Digital Processor Reviews

Of all the products I've reviewed or auditioned, a select few jump out as "best buy" recommendations. Almost universally, such products are liked by a wide range of audiophiles, and seem to match well sonically to many systems. Moreover, these products all have outstanding value; they offer a higher level of musical performance than you'd expect from the price.

It's nice to have a short list of such solid recommendations when asked for advice. The products that most readily come to mind include the McCormack DNA-1 and DNA 0.5 power amplifiers, Thiel CS3.6 and Thiel CS1.5 loudspeakers, the Theta Data Basic CD transport, and the Audio Research LS3 preamplifier—to name a few.

In digital/analog converters, the overwhelming favorite in midpriced processors had been the $2000 PS Audio UltraLink. The UltraLink brought UltraAnalog's sophisticated DAC technology to a moderate price point, and quickly established itself as the processor to beat in this competitive price category. The UltraLink's sound also brought it worldwide critical acclaim and great commercial success.

But the digital processor arena has changed considerably since the UltraLink was introduced in early 1992. Four and half years is an eternity in digital design. For example, the UltraLink used the 16-bit Yamaha input receiver chip (now known to have high intrinsic jitter), an NPC digital filter (which is significantly inferior to the new Pacific Microsonics PMD100 filter), and the UltraAnalog D20400 DAC (replaced by the D20400A). Also, since the UltraLink's heyday, designers have become more skillful in crafting a collection of parts into a more musically satisfying component.

PS Audio's new UltraLink Two processor is a complete redesign of this classic product. The Two features all new parts, a redesigned analog output stage, and more attractive cosmetics. Although the UltraLink Two seems to offer the potential of sounding better than its predecessor, today's standards of sound quality in a $2000 processor are considerably higher than when the original UltraLink reigned supreme. We'll see if the new UltraLink Two is up to the challenge.

Technology
The UltraLink Two reviewed here could be considered the third generation of the UltraLink series. The first UltraLink Two used the NPC 5803 digital filter; if you own an UltraLink II with this filter, PS Audio will install the Pacific Microsonics PMD100 HDCD decoder/filter chip found in the latest production as a factory upgrade for $398, which also includes testing to make sure the unit is performing to current specifications. Note that only the UltraLink II, not the original UltraLink, can accept the PMD100.

Because the PMD100 HDCD decoder/filter was retrofitted to an existing design, the UltraLink Two cannot decode 20-bit data. I discovered this when measuring the unit. If you feed it a 20-bit signal, the UltraLink Two truncates it to 16 bits. This becomes a drawback only if you have access to 20-bit professional sources such as the Nagra D open-reel recorder or the Sonic Solutions hard-disk editing system, or if you use an Audio Alchemy DTI•Pro or DTI•Pro 32. These devices interpolate the last four bits, converting 16-bit data to 20-bit. If you don't fit either category (professional sources or DTI•Pro user), you'll never need the ability to pass 20-bit data.

The UltraLink Two resembles the original in chassis size and shape, but the front panel is very different. A sleeker design replaces the flat front panel and small handles, and two recessed areas contain the status-indicating LEDs. One recess has four LEDs to indicate which digital input is selected; the other has three LEDs to show when the unit is locked to a digital source, if the source is HDCD-encoded, and when the polarity inversion button is engaged. It's no coincidence that the recessed areas are the same size and shape as a CD transport drawer; PS Audio uses the same front panel for their Lambda transport. Another change is the replacement of touch-sensitive buttons on the original UltraLink to the more reliable mechanical switches.

The rear panel holds four digital inputs, one each of coaxial (RCA jack), AES/EBU, ST-Type optical, and TosLink. An additional RCA jack provides digital output. Analog output is via XLR jacks (balanced) and RCA jacks (unbalanced). All digital input types are included as standard in the $2295 price. ST-Type optical was a $200 option on the original $2000 UltraLink, meaning that the UltraLink's price has risen by only $95.

There's been another big change at PS Audio since the UltraLink was introduced: the company is now 90% owned by Threshold. Randy Patton, a principal at PS Audio, acquired Threshold, then bought the remaining equity in PS Audio. The remaining 10% of PS Audio is retained by the former owners, including designer Bob Odell (who contributed his design talents and ears to the UltraLink Two).

Although the parts and design of the UltraLink Two are different from the original UltraLink, the two units look surprisingly similar inside. This is no doubt the result of PS Audio's emphasis on the power supply. The UltraLink Two features a large custom transformer with dual secondary windings, seven power-supply regulation stages, and lots of filter capacitors. Each regulation stage has more than 4000µF of capacitance associated with it, for a total capacitance of nearly 30,000µ;F—as much capacitance as found in a small power amplifier. The seven regulators are all three-pin IC types.

The digital input stage uses an UltraAnalog-designed pulse transformer and UltraAnalog AES21 input receiver. The UltraLink series went straight from the older Yamaha YM3623 input receiver to the AES21, leapfrogging the popular Crystal CS8412 chip. Digital input signals are buffered with a Schmitt-triggered inverter (which cleans up the squarewave), and buffered with a differential comparator. Relays select which input feeds the input receiver. A second pulse transformer couples the digital output signal to a rear-panel RCA jack for driving a digital recorder (a seldom-used feature).

A small daughterboard holding the PMD100 HDCD decoder/filter is mounted above the main board, evidence of the UltraLink Two's switch from the NPC 5803 digital filter to the PMD100 HDCD chip. The PMD100 performs HDCD decoding as well as 8x-oversampling digital filtering. In the UltraLink Two, the PMD100 also de-emphasizes the signal in the digital domain if the source data carries the emphasis flag. The PMD100's output dither options are unused in the Two. When decoding non-HDCD discs, the 6dB of attenuation called for by the HDCD license is performed in the digital domain by the PMD100. This technique throws away one bit of resolution, which could have been avoided by attenuating the signal by 6dB in the analog output stage, as is done in some other HDCD processors. Analog-domain attenuation is a trickier proposition than merely setting a pin on the PMD100, but doesn't introduce the potential sonic degradation imposed by digital-domain attenuation (footnote 1).

The DAC is an UltraAnalog D20400A, a slightly modified version of the D20400 used in the first UltraLink. The dual 20-bit DAC feeds an unusual hybrid output filter and buffer amplifier that is part IC op-amp, part discrete circuit. The filter is a modified third-order Bessel type based on a design called a Frequency Dependent Negative Resistor (FDNR). The filter is built around a Precision Monolithics OP-275 op-amp, but the op-amp isn't in the signal path. Instead, the filtering is passive (realized with metal-film resistors and film capacitors), with the op-amp creating a complex impedance that allows the filter to function correctly. A brief mention of the FDNR filter in Arthur B. Williams' and Fred J. Taylor's Electronic Filter Design Handbook, second edition (McGraw-Hill) shows how two differential amplifiers, a capacitor, and several resistors can simulate the action of an inductor. PS Audio claims this design is better-sounding than a passive filter, but doesn't suffer from the sonic degradation created by an additional active device in the signal path. Moreover, PS Audio claims a very low phase shift of less than 3° at 23kHz from this design.

The output filter is followed by a second PMI OP-275 (footnote 2) which acts as an output driver and inverter to generate an opposite-polarity signal to create a balanced output. The op-amp's output stages are operated in class-A by external active current sources that "pull" significant amounts of bias current through the device. The final output driver is a discrete circuit consisting of a pair of transistors that are within the overall feedback loop. These current-driving transistors are also operated with class-A bias. A muting relay between the final output and the rear-panel XLR and RCA jacks prevents noise from appearing at the analog output. Note that the UltraLink Two's balanced output is created the inexpensive way: with a simple inverter rather than four DACs and four analog output stages. The latter method is better technically, but adds substantially to the product's cost.

---

Footnote 1: It is possible to compare the PMD100's 6dB of digital-domain attenuation with no attenuation in the HDCD-equipped Mark Levinson No.30.5. Pushing a certain sequence of front-panel buttons defeats the PMD100's digital-domain attenuation. By reducing the preamplifier's volume by 6dB when the digital-domain attenuation isn't invoked, you can hear for yourself (at matched levels) the effect of throwing away one bit of resolution. I have found that the degradation is certainly audible. High-end designers shouldn't be forced to accept this compromise in sound quality; Pacific Microsonics should rescind the 6dB attenuation requirement.—Robert Harley

Footnote 2: The PMI OP-275 is also used in later production of Audio Alchemy's DAC-in-the-Box.