Overview
Soulnote recently caught our attention at a prominent Hi-Fi show in North America. The sounds being produced were so impressive that repeated visits were made to their listening room to verify our initial impressions. What we discovered with this relatively obscure Japanese brand is very special indeed – creative audio designs that strive to keep the listener engaged at a fundamental level. With three simple and distinct products ranges, Soulnote has a solution for music enthusiasts at all price and performance levels.
Design Philosophy
In the world of Hi-Fi design, following your gut instincts (not just measurements on a oscilloscope) can often yield unpredictable yet excellent results. Chief Soulnote designer Hideki Kato knows this well. Throughout his long career with companies such as Marantz, NEC & Philips, Kato was determined to find out why improvements in static performance (lab measurements) and sound quality did not always correspond. Kato named his hypothesis ‘Dynamic Performance’.
The adoption of the NOS (non-oversampling) mode for PCM digital playback as well non-NFB (negative feedback) circuit in amplifiers stem from this hypothesis. Curiously, he discovered that these design concepts may worsen the lab measurement results, but a remarkable improvement can be achieved during actual human listening tests.
While unmeasurable using conventional methods, he decided that maintaining accuracy of the original music waveform on the time axis is one of the most important considerations for music playback. At Soulnote, listening tests are heavily relied upon to determine parts selection and mechanical construction. This approach is a kind of antithesis against the supremacy of static performance, an approach not widely adopted by other manufacturers.
DAC Technology
The Soulnote D-2 incorporates discrete fully symmetric non-NFB differential amplifiers, arranged in a dual mono construction that fully isolates each channel — including the power supply rectifier. Connecting cables have been eliminated for sound signals and in the power supplies. Wiring from the transformer has also been shortened, since this is a major cause of current loops.
The D-2 uses 4 boards wrapped in 70 micron thick copper foil that eliminates noise by optimizing the printed circuit board layout. Noise countermeasures that don’t rely on filters that can impact sound quality like flight, etc. are employed throughout, and each mode keeps unwanted radiation below the noise floor.
Chassis Design
Soulnote has not limited their “hearing is believing” design approach to the internal circuit design. The chassis itself is designed to look spectacular while also maximizing performance, often in ways that are counterintuitive. The top cover, which has a particularly large influence on sound quality, features a dual-component construction that joins the base plate and the aluminum panels at three points so that they do not dampen each other. By grounding it at three points without fixing it to the aluminum portions of the housing itself, this design succeeds in pulling down the sound focus while still keeping it expansive.
In addition to the above, internal spikes ground the power supply transformers. Combining the spikes with uneven cross-sectional aluminum panels to control resonance increases rigidity, and the aluminum panels are not directly bonded in order to eliminate unwanted resonance.
Power Supply
The D-2 incorporates the newly developed power transformers with eight windings on the secondary side that are capable of regulation similar to that of very high power amplifiers. Independent rectifier circuits in each stage prevent interference. Because transformers are a source of vibration, Soulnote has used only one in each unit to simplify the vibration type and has constructed the unit body so that any unwanted vibrations are transmitted away from the body through a spike placed directly under each transformer.
Expandability & Upgrade Options
While most will use this simply as a stereo DAC, a simple flick of the switch gives you the option to make this a mono DAC, making it possible to use two units in stereo pairs — thus doubling the power margin and making channel separation practically infinite.
For even more precision from the DAC, the internal clock can be disabled by adding the Soulnote X-3 Clock Generator. The X-3 incorporates an SC Cut ultra low phase noise OXCO (thermostatic bath-type crystal oscillator) selected for its unprecedented sound quality. While this doesn’t quite get you all the way to the same performance level of the reference D-3 DAC, it’s sure nice to have this upgrade path as an option.
