Tube Power Amp Reviews

Reviewing a vacuum-tube power amplifier is like having your pants pulled down in front of a large crowd of people. I don't know how else to describe the feeling of spending a month or two luxuriating in fabulous sound, then writing a glowing review, then receiving a copy of the review as it will appear in the magazine, complete with John Atkinson's assessment of the amp's test-bench performance, which is usually miserable.

Based on my reviewing track record and my addiction to vinyl, it would not be unfair for some readers to conclude that my preferences are for noise, high levels of harmonic distortion, and roller-coaster-like frequency responses—the last due to the usually high source impedance at the terminals of most tube amplifiers, which causes response variations as a speaker's impedance varies with frequency. But it wouldn't be true.

Of course, my listening preferences have nothing to do with specs and everything to do with what sounds pleasing to my ears. I'm doing the listening, not the oscilloscope! I don't mean to downplay the importance of measurements—as JA writes in his "Measurements" Sidebar to my review of the Hovland Sapphire hybrid power amp, "Just because an amplifier uses tubes doesn't mean it also has to have hum, an ultrasonic resonance, or fairly high levels of distortion." But poor test-bench performance has seldom swayed me to dislike a product that sounds attractive to me and manages to "make music." Those clearly overpriced, underpowered, and overrated KR Enterprise amplifiers I reviewed a few years ago positively flopped on the test bench...but they sounded fabulous.

My primary job here is not to tell you what I like, of course, but what something sounds like in relation to live music and to other similar products. Over the past six months Stereophile has fed me four amplifiers: the Kora Cosmos, the Smart Devices 2X150VT, the Hovland Sapphire, and now the Music Reference RM-200. As I write this, I have no idea how the RM-200 will measure. But of the three amps already reviewed, the Smart Devices measured best and was the least expensive—and was the one I'd least prefer to listen to over the long haul. (Still, it sounds very good, and is an excellent value at $2250.)

An Industry Veteran's Latest
RAM Labs' Roger Modjeski is a familiar if perhaps somewhat shadowy figure to most veteran audiophiles. He's probably best known as one of the originators, if not the originator, of using a computer to match tubes for both bias and transconductance. Despite a well-deserved reputation for being one of the most talented and knowledgeable designers of tube amps in the business (no one I asked said anything else), Modjeski has kept a relatively low profile since founding RAM Labs in 1981, leaving others, such as David Manley, to make the big splashes.

Longtime Stereophile readers will remember a heated exchange of letters in 1991 between Modjeski and Manley over tubes and tube amplifier design. VAC's Kevin Hayes stepped in as peacemaker to write a well-reasoned letter (published in the June 1991 Stereophile). Modjeski is still naming names—in his instruction manual, he aims some serious barbs at products released by some very well-known and respected makers of tube gear.

Three of Modjeski's earlier designs—the Counterpoint SA-4, the now-discontinued Music Reference RM-9, and its replacement, the RM-9SE—are still held in high regard by audiophiles. Modjeski doesn't pump out new models every year or offer expensive upgrades to the ones he's already sold, so there's little in the way of news to attract magazine coverage. This might explain RAM's low profile. As Modjeski says in the RM-200's manual, "[Music Reference] products are designed and built with the thoughtfulness to allow reliable use for 100 years."

Introduced in 1998, the compact, easy-to-use, meticulously built RM-200 gives every indication of being such a long-lived product. With its attractive wooden frame, brushed-aluminum chassis, and transformer enclosure of polished stainless steel, the RM-200 is easy on the eyes. Its looks aren't quite as dramatic as the Hovland Sapphire's, but then, few amplifiers' looks are.

The RM-200's fully balanced hybrid design (full power is also available in single-ended use, which is how the amp was reviewed) uses a high-voltage bipolar transistor input stage and tube (6BQ7) driver and output stages. (Chinese KT88s are standard; Russian 6550s can be supplied for an additional $200.) The input device has no voltage gain. That's handled by a pair of high-voltage transistors cascoded above it in the circuit. Modjeski claims that the tight matching of input devices and a one-time trim-pot adjustment to zero out resistor tolerances result in very high common-mode rejection for the RM-200's low-noise operation—which, he says, is "close to the performance of the best input transformers."