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Equipment Measurements

November 2008

Audio Research VS115 Stereo Amplifier: Measurements

All amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test amplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

Additional Data
  • Measurements were made at 120V AC line voltage with both channels driven with balanced inputs. Output taken from the 8-ohm connectors unless otherwise noted.
  • This amplifier does not invert polarity.
  • AC line current draw at idle: 4.29A.
  • Input impedance @ 1kHz
    • Unbalanced inputs: 125k ohms.
    • Balanced inputs: 263k ohms.
  • Output impedance at 50Hz: 1.1 ohms.
  • Input sensitivity: 148.9mV.
  • Gain (8-ohm load): 19.0X, 25.6dB.
  • Output noise, unbalanced inputs, 8-ohm load, 1k-ohm input termination, Lch/Rch:
    • Wideband: 0.596mV, -73.5dBW / 0.625mV, -73.1dBW
    • A weighted: 0.129mV, -86.8dBW / 0.134mV, -86.5dBW
  • Output noise, balanced inputs, 8-ohm load, 600-ohm input termination, Lch/Rch:
    • Wideband: 0.684mV, -72.3dBW / 0.603mV, -73.4dBW
    • A weighted: 0.142mV, -86.0dBW / 0.126mV, -87.0dBW
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 115W
  • 8-ohm load at 10% THD: 126W

  • 4-ohm load at 1% THD: 129W
  • 4-ohm load at 10% THD: 160W


The Audio Research VS115 is a medium-power stereo push-pull stereo tube power amplifier utilizing two pairs of 6550 output tubes per channel. Unlike the more expensive Reference 110, which does not have an internal phase inverter topology to generate the balanced push-pull drive for the output stage, and thus has to be driven from a balanced source, the VS115 has a more standard balanced differential topology that easily and naturally allows the use of balanced or unbalanced inputs.

Chart 1 shows the frequency response of the amp with varying loads. The output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading, is reasonably low for a tube power amplifier and from this data is approximately 1 ohm. The variation of output as a function of frequency with the NHT dummy speaker load is about +0.8/-1dB.

Chart 2 illustrates how total harmonic distortion plus noise for a 1kHz test signal and SMPTE IM distortion varies with output power and output load. Typical for a tube amplifier, the amount of distortion gradually rises over most of the power range. Also typical is that distortion is higher and maximum power before clipping is greater for 4-ohm loading.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. The amount of rise in distortion at high frequencies is admirably low, although, as is typical of tube power amplifiers with output transformers, there is some increase in distortion at low frequencies.

Damping factor versus frequency is shown in Chart 4. In this design, the damping factor stays relatively constant out to a higher frequency than is usual, some 2-3kHz.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The amount of AC-line harmonics is relatively low in amplitude but numerous in number. As is frequently the case with many amplifiers tested, there are some AC-line harmonic intermodulation components on either side of the nulled-out test-signal fundamental. The principal signal harmonics are second and third with the remaining visible harmonics some 40dB below the level of the second and third harmonics.

As an aside, I listened to this amp with my Genesis Advanced Technologies 6.1 speakers -- as I do with most of the stereo amps I measure for SoundStage! I found it to be outstanding in its easy ability to make music sound believable and present in my listening room. It offers great detail and resolution without attendant harshness or edginess.

Chart 1 - Frequency Response of Output Voltage as a Function of Output Loading

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 1W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

Chart 3 - Distortion as a Function of Power Output and Frequency

8-ohm output loading
Cyan line: 100W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Chart 4 - Damping Factor as a Function of Frequency

Damping factor = output impedance divided into 8

Chart 5 - Distortion and Noise Spectrum

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


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