ModWright KWA 150 Stereo Power Amplifier (TAS 199)

Equipment report
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Solid-state power amplifiers
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ModWright Instruments KWA 150
ModWright KWA 150 Stereo Power Amplifier (TAS 199)

For someone who has lived through a decade when tube amplifiers were considered by many to have been obsoleted by transistor designs it was delicious irony to witness the renaissance of tube amplification in the 1980s. And just as satisfying is the fairly recent trend of judging solid-state designs on the basis of how closely they emulate the twin tube virtues of midrange magic and 3-D soundstaging. I’m basically a tube guy, but I recognize that solid-state (SS) amps bring the sort of current drive to the table necessary to satisfy the appetite of, for example, low-impedance planar-magnetic/ribbon speakers. Reports that, at least in its high-bias setting, the KWA 150 sounds very tube-like were sufficiently intriguing for me to take the plunge.

ModWright opened its doors for business in 2000 and is known for its tube linestages and phonostages. The KWA 150 represents its first pure solid-state design as well as its first amplifier design. For this project, Dan Wright, who happens to be mechanical engineer by degree and work experience, teamed up in 2007 with designer Alan Kimmel. This is the fellow who popularized the Mu-follower driver stage in DIY circles a few years ago. Dan explains that he sat down with Alan to draw up design concepts and physical layout. The choice of output devices and design criteria such as dual-mono layout and fully-balanced topology were all critical. The design goals were low noise, zero global feedback, direct-coupling of the signal path, low distortion, and the musicality of tube amplifiers combined with the power and strengths of SS design. For the record, a hybrid design was considered and rejected simply because of the added complexity it would have introduced.

The input is transformer-coupled using Lundahl iron. Both balanced (XLR) and unbalanced (RCA) inputs are accepted. However, Pin 1 (shield ground) of the XLR connector is unused and Pin 2 is assumed to be hot, which is in fact the most common convention. The voltage gain stage is said to be the heart of the design and has been dubbed the “Solid-State Music Stage” by Alan Kimmel. It is a proprietary cascade circuit using both bipolar and FET transistors and which, I’m told, took considerable research and development time to evolve and refine.

The output stage uses six pairs of ON Semiconductor’s highest-grade ThermalTrak bipolar power transistors per channel. These devices incorporate a thermally matched bias diode which ensures almost instant thermal bias-tracking and excellent thermal integrity. No global feedback is used. In high-bias setting, the first few watts are pure Class A with a smooth transition to Class AB. In low-bias setting (via a switch on the back panel), the amp operates strictly in Class AB. Sonically, the preferred setting is high bias, though the amp runs quite hot on this setting. However, even after the amp had been on for an hour I could touch the heat sinks without discomfort for a couple of seconds. The amp may be bridged for mono operation. This raises power output from 250 to 650 watts into a 4-ohm load.

Oh, I see that I have yet to mention the elegant and exquisitely milled aluminum chassis—as they say in Italy, Completamente moderno e bellissimo.” Push in the front logo to power on the amplifier. A soft-start circuit provides for a smooth turn-on devoid of rapid in-rush current. In addition, the amp protects itself and associated equipment from DC at input, DC at output, rail-fuse failure, shorted binding posts, over-current past clipping, and over-temperature. It should be noted that protection circuits for output short circuit and overload conditions operate outside the signal path. The speaker binding posts are my favorites, best-quality Cardas—no gold-plating (thank goodness for that) and no insulation. These hex-shaped posts allow me the pleasure of cranking down spade lug connections with a 7/16" nut driver.

Count on an extended break-in period. Dan Wright mentioned something like 200 hours, and he’s not kidding. The amp sounds good out of the box but continues to improve sonically over time. Also, it takes 30 minutes of warm-up for the amp to sound its best. I should mention that ModWright’s own LS 36.5 tube line preamplifier was used for many of the listening sessions. Dan Wright feels that this is a synergetic coupling and I agree. In fact, I had been advocating the use of tube linestages with solid-state power amps for many years. And it turns out that I’m not the only one. Spectron’s John Ulrick surprised me during a recent conversation by revealing that he also recommends at least a single tube in the signal path in conjunction with any solid-state amp. Why? Well, it would be foolish sonically not to do so.

It was not going to be a walk in the park for the KWA 150. I was determined to test it in the context of two challenging loads. First in line was the Final Sound 1000i electrostatic. This is a capacitive load with an impedance swing of about a factor of 30 from bass to treble. Many amplifiers struggle with such a load. An etched and bright treble range is often the first sign of trouble. Not so with the KWA 150. It evinced supreme transient control with absolutely no high-frequency ringing. This resulted is a slightly softer sound relative to other solid-state amps. Midrange textures came through as smooth, suave, and richly layered in the lower mids. The upper mids were also portrayed convincingly without a trace of the textural hardness and dryness endemic to most 1970s solid-state amps (e.g., Dyna 400). In particular, female vocals sang sweetly and were tightly fleshed out within the confines of a wide soundstage. The tonal balance was not quite neutral, being slightly weighted toward the lower mids. Sterility in audio terms has been defined by some as lack of distortion and coloration. That rather sounds flattering and is not at all aligned with my interpretation of the term. Think whiteness, bleached out harmonic colors, and suppressed microdynamics. Sterility in my book equals boring, uninvolving sound. The polar opposite applies to the KWA 150. It was clearly facile in humanizing the music, in reproducing its rhythmic drive and verve, and never interjecting any electronic haze into the mix. So far so good, so it was on to the next speaker load.

The Analysis Audio Omega is a planar-magnetic/ribbon speaker with a nominal impedance rating of 5 ohms, though it dips to around 3 ohms or so in the lower midrange. It requires an amp with substantial current drive in order to scale the dynamic spectrum. The KWA 150 did exceedingly well in this context, faithfully delivering an orchestral crescendo without power compression. Only rarely was I able to clip the KWA 150, and that was at fairly loud playback levels. Bass reach and pitch definition were excellent. The low output impedance guarantees minimal load interaction and an excellent damping factor. Jazz bass lines were negotiated without any confusion and exhibited a satisfying tonal weight. I was able to confirm the sonic superiority of the high-bias setting. Switching over to low bias caused harmonic colors to wilt. Textures were not nearly as rich and the sense of immediacy, of being there, was reduced as well.

The ModWright seemed to bring out the best in the Omega, including a remarkable sense of timing, plenty of spatial conviction, and admirable midrange suaveness. There was plenty of low-level detail in evidence, and while I didn’t feel that I was missing anything essential, I nonetheless suspected that it was not a resolution champ. To test that theory, I enlisted the help of the LAMM Industries M1.2 Reference Monoblocks. Yes, the LAMM is about four times more expensive, but that’s not why this is a bit of an unfair comparison. A monoblock version of any given design would most likely exhibit enhanced channel separation and reduced intermodulation distortion, so I should add the caveat that in bridged mono it’s quite possible that the KWA 150 would have performed as well as the LAMM in this area. However, with only one stereo amplifier on hand bridged mono was not an option. It became obvious, on something as simple as vocal overdubs, that the LAMM monoblocks were more resolving, and in general more revealing of a recording’s origins. The LAMM evinced a greater sense of speed and superior treble precision. By contrast the ModWright sounded warmer in the midrange, a bit more euphonic if you will, and more adept at erecting a believable soundstage. It struck me that the sonic differences between these two amps define the on-going debate between solid-state and tube amplification, with the KWA 150 sounding distinctly more tube-like.

My stance to date can be summarized as follows: No rain, no rainbow; no tubes, no tube magic. In view of the KWA 150, I’m willing to modify my position somewhat. It doesn’t quite fool me into mistaking it for a tube amp. But I can’t believe that I’m listening to a bipolar solid-state amp with suave mids and imaging magic. It’s hard for me to curb my enthusiasm in the presence of a sonic miracle. To my ears, the KWA 150 represents a smashing success, combining the musicality of tubes with the punch, power delivery, and bass reach of transistors. Factor in the performance and looks of this amp relative to its asking price and the result is a fantastic value. Kudos to ModWright for delivering the sonic goodies, while avoiding sticker shock. Buyer beware: Once auditioned, this amp is hard to walk away from.

SPECS & PRICING

ModWright KWA 150 Stereo Power Amplifier

Power output: 150W @ 8 ohm (stereo); 250W @ 4 ohm (stereo); 450W @ 8 ohm (bridged mono); 650W @ 4 ohm (bridged mono)
Bandwidth: 10Hz–100kHz (+0, -1dB)
Distortion: 0.06% THD @ 150W into 8 ohms; 0.06% THD @ 250W into 4 ohms
Channel Separation: -75dB below 20kHz
Gain: 26dB
Signal to Noise Ratio: -100dB unweighted
Input Impedance: Minimum of 15k at 50Hz; 23k at 1kHz
Output Impedance: 0.076 ohm
Damping Factor: 105 into an 8-ohm load (averaged across measurements at 100Hz, 1kHz, and 10kHz)
Dimensions: 17" x 17" x 8.5"
Weight: 74 lbs.
Price: $5995

ModWright Instruments, Inc.
21919 NE 399th Street
Amboy, WA 98601
(360) 247-6688
modwright@yahoo.com
www.modwright.com

Associated Equipment

Kuzma Reference turntable; Kuzma Stogi Reference 313 VTA tonearm; Shelter Harmony, Dynavector XV-1s, and Symphonic Line RG-8 Gold MC phono cartridges; Air Tight ATE-2 phonostage; Weiss Engineering Jason transport and Medea DAC, Concert Fidelity DAC-040 DAC, PrimaLuna ProLogue Eight CD player; Concert Fidelity CF-080 line preamplifier, Conrad-Johnson ET2 and ModWright LS 36.5 line preamplifiers; Bybee Speaker Bullets; FMS, Acrotec 6N and 8N copper, Kimber Select KS-1030, Kimber KCAG interconnects; FMS Nexus-2, Acrotec 6N and 8N copper, Kimber Select KS-1030, Kimber KCAG interconnects; FMS Nexus speaker cable