Sunday, September 24, 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 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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