Music Reference RM-200 power amplifier:
Measurements part 2
The audibility of any distortion depends on its order. Low-order harmonics are preferable because they're musically consonant—see my essay on this subject in the Stereophile Test CD 2 booklet—and more easily masked by musical fundamentals. Figs. 9 and 10 reveal that the RM-200's distortion is predominantly third-harmonic. Fig.9 was taken with a 1kHz sinewave at a low level (1W into 8 ohms from the 8 ohm tap), with just 0.011% THD. Fig.10 was taken about 3dB below clipping into 4 ohms from the 4 ohm tap, with a low-frequency tone. The third harmonic lies at -50dB (0.3%), with the second, fourth, and fifth harmonics all below -70dB (0.03%). The picture was similar with a 1kHz tone at 40W into 4 ohms (not shown), but halving the power to 20W dropped the higher harmonics to very low levels (fig.11).

Fig.9 Music Reference RM-200, 8 ohm tap, 1kHz waveform at 1W into 8 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.10 Music Reference RM-200, 4 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 39W into 4 ohms (linear frequency scale).

Fig.11 Music Reference RM-200, 4 ohm tap, spectrum of 1kHz sinewave, DC-1kHz, at 20W into 4 ohms (linear frequency scale).

Because of the amplifier's increased nonlinearity above the audioband, the high-level, high-frequency intermodulation plot (fig.12) featured reasonably high levels of high-order products; for example, -50dB (0.3%) at 21kHz. The 1kHz difference tone was at a respectable -74dB (0.002%), however.

Fig.12 Music Reference RM-200, 4 ohm tap, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 38W into 4 ohms (linear frequency scale).

Finally, figs. 13, 14, and 15 show how the Music Reference's THD+noise percentage changes with power level from each of the three transformer taps with both channels driven. With clipping defined as 1% THD, the amplifier does meet its 100Wpc specification from the 8 ohm tap into 8 ohms, giving 110W into this load (20.4dBW). However, the fairly "soft" clipping behavior means that the definition of clipping must be relaxed to 3% THD for the RM-200 to give its specified power from the 4 ohm tap (into 4 and 2 ohm loads). Nevertheless, the RAM still gives 95W into 4 ohms from the 4 ohm tap at 1% THD, which misses the specification by only a subjectively inconsequential 0.2dB.

Fig.13 Music Reference RM-200, 8 ohm tap, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

Fig.14 Music Reference RM-200, 4 ohm tap, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

Fig.15 Music Reference RM-200, 1 ohm tap, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.

Fig.15 confirms the Music Reference's excellent linearity from its 1 ohm output. However, because of the low turns ratio of this transformer tap, only moderate power is available at 1% THD+N: 62W into 8 ohms (11.9dBW), 39.5W into 4 ohms (13dBW), or 22.2W into 2 ohms (13.5dBW).

Although I was bothered by the Music Reference RM-200's poor linearity above the audioband, it struck me overall as a well-engineered amplifier. As long as it's matched with a loudspeaker whose impedance doesn't drop too far below the chosen transformer tap, its lack of noise and low levels of midband distortion are to be commended.—John Atkinson