats!

I created a detailed post, hit send & then watched it all disappear!

Anyways, which model diaphragm did you buy from GPA ? ( #34647 ? )

What was the original diaphragm model that you removed from your 902-8a drivers?

I'm not aware of any reports that Altec diaphragms will improve with age ( & usage )
/>/ really it's quite the opposite if they are driven hard.

[Edacnz]

I bought 34647 diaphragms from GPA.

Late yesterday, I began wondering if there was something else at play - room acoustics, cabling etc. So I undertook some additional testing. I wont trouble everyone with the details, but suffice to say, I was still hearing the icy, beaming sounds from the new diaphragms. I went into town and purchased some new spade connectors for the internal wiring, wondering if these might have oxidized.

((( A quick aside here: I had previously bypassed the speaker binding posts, running copper cable directly to the crossover via the speaker port, and replaced the internal wiring with stranded copper, except for the cabling to the compression drivers. These had seemed to sound better with the original diaphragms, when running the original 'tinned copper' Altec wire. )))

With the new spades in hand, I made up new leads for the HF drivers and also replaced the spades on the tinned copper Altec wire. I first tried the Altec wire, and was greeted with the same 'not nice' sound. So, for the 300th time (exaggeration) this weekend, the 19's came apart and went back together with stranded copper going to the HF.

Well.

What a difference. The iciness was gone, the 19's were listenable again. I settled in and had a good old listening session and thoroughly enjoyed what I was hearing. There is considerably more information making it through these new diaphragms (which is an interesting phenomena to deal with in itself). The sound is now smooth and liquid as it was before I swapped out the parts, but because there is so much more happening, I am now going through a reconciliation process - the sound I am hearing now is still quite different to that which I had come to love from the 19's with original parts.

I have been holding off upgrading the N1201-8A crossovers, but I think now is the time to look at these - having had the experience with the old wire sounding better with the old diaphragms encourages this further.

With all this said, there may still be a slight imbalance in the output.. I have emailed Bill and asked him his thoughts on this - I await his response. The learning-curve continues...!

Thanks for your help.

[Earl K]

Oxidation, plus poor connections can certainly wreak sonic havoc, there's no doubt.

I believe you live in New Zealand ( right ? ) so I imagine your components might have also been exposed to higher levels of oxidation (due to high humidity) than many of us are used to .

Your driver ( exposed );

[Edacnz]

Earl - yes there was some serious oxidation on some of the spades etc when I first purchased the 19s..

You shared the pic of my HF unit there - do you think that surface oxidization could have a bearing on the sound? I can't picture how it would - but maybe if that extended down into the gap.......?

What do you think?

[Earl K]

Earl - yes there was some serious oxidation on some of the spades etc when I first purchased the 19s..

You shared the pic of my HF unit there - do you think that surface oxidization could have a bearing on the sound? I can't picture how it would - but maybe if that extended down into the gap.......?

What do you think?

The pictured oxidation shouldn't be a problem ( if the HF drivers VC gap is found to be clear of oxidation ).
- I'd be more concerned about finding lots of oxidation in your Altec 416-8B woofers.

Oxidation adds resistance to the magnetic circuit ( if found where flux lines have to jump across air to metal surfaces ).

More resistance in the magnetic circuit will lower the amount of available flux lines ( within the VC's air-gap ) lowering mid-band efficiency.

That's why ( unseen internal ) oxidation can also wreak havoc to a drivers midband efficiency when oxidation is present at some mated surfaces ( ie; those found in the magnetic circuit > say between the alnico slug magnet and the top-plate or the return pot ).

This internal oxidation problem can only be addressed by someone who can completely strip the driver apart ( for rebuild ) , then re-polish all mated surfaces, recone and regauss . That's a service that's unlikely to be available on your island ( though one never knows ).

I posted your 902-8a ( exposed ) pic because obviously to me your drivers metallic surfaces are starting to oxidize ( & that should be a concern ).
- I've just described how that process can start to effect performance ( with no easy remedy ).

[Edacnz]

The pictured oxidation shouldn't be a problem ( if the HF drivers VC gap is found to be clear of oxidation ).
- I'd be more concerned about finding lots of oxidation in your Altec 416-8B woofers.

Oxidation adds resistance to the magnetic circuit ( if found where flux lines have to jump across air to metal surfaces ).

More resistance in the magnetic circuit will lower the amount of available flux lines ( within the VC's air-gap ) lowering mid-band efficiency.

That's why ( unseen internal ) oxidation can also wreak havoc to a drivers midband efficiency when oxidation is present at some mated surfaces ( ie; those found in the magnetic circuit > say between the alnico slug magnet and the top-plate or the return pot ).

This internal oxidation problem can only be addressed by someone who can completely strip the driver apart ( for rebuild ) , then re-polish all mated surfaces, recone and regauss . That's a service that's unlikely to be available on your island ( though one never knows ).

I posted your 902-8a ( exposed ) pic because obviously to me your drivers metallic surfaces are starting to oxidize.
- I've just described how that process can start to effect performance ( with no easy remedy ).

Awesome information..

Thanks a million

[Chas]

Maybe you have a radio amateur (ham) nearby? They might be able to sweep them with an AF oscillator in order to properly align them.
NB: With the appropriate hearing protection.

[4345]

I bought a single GPA diaphram for an Altec 604-8K. It was from a pair of Urei 811's. Someone had put a junkie aftermarket diaphram in one of them. The aftermarket diaphram sounded terrible. When I replaced it with the GPA diaphram, I think it sounded better than the other speaker that had the original Altec diaphram. So, I would suggest when buying GPA diaphrams for a pair of speakers to always replace both.

[Edacnz]

I bought a single GPA diaphram for an Altec 604-8K. It was from a pair of Urei 811's. Someone had put a junkie aftermarket diaphram in one of them. The aftermarket diaphram sounded terrible. When I replaced it with the GPA diaphram, I think it sounded better than the other speaker that had the original Altec diaphram. So, I would suggest when buying GPA diaphrams for a pair of speakers to always replace both.

Thanks - yes, I replaced both units, though I did so one at a time - ie I listened to the speakers with one new diaphragm installed and one old one still in the other channel. They sounded wildly different from one another, though it was impossible to conclude at that time which one sounded better.

I made some further adjustments to the positioning of the speakers last night and spent more time listening. It is clear to me that these diaphragms are getting better with more hours on them - having apparently smoothed out more even since yesterday. I invited my wife in to have a listen to the same tracks (I never tell her what to expect or what I have been up to). She remarked on the scale, ease and smoothness she was hearing vs yesterday when she listened, which was after I had added the new cables.

By the time I finished listening last night, I feel like I can confidently say the sound was significantly better than before I replaced the diaphragms, which I consider a success. I believe I may still have some room acoustics to tame however, which did not seem tom be an issue with the original diaphragms. Curious.

Thanks again for the help and suggestions.

[Chas]

^Good to hear things are improving. Bear in mind that with extended and better all around MF and HF performance, the room particularly feflective side walls, can have a major influence. I actually applied Sonex foam panels in my room to get things somewhat under control to good effect.