Let’s start with an admission. These are not Accustic Arts’ flagship mono power amplifiers. The Mono III is the flagship, and it’s a big ‘un. These are ‘just’ the new Mk 2 edition of Accustic Arts popular Mono II power amplifiers. It’s still a big ‘un, but in these days of a new kind of ‘stranger danger’, manhandling a power amp that weighs 25kg per channel as opposed to one that weighs 60kg per channel is the difference between breaking into a sweat and breaking a few vertebrae.
If you set aside the Mono III, the Mono II Mk 2 screams ‘flagship’ in its own right. The amps are substantial, taking up the low, deep rather than wide form factor of something like a pair of the latest ‘cheese grater’ Apple Mac Pro computers, but with arguably cleaner lines and lots and lots of aluminium, with a highly polished chromed brass inlay.
Each Mono II Mk 2 delivers a substantial 310W into eight ohms and will continue on down to two ohms, where it’s capable of hitting up to 750W. Much of the internal architecture is taken up with a large and fully screened toroidal transformer, hooked to 80,000µF of reservoir capacitance. It can be driven in balanced or single-ended mode at a flip of a switch.
The core technology here is the current mirror used in driver stage of the Mono II Mk 2 (and – in fairness – in Accustic Arts amps in general). This circuit enables power to be drawn from an existing current, making effectively a ‘power-controlled’ output amplifier so the 10 MOSFET output transistors in each amp deliver very high current capacity without being pushed to their performance limit. Moreover, as the quiescent current is generated by the integrated control of that current mirror circuit, there’s no need for servo-controlled DC offset monitoring.
This control system also monitors the temperature of the output stage, as well as clipping and high frequency oscillation. This control references against pre-defined values and if these values for any parameter is exceeded, the amplifier shuts down before it kills loudspeakers. This is a more sophisticated approach than either a simple fuse or an overvoltage ‘crowbar’ circuit, and the sonic performance benefits from that.