Refresh your Vintage Compression Drivers

[Tom Loizeaux]

How do you think the JBL phenolic diaphragms (1.75") hold up compared to the aluminum ones?

Thanks,

Tom

[Ian Mackenzie]

Today wile tracking down second hand 2440/41 I discover some interesting facts regards the Alinco drivers.

Thought this might be useful, no doubt most of you are aware of this......

According to a reputable Pro JBL SR installation firm here call Nova Sound, the older Alnico 2440/41 drivers can require re gaussing depending on their age and usage, particularly if the magnet has moved causing a break in the magnetic circuit.

The effect of regaussing is to top up the flux leading to more extended top end from from what I'm told.

The offer I got was a pair of 2441's re gaussed with new JBL Ti diaphragms for Aus $800 the pair (US $500). I have not decided when I will do it but figure better to get the best used article than buyer beware on Ebay.

(This guy has been in design of JBL horns for 25 years and also suggested the 2397 are an excellent device, but need the bigger driver or proper loading, the adapter does not correctly do this for 1 inch drivers.........poohy)

(the Widgets nice carvings are making me horny..muhahahah)



Ian

[Alex Lancaster]

I agree on all counts, Al-Ni-Co (Aluminum-Nickel-Cobalt), degrades magnetically not only with shocks and heat, but also with time, more than ferrite.

The re-gaussing (fancy for re-magnetizing), needs a big electromagnet, and somebody who knows what He´s doing; I posted a thread about this, but nobody could help me.

Alex.

[Don McRitchie]

Alnico does not degrade with time (it loses about 1% of its energy over a period of many hundred years). Of the common speaker magnet materials it is by far the most immune to heat. Nonetheless, when dealing with older speakers, it is pretty much expected that Alnico drivers will be substantially below their rated magnetic energy. This is strictly because of the instability of Alnico when exposed to large flux modulations.

As anedotal evidence, my web partner Steve Schell has measured the flux density of dozens of vintage drivers. From what he has told me, not one Alnico driver measured at its specified flux density (usually being around 30% down). In contrast, the ferrite drivers all met their specified flux density. This is soley due to the fact a brief exposure to a high power signal can demagnetize an Alnico driver while it will have no effect on a ferrite magnet.

[boputnam]

Hey, Don...

Widget tells that in his opinion, JBL's first foray into ferrite motors resulted in transducers that to him were sonically inferior to the later iterations. He noted in-particular, the "B" series studio monitors. Later iterations of ferrite seem to equal, if not surpass alnico motors.

Do you, Giskard or others have any fact or mythos to add to this?

[Alex Lancaster]

Don:

Thanks for the correction, I equated "time", with "use", and guess that even if You do not burn the coil, it could be subjected to very short but high electromag. peaks.

I have learned a lot in the forums in the past few months.


About the "B" series monitors and "H,J" components:

I started with JBL in 1978, I think, which was the transitition time from Alnico to Ferrite, I liked the ferrite better, because usually they handled more power, recently I listened to 4311BWX´s I sold 20+ years ago, and they still sound great for what they are.

Alex.

[Ian Mackenzie]

Alex,

I recall when leaving High school in 1978 for the big wide world I was a keen JBL addict.

Just as a short step off topic tell us your impressions when starting with JBL back then,

Ian

[4313B]

Originally posted by boputnam
Hey, Don...

Widget tells that in his opinion, JBL's first foray into ferrite motors resulted in transducers that to him were sonically inferior to the later iterations. He noted in-particular, the "B" series studio monitors. Later iterations of ferrite seem to equal, if not surpass alnico motors.

Do you, Giskard or others have any fact or mythos to add to this?

Well, I did have quite a bit to say but then I realized that it wouldn't serve any purpose. Some people are alnico fans and some people are SFG ferrite fans. I guess I'm willing to live in peace with all those alnico-V lovers...

One thing is for certain. The new alnico structures, with all the SFG ferrite goodies incorporated, are too freakin' cool!

[Don McRitchie]

I'll open the can of worms. I don’t have any technical information to support Widget's assertion. However, I can’t argue preference. The following is what was relayed to me by Greg Timbers regarding the first generation ferrite drivers.

As has been stated numerous times before, the Alnico/ferrite conversion was the result of a crises situation that cut off the supply of Alnico magnets in 1978. JBL was forced to develop replacement magnetic motors in a matter of months. Given the time constraints, JBL specified that the first generation drivers should be identical to the previous Alnicos with the exception of the motors. They would use the exact same cones, baskets, suspensions and coils. In addition, the ferrite motors would be designed to have the exact same magnetic energy of the previous Alnicos so there would be no need to redesign the systems using these drivers.

Greg related that listening tests of the first prototypes to meet the new specifications were an ear opener. They set up A/B tests of existing loudspeaker systems where the only difference was the use of a prototype ferrite bass driver in one and an original Alnico in the other. The bass response of the ferrite driver systems was noticeably different than the Alnicos and everyone agreed it sounded horrible. This led to a major effort to determine what was causing the difference.

Terry Sorenson ultimately identified the issues at play and designed the SFG motor to address them. The primary problem was the ferrite motor’s greater susceptibility to temporary flux modulations caused by the current in the voice coil. The conductive nature of an Alnico magnet means that it acts as a shorted turn to stabilize the permanent field. Ferrite, being non conductive, does not have this property. Terry determined that he could stabilize the ferrite magnets with the addition of a copper ring around the pole piece and this became a core feature of the SFG motor.

The other issue related to the non symmetric nature of the fringe flux above and below the pole piece on a ferrite motor. Because ferrite is a weaker magnetic material than Alnico, it means that a much larger magnet is required that must be housed external to the magnetic circuit. Alnicos generally have the magnet internal to the circuit with the magnet acting as part of the pole piece. The external ferrites have to have an iron core as the pole piece to complete the magnetic circuit. This iron core is permeable to magnetic flux meaning that there is a strong fringe flux field adjacent to it. For overhung coil drivers (the vast majority of bass drivers) it means that the overhung portions of the coil are exposed to a much higher flux field below the gap than above the gap. This results in distortion due to the non-linear response. The solution to this problem was to undercut the pole piece so that the iron core was moved away from the gap.

All of JBL’s first generation ferrite drivers used the SFG motor developed by Terry. According to Greg, they measured better than the Alnicos they replaced and were considered sonically superior. Initially, the new ferrites were designed as one for one replacements of the old Alnico models. The model numbers remained the same, with the change of the letter suffix to H to designate a ferrite driver, Thus the 2205, 2215, 2220, 2231, became 2205H, 2215H, 2220H and 2231H.

With the exception of the 2215H, all of these first generation drivers would be replaced by second generation versions in two to three years. However, the second generation versions had nothing to do with magnet issues. The motors for these drivers were virtually unchanged. The big differences were in suspension design and power handling. The second generation drivers saw the introduction of progressive suspensions that dealt with the issue of dynamic offset (a problem common to ferrite and Alnico drivers). It also saw the introduction of heat resistant adhesives that increased power handling. Thus, the 2235H replaced the 2231H and the 2225H replaced the 2220H and 2205H.

In summary, the first generation ferrites were considered by JBL to at least be the equal (and in some attributes better) than the Alnico’s they replaced. The second generation ferrite bass drivers are considered to be significantly better than the original Alnicos.

P.S. For those reading carefully, it would appear that this post contradicts my previous post regarding Alnico and ferrite’s relative susceptibility to flux modulation. To clarify, high power signals sent through a voice coil in an Alnico motor can generate large flux modulations that can result in permanent, partial demagnetization. A ferrite motor exposed to the same signals would have no permanent effect. However, signals within the normal operating range of a loudspeaker will cause temporary changes in both a ferrite and Alnico magnet’s field strength and this results in distortion. This flux modulation is higher in a ferrite motor than a comparable Alnico motor, unless it is engineered out. The copper flux stabilizing ring of JBL’s SFG motor was designed to do just that to result in lower levels of distortion compared to an Alnico equivalent.

[4313B]

Originally posted by Don McRitchie
I'll open the can of worms.

Thanks




I was able to A/B many, many of the alnico systems with their SFG ferrite counterparts and I can't recall preferring a single alnico version.

I was actually so upset about the whole alnico debacle that I was going to snag as many of the 2231A's as I possibly could before their impending demise. Fortunately for me I was taken aside and enlightened and I ended up walking out with only four 2231A's and four brand new 2231H's. After several weeks of goofing around with them all, the alnico's got dumped and I didn't even feel bad about it I've never looked back.

[boputnam]

Yes, Don - thanks!

OK, Giskard, now let's get back to where we were before - gimme the skinny on the favoritism you show (I think... ) for the LE14A's. They seem to hold a unique position in your AlNiCo experience.

[Alex Lancaster]

Ian:

It is off topic, hope I don't get kicked out of the Lansing C. C.,

It was great!!, at the time S. Harman had sold it to Beatrice Foods, and You would see the big execs., but I really had nothing to do with them; I was 25, and my first real speakers had been some used L19´s that were the greatest, anyway I got into the Disco business, was 2 Years out of Mechanical Engineering, although my specialty was Internal Combustion, and I had hated everything about the required Electricity classes, which I later had a lot of time to regret; Well, I wrote ,no 'net no Fax and the customs broker which I still use his services after 32 Years!, let me use their Telex, 96 Bauds mostly garbled, Ernest K. Wetzig then the main foreign sales guy answered, I went to Northridge to meet him, and there I was, the 2nd Pro dealer for Mexico.

Considering the small volume, I was treated like a king, soon the other dealer went OUB, and I was the only one.

I specially remember very fondly:

Mr. Wetzig for letting me get away with murder, and commercial tips; He even visited Guadalajara.

Garry Margolis for patiently answering my stupid questions.

John Eargle for " " " "

Murray Fancher " " " "

There were others, and You could feel the energy, care and commitment in everything.

In about a Year, Mr. Wetzig said somebody had to take the speaker repair course, which I did, and still have the diploma hanging in my office, that got us a lot of business, I taught 2 guys and were repairing 10 units a day, sometimes.

I got married in 1982, and wanted to use recorded music in a grand scale, so I went to G. Margolis with the floor plan of the church, 18th century and told Him I could get 3Kw of amps and speakers; He said: You need 30w/ch, 2 2205's and 2 2420's!, so I did and the SOTA then was a cassette tape in a TEAC 860, Burwen filter, Phase Linear pre and JBL 6233 amp., lots of people did the same later it sounded great! The cabinets were Altecs and 811 horns w/3110 xovers, a matter of availability on the Altecs.

So, lots of fun, unfortunately in 1982, Mexico went into one of its devaluations/recessions, so I closed the business a couple of years later, by then it was dead.

I hope it wasn't too long winded.

Alex.

For Don: This morning, thinking more about Alnico´s degradation, I guess just having a bar sitting there won´t affect it in 100 years, but with the magnetic poles almost shorted, except for the gap, Wouldn't it degrade sooner?

Alex.

[4313B]

Originally posted by boputnam
Yes, Don - thanks!

OK, Giskard, now let's get back to where we were before - gimme the skinny on the favoritism you show (I think... ) for the LE14A's. They seem to hold a unique position in your AlNiCo experience.

Hmmm, I started out with a huge helping of dislike for the LE14A. The LE14/1400 transducer has always seemed to be a real favorite of a certain JBL engineer. Over the years I've come around to feeling the same way but it wasn't really until the LE14H-1 rolled off the assembly line that I started to get with the program. I had the concept down early on but I just wasn't willing to give up the excitement of the 15's

My all time favorite alnico transducer would have to be the 124A. What a brutal piece of hardware.

[Oldmics]

Originally Posted By GISKARD "My all time favorite alnico transducer would have to be the 124A. What a brutal piece of hardware." With a hearty endorsement from GISKARD, eBay prices will probably climb to an astronomical level for this driver. Buckle up for the ride.Oldmics

[4313B]

Originally posted by Oldmics
Originally Posted By GISKARD "My all time favorite alnico transducer would have to be the 124A. What a brutal piece of hardware." With a hearty endorsement from GISKARD, eBay prices will probably climb to an astronomical level for this driver. Buckle up for the ride.Oldmics

K
What should we push next week