Marantz PM-KI-Pearl integrated amplifier Measurements
I measured the Marantz PM-KI-Pearl with Stereophile's loan sample of the Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com); for some tests, I also used my vintage Audio Precision System One Dual Domain.
Before I test an amplifier, I run it for 60 minutes at one-third its specified power into 8 ohms. Thermally, this is the worst case for an amplifier with a class-B or -AB output stage. At the end of this period, the Marantz's case was warm to the touch, but the grilles above its internal heatsink weren't so hot that I couldn't keep my hands on them, implying a temperature around 130°F (55°C). The amplifier's heatsinking is clearly adequate for its output power.
Assessed at the Pearl's Tape Out jacks, its phono input offered 38.3dB of gain set to MM (moving magnet) and 56.75dB set to MC (moving coil), though the latter figure will have been decreased a little by the Ohm's Law interaction between the generator's 20 ohm source impedance and the 101 ohm measured input impedance of the MC input. Set to MM, the input impedance was 48k ohms at all frequencies. The phono input preserved absolute polarity (ie, was non-inverting) in both MM and MC modes. The unweighted, audioband signal/noise ratio was excellent in both modes, at 70.4dB (MM, ref. 1kHz input at 5mV) and 53.9dB (MC, ref. 1kHz at 500µV), these figures improving, when A-weighted, to 81.1dB and 63.7dB, respectively. Channel separation (not shown) was superb, at almost 90dB over most of the audioband.
The KI-Pearl's RIAA error is shown in fig.1; the departure from perfect equalization is vanishingly low in the left channel (blue trace), but reaches a maximum of +0.4dB in the midbass in the right channel (red). Both channels start to roll out below 30Hz, reaching 2dB just below 10Hz; at the other end of the spectrum, the response above 16kHz appears to follow the so-called Neumann "fourth pole" modification (see Keith Howard's explanation of why this is unnecessary).
Fig.1 Marantz PM-KI-Pearl, RIAA error of phono input (left channel blue, right red; 0.5dB/vertical div.).
The Pearl's phono input offered very low distortion at normal signal levels, around 0.03% THD+noise, but also featured superb overload margins. The MM mode didn't reach 1% THD+N until nearly 30dB at all audio frequencies above the standard 1kHz at 5mV, while the MC mode exceeded even that, at 32dB ref. 500µV at 1kHz at all frequencies. To put this margin into perspective, the output at the Pearl's Tape Out jacks was more than 12V when the phono input reached its overload limit!
Turning to the CD input, which MF found gave the best sound from the KI-Pearl, this offered a maximum gain of 41.1dB measured at the speaker terminals, and 17.1dB measured at the Preamp Out jacks, both of which are typical for an integrated amplifier. Neither output inverted signal polarity, and the input impedance was a moderately high 20k ohms. The output impedance from the Preamp Out jacks was 217 ohms at high and middle frequencies, rising to 495 ohms at 20Hz. From the speaker terminals, the output impedance was very low, at <0.07 ohms. The modification of the amplifier's frequency response by the interaction between this impedance and that of our standard simulated loudspeaker was thus also very low (fig.2, gray trace).
Fig.2 Marantz PM-KI-Pearl, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green). (0.25dB/vertical div.)
Fig.2 also shows that the Pearl offers a wide bandwidth into higher impedances, which ties in with the excellent 10kHz squarewave response with short risetimes and flat tops (fig.3). The output does start to roll off above 16kHz into 2 ohms, however (fig.2, green trace); in addition, this graph was taken with the volume control set to 12dB. At the control's maximum setting, the Pearl's output into 8 ohms (not shown) was 3dB at 80kHz rather than the >200kHz seen in fig.2. The channel balance was excellent at all volume-control settings, however, and the control operated in accurate 0.5dB steps. When switched into circuit, the tone controls offered maximum swings of ±10dB at 50Hz and 20kHz (fig.4). Channel separation (not shown) was good rather than great at 80dB at 1kHz, while the unweighted, wideband S/N ratio, ref. 2.83V into 8 ohms and taken with the input shorted but the volume control at its maximum, was 57dB. The latter improved to 71.4dB when A-weighted.
Fig.3 Marantz PM-KI-Pearl, small-signal 10kHz squarewave into 8 ohms.
Fig.4 Marantz PM-KI-Pearl, frequency response at 1V into 8 ohms with treble and bass controls set to maximum and minimum positions (left channel blue, right red; 1dB/vertical div.).
Specified at 90Wpc into 8 ohms (19.5dBW) and 140Wpc into 4 ohms (18.5dBW), the KI-Pearl actually clipped at 103Wpc into 8 ohms (20.1dBW) and 160Wpc into 4 ohms (19dBW). This can be seen from fig.5, which plots the THD+N percentage in the amplifier's output against output power. The actual distortion is buried beneath the noise below a few tens of watts, so I plotted how the THD+N percentage varied with frequency at a relatively high level, 10V RMS, equivalent to 12.5W into 8 ohms. The results are shown in fig.6; the distortion is very low in the midrange but rises at very low frequencies, perhaps due to the influence of noise, and both at high frequencies and into low impedances. The left channel (blue and cyan traces) is less good in these respects than the right (red and magenta traces). However, unlike our first sample of Marantz's inexpensive PM5003 integrated amplifier (see Follow-Up in this issue), this behavior was not affected when the Pearl's tone controls were in circuit.
Fig.5 Marantz PM-KI-Pearl, distortion (%) vs 1kHz continuous output power into (from bottom to top): 8, 4 ohms.
Fig.6 Marantz PM-KI-Pearl, THD+N (%) vs frequency at 10V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).
At moderate levels and into higher impedances, the KI-Pearl's distortion signature comprises mostly the subjectively innocuous second harmonic (fig.7). Though some higher-order spuriae appear at higher powers and into lower impedances, these are all still very low in level (fig.8). Intermodulation distortion was also very low, even at high power into 4 ohms (fig.9; ignore the shaping of the noise floor around the two primary tones at 19 and 20kHz in this graph, which is a measurement artifact).
Fig.7 Marantz PM-KI-Pearl, 1kHz waveform at 13W into 8 ohms (top), 0.0064% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Marantz PM-KI-Pearl, spectrum of 1kHz sinewave, DC10kHz, at 72W into 4 ohms (left channel blue, right red; linear frequency scale).
Fig.9 Marantz PM-KI-Pearl, tone controls in-circuit but set to flat, HF intermodulation spectrum, DC24kHz, 19+20kHz at 106W peak into 4 ohms (linear frequency scale).
The Marantz PM-KI-Pearl's excellent measured performance is a fitting tribute to Ken Ishiwata, its progenitor and dedicatee.John Atkinson