Tuesday, September 19, 2017


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 were made at 120V AC line voltage with all three channels driven. Unless otherwise noted, all measurements were taken at the balanced inputs. The data reported below are for channel one unless otherwise noted.

Power output

  • Output power at 1% THD+N: 232.8W @ 8 ohms, 352.0W @ 4 ohms
  • Output power at 10% THD+N: 285.0W @ 8 ohms, 422.9W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 0.95A, 0.68PF, 76.0W (see Note 1)
  • Gain: output voltage divided by input voltage, 8-ohm loads (see Note 2)
    • Balanced inputs
      • Normal gain: 13.16X, 22.4dB
      • Low gain: 6.22X, 15.9dB
      • High gain: 27.73X, 28.9dB
    • Unbalanced inputs
      • Normal gain: 29.42X, 29.4dB
      • Low gain: 13.85X, 22.8dB
      • High gain: 61.84X, 35.8dB
  • Input sensitivity for 1W output into 8 ohms (see Note 3)
    • Balanced inputs
      • Normal gain: 216mV
      • Low gain: 455mV
      • High gain: 102mV
    • Unbalanced inputs
      • Normal gain: 96.1mV
      • Low gain: 204mV
      • High gain: 45.7mV
  • Output impedance @ 50Hz: 0.0135 ohm
  • Input impedance @ 1kHz
    • Balanced inputs: 105.6k ohms
    • Unbalanced inputs: 42.8k ohms
  • Output noise, 8-ohm load, balanced inputs, termination 600 ohms (see Note 4)
    • Nominal gain
      • Wideband: 0.233mV, -82.1dBW
      • A weighted: 0.076mV, -91.4dBW
    • Low gain
      • Wideband: 0.173mV, -84.3dBW
      • A weighted: 0.085mV, -90.4dBW
    • High gain
      • Wideband: 0.436mV, -76.2dBW
      • A weighted: 0.130mV, -86.8dBW
  • Output noise, 8-ohm load, unbalanced inputs, termination 1k ohm (see Note 4)
    • Nominal gain
      • Wideband: 0.275mV, -80.2dBW
      • A weighted: 0.089mV, -90.0dBW
    • Low gain
      • Wideband: 0.257mV, -80.8dBW
      • A weighted: 0.087mV, -90.2dBW
    • High gain
      • Wideband: 0.538mV, -71.4dBW
      • A weighted: 0.154mV, -85.3dBW

Note 1: As a matter of interest, the AC power draw on turn-on is up to about 210W, which slowly comes down as the unit warms up.
Note 2: Gain values were averaged for the three channels.
Note 3: Input sensitivity values were averaged for the three channels.
Note 4: Noise values for the three channels averaged for the three gain settings.

Measurements summary

The high-powered Halo A 31 is the only three-channel power amplifier in Parasound’s extensive line of Halo products.

Chart 1 shows the frequency response of the Halo A 31 with varying loads. Unusual is the uniformity of the high-frequency rolloff with changing load. The Halo’s output impedance is so low that its response with the NHT dummy speaker load would not show up.

Chart 2 illustrates how the A 31’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output for loads of 8 and 4 ohms.

The THD+N as a function of frequency at several different power levels is plotted in Chart 3. High-frequency rise with frequency is moderate, and the amount of distortion is quite low through most of the power range. Interesting is that there is a dip in distortion at around 500Hz at the higher powers.

I wonder if that dip in distortion is related in some way to the peak in the curve of damping factor vs. frequency, shown in Chart 4. Most unusual is that this curve, too, peaks at about 500Hz, then decreases and shelves off as the frequency decreases. This almost suggests that some open-loop frequency shaping was done to tune the sound -- speculation on my part.

Chart 5A plots the spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal for the Halo A 31’s balanced inputs; Chart 5B plots the same for the unbalanced inputs. The unbalanced input is quite a bit worse, with the AC line harmonics extending up into the signal harmonics. The measurements shown are of channel three, which had more transformer-induced noise than channels one and two, on the other side of the amp.

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 100W to determine lines)
Top line = 8-ohm THD+N
Second line = 4-ohm THD+N
Third line = 8-ohm SMPTE IM distortion
Bottom line = 4-ohm SMPTE IM distortion

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

Chart 3

(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 70W
Cyan line = 150W
Green line = 200W

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 inputs

Chart 5A

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

Chart 5B - unbalanced inputs

Chart 5B

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

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