SPL electronics is a 35-year-old German company that makes everything from the Vitalizer sweetener/equalizer to a well-regarded de-esser and desktop digital interfaces to mastering consoles and signal path controllers. Their model 1734 Mercury Mastering DAC is a red-faced, single space rack unit that allows for seven digital inputs (USB and two each coaxial S/PDIF, XLR AES, and optical TOSLINK). The model 1730 version of the Mercury is black, 1733 is all black, and 1734 is red. The DAC converts 16, 24, or 32 bit PCM at sample rates from 44.1 to 768 kHz and also direct-converts a DSD stream.
The Mercury includes two pairs of balanced XLR analog outputs: Fixed level, with relative dBu gain set on the front panel, and variable with output level controlled by an ALPS RK-27 "Big Blue" potentiometer (known for their channel-to-channel consistency). I mostly used it with the variable output connected directly to my Benchmark AHB2 power amplifier [Tape Op #111]. I also ran the fixed outputs to a Mara Machines MCI JH-110C tape machine to perform a record align and make a mixtape from digital sources.
SPL touts the 120V Technology in the analog output stage of this DAC. Basically, they've developed an op-amp that runs on +/-60 VDC, as opposed to the more typical +/-15 V or 18 V. This gives you a voltage swing of 120 V as opposed to 30 V or 36 V, which equates to greater dynamic range and headroom. In mastering-grade digital equipment, this matters. For one thing, the Mercury can drive +24 dBu loads all day long with incredibly low distortion (each unit comes with pages of individual test results from an Audio Precision audio analyzer). Suffice to say, dynamic range, noise floor, and distortion measurements are very impressive.
Another unique feature of this DAC is in-house designed low-pass filtering post-conversion, in the analog domain. According to the SPL engineers, "The filter is a second order low-pass. The AKM AK4490 (digital to analog converter chip used) features what AKM calls OSRD (Over Sampling Ratio Doubler). This puts the out-of-band noise up to the highest sample rates' Nyquist frequencies, and allows us to use two filters (one for DSD and one for PCM) to cope with the artifacts there – quite clever from AKM actually. Otherwise, you would need filters for each sample rate (10x PCM and 3x DSD) which is not doable." SPL founder Wolfgang Neumann is known for his analog filter designs.
The front-panel source switching worked great – no hiccups or noise bursts when switching from a 192 kHz/24 bit playback via USB to a CD playback through optical or S/PDIF input. The pushbutton switches performed and felt like they are built to last and belong in a top-notch equipment rack. There is a Mute button after the source selectors. There is also a Sync button, which not only selects the sync source for every input except USB (external word clock source sync, or using one source to sync another source) but also, when pressed and held for two seconds, selects the output reference level. Factory standard is 0 dBFS = 18 dBu, switchable to 14, 15,16, 18, 20, and 24 dBu.
The Mercury Mastering DAC is designed to fit into SPL's series of mastering studio gear built around their 120 V op-amps, and is thus aimed at the upscale mastering facility. Its design sets it up for transparent, undistorted sound with the ability to drive whatever monitoring system is present. This proved to be the case in my studio. From the get-go, it sounded like a top-drawer DAC, which to me means it basically had no signature sound – it just converted the digital signal and buffered it for playback through the power amp. It's a good "proof of work" converter – the last thing to put between your master file and your speakers in order to hear exactly what your digital master sounds like. The Mercury would also be useful for sending a digital mix to a tape master (if that's the preferred result) as it doesn't add or subtract anything from what's in the mix, and adds basically zero noise to the analog signal. The same can be said about sending a digital master to a record-cutting lathe.
Because the variable output works so well (the channel-to-channel balance is perfect at all levels I measured), it would be good to have at least one analog pass-through, so the Mercury Mastering DAC could also be used as a high-end monitor controller in a mixed analog/digital studio, but that's a minor quibble. This DAC will reproduce any pro format digital file and add or subtract nothing from the sound that's in the bits. Its very solid and attractive build quality is in line with its premium performance and price.