Pre/Power Amplifiers

Here's a fun game to play: look through the product catalogue of US manufacturer Balanced Audio Technology and try to spot the odd one out. It should only take you a matter of seconds because all of its current models, bar one, employ tubes in either their power or preamp stages. And if you still haven't realised I'm talking about the new REX 500 power amplifier, it's the one that's roughly twice as big as all the others.

A two-channel model rated at 500W a side, but with the option to be specified at purchase as a 1kW mono design, the REX 500 is described as BAT's 'flagship solid state' amplifier, but at £22,500 also counts as its flagship amp, full stop. Below it is the £14,995 tube-toting VK-90t. This is also something of a beast, claiming 2x80W from its 6C33C-B triode power tubes and weighing 45kg, yet the REX 500 goes further, tipping the scales at 63.5kg. The arc-shaped handle mounted to its rear, which echoes a design detail found on other BAT models, is therefore useful when it comes to getting the amplifier in place.

Secret Sauce
BAT's web page, and accompanying 'white paper', for the REX 500 tell us that the amplifier employs solely N-channel FETs in both its driver and power stages, and provides a hint of the thinking behind its 'SuperPak' power supplies with their parallel mix of electrolytic and hybrid silver foil/oil reservoir capacitors. There is no mention, however, of the high running temperature (55-65oC across the chassis) or the fact it's pulling 500W from the wall when idle.

Designer Victor Khomenko is very cautious about calling the REX 500 a 'Class A amplifier' and prefers the term 'heavily biased Class A/B'. Furthermore, while the amplifier does not use complementary PNP/NPN transistor pairs, neither is its design quasi-complementary in the fashion of so many early transistor amps from the 1960s. Instead, the REX 500 takes its design cues from the Circlotron triode tube circuit patented by Wiggins in the US in 1958, though others had published similar topologies earlier. In fact, all BAT's amplifiers, whether tube or transistor, have employed a modified form of the bridged Circlotron configuration.

Float Like A Butterfly...
The REX 500 follows suit with two single-ended groups of 12 N-channel FETs per channel, bridged and with floating power supplies. Moreover, because the preceding N-channel driver stage is fully balanced, both phases are already present to feed the two bridged single-ended blocks directly (ie, no additional inverter stage is required).

The circuit looks elegant enough, and benefits from a very soft transition from Class A to A/B at higher power, but requires great care in its optimisation and stability – the same can be said when attempting to measure this amplifier. Also, because the REX 500's output is floating it should not be connected to any loudspeaker with a grounded 'black' terminal...

That consideration aside, setup and operation are a breeze – once you've realised the amp has two IEC sockets to feed its twin, separate PSUs, another facet of its 'true dual-mono design'. In between these are PSU fuse holders, and above are reset buttons for each channel should you manage to trigger the REX 500's protection circuits. Inputs are offered on balanced XLR only, to match the connections on BAT's REX 3 preamplifier.

Heavy Metal
While the weight of this amp might pose some installation questions – it will surely end up on a low stand on the floor – it's just a little more than 'standard width' at 483mm wide and BAT's industrial design does help it appear at least somewhat sleek. The metal chassis, offered in silver or black, has a smart brushed finish and a slight swoop effect courtesy of the widened front fascia. BAT's logo and the standby/power button take centre-stage: the only thing upsetting its symmetry is the model number, engraved on the lefthand side. The overall impression is of a premium amp that's been very well put together.