California-based Aesthetix has carved out an enviable niche producing very-high-performance products that, while not budget-priced, nonetheless offer extraordinary value. The company’s Calypso linestage, Rhea phonostage, Janus full-function preamplifier, and Atlas power amplifiers pack a lot of innovation, superb build-and parts-quality, and great sound into the upper-end, though not stratospheric, price category. (Aesthetix does, however, offer two mega-priced, ultra-tweaky products, the Io Eclipse phonostage and Callisto Eclipse linestage.)
Aesthetix has now complemented its more affordable line with the addition of the Romulus CD player/DAC. In keeping with the company’s signature technology, the Romulus features a tubed output stage. In addition to spinning CDs, the Romulus can accept a digital input in a variety of formats including USB. For those of you who have dispensed with CD, Aesthetix’s Pandora DAC is identical to the Romulus sans disc transport.
The $7000 Romulus has a fixed output level for use with preamplifiers. For an additional $1000 the Romulus includes a variable output for driving a power amplifier directly. The large front-panel display doubles as a volume control—push the right side to increase the volume and the left side to decrease. The circuit that realizes this variable output exemplifies Aesthetix’s innovation and commitment to sound quality. Rather than simply attenuate the signal in the digital domain and accept the resolution loss (every 6dB of digital-domain attenuation is equivalent to throwing away one bit of resolution), Aesthetix has combined analog and digital attenuation in a novel circuit. Decreasing the output level attenuates the signal in the digital domain just as in other DACs with a variable output. But when you reach 6dB of attenuation, a relay engages resistors that replace the 6dB of digital-domain attenuation with 6dB of analog-domain attenuation, resetting the digital-domain level to full scale. Voîlà!—variable output without any meaningful loss of resolution at any output level. In practice, this transition from digital-domain to analog-domain attenuation is transparent to the user, save for a relay click when every 6dB threshold is crossed. The volume steps are 1dB, which is a little coarser than what is found in most DACs, but I didn’t find it a problem. The variable output circuit is, incidentally, housed on a board that plugs in vertically to the horizontal motherboard.
The Romulus incorporates other interesting design techniques, including a SPDIF receiver and clocking circuit based on a fixed-crystal clock. Most SPDIF input receivers are built around a VCXO (voltage-controlled crystal oscillator), whose output frequency can be adjusted by an external voltage. By “pulling” the clock frequency slightly the DAC can lock to the incoming clock rate. A VCXO, however, isn’t quite as precise as a fixed-frequency crystal. Aesthetix solves this problem by using a fixed-frequency crystal and running the data through a memory buffer. Presumably the buffer is large enough to “fill up” sufficiently before outputting the data so that the buffer doesn’t run empty if the Romulus clock is faster than the incoming clock, or conversely so that the buffer doesn’t overflow if the Romulus clock is slower than the incoming clock. In practice, there’s no time lag between pressing “Play” and hearing music, suggesting that a larger buffer isn’t required.
The SPDIF inputs are galvanically isolated so that ground noise on source components doesn’t get into the Romulus. They can accept datastreams up to 192kHz/24-bit. The input circuits are housed on removable boards to allow future upgrades as new interfaces become available. The same crystals used in the SPDIF input receiver are also used in the USB input circuit. It goes without saying that the USB input is asynchronous, meaning that the Romulus serves as the master clock to which the USB source must lock. Other digital inputs include TosLink optical and AES/EBU.
The 8x oversampling digital filter software is written in-house by Aesthetix, and runs on a Motorola DSP56362. Creating a custom filter is considerably more expensive and time-consuming than buying an off-the-shelf filter chip (or using the filter built into most DACs these days), but allows the designer to employ more sophisticated filtering techniques, and to tailor the filter’s sound to the context of the entire product. The digital filter has a large effect on the product’s sound, which is why many DACs that use the same filter/DAC chip sound quite similar.
The filter’s differential outputs (+ and –, or the datastream and the datastream inverted) are converted to analog by a Burr-Brown PCM 1792 chip operated differentially. With four DACs in one package, the PCM 1792 can process the L+, L-, R+, and R- separately. The downstream signal path—current-to-voltage converter, gain stage, and output buffer—is also balanced all the way to the XLR jacks. This is the right way to create a balanced analog output signal. The less expensive, but more common, alternative is to convert the digital signal to analog with one DAC and one analog signal path per channel, and then create a “balanced” signal with a phase-splitter just before the XLR jacks. Not only does this latter technique add an additional active stage to the balanced signal path, but it also doesn’t realize the benefit of differential DACs. One of these benefits is that any noise or distortion common to both DACs will cancel when the balanced signal is eventually summed, not to mention the increase in signal-to-noise ratio.