400D is a very slim (1.75'') MOSFET Class-D amplifier with a tube preamp, that can fit within one rack unit. Components are of the highest quality, including double-sided PCBs, MKP capacitors, top-drawer resistors, Neutrik connectors and dustproof pots.

A good portion of the 8.5 kg (18.7 lbs) comes from the heavy power supply transformer, being a breakthrough with its low-profile, ultra-flat configuration. This provides with a very stable power source and has a built-in thermal protection.

The power amplifier is a Class-D (PWM) type, designed with Philips UcD technology - resulting in an efficiency of more than 90%. The amplifier produces hardly any heat, so there's no cooling fan or even external fins. That makes it very suitable in the studio environment.

The double triode 12AX7 tube in the preamp is fed with DC current to avoid hum, with a built-in startup delay on the anode current for a longer tube life. It's designed to reproduce the instrument cleanly and accurately with the EQ set flat. However, the the active 5-band equalizer unit can compensate in upmost difficult acoustic circumstances. EQ controls all increase in effect as they are moved further from neutral. This brilliant feature provides with a finer control near the center and greater effect near extremes.

As a professional piece of equipment - the 400D is protected against a short circuit, clipping at the output and DC. If clipping or DC at the output occurs - the amplifier turns off automatically. Additionally, there is absolutely no pop or thump when turning the unit on or off.