Tuesday, September 19, 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.

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

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