Jeff Rowland Corus Preamp & 625 Power Amp (Hi-Fi+ 83)

Absolutes are dangerous things – especially if you are an audio reviewer. No sooner do you declare something to be the “Best” than something else will come along that’s better. Yes, absolutes allow you the luxury of effortlessly embarrassing yourself – and none more than categorical statements that this or that technology, this or that design path, can’t possibly work.

JRDG (Jeff Rowland Design Group) has never been shy of embracing high-tech solutions and the Corus and 625 combination contains enough cutting-edge technology to make any early adopters out there purr with anticipation – whilst sending the resident technophobes diving for cover. Indeed, on paper at least, these products offer a veritable laundry list of features to raise eyebrows if not serious suspicion amongst the less is more, back to the future brigade; op-amps (gasp); switch-mode power supplies (groan); milled from solid aluminium chassis (mutter); connections and facilities to trim, name or adjust every conceivable parameter, as well as incorporate the units into a (shock/horror) home cinema (Pahhh!).

But look a little closer and you’ll find other facets of these designs that should appeal to even the most resolutely retro of listeners: transformer coupled inputs and outputs (tick); ultra-short signal paths (tick); star-earthed circuitry (tick); totally dual-mono construction - even down to physical isolation of the two halves of the circuit (BIG tick).

You see, every time you try to pigeonhole these products, you uncover some detail, feature or function that allows them to worm away from such pat classification. None of the normal labels quite do the job simply because these JRDG units are genuinely different. Those neat, compact and incredibly elegant exteriors are completely at odds with the sheer range of features, functionality and technology crammed inside them.

Each unit is built into a milled from solid chassis, mated to the almost equally massive and Rowland trademark diamond burnished front panel. The complex housing that includes the beautifully sculpted heat-sinks for the power amp has to be one of the most impressively elegant examples of CNC I’ve ever seen. The pre-amp actually consists of no fewer than four separate elements (audio circuit, psu/controller, remote control handset and separate remote receiver) all milled from solid billet. Inside the main audio chassis, the various elements of the left and right channel circuitry are housed in independent pockets, carved from the solid body. The main units each have dimples machined into their bases that locate the Delrin spheres used as feet/high-frequency isolators.

Both the Corus pre and 625 power amps rely on high-speed switch-mode power supplies, incorporating JRDG’s proprietary Power Factor Correction technology. A conventional power supply places rapid, cyclic (100 or 120 times a second) current demands on the AC line, drawing current each time the voltage peaks. This results in substantial ripple and harmonic distortion that gets fed onwards into the audio circuit as well as dumped back into the mains. The PFC circuit aims to precisely synchronize constant current draw with the voltage cycles, thus eliminating this pollution and delivering a pure, high-voltage DC feed to the audio circuits.

The Corus preamplifier employs a fully complementary, ultra-short, star grounded signal path based on Burr Brown OPA1632 op-amps, built onto high-tech, four-layer ceramic circuit boards. The four balanced and two single-ended inputs and two balanced and two single-ended outputs are all transformer coupled, offering improved noise rejection/isolation as well as identical levels across all inputs/outputs irrespective of type. The power supply is entirely isolated in a small (but incredibly dense) milled aluminum outboard chassis. This is linked to the audio circuit with separate DC umbilicals for the left and right channels. Each channel of each input can be individually trimmed in 0.5dB steps, while single-ended Output 2 can also be user trimmed. The phase of each channel can be inverted – ideal for placing and checking speakers, or correcting the overall phase of recordings. Balance can be adjusted in 1.0dB steps, while the optically coupled volume control offers two-speed 0.5/1.5dB adjustment, depending how fast you spin it through its 99.5dB range.

The 625 power amp seems a model of simplicity when compared with its matching preamp, but thus it ever was. Balanced XLR sockets feed an input transformer, separate voltage and current gain stages and a class AB, bipolar output stage in a dual differential, dual mono configuration. A rear switch handles power on, with a front panel standby button, while the IEC AC input is of the large, rectangular 20 Amp variety. One other nice touch is the heavy duty, high-purity copper bus bars carrying current for both the input and output stages. Two sets of speaker connections per channel are provided, although being of the Cardas common post variety they are most at home with spades, although 4mm plugs can be pushed through at a pinch. If your cables are 4mm terminated and the 625 crosses your acquisition radar, factor in the cost (and inconvenience) of re-termination too.