Power amplifiers mono — Ubiqaudiolab Ubicube 400

The Ubicube 400 is the middle model of the Ubicube monoblock series from Ubiqaudiolab of Ljubljana, Slovenia, part of the Akrapovič Group. The maker describes it as offering the same approach as the larger Ubicube 700 in a smaller form factor, meant for listening rooms where the 700 would be too big. One amplifier drives one loudspeaker, and Ubiqaudiolab builds each unit by hand.

Key specs that matter

Into 8 ohm the 400 is rated at 400 W, rising to 700 W into 4 ohm and 1,200 W into 2 ohm. Output current peaks at 60 A, with 40 A available continuously as rms. A 268,000 µF reservoir holds 271 J. Response runs from DC to 500 kHz (-3 dB). Voltage gain is +27.75 dB, a factor of 24.4. Distortion plus noise is under 0.01% at 100 W into 8 ohm, the noise floor is more than 110 dB under full power, and damping is above 500 with an 8 ohm load. The amplifier accepts one balanced XLR source, DC coupled, with 167 kohm input impedance. The enclosure is a 333 x 333 x 318 mm cube, and the weight is 47 kg.

Engineering and use context

Each Ubicube uses a power module developed in house for its rating, so the 400 carries its own 400 W module with short signal paths, its own grounding layout and EMI/RFI shielding. The circuit is bridged. Its input stage is a symmetrical J-FET design in class A, DC coupled throughout, and the lateral MOSFET output devices are matched manually and biased high in class AB, delivering at least 10 W before leaving class A. Power comes from an unregulated switch-mode supply filtered against common-mode and differential interference, backed by Mundorf electrolytics and copper foil capacitors from Jupiter and Duelund. The housing is a solid aluminium monocoque, anodised, hand brushed and finished with a nano coating. Copper heat plates inside rely on chimney-effect airflow to cool the module. A monitored protection system limits excessive current, checks DC at the outputs, filters RF and tracks temperature, and a solid-state relay cuts the speakers off instantly. A top-mounted button switches between standby and on, and also selects the view of a level meter with peak hold covering 73 dB.

Pairing

Any source or preamp feeding it must have XLR outputs, and Ubiqaudiolab offers its Ubiquitous 300B, a preamp using a Western Electric 300B triode, as the matching control unit. The power reserve into low impedances makes the 400 a match for demanding floorstanders, and its smaller footprint fits rooms where two large monoblocks would crowd the space.

Alternatives

The Ubicube 200 is the compact monoblock in the series, rated at 200 W into 8 ohm and tuned by the maker for speakers with a sensitivity of 97 dB and above.