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Equipment Measurements

March 2007

Audio Research Reference 110 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 and on the 8-ohm outputs. Both channels driven with balanced input. Measurements made on the left channel unless otherwise noted.
  • This amplifier does not invert polarity.
  • AC line current draw at idle: 4.4A.
  • Input impedance @ 1kHz: 260k ohms.
  • Output impedance at 50Hz: 0.73 ohms.
  • Gain (8-ohm load): 16.0X, 24.1dB.
  • Output noise, 8-ohm load, 600-ohm input termination, Lch/Rch:
    • wideband: 0.24mV, -81.4dBW / 0.29mV, -79.8dBW
    • A weighted: 0.05mV, -95.1dBW / 0.14 mV, -86.1dBW
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 107W
  • 8-ohm load at 10% THD: 129W

  • 4-ohm load at 1% THD: 102W
  • 4-ohm load at 10% THD: 135W

General

The Audio Research Reference 110 is a medium-power stereo push-pull tube-based power amplifier utilizing two pairs of 6550 output tubes per channel. This unit is most unusual in vacuum-tube power-amplifier design in that it has no phase inverter and depends on receiving a balanced input signal for the output stage push-pull action. Pity the person who might use single-ended RCA female-to-XLR male adapters with pin 3 (minus phase) grounded, as is usual. I did that in my initial testing and found out that the amp responded as a single-ended amplifier putting out about 20 watts per channel at 10% distortion! Needless to say, when driven with a balanced input signal, the amplifier behaved properly. Checking with Audio Research, I found out that all of the Reference-series tube power amplifiers were designed this way. I guess there is something to be said for circuit simplification by not having a phase inverter if it is known that the amp will always be driven by a balanced preamp, which the Audio Research Reference 3 is.

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 and acceptably low for a tube power amplifier. With the NHT dummy speaker load, the variation is better than +/-1dB. One thing to note, though: There is a noticeable gentle roll-off in the audio range starting at about 500Hz with the 4-ohm loading. This is also present with the 8-ohm loading to a lesser degree and not quite noticeable in the chart with the usual scale factor that I use. This, for a perfectly flat speaker load, would have the potentially audible effect of a softening of the highs or, to put it another way, give a little more weight to the lower midrange – generally a good thing, as many amplifiers are a little too thin in this region.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. Of interest, this amplifier’s general distortion characteristic is typical of many tube amplifiers and is one of continuously increasing distortion with increasing power. As is the case with most power amplifiers, this one puts out more power, but not much more, into a 4-ohm load.

Total harmonic distortion plus noise as a function of frequency at four different power levels is plotted in Chart 3. The amount of rise in distortion at high frequencies is admirably low, although there is the typical rise in distortion at low frequencies due to the characteristics of the output transformers.

Damping factor vs. frequency is shown in Chart 4. This curve has an unusual shape and, in fact, looks somewhat like the shape of the 4-ohm frequency response. It makes me wonder if this was a deliberate design aspect of the Reference 110.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The principal signal harmonics are the second and third with all higher harmonics some 40dB or so lower. AC-line harmonics are low, although there are some line-harmonic-related sidebands around the nulled-out 1kHz test-signal fundamental -- something I have seen in quite a few of the power amplifiers that I have tested, but not all of them.

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: 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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