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Crossover mods for the AR4x


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I have about a dozen AR4x woofers here, so we'll be able to nail that part well once it's set up.

Tweeters, not so many. :)

Ken dissected some defectives, and will post pics and an analysis when he gets time, he says.

[The silicone blob was intact in one of them.... :) ]

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If you wanted to stick with a first order network, this was a little better than stock. It is 10uF and reversed phase (reversed from as sold, but actually in-phase with the woofer). The overlap between drivers was less and and the 1200 Hz bump is gone but there is still a broad depression above.

First of all, great thread Dave! I have a simple question.

Since your suggestion here is so simple here, why didn't the original designers do the same thing? You mention that cost was a big factor in the design, and this wouldn't cost any more for materials or labor. The only thing I can think of is 20uf caps were maybe more common or cheaper at the time.

I'm not saying the original design was better, I will probably try these when the designs are complete. :) This is just the first thing that came to my mind when I read this. Thanks!

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First of all, great thread Dave! I have a simple question.

Since your suggestion here is so simple here, why didn't the original designers do the same thing? You mention that cost was a big factor in the design, and this wouldn't cost any more for materials or labor. The only thing I can think of is 20uf caps were maybe more common or cheaper at the time.

I'm not saying the original design was better, I will probably try these when the designs are complete. :) This is just the first thing that came to my mind when I read this. Thanks!

First off, I don't want to sound critical of what was a very successful design for a long period of time.

We can only speculate while looking at the system several decades later. We do know that the AR design preference at the time was to optimize the individual driver and crossover bandpasses in a reverberent chamber. This would emphasize the power response of each driver while downplaying the axial response. When looking at the woofer and its crossover, and the tweeter and its crossover, in a reverb room, the primary difference would be the woofer's response. As any woofer rolls off considerably off axis, it would effectively have a lower crossover point in a reverb room. This would downplay the overlap issues and might suggest a larger capacitor value as optimum.

AR also prefered a balance that followed measurements of concert halls (room R converted to a response curve). This, along with optimizing woofers to be flattest in 2 pi (half space), is the reason why every early generation product seems to have a family curve that crowns at mid frequencies when the system is measured in a free field. The 2pi-4pi difference gives an uphill trend in the woofer range and the down-tilt in the treble matches the concert hall trend.

None of these are matters of "right philosophy, wrong philosophy", but are typical af a general industry-wide evolution in design approach over the years. We certainly saw the same thing within AR: the AR 9 is designed to a flatter free field response and has key features such as the acoustic blanket, that only impact the direct response.

Its also worthwile pointing out the contribution of test equipment over the years. Much of the Linkwitz/Reilly approach to crossover design, which is the basis of my discussion of "adding an order to get the phase more in line" would have been, as a practical matter, impossible in the 60's. The essential component of measuring phase response couldn't be done on a frequency sweep basis. The first tool for doing that that I recall was the B&K phase meter of about 1980. It still didn't allow removal of excess phase, the 1 or 2 meters of air path delay, so it wasn't a great help. (My rusty recollection.) It really took MLSSA, TEF and FFT methods to give easily made phase curves.

I don't think capacitor cost was a factor. But I'm sure that a more complex crossover, certainly a third order network, would have been rejected for reasons of cost. This is again typical of the times. JBL made a lot of money selling L100's with 2 caps and 2 L-Pads, nothing else. An inductor on the woofer was already an extravegence on AR's part, in a budget system.

Regards,

David

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First off, I don't want to sound critical of what was a very successful design for a long period of time.

We can only speculate while looking at the system several decades later. We do know that the AR design preference at the time was to optimize the individual driver and crossover bandpasses in a reverberent chamber. This would emphasize the power response of each driver while downplaying the axial response. When looking at the woofer and its crossover, and the tweeter and its crossover, in a reverb room, the primary difference would be the woofer's response. As any woofer rolls off considerably off axis, it would effectively have a lower crossover point in a reverb room. This would downplay the overlap issues and might suggest a larger capacitor value as optimum.

AR also prefered a balance that followed measurements of concert halls (room R converted to a response curve). This, along with optimizing woofers to be flattest in 2 pi (half space), is the reason why every early generation product seems to have a family curve that crowns at mid frequencies when the system is measured in a free field. The 2pi-4pi difference gives an uphill trend in the woofer range and the down-tilt in the treble matches the concert hall trend.

None of these are matters of "right philosophy, wrong philosophy", but are typical af a general industry-wide evolution in design approach over the years. We certainly saw the same thing within AR: the AR 9 is designed to a flatter free field response and has key features such as the acoustic blanket, that only impact the direct response.

Its also worthwile pointing out the contribution of test equipment over the years. Much of the Linkwitz/Reilly approach to crossover design, which is the basis of my discussion of "adding an order to get the phase more in line" would have been, as a practical matter, impossible in the 60's. The essential component of measuring phase response couldn't be done on a frequency sweep basis. The first tool for doing that that I recall was the B&K phase meter of about 1980. It still didn't allow removal of excess phase, the 1 or 2 meters of air path delay, so it wasn't a great help. (My rusty recollection.) It really took MLSSA, TEF and FFT methods to give easily made phase curves.

I don't think capacitor cost was a factor. But I'm sure that a more complex crossover, certainly a third order network, would have been rejected for reasons of cost. This is again typical of the times. JBL made a lot of money selling L100's with 2 caps and 2 L-Pads, nothing else. An inductor on the woofer was already an extravegence on AR's part, in a budget system.

Regards,

David

Big woops.

Crossover design has been turned from a black art of endless trial and error into an exact science by professionals using advanced technology. Speaker designers can try out an infinite number of combinations using active crossovers and equalizers until they find filter frequencies and shapes they like whether their criteria are measurements or the way the results sound and then use software to desigjn a passive network that gives equivalent results. Yes, you have to take into account the complex impedence of the load (the drivers) to design a network for the filter design to really work equivalently to the active one. That's what makes an actively crossed over and equalized speaker a simpler project even though it may be more expensive, larger, and more complex. It is also tempting to keep playing with it indefinitely. What a very complex passive crossover network will do to a particular amplifier that is less than stable with all loads is anyone's guess.

I expect that in the future, we will see digital active crossover networks that will have infinite slopes. This is possible without gaps or overlaps equivalent attempts at analog crossover networks can suffer. It will be interesting to see if they sound horrible. One thing they will not have is phase interference in their crossover regions.

The problem for AR4x exemplifies the problem of designing a full range two way system and even three way systems clearly. It can be done but it takes extraordinary effort. The problem centers on the fact that you are combining two resonant devices which generally have a range in which their response is relatively uniform of only 2 to 2 1/2 octaves to try to create what in effect is a single device that is not resonant for ten octaves. Something's gotta give. When you add a subwoofer to a two way system, it isn't a two way system anymore, it becomes a three way system. The irregularity of response and directionality of response at the high end of the woofer and the lack of power handling capacity and frequency falloff at the low end of the tweeter becomes obvious. The attempt to get them to meet in the middle usually involves a compromise that costs performace at their other extremes, lack of deep bass power for the woofer lack of output and dispersion at the high end of the tweeter. The use of multiple similarly sized small woofers 6 to 8 inches in diameter, special long throw small woofers with light cones, and exotic tweeters is usually not entirely satisfactory but adds enormously to the cost. Even in AR's 12" 3 way design the problem of marrying a 2" dome midrange to a 12" woofer woofer that responds down to 30 hz on its low end was not satisfactorily resolved until AR9 added another driver. This is why many regard AR5 as an overall better performer than AR3a which is identical except for AR5's 10" woofer that's a better match for the dome midrange. The cost as expected is the lowest bass frequency capability.

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Dave also has yet to post any listening impressions. So while all these different configurations and their curves are interesting to look at, we still don't know how the end result is going to sound when actual music is played through it.

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I think that will depend almost entirely on what is played through them. As properly functioning amplifiers and CD players sound pretty much alike, the variable is the program material. My experience is that the variations in the way recordings are made is equal in variety and degree to the variations in speaker performance. Room acoustics and speaker placement are also a major variable factor.

Yes, but all of these things are outside of the changes Dave is making in his speaker. The question is. when the modded speaker is A/B'd next to the original under the same conditions and with the same program material, will it sound better than the original?

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Yes, but all of these things are outside of the changes Dave is making in his speaker. The question is. when the modded speaker is A/B'd next to the original under the same conditions and with the same program material, will it sound better than the original?

Genek you ask a frustrating question. Better playing which recording and better in who's opinion? Will it sound more accurate or less accurate to an original acoustic musical instrument? That again depends on the other variables of the recording, the room placement, and the room acoustics.

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Genek you ask a frustrating question. Better playing which recording and better in who's opinion?

In this case, I would say the answer is in Dave's opinion.

I would be a terrible person to use as an evaluator, because my listening preferences are decidedly bent toward the classic AR sound; I don't even care much for most of AR's own products after the ADD series. I might perceive an improvement from flattening out dips and bumps, but if the resulting speaker has a modern "flat and accurate" sound, I either won't like it or will be reaching for the HF control to dial that classic AR rolloff back in.

Will it sound more accurate or less accurate to an original acoustic musical instrument? That again depends on the other variables of the recording, the room placement, and the room acoustics.

If the original and modded speakers both have the same drivers and cabinet and are both being used to play the same recording from the same location in the same room, it seems to me that variables other than the crossover mods should have a minimal effect on the question. What would cause this not to be the case?

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In this case, I would say the answer is in Dave's opinion.

I would be a terrible person to use as an evaluator, because my listening preferences are decidedly bent toward the classic AR sound; I don't even care much for most of AR's own products after the ADD series. I might perceive an improvement from flattening out dips and bumps, but if the resulting speaker has a modern "flat and accurate" sound, I either won't like it or will be reaching for the HF control to dial that classic AR rolloff back in.

If the original and modded speakers both have the same drivers and cabinet and are both being used to play the same recording from the same location in the same room, it seems to me that variables other than the crossover mods should have a minimal effect on the question. What would cause this not to be the case?

Perhaps I'm missing something here. I thought the ONLY mods he's making are to the crossover network. Is something else being changed? Not the drivers, not the enclosure, not the stuffing? What else is there?

Changes to the crossover network will change the frequency response of the system. It will mitigate recordings that have FR anomolies when played through the original design, exaggerate them on others. For other recordings with complimentary anomolies the reverse will be true. This is why I find this type of generalization impossible.

Here's an example. Howard is always saying when comparing two speakers they should be exactly level matched. Consider one speaker with exaggerated bass and weak treble and another with thin bass and bright treble. At what frequency do you level match them, arbitrarily 1000 hz? When playing a violin, the second speaker will sound louder. When playing a tuba the first one will sound louder. A sound reproducing system to be "high fidelity" must make provisions for the variables that will be encountered in using it. That doesn't sound very simple does it? Especially when the trend is to take all control that might affect the sound away from the consumer, to engineer it out in so called high end systems.

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Perhaps I'm missing something here. I thought the ONLY mods he's making are to the crossover network. Is something else being changed? Not the drivers, not the enclosure, not the stuffing? What else is there?

Nothing, that was my whole point. If there's no change in the drivers, stuffing, cabinets, etc., and the original and modded-crossover speakers are both placed identically and used to play the same recordings, it should be possible to assess what changes the crossover mods have made in the way the speakers sound, and for a listener to make a subjective decision as to whether one sounds "better" than the other. It should even be possible for a listener like me to do it, as long as the modded speaker has its HF control tuned as close as possible to the original so that the only measurable change is the leveling of the response curve. All the other items you cite as affecting sound should not factor into a comparison if they are the same for both speakers.

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Nothing, that was my whole point. If there's no change in the drivers, stuffing, cabinets, etc., and the original and modded-crossover speakers are both placed identically and used to play the same recordings, it should be possible to assess what changes the crossover mods have made in the way the speakers sound, and for a listener to make a subjective decision as to whether one sounds "better" than the other. It should even be possible for a listener like me to do it, as long as the modded speaker has its HF control tuned as close as possible to the original so that the only measurable change is the leveling of the response curve. All the other items you cite as affecting sound should not factor into a comparison if they are the same for both speakers.

Why would the results be different from using an equalizer to make the same change?

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Why would the results be different from using an equalizer to make the same change?

I wouldn't necessarily expect them to be. But this thread isn't about what happens to the sound when the response is shaped with an equalizer, it's about what happens to the sound when the crossover is modded.

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Dave also has yet to post any listening impressions. So while all these different configurations and their curves are interesting to look at, we still don't know how the end result is going to sound when actual music is played through it.

All those pretty curves and you want me to listen to it too? Geez! I'm workin on it! I'm workin on it!

David

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Part 4, Bumps in the Road

So far we have one prototype that looks very promising with significantly flatter response and good on-axis summing. In actuality it was a mess of wires and alligator clips and dangling parts.

What I need to do is build it up in a more permanent basis. At the same time, in order to do a listening comparison I'll need to get one speaker working with the revised crossover and one to the original. I decided to mount the tweeter crossover parts on a board and the 2 woofer circuit elements directly to the woofer terminals. I had a small double crossover (passive subwoofer crossover) board with solder terminals and a symmetrical layout, so it could be cut in half and used for both crossovers.

Parts were wired point to point and then glued down with a silicon caulk. For no particular reason I wired it from back to front (right to left). The drawn schematic is left to right in the normal way.

post-102584-1268604519.jpg

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With two boards done I could hang one outside the system I had been working on and place the second inside the other cabinet. The #2 modified system will be listened to with the new network and the first one (with its connection points outside the box) modified back to AR's original network.

A quick measurement of system #1 showed the tweeter reverse phase from what I expected. In fact I'm now totally confused about its current phasing vs. the factory network. I had thought that AR had the tweeter out of phase with the woofer. Earlier measurements showed most midrange output with that phasing, although a smaller tweeter cap and tweeter and woofer in phase did give a little better response IMHO. The third order network pretty clearly needs the woofer and tweeter in the same phase. Usually a battery to the tweeter terminals will reveal which is the plus terminal but I just couldn't see enough motion to tell. I'll keep digging and see if I can clear up the confusion.

Anyhow, with the right polarity the new board mounted network looked great for system number one. For number two, it was immediately obvious that the second tweeter wasn't a great match for the first one. Sensitivity was 2 to 3dB down all across the board. Response shape was good and the driver isn't defective (it sweeps clean) but I'll have to compensate for the lack of sensitivity. The woofers are quite close, within a dB, and good enough for me.

The orange curve is a built up network for system#1. Blue is system #2.

post-102584-1268606065.jpg

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To get the treble end of both systems to match I'll probably have to adjust more than one component. Lets start with the padding resistor. If I put 3.3 ohms in parallel with the current 4 ohms that will give an effective pad of about 1.8 ohms. This is a pretty big change but we have a long way to go.

As an asside, if you were a speaker company you would most likely buy drivers that were guaranteed to have broad band sensitivity held to about +- 1.5dB from an approved sample (bigger deviations at spot frequencies would usually be allowed as well). You can do better but you will usually have to pay a premium for it.

The smaller resistor value pulled up the top end of the tweeter nicely, but we are still pretty far off at lower treble frequencies. Orange is sytem #2 with no change, blue is the smaller total resistance, and the top burgundy curve is system #1 that we are trying to match.

David

post-102584-1268607070.jpg

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I tried changing the first cap (was 22uF) and the second cap (was 10 uF) and in the end even changing both a significant amount didn't get me quite to parity. This all makes you think about restorers that are matching their parts to 1% or better. Good idea if you drivers are really close, but what does it get you when tweeters are 3dB off?

This curve shows the front 22uF cap with +5.6uF and the 10 + 4uF. We are now about within 1dB of the other system but I've run out of time for today. I'll tweek a little more when I get a chance and, I promise, next time I'll do a proper listening comparison!

David

post-102584-1268608073.jpg

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First off, I don't want to sound critical of what was a very successful design for a long period of time.

We can only speculate while looking at the system several decades later. We do know that the AR design preference at the time was to optimize the individual driver and crossover bandpasses in a reverberent chamber. This would emphasize the power response of each driver while downplaying the axial response. When looking at the woofer and its crossover, and the tweeter and its crossover, in a reverb room, the primary difference would be the woofer's response. As any woofer rolls off considerably off axis, it would effectively have a lower crossover point in a reverb room. This would downplay the overlap issues and might suggest a larger capacitor value as optimum.

AR also prefered a balance that followed measurements of concert halls (room R converted to a response curve). This, along with optimizing woofers to be flattest in 2 pi (half space), is the reason why every early generation product seems to have a family curve that crowns at mid frequencies when the system is measured in a free field. The 2pi-4pi difference gives an uphill trend in the woofer range and the down-tilt in the treble matches the concert hall trend.

None of these are matters of "right philosophy, wrong philosophy", but are typical af a general industry-wide evolution in design approach over the years. We certainly saw the same thing within AR: the AR 9 is designed to a flatter free field response and has key features such as the acoustic blanket, that only impact the direct response.

Its also worthwile pointing out the contribution of test equipment over the years. Much of the Linkwitz/Reilly approach to crossover design, which is the basis of my discussion of "adding an order to get the phase more in line" would have been, as a practical matter, impossible in the 60's. The essential component of measuring phase response couldn't be done on a frequency sweep basis. The first tool for doing that that I recall was the B&K phase meter of about 1980. It still didn't allow removal of excess phase, the 1 or 2 meters of air path delay, so it wasn't a great help. (My rusty recollection.) It really took MLSSA, TEF and FFT methods to give easily made phase curves.

I don't think capacitor cost was a factor. But I'm sure that a more complex crossover, certainly a third order network, would have been rejected for reasons of cost. This is again typical of the times. JBL made a lot of money selling L100's with 2 caps and 2 L-Pads, nothing else. An inductor on the woofer was already an extravegence on AR's part, in a budget system.

Regards,

David

Thanks for the explanation, although I don't understand all the technical details, I get the gist of what you said. I can certainly see the difference on the curves. :)

I did do an A/B comparison with the old block cap hooked up in one and a new in the other. Sometimes there wasn't much difference in sound, but other times it was very apparent, such as acoustic guitar and some vocals. I would say the one with the new cap sounded a bit 'boxy' compared with the old cap. I'll probably keep these relatively stock, but would definitely try the mod if I run into a second set of these. :D

Interestingly enough, my 4x's have the tweets in phase with the woofers(green wire attached to wiper, and yellow to #2) they're not the super early cross hatch woofers, but must be shortly after. I'll probably start another thread soon to document some of the differences on mine. :)

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Déjà vu.

Westies are stuck at the gate 'til we scrounge up more AR4x tweeters out here:

Z-

I don't know where they are at this instant, with all the moving going on... but was there anything in what you gave me that might help David get a matched pair of tweeters?

-k

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Déjà vu.

Westies are stuck at the gate 'til we scrounge up more AR4x tweeters out here:

Your pair looks like my pair (she said with a wink). I was thinking of fixed resistors, and probably will keep that with what I build up.

But for the sake of the experimenters that want to try their own, it looks like some variable resistance to be set by ear will be called for. If I remember right the shape worked well with series resistance only (rather than a potentiometer connection) so the 15 ohm pot with another resistor around it might be the best answer.

Second topic: I was looking at some of your threads at AK and other threads here and saw the inductor #4, inductor #5 issue. I'll have to check to see what I have but this adds another complication for those wanting to duplicate this. I wonder if that was a conscious re-voicing or to compensate for a woofer change.

Anyone?

David

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Second topic: I was looking at some of your threads at AK and other threads here and saw the inductor #4, inductor #5 issue. I'll have to check to see what I have but this adds another complication for those wanting to duplicate this. I wonder if that was a conscious re-voicing or to compensate for a woofer change.

Anyone?

David

I do know AR eventually changed from fabric to foam surrounds (I just refoamed a pair). I hope John O'Hanlon chimes in here soon. He has a good knowledge of the 4x. There is an old thread here in the AR area regarding 'speaker box stuffing study' where John listed recommended stuffing amounts (not density) for the #4 and #5 coil versions. His moniker is 'johnieo' if you do a search.

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I don't know where they are at this instant, with all the moving going on... but was there anything in what you gave me that might help David get a matched pair of tweeters?

Nope, they'd be the dead ones you dissected, and we're counting on you to figure out and tell us what's going on here.

I just gotta go out and round up some more, is all. It's important to know the variability of the stock tweeters in AR4x today, as Dave has found this to be a significant factor.

[Did I send him the "good" one or the "bad" one? :) ]

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Your pair looks like my pair (she said with a wink). I was thinking of fixed resistors, and probably will keep that with what I build up.

Second topic: I was looking at some of your threads at AK and other threads here and saw the inductor #4, inductor #5 issue. I'll have to check to see what I have but this adds another complication for those wanting to duplicate this. I wonder if that was a conscious re-voicing or to compensate for a woofer change.

David

Hello David:

More than one issue in your above comments. First, AR apparently revoiced all their speakers in the late 1960s. In about June of 68, the AR-3a woofer was replaced with the new ceramic magnet based woofer, and this of course used a new cone and surround. As well, the stuffing was reduced from 28-30 oz to 18 oz of FG and its inductor changed from #7 to #9 (About serial number 38,500). Be aware that the AR published data were taken on the new ceramic magnet style woofer. Ditto for the AR-4x. Its coil was changed from #4 to #5 and FG stuffing reduced from 18 to 12 oz somewhere between June and Dec 68 (between serial numbers 175,000 and 215,000). At some point about 1971, near the end of its production run, old woofers with cloth surounds were eventually replaced with woofers with foam surrounds. I measured the system Q of several old AR-3a and old 4x and found it to be reduced from about 1 to about 0.75, if my memory serves me. These numerous changes are why I doubt anyone can "hear the sound of an Alnico magnet" ;)

My gut feeling is that AR was responding to competition by reducing Q so as to gain another hertz or two of low frequency amplitude.

Some years ago, I had a pile of 4x tweeters and sent them to Roy. He had an equal-sized pile and paired them by output level as hi, medium, and low and returned same number to me as pairs. Since the units all have level controls, it did not make much difference which pair we used--loud or soft--as we could adjust the level control.

My personal solution to this was to rebuild all AR-4x according to one style--#5 coil, 12 oz Fiberglass, cloth surrounds, potentiometer level control. In this way replicate the "new" 4x style. Even if the level of the weak units was not extreme with control full up, they made fine surround speakers for a small-room home theater. Also, I have used either dilute Permatex or RTS (Roy's tacky stuff) to coat the cloth surrounds, if the originals were dried and leaky. And again, my capacitor choice has been polymer dielectric, so my kids will not have to carry them to the curb after I am gone--they don't know a capacitor from a magic marker!

Personally, I would doubt that you could ever make a new crossover without level controls as a subsititute for any old AR speaker that used level controls in its original design. They were intended to deal with room and placement differences; today we have the added distraction of decades of variable level decay in any vintage AR speaker. I doubt that any one tweeter pair you might obtain would be at all representative. Also, the output near crossover (1,200 Hz) will come from both drivers, so wouldn't the change in crossover order affect the overall speaker's character in the octave near crossover?

Cheers,

John

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