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Digital technology is evolving so fast that it is difficult to keep track of improvements and the resulting changes in product lines. This is particularly true at the top of the high end, where each improvement in one manufacturer's products leads to a competitive innovation in others. Theta Digital's new top-of-the-line D/A converter, the DS Pro Generation V, is a good case in point. There are some significant changes in this unit, and I was a bit stunned to see that it is claimed to have 125 million instructions per second (or MIPS) of computing power, more than most computers had in the days when NASA was still capable of putting a man on the moon.
The Generation V, which sells for $5,595, differs significantly from other Theta D/A converters in a number of ways. Advances in the analog stages of D/A converters and CD players have proved to be as important as advances in their digital technology, and the Generation V is the first Theta product to use fully discrete, Class-A analog circuits, with very fast rise-times and low settling times.
Theta indicates this change to discrete components was not easy. IC-based and op-amp-based designs have inherent advantages in the analog circuits of digital audio components. Because of smaller geometries, the best of the IC-based designs are faster and tend to settle more quickly. This is particularly important in the current-to-voltage converter stage following the DAC chip. For example, the UltraAnalog DAC, which is well known in the industry for its resolution and performance, uses ICs for current-to-voltage conversion.
The other advantage of IC-based designs is d.c. precision. It is possible, using premium ICs, to design circuits that use no blocking capacitors or servos to keep the output at 0 V d.c. The cost of such circuits is that they normally have very high open-loop gain and require a great deal of feedback-which, in turn, sacrifices soundstage openness, air, apparent transient speed, upper octave detail, and sometimes midrange warmth.
With the Generation V, Theta believes it has overcome the shortcomings of discrete designs. The circuit topology uses a cur rent-feedback amplifier for the current-to-voltage stage and discrete 6 buffers for the filter/output section. The new analog section uses a balanced differential design throughout the whole unit, as well as a new and much better circuit board. The analog circuit is Class A, to eliminate crossover distortion, and it uses a current-feedback design with a very large open-loop bandwidth and very low feed back. Theta claims this yields ultra-fast slew rates and extremely low peak error and settling time.
The Generation V has balanced as well as single-ended outputs. As is usual with Theta, the balanced outputs are derived from two DACs per channel. Theta feels this is the best (albeit most expensive) means of generating a balanced analog signal. Furthermore, the single-ended output is summed from the balanced signals, which means that these outputs also have the advantage of using two DACs in each channel.
Much of the digital section from Theta's Generation III has been retained because the company found, after considerable experimentation, that it could not do much to improve performance without spending so much money that the retail price would rise to $15,000 or more. The Generation V does, however, use the latest Crystal Semiconductor EBU receiver and proprietary jitter-reduction circuitry. It has three digital signal processors and uses Theta's own digital filter algorithm to reach 20-bit resolution. It also uses a process Theta calls "digital domain balancing." Unlike most D/A converters, which provide a balanced analog output by balancing the signal in the analog domain, the Generation V creates a balanced signal in the digital do main. Eliminated is the "inversion stage," which normally comes after the main analog stage and which reduces sound quality because the negative phase of the signal goes through two analog stages while the positive phase goes through only one. In contrast, the Generation V creates the positive and negative signals in the digital do main and passes them on to a separate DAC chip for each phase of each channel; as a result, the analog output is symmetrical for each channel and phase.
The Generation V has a new power sup ply that uses 14 regulators. It delivers twice the peak-to-peak voltage swing of the sup ply in the Generation III, and the analog portion is dual mono. All this is said to greatly increase dynamic range and channel separation. There are four separate power transformers to help isolate each stage.
As for features, you have a "Phase" switch and front-panel selection of three inputs, an AES/EBU input and two RCA coaxials. One of the main RCA inputs can optionally be replaced by an AT&T, a Toslink, or Theta's proprietary single-mode fiber-optic input. You also have separate digital tape inputs and outputs plus monitor switching. The single-mode digital input, incidentally, can only be used with Theta CD transports having the optional single-mode output. Theta feels this connection offers a significantly cleaner transfer of information than the conventional AT&T variety.
So much for the manufacturer's technical rationale. What do you actually get in terms of sound quality? The answer is a considerable refinement over that of the Generation III. The sound of the Generation V has the musical sophistication of top-of-the-line Krell and Mark Levinson D/A converters, although it is different in dynamics, timbre, harmonics, transients, noise floor, and many other qualities.
Past Thetas had more apparent upper octave energy and more aggressive dynamics than many competing D/A converters but at the cost of a slight aggressiveness and hardness. The Generation V has the sweetness, superior harmonics, and air that are associated with the best Class-A circuits, and its sound is distinctly sweeter and more refined. This change may at first seem a step backward to those who are used to the punch of the Generation III, but extended listening will reveal that the new model is more realistic in its string, piano, and guitar sound, as well as in fine percussion detail such as skin sounds and cymbal.
The bass, for example, is now much tighter and better controlled. This initially may seem like less bass, but it is actually bass that is more musically correct. In terms of sheer bass energy, the Generation V is somewhere between the high levels of units from Krell and Wadia Digital and the slightly leaner sound of Mark Levinson units. The differences, however, occur only in the deep and mid-bass and can vary ac cording to signal source. As to which of these bass sounds is "right," well, what is the "right" seat in the concert hall? Buy according to your taste.
The Generation V still has a bit more apparent upper midrange and treble energy than most other top-of-the-line D/A converters, and this preserves the dynamic and involving character of previous Theta units. Yet massed strings and winds are now cleaner and subtler, and the soundstage has expanded to the rear. You get a more natural mix of musical detail and a more open hall. There is far less tendency to move the apparent listening position slightly forward. Guitar, harpsichord, soprano voice, and complex musical passages are all more realistic. The Generation III had a slight tendency to exaggerate low level dynamics, which was exciting but not accurate; the Generation V corrects this, and major shifts in musical dynamics are now more natural. At the same time, peak orchestral and rock passages open up, and there is more information and subtlety.
Without question, Theta Digital's DS Pro Generation V is one of a handful of top-of-the-line D/A converters that currently define the best in digital sound. I can assure you that the Generation V is much better than the Generation III in terms o musical realism, and the Generation III was very good.
-Anthony H. Cordesman
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CHOOSING A D/A CONVERTER
The choice between the best D/A converters is highly personal. Although I do hear more similarity in sound between top converters than I have in the past, there remain subtle differences in harmonics, dynamics, air, and soundstage that define the musicality of each unit. These differences are most readily apparent with guitar, piano, violin, and other familiar solo instruments with complex harmonics. You can also hear the differences on the bands of various Chesky and Opus 3 test CDs that demonstrate depth, imaging, and low-level resolution, and they will show up as well during prolonged listening to orchestral or choral music or to acoustic rock or jazz. The problem is that all of these different sounds are now musically credible.
My suggestion is that you audition a range of top units carefully over time. Try to avoid listening to dramatic musical events or audiophile spectaculars. Concentrate on CDs you know well, zeroing in on the sound of solo instruments, the comparative resolution of soft passages, and the way each unit handles the soundstage, particularly in terms of depth and hall effects. If you are on a budget, listen to middle-of-the-line D/A converters by top manufacturers as well. Digital technology has improved to the point where many of these units come very close to the sound of the outright best ones, and there is no point in paying for diminishing returns at the cost of imbalances in the relative quality of your other components.
I also caution you to use CDs from different labels and to do your auditioning with your particular CD transport (or try different models). There also are subtle sonic differences in the interactions between particular D/A converters, transports, connections, and sometimes cables, as well as the A/D processing in given source material.
You need to be sure of cause and effect, and that you will hear the same differences at home that you hear in a dealer's showroom. The best approach may be to contact your dealer and either bring in your transport and cable or arrange a loan for auditioning in your home system. Above all, be sure to audition the sound you will actually get in your system if you are concentrating on choosing between nuances in the best digital equipment.
-A.H.C.
Theta DSPro Prime D/A Converter (Auricle) (Aug. 1992)
Theta DS Pre Preamplifier (Auricle, Sept. 1989)
Vacuum Tube Logic Straight-Line D/A Converter (Nov. 1992)
(adapted from Audio magazine, Feb. 1995)
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