Power output with 1kHz test signal
- 8-ohm load at 1% THD: 9.0W
- 8-ohm load at 10% THD: 56.0W
- 4-ohm load at 1% THD: 7.4W
- 4-ohm load at 10% THD: 66.0W
The Esoteric A-100 is a medium-power stereo tube integrated
amplifier. A pair of KT88 output tubes per channel provides a modest power output of about
45Wpc. The technology of the circuitry is said to provide automatic tube bias correction
under various conditions. For such a technical story of the units sophistication,
the measurements of this design are not particularly impressive. It would appear that the
amount of overall feedback is low given the high output impedance and consequent low
damping factor. One thing of note is that the two-position bias-control switch didnt
seem to make a difference in distortion or AC-power drawn as would be the case if the
actual output tube bias were changed as is suggested in the owners manual.
As a point of interest, the integrated amplifier sounded
extremely refined, dynamic and musically realistic when driving my Genesis Advanced
Technologies 6.1 speakers. However, I did have to reduce the bass level of the active
woofers somewhat to get a more correct balance due to the rising impedance with decreasing
frequency of these speakers.
Chart 1 shows the frequency response of the A-100 with
varying loads. The high-frequency response is impressively wide, with an approximate 3dB
down point of 80kHz and nicely controlled high-frequency roll-off. However, output
impedance as judged by the closeness of spacing between the curves of open-circuit, 8-ohm,
and 4-ohm loading is quite high. As a consequence, the NHT dummy-speaker load has a
variation of a bit more than +/-2dB. In my opinion, this is too much and will produce
audible coloration with many speaker loads. The frequency response was quite independent
of volume-control setting. Volume-control tracking was generally within about 0.5dB down
to 60dB, where it increased to about 2dB.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load.
Amount of distortion is typical of many tube power amplifiers with low overall amounts of
feedback. Interestingly, the 4-ohm loading for the 1kHz test signal produces quite a bit
lower distortion up to a few watts and a bit more maximum power at the 10% distortion
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. Distortion does rise at low frequencies
and relatively more at lower powers. At the 45W output level, distortion rises to greater
than 10% below about 200Hz.
Damping factor vs. frequency is shown in Chart 4 and is, as
mentioned above, quite low, resulting in poor output regulation with changing load. It is
quite uniform over most of the audio range, however.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5. The magnitudes of the AC-line harmonics are
reasonably low and simple. There is evidence of the line harmonic intermodulation of the
1kHz signal, as seen in quite a few amplifiers. Signal harmonics consist of a tapering-off
spectrum of even and odd harmonics.