Quad Artera Play Linestage/DAC/CD Player and Stereo Power Amplifier
Robert E. Greene offers an in-depth commentary and analysis of the Quad Artera Play linestage/DAC/CD player and Stereo Power Amplifier reviewed by Paul Seydor in Issue 268 (December, 2016).
Like many people who are interested in how amplifiers actually work, I have long been fascinated with the Quad “current dumping” amplifiers, starting with the 405 model and the 306 and 606 models, which I reviewed in Issue 71. In this review, I concluded with the thought that if high-end people did not think the Quad amplifiers were perfect within their power limits, as Quad asserted, then it was really obligatory for the objectors to explain what was wrong with them.
This challenge arose not just from measured performance and Quad’s own assertions that the amplifiers were transparent to signal in audible terms. There were those demos by Quad where they strung many amplifiers together, with suitable attenuation in between and challenged listeners to tell the difference between a chain of a number of them and a single amplifier. No one ever could tell. This struck me as a challenge few manufacturers would like to submit their own product to. Moreover, Quad was on record as checking things that other manufacturers tended to roll right over. For example, they eschewed fuses—at a time when fuses in the signal path were very common—in favor of a thermal protection mechanism, which detected excessive heat production internally and if it were found, would simply turn the amplifier off. The reason was that, as Peter Walker said in a TAS interview, “You can hear a fuse.”
Altogether, I got the idea that Quad was trying to make its amplifier designs as accurate as possible and was checking this by demonstrably valid listening tests, at a time when many high-end designers were trying to check their amplifiers by listening to commercial recording to see if they could hear “soundstage” and other such comparatively indirect methods. This still goes on, People seem to have a checklist of things they think ought to happen in listening to commercial recordings but as often as not these expectations do not have much logical basis. (A larger soundstage is not always better. Maybe the recording does not have a large soundstage!)
Still, for all my enthusiasm for the Quad amps at the time (and I in fact bought the 306), I was concerned about two aspects of the designs. First was that they had limited ability to deliver current and thus did not react well to low- impedance loads. The second source of concern was that they were band-limited, with a quite severe roll-off in the bass and a considerable roll-off on the top as well. These had to my ears some audible effect, though it was hard to be sure that the effects were not just the superior behavior of the amplifier otherwise. But on the Hafler differential input/output test, the phase shifts associated to the band limiting made for a larger differential between input and attenuated output then wider bandwidth designs exhibited. Though it was hard to square the possible audible importance of this with the daisy-chain inaudibility, the point remained a concern to me.
I was so impressed with the design in principle that I seriously considered trying to get a license from Quad to design and manufacture myself a version of the amplifier with higher current capability and wider bandwidth. I gave this idea up because I realized that I am really more a critic and theoretician than a businessman. Anything I cooked up would likely have remained a prototype! I always hoped, however, that Quad itself would make such a version of the current dumping design.
I had to wait a long time. But here it is. The Artera Stereo is essentially flat to 1Hz and has a -6dB upper limit of just over 100kHz, -0.35dB or so at 20kHz (figures are specifications by Quad, supplemented by Paul Miller’s measurements for Hi Fi News). This is hardly the proverbial DC-to-light territory (some amps go out to megahertz—though they usually do so at the price of other problems), but it is plenty to get largely rid of what I had supposed were the audible consequences of the band limits of the older “current dumper” designs, as indeed they turned out to be. And the Artera Stereo does a respectable job of even 1-ohm loads—which fortunately are rare nowadays. Certainly for 2 ohms on up, one will be in good shape, with the current capacity being 12 amps—not enough to drive trucks but almost surely enough for domestic audio with ordinary speakers in rooms of plausible size.
In short, the Quad Stereo is the amplifier I would have hoped to build myself, back when I was dreaming dreams of current dumping.
Does it sound “good”? Of course it does—if the material is good, the sound is good. This is a stable, clean, essentially neutral amplifier.
Sometimes when one waits for something a long time, when it arrives it can seem not so important as when one first was waiting for it. In the decades since Issue 71, when I first encountered in the Quad current dumpers, amplifier design has come some distance, though not in fact quite as much a distance as reviewing sometimes suggests. And the Quad Artera Stereo, which might have swept all before it in 1985, has serious competition today, even at its reasonable price. For one thing, for not much more money, there is the Benchmark, which has slightly less power but even better distortion specifications and is considerably quieter and has wider bandwidth (-3dB at 200kHz, -0.17dB at 20kHz).
Is this wider bandwidth going to be audible? Yes, just barely in in-band response terms. When the Quad arrived. I put on a recording I had been listening to a lot with the Benchmark amp. (I did not switch to the Quad Play—the input to the Quad amp was the same as the input to the Benchmark amp had been). And yes, the top sounded just slightly more subdued—but slightly is definitely the word. The Benchmark costs a bit more but not a lot more. For people who need more power, there is the Sanders Magtech, which costs roughly twice as much but will drive anything at all (and sounds superb). And of course others will have their own favorites.
Sentimentally, nostalgia in action, I wish that I could say that the Quad Stereo was not just really good but was a one- and- only amplifier in its power range. But in fact there are other really good amplifiers, really good in the sense of being really truthful, the only sense in which I am interested in really good myself, in roughly the same power range, the Benchmark being an obvious competitor for example. Interestingly, the Benchmark uses a feed-forward circuit that in general terms is related to how the Quad works. In any case, the Quad is not alone. But it is surely a very fine amplifier indeed.
For what it is worth. PS and I part company on its relationship to the Quad 909 (and all other previous current dumpers). I think the Stereo brings the current dumper design to a higher level than before without any question, though I admit that if one likes the character of the earlier ones, then indeed this specific character is diminished. The Stereo is less idiosyncratic, but it is also better in my view. To me, the 909 just sounds rolled off in a way that the Stereo does not.
The Stereo retains the effect, all to the good, of not emphasizing the problems that lots of out-of-band energy can cause, high frequency transducers being as they are, namely often badly behaved out of band. (This can cause audible problems lower down). This can be a real advantage, depending on which speakers one is using. The Stereo sounds calm and controlled in a way that not all amplifiers of extremely wide bandwidth do.
To go into a bit more detail: Of course ideal stereo has infinite bandwidth at all stages for the minimum phase parts of the chain, because any roll-off inevitably induces some phase nonlinearity in the audible band, however slight it may be. And it is really and truly inevitable—this a mathematical matter. But in practice, any, perhaps even most, speakers behave rather better if one does not drive them hard out of band. Tweeters get nasty way high up in many cases, and the nastiness can intermodulate down. It has been remarked on occasion that CD digital in a sense has an advantage over wider bandwidth systems for that reason—that the tweeters are not exerting themselves way on out and opening up possible trouble in the audible band. Life is full of compromises.
In practice, I have had quite often the experience that the older current dumpers could make the top end of metal dome tweeters in particular a lot easier to take, presumably because they supplied less energy to the nasties in the tweeter, out of band. If you are doing research on the effects of digital filters or the like way out there then you better have as broad a bandwidth in amplification as available. But if you are listening to music using a tweeter which is less than perfect forever on up—and that would be all of them, wouldn’t it?—then a certain roll-off way on out there might actually be an advantage. In any case, the Stereo does go out quite far, but it seems not to exacerbate tweeter problems too much, presumably because its top end is well behaved not only in band but also out of band and is not excessively extended.
The Quad Artera Stereo is a very fine amplifier indeed at a reasonable price. I would definitely put it on my short list in this power range. And if it has just a feathery touch of the old Quad current dumper sound, well, that might, as noted, be a good thing! I am happy to see the dream of my comparative youth come true. And I enjoyed my time with the Stereo, every minute of it.
The situation of the Artera Play—the preamp, DAC and CD player combined unit that is the companion piece of the Stereo—is more complicated, but at the same time perhaps more interesting. The Stereo has an unusual circuit, but in practice it behaves just as one would expect a stable, low distortion, medium-bandwidth amplifier to behave. But the Play unit is actually operationally different from the ordinary CD /general digital unit.
By operationally I do not mean how you get it to work (primarily by remote control). But the Play offers a feature that is all but unique, though it may someday become common.
Namely, it offers a choice of the filtering for its output. Digital signals when being converted into analog signals to drive analog-input amplifiers need to be filtered or at least such is the prevailing wisdom. (In principle, one could let one’s ears do the filtering if one did not mind driving one’s tweeters really hard at out of band frequencies) Ordinary digital devices contain filtering, but it is not usually discussed in public. It is built in and the user has no control over it so a lot of users may not even know it is there unless they have read somewhere that it is and indeed, in the usual perception, has to be.
With the Play, you are in the business of choosing a filter. One of them, filter 4 “narrow” is CD standard on the player output according to Quad when playing CDs. (But if you use the digital Airport Express input, it truncates the top end aggressively to save you from the effects of high jitter.)
Quad does not give one exact information in the manual on what the other filters actually do but rather gives one a verbal description in general terms of how they operate and sound. As PS noted in his review, these descriptions are in quite general terms and one needs to listen for oneself. Quad was forthcoming about what they did when I asked. And in practice, I think that people will find experimenting with the filter choices interesting and perhaps useful.
Attention has been focused recently on filtering on account of, first, the “apodising filter” that RH was so enthusiastic about some issues back and more recently MQA, which as I understand incorporates what amounts to apodising filtering. The idea is mathematical but in outline one can think of it as giving up a little frequency response flatness and reducing slightly out of band rejection of undesirable digital products in exchange for a cleaner impulse response and reduced “pre-ringing” and “post-ringing”—sound before or after the actual sound. (Even though this pre and post sound is beyond the usual idea of the audible band, there is some evidence to suggest that it is even so audible at least in extreme cases.) On high-bit-rate PCM, filters can be essentially flat and phase linear in the audible band and their impulse response will be good in terms of what is in the audio band or anywhere near it. In CD digital, one has to give up some of the extreme top and on occasion some of the suppression of out of band products to get a cleaner-looking impulse response. This is related to what Wadia was doing in the 1990s and after. The Wadia players had a response that started to roll within the audible range though it did not roll far down until considerably above the usual limit of one half the sampling frequency. But in exchange as it were, they offered cleaner impulse behavior.
Quad did supply some frequency response and impulse response graphs upon my request. But it is not easy to connect this sort of information up to audible performance directly. Roughly, the “fast” filter is the one with the earliest in-band roll-off at the top but the best-behaved impulse response (to look at) and a lot of out of band rejection. Here we are speaking of being down by what looks to be on Quad’s curves close to 3dB at 20kHz, enough to soften the top a bit, though that may be all to the good with a lot of speakers with rising top ends. The “smooth” filter is maximally flat in the audible range but allows through quite a lot of energy above the nominal stopping frequency of 22.05kHz. And the “wide” filter has minimal pre and post ringing but less out of band suppression. The “narrow” filter when used on CDs played by the unit or USB digital input at CD standard is, as noted, Red Book standard.
So which one is “right”? In trying to decide that, one has a problem because digital audio at CD standard in particular involves a certain compromise from the start. The upper limit, the sampling rate/2 which is 22.05kHz, is too close to the audible band for a transparent filter to be constructed at all easily, or perhaps at all if the filter is to suppress all out of band products to an extreme. Roughly, what happens is that if one wants impulses to be clean—minimal pre and post echo—then one has to give up a bit of frequency response flatness and perhaps some out of band rejection, as Wadia did.
This is a tricky subject, but most people have decided that one really ought to have a higher sampling rate so that a filter could be flat within the sub-20kHz frequencies (and phase linear there) and have a more gentle roll-off than the kind needed to be out of there by half the sampling rate. To suppress out of band products almost completely is of course not going to require such severe filtering near the audible band if half the sampling rate is considerably higher than 22kHz! But for CD one is stuck with the issue. As noted above, the out of band products are not as such a problem—your ears will filter them as will many speakers, though in principle they put some stress on the system,
In tonal terms, small droops up close to 20kHz have rather little effect, and are also consistent with live music (air absorbs frequencies up there and reverberation tends to have almost no energy up there at all), although the amount in the ”fast” filter is audible. It is beginning to seem likely that the improvements wrought by the cleaner impulse response and perhaps some other things outweigh the loss from a reduced output at 20k and also justify letting through some energy that is out of the band where it really represents direct audio information—the stuff above 22kHz is not really part of the audio signal that was originally recorded.
Technical audio people tend to believe in flatness as a matter of religion, and Red Book specifies at the same time the deep suppression of output beyond 22.05kHz. But there is a growing body of anecdotal evidence—and sometimes more than anecdotal perhaps—that better impulse response and phase shift may be more significant in audible terms than a slight top end loss, audible though the latter will be, or out of band energy, which in theory is not directly audible. The reader interested in this general situation might find it enlightening to go back and read people’s reaction to Wadia’s work. One might also think back to years ago when Onkyo introduced a CD player that allowed a lot of out of band energy through (apparently on the grounds that the ear/brain liked some out of band energy but did not much care what it was as long as it was roughly related to the signal). For whatever reason, that CD player actually sounded very good).
So in practice, one is almost inevitably compromising a bit on CD digital, whatever one does.
The thing is, however, that one cannot really judge this in the abstract by listening to material already recorded. One can of course judge what one likes. PS liked the “fast” filter. I thought it made piano music too transient–emphasized, with too much attack, not in the sense of edge but of transient weight as it were. And so it will go—different people will like different things. Perhaps out of habit, I actually preferred my usual (Benchmark DAC1) Red Book CD standard to the Quad’s alternative filters most of the time. Of the Quad alternative filters, I preferred “smooth” almost always. But it is important to realize that such preferences are essentially personal and recording- dependent as well.
All that really counts in universal terms is how a D-to-A arrangement, when combined with an ideal A to D, preserves the signal. One can really only check this completely with a live mike feed and a comparison of the live feed with the A-to-D and D-to-A concatenated. Anything else is just choosing flavors in a context where what is right is not truly knowable and personal preference rules. Still, you may enjoy playing around with the filter choices just to get some understanding that the issue exists.
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