Ray Audio KT150 Vacuum Tube Power Amplifier – HiFiReport
My previous audition of the Tuku vacuum tube preamplifier marked my first extended experience with Walker Ray Audio equipment. That encounter was genuinely “astonishing.” I was captivated by the rich, natural timbre, the unpretentious directness, the precise imaging and soundstage, and the vivid dynamic contrasts. It was abundantly clear that the Tuku not only boasts a unique design, but also that the output transformers, meticulously hand-wound by the principal, Mr. Lien Chia-Chen, are truly exceptional.
This time, I’m reviewing the KT150, a power amplifier where the output transformer assumes an even more pivotal role. How does it perform? To put it simply, within its price bracket, I believe you’d be hard-pressed to find a vacuum tube power amplifier that surpasses the KT150 in sonic vibrancy, comprehensiveness, allure, and sheer driving force.
Delivering a Highly Practical 100 Watts of Power
While some Ray Audio models bear names like the Tuku preamplifier and the Takeda No. 1 isolation transformer, most are directly named after the vacuum tube they employ. True to form, the KT150 power amplifier is indeed based on the KT150 tube. It utilizes two KT150 tubes per channel in a push-pull configuration, delivering a very practical 100 watts of power.
The term “practical” here signifies its versatility in pairing with a wider range of loudspeakers and its excellent performance across diverse musical genres. The KT150 submitted for review is the XLR version, specifically designed for front-end equipment with balanced outputs, such as Ray Audio’s own Takeda or Tuku preamplifiers. Consequently, the KT150 XLR version features only XLR inputs, omitting RCA inputs.
Why this design choice? Because the KT150 employs a push-pull amplification topology. Even with a single-ended input signal, it must undergo a phase-splitting process to create positive and negative half-wave signals for amplification before being recombined by the output transformer to drive the speakers. Some preamplifiers primarily emphasize balanced circuitry, and users often prefer balanced connections to power amplifiers. In such cases, the single-ended inputs on the power amplifier may remain unused. Therefore, the KT150 XLR version omits the single-ended input phase-splitting circuitry, streamlining the circuit path for potentially enhanced sonic purity.
If your front-end equipment only offers RCA outputs, you can opt for the RCA version of the KT150, which includes both XLR and RCA inputs for broader compatibility.
Circuit Simplicity as Priority
Ray Audio consistently prioritizes circuit simplicity over complexity, a principle embodied in the KT150. Upon entering the KT150, the balanced signal for each channel immediately encounters a 12AT7 signal tube for voltage amplification.
Mr. Lien Chia-Chen chose the 12AT7 for the first amplification stage due to its exceptional bandwidth, fast response, and modern sonic character, which aligns well with the diverse musical tastes of contemporary streaming listeners. Moreover, the 12AT7 tube offers a vast selection, from new and vintage to budget-friendly and high-end options, ensuring effortless tube rolling.
After amplification by the 12AT7, the positive and negative balanced signals are then fed into the 6SN7 driver stage. The 6SN7 is not only an ideal driver tube but also shares the 12AT7’s wide range of available options.
The voltage amplification and driver stages in the KT150 are directly coupled, eliminating coupling capacitors to avoid signal degradation, coloration, and other issues associated with capacitors. However, a coupling capacitor between the driver stage and the KT150 power tubes is unavoidable for circuit safety. Here, Mr. Lien specifically selected Mundorf capacitors for their comprehensive performance, aiming for balanced frequency response and consistent speed across all frequency bands, thereby highlighting the strengths of the KT150’s circuit design and output transformer performance.
Mr. Lien mentioned that he experimented with numerous coupling capacitors, each with its own merits and drawbacks. Ultimately, the premium Mundorf capacitor was chosen for its superior all-around and balanced performance.
Output Transformers with Superior Bandwidth
For the crucial output transformers, Mr. Lien naturally employs his own creations. He stands out as one of the very few tube amplifier designers capable of in-house winding high-quality output transformers. This capability allows him to tailor transformers specifically to the characteristics of different vacuum tubes, desired sonic signatures, and even the unique requirements of individual customers.
The output transformers in the KT150 boast impressive specifications, with a frequency response of 5Hz – 50kHz / -3dB. The 0dB range spans from 10Hz to over 40kHz, demonstrating exceptional bandwidth and flatness. Mr. Lien explained that achieving 0dB at 5Hz is possible, but it would compromise high-frequency performance. Therefore, he opted for 0dB at 10Hz, allowing the high frequencies to extend to 50kHz before reaching -3dB, thus balancing both low-frequency and high-frequency performance.
My listening experience corroborates this. The high frequencies are vibrant and luminous, while the low frequencies are both solid and agile, with ample presence. The overall sound is open and well-rounded.
The chassis is unequivocally robust. The KT150’s chassis is constructed from galvanized steel, commonly known as “zinc plate,” with a substantial thickness of 1.6mm. This provides excellent rigidity, oxidation resistance, and long-term durability. The surface is finished with a matte black powder coating, offering a clean, understated aesthetic that is both dirt-resistant and scratch-resistant. The front panel is enhanced with a 1cm-thick solid aluminum plate, and the top cover also features a black aluminum plate, with a fine hairline finish adding to the overall sense of quality. Internally, except for the power input and bias adjustment sections which utilize circuit boards, the amplifier is meticulously hand-wired, showcasing a high level of craftsmanship.
The KT150’s power tube bias is manually adjustable. The top cover is pre-drilled with holes for multimeter probes and adjustment screws, enabling users to fine-tune the bias themselves using a multimeter and a small flat-head screwdriver. When should you adjust the bias? Mr. Lien recommends adjusting the bias every three months for new tubes as they undergo initial burn-in. After a year, once the tubes are fully broken in, bias adjustments can be performed every six months, coinciding with seasonal changes. This is because changes in temperature can affect electricity demand and household voltage, necessitating bias adjustments.
In-House Wound Output Transformers: No Simple Feat!
The listening session took place in my home, using the following equipment: Intel NUC Roon Core, Auralic Aries G1 streamer, Holo Audio Spring 3 DAC, Holo Audio Serene preamplifier, Aurum Cantus Grand Harmony speakers, and NuPrime EVO One as a reference power amplifier.
The KT150 resided in my system for an extended period, during which I primarily used it for music listening, seldom reverting to my Class D EVO One. The KT150 possesses the characteristic transparency, crystalline clarity, and vibrant tonality of tube amplifiers. Its sound is clear yet remarkably smooth, making listening lively and comfortable, compelling me to keep playing music.
Taking “Tutti!”, a renowned audiophile symphonic recording, as an example, track 6, Mozart’s Piano Concerto No. 21, “Andante,” the dense texture of the opening string ensemble was highly satisfying. The lines were clear, and the smoothness was just right. The sheen of the strings was particularly impressive – beautiful, natural, and accurate. The overall body was excellent, the sound full and the tonal balance superb. The subsequent piano timbre was sufficiently lush, crystalline, and transparent, fully showcasing the merits of the RR recording, making for a thoroughly enjoyable listening experience.
Furthermore, the soundstage’s vertical and horizontal dimensions, as well as the front-to-back layering of the orchestra, were rendered with excellent precision. The extension at both frequency extremes was effortless and natural, unequivocally demonstrating the exceptional quality of Mr. Lien’s in-house wound output transformers. Switching back to track 1, Rimsky-Korsakov’s “Dance Of The Tumblers,” a favorite among audiophiles for testing system transients and dynamics. Through the KT150, the sense of impact was not as immediate and rapid as some amplifiers; the pace was slightly slower and more relaxed, yet still effortless and unconstrained. The overall structure, layering, and separation were remarkably good, and the bass drum’s deep extension felt remarkably effortless and natural. This demonstrates the KT150’s impressive driving power and control. Who knew 100 watts could be so effective? Listening to RR’s symphonic pieces through the KT150 was both solid and airy, the sound was alive, truly excellent! If I were to point out a minor drawback, it would be a slightly slower sense of pace, but this is only in comparison to Class D amplifiers.
Clean, Clear Sound with Rich Timbre
Next, I listened to piano music to further assess the KT150’s tonal characteristics. I selected the 2017 Hong Kong High-End Audio Visual Show commemorative CD, featuring several piano solo pieces from Steinway Records. Driven by the KT150, the advantages of the Aurum Cantus speakers, with their air-motion tweeter and ribbon super tweeter, immediately became apparent. The high frequencies were ethereal, making the lingering resonance accompanying the piano’s high notes particularly distinct. The timbre was not only pure but also possessed a pleasing liquidity.
The KT150’s sonic character leans towards smoothness and airiness, rather than heaviness. The brilliance is excellent, with a touch of warmth, akin to “gentle sunlight in fresh air.” The sonic background is clean and clear, effortlessly revealing the textures and nuances inherent in the recording. The tonality is remarkably neutral, faithfully reflecting the characteristics of the source components, making it difficult to pinpoint any specific sonic bias. Consequently, it’s not easily used for system voicing or achieving a particular tonal color. The soundstage exhibits excellent openness, and the piano’s image and scale are appropriately rendered. Even at high volumes or during fortissimo passages, it maintains ample headroom, avoiding any sense of strain. In simple terms, it sounds natural! This kind of sound, rich and engaging, is something I honestly haven’t encountered in amplifiers at this price point.
Effortlessly Rendering Passionate Passages
Moving on to “Some Other Time,” an album featuring cellist Zuill Bailey and pianist Lara Downes performing works by several American composers. Despite its simple instrumentation of cello and piano, this recording boasts considerable dynamic range (a dynamic range of 15 as indicated by Roon). Several tracks exhibit significant dynamic shifts and passionate intensity.
The KT150 possesses a remarkable smoothness, rendering strings with exceptional gentleness and fluid lines. The sheen and texture are excellent. Yet, through the KT150, I could fully appreciate the passion and intense emotional fluctuations of the music and performance. Even subtle nuances were clearly presented, such as the vibrato at the end of notes and the minute contrasts in softer passages, all vividly rendered.
Perfect for Vocal Music
How does the KT150 perform with vocals? I played Jacky Cheung’s Cantonese album “Private Corner,” where he explores jazz styles, showcasing his unrestrained vocal artistry. Track 1, “Mini,” is a ballad. The piano intro sounded bright, crisp, and refreshing, instantly captivating. The piano revealed excellent high-frequency extension, and the bass demonstrated appropriate low-frequency presence, with sufficient body and clear articulation, exhibiting both agility. In essence, the performance from high to low frequencies was consistently high-quality, with outstanding openness.
Jacky Cheung’s vocals here are not particularly thick, as the KT150 is not inherently a thick-sounding amplifier. Instead, it is neutral, linear, and balanced, faithfully reflecting the system’s characteristics. The key is that the sound is exceptionally agile and lively, with a pleasing smoothness, making listening both relaxed and comfortable, but definitely not muddy, slow, or overly lush.
Through the KT150, I not only heard the rich tonal qualities of Jacky Cheung’s voice but also his exceptional vocal technique. I could also appreciate his unrestrained vocal delivery, his effortless control, and his vivid dynamic expression, all imbued with infectious musicality. Indeed, it was the infectiousness that compelled me to listen to song after song, including tracks by Penny Tai, Sandy Lam, Tsai Chin, Chen Jiannian, Jonathan Lee, Tom Waits, Jennifer Warnes, and others. Through the KT150’s rich expressiveness and excellent detail retrieval, coupled with its focused and palpable imaging, I could clearly discern and meticulously appreciate the unique vocal signatures of each singer. Each vocalist possessed a distinct timbre, texture, and vocal style, along with unique phrasing and enunciation. The KT150 naturally presented the distinctiveness of each singer, making the listening experience particularly engaging.
The KT150 delivers such performance at such a price point, it is more than just a bargain.
Definitely Capable of Challenging High-End Imports
During the listening session, I also compared the KT150 with the EVO One Class D power amplifier. The EVO One boasts significantly higher power output and a denser, more solid, and grounded sound. It exhibits superior control in large-scale orchestral pieces and at high volumes. However, when it comes to sonic elegance and brilliance, and the liveliness of musical expression, the KT150 takes the lead. This is the advantage and allure of tube amplifiers, and the KT150 exemplifies it beautifully.
Regarding power, 100 watts is already highly practical, effortlessly driving all bookshelf speakers and most small to medium-sized floor-standing speakers. If 100 watts is excessive, Ray Audio also offers the KT120 power amplifier, delivering 70 watts, which is also highly practical.
As for the price, unless you venture into significantly more expensive imported amplifiers, I believe the KT150 is unrivaled in its price range, offering exceptional value for money. It even possesses the capability to challenge amplifiers in higher price brackets. Your best course of action is to audition it as soon as possible; early purchase, early enjoyment, that’s the right decision.
The KT150 will be showcased at the upcoming TAA High-End Audio Show, located in room 331 of the hotel. It will be paired with the Swiss STENHEIM Alumine Five speakers, a system in the million-dollar range, providing an excellent opportunity to experience the KT150 firsthand.
Specifications
* Type: Vacuum Tube Power Amplifier
* Power Output: 100 watts per channel into 8Ω
* Tubes: KT150 × 4, 6SN7 × 2, 12AT7 × 2
* Inputs: XLR × 1
* Outputs: 8Ω / 4Ω
* Frequency Response: 10 Hz – 50 kHz
* Dimensions: 405 × 220 × 395 mm (W×H×D)
* Weight: 25.8 kg
