Solid State Power Amp Reviews

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.

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.

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.

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).

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.

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