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Equipment Measurements

May 2002

Opera Audio M800 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
  • Amplifier was measured in both Ultralinear and Triode modes of operation.
  • Measurements were made with 120V AC line voltage.
  • Power output plotted with one channel driven (this is a mono amplifier).
  • Output tube plate current set to 35mA.
  • Gain, triode/ultralinear: 24.2x, 27.7dB/28.5x, 29.1dB.
  • Output noise, 8-ohm load
    • Ultralinear: wideband 0.48mV, -75.4dBW; A weighted 0.145mV, -85.8dBW.
    • Triode: wideband 0.445mV, -76.1dBW; A weighted 0.135mV, -86.4dBW.
  • AC line current draw at idle: 1.25A.
  • Output impedance
    • Ultralinear: 2.1 ohms.
    • Triode: 1.4 ohms.
  • This amplifier does not invert polarity.
Measurements Summary

Power output with 1kHz test signal (triode)

  • 8-ohm load at 1% THD: 13W
  • 8-ohm load at 10% THD: 40W

  • 4-ohm load at 1% THD: 4W
  • 4-ohm load at 10% THD: 43W

  • 16-ohm load at 1% THD: 19W
  • 16-ohm load at 10% THD: 32W

Power output with 1kHz test signal (ultralinear)

  • 8-ohm load at 1% THD: 3.7W
  • 8-ohm load at 10% THD: 70W

  • 4-ohm load at 1% THD: 0.9W
  • 4-ohm load at 10% THD: 72W

  • 16-ohm load at 1% THD: 15W
  • 16-ohm load at 10% THD: 32W

General

Chart 1 shows that this amplifier has a relatively wide bandwidth in excess of 10Hz-100kHz. However, like many tube amplifiers, the output regulation is not too good due to the relatively high output impedance -- shown by the drop in level from open circuit to 4-ohm loading. The "Triode" mode has lower output impedance and, therefore, better output regulation. Chart 2 shows that the "Triode" mode has lower distortion than the "Ultralinear" mode, but with about half the power available. Power output and distortion are best with the rated load on either of the output taps. With a load of half of the rated value, the power output is about the same, but at higher distortion. With a load of twice the rated value, distortion is lower but so is output power. This amp can deliver about 35W in "Triode" mode and 60W in "Ultralinear" mode at about 2-3% distortion over most of the audio range into the rated load. Chart 3, which shows distortion versus frequency, shows the "Triode" mode as having the lowest distortion, and unusually and desirably, the distortion amount is fairly constant with frequency. However, in the "Ultralinear" mode, the distortion at low frequencies rises quite a bit at low frequencies. Damping factor in the two modes is illustrated in Chart 4 with the "Triode" mode having the highest damping factor (lowest output impedance). In Chart 5, the higher harmonics are lower in the triode mode.

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

Triode mode with data taken on the 8-ohm tap

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

Ultralinear mode with data taken on the 8-ohm tap

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

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

Triode mode with data taken on the 8-ohm tap

Top line: 8-ohm SMPTE IM
Second line: 4-ohm THD+N
Third line (red): 8-ohm THD+N
Bottom line: 16-ohm THD+N

Ultralinear mode with data taken on the 8-ohm tap

Top line: 8-ohm SMPTE IM
Second line: 4-ohm THD+N
Third line (red): 8-ohm THD+N
Bottom line: 16-ohm THD+N

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

Triode mode with data taken on the 8-ohm tap

8-ohm output loading
Green line: 35W
Blue line: 20W
Magenta line: 5W
Red line: 1W

Ultralinear mode with data taken on the 8-ohm tap

8-ohm output loading
Green line: 35W
Blue line: 20W
Magenta line: 5W
Red line: 1W

Chart 4 - Damping Factor as a Function of Frequency

Triode and Ultralinear modes both plotted

Damping factor = output impedance divided into 8
Red line: Triode
Magenta line: Ultralinear

Chart 5 - Distortion and Noise Spectrum

Triode mode with data taken on the 8-ohm tap

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

Ultralinear mode with data taken on the 8-ohm tap

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

 

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