Music Reference RM-200 power amplifier Measurements

Sidebar 3: Measurements

One amplifier, three separate transformer outputs, balanced or unbalanced drive, and two choices for output tubes—to measure every combination offered by the Music Reference RM-200 was not practicable, so I used balanced drive and what I assumed were the Chinese output tubes MF preferred (they were only identified as "RAM Labs" tubes).

The amplifier was non-inverting, with pin 2 of the XLR jack wired as positive. Its single-ended input impedance measured 15.4k ohms at 1kHz, this rising slightly at the edges of the audioband and doubling when driven balanced. The voltage gain into 8 ohms was a normal 27.75dB from the 8 ohm output transformer tap. This dropped to 24.8dB from the 4 ohm tap, but to a low 19.2dB from the 1 ohm tap. However, the 1.24V required to drive the RM-200 to clipping from this tap is still well within the output capability of any normal preamplifier.

The tradeoff for the low gain from this tap is a usefully lower output impedance than is usual for a tube design. The RM-200's 1 ohm output offers a source impedance of 0.35 ohm over most of the audioband, rising slightly at very low and very high frequencies. As a result, any modification of the amplifier's response due to the interaction between the amplifier's source impedance and the manner in which the loudspeaker's impedance changes with frequency is small. The output impedance from the Music Reference's 4 ohm tap is 0.75 ohm in the midrange, rising to 1 ohm at 20kHz. Even so, the response modification with our simulated loudspeaker load is still a fairly modest ±0.55dB (fig.1). The 8 ohm tap has a much higher source impedance, at 1.25-1.6 ohms, resulting in ±1dB response variations.

Fig.1 Music Reference RM-200, 4 ohm tap, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (right channel dashed, 0.5dB/vertical div.).

The RM-200's frequency response is flat within the audioband, and down by just 0.2dB at 20kHz. Despite the steeper-than-usual ultrasonic rolloff in fig.1, the shape of a 1kHz squarewave (fig.2) is excellent. However, note the slight overshoot on the wave's leading edges in this graph. Though this can also be seen in the amplifier's reproduction of a 10kHz squarewave (fig.3), it appears to be critically damped, which will count against any potential stability problems.

Fig.2 Music Reference RM-200, 8 ohm tap, small-signal 1kHz squarewave into 8 ohms.

Fig.3 Music Reference RM-200, 8 ohm tap, small-signal 10kHz squarewave into 8 ohms.

Overshoot on squarewaves is often associated with an ultrasonic frequency-response peak. I couldn't find any such peak with the RM-200's output from its 8 and 4 ohm taps. However, the expanded-range graph of the 1 ohm response (fig.4) shows a slight peak in the right channel's output at 180kHz, and the implication of a left-channel peak above the 200kHz limit of the Audio Precision System One measurement set. As implied by the squarewave response, these peaks are well-suppressed.

Fig.4 Music Reference RM-200, 1 ohm tap, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (right channel dashed, 5dB/vertical div.).

The Music Reference amplifier was very quiet, with an A-weighted signal/noise ratio of 95.4dB ref. 1W into 8 ohms from the 8 ohm tap. Even the wideband, unweighted S/N ratio was a still excellent 87.8dB. Channel separation was better than 90dB below 1.5kHz, but above that frequency decreased (at the usual 6dB/octave rate) to a still good 62dB (L-R) and 65dB (R-L) at 20kHz (fig.5).

Fig.5 Music Reference RM-200, channel separation (R-L dashed, 10dB/vertical div.).

Figs. 6, 7, and 8 show how the RM-200's small-signal THD+noise percentage varies with frequency from the 8, 4, and 1 ohm output taps, respectively. Three points can be noted from these graphs:
• When its output transformer is matched to or less than the load, the RM-200 offers superbly low distortion from the upper bass through the midrange to the mid-treble.
• With lower-impedance loads, the distortion rises significantly with the degree of mismatch between the output tap and the load.
• There is a modest rise in THD in the low bass, presumably due to the transformer core starting to saturate. At the other end of the spectrum, however, the distortion rises to more than 1%. Yes, this is more than an octave above the audioband, and the amplifier's response is rolling off rapidly in this region, but I still would have preferred not to see this.

Fig.6 Music Reference RM-200, 8 ohm tap, THD+N (%) vs frequency at 2.83V into (from bottom to top at 1kHz): simulated loudspeaker load, 8 ohms, 4 ohms, 2 ohms (right channel dashed, 5dB/vertical div.).

Fig.7 Music Reference RM-200, 4 ohm tap, THD+N (%) vs frequency at 2.83V into (from bottom to top at 1kHz): simulated loudspeaker load, 8 ohms, 4 ohms, 2 ohms (right channel dashed, 5dB/vertical div.).

Fig.8 Music Reference RM-200, 1 ohm tap, THD+N (%) vs frequency at 2.83V into (from bottom to top at 1kHz): simulated loudspeaker load, 8 ohms, 4 ohms, 2 ohms (right channel dashed, 5dB/vertical div.).

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