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Equipment Measurements

February 2002

Tenor Audio 75 Wi Mono Amplifiers: 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.
  • Input signal was applied to the "direct" input.
  • Plate current was adjusted to 300mA per tube when warmed up.
  • Power output plotted with one channel driven (this is a mono amplifier).
  • Gain: 33.3x, 30.5dB.
  • Output noise, 8-ohm load, 1000-ohm input termination, volume control set for 1W/8 ohms with
    1kHz 500mV input signal to "input 4" in normal (not direct) mode: wideband 0.291mV, -79.8dBW; A weighted 0.078mV, -91.2dBW.
  • AC line current draw at idle: 5.0A
  • Output impedance (measured by an injection of a constant 1A of current at 50Hz): 0.76 ohms.
  • This amplifier does not invert polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 79W
  • 8-ohm load at 10% THD: 120W

  • 4-ohm load at 1% THD: 25W
  • 4-ohm load at 10% THD: 90W

  • 16-ohm load at 1% THD: 89W
  • 16-ohm load at 10% THD: 120W


The Tenor 75 Wi appears to be a robust, well-designed output-transformerless power amplifier. Ironically, many of its characteristics, like distortion and output impedance, are typical of good tube amplifiers having output transformers. As can be seen in Chart 1, frequency response is wide, with a -3dB high-frequency point in excess of 200kHz. There is some small rise at very low frequencies. This very-low-frequency rise has effects on the low-frequency damping factor characteristic, as can seen in Chart 4. Output impedance is fairly low, meaning that the frequency response delivered to a connected speaker will be affected minimally. The response with the NHT dummy speaker load was within about +/- 0.5dB. Chart 2 shows that output power at clipping is fairly constant at 70-80W with loads over a 4-16-ohm range. Harmonic distortion is reasonably low and stays below 1% for an 8-ohm load to better than 80W output. Harmonic distortion increases only moderately with frequency, as shown in Chart 3. As predicted from Chart 1, output impedance is fairly low, and this translates to a damping factor of over 10 over most of the audio frequency range. Finally, in Chart 5, the harmonic-distortion residual spectrum shows the AC-line harmonics to be reasonably low and the signal-distortion components are dominantly second and third harmonic.

Chart 1 - Frequency Response of Output Voltage as a Function of Output Loading

Magenta line: open circuit
Green line: NHT dummy speaker load
Red line: 8-ohm load
Blue line: 4-ohm load

Chart 2 - Distortion as a Function of Power Output and Output Loading

(line alignment at 10W)
Top line: 8-ohm SMPTE IM
Second line: 4-ohm THD+N
Third line:  8-ohm THD+N
Bottom line: 16-ohm THD+N

Chart 3 - Distortion as a Function of Power Output and Frequency

8-ohm output loading
Cyan line: 85W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Chart 4 - Damping Factor as a Function of Frequency

Damping factor = output impedance (measured by an injection of a constant 1A at all frequencies) divided into 8

Chart 5 - Distortion and Noise Spectrum

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


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