Hi Todd
You wrote, "The T/S's on my site are the most accurate available". What is/are "T/S's"?
Please excuse my ignorance, a lot of what I do is based on the "LAR" principle (looks about right) LOL. Oh... throw in a little common sense along with a lot of questions directed at knowlegable people and researching published material.
I think I just figured it out.... Thiele/Small commonly refers to a set of standard parameters that define how a loudspeaker driver performs. OK, now, what did they do to make the speaker sound different. Again, if everything is the same except the magnet material but, it still has the same flux density. Why is there is difference in the T/S and the sound? What else is different?
Jammin,
You got it about the T/S parameters. Quite a lot different between the -8B and -8C.
The cone number doesn't tell the whole story. The cone number only relates to the actual cone. Not to the spider or VC assembly. These could be different.
The pole piece might also be different. And the gap might be different. And the position of the VC in the gap might be different.
As I said, I have measured stock well functioning -8B's and the re-coned -8C's and have verified the T/S parameters in Todds list.
About sounding different. Well, the T/S parameters dictate the box size and tuning required for optimum performance. Therefore the -8B wants a bigger box than the -8C. Given an optimal enclosure for each, properly tuned, the -8B will go lower. So that will affect to sound down low. Up above that, my guess is that the two drivers will sound the same.
Another thing to remember is that Altec more than likely changed cone material, thickness, and weight over the years. So who really knows what was going on. That info is probably lone gone.
The new cones made by GPA are not the same construction as the originals. I measured thickness and weight. The GPA cones are thinner and lighter. It's obvious when you have the raw cones side by side.
I'm not bashing GPA's cones, just stating that they are different than original stuff.
I'm going to do my own re-cone on the -8B's I have with GPA parts. I just need to go buy the glues from the local reconer and get going on it. Should be fun!
Ron
Hi RonSS
Thanks for your input. I really like the 8-B's and I like the lows that come from them.
Howdy Folks!
I recently discovered this thread on the heritage website, it's the most conclusive and definitive account of the behavior and sound of alnico vs. ferrite.
Here is a post from Greg Timbers, some of you will most certainly know who he is. The topic was "recharging alnico magnets". I thought this post and particularly his statement about why alnico motors sound the way they do was very relevant to this thread even tho it specifically refers to JBL product.
<font size="2" face="Verdana, Helvetica, sans-serif">For those interested and who don't normally travel the heritage website, here's a link to the entire thread:Ferrite magnets do not demagnitize with time or drive. They are affected by temperature but that is reversible. They will return to normal when they return to room temperature. Ferrite is basically a lousy magnet material for speakers but it is cheap and readily available. JBL has done a ton of things within the magnetic circuit to make the material behave in a more stable manner. At 100 degrees F, a Ferrite motor will be down about 1.5 dB in level which means the midband of the woofer will be lower by that much and there will be increased output around the system resonance. The TS parameters will be completely different - as though the BL was reduced by about 18%.
Alnico magnets, by their nature are easy to demagnitize with drive. They will not change with time and their dependence on temperature is really small - maybe 1% at 100 deg.F. Alnico stability and resistance to back EMF is really good. This is why they make the best sounding magnetic structures. Unfortunatelly, given a big enough pulse of magnetic energy, they will demagnitize by up to 3 dB. The sensitivity to demagging is dependent on the specifics of the magnetic circuit and the length of the coil providing the field. Underhung woofers (LE15 and such) midranges, tweeters and compression drivers do not have sufficient back EMF fields to push the operating point of the structure below the knee. They are essentially stable regardless of input signal. The short gap-long coil speakers are the ones that have a problem. A 2235 can take a hit of up to 3 dB if a big enough hit of current takes place. 1.5 dB to 2 dB is more common. The effect does not get better or worse with time, it solely depends on how much current is driven through the coil. The more current, the more field. Once the field is bigger than a certain number, some amount of demagnitizing occurs. It is perminent (until externally recharged) and will only increase if a larger sustained current hit occurs.
Therefore, if you have a qualifying alnico woofer and you have played it loudly you have some damagging. You can have the unit recharged and it will be fine until you play it again. Exceed the critical level and it will start happening. If you never do, it won't ever demag. Most of these designs trace back to the 50's and 60's where 15 - 30 watt tube amps were the rule. They didn't have the current capability to hurt anything. With the advent of big solid state amps, the current levels went up and the problems started to surface.
Most of the qualifying 4" motors will loose 1 - 1.5 dB unless they are pummeled. Some of the older 3" with really short magnets, like the 2213A and 123A will typically be around 3 dB down. They go really easily. The old decade woofers (116A and 127A) only had to see an amplifier in the room and they got really nervous. FYI, the new 1500Al used in the S9800 can take continued pulses of 5000 watts and loose no more than 1%. The test can only be done a few times before the coil is destroyed, but the magnetic assembly is totally stable.
[ame="http://audioheritage.csdco.com/vbulletin/showthread.php?t=10280"]http://audioheritage.csdco.com/vbulletin/showthread.php?t=10280[/ame]
For me and my simple mind, Greg's post closes the case on the alnico vs. ferrite debate, as well as answering some other questions about both types of magnet.
I thought it was a most interesting read and wanted to be sure that the members of this board who don't frequent the heritage site had the opportunity to see it.
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