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Equipment Measurements

June 2008

Esoteric A-100 Integrated 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 being driven.
  • Measurements made on left channel and into the "RCA 1" input and on the 8-ohm output unless otherwise noted.
  • This integrated amplifier does not polarity.
  • AC line current draw at idle: 1.96A
  • Input sensitivity for 1W output into 8 ohms, volume at maximum, Lch/Rch: 72.0mV/67.8mV
  • Input impedance @ 1kHz
    • RCA 1 input: 10k ohms
    • XLR input: 20k ohms
    • Direct input: >350k ohms
  • Output impedance at 50Hz: 4.7 ohms
  • Gain, output voltage divided by input voltage, volume at maximum, Lch/Rch: 39.3X, 31.99dB/41.7X, 32.4dB
  • Output noise, 8-ohm load, 1k-ohm input termination, Lch/Rch
    • Volume control at reference position
      • wideband: 0.63mV, -70.0dBW/0.68mV, -72.4 dBW
      • A weighted: 0.30mV, -79.5dBW/0.16mV, -85.0dBW
    • Volume control full clockwise
      • wideband: 0.63mV, -70.0dBW/0.65mV, -72.8dBW
      • A weighted:0.28mV, -80.1dBW/0.20mV, -83.0dBW
    • Volume control full counterclockwise
      • wideband: 0.60mV, -73.5dBW/0.76mV, -71.4dBW
      • A weighted: 0.29mV, -79.8dBW/0.32mV, -78.9dBW
Measurements Summary

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 unit’s 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 didn’t 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 owner’s 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 point.

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.

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 10W 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: 45W
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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