LUXURY & PRECISION's high-power portable tube amplifier "EA4," featuring an all-Japanese vacuum tube, has evolved into a next-generation portable headphone amplifier for all those who love high-grade audio.
This article traces the changes from its exterior to its internal structure, showing how the EA4 achieves both sound quality and practicality.
Evolutionary Path: Internal Structure Edition
1. Fully Discrete Circuitry
The initial version of the EA4 utilized a PCB slot structure and an enclosed tube anti-vibration structure on all four sides to absorb tube shocks. While the subsequent stage featured a fully discrete headphone amplifier structure, actual tests revealed that the anti-vibration effect of the tube was not ideal, and there were challenges with power supply and noise control for the fully discrete output stage.
2. Tube Shock Absorption Solution
In the second version, the PCB circuit design and related structures were completely revamped. This significantly improved the instantaneous output capability of the amplifier's power circuit, and the tube shock absorption structure was largely finalized. However, in this version, the maximum output remained at approximately 3500mW, and noise levels were higher, failing to meet the target.
3. Increased Output and Noise Reduction
The basic PCB layout of the third version was largely the same as the second, but it featured a more powerful and efficient power supply chip and a refreshed circuit design. Eight 10x10.5mm high-capacity capacitors were added to the amplifier circuit to enhance sustained output capability and improve bass power. Furthermore, overcurrent protection and DC output protection functions were added, and the heat dissipation solution was changed, allowing high-power transistors to directly dissipate heat to the exterior via thermal pads. As a result, the EA4's maximum output reached approximately 4.5W, and the noise floor was suppressed to within 20uV (measurement conditions A-weighted 20K BW AES17).
4. Significant Improvements to the Supply Circuit for Output and Noise Optimization
In the fourth version, the power supply was improved from a single-line to a dual-line system. The output transistor's emitter resistor was changed from a single resistor to three parallel resistors, with a total of 24 emitter resistors added to enhance output stability. The amplifier circuit, replacing the previous version's eight high-capacity capacitors, now features sixteen 10x12.5mm and 10x16mm high-capacity capacitors, enabling it to store extreme power. This increased the maximum output to 5500mW, and the noise floor was reduced to within 16uV (measurement conditions A-weighted 20K BW AES17).
5. Final Adjustments, Expanded High and Low Frequencies
In the latest version, sound quality improvements were made. To make the high-frequency range clearer, the eight tantalum capacitors used in the previous version were replaced with twelve large Panasonic film capacitors. Furthermore, the supply circuit was improved, and the EA4's maximum output exceeded 6500mW. While the maximum output increased by over 1000mW compared to the previous version, the noise floor was successfully reduced to within 14uV (measurement conditions A-weighted 20K BW AES17).
While we've showcased representative versions with significant changes to the EA4 circuit board, many versions were actually shelved because they did not achieve sufficient improvements in sound quality or performance. Especially for purely analog devices like headphone amplifiers, circuit design is the most crucial factor determining sound quality, allowing no compromise.
Evolutionary Path: Exterior Edition
1. Improved Rear Design for High Heat Dissipation
The most significant change in the EA4 is its design modification for heat dissipation. To maintain a high output of 6500mW@32Ω, efficient heat dissipation is essential. Therefore, the final design of the EA4 incorporates a "groove structure" to efficiently release heat to the outside.
2. Easy-to-Read Volume Display Prevents Accidental Operation
Reflecting user feedback, the EA4 includes a dedicated volume display. This prevents the risk of accidental high-volume playback even when using small earphones. With a custom-made electrometer, users can intuitively check the volume while balancing operability and visibility.
3. Optimized Control Panel for Enhanced Functionality
The EA4 also considers convenience in daily use. To prevent accidental operation in a bag, the power switch has been moved to a push-button switch on the rear, improving usability.
4. Ergonomic Power Design and Charging Port Placement
To prevent accidental operation and enable intuitive control, the placement of the power and charging ports has also been carefully considered. The long-press power switch on the rear and the position of the charging port enhance visibility during charging, prioritizing ease of use.
Evolutionary Path: God in the Details Edition
We have introduced the evolution of the EA4's exterior and internal circuitry, but now we will present a design element that, while seemingly inconspicuous, was crucial for developing a truly superior product.
That is the aluminum nitride thermal pad.
The image above shows the eight aluminum nitride thermal pads inside the EA4. One reason the EA4 can achieve its characteristic high output as a flagship portable headphone amplifier is this high-performance thermal management. To achieve high output within the small and limited constraints of a portable amplifier, two points are crucial:
1. An amplifier circuit that achieves high output without distortion.
2. A mechanism to efficiently dissipate the large amount of heat generated by the high output.
Without this, internal components can overheat and be damaged. Therefore, effective heat conduction and increased heat dissipation surface area are necessary.
The basic design uses eight large TO252 transistors as a discrete circuit. The spacing between each large transistor is widened to promote heat dissipation and reduce interference. And the often-overlooked part is the thermal pads attached to these transistors. These thermal pads are the decisive factor in how efficiently heat can be conducted from inside the EA4 to the outer casing and quickly dissipated.
Among the many insulating and stable heat-conductive materials, 'aluminum nitride' has particularly excellent thermal conductivity. While typical silicon pads have a thermal conductivity of 3-4 W/m·K, aluminum nitride pads have 10-15 W/m·K. Furthermore, through careful selection, the eight white aluminum nitride pads adopted in the EA4 achieve a thermal conductivity of 18.6 W/m·K.
Heat is transferred to the exterior through these pads. The EA4's metal exterior acts as a giant heat sink, and for this reason, copper, which is a metal with the highest class of thermal conductivity, was chosen as the exterior material for the 10th-anniversary edition.
As a result of testing with the EA4, heat dissipation efficiency was significantly improved, and during long-duration high-current endurance tests, the temperature rise rate of the casing was greatly improved, with the maximum steady-state temperature decreasing by approximately 5-10℃ compared to before. Thanks to these comprehensive heat dissipation measures, even in the limited space of the EA4, packed with many components, a maximum output of 6500mW at 32Ω was achieved, and the 10th-anniversary limited edition reached 7500mW@32Ω.
We hope that this glimpse into the development history has conveyed LUXURY & PRECISION's commitment to development and sound quality, as well as our sincerity to you.
*Editor's note included