You should know that my close encounter with the Monarchy NM24 DAC was not a random occurrence, but rather the result of a conscious choice driven by specific selection criteria—basically, a couple of factors that I’ve come to associate with a high probability of natural sound reproduction. In a nutshell, these are inclusion of an R2R DAC and a tube buffer stage. Think of these criteria as my personal “first cut” assessment of the universe of commercial DAC designs. Of course, these are not the only important design factors; two others that come to mind fairly quickly are power supply and time-base jitter. Over the years, my search for analog-like sound from digital sources has proven to be a frustrating quest. The low point was the advent of the single-bit delta-sigma DAC and its adoption by many mainstream manufacturers—an event which, in my estimation, plunged digital audio into a Dark Age for several years, until decent multi-bit delta-sigma DACs emerged. Even today my preference is for one of the classic R2R DAC chips. DIYers are still flocking to the ancient and long-out-of-production Philips TDA1541 16-bit/44.1kHz and TDA1543 16-bit/96kHz DAC chips as low-cost tickets to natural sound quality. The NM24 uses the Burr-Brown (Texas Instruments) PCM1704 24-bit/96kHz DAC, the undisputed king of all R2R DACs. The PCM1704 employs a sign-magnitude architecture in which two DACs are combined in a complementary arrangement, sharing a common reference and a common R2R ladder for bit current sources. Ladder resistors are laser trimmed at the factory to achieve an accurate match between the two DACs. The result is exceptionally linear and low-harmonic-distortion output. Sadly, Texas Instruments has put the PCM1704 on end-of-life status probably due to cost of manufacture and dwindling demand, so I would not be surprised to see it discontinued by the end of 2015.
The key factor in the application of the Philips TDA-series chips is turning off the companion digital filter. To paraphrase the immortal words of the classic Schoolhouse Rock jingle, “Zero is my Hero.” I can emphatically state that zero oversampling is my hero. Turning off the digital filter on my TDA1541-based DIY DAC transforms it from a sonic ugly duckling to a beautiful swan. From an engineering standpoint, oversampling the datastream to push ultrasonic image spectra further away from the audio bandwidth followed by a digital filter makes perfect sense. But it clearly doesn’t sound good in the context of a 16-bit DAC, possibly because oversampling re-quantizes the data, which is then truncated back down to a 16-bit word length.
But as I’ve discovered recently, oversampling isn’t always a bad thing, and the Monarchy NM24 does put it to good use without adversely impacting sound quality. The PCM1704 is preceded by the DF1704—an 8x-oversampling digital interpolation filter. For a 44.1kHz Red Book CD, that would push the image spectra to beyond 350kHz. Unusually, the NM24 features two sets of DAC outputs, which can be used simultaneously. One is tube buffered, while the other is buffered by an Analog Devices AD811 video op-amp. A third-order analog low-pass filter rolls off the image spectra at the input to the tube buffer. However, the image filter is omitted for the solid-state buffer. Monarchy Audio’s C.C. Poon explains that there were several considerations involved in this decision. Aside from space and cost limitations, he does not consider the image filter to be an essential part of the circuit, as most users simply use the tube output and forget about the solid-state output. Technically speaking, the main reason for attenuating the ultrasonic image spectra is in order to protect downstream components, e.g., tweeters, preamp, and/or power amp from potential non-linear distortion. I’ll have more to say about the solid-state DAC later.
The tube DAC’s buffer is a series-regulated push-pull (SRPP) stage that uses a 6922 dual-triode per channel. I’ve maintained for over 20 years that a tube buffer is an effective antidote for residual digital nasties. Defanging a delta-sigma DAC is no job for a solid-state buffer, and besides, I’m fed up with having to listen to the sonic signature of a plain-vanilla op-amp. It’s no coincidence that the two delta-sigma DACs I reviewed and raved about in the recent past are tube buffered. As a reminder, these were the EAR-Yoshino 192 DACute and the Ayon Stealth. These two are in the $6k to $10k price range, and I think that this is the sort of investment it takes to prime a delta-sigma DAC to sound musical. Otherwise these modern DACs tend to sound bright and/or lack convincing timbral accuracy. On the other hand, the R2R DAC-based Monarchy sings sweetly with realistic tonal colors for a fraction of the cost.
The NM24 is much more than a basic DAC. It includes a volume pot and a line-level gain stage, comprising yet another 6922 dual-triode configured as an SRPP. Since there’s only one line-level input, there’s no input selector. A switch on the front panel allows the internal connection of either the tube DAC output or the line-level input to the linestage. With the tube DAC switched to the linestage, the NM24’s line output can be connected directly to a power amp. However, I suspect that many of you would be interested in using the unit as a standalone DAC by connecting the tube DAC output to an external preamp. That was my preferred connection scheme. There are only two digital inputs (TosLink and coaxial), so you’ll need a good external asynchronous USB link for computer audio. A 3.5mm stereo jack is provided on the back panel to connect the linestage outputs to your favorite headphones. The linestage output impedance is 300 ohms, so any headphones with that sort of input impedance should be compatible. The operating instructions specifically mention the Sennheiser HD650 (300 ohms) as well as the Beyerdynamic DT990 (250 ohms). I did briefly try such a connection with my Sennheiser HD600 ’phones. OK, so these are not the most transparent cans on the market, but they sounded way better than they have in many years.
Monarchy Audio encouraged me to try its AC-Regenerator ($850) in conjunction with the NM24, which apparently benefits considerably from a clean AC supply. This seemed to me to be the next best thing to a battery supply, so I tried it and liked it a lot. The AC Regenerator rectifies the AC mains voltage and uses a power amplifier to generate a pure AC signal. Precision voltage and frequency selections can be dialed in on the front panel. Line frequency selections are 50, 60, 120, and 400Hz. C.C. Poon recommended settings of 117VAC and 400Hz, which I did settle on. Two outlets are provided on the rear panel that can accommodate a combined power draw of 100 watts. You might legitimately ask, why use such a frequency as high as 400Hz? Well, operating a power supply at higher frequencies does improve its efficiency and reduces residual AC ripple in the filter circuit. Aircraft power supplies operate at 400Hz for this reason. I wondered if silicon rectifier diodes would squawk about being switched on and off at higher frequency than 60Hz. It turns out that even a garden-variety rectifier diode has a settling time of about 30 microseconds, which means that it would be comfortable at 400Hz and even higher line frequencies. It is surprising that even with its massive twin toroidal power transformers and plenty of voltage regulation that the NM24 would benefit from clean re-constituted AC. In particular, the sonic effect of switching from a setting of 60 to 400Hz was quite audible and could be best described as a smoother, more relaxed presentation, as if harmonic textures were cleansed of a layer of electronic hash. Image focus was superior as well. This was not an isolated case; both my DIY DAC as well as the April Music Eximus DP1 benefitted similarly when mated to the AC-Regenerator.
Let’s talk tonal balance. It is helpful to contrast the NM24 with a similarly priced, popular commercial DAC that sits at the opposite end of the tonal spectrum. The Oppo Digital BDP-105 has garnered rave reviews. Here’s what Chris Martens had to say about the BDP-105: “If you buy the notion that some source components try for a softer, smoother, and thus ostensibly more ‘musical’ presentation, while others aim for maximum musical information retrieval, then I would say the Oppo falls squarely in the information-retrieval camp (as do a great many other high-performance solid-state players).” I did not audition the Oppo in my own system, but I do recall that the Oppo sounded more detailed than it had a right to, and I don’t think that it was a function of the 32-bit Sabre DAC. After all, a Red Book CD’s resolution is fixed at 16-bit and that’s all there is to it, whether a 16-, 18-, 20-, 24-, or 32-bit DAC is used. To my ears, the Oppo had sounded slightly bright through the upper midrange, which emphasizes low-level detail and at the same time projects female vocals a bit forward. After a significant exposure to the Oppo, the NM24’s middle-of-the-hall perspective may sound rolled-off to you, but it sounded far more natural to me. An appropriate analogy that comes to mind is that of a moving-magnet vs. a modern moving-coil cartridge. The NM24 sounded much like a good mm. Think suave midrange textures without a trace of brightness and you’ll get the big picture: an overall presentation that was a bit darker and far more relaxed than the Oppo.