Overview

The fx-audio D302J++ is a full-digital amplifier centered around the STMicroelectronics STA369BW IC. An update from the previous D302J+, it features an upgraded USB receiver IC, the SAVITECH BRAVO-HD SA9123L, enabling 24bit/192kHz support via USB. This compact desktop audio solution integrates a 4-input system (USB Type-B, optical digital, coaxial digital, analog RCA), a 15W×2ch (4Ω) speaker output, and a headphone output into a single unit. Sold by North Flat Japan (NFJ) and released in March 2023, this relatively new model adheres to a conventional design approach.

Scientific Validity

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The nominal specification for the STA369BW chip used in this unit is a THD+N of 0.08% (1kHz, 1W, 4Ω). While this figure avoids the “problematic level” (≥0.1%) in our measurement criteria table, it falls far short of the “transparent level” (≤0.01%). The theoretical advantages of a full-digital architecture are not fully realized at this level of implementation. The 15W×2ch output is suitable for near-field small speakers but lacks the power to drive less efficient speakers or fill a large room. The 24bit/192kHz support has weak scientific evidence for providing significant auditory improvement and is largely a marketing point. Overall, from a high-fidelity perspective, it does not meet modern standards.

Technology Level

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The technical composition of the D302J++ is a typical design based on a combination of off-the-shelf, standard ICs. While the core STA369BW is a proven IC, it is not technologically advanced compared to newer, higher-performance Class-D amplifier ICs. The USB receiver IC (SA9123L) and ADC (PCM1808) are also standard components, and there is no unique or innovative approach in the circuit design. Its reliance on an external AC adapter is effective for cost reduction but shows a lack of consideration for power supply quality, which is critical for audio performance. This suggests a lack of thoroughness in the design. Its technical originality and advancement are below the industry average.

Cost-Performance

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We will evaluate the D302J++ against its current market price of 12,980 JPY by looking for functionally equivalent or superior alternatives. This unit features speaker outputs and a versatile 4-input system (USB, optical, coaxial, and analog). A product that meets these criteria is the Sabaj A10a, available for about 11,000 JPY. The A10a also has multiple inputs and features a more powerful amplifier. Cost-performance is calculated by dividing the competitor’s price by the review target’s price. 11,000 JPY ÷ 12,980 JPY ≈ 0.847 Therefore, the score is rounded to 0.8. The existence of a more affordable and higher-performance alternative means this product does not achieve the highest cost-performance rating.

Reliability & Support

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The product is sold by North Flat Japan (NFJ) with a 6-month warranty period. While support from a domestic distributor is a plus, the warranty period is short compared to the industry standard of one year or more. The fx-audio brand itself is widely available in the low-price segment, but objective data on long-term reliability is limited. The fact that the power adapter is sold separately (or the quality of the included one varies) could also be a source of trouble depending on the user’s choice. Considering these factors, reliability and support are rated as average for the industry.

Rationality of Design Philosophy

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The product concept of packing various functions into a compact body is rational and meets the demand for desktop audio. However, its value is limited if the performance of individual functions is mediocre. Touting high-resolution support while the basic performance of the amplifier, which governs sound quality, is not high by modern standards, shows a lack of coherence in the design philosophy. Furthermore, similar functionality can be achieved by combining a PC or smartphone with an inexpensive USB DAC. The product lacks the performance or functional superiority to justify its existence as a dedicated device, leaving its design philosophy at an average level.

Advice

The D302J++ may suit the specific needs of users who want to enjoy various digital and analog sources with speakers in a limited space and budget. However, if sound quality is the top priority, better options exist. In the same price range, there are products with superior measurement performance, such as the Sabaj A10a or SMSL A100. If an analog input is not necessary, the range of choices expands even further. This product’s measured performance does not reach a “transparent” level and is not recommended for users pursuing high audio performance. When considering a purchase, it is key to clarify whether you prioritize multi-functionality or pure sound quality to avoid disappointment.

(2025.7.22)