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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

All amplifier measurements are performed independently by Warkwyn Associates. 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 were made at 120V AC line voltage with both channels being driven (stereo mode). Measurements made on left channel through the balanced input for stereo and mono modes unless otherwise noted.

Power output (stereo mode)

  • Power output at 1% THD+N: 333.3W @ 8 ohms, 499.4W @ 4 ohms
  • Power output at 10% THD+N: 365.2W @ 8 ohms, 576.0W @ 4 ohms

Power output (mono mode)

  • Power output at 1% THD+N: 1200W @ 8 ohms, 1953W @ 4 ohms
  • Power output at 10% THD+N: 1453W @ 8 ohms, 2303W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 1.29A, 0.62PF, 98W
  • Gain at 1W at into 8 ohms at 1kHz, unbalanced and balanced inputs
    •      Stereo and mono modes: 39.9X, 32.0dB
  • Input sensitivity for 1W into 8 ohms, unbalanced and balanced inputs 
    •      Stereo and mono modes: 71.1mV
  • Output impedance @ 50Hz
    •      Stereo mode: 0.0093 ohm
    •      Mono mode: 0.0182 ohm
  • Input impedance @ 1kHz
    •      Stereo mode, unbalanced input: 8.58k ohms
    •      Stereo model, balanced input: 26.6k ohms
    •      Mono mode, unbalanced input: 5.33k ohms
    •      Mono mode, balanced input: 15.7k ohms
  • Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch
    •      Wideband: 0.133mV/0.142mV, -86.5dBW/-86.0dBW
    •      A weighted: 0.101mV/0.099mV, -88.9dBW/-89.1dBW
  • Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
    •      Wideband: 0.129mV/0.138mV, -86.8dBW/-86.2dBW
    •      A weighted: 0.095mV/0.093mV, -89.5dBW/-89.6dBW
  • Output noise (mono mode), 8-ohm load, unbalanced inputs terminated with 1k ohms
    •      Wideband: 0.164mV, -84.7dBW
    •      A weighted: 0.121mV, -87.4dBW
  • Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms
    •      Wideband: 0.160mV, -84.9dBW
    •      A weighted: 0.119mV, -87.5dBW

Measurements summary

All charts: A = stereo mode, B = mono mode.

Charts 1A and 1B show the frequency response of the Hegel H30 with different resistive loads. With loads of 4 and 8 ohms, there are small deviations in output response at 20kHz in both stereo and mono modes, but the dummy-speaker load shows very little deviation at 20kHz, which reflects real-world load conditions.

Charts 2A and 2B illustrate how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals. The results show very low and uniform distortion levels in both modes until clipping is reached.

Charts 3A and 3B show THD+N as a function of frequency at several different power levels into 4 ohms. The high output power achieved in stereo and mono modes with low THD+N is admirable. (Note that the decrease in distortion above 10kHz in these charts is a result of a 22kHz cutoff filter, which helps to improve the accuracy of this test below 10kHz.)

Charts 4A and 4B show the H30’s damping factor vs. frequency. The result in mono mode is about half that in stereo mode, but has a similar shape.

Charts 5A and 5B show the spectrum of harmonic distortion and noise residue for a 10W, 1kHz test signal.

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

Chart 1A

Chart 1A

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

Chart 1B

Chart 1B

Mono mode
Red line = open circuit
Cyan line = 8-ohm load
Blue line = 4-ohm load
Magenta line = dummy-speaker load

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

Chart 2A

Chart 2A

Stereo mode
(Line up at 20W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N

Chart 2B

Chart 2B

Mono mode
(Line up at 20W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N

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

Chart 3A

Chart 3A

Stereo mode, 22kHz cutoff filter
(4-ohm loading)
Cyan line = 2W
Green line = 20W
Blue line = 100W
Red line = 200W
Magenta line = 400W

Chart 3B

Chart 3B

Mono mode, 22kHz cutoff filter
(4-ohm loading)
Cyan line = 2W
Green line = 20W
Blue line = 600W
Red line = 1400W

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 a 4-ohm load

Chart 5B

Chart 5B

Mono mode
1kHz signal at 10W into a 4-ohm load

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.

Measurements of the Pathos Lògos were made at 120V AC line voltage, both channels driven, with the left channel measured at the balanced inputs, unless otherwise noted. Measurements were made using the old IHF integrated-amplifier standard, in which the volume control is set to a nominal 5W output into 8 ohms with a 500mV analog input.

Power output

  • Output power at 1% THD+N: 155.0W @ 8 ohms, 237.4W @ 4 ohms
  • Output power at 10% THD+N: 195.5W @ 8 ohms, 300.9W @ 4 ohms

Additional data

  • This amplifier inverts polarity.
  • AC-line current draw at idle: 1.41A, 0.76PF, 128.0W
  • Gain: output voltage divided by input voltage
    •      Unbalanced inputs: 76.4X, 37.7dB
    •      Balanced inputs:  36.5X, 31.1dB 
  • Input sensitivity for 1W output into 8 ohms
    •      Unbalanced inputs: 37.0mV
    •      Balanced inputs: 77.5mV
  • Output impedance @ 50Hz: 0.053 ohm
  • Input impedance @ 1kHz, unbalanced and balanced inputs: 30k ohms
  • Output noise, reference conditions, balanced inputs, termination 600 ohms, Lch/Rch
    •      Wideband: 2.02mV/2.35mV, -62.9dBW/-61.6dBW
    •      A weighted: 0.31mV/0.54mV, -79.2dBW/-74.4 dBW
  • Output noise, reference conditions, unbalanced inputs, termination 1k ohm, Lch/Rch
    •      Wideband: 0.81mV/1.5mV, -70.8dB/-95.5dBW
    •      A weighted: 0.21mV/0.21mV, -82.6dBW/-82.6dBW
  • Output noise, 8-ohm load, volume at maximum, balanced inputs, termination 600 ohms, Lch/Rch
    •      Wideband: 5.8mV/6.6mV, -53.7dBW/-52.6dBW
    •      A weighted: 0.92mV/1.5mV, -69.7dBW/-65.5dBW
  • Output noise, 8-ohm load, volume at maximum, unbalanced inputs, termination 1k ohm, Lch/Rch
    •      Wideband: 2.7mV/6.4mV, -60.4dBW/-52.9dBW
    •      A weighted: 0.73mV/0.81mV, -71.8dBW/-70.9dBW
  • Output noise, 8-ohm load, volume at minimum, balanced inputs, termination 600 ohms, Lch/Rch
    •      Wideband: 0.77dBW/0.80mV, -71.3dBW/-71.0dBW
    •      A weighted: 0.20mW/0.20mV, -83.0dBW/-83.0dBW
  • Output noise, 8-ohm load, volume at minimum, unbalanced inputs, termination 1k ohm, Lch/Rch
    •      Wideband: 0.76mV/0.79mV, -71.4dBW/-71.1dBW
    •      A weighted: 0.20mV/0.21mV, -83.0dBW/-82.6dBW

Measurements summary

The Pathos Lògos MKII is a medium-power stereo integrated amplifier with a vacuum-tube input stage. It can be ordered with a built-in DAC module, but the review sample lacked this option.

Chart 1 shows the Lògos’s frequency response with varying loads. The frequency response has a high-frequency bandwidth of about 199kHz, depending on the load. The output impedance is low in the audioband, and the variations due to the NHT dummy load did not appear and therefore are not shown here. Note the mild low-frequency rolloff beginning at about 50Hz. To test volume-control tracking and frequency response as functions of the volume setting, a measurement test (results not shown) was set up to establish a reference point. The volume was set to its maximum level at 5W output. The responses of both channels were then measured as the volume was reduced in 10dB increments. Volume tracking was excellent right down to -60dB, the response shape remaining constant at all levels -- something that is not always the case, especially at very low volume settings.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load into 8 and 4 ohms. Not usually seen, although low, the distortion of this amp is quite constant over a very wide power range.

The Lògos’s THD+N as a function of frequency at several different power levels is plotted in Chart 3. The amount of rise in high-frequency distortion is significant, especially with increasing power level.

The Lògos’s damping factor vs. frequency, shown in Chart 4, has a shape typical of most power amplifiers, except that the high-frequency rolloff starts quite a bit higher than the usual 0.5-1kHz.

A spectrum of the Lògos’s harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Chart 5. The magnitudes of the AC line harmonics are low in amplitude compared to the signal harmonics. The signal harmonics are reasonably low in amplitude, with odd harmonics dominating, and with a complex and decreasing series of odd and even harmonics.

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

Chart 1

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

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

Chart 2

(Line up at 30W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N

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

Chart 3

(4-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 30W
Cyan line = 70W
Green line = 110W

Chart 4 - Damping factor as a function of frequency

Chart 4

Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

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

All amplifier measurements are performed independently by Warkwyn Associates. 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.

Note: Measurements were made at 120V AC line voltage and through the balanced inputs with both channels driven unless otherwise noted. All measurements made with the Stanford Research Systems SR1 audio analyzer.

Power output

  • Output power at 1% THD+N: 90.8W @ 8 ohms, 135.8W @ 4 ohms

Additional data

  • This integrated amplifier does not invert polarity.
  • AC line current draw: 0.27A, 0.61PF, 20.0W
  • Input sensitivity for 1W output into 8 ohms, balanced and unbalanced inputs: 77.4mV
  • Input impedance @ 1kHz:
    • Unbalanced inputs: 27.3k ohms
    • Balanced inputs: 100.7k ohms
  • Output impedance at 50Hz: 0.022 ohms
  • Gain, output voltage divided by input voltage: 36.56X, 31.26dB
  • Output noise, unbalanced inputs, 8-ohm load, 1k-ohm input termination, Lch/Rch
    • Volume control at reference position
      • wideband: 0.168mV, -84.5dBW / 0.111mV, -88.1dBW
      • A weighted: 0.111mV, -88.2dBW / 0.083mV, -90.1dBW
    • Volume control full clockwise (maximum)
      • wideband: 0.094mV, -89.5dBW / 0.083mV, -90.1dBW
      • A weighted: 0.066mV, -92.6dBW / 0.060mV, -93.5dBW
    • Volume control full counterclockwise (minimum)
      • wideband: 0.080mV, -91.0dBW / 0.080mV, -91.0dBW
      • A weighted: 0.057mV, -92.6dBW / 0.060mV, -93.5dBW
  • Output noise, balanced inputs, 8-ohm load, 600-ohm input termination, Lch/Rch
    • Volume control at reference position
      • wideband: 0.171mV, -84.4dBW / 0.121mV, -87.3dBW
      • A weighted: 0.115mV, -87.8dBW / 0.092V, -89.8dBW
    • Volume control full clockwise (maximum)
      • wideband: 0.150mV, -85.5dBW / 0.151mV, -85.4dBW
      • A weighted: 0.115mV, -87.8dBW / 0.117mV, -87.7dBW
    • Volume control full counterclockwise (minimum)
      • wideband: 0.081mV, -90.9dBW / 0.080mV, -91.0dBW
      • A weighted: 0.057mV, -93.7dBW / 0.060mV, -93.5dBW

    Measurements summary

    The Hegel H80 is a medium-powered, solid-state integrated amplifier with analog and digital inputs. Its 31dB of voltage gain is typical for a modern integrated amplifier. Measurements were performed through the balanced analog inputs and coaxial digital input, as noted.

    Chart 1A shows the H80’s frequency response through the analog inputs with varying loads. The close spacing of the lines from 10Hz to nearly 20kHz indicates a low output impedance that will minimally interact with most loudspeakers. In the lowest frequencies, the H80’s output does taper off slightly beginning at about 70Hz, but is down by only 0.5dB at 10Hz. The measurements reveal some load dependence in the upper frequencies, with the open-circuit test flat to nearly 50kHz, the 8-ohm loading down by 0.1dB at 20kHz, and the 4-ohm loading down by 0.2dB at 20kHz, though the latter drops are quite small.

    Chart 1B shows the frequency response into varying loads when fed a 24-bit/48kHz signal through the coaxial digital input. As with the analog inputs, the lines are closely spaced, meaning there will be little variation, regardless of load. The lowest frequencies show the same kind of subtle rolloff as through the analog inputs, but all load conditions, even the open circuit, show a slight rolloff of 0.3-0.5dB by 20kHz, likely due to the influence of the antialiasing filter.

    Chart 2 shows how total harmonic distortion (THD) plus noise and intermodulation distortion (IMD) vary in relation to power output. THD and IMD levels stay comfortably below 0.05% at output levels short of clipping.

    Chart 3A shows distortion in relation to power output and frequency for the balanced analog inputs. Distortion remains below 0.02% from 20Hz to about 12kHz for power-output levels of 1 to 70W, and falls below 0.01% above about 12kHz for those same power levels. Chart 3B is the same test, but with a 24/48 signal fed through the coaxial input of the DAC section. The 1W distortion level is slightly higher than through the analog inputs, but still less than 0.03% throughout the audioband. For higher power levels, the distortion remains below 0.02% from 20Hz to 10kHz, and is less than 0.01% for higher frequencies.  (Note that the decrease in distortion above 10kHz in these charts is a result of a 22kHz cutoff filter, which helps to improve the accuracy of this test below 10kHz.)

    Chart 4 shows damping factor vs. frequency. The H80’s damping factor is usefully high from about 100Hz to 4kHz. The decrease in damping factor into higher frequencies is typical for a solid-state amplifier, but the rolloff below 100Hz is unusual, and may be related to the H80’s tapering response into lower frequencies seen in Chart 1A. The shape of this curve is similar to that of Hegel’s H20 stereo power amplifier, which was measured in 2011.

    Chart 5A shows the spectrum of harmonic distortion and noise residue of a 1kHz test signal at 10W fed through the balanced analog input. A series of power-supply-related harmonics are visible, as are intermodulation components of signal harmonics in line harmonics, but all lie below 0.0001%. The highest signal harmonic is the third, at about 0.002%, with higher-order harmonics visible to beyond 10kHz.

    Chart 5B shows the same test with the signal fed through the coaxial digital input. Noise from the power supply is higher than through the analog input, with the second harmonic reaching 0.0017%, but any intermodulation components remain below 0.0001%. With the digital input, the second signal harmonic is the strongest, at about 0.004%, and a long series of higher harmonics are still apparent.

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

    Chart 1A - balanced analog input

    Chart 1A

    Black line = open circuit
    Magenta line = dummy-speaker load
    Cyan line = 8-ohm load
    Blue line = 4-ohm load

    Chart 1B - digital input @ 48kHz

    Chart 1B

    Black line = open circuit
    Magenta line = dummy-speaker load
    Cyan line = 8-ohm load
    Blue line = 4-ohm load

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

    chart2

    (Line up at 10W to determine lines)
    Top line (red dashed) = 4-ohm SMPTE IM distortion
    Second line (black dashed) = 8-ohm SMPTE IM distortion
    Third line (red) = 4-ohm THD+N
    Bottom line (black) = 8-ohm THD+N

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

    Chart 3A - balanced analog input, 22kHz cutoff filter

    Chart 3A

    (8-ohm loading)
    Black line = 1W
    Green line = 10W
    Blue line = 20W
    Red line = 40W
    Magenta line = 70W

    Chart 3B - digital (coaxial) input @ 48kHz, 22kHz cutoff filter

    Chart 3B

    (8-ohm loading)
    Black line = 1W
    Green line = 10W
    Blue line = 20W
    Red line = 40W
    Magent line = 70W

    Chart 4 - Damping factor as a function of frequency

    Chart 4

    Damping factor = output impedance divided into 8

    Chart 5 - Distortion and noise spectrum

    Chart 5A - balanced analog input

    Chart 5

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

    Chart 5B - digital (coaxial) input at 48kHz

    Chart 5B

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

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 and balanced inputs of both channels were made at the 120V AC line voltage, both channels driven. Since the Parasound Halo A 23 is switchable from stereo to mono mode, measurements were made in both modes. (The suffix “A” indicates a stereo-mode measurement. The suffix “B” indicates a mono-mode measurement.) Unless otherwise noted, the data reported below are for the unbalanced inputs and the left channel.

Power output (stereo mode)

  • Power output at 1% THD+N: 131.5W @ 8 ohms, 193.5W @ 4 ohms
  • Power output at 10% THD+N: 160.0W @ 8 ohms, 227.5W @ 4 ohms

Power output (mono mode)

  • Power output at 1% THD+N: 384.6W @ 8 ohms
  • Power output at 10% THD+N: 449.3W @ 8 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 0.6A, 0.68PF, 47.6W
  • Gain: output voltage divided by input voltage, 8-ohm load
    •      Stereo mode: 27.55X, 28.8dB
    •      Mono mode: 54.0X, 34.7dB
  • Input sensitivity for 1W output into 8 ohms
    •      Stereo mode: 102.8mV
    •      Mono mode: 52.4mV
  • Output impedance @ 50Hz
    •      Stereo mode: 0.01 ohm
    •      Mono mode: 0.29 ohm
  • Input impedance @ 1kHz
    •      Stereo mode, unbalanced inputs: 34k ohms
    •      Mono mode, unbalanced inputs: 34k ohms
    •      Mono mode, balanced input: 66.5k ohms
  • Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch
    •      Wideband: 0.220mV/0.197mV, -82.2dBW/-83.1dBW
    •      A weighted: 0.069mV/0.060mV, -92.3dBW/-93.5dBW
  • Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
    •      Wideband: 0.272mV/0.221mV, -80.3dBW/-82.1dBW
    •      A weighted: 0.100mV/0.073mV, -89.0dBW/-91.8dBW
  • Output noise (mono mode), 8-ohm load, unbalanced inputs terminated with 1k ohms
    •      Wideband: 0.362mV, -77.9dBW
    •      A weighted: 0.130mV, -86.8dBW
  • Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms
    •      Wideband: 0.371mV, -77.6dBW
    •      A weighted: 0.135mV, -86.4dBW

Measurements summary

The Parasound Halo A 23 is a stereo power amplifier of moderate power output, and the lowest-powered amp in the Halo line. As the A 23 can be switched from stereo to bridged mode, both modes were measured. In the charts, the suffix “B” indicates measurements taken in bridged mode.

Charts 1A and 1B show the frequency response of the A 23 with varying loads. There is more high-frequency rolloff in bridged mode due to the series-connected nature of this mode. This also causes the high-frequency deviation with load to show up more. Since the A 23’s regulation is so good, the IHF dummy load wouldn’t show any significant variation in the audioband.

Chart 2 illustrates how the A 23’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals for loads of 8 and 4 ohms. Chart 2B is for bridged mode into 8 ohms -- the A 23 is not specified for bridged use into loads of 4 ohms. Interestingly, the curves for 1kHz THD+N and SMPTE IM distortions are almost an overlay.

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

The Halo A 23’s damping factor vs. frequency, shown in Chart 4A, is of a value and nature typical of many solid-state amplifiers: high up to 1-2kHz, then rolling off with increasing frequency. Somewhat puzzling was the measurement of damping factor in bridged-mono mode (4B). Usually, this is about half the damping factor in the individual channels of a stereo amp in stereo mode, but in the case of the A 23 it was much lower. I checked this with an alternate method, comparing the open-circuit voltage vs. the voltage when loaded with 4 ohms, and got the same result.

A spectrum of the residue of harmonic distortion and noise of a 10W, 1kHz test signal is plotted in Charts 5A and 5B. The AC-line harmonics are complex but relatively low in magnitude. Signal harmonics are low enough in amplitude, and consist mostly of a descending series of odd harmonics. Things are similar in bridged mode (Chart 5B), but with magnitudes about doubled.

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

Chart 1A

Chart 1A

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

Chart 1B

Chart 1B

Mono mode
Red line = open circuit
Magenta line = 8-ohm load
Blue 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 20W to determine lines)
Top line = 4-ohm THD+N
Second line = 8-ohm THD+N
Third line = 4-ohm SMPTE IM distortion
Bottom line = 8-ohm SMPTE IM distortion

Chart 2B

Chart 2B

Mono mode
(Line up at 300W 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 = 70W
Cyan line = 100W
Green line = 125W

Chart 3B

Chart 3B

Mono mode
(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 0W
Cyan line = 100W
Green line = 300W

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

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.

Note: Measurements were made at 120V AC line voltage with both channels being driven. Measurements made on left channel through the balanced inputs unless otherwise noted.

Power output

  • Output power at 1% THD+N: 198.4W @ 8 ohms, 285.5W @ 4 ohms
  • Output power at 10% THD+N: 246.1W @ 8 ohms, 365.1W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 2.49A, 0.76PF, 233W
  • AC-line current draw at standby: 0.28A, 0.64PF, 21.8W
  • Gain: output voltage divided by input voltage, unbalanced and balanced inputs: 17.5X, 24.9dB 
  • Input sensitivity for 1W output into 8 ohms, unbalanced and balanced inputs: 161.6mV
  • Output impedance @ 50Hz: 0.153 ohm
  • Input impedance @ 1kHz, unbalanced and balanced inputs: >800k ohms
  • Output noise, 8-ohm load, balanced inputs, termination 600 ohms, Lch/Rch
    •      Wideband: 0.58mV/0.58mV, -73.8 dBW/-73.8dBW
    •      A weighted: 0.085mV/0.085mV, -90.4dBW/-90.4dBW
  • Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm, Lch/Rch
    •      Wideband: 0.58mV/0.58mV, -73.8 dBW/-73.8dBW
    •      A weighted: 0.085mV/0.084mV, -90.4dBW/-90.5dBW

Measurements summary

The Ayre Acoustics VX-5 is a medium-power stereo amplifier. Like previous Ayre power amplifiers, it is fully balanced and uses no overall negative feedback.

Chart 1 shows the VX-5’s frequency response with varying loads: very flat throughout the entire test range of 10Hz-200kHz. The output impedance is low, but still high enough that you can just see the effects of changes in load on the vertical scale used for FR charts. The response to a dummy NHT speaker load is barely discernible between the limits of an open circuit and a 4-ohm load, which indicates that the impedances of most speakers won’t materially affect the VX-5’s frequency-response output.

Chart 2 illustrates how total harmonic distortion plus noise (THD+N) vs. power varies with 1kHz and SMPTE intermodulation (IM) test signals and amplifier output load for 8- and 4-ohm loads. The amount of distortion and how it rises with output level is similar to some other MOSFET power amps I have measured recently.

THD+N as a function of frequency at different power levels is plotted in Chart 3. The amount of rise in distortion at high frequencies is reasonably low in this design.

Damping factor vs. frequency, shown in Chart 4, is of a quality rarely seen in power amplifiers: flat throughout the audioband! I can remember only a very few other designs that achieved this.

A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Chart 5. The magnitudes of the AC-line harmonics are very low and simple except for a few clustered around the suppressed 1kHz test signal. The dominant signal harmonic is the third, a testament to the circuitry’s basic symmetry.

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

Chart 1

Red line = open circuit
Magenta line = 8-ohm load
Blue line = 4-ohm load
Cyan line = NHT dummy load

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

Chart 2

(Line up at 10W to determine lines)
Top line = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 4-ohm THD+N
Bottom line = 8-ohm THD+N

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

Chart 3

(4-ohm loading)
Red line = 2W
Magenta line = 20W
Blue line = 100W
Cyan line = 200W
Green line = 250W

Chart 4 - Damping factor as a function of frequency

Chart 4

Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

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

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 were made at 120V AC line voltage with both channels being driven (stereo mode). Measurements were made on the left channel through balanced inputs unless otherwise noted.

Power output (stereo mode)

  • Power output at 1% THD+N: 325.8W @ 8 ohms, 542.8W @ 4 ohms
  • Power output at 10% THD+N: 400.7W @ 8 ohms, 671.2W @ 4 ohms

Power output (mono mode)

  • Power output at 1% THD+N: 1100W @ 8 ohms, 1520W @ 4 ohms
  • Power output at 10% THD+N: 1430W @ 8 ohms, 2220W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 0.77A, 0.6PF, 54W
  • Gain: output voltage divided by input voltage, 8-ohm load
    •      Stereo mode: 35.9X, 31.1dB
    •      Mono mode: 71.9X, 37.1dB
  • Input sensitivity for 1W output into 8 ohms
    •      Stereo mode: 78.8mV
    •      Mono mode: 39.3mV
  • Output impedance @ 50Hz
    •      Stereo mode: 0.002 ohm
  • Input impedance @ 1kHz
    •      Stereo mode, unbalanced inputs: 47k ohms
    •      Stereo mode, balanced inputs: 95k ohms
    •      Mono mode, balanced input: 47.5k ohms
  • Output noise (stereo mode), 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch
    •      Wideband: 0.148mV/0.155mV, -85.6dBW/-85.2dBW
    •      A weighted: 0.051mV/0.053mV, -94.9dBW/-94.5dBW
  • Output noise (stereo mode), 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
    •      Wideband: 0.176mV/0.152mV, -84.1dBW/-85.4dBW
    •      A weighted: 0.050mV/0.050mV, -95.0dBW/-95.0dBW
  • Output noise (mono mode), 8-ohm load, balanced inputs terminated with 600 ohms
    •      Wideband: 0.217mV, -82.3dBW
    •      A weighted: 0.070mV, -92.1dBW

Measurements summary

Simaudio’s Moon Evolution 870A is a new high-power stereo amplifier design that replaces the Moon Evolution W-8. As the 870A is also capable of operating in bridged mono, some essential measurements were made in that mode as well.

Chart 1 shows the 870A’s frequency response with varying loads. Because the amp’s damping factor and resultant very low output impedance cause no noticeable change with output loading, the curve shown is the same for an open circuit, for loads of 8 and 4 ohms, and for an NHT dummy speaker. The results were substantially the same for mono mode (not shown).

Chart 2A illustrates how total harmonic distortion plus noise (THD+N) vs. power varies with 1kHz and SMPTE intermodulation (IM) test signals and amplifier output for loads of 8 and 4 ohms. Chart 2B shows the results for mono mode. Here, the IM curves reveal some abrupt increases in distortion at higher powers, which sometimes indicates HF instability with lower-impedance loading.

Charts 3A and 3B show THD+N as a function of frequency at different power levels for, respectively, the stereo and mono modes. The amount of rise in distortion at high frequencies is admirably low.

Damping factor vs. frequency is shown in Chart 4. This is extremely high, exceeding 500 at 20kHz. The damping factor in mono mode (not shown) was similar in shape but about half the magnitude.

A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Chart 5. The magnitudes of the AC-line harmonics are relatively low; the signal harmonics are predominantly the second and third. The spectrum for the mono mode (not shown) is similar regarding the signal harmonics, but the AC-line harmonics have largely canceled, due to the differential subtraction action of similar signals.

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

Chart 1

Stereo mode
Red line = open circuit, 8-ohm load, 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 = 4-ohm SMPTE IM distortion
Second line = 8-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 = 4-ohm SMPTE IM distortion
Second line = 8-ohm SMPTE IM distortion
Third line = 8-ohm THD+N
Fourth line = 4-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 = 100W
Green line = 250W

Chart 3B

Chart 3B

Mono mode
(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 30W
Cyan line = 100W
Green line = 300W
Yellow line = 900W

Chart 4 - Damping factor as a function of frequency

Chart 4

Stereo mode
Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

Stereo mode
1kHz signal at 10W into a 4-ohm load

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 were made at 120V AC line voltage with both channels being driven. Measurements were made on the left channel in the stereo mode unless otherwise noted.

Power output (stereo mode)

  • Power output at 1% THD+N: 117.5W @ 8 ohms, 209.5W @ 4 ohms
  • Power output at 10% THD+N: 146.5W @ 8 ohms, 270.0W @ 4 ohms

Power output (mono mode)

  • Power output at 1% THD+N: 405.4W @ 8 ohms, 671.0W @ 4 ohms
  • Power output at 10% THD+N: 504.2W @ 8 ohms, 820.0W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at high bias: 1.63A, 0.71PF, 138W
  • AC-line current draw at low bias:  0.92A,  0.72PF, 81W
  • Gain: output voltage divided by input voltage, 8-ohm load
    • Low bias
      •      Stereo balanced inputs: (Lch/Rch): 23.3X, 27.4dB / 23.2X, 27.3dB
      •      Stereo unbalanced inputs (Lch/Rch): 31.6X, 30.0dB / 31.3X, 29.9dB
      •      Mono balanced input: 22.2X, 26.9dB 
      •      Mono unbalanced input: 30.4X, 29.7dB
    • High bias
      •      Stereo balanced inputs: (Lch/Rch): 23.8X, 27.5dB / 23.7X, 27.5dB
      •      Stereo unbalanced inputs (Lch/Rch): 32.2X, 30.2dB / 32.0X, 31.1dB
      •      Mono balanced input: 23.0X, 27.2dB
      •      Mono unbalanced input: 31.6X, 30.0dB
  • Input sensitivity for 1W output into 8 ohms
    • Low bias
      •      Stereo balanced inputs: (Lch/Rch): 121.4mV / 121.9mV
      •      Stereo unbalanced inputs (Lch/Rch): 89.5mV / 90.4mV
      •      Mono balanced input: 127.4mV
      •      Mono unbalanced input: 89.5mV
    • High bias
      •      Stereo balanced inputs: (Lch/Rch): 118.8mV / 119.3mV
      •      Stereo unbalanced inputs (Lch/Rch): 87.8mV / 88.4mV
      •      Mono balanced input: 123.0mV
      •      Mono unbalanced input: 89.5mV
  • Output impedance @ 50Hz
    •      Stereo low-bias mode: 0.279 ohm
    •      Stereo high-bias mode: 0.180 ohm
    •      Mono low-bias mode: 0.571 ohm
    •      Mono high-bias mode: 0.375 ohm
  • Input impedance @ 1kHz
    •      Stereo balanced inputs: 10.3k ohms
    •      Stereo unbalanced inputs: 67.2k ohms
    •      Mono balanced input: 3.6k ohms
    •      Mono unbalanced input: 69.0k ohms
  • Output noise, stereo mode, 8-ohm load
    • Low bias
      • balanced outputs terminated with 600 ohms, Lch/Rch
        • Wideband: 0.18mV / 0.11mV, -83.9dBW / -88.2dBW
        • A weighted: 0.072mV / 0.051mV, -91.9dBW / -94.9dBW

      • unbalanced inputs terminated with 1k ohms, Lch/Rch
        • Wideband: 0.30mV / 0.21mV, -79.5dBW / -82.6dBW
        • A weighted: 0.10mV / 0.074mV, -89.0dBW / -91.6dBW
    • High bias
      • balanced outputs terminated with 600 ohms, Lch/Rch
        • Wideband: 0.29mV / 0.12mV,  -79.8dBW / -87.5dBW
        • A weighted: 0.11mV / 0.056mV, -88.2dBW / -94.1dBW
      • unbalanced inputs terminated with 1k ohms, Lch/Rch
        • Wideband: 0.44mV / 0.27mV, -76.2dBW / -80.4dBW
        • A weighted: 0.15mV / 0.087mV, -85.5dBW / -90.2 dBW
  • Output noise, mono mode, 8-ohm load
    • Low bias
      • balanced outputs terminated with 600 ohms
        • Wideband: 0.19mV, -83.5dBW
        • A weighted: 0.080mV, -91.0dBW
      • unbalanced inputs terminated with 1k ohms
        • Wideband: 0.53mV, -74.5 dBW
        • A weighted: 0.18mV, -83.9 dBW
    • High bias
      • balanced outputs terminated with 600 ohms
        • Wideband: 0.26mV, -80.7dBW
        • A weighted: 0.097mV, -89.3dBW
      • unbalanced inputs terminated with 1k ohms
        • Wideband: 0.90mV, -69.9dBW
        • A weighted: 0.29mV, -79.8dBW

Measurements summary

The Liberty Audio B2B-100 is a medium-power stereo power amplifier that can be switched between stereo and mono modes. Additionally, it has a switch for operating at low (Low) and high (Hi) biases.

Most of the measurements were affected by the choice of bias level; to avoid having to show all combinations, I concentrate here on the Hi bias setting and show, on the same or several charts, the effects of the two bias settings.

Chart 1 shows the B2B-100’s frequency response with varying loads in stereo mode and with Low bias. With Hi bias, the curves are closer together simply because the output impedance is lower at the higher bias.

In mono mode, the shapes of the curves are similar, with greater separation between curves because the two stereo outputs are in series with the load, which raises the output impedance. In general, the output impedance of the amp, especially in the Low bias setting, would potentially cause the frequency-response variations with some speakers to be audible.

Chart 2 illustrates how the B2B-100’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load for 8- and 4-ohm loads. This is for the stereo mode and Hi bias. The amount of distortion and how it rises with output level is similar to some other MOSFET power amps I’ve measured recently, and suggests low amounts of overall loop feedback. The effects of the two bias settings are most visible in the mono mode, where the effective load on each half of the B2B-100 is effectively halved. Chart 2A shows the four load conditions in Hi bias mode, Chart 2B in Low bias.

Chart 3 plots THD+N as a function of frequency at several different power levels. The shape of these curves is a bit unusual -- the distortion is lower below 100Hz, rises in level to about 1kHz, then stays pretty constant up to 20kHz. It’s almost as if some internal open-loop response shaping is taking place.

Damping factor vs. frequency is shown in Chart 4 for the two bias settings, the higher damping factor being for Hi bias.

A spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal is plotted in Charts 5A and 5B for the two bias settings. With Hi bias, the higher signal harmonics disappear into the noise much more quickly. The magnitudes of the AC line harmonics are quite high, and largely a result of flux leakage from the power transformer.

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

Stereo and low-bias modes

Chart 1

Red line = open circuit
Magenta line = 8-ohm load
Blue line = 4-ohm load
Cyan line = NHT dummy-speaker load

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

Stereo and high-bias modes

Chart 2 

(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 2A - Mono and high-bias modes

Chart 2A 

(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 - Mono and low-bias modes

Chart 2A 

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

Mono and low-bias modes

Chart 3

(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 60W
Cyan line = 400W

Chart 4 - Damping factor as a function of frequency

Stereo mode

Chart 4

Damping factor = output impedance divided into 8
Red line = low bias
Magenta line = high bias

Chart 5 - Distortion and noise spectrum

Chart 5A - stereo and low-bias settings

Chart 5A

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

Chart 5B - stereo and high-bias settings

Chart 5B

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

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