SONNET DIGITAL AUDIO MORPHEUS DAC REVIEW PART ONE – M & S

Sonnet Digital Audio Morpheus has just landed at Mono & Stereo for a review. Cees Ruijtenberg surely needs no introduction to the audiophiles and high-end audio community. He’s a well known and respected audio designer and the very man behind the Metrum Acoustics’s products.

I have a pretty long tracked experience with Metrum Acoustics Adagio DAC and really liked it’s sonic balance and potent core. It will be most interesting to explore what Ruijtenberg’s new venture and his latest DAC provides in the digital domain.

TECHNICAL HISTORY

The creation of the Sonnet Morpheus DAC started somewhere before 2009 when Ruijtenberg was working for various industries (medical and industrial companies). In most cases, new designs were based on Ultrasound equipment. Ultrasound is widely used in the chemical, electronics and food industry. The frequencies that this device is using are far above human hearing, starting 35 kHz and ending at around 10 MHz!! In many applications, such products come with many critical demands and among many of these attributes speed and precision are always fixed standouts.

Besides Ruijtenberg’s work as an electronic and acoustic designer, he was already an audiophile for many years. As a result, lots of audio products were made for his own use and in many cases, they were based on electronics and part that he used for his daily job.

Ruijtenberg found out that good audio performance was foremost based on speed rather than the lowest possible distortion! Interestingly and intriguingly, a closer observation of most audio op-amps’ specs shows clearly how bandwidth is always based on a lot of feedback. For instance, the old NE4434 op-amp has an open-loop bandwidth of around 10 Hertz and can achieve nice specs by using a lot of feedback. The new designs used today are still not so better in this respect and they’re focusing on low noise, distortion and output impedance.

THE INITIAL GOAL

“Of course designing audio was based on my interest in music and have visited many live performances to create my own reference. Comparing acoustical instruments in a live environment with recordings made for CD such recordings lacks the natural timbre and speed as perceived in the concert hall. In most cases, there was a kind of over-detailed reproduction which I never heard in real live situations.”

FILTERING

While Ruijtenberg made many DACs for different industries he never created specific audio related. Besides speed and resolution, the industrial DACs do not use any filtering and is up to the designer to implement it.

Ruijtenberg was not aware of how much of the sonic signature digital filters can have. It was an article from Ryohei Kusunoki (Sakura systems) that triggered Ruijtenberg’s interest. Kusunoki profoundly investigated these phenomena and he found that by removing the heavy filtering created by digital over-sampling can produce far more life/live-like reproduction. This was highly interlinked with reproducing the right timbre.

Ruijtenberg started to experiment around this concept, but instead of using TDA1541 as Ryohei, he has implemented high-speed 16-bit non-audio related DACs.

Based on his prolonged R & D Ruijtenberg has built the first commercial DACs. “Quad” (two DACs per channel), “Octave” (four DACs per channel) and “Hex” (8 DACs per channel) were made under Metrum Acoustics brand name…

All these designs shared the same important feature. The filtering was not based on oversampling techniques, but on the implementation of mild filters. Human hearing is closely connected with the human perception of the sound. It uses extremely sharp filtering caused by the ear itself. (https://en.wikipedia.org/wiki/Cochlea)

But… The industrial DACs used in these designs still were still not a perfect fit. They were limited to 16 bits, having built-in current to voltage converters and they had far too short life cycle. Ruijtenberg was forced to look further and find a different solution…

THE SPEED!

As already mentioned the speed was one of the key demands of the new design. Some R2R ladder networks can have a usable bandwidth of 10 MHz in parallel mode, but again, they’re limited to 16-bit resolution.

Ruijtenberg followed the interesting idea of splitting the 24-bit format into two 12 bit formats, that would be separated through two 16 bit DACs used in the most linear region of the DAC. The further step and challenge were to combine these two streams coming from both DACs in the analog domain.

So basically there were two major challenges… Splitting up the 24-bit domain into two digital 12-bit streams and blend them together in the analog domain. As summing in the analog domain was the most difficult part, the digital part had to be optimized.

This reburied a specific highly complex algorithm. To match both streams without having any timing problems a very fast FPGA that handles speeds up to 400 MHz was chosen.

The seamless combining of the analog data demanded high-speed amplifiers to perfectly join both of the signals. These devices have an open-loop gain of 100kHz, which translates to being 10.000 times faster than the old and famous NE5534 op-amp. The slew rate of these amplifiers is 200V/uSec. A possible drawback with this speed is a bit higher distortion if compared to audio amps. 0.003% THD is not an ultra-low spec, but it’s still fine and most importantly executable.

This particular technology was used during Ruijtenberg’s Metrum period and it was also used in the Transient DAC TWO. Based on this module several products like Amethyst, Jade, Onyx, Pavane, and Adagio were being made.

There are a few more advantages intimately related to this approach. One is switching crazily specced switching noise that at theoretical -170dB noise simply cannot be measured.

Another one is the changing of the reference voltage. This creates a lossless variable gain providing a range of about 60dB. By this approach, the DACs can be connected directly to the outputs without the intervention of potentiometer or buffer amplifiers and this is exactly why I’ve liked and love Metrum Acoustics Adagio’s imprint-less nature so much.

With Pavane/Adagio three transformers were used to feed the digital front end and both mono boards, but this calls for quite some power handling.

MEET THE SONNET MORPHEUS

For the Sonnet Morpheus new demands like low power requirements, reduction of price and performance that would at least match or surpass Adagio. Instead of using two R2R ladders with an FPGA, four new R2R ladders controlled by one FPGA and a new algorithm are used in Morpheus.

With 16 R2R ladders (like Adagio/Pavane) the power requirement is much lower and a single 15VA transformer can be used instead of three. The entire design consumes less than 8 Watts, with only 1.5 Watts draw for the front panel and display.

This approach also introduces a few more advantages. Lower currents mean lower noise and radiation on the motherboard. This provides far better noise floor, as low as -155 dB related to 2Volt RMS and -120dB channel separation over the entire spectrum that is comparable with previous designs. This prevents a need for splitting the boards, which again resulted in a price reduction.

Due to the clustering of the R2R ladders close around the FPGA (very short copper tracks), the sound becomes warmer (more analog sounding). High-speed information that is coming from the FPGA’s and is sent to the ladders is very critical and special care is needed to avoid reflections over the tracks.

This is not exactly new info, but surely an important technical attribute. Most importantly the high speeds have introduced unexpected, yet nicely sounding results that far closely reflect natural timbre and life-like dynamics.

Due to the overall optimization, the aluminum and glass parts could be removed. This brought a further cost cut and an additional benefit for the end-user as the price could suddenly be halved compared to previous top-tier designs.

The Sonnet Digital Audio Morpheus DAC encapsulates the in-depth researched contemporary high-tech approaches and decades of Ruijtenberg’s know-how in a state-of-the-art DAC with a slimmer chassis and much more user-friendly price! In the era of most highly demanding audiophiles and music lovers this more than just a smart move. It’s needed and worthy of highlighting!