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
The Rogue Audio Atlas is a medium-power push-pull stereo
tube amplifier utilizing one pair of KT77 output tubes in each channel. I dont
recall seeing KT77 tubes before, but their plate structure looks like those of EL34 output
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
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