Marantz SM-11S1 Reference power amplifier Measurements
As I usually do with power amplifiers, I preconditioned the Marantz SM-11S1 Reference by running both channels at one-third the maximum measured power into 8 ohms for one hour. Thermally, this maximally stresses an amplifier with a class-B or -AB output stage. At the end of that period, while the wire grilles set into the top panel were hot, the chassis was barely warm. The Marantz has adequate heatsinking for an amplifier of its rated power. In addition, the measured THD+noise percentage didn't change through the preconditioning, indicating a stable design topology. The LCD power meter on the front panel was quite accurate, indicating "48.8W" with the amplifier driving a measured 48.5Wpc into 8 ohms. The meter also takes impedance into account; halving the impedance from 8 to 4 ohms almost doubled the indicated power.
Though it can be bridged for mono operation, I tested the Marantz in stereo mode, which is how Michael Fremer listened to it. The SM-11S1's gain can be adjusted over a range of ±6dB. The amplifier was set to its factory default for the review, indicated on the meter as "Gain = 0"; in this condition, the voltage gain into 8 ohms was significantly lower than the norm, at 22.35dB from the balanced inputs and 22.7dB from the unbalanced inputs, both figures slightly lower than the specified 23.0dB. The unbalanced input preserved absolute polarity; the balanced input inverted polarity, however, confirming that the XLR jacks are wired with pin 3 rather than pin 2 hot (the latter is the AES convention). The unbalanced input impedance was a little lower than specified, at around 18k ohms across the audioband, dropping very slightly and inconsequentially to 17.2k ohms at 20kHz. The balanced input offered twice these values, as expected.
The SM-11S1's output impedance was very low, at 0.08 ohm at low and middle frequencies, rising slightly to 0.11 ohm at 20kHz. As a result, the modification of the amplifier's frequency response by the interaction between this impedance and that of the loudspeaker is below ±0.1dB (fig.1, red trace). The amplifier's output into 8 ohms is down by less than 2dB at 200kHz (fig.1, yellow and magenta traces), but the small-signal bandwidth does decrease slightly into lower impedances. Even so, in the worst case, into 2 ohms (fig.1, blue trace), the output at 20kHz is down by just 0.2dB. That this is indeed a wide bandwidth design is confirmed by the SM-11S1's excellent reproduction of 1kHz and 10kHz squarewaves (figs.2 and 3, respectively).
Fig.1 Marantz SM-11S1, frequency response at 2.83V into: simulated loudspeaker load (green), 8 ohms (red), 4 ohms (blue), 2 ohms (magenta). (1dB/vertical div.)
Fig.2 Marantz SM-11S1, small-signal 1kHz squarewave into 8 ohms.
Fig.3 Marantz SM-11S1, small-signal 10kHz squarewave into 8 ohms.
Channel separation (not shown) was excellent, at better than 90dB in both directions below 2kHz, though this decreased to 70dB at 20kHz. The A-weighted signal/noise ratio, ref. 1W into 8 ohms and measured with the input shorted, was to specification, at 101.6dB. It worsened to 63.8dB with a wideband, unweighted measurement, due to the presence of some low-level HF noise.
Specified as delivering 110Wpc into 8 ohms or 220Wpc into 4 ohms (both equivalent to 20.4dBW), the SM-11S1 easily exceeded those figures on the test bench, not clipping (defined as 1% THD+N) until 140Wpc into 8 ohms (21.5dBW) and 230Wpc into 4 ohms (20.6dBW). The manner in which the THD+N percentage changed with output power into these two loads is shown in fig.4; the distortion is extremely low at all levels below clipping, and actually lies beneath the noise floor below 10W. The amplifier went into protection with sustained delivery of more than 230Wpc into 4 ohms, so I didn't repeat this test into 2 ohms. Plotting how the THD+N figure changes with frequency (fig.5) revealed a large increase in measured THD into 2 ohms (cyan trace), suggesting that this amplifier is indeed uncomfortable driving loads of less than 4 ohms.
Fig.4 Marantz SM-11S1, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 4, 8 ohms.
Fig.5 Marantz SM-11S1, THD+N (%) vs frequency at 13.15V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right magenta), 2 ohms (cyan).
To be certain that I was looking at distortion rather than noise, I performed this test at the relatively high voltage of 13.15V, which is equivalent to 21.5W into 8 ohms. however, the Marantz offers very low levels of distortion: <0.002% in the midrange and bass into both 8 ohms (fig.5, blue and red traces) and 4 ohms (green and magenta traces). The distortion plots show the usual rise in the treble, due to the amplifier's finite open-loop bandwidth reducing the amount of corrective negative feedback at higher frequencies, but the level remains below 0.01%. The left channel was very slightly worse in this respect.
Into higher impedances, the content of that distortion is predominantly the benign second harmonic (figs. 6 and 7), though the third harmonic make a stronger appearance as the output current increases (fig.8). Even so, both the second and third harmonics lie at or below –100dB (0.001%), and though some higher-order harmonics can be seen, these are 10–20dB lower still. Despite the decrease in linearity seen above 5kHz in fig.5, the levels of intermodulation products produced with the amplifier driving the demanding 1:1 mix of 19 and 20kHz tones at a level just below visible waveform clipping on an oscilloscope were very low (fig.9). As expected from fig.5, however, the level of the 1kHz difference tone in the left channel (blue trace) was a little higher than in the right (red).
Fig.6 Marantz SM-11S1, 1kHz waveform at 48.5W into 8 ohms (top), 0.0103% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.7 Marantz SM-11S1, spectrum of 1kHz sinewave, DC–1kHz, at 63W into 8 ohms (linear frequency scale; left channel blue, right red).
Fig.8 Marantz SM-11S1, spectrum of 1kHz sinewave, DC–1kHz, at 123W into 4 ohms (linear frequency scale; left channel blue, right red).
Fig.9 Marantz SM-11S1, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 60W peak into 8 ohms (linear frequency scale; left channel blue, right red).
Overall, the Marantz SM-11S1 Reference offered a very clean bill of health on the test bench. It was a pleasure to measure an amplifier so competently designed.—John Atkinson