3a x-o change from 575 to 525Hz

[Steve F]

There has been a lot of interesting discussion on the Forum about the change in the 3a’s (and 5’s) crossover specification from 575 to 525Hz (in the 5, from 650 to 550Hz). These changes were first published in 1974, and according to a letter I received from AR (though not from Roy Allison, who had departed at the end of 1972), the new specs reflected, as they put it, "changes that had been made some time ago."

Tom Tyson has reported that Roy Allison doesn’t think the 3a’s x-o ever changed (although the 3a did continue in production for another three years after Allison left AR).

There is a very plausible explanation for the 3a’s and 5’s change in specs--an explanation that would be consistent with both Roy’s contention that the crossover never changed AND the accuracy of the newer specs.

Consider this:

As we all know, there is a difference between the electrical crossover and the acoustic crossover of a speaker system. If you run the voltage curves through the crossover network, they'll show a very definite characteristic whereby the "crossover" frequencies of the low-, mid-, and high-frequency sections are readily apparent, with no ambiguity whatsoever.

However, drivers do not behave as precisely and predictably in actuality as do electrical circuits. Even though the crossover may be "telling" the woofer to roll off by 3dB at, say, 600Hz, the woofer may in fact continue to respond acoustically until 650 or 700Hz. Drivers are imperfect devices: they have resonances, inertia, variable compliance, stored energy, etc. that makes them not "start and stop" exactly where the electrical "crossover frequency" would indicate.

Therefore, the result is that the driver’s acoustic curves overlap and "crossover" at different points than the electrical circuit of the crossover network. When Engineering finishes a speaker, they go to Marketing and show them the curves and other data. Marketing then interprets the data and decides how to rate the published specs for the product’s frequency response, x-o frequency, impedance, everything.

The x-o voltage curves would say one thing, and the actual acoustic output of the drivers and where they overlapped would say something else. They were similar, but not identical by any means. Usually, the engineers would say to the head marketing person, "OK wise guy, where do you want to say it crosses over?" It was never clear-cut. It was always open to interpretation. I know this first-hand from over 25 years’ experience.

Therefore, I have the feeling that the re-specing of the x-o frequencies from 575 to 525 (3a), and 650 to 550 (5) were just a rethinking of how to characterize the existing—unchanged—data. This strikes me as even more likely to be true if the actual schematics and layout of the two crossovers never changed, as Roy indicated they never did.

Anyway, that's my take. It certainly seems plausible, especially since marketing people like lower crossover frequencies and this would have been an easy way for AR to appear to have made a small improvement in the speakers at a time (1974) when the company was under tremendous competitive pressure from Advent, EPI, JBL and others, and AR was failing quite badly in the marketplace.

Steve F

[johnieo]

Steve: Thanks for your comments; your experience is appreciated. However, we still have to deal with the reality that numerous early AR-3a speakers contained a different woofer inductor. AR-3a design modifications such as woofer change and stuffing weight should not have affected the electrical x-o frequency; not so for the inductor change.

The woofer inductor change from #7 (1.9 mH) to #9 (2.85 mH) would definitely affect x-o frequency in the proper direction and magnitude. There have been numerous pictures of parted-out, early AR-3a x-o boards on e-Bay and other sites where the “#7” coil marking was clearly visible. Forum members have described many early cabinets containing the small coil—too many and over too long a time period for this to have been an assembly error.

We may never learn more than AR’s admission that it did change the crossover frequency. The data show that the coil change involved s.n.’s as early as the four thousands and as recent as twenty-eight thousands (July ‘69). Some of us have proposed that this change was linked to the woofer change. This may be incorrect; the coil may have been changed as early as mid-’69 and for a different reason. Perhaps all early cabinets should be refitted with the large inductor?

Manufacturers continually make small changes to products. It would not have been in AR’s best interests to announce publicly that they changed the response of a best-selling, highly rated speaker.

[Guest Furtsurfer]

Very interesting debat.

If a member can explain the following:

-In earlier ads for LST, the crossover frequency is 575 kHz for bass to midrange, and then 525. OK, no problem (the dates?? and/or serial numbers).

-Th first 3a's are also 575 kHz and then 525.

-The 3a/Improved (here in Europe) is produced with a crossover for 575 kHz!! (a return to a 3a first version!). I have this model.

Furtsurfer

[Guest Bret]

>AR-3a design modifications such as woofer change and stuffing weight should not have affected the electrical x-o frequency; not so for the inductor change.<

John - I understand the comment about electrical crossover frequency; but let's not forget other things suggested by Steve F.

The crossover frequency that AR was quoting should have been the *effective* middle of the crossover frequency, shouldn't it?

By changing driver motors or just cone materials you could affect the entire crossover point; even if you didn't touch the passive crossover.

You know I don't think I have to know everything and maybe this is a walk down a primrose path rather than a tip-toe through the tulips, but the Tonegen woofer on the same passive crossover resulted in a very different acoustic crossover point and character than the original woofer. The Tonegen had a lot more top-end output that seemed to overlap the midrange's low point by quite a lot even though the electronics were telling it to hush. (in other words, 6db down did not occur at the same frequency, it was nevertheless 6db down for that specific driver)

So the inductor change *might* have been compensation for a change in the driver and *not* be perfectly correlated to a change in crossover frequency. I realize that it also *might* be, but I don't think I'm hunting pink elephants in my pajamas.

They could change the woofer and the choke and not change the speaker system's spec. In fact, the change in choke could be to bring the system back to original spec after nothing more than a woofer cone materials' change.

They could change nothing but a driver and change the crossover point while leaving the passive crossover alone, too.

It's the mechanical mechanism's effect on tonality and performance that makes the Tonegen seem "out-of-place" in a system where the crossover is untouched. Not only does making the spider "more floppy" increase the extremes of deep bass response, it seems (to my ear) to reduce the driver's output at the top of its crossover range. They might have had to "goose" the earlier driver's top-end output to get a flat system response. Alternatively, they might have had to more quickly quieten a slightly different driver for the same result.

In any case, I'm just repeating, differently, what Steve F has already said. It just bears repeating.

Bret

[Guest Bret]

>Therefore, I have the feeling that the re-specing of the x-o frequencies from 575 to 525 (3a), and 650 to 550 (5) were just a rethinking of how to characterize the existing—unchanged—data.<

Absolutely lucid. It could also be a real change; the result of changing nothing but cone materials or motor strength or spider compliance or foam edging, right?

Bret

[RoyC]

Hi Bret,

We both experienced the Tonegen woofer and I totally agree with your description of it's character. In fact my reworked Tonegens now have LESS high frequency output than the AR originals....but are much more satisfying than they were.

I also experienced the sound of the #7 coil with an alnico and ceramic woofer. I had taken out the alnico woofer from an old '3a I had just acquired to investigate an intermittent rattle and dropped in a woofer from a '71 AR-3a. The sound in the midrange was immediately noticable in a "bad" way and different enough to make me wonder what was going on. It prompted me to check out the crossover and discover a 1.9mh coil where the 2.85mh #9 was supposed to be. I searched the website and found references to the anomaly and later compared notes with John. Although I find everything you and Steve suggest plausible, the sound with a ceramic woofer and #7 is not in keeping with the best attributes of the AR-3a (in my opinion). If I were at the helm in AR land I would have replaced that sucker as soon as possible.

Anyway, time to get my jammy's on and keep an eye out for those pink elephants :-).

Roy

[Guest Bret]

> the sound with a ceramic woofer and #7 is not in keeping with the best attributes of the AR-3a (in my opinion).<

I haven't heard it so commenting about the sound would be useless.

But this does sorta smack of a situation where the reps complain that the customers aren't happy with the new woofer. (or a consumer, or someone)

A different someone with a slide rule,three masters degrees, a pocket-protector collection that makes Emelda Marcos' shoe collection look amateuristic, who thinks that selling things and listening to customers is FAR beneath them (although it feeds them and keeps them in HP-41Cs) says to the marketing team (who obviously are all completely ignorant sub-educated, "little people" and don't deserve air to breathe), "Golly-gosh boys and girls; the new woofer is a reasonable replacement for the old one, just look at the graphs and numbers! Nobody can hear that difference," and then mutter under their collective breath, "Why do we have to have marketing people and customers anyway?"

Finally someone says, "But have you HEARD it?!" to which the overly self-important respond, "Well, no, but we have the adding-machine tape to prove. . ."

"Well then, here, LISTEN !"

"Ohmygosh does that suck. Let's change an inductor."

Inconvenient facts sometimes smack quad-digit IQs around.

[RoyC]

LOL