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All amplifier measurements are performed independently by BHK Labs. All measurement data and graphical information displayed below are the property of the SoundStage! Network and Schneider Publishing Inc. Reproduction in any format is not permitted.

Notes: Measurements were made at 120V AC line voltage with both channels being driven. Measurements were made on the left channel through the balanced input unless otherwise noted. The Audio Precision AUX-0025 measurement filter was used unless otherwise noted.

Power output

  • Power output at 1% THD+N: 187.5W @ 8 ohms, 348.8W @ 4 ohms
  • Power output at 10% THD+N: 272.7W @ 8 ohms, 448.8W @ 4 ohms

Additional data

  • This amplifier does not invert polarity.
  • AC-line current draw at idle: 0.14A, 0.63PF, 10.0W
  • AC-line current draw in operation: 0.59A, 0.67PF, 48.8W
  • Gain: output voltage divided by input voltage, 8-ohm load
    •      Unbalanced inputs: 39.5X, 31.9dB
    •      Balanced inputs: 39.5X, 31.9dB
  • Input sensitivity for 1W output into 8 ohms
    •      Unbalanced inputs: 71.6mV
    •      Balanced inputs: 71.6mV
  • Output impedance @ 50Hz: 0.018 ohm
  • Input impedance @ 1kHz
    •      Unbalanced inputs: 135k ohms
    •      Balanced inputs: 135k ohms
  • Output noise, 8-ohm load, balanced inputs terminated with 600 ohms and unbalanced inputs terminated with 1k ohms, without AUX-0025 filter, Lch/Rch
    •      Wideband: 471mV/477mV, -15.6dBW/-15.5dBW
  • Output noise, 8-ohm load, balanced inputs terminated with 600 ohms and unbalanced inputs terminated with 1k ohms, with AUX-0025 filter, Lch/Rch
    •      Wideband: 0.87mV/0.91mV, -70.2dBW/-69.9dBW
    •      A weighted: 0.29mV/0.34mV, -79.8dBW/-78.4dBW

Measurements summary

The Rogue Audio Medusa is a high-power hybrid stereo power amplifier with a vacuum-tubed front end coupled to a pair of Hypex switching-amplifier modules.

Chart 1 shows the Medusa’s frequency response with varying loads. The amp’s high-frequency response is rolled off before the cutoff frequency of the Audio Precision AUX-0025 measuring filter used for measuring switching amplifiers. As a result, the high-frequency response remained virtually unchanged, regardless of whether or not it was sent through the filter. The -3dB point is about 26kHz. Further, the -3dB down point at 26kHz is independent of load -- a feature of the Hypex modules.

As can be seen, the Medusa’s output impedance is so low that the variations with the NHT dummy speaker load don’t show up on the response plot.

Chart 2 illustrates how the Medusa’s total harmonic distortion plus noise (THD+N) vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load for loads of 8 and 4 ohms. The amount of distortion, and how it rises with output level, are similar to those of tube power amplifiers, and are no doubt caused by the Medusa’s tubed front end.

THD+N as a function of frequency at several different power levels is plotted in Chart 3. The degree of increase in distortion at high frequencies is admirably low.

Damping factor vs. frequency, shown in Chart 4, is of a value and nature typical of many solid-state amplifiers: high up to about 1kHz, then rolling off with increasing frequency.

Chart 5 plots the spectrum of the harmonic distortion and noise residue of a 10W, 1kHz test signal. The magnitude of the AC line harmonics is relatively complex, with 120Hz being the dominant one. Not surprisingly, the dominant signal harmonic is the second, coming from the tube circuitry. All higher harmonics quickly disappear into the noise level.

Chart 1 - Frequency response of output voltage as a function of output loading

Chart 1

Red line = open circuit
Magenta line = 8-ohm load
Blue line = 4-ohm load

Chart 2 - Distortion as a function of power output and output loading

Chart 2 

(Line up at 100W to determine lines)
Top line = 8-ohm SMPTE IM distortion
Second line = 4-ohm SMPTE IM distortion
Third line = 8-ohm THD+N
Bottom line = 4-ohm THD+N

Chart 3 - Distortion as a function of power output and frequency

Chart 3

(8-ohm loading)
Red line = 1W
Magenta line = 10W
Blue line = 70W
Cyan line = 200W

Chart 4 - Damping factor as a function of frequency

Chart 4

Damping factor = output impedance divided into 8

Chart 5 - Distortion and noise spectrum

Chart 5

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

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