Audio Research D-100 power amplifier

We cannot recall when any new products have generated as much of a stir among perfectionists as the new solid-state equipment from Audio Research. Preceded by rumors of "a new kind of amplifying device—a cross between a tube and a transistor"—the announcement of ARC's new power amp and SP-4 preamplifier elicited very mixed reactions from loyal ARC customers, some of whom gleefully anticipated a virtual revolution in audio electronics, others of whom felt betrayed by the company which, having originally convinced them that "Tubes Are Better," suddenly seemed to be doing an about-face and espousing the views of the Enemy—the "Soiled-State"—forces.

Now that the first of the "new breed" of ARC products has been out for a few weeks—This is being written in mid-October 1976—it is making even bigger waves than the initial, highly provocative scuttlebutt. Reactions to the D-100 have varied from ecstatic to strongly negative, with the ecstatics outnumbering the detractors by a small but definite margin, And curiosity about the new ARC equipment continues at an even higher level than before it reached dealers' showrooms, spurred by the company's Pentagon-style secrecy about their new circuitry.

Inside the D-100
oooiiouyeeee We received two D-100s for testing, for a double-barreled reason: because new products tend, more often than not, to fail the moment Stereophile tries to test them and, barring such a likelihood, so that we could biamplify with them. We are happy to say that we had the option to biamplify.

Of course, the first thing we did when we decartoned our first D-100 was to remove the top panel and take a gander at the innards. We were impressed. Most of its internal volume was occupied by four very fat filter capacitors and a massive power transformer. The rest left no doubt about the nature of this beast: It is unabashedly solid-state, with what appear to be a couple of dozen (at least) output transistors in an efficient-looking heatsinking arrangement, and several smaller devices that are unmistakably bipolar transistors of conventional appearance. But there were also several little black boxes—encapsulated modeules whose outer appearance gave no hint as to their contents except to make it clear that no conventional vacuum tubes were contained therein.

ARC will shed no light on the contents thereof. In fact, we were informed that considerable time and expense went into the development of an encapsulation system that would defy identification of the contents as effectively as anything short of a booby-trap that would remove the hands of anyone who tried to pry into its inner workings.

Reactions of other manufacturers to this unprecedented (in audio) secrecy varied from a disinterested pooh-pooh to intense curiosity. Some asserted that the secrecy was intended only to conceal the fact that ARC had not in fact come up with anything new and remarkable, and was possibly even concealing the fact that they had swiped some other manufacturers' patented circuitry. Others, noting that some of the claimed performance figures were superior to what anyone had previously attained, suspected that the modules might indeed contain unique new active devices as rumored.

There was a strong consensus on one point, though: Whatever the reason for the secrecy, be it patent theft or protection of an original invention, the "cracking" of ARCs modules offers the kind of challenge dear to the heart of American technology. Many companies may have many different reasons for active curiosity, whether it is a suspicion that their own patents may have been violated or that here is a new circuit worth stealing, but it is certain that people are going to keep trying to find out what is in ARCs modules until, eventually, someone may succeed. If that happens, it is anyone's guess what the consequences may be.

There are no protection circuits or power-limiting circuits in the D-100. These omissions were, ostensibly, to retain the best possible sonic performance, and there are enough output transistors in each channel so that, if anything is severely overloaded, the only thing that pops is the power-supply fuses. The only hitch here is that those fuses are located inside the chassis, and you must remove the cover plate (via 8 screws) to get at them. (Spares are provided.)

Physically, the D-100 looks like a scaled-down version of the D-76A and weighs about 5 lbs less. And sonically, the D-100 has thus far proven to be as controversial as its PR image.

Sound Quality
When the Dual-150 first came out, our telephone rang incessantly for about a week, as subscribers called to tell us it was fantastic, the best amplifier made, and so on. (We rarely get a product before dealers have it.) A few subscribers said they were not altogether enchanted by the D-150, but there was an overwhelming consensus, and it was favorable. Not so with the D-100. In fact, the first five readers who telephoned us about it were disappointed with it, citing "thin bass," "receding middles," "lack of depth," and either "too many highs" or "not enough highs." Only by the sixth call did we start to hear nice things about the D-100 (which we were not slated to get, ourselves, for another week), and some of those favorable comments ranked the amplifier above anything else available.

After a while, a pattern of sorts started to emerge. It seemed that the first five readers who reported dissatisfaction with the D-100 had tried it driving the Fulton J-Modular speakers. Most people who tried it with other speakers were favorably impressed, some to the point of ecstasy.

So, what did we find out from our own tests? Well, since we had been previously of the opinion that the D-150 was the best-sounding amplifier we had heard to date, that was what we first compared the D-100 with. We will admit that we became instantly nonplussed.

We—and apparently a lot of other perfectionists—had rather expected the D-100's sound to fall about midway between that of the D-150 and an excellent solid-state amp like Son of Ampzilla. We expected highs that were a hair crisper than the D-150 but sweeter than SOA. We expected bass tighter than the D-150 but, perhaps, not quite as dry as that from SOA, and we expected even more depth and inner detail than from the D-150.

Instead, what we heard was an amplifier that seemed to excel in every respect, yet sounded rather unlike anything we had ever heard before. We heard immediately what it was that J-Modular owners were complaining about. Driving that system (full-range, one stereo amp), the D-100 virtually dried up the low end, leaving thinness where once there was richness. There was less depth than from the D-150, the middle highs seemed rather more distant, highs were (perhaps by contrast) rather more abundant, and extreme highs—above about 10kHz—seemed muted. In other words, with that speaker system, the D-100 was a disappointment.

On the Js, the only respect in which the D-100 was obviously superior to the D-150 was in inner detail—the "definition" that has always been ARC's secondary trademark. Our auditions with other loudspeakers confirmed the fact that the D-100 outperformed everything else we have tried in that one respect. In other respects, it was a little harder to assess the D-100's performance, for the best loudspeaker systems we had on hand were ones that had apparently been designed to work best with tubes, while the D-100 had the unmistakable characteristics of transistors. As a result, the best systems—like the Js—tended to sound overly dry at the bottom with the D-100 and, with the exception of the Js, a little bit toppish with the D-100.

With all other speaker systems tested, there was practically no contest. The D-100 wiped out just about every™ thing else in and below its price class. The only solid-state amplifiers that were judged comparable were the now-discontinued Marantz 500, which came a close second on all counts, and some of the super-powered amps like the BGW 1000 and the Dunlap-Clarke 500, both of which were judged to have somewhat more low-end impact and, of course, the potential for higher (ear-shattering in fact) listening levels, but had substantially less definition and a somewhat dry, more typically solid-state-type high end.

On those speaker systems which we have found to be complemented by tubes—full-range electrostatics (including the mid and high sections of the Infinity SS-1A), the FMIs, and as another example, the BBC Mini-Monitors reported elsewhere in this issue—our listening group was unable to reach a consensus. All agreed that, with the D-100, bass was deeper and tighter but sparser, highs were a shade less bright but more sharply-etched (yet without hardness), and overall definition was better than with any tube amplifiers. But there was no agreement as to preference. Some listeners found the D-100's sound to be cleaner, leaner and generally more satisfying; others felt its sound to be rather antiseptic and clinical. One likened it to the taste of distilled water: wet, but bland and uninteresting. The same listener subsequently admitted that the D-100 was probably a more accurate amplifier than the D-150, but he felt the D-150's distortions of the sound (which they must be if the D-100 is more accurate) yielded a more musically satisfying listening experience. Ye Editor & Publisher concurs.

Bridged mono operation
Incidentally, the D-100 is one of the few available power amplifiers which is not only switchable into a parallel mode (for mono operation of both strapped channels), but will actually deliver almost four times its 100Wpc stereo-rated power into an ohm load when so used. The audible result of this strapped operation of the D-100 is, interestingly, not too dissimilar from its sound as a stereo amp, except that the low end becomes even tighter (and thinner on systems that were designed for tubes) and the overall sound takes on an impact and effortlessness at ear-shattering levels that must be heard to be appreciated. In the strapped mono mode, though, these are literally killer-amplifiers, in the same league as the biggest BGW model as a potential speaker-buster.

Let's review for a moment what each of the current-model Audio Research power amplifiers has to offer in terms of sound. The D-76A is the most like what the typical perfectionist means when he refers to "tube sound." It is rather soft and sweet at the extreme high end, slightly bright and a bit forward, with very good inner detail through the middles, and almost juicily plump at the bottom. Lows are rather fat, rich and warm, and the overall sound has a limpid liquidity which perfectly complements the sound of most full-range electrostatics or (for biamping) electrostatic tweeters. With speakers of comparable transient response, it is a superb reproducer of string tone and the characteristic sounds of most other instruments, but it tends to round off the attack transients of "hard" percussive sounds. It is, usually, not very satisfactory for driving any speakers except electrostatics or dynamics which were designed for use with similar amplifiers. And it does not have the power to flap walls when driving speakers of average efficiency.

The D-150 is as close as any amplifier has come to bridging what most audiophiles felt to be the gap between tube sound and solid-state sound. It is superior to the D-76A in every way, yet still retains some vestiges of the D-76A sound—a slightly sweet (yet crisp) high end, superb inner definition, superb rendition of depth, and that liquid transparency that has made tube freaks of so many persons who are familiar with the sound of live, un-amplified music.

The D-100 does not sound like a better D-150. Its sonic characteristics are strictly in the solid-state camp, yet it has none of the irritations—the dryness and rough highs—that have repelled the musically oriented in the past. In fact, it probably has as little sound of its own as any amplifier currently available, which may or may not appeal to any specific listener. Its major problem, in fact, would seem to lie more with the loudspeakers that are available for use with it than with any of its own behavioral idiosyncrasies, so it should spur some rethinking of loudspeaker design parameters.

Yet we still find it difficult to assess this amplifier. Logic, reason, and observation prompt us to give it a "Class-A Recommended" rating, but the gut reactions of some of our listeners leave us with the disquieting feeling that something is wrong somewhere, either with our standards of judgment, or with the D-100. The fact of the D-150's incredible musicality, despite observable imperfections, refuses to lie down and stop nagging at us, and even some of those of our listening panel who profess to prefer the sound of the D-100 have requested that we go back to the D-150 from time to time when evaluating other components "to regain their perspective." Perhaps it is just that, like an old shoe, the D-150 is familiar and comfortable, but perhaps too it has other sonic virtues which the "more-perfect" sound of the D-100 cannot yet provide us.

Interestingly, all of us continue to luxuriate in the sound of the D-150—the amplifier which until now we had felt to have no sound of its own. Yet when listening to the D-100, none of us comments on how gorgeous it sounds, and no one complains about anything it does, either. It just doesn't seem to be there at all, which should tell us something. Perhaps our dilemma at this point is best summed up by one of our listeners who quipped, jokingly of course, "Maybe I don't really like high fidelity, maybe I like tubes."

Meanwhile, it should be obvious by now that after several weeks of listening to the D-100, we still haven't really placed it in the order of things. We are listing it as "state of the art" in our Recommended Components section, not because we are in love with it but because we can't find anything the matter with it. Which is one hell of a state of affairs. Maybe by our next issue we will be prepared to climb out on a limb about the D-100, but maybe not.

We'll say one thing, though, as our parting volley: If you own a D-150 now, don't be in any hurry to unload it for a D-100. We suspect that the D-100 is the better amplifier, but you bought Audio Research equipment because of its sound (why else?), and you may or may not prefer the sound of the D-100 to the D-150.

Audio Research Corporation
3900 Annapolis Lane N.
Plymouth, MN 55447-5447
(763) 577-9700