Editor’s Note: This review was commissioned and originally published before Ken Ishiwata’s recent departure from Marantz!
I thought I was doing well to get 30 years under my belt in this business but Ken Ishiwata is way ahead of me, as the precious gem in this amplifier’s name suggests he has been with Marantz for 40 years. And then he was with Pioneer for 10 years before that. Clearly, this is not a young man’s game anymore.
The PM-KI Ruby looks and functions as an integrated amplifier with all the convenience and reduction in cabling that that promises, yet under the skin, it has been built like a pre/power combination from the mains transformer onwards. This is because both the active preamplifier stage and 100 Watt output stage have their own separate power supplies, but it doesn’t weigh a ton (or even a tonne) because the output stage is a Class D type with a switching power supply. This is an approach that was once the preserve of Chord Electronics and Linn but its popularity is growing rapidly for a number of reasons. One of the biggest reasons for larger-scale manufacturers is legislation that prohibits inefficient electronics from being sold in key territories including the EU. Class D is also on the rise for economic reasons; you can make a far more powerful amplifier for a given budget than you can using conventional Class A/B technology. But the most important reason unless you are a fervent polar bear enthusiast is that Class D has been proven to sound remarkably good by a few key players and this trend is likely to continue.
I was highly impressed with the Kii Three active loudspeaker last year and that has Class D amplification onboard. It was designed by Bruno Putzeys, a master of this art, so when Ken Ishiwata mentioned that PM-KI Ruby differs from most Class D designs by virtue of putting the feedback after the output filter and credited this design to a ‘Belgian engineer’, Bruno’s name sprang to mind. But that’s just a guess; I suspect that there are some other canny audio engineers in Belgium. Ishiwata himself lives in Antwerp (and works in Eindhoven, Holland. Go figure!) so while he started his career at Marantz as a bridge between manufacturing in Japan and their European base he now spans a somewhat smaller gap that nonetheless appears to be bearing audio engineering fruit.
The PM-KI Ruby is an analogue amplifier; all digital duties are handled by the partnering SA-KI Ruby SACD/CD player and DAC reviewed in the January issue (168). It only has single ended inputs, which differentiates it from the rather more pricey PM-10 (externally and internally, the PM-10 is a balanced amplifier). But there are two pairs of inputs that stand out by virtue of silver plating instead of the ubiquitous gold finish, silver being a better conductor but one that’s rarely used because of its tendency to tarnish. The second pair of silver terminals is for a phono stage, and not just an MM one at that. In fact it’s a rather special part of the amplifier that Ishiwata is particularly proud of; it’s a discrete, cascaded design with an initial gain stage for MC that acts like a head amp before the EQ stage, which is a very rare approach outside of tube preamplifiers. Ken has long been a tube fan so it’s not surprising, but the fact that it’s a superior phono stage to the one in the PM-10 according to Ken is surprising.