AR-9 woofer anomaly

[Mach3]

So about three years ago I bought a set of AR-9s from a fellow in New York. Eventually got around to rebuilding the crossovers (all film caps and new electrolytics for the bass section - and all new wiring). The things sound MAGNFICENT. I have been to an RMAF and did not hear a better sounding speaker. I would not take $10k for these speakers.

BUT - and it is a big butt - when I was rebuilding these units I noted that the woofers had a square magnet structure. But they fit the hole in the cabinet (and the cabinet is that of an AR-9 being 53" tall) so I assumed they were original AR-9 woofers (the seller did share that he had these units refoamed - and the surrounds were in fact brand new).

Unfortunately pictures I have seen of the original 12" AR woofer all show a circular magnet structure - and for the 10" woofer a square magnet structure.

So my question is; is it possible that somebody has placed a 10" AR woofer magnet structure on my 12" woofers? Is this even possible? Or did the factory itself have a reputation for swapping the magnet structures back and forth? The drivers fit the holes in the cabinet perfectly - as in no room to spare. So the speaker baskets are in fact that of the AR 12" speaker.

I do know this - I have prodigous bass - flat down to 30 Hz (measured it with various devices) IN THE ROOM. I do NOT seem to be lacking in the bass department.

I do wonder - has somebody been messing around with these drivers? I would imagine that a profit could be turned somehow by changing out magnet structures. Generally if a profit can be made somebody will be out there doing it. Human nature and all of that.

Next question: If in fact I do have a "modified" woofer driver (or actually FOUR OF THEM) can anybody suggest a good replacement? ABTech? Simply Speakers? Both of which advertise that they are selling OEM drivers.

And insight would be appreciated.

Thanks

[fordf250]

Hello Mach 3-there were several series of OEM drivers used the AR3/9 series of products. Woofers, with square magnet motor structures are included (see pictures, in "restoring your AR3 series speakers-sect. 23"). I had the square magnet drivers in my AR9's, and I purchased them new, in 1980.

[ironlake]

It seem s reason the the round magnet would keep the magnetic field the same for 360 degrees where the square magnet would lose some power as it goes away from the voice coil on the corners. Not sure on this one. This happened in the change over from the 1st masonite advent woofer to the second all steel woofer with round magnet.

[soundminded]

When I bought my AR9 speakers as demos in 1984, the woofers had round magnets. The retailer had to replace all of the drivers. The repalcements came from the factory. The replacements had square magnets. I think they were manufactured by Tonegen, probably the last production variation of that design. The materials and precise configuration of the standard AR 12" driver changed over the years but they are for all practical intents and purposes the same driver and interchangeable.

This brings up an interesing point I was going to start a thread on anyway. Do the cloth surround acoustic suspension drivers sound different/better than the foam surround drivers? I think they might. I might just one day go "shopping" on e-bay for early versions with cloth surrounds.

[Carlspeak]

I have refoamed and reconed many AR 11" woofers. Most had round magnets, but some had square magnets, pole and back plates. I just figured it was less costly to make square magnets and plates due to the inherent waste reduction afforded cutting square shapes vs round ones.

Aren't most or all of the cloth surround woofers with Alnico magnets vs ceramic that replaced them? I'm not 100% sure on the timing of the magnet change to the surround change. TT, Roy?

With regard to the sound of one vs the other, there seems to be much sentiment for the sound of Alnicos. Then again, don't we all have our own preferences?

[Steve F]

I've heard that one of the main reasons for the move to foam surrounds was that foam is more uniform in its movement than cloth, which can "pucker" slightly as it travels. Theoretically, foam's more linear motion "should" result in a very slightly lower THD , as well as superior unit-to-unit manufacturing consistency. Certainly in 1968--when the AR-5's 10" woofer was introduced with the industry's first foam surround--AR and the foam vendor didn't know about the limited lifespan and foam rot.

There may be other characteristics of the older 12" woofers that make them better, however. For instance, some of the very early ones had cast aluminum baskets instead of the cheaper, lighter stamped steel baskets on later variants, and again--"theoretically"--a cast aluminum basket will hold the various moving parts in tighter alignment with less tendency to warp or "wobble."

I'd be surprised, however, if a listener could reliably discern in a blindfold test the difference between a cast/cloth 12" woofer and a stamped/foam 12" woofer, assuming all the T-S parameters were within normal tolerances.

Steve F.

PS--Mach3: Nice B-70 avatar! Still looks futuristic, even though it last flew in 1965-- two years before the 3a was even introduced!

[Mach3]

Thanks everybody for your kind and thoughtful posts. I am relieved to say the least (and at least $600 richer as I am no longer facing replacing 4 woofers).

In re the "Valkyrie" - the last of the "lets fly higher and faster" bomber school. That thing - which was stunningly beautiful - even today - could cruise at Mach 3 (not burst speed mind you - CRUISE speed) at an altitude of 80k feet. It was designed to fly at 100k feet but that would have required an exotic fuel (Boron methylcalate as I recall - probably wrong on that) to supply oxygen to the burn.

The "official" reason for canceling the B-70 program was that Soviet SAMs would have been able to effectively shoot it down. Later analysis done after the Berlin Wall fell seem to indicate that this was probably not so true. In fact the Soviets did not believe that the B-70 was vulnerable to SAMs - hence their development of the Mig-25 Foxbat which only had one purpose - fly to 80k feet and knock down an incoming flight of B-70s.

Regardless - a stunningly beautiful aircraft that was designed in the late '50s and built in the early '60s' - slide rules and all that.

[RoyC]

I'm not 100% sure on the timing of the magnet change to the surround change. TT, Roy?

With regard to the sound of one vs the other, there seems to be much sentiment for the sound of Alnicos. Then again, don't we all have our own preferences?

The changeover to the foam surround and the ceramic magnet appears to be simultaneous around 1970. The earliest foam surround woofer retained the damping ring on the cone, and continued to use the #7 inductor for a very short while. This appears to have changed to the ring-less cone and #9 inductor within months in1970.

The earliest ceramic magnets had indentations around the perimeter and were used well into 1971, and possibly early 1972. The most common completely round ceramic magnet showed up in 1972. The only square magnet version was ceramic, and appears to have been used for a short time in the early 80's, and shows up often as a replacement woofer. When Tonegen started manufacturing them in the mid 80's the magnet went back to the round shape.

The cloth surround/alnico magnet version was tough and consistent. There are several iterations of the foam surround/ceramic magnet woofer, and the TS parameters did gradually change somewhat. Consequently, AR-3a's using them varied slightly in character over the years, and were affected by the changing nature of the foam as it decomposed.

Attached are photos of the early foam surround cone (early 1970) and early ceramic magnet (1971).

Roy