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
General
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.
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