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Everything posted by johnieo

  1. IRa.ra: At this point, Iwould replace the capacitors. No elecrolytic made in those days was intended to last this long! The Sprague and Compulytics made in the 1970s were well made with low ESR. If one looks in the old Handbook of Components for Electronics or the Electronic Engineer’s Master (EEM) available in your friendly nearby University library, one will see ESR values of order 0.016. The demand for switching high currents in capacitors while minimizing heat generation was the driving force behind these quality capacitors. Bridge measurements that I made in 2005-6 showed the capacitance values to be close to original value, but the ESR to have increased considerably. To evaluate these capacitors one needs a bridge that will measure both C and ESR in the octave or so frequency range near the crossover frequency of interest. Cant be done with large capacitors on a small L/C/ESR meter. Cheers and enjoy refurbishing these--its a fun hobby. John
  2. The correct numbering of the International Division meetings may be obscured in the dust of crumbling surrounds. I do recall meeting at Organ Stop Pizza, Mesa, AZ in 2009 with Richard Lowe (Melbourne) and Ken Kantor. That meeting followed one between Richard and Klaus in Denmark (2008??). Perhaps there were several others? As a physicist, I would accurately call the most recent Rome meeting number "N + 1" Finally to complete Kent's meeting definition---photo and sustenance---we might consider including a pint or a maß (for those so inclined) hoisted in toast to Edgar Villchur ... provided an AR 12-in woofer was not chosen as the "designated driver." Cheers, John
  3. Hi Ligs: Caps in parallel: Add the individual C values to find total C, as you know. The max voltage of the group is that of the capacitor with the lowest voltage rating---they all have the same applied voltage, and the unit with the lowest rating will fail first. However, there is an issue if we are using electrolytic caps-- in case of electrolytic caps, one usually uses caps at some max voltage near to, but below their rated value. For example I might choose caps with a 50-V rating to use with say, 25-35 V max applied. So in the case of this max applied voltage I would likely not use an electrolytic with a 450-V rating in parallel with one having a 50-V rating, as one would operate at a very low fraction of its rating and not be formed properly. All electrolytic caps used in a parallel group do not all have to have the identical max voltage rating but should be similar-- perhaps, say, between 50--100 V for that example. Caps in series: As you and others know, the total C is found from the reciprocal rule. 1/Ctotal = 1/C1 + 1/C2 + etc. It is not prudent to ever put caps in series because there is an added complication: capacitors have leakage resistance, and it affects the way the voltage is divided between the two devices. Capacitors have large (parallel) leakage resistances. Lets for example say that we have two 100 uF caps in series. Ideally we have 50 uF total, with half the voltage across each device. However, that assumes ideal capacitors with infinite parallel resistance. Nothing is ideal in real life! Let's assume that one of the 100-uF capacitors has a leakage resistance of 2 MOhms and the other has a leakage resistance of 4 MOhms. The DC voltage across the two capacitors in series divides according to their resistance values, not their C values. So in this case 2/3 of the applied voltage would appear across the unit with 4MOhms leakage and 1/3 of the voltage would appear across the device with the 2 MegOhm leakage. Were we to apply a large voltage to these units, one of them would fail well before the other! Since leakage R can change with age, it is not wise to ever connect caps in series. So the practical answer to what is the voltage rating of series caps? Don't do it! hope this helps, john
  4. "However, as much as I do like these 10 uf surplus caps and am pleased that Kent has directed us to them with high endorsement, I would be totally uncomfortable combining this many capacitors into one cabinet for a single driver - - - many of you are certainly superior to me at circuitry assembly, but this recommended scenario would be nothing short of a wiring nightmare in my hands." Hello: Wiring ten 15-uF caps in parallel is really simple as the ten can be combined outsidet the cabinet and installed as one unit. It does have one advantage. When connecting ten in parallel, the total capacitance becomes 10x15=150uF, as expected, but the equivalent series resistance becomes ESR/10. Levenson used ten small film caps in his cello amati likely for this reason--reduced ESR. That said, there is absolutely no problem with using non-polar electrolitics other than giving one's grandchildren a headache in some years. The reason most old electrolyics failed was a poor end seal that allowed water in the dielectric to evaporate. First C increases as the aluminum etches faster, then it becomes very small, as the device dries completely. The worst cpacitors were encapsulated in PVC tubing (usually black), and then end sealed with pvc (usually red). The plug did not stay sealed to its shell. The best were made by Sprague and Cornell-Dublier; both firms used a high quality elastomer with a strong crimp seal--many are still within spec (both C and ESR) thirty-five years after fabrication! Still, its a good idea to replace all at this age. Today inexpensive electrolytics are rated anywhere from 2000--7000 h. I have fixed many a modern widget for a few cents worth of small electrolytics! JohnO
  5. Do you have to be a member to get the custard pie? Nope, 80 cents will do. BTW, how does one qualify for membership in the society? Article 0 (dated 23June2013) of the society bylaws states: "membership is attained by appearing in a photo with at least one other card-carrying CSP member." Cheers, JohnO
  6. The third 2013 meeting of the AR Restoration Society was held at Wong's on Mott St. in the City. Minh gave a "chopsticks-on" demonstration entitled Selecting and Enjoying Fine Chinese Cuisine. This was followed by a "hands-on" session on how to select dessert at a Chinese bakery. This was the most popular meeting, with a 50% increase in attendance. At the suggestion of JKent, no continuing education credits will be given, lest the tax-exempt status of the Society be jeopardized. Respectfully submitted, JohnO, recording secretary
  7. The second 2013 meeting of the AR Restoration Society was held 16 June in Highland Lakes, NJ. JKent gave a fine paper entitled: Purchasing AR-4x and AR-90 in northern New Jersey. Respectfully submitted, JohnO, Recording Secretary
  8. The first 2013 meeting of the AR Restoration Society was held 15 June in Latham NY. All in attendance listened attentively while Roy C presented a paper entitiled Restoring AR Speakers Found in the Dumpster.See attached image. Respectfully submitted, John O, recording secretary
  9. Here's a link to an article in the current issue of IEEE Spectrum that discusses yesteday's technology. Not about AR, but about its era. Enjoy! http://spectrum.ieee.org/geek-life/tools-toys/oldies-but-goodies/?utm_source=techalert&utm_medium=email&utm_campaign=091312 Cheers, John in Tucson
  10. Most of the AR community knows that Villchur left speaker research and devoted his time to the development of hearing aids. He proposed two radically new design ideas: multi-channels and the use of volume compression in channels. It is also known that he sarted Resound hearing aid company, because none of the firms of that day would accept his design ideas. Today, Villchur's two design concepts are incorporated in all hearing aids. Friday, my audiologist told me that there is yet one more unrecognized tribute to Villchur's legacy; the purchase of Resound by Danavox resulted in a name change. After Great Nordic, the parent company of Danavox, purchased Resound they changed its name from Danavox to Resound; Mr. Villchur would have been pleased. Cheers, John
  11. Hello Tim: First, it is worth double checking to ensure that there is no insulation on the two mid wires you are checking for continuity. Scrape the end of each a bit with the side of a knife or fine sandpaper, then check again--as well, check your ohmmeter by shorting probes. If for sure it is dead, you might consider Vintage AR (Larry LaGace) / Ljlagace at aol dot com / Albany, NY / 518-869-3665 /. He has been able to repair some mids, if the break occured where he can access it. If not, I would purchase a replacement from him rather than an unknown eBay source, as he checks the output level of each driver he sells. Cheers, John
  12. As all have mentioned, Ar stuffed the early versions "uniformly." The only one I have worked on that was not stuffed uniformly was the AR-3a-Limited (the Asian version, not to be confused with the Europen Improved).) Alex Barsotti, who worked on that model, said it was not stuffed with fiberglass but with 18 ounces of the the two-diameter, red/blue polyester like that used in the improved. In particular it was to be stuffed "mainly in the woofer half of the cabinet." Who knows, there may be a few additional unique models around? Cheers, John
  13. Hello npt3: Most woods bleach lighter with age (UV exposure); walnut and cherry darken with UV exposure, so its original color would have been ligher than you see today. If you choose to bleach (not usually needed) do it uniformly. Then try the Sal Martino method: apply Danish oil, wipe, wait 24 hrs, then wet polish with 240 grit ("wet-or-dry" paper)/Danish oil, dry 24 hrs, then wet polish with 350 grit/Danish oil. Dry a day, then wax with something quality like Liberon Black Bison paste wax. I begin with light or medium walnut Watco, as the finsh darkens a bit after multiple coats. Cheers, John
  14. Chip: There is no need to remove the crossover boards. They are hot glued and gun stapled to the cabinet backside. If they were destroyed on arrival, then one would use tempered Masonite, but if so, would need to restore lettering visible from exterior. Best to keep original. Simply perform all re-wiring and component replacement through the woofer opening. The AR-3a restoration manual (in AR section of library) describes techniques that apply to many facets of restoring other early AR models. AR finishes were usually oil stained and pehaps (?) over coated with clear lacquer on some versions. Additionally, what "accidents" did your cabinet surfaces befall? Attempting to remove the original finish on an old, thin venier may not result in a surface that will take, uniformly, a darker oil stain. You might not be happy with the result. We do not feel that all pieces of furniture in a room have to be the same color, just that their styles blend. Cheers and good luck! John
  15. Steve: As you noted, the alpha charaters after the number mean a great deal, and well beyond the classics of 1970 era. Long ago you posted on the AR-18 series; I saved your notes, because I was restoring some. --abbreviated here: AR-18 great speaker, basically AR-7 in a new box AR-18s changed cabinet to bull nose design AR-18b, moved tweeter to center AR-18bx, switched to cheap woofer AR-18bxi, switched to cheap woofer and cheap tweeter There were other changes in crossover frequency and circuitry as well. I don't know how these alpha designations apply to the later series, such as 9 and 94, etc., but they are extremely important when replacing parts in the 18. A good long-term project for someone would be to list in spreadsheet format the drivers for all the iterations of AR speakers! That would require a lot of coffee Cheers, John
  16. Hi Carl: Yes, agreed, but there is one added variable to watch when making these measurements--the free resonance of the woofer. If it is of order 21 Hz, then all is fine with a cabinet resonance is 37-38 Hz. However, I had a couple of Alnico woofers with Fs = 111-13 Hz. These also produced low cabinet resonance, but 6 months after putting them in to regular use, the magnet began to hit the backplate on loud passages. Problem: really old soft spider. After replacing the decayed spider, performance was restored, even though the new spider was a tad stiffer than the old. Since then I realized that 11-14 Hz woofer resonance was a measure of spider decay. That was easily verified by adding increasingly larger loads to the woofer cone and noting that it hit the backside with about 1/3 the load of a good woofer. As we expect, materials decay is the major issue in restoring these great speakers. Let's see- a 1960 AR-3 woofer is now 50 years old! Cheers, John
  17. Yes, the Rayl - a unit of acoustical impedance (pressure divided by velocity) is almost like viscosity, which is pressure divided by a rate of velocity change in the perpendicular direction. It is my impression that drag induced vibration is important. The "average" free length of FG is about 60 um; 250 for PE. The boundary layer is the distance from a surface at which the air velocity goes from its max value to zero (at surface), so if the BL is of order 200 um, it means that there is a lot of drag on the FG fibers and little on the PE -- hence my crude analogy to air flowing through fly screen versus chicken wire. It's my feeling that the oscillating pressure waves set FG fibers in motion and their vibration acts like an energy storage and release mechanism. It would appear that PE is too soft to do much at all. I have a very large bag of about 5-um-diameter melt blown PP fiber -- the kind used to soak oil, I believe. If you would like some I'll be happy to send you lots! Have never been able to think of a way to card it. It is in the form of clumps of medum length fibers. Want some Carl? just PM me your mailing address, I lost it! Cheers, John
  18. Thanks, Dave> Yes we did measure parameters as a function of FG and PE and found the suffing quantity that produced the lowest resonant frequency. Adding more stuffing simply raised the resonant f, because the volume of air in the cabinet decreased too much. The issue was simply that PE did not produce as low a minimum resonance as did FG. When you say theoretical, one has to consider what theory. If adiabatic/isothermal, I don't think the percent realized is meaningful, as I doubt that this theory is the cause of our measured effect! It has been noted that "theory" says the cabinet must be stuffed uniformly, but in practice, it has more effect near the woofer. I recall Alex Barsotti telling me that the AR-3a Limited (Asian) had 18 oz of mixed diameter (17- 34-um PF) and that it was located mostly near the woofer. Absolutely! Plots of the real part of the acoustical impedance versus frequency illustrate this in spades! Z(FG) -- both real and reactive -- are very high at low frequency and decrease as f is increased (only plotted to 700 Hz), wherease Z(PE) is quite small except for a resonance at 400 Hz, indicating a standing wave as you note. As we all know, this is why FG is used in acoustical tile Cheers, John
  19. Hello Carl: Thanks for reviving this thread. Yes, I agree that density is important. However, I think the value of the opimimum density would depend on the kind of stuffing, and as well, the woofer's design. To go back to your first question- I do think the coil change and stuffing change were made at about the same time, but perhaps not for the same reason? I think the stuffing reduction was made to increase the Q a bit to enhance AR in the showroom. Could the coil change have been made to deal with small woofer changes near crossover? e.g. the new 3a with foam surround used the #9 coil, whereas the cloth surround used the #7 coil, the AR-3a-Limited (Asian) used a stiffer woofer and a 2.6 mH coil. Back to stuffing... One thing to keep in mind about rock wool in paticular is that it is refinery slag and contain large lumps of glass. These make RW heavier per unit volume than uniformly-spun FG. These lumps most likely don't enhance cabinet acoustics. In last couple of years I have been making some back of envelope calculations about the roll of fiber properties in determining cabinet resonance. Historically, the role of fiber stuffing is said to be due to an adiabatic/isothermal conversion, wherein the pressure change caused by cone movemennt in and out is mitigated by the fiber's absorbtion of heat on the in stroke and release of heat on the out stroke, thus supposedly keeping the cabinet temperature constant. Next we observe that the replacement of fiberglass with polyfill resulted in a slight increase in the low frequency cabinet resonance--e.g. compare AR-3a to AR-11--using same woofer; PE not as good as FG for reducing low frequency cabinet resonance. Then some quick calculations of how fast heat can flow into and out of polyfill and FG shows that the thermal diffusion constant is much faster in FG than PF, but both are fast enough to account for keeping temperature constant in the 20-60 Hz range. Also, if one calculates the surface area of the two times the indiffusion depth, there is sufficient heat capacity to absorb and the required calories in either material for the small temperature change. Therefore, the notion of thermodynamic effects causing the box to appear larger as its physical size doesn't appear to be on solid theoretical ground. Now we look at the formulas I used to calcluate the resonance of a cabinet. These formulas were developed as part of an Air Force contract on noise reduction in fighter cockpits in the mid 1980s at UT Austin by a well known acoustics professor. The main variables in these formulas are (1) porosity of the cabinet (how much space is air and how much is fiber), (2) density of the material, and (3) fiber diameter. The results for propagating a plane wave from a cone to a reflecting wall located at the back of the speaker cabinet show that the acoustic impedances are: Z(FG) = 2400 - j1000 Rayls/m, Z(PF) = 115 + J50 Rayls/m. This says that both fibers are absorbing and storing energy on each cycle. The real part of the impedance represents loss (heat dissipation), whereas the reactive part represents energy storage and release (fiber vibration). But, note that FG is 20 times more effective than PF in both absorbing and storing energy! These differences in energy storage and release in the form of vibrations of individual fibers can be viewed in a different way by looking at the vibration frequency of a beam. Each "free" part of a fiber (between the points at which they touch) represents a miniature "beam" with a resonant frequency that is determined by the fiber's length, stiffness, and diameter. FG is much stiffer and of smaller diameter than PE. The "average" length of a free FG fiber is 60 um, whereas the same for PE is ~250 um. the boundary layer for air drag at 40 Hz is about 200 um, so our stuffing situation is crudely analogous to the difference in air resistance of wind blowing through fly screen versus chicken wire -- the widely spaced PE does not vibrate (absorb and release) nearly as much energy as does the FG. Said another way, the difference in natural resonance frequency between a glass fiber and a PE fiber can be observed by suspending pieces of thread and FG in one's fingers, tweaking them and observing their natural resonance!! My current view is that the mechanical properties of ordinary fibers - properties such as Young's Modulus (stiffness), fiber diameter, fiber density, and cabinet porosity deterimne how well it vibrates in sympathy with the acoustic signal. Therefore it would be interesting to examine other non-glass fibers of similar diameter as PE, but with increased stiffness. Hmmm, got any other fibers to test, Carl? The natural or uncompressed fiber density which you descrbe, Carl, is important and is contained in the above terms (diameter, density, cabinet porosity). It is also likely that the optimum density depends on the woofer, so that the best density for an AR-4x is probably not the same as for a 3a--but I know little about woofer design! Thus, I have tended to use units of "pounds per model x cabinet". :-)
  20. Hello Dave: Thanks for your comments. No, I was not worried about crossover frequency change but whether a slope change in the network would change the character of the original (the ratio of power from each driver). But maybe that is what you wish to accomplish? Certainly Q does not affect sound in the crossover range. In addition, some have worried about the ESR of a capacitor without regard for two issues--the frequency at which their meter measures ESR, and the role of the capacitor in the crossover. For example, the impedance of a 20 uF woofer capacitor is important in the octave near crossover, so if we measure ESR of an aging cap at 8 kHz, and find it has changed, it matters not. However, if it is measured at 1 kHz, and has changed, that is a different matter. Ditto for aging caps used at low crossover frequencies, that issue is changing dielectric absorption, not changing high frequency resistance. Will be interesting to see your final results. Cheers, John
  21. Well ... my memory did not serve me at all. Let's try that again: "... found it to increase from 0.75 to about 1." sorry, neurons rot like foam. John
  22. 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
  23. It is impossible to conduct a meanignful experiment when changing more than one variable. When Alnico-magnet drivers were replaced with ceramic drivers, many other changes were made. For example in the AR-3a and the AR-4x the stuffing weight, and woofer inductor values were changed. I do not know of any early transition from Alnico to Ceramic that did not change at least one or two other parameters. We cannot change many variables and attribute differences to the one of our choice. That has nothing to do with your enjoyment of the old speakers! They are wonderful! Cheers, John
  24. Hi Jim: The blue capacitors are electrolytic; Sprague if I recall. They were made with an excellent hermetic seal and had a longer useful life than most brands. The tan box--the tweeter cap--contains an oil-impregnated paper capacitor, whose hermetic seal is shot by now. Have looked at many of them and most of the time, their capacitance remain in spec, but their dielectric loss and ESR both had increased significantly. Capacitor recommendations, as you well know, draw more rapid and sharp responses than piranha going for food in a jungle pool. Good quality non-polar electrolytic work well. For me, polypropylene have worked well with a small series R in certain crossover locations. We pick them, because they will outlive my grandchildren! In the 70s Sprague and Cornell-Dublier made excellent hermetic seals; I have no forty-year encapsulation life test data on npes made in 2009 I am not sure if any mass production electronics today is made to last that long. Cheers, John
  25. Carl: I understand that you repair to suit a customer's needs, but sometimes the customer may not understand that the old cloth surround will last much longer than the new foam surround. As a business person, you need to respect your customers wishes. In my experience, the few wrinkles in the cloth surrounds on AR-4x and AR-3, 3a drivers are not an issue; they are cosmetic. The real issue I have faced is aging of their spiders, but not in all units, just in a few. An example of this was a pair I restored for a good friend (one of two pair that were not for my family!) This pair of woofers came from AR-3 manufactured in 1966. When restored in March 2007 as AR-3a Limited units, they had a very low Fc, ~37 Hz, and could be played at high power levels with no problems. In June 2009 the voice coil of one unit began to hit its back plate; my friend enjoys listening at high levels. To verify the cause, I placed small copper weights on each cone and plotted cone displacement versus load and found one driver quickly and linearly reached the back plate, whereas the other had a much lower (and linear) slope, but did not hit the backplate when it reached its maximum excursion. The solution was to have Bill LeGall of Millersound replace both spiders. When finished, Fc was about 40-41 Hz. One of the cues to a weak spider is low Fc and Fs which these units exhibited, even though they did not bottom out when first assembled. I suspect that two years of high listening levels hastened the end of life of an elderly spider. Given the choice, I would prefer to replace the spiders and keep the cloth surrounds--coated with a bit of Roy's excellent tacky butyl rubber compound. These should now be good for many more years of listening enjoyment with no worry of crumbling foam! Cheers, John
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