Tube Power Amp Reviews

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com) to measure the RM-200 Mk.II. Before performing any tests, I checked that the output tubes were all correctly biased. However, the left channel's bias couldn't be optimized and the distortion in that channel was 100 times higher than in the right. I performed a number of tests on the correctly operating right channel while awaiting word from Music Reference's Roger Modjeski. It turned out that each output tube is protected by a 250mA slow-blow fuse; if a single fuse blows, while that channel will still pass audio, it will be with high distortion.

I checked the four fuses and, yes, one of them was open-circuit. Michael Fremer felt this must have happened at the end of his auditioning, when he inadvertently drove the amplifier with no load and one of the output tubes started to glow cherry red. I replaced the fuse with the nearest equivalent I had in my parts bin, a 500mA fast-blow type, reset that channel's bias, and continued the testing.

I used a balanced test signal exclusively, and carried out a full set of measurements from each of the RM-200 Mk.II's four output transformer taps (though I have reproduced in this sidebar only a limited set of graphs). The Music Reference's voltage gain into 8 ohms was 29.7dB from the 8 ohm tap, 26.5dB from the 4 ohm tap, 24.3dB from the 2 ohm tap, and 20.9dB from the 1 ohm tap. This is just what I would expect, as lowering the nominal value of the transformer tap increases its current capability at the expense of voltage gain. The amplifier preserved absolute polarity (ie, was non-inverting). The input impedance was to specification, at close to 29k ohms across the audioband.

The output impedance depended on both frequency and the output-transformer tap. The highest impedance was, as expected, from the 8 ohm tap, at 1.15 ohms at 209Hz, 1 ohm at 1kHz, and 1.4 ohms at 20kHz. This is fairly low for a classic tube amplifier, and, as can be seen from the gray trace in fig.1, this resulted in ±0.6dB variations in response with our standard simulated loudspeaker (see www.stereophile.com/reference/60/index.html), and an output that gently rolled off above the audioband. The impedance followed a similar pattern with frequency from the other taps, but was lower. At 1kHz, the RM-200 Mk.II's output impedance was 0.8 ohm from the 4 ohm tap, 0.44 ohm from the 2 ohm tap, and 0.29 ohm from the 1 ohm tap. The latter gave rise to small (±0.2dB) variations in response with our simulated loudspeaker (fig.2, gray trace), but now an ultrasonic resonance appears. This can also be seen by comparing the 10kHz squarewave response from the 8 ohm tap (fig.3) with that from the 1 ohm tap (fig.4). Though there is no overshoot from either tap, there are two cycles of well-damped ringing from the 1 ohm tap. The 1kHz squarewave from the 8 ohm tap was superb (fig.5), the flat top of the waveform indicating extended low-frequency response.

Though I have shown only the right channel's behavior in these graphs, the two channels matched quite well in response once I replaced the blown output-tube fuse, the main difference being a 0.3dB difference in gain. The two channels then also matched well with respect to noise, the unweighted, wideband signal/noise ratio with the input shorted measuring 77dB ref. 1W into 8 ohms from the 8 ohm tap. (This is a slightly smaller ratio than I measured for the original RM-200 and is equivalent to a voltage of 0.4mV. Music Reference's Roger Modjeski feels strongly that amplifier noise specification should be the absolute level of the noise rather than a ratio referenced to a possibly undefined or arbitrary level.) The spectral content of the noise was primarily a small amount of 60Hz hum; A-weighting increased the ratio to an excellent 87.7dB. Channel separation (not shown) was excellent, at 100dB in both directions in the midband, decreasing slightly to 90dB L–R and 78dB R–L at the top of the audioband.

Fig.6 plots the THD+noise percentage against output power from the 4 ohm transformer tap. I haven't shown the graphs for the other three taps, but the pattern there was very similar: the amplifier clips at its specified power of 100Wpc when the load impedance is equal to the nominal tap value; the lowest distortion, but also reduced maximum power, occur when the load impedance is much higher than the tap value; and the measurement below 2W or so is dominated by noise. The lowest level of distortion is offered by the 1 ohm tap, but at the expense of maximum power—this tap clips at just 20Wpc into 8 ohms (13dBW).

The highest distortion occurs from the 8 ohm tap: fig.7 plots the right channel's THD+N percentage at 3V into loads ranging from 2 ohms (red trace) to 16 ohms (green). Though the distortion rises at the frequency extremes, it is overall very low when the load impedance is equal to or higher than the nominal tap value. Fig.8 repeats the plotting from the 1 ohm tap. The midrange THD+N is one-fourth what it was from the 8 ohm tap, and the distortion into 2 ohms (red trace) is still superbly low in absolute terms. The rise in THD at low frequencies in both of these graphs is moderate for a tube amplifier, which is a tribute to the quality of Music Reference's output transformers.

At low powers, the spectral composition of the RM-200 Mk.II's distortion in the midrange is predominantly the subjectively benign second harmonic (fig.9), joined at higher powers by the third and fifth harmonics (fig.10). But even then, the distortion components are respectably low in absolute terms. However, a series of sidebands spaced at 60Hz can be seen around each spectral component in this graph. I suspect that these arise from magnetic interference from the AC transformer. Intermodulation distortion was also low, and lowest from the 8 ohm tap. Fig.11, for example, plots the spectrum of the RM-200 Mk.II's output from its 8 ohm tap while the amplifier drives an equal mix of 19 and 20kHz tones at 12W peak into 8 ohms. The 1kHz difference component lies at –70dB (0.03%) in both channels, with the higher-order components at 18 and 21kHz a little lower. While the difference component remained low in level from lower-impedance taps, more higher-order components were evident. And again, the main spectral components are accompanied by 60Hz-spaced sidebands.

As did the original version of the Music Reference RM-200 amplifier, the Mk.II edition offers superb measured performance for a tubed design, particularly in terms of midband distortion and the ability to drive low impedances. And I remain impressed by the quality of its output transformers.—John Atkinson