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.