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Equipment Measurements

July 2005

Audiopax Stereo Eighty Eight 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 with both channels being driven. All measurements made with "timbre" control in mid position unless otherwise noted.
  • Gain: 4.25x, 12.8dB.
  • Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.292mV, -79.7dBW; A weighted 0.040mV, -97.0dBW.
  • AC line current draw at idle: timbre control counterclockwise, 0.86A; timbre control at mid rotation, 1.0A; timbre control clockwise, 1.37A.
  • Output impedance at 50Hz: 3.1 ohms.
  • This amplifier inverts polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 0.75W
  • 8-ohm load at 10% THD: 14.6W

  • 4-ohm load at 1% THD: 0.17W
  • 4-ohm load at 10% THD: 8.7W


The Audiopax Stereo Eighty Eight is a low-/medium-powered single-ended tube amplifier utilizing a single KT88 output tube in each channel. The design is unusual in that it has a control for adjusting the "timbre" of the sound for different speakers and situations. This control appears to vary the quiescent current in the output tube. It turns out that one measurable effect of this control is to vary the damping factor of the output. Gain and distortion characteristics are also affected. Gain of the unit is somewhat lower than usual for power amplifiers. Rated power is 15W into an 8-ohm load.

Chart 1 shows the frequency response of the amp with varying loads. As can be seen, the output impedance, as judged by the closeness of spacing between the curves of open circuit, 8-ohm, and 4-ohm loading, is of a typical value for tube amplifiers. The variation with the NHT dummy load in the audio range is on the order of +/-2dB.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. This design, with its single output connection for speaker load, puts out about the same power into 4- or 8-ohm loads, although the distortion is greater with 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, but the increase in distortion at low frequencies is quite pronounced.

As an illustration of one of the parameters that are affected by the timbre control, damping factor is plotted in Chart 4 as a function of the timbre control set at counterclockwise, mid-rotation, and clockwise positions. This measurement was done at a much lower current (.125A) than the usual 1A in order to maintain signal linearity at the lowest timbre setting.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The magnitude of the AC-line harmonics is very low, and the signal harmonic spectrum has the desirable characteristic of tapering off as the harmonic number increases.

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 2W 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: 14W
Blue line: 10W
Magenta line: 5W
Red line: 1W

Chart 4 - Damping Factor as a Function of Frequency

Damping factor = output impedance divided into 8
Red line: timbre control fully clockwise
Magenta line: timbre control at mid position
Blue line: timbre control fully counterclockwise

Chart 5 - Distortion and Noise Spectrum

1kHz signal at 10W into an 8-ohm load


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