Theta DS Pro Generation III digital processor

A visiting manufacturer recently expressed the idea that digital processors and transports are the worst value in high-end audio. He contended that, because they all sound bad, their differences and degrees of imperfection are meaningless. In his view, the very best digital differed very little from the worst. His advice? Buy a moderately priced CD player and enjoy your LPs.

Based on my experience with a wide range of digital processors, I hold the opposite opinion. Sure, the best digital processors are expensive, but the difference in the musical experience between superlative and mediocre converters is, for me, vast.

This view was dramatically strengthened the past two months as I auditioned seven digital products. They ranged from the outright unmusical to the state of the art. The difference in the quality of the reproduced sound—and, more important, the musical experience—made by changing only the digital converters, was enormous. It wasn’t just a case of degree; the best converters produced a total immersion in the music, while the poor converters rendered the system uninvolving.

The digital processors that solidified this position were the $14,000 Mark Levinson No.30 and the new Theta DS Pro Generation III reviewed here. I recently gave another listen to the No.30, borrowed to compare with the Gen.III for this review. Putting these processors in my system transformed digital playback from the ordinary to the truly musical. On some discs, the quality of the musical experience was extraordinary—despite being digital.

With that foreshadowing of what I think of Theta’s DS Pro Gen.III, I should add some background. The Gen.III replaces—what else?—the Gen.II. The price has increased from $3500 to $4000, reflecting the new unit’s higher parts cost. Gen.II owners can upgrade to Gen.III for $1250 ($1500 if you bought it secondhand). The only thing saved is the chassis and DSP chips; the entire electronics are gutted and replaced.

Although I still prefer analog, I think many of digital’s critics haven’t heard CDs made with state-of-the-art A/D converters and the best digital playback hardware available—playback hardware which now must include the very special Theta DS Pro Generation III.

Technical description
The Generation III’s appearance is traditional Theta: plain black box with front-panel toggle switches. The ¼”-thick front panel holds four switches and two LEDs. The former select among the unit’s three digital inputs, invert absolute polarity, engage the digital tape input, and turn the unit on or off. The LEDs indicate power on and lock to an incoming digital signal.

The rear panel is similarly minimal. Three RCA jacks provide digital inputs, with space for an optional ST-type optical jack. A digital output appears on an RCA jack for driving a DAT machine, CD recorder, or any other device that requires a digital input. For an additional $75, the Gen.III can be ordered with a Toslink optical input jack instead of one of the RCA coaxial inputs. The Toslink input is significantly inferior to either coax input or ST-type optical: it is offered only for compatibility with sources having only Toslink output, such as laserdisc players. The analog output is provided on RCA jacks; balanced outputs on XLR jacks are available as an option. An IEC line-cord jack and line fuse finish off the rear panel.

Looking at the Gen.III’s circuitry required removing both the top cover and bottom panel. The unit is a split-level design, with the power supply mounted upside-down on a piece of sheet metal that horizontally bisects the chassis. The power supply is mounted on a single printed circuit board and contains four sections, each fed by a separate transformer. The four supplies are: +5V to the main digital supply for the DSP board, +5V to the digital portion of the DAC circuit, and ±15V to the analog stage. The analog supply uses can-type regulators instead of the more common TO-220 plastic package, a difference that reportedly had an effect on the unit’s sound. Twenty-one 4700µF electrolytic caps provide smoothing and filtering. Additional filtering is provided on the analog board next to the circuits these rails supply.

On the top side of the bisecting plate, the large digital board (9″ by 10.5″) consumes most of the real estate. The pcb is surrounded by a shield to prevent radiated noise from getting into the analog stages. The DACs and analog output circuitry are mounted on a pcb adjacent to the digital board. In the unbalanced configuration, one board—which contains both left and right channels—is used. The balanced version uses two identical boards, one processing both phases of one channel’s balanced signal, the other processing both phases of the other channel.

It is important to note that the Gen.III’s balanced outputs are not merely XLR connectors with a phase splitter in front of them. This is a typical method of achieving a balanced signal: take the analog output and split it into two polarities just before the XLR jack.

Theta takes a different—and much more expensive—approach. They split the signal in the digital domain before the DACs, and convert the signals with four DACs (± left channel, ± right channel) instead of just two. In addition to doubling the number of DACs, each of the four outputs must have its own current-to-voltage (I/V) converter and output stage. The advantage of this approach is that any radiated noise, distortion, or converter artifacts common to both halves of the balanced signal will cancel. This is why the balanced option is a whopping $1000; both the whole DAC and the analog output stages are doubled.

To my knowledge, only Theta, Madrigal (in the Mark Levinson No.30), Meridian, and Kinergetics balance the signal before the DACs. According to designer Mike Moffat, this technique greatly reduces the amount of clock noise riding on the signal. Note that a balanced Gen.III’s single-ended outputs do not benefit from the fact that it is balanced: there is no summing circuit before the RCA jacks as there is in the Meridian and Kinergetics designs. However, anyone who spends $1000 for the balanced version will likely use the balanced outputs exclusively.

The heart of the Gen.III—and all Theta processors—is the software-based digital filter. Rather than use an off-the-shelf digital filter chip, Theta designed their own filter with Digital Signal Processing (DSP) chips and custom software. This software is the list of instructions that tells the DSP chips what to do to the audio samples passing through the DSP. By varying the software, the designer can optimize those filter parameters he considers most important—and have greater control over the product’s sound. Only Theta, Wadia, and Krell use custom software-based digital filters (footnote 1).

The Gen.III’s filter differs from the Gen.II’s in that the III uses three DSP chips instead of two. This third DSP increases the filter’s mathematical processing power, resulting in improved filter performance. Three Motorola 56001 DSP chips running on a 30MHz clock is a formidable amount of number-crunching power (footnote 2).

The software is held in two Erasable Programmable Read-Only Memory (EPROM) chips located next to the DSP chips. The 8x-oversampling filter is optimized for time-domain performance. Conventional digital filter chips are optimized for frequency-domain performance—flattest passband and maximum stopband attenuation.

A new input receiver chip, the Crystal 8412, replaces the Yamaha YM3623 used in previous Theta processors. This receiver reportedly has much less jitter than the Yamaha chip. The Gen.III’s input switching and other “glue logic” is performed by a programmable gate array. This is an integrated circuit that can be configured any way the designer wants. A second gate array processes the clock signals, converting the 44.1kHz, 16-bit clock to an 8x-oversampled, 20-bit clock. These two gate arrays greatly reduce the number of individual chips on the board. In addition to reducing the parts cost and real estate, the gate arrays radiate far less digital noise than do rows of discrete logic chips.

The DAC and analog output stage are mounted on a single pcb next to the digital board, but separated by an RF shield. Two identical boards are used in the balanced version. The DACs are Burr-Brown PCM63s, the 20-bit “Colinear” device that has excellent low-level linearity and needs no external MSB trimmer. This DAC is quickly becoming a favorite of high-end designers. The current-to-voltage converter is an AD841 op-amp, a longtime favorite of Mike Moffat’s. The resistors associated with the 841 are mounted atop the IC to keep the pcb traces as short as possible. The output buffer is the Precision Monolithics BUF-03, used in other Theta products and in Corey Greenberg’s buffered passive preamp.

The output stage is direct-coupled, with an op-amp providing DC servo action. A pair of trim pots are set at the factory for lowest DC offset at the BUF-03. De-emphasis is performed in the digital domain by the DSP filter. The analog low-pass output filter is a combination of a 6dB/octave rolloff around the I/V converter and an 18dB/octave rolloff around the BUF-03.

Parts and build quality are very high: the DAC/analog output board is made of Teflon, and every resistor on the board is the very expensive (several dollars each) Vishay bulk-foil types. In addition, the layout and execution are first-rate.

Footnote 1: See my interview with Theta’s Mike Moffat in this issue. He provides a fascinating look at the effects of DSP-based filters and software on a digital processor’s sound.

Footnote 2: For comparison, a particular Macintosh-based professional digital audio editing system uses just one 56001. The system does cut-and-paste editing, level adjustment, equalization, and other digital-domain processing, all on the single DSP chip.

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Theta Digital/ATI

1749 Chapin Road

Montebello, CA 90640

(323) 278-0001

www.thetadigital.com

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