Driver Magnet Size

[jimd]

Does anyone out there know the magnet size/weight of the smaller 175/2410 and the larger 275/2420/85 1" throat drivers? I was once told they all have the same size magnet, just different magnetic pots, I wonder about that.
While ALTEC would tell you these things in their spec sheets, JBL only listed the magnetic structure weight, not the true magnet size, which is what I want to know. So, if you have that information, please let me know, JIM

[speakerdave]

According to my listing from the late 70's the LE175 and LE85 have different flux densities, 16000 Gauss vs. 19000. Since they use the same diaphragms, and therefore have the same gap geometries, that means the magnets on the LE175 are smaller. The field intensity is the number that matters here, I think.

David

[jimd]

Dave, this my not be the case. They may have the same magnet, the larger structure of the 275/2420/85 may be more efficient and raise the flux density.

The ALTEC 802 had 1.2Lbs of alnico ring magnet. The LE 175 is exactly the same exterior dimension as the 802. The two drivers, except for cosmetics are nearly identical in performance. This is not to be unexpected since the diaphragm dies for both companies were made by Mr. Lansing. The suspension tangents in the surrounds of the respective drivers do run in opposite directions, however this is only because Mr. Lansing wanted to distance his later driver from that of his former employer, ALTEC. The 2420 size pot was an attempt to improve on the smaller unit, and was, from my experience, an almost unpreceptable difference.

So, I continue my quest for the magnet weights of the two drivers. JIM

[Steve Schell]

I believe that the 2420 does have a larger Alnico V magnet than the D175, but this is going from memory. Generally in a compression driver, when it is well designed like a JBL, the entire return path will have at least three times, and often four or five times the cross sectional area of the inner pole piece at the voice coil gap. This places the operating point of the material in the return path well down the BH curve and the material used is not terribly critical as long as it is quite low in carbon and has low reluctance. When this is the case (usually is), the rest of the circuit is far from saturation and adding more material would not materially alter the flux density in the gap, which is pretty much determined by the magnetomotive force of the magnet and its ability to saturate the pole tips. If there is enough magnet then exotic pole tip materials like Permandur will permit a higher saturation flux density, but I don't think this is the case with any of these JBL drivers.

[jimd]

Thank You Steve,

You are probably right, I wonder if anyone anywhere knows the magnet size short of dissembling a driver and measuring it. Might be my only option. JIM

[subwoof]

I have a single 2420, LE175 , 2440 and K140 alnico magnets that are weak in my "later" pile in the shop corner...

I will bring them home tonight and post pics / dim.

BTW the 2420 mag is bigger that the 2410 if I remember.

:cheers:

sub

[jimd]

Sub,

Thank you for your offer. Too bad you live so far away, I have access to a magnet charger. Was just about to think about buying a dead one to take apart myself,if you want to sell one, let me know.

On another note, as most of you know, I grew up working with ALTEC products and know them inside out. I have had some contact with JBLs, but not near as much, and as such am not near as versed in the product. I know, the 275 and LE85 are identical. The 2420 is similar and has a different back cap, what is the difference between the 2420 and 2421? Never too late to learn, JIM

[edgewound]

The only difference in the 2420 and the 2421 is the stock diaphragm. 2420 had half roll suspension, 2421 has diamond embossed suspension.

[Earl K]

The only difference in the 2420 and the 2421 is the stock diaphragm. 2420 had half roll suspension, 2421 has diamond embossed suspension.

- That's half right .

- The 2420 (like the 2410 ) had a tangential ( swirled ) surround .

- The 2440 has the half roll suspension . ( Also, all the phenolics use a form of the half roll suspension )

:)

[edgewound]

- That's half right .

- The 2420 (like the 2410 ) had a tangential ( swirled ) surround .

- The 2440 has the half roll suspension . ( Also, all the phenolics use a form of the half roll suspension )

:)

Sorry:o: ...pre-senior moment...I guess it's better to be half right than totally wrong:p . Thanks for the correction, Earl...shoulda confirmed before I hit the submit button.

[subwoof]

The magnets are the same size. Surprise surprise...

:cheers:

sub



Dave, this my not be the case. They may have the same magnet, the larger structure of the 275/2420/85 may be more efficient and raise the flux density.

[Earl K]

Wow,

- Great info Sub !

- I think anyone interested in magnetic circuit design will find this all very intriguing .

- Are you able to get an accurate weight on those magnets ?


:)

[Earl K]

Sorry ...pre-senior moment..

Think nothing of it . :)
I have those moments all the time, just about always before my second cup of coffee. ;)

:p

[subwoof]

Well dealing with 75lb+ amplifiers and even heavier speakers, a less than 10lb accurate scale is something I have yet to install in the shop.

However I will pop by the post office and use the "accurate" one in the customer service area.

Assuming the residual magnetism doesn't set off alarms or ruin the scale, we shall know soon.

sub

[Steve Schell]

Well gee whiz, I stand corrected. Thanks for the great picture, subwoof.

This brings about a new series of questions. What is the spec for flux density for the two driver types, and what do they actually measure in practice? I've got a whole bunch of rusty surplus 2420 bodies, though I'm a bit thin on 175s; only have a couple of them in vintage systems. I will have to measure their flux density and report back.

The ALE company in Japan makes a line of Alnico compression drivers, several of which seem to have really humongous magnet structures. Their best line features the use of Permandur for the entire return path, which up to now has seemed to me to be a pure extravagance. Maybe not!

[jimd]

A friend of mine once told me the two drivers had the same magnet, I wasn't sure, but now have to have to admit he was right. I am willing to bet the magnet will weigh 1.2 lbs, the same as the Altec 802! Children of a common parent.

Like I earlier said, I do not have the experience with these drivers I have with the Altecs. In you opinions which is the better sounding unit, the 2420 or 2421? I also wonder if the two diaphragm are made of the same thickness of aluminum?

JIM

[jbl_man_uk]

.....Interesting thread...i have several 2420`s and 2410s here,but strangely the smaller bodied 2410 is louder in every case...u would have thought the larger pot and thus magnetic return structure would have made the 2420 the louder....obviously i swaped the diaphramms about to make sure that wasnt causing it...and tried it on several different units incase some of the 2420 were down on gauss...but every time the smaller 2410 was louder.:blink:

[speakerdave]

The magnets are the same size.

Imagine my surprise!

[jimd]

Dear Subwoof,

Replied to your PM, are ypu still out there?

Jim

[Earl K]

Dave, this my not be the case. They may have the same magnet, the larger structure of the 275/2420/85 may be more efficient and raise the flux density.

This was truly one of those eureka moments ( at least for me ) in the history of this forum .

Steve Schell ( anyone ), would you care to speculate on who was responsible for the redesign of the magnetic return circuit that resulted in getting more ooomph out of the 175 ( & its standard sized 1.2 lb alnico ring magnet ) ?

- I'm going to go with Bart Locanthi and those mid-fifties collaborations with Ampex ( Magnetics ) , as the design team responsible for filling in the ( product ) lineup ( & thus creating the 275 ) between the 175 & the 375 .

<> cheers :)

[Don McRitchie]

It was Bart. He was responsible for the 275 and 375 which ustilized cast pot structures. He was also responsible for JBL converting their bass drivers from machined pots to cast pots around the same time. Later at Altec in the early 70's, he did the same thing with the 416 and I believe the 515 so that they were redesigned with cast pots.