phenolic vs symbiotik conundrum

Hi and Happy New Year

My Altec saga continues. I have been working with a pair of 803 multicells which came with 291A drivers.
It would seem, that as I have become spoiled by 24db/oct linkwitz riley crossovers and their amazing transparency, I have now become spoiled by the speed and transparency of aluminum!

Just tried a pair of 390 drivers after a month of fine tunnning the 291's. My hope was to add a bit more body to the bottom, but I did not expect to sacrifice anything on top.
For reference the crossover is in the midrange decade 350/3500 24 db/oct.

To get lower, 390's use a phenolic diaphragm and rolled suspension.

But I must say, while single voices and single notes of instruments are very good.
Grouped harmony, guitar and piano chords etc are just plain distorted by comparison with the 291's. all the air, delicacy and lighting fast transient response is gone. Bummer!
Funny, because I thought crossing at 3500 would not affect the perception of speed all that much.

So, Unless there is something wrong with these drivers, I suspect I have to move my attention to the midbass horn. because trying to get the 803 down to 350hz is just riddled with too many compromises.

thought I would share that with the group.
I would appreciate any comments about this experience.
Also, are there any published distortion specs on these drivers? THD, and IM perhaps.
It would be good to compare the 291 vs 390(290) regards distortion.

Kevin

Jan 2 2011

Kevin,
Very interesting experiment. I've been toying with either the 390's or the 399's mated to the 805 multi & this has been exactly what has been going through my mind.....will I lose the detail by going to the phenolic diaphragm.
Do you think this is purely because of the fram or would the drivers have an influence here?

Regards,
John

If you wanna go low, seriously the 390 is a wrong choice.

The 390 is a "pancake' lighter version of a 290.

Get the last Alnico or ferrite 290 drivers that weigh 30 pounds.

The 390 is for more modern designs, that don't expect to go to the absolute bottom limit of the driver.

Regardless you will lose the top end, and have to put it back VIA a quality device.

Hi,
I did a sine sweep and that revealed lots of problems with these drivers.
first the second harmonic is nearly as loud as the fundamental below 1000hz.
You can hear it singing above the fundamental. My 291's with symbiotik's produce nice whole tones.

Further there is an array of all sorts of noises that occur at very low level. this explains why guitar chords were fuzzy. Very strange. they are brand new drivers. and both exhibit nearly identical characteristics. It is a shame they have to go back. To bad too because i feel I will never be able to properly comment on the performance of this driver.

But I do feel that the driver is too slow. As I noted in my first post, you get spoiled by the sheer speed of the aluminum. I mean, there are comments out there about the loss of speed going from aluminum to Pascalite, so going to a rolled suspension does not surprise me now. But like most things you have to experience it first hand to tell.

My goal was for a warmer mid with little change in the top end. Afterall, I roll these off above 3500, I did not expect the loss of transparency. At good volume, vocals were fuller and warmer, but the the tradeoff on speed and transparency is not worth the gain at the bottom.

Yes, I am of the mind that if you really want to experiment with the phenolic, you need to find a good used or NOS 290. Not so much because it is bigger or more robust though , but just that it is vintage, and likely made with more care.

Hope that is helpful
Kevin
010311

The model drivers I mentioned had a casting added to damp resonances.

The late model drivers weigh more than they early models because of that casting.

So you completely misunderstood.

I pointed to a series of drivers designed to reduce the issues you were talking about.

This paper discusses increased magnet size

http://alteclansingunofficial.nlenet...ers/TL_231.pdf

Here is the back of a 288C sheet...20 pounds.


http://www.voiceofthetheatre.com/images/288C.2.jpg

And the back of a 288G sheet 29 pounds

http://www.voiceofthetheatre.com/images/288G.2.jpg


To sum up....you are using a lightweight version of the driver designed for higher crossover points used in modern systems, I attempted to direct you to designs optimized for lower crossover points.

The pot casting to damp resonances was added to late Alnico 288, 291, and 290. It was retained when the switch was made to ferrite.

Essentially you are arguing that a 29 pound driver wont resonate less than a 20 pound one.

Hi,
The only points I am making are:
The drivers are brand new made less than 30 days ago, that they have a low frequency specified rating of 300Hz,
and that they were reccomended for my application.
And yet at the very low power that I operate them at -less than 90dB measured at the horn mouth- they are producing a signficant "double tone" at all frequencies below 1000hz
also, as you lower the level down, you hit a point where a "chorus" of tones appear.
Both drivers do this, so that to me indicates that they are defective in some way.

I am unable to accept this as "normal" operation.

Perhaps if I were running these at very high levels, true resonances - that are the result of build design- will apear, but I would not expect that at these low levels.

Kevin
010311

Remove the rear cover and see if it still happens.

GM

Quote:

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Originally Posted by GM

Remove the rear cover and see if it still happens.

GM

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Interesting idea.

I'm wondering,,,the big transformer cans...would there be any merit to trying them without the inside cover, possibly filling them with different types? probably not...just my wandering attention span again....

I would think obviously the driver would have to be derated, or am I wrong again?