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AR91 repair story - new mids and tweeters identified


Carlspeak

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I received a pair of the subject speakers to 'get working again'. Evidently, the mids and tweeters were blown which subsequent testing with my VOM confirmed. Since no OEM drop ins are available, I was tasked with finding something comparable so the speaker sounded close to original.

After wading thru the myriad of tweeters and mids available, I settled on a Vifa BC25TG15-04 (4 ohm) and a HiVi DMB-A dome midrange. The Vifa was rated at 5 ohms nominal. Upon receipt I was pleased to discover they OD of the midrange fit the cabinet perfectly. All I had to do was salvage the original plastic decorative face plate and drill new mounting holes to accept the different hole pattern. The tweeter OD was smaller than original and thus I had to fabricate two masonite adapter plates so I could have something for the mounting screws to fasten to. For both the tweeter and midrange, I was able to salvage the Velcro dots and reuse them for attaching the original, contoured foams.

Below you see the fruits of my labors. The crossover remained unchanged and the speakers sound great. I have them set at -3 dB switch position for both mids and tweets for best sound.

When I removed the woofers to inspect the crossovers to see if there had been any damage during the event that cooked the mids and tweets, I discovered the woofers had been refoamed incorrectly. Someone had used a thick, 12 inch foam and added a cardboard spacer ring (gray in pic below) to bridge the gap between the cone OD and the ID of the 12 inch foam. I convinced the customer they needed refoaming with proper foam which is also thinner and more flexible. Upon completion, I tested one woofer and found the Fs was reduce by 6 hz from 28 hz down to the normal 22 hz for fresh, un-broken in foam. The picture below shows the woofer before refoaming.

post-100237-0-28503900-1350082127_thumb.

post-100237-0-48110600-1350082130_thumb.

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Wow--that cardboard spacer seems pretty flaky. The old cut 'n paste method would have been better!

The repair with your driver subs looks great! Any way of comparing the new to the original? (no--I won't drive up to CT with my 91s ;) ).

Kent

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Wow--that cardboard spacer seems pretty flaky. The old cut 'n paste method would have been better!

The repair with your driver subs looks great! Any way of comparing the new to the original? (no--I won't drive up to CT with my 91s ;) ).

Kent

Couldn't compare the old. I got the speakers with the mids and tweets D.O.A. The new sounds great. The customer agreed and now has them. I don't any more.

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How do you decide on a replacement for driver that is no longer made? My Warfedale W-35 need both mid-range and replacement tweeters. These BIC drivers have not been available for several decades. The NHT woofers have worked out very well, thanks JKent, But the rest?

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How do you decide on a replacement for driver that is no longer made? My Warfedale W-35 need both mid-range and replacement tweeters. These BIC drivers have not been available for several decades. The NHT woofers have worked out very well, thanks JKent, But the rest?

Finding replacement drivers from mfgr's other than OEM isn't easy (other than checking size for proper fit into a cab.). It requires considerable experience with speakers, design and technical know how.

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Nice work, Carl...

I've had my eye on that mid for awhile now, and agree it was the best choice. I have tried that Vifa tweeter in some other AR models, and found the sensitivity to be a bit on low side. I'm surprised you didn't make any crossover adjustments with two new drivers in the mix.

Removing the original mids' donut ring faceplate covers was a good way to go. It is easy to do. I recently found backwired AR 4 ohm mids, from the screen-covered versions found in the AR-3a/11/10pi/LST models to the donut ring clad versions of the AR-9/91/92/58 era, to ALL measure pretty much the same when the screens and donut rings are removed. In other words, they are interchangeable! I had some bare original Cello Amati AR mids here as well, which also proved to be identical. I have converted a number of them in one direction or another by removing or adding the faceplate enhancements.

Roy

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I could have done a few tweaks on the xo, but that would have added considerable cost to the customer.

One surprise I did find was the box Q with the refoamed woofers installed. It averaged about 0.5! And bass did sound lean.

Additionally, Fc was a very nice 37 hz.

The boxes were stuffed pretty much to the max with crimped polyester. I pulled up to about 1/2 of it out of one box trying to raise Q closer to 0.7 or 0.8 in a few tries and the best I got was 0.54. I was quite surprised at the low Q considering all of the measurements I'd done over the years on numerous other AR models that ranged from 0.7 to over 1. Perhaps the British affiliation with AR Teledyne had something to do with the overdamped AR 91?

I estimate the AR 91 box volume around 1.6 Cu. Ft. The AR dwg. calls for 20 oz of stuffing. At 0.78 lb/cu. ft. rate, that seems about right for PET based on my studies. So why such a low Q? Perhaps the series coil with the woofer has something to do with it.

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One surprise I did find was the box Q with the refoamed woofers installed. It averaged about 0.5! And bass did sound lean.

Additionally, Fc was a very nice 37 hz.

Interesting, Carl...I agree, that seems very low. Other than Fs, what were the measurements of the woofer out of the cabinet?

The cabinet volume of the AR-91 was about the same as the AR-3a, and the AR-91 is often considered to be one of the last versions of the 3a, preceded by the AR-11 and 10pi.

Early AR-3a system Q, using the alnico magnet/cloth surround version of this woofer, was very low (.6 to .7), and was highest (around 1.0) around 1970/71 with the introduction of the foam surround/odd shaped ceramic magnet. It gradually went back down to near .7 by the second generation of the AR-11. The use of poly-fill instead of fiberglass contributed to lowering system Q of the AR-11 in the very late 70's. (The first version of the AR-11 used fibgerglass, and the second version used poly-fill stuffing).

Changes to the AR 12 inch ceramic magnet woofer included increasing magnet strength, lighter cone, and different spider material. By the time of the AR-91, the most important role of the 12 incher was in a dual configuration in the AR-9. Increased power handling was probably the main reason for changes made to this woofer. I doubt anyone knows precisely what or when changes were made, but the trend is clear.

I think your findings challenge the notion of those who speculate the AR-91 was the "ultimate" AR-3a. I don't think it was. The 3a design was no longer a priorty, and the parts had been tweaked for other designs.

Roy

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Below is a WT2 test on the AR 91 woofer after I refoamed it correctly. To my recollection, the T/S parameters weren't all that different from my other AR3a woofer tests.

...just took a quick look at some measurements I did awhile back. Your Q measurements are a bit lower than my measurements of 1971 to 1978 woofers, but not unusual for post 1980 versions (including the later Tonegen replacement version). 1979 appears to have been a transition year based on some recent measurements.

Qms of the older woofers are typically above 5, with the earliest woofers near 10. Qts is typically around 3. Qes is pretty much the same across the board. There is no doubt that the earlier ceramic magnet specimens had higher out-of-cabinet Q. Zmax is also higher with the earliest woofers (usually around 100 ohms). Fs for all of the above ranged from 18 to 21hz.

An interesting side note related to the AR 12 inch woofer...Since I have been working on woofers for Larry/"Vintage AR" I have been seeing many bad spiders in post 1980 woofers (but not the Tonegen made versions, which are very well built). The spider material is different, being thinner, but less flexible than the earlier woofers. They tended to sag, and stay that way over time.

The Tonegen spider material is more like the earlier 1970's woofers, but somewhat less compliant.

Roy

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I still suspect the woofer series inductor coil is the culprit. The AR 91 has a coil 1/2 the inductance (1.37 mH) as the AR3a (2.85 mH). Parallel caps are similar with a 100 uF cap in the 91 and a 150 in the 3a. I tried doing some modeling using Bass Box Pro and Xover Pro to no avail.

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I still suspect the woofer series inductor coil is the culprit. The AR 91 has a coil 1/2 the inductance (1.37 mH) as the AR3a (2.85 mH). Parallel caps are similar with a 100 uF cap in the 91 and a 150 in the 3a. I tried doing some modeling using Bass Box Pro and Xover Pro to no avail.

It certainly isn't helping matters...Whatever the cause, the low Q is obviously original to the 91, and, for better or worse, does not sound like its older cousins.

Hey, maybe AR didn't want it to sound too "big". After all, there were more expensive models to be sold. :)

Roy

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I still suspect the woofer series inductor coil is the culprit. The AR 91 has a coil 1/2 the inductance (1.37 mH) as the AR3a (2.85 mH). Parallel caps are similar with a 100 uF cap in the 91 and a 150 in the 3a. I tried doing some modeling using Bass Box Pro and Xover Pro to no avail.

Raises the question: Would an inductor mod "improve" the 91?

Kent

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So here's the question,could the 91 bass sound be made to sound like the 3a bass sound?

I have both the 91 and 3a and there is a very different bass sound.The best way I can explain it is the 91 comes out of the front of the speaker and the 3a comes out the back,the 3a going lower. How's that for technical terms. My new to me AR-11's are much more like the 91's then the 3a's.

Kent beat me to it.

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Carl

What Roy posted about the spiders in the post 80 woofers is making me wonder if my 91's may not be good to compare to. I think I'm going to switch a woofer from one of the AR-11's to one of the 91's and see if I can hear a difference. I will let you know tomorrow. The serial # on the 11's are 1014 and 1015. Am I correct in thinking these would be early versions? I just checked the 91 serial #'s, they are 310 and 5758.

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I have compared all versions of the AR 12 inch woofer many times in the AR-3a (early and later), as well as the AR-11.

The early to mid 70's woofers with the measurements I mentioned above consistently produce a higher system Q (.8 to 1), and consistently sound more reverberant. The numbers I mentioned above may not seem much different from the later woofer Carl measured, but they reflect significant differences percentage-wise. They may also suggest differences we are not measuring. The early 80's and Tonegen versions simply do not produce the same subjective reverberant response in the models above, regardless of stuffing and crossover differences.

If the goal is to bring the subjective bass response of the AR-91 closer to that of the AR-3a, I would first try pre-1980's AR woofers, and switch out the poly-fill for fiberglass. Switching out the crossover inductor would lead to the need for other crossover changes, and would not make a large enough change in bass response unless earlier woofers are also employed, imo.

About 9 years ago I dropped off AR woofers for Bill LeGall of Millersound to work on and compare. http://www.millersound.net/about.htm

His opinion was reduced cone mass, stiffer spiders, and (possibly) higher magnet strength of the later woofers accounted for all of the differences.

Roy

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So switching out my 91 woofers to 11 woofers should be quite noticeable?

It depends on when your woofers were manufactured, and their condition. There should be a date stamped on the back of the woofer magnets. If you have the "A" version of the AR-11 along with the original woofers, it is worth a try. Fiberglass stuffing will also raise Q. The combination of the two should be noticeable. Post a photo of your woofers...

Something else to consider is that the tonal balance of the AR-91 leans much more toward the higher frequencies than the AR-3a. The same subjective volume level will not sound as "bassy" with the AR-91. One more thing...Listening space has a great deal to do with perceived bass response. Make sure the speakers are being compared in the same place in the room before deciding a woofer change may be needed,

Roy

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Roy

I get what you're saying.The 3a's rattles the glass at lower volumes then the 91's. I am going to swap one of the 11's woofers into a 91 just for kicks. I'll post what I hear.I'll also check the date on the woofers.

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So switching out my 91 woofers to 11 woofers should be quite noticeable?

Or... Add a big-a** subwoofer. I use an Original VMPS. Thunderous.

Kent

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Ok I did the speakers swap.

The date code on the 91 woofer is 5618108 the serial # 200003-1,the 91 serial # is 310.

The date code on the 11 woofer is 561 7726 the serial # 200003,the 11 serial # is 1015.

I set them side by side in the middle of the room,set the receiver on mono and went back and forth with the balance control and there is a difference. This is a listening test only, I have no equipment to measure anything.

The 11 woofer is much more "there" then the 91 woofer even at lower volumes. The 91 woofer is not as crisp as the 11.

Could it be the weak spiders Roy was talking about.

Both woofers have had new correct surrounds done in the last year.

I will have to say the 11 woofer is a noticeable improvement over the 91 woofer. I don't know if the 11 goes lower then the 91 but it does sound better.

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How's your 91 bass sound compared to your other AR's? Harry M thinks his is not as good as his 3a's.

I'm not sure. They are in different rooms and the 91s are not positioned optimally. Also, I have a subwoofer set up with the 3a's. Absolutely not needed but I had it left over from years ago when my setup consisted of a pair of Allison: Fours and a VMPS Original sub. The sub was just sitting there, so I hooked it up with the 3a's, crossing over at 40 Hz.

Kent

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One surprise I did find was the box Q with the refoamed woofers installed. It averaged about 0.5! And bass did sound lean.

Additionally, Fc was a very nice 37 hz.

The boxes were stuffed pretty much to the max with crimped polyester. I pulled up to about 1/2 of it out of one box trying to raise Q closer to 0.7 or 0.8 in a few tries and the best I got was 0.54. I was quite surprised at the low Q considering all of the measurements I'd done over the years on numerous other AR models that ranged from 0.7 to over 1. Perhaps the British affiliation with AR Teledyne had something to do with the overdamped AR 91?

I estimate the AR 91 box volume around 1.6 Cu. Ft. The AR dwg. calls for 20 oz of stuffing. At 0.78 lb/cu. ft. rate, that seems about right for PET based on my studies. So why such a low Q? Perhaps the series coil with the woofer has something to do with it.

Your observation regarding the low "Q" would suggest that the units were functioning true to design intention, as previously outlined by Tom Tyson:

>Wasn't the AR-91 the logical extension, and evolutionary

>descendant of the AR-3a?

>

>With all of its drivers derived from the AR-9 (as well as the

>Acoustic Blanket, and vertical driver alignment), does this

>make the 91 AR's most refined 3-way?

>

>Is this system undervalued by AR enthusiasts?

Others have commented on the AR-91 (and this applies to the AR-92 as well), but there are some engineering highlights that have not been mentioned.

When the AR-9 was introduced, AR paid careful attention to the "boundary effect" (originally identified and characterized by Roy Allison), whereby the woofers were mounted on each side, close to the floor-wall intersection when the speaker was placed, as intended, back-against-the-wall. Unfortunately, AR did not give credit to Roy Allison for this development.

The AR-91 (and AR-92) also attacked the problem of boundary interference, and each enclosure placed the woofer close to the floor with the speaker systems also intended for against-the-wall placement. The compromise was that the AR-91 woofer was front-mounted, but the boundary-interference effect was somewhat reduced. The AR-91's crossover also was modified to further minimize the effect. The AR-91's cabinet volume was somewhat larger than the AR-3a's, and system resonance was dropped to 40Hz (-3dB was 35Hz) with the "Q" set to 0.56. The result was a slightly overdamped system offset somewhat by the crossover and by the fact that the woofer was placed relatively close to the floor.

The AR-91 was also a "vertical" speaker, and by vertically stacking the woofer, midrange and tweeter, the interference effects were shifted into a vertical plane, rather than horizontal, and thus the frequency response from both speakers, as heard in the listening position, was more stable. The AR-91 tweeter was identical to that used in the AR-9 and AR-90, but the midrange unit was modified. This unit has a different part number as well. The 1-1/2-inch midrange unit used in the AR-9 and 90 used Ferro Fluid on *both* sides of the voice coil, and sealed off the cavity under the dome, thus raising the resonance frequency of the dome; but in the AR-9 and 90, the crossover was set quite high and this was not an issue. In the AR-91 the crossover was 700Hz, so the voice coil in the midrange used Ferro Fluid on the *inside* of the voice coil only, and thus the cavity beneath the dome was larger, lowering the resonance frequency of the dome. It could then operate within the 700Hz crossover range more effectively. The small "semi-horn" appendage on the front did not affect the lower cutoff frequency of the dome, as it was designed to help maintain efficiency in the upper level of the operating range. It did nothing below 3kHz according to Tim Holl.

The result of all this was an improvement in many ways over earlier 3-way AR speakers, and a speaker not always recognized for its fine performance. As Steve F remarked, it was somehow overlooked in the audio press.

--Tom Tyson

See also the following comment concerning the AR-58s (which had the same 1.37 mH coil in place):

The AR58s was originally described as a "bookshelf" speaker (it would have to have been placed vertically on a bookshelf for optimum performance), but with the advent of AR "Verticals," it was clearly intended as a floor-or-stand-mounted speaker, and it was "tuned" (highly damped) more for floor and stand use because of its fairly low "Q" of 0.5 or 0.6 -- designed to attenuate the bass at resonance slightly to keep it from sounding bass-heavy when placed on the floor. Besides, people were getting away from "bookshelf" speaker placement by this time and usually placing the speakers on the floor or on a stand/shelf.

--Tom Tyson

Given the premise and direction of this topic, it would seem perhaps better placed in the "Mods, Tweaks, and Upgrades to the Classics" section of the forum.

Robert_S

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