August 2002
Audio Research
VS55 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.
 Measurements were made with 120V AC line voltage.
 Output tube plate current adjusted to 65mA per tube when
warmed up.
 Power output and distortion plotted with both channels
driven.
 Gain: 18.3x, 25.2dB.
 Output noise, 8ohm load, 1kohm input termination: wideband
0.40mV, 77.0dBW; A weighted 0.088mV, 90.1dBW.
 AC line current draw at idle: 1.9A.
 Output impedance at 50Hz: 0.89 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 50W
 8ohm load at 10% THD: 58W
 4ohm load at 1% THD: 33W
 4ohm load at 10% THD: 70W
 16ohm load at 1% THD: 33W
 16ohm load at 10% THD: 38W
General
Measurement results for this design are typical of other
well designed similar powered units. As can be seen in Chart 1, output regulation with
load change is reasonable and the response variation with the NHT dummy speaker load is
within +/ 0.9dB over the audio range. Highfrequency bandwidth with a resistive load is
about 50kHz. The tendency to peak with an open circuit load could possibly give a slight
high frequency rise above 10kHz with speakers with a high frequency increasing impedance
load  typical of most dome tweeters. Harmonic distortion is reasonably low in the
critical "first watt" region for 8ohm and higher loading on the 8ohm tap, but
does increase with 4ohm loading. On the other hand, 4ohm loading on the 8ohm tap does
increase the power output at onset of clipping from 50 to 60 watts. While the 16ohm power
on the 8ohm tap is down to about 28 watts, the overall matching of the output transformer
to favor some power increase with low loading on an output tap is a good choice. The
amount of distortion rise with frequency in Chart 3 is quite typical of many power
amplifiers, both tube and solid state. In the spectrum of harmonic distortion residue of a
1kHz signal at the 10W power level in Chart 5, the odd harmonics are dominant with the
even harmonics quite a bit lower indicating good pushpull balance. Although the magnitude
of the AC line harmonics is similar to other amplifiers, the noise floor is quite low
between the harmonics. Of interest, and technically not desirable, is the presence of a
number of 120Hz spaced sidebands about the nulled out fundamental 1kHz signal. The damping
factor in Chart 4 stays up at low frequencies but does start to decrease above 1kHz, again
typical of many power amplifiers both tube and solid state.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Green line: NHT dummyspeaker load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 20W to determine lines)
Top line: 8ohm SMPTE IM
Second line: 4ohm on 8ohm tap THD+N
Third line: 16ohm on 8ohm tap THD+N
Bottom line: 8ohm on 8ohm tap THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
8ohm output loading on 8ohm tap
Green line: 50W
Cyan line: 30W
Blue 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 8ohm load
