Music Reference RM-200 Mk.II power amplifier
In his "Manufacturer's Comment" in response to my review of the original Music Reference RM-200 power amplifier in the April 2002 issue, designer Roger Modjeski admitted that being a manufacturer was not his first choice. "Frankly, I'd rather consult than produce," he claimed. "I'd rather be making a living doing stand-up comedy," I said to myself after reading his comment.
But a decade later here I still am, and Modjeski is still producing his own Music Reference brand of electronics and marketing computer-matched tubes sold under the RAM Tube namenot that both of us aren't also doing other things. His website is worth visiting.
The original RM-200's sonic performance, excellent measurements, and reasonable price made it an ideal reference tube amp. Through a decade's steady use in my listening room it has proven to be 100% reliable and sonically competitive with far more expensive amplifiers.
Mk.II: Similar but Different
The RM-200's basic physical and electrical architecture remain unchanged in the Mk.II edition. The fully balanced design features a high-power, bipolar, solid-state input stage and tubed driver and output stages.
Describing the circuit for me when I prepared the original review, Modjeski explained that the RM-200's input device produces no voltage gain; that's done by a pair of tightly matched high-voltage transistors cascaded above it in the circuit. He also claimed that careful matching of the input stage's devices, and a onetime factory adjustment of a trim pot to zero out resistor tolerances, result in high common-mode rejection and low-noise operation that is "close to the performance of the best input transformers." The RM-200's signal/noise ratio of 95.4dB (A-weighted), as measured by John Atkinson, backed up this assertion.
Though the RM-200 has only six tubestwo matched pairs of KT88 (standard) or 6550 (optional) output tubes, and a pair of 6BQ7 driversthe amp is claimed to deliver more than 100Wpc. (For more details about the RM-200's circuits, read my April 2002 review.)
The RM-200 Mk.II's compact, tray-like chassis is framed in wood. Atop this, a vented enclosure houses the power and output transformers, its brushed-satin finish matching that of the top plate and replacing the original edition's polished stainless steel. This enclosure provides some protection for the output tubes, but it's best to locate the RM-200 far from where dogs and children could easily get at it, or where something could fall on and shatter the tubes.
Easily accessible on the chassis' top surface, just behind the transformer enclosure, are, from left to right: two XLR balanced inputs, five brass output terminals per channel, a line fuse, and an IEC AC jack; under the 10 output terminals is a row of four bias test points, one for each output tube. The output-tube fuses are now in a more easily accessed location next to the tubes themselves. A fifth fuse protects the power transformer's screen and driver-supply windings.
The biggest difference between the original RM-200 and the Mk.II is a capacitor-forming function suggested by Richard Vandersteen, who uses batteries to "form" the crossover coupling caps in some of his loudspeaker models. Other than when you won't be using the RM-200 Mk.II for an extended period of time, the "forming" function should be left switched on. When it's turned on, the dielectric of the RM-200's capacitors "forms" at a voltage 10% higher than the normal operating voltage. Modjeski claims that this higher voltage "forms" the caps far faster than in conventional amps; even from a fully "off" state, the caps "form" in about 20 minutes. Because the tubes don't light up and the power draw is minimalabout 10Wthis mode generates very little heat. Compared to leaving your amps continuously on, which some audiophiles do, this feature greatly extends tube life, saves electricity, and is more environmentally responsible even if you don't care about the money.
Modjeski says the Mk.II uses better output transformers than the originalanother major upgrade. Add $1000 and he'll hand-wind even better ones for you. An optional bias balance control ($800) precisely balances the biases of each pair of output tubes, and lets you use unmatched pairs of tubes. (Otherwise, matched output pairs are required; the standard RM-200 Mk.II's single potentiometer per channel simultaneously biases both tubes.)
Modjeski claims that the RM-200's already low noise floor has been dropped another 6dB in the Mk.II, and that the new version's high-frequency distortion measures half as much. The driver stage and the power supply have also been updated.
Set It and Forget It?
In writing about the original RM-200, I described the reviewing of tube amps as akin to having one's pants pulled down in front of a large crowd of people. At least, that's how it feels when such a review is published in Stereophile, where a reviewer's descriptions of what he's heard are accompanied by measurements the reviewer doesn't see until after the article has been edited, fact-checked, and laid out.
When you salivate over the sound of a tube amp that turns out to measure poorly, you can look pretty small in the eyes of those who believe that measurements tell the whole story. If the RM-200 Mk.II's measurements are as good as or even better than the RM-200's (as Roger Modjeski promises they will be), I can look "big." In his measurements of the original RM-200, John Atkinson noted the differences in performance among the amp's 8, 4, and 1 ohm speaker terminals. (The Mk.II adds a 2 ohm tap.) JA noted that even though the voltage gain drops from 24.8 to 19.2dB between the 8 and 1 ohm taps, the 1.24V needed to drive the amp to clipping from the 1 ohm tap is well within the range of any normal preamplifier.
The advantage of the lowest-impedance tap is that the lower output impedance minimizes the effects on the amplifier's frequency response of the loudspeaker's impedance changes relative to frequency; the disadvantage is that this tap delivers the least power.