Wednesday, November 22, 2017

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All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Notes: Measurements of the unbalanced (UB) and balanced (B) inputs of both channels were made at the 120V AC line voltage, both channels driven. Since the Luxman M-900u is switchable from stereo to mono mode, measurements were made in both modes. (The suffix “B” indicates a mono-mode measurement.) Unless otherwise noted, the data reported below are for the balanced inputs and the left channel.

Power output (stereo mode)

  • Power output at 1% THD+N: 212.5W @ 8 ohms, 368.2W @ 4 ohms
  • Power output at 10% THD+N: 259.2W @ 8 ohms, 445.4W @ 4 ohms

Power output (mono mode)

  • Power output at 1% THD+N: 749.0W @ 8 ohms
  • Power output at 10% THD+N: 904.3W @ 8 ohms

Additional data

  • Input/output polarity
    •      Unbalanced inputs: noninverting
    •      Balanced inputs (switch set for Normal, pin-3 hot): inverting
  • AC-line current draw
    •      Operating: 276.0W, 3.02A, 0.74PF
    •      Standby: 1.4W, 0.03A, 0.38PF
  • Gain: output voltage divided by input voltage, 8-ohm load
    •      Stereo mode B inputs: 27.4X, 28.8dB
    •      Stereo mode UB inputs: 28.2X, 29.0dB
    •      Mono mode B input: 55.6X, 34.9dB
    •      Mono mode UB input: 55.0X, 34.8dB
  • Input sensitivity for 1W output into 8 ohms
    •      Stereo mode B inputs: 103.2mV
    •      Stereo mode UB inputs: 100.0mV
    •      Mono mode B input: 50.8mV
    •      Mono mode UB input: 51.4mV
  • Output impedance @ 50Hz
    •      Stereo mode: 0.012 ohm
    •      Mono mode: 0.024 ohm
  • Input impedance @ 1kHz
    •      Stereo mode B inputs: 34.0k ohms
    •      Stereo mode UB inputs: 52.0k ohms
    •      Mode mode B input: 34.0k ohms
    •      Mode mode UB input: 52.0k ohms
  • Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
    •      Wideband: 0.532mV/0.546mV, -74.5dBW/-74.3dBW
    •      A weighted: 0.164mV/0.166mV, -84.7dBW/-84.6dBW
  • Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch
    •      Wideband: 0.167mV/0.154mV, -84.6dBW/-85.3dBW
    •      A weighted: 0.043mV/0.040mV, -96.4dBW/-97.0dBW
  • Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms
    •      Wideband: 0.750mV, -71.3dBW
    •      A weighted: 0.231mV, -81.7dBW
  • Output noise (mono mode), 8-ohm load, unbalanced inputs terminated with 1k ohms
    •      Wideband: 0.656mV, -72.7dBW
    •      A weighted: 0.200mV, -83.0dBW

Measurements summary

Charts 1A and 1B show the frequency response of the Luxman M-900u with varying loads. Otherwise essentially identical to the stereo mode in the audioband, the mono response shows a bit more dependence on load above that range. The M-900u’s output regulation is so good that the IHF dummy load showed no significant variation in the audioband.

Chart 2A illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation (IM) test signals and amplifier output load for loads of 8 and 4 ohms. Chart 2B is for mono mode and a load of 8 ohms, as the M-900u is not specified for a mono load of 4 ohms.

THD+N as a function of frequency at several different power levels is plotted in Charts 3A and 3B. High-frequency rise with frequency is moderate, and the distortion is quite low over most of the power range in both stereo and mono modes.

The plot of damping factor vs. frequency (Chart 4A) is of a value and nature typical of many solid-state amplifiers: high up to about 1-2kHz, then rolling off with increasing frequency. The damping factor in mono mode (4B) is about half that in stereo mode, with the same curve shape. This is normal; the channels’ separate output impedances are in series with the load in mono or bridged mode.

The M-900u’s spectrum of harmonic distortion and noise residue of a 10W, 1kHz test signal is shown in Charts 5A and 5B. The AC line harmonics are relatively low in magnitude and simple in nature. The signal harmonics in stereo mode consist of a single second harmonic of low amplitude. In mono mode, the single second harmonic seems to have been canceled and does not show at all!

Chart 1 - Frequency response of output voltage as a function of output loading

Chart 1A

Chart 1A

Stereo mode
Blue line = open circuit
Red line = 8-ohm load
Magenta line = 4-ohm load

Chart 1B

Chart 1B

Mono mono
Blue line = open circuit
Red line = 8-ohm load
Magenta line = 4-ohm load

Chart 2 - Distortion as a function of power output and output loading

Chart 2A

Chart 2A

Stereo mode
(Line up at 100W to determine lines)
Top line = 8-ohm SMPTE IM distortion
Second line = 4-ohm SMPTE IM distortion
Third line = 8-ohm THD+N
Bottom line = 4-ohm THD+N

Chart 2B

Chart 2B

Mono mode
(Line up at 100W to determine lines)
Top line = 8-ohm SMPTE IM distortion
Second line = 8-ohm THD+N

Chart 3 - Distortion as a function of power output and frequency

Chart 3A

Chart 3A

Stereo mode
(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 30W
Cyan line = 120W
Green line = 150W

Chart 3B

chart3b

Mono mode
(8-ohm loading)
Red line = 1W
Magenta line = 20W
Blue line = 60W
Cyan line = 120W
Green line = 600W

Chart 4 - Damping factor as a function of frequency

Chart 4A

Chart 4A

Stereo mode
Damping factor = output impedance divided into 8

Chart 4B

Chart 4B

Mono mode
Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5A

Chart 5A

Stereo mode
1kHz signal at 10W into an 8-ohm load

Chart 5B

Chart 5B

Mono mode
1kHz signal at 10W into an 8-ohm load

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