Dynaco A-25 loudspeaker
Dynaco warned us in advance that the A-25s were "something special," but since we have heard exactly the same kind of ballyhoo about every other new product we've ever tested, we could be forgiven for being just a little skeptical. Of course, Dyna's stuff had been pretty good in the past, but how much could anyone really do with a speaker just a little bit bigger than a large bread box. Nice highs, maybe, but perfectionist-type bass?
Not a chance!
According to the laws of physics, small dimensions are just not conducive to deep-bass reproduction. Small cones tend to be lighter than large cones, and thus resonate at a higher frequency. And a woofer's output normally falls off progressively below its resonance point. Enclosing the woofer tends to raise its resonant frequency even further, and the smaller the enclosure, the more the resonance is raised, and the more pronounced it becomes. And as if that weren't enough, a small cone can't get a big enough "bite" on the air to produce long-wavelength pressure changes. Hence, the speaker 's efficiency diminishes as the frequency goes down. These are the hard facts of audio life, but while there is no known way of repealing the physical laws involved, there are ways of circumventing them.
For example, it was learned many years ago that the dwindling low-end efficiency of a cone could be offset to a large extent by letting the enclosure resonate at a lower frequency than the cone, and cutting a hole in the front of cabinet to let its internal pressure augment that from the front of the cone. The hole also relieved some of the internal pressure buildup (making the enclosure behave as though it were some what larger), and it was found subsequently that if the passage of air back and forth through the hole were limited to some extent by the addition of acoustic resistance (via a couple of layers of burlap, for instance), the sharpness of the two resonant peaks could be reduced to the point where the whole low end was acceptably smooth. This "bass reflex" systemgenerally attributed to Jensen Loudspeakers, Inc.was the accepted way of producing "small" (that is, smaller than large horn-type) speaker systems until Ed Villchur spawned the acoustic-suspension system.
By the end of the 1940s, some designers had found that stuffing an enclosure full of fiber-glass had the effect of making it behave like a larger enclosure, and also helped to smooth out resonances. If the enclosure were totally sealed, only one resonance would develop, and if the proper design parameters were chosen, the resulting system could give smoother bass and just as good low-end range as a bass-reflex system of comparable size.
Because of the lack of augmentation from internal pressures, the whole bass range was slightly less efficient than from a bass-reflex system, but since higher-powered amplifiers were becoming increasingly common, it was no great disadvantage to have to reduce the system's upper-range efficiency slightly in order to match the low end.
Villchur's acoustic suspension system just carried the stuffed "infinite baffle" principle (used in Bozak speakers at that time) a step further. The suspension of the woofer cone was made exceedingly flexible (to reduce its free-air resonance to an extremely low frequency), and the air pressure in a small enclosure was used as the main source of "restoring force" (the force needed to return the cone to its "at-rest" position between vibrations). The combination of a heavy cone, a small sealed box, and just the right amount of internal stuffing yielded a broad low-end resonance that could maintain a fairly flat low end down to around 50Hz, and although the efficiency was now even lower in the bass range than that of the larger completely enclosed systems, 50W amplifiers made it practical to reduce the entire upper-range efficiency even further, to match the low end.
The first AR system (the AR-1) and its successor, the AR-5, were the ideal size for no-holds-barred low-end response, and the slightly smaller AR-2 carried the principle down to its practical limit of miniaturization. Smaller systems ran into the same inflexible laws of physics as before (before the fiber-glass stuffing, that is), and in order to cover up the resulting lack of deep bass from them, designers found it necessary to underdamp their mid-bass resonance to give "fullness" to their sound. So while these ultra-compact systems sounded balanced, they also had a characteristic boominess and a conspicuous absence of output below their boom range. Clearly, the acoustic suspension system was no longer enough.
In view of the history of the small loudspeaker, it is surprising that nobody thought of Dynaco's "aperiodic" system a long time ago. The bass-reflex system allowed for a significant reduction in cabinet size without undue sacrifice in low-end range, without any stuffing in the cabinet. Eventually, it must have occurred to someone to add a reflex port to a stuffed enclosure.
We don't really know whether or not the A-25 qualifies as a genuine bass-reflex system, but its construction and behavior suggest that that is exactly what it is, although this particular variety works the way earlier versions should have but never quite did.
As before, the hole in the A-25 enclosure relieves some of the internal pressure buildup and feeds some out through the front to augment the woofer's front radiation, and a "plug" of acoustically resistant material in the hole reduces the amplitude of the system resonances and spreads them out to the point where they become virtually a smooth low-end rise through the range where the woofer would normally fall off. The result is not quite an "aperiodic" system, but is at least "essentially nonresonant," to quote from Dyna's poop sheet.
The A-25 differs from other acoustic-suspension-type systems in another respect, too. Most of these use rather heavy cones, to keep the system resonance as low as possible, concomitant with the other design parameters. The A-25'8 cone is considerably lighter, and this plus a lower-than-usual crossover frequency (1500Hz as opposed to about 2kHz) should, at least in theory, assure better transient response than is typical of such systems.
The tweeter is described as a non-rigid hemispheric (dome) type, whatever that may be, and tweeter/woofer balance is adjustable by a five-position switch at the rear of the enclosure. The switch knob, by the way, is quite small and relatively difficult to turn. It won't challenge the average adult, but it will frustrate any child who's too young to have learned to keep his mitts off things.
We compared the A-25s with two systems of comparable pricethe Acoustic Research AR-4x ($57 each) and the KLH Model Seventeen ($70 each)and some top-rated higher-priced systemsJanszen Z-600s, Acoustic Research AR-5as, and a single pair of KLH Model Nine panels. Readers of previous Stereophile reports will recall that we have faulted many an otherwise-excellent loudspeaker system for coloring the critical musical range, because we feel that if a speaker distorts instrumental timbres, no amount of dispersion or bass or treble range can make it any more accurate a reproducer of music.
The speakers we put up against the A-25s were all outstandingly good in this respect, so we were most curious to see which, if any, the A-25s could match. Would you believe, the A-25s beat out all of them!
This is one of the very few speaker systems we have ever heard that seemed to have virtually no sound of its own. Brasses, strings, woodwinds and most percussion instruments were reproduced equally naturally and with nary a trace of hollowness or nasality or steeliness, and it was just not possible to characterize the sound as Row-A or Row-G or Row-M. In these respects, it was slightly better than the best of the other systems we compared it with.
Dynaco has the center position of the balance switch indicated as the Flat position, so we started our tests with the balance set accordingly. We found no reason to change this, in the three rather acoustically different rooms we listened in, so the following comments apply to the systems with that balance setting.
Treble dispersion was excellent: estimated at about 100°, and without any significant interference effects between drivers. As a result, stereo imaging was excellent, and good stereo spread was obtained even when sitting to the left of the left-hand speaker (and vice versa).
Efficiency was typically low for a compact systemaround 1%but power-handling ability was considerable. These speakers were able to put out rather more clean sound than some slightly-higher-efficiency systems, including the Janszen Z-600s, which tended to get a bit muddy at equivalent levels. Dynaco's literature makes a strong point about the smoothness of the A-25's impedance curve, explaining that solid-state amplifiers are less tolerant of load impedance variations than were tubed amplifiers. This is true, but we did not think the problem was quite as acute as the A-25 proved it to be. By actual comparisons, the AR-5a is only slightly less efficient than the A-25. But we were able to get almost 4dB more clean signal from the A-25s than from the AR5s, which do have rather more variable impedance.
At the high end, the A-25s were good but not really outstanding. Generally, the impression was one of considerable smoothness but with a very subtle roughness up around 10kHz and a mildly soft quality which we found much more agreeable than the hardness which passes for hif-fi in a lot of other small systems. We judged the A-25s about equal to the AR-4xes at the top and somewhat smoother than the KLH Seventeens. At the low end, though, it was a different story.
Unlike most small systems, which need all the low-end augmentation they can get, the A-25s tend to put out too much bottom in most rooms when placed on the floor or in the room corners. Best results were obtained in most instances with the speakers a couple of feet above floor level, which is convenient in view of the fact that these are, after all, supposed to be bookshelf systems.
Both the AR-4x and the Seventeen have a noticeable amount of the mid-bass heaviness that seems almost to be an innate characteristic of ultra-compact systems, so whatever output they may have in the extreme low-bass range is rather effectively masked by the upper-range weight.
By contrast, the A-25s seemed at first to be deficient through the entire low end, at least until some really deep stuff came along. When it did, what came out of the A-25s simply defied belief, for they went deeper even than two of our "standard" systems, the Z-600s and the KLH Nines.
We knew that a single pair of Nines, with the panels separated, start to roll off below about 50Hz, and that the Z-600s in most rooms start to dwindle below 40Hz. But we were certainly not prepared to find these piddling little Dyna systems going flat down to 55Hz and rattling windows at a hair below 50Hz! And this with a degree of detail and tightness that rivalled the Nine's and ran circles around the Z-600's.
The AR-3a, of course, is practically in a class by itself when it comes to low-end range. With virtually flat bottom down to around 25Hz, nothing short of some monster systems can equal it in this respect. Certainly, the A-25s couldn't. But in the matter of transient response, particularly through the woofer's range, the AR-5a has left something to be desired, and it is here where the A-25s offer the AR-5as some real competition.
In test after test, the A-25s revealed more bass detail than the AR-5s and, in most cases, produced a more natural bass/treble balance than the larger ARs. Some listeners have complained about a certain "heavy" or "thick" quality about the AR-5a's sound. The A-25s had virtually none of this, and neither did they have any of the mildly distressing "crinkling-paper" sound that is so common in small acoustic-suspension-type systems.
In other words, as ridiculous as this may sound in view of the price difference, we would opt for a pair of the A-25s over a pair of AR-5as.
As we mentioned, the AR-5a's real point of superiority is in low-end range. But since both systems more than span the low-end range on commercial recordings, the AR-5a's superiority down there strikes us as being somewhat academic.
Only in one respect did we find the A-25s to be clearly inferior to two of the systems we compared them With: transparency. By comparison with the liquid clarity of the electrostatic systems, the A-25s had a dry, almost grainy quality and a somewhat dead sound. There was however practically no sense of restricted high end, so we attribute the difference mainly to the simple fact that nobody has yet managed to design a dynamic loudspeaker driver whose transient response can challenge that of an electrostatic.
Actually, we have heard some dynamic systemslike the Altec A-7-500 and some of the Hartley unitsthat had more apparent transparency than the A-25s, but these were much higher-priced and had a few minor shortcomings of their own. Actually, although we were not able to make direct comparisons, the A-25s reminded us more of the KLH Model Twelves than anything else we've heard, although our recollection was that the Twelves had a shade more impact and detail at the bottom, a somewhat better-detailed high end, and a subtly drier sound than the A-25s.
Summing up, then, we feel that these A-25s are better than anything else we've ever encountered for less than $200 each, and are worth consideration in any but the highest price category. We will even go so far as to say that they are quite probably the best buy in high fidelity today.
If you're looking for some small loudspeakers for a second, remote listening location, these would seem to be the logical choice. For a main, price-no-object speaker setup, you can buy better transients and more efficiency and, perhaps, deeper bass with equivalent or better detail, but you'll have a hard time buying more musical naturalness at any price. Just one other suggestion, though: don't tell your status-conscious friends how much they cost. They don't sound quite as good when you know.J. Gordon Holt
The aperiodic design is not a bass-reflex approach, since there is no acoustic output through the port. The characteristics of the "plug" in the port are quite critical, necessitating individual adjustment of each system. This added acoustical impedance damps the woofer, improving its response to transient signals. Examination of the woofer cone motion shows that, with this aperiodic design (on which patents are pending), the cone follows the input signal all the way down to DC with far greater precision than is the case with either bass-reflex or acoustic-suspension designs.Dynaco