September 2003

Orpheus Laboratories Three S 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.

• Measurements were made with 120V AC line voltage.
• Power output and distortion plotted with both channels driven.
• Test signal applied to unbalanced inputs unless otherwise noted.
• Gain, unbalanced input/balanced input: 34.9x, 30.9dB/17.2x, 24.7dB.
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.348mV, -78.2dBW; A weighted 0.087mV, -90.2dBW.
• Output noise, 8-ohm load, balanced input, 600-ohm input termination: wideband 0.295mV, -79.6dBW; A weighted 0.080mV, -91.0dBW.
• AC line current draw at idle: 0.367A.
• Output impedance at 50Hz: 2.89 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 40W
  
  
• 4-ohm load at 1% THD: 62W

General

The Orpheus Three S design purports to deliver constant power to a varying impedance speaker load instead of the usual constant voltage. With either a constant voltage or constant current source to a speaker, the delivered power will not be constant as a function of frequency. Some output impedance between zero and infinite could deliver a constant power. In measuring the Atma-Sphere MA-1 Mk II amp some while ago, I found that that design had an output impedance of about 10 ohms. Designer Ralph Karsten went to considerable trouble to show that his amp delivered quite constant power into my NHT dummy speaker load. In short, the Orpheus Three had an output impedance of about 2.9 ohms measured both from the data of Chart 1 and with the 1A current-insertion method to generate Chart 4. Therefore, the amp did not generate constant power as a function of loading. As I have stated previously, not constant power, but constant voltage out of a power amplifier as a function of varying speaker impedance loads is what the majority of speaker manufacturers assume to drive their speakers.

Measurements were made through the unbalanced inputs. Results were essentially the same through the balanced inputs. Chart 1 shows the frequency response of the amp with varying loads. As discussed in past measurements, a high output impedance will have the effect of causing the designed frequency response for a speaker that was designed for constant voltage to vary by as much as the voltage varies when driving that speaker. As can be seen in Chart 1, in the case of the NHT dummy load, that variation is almost +/- 2dB. Chart 2 illustrates how total harmonic distortion plus noise versus power varies for a 1kHz and SMPTE IM test signals and amplifier output load. As can be seen, attainable power is greater for the 4-ohm load, as is usual for most power amplifiers. Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. Of interest, all powers levels below clipping seem to have the same distortion at 20kHz. Damping factor versus frequency is shown in Chart 4. A spectrum of the harmonic distortion and noise residue is plotted in Chart 5. Both the AC line and signal frequency have a rich series of harmonics.

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

(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

4-ohm output loading
Red line: 1W (overlaps with magenta)
Magenta line: 10W (overlaps with red)
Blue line: 30W
Cyan line: 50W

Damping factor = output impedance divided into 8

1kHz signal at 10W into a 4-ohm load

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