ET400A Class D 350Wx2 Hifi Digital Power Amplifier, New Housing Pluggable Buffer Stage IC. 

From the main board and power supply produced in Denmark of the same type as NAD,
DIY assembles the new aluminum alloy chassis.
Features:
- Latest Class D technology, high-precision THD+N and IMD indicators for better hearing
- Each power amplifier, starting from its board, has been tested by devices for AP before delivery, and the performance is 100% guaranteed
- Using open-loop LLC architecture power supply, full-power ZVS work and rectifier ZCS work, so that the noise is lower
- At full power, the efficiency of the whole machine reaches 93-94%. The static power consumption of the whole machine is 16W (measured at 220V)
- OPA1612 high-precision op amp
- Original film capacitors for WIMA have low losses and small size
- Taiwan-made gold-plated RCA and SWISS imported XLR input socket (for NEUTRIK), carbon fiber gold-plated and terminals
- The power supply capacitors use original LS (primary) and RX (secondary) high frequency capacitors for Nover
- The power amplifier capacitor adopts Japan's ZLH series high-frequency long-life capacitor for Rubycon
- Thickened all-aluminum chassis
- Waterproof stainless steel power switch with light

Specifications:
- Standard chassis: 430 x 220 x 60mm/16.9 x 8.7 x 2.4"
- Power 1%: 200W*2 at 8 ohms; 350W*2 at 4 ohms
- Distortion: Less than 0.00025% at 1K 100W
- SN: over 125dB, measured 127dB
- Damping coefficient: more than 800 in the full frequency band, 1400 measured at 1K
- Frequency response: -0.02dB at 20-20K
About the buffer plate:
Currently, the buffer board is with a gain of 14dB. Unplugging the op amp, it is equivalent to a straight-through power amplifier. Without the buffer board, that is, the gain will be smaller than 14dB. While unplugging the op amp, there are 4 short-circuit pins inside, and it is necessary to jump to the other two pins of the three pin headers. When plugging in the op amp, it is necessary to jump back, otherwise it will affect the distortion of the whole machine.