Tuesday, September 26, 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.

Note: Unless otherwise noted, measurements were taken at the balanced left-channel input, at a line voltage of 120V AC.

Power output

  • Power output at 1% THD+N: 60.4W @ 8 ohms, 39.6W @ 4 ohms
  • Power output at 10% THD+N: 79.3W @ 8 ohms, 104.5W @ 4 ohms

Additional data

  • Input/output polarity: noninverting
  • AC-line current draw
    • Operate: 242W, 2.4A, 0.84PF
    • Standby: 1.2W, 0.03A, 0.29PF
  • Gain: output voltage divided by input voltage, 8-ohm load (Lch/Rch)
    • Balanced inputs: 17.55X, 24.9dB / 17.43X, 24.8dB
    • Unbalanced inputs: 34.45X, 30.7dB / 34.11X, 30.7dB
  • Input sensitivity for 1W output into 8 ohms (Lch/Rch)
    • Balanced inputs: 161.1mV / 162.2mV
    • Unbalanced inputs: 82.1mV / 82.9mV
  • Output impedance @ 50Hz: 0.99 ohm
  • Input impedance @ 1kHz
    • Balanced inputs: 196k ohms
    • Unbalanced inputs: 92k ohms
  • Output noise, 8-ohm load, balanced inputs terminated with 600 ohms, Lch/Rch
    • Wideband: 231.3uV/85.5uV, -81.8dBW/-90.3dBW
    • A weighted: 33.8uV/29.8uV, -98.5dBW/-99.6dBW
  • Output noise, 8-ohm load, unbalanced inputs terminated with 1k ohms, Lch/Rch
    • Wideband: 970.0uV/1000.0uV, -69.3dBW/-69.0dBW
    • A weighted: 200.2uV/200.2uV, -83.0dBW/-83.0dBW

Measurements summary

The VT80 is a power amplifier in Audio Research’s new Foundation Series, which is said to have a new auto-bias arrangement for the output tubes. These tubes are large KT120s, each operated conservatively at a plate dissipation of about 25W.

Chart 1 shows the frequency response of the VT80 with varying loads. An output impedance of about 1 ohm, which is typical of tubed power amps, causes considerable variation of the output level with load. With the NHT dummy speaker load, the variation in output level with frequency is about +0.5/-1.0dB. The high-frequency, -3dB bandwidth is about 60kHz.

Chart 2 illustrates how the VT80’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE intermodulation test signals and amplifier output load for 8- and 4-ohm loads at the 8-ohm output tap. The distortion in this plot starts to rise quickly above 10-20W, depending on the load. Interestingly, the distortion with a 4-ohm load at the 8-ohm tap begins to rise at about 10W, and at 20W for an 8-ohm load on the 8-ohm tap. Note that a load of 4 ohms on the 4-ohm tap would produce a level of distortion similar to that produced by an 8-ohm load on the 8-ohm tap in the chart.

Chart 3 plots the VT80’s THD+N as a function of frequency at several different power levels. The increase of distortion with frequency is reasonable, and the very-low-frequency region shows more distortion at higher power levels.

Chart 4 plots the VT80’s damping factor vs. frequency. The quite low damping factor is typical of tubed power amplifiers, is constant over quite a wide frequency range, and begins to decrease at about 4kHz.

A spectrum of the residue of harmonic distortion and noise of a 10W, 1kHz test signal is plotted in Chart 5. AC-line harmonics are quite complex in frequency content. The signal harmonics are dominated by the second and third harmonics, with decreasing amounts of lower-level higher 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
Cyan line = NHT dummy speaker load

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


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

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

Chart 3

(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 30W
Cyan line = 60W
Green line = 75W

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

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

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