Music Reference RM-200 power amplifier Page 2
In describing his circuit to me, Modjeski advocated the use of low screen voltages and higher-than-usual plate voltages in the output stage. The RM-200 runs the screens and plates of its carefully matched set of four output tubes at 350V and 750V, respectively. The bias and transconductance of each tube is noted on its box; the owner is instructed to note these values in a grid on the last page of the manual, should the need to order replacement tubes arise.
Modjeski also made a point of telling me that he's worked to cure the problem of grid leakage, or "gas," as some companies call it. He claims that tubes no longer usable in other brands because of grid leakage (up to 1000µA) will work fine in his amps, with no drift in bias or loss of performance—which is one reason he claims far longer tube life than many other manufacturers.
The RM-200 is fully balanced from input to output. About 10dB of feedback is returned to the input via four Roederstein resistors, these arranged in what Modjeski calls a "current summing" circuit. This is said to ignore the nonlinearity of the input amplifying device, relying instead on the linearity of the four resistors. "Additionally, there is high-frequency feedback taken from the plates through small handmade Teflon capacitors that are adjusted in value for each output transformer to give total stability for all reactive loads. This amplifier will not oscillate, even into the small cable capacitance of 0.01µF that is often encountered in audiophile cables."
The details of the power supply's design could eat up a few more pages; all I'll say here is that Modjeski recommends against line conditioners—he claims his designs are "line-voltage tolerant" and do not need protection against voltage spikes. "Save your money and buy music," he says. He also told me that "There are no regulators to ever go wrong, as there are none in the amp. I don't need them and neither do you!" There are plenty of fuses, though: seven, including one for each output tube, one for the mains line by the IEC jack, and one each to protect the lower-current screen winding and driver-supply winding of the power transformer.
Setup and Use
Connections to the top-mounted jacks and terminals were fast and easy. A pair of supplied XLR-to-RCA adapters permitted single-ended connection to my Hovland HP-100 preamplifier. Five rugged, solid-brass speaker terminals per channel offer 1, 4, and 8 ohm connections in a somewhat unorthodox but well-labeled configuration. A bias-adjustment point for each tube is located next to the terminals. To check bias, insert a voltmeter pin into the point and touch the other pin to the 8 ohm tap closest to the test point. A single bias pot per channel, located toward the front of the chassis, adjusts both tubes simultaneously. I found the bias very close between each channel's two tubes, and very stable. The only time I had to adjust bias was when I switched from KT88s to the 6550s.
I placed the RM-200 on a Grand Prix Audio stand fitted with a Symposium Ultra isolation board, and disabled the amp's rubber feet with four sandwiched sets of Symposium Roller Blocks. On first powering up the RM-200, I noted mechanical hum emanating from its power transformer. Roger Modjeski, surprised by this, told me it was unusual, and that the RM-200 should be quiet, both physically and electrically. I'll take what I heard to be an aberration. In any case, it was never loud enough to be heard along with the music, even when I listened at very low volumes.
Live at Carnegie Hall
Since I'd just finished reviewing the Hovland Sapphire and found it lacking in the re-creation of the depth I knew to be encoded on some of my favorite LPs, I began my listening with a series of albums whose titles end in ...at Carnegie Hall, including Tony Bennett... (Columbia C2S 823), The Weavers' Reunion... (Analogue Productions APF 005), and Harry Belafonte... (RCA/Classic LSO-6006, 45rpm; RCA FTO-6000, open-reel).
The RM-200 restored the depth and air to these recordings, and thus a greater sense of actually sitting in Carnegie. You hardly need surround sound for that, though the two-channel perspective is a bit odd: the performers appear to face you bit with their backs to the audience. This has never been bothersome, though when you hear a really good multichannel recording—such as Chesky's Swing Live SACD, featuring Bucky Pizzarelli (SACD223, the March issue's "Recording of the Month")—the potential for something truly better becomes clear.
I've played that damn "Matilda" track from the Belafonte album hundreds of times since 1959 (gasp!). This time, I found that the RM-200 did an outstanding job of suggesting Carnegie Hall—width, depth, height, and air—while placing solid, convincing, individual images on the stage, well-delineated from front to back. Instrumental groupings on either side of the stage seemed unusually well-sized and focused in space, and with the clearest sensation of a great distance between them and the audience than I'd previously experienced. Belafonte himself was convincingly focused, without etch, well forward of the instrumentalists, and thus even farther from the audience; that, too, was effectively communicated. As he moved around the stage kibitzing with the musicians and the audience, his movements fore and aft were dramatically apparent.