Rogue Audio DragoN monoblock power amplifier Measurements

Sidebar 3: Measurements

I performed a complete set of measurements on one of the Rogue DragoNs (serial number DRAM-0551) with my Audio Precision SYS2722 system.

As the DragoN has a class-D output stage, I inserted an Audio Precision AUX-0025 passive low-pass filter between the test load and the analyzer. This filter mitigates noise above 80kHz and eliminates noise above 200kHz that would otherwise drive the SYS2722's input circuitry into slew-rate limiting. Without the filter, there was 408mV of ultrasonic noise with a center frequency of 416kHz at the amplifier's output terminals. I used the Audio Precision filter for all the tests other than frequency response. The Rogue remained cool during the testing; the temperature of the top panel after three hours was 80.4°F (26.9°C).

The Rogue amplifier's single-ended and balanced inputs preserved absolute polarity, ie, the XLR jack is wired with pin 2 positive. The DragoN's balanced input impedance was a very high 178k ohms at 20Hz and 1kHz, dropping inconsequentially to 157k ohms at 20kHz. The single-ended input impedance was lower but still high at 29k ohms at 20Hz and 1kHz, 24k ohms at 20kHz.


Fig.1 Rogue DragoN, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (blue), 4 ohms (magenta), and 2 ohms (red) (1dB/vertical div.).


Fig.2 Rogue DragoN, small-signal 10kHz squarewave into 8 ohms.

While the single-ended input's voltage gain at 1kHz was 30.35dB, the balanced input's gain was 6dB lower rather than the usual 6dB higher. The output impedance, including the series impedance of 6' of spaced-pair cable, was very low, at 0.08 ohms at 20Hz and 1kHz, and 0.095 ohms at 20kHz. As a result, the variation in the frequency response with our standard simulated loudspeaker (fig.1, gray trace) was minimal. Even without the auxiliary Audio Precision low-pass filter, the response into resistive loads rolled off in the top octave, lying at –1.5dB at 20kHz and at –6dB by 65kHz. Into 2 ohms (red trace), there was a hint of an ultrasonic resonance at 60kHz. The ultrasonic rolloff meant that the Rogue's reproduction of a 10kHz squarewave into 8 ohms featured lengthened risetimes (fig.2), but there was no overshoot or ringing. Repeating this waveform capture without the AUX-0025 filter didn't change the basic shape of the trace, though it was now overlaid with ultrasonic noise.


Fig.3 Rogue DragoN, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).

The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms), taken with the single-ended input shorted to ground and the auxiliary filter in-circuit, was a good 71.1dB. This ratio improved to 77.7dB when the measurement bandwidth was restricted to the audioband, and to 86.1dB when A-weighted. Spectral analysis of the low-frequency noisefloor while the Rogue drove a 1kHz tone at 1W into 8 ohms (fig.3) revealed a higher-than-usual level of random noise, and spuriae at 60Hz and its odd-order harmonics. These will be due to magnetic interference from the power transformer, most likely being picked up by the steel pins of the ECC802S input tube.


Fig.4 Rogue DragoN, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.5 Rogue DragoN, distortion (%) vs 1kHz continuous output power into 4 ohms.

Rogue specifies the DragoN's maximum power as 325W into 8 ohms (25.1dBW) and 525W into 4 ohms (24.2dBW). We define an amplifier's clipping power as being when the THD+noise reaches 1%. With that criterion, the DragoN didn't meet its specified powers, clipping at 127W into 8 ohms (21dBW, fig.4) and 226W into 4 ohms (20.5dBW, fig.5). Only if I relaxed the definition of clipping to 3% THD+N did the amplifier exceed the specified powers, delivering 345W into 8 ohms (25.4dBW) and 590W into 4 ohms (24.7dBW).


Fig.6 Rogue DragoN, THD+N (%) vs frequency at 12.67V into: 8 ohms (blue), 4 ohms (magenta), and 2 ohms (red).

The shape of the traces in figs.4 and 5 indicates that while actual distortion lies below 0.1% below 1W into 8 ohms and 2W into 4 ohms, it rises in a linear manner as the power increases. This suggests that there is only a limited amount of corrective negative feedback. However, when I examined how the THD+N percentage varied with frequency at 12.67V, which is equivalent to 20W into 8 ohms, 40W into 4 ohms, and 80W into 2 ohms, it was high across the band but didn't increase into lower impedances (fig.6). The Hypex NCore class-D output modules have very low distortion (footnote 1), so the distortion shown in this graph must therefore be introduced by the tubed input stage.


Fig.7 Rogue DragoN, 1kHz waveform at 50W into 8 ohms, 0.66% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.8 Rogue DragoN, spectrum of 50Hz sinewave, DC–1kHz, at 20W into 8 ohms (linear frequency scale).


Fig.9 Rogue DragoN, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 4 ohms (linear frequency scale).

Fortunately, the distortion waveform was predominantly the subjectively innocuous second harmonic (fig.7), with higher-order harmonics more than 30dB lower in level (fig.8). This graph was taken into 8 ohms; the spectrum into 4 ohms was identical. When tested with an equal mix of 19 and 20kHz tones at 100W into 4 ohms, the second-order difference product at 1kHz lay at a high –50dB (0.3%, fig.9). The level of this product didn't change significantly into higher impedances or at lower powers, which again suggests that the nonlinearity stems from the tubed input stage.

The Rogue DragoN's measured performance is dominated by the use of a tube in the input stage. This amplifier offers high power but with high levels of second-harmonic distortion.—John Atkinson


Footnote 1: See, for example, figs.19 and 20 here.

COMPANY INFO
Rogue Audio, Inc.
545 Jenna Dr.
Brodheadsville
PA 18322
info@rogueaudio.com
(570) 992-9901
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COMMENTS
georgehifi's picture

First off I like Rogue and had way back a pair of M120/180's
I have two mono blocks with Hypex NC500 and massive linear supply, they sound great into the right speakers (benign 4ohm load), but to put a tube stage in front to add all that 2nd harmonic distortion I just can't see why. Unless to make something in the system that sounds hard/harsh soft???
This observation says it all in a polite way. "The presentation was thicker and lusher than I'd previously heard"

And as I think it has a opamp buffers after the tubes in the stereo to drive the low 2kohm input impedance NC500 with https://shorturl.at/drwEP
Monoblock https://ibb.co/gZPb3gC
Cheers George

rschryer's picture

It's a pleasure to meet you. Welcome aboard.

13stoploss's picture

Thank you!

avanti1960's picture

Magnepan LRS+. The sound was natural, ultra smooth and refined, powerful and with life like vocals. Excellent amp to speaker synergy. Memorable system.

Glotz's picture

Thanks for input. Helpful to get what others heard for themselves. I may look to this amp for my 1.7i's.

hiendmmoe's picture

I heard these amps and was more impressed than this reviewer seems to be.
They touch me in away some much more expensive amplifiers haven’t been able too.

MikeSTL's picture

“Black No. 1 (Little Miss Scare-All)” from Bloody Kisses Is an amazing recording. It should serve as a reminder that contemporary music of all types can be well recorded. This song definitely does not fit into my preferred listening music, but I alway happy to get new track recommendations. Too often reviewers here are testing equipment on tracks that were recorded sixty or more years ago. Let it go! I wish other reviewers were as willing to find new standards from the last decade or two.

Killing For Company's picture

I have to say- I VERY much appreciated reading a hi-fi review that covered music that I will actually play through my system. Type O, Portishead, Agalloch - THANK YOU. At best I usually am left trying to interpret or translate when reading hi-fi reviews. This was a breath of fresh air as a younger and aspiring audiophile who first started as a physical media collector.

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