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I suspect few readers think of Audio as a "religious" magazine. First, audio really isn't quite that serious. Second, anyone who reads Audio has to share a common and pragmatic interest in the finest in music and sound quality.
At the same time, it seems there are three schools of opinion regarding audio reviewing. These sometimes are taken far too seriously and are then pushed to the point of becoming cults.
One school holds that only technically measurable differences matter in judging the quality of audio equipment; the second holds that only subjective differences matter, and the third holds that some technical and subjective differences do matter but can only be validated through controlled listening tests.
As a largely subjective reviewer, I have to confess to belonging more to the second school than the first or third-although I read my more technically inclined colleagues with considerable attention and take their results very seriously indeed. I also pay close attention to the ongoing debate over whether controlled listening tests really work. My interest in audio equipment, however, is largely in what it can deliver in terms of providing a more convincing illusion of a live performance not in its terms or technology. I also believe that sound quality can only be determined by weeks or months of careful listening to a wide range of performances, and I believe in the kind of comparative listening that takes hours, not minutes or even moments.
Now, all of this introduction would be little more than a statement of personal philosophy if the value of the product I am about to review was not so dependent on subjective sound quality. The Theta DS Pre is one of the most sophisticated consumer audio products now available, but it does not necessarily measure any better than many other units which perform a similar function.
The Theta's whole purpose is to pro duce subjectively superior results in the process of digital-to-analog con version. It also is only fair to point out that D/A conversion is an area where experts and reviewers are still sharply debating whether any serious differences are audible and what aspects of digital sound, if any, need further improvement.
Let me preface my remarks on the sound of the Theta DS Pre by establishing that it is not merely the result of a minor "tweak" or improvement in existing D/A systems. The Theta DS Pre is based on some major innovations in technology-as might be expected with any unit that costs $4,000. The DS Pre is the equivalent of a sophisticated computer system and uses a proprietary, composite algorithm produced with the aid of a supercomputer. Ac cording to the manufacturer, this al lows digital decoding to be optimized for time domain, group delay, transient ability, and frequency response. In contrast, virtually all other decoders optimize only for frequency response.
The digital-to-analog conversion process in the DS Pre is programmable. It can be used with both CD players and DAT machines, and its performance can be steadily upgraded or optimized by changing the ROM chips in each channel. The design also minimizes the amount of analog circuitry involved. Rather than try to correct or alter the sound once conversion has taken place-which is what goes on in many CD players-the DS Pre does this during the actual digital conversion process.
The unit's main designer, Mike Moffat, describes the design philosophy behind the Theta as follows: "All over-sampling, digital playback devices and recording devices-utilize digital filters to filter the signal at half the sampling rate, as well as to actually generate the oversampling itself. It takes a microprocessor to compute the necessary interpolated samples. All micro processors need some kind of program or software to run.
"So far, all of the CD players and R-DAT machines available to the public have utilized a simplified digital filter, which consists of a simple processor with a program masked onto a memory portion of a chip. The only way to change the performance of the digital filter, which is critical to the sound of the decoded music, is to replace the CD player or the R-DAT machine.
When the best CD players cost over $5,000, this is a less than ideal way to deal with progress, as better and faster filters are developed.
"My research has indicated that different methods of processing in the digital domain of the filter have more impact on the final decoded sound than what can be achieved by the relatively trivial improvements to be gained in the analog section of what has to be a flawed medium.
"The Theta DS Pre uses a separate, digital signal processor for each channel, which is programmable by the user or the dealer by simply changing ROM chips. There are at least 50 different ways to design and optimize digital signal processors or filters. They all sound different. They all perform wonderfully when measured on normal test equipment. The Japanese and European companies which produce dig ital audio devices all use the same filter design, simply for the reason that it was one of the latest methods published in the Bell Laboratories and IEEE references. Unfortunately, it is the one that sounds the worst." The Theta preamp also has a number of other technical features worthy of note. The unit uses non-switching eight-times-oversampling filtration and conversion with only a single-pole filter. There is said to be less than 5° of phase shift from 20 Hz to 20 kHz. Interference from digital control signals is prevented by an exceptionally heavy solid-steel chassis, which is plated with copper and then with zinc. There are seven power supplies, to prevent current draw in one area from affecting another.
The DS Pre has a wide range of control features that have practical value and greatly improve Vie flexibility with which this preamp can be used.
The rear panel has two switchable digital inputs, and a source-monitor switch on the font panel can be used to directly compare the decoded output of the Theta with that of the original signal source or of some other digital input.
The channels can be reversed, and there is an absolute-phase switch.
While the DS Pre is not a true preamplifier, it has a number of analog controls wh.ch allow it to perform some of the functions of a preamp. The audio output is switchable between a fixed line output and a volume control with right- arid left-channel trim pots to pro vide control of balance. There is a "Mute" switch, and a "Normal/Thruput" switch makes it extraordinarily easy to compare the sound of the Theta with that of the D/A and analog circuitry in any CD player or R-DAT unit which the DS Pre is used with. In other words, this unit has all the controls needed for it to be used to drive power amplifiers directly. In a truly simple audio installation, you could combine CD and R-DAT units with a high-level source such as a tuner and do without a conventional preamp entirely.
All of this technology and the features would be unimportant if the Theta did not produce impressive sound quality. "Gilding the lily" has no more benefit in audio than in any other art form, neither, for that matter, does "gilding the ear." A unit such as the Theta preamp only has meaning to the extent that it is capable of producing audibly superior and more enjoyable music reproduction.
The DS Pre does this in spades! It not only reproduces music far better than the D/A conversion stages of any CD player or R-DAT unit I have yet heard but also promises that digital sound, in addition to its own benefits, can surpass any of the claimed benefits of analog. In fact, this Theta preamp provides such superior reproduction of the finer details in recorded music that we may not be able to tell what digital can do until the producers and manufacturers of CDs and R-DAT cassettes begin to use equally good equipment during studio monitoring and production.
Let me be more specific. I have used the Theta DS Pre with over a dozen top CD players and two R-DAT units. Given the rapid change in digital products, there isn't much point in naming names, but the units I used have had very favorable mention in this and other top audio magazines. In every case, the Theta improved the depth and imaging of the soundstage.
With the exception of a few CDs that were one- or two-dimensional to the point where the recording engineer appeared to have performed an act of malice, this improvement in the soundstage was clearly perceptible, and in many cases, it was striking. For example, in listening to the Proprius recording. Jazz at the Pawnshop (PRCD 7778), I found for the first time that the CD could equal or surpass the analog record in revealing the details of the soundstage.
The Theta preamp also added to many recordings a degree of subtlety and transparency that was sadly lacking in virtually all the CD players I auditioned. Audiophiles may disagree as to whether or not phonograph records have an added degree of "air," or mix of soft harmonics and overtones, that is lacking in CDs. I doubt, however, that most audiophiles who hear the Theta preamp will argue about whether or not it reveals more "air" than the D/A con version stages in virtually all competing CD or R-DAT units. You can hear the difference on everything from Willie Nelson to Gregorian chants, and it is particularly impressive in choral works, opera, and complex vocal music.
There is a consistent improvement, ranging from slight to immediately impressive, depending on the recording and on the competing D/A circuit.
I am not a supporter of those who claim that most digital equipment's up per octaves drive you out of the room, are lacking in musicality, or somehow produce a consistently unpleasant edge. A bad recording is a bad re cording, and the better high-frequency performance of CDs can scarcely be blamed for revealing this. Nevertheless, I often do find that many digital recordings seem to have upper octave information which is slightly a-musical, and they sometimes contain smeared or hardened music. The DS Pre does not save bad recordings, but it does do a great deal to show that many recordings actually have far better and more musical upper octave detail than I had previously believed. The Theta preamp often makes a particularly impressive improvement in the reproduction of violin, piano, vocal sibilants, and upper octave woodwinds.
Finally, I found that this preamp produced a more convincing and dynamic mix of bass, upper bass, and lower midrange. I had previously found this part of the musical spectrum to be a bit flat on many CDs, and to lack natural musical life and interest. The Theta may not always produce more powerful bass, but it consistently produces more musically detailed bass. It also reproduces the natural richness of the transition from bass to midrange something I find critical to enjoying live music but often miss in recordings.
At this point, I have to return to the three cults, or schools, of audio reviewing discussed at the beginning of this review. If you already believe in subjective listening, you can test the validity of my praise of the Theta simply by auditioning it at any high-end dealer who carries it. The benefits of the DS Pre are not all that subtle. They should show up clearly after listening to a few of your favorite CDs on any system that is set up to provide a revealing soundstage and that is quick and extended enough to be called transparent. Further, if your CD player has a digital output, you can plug it into the Theta and immediately compare the improvement it makes in sound quality, few products provide an easier way to make sure you do not get carried away with reviewer claims or technical hype.
If you sincerely believe that only technical measurements matter, you've got a problem. There is no way I can possibly prove that the DS Pre really sounds better, although the summary of its design philosophy and technical features may at least persuade you that it could. You may then have to compromise your beliefs a bit to enjoy this preamplifier.
Similarly, if you only believe in con trolled listening tests. I am afraid my proof of the Theta’s merits is equally lacking. It is easy to "A/B" this unit-it has A/B switching built in. I also ran enough blind tests to confirm that after extended auditioning, non-audiophiles hear a real improvement when listening to carefully chosen passages and to their own favorite CDs when com paring the Theta DS Pre to their favorite player. What I cannot offer, how ever, is a statistically valid sample to show that the Theta is better-if for no that I know of no way to determine how to structure an experiment in which the exposure to given lengths and types of music would really determine what is statistically relevant in terms of perception. This re view is not the place to get into statistics, but it is very easy to measure the statistical validity of a large sample from a given experiment. What is difficult is to structure the experiment so that the actual test is relevant. All my experience with audio equipment indicates that controlled or blind listening to short periods of music does little to tell most people about the sound character of a given piece of equipment, and it quickly confuses most listeners.
Similarly, tests of large numbers of listeners--to determine what levels of distortion and frequency can be perceived-have tended to produce results that minimize hearing abilities and maximize the levels of distortion that matter. This occurs, in part, because the tests often are not relevant to the results of prolonged listening to music.
All I can suggest is that, even if you do firmly reject subjective, open, long term listening as a major criterion for selecting equipment, you should still audition the Theta preamp so you can play the role of devil's advocate. At least you will be able to say that you personally tried to figure out why subjective reviewers get so enthusiastic about "nonexistent" differences in sound quality, and that you cannot see any reason why there is a debate about digital sound. Further, if you do hear the benefits I claim for the Theta DS Pre, this may prove an occasion for joining one of the few cults that are harmless. You give up nothing, and you may well find that the sport of audio becomes far more enjoyable--and that your system can reproduce far more musically convincing sounds.
-Anthony H. Cordesman
Also see:
Theta DSPro Prime D/A Converter (Auricle) (Aug. 1992)
Vacuum Tube Logic Straight-Line D/A Converter (Nov. 1992)
Vimak DS-2000 D/A Converter and Preamp (Nov. 1992)
(adapted from Audio magazine, Nov. 1992)
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