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"One more impressive model to mention would be the Pioneer DV-AX10. This was the world’s first real universal player capable of playing CD, SACD, DVD and DVD-A. Not only was the DVD video quality remarkable but also the CD sound was full of musicality. The DV-AX10 garnered the gold medal of Japan's HiVi monthly magazine from amongst all of 1999's A/V products reviewed in Japan. As a general manager of the DVD engineering department I felt delight and honor. And here I met one fabulous engineer, Mr. Banno who struggled to manage an engineering team of about 40 and chased a breakthrough across a range of tough issues. And would you believe it, 10 years later Mr. Banno and I each day are in hot pursuit of a good-sounding D-class amplifier here at SPEC Corp. He and I experienced so many types of sound during the AX10's R&D and it seems to us that those experiences led directly to the development of our D-class amplifiers.

"Around the end of the 1990s and by the beginning of the millennium, Pioneer had reached its zenith where we thus could enjoy the rare opportunity to develop as high-end a DVD player as the AX10. We also had a well-designed spacious listening room and could select high-priced speakers, preamps and power amplifiers as reference for our development of the AX10. I remember that I bought a pair of B&W801, then the reference of specialist audio shops in our domestic market. Of course B&W 801 were driven by high-power semiconductor amplifiers of various famous domestic and foreign brands. Yet Mr. Banno and I were never moved by the sound of this high-end audio system. The sound was like watching miniature paintings. We could hear all the details but were never touched by the music. We felt the sound was very precise but one-dimensional and lacked any real dynamics or rhythm. When I returned home and listened to music over my old high-efficiency system, I was once again intoxicated by the playback which was so transparent, natural and organic. I felt that the difference came from the distinction of certain dynamic characteristics of these two systems, not merely specific static qualities. Yet at the time I couldn’t clearly understand the reasons for my dissatisfaction with the standard approach.

"It was unexpected but my career in the engineering department was over by the spring of 2000 when I was transferred to a subsidiary production company at Semarang/Indonesia for management reconstruction as their director. I thus had to leave behind the sound of my old system and experienced many hardships about all aspects of expat life in an equatorial foreign country. I think those hardships, unfamiliar circumstances or the nature of the rainforest fostered some inner strengths and musical hearing abilities. In any case Indonesia has since become my second home.

"Getting back to Japan, toward the end of my career at Pioneer I was put in charge of their R&D center. I could choose a good sound system with an amplifier and speakers of my own preference. I well remember one day in October 2006 when I had opportunity to attend a demonstration of new D-class power devices and a prototype amplifier which used them. I was very impressed with the sound. It was only a prototype so the sound wasn't truly sophisticated yet its rich mid to low ranges made it feel like a good tube amplifier with very similar musicality. I noticed an obvious difference to the sound of traditional transistor amps. On another business trip on December 9th, 2006, I had the chance to talk to the person who had developed that amplifier in Los Angeles. As a matter of fact he turned out to be my junior colleague from Pioneer and a personal audio friend.

"Mr. Honda had emigrated to the US with his family and found a job with International Rectifier, a semiconductor company specializing in a power management technique called IR. After around 6 years working there, he had developed new D-class devices. I felt he was not only a nice guy but also a brilliant engineer with a certain genius for audio circuitry so a joint dinner at a seaside restaurant became a true delight. We talked about our private lives and of course hifi, especially his new parts. It was a great night for both of us and this meeting became the start of my search for the full musical potential of these brilliant IR class D devices. On February 6th 2010 we started our new company SPEC Corporation.

"I already had a lot of know-how on how to work with such parts to improve their musicality. There are two major areas for tweaking. One is the low-pass filter in the final stage of the D-class circuit, the other major determinant is the quality of the power supply. Today I'd like to focus on the low-pass filter. The role of this filter is very important for switching amplifiers because it filters out the PWM switching carrier frequency of about 400kHz in an efficient manner to pass along pure analog current to the speakers. This circuit is ultra simple—composed of a single inductor and capacitor—but the quality of the analog current and its sound quality depends most highly on the quality of these two basic parts. Here I must add that the switching abilities of these new IR amplification devices are near perfect. IR states power transfer efficiency approaching 96%. So yes, here we have a new generation of audio power parts. I firmly believe this is very good news for music lovers around the world.

"I would like to get back at the low-pass filter. By the end of 2009 I had reached the conclusion that the best capacitor to use in this filter was the vintage oil-filled capacitor marked 'hermetic seal' from WEST CAP, part N°. CPV09 0.47/600 made for the US military in 1967. Of course as coupling capacitor it also remarkably improved the sound of my old DA30 non-feedback SET. Its sound gained in musicality and deep bass with no trace of grain in the mid and higher ranges. But we couldn’t use these NOS parts for our new D-class amplifier. It was hard to find a solution for this vexing problem, so hard in fact that I can only explain the ensuing find as divine intervention. It turned out that the original capacitor manufacturer had survived in Tucson, Arizona. They'd changed their name from West Cap to Arizona Capacitors Inc. in the early '90s. Arizona Capacitors took over from West Cap huge production facilities and endless engineering drawings. As their Japanese distributor our company has worked together with Arizona Capacitors selling quality custom oil-filled capacitors since the summer of 2011. Japanese tube amplifier fans remember the name of lovely sounding vintage caps from Sprague and West Cap. Our distributing business has grown because of the high tonal quality of these oil-filled capacitors. And finally we were able to adopt these custom oil-filled capacitors for all our amplifiers. This was a real miracle and I thank the Lord for having blessed me so many times by coming across these tonally pure parts.

"If I were to say something about the true potential of D-class amplifiers, I'd say that it's important how they control the inevitable back electromotive force from the speakers. In D-class amplifier the current of the back EMF is directed back at the power supply whereas in a traditional solid-state amplifier the current forcibly enters their feedback loop. That disturbs the precise phase transmission of dynamic music signals. That's why our RSP-101 improves the sound and helps protect amplifier circuit precision especially of solid-state amplifiers with deep feedback." - Shirokazu Yazaki. About today's review subject, he had this to add: "With the REQ-S1 we improved our domestic REQ-77S version to let you hear the performer's very mind and soul. We also adopted the very latest parts of ultra low-noise high-gain op amps within very simple circuitry whose very simplicity means that we more easily hear the quality differences between passive parts like capacitors and resistors. This fact is the same as for our class-D amplifiers and the RSP-501EX. Put simply, the experience of developing the class-D amplifiers RSP-101, 301 and 501EX led us directly to the REQ-S1. For its power supply we have adopted the newest SiC (silicon carbide) diodes. These are also in the RSA-M3EX. They are free from switching noise and have an excellent sonic character of a very transparent mid to high end with rich and powerful mid to low registers."

The REQ-77S launched for sale in Japan in March 2013 and its European version followed in April. Our review unit with serial number 0001 would be a world premier. At Munich HighEnd 2013 SPEC Corp. showed its new mass-loaded AP-5 turntable [the speaker end of that system is shown below - Ed].

Here are Shirokazu Yazaki, director (right) and Tsutomu Banno, the main engineer and designer of SPEC Corp (left) at the same show.