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soundminded

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  1. I explained how and why I re-engineered my pair. Clearly 901 offers something that is valuable to many people or it would not have lasted on the market as long as it did and it would not have sold so many units. IMO the original and series II were the best design. The criticisms of 901 are actually valid. However, being an engineer and a tinkerer I don't give up on something I like for one reason that attracted me to buy it in the first place because of something(s) I don't like and I try to see why I don't like it and what I can do about it. I'm not in this industry but I have many relevant skills I was able to bring to bear. No highs, No lows, it's Bose. There's more than an element of truth in this for 901. Why are there no lows? Because the equalizer was designed incorrectly. Further equalization fixed that but power requirements were far greater. Why were there no highs? Because what is now called a midwoofer has too much inertial mass to produce much in the way of high frequencies no matter how hard it is driven and what little it does produce would beam in one narrow direction from a 4" driver. The fix for that was to turn it into a two way biamplified speaker system. This was not nearly as easy as it sounds. The first half hearted try ended in failure. I have to admit it wasn't much of a try. The second try about 10 years later took four years to get it to where I wanted it. I've re-engineered many of my other speakers to because I don't like them for other reasons. This one though was by far the most challenging. The results were well worth the effort and the envelopment effect and other attractions of this speaker were not compromised, in fact they were enhanced.
  2. It's a tough problem. There are still a lot of AR 12" woofers out there that can be restored. The original larger Advent woofer may be comparable. Others used it including John Dahlquist in his famous DQ-10. Finding a paper cone woofer manufactured today that is a close match for the AR woofer is a tough one. There are some dome midranges around but you need to find one with a low Fs. It should be no higher than 1/2 the crossover frequency. The tweeter is the real killer. I don't know of any dome tweeters made today that come remotely close to the AR3a 3/4" tweeter for dispersion. Even the tweeter in the AR9 is no match in dispersion for the AR3a because the dome is slightly recessed. Perhaps an array of 3/8 inch mylar tweeters could work. For the crossover network you're on your own. I'd likely buy an active crossover network and triamplify it. I think the problem with a poly woofer if you can find one that's comparable to the AR 12" woofer is at the high end. It could be useful if you wanted to duplicate something like an AR9 where the crossover frequency is 200 hz. You have a wide choice of 8" paper cone lower midrange drivers usually sold as woofers.
  3. Hi, I'm back again. Sorry to disappoint you folks who hoped I was gone forever. I think the real value of Bose 901 for me was what I learned from the design. Despite its flaws, its claims some of which were valid and some absurd, it was a novel idea and a bold departure from the conventional wisdom of the day. I'll never forget the first time I saw one in a store window with its price tag. I thought who would ever be stupid enough to buy such a thing. Turned out I was and I've never been sorry about it. It was a lot of fun experimenting with it, understanding its unique qualities that made it so attractive to so many people and understanding and correcting its flaws that made it one of the most maligned products by audiophiles I've ever seen. Now for many newbee audiophiles who never heard of it let alone heard a pair it's something from ancient history that at worst gets a shrug. RIP Bose 901 and RIP Dr. Bose who built a privately owned billion dollar company from scratch starting with this product. I'll bet you laughed all the way to the bank.
  4. 12>5>6>17 among these 6 came first. Unequalized it's a better speaker than AR2ax. Equalized AR2ax beats it every which way. I own both. 17 is a 6 with a smaller woofer. This speaker has a cult following. Its shortcoming is its deep bass. Add a subwoofer and it's a very good speaker. 5 is a 6 with a beefed up woofer and two 5" high quality full range drivers used also in table radios and small package units used here as midrange units. Unequalized octave to octave balance is poor. Equalized it's probably an excellent speaker with bass that matches AR3a. 12 is 5 in a much larger cabinet for deeper bass and has an elaborate crossover that allows -,0,+ level controls for 4 frequency ranges. This was in the day before equalizers were available to consumers. There are variants of the woofers and tweeters between the various units but in some cases they are very similar. One nice thing about them is they didn't use foam woofer surrounds but they can need to be resealed. Roy has the magic goop. Use anything that hardens or cures and you've ruined the woofer surrounds. Among AR speakers AR3a was highly regarded but like most AR speakers I heard they didn't seem to have reasonable tonal balance. Equalization can probably fix that. LST is like an AR3a on steroids with 4 midranges and 4 tweeters to improve power handling and horizontal dispersion. 10pi and 11 used an improved tweeter having ferrofluid cooling. AR9 is considered the cream of the crop. It has by far the best bass and is a 4 way system. It took quite a bit of re-engineering mine to get the treble right. AR9 is in a class by itself. There is no other speaker I've ever experienced that can seem to dominate a room like AR9. Most peculiar. Unfortunately it does not have the high frequency dispersion of earlier AR TOTL models. This was pandering to those who wanted narrower dispersion. AR and KLH speakers are not known for "imaging." If that matters to you, you'd probably want to avoid them. This seems to be the be all end all for contemporary audiophiles. Frankly I don't care if the trombone player sits two feet to the left or right of the tuba player. I expect a tuba to sound like a tuba, a trombone to sound like a trombone. There are a lot of very expensive speakers out there today that can't seem to manage that.
  5. Any frequency response that can be achieved through modifications at the crossover network level can be achieved at the preamplifier signal level with active equalization, invariably with much greater control. Further control over FR and relative levels can be achieved with active crossover networks and bi-amplification. The advantages are many including not presenting a risk to the power amplifier by creating a very low impedance load that could destabilize or damage it. IMO the current design trend for very complex crossover networks to replace better control offered at the low signal level so that amplifiers can be operated without such signal processing is an unfortunate choice. So called room correction systems are nothing more than automatic equalization with a different name. Unlike manually operated equalizers they don't offer the same degree of user control or flexibility.
  6. From the "for what it's worth" department, the best speaker in the world of the month award for January 2005 goes to.....(fanfare) TA-DA....NHT 3.3. I've always wondered what AR's bottom line would have looked like if a certain somebody hadn't blown out about a zillion LST tweeters every....xxxx and gotten free replacements What if that certain someone had bought Cerwin Vega speakers instead. I'm sure this is a very fine speaker and and excellent value but if you get them, be warned by the design engineer, it doesn't matter what part of a room you put them in so long as you adjust their position precisely with a micrometer
  7. I haven't heard the NHT 3.3 however I do have some observations about AR9 which I've owned for 29 years. AR9 as manufactured did not have accurate treble. There was no amount of FR tweaking between level controls and an equalizer that could get it accurate. Only substantial modification of the system's high frequency dispersion using a 11 outboard indrect firing tweeters per channel and careful tonal rebalancing could mitigate AR9s problems. Additional equalization is still required. AR9 recognized the problem that a 3 way system has covering the entire aludible range. Most drivers have a useful range of about 2 1/2 octaves to 3 octaves at most (IMO.) Something's gotta give and with AR speakers the region between the woofer and LMR always did one way or another. AR speakers also always had a tendency to a high end rolloff starting with AR3 and it never seemed to be satifactorily resolved to my ears. The modern trend is towards brighter more directional high frequencies. IMO this is a mistake, AR got it right with wide dispersion. AR9 does not have HF dispersion equal to older models as can be seen by the small recess between the tweeter dome and the front faceplate. I think it was a misguided concession to the more modern trend but it was earlier on in the late 1970s and therefore did not fully compete with modern speakers that can sound shrill and require you to sit where X marks the spot. Tonegen 1259 was patterned after the AR 12" driver I think with Ken's help. It's Vas is nearly twice as great requiring a larger box. With side firing woofers room placement is even more critical for bass response than it usually is for other speakes. Ken positioned Stereophile's test pair in their room using a micrometer :-) I'm not surprised without further optimization of AR9 it came out second best to NHT 3.3's treble, it would have been surprising if it didn't. However, given a choice between the two and knowing how to improve AR9 I would not trade. AR9 when restored (were the comparison speakers restored before auditioning?) it's one of the most potent bass makers I know of, far more capable even with a good 60 wpc amplifier than anything required in my 400 square foot 4000 cubic foot live listening room.
  8. Having two pairs of AR3as allows for some interesting experimentation with placement. Were I in such a fortunate circumstance the first thing I'd try is placing each pair side by side towing the outer speakers outward and the inner speakers inward in an arrangement similar to LST. This would further increase AR3a's already excellent lateral dispersion. Then I'd try them with the inner pair toed out and the outer pair toed in which should have a similar effect except you'd never be far off axis of at least one midrange and tweeter for each channel. Then I'd pull them away from the walls and try them as a direct/reflecting pair. If you have four separate channels of amplification and at least one equalizer you can experiment with the front facing and backward facing pair at different loudnesses and equalizations to copensate for the absorption/reflection properties of the reflecting wall. You could angle the backwards facing pair toed in or out to determine how different dispersions affect the overall sound field. Two pairs of AR3as will not achieve the same low frequency reach as a single pair of AR9s, they are tuned differently but they will double the power handling capacity to the point where any perceived loss of bass by pulling the speakers away from the walls and rasing them on stands could be compensated for with equalization without overdriving them (not easy to do in any event.)
  9. This scheme is not advisable although there probably isn't any risk of damage to your amplifier. The voltages are not likely to divide uniformly at all frequencies if for no other reason than differences in the resistances of the potentiometers will make each speaker's complex impedance different from the others. It will also vary due to other seemingly minor differences such as different actual capacitance values. The effective amplifier damping factor will be below one for each of them. The best method is to use multiple amplfiers, one amplifier channel per speaker. If you have two pairs this means two stereo amplifiers or four monoblocks. Adjust the settings one pair at a time to try to get them to match each other as closely as possible.
  10. There seems to be a discrepency between the second and third thumbnails in the original posting. One shows a 40 mfd series LF blocking capacitor for the midrange array and the other shows it at 50 mfd. Otherwise all components in the midrange bandpass filter seem the same. Does anyone have an explanation for the difference? It should have at least some audible effect. LST appeared in an era before low cost consumer equalizers were available. IMO that is a much better way to adjust the electrical signal than the level controls on the speaker itself. Amplifiers were much more expensive and active crossovers also not widely available. 10 pi seems to have been designed with these limitations of that era also in mind. The Amati version demonstrates that the price for high end consumer products in general and audio equipment in particular does not reflect actual production cost but is more closely related to brand name perception. Jack up the price high enough and they will come seems to be their motto. In that era the most expensive speakers on the market were comparable to the price of a new car. Today it's comparable to the price of a new house. Things haven't improved that much but who am I to say how people should spend their own money.
  11. I still fail to see anything innovative here. 24 bit 192 khz digital audio signals, I've had that in Toshiba DVDs for four years and has been available in DVD players and recorders for as low as $30. DSP through a digital switching amplifier, old hat. (BTW on direct comparison to 1 bit 8x oversampling JVC circa 1991 it sounds identical.) Automatic room equalization, very commonly offered now even in low cost receivers from among others Sherwood and Pioneer. Heil air motion transformer tweeters, around since the 1970s, now being manufactured again and distributed by Parts Express. Looks like the subwoofers are a compact version of the AR90. By making the cones more massive, a lower system F3 can be obtained in a much smaller enclosure, a direct application of Newton's second law of motion applied to forced oscillation as simplified in the Thiel-Small cookbook. What would interest me is hearing an LvR comparison again. Others have tried it without the insight and skill to pull it off as successfully as Roy Allison did at AR. I think VMPS is one example. As for reproducing the sound of a symphony orchestra in an apartment, that is absurd on the face of it. Without recreating the effects of the acoustics of the hall it is normally heard in, even if it could be done it is not a pleasant sound. I know, I've heard many rehearsals of orchestras in practice rooms that are much larger than any room in an NYC apartment and it is not the kind of sound knowledgeable people pay good money to hear.
  12. I don't see anything particularly innovative about this equipment unless you consider the ratio of the price paid for the assembled merchandise received compared to the cost of the parts that that contribute to its function has reached a new paradigm. Everyone claims to have the best sound reproducing system in the world. Geddes, Linkwitz, Von Schweikert, Glasgal, and you could go on and on all make or imply that claim. Of course the truth is that I actually have the best one but that's an entirely separate story.
  13. I don't know what puzzles me more, the prices asked for this class of equipment which seems outrageous when you consider the cost of DIY versions of them, especially the speakers or where there is a market for it. Considering how much of it is being offered the market there must be people somewhere buying it but I can't imagine where. People I know who can afford it aren't interested. I thought it was crazy when a home audio system cost as much as a new car, then a new luxury car. Now some of them cost as much or more than houses. Do they really perform better than mine? I'm not convinced by a long shot.
  14. Hi Vern. It's been a long time since I've seen your posts around here. I didn't try these other tweeters. They didn't do what I was looking to try anyway. And of course they are much more expensive. The mylar tweeters seem to do exactly what I want them to. They are small, cheap, and easy to find. The last batch I bought were so cheap I got 400 of them for $20. That's on top of about another 175 sitting in my basement that ran about 28 cents each. What I'm going to do with all of them I don't know. But if I burn a buch of them up, at least there won't be much to cry about. I've had 4 Tonegen ribbons in my basement for the longest time too. I don't know what I'll do with those either. They were part of a large batch of drivers I bought at a low fixed price. The rest weren't much to crow about.
  15. When I refer to absolute vector distortion, I'm referring to the overwhelming majority of sound propagated by a source. There are other types of vector distortion. (BTW, when you invent something, you have the liberty of putting whatever name you like on it.) Here's a simple example of what I mean. Next time you are at a piano bar or someone is playing a piano in a hotel lobby or a buskar (street musician) is playing a violin, or whatever, walk around the musician and the instrument. Notice that the tone of the instrument hardly changes at all. The rare exception may be a singer or a piano with its lid propped open. You'll lose some high frequencies when you're behind those. Now put your speaker in the middle of a room or better yet in your back yard and walk around it. Notice how the loudness changes. Much louder when you are in front of it, not nealy as loud when you are behind it. Because the vector propagation in different directions is so qualitatively different, in your listening room, the speaker can never sound like the musical instrument. This is because the sound propagated in different directions is reflected by the room differently in the two cases. Important reflections which give life to the musical instrument are quantatively absent and qualitatively different from the speaker. The sound coming out of it will always sound like its coming out of a box because most of it reaching you, especially at high frequencies is. Except for Bose 901. It is just about the only speaker where the sound actually seems to be alive in the room with you as though it was coming from actual musical instruments. This was IMO the reason it was so popular. Too bad its sound was so badly flawed in other ways that prevented it from reproducing the timbre of musical instruments accurately. The tendency of the 4" driver to beam or for 1" tweeters to beam for that matter compared to the midrange and woofer in ways real musical instruments don't propagate their high frequencies radically differently from other tones gives rise to another form of geometric distortion. My model explains these and many more. It takes a novel approach to the phenomenon of acoustics. It is this approach which is at the heart of my discoveries about sound and other inventions that arose from it including the one I patented. That idea may yet have the possibility of a new lease on life. That's why it remains unpublished. BTW, without these enhancements, the Alison speakers are probably a better bet for most people IMO. They will at least play the top octave of the audible spectrum. That's very important if you can hear it.
  16. I have probably posted more about it than I should have. However, since I did let the cat out of the bag so to speak, I'll review what I did. If you try this, be warned, I found it very time consuming and tedious. On the other hand, the results were more than well worth it. For its type and purpose which I have clearly defined, it's the best setup I've encountered and will beat anything else I have or heard including my modified AR9s (except for AR9's extraordinary deep bass capabilities even with modest amplifier power.) The problem and solution are based on my own observations and still proprietary mathmatical model of acoustics so I will not tell all but I will give you the general idea which you can try for yourself. Bose 901 cannot reproduce concert hall acoustics by itself no more than any other current sound system can. There are many reasons for this but the fact that they don't exist on recordings either qualitatively or quantitatively would be sufficient by itself. So the first thing to accept is that you will not be hearing a reasonable facsimile of a live performance at a large public venue from them. The goal of the sound system I devised using them is specific, to reproduce the sound of unamplified musical instruments and human voices as they would be heard in the same room you're in. This restricts their usefulness in this regard to solo instruments and small groups. Although they will of course play any kind of recording, this works best for classical music and jazz. One critical issue is that they will be played at the correct SPL. Bose 901 has a distinct advantage over all other speaker systems in that it has what I call inherently low absolute vector distortion. This means that sound waves from a single instrument arrive at the listener from a multitude of directions just the way they do from live musical instruments. This is exactly the opposite of the current idea of producing highly directional loudspeaker systems. Bose 901's main problem as I see it is that it does not produce flat frequency response. The original model and series II are acoustic suspension speakers which have the capability to produce very low tones down to the limits of audibility better than most other speakers. Within their power handling capability, the can give AR9 a run for its money. But power requirements are enormous, far greater than Dr. Bose indicated for most rooms. For this reason they work best in multiple pairs with huge amplifiers. (I use only one pair in a 14 x 14 room with a cathedral ceiling and 138 wpc is marginal. (AR9 does very well with 60 wpc in a room twice as large.) My pair like most I think have a significant peak of around 7 db at around 500 hz. Starting at this point and going down in frequency the speaker falls off at 12db per octave like any AS speaker system but the equalizer only supplies 6db per octave boost. So a cut is needed at 500hz and slightly above and then rising output as frequency lowers by another 6db per octave. If you have series III or later, you may not hear the lowest octave and might overload the system if you try. From what I can tell, the ported system is likely cut off at an octave above the AS version. This additional equalization can bring the bass into far flatter and more extended performance without playing it at deafening levels. 901's other problem for any version is that a 4" driver we call a midwoofer today makes for an awful tweeter. Not only is output of the highest octave very limited due to the high inertial mass of the cone (Gordon Holt's observation with which I fully agree) but what little output it produces in this range beams directly on axis. This is a consequence of the cone diameter. To correct this problem, I used an array of small mylar tweeters similar to Audax's 3/8" version. They are very inexpensive and easy to obtain. I used 8 ohm versions. I used 6 per channel, three on the back facing left, right and center, two facing the ceiling, and one in front directly above the 4" driver. I crossed them over at 6db per octave at around 9khz and drive them with a separate amplifier. Equalization for each recoring is critical to obtain correct tonality. I'd say only about 5% of the tweeter energy is directed forward owing to the crossover design and the reflected energy does not have the same FR as the direct energy. This is done to compensate for the differential absorption of the sound as a function of frequency before it reaches the listener. The goal is to have not only the direct sound flat but all of the reflections too. This eliminates two additional forms of geometric distortion my model predicts that other speaker systms can't come close to preventing unless you are unfortunate to be in the world's worst possible listening space, an anechoic chamber. When properly adjusted, the ability of this configuration to accurately reproduce the sound of musical instruments and voices with remarkable presence from recordings is astonishing, far better than I would have expected. The success of this arrangement is proof that the theory that Redbook CD from inexpensive CD players is adequate for any music is compelling, it's all you need. No exotic audiophile amplifiers, wires, or other peculiar devices that are popular but carry no weight with mainstream electrical engineers are necessary either and would serve no useful purpose. In fact they would probably detract from this system if they have any effect at all. For best results, each recording will need its own equalization owing to the variables of spectral balance inherent in recordings. Multiple equalizers are desirable to facillitate this, my setup for this system has four inclding the Bose equalizer. Fortunately they are not expensive. BTW, my first effort at this in the early 1990s was a complete failure and my second effort even when I had much more experience re-engineering other speakers and knew what had to be done took nearly four years from 2004 to 2008 before I considered it entirely successful.
  17. Hi Hoho;

    I don't quite understand what you mean by "thinness" in the mid treble. Usually the term is used to describe lack of bass.

    Transient response is one way of looking at the relative output of high frequencies, frequency response is another. They are virtually interchangeable. If you can't hear high frequencies, you won't perceive transient a...

  18. The gasketing material is only sticky on one side and can be used to make a coveer for the metal outer frame. It is cheap, easy to use, easily removed at any time just by peeling it back if necessary to access the screws. The entire assembly is invisible in normal operation as this is all covered by the grill cloth. Painting is not recommended. Even one drop of paint on the suspension will compromise its compliance and performace with no change in appearance in normal use as compensation.
  19. Parts Express has exactly what you are looking for. http://www.parts-express.com/pe/showdetl.c...tnumber=260-542 When you peel away the brown paper, the gasketing material is black and sticky on one side. I used it on my own AR9s, AR2axs and other speakers as well.
  20. Why would the results be different from using an equalizer to make the same change?
  21. 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.
  22. 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.
  23. 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.
  24. You can take a idea and develop it to its most absurd limits. The idea of a two way 8" acoustic suspension small bookshelf louspeaker went from the $57 AR4/AR4x to one costing hundreds of dollars probably Pete Snell's model K to one costing many thousands of dollars such as Peter Qvortrop's Audio Note Model K. It would be interesting to see how the best version of Audio Note's expression of this idea which I think costs over $15,000 stacks up against AR4x whether in its original form, as tweaked here, or as tweaked using other means such as a graphic equalizer. There might not be much difference. http://www.audionote.co.uk/products/speakers/an-k_01.shtml A somewhat larger 2 way 8" ported design model E sold in the past in versions costing up to $125,000 per pair. This in part due to two outboard capacitors in separate enclosures made of pure silver. http://www.audionote.co.uk/products/speakers/an-e_01.shtml The use of Russian birch for the cabinets is claimed by the manufacturer to enhance the sound by acting like a sounding board in the construction of a musical instrument such as a guitar due to the way it resonates. I've heard it and wasn't impressed.
  25. It seems like an awful lot of trouble to go to to improve what was Acoustic Research's least expensive loudspeaker of the time, clearly their maximum compromise of performance for the sake of economy. When AR set about building a better model, they took an entirely different route, they built three way systems recognizing the inherent difficulties of matching only two drivers to cover ten octaves.
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