December 2005

Rogue Audio Tempest II Integrated Amplifier: Measurements

All integrated-amplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test integrated 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 at 120V AC line voltage, with volume control fully up unless otherwise noted.
• Gain
  • Ultralinear: 19.7X, 25.9 dB.
  • Triode: 19.0X, 25.6 dB.
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination
  • Ultralinear, volume control fully up (worst case): wideband 2.75mV, -60.2dBW; A weighted 0.68mV, -72.4dBW.
  • Ultralinear, volume control fully down (CCW): wideband 1.08mV, -68.4dBW; A weighted 0.29mV, -79.8dBW.
  • Triode, volume control fully up (worst case): wideband 2.57mV, -60.8dBW; A weighted 0.65 mV, -72.8 dBW.
  • Triode, volume control fully down (CCW): wideband 1.02mV, -68.9dBW; A weighted 0.27 mV, -80.4 dBW.
• AC line current draw at idle: 2.03A.
• Output impedance at 50Hz
  • Ultralinear: 0.95 ohms.
  • Triode: 0.92 ohms.
• This integrated amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD (ultralinear): 69W
• 8-ohm load at 1% THD (triode): 41W
• 8-ohm load at 10% THD (ultralinear): 74W
• 8-ohm load at 10% THD (triode): 47W
  
  
• 4-ohm load at 1% THD (ultralinear): 18W
• 4-ohm load at 1% THD (triode): 24W
• 4-ohm load at 10% THD (ultralinear): 79W
• 4-ohm load at 10% THD (triode): 49W

General

The Rogue Audio Tempest II is a medium-power stereo push-pull tube integrated amplifier utilizing one pair of Electro Harmonix KT88EH output tubes in each channel. This amp has the usual passive-selector-switch/balance/volume-control arrangement ahead of the power amplifier proper. Additionally, it has jumpers that can be changed to access directly the power amplifier input, bypassing the passive front end for external preamp use.

Charts 1A and 1B shows the frequency response of the amp with varying loads for both ultralinear and triode modes. The output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading over most of the audio range, is somewhat lower than typical for tube power amplifiers. The variation with the NHT dummy load in the audio range is about +/-0.6dB. The ultralinear mode has a bit more high-frequency bandwidth than the triode mode.

Charts 2A and B illustrate how total harmonic distortion plus noise versus power varies for 1kHz and SMPTE IM test signals and amplifier output load for the ultralinear and triode modes. Unusually, this design, with its single output connection for speaker loads, produces about the same power at the point of clipping for both 4- and 8-ohm loading, albeit at higher distortion prior to clipping. The amplifier has a 4-ohm output winding that can be connected to the hot output terminal instead of the 8-ohm tap if desired for lower-impedance speaker loads. This behavior would suggest equal power into 4- or 2-ohm loads with the 4-ohm windings connected to the output terminals.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3 for the ultralinear mode. Appearance for the triode mode was virtually the same except for the two higher powers that were 20W and 35W. Amount of rise in distortion at low and high frequencies is quite pronounced, but not atypical for many tube power amps. Excessive distortion at higher powers and high frequencies prevented a run at rated power both in ultralinear and triode modes.

Damping factor versus frequency is shown in Chart 4 and rolls off at low and high frequencies quite a bit more than other measured tube power amplifiers. This can be inferred in Chart 1 from the increased spacing between the curves at low and high frequencies.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. AC-line harmonics are quite numerous but reasonably low in magnitude, and intermediation components of line harmonics with signal harmonics are also reasonably low but visible. The decay of signal harmonics with frequency is nice and uniform.

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