February 2006

Rogue Audio Atlas 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 at 120V AC line voltage with both channels being driven.
• Gain: 20.8x, 26.4dB.
• Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.800mV, -71.0dBW; A weighted 0.418mV, -76.6dBW.
• AC line current draw at idle: 1.82A.
• Output impedance at 50Hz: 0.73 ohms.
• This amplifier does not invert polarity.

Power output with 1kHz test signal

• 8-ohm load at 1% THD: 78W
• 8-ohm load at 10% THD: 89W
  
  
• 4-ohm load at 1% THD: 29W
• 4-ohm load at 10% THD: 75W

General

The Rogue Audio Atlas is a medium-power push-pull stereo tube amplifier utilizing one pair of KT77 output tubes in each channel. I don’t recall seeing KT77 tubes before, but their plate structure looks like those of EL34 output tubes.

Chart 1 shows the frequency response of the amp with varying loads. As can be seen, the out-of-band high-frequency response has some complex peaking going on. The output impedance over most of the audio range, as judged by the closeness of spacing between the curves of open circuit, 8-ohm, and 4-ohm loading, is lower than typical for tube power amplifiers. The variation with the NHT dummy load in the audio range is about +/-0.5dB.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. This design, with its single output connection for speaker loads, is more optimized for 8 ohms than 4 ohms. As can be seen, the power attainable is greater for 8-ohm loading for a given distortion amount. For lower-impedance speakers, the amp has 4-ohm taps that can be connected to the hot output terminals instead of the default 8-ohm taps.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. Amount of rise in distortion at low and high frequencies is quite pronounced, but not atypical for many tube power amps.

Damping factor vs. frequency is shown in Chart 4. It rolls off at low and high frequencies quite a bit more than for other measured tube amplifiers. This can be inferred from Chart 1 where the spacing between the curves increases for 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 intermodulation components of line harmonics with signal harmonics are also reasonably low but visible. The principal signal harmonics are second and third, with the remaining harmonics more than 20dB below the level of the second and third harmonic.

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

(line up at 20W 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

8-ohm output loading
Cyan line: 60W
Blue line: 30W
Magenta line: 10W
Red line: 1W

Damping factor = output impedance divided into 8

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

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