˻ PREMIERE ˼

⌈ SPEC Corporation is a Japanese company founded in 2010 by Mr. SHIROKAZU YAZAKI. Today, its chief engineer and “brain” of the team is Mr. BANNO TSUTOMU. This company specializes in the production of amplifiers operating in analog class D. To make it easier for them to work with loudspeakers, the company has developed a specific corrective circuit, called Real-Sound Processor. We are testing its latest version RSP-W1EX. ⌋

SPEC IS A COMPANY SPECIALIZING in the design, construction, and sale of electroacoustic amplifiers operating analog class D, with PWM modulator. Although the technology it uses is similar to that found in many other amplifiers of this type, SPEC equipment differs in details.

Already back in 2010, the brand's founder and chief engineer at the time, Mr. SHIROKAZU YAZAKI, presented his first amplifier, with which he - in a way - “designed” the company's development for the following years. The idea was that he constructed the device based on a linear power supply - a unique thing in class-D amplifiers at the time - selected oil and mica capacitors in the output filter and the power supply itself, and made the “heart” of the device a PWM modulator developed jointly with American company International Rectifier, the IRS2052SM chip.

As Mr. Yazaki wrote on the occasion of the RSP-901EX review, the previous flagship corrective circuit design from this manufacturer, the progress in amplification technology looks to him like this: from tube amplifiers, referred to as “generation 1,” through classic solid-state amplifiers, i.e. “generation 2,” to the new class-D amplifiers, which are the 3rd generation of amplifiers in general. This, he adds, is something of an “evolution” - from simple to complex “organisms,” at least as far as sound is concerned.

When I first encountered the company's amplifier, the RSA-V1 model, in 2012, shortly after its inception, I was surprised by the incredibly simple gain path. Years later, Mr. BANNO TSUTOMU, currently the company's chief engineer, explained to me that this was a derivative of both his and Miss Yazaki's love of tube amplifiers. And if you know how they are built, the “simplicity” of the circuit is the first thing that comes to mind. That's why every detail, even a small component, is so important in them, because with their transparency, they communicate every error even more emphatically.

However, sometimes it is also the case that things need to be made a little more complicated, so that simplicity does not turn into blandness or even arrogance. These, as you can check in dictionaries, are all synonyms, but their meaning changes depending on the context. And the context for the speaker behavior-correcting system tested this time is brand’s own amplifiers, such as the four-box model → RSA-EX1000. Indeed, these little boxes are designed to make their job easier by controlling the backward electromotive force generated by the speakers.

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A few simple words…

BANNO TSUTOMU
Chief engineer

photos by SPEC/原澤 直希

The REAL SOUND PROCESSOR counteracts the back electromotive force of the driver and inhibits the movement of the diaphragm, helping to drive it accurately. This support improves the start and stop response of the driver's diaphragm. It can also dampen their unwanted vibrations, improving reverberation rendering, clarity and energy transfer.

The electrical circuit of the RSP is composed of resistors and capacitors, but the end result (sound) cannot be expressed by a fixed circuit value alone. Component selection is important - it is not enough just to match the electrical characteristics. The RSP-W1EX is a processor developed for woofers and works with even lower frequency signals than the RSP-AZ9EX. As a result, the bass is clearer and more powerful. The capacitor responsible for the bass performance characteristics is a large PP one.

It makes direct contact with the spruce wood base. The spruce base and large walnut cabinet vibrate together with the music signal to bring out rich low tones. Combined with the RSP-AZ9EX, an even wider frequency response can be achieved. A custom-made box is also available for placing the RSP-W1EX and RSP-AZ9EX, one on top of the other, (sold separately). BT

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▌ RSP-W1EX

The RSP-W1EX IS A PASSIVE CORRECTING DEVICE from SPEC. At the same time, it is the first such product prepared by Mr. Banno-san. We call them “filters”, but they are not really filters, because they do not “filter” anything, but “correct”. SPEC itself, by the way, points this out - “filter” connotes subtracting something from the signal, and RSP does not affect the signal.

These products fulfill two functions: one is the sound improvement stemming from the reduction of impedance fluctuations as a function of frequency in the midrange and tweeter, and the other is the ability to absorb, to some extent, the aforementioned electromotive force generated in the driver’s coil and eliminate the inductive component of the system composed of the driver’s coil and crossover components.

Connecting the RSP is trivially easy. On its rear panel there are speaker sockets, which we connect to the speaker sockets of the speakers, remembering the proper connection of “plus” (red color) and “minus” (black color). This is a parallel connection that we can make with short or even very short speaker cables / jumpers. As Mr. Banno, responsible for this design, once mentioned, it's also worth trying the option with two “processors” per channel - one on the speaker side and the other on the amplifier side. This circuit does not load the amplifier output, so it can work with both solid-state and tube devices.

The RSP-W1EX is already the fifth RSP model that SPEC offers, not counting the special limited edition Urushi; more about that version → HERE. We tested them all, starting with → RSP-101 in October 2011, next → RSP-501EX ˻PL˺, → RSP-901EX, and → RSP-AZ9EX ˻PL˺. We tested them frequently, we should add, as one of the first in the world. And all of them worked in our reference system, with the AZ9EX model prominently displayed on the Harbeth M40.1 speaker stand until today.

It has been as long as five years since the test of the latter. It seemed that this was the final, best, polished circuit that could be made. But, after all, this is an audiophile world, a perfectionist world, and thus one that never stops searching. Reaching for a test of the RSP-901EX model, I found a confession from Mr. Shirokazu Yazaki, the company's former head and chief engineer:

To tell you the truth, I thought the RSP-901EX would be our last and best RSP - at least until last summer. That's when we managed to make even better sounding capacitors, Green Cactus, prepared by Arizona Capacitors, Inc. but also our own mica capacitors for audio applications. For a long time, however, I have been looking for resistors whose sound would satisfy me. Finally I succeeded, I found resistors with an outstanding complex sound.

And? - And the situation repeated itself once again, and out of Mr. Banno's brain came the RSP-W1EX model. With the latest “processor” - let me remind you that this is how SPEC's people not only say, but also think about the products we test - things look different. The previous version is still considered by the designers of this company with the best they could prepare. However, the correction circuit can be designed a bit differently, sacrificing some elements of sound for the sake of others. In this case, it was about improving low-frequency performance.

RSP is simply a bunch of capacitors and resistors in a specially made wooden housing. But it's about carefully selected resistors, and capacitors, as well as their values. Successive generations of RSP filters differ precisely in these three elements. In the RSP-W1EX, the main capacitor has been changed to one with higher capacitance, and the whole thing is housed in a larger enclosure. The capacitor is really big and is specified for 1500 V (1.5 kV) DC. It is made using the Metallized Film Capacitor technique, that is, with a plastic ribbon on which, on two sides, metal, usually aluminum, has been vaporized. The internal connections are made directly, without the involvement of a printed circuit board.

But the housing itself has also changed. Not only is it larger, but the material is different. Previously, Manchurian walnut wood was used, while the new version is American walnut, with a base made of spruce wood. SPEC says that this “hybrid design” was borrowed from the design of the company's amplifiers, sitting on just such wooden bases. This chassis is manufactured by the Japanese company Shirakawa, a famous furniture manufacturer from Hida-Takayama, a well-known historical city in Japan. As the press materials say, “The Real Sound Processor is also a musical instrument.”

Banno-san mentioned earlier the possibility of combining two SPECs, the RSP-W1EX and RSP-AZ9EX. The former can then be connected to the woofer section of the speakers, and the latter to the midrange/treble section. Or, as in my case, connect them in parallel to the single terminals of the speakers. To help users to present them in a nice way, the manufacturer has prepared a special “case” called RSP-W1-CEX. It is 115 wide, 146 high and 105 mm deep; it weighs about 225 g (pc.) We can place both filters in it, one on top of the other.

As usual, the packaging, workmanship and finish are excellent. This is real handmade product straight from Japan.

▌ SOUND

HOW WE LISTENED • Real-Sound Processor RSP-W1EX was tested in the “High Fidelity” reference system. It is the fifth in a series of products of this type, our system worked with. So it's safe to say that we know them quite well and hold them in equally high regard.

The new RSP was compared to the RSP-AZ9EX, connected to the terminals of Harbeth M40.1 speakers driven by a Soulution 710 solid-state amplifier. The signal to both RSPs was routed through vintage cables, dating back to the 1940s Western Electric WE16GA (KS13385L-1), manufactured by A.I.W. (American Insulated Wire Corporation) in the USA.

This is the cable used by Mr. Yamazaki to wire his tube amplifiers. It is made of braided copper covered with a very thin layer of tin; more → HERE. My cable is terminated on both sides with Acoustic Revive RBN-1 banana plugs. Not cheap, costing 1600 zloty for four pieces, but they are really excellent. They have a vibration-damping design, in which a brass body is combined with A2017S aluminum alloy elements of various types.

We conducted a separate test with a complete system, consisting of the RSP-W1EX, RSP-AZ9EX and RSP-W1-CEX. The two RSPs were connected by short runs of the same WE16GA cable that routed the signal from the speakers. This time the cables were connected directly to the sockets, without using additional plugs. Maybe someday...

During the test, music was played from a Lumin T3 file player and a Rega P3 RS Edition turntable, with an Nd5 cartridge. The test was a AA/BB/AA comparison with A and B known, and the length of the excerpts played was two minutes.

» RECORDINGS USED FOR THE TEST ⸜a selection

⸜ GENESIS, Back in N.Y.C. (2025 Remaster), Charisma/Craft Recordings/Tidal, SP, FLAC 24/96 ⸜ 2025.
⸜ MIDNIGHT GENERATION, Don’t Wait Up, Midnight Generation/Tidal, FLAC 24/44,1 ⸜ 2025.
⸜ AMERICA, America, Warner Records/Tidal, FLAC 16/44,1 ⸜ 1971/1987.
⸜ JUDY GARLAND, Changing My Tune (Audiophile Edition), Phonogram Recordings/Tidal, FLAC 16/44,1 ⸜ 2024.
⸜ WES MONTGOMERY & WYNTON KELLY TRIO, Smokin' At The Half Note, Verve/Universal Music K.K. [Japan] UCJU-9083, 200 g LP ⸜ 1965/2007.
⸜ The Famous Sound Of Three Blind Mice Vol. 1, Three Blind Mice/Impex Records ‎IMP6027, 2 x 180 g LP ⸜ 2018.
⸜ JEAN-MICHELE JARRE, Magnetic Fields, Polydor POLS 1033 | 2311 075, LP ⸜ 1981.

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THE PLAN WAS TO listen to the new RSP longer, not even comparing it with the older version, and only after some time sit down for proper listening, a proper test. As usual, plans are one thing, and true life is another. The thing is, the differences between the RSP-W1EX and the RSP-AZ9EX, are so great that I couldn't sit still without at least a few words written down. And once I started, I couldn’t stop until it was finished.

RSP-W1EX • The RSP-W1EX was conceived as an aid to the woofer. And indeed, the bass is slightly deeper and slightly richer in tone with it. But that's not why the change was so big. It was about something more - the way the music was presented. With the new RSP it was a much orderly, smoother, and warmer playing. Both the GENESIS listened to right at the beginning from the new remaster of Back in N.Y.C.. (2025 Remaster) and the almost disco-like Don't Wait Up by MIDNIGHT GENERATION, sounded denser, fuller - I have to say it - lower.

But this “extension” into lower regions of the band was not about pumping up the air at the bottom of the band. It wasn't a “subwoofer” effect. It was about something like being able to easier understand what's going on down there. It's higher resolution, that's how I interpret this change, at the bottom of the band with the new SPEC product. And if the bottom end is more resolving, and thus richer internally, then the rest of the bandwidth is much better as well. It's an effect I experience whenever I hear a well-tuned subwoofer.

I didn't do it deliberately, but both tracks I listened to at the beginning are rich in low sounds. This is particularly striking when listening to Genesis, as I'm used to this album sounding rather light. The new remaster, prepared by Miles Showell at Abbey Road studios, brought out incredible energy in the lower end. It was a good energy, not an “annoying” one. The tested RSP in combination with the Soulution 710 and Harbeth M40.1 gave a smoother, more “polished” sound with a lower center of gravity. But also less dynamic, a bit sedated.

It wasn't a flaw, I think, that plenty of systems in fact beg for such an adjustment, pouring a load of tense, dirty sounds on music listeners. Maybe dynamic, maybe selective and even detailed. But still - unpleasant. The RSP-W1EX will change that. But it will change it in any system. And if everything is fine with it in terms of timbre we can perceive it’s effect as calming.

The change I'm talking about, however, was not unpleasant, even in my case, with my reference system. And after some time, without a direct comparison with the RSP-AZ9EX, it became apparent that listening with it inspired an incredible tranquility in me. I was calm, but also focused. It's as if the RSP brings out something in the recordings that makes up the “deep background,” that is, the part of the sound that is masked by what hits louder, which is, after all, the essence of the sound.

That's why A Horse With No Name by AMERICA, a track from their debut album, with the older SPEC “processor” sounding somewhat bland, with the new one gained some depth, got a kind of “support” making it sound stronger, denser, simply louder. So it was that the dynamic, extended in bandwidth songs played softer, gentler, and those with a narrower bandwidth - stronger, clearer. Which I confirmed by listening to JUDY GARLAND from the Changing My Tune (Audiophile Edition).

As I said, the bass is smoother and denser with the new RSP. And yet the strong “pops”, which in the Don't Tell Me That Story track were previously a bit bloomy, here were quickly extinguished, although their energy did not suffer. These are, granted, distortions, but ones that are already part of the music, without which the recording is simply different. And therefore they are an important part of the presentation. And with the new “box” from Japan, they were better connected to it, not drawing as much attention to themselves as before.

This will be the case, by the way, not only with digitally music. Perhaps even more important was how the SPEC corrected sound coming from LPs. Interesting, but it did it with them more delicately, not in as spectacular a way as with music played from a file player. But after a while, after several comparisons, it was apparent that the change had a deeper and more structural rather than punctuated effect.

In the sense that when WES MONTGOMERY's guitar sounded on the Smokin' At The Half Note album where he plays on with the WYNTON KELLY TRIO, it had with the Japanese RSP thicker background, stronger sustain. The cymbals seemed a bit smoother than before and warmer, but along with that they were also internally better ordered. So was the large ensemble from the Midnight Sunrise recording from The Famous Sound Of Three Blind Mice Vol. 1, released in 2018 on the Impex label.

So I wasn't particularly surprised when deeper, denser, warmer was also JEAN-MICHEL JARRE's Magnetic Fields album, from the original UK pressing. It was with it that I repeated what I had heard earlier with the America's album, namely, a subjectively louder sound. So, on the one hand, with the RSP-W1EX we would get denser bass, warmer treble and softer dynamics, and on the other hand more “tangibility” of the sound and its stronger “presence”.

W1EX + AZ9EX + W1-CEX • So it was with all the more curiosity that I put together a system that, according to the SPEC people, is the “definitive” one: the RSP-W1EX and RSP-AZ9EX connected in parallel in the RSP-W1-CEX box. This time I decided to listen to all the tracks selected for this comparison at once, without recording impressions in between. I wanted to immerse myself in the sound and see what would come of it.

And I didn't even know I wanted to hear what I ultimately heard. That is, great resolution. Because that, ultimately, is what it's all about. The change seemed really big to me. Objectively, it may only be a few percent improvement, but it's the percentage that's the hardest to improve, that anyone who lives for and with music dreams about.

The complete RSP system produces a sound that is softer on the one hand than the RSP-AZ9EX alone, and more resolving and energetic than with the RSP-W1EX alone. Tracks that play with energy will get better differentiation, making them seem to play quieter by 1, maybe even 1.5 dB. Turn up the volume and you get more of everything, but without the impression of obtrusiveness or exaggeration. The quieter ones, on the other hand, gain presence. It's like they've been turned up 1 dB louder. But we don't have to turn them down, because from now on they have a fuller and richer tone.

The three RSP components put together offer an effect comparable to changing cables for the better ones. It's a change, like from a good but simple power cable to a sophisticated product. At least in my perception. Looking at it objectively, the change was clear, important, significant, but not as dramatic as the above description would suggest. And yet it is a true description - true for me, here and now. It is true by how it transforms the sound of the system I know so well, after all.

▌ Summary

SPEC'S REAL-SOUND PROCESSOR RSP-W1EX produces a different sound than the RSP-AZ9EX model. It is deeper, has more weight, but it’s also more plastic, smoother, warmer and slightly more withdrawn in terms of energy with strong recordings, and stronger and more “present” with those that were previously withdrawn. Its plasticity is thus higher than the previous version, but in turn the attack is better with the RSP-AZ9EX.

In a system we get not so much a combination of these qualities, that's not the point, but something greater than them playing solo. It's much better resolution, energy, tangibility. And above all, better differentiation. And that's without the negative effects such as hardening of the attack, nervousness or emphasized top end. The latter is stronger, because both the noise from the beginning of the song by America, and the cymbals from Wes Montgomery's recording - all were clearer and more forceful. But it was also smoother and more vivid.

This is the best version of the Real-Sound Processor that I know of. And not just by a little margin, but truly, clearly the best. So it stays in our system as an important component of it. ●

▌ Technical specifications (according to the manufacturer):

Required speaker’s impedance: 10 Ω or less
Maximum amplifier’s output: 1 kW or less
Dimensions (W x H x D): 100 x 85 x 117 mm
Weight: around 360 g/pc

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THIS TEST HAS BEEN DESIGNED ACCORDING TO THE GUIDELINES adopted by the Association of International Audiophile Publications, an international audio press association concerned with ethical and professional standards in our industry, of which HIGH FIDELITY is a founding member. More about the association and its constituent titles → HERE.

→ www.AIAP-online.org