In WATATSUMI, the amount of components was increased to achieve a significant improvement in sound quality.
To maintain a size comparable to previous models, the chassis is CNC-machined from a solid block of aluminum, allowing for a higher-density and more flexible internal layout.

The battery capacity has been increased by over 10% compared to previous models, and the battery decoupling capacitors have been expanded to 4000μF—four times the previous amount.

To effectively position the larger battery, enhance noise resistance in the circuitry, and improve heat dissipation for the power supply and power amplifier, a custom CNC-machined aluminum shielding case was adopted.
The power supply and power amplifier circuits mounted on the PCB are thermally coupled to the shielding case, and the structure that dissipates heat through side vents follows the same design as FUGAKU.

While the amplifier generally does not generate significant heat, the power amplifier that drives earphones and headphones can produce temporary high heat when outputting large currents.

Since amplifier performance in terms of distortion and noise can fluctuate with temperature changes, a heat dissipation structure was implemented to suppress transient thermal variations and maintain consistent sound quality under all conditions.

In WATATSUMI, the toggle switch for input signal selection has been eliminated and replaced with an automatic selection function that detects plug insertion and removal.
To prevent noise during plug insertion and removal, the 4.4mm output jack is also monitored, and the output relay is turned off when no earphones or headphones are connected.

Additionally, to prevent accidental operation or damage to the volume knob, a protective structure has been added around it. This guard is integrated into the aluminum chassis and CNC-machined as a single piece.

The volume knob has been changed from brass to a titanium alloy, and its durability has been further enhanced with a PVD coating.
Accordingly, the chassis screws have also been replaced with ones made of pure titanium.

The battery indicator has been upgraded from three levels to four, allowing for more precise monitoring of the remaining battery charge.

Ultrasuede is partially applied to the bottom of the unit to prevent damage to both the device and surrounding surfaces.
A forged carbon plate, similar to that used in FUGAKU, is mounted on the top of the unit, contributing to both design aesthetics and weight reduction.

The power supply, as with previous models, consists of a total of six independent rails for both left and right channels.
The major changes in the power supply circuit are the increased number of decoupling capacitors and the redesigned PCB layout.

The power supply and its corresponding amplifier circuit are grouped as independent sets. In the layout, each power “island” is paired with its corresponding amplifier “island” and connected through a broad copper pattern, resulting in low-impedance wiring.

The circuit that converts battery voltage into dual power supplies operates via switching, which necessitates careful handling of high-frequency regions.
If the layout is not properly designed, it can lead to increased switching noise and greater electromagnetic radiation into the surrounding space.
In WATATSUMI, the layout has been optimized to maintain the same low-noise performance as previous models.

One of the major differences from the original TSURANAGI is the configuration of the input amplifier and the layout of the output amplifier.

With the growing presence of low-impedance earphones that are sensitive to amplifier residual noise, we have been exploring ways to further reduce residual noise.

During the development of FUGAKU, we arrived at a circuit that achieves lower noise while maintaining common-mode rejection ratio (CMRR), high input impedance, and low distortion. Upon early adoption, it delivered a sound quality we were fully satisfied with, leading us to implement the same input amplifier in WATATSUMI.

To clearly distinguish between different circuits, we decided to assign names to each block.
The input amplifier installed in this model is BIS2.0.

As a result, residual noise has been reduced to approximately one-fourth compared to TSURANAGI and about half compared to TSURANAGI-V2.
At the same time, the gain has been set to 6dB, which is an increase of 1.5dB compared to the 4.5dB gain of TSURANAGI-V2.

Noise reduction is achieved through the input amplifier, while distortion reduction is realized by optimizing the output amplifier layout and lowering the output impedance.

The output amplifier continues to use a fully differential OPAMP combined with a current feedback amplifier, while the capacitive load compensation circuit has been redesigned with lower impedance to prevent oscillation. This results in lower distortion during high current output and improved sense of immediacy in sound.
The output amplifier installed in this model is BOS1.5.

The DC servo circuit used in FUGAKU has been implemented, achieving lower power consumption and improved offset cancellation while maintaining low noise and low distortion.

The PCB remains a 6-layer board as before, but the copper foil thickness has been doubled to reduce wiring resistance.
As a result, it particularly contributes to improved low-frequency response and texture.

Unlike the TSURANAGI series, which used a mirrored layout between left and right channels, WATATSUMI adopts a revised component placement and routing. The layout is fundamentally left-right symmetrical, with some sections mirrored between the channels.

The input amplifier, electronic volume, and voltage amplifier are positioned at the center, with the output amplifiers placed symmetrically on the left and right sides.
The single-point ground, which serves as the circuit’s reference, is placed at the top center, and the layout ensures it is as equidistant as possible from the left and right circuits while also minimizing the distance from the output amplifiers.

By lowering wiring impedance, implementing a symmetrical layout, and reviewing circuit components, THD+N during high current output has been reduced.
As a result, source fidelity has been improved, delivering a pleasant and stable sound quality even at higher volume levels with minimal distortion.

As a general concept, we believe that lower noise levels make it easier to perceive subtle sounds, while lower distortion improves the ability to faithfully reproduce the tonal character of the source.
Of course, other factors also affect sound quality, but those are omitted here for simplicity.

The diagram above shows the measured THD+N of the output as the input level to the amplifier is gradually increased.
It is a commonly used metric for evaluating an amplifier’s noise and distortion performance together.

In the graph, moving to the right indicates a larger output signal, while moving downward indicates lower noise and distortion (THD+N).
Basically, as the output signal increases, the gap between the signal and noise widens, resulting in lower THD+N. However, it eventually reaches a limit.
This is because distortion increases as the amplifier’s output current grows. As the signal becomes even larger, the output signal begins to exceed the power supply voltage, causing waveform clipping and a rapid rise in distortion.
The key point is how linearly and distortion-free the THD+N can decrease in the region just before clipping occurs.

Conversely, in the region where the output signal is small, noise becomes more dominant than distortion, and in WATATSUMI, the characteristics of the input amplifier play a major role in this range.
It can be seen that with each new generation, the noise level has been reduced by half.

従来のTSURANAGIの仕様は16Ω負荷時に700mW+700mW(THD+N < 0.01%)としており、700mW以下において0.01%未満を実現するという意味でした。
WATATSUMI maintains excellent performance with THD+N remaining below 0.002% even at 2000mW output.

Generally, the rated power is defined as the amount of power an amplifier can continuously and stably output with THD below 0.1%. However, we feel that 0.1% distortion already results in noticeable sound degradation, making it too rough a benchmark when used as an indicator of sound quality.

Therefore, at Brise Audio, we specify the maximum output at which THD remains below 0.01%—one-tenth of the conventional 0.1% threshold—as our standard in the specifications.

In WATATSUMI, both noise and distortion have been significantly reduced compared to previous models, resulting in noticeably improved reproduction of subtle details and greater accuracy during high-output playback.

While WATATSUMI uses pure silver wire internally, it shares the same role as TSURANAGI as an amplifier for evaluating connected cables.

It is designed with a neutral sound signature to bring out the full potential of any connected cable or player.

Pure silver is said to have approximately 5% higher electrical conductivity than pure copper.

In current-carrying paths such as amplifier outputs, losses due to wiring resistance become more noticeable, making silver's high conductivity a significant advantage.
In addition, by using pure silver wire with a greater conductor volume than SHIROGANE, output resistance has been minimized to the utmost limit.