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
- Test signal applied to unbalanced inputs unless otherwise
- Gain, unbalanced input/balanced input: 34.9x, 30.9dB/17.2x,
- 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
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.
- Frequency Response of Output Voltage as a Function of Output Loading
Magenta line: open circuit
Red line: 8-ohm load
Blue line: 4-ohm load
Green 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
4-ohm output loading
Red line: 1W (overlaps with magenta)
Magenta line: 10W (overlaps with red)
Blue line: 30W
Cyan line: 50W
|Chart 4 - Damping Factor
as a Function of Frequency
Damping factor = output impedance divided into 8
|Chart 5 - Distortion and
1kHz signal at 10W into a 4-ohm load