Guest AR-2-D2 Posted July 31, 2005 Report Share Posted July 31, 2005 HiMy first post here. I have read many of your old posts with a lot of interest.I purchased a pair of AR-2ax speakers a while ago (European manufactured (US drivers) from the 70's ). One of the speakers had a refoamed woofer but was otherwise intact and sounded very very nice. The other one had a missing mid-driver and the original foam (10" woofer) was rotten. I will try to refoam the woofer and get a working original mid for it someday.Anyway I have started to do some preliminary testing just for fun and to see if there are any hidden problems. Disclaimer: I am aware of 1. the limited value given that these are just my speakers and 2. that they are rather old. 3. The measurements are of the quick and dirty variety and the data are (so far) absolutely not intended to be highly accurate or anything but perhaps someone here could find them slightly interesting or I can redo them or run additional tests per request. Any suggestions of what could be interesting in this area or should I just forget it? Just as an example I attach a graph below of my measured impedance for the 10" 4-bolt 70s woofer in the sealed original cabinet (caution! with the series coil still in circuit) and some preliminary measurements in free air (just the driver, note! not really properly mounted for this kind of test). Preliminary data show that the Free air Fs of the refoamed driver is about 28 Hz and resonance at 54 Hz in the sealed box (+ or - a couple of Hz). The free air Fs of the other driver with rotten foam was surprisingly slightly higher.I have much more of these "quick and dirty"-measured stuff of the other 2ax drivers if any want some of it.Best regards / Bo10" woofer impedance: Link to comment Share on other sites More sharing options...
Guest AR-2-D2 Posted July 31, 2005 Report Share Posted July 31, 2005 The first test-attachment worked :-) so I do another one.The midrange has two 3 ohm resistors attached which are connected in series with the driver. Resonance is at about 1250 Hz or slightly lower.When connected in the system the attenuator is also normally in circuit so the actual impedance that the crossover "sense" is different and most probably depending on the attenuator setting. The crossover frequency for the mid is usually stated as 1400 Hz in speakers from this period which do not make real electrical sense given the small capacitor value but there may be some additional acoustic roll of in this driver. I have not tested any frequency response for the midrange driver yet.On the other hand the tweeter (not shown) is spot on 5 kHz electrically with the attenuator up full (no attenuation).I have so far only used "speaker workshop" software and my Echo Mona soundcard for these impedance measurements. I should probably use a proper amp as a driver for more reliable results. Best regards / BoThe midrange impedance (below): Link to comment Share on other sites More sharing options...
Guest AR-2-D2 Posted July 31, 2005 Report Share Posted July 31, 2005 Well I do not know if anyone is interested but I did repeat measurements on 3/4 tweeters today and think I found a potential problem. I tested a known vifa tweeter for reference just to be sure that my measurements was working.Both AR-2ax tweeters seem to work and give sound but they clearly show differences in the impedance curve and resonance.Perhaps someone has any suggestions about which one is closest to normal function or if they both indicate problems?One of the tweeters show a resonance peak at about 2630 Hz but has additional resonance lower down in the spectrum (7-800 Hz). This is off course far below crossover frequency so it may not be a big problem but is it normal for this type of tweeter? I think it looks a bit strange. The other one seems almost over damped and has no signs of the lower frequency resonance that the other has. Perhaps this one is a normal one and the damping is intended? If this is the best working one I wonder if experimenting with new Butyl rubber dots on the very resonant tweeter could help restore damping?I will appreciate any suggestions or solutions as >I really do not know or have seen any similar graphs for these tweeters before.Btw are those the same tweeters as in AR-3a ?I interpret the data as sound and proper DC resistance is not enough proof of a functional tweeter or to be sure that those old AR-tweeters work as intended. Best regards / BoImpedance of two 70's AR-2ax tweeters: Link to comment Share on other sites More sharing options...
kkantor Posted August 1, 2005 Report Share Posted August 1, 2005 The Vifa tweet looks about right, but neither of the AR's do. - No crossover was in place, correct?- Was there any background noise in the room as you were testing them?- Where they positioned and held in a way that they could not vibrate?- Do they sound clean if you run a 2 Volt sinewave through them at gradually increasing frequencies between, say, 400 Hz and 4000 Hz? Link to comment Share on other sites More sharing options...
Guest AR-2-D2 Posted August 1, 2005 Report Share Posted August 1, 2005 >The Vifa tweet looks about right, but neither of the AR's do.> >>- No crossover was in place, correct?>Hi KenI am very honored to have your response and thanks for the good suggestions You are right, no crossover.>- Was there any background noise in the room as you were>testing them?>There was a quiet computer running in the room. Measured at night so fairly quiet conditions.>- Where they positioned and held in a way that they could not>vibrate?>They were factory mounted in the baffle with the crossover disconnected so not optimal with regard to reflections but should at least not vibrate if AR's reputation is right . >- Do they sound clean if you run a 2 Volt sinewave through>them at gradually increasing frequencies between, say, 400 Hz>and 4000 Hz?Sinus do sound fairly clean as long as my ear can follow. But on that graph there I only collected data from 200 different frequencies over the spectrum so the curve was obviously not overly correct. I have since yesterday taken out one of the tweeters from the baffle and tried a much slower sinus-sweep over time and reading from many more data points. It turns out that this tweeter strangely almost shorts the signal completely, but only between 18-18.7 KHz. It was not possible to see this before because of to little data collected . So I guess that this tweeter is not useable in this state. I will take out the other tweeter also and test further to see if there is a similar problem with that one.Best regards / Bo Link to comment Share on other sites More sharing options...
Guest AR-2-D2 Posted August 1, 2005 Report Share Posted August 1, 2005 >>- Do they sound clean if you run a 2 Volt sinewave through>them at gradually increasing frequencies between, say, 400 Hz>and 4000 Hz?Thanks again Ken and also a correction of my last message. I do not think I have really listened as low as 400 Hz as I was a bit afraid they could not take low frequencies with too much voltage. I will try that later on.I think I have found out some very interesting though ;-). The chassis and magnet system is sitting inside the mounting plate in a cavity . The chassis edge and remaining cavity is covered with some adhesive plastic film. The film has shrunken somewhat over the years. One would think the covering film was put there only for cosmetic reasons but....I carefully lifted of some of this film and voila...suddenly "tweeter A" which did not have the low resonance like "tweeter B" now also show a similar phenomena. Perhaps the tweeters got some opening on the backside and some sealing compound there has not survived the years or otherwise this plastic film is crucial for function (but I doubt that). I will try to remove the chassis from the mounting plate bracket and look for something on the backside.I attach a picture of opening and the old/new graph below. To be continued...Best regards / Boopening old/new graph below Link to comment Share on other sites More sharing options...
Guest AR-2-D2 Posted August 1, 2005 Report Share Posted August 1, 2005 >>To be continued...Update!Pictures from the inside of the tweeter (below message) !!! Well, I removed the 3/4" tweeter chassis from the plastic mounting bracket/cup.Attachment 1 show that sealing should have been provided on the backside by a glued fiber plate (marked with square) and in addition they tried to fill the hole assembly with epoxy just to be sure that everything was airtight. But no....The red line in attachment picture 1 show parts where the epoxy never covered properly. In addition, attachment 2 show that the fiber plate was not centered optimal and that this plate was never glued properly all away around (glue missing blue line) . In this area the epoxy is also missing which makes the problem worse. Therefore, probably this tweeter was never completely sealed or airtight even from the factory. I think both tweeters were made in 1974. May be they were manufactured on a bad monday after a heavy weekend party ;-) It was the 70's...remember I will now continue and try to remove the fiber plate and look what I find there and also try to seal the tweeter properly and re-run the impedance measurements.Best regards / Bo Link to comment Share on other sites More sharing options...
RoyC Posted August 2, 2005 Report Share Posted August 2, 2005 Bo,Tom T. informed us with some very interesting posts and photos that the manufacturing quality of these tweeters did indeed suffer beginning in the mid 70's. Take a look at Bret's thread entitled "Roy's 1975 Tweeter" of a couple of months ago.I also (subjectively) experienced inconsistent performance among a number of these tweeters of both the 4 and 8 ohm variety, and posted photos showing degradation of the foam damping material under the domes.Your measurements and observations are certainly consistent with what is a growing concern regarding these old "one of a kind" drivers.Roy C. Link to comment Share on other sites More sharing options...
Guest AR-2-D2 Posted August 2, 2005 Report Share Posted August 2, 2005 >Bo,>Tom T. informed us with some very interesting posts and photos>that the manufacturing quality of these tweeters did indeed>suffer beginning in the mid 70's. Take a look at Bret's thread>entitled "Roy's 1975 Tweeter" of a couple of months ago.>>I also (subjectively) experienced inconsistent performance>among a number of these tweeters of both the 4 and 8 ohm>variety, and posted photos showing degradation of the foam>damping material under the domes.>>Your measurements and observations are certainly consistent>with what is a growing concern regarding these old "one of a>kind" drivers.>>Roy C.Roy C, I Thank you very much for directing me to this interesting thread. Actually, deteriorated foam and dust particles moving beneath the paper dome make a lot of sense to me. May be that I have just moved around the tweeter changes positioning of this dust and therefore can cause some slightly different impedance patterns. Everything is visible there...Both drivers now got a similar peak at about 2500-2700 Hz though so I guess that can possibly be the normal Fs for the tweeter.Both tweeters also show a lot resonance going on beneath 1 kHz. The completely opened tweeter also has an additional sharp peak at 3.5 KHz now. Air can reach to the center off the backside but I am not so sure anymore if this really can influence performance. There seem to be epoxy glued parts there in the middle so may be that the air cannot get through there anyway. I will certainly try to seal it to check if it changes anything.Per suggestion from Ken I carefully listen to sinewave 400-4K Hz slow sweep and I could actually hear a lot of distortion going on now in some ranges.All epoxy/glue makes it very difficult to disassemble this tweeter without damaging it for good. I will try though.Best regards / Bo Link to comment Share on other sites More sharing options...
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